Topics: Statistics The Human Visual System Color Science Radiometry/Photometry Geometric Optics...

Topics:

Statistics The Human Visual System Color Science Radiometry/Photometry Geometric Optics Tone-transfer Function Image Sensors Image Processing Displays & Output Colorimetry & Color Measurement Image Evaluation Psychophysics

Some Radiometry / Photometry References Grum, F. and Becherer, R. (1979). Optical Radiation

Measurements, Vol. 1 Radiometry. Academic, N.Y. Mooney, W. (1991). Optoelectronic Devices and

Principles. Prentice Hall, N.J. Stimson, A. (1974). Photometry and Radiometry for

Engineers. Wiley & Sons, N.Y. Walsh, J. (1958). Photometry. Constable & Company,

LTD. Wolf, W, (1998). Introduction to Radiometry. SPIE

Press.

Outline Definition of Terms and Units

Radiometry Solid Angle, Point Sources, Projected Area Sources of Radiation

Gas Discharge Fluorescent Blackbody Radiation Incandescent

The Sun and daylight SOURCES LABORATORY

Outline Radiance

Radiance invariance Lamberts Cosine Law and Lambertian Surfaces Relationship between radiance and exitance Relationship between flux and intensity

Photometry and Color Temperature HVS short overview Photometric terminology Planckian locus Color temperature Distribution temperature

Outline Spectral radiometry

Diffraction gratings Spectrometers Wavelength selectors

Absorption and interference filters IR rejection ND filters Monochromators

CALIBRATION LABORATORY Irradiance Variation with Distance and Angle

Point source (inverse square law) Line source Plane source

Outline Irradiance Variation with Distance and Angle

Point source (inverse square law) Line source Plane source

INV SQ LAW LABORATORY

Outline Radiometry and Imaging Systems

Lens falloff and exposure Irradiance onto the focal plane

Non-Point Sources How to handle non-point source calculations Some governing formula’s

Integrating Spheres (if time permits) Governing sphere equation Applications of integrating spheres

Radiometric Quantities and Units

Emmett Ientilucci, Ph.D.Digital Imaging and Remote Sensing Laboratory

Chester F. Carlson Center for Imaging Science

Overview

What is Radiometry? Where does it come from? In the early days Radiometry was “illumination science” (which

is still alive today) In this early era much arcane terminology was developed

Who uses Radiometry? Physicists, vision and psychophysics', EE, etc

They all use different words to mean the same thing You will learn the internationally accepted terminology

What is Photometry?

Radiometric / Photometric Definitions

What is Radiometry?

Measurement or characterization of EM radiation and its interaction with matter

It is the measurement of “how much”

The science of putting “numbers” on things you do in the science of imaging.

What is Photometry?

Measurement or characterization of EM radiation which is detectable by the human eye

Why Develop These Concepts?

Given an optical system, for example Camera, telescope, etc And any optical radiation source, a surface, detector, etc.

Can calculate “how much” radiation gets to the detector array or film in the image plane

e.g., Can calculate the value of the Signal-to-Noise (i.e., SNR) or exposure

2 Second Review …

We will be using units such as micro-meters (um) nano-meters (nm) and will be using the MKS (meter, kilogram, second)

system.

We will be ‘counting’ counting photons (i.e., the particle nature of light)

What is Radiometry?

EM spectrum of interest This is not universal terminology Most remote sensors use this terminology For example, NIR : Where Si detectors respond

Radiometry Definitions: Summary

Units can be divided into two conceptual areas Those having to do with energy or power

Energy, Q (joule or [J] ) Power or flux, (watt or [W] )

Those that are geometric in nature Irradiance, E [W/m2] Exitance, M [W/m2] Intensity, I [W/sr] Radiance, L [W/m2 sr]

Photometry Definitions: Summary

Units can be divided into two conceptual areas Those having to do with energy or power

Energy, Q (lumen second or [lm s] or Talbots) Power or flux, (lumen or [lm] = [cd sr] )

Those that are geometric in nature Illuminance [lm/m2 = lux or lx] Emittance [lm/m2 = lux or lx] Intensity [lm/sr = candela or cd] Luminance [lm/m2 sr = cd/m2 = nit]

Sometimes called Luminosity

Systeme International d’Unites (SI)

SI developed in 1960 7 SI Base Units

Kilogram [kg] Second [s] Meter [m] Ampere [A] Kelvin [K] Mole [mol] Candela [cd]

All others are SI derived units Previous slide: Radiometric definitions are all

SI derived units

Radiometric Definitions

Photon Energy, q We think of energy as being transferred in terms of

energy packets or quanta The energy carrier is a “photon” Each photon carries energy,

Shorter wavelength photons carry more energy than longer wavelength photons

h = Planck Constant (6.6 E-34 J s)c = Speed of Light (3 E8 m s-1) = Wavelength (meters)


Radiant Energy, Q Since we are counting photons, lets sum them up!

