Lighting Dan Heat

INDUSTRIAL HYGIENE

ENGINEERING

Recognition, Measurement, Evaluationand Control

Second Edition

Edited by

John T. Talty, P.E.

National Institute for Occupational Safety and HealthCincinnati, Ohio

Reprint Edition

NOYES DATA CORPORATIONPark Ridge, New Jersey, U.S.A.

Copyright ©1988 by Noyes Data CorporationLibrary of Congress Catalog Card Number 88-17863ISBN: 0-8155-1175-2Printed in the United States

Published in the United States of America byNoyes Data CorporationMill Road, Park Ridge, New Jersey 07656

1098

Library of Congress Cataloging-in-Publication Data

Industrial hygiene engineering: recognition, measurement, evaluation,and control! edited by John T. Talty. -- 2nd ed.

p. cm.Includes bibliographies and index.ISBN 0-8155-1175·2 :1. Industrial buildings-- Environmental engineering. 2. Sanitary

engineering. 3. Industrial hygiene. I. Talty, John T.TH60S7.153153 1988628.S"··dc19 88-17863

CIP

SECTION 3 THERMAL STRESS

1. Heat Exchange and Its Effects on Man

Heat Exchange

Heat is a form of energy, whi Ie cold is the absence of heat or the absenceof this energy. Quite often there exists a confusion between the terms "heat"and "temperature." Heat itself is a measure of the energy in terms ofquantity. Temperature on the other hand, is a measure of the intensity of theheat or the hotness of an object.

A review of two examples wi I I help to clarify the di fference between heatand temperature. Consider a large block of iron and a sma I I block of ironthat are being heated with the same amount of energy. If the source of energyis removed after a given amount of time, the temperature of the sma I ler blockwi I I be higher than that of the larger block. The same amount of heat hasbeen transferred to the blocks of iron, but the temperature readings differ.On the other hand, suppose a sma I I quantity of water and a large quantity ofwater are both brought to the boi ling point. The temperature of bothquantities of water wi I I be the same; i.e., 100°C or 212°F. However, it wi I Itake a longer period of time to bring the large quantity of water to theboi ling point; thus, the amount of heat required wi I I be greater. In thiscase, the temperature of each body of water is the same, but the amount ofheat required to raise the body to this temperature is different.

Temperature is measured in terms of degrees. This measurement may be indegrees Fahrenheit, degrees Celsius, degrees Kelvin, or degrees Rankine.Conversion from one temperature scale to another can be accompl ished using thefo~lowing formulas:

(3.1.1)(3.1.2)(3.1.3)(3.1.4)

OF 9/5°C + 32°C 5/9(OF - 32)OK °C + 273OR = OF + 460

Note: The actual values for the constants in (3.1.3) and (3.1.4) are273.16 and 459.69, respectively. However, the rounded values areaccurate enough for most work.

The measurement of heat energy is in terms of either calories or BritishThermal Units (BTU). The calorie is defined as the quantity of heat necessaryto raise the temperature of 1 gram of water 1°C. Since there is some variancebetween quantities of heat required depending on the beginning and endingtemperature, the calorie is based on a standard temperature of 16.5°C to17.5°C. The BTU is the quantity of heat necessary to raise the temperature of1 pound of water 1°F.

326

Thermal Stress 327

Not all materials require the same amount of heat to raise theirtemperature one degree Celsius or Fahrenheit. The concept of speci fic heathas been conceived to handle this fact. The specific heat of a material isthe quantity of heat that is necessary to raise 1 gram or 1 pound of thesubstance 1°C or 1°F. Again. this figure is based on a standard of 16.5°C to17.5°C. The heat capacity is related to the specific heat in that it is thequantity of heat necessary to raise the temperature of a given material 1°.Heat capacity is stated as the amount of heat necessary to raise a unit massof a substance 1 degree (OC or OF resulting in calories or BTU's).

As heat is added or removed from a substance. there is a point when thesubstance wi I I undergo a change in phase. That is. the substance. if a sol id.wi I I become a liquid or, if a I iquid. wi 1I become either a sol id or a gas.For example, as heat is added to water. it reaches a point cal led the boi lingpoint where the water begins to change its phase to a vapor. On the otherhand. as heat is removed from water, it reaches a point where it freezes andbecomes a sol id. The change of phase does not occur instantaneously. Aquantity of heat is required to cause this change in phase. As this phasechange is occurring. the temperature does not vary, and the heat appears to belost in the substance. Two concepts have been introduced to quanti fy thisoccurrence. The heat of vaporization is the quantity of heat that is requi redto vaporize one unit mass of a I iquid without changing its temperature. Theheat of fusion is the quantity of heat necessary to melt one unit mass of asol id without changing its temperature. The concept of enthalpy or storedenergy is somewhat simi lar to that of heat absorbed during a change of phase.

Figure 3.1.1

Change of phase.

TEMP

1()()OWATER.srJ

/

E:; OF VAPORIZATION

ICE·WATER

HEAT OF FUSION

HEAT ADDED

328 Industrial Hygiene Engineering

Since heat is energy. a hot body has more energy than a cold body, giventhe same mass and material for both bodies. The flow of energy is from thehighest to the lowest level. Thus, a hot body gives off heat or energy to acolder body. A process in which heat is given off is termed an exothermicprocess. The converse, or a process in which heat is absorbed, is cal led anendothermic process.

Methods of Heat Exchange. There are three basic methods for the transferor exchange of heat between materials. The first method of exchange is byconduction. In conduction heat passes from one part of the body to another.If two bodies are in di rect contact, the heat wi I I pass from one body di rectlyto the other as if the two bodies were one single body. Conduction takesplace only if a difference in temperature exists between the two bodies orparts of a single body. The conduction of heat is different for di fferentmaterials. Metals usually conduct heat wei I; solids are generally betterconductors of heat than liquids; and gases are the poorest conductors. Forthe most part, conduction is of I ittle importance when considering problemsinvolving hot environments and heat stress situations since the worker must bein contact with the surface for conduction to take place.

The second method of heat transfer is convection. Convection is a processwhere the transfer of heat occurs as a result of the movement of a fluid pasta source of heat. The rate of convection is affected by the characteristicsof the fluid that is moving past the source of heat, the surface of the heatsource, the position of the source surface, the velocity of the fluid, and therelative temperature of the source and fluid. In most situations, the fluidis air; and the heat is transferred to the surrounding envi ronment by inducedair current movement. Convection itself causes the movement of the fluid. Asthe air is heated, it expands and becomes I ighter. The I ighter air rises awayfrom the hot source, and colder air flows in to replace the heated ai r. Theheated air mixes with the environment to cause a general increase intemperature. If the ai r and source are the same temperature. no movement wi I Ibe induced. The transfer of heat can be increased by increasing the flow inthe fluid using mechanical means. Convection is of major signi ficance as amethod of heat transfer in a hot environment. Thus, the industrial hygieneengineer must be very concerned with convective heat transfer.

The third method of heat transfer is radiation. Radiation di ffers fromconvection and conduction in that no fluid or solid need be present for theheat to be transferred from one object to another. The heat energy istransferred from a hot body to its surroundings in the form of electromagneticwaves or infrared radiation. An example of radiant heat is the thermal energythat is transferred to the earth from the sun. Generally, the wavelength ofradiant heat is not visible; however, as an object becomes hotter. the wavelength shortens and enters the visible spectrum. When an object is termed"red hot," th is means that it is hot enough to emanate rad iat ion in the redspectrum which is approximately 700 DC. The color of I ight from the hot objectindicates its approximate temperature. Thus. white hot is an extremely hightemperature·(approximately 1200DC).

Figure 3.1.2

Conduc t ion.

HOT

COLD

Thermal Stress 329

Figure 3.1.3

Convec t ion.

~A:"RL::JV

Figure 3.1.4

Rad iat ion.

~ -HOT ............... - HEAT

SOURCE

~ -Radiant heat may be either reflected or absorbed. Highly pol ished

surfaces. such as aluminum, are generally good reflectors. On the other hand,black bodies are good absorbers. A common example of this phenomenon can beseen by the fact that dirty snow, which is darker, melts faster than clean,white snow which reflects more of the radiant energy.

The rate of heat exchange by radiation depends on a number of factors.The difference in absolute temperature of the surfaces of the body and itssurroundings affects the rate of heat exchange by radiation. If a body andits surroundings are the same temperature, then no radiant heat energy wi I I betransferred from the body to its surroundings. A second factor is therelative emissivity of the body and its surroundings. Emissivity is the ratioof the energy radiated by a given surface and that which would be radiated bya perfect black body at the same surface temperature. This leads to a generalstatement of Kirchoff's law of radiation. This law states essentially that abody is as effective as a radiator as it is as an absorber. Thus. a poorabsorber is likewise a poor radiator; a good absorber is a good radiator; anda poor absorber is a good reflector. This information is of sign; ficant valuewhen determining control methods for radiant heat stress.


Sources of Heat

Two major sources of heat that are of concern are the environment itselfand metabolic heat generated by the workers within the environment. In orderto provide adequate control of heat within the workplace. it is necessary toidentify the heat sources.

Often the cl imate and solar radiation generate signi ficant amounts of heatwithin a workplace. Workers who are required to perform their job functionsout of doors in hot cl imates are subjected to heat and humidity present in theair as wei I as solar radiation. These factors are also present wi thin plantstructures themselves. Hot outside ai r entering a hot plant wi I I provide norei ief from the heat within the plant. Also, solar radiation absorbed by theplant roof can add an additional heat load within the industrial bui Iding.

In many cases, the industrial process adds signi ficant heat to theworkers' environment. The air temperature may be increased as convectionalcurrents pass by hot processes. Radiation emanating from high-temperatureprocesses can provide an addi tional heat load. Steam that is used in manyprocesses adds not only heat but also humidity to the air. Mechanical andelectrical equipment can generate large quantities of heat in their no~mal

operation. Finally. normal plant faci I ities, such as illumination and steamdistribution piping, can also be a signi ficant factor in increasing theoveral I heat load in the envi ronment.

A second source of heat that is of concern is metabol ic heat. Heat is anormal by-product of the body's activity. As the eel Is work to perform theirfunctions, heat is generated. This heat is generally termed basal heat. Ifthe individual is involved in physical work, additional heat is generated as aby-product of the muscular activity. The basal heat and work heat must bedissipated into the atmosphere. or they can present a hazard to the worker.If the environmental conditions in the workplace do not provide appropriaterei ief to the worker for this metabol icand work heat, an accumulation of heat wi I I occur in the worker's body. Suchan accumulation of heat can result in various physiological reactions that canbe harmful to the worker's health. The resulting heat-induced illnesses wi I Ibe further discussed later in this chapter.

Physiological Responses to Extreme Temperatures

The hypothalamus, located in the base of the brain, is the regulatorycenter that controls the response of the human system to heat. Thehypothalamus acts to attempt to maintain a thermal balance within the body.This balance is maintained with a deep body or core temperature ofapproximately 37°C (98.6°F). The skin temperature normally varies between33°C and 34°C (91.4°-93.2°F), but it may be near the core temperature or be10° lower than the core temperature if the individual is exposed to extremetemperature ranges. The oral temperature that is fami I iar to al I ranges from36°C to 37°C (97° to 98.6°F).

As the body begins to bui Id up heat. the hypothalamus initiates certainphysiological reactions. The heart rate increases. and the blood vessels

Thermal Stress 331

di late to increase circulation in the body. This increased blood flow carriesheat away from the inner core of the body to the skin surface where heat isdissipated into the surrounding environment. Increased respiratory activityalso occurs. In this manner, exhaled ai r carries heat away from the body.Along with this increased circulation and respiration, the body begins tosweat. The heat of vaporization, that is, the heat necessary to cause aI iquid to vaporize, requires a significant expenditure of heat energy. As thebody sweats and the sweat is evaporated into the atmosphere, the vaporizationrequires significant amounts of heat. In this manner, heat generated withinthe body is dissipated without raising the body temperature.

On the other hand, the absence of heat, or cold, can also present aproblem. In general, man's tolerance to cold is less than his tolerance toheat. Clothing makes up for this lack of tolerance and al lows working intemperatures that are far below that which a nude human would be able totolerate. The general physiological response that occurs 'when a human isexposed to extreme cold is shivering. This shivering creates muscle activityand results in the generation of heat. Shivering is the method that the bodyuses to generate heat to maintain its core temperature in equi I ibrium. Whenthe body is exposed to cold. the blood vessels contract to restrict the flowof blood to the surface, thus conserving heat within the core of the body.Humans wi I I also attempt to remain active during exposure to cold, though theymay not be aware of it. thus generating additional heat to maintain thermalequi librium. The lethal lower core temperature for the human body is 26°C(78°F) .

Stress and Strain

Stress is the acting force on the body. Thermal stress is either thepresence of excess heat or the absence of sufficient heat. Stress may bethought of as the cause of a given human response.

Strain, on the other hand, is the result of stress. Strain may be thoughtof as the cost or consequence in the human body of a given stress being placedupon it. When the response of the human system is abnormal. this is a resultof some strain that has been experienced. Strain can be measured in terms ofthe physiological response, in terms of heart rate, respiratory rate, etc.; orit may be indicated from the disorders that arise.

Indicators of Thermal Strain

As was discussed above, the objective of the physiological response of thehuman system is to maintain the core temperature in equi I ibrium. If the coretemperature drops below 26°C or rises above 41°C (106°F), damage and potentialdeath will occur. In particular, as the temperature rises above 41°C, theregulating abi lity of the hypothalamus is depressed. Thus. the body's abi I ityto regulate its temperature is depressed, resulting in a vicious cycle inwhich the core temperature continues to rise. Only external action, such asan alcohol bath or immersion in cold water, wi I I prevent death to such anexposed individual.


Figure 3.1.5

Stress vs. strain.

STRESS

-----..~,...----STRAIN

A chain of events is initiated when the human system is exposed to heatstress. The first response of the body is for the regulatory functions tobegin to work. If these functions can work properly, and if excessive heatstress is not present, the body wi I I attain thermal equi librium. On the otherhand, if the heat stress is such that the regulatory functions cannot controlthe bui Idup of heat, the result is a heat disorder.

There are generally four categories of heat disorders that are ofinterest. The first disorder that is recognized by many medical personnel istermed heat syncope. Because of excessive pooling of the blood in theextremities resulting from the body's attempt to dissipate heat by increasedcirculatory activity and di lation of the blood vessels, the brain does notreceive an adequate supply of oxygen. The result is that the exposedindividual loses consciousness. This reaction is simi lar to heat exhaustionexcept that it is I ikely to occur much more quickly without any accompanyingphysical exertion on the part of the worker. Heat syncope is directly relatedto the circulatory response of the affected individuals.

A second disorder is heat exhaustion. As the worker performs physicaltasks in the hot environment, profuse sweating occurs, and the circulatory andrespiratory activity is increased. If the worker sustains the physicalactivity for an extended time period, the body wi I I become dehydrated and/orthe circulatory system wi II become overworked. Then the worker wi I Iexperience fatigue, nausea, headache, and giddiness. The skin wi I I be moistand clammy, indicating that sweating is sti I I present, but the circulatorysystem may cause a pool ing of blood that leads to fainting. The skin mayappear either pale or flushed.

The third disorder that may occur is heat cramps. Heat cramps are aresult of profuse sweating that dissipates body salt along with the loss offluids. The general sign of such a disorder is a painful muscle cramp spasm.Heat cramps are generally caused by sweating and hard work without adequatefluid and salt replacement.

Finally, the most severe heat disorder is heat stroke. Heat stroke is afai lure of the body's thermal regulatory system. Unless control ledimmediately, heat stroke can result in an increased body temperature beyondwhich eel I damage occurs. Death is likely unless external action is taken tocontrol the rising temperature. Heat stroke is evidenced by hot, dry, red

Thermal Stress 333

skin, rapid pulse, and an absence of sweating. Figure 3.1.6 illustrates theprogression of the human physiological response to heat stress.

In terms of treatment, heat cramps are generally easi Iy prevented byproviding adequate salt for the worker. This may be either in the form ofsalt tablets or in a one-tenth percent (0.1%) salt and water solution that theworker should ingest frequently. For heat syncope, the worker can beaccl imatized (to be discussed later in this chapter) or encouraged to remainsomewhat active to stimulate return circulation to the heart. If a worker issuffering from heat exhaustion, it is adequate to remove the worker from thesource of heat and provide fluid and salt replacement along with adequate restto al low the body to recover. Provision of salt should be done with care,since an excess of salt can be harmful to individuals suffering fromcardiovascular disorders.

Heat stroke requires immediate positive action. The worker should beremoved from the heat and action taken to cool the body, either through coldcompresses, immersion in cold water, or an alcohol bath. Fluids should bereplaced as soon as possible, since one of the initial causes of heat strokeis the dehydration of the body.

F igu re 3.'.6

Effects of heat stress.

..... Loss of water -

..... Fatigue of sweat glands

- Prickly heat

Sweating _ EvapOrative heat loss

- Loss of salt _

Increasedheat lossby convec:tlonand rad'atlon

-HEAT CRAMP

DEHYDRATIONl

_ Reduced sweating

l

Increased- heat flow

from coreto surface

Inadequatesupply to - HEAT EXHAUSTIONvital areas

;'

Augmentedcirculation-

Reducedheat gain

;'

Body andsIIln temperaturerises

/

Heat stress -

•

Heat gaon mayexceed heat loss,f heat loadis excessive

Inadequate evapOrativehe81loss

lFunher roseIn body temperature

lDisturbances In temperatureregu latlon and other vllalfunctions of the body

lHEAT STROKE


Factors in Heat Stress

For a given temperature, the thermal stress that is placed on anindividual varies. There are a number of factors, both environmental andindividual. that cause this variance.

The environmental factors that affect stress are the movement of air, themoisture content of the air, and the radiant heat load. Air movement isimportant in reducing heat stress. As the air moves past the worker, itcarries away vapor from the evaporated sweat on the worker's skin, thuscool jng the body. Without such air movement, the body would become insulatedby the surface sweat, and sufficient cool ing could not take place.

The moisture content of the air, or humidity, is also important. If thevapor pressure in the environment is high, then sweat does not evaporate. Asa result, the cool ing that normal ty occurs during the evaporation of sweatdoes not take place, and heat bui Ids up in the body.

The radiant heat load is in addition to that which results from convectionand metabolic heat. This heat load can be high enough to produce an excessivethermal stress on the individual even in cases where adequate air movement ispresent to al low for evaporative cooling.

The thermal stress that occurs in workers varies between given workers asa result of certain individual factors. The surface to weight ratio for aworker is of importance since heat loss is a function of area, and heatproduction is a function of weight. Because of this, obese or stockyindividuals are affected by heat more than slender individuals, since theyproduce a greater amount of heat and have a proportionally sma I ler area inwhich the heat can be dissipated.

~ is another factor that can affect the physiological response to heatstress. Workers in the range of 40 to 65 years of age are not so tolerant ofheat as are younger workers. This may be somewhat as a result of the factthat young workers have a better respiratory and circulatory response system.

Workers who have a history of cardiovascularsubjected to strain resulting from heat stress.reduced capacity of the cardiovascular system tothe body.

disease are especiallyThis is a result of thereact to dissipate heat from

The physical fitness of the worker is also important. Through improvedconditioning. the individual develops an increased cardiovascular responseand, in addition, an increased efficiency of muscle use. This increasedmuscular efficiency results in lower heat generation when performing aparticular task.

The alcohol ic habits of the worker wi I I also affect his tolerance toheat. Alcohol dehydrates the body, and as a result, dehydration can occurmore quickly when the worker is subjected to heat stress.

Accl imatization. Experiments have shown that individuals respond to heatstress with strain at the first exposure. If these individuals are exposed

Thermal Stress 335

regularly to heat stress, the amount of strain is reduced. After one to twoweeks of exposure to above normal temperatures, no strain is present. Theseexperiments have led to the practice of accl imatizing workers. Duringacclimatization, the worker is gradually exposed to longer periods of heatstress unti I the possibi I ity of heat strain is minimized. In general,acclimatized workers exhibit an increased sweat rate (thus, more efficientcooling) with a lower salt loss than those workers who are not accl imatized.

However, accl imatization is lost quickly. In fact. experiments have shownthat there is some loss of accl imatization after a weekend away from work.After two weeks away from work, this loss is substantial. Therefore. in orderfor accl imatization to be effective in reducing heat strain, it must bereinforced regularly.

On the other hand, humans do not generally become accl imatized to coldtemperatures. There is no significant differences between the tolerance tocold of Eskimos in Alaska and that of native southern Americans.

Other Effects of Heat Stress. Aside from the physiological effects ofheat stress. there are other effects that may result. Psychologically, theindividual exposed to heat stress becomes edgy and develops a lassitude towardaccomplishing a given task. The performance efficiency of these individualsis lowered. resulting in a potential for increased accidents.

Excessive heat stress can also have an effect on the morale of theworker. As a result. di fficulties in hand I ing workers in heat stress areasmay be significantly greater than those experienced with workers performingunder normal temperature conditions. Also. the performance of individualsexposed to heat stress may be decreased not only as a response to heat butalso as a response to the lowered morale of the worker.

Summary

Often within the industrial environment the worker is subjected to extremetemperatures. If these temperatures are above normal, the worker can besubjected to developing illnesses such as heat cramps, heat syncope, heatexhaustion, and heat stroke. In these cases, the thermal load is such thatthe body's thermal regulatory functions cannot act to dissipate the heatbui Idup rapidly enough.

Strain resulting from a given thermal stress differs between individuals.Important factors, such as the physical bui Id, age, condition, and alcohol ichabits of the worker act to cause these differences.

Repeated exposure to thermal stress can accl imatize the worker. Theaccl imatization helps to lower the strain experienced for a given thermalstress.

2. Thermal Measurement

Introduction

It is not simple to determine the thermal stress to which the worker issubjected. Obviously the temperature of the work environment is significant.However, temperature alone does not determine thermal stress. The presence ofwater vapor in the air (humidity) must also be considered, since humidity is adetermining factor in the rate of sweat evaporation. Also, air movement inthe workroom environment must be considered. Without adequate air movement,evaporated sweat cannot be carried away from the worker, and the vaporpressure around the worker increases, thus reducing the evaporative coolingthat can occur. In addition, the industrial hygiene engineer must determineif a radiant heat load is present, since radiant heat can be a major factor inthe thermal stress present in an environment. Also to be considered are theproblems associated with the metabol ic heat generated by the workers'activities and the individual differences of the workers in response to heatstress. It is obvious, then, that a simple measurement of the temperature ofthe ambient air in the environment is not sufficient to ·determine thermalstress.

The temperature reading with which we are al I fami liar is taken using adry-bulb thermometer. It is obvious from the discussion above that thedry-bulb thermometer alone is not adequate for determining the level ofthermal stress. Consideration must be given to the presence of water vapor,air movement, radiant heat, and worker activity in order to determine thetotal thermal stress present in the environment. The instrumentation shouldalways be located so that the readings obtained are representative of theenvironmental conditions to which the workers are exposed. The sensors shouldbe located at chest height of the worker, and due consideration should begiven to the location of radiation sources and the direction of air movement.

Measurement of Air Temperature

Air temperature may be measured by a variety of instruments, each of whichmay have advantages under certain circumstances. Mercury (oralcohol)-in-glass thermometers, the usual common glass thermometer, is oftenused for determining air temperature. Because of its very common nature,sometimes the simplest of precautions are neglected.

Thermometers may be in error by several degrees. Each thermometer shouldbe cal ibrated over its range in a suitable medium (usually atemperature-control led oj I bath) against a known standard, e.g., NationalBureau of Standards--certified, thermometer. Only thermometers with the

336

Thermal Stress 337

graduations marked on the stem should be used. Those with scale markings on amounting board can be off by 10 degrees; further the stem can shift relativeto the mounting board. It seems superfluous to specify that the range of thethermometer should be selected to cover the anticipated environment where themercury wi I I not break the capi I lary glass tube; this is not an unusualoccurrence in practice.

Sometimes the I iquid column in a thermometer wi I I separate. Beforereadings are taken, the continuity of the column should be checked. Separatedcolumns may be rejoined by shaking. or by heating in hot water (never aflame!). When measurements are taken. it is important that the dry-bulbthermometer be sheltered from any source of radiant heat since the measurementthat is desired is that of the ambient air. For example. in outdoormeasurements. an unshielded dry-bulb thermometer may be several degrees higherthan a shielded dry-bulb thermometer.

The second method that can be used to measure the temperature of theambient air is a thermoelectric thermometer. The operation of this device isbased on the fact that when two dissimi lar metals are joined, and thetemperature of the junction is changed. a sma I I voltage is generated. Twojunctions in a circuit, with one held at a known temperature ("referencejunction") form the basic elements of a thermocouple. The current flowing ;nthe circuit resulting from the voltage (electromotive force) generated may bemeasured directly by a galvanometer, or the electromotive force ~ay bebalanced by a known source potentiometrically. The latter technique ispreferred. as the length of the thermocouple (hence its resistance) becomes ofno consequence when the current flowing becomes zero. Each thermocouple usedwith a current measuring device must be calibrated individually. Figure 3.2.1shows a schematic arrangement of the components in a thermocouple system.Instruments of this type must be calibrated to assure accuracy in measurement.

Figure 3.2.1

Thermocouple.

COLD JUNCTION

COPPER

A thermocouple has certain advantages over a mercury-in-glass thermometer:

1. Provides a method for obtaining the surface temperature of an objectwhere the bulb of a thermometer would not be appropriate, e.g., skinsurface.

2. Thermojunctions may be placed at the measurement site and readremotely over long distances. if desired.


3. Simultaneous readings from several locations may be read at one placeusing one potentiometer with a rotary selector switch in the ci rcuit.

4. Adaptable for use when continuous monitoring and recording arenecessary.

5. Equi I ibrium time with changing temperatures is vi rtual Iyinstantaneous, whereas mercury-in-glass thermometers may requireseveral minutes to reach a steady state.

