ANALYSIS OF DAYLIGHTING USING DAYLIGHT FACTOR AND ... · Malaviya National Institute of Technology,...

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International Journal of Civil Engineering and Technology (IJCIET)

Volume 9, Issue 10, October 2018, pp. 949–960, Article ID: IJCIET_09_10_096

Available online at http://www.iaeme.com/ijciet/issues.asp?JType=IJCIET&VType=9&IType=10

ISSN Print: 0976-6308 and ISSN Online: 0976-6316

© IAEME Publication Scopus Indexed

ANALYSIS OF DAYLIGHTING USING

DAYLIGHT FACTOR AND LUMINANCE FOR

DIFFERENT ROOM SCENARIOS

Badri Narayan Mohapatra

Research Scholar Electronics and Communication Engineering Department,

Centurion University of Technology and Management, Paralakhemundi, Odisha, India

M. Ravi Kumar

Malaviya National Institute of Technology, Jaipur, Rajastan, India

Sushant K. Mandal

Electronics and Communication Engineering Department,

Centurion University of Technology and Management, Paralakhemundi, Odisha, India

ABSTRACT

Consciousness of the advantages of utilizing desired day lighting, and plans of

many buildings taking daylight into consideration has risen in recent years.

Architectural impact makes daylight a critical part of designing. The inclusion of day

lighting in designing, through design tools, results in improved daylight, and

therefore reduces visual discomfort. From energy point of view, the main objective of

windows is to provide “usable" daylight to the interior of offices, allowing the electric

lighting to be replaced by natural light and reducing the cost of electric load. The

determination of daylight-related performance indicators of illuminance, and daylight

factor were identified by software (VELUX). VELUX is useful for daylighting design,

and analysis. It is used for simulation of building models to get information on

daylight factor, and illuminance distribution. Some of the simulation results are

validated with a theoretical model. Daylight factor and illuminance distribution

corresponding to different room scenarios and various window configurations are

analyzed. Simulation studies conducted in this article establish that windows have

potential to reduce electrical energy for illumination, and maintain a well-daylit

environment throughout the office room.

Keywords: Daylighting, Daylight Factor, Window, Illuminance, Strip and Punch

Window and Double Window.

Cite this Article: Badri Narayan Mohapatra, M. Ravi Kumar and Sushant K. Mandal,

Analysis of Daylighting Using Daylight Factor and Luminance for Different Room Scenarios,

International Journal of Civil Engineering and Technology, 9(10), 2018, pp. 949–960.

Analysis of Daylighting Using Daylight Factor and Luminance for Different Room Scenarios


http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=9&IType=10

1. INTRODUCTION

Reduced exposure to daylight for long duration is found to cause mood swings and

depression [1, 2]. Daylight is a valuable natural resource (Sun) and plays a role in passive

solar building design. Appropriate daylighting design provides a connection with

surroundings, ensuring an adequate illuminance level with high uniformity and a glare-free

environment, and saves energy [3]. Sun is the source that has been emitting energy for about

4.5 billion years [4].Commercial buildings claiming energy efficient, sustainable, or green

most likely refer to daylight as a plan to reduce energy consumption and improve quality of

indoor environment [5]. Students in daylit schools may also experience health benefits, like

better immunity, growth, and eye- sight [6].

Simulation analysis and field survey show that daylighting can give extensive savings in

energy, from 30 to 70 percent of electrical light consumption [7]. Daylight is a comfortable

source of light for occupants even with variations in luminosity [8]. Choice of windows has

an impact on energy consumption in future, which depends on initial phase of building design

[9]. Daylight in the working environment improves the quality of space, and productivity of

employees. Natural light is a free energy resource as well. On one hand, a proper design of

daylight such as distributed top lights can reduce the electrical lighting consumption [10].

VELUX is a software program used for analysis of daylighting [11].

This article discusses factors like daylighting feasibility, daylight factor, and preferred

sitting arrangement for maximum productivity in Section 2. Different components involved

in VELUX are elaborated, and simulation results for bare office rooms are presented in

Section 3. The effect of room depth on daylight factor, and lux distribution of three different

office rooms consisting of a central meeting table with depths of 3mt, 4mt, and 5mt is

explored in Section 4. The effect of window configurations in large office rooms consisting

of desks and meeting table is verified by variations in daylight penetration for punched, strip,

and full windows by images in rendered form and with data interpretation form in Section 5.