The total energy (Q) in a beam is a function of: Frequency or wavelength of the photons, or Number of photons (n), of a particular frequency or

wavelength, i


Radiant Energy, Q Example: HeNe LASER (632 nm)

Example: 632 nm AND 1000 nm sources


Radiant Flux or Power, We are looking at light continuously hitting a surface.

How many photons over a period of time?

DEF: Quantity of energy propagating onto, through, or emerging from, a specified surface of a given area in a given period of time

2

Time Interval


Radiant Flux or Power DEF: Quantity of energy propagating onto, through, or

emerging from, a specified surface of a given area in a given period of time

- P = iv, electrical power [watts] dissipated by a resistor.- Some of this power is turned into light

- That is, electrical power is converted to radiant power


Irradiance, E Change of power ONTO a surface. Rate at which radiant flux, is delivered onto a surface

(e.g., a detector surface)

- These are spatially dependant terms !!- However, we will often not write this. Will look at bulk average

Projected Area


Radiant Exitance, M Rate at which radiant flux, F is delivered away from a

surface (e.g., a diffuser, reflected surface)

- If the surface is opaque, then there is flux onto the surface- Since we can see it, there must be flux OFF the surface- M can be related to E via the transmission, “tau”


Radiant Intensity, I Rate of change at which radiant flux, is incident on,

passing through, or emerging from a point in space in a given direction

“steradian”

- No spatial dependency here- Associated with sources (points sources)

Point Source

Point Source or Point Location A point is a location in space that occupies no space.

How do you get photons through a point?

Really, we will be looking a photons that emerge from a very small source that approximates a point source

Approximations: A point source is “small” compared to how far we are away from it R.O.T.: Should be 10x the radius of source away, for example

(about a 1% error in this assumption)….I will show you later…

Errors in Point Source Approximations, In General

About 0.002% error in assuming Sun is a point source

If the sensor is 10xthe source radius away,we have a 1% errorin our assumptions.


Radiant Intensity, I Rate of change at which radiant flux, is incident on,

passing through, or emerging from a point in space in a given direction

“steradian”

- No spatial dependency here- Associated with sources (points sources)

Plane Angle

Plain angle or linear angle, Length of arc, s divided by the radius, r Plain angle is dimensionless However, to aid in

communication it is assigned the SI unit of measure … radian

s

ss

r=1

C = 2r

For any s :

2 radians in a full circle

Plane Angle

So… A straight line or even a curved line can subtend

the same angle as an arc on the circle

So…DEF: A plane angle is the projection of a line on a unit circle, and the line need not be straight

Solid Angle

Solid angle, 3D equivalent of a plane angle Projection of a area (or a closed curve in space) onto a unit

sphere “square radians” or steradian

SA = 4r2

r=1

dAFor any A

4 steradians in a sphere2 steradians in a hemi-sphere- Lets derive it !

d

Solid Angle

Solid angle, Sphere of radius, r has a (surface) area SA = 4r2

SA = 4r2

r=1

dA

d

4 steradians in a sphere

2 steradians in a hemi-sphere

Example: Def, Sphere, Pt Source


Radiance, L One of the problems with M, is we don’t know

where any of the flux goes ! They might go straight or make a left turn?

Same thing with irradiance, E

So we combine the concepts if irradiance with Intensity

This is called “radiance”


Radiance, L Function of both position and direction

DEF: Flux, incident on, passing through, or emerging in a specified direction from a specified point in a specified surface

Formal Definition

[W/m2 sr]


Radiance, L Element ofFlux d2

Element ofSolid Angle d

Element ofArea dA0


Radiance, LElement ofFlux d2

Element ofSolid Angle d

Element ofProjected Area dA = dA0 cos

Element ofArea in the Surface dA0

The “area” in the units (m2) is now with respect to the projected area dA.