Another method that can be used to measure the ambient air temperature isthe thermistor. Thermistors are semiconductors which exhibit substantialchange in resistance in response to a sma I I change in temperature. As theresistance of the thermistor itself is measured in thousands of ohms. theresistance imposed by lead wires up to 25 meters or so is immaterial,permitting remote readings as with thermocouples. Readout equipment isbattery-powered, relatively I ight. and portable which is convenient for fieldstudies. The advantages of thermistors are:

1. Simple to use with minimum training.

2. Less bulky and compl icated to use than thermocouples.

3. Requires no reference junction.

4. Output signal may be recorded.

5. Variety of probes avai lable for special applications.

Thermistor probes. though they are called "interchangeable," requireindividual cal ibration before use. Cal ibration of thermistor beads wi I I shi ftsomewhat with age, requiring annual or biennial recalibration. The advantagesof the thermistor thermometer make it the instrument of choice for field usewhen mercury-in-glass thermometers are inappropriate.

Measurement of Radiant Heat

The standard method for measuring radiant heat is the black globethermometer (Vernon Globe). A black globe thermometer is constructed of a6-inch diameter thin-copper sphere that is painted matte black. A hole isdri I led in the sphere into which a rubber stopper can be placed. Amercury-in-glass thermometer, having a range of 30° to 220°F with 1°Fgraduations and accurate to : 1°F is inserted through a rubber stopper in ahole in the top of the shel I and the thermometer bulb is located at the centerof the globe. Where it is desirable for quicker readings. a thermocouple orthermistor can be used in place of the mercury-in-glass thermometer.

The black globe acts to absorb the radiant heat that is being emitted froma source. The thermometer inside the globe reaches equi librium after a periodof time, generally between twenty and thirty minutes.

Thermal Stress 339

Figure 3.2.2

Globe thermometer.

~ RADIANT HEAT

MERCURY·iN·GLASSTHERMOMETER

The globe itself is subject to convective ai r temperatures on its outersurface. The convection acts to reduce some of the heat that is absorbed bythe globe. Thus. it is not the actual radiant energy that is being measuredbut some lesser amount of energy. The energy being measured is generallytermed the "mean radiant temperature." The mean radiant temperature can becalculated after equi I ibrium has been reached between the convective heat losson the outside of the globe and the radiant heat gain inside the globe. Themean radiant temperature is calculated as fol lows:

(3.2.1) Tw = [(Tg + 460)4 + (0.103 x 109vO.5) (rg -Ta )]0.25_460

whereTw =Tg

vTa

the mean radiant temperature OFthe measured globe temperature atvelocity of the air in ft/mintemperature of the air from a dry

equi I ibrium in of

bulb reading in OF

Notice that the formula above takes into account both the measured globetemperature and the dry-bulb temperature. In this way. the formula accountsfor heat energy lost by convection around the globe.

Measurement of Air Velocity

As noted previously. heat transfer by convection and by evaporation arefunctions of movement of the ambient air. Whi Ie the units associated with ai rmotion--distance per unit time--suggest movement of the mass of air past apoint, turbulent air with little net mass movement will be as effective inheat transfer as linear movement.

Directional instruments, useful in venti lation engineering or meteorology,are usually not appl icable for assessment of heat stress. On the other hand,instruments which depend upon a rate of cool ing of a heated element providereadings meaningful in terms of "cooling power" of the moving air, and arethus the instruments of choice.


One useful instrument of this type is a thermoanemometer. There areseveral variations of this avai lable. One measures air motion by the rate ofcool ing of a heated thermocouple at the tip of the probe. One thermojunctionis heated by a constant current suppl ied to a heater wire; the other junctionis located in the air stream. The ai r speed governs the rate of heat removalfrom the heated thermocouple, which in turn determines its mi I I ivolt output.The scale is cal ibrated directly in feet per minute. The low mass of thethermocouple permits almost instantaneous response of the instrument.Batteries supply the power to the unit, thus making it portable andself-contained. The heater supports and the thermocouple restricts airflowsomewhat; in order to obtain the maximum reading, the probe should be sl ightlyrotated.

Another version of the thermoanemometer has two matched thermometers whichare mounted about 5 em apart in the environment. One of the thermometer bulbsis wrapped with a fine resistance wire. Current from a battery passingthrough the wire heats the bulb. The second thermometer is bare. Thetemperature differential between the heated and the unheated thermometersdepends on the current through the wire (adjustable), and the air speed. Thevoltage is set between 2 to 6 volts, depending on the range of air speedencountered. At high air speeds, greater heat input is required to obtainsufficient differential between the thermometers for rei iable readings.Knowing this temperature di fferential and the voltage, the operator may findthe air speed from the cal ibration curves supplied with each instrument.Achieving equi I ibrium requires 2 to 5 minutes. On the one hand, this providesan integrating effect in turbulent air, but on the other hand makesdetermination of air speed at many locations tedious. Its design, however,assures relatively non-directional response.

The Anemotherm, which is simi lar in operation to the fi rstthermoanemometer mentioned, uses a heated resistance wire instead of a heatedthermocouple ci rcuit as one leg of the wheatstone bridge. The Anemotherm canbe used to measure temperature and static pressure also.

The Kata thermometer was developed to determine the cool ing power of airas a measure of efficiency of venti lation in factories, mines, etc. It isessentially an alcohol-fi I led thermometer with an outsized bulb. The bulb isheated in warm water unti I the column rises into the upper reservoir and isthen wiped dry. The instrument is suspended in the air stream (it may be handheld, provided the body of the operator does not interfere with the flow ofair); the fal I of the column from the upper to the lower mark etched on thestem is timed with a stopwatch. The cooling time of the Kata is a function ofair speed and air temperature; the air speed is determined from nomogramsaccompanying the instrument.

Measurement of Humidity

The amount of water vapor in the air (humidity) controls the rate ofevaporation of water from skin surface and from other moist tissues, e.g ..lungs, respiratory passages, conjunctiva of the eyes, etc. Water, like otherI iquids, wi I I tend to saturate the surrounding space with vapor. In anenclosed vessel, the amount of water vapor per unit volume in the space above

Thermal Stress 341

Figure 3.2.3

Kata thermometer.

TIMINGUARKS

the water is dependent only on the temperature of the system (assumingconstant pressure). In accordance with Dalton's law of partial pressures,presence or absence of other species of gases in the space wi I I have no effecton the amount of water vapor present. I f all other gases are evacuated. thepressure developed is termed the true vapor pressure (or saturation pressure)of the liquid at the existing temperature. If the temperature is raised.saturation vapor pressure wi I I increase. When the vapor pressure equals totalatmospheric pressure. boi I ing occurs. In an open vessel where ambient aircurrents carry away the water vapor, continuous evaporation takes place.

"Relative humidity" (RH) is defined as the amount of moisture in the aircompared with the amount that the air could contain at saturation at the sametemperature. It is usually expressed as a percentage. Thus, the amount ofmoisture in the air at 50% RH wi I I vary depending on the air temperature.Since it is the amount of water vapor 'in the air ("absolute humidity") whichinfluences evaporation, the relative humidity cannot be used directly tocompute evaporative loss.

As an example. water vapor in air saturated at aoe exerts a vapor pressureof about 5 mm Hg. This condition might prevai I on a winter's day withfreezing drizzle. When this air is inhaled into the lungs, it passes overmucous membranes coated with I iquid water at 37°C, corresponding to a vaporpressure of about 45 mm Hg. With this gradient of 40 mm Hg, evaporationoccurs, quickly saturating the air, now warmed to 37°C. Thus air at 100% RHenters at aoe. and air at 100% RH leaves at 37°C. yet evaporation hasoccurred, and the moisture content di ffers greatly from inhaled to exhaled


ai r. On exhalation, the air cools and the new moisture burden condenses out,creating a visible cloud.

Given the relative humidity and the temperature, the water vapor pressuremay be determined. In fact, any two properties (temperature. total heatcontent. dew point, relative humidity. etc.) completely define thethermodynamic state of the air-water vapor mixture. The psychrometric chartis a convenient graphical representation of the mathematicalinterrelationships of these parameters.

The Psychrometric Chart. The wet-bulb thermometer does not di rectlymeasure the presence of humidity in the air. To determine this, it isnecessary that a psychrometric chart be used. The psychrometric chart isdesigned to give the relationship between the temperature of the air asmeasured on dry and wet bulb thermometers. the relative humidity, the vaporpressure, and the dew point. The dew point is the temperature at which theai r becomes saturated without a gain or loss in moisture. The psychrometricchart is constructed assuming standard barometric pressure. Conversion tononstandard conditions must be made as required.

Multiple charts are generally avai lable to simpl ify reading at varioustemperature levels. Not al I psychrometric charts are constructed providingexactly the same information. However, the simi larities are such that it isfairly easy to transfer from one type of chart to another.

The basic information on a psychrometric chart is presented in Figure3.2.4. Dry-bulb temperature in degrees Fahrenheit is presented on theabscissa, whi Ie grains of water or vapor pressure in mi I limeters mercury ispresented on the ordinate. The wet-bulb temperature can be found byconsulting the left-hand margin of the graph section.

As was mentioned previously. you only need two properties to enter thechart and obtain the remaining properties. Generally speaking. however, thetwo most frequently used properties are the wet-bulb temperature and thedry-bulb temperature. These readings are found on the chart, as wei I as theadditional data concerning the relative humidity, vapor pressure, and dewpoint. At saturation, the dry-bulb, wet-bulb. and dew-point temperatures areequal.

An example is presented here to illustrate the use of the psychrometricchart. Assume a wet-bulb temperature reading of 75°F and a dry-bulb readingof 1QOoF are obtained. The psychrometric chart in Figure 3.2.4 shows theintersection of the vertical line from 100°F and the diagonal from a wet-bulbtemperature of 75°F. At this point. a relative humidity of 30% exists with adew point of 62.5°F. The vapor pressure exerted by the water vapor in the ai rin this situation is 15 mm Hg. which corresponds to approximately 90 grains ofwater per pound of dry air. Other types of psychrometric charts listadditional information, such as enthalpy at saturation and pounds of water perpound of dry air.

Equipment for Measuring Humidity. A sl ing psychrometer is one of the mostpopular instruments used for measuring humidity. This instrument consists of

Thermal Stress 343

Figure 3.2.4

Psychrometric chart.

j 48

45

a:

~"<> 39a:0 36

'"... -< :z:0 33 '00z 30 e::;) e0Q, 27 ...a: a:w 24

::;)Q, ena: en

21 ww a:... Q,ce~ 18 a:

0...15 Q,

0 ..c(

en >z

r<a:

"

220

200

120

300

280

260

240

140

180

160

100

40

80

60

20

o140 lS0130100 110 120908070605040

100

t. '/ If..... 1/ / / i----95/: / / /~ r---.. / /11't. 1/ V / V"'--- I"--.... /

I9O/:' / / / I'-,. / / ........

........... t;/'" ::-~ V jr--........ V VDP -~:1-- SH. VP

~ 0,0 c"".. 7'-. 'sf.. '/ /'1'---- /",. 85f-.../ qj ",0 "" ~

§' j:/ r'7 roC - ~ r ...........l! / ...........,.:io,p 80 '/ ...,z ",0"_0 "-DB

Q'<' ~G '/ ~ "" ......... l.,.(."CI--

'11.;:,....'11. ~S</// /V'--Z/ 1/.;: - ... ...........C I'-.....

to 70s<:/ / ;o<.....V 1/ [:>.( / .........,~ ~ / ~0-

,~V//~ V-......< /><--r---.. .........~60

S5~~kV></ ><.................1,..><1'--- ::: ><50

40 45Ai§: ~ ~~V<"<' ........ ~V..................... ...-"<r--.. I'-- ......

~ k~~~~--'~~~~ ................... ....................

--... ................... ......... ......... ....... ......... N

DRY BULB TEMPERATURE. F

two mercury-in-glass thermometers clamped in a frame which is fastened to aswivel handle. A cotton wick Shielded from radiation and dipped in disti I ledwater covers one thermometer; the other is bare. The terms "wet-bulb" and"dry-bulb" temperatures originated from this type of instrument. When it israpidly whi rled (so that the velocity of air past the thermometers is between15 and 16 feet per minute) water evaporates from the wick, cool ing the bulb.The rate of evaporation from the wick is a function of the vapor-pressuregradient, determining in turn the depression of the wet-bulb thermomet~r

reading below the dry bulb. The vapor pressure can be read directly from thepsychrometric chart or tables.

To ensure that correct readings are obtained, a few simple precautionsshould be observed when using the sl ing psychrometer. Usually one minute ofswinging adequately cools the wet bulb to its lowest reading. It is advisableto check the reading, and then swing again for a few seconds. Repeat thisprocedure if the temperature continues to fal I. You need to achieve theminimum wet-bulb temperature. Make sure there are no obstructions in the pathof the swinging thermometers. The useful life of the wick can be extended byusing disti I led water only. Remember, thermal radiation can cause ratherlarge errors in both dry- and wet-bulb temperatures taken with a sl ingpsychrometer.


There are also several types of aspirated psychrometers avai lable,battery-powered for field use, as wei I as conventional laboratoryinstruments. These accomplish the same end as the sling psychrometer; airmotion across the thermometer bulbs is created mechanically rather thanwhirl ing by hand.

Another device used for measuring humidity in the atmosphere is the hairhygrometer. Human hair absorbs and desorbs moisture with changes inatmospheric humidity. The length of hair under tension changes in turn withits moisture content. This motion is transmitted through a system of leversto a pointer indicating the relative humidity. Fi I fed with a pen, the pointerrecords the relative humidity on a revolving drum.

Figure 3.2.5

Wet bulb thermometer.

SUlllllary

DISTILLEDWATER WICK

As has been discussed in this chapter, a measurement of dry-bulbtemperature alone is not sufficient to determine the level of heat stress.Other important factors are air movement, the absolute humidity of the air,and the radiant heat load in the environment. These factors can be measuredusing the equipment discussed in the chapter. One other important factor indetermining the thermal stress is individual differences of the workers.Methods for measuring such individual differences do not exist at the presenttime. Only experience and past history can be used to estimate their effects.

Measurement alone does not determine the level at which a thermal stresswi II exert a strain on the human body. The measurements obtained must beconverted in some manner to a stress level that indicates the point at which aphysiological strain wi I I be encountered for most individuals. Variousattempts have been made to develop such a method. These attempts wi I I bediscussed in the next chapter.

3. Thermal Stress Indices

Int roduct jon

Excessive thermal stress can result in a physiological and psychologicalstrain on the exposed worker. The amount of thermal stress that is present ina work environment is a function of certain environmental measures. such asthe temperature of the air, the humidity of the air, the radiant heat load.and the air movement present. These measures have been discussed in theprevious chapter.

In addition. stress is a function of certain physiological conditionsinvolving the worker. The amount of acclimatization to which the worker hasbeen exposed affects the stress to which a specific worker is beingsubjected. The worker's metabol ic rate and work rate are also important. Inaddition. the body surface area-to-weight ratio, as previously discussed. canaffect the worker's stress level.

Other important factors have been discussed previously: the worker'sclothing, the worker's age, sex, and physical condition. It has been shownthat older workers are more subject to strain resulting from thermal stressthan younger workers. The worker's sex is also a factor since experimentshave shown that tolerance to heat is higher among males than females. Theworker's general health and physical condition are also factors that affectthe stress that is placed on an individual worker. Since each workerrepresents a different mixture of the various factors, there are individualvariations in the abi lity to withstand heat stress. This alone presents somedi fficulties when attempting to determine how much heat wi I I be hazardous to agiven group of workers.

On the other hand, the physiological strain that results from thermalstress is a function of the circulatory capacity of the individual, hiscapacity for sweating, and tolerance to elevated body temperature. Inaddition, the exposure time is an important factor in determining the strainthat is felt by an individual. The human body can withstand high temperaturesfor short periods of time without causing harmful effects to the health of theexposed individual.

In order to determine the amount of thermal stress above which workersshould not be exposed. it is necessary to develop a method that relates stressto strain. That is, it is necessary to state in some manner strain as afunction of the stress variables:

Strain = f(Stress)

345


Attempts have been made to develop such a thermal stress index for thisrelationship. Although many indices have been developed. none are entirlysatisfactory. In developing criteria for a thermal stress index, it isimportant that the fol lowing factors be considered:

1. The index that is developed should be quantitative and yield scalarvalues relating to stress and strain.

2. The index should be calculated from avai lable data concerning theconditions that are present in the environment.

3. The index should be tested and proved applicable through use.

4. AI I important factors should be included in the index.

5. The method should be simple to use and not lead to rigorouscalculation or difficult measurements.

6. AI I factors included should be related to physiological strain in aweighted manner.

7. The method should be applicable and feasible for determiningregulatory I imits or threshold limit values for exposure to heatstress.

In the fol lowing discussion. various heat stress indices wi 1 I bepresented. None of the indices totally meets the criteria outlined above. Insome cases the calculations and measurements are difficult to obtain. Inother cases not al I factors are included. However, the indices are the bestthat are avai lable and are the tools that the industrial hygiene engineer hasavai lable to determine if thermal stress is present in the work envi ronment.

The ultimate test of val idity of an environmental heat stress index is itsabi I ity to provide a number which can be used to accurately predict how peoplewi I I respond to environmental conditions being measured. Numerousinvestigators have conducted studies relating human response to variousenvi ronmental heat levels. Unfortunately the investigators have used severaldi fferent indices to describe the levels to which the subjects were exposed.

Effective Temperature

The Effective Temperature (ET) is a widely used index that is related tothe comfort that is felt in a given atmosphere by individuals subjected tothis environment. The Effective Temperature was developed by the AmericanSociety of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE) in1923 and was revised in 1950.

The Effective Temperature combines into a single value the temperature ofthe envi ronment, the humidity of the ai r, and the ai r movement. The Effect iveTemperature scale was developed from empirical data gathered from individualswho indicated the thermal sensation they felt upon entering a givenatmosphere. The individuals involved in responding to the environment were

Thermal Stress 347

either sedentary or normally clothed or stripped to the waist performing lightwork. The Effective Temperature scale has been used extensively in the fieldof comfort venti lation and air-conditioning work.

The relationship of Effective Temperature to the wet bulb temperature, thedry bulb temperature, and the air velocity has been plotted for both sedentaryindividuals and those performing light work. Figure 3.3.1 illustrated therelationship between Effective Temperature and the factors I isted forindividuals performing light work.

Figure 3.3.1

Effective temperature.

120 120

~110 ~

QQ

WIUcz:a: :::l:::l 100 ~

~ 4(4( cz:a: IUIU Ii.Ii. ::il::il 90 IUW ~~

CD CD...I... :::l:::l CDCD 80~> IUa: ~Q

70

700_'!JO

~ 600 60" 500~~ 400

300~~ 200

0-~ 50

~~1oo EXAMPLE:

~ SOO DB & 65° WB

~ 50-40 610 ET at 600 FPM 40~

'0~ 0-30 I) 30~ '!i

~~?


As an example of how to use the graph. consider a dry-bulb temperature of80°F and a wet-bulb temperature of 65°F in an atmosphere in which the air ismoving at 100 feet per minute. Drawing a line between the dry-bulbtemperature and the wet-bulb temperature on the vertical graphs. one candetermine the point at which this I ine intersects with the diagonal linesindicating air velocity. Reading diagonally down the Effective Temperaturescale, a value of 68.5°F is obtained. These values indicate the degree ofwarmth felt by individuals in an envi ronment with the conditions listed.

Somewhat akin to the Effective Temperature is the ASHRAE comfort chart(Figure 3.3.2). This index presents the subjective feel ing of warmth of anindividual after being in an environment for three hours. It is based upon

Figure 3.3.2

Comfort chart for sti I I air.

80u.wa::)l-e(a:wCL.~Wl- SO=~:)

=I-W~ 50

50 60 70 80 90DRY BULB TEMPERATURE F

100

Thermal Stress 349

responses from sedentary individuals wearing I ight clothing in both summer andwinter. This index is quite often used for comfort venti lation determinations.

The Effective Temperature has certain problems when one considers its useas a measure of thermal stress. In the first place, the Effective Temperaturerequires a radiation correction. The accuracy of this correction has beenquestioned. In addition, the Effective Temperature does not consider varyingwork rates and the resulting varying metabol ic heat-generation rates.Experience has shown that the Effective Temperature exaggerates stress in hot,dry conditions at air velocities of 100-300 fpm. On the other hand, theEffective Temperature underestimates stress at low air movement with hot, wetconditions. As a result, the Effective Temperature has not proved ofsignificant value in determining the level of heat stress present in a workenv i ronmen t .

Heat-Stress Index

The Heat~Stress Index (HSI) was developed by Belding and Hatch in 1965.The Heat-Stress Index considers the radiant heat load, convective heat load,and metaboJ ic heat generated by the worker. The heat stress relationship isstated below.

M : R : C

the required sweat-evaporation rate todissipate the heat load in BTU/hr

M= the metabol ic heat produced in BTU/hrR = the radiant heat load in BTU/hrC = the convective heat load in BTU/hr

Ereq

Ereq =

(3.3.1 )where

Using this relationship, a Heat-Stress Index is developed. The strainrelationship is stated as

(3.3.2) HSIEreq

x 100

whereHSI = the Heat-Stress Index

Emax = the maximum evaporative heat loss in BTU/hr

From the relationship for the Heat-Stress Index stated above, it can been seenthat if the ratio Ereq/Emax = 1, the environment wi I I not provide relieffrom heat stress.

Emax , or the maximum evaporative cooling that is possible in theenvironment, can be determined by making measurements of environmentalconditions such as the air velocity and vapor pressure. On the other hand,under hot, dry conditions Emax is confined to man's abi I ity to sweat whichis never more than 2400 BTU's per hour or one liter per hour.

Formulas have been empirically developed for calculating the variables inthe heat stress relationship. These formulas are presented below.


Radiant Heat Load

(3.3.3) R = 17.5 (Tw - 95)

Convective Heat Load

(3.3.4) C = 0.756vO.6(Ta - 95)

Maximum Evaporative Capacity

(3.3.5)

whereTwTa

vPWa

Emax = 2.8vO.6(42 - PWa )

= the mean radiant temperature ofthe dry-bulb temperature of the ambient air ofthe air velocity ft/minthe vapor pressure of water in the air measured in

mm Hg

In order to evaluate the various levels of the Heat-Stress Index, Beldingand Hatch also presented an interpretation of these levels to thephysiological impl ications of an 8-hour exposure to various levels of theHeat-Stress Index. A summary of the Belding Hatch information is presented inTable 3.3.1.

Table 3.3.1

Heat-Stress Index implications of a-hour exposure.

-20 to -10

o

+10 to +30

+40 to +60

+70 to +90

+100

Mi Id cold strain. Frequently exists in heat recovery areas.

No thermal strain.

Mi Id to moderate heat strain. Subtle to substantialdecrements in performance may be expected whereintellectual forms of work are performed. In heavy work,I ittle decrement is to be expected unless worker isphysically fit.

Severe heat strain, involving threat to health unless menare Jhysically fit. Acclimatization requi red. Notsuitable for those with cardiovascular or respiratoryimpairment. Also not suitable where sustained mentaleffort required.

Very severe heat strain. Personnel should be selected by(1) medical examination and (2) trial on the job afteracclimatization. Slight indisposition may render workerunfit for this exposure.

The maximum strain tolerated by fit, accl imatized young men.

Thermal Stress 351

Though the Heat-Stress Index considers al I the environmental factors andthe work rate, it is not totally satisfactory as an index for determining theheat stress on an individual worker. The Heat-Stress Index requires that ameasurement of the air velocity in the workplace be made. In actual practice,such measurements are difficult to obtain with accuracy since workers movearound and the turbulence of the workplace atmosphere is such that di fferingvelocities exist in different areas. In addition, the procedure is relativelycomplicated and requires that the metabol ic rate of the worker be estimated.This estimate can be determined using tables of the metabol ic rate for giventypes of activities.

In 1966, McKarns and Brief developed nomographs that can be used toestimate the Heat-Stress Index. These nomographs give an allowable exposuretime. The allowable exposure time (AET) is defined as the time necessary toraise the body temperature 2°F. The formula from which this allowableexposure time is developed is as fol lows:

250 x 60(3.3.6) AET =------------

Ereq - EmaxIn addition, the minimum recovery time (MAT) from exposure to heat stress

can be calculated using the formula

250 x 60(3 .3 .7) MRT =

Ereq - EmaxIn order to illustrate the McKarns and Brief nomograph, consider the fol lowingexample and the accompanying charts:

Procedure

GivenTgTaTwb

vM

= 120°F= 100°F

78°F40 feet per minute1500 BTU's per hour

Step 1.

Step 2.

Determine the convective heat load (C). Connect Column Ior air velocity with Column I I or air temperature. Readconvection or C in BTU's per hour in Column I I I. C thenhas a value of 35 BTUh. Since the air temperature is above95°, this is a positive value. Below 95°, the left-handside of the Column I I is read and indicates a negativevalue.

Determine the maximum evaporative cool ing (Emax )' Thedew-point temperature must be calculated from apsychrometric chart. Given the values stated, thedew-point temperature determined is 68°F. Connecting theair velocity with the dew-point temperature in Column IV,the intersection of Column V for Emax is read. TheEmax value obtained is 610 BTUh.


Figure 3.3.3

Heat st ress nomograph.

Chart 1

v aor (Ipm) Emax (Bruhl C (Bruhl K DP I 'F) fa ("F) t9

.r a (oFI

500 1500 r1000 200 50 130 100400

800 150 60 8024002000 600 70 1701 120 60

3001500 115 40

110 30200

160 20

140 15

12010

100 80

80 6070 92 191\ 99 50

6040

,92) 9850 93 30

25

40 193) 97 2040 10 1 510

30

20 05 94 105 (94) 96

Step 3.

Step 4.

Step 5.

Step 6.

Determine the constant value, K. Connecting Column I orair velocity with the temperature difference between theglobe thermometer and the ambient air in Column VI andreading Column VII for the constant value K, the valueobtained for K = 13.

Determine the mean radiant temperature. Locate the valuefor K in Column VI I of the second chart. Connect thisvalue with the globe thermometer reading in Column VI I I.Read IX for the mean radiant temperature. In this case,Tw = 135°F.

Determine the radiant heat load (R). Fol lowing thediagonal line upward from 140° to Column X, a radiant heatload of approximately 700 BTUh is obtained.

Determine the sum of the radiant heat load and themetabol ic heat load. An estimate of the metabol ic heatrate must be made. Assume in this case that the metabol icheat rate is estimated to be 1800 BTUh. Connecting Column

Step 7.

Step 8.