The article is finally concluded with directions of future research in Section 6.

2. DAYLIGHTING

Use of sunlight to reduce electric lighting is daylighting. High performance buildings should

meet low energy design objectives, and maximize occupant comfort along with productivity.

Inspection of artificial lighting layouts is easy to implement with the help of some software

programs. With the help of such programs, experts can find best performance of building

components like glazing, blinds, and shades among others.

2.1. Daylighting Feasibility

Day lighting would be effective when daylight falls on windows throughout the day. Higher

glazing should provide good day lighting. Orientation, door height, sky views, ground

reflectance, and interior design can be treated together for feasibility at each portion of a

room. The appropriate choice of glazing in a facade depends on many factors. They include

location, orientation, climatic condition, window transmittance, usage of the building,

required user comfort, and the architectural concept [12]. The Feasibility Factor ( FF ) is

expressed as,

WTOFWWRFF ** (1)

Badri Narayan Mohapatra, M. Ravi Kumar and Sushant K. Mandal


In the above expression, WWR is window to wall ratio, OF is obstruction factor, and

WT is visible window transmittance. For maximum daylight effectiveness, sitting

arrangement, fade, and windows play an important role. Light colored surfaces will reflect

more daylight than dark surfaces. Spaces with windows on two sides often have better day

lighting distribution.

2.2. Windows

For a fixed window height, if the window is placed higher, the daylighting zone is deeper.

The typical depth of a day lighted zone is 1.5 to 2 times the head height of window [13].

More control is required for large windows to mitigate the effects of solar heat gain.

Similarly, if the glass area is more, then required visible window transmittance reduces.

South facing windows provide good access to strong illumination. North facing windows

provide quality consistent daylight with minimal heat gains. In case of East and West facing

windows, shading is difficult, and hence maximum people choose north or south facing

windows. The amount of daylight will vary throughout the day. Integrating personal shading

or glare control systems into workspaces [14] are some of the steps to mitigate excessive

daylight. Three different sitting arrangements are shown in Fig. 1 [13]. Among the three,

person sitting as shown in the left side encounters shading on desk, and the person sitting on

the right has a tiring effect on eyes due to excessive daylight. These two positions limit

productivity of person. The preferred sitting position is as shown in the middle.

A daylight design approach increases the potential to meet credit requirements for a

visual connection to the outdoors. Credits are given for daylighting in different versions of

LEED (Leadership in Energy and Environmental Design) Rating Systems, and that relates to

decisions on envelope and room design. This credit system defines parameters, and their

range to meet requirements. Windows positioned at 30 inches or more above the floor should

meet the following range criteria; 0.150 < VLT *WFR < 0.180, where VLT is visible light

transmittance, and WFR is ratio of window-to-floor area.

Figure 1 Three different sitting arrangements among many possible combinations. The middle case

should be preferred for maximum productivity.

2.2. Daylight Parameter

Illuminance is the amount of light that reaches a surface, before it is reflected [15]. The unit

of measuring illuminance is lux. Some simulation results show illuminance of rooms.

Daylight factor is defined as a ratio of indoor to outdoor illuminance for an overcast sky,

and is written in percentage form. If illuminance at a point in a room is 180 lux, and outdoor

illuminance is 20000 lux then, daylight factor at that point is 0.9 percent.

Luminance is the amount of light that is reflected off surfaces. Luminance is basically

what we see and can also be described as brightness [15]. The unit of measuring luminance is

candela/(mt)2.



3. DAYLIGHT FACTOR OF AN OFFICE BLOCK

VELUX daylight visualizer [16] is a design program which is typically used in the initial

stage. VELUX uses the model of photon map to display appropriate results. Algorithms are

normally classified according to direction of propagating rays. The algorithms may compute

the ray traversal in any of the following ways. Ray traversing from light source is referred as

forward tracing, from observer's eyes is referred as backward tracing. Bidirectional ray

tracing refers to rays traversing from light source as well as observer. The different types of

ray tracing are useful for lighting calculations, and to obtain rendered images [16]. Daylight

simulation consists of various individual components as shown in Fig. 2. Most basic among

them for daylight simulation is a `3D CAD model' of a room or building which is to be

analyzed. The reflectance of light from surfaces is dependent on types of `materials' used, and

its thickness on the surface. Change in reflectance would impact daylight factor as well.