Radiance, L- Radiance can be used for a source or a receiver- It is a much more universal definition

- It does not change with distance (as we will derive later)

- e.g., If I know L headed in a certain direction (lossless), then the radiance leaving that surface will be the same radiance hitting any surface along that direction

-We need (x,y,,) dependency because the radiance can be different in any location on a surface and different in any direction

- We will devote an entire section to Radiance

Sources of Radiation



Classes of Emitted Radiation

Line spectra sources Continuous spectra sources Monochromatic sources

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Methods of Producing Radiation

Gas Discharge Sodium street lamp

Incandescence The Sun, tungsten light bulb

Chemical Fluorescence, phosphorescence

Electroluminescence Semiconductors and LED’s

Sodium Vapor Lamp

Gas Discharges

Gases are not good conductors at STP However, in some cases, insulation properties can break

down… e.g., Plug in a power supply. Current across air !

Contents of Gas Discharge lamp Noble gas such as

Argon, neon, krypton, xenon, etc. Combination of Nobel gases And/or materials such as

Sodium or mercury

Gas Discharges

The gas can be under low or high pressure 5-15 torr (low pressure) 760 – 1520 torr (1-2 atm), (high pressure)

Apply a high voltage 1000 – 5000 volts Creates an electric field EF creates free electrons across tube

Gas Discharges

Conditions: (Overview)1. Create an electric field between electrodes

2. Free electrons will accelerate away from (-) electrode

3. Free electrons can collide with gas molecules

4. e- can transfer energy to molecule (excited)

5. Light is generated from de-excitation

Gas Discharges

Comments:1. We get non-continuous line spectrum

2. The type of gas in the tube determines the glow Neon emits a red glow, naturally Mercury emits blue Sodium emits yellow Helium produces a pale yellow Argon yields purple

Gas DischargesArgon

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Low Pressure Sodium Lamps Very efficient (ratio of electrical power in to radiant power out) Entire output in visible consists of

589.0 and 589.6 nm (poor color rendering)

Efficacy [lm / W] Term to help characterize “human perceived brightness”…. We use the term efficacy (not to be confused with efficiency)

Ratio of emitted luminous flux [lm] to radiant flux [W]

The “lumen is a measure of perceived brightness” of light The maximum efficacy for any source is 683 lm/W

Since DEF of a lumen is 1/683 watts at 555 nm.

Gas Discharges

Example A monochromatic source that emits at 1000 nm

Has an efficacy of zero (since there is no useful radiation in the visual portion of the spectrum)

Sodium Source Very efficient and large “efficacy” 183 – 200 lm/W

Gas Discharges

This makes them ideal for mass street lighting Cheap !! But poor color rendering …

Low Pressure Sources Sodium gas lamp Mercury gas lamp (and also a fluorescent tube) Neon tube

Gas Discharge

This makes them ideal for mass street lighting

- Cheap !!- But poor color rendering …

Gas Discharges

High Pressure sodium (and mercury) Lamps (high) pressure 1 -2 atm (760 torr = 1atm)

Proximity of atoms is closer Concentration of excited molecules is much higher Lines of emission becomes broader (line broadening) Much brighter than (low) pressure lamps

Better color rendering (i.e., pale pink-orange) Good for flood lighting, parking lot lighting

Efficacy of about 150 [lm/W]

Gas Discharge

Gas Discharge

Mercury Lamps (low and high pressure) Low pressure produces lines at

253.7, 365.4, 404.7, 435.8, 546.1 and 578.0 nm

We will see that 253.7 and 365.4 nm are used in a fluorescent lamp to excite phosphors That is, a fluorescent lamp is basically a better quality

(mercury) gas discharge lamp

Gas Discharge

Mercury Vapor Source

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Metal Halide lamps (high pressure) Same as high pressure mercury lamps …add some metal halides to the mix

Extra lines can be provided in the spectrum Improved color rendering Efficacies around 80 [lm/W]

Applications Floodlighting in stadiums

High Intensity Discharge (HID)

Basically…class of lamps with large luminous output High pressure Mercury vapor lamps High pressure Metal halide lamps High pressure sodium lamps

Efficacy [lm/W]

Applications

Xenon Arc Lamps

http://en.wikipedia.org/wiki/Xenon_arc_lamp

Xenon Arc Lamps

http://en.wikipedia.org/wiki/Image:Xenon_short_arc_1.jpg

15,000 W Xenon short-arc lamp used inIMAX projectors.