Thermal Stress 353

Figure 3.3.3 (continued)

Chart 2

K RfBluh) Iw lFl M·R fBluhl Ig(FI E,eq IBtuh) AET M (Btu h) C IBtuhl E max (Btuhl

Im'ni

2750

'000 f2505000

lBO 65002 1 41 140080 02500 240 6000 212 2300

4500 170 550023 4 1200 20070 2100 13

+1000400

4000 500060 160 3-1 2

1900 1 6004500 4

3500 . 800 80050 150 1700

30001000

40 1500 + 1200I

2500... 400

1300 rf40030

-EfOO130 1600

20 120 i" 18001500 900 .

!'OOO10 1000 12001101000

700 t 22005000 0 95

100500 1-400 1 2400500 0

~ X :rx: XlI ~ XlJl :mxr ][ JI.

X for R and Column XI for the metabolic heat load andreading Column XI I for M + R, a value of approximately 2400BTUh is obtained for M + R.

Determine the required evaporative rate (E reg ). Usingthe value obtained in Chart 1 for C. or 35 BTUh. connectColumn XI I for the value of M + R to Column I I I on Chart 2for C = 35 BTUh. Read Column XII I for Ereq . The valueobtained is approximately 2500 BTUh.

Determine the allowable exposure time (AET). The allowableexposure time is calculated by connecting the value forEreq in Column XI I I with the value obtained in Chart 1,Column V, for Emax on Column V of Chart 2. ReadingColumn XIV for the al towable exposure time, the valueobtained is approximately 8 minutes.

The minimum recovery time can then be calculated using the formula previouslystated. The values of Emax and Ereq in calculating MAT are based uponthat which the worker would experience in the rest area.


The Predicted Four-Hour Sweat Rate

The Predicted Four-Hour Sweat Rate, commonly denoted as P4SR, is basedupon the sweat loss in Ii ters for various environments. A graph is used tocalculate the four-hour sweat rate. Using the graph, determine the value forP4SR in the fol lowing example. The example is based upon sedentaryindividuals.

Twb 80°FT9 95°F

v 50 feet per minuteM 150 K calories per meter2 per hour

To obtain the P4SR, fol low the line intersecting the Twb to thevelocity I ine. This point is then connected to the Tg . and the intersectionof the connecting line with the P4SR for the appropriate veloci ty yields theresulting P4SR. In the example, Twb = 80°F, and the intersection of theTwb and v = 50 ft/min is c?nnected to a value.of Tg = 95°F. The connectingline Intersects the P4SR line for v = 50 ftlmln (v = 10 - 70 ft/min) at .6liters. I f the workers are involved in a work activity in which a metabol icrate is estimated. the sma I I chart in the upper left-hand quadrant is used,and a correction for Twb is made for the appropriate estimated metabol icra te.

The P4SR index is based upon young men working in shorts and, as such.has I imitations when appl ied in the industrial environment. The index alsorequires an estimate of the metabol ic rate and a measurement of ai r velocity.

The Wet-Bulb Globe Temperature Index

The wet-bulb globe temperature index, commonly designated as WBGT, isbased upon a measurement of the globe thermometer reading, a dry-bulbthermometer reading, and a natural wet-bulb thermometer reading. The naturalwet-bulb thermometer (Tnwb) reading is obtained using no artificial airmovement with only evaporation in the ambient air occurring.

Two formulas have been developed, one for outdoor use and the other forindoor use. The formula for indoor use does not involve a dry bulb reading.These formulas are presented below:

Outdoor Use(3.3.8)(3.3.9)

WBGT = O.7Tnwb + 0.2Tg + 0.1TaWBGT = O.7Tnwb + 0.3Tg

The WBGT formula is easy and simple to use. It is the basis for the ACGIHguide for a heat stress TlV. It is also the basis for the NIOSH recommendedstandard.

Although the WBGT is easy and simple to use, it does not in itself includea factor for the rate in which the individual is working. In addition, it isnot possible to determine an allowable exposure time directly from the WBGT.However. in the fol lowing section discussing the ACGIH TlV guidel ine. certainsteps have been taken to el iminate these di fficulties.

Figure 3.3.4

Predicted four hour sweat rate.

Thermal Stress 355

BO

·~LCD 5~4

~ 302~1

o50 100 150 200

MET RATE C/M2/Hr.

...IXl

~

'" Z

IS) ~

~ ......Z

~~<1 97...IS) ...

, <"' ~<Sl lSI Z:; :; ~

\\ '"<Sl

~

<Jl96

~

95 ....a:::l...C[a:

94 ....C1.~

93......CD...

92 ::lCD

91......~

9089888786848280


t 1 + tz + . . . t n(3.3.10) WBGTavg . =

Whenever a worker is exposed to different heat loads for various timeperiods during his work schedule, a time-weighted WBGT must be used. Aformula for calculation of this time-weighted WBGT is presented below.

WBGT1<t1) + WBGT2<tZ) + . W8GT~<tn)

The ACGIH Guide for Assessing Heat Stress

The ACGIH has developed a guide for determining heat stress and has setforth a TLV based upon the work rate of the worker exposed to this heat stress.

The basis for measurement of this index is the WBGT temperature. Theprocedure required for such a measurement is as fol lows:

A. Ory- and wet-bulb thermometer in the range of -sooe to +sooe with anaccuracy of ~O.soC.

B. The dry bulb is to be shielded from the sun when outdoor measurementsare being made.

C. AI low 1/2 hour for the wet bulb to reach equi librium.D. The wick should be entirely wetted 1/2 hour before reading.E. The globe is to be a 1S-centimeter (6 inches) diameter hoi low copper

sphere painted matte black.F. the globe thermometer range should be -SoC to 1OQ°C with an accuracy

of ~O.SoC.

G. Allow 25 minutes for the globe thermometer to reach equi I ibrium.

Table 3.3.2 sets forth the criteria for determining the work load of theexposed worker. This table can be used to estimate the metabolic rate of theworker. Another method that is much more time consuming is to measure theworker whi Ie he ;s performing the job.

Sample Calculation: Using a heavy hand tool on an assembly line

A. Walking Along 2.0 kcallminB. Intermediate value between heavy work wi th

two arms and light work with the body 3.0 kcallmin5.0 kca.l/min

C. Add for basal metabo Ii sm 1.0 kcal/minTotal 6.0 kca llmi n

Adapted from lehmann, G. E., A. Muller and H. Spitzer: Der Kalorienbedarfbei gewerblicher Arbeit. Arbeitsphysiol. 14:166, 1950.

A permissible heat exposure threshold I imit value has been presented basedon the amount of time the worker is involved in continuous work and the levelof work that the worker is performing. This TLV is presented in Table 3.3.3.The fly is applicable only for accl imatized workers and assumes that theworker. are wearing I ight summer clothing. If the rest area is maintained ata t..,.rature below 24°C WBGT, then the amount of rest that is required may ber~ by 25%.

Thermal Stress 357

Table 3.3.2

Assessment of work load.Average values of metabol ic rate during different activities.

A. Body Position and Movement kcal/min

Si tt ing 0.3Standing 0.6Walking 2.0-3.0Walking up hi II add 0.8

per meter (yard rise)

Average RangeB. Type of work kcal/min kcal/min

Hand worklight 0.4 0.2 - 1.2heavy 0.9

Work wi th one arm

light , .0 0.7 - 2.5heavy , .8

Work wi th both arms

light 1.5 1.0 - 3.5heavy 2.5

Work wi th body

light 3.5 2.5 - 15.0moderate 5.0heavy 7.0very heavy 9.0

Light hand work: writing. hand knittingHeavy hand work: typewritingHeavy work with one arm: hammering in nai Is (shoemaker. upholsterer)Light work with two arms: fi I ing metal. planing wood. raking a gardenModerate work with the body: cleaning a floor. beating a carpetHeavy work with the body: rai Iroad track laying. digging, barking trees


Table 3.3.3

Permissible heat exposure Threshold Limi t Values.(Values given are in °c WBGT. )

Work LoadWork-Rest Regimen Light Moderate Heavy

Continuous Work 30.0 26.7 25.0

75% Work--25% Rest. Each hour 30.6 28.0 25.9

50% Work--40% Rest. Each hour 31.4 29.4 27.9

25% 'Nork--75% Rest, Each hour 32.2 31 . 1 300

The Wind-Chi I I Index

Many workers are exposed to extremely cold temperatures. In general, thestrain produced in the worker by the stress of the cold envi ronment is basedon many factors. Little work has been done in this area. However, awind-chi I I index has been developed that indicates where danger areas may bepresent.

Since convection causes heat loss and helps to reduce thermal stress, itstands to reason that, as the wind increases addi tional heat is lost from thebody. Thus, when the temperature is cold. this additional heat loss canpresent a problem to the workers. A wind-chi I I index has been developed thatequal izes the temperature and wind factors for these two conditions. Table3.3.4 illustrates values for the wind-chi I I index.

Summary

Significant work has been done to develop a relationship between thermalstress and physiological strain. Among the indices that have been developedare the Effective Temperature, the Heat-Stress index, the P4SR. and theWBGT. None of these indices are perfect and at best provide only an estimateof the relationship between thermal stress and physiological strain. However,unti I such time as a better method is developed. the industrial hygieneengineer has the indices that have been developed to evaluate thermal stressin the industrial environment.

Thermal Stress 359

Table 3.3.4

Equivalent temperatures (OF) on exposed fleshat varying wind velocities. 1

Wind Velocity, mph0 1 2 3 5 10 15 20 25

23 47.5 53.5 57 60 65 67 68 69.5-11 20 34.5 39 44.5 52 55 57 59-27 0 11 18.5 28 38 42.5 45 47-38 -23.5 -9 0 11 25 30.5 34 36-40* -40* -40 -16.5 -5 11 18 23 25

-40* -40 -19 -2 6 11 14-40* -35 -15 -6 0 3

-40 -29 -18 -12 -8-40* -40 -30 -23 -18

-40* -40 -35 -30*Less than value indicated -40* -40* -40*

1Adapted from Consolazio, Johnson, and Pecora, PhysiologicMeasurements of Metabol ic Functions in Man, McGraw-Hi I I Book Company,New York, 1963.

4. Methods for Controlling Thermal Exposures

Introduct ion

In the preceding chapters, the discussion centered about the fact that manat work in an industrial environment can be exposed to thermal stress. Theindustrial process generates heat by convection and radiation, whi Ie theworker in the environment generates body heat as a result of metabolism andgeneral physical activi ty. The industrial hygiene engineer must developmethods to control the exposure to thermal stress in order to protect theworker from physical strain. There are three general methods that areavai [able to accompl ish this task. These methods are:

1. Administrative controls2. Modifying the thermal environment3. Personal protective equipment

Before undertaking a method for control ling thermal stress, it isimportant to determine the type of thermal stress that is present in theenvironment. Since the methods of control differ for radiant heat andconvective heat, it is important to determine the relative heat load from eachof these sources. In addition, it is important to identify the source of heatand measure its intensity. As discussed in the previous chapter, in mostcases, the WBGT method is the simplest method for such measurement whi Ie theHSI method takes into account the total heat load on the worker. Theindustrial hygiene engineer should consider the work rate at which the workeraccomplishes his tasks. This work rate wi I I determine the metabol ic heatgenerated by the worker. Incidental envi ronmental heat such as that resultingfrom the operation of equipment in the area, I ighting, the general climaticconditions, and heat and steam distribution lines should be considered.

After the industrial hygiene engineer has identified the source of heat,measured its intensity, and determined the type of heat stress to which theworker is being exposed, the work of developing controls can begin. Theremainder of this chapter wi I I discuss the various methods of control that areavai lable to control thermal stress. It is unlikely that any single methodwi I I be totally satisfactory. In most cases, it may be necessary to combinevarious approaches to control the thermal environment of the worker.

General Administrative Methods for Reducing Heat Stress

Decreasing the Work Required. One obvious method that can be used tocontrol the exposure to heat stress is to decrease the amount of work requi red

360

Thermal Stress 361

on the part of the workers exposed to the heat. As the physical activity ofthe worker lessens, the metabol ic rate is lower and the possibi I ity of heatstrain developing is thus lessened. Quite often this approach does not resultin a significant reduction in heat stress as is evident from the discussionrelated to the Heat Stress Index. However, in many situations where amarginal exposure exists, this reduction may be sufficient to el iminate thepossibi I ity of heat strain occurring.

In order to determine if the physical activity of the worker can bedecreased, it is necessary to observe the jobs being performed. During thisobservation, the amount and type of physical activity of the workers can benoted. Analysis can then be made to determine if these activities can bemodified in some manner to reduce the physical exertion of the worker. Can aparticular tool be modified to reduce the amount of muscular activity? Canthe total procedure be automated in a manner that al lows the worker to beremoved from the source? Can various procedures be implemented that decreasethe worker's activity?

Modifying the Worker's Exposure to Heat Stress. In addition to decreasingthe work required. it may be possible to determine methods for modifying theexposure of the worker. One general method avai lable is to provide rei ief tothe workers on a regular schedule. By providing rest areas where the workercan escape the heat and cool down, the possibi lity of heat strain developingis signi ficantly lessened. Such rest areas should preferably be airconditioned at or below 75°F (24°C). These rest areas should be located nearthe workplace to faci I itate their regular use. A rest area located at somedistance from the workplace is not likely to be used by the worker because oftime lost going to and from the area.

Another method for modifying the exposure is to schedule the performanceof hot jobs. Where possible, hot jobs should be scheduled in the cooler partof the day. Thus, the environmental heat load wi I I be lessened. I f suchscheduling is not possible, it is wise to balance the work load throughout theday. It is better to have the worker intersperse hard, physical tasks withtasks of a less physical nature than to attempt to do al I the difficultphysical tasks at once.

A supply of cool water (SOOF to 60°F) at or near the workplace isimportant in reducing the possibi I ity of heat dehydration, resulting in heatstroke. The supply of water should be in close proximity to the workplace toencourage its use. By placing a 0.1% salt solution in the drinking water, thepossibi lity of the worker developing heat cramps is significantly lessened.This method is preferred to the use of salt tablets since it assures that saltreplacement occurs when the fluid is ingested.

Screening of Workers. Since it is known that certain individualcharacteristics of workers make these workers more susceptible to heat strainresulting from stress, it is important that a screening procedure bedeveloped. Such screening can occur prior to employment or placement in a hotarea. Such things as illness, particularly that involving the cardiovascularsystem, and general physical condition are important.


After the workers have been placed in a hot job. it is desirable thatperiodic examinations of the exposed workers be scheduled. The purpose of theexamination is to determine if the worker's physical condition has changed insuch a manner that exposure to heat stress wi I I be harmful.

Education and Training of Workers. Before a worker is placed in a heatstress environment, education and training should be provided to assure thatthe employee is aware of the thermal hazards involved in the job. Thistraining should include such items as:

1. The effects of accl imatization.2. Need for I iquid replacement.3. Need for salt replacement.4. Recognition of the symptoms and treatment of heat disorders.5. The effects of alcohol, lack of sleep, illness. etc .. on heat

tolerance.6. The appropriate clothing to wear on the job.7. The need for rest away from the workplace.

By participating in such training and education, the worker can become moreaware of the dangers involved in exposure to thermal stress .. In this way theworker wi I' act to pol ice his own activities in such a manner to reduce th~

possibi Ii ty of strain occurring.

Accl imatization of Workers. Whenever a worker is being placed in a hotenvironment for the first time, it is important that an acclimatizationprocedure be used. Data from experimental studies indicate that a properlydesigned acclimatization program wi I I reduce the possibi lity of straindeveloping in workers exposed to heat stress. Generally, a two-week programof acclimatization is required. During this period, the worker isprogressively exposed to the hot environment and physiological adjustmentsoccur in the body to reduce the strain experienced. The NIOSH recommendationsfor a standard for work in hot environments suggest that the unaccl imatizedemployee be acclimatized over a period of six days, with 50% of theanticipated total work load and time exposure on the first day. Each dayfol lowing the first day, a 10% increase in exposure is scheduled. bui Iding upto a 100% total exposure on the sixth day. In addition, the recommendedstandard recognizes the fact that accl imatized employees tend to lose theeffects of acclimatization after a layoff. The recommended standard statesthat after nine or more consecutive days leave, the employees should undergo afour-day acclimatization period with dai Iy increments of 20%. beginning with a50% exposure on the first day. In addition. if the employee has been awayfrom work four days or more because of illness, the same four-dayreacclimatization period should be instituted. (Criteria for a RecommendedStandard ... Occupational Exposure to Hot Environments. USDHEW, HSMHA,NIOSH. U. S. Government Printing Office, HSM-72-102-69. Washington, DC, 1972.)

Other Administrative Controls. Other administrative controls that can beused to reduce the exposure to thermal stress include the monitoring of thehot workplace to determine the level of thermal stress present. This isbasically an identification task that determines where further controls arenecessary. However, before a control can be instituted, it is necessary todetermine where such a control is required.

Thermal Stress 363

Tab Ie 3.4.1

Summary of administrative controlsfor control of heat exposure.

Decrease' the Work RequiredModify the Worker's Exposure

• rest• schedu ling

Screen WorkersEducation and TrainingAcc I imat i zat ionRecord-keeping

An additional method for helping to identi fy where heat exposure exists isto keep historical records of heat illnesses as they occur. These records canpoint out workers who have a low tolerance to heat exposure. Also, theserecords can point out areas of extreme thermal stress that can then be thesubject of further investigation and control efforts.

Modifying the Thermal Environment for Radiant Heat

One of the major sources of heat stress in the industrial environment isradiant heat. The reduction of the radiant heat load in a work area can makea significant contribution to the control of thermal exposures. There arethree general methods that are avai lable to reduce the radiant heat load.These methods are:

1. Lower the radiation level.2. Shield or isolate the worker.3. Provide the worker with protective clothing.

Lower the Radiant Heat Level. The surface of a hot body often radiatessignificant amounts of heat. If the surface can be treated with a material oflow emissivity, the amount of radiant heat can be significantly reduced. Inorder to accompl ish this. the surface can be painted with a reflective orshiny paint, preferably an aluminum-type paint. In addition to reducing theradiant heat level exterior to a hot source, this approach conserves the heatinside the body where the process requires this heat.

A second method for reducing the radiant heat load is to insulate theradiant source. A thermally conductive material can be placed on the outsideof a hot body that wi I I in turn reduce the radiant heat load and cause it tobe converted to heat that can be carried away by convection. An alternatemethod of insulating the outside of the radiant source is to provide a waterjacket through which water is circulated. The water absorbs the heat energyand carries it away before entry to the workroom environment.


Shielding for Control of Radiant Heat. One of the most effective ways toreduce the level of radiant heat to which the worker is exposed is to useshielding. By applying shielding between the worker and the source, infraredradiation is intercepted. Since radiant heat travels in a straight line. itis important that the shield be located between the worker and the source andextend in such a manner that the entire worker is protected from any straightline infrared radiation.

There are basically three types of shielding methods avai lable. These arereflective shielding, absorbing shielding. and heat-exchange shielding.Examples of reflective shielding include aluminum sheet. aluminum foi I.aluminum paint on the surface of another metal. special reflective glass. wireor chain mesh. or flexible material. Depending upon the application. one typeof material wi I I have an advantage over the other. For example. aluminum foi Iis relatively inexpensive but does not hold up wei lover time and is easi Iytorn. Reflective glass. wire, or chain mesh provides the operator with theabi I ity to observe the process whi Ie limiting the exposure to radiant heat.However. neither material is as effective in reflecting radiant heat asaluminum sheeting.

Figure 3.4.1

Shielding.

tCONVECTION

HEATSOURCE

The shield should be located with space between the shield and the sourceof heat. This space wi I I act as a chimney that wi II carry away heat that isreflected back towards the source. Otherwise. the possibi I ity of overheatingof the structure of the source is possible. Whatever surface is used must bekept clean in order that it acts effectively as a reflector. A thin fi 1m ofdirt can substantially reduce the reflective abi I ity of aluminum sheeting.

A second method of shielding is to place an absorbing surface between theworker and the source of infrared radiation. A flat, black surface wi I Iabsorb the infrared radiation. The radiant heat is then converted toconvective heat by air passing over the surface. However, in thesesituations. some reradiation occurs. and this method is not as satisfactory asusing reflective shielding.

The absorptive shield may be combined with a water-cooled jacket. Wateris circulated inside the shield and carries away the heat bui Idup in theshield. This approach. though effective. tends to be relatively expensive forinstallation as compared to reflective shielding.

Personal Protective Equipment. In certain situations it is desirable thatthe worker enter a hot environment with a high radiant heat load to perform

Thermal Stress 365

maintenance functions. In these situations the fixed shield is not feasible.It is necessary to provide protection to the worker from the radiant heat loadduring exposure. Personal protective clothing provides a solution to thistype of situation. Though this clothing is somewhat restrictive to motion anddoes not breathe, thus prohibiting evaporative cooling, it does provide theworker with a short protection against the radiant heat load.

Where it is necessary that longer exposures occur or where additionalconvective heat loss is required, refrigerated suits can be used. Generallythe refrigeration involves either water cool ing, compressed ai r passingthrough a vortex and thus expanding, or refrigerated air. Though these uni tsare very restrictive to activity, they are useful for emergency entry intoextremely hot areas.

In situations where only one portion of the worker's body is exposed toradiant heat, experiments have shown that partial protection using reflectiveclothing can be effective. Such items as aprons, gloves. hats, and faceshields can be used where the worker must be exposed to radiant heat for shortperiods of time. However, again these methods do not replace the morelong-term controls such as shielding.

Modi fying the Thermal Envi ronment for Convective Heat

Where the worker is exposed to convective heat stress, the industrialhygiene engineer should consider the methods of substitution and isolation aspossible controls. An investigation should be made to determine if it ispossible that the process can be changed to eliminate the heat requirements.Can another process be substituted that generates less heat? Can the workerbe isolated from the heat source and thus protected? An example of isolationis the provision of a separate air-conditioned control room for the workers.Another method is to isolate the hot processes in an area away from otheractivities, thus central izing the source of heat in an area where control canbe implemented.

One other method for control ling convective heat is to determine if theheat itself can be removed from the work area. This can be accompl ished byinsulating the source of heat as was discussed in the section on radiantheat. In addition, vents and local exhaust hoods can carry away significantportions of the convective heat generated.

General Oi lution Venti lation. In many situations involving convectiveheat loads, general di lution venti lation can be used to reduce the thermalstress encountered. Recirculation of air from man-cool ing fans can increasethe convective heat loss of the workers. However, it should be rememberedthat such a recirculation of ambient air does not reduce the temperature.Thus, if the temperature is above 95°F, recirculation wi I I only increase theheat gain of the workers rather than accompl ishing the desired result ofreducing this heat gain.

Where the general heat load of a work area is high. thermal draft mayassist in providing protection to the workers. Since hot ai r rises andescapes through venti lators in the roof or is drawn out by fans. effective


cool ing can be accomplished by supplying air at the outside temperature.Again, this ai r must be of significantly lower temperature in order to beeffective in lessening the heat 10ad. This air supply can be distributedthroughout the entire work zone. Where spot cool ing is required in spec; fiehot areas, the supply then can be di rected to these areas.

Removing Heat from the Air

In certain situations, the outside air is not sufficiently cool to providerei ief from the hot environment. Thus, it is necessary to cool the outsidesupply air. This can be done either through evaporative cool ing or throughrefrigeration of the air.

Evaporative cool ing involves passing the air through a water spray. Asthe water evaporates into the air from the spray, heat is removed from the airbecause of the required BTU's necessary to accomplish this evaporation. Theair thus becomes cooler as it is suppl ied to the worker. A totally efficientunit wi I I reduce the dry-bulb temperature of the air to that of the incomingwet-bulb temperature. To further increase the cooling capacity of theevaporative cooler, water below the wet-bulb temperature can be uti I ized inthe spray system.

The second method avai lable is to refrigerate the air. Direct expansionrefrigeration units can be used to supply a general area or for spot cool ingof a local enclosure or area. When a large industrial area must be cooled,this method tends to be expensive for installation and operation.

Protection from Climatic Conditions. The cl imatic conditions of the plantlocation can add an additional heat load to the interior environment.Radiation from the sun as well as hot and humid air in the environment canmaterially increase the potential for heat stress. Reflective glass can beplaced in the windows that wi I I reduce the radiation level passing throughinto the work environment. Water sprays can be placed on the roof that removeheat during evaporation. In addition. adequate insulation in the wal Is andbelow the root can result in savings when the interior plant requi res thatcool air be suppl ied.

Modi fying the Environment for Moisture

In some cases it is necessary to change the water vapor pressure in theair. A reduction in humidity al lows for evaporative cool ing in the worker'sbody to take place. On the other hand, an increase in humidity may benecessary to reduce the potential for static electricity bui Idup. In anexplosive atmosphere, such static electricity bui Idup can result in theexistence of a potential hazard.

The moisture content in the air can be increased by applying steam jethumidifiers and air washers to the incoming air. In these cases, a watervapor is introduced to the incoming air that increases the vapor content ofthe plant atmosphere.

Thermal Stress 367

Lowering the moisture content can be accompl ished in a number of ways.Direct expansion refrigeration results in a lowering of moisture content sincethe air is cooled and reaches its dew point. In addition to cool ing the ai T,the water vapor contained in the ai r condenses, thus lowering the humidity.

Chi I led water can be sprayed into the incoming ai r. By lowering thetemperature of the air, water vapor within the air condenses. A second methodfor introducing chi lied water is through coi Is. As the ai r passes over thechi I led water coi Is, it is cooled, thus lowering the dew-point temperature andcausing water vapor to condense.

Absorption of water vapor as the air is passed through certain sol ids andI iquids can result in lowering the humidity. Certain problems exist whenusing sol id absorption materials since, once the solids have absorbed thewater vapor, the process cannot be reversed. Liquids can be heated to releasethe water vapor, and thus recycling of the liquid can occur. In general theI iquids are sprayed through the incoming air, gathered, heated. and thencooled and recycled back to the incoming air. One problem with such a system.however, is that there is a sl ight increase in the temperature of the ai r as aresult of the dehumidi fication process.

Sol id adsorption is another method for removing moisture from the air.Sol ids such as si I ica gel are often used as an adsorption medium. Heating ofthe adsorption material regenerates its adsorption properties. As in the casewith absorption, heat is generated during the adsorption process, thus raisingthe heat of the incoming air. In many cases it is advantageous to include arefrigeration of the air prior to the adsorption process. This not onlymaintains the air at a comfortable incoming temperature but also removes someof the moisture prior to the adsorption process.