Discomfort may be caused by glare from `luminaires' with a non-uniform source luminance.

`Sky model' variation would change the amount of daylight falling on the windows, which in

turn would change daylight factor. A CAD model, materials, luminaires, and sky model

together describe the scene description. It is possible to explore various combinations by

changing 3D model, thickness of materials, luminaires, and sky model. Renderer gets

quantitative data from measuring points, and qualitative analysis from the `camera' position.

Diffrerent predictions can be obtained by varying each of the parameters.

Figure 2 Different components involved in simulation of daylight using VELUX

VELUX Daylight Visualizer 2 is useful for assessing the orientation of buildings, and

illuminance levels [17]. For the simulation, out of 15 sky conditions we are only considering

a fixed sky, i.e., OVERCAST sky condition, a fixed location (Bhubaneswar), and a fixed time

of the year (March 21), so that it would be easy to know variations in daylight factor.

Figure 3 Investigated office building designed on sketchup, and exported as 3D model



Figure 4 Visualization of one block out of thirty identical blocks of the building shown in Fig.3 .

We first consider the sketchup visualization of an office building in Bhubaneswar where

we consider one identical block among thirty units in that whole office which is shown in

Figures 3 and 4. The primary reasons for considering

Bhubaneswar as the preferred location are frequent load shedding and bright sunlight for

more than 10 months in a year.

Figure 4 shows office rooms connected through a central aisle. The value of daylight

factor is very less (almost negligible) in the aisle as compared to the rooms with windows

exposed to daylight. The office is first considered to be empty without any chairs or tables.

After initial analysis of simulation results, further simulations are carried out to analyze

daylight factor with the presence of table, and chairs.

Figure 5 Three different office rooms with depths of 3mt, 4mt, and 5mt

4. EFFECT OF ROOM DEPTH ON DAYLIGHT FACTOR

In this section, we have used the same VELUX daylight visualizer as in the previous section,

and have validated some of the simulation results with a mathematical model. Three different

office rooms with increasing depths are considered with a meeting table in the middle of

every room as shown in Fig.5. Distance between window and its opposite wall is referred as

depth. The terminology is relative to daylight penetration depth in a room. As indicated in

Fig. 5, Case 1 refers to room with depth of 3mt, Case 2 refers to room with depth 4mt, and

Case 3 refers to room with depth 5mt.

Other parameters that are used for simulation are the following. FR, and Fr are floor

reflectance, and roughness, CR, and Cr are ceiling reflectance, and roughness, TR, and TW

are thicknesses of roof, and wall respectively. Parameter values, and conditions that are

considered for simulations are

FR = 0.35, Fr = 0.5, CR = 0.84, Cr = 0.3, TR = 400mm, and TW = 300mm.

4.1. Simulation Results

Output can be obtained in two forms, text-only (quantitative) and graphical representation

(qualitative) [16]. The obtained quantitative values can be used for different forms of

analysis. Qualitative output may be used for purposes like identifying lighting preferences of



different age group users [18], assessment of user preferences for positioning of luminaires in

rooms [19], and visual stimuli simulation for patients [20]. Interactive, rendered, and data

interpreted images are qualitative outputs.

ISO-luminal contours were used to gather the initial data, from which higher level

features were extracted. Images in the form of ISO-contours can be obtained from

simulations, and are shown in figures 6, and 7. Figure 6 shows the daylight factor contours of

a room having dimensions of Case 1 (Table 1) ranging from 1 to 8, which are represented by

different colours.

Figure 6 ISO-contour image for room of 3mt x 3mt (Case 1 - with 1.25mt high and 1.8mt wide

window) indicating the distribution of daylight factor.

Figure 7 is similar to 6 for Case 3 (Table 1). From Fig. 7, it is found that the depth of

daylight penetration is very low beyond 3mt.

An attempt is made to increase the penetration of daylight into rooms by varying

dimensions of window. Results obtained by variation in window dimensions are analyzed to

find the best possible design of windows. The daylight factor images inside a room for

different models (variation in width and height) of windows are shown in figure 8 (where

height is 1.5mt and width is 1.8mt) and figure 9 (where height is 1.25mt and width is 2.16mt).

The approximate depth of penetration with a minimum daylight factor of 1 (indicated by blue

contour) is 3.6mt, and 3.3mt in figures 8, and 9 respectively. Increasing the height, rather

than width of a window gives better daylight depth of penetration. It is to be noted that, the

area of window in figures 8, and 9 is kept the same for proper comparison.