-Filled with xenon gas-At very high pressure-Large current draw (450 W: 18V at 25 A)

Xenon Arc Lamps

http://ncr101.montana.edu/Light1994Conf/5_9_Kofferlein/Fig%20Pay%201.jpg

-Contains spectral lines-Also fair amount of incancescence-Very bright-Color rendering resembles “daylight”

Efficiency

Tungsten 5% of energy consumed by 100 watt bulb is converted to

visible light The rest is in the IR and/or heat !!

Discharge Lamps 30% of the energy consumed is converted to visible light

Fluorescent lamps can approach 50%

Fluorescent Lamps

Conditions: Low pressure mercury gas in tube Inside is coated with phosphors that can be

excited by UV lines of the mercury spectrum

Phosphors selected so as to provide additional light at long wavelengths i.e., Light does come from gas discharge but mainly

from phosphors

Fluorescent Lamps

Process: Contains Hg and inert gas such as Argon

We excite the Hg vapor in Ar gas Power on: Cathode heats up to emit electrons Electrons migrate across due to potential difference e- ionize the argon gas

Fluorescent Lamps

Process: Electron and ionized argon atoms move through tube

This allows higher currents to flow Which, in turn, (vaporizes) and ionizes the mercury De-excitation of the Hg gas emits UV radiation

A black light !!

Fluorescent Lamps

Process: Efficiency is mainly due to the fact that

65% of total light is at the 253.7 nm line. UV radiation absorbed by phosphor coating Phosphor re-radiates at longer wavelength

440 and 546 nm

Fluorescent Lamps

Fluorescent and Mercury Sources

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Spectral Line Broadening

In Gases We have seen that when gas pressure is raised, spectral

lines in the emission are broadened This is due to the effects of the neighboring atoms in the

gas

… and In liquids and solids….. Atoms are much more closely packed than in a gas Line broadening effects are so great that emission is

continuous throughout the spectrum

Incandescent Light Sources

The most important natural incandescent light source is the Sun

The most important man-made incandescent source is the tungsten filament lamp

The amount of power radiated by incandescent sources depends on their temperature

Incandescent and Fluorescent

Blackbody Radiation: Solids

Max Planck (1901) Derived an expression for the spectral radiant

exitance, M, from a “blackbody”

k = Boltzmann gas constant (1.381 x 10-23 [J K-1])h = Planck constant (6.63 x 10-34 [J s])c = speed of light (3.0 x 108 [m s-1])

Max Karl Ernst Ludwig Planck, 1901


- A “blackbody” is an idealized surface where the reflectivity is zero and the absorpitivity is one

- That is, it has the property that all incident EM flux can be perfectly absorbed and can be re-radiated- The spectrum emitted by a solid, when heated, resembles a blackbody

Similar to your:

-Toaster-Stove-Hair dryer-Etc…-(those these are not “Ideal” blackbodies)

Sources: Solid

http://www.csr.utexas.edu/projects/rs/hrs/pics/

Sources: Solid and Gas

Incandescent Lamp

Fluorescent Lamp


Can you see energy atT = 300 K?

- Is the Sun a perfect BB?- What about things that “almost” look like BB’s……….


Emissivity In reality, objects are NOT perfect absorbers To describe this phenomenon, we introduce the

concept of emissivity, ()

Ratio of the spectral exitance from an object at temperature T to the exitance from a blackbody at that SAME temperature

( )M (T)

MBB (T )- Ranges from 0 to 1- [unitless]


Emissivity i.e., how “poor” does it resemble a BB. The Sun has an emissivity < 1

Can’t have negative emissivities That would suck photons out of existence !!

Can’t have greater than one (>1) That would mean you are radiating better than an ideal

BB!

( )M (T)



Emissivity Objects whose emissivity is constant with wavelength are

called Gray bodies

Objects with spectrally varying emissivities are called Selective Radiators

( )M (T)



Transmissivity, transmittance, transmission Ability of the material to allow the flux

to propagate through it

MEi

E M

- Ranges from 0 to 1- [unitless]


Reflectivity, Reflectance Ability of the material to turn incident flux back into the

hemisphere above the material

E

M

r MrEi

r



Absorptivity, absorptance Ability of the material to remove EM flux from the system by

converting incident flux to another form of energy (e.g., heat)

How much EM energy is “lost” to the system i.e., How much of the exitance is lost (absorbed) by the

system

E

M

MEi



In General Conservation of energy requires all the incident flux to be

absorbed, transmitted or reflected. We have:

(I don’t really know what else to do with the photons !)