Modi fying the Environment for Cold

During the winter months in cold cl imates. it is necessary to add heat tothe air within an industrial faci lity. Heating the air can be accompl ishedthrough general heating of the make-up air or, in cases where general heatingis not feasible, through the use of local heating with unit heaters andradiation panels. A further discussion of this topic is beyond the scope ofthis book. The interested reader is referred to the ASHRAE Engineers Guideand Data Book or the AIHA publ ication, "Heating and Cool ing for Man inIndustry."

Personal Protective Clothing. Workers are often subjected to extreme coldtemperatures when working out of doors or in refrigerated areas. Generalheating cannot provide protection in these cases. Thus, the best approach isto provide personal protective clothing for the workers.

In general, individuals do not become acel imatized to cold. In fact.man's tolerance to cold is somewhat less than one might expect. Particularlydangerous is the exposure of the extremities and respi ratory passages. Thusthere is a need to protect the worker from exposure to extreme cold that canresult in frostbite and even death if the exposure lasts over a long period oftime.


Where heavy work is required, there is an additional problem ofevaporation of sweat. The body can become overheated as the sweat collects onthe skin and cannot evaporate. During rest periods after heavy activity, thesweat that is collected evaporates and exposes the body to excessive cool ing.Therefore, it is important that the worker dress for the activity in which heis involved. Air that is held between layers of clothing provides additionalinsulation against heat loss and, as a result, multi-layered clothing ispreferable. The outer layer should be wind resistant to protect the workeragainst convective heat loss. Layers should be such that they can be removedduring heavy work periods and added during rest periods or periods of lighterphysical activity. Protection of the hands and feet can be accompl ished byusing mittens and insulated boots. Mittens are preferred over gloves sincethey provide better protection of the hands. Boots should be waterproofleather as opposed to rubber since the leather wi I I breathe and al low forevaporation of moisture on the feet.

Summary

When control ling the work environment for thermal stress. the industrialhygiene engineer must consider both radiation and convection heat loads. Thegeneral control methods avai lable include decreasing the work load. modifyingthe exposure, screening the workers, education and training of the workers,accl imatization. monitoring the workplace, and maintenance of historicalrecords of heat illness. For control of radiant heat, action can be taken tolower the radiation level by insulation or surface treatment, by placingshields between the source and the worker, and by providing personalprotective equipment to the worker. For control of convective heat, theprocess may be modified, the source can be isolated from the work area, localexhaust can be used to remove the heat, general di lution can be used, or theworkroom can be air conditioned. The industrial hygiene engineer must alsotake action to control exposure to high humidity and cold.

5. R~e~nces

American Conference of Governmental Industrial Hygienists. Threshold LimitValue for Chemical Substances and Physical Agents in the WorkroomEnvironment with Intended Changes for 1976. Cincinnati: AmericanConference of Governmental Industrial Hygienists, 1976.

Committee on Industrial Venti lation. Industrial Venti lation:A Manual of Recommended Practice, 13th ed. Lansing: American Conferenceof Governmental Industrial Hygienists, 1974.

American Industrial Hygiene Association. Heating and Cooling for Man inIndustry. Akron: American Industrial Hygiene Association, 1974.

Baumeister, Theodore, ed. Marks Standard Handbook for Mechanical Engineers,7th ed. New York: McGraw-Hi I I Book Company, 1967.

Giever, Paul M., ed. Air Pollution Manual Part I--Evaluation, 2d ed. Akron:American Industrial Hygiene Association. 1972.

Hewitt, Paul G. Conceptual Physics ... A New Introduction to YourEnvironment, 2d ed. Boston: Little, Brown &Co., 1974.

Horvath, Steven M. and Roger C. Jensen, eds. Standards for OccupationalExposures to Hot Environments. Proceedings of Symposium, February 27-28.1973. U. S. Department of Health, Education, and Welfare, Publ ic HealthService. Center for Disease Control, National Institute for OccupationalSafety and Health. Cincinnati: U. S. Government Printing Office, 1976.

International Labor Office. Encyclopaedia of Occupational Safety and Health,2 vols. New York: McGraw-Hi I I Book Company, 1971.

Jensen, Roger C. and Hems. Donald A., Relationships Between Several ProminentHeat Stress Indices, OHEW (NIOSH) Publication No. 77-109, Cincinnati,

Ohio: NIOSH, Division of Biomedical and Behavioral Science, October,1976.

McElroy, Frank E.• ed.7th ed. Chicago:

Accident Prevention Manual for Industrial Operations,National Safety Counci I, 1975.

Mutchler, John E., Delno Malzahn, Janet L. Vecchio and Robert D. Soule. AnImproved Method for Monitoring Heat Stress Levels in the Workplace. ~ S.Department of Health, Education, and Welfare, Pub I ic Health Service,Center for Disease Control, National Institute for Occupational Safety andHealth. Cincinnati: U. S. Government Printing Office, 1975.

Olishifski, Julian B. and Frank McElroy, eds. Fundamentals of IndustrialHygiene. Chicago: National Safety Counci I, 1971.

Patty, Frank A. Industrial Hygiene and Toxicology, 2d ed., 2 vols., New York:Interscience Publishers. Inc., 1958.

369


Schaum. Daniel. Col lege Physics. 6th ed. New York: McGraw-Hi I I Book Company,1961.

Theory and Problems of Col lege Chemistry, 5th ed.New York: McGraw-Hi I I Book Company, 1966.

U. S. Department of Health. Education, and Welfare, PublicNational Institute for Occupational Safety and Health.Environment: Its Evaluation and Control. Washington:Pr i nt ing Off ice. 1973.

Health Service,The Indus tr i a I

U. S. Government

SECTION 5 INDUSTRIAL ILLUMINATION

1. Light

Introduction

The purpose of industrial I ighting is to provide an efficient andcomfortable seeing of industrial tasks and to help provide a safe workingenvi ronment. Adequate lighting is not for safety alone. Lighting adequatefor seeing production and inspection tasks wi I I be more than is needed forsafety. Light is more for comfort and convenience than for safety.

I t has been shown that adequate industrial lighting results in manybenefits; for example:

I. Promotes reduced production and inspection mistakes.2. Increases production.3. Reduces accidents.4. Improves morale.5. Improves housekeeping.

Wha tis Li gh t?

The nature of I ight is not easy to understand. The question. "What isI ight?" has been extremely elusive throughout the history of science.

Near the end of the seventeenth century, there were two theories toexplain the nature of I ight; the particle or corpuscular theory and the wavetheory. In the nineteenth century. the discovery of interference anddi ffraction--the bending of light as it passes through different media--madethe wave theory of light the predominant theory; i.e., interference anddi ffraction could not be explained adequately by the particle or corpusculartheory.

In the late 1800's, I ight was thought to be electromagnetic waves, whichat certain frequencies could be seen by the human eye. This conceptual izationof light was primari Iy used to explain the propagation of light. Toconceptual ize the propagation of light, an understanding of electrical forcesand fields and magnetic forces and fields is needed.

To illustrate electrical forces, the simple case of an electrical chargeat rest wi I I be discussed. Each atom has a positively charged core. thenucleus, which is surrounded by negatively charged electrons. The nucleusconsists of a number of protons, each with a single unit of posi tive chargeand one or more neutrons (except for hydrogen). A neutron is a neutral

494

Industrial Illumination 495

particle. Normally, an atom is in a neutral or uncharged state because itcontains the same number of protons as electrons. If, for some reason, aneutral atom loses one or more of its electrons, the atom wi I I have a netpositive charge and is referred to as an "ion." A negative ion is an atomwhich has gained one or more additional electrons. That is, an object whichhas an excess of electrons is negatively charged; and an object which has adeficiency of electrons is positively charged. Objects can be electricallycharged in many different ways.

I t is known that objects with the same charge repel each other, whi Ieobjects with opposite charges attract each other. That is. some force existsbetween charged objects. In 1784 Charles Augustine de Coulomb found that theforce of attraction or repulsion between two charged objects is inverselyproportional to the square of the distance separating them. Coulomb's law canbe stated as:

The force of attraction or repulsion between two point charges isdirectly proportional to the product of the two charges and inverselyproportional to the square of the distance between them.From Coulomb's law, the fol lowing may be written:

qq'F CL

r2or

kqq'F =

r 2

where F denot~s the magnitude of force: q and q' represent the magnitude oftwo charges; r represents the distance between the charges; and k representsCoulomb's constant, a proportional ity constant that takes into account theproperty of the medium separating the charged bodies. The uni ts attached toq, q'. and k are of no concern for conceptual development. This relationshipcan be graphically illustrated by Figure 5.1. I.

Electrically charged bodies then exhibit a force. The presence of anelectrically charged object alters the space around it. This alteration inthe surrounding space can be described by introducing the concept of fields.An electrical field is said to exist in a region of space in which an electriccharge wi I I experience an electrical force. The strength of the electricalfield at any point wi I I be proportional to the force a given chargeexperiences at any point: i.e., the strength of an electrical field can berepresented by the force per unit charge. The electrical field intensity, E,is then defined at a point in terms of the force, F, experienced by anarbitrary positive charge, +q, when it is placed at that point. Thus

FE

+q


Figure 5.1.1

I I lustration of Coulomb's law.

F

Q Q'

~-----vI

-8+ 8--+F + + + +.. + F.. ..

+ • + ...

~r~

Since the electrical field intensity is defined in terms of a positive charge.its di rection at any point would be the same as the electrostatic (at rest)force on a positive charge at that point. (See Figure 5.1.2.)

Figure 5.1.2

Oi rection of electrical fields.

+0 \0.. + () F

.. ..+ + )-0

\0- - F (!)- -)

On this basis, the electrical field in the vicinity of a positive charge wouldbe outward or away from the charge. whi Ie in the vicinity of a negative chargethe di rection of the field would be inward or toward the charge, (See Figure5. 1.3. )


Fi gu re 5. 1.3

Direction of electrical fields.

ELECTRIC FIE~D OF ELECTRIC FIELD OFNEGATIVE CHARGE POSITIVE CHARGE

If an electrically charged particle is brought into the field created byanother electrically charged particle, then

F kqq'/r2 kqE=-= =-

q' q' r 2

where E denotes the electrical field intensity, F denotes force,r denotes the distance between the two emitted charges, q and q'denote the magnitude of the charges, and k denotes Coulomb's constant.

Magnetic forces and fields are simi lar to electrical forces and fields.The magnetic law of forces states: Like magnetic poles repel each other whi Ieunlike magnetic poles attract each other. In the eighteenth century, deCoulomb discovered that (1) the force of attraction or repulsion between thepoles of two magnets is inversely proportional to the square of the distance,r, between the poles: and (2) the force of attraction or repulsion between twopoles is along a line joining the two poles and directly proportional to theproduct of the pole strengths, P1 and P2. These two statements may becombined to form a mathematical statement:

kP1P2F =--

r 2

where F denotes the force between two poles of strength P1 and P2which are separated by a distance, r. The value of the proportionalconstant, k, depends upon the units chosen and the medium surroundingthe magnets.

Every magnet is surrounded by a space in which the magnetic effects arepresent. These regions are called "magnetic fields." The strength of themagnetic field at any point is referred to as the magnetic field intensity, H,and is defined in terms of the force exerted on a unit north pole; i.e. themagnetic field intensity, H, at any point is the magnetic force per unit northpole placed at that point:

FH =

P


where H denotes the magnetic field intensity, F denotes force, and pdenotes the magnitude of the unit north pole.

A more useful expression for computing the magnetic field intensity isgiven by

H '"F

p'

kpp'/r 2

p'

kp

where p' denotes a test pole placed distance r from the pole p, and theremaining symbols are defined as before.

Magnetism is bel ieved to result from movements of electrons within theatoms of substances. That is, magnetism results from a change in motion. Themagnetic polarity of two atoms stems primari Iy from the spin of electronsabout their own axis and is due also to their orbital motions around thenucleus. (See Figure 5.1.4.) As can be seen, magnetism is closely related toelectrical phenomena.

Figure 5.1.4

A charge in motion.

If compass needles were placed around an electrical current, a magneticfield would be produced. (Figure 5.1.5.) If a moving charged particlecreates a magnetic field, wi I I a moving magnetic field create an electricalfield? The answer is "yes." (Figure 5.1.6) This figure consists of a wi rewith some loops in it and a regular bar magnet. I f the magnet is moved up anddown, the meter to the right wi I I register an electrical current or electricalfield. A magnetic field is created by the movement of a charged particle, andthe movement of a magnetic field creates an electrical field.

The upper part of Figure 5.1.7 indicates that when a wire with no initialcurrent is moved downward, the charges in the wire experience a deflectingforce perpendicular to their motion. Since there is a conducting path made bythe wire in this direction, the electrons fol low it. thereby consti tuting a


Figure 5.1.5

A moving charge creates a magnetic field.

MAGNETIC FIELD

f3f3f3f3(Jf3f3f3f3<:URRENT CARRYING

CHARGE

Figure 5.1.6.

A moving magnetic field createsan electrical field.


Figure 5.1.7

Moving charges experience a force that isperpendicular to the magnetic field I ines they traverse.

WIRE MOVING DOWNWARD

MAGNETIC~IELO

STATIONARY WIREAND MAGNETIC FIELD

MOVINGCHARGES(CURRENT)

current. In the lower half of the diagram where the magnet is stationary andthe wire is stationary, when a current moves through the wire to the right.there is a perpendicular upward force on the electron. Since there is noconducting path upward. the wire is tugged upward along with the electrons ofthe charged particles. The relationship between magnetic fields andelectrical fields can be summarized by saying that moving charges experience aforce that is perpendicular to the magnetic field lines that they traverse.

The fact that a magnetic field induces an electrical field and a movingelectrical field produces a magnetic field is how an electromagnetic wave isproduced. Consider an electrical charge vibrating back and forth at a certainfrequency. The charge, because it is a moving charged particle. wi I I producean electrical field around it. The moving charge. if it vibrates back andforth. creates a magnetic field. However. the magnetic field is also changingor moving; and the moving magnetic field produces an electrical field. Thus.the two fields are mutually induced. The changing magnetic field induces an


electrical field which induces a magnetic field, etc., and electromagneticwaves are produced.

Consider first the initial magnetic field induced by the moving charge.This changing magnetic field induces a changing electrical field, which inturn induces a magnetic field. The magnitude of this further induced magneticfield depends not only on the vibrational rate of the electrical field butalso on the motion of the electrical field or the speed at which the inducedfield emanates from the vibrating charge. The higher the speed, the greaterthe magnetic field it induces. At low speeds, electromagnetic regenerationwould be short Jived because the slow-moving electrical field would induce aweak magnetic field which in turn would induce a weaker electrical field. Theinduced fields become successively weaker, causing the mutual induction to dieout. But what about the energy in such a case? The fields contain energygiven to them by the vibrating charge. I f the fields disappear with no meansof transferring energy to some other form, energy would be destroyed.Low-speed emanation of electrical and magnetic fields is incompatible with thelaw of conservation of energy. At emanating speeds too high. on the otherhand, the fields would be induced to ever-increasing magnitudes wi th acrescendo of ever-increasing energies--again clearly in contradiction with theconservation of energy. At some critical speed. however, mutual inductionwould continue indefinitely with neither a loss nor a gain in energy. This.critical speed without loss or gain of energy is 186.000 mi les per second--thespeed of I ight. Thus, energy in an electromagnetic wave is equally dividedbetween electrical and magnetic fields that are perpendicular. Both fieldsosci I late perpendicular to the direction of the wave propagation. (Figure5. I .8)

Figure 5.1.8

Representation of electromagnetic wave.

VIBRATING ELECTRON

oELECTRICAL FIELD

MAGNETIC FIELD

"OR"

oELECTRICAL FIELD

MAGNETICFIELD


Electromagnetic Spectrum

In 1885 H. R. Hertz showed that radiation of electromagnetic energy canoccur at any frequency. AI I electromagnetic waves travel at the same speed ina vacuum. The waves are di fferent from one another in their frequency andwavelength. The relationship between velocity, frequency, and wavelength isas follows:

C = f).,

where C denotes velocity, f denotes frequency, and A denotes wavelength.

I f the speed or velocity of electromagnetic waves is constant, then when thefrequency changes, the wavelength must change. The higher the frequency ofthe vibrating charge, the shorter the wavelength.

Figure 5.1.9 shows the electromagnetic spectrum. It extends from radiowaves to gamma waves. In al I sections of the electromagnetic spectrum, thewaves are the same in nature; they differ only in frequency and wavelength.

Figure 5.1.9

Electromagnetic spectrum.

VISIBLE

INFRARED ( LIGHT X RAYS

,--__~RA~D~IO~W=A~V::::ES=----------___;==;;;:~;----------...::'"......---,' ULTRArVI-:O-L=ET:-"';;';'-';';G:-A~M~M:-::A-·-MICROWAVES r----"'-----.",-----"'"...---,

Electromagnetic waves in principle can have any frequency from zero toinfinity. The classification of electromagnetic waves according to frequencyis ca II ed the "e Iect romagnet ic spect rum. " Elect romagnet i c waves withfrequencies of the order of several thousand hertz (ki locycles/sec) areclassified as radio waves. The VHF (very high frequency) television bandstarts at about 15 mi Ilion Qertz (megacycles/sec). Sti II higher frequenciesare called "microwaves" followed by infrared waves often cal led "heat waves."Further sti I I is visible I ight which makes up only one percent of the measuredelectromagnetic spectrum. Beyond light. the higher frequencies extend intothe ultraviolet, X-ray, and gamma-ray regions. There is no sharp distinction


between these regions which actually overlap each other. The spectrum issimply broken up into these arbitrary regions for classification.

The Quantum Theory

Around 1885 I ight was thought to be wave-I ike and not particle-I ike.However, in 1887 Hertz noticed that an electrical spark would jump morereadi Iy between charged fields when their surfaces were illuminated by theI ight from another spark. This observation is commonly known as the"photoelectric effect." The arrangement for the photoelectric effect can beseen in Figure 5.1.10.

Figure 5.1.10

Photoelectric effect.

Light shining on the negatively charged photosensitive metal plateliberates electrons, which are attracted to the positive plate, producing ameasurable current. This photoelectric effect could not be explained by theelectromagnetic wave theory of light. The brightness of light in no wayaffected the energies of the ejected electrons. I f light was accepted to beelectromagnetic radiation, then the stronger electric fields of bright lightwould surely interact with electrons, causing them to eject at greater speedsand, thus, greater energies. Yet. this was not the case. No increase inelectron kinetic energy was detected. A weak beam of ultraviolet lightproduced a given number of electrons but much higher kinetic energies. Thiswas puzzling; the wave theory of light could not explain this phenomenon.

In an attempt to bring experimental observations into agreement withtheory, Max Planck publ ished his quantum hypothesis. He found that the


problem with the electromagnetic theory of I ight lay with the assumption thatenergy is radiated continuously. He postulated that electromagnetic energy isabsorbed or emitted in discrete packets or quanta, referred to as photons.Planck postulated that the energy states of electrons in atoms are quantized;i.e., electrons can only vibrate with certain discrete amounts of energy. Anelectron farther away from the nucleus has a greater potential energy withrespect to the nucleus than the electron nearer the nucleus; the furthermostelectron is at a higher energy level. When an electron in an atom is raisedto a higher energy level, the atom is said to be excited. The higher level ofthe electron is only momentary. The electron loses its temporari Iy acquiredenergy when returning to a lower energy level. (Figure 5. 1.11)

Figure 5.1.11

Energy levels.

ENERGYLEVELS

Radiation occurs when an electron makes the transition from a higherenergy state to a lower energy state; i.e .. when the atom becomes de-exci ted.This energy is in quanta. The electron moves in discrete steps from higherlevels to lower levels. Each element has its own number of electrons; eachelement also has its own characteristics of energy levels. An electron'sdropping from a higher energy level to a lower energy level in an excited atomemits energy in photons or quanta with each jump. So for each element. theamount of energy wi I I be different.

Planck postulated that the energy of the resulting quanta of radiationwould be equal to the difference in the energy state of the atom. Further, hepostulated that the frequency of the emitted radiation is proportional to thisenergy difference. Planck's equation can be written as

E -= hf

where E denotes the energy of the photon. f denotes the frequency ofradiation, and h is the proportionality factor cal led Planck's constant.

Thus, a photon or quantum of infrared radiation has a very sma I I energy; aquantum of green light, a sma I I energy: and a quantum of ultraviolet I ight, a


larger energy. The greater the radiation frequency of the quantum, thegreater the energy.

With Planck's theory, the nature of I ight was seen to be dual istic--it hadwave-I ike properties and particle properties (quanta or photons being releasedupon de-excitation). It is customary to discuss the wave-like properties ofI ight when the propagation of I ight is discussed, and it is customary to usethe particle theory when the interaction of I ight with matter is discussed.Light may be thought of as radiant energy transported in photons which arecarried along by a wave field.

The Emission Spectra

Every element has its own characteristic pattern of electron ievels. Anelectron dropping from higher to lower energy levels in an excited atom emitsa photon with each jump. Many frequency characteristics of an atom areemitted corresponding to the many paths the electron may take when jumpingfrom level to level. These frequencies combine to give I ight from eachexcited atom its own characteristic color. This unique pattern can be seenwhen the light is sent through a prism.

Each component color is focused at a definite position according to itsfrequency and form. If the I ight given off by a sodium vapor lamp isanalyzed, a single yellow line is produced. I f the width of the ray of yel lowI ight could be narrowed, it would be found that the I ine is composed of twovery close lines. These I ines correspond to the two predominant frequenciesof light emitted by the excited sodium atoms. The rest of the spectrum isdark. (There are many other I ines too dim to be seen with the naked eye.)

This situation is not unique in sodium. Examining the I ight from amercury vapor lamp reveals two strong yel low I ines close together (but indi fferent positions than those of sodium), a very intense green I ine. andseveral blue and violet lines. Simi lar but more compl icated patterns of linesare found in I ight emitted by a neon tube. The I ight emitted by every elementin a vapor state produces its own characteristic pattern of Iines. Theselines correspond to the electron transitions between atomic energy levels andare characteristic of each element as are the fingerprints of people.

Incandescence

Light emitted from a neon tube is red because the average difference inneon energy level is proportional to the frequency of red I ight. Lightemitted by a common incandescent lamp, however, is white. AI I frequencies ofvisible radiation are emitted. Does this mean that tungsten atoms making upthe lamp fi lament are characterized by an infinite number of energy levels?The answer is a definite "no." If the fi lament were vaporized and thenexcited, the tungsten gas would emit a finite number of frequencies, producingan overal I bluish color. The frequency of I ight emitted by atoms depends notonly upon the energy levels within the atom but also on the spacings betweenneighboring atoms themselves. In a gas. the atoms are far apart. Electrons


undergo transition between energy levels within the atom quite unaffected bythe presence of neighboring atoms. But when the atoms are closely packed, asin a sol id, the electrons of the outer orbits make transitions not only withinthe energy levels of the parent atoms but also between the levels ofneighboring atoms. These energy level transitions are no longer wei I definedbut are altered by interactions between neighboring atoms, resulting in aninfinite variety of energy level differences, thence, an infinite number ofradiation frequencies. And this is why tungsten fi lament I ight is white.

Mercury vapor I ight is bright and less expensive than incandescent lamps.Most of the energy in incandescent lamps is converted to heat. whi Ie most ofthe energy put into mercury vapor lamps is converted to I ight. As thefi lament in a tungsten fi lament lamp becomes heated. wider energy leveltransitions take place. and higher frequencies of radiation are emitted. Ahotter fi lament produces a whiter light.

Fluorescence and Fluorescent Lamps

Atoms absorb I ight as wei I as emit I ight. An atom wi I I most stronglyabsorb I ight having the same frequency or frequencies to which it is tuned.the same frequency it emits. For example. when a beam of white I ight passesthrough a gas. the atoms of the gas absorb selected frequencies. Thisabsorbed energy is reradiated in at I directions instead of in the directionsof the incident light.

Some atoms become excited when absorbing a photon of I ight. UltravioletI ight has more energy per photon than lower frequency light. Many substancesundergo excitation when illuminated by ultraviolet I ight. When a substanceexcited by ultraviolet light emits visible I ight upon de-excitation, thisaction is cal led fluorescence. What happens in some of these materials isthat a photon of ultraviolet I ight collides with an atom of the material andgives up its energy in two parts. Part of the energy goes into heat.increasing the kinetic energy of the enti re atom. The other part of theenergy goes into excitation, boosting the electron to a higher orbit. Uponde-excitation. this part of the energy is released as a photon of light.Since some of the energy of the ultraviolet photon is converted to heat. thephoton emi tted has less energy and, therefore. lower frequency than theultraviolet photon. That is, the secondary photon of I ight that is releasedis of less energy than the primary photon since some energy goes to heat;thus. it is of a lower frequency. Light emitted from fluorescent lamps isproduced by primary and secondary excitation processes. The primary processis excitation of a gas by electron bombardment; and the secondary process isexcitation by ultraviolet photons. fluorescence. The common fluorescent lampconsists of a cylindrical gas tube with electrodes at each end. (See Figure5. 1.12.) As in the neon sign tube. electrons are boi led off from theelectrodes and forced to vibrate back and forth at high speeds within the tubeby an AC voltage; and the tube is fi I led with very low-pressure mercury vaporwhich is excited by the impact of high-speed electrons .. As the energy levelsin the mercury are relatively far apart, the resulting emission of I ight is ofvery high frequency. mainly ultraviolet light. This is the primary excitationprocess. The secondary process occurs when the ultraviolet light impinges


upon a thin coating of powdery material made up of phosphors on the innersurface of the glass tube. The phosphors are excited by the absorption of theultraviolet photon and give off a multitude of lower frequencies in al Idi rections that combine to produce white I ight. Different phosphors can beused to produce different colored lights.

Figure 5.1.12

Fluorescent lamp.

...•

5. Upon de-excitation. a photon IS released.producing visible light.

PHOSPHORCRYSTALS\o®--_o_-;;@

ELECTRODE

GLASS TUBE "\

1 Electrode emits electron at high speed.2. Collides with atom (usually mercury atom).3. Collision causes excitation. Upon de-excitation.

a photon is released (ultraviolet light).4. Ultraviolet photon hits phosphor crystals where

excItation takes place.