From simulations, it is clear that, if the window size is increased, then daylight factor also

increases. Results indicate that the geometry of a window affects the daylight distribution in a

room. Since the height of window gives better daylight performance, the penetration of light

by a window shifted upwards by 0.25mt is also analyzed. As shown in figure 10, approximate

depth of penetration with a minimum daylight factor of 1 is 3.3mt. Hence, it can be stated that

maximum penetration of daylight is obtained by increasing the height of windows.

Dimensions of rooms, and the various window configurations used in this simulation

study are shown in Table 1. The approximate Daylight Factor Penetration Depth (DFPD) for

DF=1 is measured from the simulation results, and given in the last column.



Figure 7 ISO-contour image for room of 3mt x 5mt (Case 3 - with 1.25mt high, and 1.8mt wide










Table 1 Room, Window Size and position of meeting table

Rooms Room Size Window Size DFPD(mt)

(DF=1) depth width Height Width Height

Case1 3mt 3mt 2.8mt 1.8mt 1.25mt No

Case2 4mt 3mt 2.8mt 1.8mt 1.25mt 3.7mt

Case3 5mt 3mt 2.8mt 1.8mt 1.25mt 3mt




When a luminance image has been rendered, it is possible to view values at specific

points [15] as shown in Fig. 11. Lux distribution for Case 1 (Table 1) is shown in Fig. 11. .

Since the meeting table is located in middle of the rooms, for Cases 1, 2, & 3, table is at

distances of 1.5mt, 2mt, and 2.5mt from window. Hence, luminance value at the meeting

table varies for the three rooms. Observing the simulation results, it should be noted that,

daylight factor, and lux values decrease with increasing depth of rooms.

Close inspection of Fig. 11, reveals numbers spaced apart in a grid. Two values are

visible on the meeting table which is seen in red colour. First, and second numbers on the

meeting table are considered as variables X, and Y . Considering minimum luminance value

(Z) required for proper visibility to be 500 lux, the difference in luminance (DL) can be

calculated as shown in Table 2.

Figure 11 Illuminance (lux distribution) image for room of 3mt x 3mt indicating the distribution of

daylight factor, and the values of illuminance at specific points

Table 2 Calculation of luminance requirement

Rooms X(Lux) Y(Lux) (X+Y)/2 Z(Lux) DL(Lux)

Case1 718.8 749.4 749.1 500 249.1

Case2 164.9 168.8 165.35 500 -334.65

Case3 142.0 143.6 142.8 500 -357.2

From simulation result, obtained luminance values rendering on meeting table are written

in Table 2. It can be understood that for an office, 500 lux is the average luminance

requirement. When the value goes below 500 lux,

electric light may be used to compensate for the low level of lighting. When the value

goes above 500 lux, glare control at windows should be used to avoid inconvenience on the

eyes of persons.

According to [21], the average daylight factor (DFav) is given by



)1( 2RA

mADF

w

av

(2)

In the above equation, wA is area of window (in mt2), is transmittance of glazing

material, is vertical angle of sky as seen from centre of window, m is maintenance factor

based on glazing angle, and cleanliness, A is total internal surface area (in mt2) of space

(walls, doors, ceilings and windows), and 2R is the area weighted average reflectance of all

surfaces making up A. The value of maintenance factor typically varies from 0.5 to 0.9.

Values of parameters involved in calculating the daylight factor are the following.

Reflectance for glass is considered to be 0.1. AW = 1.25mt x1.8mt = 2.25mt2, = 1.47, m =

0.9, varies according to the distance of measuring point from window, and the values of

for different measuring points are obtained as shown in Fig. 12. Figure 12 indicates the

calculation of angles ( ) for every 1mt separation in the office room.

Figure 12 Calculation of with change in point of measurement to find daylight factor.

Table 3 Results of daylight factor obtained from the mathematical model for rooms shown in figure 5.