For an opaque material or surface we have:

r 1

r 1


In General (with out proof…) There’s a law from thermodymanics that comes into play

regularly when dealing with radiative surfaces

Kirchhoff’s law (kirk-offs) states that emissivity must be numerically equal to the absorptance surface in thermodynamic equilibrium (TDE)

That is, good absorbers are good emitters (in TDE) TDE, T is constant. Therefore I can write….

r 1Gustav Robert Kirchhoff1861


…And for opaque objects I have

This is a very useful substitution… Because it is often easier to measure the reflectivity

of an object than its emssivity

When are you not in TDE? i.e., When can’t I use Kirchhoff’s rule? When something is heating and cooling However, in “normal” operating conditions in the world,

you can set =

r 1


Total exitance from a blackbody Is the area under the Planck function curve

Stefan-Boltzmann Law (or Stefan’s Law)

= Stefan-Boltzmann constant

Joseph Stefan, 1879 Ludwig Eduard Boltzmann, 1884


Total exitance from a blackbody Area under the Planck function curve

But when would we use this result?

In imaging, we never look at ALL the photons over the entire EM spectrum ?

I really care about an interval from 1 to 2

So, I need to solve the Planck equation from 1 to 2 Anyone know this closed form solution?


Exitance within a bandpass Solved numerically for no closed form

solution exists

I will demonstrate how to do this through example in a minute……but first..


Wien Displacement Law Where is the peak in the Planck function ??

This says that the peak exitance from the Sun will be approximately 500 nm (yellow-green)

Wilhelm Wien, 1893

A = Wien displacement constant (2898 um K)

Integrate Planck Function

Trapezoid:(a+b)/2 x wa b

w


So the exitance in an interval , is

And the total exitance summing all intervals is

Tungsten Lamps

Make an incandescent source… Heat a conductor by passing electrical current through it

(above 1000 K)!! In air, you get rapid oxidation of the material In vacuum…much more stability

Most stable conductor today is “tungsten” Spectral emissivity is constant in visible

i.e, it looks like a Planckian radiator Melts at 3683 K

Greater temperature = more light = more evaporation? = early failure

Tungsten Lamps

Thermionic Emission (Thomas Edison, 1883) Flow of electrons from a metal surface caused by thermal

vibrational energy overcoming the electrostatic forces holding electrons to the surface

So, - Some e- de-excite (produce light)- Some leave the surface altogether

Tungsten Lamps

Use inert gas such as Argon to reduce effects of Thermionic Emission Inert gas will not react with other elements

Tungsten Lamps

As tungsten evaporates it “may” collide with an argon atom and bounce back to the filament

Tungsten-Halogen Lamps

Fill envelope with a halogen gas Iodine, bromine

This create a reaction such that the tungsten is preferentially re-deposited on the filament Preferentially because the filament thickness actually

varies, thus the temperature has spatial variation across the filament

Therefore we can run this at higher temperatures !! But need to use different glass casing (i.e., quartz)

Tungsten, Tungsten-Halogen

Tungsten (2650 to 3200 K). Typically 2800 K 10 to 25 [lm/W]

Tungsten-Halogen (2850 to 3300 K). Typ 3200 K 15 to 35 [lm/W]

Efficacy [lm/W]

LED’s

Group IV semiconductors are Indirect bandgap Si (1.12 ev = 1.1um), Ge (0.66 eV = 1.9um), SiC Recombination is a non-radiative transition

i.e., no optical emission under forward bias

Direct band gap materials Recombination produces optical radiation

i.e, Near UV, VIS, NIR, under forward bias

LED’s

Blue LED’s

Wide bandgap material GaN (3.4eV = 364nm), used in Blu-ray DVD players (PS3)

First blue LED 1971: very fragile 1980’s: New, more robust, process 1993: Shuji Nakamura, new process

This facilities the creation of white LED’s 450, 470 nm Blue LED

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White LED’s

White LED’s (1996) Modified blue LED’s Uses a phosphor (inside) coating Re-emits at 580 nm Mix of blue and yellow gives appearance of white light

White Light LED

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LASER

HeNe LASER

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SOURCES LABORATORY

Radiance



Constancy of Radiance

Radiance does not change with distance. As you move away, the sampled area increase effectively cancelling any inverse square law losses.

Diffus

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Surfa

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Detector measuring radiance[W / m^2 sr]

- Lossless Medium

Responsivity and Radiance

Silicon Response

Lamberts Law and Lambertian Surfaces

Johann Heinrich Lambert (1728-1777)

Relationship between L and M

For a Lambertian surface ONLY M = L [W/m2] = [W/m2 sr] * [sr]

That is, if you know the radiance from a LAMBERTIAN surface, the exitance (M) can be found by scaling the radiance by a factor of steradians.