\ I I I I VISIBLE~", "" LIGHT~ -- . -

Comparing Di fferent Light Sources

The process of excitation and de-excitation (and the release of photons)explains how I ight is emitted. Mercury vapor lamps. fluorescent lamps. andincandescent lamps al I work on the same principle.

Incandescent lamps produce extremely white I ight. The hotter thefi lament, the whiter the light. However, the hotter they burn, the weakerthey get and the more wear and tear on the fi lament. thus decreasing the lifeof the lamp.

High-pressure mercury lamps are about twice as efficient as fi lament lampsbecause less energy ;s converted into heat. One of the disadvantages ofhigh-pressure mercury lamps is the delay in starting and restarting them.Several minutes are required for the lamps to reach ful I brightness. In casesof power interruption. the lamps wi II not restart unti I the arc tube hascooled sufficiently for the mercury vapor to condense (about five minutes).This disadvantage can be overcome by instal I ing a fi lament lamp along wi th amercury lamp.


Fluorescent lamps (low-pressure mercury lamps) are about three times asefficient as fi lament lamps. A coating on the glass fi Iters out the radiationthat would be harmful to the eyes or to the skin. In the past, fluorescentlamps were started with a starter that heated the electrodes at the end of thetube. Modern lamps are started with a ballast; they have sufficient voltageto start the lamp immediately. Fluorescent bulbs give long life. about 7500hours. The lamp is affected by the number of starts. lamps wi I I last longerif started less frequently.

When deciding what type ot lamp to use, one has to look at both efficiencyand cost. Where costs are low, a shorter lamp Ii fe would be sufficient.Where lamps are costly or the labor costs of replacing them are high. alonger-I ife lamp is more economical.

Incandescent ti lament lamps come in various shapes and sizes. The lampbulbs are designated by a letter code fol lowed by a numeral. The letterindicates the shape (straight. S; flame, F; globe. G; general service. A;tubular. T; pear shape. PS; parabol ic, PAR; and reflector, R). The numberindicates the size--the diameter ot the bulb in eighths of an inch. Thus. aT-12 lamp is a tubular lamp that is 12/8 inches or 1.5 inches in diameter.Incandescent lamps also come with different kinds of bases: disc. candelabra.intermediate. mogul. bayonet, bipost. etc.

Mercury vapor lamps are designated by ASA nomenclature; e.g .. H33-I-Cl/C.where H denotes mercury; 33-1. the ballast number; Cl, arbitrary lettersdesignating physical characteristics ot the lamp such as bulb size, shape.material. and finish; and C indicates the color of the light.

2. Light and Seeing/Design of a Lighting System

The last chapter discussed the nature of I ight. Light was seen as radiantenergy transported in photons that are carried by a wave field. In thischapter, the eye is discussed. Also discussed are some objective factors inthe seeing process. The chapter ends by introducing some terms used byillumination engineers.

Behavior of Light

In the last chapter, the origin of Iight was discussed. This chapterdiscusses briefly the behavior of I ight after it leaves the source. Threebasic characteristics of I ight wi I I be discussed. The first characteristic is:

Light travels in a straight line unless it is modified orredirected by means of a reflecting, refracting, or diffusingmedium.

When I ight travels. it travels in a straight I ine. When it is incident upon asurface. part of the I ight is reflected. On a metal I ic surface, almost 100percent of the light is reflected; whi Ie on a clear glass surface, only asmal I portion is reflected. The ratio of light reflected from a surface tothat incident upon it is cal led reflectance.

The law of reflectance is simply stated as: The angle of incidence isequal to the angle of reflection. (See Figure 5.2.1.)

Figure 5.2.1

Law of reflection.

INCIDENTRAY

MIRRORa=b

509

REFLECTEDRAY


Reflections may be of several types; the most common are specular (seeFigure 5.2.2), diffuse (see Figure 5.2.3), spread reflection (see Figure5.2.4), and mixed reflection (see Figure 5.2.5), which is a combination ofdi ffuse and spread reflection.

Figure 5.2.2

Specular reflection.

Figure 5.2.3

Oi ffuse reflection.

\

~p--Figure 5.2.4

Spread reflection.

Figure 5.2.5

Mixed reflection.


Refraction is the "bending" of light as it passes from one transparentmedium to another. The speed at which light travels through these materialsis what makes I ight bend. The speed of I ight is consistent in space.However, I ight has a lesser speed in a transparent medium. In water, lighttravels 75 percent of its speed compared to a vacuum; in glass, about 67percent, depending upon the type of glass; in a diamond, about 41 percent.When light emerges from these media, it again travels at its original speed.This concept may be troublesome because from what is known about energy, thismay seem like strange behavior. If a bullet is fired through a board, thebul let slows when passing through the board and emerges at a speed less thanits incident speed. It loses some of its kinetic energy whi Ie interactingwith the fibers and spl inters in the board. But things are di fferent withI ight. To understand the behavior of light, the individual photons of lightthat make up a beam and the interaction between the photons and the moleculesthey encounter must be considered. Incident photons interact with theelectrons of molecules. Orbiting electrons can be thought of as attached toI ittle springs. These electrons wi I I resonate at certain frequencies and canbe forced into vibration over the range of frequencies. This range varies fordifferent molecules. In clear glass. for example, the range extends over theenti re visible region. When a photon is incident upon a transparent mediumsuch as glass, it is absorbed by a molecule at the surface. An electron inthe absorbing molecule is set in vibration at a frequency equal to that of theincident photon. This vibration then causes the emission of the second photonof identical frequency. It is a di fferent but indistinguishable photon. Thesecond photon travels at 186,000 mi les per second unti I it quickly is absorbedby another molecule in the glass, whereupon an electron is set in vibrationre-emitting a di fferent but indistinguishable photon of its own. Thisabsorption/re-emission process is not an instantaneous event. Some time isrequired for the process; and, as a result, the average speed of light throughthe material is less than 186,000 mi les per second. That is, the photon thatenters the glass is not the same photon that leaves the glass.

Light bends when it passes obliquely from one medium to another. This iscalled refraction. It is the slowing of light upon entering the transparentmedium that causes the refraction. (See Figure 5.2.6.)

The straight line travel of light can also be altered by a diffusingmaterial. Light travel ing through a transparent or translucent material issaid to be transmitted, such as light traveling through a clear glass plate.When light leaves the material, it may become diffuse. (See Figure 5.2.7.)The degree of diffusion depends upon the type and density of the material.Most luminaires are made so that the I ight leaving the luminaire becomesdi ffuse.

The second characteristic of I ight is:

Light waves pass through one another without alteration of either.

That is, a beam of red I ight wi I I pass directly through a beam of blue lightunchanged in direction and color.


Figure 5.2.6

Refraction of light.

Figure 5.2.7

Oi ffusing glass.

The third characteristic of I ight is:

Light is invisible in passing through space unless some medium,such as dust or water, scatters it in the direction of the eye.

The scattering of light is simi lar to the phenomenon of resonance in sound andforced vibration. Atoms and molecules behave I ike tuning forks andselectively scatter waves of the appropriate frequency. A beam of light fal Isupon an atom and causes the electrons to vibrate. The vibrating electron inturn radiates I ight in different directions. An example of scattering is asearchlight beam sweeping across the sky at night. Such beams are seen bylight being scattered by particles (dust or water droplets) in the atmosphere.

The Human Eye

Figure 5.2.8 shows the structure of the eye. Light passes through thecornea (a protective coating over the front of the eye). Light next passes


Figure 5.2.8

The human eye.

OPTICNERVE

LID

IRIS

CIUARYMUSCLE

through the pupi I. an opening in the iris that can be widened or narrowed tolet more or less I ight in by contractions in the muscles of the iris. thecolored portion of the eye. light passes through the pupi I into the lens. atransparent capsule behind the iris whose shape can be changed in order tofocus objects at various distances. The lens is control led by the ci Iiarymuscle. which is ring shaped and changes the curvature of the lens. The lightis then focused through the lens into the inner I ining of the back of theeyebal I. the retina. There the I ight stimulates receptor cells that transmitthe information to the brain via the optic nerve.

More than six mi I lion cones and 100 mi I I ion rods are distributed in theretina. Rods are sl im nerve cel Is--receptors--which are sensitive to lowlevels of-i-'-Iumination. Rods have no color response. They are found only onthe outside of the foveal region. increasing in number with the distance fromthe fovea. The outer portion of the retina is composed chiefly of rods whichdo not afford distinct vision but are highly sensitive to movement andflicker. When I ight strikes a rod. it causes the breakdown of a chemicalrhodopsin (visual purple). This photosensitive chemical triggers activity inthe optic nerve and. subsequently, in the brain.

Cones are the receptors that make possible the discrimination of finedetai I and the perception of color. Cones are insensitive at low levels ofillumination. The cones are found mainly near the center of the retina, withthe greatest concentration at the fovea. A few cones are mixed with rods aJ fthe way to the outer edges of the retina. but the center of the eye is themost color-sensitive portion. The cones also contain a photosensitivechemical that breaks down when struck by I ight waves.

The eye has the abi lity to adapt to a wide variety of illuminationlevels. Adaptation involves a change in the size of the pupi Is along withphotochemical changes in the rods and cones. In dim' ight--Iow levels ofi I lumination--the chemicals in the rods and cones are bui It up faster thanthey are broken down by I ight stimulation. The greater the concentration ofthese chemicals, the lower the visual threshold. Thus. the adaptation to


darkness is a matter of bui Iding up a surplus of rhodopsin in the rods andother chemicals in the cones.

The cones adapt quickly in the dark (10 minutes or so), but the rods adaptslowly and continue to adapt even after 30 minutes or more of darkness. Theseare only rough estimates since the length of time of adaptation depends uponthe previous state of adaptation and the magnitude of the change. Whencompletely adapted, the rods are much more sensitive to light than the cones.Thus, to see a dim light in pitch darkness, one should not look di rectly at itsince the center of the eye contains only the less sensitive cones. Bylooking away from the object, the image wi I I fal I on the edge of the retinawhere the rods are. This manner of viewing affords a higher I ikel ihood ofseeing the dim light. Since the rods work in dim light and the cones do not.vision in very dim light is entirely colorless. Although vision is colorlessin dim I ight, the eye becomes relatively sensitive to energy at the blue endof the spectrum and almost bl ind to red.

Visual acuity is the abi lity to discriminate the detai Is in the field ofvision. The normal field of vision extends approximately 180 a in thehorizontal plane and 130 a in the vertical plane (60 a above the horizontal and70 a below). One way the abi I ity to discriminate detai I in the field of visioncan be measured is by using the fami liar eye chart. Standard perfect visionis often cal led "20/20 vision." If a person stands 20 feet away from the eyechart and sees the material on the chart clearly, he or she is seeing normallyand is said to have 20120 vision. If the person does not see normally. someof the material wi f I be blurred. I f a person standing 20 feet away from thechart sees what a person with normal vision sees at 50 feet, the person has20/50 vision. If a person has 20/10 vision, he sees things 20 feet away assharply as a person with normal vision sees them at 10 feet.

Part of the retina, the "blind spot." has no visual acuity. This spot isthe point at which the nerves of the eye converge to form the optic nerve.The optic nerve extends through the back wal I of the eyebal I and connects theeye to the brain. People are usually unaware of the bl ind spot; theycompensate for this blind spot in their vision primari Iy by moving their headsand making use of their other eye.

The four most common causes of defective VISion are astigmatism. theinabi lity to bring horizontal lines and vertical lines into focus at the sametime; myopia, where objects focus in front of the retina--nearsightedness;hypermetropia, where objects focus behind the retina--farsightedness; andpresbyopia. loss of elasticity of the lens with age. AI I of these visualdefects can usually be corrected by properly fitted corrective glasses orlenses.

Variables in the Seeing Process

What makes an object easy to see? Investigations have shown that adequateseeing depends upon at least four variables. These are the size of theobject, the contrast of the object with its background, the brightness of theobject. and the time available to see the light.


The obvious factor in seeing an object is its size. The size of theobject depends upon the visual angle. The larger an object in terms of itsvisual angle--the angle subtended by the object at the eye--the more readi Iyit can be seen. The fami I iar eye test chart illustrates this principle. Theperson who brings a sma I I object closer to his eyes in order to see it moreclearly is unconsciously making use of the size factor by increasing thevisual angle. (See Figure 5.2.9.)

Figure 5.2.9

Size of object--visual angle of object.

-."..----~~:::----,------

----...1.LARGE----_....VISUAL -ANGLE

~ :::::------;--------M\-------.l.----

SMALL ----.VISUAL -ANGLE

OBJECTAPPEARSLARGE

OBJECTAPPEARSSMALL

Along with the size of the object is visual acuity. Visual acuity,expressed as the reciprocal of the visual angle in minutes, is a measure ofthe sma I lest detai I that can be seen. Since visual acuity increases markedlywith increase in illumination, I ight is sometimes said to act as a magni fier,making visible sma I I detai Is that could not be seen with less light.

The second factor involved in seeing objects is contrast. Contrastprimari Iy refers to two factors--color contrast and brightness contrast.Color contrast refers to the contrast in color between the object to be seenand its immediate background. For a given set of conditions. visibi I ity is atits highest when the contrast is at a maximum. Black print on white paper ismuch more visible than the same print on grey paper. (See Figure 5.2.10.)Brightness contrast is the contrast in brightness between the object and itsimmediate background.

The third factor in seeing is brightness. The luminance or brightness ofan object depends upon the intensity of the I ight striking it and theproportion of that light reflected in the direction of the eye. A white


surface wi I I have a much higher luminance than a black surface receiving thesame illumination. However, by adding enough light to a dark surface, it ispossible to make it as "bright as the white one." The darker an object orvisual task, the greater the illumination necessary for greater i I luminanceand, under like circumstances, for equal visibi lity. In addition. thebrightness between the object and its immediate background should beapproximately the same. Different ratios in brightness between the object andthe background can cause problems for the viewer.

The fourth factor in seeing is time. Seeing is not an instantaneousprocess--it requi res time. The eye can see very sma I I detai J under very lowlevels of illumination if sufficient time is al lotted and eyestrain isignored. However, more I ighting is required for quick seeing. The timefactor is particularly important when the visual object is in motion. HighI ighting levels actually make moving objects appear to move more slowly andgreatly increase their visibi I ity.

Size, luminance, contrast. and time are mutually interrelated andinterdependent. Within I imits. deficiency in one can be made up by anadjustment in one or more of the others. In most cases, size is a fixedfactor of the visual task. with luminance, contrast, and time subject to somedegree of modi fication. Of these. luminance and contrast are usually mostdirectly under the control of the illuminating engineer. Properly employed.they can be of tremendous aid in overcoming unfavorable conditions. sma I Isize. and I imi ted time for seeing.

To see the interrelation among the four factors, consider the fol lowing:

A. Sma I I objects must have a high contrast to be seen.B. Low contrast objects must be large in size.C. As brightness increases. contrast and size can be decreased.D. When more time is avai lable for seeing objects, the size can be

smaller and the contrast lower.E. In most situations, the object is fixed. contrast is usually fixed,

and the time for seeing is fixed; thus, brightness is most often thevariable under the control of the engineer.

Terminology Used in the Science of Light

There are some terms related to the science of light. with which everyoneworking with light should be fami liar.

Luminous Flux (F). Luminous flux is the total radiant power emitted froma light source that is capable of affecting the sense of sight. Actually, itis more precisely defined as the time rate of flow of I ight (or luminousenergy). In ordinary practice, the time element can be neglected, andluminous flux is commonly considered a definite qual ity. Luminous fluxismeasured in units of a lumen where one lumen is defined as the luminous fluxemitted from ai/50 cm2 opening in a standard source and included within asolid angle of I steradian. The standard source consists of a hoi lowenclosure maintained at the temperature of solidification of platinum, about/773°C. A sol id angle in steradians is given by:


A cos en =

R2Where n denotes a solid angle, A denotes surface area, and Rdenotes distance (or radius of a sphere), where R isperpendicular to the surface, A, and 9 represents the angle fromthe center of the sphere to the surface area (A).

A sol id angle can be graphically illustrated as in Figure 5.2. '1.

Figure 5.2.10

Contrast.

CONTRAS :"VISUAL 0THE 1M MEBACKGROIMPORTA

Figure 5.2.'1

Definition of a solidangle in steradians.

If e = 0°, that is, the surface area is perpendicular to the center of thesphere, then cos e = 1 and n = A/R2.

In the definition of a lumen, then, one steradian can be defined as thesol id angle subtended at the center of a sphere by an area A on its surfacethat is equal to the square of its radius R. It can be shown that there are4w steradians in a complete sphere.

An =

R2

4wR2=

R24w steradians

A lumen is often defined as the flux fal ling on a surfacearea, every part of which is one foot from a point SOurceintensity of one candela lcandlepower) in al I directions.is a term not yet defined.

one square foot inhaving a luminous

Luminous intensity


Luminous Intens; ty. Luminous intensity of a I ight source is the luminousflux emitted per sol id angle:

Intensity = Flux/ n

The unit for intensity is the lumen per steradian cal led a candela orcandlepower. as it was cal led when the international standard was defined interms of the quantity of light emitted by the flame of a certain candle.(This initial standard was replaced by the platinum standard.) This standardcandle had a luminous intensity in a horizontal direction of approximately onecandela. I f a I ight from a candle with the candlepower of I fel I on a surfaceof one square foot, where every part of the surface was one foot from thecandle. the amount of flux would be one lumen. Note from the above equationthat

Flux = Intensity x n

I f the light source is an isotropic source (one which emi ts I ight uniformly inall di rectionsl, then the total flux emi tted would be

Flux = 4~ Intensity

since the total sol id angle for an isotropic source is 4~ steradians.

Example

If a spotl ight one foot away from a wal I had a bulb with a candlepower ofI candela and the beam covered an area of I square foot of the wal I. whatwould the luminous intensity of the spotl ight be?

So Iut ion

Total flux emitted by the I candela bulb is

Flux = 4~ intensity(4~)(1 candela)

12.56 lumens (flux is measured in lumens)

The I ight is concentrated into a sol id angle given by

A

R2

1 f t 2

(I ft)2

I steradianAnd the intensity of the beam is given by

Intensity Flux/Q


12.56 lumens

I steradian12.56 candela (intensity is measured in candela)

Note that the unit of intensity, candela (lumens/steradian) and the uni t offlux (lumen) are the same dimensionally because the solid angle in steradiansis dimensionless. Also note that flux and intensity wi I I be equal when n =I steradian (i .e., when A = R2).

I Ilumination. I I lumination is the density of luminous flux on a surfacearea; i.e.,

I I lumination = Flux/Area

When flux is measured in lumens and area is measured in square feet, thenillumination is expressed in lumens per square feet. The lumens per squarefeet is sometimes cal led the foot-candle. I f a lumen is defined as the fluxfal ling on a surface area of one square foot, where every part of the surfaceis one foot from a point source having a luminous intensity of I candela(candlepowerl, then it is obvious that one lumen uniformly distributed over.one square foot of surface provides the illumination of one foot-candle.Visually, a foot-candle is the illumination at a point, X, on a surface whichis one foot from and perpendicular to a uni form point source of one candela.(See Figure 5.2.12.) In this special case where the incident light isperpendicular to the surface, it can be shown that

IntensityI I I um i na t i on

Distance2

The relationship

Lumens (f lux)I I lumination = ------------

Area

i~ important and wi I I be used during the discussion of the lumen method ofdesigning a lighting system.

Luminance. Luminance (sometimes cal led photometric brightness) is ameasure of the brightness of a surface, when viewed from a particulardirection, emitting or reflecting one lumen per square foot. Luminance isdirection-specific and is often measured in foot-lambert units.

Reflectance. Reflectance is a measure of how much I ight is reflected froma surface. It is the ratio of luminance to illumination.

LuminanceRef lectance =

I I I um i na t ion


Figure 5.2.12

Foot-candle.

The Measurement of Light

T1 FOOT

L , SQUARE FOOT

Foot-candle Measurements. I I lumination measurements are most commonlymade with one of several types of foot-candle meters embodying aI ight-sensitive, barrier-layer cel I. (See Figure 5.2.13.) This type of cel Iconsists essentially of a fi 1m of I ight-sensitive material mounted on ametal-based plate and covered by a very thin translucent layer of metalspattered on its outer surface. Light striking the cell surface causes thesemiconducting, light-sensitive material to emit electrons that are picked upby the metal col lector in contact with the translucent front electrode. Apotential difference is thus set up between the collector and the base plate;and, when a microammeter is connected between them, it measures the currentgenerated by the eel I. Since the current is proportional to the intensi ty ofthe incident I ight, the meter can be calibrated to read directly infoot-candles. Portable meters are made in a number of types and wi th a widerange of sensitivity for various appl ications. Although portable,light-sensitive cel I meters are simple and highly convenient to use, most ofthem are not designed to be precision instruments. Careful hand I ing andfrequent cal ibration wi I I help to maintain rei iabi Iity. Ordinary measurementsmade in the field should not be expected to have an accuracy greater than! 5%under the most favorable conditions. In addition, al I light-sensitive eel Ishave certain inherent characteristics that the user must understand if he orshe is to obtain the best possible results:

A. The instrument must be color corrected because the response of thelight-sensitive cel Is to the various wavelengths of the visiblespectrum is quite different from that of the human eye.

B. They must be cosine corrected; that is, adjusted for the angle of theref lected light.

C. All light-sensitive cel Is exhibit a certain amount of fatigue; thatis, a tendency for the meter indication to drop off slowly over aperiod of minutes unti I a constant reaaing is reached. This effectis most noticeable at high foot-candle values, particularly if the


Figure 5.2.13

I I lumination meter.

I.IGHTSENSITIVEMATERIAl.

BASE PLATE

MICROAMMETERINSTRUMENTS NEED TO BE

{AI COLOR CORRECTED{Bl COSINE CORRECTED TO

COMPENSATE FOR UGHTREFLECTED FROM THELIGHT DETECTING CELLSURFACE

cel I has just been previously in the dark for some time or exposed toa much lower level of illumination. Before any measurements arerecorded, therefore. the meter should be given as long an adaptationperiod as may be necessary at the foot-candle level to be measured.In addition, there is a constant need to have the instrumentcal ibrated.

Brightness or luminance is measured using a photoelectric tube. Theinstrument is aimed at the surface to be measured. and a lens focuses theimage on a sma I I area on the tube which produces a current proportional toluminance. The current is read on a microammeter calibrated in foot-Iamberts.

Brightness is also measured by a visual luminance meter. This uses anoptical system to bring the eyes of the observer side by side to the surfaceto be measured and a comparison field inside the meter.

Luminance can be measured with a foot-candle meter providing thereflectance of the surface is known. This is because i I luminance equalsluminance divided by reflectance.

Reflectance can be measured by a cel I-type foot-candle meter. There aretwo procedures that can be used. The more accurate procedure requi res a piece


of matte material at least one foot square, the reflectance of which isknown. White blotting paper, at about 80 percent reflectance, is suitable.The blotting paper is placed against the surface to be measured, and the meteris held two to four inches away with the cell facing the paper, reading A.The blotting paper is then removed without moving the meter, and the reading Bis noted. The reflectance of the surface is reading B divided by reading Atimes 0.80. In al I measurements of this sort, special care must be taken tomaintain al I conditions, especially the position of the meter, constant forboth readings of a pair.

Light Survey Procedures

"How to Make a Lighting Survey," developed by the Illuminating EngineeringSociety, provides detai led information on conducting a lighting survey. A setof instructions and a form are included with this information.

The survey includes reporting on the fol lowing information:

I. Description of the illuminated area:

a. Room dimensionsb. Colorc. Ref Iectanced. Conditions of room surfacee. Temperature surrounding the lights

2. Description of the general lighting system:

a. Quantitiesb. Conditionsc. Wattagesd. Lampse. Distributionf. Spacings9. Mount ings

3. Description of any supplementary lighting that might be used.

4. Description of instruments to be used.

5. I I lumination measurement:

a. Operator must be aware not to cast shadows.b. Operator must be careful not to reflect additional light from

clothing.c. Test surfaces should be as close as possible to the working

plane. If there is no definite working plane, take measurementson a horizontal plane 30 inches above the floor.

6. Luminance measurements.


Evaluation of Results. Data resulting from a I ight survey can be used tocompare the illumination levels for campi iance with the recommended levels; tocompare the luminance with compl iance luminance levels; to determine luminanceratios for visibi lity and safety; to determine indications of comfort andpleasantness in the area; to determine deficiencies in the area; and todetermine a maintenance schedule--that is, a good housekeeping schedule.

3. Lighting Design

Introduction

In the last chapter. the four factors of seeing industrial tasks--size.contrast. brightness. and time--were discussed. Also discussed were commonlighting terminology and the behavior of I ight when it leaves the source.

This chapter discusses the design of a I ighting system. The design of anyI ighting system involves the consideration of many variables: What is thepurpose of the I ighting--is it lighting for critical seeing, lighting forsel ling. or lighting for decoration? How severe is the seeing task, and forwhat length of time is it to be performed? What are the architectural anddecorative requirements, together with the constructional I imitations. of thearea? What economic considerations are involved? The answers to suchquestions as these determine the amount of light that should be provided andthe best means for providing it. Since individual tastes and opinions vary.especially in matters of appearance, no one solution of a I ighting problemwi 1I be the most desirable under all circumstances. However. there arecertain basic rules governing adequate lighting and the qual ity of thatI ighting. Two factors that must be considered in designing a lighting systemare the quantity of I ight and the quality of light reaching the seeing task.

Quantity of Light

The most obvious consideration in designing alighting system is theadequacy of the I ight on the seeing task. Research has shown thatillumination of thousands of foot-candles is required to see dark.low-contrast tasks as easi Iy as I ight-colored tasks of high contrast under lowlevels. However, there are other factors involved. These factors suggestthat for any task the minimum number of foot-candles is 30.

The Illuminating Engineering Society published a document entitled illESLighting Handbook--Application Volume". 1981, which contains a complete tableof i I luminance ranges recommended for certain kinds of tasks. A page fromthis document can be found in Table 5.3.1.

Evidence providing a sound basis for definite i I luminance recommendationsis not easy to obtain. Much work in this field has been done over a period ofmany years. using various methods and various criteria of visual performance.On the basis of such research, the I I luminating Engineering Society has madeilluminance recommendations for a wide variety of representative industrialoperations and other visual activities.