RD(mt) RW(mt) RH(mt) Surface

area (mt

2)

𝝷 (deg) DFavg

1 3 2.8 28.4 64.538 7.516

2 3 2.8 40 46.397 3.83

3 3 2.8 51.6 34.99 2.24

4 3 2.8 74.8 27.699 1.224

5 3 2.8 86.4 19.29 0.738

Table 3 shows the variation of daylight factor with change in room depth using the

mathematical model given in Eqn. (2).Here in table 3 RD,RW,RH are room depth, room

width and room height. For Case 1, values of daylight factor for increasing room depth are

given in first three rows of Table 3. Similarly for Case 2, values of daylight factor for

increasing room depth are given in first four rows of Table 3, and all rows are applicable for

Case 3. It is to be noted that as the room depth increases, the daylight factor decreases.

Figure 13 Configuration of punched (left) and strip (right) windows in large rooms.

Since wall is present at depths of 3mt, 4mt, and 5mt, in cases 1, 2, and 3 respectively,

corresponding values of daylight factor cannot be obtained by using VELUX.



5. PUNCHED, STRIP AND FULL WINDOW

As seen from the previous sections, daylight penetration is roughly a fraction of the outside

illuminance. So, this section explores the types of windows to be used for very large rooms

with the presence of office furniture. For this, we now consider large rooms having length

(12mt) and width (9mt) as shown in Fig. 13. Three office desks and a meeting table are

positioned as shown in Fig. 13. Since the rooms are very wide, three windows are used. Two

configurations (punched, strip) are considered in the simulation. Parameters used are the

following. Window width is 1.8mt, height is 1.25mt, and windows are placed at 0.85mt above

the ground level.

In the simulation, a high beam camera is used to focus on room, window, and tables

which is shown in Fig. 14.

Figure 14 Position of camera with three office desks and a meeting table.

Figure 15 Daylight factor distribution for punched (left), and strip (right) windows of large office

rooms.

The results from VELUX show daylight factor in the form of rendered images in Fig. 15

for the large offices. Position of windows may vary as per the designer/ architect, based on

their idea for proper daylighting inside a room. Here we investigate different window

configurations without varying the area (2.25mt2). Daylight factor data from physical model

studies of considered case Fig. 13 can be presented graphically in the form of contours having

equal daylight factors plotted over a building floor plan. The obtained results indicate that

penetration depth of daylight is more in case of strip windows as compared to punched ones.

We can also observe that strip windows provide more uniform daylight in the room.

Figure 16 Daylight factor distribution for punched (left), and strip (right) configurations using double

windows in large office rooms.



Figure 17 Images with data interpretation indicating illuminance values for large office rooms shown

in Fig. 13.

For the same window area of (2.25mt2), single windows are replaced with double ones in

punched and strip configurations. The variations in results are shown in Fig. 16. It is observed

that daylight factor is slightly more (almost negligible) when the single windows are replaced

with double ones.

Images with data interpretation for the large offices are shown in Fig. 17. Illumiance

values throughout the room for Fig. 15 are indicated in Fig. 17. The change in daylight factor

provides information on the impact of illuminace distribution on the floor space. Beside the

stage, output also depends on kind of visual observation (eye or camera) used for studying

qualitative, and quantitative results.

Figure 18 Images in rendered form (left), and with data interpretation (right) for large offices with a

single large window.

Figure 18 shows the obtained results as rendered images on left, and in data interpretation

form on right when there is only a single large window. The size of window considered in

this simulation is a width of 8.775mt, and height of 1.6mt occupying an area of 14.04mt2. It is

to be noted that, even though this window is more than 6 times the area occupied by punched,

or strip configurations, the daylight factor penetration is roughly double that for the punched

case, and 1.5 times that of the strip case.

6. CONCLUSIONS

Overall daylight factor depends on an integral design approach considering various

parameters like location, time of year, sky condition, room, and window size, roof, and wall

thickness, maintenance factor, sitting arrangement, and situation of the room (whether facing

south, north, east, or west). The distribution of daylight factor in a room also varies during

summer, equinox and winter. Lighting simulation programs are useful after finalizing

parameters like window size, and others. Penetration of daylight in a room can be chosen by

appropriately selecting the height of a window. This type of simulation study shall be helpful

for architects before initiating a plan for building and placement of interior items so that

maximum daylight could be used and the load on electric lighting shall be reduced at least

during daytime. Simulation results also indicate that strip windows provide more comfort as

compared to punched window. The use of full width windows does not increase the daylight

penetration substantially.

The work could be extended to find the best possible window configuration for large

office rooms. New means may be found to direct daylight towards the farther regions from



windows. Scope for further research includes new light transport models enabling simulation

of many more physical objects, phenomena.

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