Photometry and Color Temperature



Photometry

Radiometry for the human visual system (HVS)

Photometry (predates Radiometry)

- The eye is just another detector

-You do the radiometry problem first, then turn it into something the HVS sees.

-Q. But how do we get a signal out of your eye?

-A. We ask people “how much”-Not very calibrated !

-Visual response is in terms of perceived brightness

Photometry

-HVS responses to radiance- It’s an “apertured” system-Solid angle is formedby the pupil

Detectors

-Cones (RGB)-Rods (BW)

-Flux perceived by HVS quantified in terms of Lumens

Photometry Units

Energy or Power Energy, Q (lumen second or [lm s] or Talbots) Power or Luminous flux, , (lumen or [lm] = [cd sr] )

Those that are geometric in nature Illuminance [lm/m2 = lux or lx] Emittance [lm/m2 = lux or lx] Intensity [lm/sr = candela or cd] Luminance [lm/m2 sr = cd/m2 = nit]

Sometimes called Luminosity Used to characterize brightness

Photometry Units

Other arcane units sometimes used Illuminance

1 metre-candle = 1 lux 1 phot = 1 lm/cm2 = 104 lux 1 foot-candle = 1 lumen/ft2 = 10.76 lux 1 milliphot = 10 lux

Luminance 1 stilb = 1 cd/cm2 = 104 cd/m2 = 104 nit 1 cd/ft2 = 10.76 cd/m2 = 10.76 nit

Photometry Units – Illuminance/Exitance

1m21ft21cm2

Source of 1 [cd](1 lm/sr)

1 lux = 1 lm/m2

1 fc = 1 lm/ft2

1 phot = 1 lm/cm2

(intensity)

Normalized Std. Response Functions

(average) Human visual response functions Developed in 1931 Based on many observations

i.e., standard observed response functions

http://retina.umh.es/Webvision/Psych1.html

V()V’()

Pea

k N

orm

aliz

edTo calculate the “output” ofthe HVS (in terms of perceivedflux), we need the VRF’s.

Standard Response Functions

http://retina.umh.es/Webvision/Psych1.html

Lum

inou

s E

ffic

acy

[lm /

W]

-Flux perceived by HVS quantified in terms of lumens (lm)

-Can we map flux in (W) to flux out (lm)?

-Scaled by luminous efficacy scalar [lm/W]

-Scales radiometry flux levels to be consistent with historical scales of perceived brightness

-Now looks like a “responsivity”-How many lumens do I get out per watt in….

Photometry

Can take any radiometry problem and make it a photometric problem using the relation

which calculates the luminous flux.- [lm] = [W/um] *[lm/W]*[um]

- k = 683 [lm/W] (photopic) - k’ = 1700 [lm/W] (scotopic)

Photometry

Can also do for: Illuminance Emittance Intensity Luminance [lm/m2] = [W/m2 um] *[lm/W]*[um]

Plotting Color in 2D

-Given a radiance L, we can find-Tristimulus Values

-From the tristimulus values we can find-Chromaticity coordinates (x, y)

-Calculate x,y for monochromatic radiation-Which produces this “horseshoe shape”

-Horseshoe shape is often called “spectrum locus”

-Region inside is called the “color gamut”

-A given system will be capable of displaying some subset of this gamut

http://www.ecse.rpi.edu/~schubert/Light-Emitting-Diodes-dot-org/chap17/chap17.htm

Displaying Sources

-Chromaticity coordinates of blackbodies form a (Planckian) locus in the chromaticity diagram

-We have a way to describe its color

http://en.wikipedia.org/wiki/Planckian_locus

Displaying Sources

-A fluorescent source is NOT a blackbody.

-But we can STILL compute chromaticity coordinates

-But this will NOT be on the BB locus

-We compute the closest BB radiator. (perceived not Euclidian closest)

-And give it a…- (correlated) color temperature (CCT)

http://en.wikipedia.org/wiki/Planckian_locus

5000K Fluorescent Bulb

Can “look” the same to the eye

Distribution Temperature

Has nothing to do with color and the CIE diagram

Distribution temperature is for curves that look like a blackbody

Topics: Statistics The Human Visual System Color Science Radiometry/Photometry Geometric Optics...

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