524


The illumination recommended in the IES Lighting Handbook is to be providedon the work surface, whether it is horizontal, vertical. or obi ique. Wherethere is no definite area, it is assumed that the i J lumination is measured ona horizontal plane 30 inches above the floor. The values given are not to beconstrued as initial foot-candles provided by a new installation; they are

Table 5.3.1

Currently recommended i I luminance categories and i I luminancevalues for lighting design--target maintained levels.

illumInance Categones and illumInance Values for Generoc Types ot ACIlYltles In Intenors

UkJm"'.anceRanges of Illuminances

TyCM' of ActivityCaleQOry

Fle-fetenee Wor".PlaneLux FoolCan<lles

Public spaces wllh dark surroundIngs A 20-30-50 2-3-5---_..---- ,--S,mple onentahon tor Short temporary B 50-75-100 5-75-10 General IIgn1tng

VISits tnrougnoul scaces

WorkIng spaces where v,sual tasks are C 100-150-200 10-15-20only occaSIonally pertormed

Pertormance of vIsual tasks of h,gh con- 0 200-300-500 20-30-50trast or large sIze

Pertormance of vIsual lasks 01 me<llum E 500-750-1000 50-75-100 lIIum,nance on taskcontrast or small SIze

Pertormance 0' v'sual tasks ot low con- F 1000-1500-2000 100-150-200'rast or very small SIze

Pertormance ot v'sual tasks ot low con- G 2000-3000-5000 200-300-500IraSI and very small Size over a pro-longed perIod illumInance on task

oblalned by a com-Pertormance 0' very prolonged and ex- H OOסס5000-7500-1 500-750-1000 bInaI,on of general

actIng v'sual tasks and local (suCcle-

Pertormance of very specIal v'sual taSks 10000-15000-20000 1000-1500-2000 mentary IIgnhng)

01 extremely low contrast and smallsIze

II. CommercIal. Institutional. Resident'al and Public Assembly Interoors

Areal ActI..tyilluminanceCalegory

Air tenninal. (see TranacMl<tlttion tenni.....)

~

Art galleri.. (see MuHUIIla)

AuditoriumsAssemblySocIal activ,ty

Bank. (alSO see Reading)Lobby

GeneralWrit,ng area

Tellers stations

C'

C'B

CoE'

BarMr UIoPS and beauty parlots E

Churc,," and .ynagogUft (see cage 7-2'-ClUb and lodger_

Lounge and reading 0Conf__r_

ConferrIng 0Cntlcal seetng (reter to indIVIdual task)

CourtroomaSeating area CCourt actIvity area E

~anc. hall. and diac:ottlequ.. B

By permission of the I I luminating Engineering Society. Source: IES LightingHandbook, 1981 Application Volume.


recommended minimum foot-candles at any point on a task at any time. Thismeans that the installation must be so designed that the collection of dirt onluminaires, lamps, wal Is and cei lings and the normal depreciation of lightoutput of the lamps themselves wi I I not at any time lower the illuminationbelow the recommended levels. In order to ensure the minimum levels, oneshould design a lighting system with higher levels than those indicated in thetables. One can look at the minimum levels specified in the document as beingthe levels on the task when the lighting systems on the room surfaces havedepreciated to their lowest level before maintenance procedures are affected(cleaning, relamping, painting, etc.). In addition, the recommended levels donot take into account the wearing of goggles. I f goggles are worn, the levelsof illumination should be increased in accordance with the absorption of thegoggles. The levels specified in the document again should be viewed asminimum levels. They are not to be construed as standards. They aresuggested levels. However, one can use the tables to identify what level-what quantity of i I lumination--is needed for any speci fied task.

The quantity of light also depends upon the distribution of theluminaires. In light for seeing or light for production and inspection. it isusually desirable to position the luminaire to provide reasonably uni formgeneral illumination over the entire area. The ratio of maximum foot-candlesunder the luminaires to the minimum between them should never be greater than2:1; and for best results. it should be nearer to unity. Units with widedistribution characteristics can be spaced farther apart for the same mountingheight than those with more concentrated distributions. Maximumspacing-to-mounting height or cei ling-height ratios for various types ofequipment are supplied by the manufacturers.

Quality of Light

In addition to the quantity of light. one must consider the qual ity ofI ight when designing a lighting system. Quality of I ight refers to glare,brightness ratio, diffusion. and color.

Glare is the effect of brightness di fferences within a visual fieldsufficiently high to cause annoyance, discomfort. or loss of visualperception; whi Ie brightness is the intensity of I ight emitted. transmitted,or reflected from a given surface. Basically. there are two kinds of glare:glare caused by a bright light source which is sometimes cal led direct glare;and glare by bright reflections, sometimes cal led reflected glare. There aregenerally considered to be two forms of glare--discomfort glare and disabi lityglare--each of which may be caused by a bright I ight source or by brightreflections on room surfaces. Discomfort glare, as its name implies. producesdiscomfort and may affect human performance but does not necessari Iy interferewith visual performance or visibi lity. In some cases, extremely brightsources can even cause pain. Disability glare does not cause pain but reducesthe visibi I ity of objects to be seen. An example is the reduced visibi I ity ofobjects on a roadway at night caused by glare of bright oncoming headlights.

Direct glare results from high brightness light sources or luminai res inthe field of view that are not sufficiently shielded or cover too great an


area. It is also possible that direct glare can result from improperly shadedwindows.

An industrial environment, then, wi I I be relatively comfortable if thereis no direct glare; and seeing wi II be unimpaired if there is no disabi Ii tyglare. The effects of direct glare can be avoided or minimized by mountingthe luminaires as far above or away from the normal lines of sight aspossible. In general, this can be done by shielding luminaires to at least 25degrees down from the horizontal and preferably down to 45 degrees. In otherwords. brightness of bare lamps should not be seen when looking in the rangeof sight straight ahead up to 45 degrees above the horizontal.

Direct glare from windows can be minimized by properly shielding thesources of day I ight with adjustable shades, blinds, or louvers.

Reflected glare is bright areas on shiny surfaces that become annoying.Light sources between the vertical and 45 degrees from the vertical contributeto reflected glare. Luminaires with lens that polarize light (lens thattransmit I ight waves which vibrate in only one direction) tend to reducereflected glare in many cases. Reflected glare can also be reduced byadjusting the position of the seeing task and by control I ing the distributionpattern of lighting fixtures.

In addition to the above information about glare. the fol lowing should beconsidered:

I. Glare is influenced by characteristics of the room and the use ofluminaires. This is particularly true when considering reflectedglare.

2. Luminai re brightness that is comfortable in a smal I office where theI ighting units are out of range of vision may be excessive in largerrooms where the luminaires farthest away may approach the I ine ofvision.

3. Luminaires that do not have objectionable high brightness may, ifmounted in large groups. present a total picture that isuncomfortable. This usually results when some type of fluorescentluminaires are mounted across the line of sight in an area withrelatively low cei lings.

4. The color of wal Is and cei lings is extremely important. This isparticularly true with reflected glare. Since specular reflection isdirectional. it is frequently possible to prevent reflected glare bypositioning the I ight source. the work surface, or the worker so thatthe reflected I ight wi I I be directed away from the eyes. Reflectedglare may also be control led by means of large-area, low-brightnesssources, and by using light colors with dul I. nonglossy reflectedfinishes on furniture and working surfaces.

The next concept to consider when dealing with quality of light is thebrightness ratio or brightness contrast. Brightness is sometimes cal led


luminance or photometric brightness. The brightness ratio refers to thedi fferent levels of brightness in the area of a task and the immediatebackground surrounding the area. Even though the differences in brightnessbetween a surrounding area and the task may not be severe enough to causeglare, the differences in brightness may be detrimental to the lightingquality. If there is a difference in brightness between the task area and thesurrounding background, the eyes wi I J continuously have to adapt between thetask and the surrounding area. It takes some time for the eyes to do thiskind of adaptation; and. therefore, the visibi lity of the task wi I I beaffected. In addition, brighter surroundings tend to attract the eyes awayfrom the task. The eyes function more efficiently and comfortably when theluminance within the visual environment is not too different from that of theseeing task. To reduce the effect, maximum luminance ratios are recommendedas shown in Table 5.3.2. A ratio of the brightness of the task to that of theimmediate surroundings of 3 to I is generally acceptable. Ratios no greaterthan 10 to J anywhere in the field of vision are desirable, and 30 to I or 40to I is the maximum permissible.

As an aid in achieving these reduced luminance ratios, the reflectanceupon room surfaces and equipment should be as I isted in Table 5.3.3.

The third factor influencing the qual ity of I ight is diffusion. Diffusionis I ight coming from many directions as opposed to I ight coming from onedirection. Oi ffusion is measured in terms of the absence of sharp shadows.The degree or absence of diffusion depends upon the type of work beingperformed. Perfectly di ffuse light is ideal illumination for many criticalseeing tasks; for example, in schools and offices. Where polished metalsurfaces must be viewed. a highly diffused light is essential to preventannoying specular reflections. In other cases, direct lighting may be moreimportant or desirable than di ffuse lighting; for example, surfaceirregularities that are almost invisible under diffuse Iight may be clearlyrevealed in I ight directed at a grazing angle. Diffusion is achieved bymultiple I ighting sources, by having a large number of low brightnessluminaires. by indirect or partially indirect I ighting in which the cei lingsand wal Is become secondary sources, and by light-colored, matte finishes oncei lings, wal Is. furniture, and even on the floors. A quality lighting systemwi I I have luminaires spaced so that the ratio of the intensity below theluminaires to the intensity between the luminaires is 1 to 1. Ratios of 1.5to 1 are acceptable, and the maximum is 2 to 1.

The fourth factor to consider when discussing the qual ity of I ight iscolor. Color is the sensation produced in the eye in response to I ight incertain portions of the dichromatic spectrum (wavelengths of 3800 to 7200angstroms; to convert angstroms to inches multiply by 3.937 x 10-9 ). Theeye is more sensitive to energy emitted at certain wavelengths than atothers. The effectiveness of energy emitted at a given wavelength inproducing a response in the eye is indicated by the relative luminosityfactor. (See Figure 5.3.1.>

A source of I ight might be emi tted in (1) a narrow band of one or twofrequencies (line spectrum), (2) a continuous spectrum containing variousquantities of al I frequencies in the visibi lity spectrum (such as tungsten


Table 5.3.2

RecOllll1ended maximum luminance ratios.

Env i ronmen ta ICIass i fica t ion

A B CI. Between tasks and adjacent

darker surroundings 3 to 3 to 5 to

2. Between tasks and adjacentlighter surroundings 1 to 3 1 to 3 1 to 5

3. Between tasks and moreremote darker surfaces 10 to 1 20 to 1

4. Between tasks and moreremote lighter surfaces I to 10 1 to 20

5. Between luminai res (orwindows. skyl ights. etc. )and surfaces adjacent to them 20 to 1 *

6. Anywhere within the normalfield of view 40 to 1 * *

*Luminance ratio control not practical.

A. Interior areas where reflectances of entire space can be control led inI ine with recommendations for optimum seeing conditions.

8. Areas where reflectances of immediate work area can be control led. butcontrol of remote surroundings is limited.

C. Areas (indoor or outdoor) where it is completely impractical to controlreflectances and difficult to alter environmental conditions.

Source: The Industrial Environment: Its Evaluation and Control.

lamps). or (3) an equal energy spectrum containing equal amounts of energy ateach wavelength in the visible spectrum.

Color is often described by a temperature. where the temperature comparesthe color of a light source with the color of a black box heated to varioustemperatures (usually measured in degrees Kelvin). Designation of color bytemperature is usually limited to colors with continuous spectrumcharacteristics because black bodies do not emit colors comparable to thosewith line-band rad iat ion.


Table 5.3.3

Recommended reflectance valuesapplying to environmental classifications

A and B.

Cei ling

Walls

Desk and bench tops.machines and equipment

Floors

Ref lectance"(percent)

80 to 90

40 to 60

25 to 40

Not less than 20

"Reflectance should be maintained as near as practicalto recommended values.

Source: The Industrial Environment: Its Evaluation and Control.

Figure 5.3.1

Relative luminosity factor.

>..enoz~:::l~

w>>=c(~

wa:

100

75

50

25

4000 5000 6000 7000

WAVE LENGTH (ANGSTROMS)


Objects appear to be a certain color because they have the abi I ity toabsorb I ight energy of particular wavelengths. The characteristic of thereflected I ight determines the color of the object.

For performance of ordinary visual tasks. no one color I ight source hasany advantage over any other. However, color must be considered a key factorin special ized appl ications. For example, minor color differences are bestdistinguished when an object is viewed under a light with low energy in thespectral region of the object's maximum reflectivity.

Color may also be important for psychological reasons. Certain colorsconvey a feel ing of warmth. and others appear cool. The design of a lightingsystem must recognize psychological and traditional factors in achievement ofqual ity for a particular seeing task.

Luminaire Classi fication

Luminai res are designed to control the source of I ight so that it can bebetter used for a given seeing task. The materials used in luminai res aredesigned to reflect. refract. di ffuse, or obscure I ight. Luminaires areclassified into two general types. general and supplemental. General lightingluminaires are subdivided as shown in Figure 5.3.2.

Indi rect Lighting. Ninety to 100 percent of the light output of theluminaire is directed toward the cei ling at an angle above the horizontal.Practically al I the I ight effective at the work plane is redirected downwardby the cei I ing and. to a lesser extent, by the side wal Is. Since the cei lingis in effect a secondary lighting source, the illumination produced is quitedi ffuse in character. Because room finishes play such an important part inredirecting the light. it is particularly important that they be as light incolor as possible and be carefully observed and maintained in good condi tion.

The cei I ing should always have a matte finish if reflected images of lightsources are to be avoided. For comfort. the cei I ing luminance must be withinthe prescribed limits. Di ffuse I ighting is usually desirable because it giveseven distribution and minimum shadows and minimum reflected glare.

Semi-Indirect Lighting. Sixty to 90 percent of the I ight output of theluminaire is directed toward the cei ling at angles above the horizontal whi Iethe balance is directed downward. Semi-indirect I ighting has most of theadvantages of the indirect system but is sl ightly more efficient and issometimes preferred to achieve a desirable luminance ratio between the cei lingand the luminaire at high-level instal lations. The diffuse medium employed inthese luminaires is glass or plastic of lower density than that employed inindirect equipment.

General Diffuse or Indirect Lighting and Direct-Indirect Lighting.to 60 percent of the light is directed downward at angles below thehorizontal. The major portion of the illumination produced on ordinaryworking planes is a result of I ight coming directly from the luminai reois. however, a substantial portion of the light di rected to the cei ling

Forty

Thereand


Figure 5.3.2

Classififcation of luminaires.

SEMI· INDIRECT

IfINDIRECT

DIRECT· INDIRECT

SEMI· OlRECT

GENERAL DIFFUSE

the side wal Is. The difference between the general diffuse anddirect-indirect I ighting classification is the amount of light produced in ahorizontal direction. The general diffuse type is exemplified by theenclosing globe (lamp) which distributes I ight nearly uniformly in al Idirections, whi Ie the direct-indirect luminaire produces very I ittle light ina horizontal direction due to the density of its side panels.

Semi-Direct Lighting. Sixty to 90 percent of the light is directeddownward at angles below the horizontal. The light reaching the normalworking plane is primari Iy the result of the light coming directly from theluminaire. not from the cei I ing or from the walls. There is a relativelysma I I indirect component, the greatest value of which is that it brightens thecei I ing around the luminaire, with the resultant lowering of the brightnesscont rasts.

Supplementary Lighting. The supplementary lighting category of luminai reis also subdivided. These luminaires are used along with the general lightingsystem but are localized near the seeing tasks to provide the higher levels or


quality of light not readily obtainable from the general lighting system ..They are divided into five major subtypes, from S-I to S-V, based upon theirI ight distribution and luminance characteristic. Each has a specific group ofappl ications, as shown in Figure 5.3.3.

Figure 5.3.3

Supplementary luminaires.

a b c d e

Exempt.. of Placement 0' Supplementary Luminaires (al L.umlna,r.'ocatedto J)levent reflected glare.. · reflected light does not COInCide with angle ofView Ib) R.flected hght COIncideS with angf. of View CC~ Low angl. hght1no toemp"aStze surface Irregularities. (dl uroe·." •• surface and paltern a'.r.flected toward 1". eye. ca' Translllum,nation from diffuse sources

Lighting Systems or I I lumination Methods

The illumination produced by anyone of the five types of luminairesystems may be further classi fied according to the distribution of lightthroughout the area. Whether the lighting is general. localized general. orsupplementary depends upon the location of equipment and its distributioncharacteristics.

General Lighting. General lighting is the arrangement of lightingequipment so that a uniform level of illumination is produced. Factorsaffecting uniform distribution of light are:

a. the physical characteristics of the roomb. the level of i Iluminat"ion desi redc. the appearance of the finished installation

Uniform I ighting can be obtained using the lumen method (described later)which gives the number of luminaires needed to provide a certain quantity oflight. After the number of luminaires is computed, the approximate locationcan be made so that the total number of .Iuminaires can be adjusted to beevenly divisible by the number of rows. The exact distance between fixturesis determined by dividing the length of the room by the number of luminairesin a row, at lowing for about one-third of this distance between the wal I andthe first unit. In a simi lar manner, the distance between the rows is thewidth of the room, divided by the number of rows, with about one-thi rd of the


distance left between the side of the wal I and the first row. In high-cei lingindustrial areas, these recommended distances may be up to one-half of theluminaire spacing.

Localized General Lighting. Localized general I ighting is the positioningof general I ighting equipment with reference to particular work areas wherehigh intensities are necessary, with the spi I I light from the same luminairesusually providing sufficient illumination for adjacent areas. Luminai res of adirect, semi-direct, or direct-indirect type are usually employed for thispurpose since a substantial direct component is essential where it isdesirable to concentrate most of the Iight on the restricted area beneath theluminaire; that is, the work plane.

Supplementary Lighting. Supplementary I ighting is the provision ofrelatively high intensity at speci fic work points by means of direct lightingequipment used in conjunction wi th general local ized illumination. I t isfrequently necessary where specialty-seeing critical tasks are involved andwhere it is not feasible to provide the desi red intensity by either generallighting or localized general lighting. It is also used where light ofdirectional qual ity is required for certain inspection tasks. Equipment usedfor this purpose varies in distribution characteristics depending upon thearea to be covered, by the distance from the equipment location to the work.point. and the foot-candles required. When using supplementary I ighting, caremust always be exercised to keep a reasonable relationship between theintensities of the general illumination and the supplementary I ighting, sincean excessive luminance ratio between the work point and its surroundingscreates an uncomfortable seeing condition.

Other Factors to Consider When Designing a Lighting System

When designing a I ighting system, one must consider the quanti ty of light.the quality of light, the type of light (that is, direct or indirect), and thetype of lighting system (e.g., supplementary, general localized, or general).In addition, other factors should be considered. Such factors are:

I. The choice of I ight source• Fi lament• Mercury vapor• Fluorescent

2. Heat produced from the source

3. Efficiency of the lamp or I ight source

4. Electrical features• Use equipment that conforms to the industrial specifications• Use adequate type of wiring and circuits

5. Mechanical structure of the support of the fixtures

6. Appearance/decoration


One of the other characteristics to consider when designing a lightingsystem is the maintenance of the luminaires. There are three things to lookat in maintenance of luminaires.

1. The light source

2. The luminaire

3. The room surface

The life of the I ight source is a maintenance problem. Length of life isdifferent for filament, mercury vapor, and fluorescent lamps. Filament lampshave the least length of I ife. Mercury vapor and fluorescent last longer;however, near the end of their life, they produce only about 75 percent oftheir original output. It is frequently found to be economical to establish areplacement program in which new lamps are instal led before the old ones havereached the end of their I ife; that is, the 75 percent of thei r originaloutput. Such a program can be best carried out by systematically replacingthe lamps in a speci fic area after they have burned a predetermined number ofhours. This procedure is conmonly termed "group relamping." This method hasan advantage in that it results in less variation in the illumination leveleffectiveness in the area.

The second factor to consider in maintenance of the system is theluminaires. Luminaires do not function efficiently when they are covered withdirt. The amount of dirt depends upon the characteristics of the environment,the room area, and the type of activity being conducted in the room. Thereason dirt interferes with the luminaire is that the light must travelthrough the layer of dirt; and because dirt changes the distributioncharacteristics of the equipment, it is necessary to take this intoconsideration when the lamp is to provide a direct beam of light--the dirtmakes the resulting light di ffuse. Because maintenance of the luminaires andthe dust accumulating on them is a problem. one must consider these problemsbefore selecting the type of luminaire. One should look for such things ashow difficult the unit is to handle. its weight, its size, its accessibi lityor inaccessibi I ity. One must also look to see if the luminaire is hinged orotherwise secured to the main body of the fixture. For purposes of cleaning,it is an advantage to be able to remove the lamps and reflecting equipmentreadi Iy. The normal cleaning heights of the luminaires must also beconsidered. They can be cleaned at regular heights usually from stepladders.However, where cei lings are high or the floor area beneath the luminai res isinaccessible. telescoping ladders with extension platforms may be required.These are frequently necessary for use in machine shops and largeauditoriums. In these conditions. the possibi I ity of catwalks or messengercables should be considered. The cleaning schedule depends upon how di rtythe environment is. I f the cleaning coincides with the replacement schedule,the labor costs wi I I be reduced.

The third factor to consider in maintenance is the· room surface. I f theI ighting system is indirect lighting, the dirt on the room surfaces may affectthe quality of I ight. A reduction in the reflection factor due to di rt has


less effect in a direct system than in an indirect or partly indirect system.The necessity for cleaning or refinishing room surfaces varies withconditions. In areas where the dirt sticks to the surface. wal Is and cei lingsshould be reconditioned once or twice a year. Where the dirt condition isless severe or where air-cleaning systems are employed, room surfaces may bepermitted to go several years between servicing.

Lumen Method of Lighting Design

Introduction

The fol lowing characteristics of designing a good I ighting system havebeen discussed: the quality of light; the quantity of light; luminaireclassification; illumination system; and the maintenance of luminaires, roomsurfaces, and light sources.

There are two types of calculations that can be used in designing alighting system. One is the lumen method; the other is the point-by-pointmethod. The lumen method is appropriate for general ized lighting only. It isa way for computing the average illumination throughout the entire room. Theprocedure is not very good when general localized or supplementary lighting isused. In these cases, one must compute the illumination at the point wherethe actual seeing task is located. The average illumination throughout theroom is meaningless in this case. In the case of generalized localizedI ighting or supplementary lighting. the point-by-point method is used tocalculate the quantity of i (Iumination. The point-by-point method wi I I not bediscussed in this text.

The first step in the lumen method is to analyze the seeing task and theworking environment. One should ask himself the fol lowing questions:

1. How should the task be portrayed by the light?2. Should the I ighting be diffuse or directional or some combination of

both?3. Are shadows important?4. Is color important?5. What is the area atmosphere and, therefore, the type of maintenance

characteristics that wi I I be needed?6. What are the economics of the lighting system?

From the answers to the above questions, one can determine the followingtwo things:

1. What level of illumination is needed for the task. (This can bedetermined by looking at the recommended standards.)

2. The type of luminaires that are needed (that is, direct or indirect)and the required maintenance considerations.

Once the type of luminaires that are going to be used and the amount ofillumination needed have been determined, then it is possible to calculate the


number of luminaires needed to produce that illumination using the techniquethat follows.

The Formula

The lumen method is based on the definition of a foot-candle equal ing onelumen per square foot; thus

Lumens striking areaFoot-candles =

Square feet of area

By knowing the initial lumen output of each lamp (publ ished by the lampmanufacturer>, the number of lamps installed in the area, and the square feetof area, one can calculate the lumens per square foot generated initially inan area. However, this value differs from the foot-candles in the area. Thisdifference occurs because some lumens are absorbed in the luminaire and alsobecause of such factors as dirt on the luminaire and the gradual depreciationin lumen output of the lamp. (Recal I that fluorescent lamps operate at onlyabout 75 percent of their original output at the end of their life. > Thesefactors plus others are taken into consideration in the lumen method formulawhich is as fol lows:

Lamps per luminaire x lumens per lamp xcoefficient of uti I ization x light loss factor

Foot-candlesArea per luminaire

By manipulating this equation, one can determine the number of luminairesneeded. This is as follows:

Foot-candles x areaNumber of luminaires =

Lamps per luminaire x lumens per lamp xcoefficient of uti I ization x I ight loss factor

Or one can calculate the number of lamps needed:

Foot-candles x areaNumber of lamps =

Lumens per lamp x coefficient of uti I ization xI ight loss factor

And the number of luminaires can be computed as fol lows:Number of lamps

Number of luminaires = -----------------Lamps per luminaire

Knowing the type of luminaires and the number or quantity of ligh! needed,the next step is to use the formula: that is, to apply the formula to theproblem to determine the number of luminaires that are needed to cover thearea. To use the formula, it is necessary for the user to understandconceptually some of the quantities in it; for example. the coefficient ofutilization and the light loss factor.


Cae ff ic ien t 0 f Uti I iza t j on

The coefficient of uti I ization is the ratio of the lumens reaching thework area (assume a horizontal plane 30 inches above the floor) to the totallumens generated by the lamps. It is a factor that takes into account theefficiency and distribution of the luminai res, luminaire mounting height. roomproportions, and reflectance of wal Is. cei I ings and floors. (Note: Becauseof multiple reflections within a room. some I ight passes downward through theimaginary work plane more than once. Under some circumstances, this may causethe coefficient of uti I ization to be larger than 1.)

In general, the higher and narrower the room. the larger the percentage ofI ight absorbed by the walls and the lower the coefficient of uti I ization.Rooms are classi fied according to shape by ten room-cavity ratio numbers. Theroom-cavity ratio is computed using the fol lowing formula:

5h (room length + room width)Room-cavity ratio = ---------------------------

room length x room widthwhere h is the height of the cavity.

A more conveniently used formula is the fol lowing:

10hRoom-cavity ratio =--------- x Gaysunas ratio

room width

The Gaysunas ratio comprehends the influence of room length and varies withthe ratio of the room length to the room width. The Gaysunas number isselected from Table 5.3.4.

Table 5.3.4

Gaysunas ratio.

Room LengthRoom Width

1.001.251.502.002.503.004.005.00CD

Source: Westinghouse Lighting Handbook.

GaysunasRat io

1.09/105/63/47/102/35/86/10

1/2


One must compute the room-cavi ty ratio before looking up the coefficientof uti I ization. A room can be broken up into three cavity areas: thecei ling-cavity area, the distance between the cei I ing and the luminai re plane;the room-cavity area, the distance between the luminai re plane and the workplane; and the floor-cavity area, the distance between the floor and the workplane.

In effect, then there can be a cei ling-cavity ratio. a room-cavity ratio,and a floor-cavity ratio. AI I these cavity ratios are computed using theformula given previously.

Assume a room 20 feet by 40 feet by 12 feet high with the luminai resinstal led 2 feet from the cei ling and the work plane 2,5 feet from the floor.The cei ling-cavity ratio would be equal to

10hCei I ing-cavi ty ratio = x Gaysunas ratio

room width

where h is the distance between the cei I ing and the luminai re plane: inthis case, 2 feet.

From the table of Gaysunas ratios, the Gaysunas ratio for room length/roomwidth of 2.00 is 3/4. Substituting 2 feet for hand 3/4 for the Gaysunasratio in the formula above, the cei I ing-cavity ratio would be equal to:

10 x 2 ftCei ling-cavity ratio =----

20 ft.

= 0.75

The floor-cavity ratio would be equal to

10hFloor-cavity ratio =

(3/4)

x Gaysunas ratioroom width

where h is the distance of the work plane from the floor; in this case,2.5 feet. The Gaysunas ratio remains the same.

Substituting in the formula, the floor-cavity ratio would be equal to

10 x 2.5 ftFloor-cavity ratio =

20 ft

= 0.94

(3/4 )

The room-cavity ratio also can be computed using the same formula:

10hRoom-cavity ratio = x Gaysunas ratio

room width

where h equals the distance of the room cavity, the distance between thework plane and the luminaire plane. In this case, h would be 7.5 feet,and the Gaysunas ratio would remain the same.


Substituting these values in the formula gives

10 x 7.5 ftRoom-cavi ty rat io = (3/4)

20 ft

= 2.81

It should be pointed out that tables are avai Jable to compute the cavityratios. One of these tables is shown in Table 5.3.5. The calculations justcompleted can be determined using this table. For example. from the table forroom width 20 feet, room length 40 feet and cei I ing cavity 2 feet. thecei I ing-cavity ratio is equal to about 0.75.

The cei ling-cavity ratio is used to look up the coefficient ofuti I ization. However, before the room-cavity ratio can be used for thispurpose, the reflectance of the cei I ing and of the wal Is must be known. Theyare two cei ling reflecting values that are important: the actual reflectanceof the cei J ing and the effective cei I ing reflectance. The effective cei lingreflectance is actually an adjustment of the actual cei ling reflectance, usingthe cei ling-cavity ratio. For example, a room is 20 feet by 40 feet by 12feet high. the actual cei I ing reflectance is 80 percent, and the reflectanceof the wal Is is 50 percent. Although the actual cei I ing reflectance is 80percent. this must be adjusted for the fact that the lights wi I I be instal led2 feet from the cei I ing. This wi I I change the reflectance. To compute theeffective cei I ing reflectance, a table such as the one provided in Table 5.3.6must be used. For the example where the cei ling-cavity ratio is 0.75 andtheactual cei ling reflecfance is 80%--which on the table is cal led the basereflectance--and the wal I reflectance is 50%, the effective cei lingreflectance would be between 69% and 71%, or about 69.68%.

Once the effective cei I ing-cavity reflectance is known, the coefficient ofuti I ization can be determined from a table. such as is shown in Table 5.3.7.Notice that to look up the coefficient of uti I ization the fol lowinginformation must be known: the type of luminaire, the type of distribution ofthat luminaire, the effective cei ling-cavity reflectance, the wal Ireflectance, and the room-cavity ratio. For example, using the firstluminaire on the table. the coefficient of uti lization can be found for aneffective cei I ing-cavity reflectance of 80%, war I reflectance of 50%. and aroom-cavity ratio (RCR) equal to 5. The coefficient of uti lization is 0.50.(Note: The cei ling-cavity reflectance in the table refers to the effectivecei I ing-cavi ty reflectance.) The categories of luminai res as they areindicated on the table wi I I be discussed later.

For cei ling-mounted luminaires or recessed luminaires. the cei ling-cavityreflectance is the same as the actual cei ling reflectance; that is. one doesnot need to compute the cei ling-cavity ratio and adjust the cei lingreflectance by the cei ling-cavity ratio. But for suspended luminaires. it isnecessary to determine the effective ceiling-cavity reflectance as has beendone in the example.

Table 5.3.5

Room-cavity ratios.

Room Dimension. CavilW Dep'h

Width length 1.0 1.5 2.0 2.5 3.0 3.5 4.0 5.0 &.0 7.0 • II to 11 12 14 1& 20 25 30

• • 12 19 25 3 I 3 7 44 50 62 75 88 100 112 12510 1 1 I 7 22 28 34 39 45 56 67 79 90 101 113 12414 10 15 20 25 30 34 39 49 59 69 78 88 97 107 11 720 09 13 1 7 22 26 3 I 35 44 52 6 I 70 79 88 96 105 12230 08 12 16 20 24 28 32 40 41 55 63 7 1 79 87 95 11040 07 I I 15 19 23 26 30 37 HI 53 59 65 74 8 I 88 103 II 8

10 10 10 15 20 25 30 35 40 50 60 70 80 90 100 110 12014 09 13 I 7 2 I 26 30 34 43 5 I 60 69 78 86 95 104 12020 07 I I 1 5 19 23 26 30 37 4S 53 60 68 75 83 90 105 '2030 07 10 I 3 1 7 20 23 27 33 40 47 53 60 66 73 80 94 10640 06 09 1 2 16 19 22 25 3 1 37 44 50 56 62 69 75 87 100 12560 06 09 12 I 5 I 7 20 23 29 35 41 47 53 59 65 7 I 82 94 11 7

12 12 08 12 I 7 2 I 25 29 33 42 50 58 67 75 84 92 100 11716 07 11 15 18 22 25 29 36 44 51 58 65 72 80 87 102 11624 06 09 12 16 19 22 25 31 37 44 50 56 62 69 75 87 100 12536 06 08 I I 1 4 1 7 19 22 28 33 39 44 50 55 60 66 78 88 11050 05 08 10 I 3 1 5 18 2 1 26 3 I 36 4 I 46 5 I 56 62 72 82 10270 05 07 10 12 1 5 I 7 20 24 29 34 39 44 40 54 58 68 78 97 122

14 14 07 11 1 4 18 21 25 29 36 43 50 57 64 7 I 78 85 100 11420 06 09 12 , 5 18 2 I 24 30 36 42 49 55 6 I 67 73 86 98 12330 05 08 10 I 3 16 18 2 I 26 3 I 37 42 47 52 58 63 73 84 10542 05 07 10 12 I 4 I 7 19 24 29 33 38 43 4 7 52 5 7 67 76 95 11960 04 07 09 1 I I 3 I 5 18 22 26 3 I 35 39 44 48 52 61 70 88 10990 04 06 08 10 12 1 4 16 20 25 29 33 37 4 I 45 50 58 66 83 103 124

17 17 06 09 1 2 15 18 21 23 29 35 4 I 47 53 59 65 70 82 94 1172S 05 07 10 12 15 I I 20 25 30 35 40 45 50 55 60 70 80 100 12535 04 07 09 I I I 3 1 5 I 7 22 26 3 I 35 39 44 48 52 6 I 70 87 10950 04 06 08 10 I 2 I 4 16 20 24 28 3 I 35 39 43 45 54 62 77 97 I I 680 04 05 07 09 11 12 14 18 2 I 25 29 33 36 40 43 5 I 58 72 90 109

120 03 05 07 08 10 1 2 13 I 7 20 23 27 30 34 37 40 4 7 54 67 84 10120 20 05 07 10 1 2 I 5 I 7 20 25 30 35 40 45 50 55 60 70 80 100 125

30 04 06 08 10 I 2 I 5 I 7 2 I 25 29 33 37 4 I 45 49 58 66 82 103 12445 04 05 07 09 11 I 3 I 4 18 22 25 29 33 36 40 43 5 I 58 72 9 I \0960 03 05 07 08 10 12 I 3 I 7 20 23 2 7 30 34 37 40 47 54 67 84 10190 03 05 06 08 09 I I 12 15 18 2 I 24 27 30 33 36 42 48 60 75 90

.~~-e-J1.L ~." 06 07 08 10 11 14 1 7 20 23 26 29 32 34 40 46 5 7 72 86-- f-'24 24 04 06 08 10 1 2 I 5 I 7 21 25 29 33 37 4 I 45 50 58 67 82 103 124

32 04 05 07 09 11 1 3 1 5 1 8 22 26 29 33 36 40 43 5 I 58 72 90 11050 03 05 06 08 09 I I 12 15 18 22 25 28 3 1 34 3 7 4 4 50 62 78 9470 03 0 .. 06 07 08 10 11 1 .. 1 7 20

~~25 28 30 33 38 4 4 55 69 82

100 03 04 05 06 08 09 10 I 3 16 18 24 26 29 3 I 3 7 .. 2 52 65 79160 02 04 05 06 07 08 10 1 2 14 1 7 1 9 2 1 24 26 28 33 38 4 7 59 7 I

By permission of Norlh American Philips l/ghling Corporal/oil.

:r0.c:en....,~

=c:~.::lQl~.o::l

~-

Table 5.3.6

Effective cavity reflectance.

--Base Rell. 00 90 80 70 60 50

.-

Wall Rell 00 908070503010 0 9080 70 50 30 10 0 90 80 70 50 30 10 0 90 80 70 50 30 10 0 90 80 70 50 30 10 0

-'-T0.2 ~\l1I8 86116 65 114 82 7978711 77 76 7~ 72 7069611 67 66 65 b4 60 5'.1 59 ~II 56 55 53 50 50 49 411 4 7 ~6 4404 88878684817976 7977 76 7~ 72 7068 696867656361 58 1;0 59 59 57 54 52 50 50494847 45 44 420.6 87868480 77 7~ 73 78 76 7'> 11 68 li5 63 6967 65 63 59 57 54 tiO 511 57 55 5 I 5046 5048 47 45 43 41 3808 87858277736967 78757369656157 6866 64 60 56 53 ~o '>9575654484643 5048 47 44 4038 361.0 868:1 80 75 696462 77 74 72676<'5755 6865 62 58 53 50 47 ')957 ~5 ., 1 454341 50484643383634

1.5 85110 7668 61 5551 75 72 b1l61 54 40J 46 67 b2 59 54 464240 59 55 52 46 40 37 34 504745403431262.0 83 77 72625347 43 i 4 1;9 64 56 48 4 1 38 66605649403633 58 54 50 43 35 31 29 50 46 43 37 3026242.5 8:' 15685747 4036 736761 ~14235J2 656054453631 29 585347 39 30 25 23 50464\ 3527222 \3.0 80 7264 52 42 14 30 72655847 37 1027 64 58 52 42 32 2724 57524637282320 5045403224 19 173.5 7'1706148373126 71 63 55 n 33 26 24 63575038292321 575044 35252017 50443930 n 17 15

JJ776\15844332522 7061 53 40 JO 22 20 63 55 48 36 26 20 17 57 49 42 32 23 \ 8 14 5044 J8 28 20 15 12

5.0 75 59 53 38 28 20 16 6858483525 18 14 61 5244 31 22 16 \2 56 48 4028 20 14 1 I 50423525 17 12096.0 73614934241611 66 55 44 31 22 15 10 6051 4\ 2819 1309 55453725171107 50 42 34 23 15 10068.0 68554227181206 62 50 38 25 17 11 05 51463523\51005 53423322 140804 49403019120703

10.0 65 51 36 22 150904 59 46 33 21 14 08 03 55433119120803 5139291811 0702 47372717 \00602--

Ba.. Rell. o. 40 30 20 10 0-.

Wall Rail. 00 90 80 70 50 30 10 0 90 80 70 50 30 10 0 90 80 70 50 30 10 0 908070503010 0 90 80 70 50 30 10 0

0.2 40 40 39 39 38 36 36 31 31 30 29 29 28 U 212020201919 t7 11 11 11 '0100909 0202 02 0 I 0 I 00 00.4 4 I 40 39 38 36 34 34 3\313029282625 22 21 20 20 19 18 16 1211 11 11 100908 04 03 03 02 01 00 00.6 4 I 40 39 37 34 32 31 32 31 3028 26 25 23 23 21 21 19 18 17 15 13 13 12 I I 10 0808 050504 30020 I 00.8 4 1 40 38 36 33 31 29 32 31 3028 25 23 22 24 2221 19 18 16 14 151413 11100807 07 06 05 04 02 01 01.0 4230383432 29 27 3332 :10272422 20 25 23 22 19 17 15 13 16 14 13 12 100807 08070604020\ 0

Ui 4239 J7 J<! 28 24 22 34 33 3025 22 18 17 2624221816131\ 181b 15 12 100706 11 10 08 06 03 01 02.0 ~2393ti31252119 35 33 29 24 20 16 14 282523 18 '5 1\ 09 20 18 16 I3 09 06 05 1412\007040\ 0

I 2.!> 43 39 35 29 23 18 15 36322924181412 29262318\41008 22 20 I 7 13090504 161412080502 03.0 4)193527:'11613 37 J3 29 22 I 7 \2 10 30272317\30907 24 21 18 IJ 09 05 03 18 16 13090502 03.5 44 39 34 26 20 14 12 3833292115 \009 32 27 23 17 12 08 05 26 22 19 13090503 2017 15100502 0

40 443113325181210 311332821140907 33282317 11 0107 27 23 20 14 09 04 02 2218 \5 100502 050 45383122 15 1007 39 33 28 19 I3 08 05 35292416100604 30 25 20 14 08 04 02 25 21 \ 7 I \ 06 02 06.0 44 37 30 20 13 08 05 393327 \811 0604 36302416100502 3 I 26 21 14 08 03 0 \ 2123\11120602080 44 35 28 18 I 1 0603 40 33 26 16 09 04 02 37302315080301 33272113070301 3025 20 120602 0

10.0 43 ]4 25 15080502 4032 24 14 08 OJ 01 37292213070301 34282\ 12070201 31 25 20 12 06 02 0

By permission of North American Phi lips Lighting Corporation.

~I\.)

:l0c:en.....,0;'

:::r:-<lQCD':lCl>

m:l<e.:lCl>Cl>..,:llQ

Table 5.3.7

Coefficients of uti lization.

COEFFICIENTS OF UTILIZATIONi I

R.II.cllnc..

S~8Clng I Caillng I 80·t. I 50~ I 10"- I 0%LUMINAIRE I DISTRIBUTION I 0110

ClIVllV • •

Elc..d Wilt. 50% 30% 10% 50"1. 30"1. 10"1. 50% 30"1. 10% 10%

RCR Coalllclanli 01 Utlllllllon

Calegory III~ I I 85 82 79 79 77 75 73 72 71 69

0 2 74 69 65 70 66 62 65 62 59 58, 3 65 60 54 62 57 53 57 54 51 4913 . 4 58 51 46 55 49 45 51 47 44 42

- Mounhng 5 50 44 38 47 42 37 45 40 36 356 44 38 33 43 36 32 40 35 32 30, Helghl 7 40 33 28 38 33 28 36 32 27 26

l'8 36 29 24 34 28 24 32 27 23 229 33 25 20 31 25 20 29 24 20 18

VenUlaled Dome Relleclor 10 29 22 18 28 22 18 26 21 18 17

Calegory I

&1 \ 08 , 05 102 1 10 99 91 94 93 91 89

° 2 98 93 89 93 89 86 88 85 82 80

t 3 89 83 78 85 80 76 80 76 73 7,

, 5 . 4 81 74 68 77 72 67 73 69 65 64- Mounhng 5 73 66 60 70 64 '59 56 62 58 56, 6 61 59 53 64 58 52 61 56 52 50Helghl 7 60 52 47 58 51 46 55 50 46 45100 8 54 46 40 52 45 40 49 44 40 38

R·52 Fllamenl Retleclor Lamp 9 48 40 35 46 39 35 44 38 34 33 I 3'"Wide O'SI -500· and 750·Wall 10 43 36 30 42 35 30 40 34 30 28 a.c:<II...

Calegory I

~1 1 10 108 105 , 04 102 100 97 96 95 93 I .,

0 2 102 98 94 97 94 91 91 89 88 86 iii', 3 95 90 85 91 87 83 86 83 81 796 . 4 88 82 78 85 80 76 81 77 75 73

5 82 76 71 79 74 70 76 72 69 67 c:- Mounhng 36 77 70 66 74 69 65 72 68 64 63, Helghl 7 71 65 61 69 64 60 67 63 60 58 :;III

100 8 66 60 56 65 59 55 63 58 55 54 ...R·57 Fllamenl Relleclorlamp 9 62 55 5' 60 55 51 59 54 50 49 o'

Narrow Oisl -500- and 750-Wall 10 58 51 47 56 5\ 47 55 50 46 45 :J

~Vol

U1~

Table 5.3.7 (Continued)~

3"a.c:<II...C...."'~ 81 78 76 76 74 72 71 69::!.

I 68 67 Ql

0 2 73 69 65 69 66 63 64 62 60 593 65 60 56 62 58 55 58 55 53 51 :I:

t 4 59 53 49 56 52 48 53 50 47 45 -<12 . cc

5 53 47 43 51 46 42 48 44 41 40 iii'- Mounllng 6 48 42 38 46 41 37 44 40 37 35 :J

l Helghl 7 43 37 33 41 36 32 39 36 32 31 (l)

8 39 33 29 38 32 28 36 32 28 27 mVenl,laled Porcelain Enamel low Bay

16 9 36 30 26 34 29 25 33 28 25 24 :Jcc

400·W Phos Coaled Vapor lamp 10 32 27 23 31 26 23 30 25 22 21 :l(l)(l)....

Calegory III ~ I 1 93 90 88 85 83 82 76 75 74 72 I :l2 86 82 79 79 77 74 72 70 69 67 10• 3 79 75 71 74 70 68 68 65 64 62

t 7 . 4 74 69 65 69 65 62 64 61 59 57

- Mounllng 5 68 63 59 64 60 57 60 57 54 536 63 58 54 60 56 52 56 53 50 49

i Helghl 7 59 53 49 56 51 48 52 49 46 45JJ 8 55 49 45 52 47 44 49 45 43 41

18" Venldaled Alum. High Bay Cone 9 50 45 41 48 43 40 45 42 39 38Disl 400-W Clear Vapor lamp 10 47 41 38 45 40 37 42 38 36 35

Calegory III ...... I 88 86 84 80 79 77 71 70 69 67.. 2 81 77 74 75 72 70 67 65 64 623 74 70 66 69 65 62 62 60 58 56

t 12 • 4 68 63 59 64 60 57 58 55 53 51

- Mounllng 5 63 57 53 59 55 51 54 51 49 476 58 52 48 54 50 46 50 47 44 43

1:. ~ Ii He'ghl 7 53 47 43 50 45 42 46 43 40 39,. 8 48 43 39 46 41 38 42 39 36 35

18" Venhlaled Alum H'eh Bay Spread 9 44 39 35 42 37 34 39 35 33 31Disl 400·W Coaled apor lamp 10 41 35 31 39 34 30 36 32 28 28

C"-"A I 86 83 80 78 76 73 68 67 65 632 77 72 68 70 66 63 61 59 57 55

II 3 68 62 57 62 58 54 55 52 49 47t 13 • 4 61 55 49 56 51 47 50 46 43 41

5 55 48 42 50 45 41 45 41 38 36- Mounltng 6 49 42 37 45 39 35 40 36 33 31i Helghl 7 43 36 31 40 34 30 36 31 28 2611 8 39 32 28 36 30 26 32 28 25 23

24'- Venilialed Porcelain Enamel 9 35 28 24 33 27 23 29 25 22 201000·W Phosphor Coaled Vapor lamp 10 32 25 21 29 24 20 26 22 19 17

CJ1.j::.en

Table 5.3.7 (Continued):i0..CVI......,

1 90 811 86 81 80 78 71 70 70 67 ~

II2 83 79 76 76 73 71 67 66 64 623 77 72 68 70 67 64 63 6\ S9 S7 I

t 4 71 66 62 66 62 S9 59 57 55 53 -<1 3 • <0

- Mounltng 5 65 60 56 6\ 57 53 55 52 50 48 (\)

6 60 55 50 56 S2 48 52 48 46 44 :::l

~Height 7 55 50 46 52 47 44 48 44 42 40 ro

'S 8 SI 4S 41 48 43 40 44 41 38 37 m:::l

24" lIenltlaled Alum High Bay I 9 47 41 38 44 40 37 41 38 35 34 <0

lOOO·W Phos Coaled lIapor Lamp 10 44 38 34 4\ 37 33 38 35 32 31 :::lroro

Calegory III I 1 88 84 81 79 77 74 69 68 66 64 I ~.

~:::l

2 77 71 66 70 65 62 61 59 S6 54 <0

" 3 68 61 56 61 S6 S2 S4 51 48 46t 13 • 4 60 S2 47 S4 49 44 48 44 4\ 39

- Mounltng S S2 45 39 48 42 37 43 38 35 336 47 39 34 43 37 32 38 34 30 28

~H8Ighl 7 42 34 29 38 32 28 34 30 26 24

"8 37 30 25 34 28 24 31 26 22 21

2 T·12 Lamps - Any Loadln8 9 33 26 21 31 25 21 28 23 19 18ForT·10Lamps-CU· 12 10 30 23 19 28 22 18 25 20 17 IS

Calegory II ~ 1 88 8S 81 77 7S 73 6S 64 62 S9

"2 77 71 67 68 64 60 S7 S5 S3 503 68 61 S6 60 SS SI SI 48 45 42

t 1 3 • 4 60 53 47 53 48 43 45 42 38 36- Mounllng 5 53 4S 40 47 41 36 40 36 33 30

6 47 39 34 42 36 31 36 31 28 26~

Height 7 42 34 29 38 31 27 32 28 24 2271 8 38 30 25 34 28 23 29 24 21 19

2 T·12 Lamps - Any Loadln8 9 34 26 22 30 24 20 26 21 18 16ForT·10lamps-CU .1 2 10 31 24 19 26 22 18 24 19 16 14

-1 84 81 78 74 72 70 61 60 S9 562 75 70 6S 66 62 59 S5 S3 51 48

" 3 66 60 56 59 54 51 49 47 44 42t 13 • 4 59 52 47 S2 47 43 44 41 38 36

- MOunllng 5 52 45 40 46 41 37 39 36 33 316 47 40 35 42 36 32 36 32 29 27

~H8Ight 7 42 35 30 37 32 28 32 28 25 23

2 T·12 lamps - Any loading I II 8 38 31 26 34 28 24 29 25 22 20Center Shield For T·1 0 lamps 9 34 27 22 30 25 21 26 22 19 17

-CU·102 10 31 24 20 27 22 18 23 19 t7 15

Table 5.3.7 (Continued)

COEFFICIENTS OF UTILIZATIONI I

ReUec••nce.

s~ec'ngI Ceiling I 10% I 50% I 10% 10%

lUMINelRE I DISTRIBUTION I ottoCawlly

bCHdWa"a 50% 30% 10% 50% 30% 10% 50% 30% 10% I 0%

RCR Coelf'clan'e of Ullllzatlon

Catego,y III ~ I I 86 83 80 78 16 73 69 67 66 64

I2 75 70 66 69 65 61 61 58 56 543 67 60 55 61 56 52 54 51 48 46

t 1 3 • 4 59 52 47 54 <49 44 48 45 41 395 52 45 39 48 42 38 43 39 35 33- Mounting 6 46 39 34 43 31 32 38 34 30 28

l He.gh' 7 41 34 29 38 32 28 34 30 26 25I. 8 37 30 25 34 28 24 31 26 23 21

3 T·12 Lamps - 4300,800 MA 9 33 26 22 31 25 21 28 23 20 18Fo,T·,OLamps-CU·'02 10 30 23 Ig 28 22 18 25 21 17 16

Category II ~ 1 85 82 79 76 13 11 64 63 62 592 75 70 65 67 63 59 51 55 52 50

II 3 66 60 55 59 54 50 51 48 45 42t 13 • 4 59 52 46 52 47 43 45 41 38 36- Mounung 5 51 44 39 46 40 36 40 36 33 30

6 46 39 33 41 35 31 36 31 28 26~

He,ghl 1 41 34 29 37 32 27 32 28 24 23.. 8 37 30 25 33 27 23 29 24 21 193 J. 12 Lamps - 4300,800 MA I 9 33 26 21 30 24 20 26 21 18 16Fo. T· 10 Lamps - CU· I 02 10 30 23 19 27 21 18 23 19 16 14 I :r

Coc:

I 10 66 63 62 59 57 52 51 4941 I II>...

Il~\tf~\\\ I

2 60 54 50 53 49 46 45 42 40 37 ..,3 52 46 41 46 41 38 39 36 33 31 iii'

t -1 5 • 4 46 39 34 41 36 32 35 31 28 26

5 40 33 28 36 30 26 31 27 24 22Mounting 6 36 29 24 32 26 22 27 23 20 18 c:

~

~Helghl 7 32 25 21 29 23 19 25 21 17 16 ~... 8 29 22 18 26 20 17 22 18 15 13 ::l

ll)

2 T·I2 Lamps - 430 MA I 9 26 19 15 23 18 14 20 16 13 11 ...For 800 MA - CU· 96 10 23 17 13 21 16 12 18 14 11 10 O·

::l

U1~-..J

Table 5.3.7 (Continued)

COEffICIENTS Of UTILIZATION

Re"ectenc..

·C""Ceiling 10% 50% 10% 0%

LU..,NAIAE DISTAl.UTION to Cnlt,

EICHel Welle 50% 30% 10% 50% 30% 10% 50% 30% 10% 0%

ACA Coenlclenle 01 UlIlIzellon

I 66 64 62 62 61 59 58 57 56 55

• 2 60 56 53 56 54 52 53 51 49 483 54 50 46 51 48 45 48 46 44 43t I 2 • 4 49 44 4t 46 43 40 44 41 39 385 44 39 35 42 38 35 40 37 34 33- Mounllng 6 40 35 31 38 34 31 36 33 31 29I Helghl 7 36 31 28 35 30 27 33 30 27 26

4 T·12 lamps - 430 MA U 8 32 28 24 31 27 24 30 26 24 23PlIsmallc lens 2' Wide - 9 29 24 21 28 24 21 27 23 21 20

ForT·IOlamps-CU·102 10 27 22 19 26 23 19 25 21 18 17

I 60 58 56 56 55 54 52 51 50 49

• 2 54 51 48 51 49 47 48 46 45 443 49 45 42 46 43 41 44 41 40 39t 12 • 4 44 40 37 42 39 36 40 37 35 34

- Mounllng 5 40 35 32 38 35 32 36 33 31 306 36 32 29 35 31 28 33 30 28 27I Helghl 7 33 28 25 32 28 25 30 27 25 24

6 T·12lamps - 430 MA II 8 30 25 22 28 25 22 27 24 22 21Prismaile lens 2' Wide - 9 27 22 19 26 22 19 25 21 19 18

For T·l 0 lamps - CU. 1 03 10 24 20 17 23 20 17 22 19 17 16

~

a.c~~

c3:i'1lI....o'~

~«)

01010

:la.c'"....:::!.~

I-<'9.(1)

Table 5.3.7 (Continued) ::lC1l

m::l'9.::lC1l

c.~1 S9 S7 SS SS S4 S2 SI SO 49 48 I C1l

:::!.2 S3 SO 47 SO 48 46 47 45 44 43 ::l• 3 48 44 41 45 42 40 43 40 39 38 <C

• • • t 1.3 • 4 43 39 36 41 38 35 39 36 34 33- Mounllng 5 39 35 31 37 34 31 35 32 30 29

~Helghl 6 35 31 28 34 30 28 32 29 27 26

7 32 28 25 31 27 25 29 26 24 238 Y·12 lamps - 430 MA is 8 29 25 22 28 24 22 27 24 21 20Pnsmallc lens 4' • 4'- 9 26 22 19 2S 21 19 24 21 19 18

ForY·l0lamps-CU.102 1O 24 20 17 23 19 17 22 19 17 16-

-~

1 S6 S4 S2 S2 SO 49 47 46 45 442 50 47 45 47 44 42 43 41 40 39

I 3 45 41 38 42 39 37 39 37 35 34t 1.2 • 4 41 37 34 38 35 32 35 33 31 30- MounllOg S 37 32 29 34 31 28 32 29 27 26

6 33 29 26 31 28 25 29 27 24 23~

Helghl 7 30 26 23 29 2S 22 27 24 22 204 T·12 lamps - 430 MA I 'I 8 27 23 20 26 22 20 24 21 19 18

Pnsmallc lens 2' W,de - 9 2S 20 18 23 20 17 22 19 17 16ForT·l0lamps-CU .102 10 22 18 16 21 18 15 20 17 15 14

Table 5.3.7 (Con t i nued )

COEFFICIENTS OF UTILIZATION, IReflectance,

SpacingCeiling I 80". I 70% I 50"1. 10%

LUMINAIRE I DISTRIBUTION I Nollo I CavHveaceed

~50% 30% 10"1.150% 30% 10% l50·... 30"1. 10"1. I 0%

RCR Coelllc,enl' 01 Ullllllllon

\ 68 65 63 65 63 6\ 61 60 58I 2 60 56 53 58 55 52 55 52 49

t 3 54 49 45 52 48 45 50 46 43\ 2 . 4 49 43 40 47 43 39 45 41 38

- Mounhng 5 44 38 34 43 38 34 40 36 33

~6 40 34 30 39 34 30 37 32 29Helghl 7 36 31 27 35 30 26 33 29 26

H 8 32 27 24 32 27 23 30 26 232 T·12 lamps - 430 MA

I9 29 24 21 29 24 20 27 23 20

I' Wide Plismalic Wrap,Around \0 27 22 \8 26 21 18 25 21 \8

1" , \ 66 64 61 64 62 60 61 59 57

• /' \- - I \ 2 59 55 52 57 54 51 55 52 49l' {/y" .•_ I 3 53 48 45 52 48 44 49 46 43t .' ... f" '"'" ~ 1 3 ' 4 48 43 39 47 42 39 45 41 38

- Htfj" Mounlonq 5 43 38 34 42 37 34 40 36 336 39 34 30 J8 34 30 36 32 29

~Helghl 7 35 30 26 34 30 26 33 29 26S. 0' cr' .... 8 32 27 23 31 26 23 30 26 23

4 T·12 lamps - 430 MA I < I 9 28 24 20 28 23 20 27 23 202' W,de Prosmaloc Wrap,Around \0 26 21 18 25 21 \8 25 20 17 I 5"

a.c::I 83 79 75 79 76 72 73 70 67

I<II~

2 71 65 60 68 62 57 62 58 54..,

11

~.3 62 55 49 59 53 47 55 49 44

0;'

t 16 . 4 55 47 4\ 52 45 39 48 42 37Mounlong 5 48 40 34 46 38 33 42 36 3\ c::- 6 43 35 29 4\ 33 28 38 31 26 3t He,ghl 7 38 30 25 36 29 24 34 27 23

8 34 26 2\ J3 25 2\ 30 24 \9 ~

"Q)

9 30 23 18 30 23 18 27 21 \ 7 ~

2 Lamp SlllP - Any LoadIng I \0 28 2\ 16 27 20 15 25 19 15 O·~

U1U1.....

Table 5.3.7 (Con t i nued )0101l\)

I .64 62 60 .6J 61 .59 .60 .59 .57 :;-Cl.

• 2 58 .55 52 57 .54 .51 .55 .52 .50 cJ 52 .48 .45 51 .47 .44 49 46 .44 CIl

t.....

12 . 4 .47 .42 .J9 46 .42 .39 .45 .41 .38 ..,- Mounlong 5 42 37 .J4 .42 .J7 .J4 .40 .J6 .34 ~

6 38 33 JO .38 .3J .30 .J7 .J2 .30 :I:i Helghl 7 .35 .JO 26 .J4 .30 26 .3J 29 .26 <.. 8 Jl .26 23 Jl 26 .2J .30 .26 .2J <9.I lamp .- An~ loading I 9 28 .23 20 .28 .23 .20 .27 .23 .20 III

2' W,de. '1' Deep /lsmallC Lens ,0 .26 .21 18 .25 .21 .18 .25 .21 .18 :lIII

m:l

Calegory VI II I I 68 .65 .62 59 .56 .54 .42 .41 .39 <9.- 2 59 .54 .51 .51 48 .44 .37 .J5 .32 :lII 3 52 .46 42 .45 40 J7 .32 .29 .27 III

tIII

4 46 40 .35 .40 .J5 .31 28 .25 .23..,

15 • :3- Mounhng 5 40 .34 .30 .35 .30 26 .25 .22 .20 106 36 .30 .26 .31 .27 23 22 .20 .17

i Helghl 7 32 .26 .22 28 23 19 .20 .17 .14S 8 29 .23 .19 .25 20 17 18 .15 .13

2 lamp - An! loading 9 26 .20 17 .23 18 15 .17 .13 .11Opaque Illes 10 .24 18 .15 .21 16 .13 .15 .12 .10

Calegory VI -- .!I fo, (iVllleS1n.1 .'e p.'n1ed

•1 whIle uu 10'1. 60 .58 56 .58 .56 54

0 2 elleel,ve ee,hng .53 .49 45 51 .47 43

t 1.510 3 e.v,ly 47 .42 37 .45 41 362.0 • 4 relleel.nee .41 .36 32 39 35 31

- Mounhng 5 " fu, c.v'hes 37 31 27 .35 .30 .26, He'ghl 6 IUI.fe 33 27 23 31 .26 .23above 7 oDSUueled or 29 24 20 .28 .23 .20

69- Ollluser 8 h.ve lower 26 .21 18 25 .20 .179 felleel.nees uu 23 .19 .15 23 18 .15

10 50'~ ellecllve 21 .17 .13 21 .16 .13lumInous Ce,long - 50'10 TransmIssIon CIlllng e.v,ly

80% Cavlly Relleclance felleel.nee

Calegory VI t«///d/.////r'(/t~~ I 42 40 .39 J6 .J5 J3 25 .24 .232 37 34 32 32 .29 .27 22 .20 .19, 3 32 29 .26 .28 25 23 19 .17 .16

/ Cove 12 10 18 Inches belOw 4 29 25 .22 25 22 19 .17 .15 .13ceIling Relleclors w,lh 5 25 21 18 22 19 .16 15 13 .11lIuorescenllamps Increase 6 23 19 .16 20 16 14 .14 .12 .10coel/,e,enls 01 ul,lIzaloon 7 20 17 .14 17 14 12 .12 10 .095 10 10~~ 8 18 15 .12 16 13 .10 .11 09 .08

9 17 13 10 15 11 .09 '0 08 .07Cove Wllhoul Relleclor I I I 10 15 12 09 13 10 .08 .09 .07 .06

By permiSSion 01 Nor Ih Amer Ican Phi Ilp~ I qlhl tllg Corpordl iOIl


NOTE: The coefficients of uti lization determined in the examplespresented wi II be appl icable for areas having 20 percent floor-cavityreflectance. If the actual floor-cavity reflectance differs sUbstantiallyfrom 20 percent, a correction may be necessary depending upon the accuracydesired. Correction factors for floor-cavity reflectance of 10 percent and 30percent are given in Table 5.3.8. The effective floor-cavity reflectance isdetermined in the same manner and using the same tables as were used indetermining effective cei ling-cavity reflectance. For 30 percent effectivefloor-cavity reflectance. multiply by the appropriate factor found in thetable. For 10 percent effective floor-cavity reflectance. divide by theappropriate factor found in the table.

In the calculations included in this chapter, correction wi I I not be madefor floor-cavity reflectance. The floor-cavity reflectance wi I I be assumed tobe 20 percent. However, you should be aware that such adjustments can andshould be made, depending upon the accuracy required.

Table 5.3.8

Correction factor for effective floot-cavity reflectancesother than 20 percent.

10

10

305010

50

10 50 30Wall ReflectancePercent

Percent Effective Cei lingCavity Reflectance

70

50 301030

80

50RoomCavi tyRat io

1.08 1.08 1.07 1.07 1.06 1.06 1.05 1.04 1.04 1.01 1.01 1.01

2 1.07 1.06 1.05 1.06 1.05 1.04 1.04 1.03 1.03 1.011.011.01

3 1.05 1.04 1.03 1.05 1.04 1.03 1.03 1.03 1.02 1.01 1.01 1.01

4 1.05 1.03 1.02 1.04 1.03 1.02 1.03 1.02 1.02 1.01 1.01 1.00

7 1.03 1.02 1.01 1.03 1.02 1.01 1.02 1.01 1.01

5 1.04 1.03 1.02 1.03 1.02 1.02 1.02 1.02 1.01

6 1.03 1.02 1.01 1.03 1.02 1.01

8 1.03 1.02 1.01 1.02 1.02 1.01

9 1.02 1.01 1.01 1.02 1.01 1.01

10 1.021.011.01 1.021.011.01

1.021.021.01

1.02 1.01 1.01

1.02 1.01 1.01

1.02 1.01 1.01

1.01 1.01 1.00

1.01 1.01 1.00

1.011.011.00

1.011.011.00

1.01 1.01 1.00

1.01 1.01 1.00

Source: Westinghouse Lighting Handbook


The Light Loss Factor

The other factor in the formula that needs to be explained is the lightloss factor.

From the day the new lighting is energized. the illumination is in theprocess of continually changing as the lamp ages. as the luminai re accumulatesdirt and dust. and as the effect of other contributing factors is felt. Somecontributing loss factors may. in some instances, tend to increase theillumination; but their net effect is nearly always to cause a decrease inillumination. The final I ight loss factor is the product of all contributingloss factors; it is the ratio of the illumination when it reaches its lowestlevel at the task just before corrective action is taken to the initial levelif none of the contributing loss factors were considered. In this context.the initial illumination is that which would be produced by lamps producinginitially rated lumens. (NOTE: Lamp manufacturers rate fi lament lamps inaccordance with lumen output when the lamp is new. Vapor discharge lamps.including fluorescent, mercury and other common types. are rated in accordancewi th thei r output after 100 hours of burning.)

There are eight contributing loss factors that must be considered. Someof these must be estimated; others can be evaluated on the basis of extensi.vetest data or published information. Of the eight factors. only four factorscan be obtained from published information. The remaining four factors haveto be estimated. Only the four common factors that can be located in testdata or by manufacturers' published data wi I I be discussed here. These fourfactors wi I I be considered the only four factors contributing to the lightloss factor.

1. Bal last performance. Fluorescent lamps as wei I as some other lampsinclude a bal last which serves as (I) an autotransformer to step up supplyvoltage (e.g., 120. 208. 240, 277, etc. volts) to the necessary starting value(e.g .. 255 or 500 volts) and (2) a choke to limit the current through thelamp. Bal last consists of a core and coi I which stabi I ize the operation ofthe lamps; a power capacitor which corrects the power factor and reduces theload on the electrical distribution system; a radio interference-suppressingcapacitor which reduces feedback of radio frequency energy to the power line;and a compound which fi I Is al I voids inside the ballast case, improving heatdissipation and reducing sound.

The Certified Ballast Manufacturers (CaM) Association specification forfluorescent lamps requires the bal last to operate a fluorescent lamp at 95percent of the output of the lamp when operated on a reference bal last. Areference bal last is the laboratory standard used by lamp manufacturers inestablishing lamp ratings. For ballast bearing the CeM label. use a factor of0.95. For bal last not bearing the CBM label, lumen output is usually lower.Lamp life is also usually shortened. Consult with the bal last manufacturerfor this I ight loss factor

2. Luminaire reflectance and transmission changes. This effect isusually sma I I but may be significant over a long period of time for luminaireswith inferior finishes or plastic. Comprehensive data are usually notavai lable.


3. L~p lumen depreciation. The gradual reduction in lumen output of alamp as it burns through I ife is more rapid for some lamps than for otherlamps. The contributing light loss factor for fluorescent lamps is usuallyexpressed as a ratio of the lumen output of the lamp at 70 percent of ratedlife to the initial (100) hour value. Since the Ii fe is influenced by burninghours per start, the contributing loss factor is usually expressed as afunction of burning hours per start, even though it is actual fy a function oflamp burning hours. The lamp lumen depreciation for fluorescent fi lament andmercury lamps can usually be found in manufacturers' data. As an example,Table 5.3.9 is a I isting of fluorescent lamps.

Table 5.3.9

Fluorescent lamp data.

Lamp LumenDepreelation (LLD) :

J

lamp Ordering Approll. Base RatedAbbreviation ' Watts Hours per Start Initial

Lumens

6 t2 18

Pre-He.t-Rap. St.F40CW 40 Med Blpln 88 87 86 3150F40TlO,CW:99 40 Med Blpln 86 84 83 3200FB40CW/6 40 Med Blpln 85 83 81 2950

Slimlin.F48TI2'CW 385 Single Pin 88 87 86 3000F72TI2CW 56 S,ngle Pin 88 87 86 4550F96TI2'CW 735 SIngle Pin 88 87 86 6300

High OutputF48TI2 CW HO 60 Rec DC 85 84 83 4200F72T 12,CW;HO 85 Rec DC 85 84 83 6650F96TI2'CW,HO 110 Rec DC 85 84 83 9000

Very High OutputF48TI2:CWiVHO 110 Rec DC 80 79 78 6900F72Tl2 CW,VHO 160 Rec DC 80 79 78 11 100F96T12'CWiVHO 215 Rec DC 80 79 78 15.500F96T 12 CWiVHOIl 215 Rec DC 80 79 78 15.500

,j\ For Standard Cool White lamp Other COlors musI have proper deSIgnallons'11 lamp Lumen Depreciation values apply to Standard Cool WhIt", Siandard Warm While and While

lamps at 70·'. ot rated hie,;, Values apply to Standard Cool White For other COlors mu'ltply by the fOllOWIng factors While and

Siandard Warm White. 1 04. Dayhght. 86 Cool Green 92 Warm Wh,le Deluxe and Cool WhileDeluxe. 75

By permisssion of North American Phi lips Lighting Corporation.


Using Table 5.3.9, it is found that a fluorescent lamp--a high-output lampF96TI2/CW/HO--would have a lamp lumen depreciation of 0.85 if it burns sixhours per start; 0.84 if it burns 12 hours per start; and 0.83 if it burns 18hours per start.

4. Luminaire dirt depreciation. This factor varies with the type ofluminaire and the atmosphere in which it is operating. Luminaires are dividedinto six categories. The category for each luminaire has its own set of dirtdepreciation curves. The dirt depreciation curves are as fol lows:

Figure 5.3.4

Dirt depreciation.

CATtGOlW I c..'tGO"" If

~- -~r.~"'- -.1'" "":~~ -jI.I~- r-

~.l

i '-. !'I'-tt

i1 'I·......

I"r--. ~~ l![

~ '- I ,.-l":- :...~ ·t~(.~

I...... .~-....fu.~~ • L.. .. ~ r-r-

iI

. 0 , '2 .. 2. lO 3' .2 .. ,.. to 0 1 'I .. 2. )0 _ ., .. ,.. to_'WS _'WS

C:"'CGOft "I CATlGOlW IY

~~I.....

\'l'\ -, r-l-t-, t-

\ .... r'. ....... '-

1\ I'". t'."- f' t .....1"., • • •• • 60011 .. 2111_2

_'WSCATI:GOIPI WI

~~ """-~~ t..... ~:- t"--~'.

r-.... .... -....~ ...]....

i"'"

o 1 ~ .. ~ ~ ~ ~ .. ,.. toWON''''S

CArtGOllt v000

f't"-......I~~ l'''': '. r-~-" ~.

f\.. l". ...... r-:-..... '-!-.....~ ...~.. ,....

60

i

~ r:--..tc \~~ "i""'-t"r-1,\' "- '.r, ,

r'<. ,

!'..I'

"" ......

1'...O. Il4~~~~~~~ O. ~~U~~q4e~~

IrII(WItTtotS ...:)NfMS

By permisssion of North American Phi lips Lighting Corporation.


After determining the category, the luminai re dirt depreciation factor can beread from one of the five curves for each category. The point on the curveshould be selected on the basis of the number of months between. cleaning theluminaires. The particular curve selected should be based on dirt content andatmosphere. For example, in category I I, if cleaning were every 24 months andthe conditions were very dirty, the luminaire dirt depreciation factor wouldbe about 0.75.

The total I ight loss factor is determined by multiplying the separate fourfactors together. For example. if a lamp had a bal last performance of 0.95.luminaire reflectance of 0.98, lamp lumen depreciation of 0.85, and luminairedirt depreciation of 0.70. the combined I ight loss factor would be 0.55.

Using the Formula. Recal I that the formula for the number of lamps is asfollows:


Lumens pe r Iamp x coe f f ic i en t of uti I iza t ion xI ight loss factor

where the number of luminaires is as fol lows:

Numbe r a f IampsNumber of luminaires =

Lamps per luminaire

Problem. Assume a smal I office 20 feet by 40 feet with 12.5 foot cei lingsis to be illuminated for regular office work. The reflectance of the cei lingis 80% (actual reflectance, not effective reflectance>, and reflectance of thewal Is is 50%. The luminaires wi I I be instal fed 2 feet from the cei I ing, andthe work plane is 2.5 feet from the floor. The luminaires wi I I have opaquesides and are category VI lamps. Assume that the environment wi II beconsidered clean and that the luminaires wi I I be cleaned every 12 months. Thebal last wi I I meet the requirements of the Certi fied Bal last Manufacturers.From this information, compute the number of lamps and the number ofluminaires that wi I I be needed to provide a sufficient quantity of light.

Solution. The first task is to determine the number of foot-candles thatare recOlllllended for regular office work. According to "Recommended Levels ofIllumination," publ ished by the Illuminating Engineering Society. regularoffice work requires a minimum of 100 foot-candles.

The second step is to determine the coefficient of uti I ization. To dothis, the room-cavity ratio and the cei I ing-cavity ratio must be computed.The room-cavity ratio would be

Room-cavity ratio =10h

x Gaysunas ratioroom width

Using 8 feet for h and a Gaysunas ratio of 3/4, the room-cavity ratio is 3.0.(If the table is used to compute the room-cavity ratio. the room-cavity ratiofrom the table is 2.9 with an 8-foot cavi ty depth.)


The cei Iing-cavity ratio is computed as fol lows:

10 x 2 f tCei I ing-cavi ty rat io = ----

20

= 0.75

(3/4)

Using the room-cavity ratio and the cei ling-cavity ratio, the effectivecavity reflectance of the cei ling can be found using the Effective CavityReflectance Table, Table 5.3.6. The table shows that for an 80% base (actualcei I ing) and a 50% wal I reflectance, the cavity reflectance is between 69% for0.8 cavity ratio and 71% for 0.6 cavity ratio; so for this room, thecei I ing-cavity reflectance is approximately 70%. Using the effective cei lingreflectance as 70% and the wall reflectance of 50%, use the Coefficient ofUti I ization Tables (Table 5.3.7) to look up the coefficient of uti I ization.This is done by finding the luminaire that is in category VI with a 75%distribution of light hitting the cei I ing for a room-cavity ratio of 3.0. acei I ing effective cavity reflectance of 70% and a waf I reflectance of 50%.The coefficient of uti lization is then 0.45.

The next step is to determine the light loss factor. The firstconsideration in determining the I ight loss factor is the bal lastperformance. Although the ballast performance is not indicated in the datagiven. it has a CBM label, so assume the bal last performance is 0.95. Thesecond factor to consider in computing the light loss factor is the luminancereflectance and transmission changes which were not given. Assume that to beabout 0.98. Next, consider the lamp lumen depreciation. Assume that theluminaire takes a F96TI2/CW/HO lamp that wi II burn 12 hours per start. Usingthe lamp data for fluorescent lamps, Table 5.3.9, the lamp lumen depreciationfactor would be 0.84. The next factor to consider is the dirt depreciationfactor. This can be computed from the appropriate dirt depreciation curves.For a category VI lamp for a 12 month replacement. it would be around 0.86under the clean condition. This means that the total light loss factor wouldbe

0.95 x 0.98 x 0.84 x 0.86'= 0.67

The next thing needed in order to use the formula is the lumens per lamp.This can be found using the lamp data provided in the lamp data table (Table5.3.9). For F96T12/CW/HO lamp, the rated initial lumens is 9000.Substituting these values in the basic formula gives

Foot-candles x areaNumber of lamps = ---------------------

Lumens per lamp x coefficient of uti lization xlight loss factor

100 foot-candles x 20 ft x 40 ftNumber of lamps

9000 lumens x 0.45 x 0.6729.48 lamps or 30 lamps

The number of luminaires can be computed using the fol lowing formula:


Number of lampsNumber of luminaires =

Number of lamps per luminaire

Substituting in the equation gives approximately 15 luminaires (use 14).Fourteen 2-lamp luminaires can be instal led in seven rows of two luminairesmounted crosswise in the room. Ordinari Iy, it is preferable for theluminaires to be mounted so that the lowest candlepower is projected in thedi rection of most of the workers in the area. This may require that someluminaires be mounted paral lei to the line of sight of most workers. Otherluminaires should be mounted perpendicular to the line of sight. Thisparticular luminaire has low candlepower from al I viewing directions. but itdoes tend to create a high cei ling brightness. However, the bright cei ling isshielded from view by the luminaires if they are mounted perpendicular to theI ine of sight. Since in this room the predominant I ine of sight is mostI ikely to be paral lei to the length of the room, it is suggested that theluminaires by mounted perpendicular to the room length.

Summary of Steps Involved in Computing Lumen Method

Step I. Determine the required level of illumination using the recommenationsof the I I luminating Engineering Society.

Step 2. Determine type of luminaire.

a. Distributionb. Categoryc. Lumensd. Lamp lumen depreciatione. When replacedf. Environmental conditions

Step 3. Determine coefficient of uti lization

a. Compute room-cavity ratio.b. Compute cei I ing-cavity ratio.c. Compute floor-cavity ratio.

NOTE: Use either

5h (room length + room width)

room length x room widthor

10hx Gaysunas ratio

room width

d. Determine effective cavi ty reflectances using Effective CavityReflectances Table.

Compute cei I iog-cavity reflectance(If floor-cavity reflectance other than 20%,use correction table.)


NOTE: Compute floor-cavity reflectance same as room-cavityreflectance to determine if less than 20%.

e. Use effective cei ling-cavity reflectance. room-cavity reflectance.wal I reflectance. and look up coefficient of uti lization in tablefor selected luminaire.

Step 4. Determine light loss factor (llF).

a. Sal last performance (0.95 if certified bal last) =b. luminaire reflectance and transmission changes =c. lamp lumen depreciation = ___d. Luminaire dirt depreciation (determined from llD curves)e. llF - a x b x c x d =

Step 5. Use formula.


lumens per lamp x coefficient of uti I ization xI ight loss factor

NOTE: Coefficient of uti lization was computed in Step 3. light I~ss

factor computed in Step 4.

Step 6. Determine number of luminaires.

Step 7. Determine layout.

a. Spacing not to exceed (see Coefficient of Uti I ization Table).b. Draw layout.

4. References

Baumeister. Theodore, ed. Mark's Standard Handbook for Mechanical Engineers.7th ed. New York: McGraw-Hi I I Book Company, 1967.

Hewitt, Paul G. Conceptual Physics ... A New Introduction to YourEnvironment. Boston: Little. Brown, and Company. 1974.

North American Phi lips Lighting Corporation, Lighting Handbook. Bloomfield.NJ: North American Phi lips Lighting Corporation, 1984.

Patty, Frank A. Industrial Hygiene and Toxicology. 2d ed .. 2 vol. New York:Interscience Publ ishers, Inc., 1958.

Trippens. Paul E. Applied Physics. New York: McGraw-Hi I I Book Company. 1973.

U. S. Department of Health, Education and Welfare. Publ ic Health Service.Center for Disease Control. National Institute for Occupational Safetyand Health. Recognition of Occupational Health Hazards. Student Manual.1974.

The Industrial Environment: Its Evaluation and Control.Washington: U. S. Government Printing Office, 1973.

Westinghouse Electric Corporation, Lighting Handbook. Bloomingfield. NJ:Westinghouse Electric Corporation, Lamp Division, 1974.

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