Light pollution index (LPI): an integrated approach to study light pollution with street lighting and facade lighting.
Rather than being complementary, street lighting and facade lighting are typically designed independently of each other. This is a problem when light from one spills onto the other's area of influence. Figure 1 illustrates: a) Street lighting spilling onto building facades, which influences the required light levels and the color of the light on the facade and, b) Street lighting that ends up with higher light levels because of the reflected light from the facade. These are the sort of problems that occur when facade and street lighting are designed separately.
The three major scenarios where the study of street and facade lighting in unison is necessary are:
a. New buildings or upgrades of older buildings where the facade lighting is designed within a city environment that has pre-existing street lighting.
b. Heritage City preservation, conservation and restoration activities with an emphasis on energy efficiency and sustainability while retaining the effect of the existing facades and street lighting.
c. Upcoming, planned residential and commercial neighborhoods.
If the street and facade lights are designed and controlled by the same entity, such as a developer or municipality, the mutual benefits/harms can be coordinated and controlled to:
a. Provide the required light levels onto the street.
b. Retain the aesthetic aspirations of the facade lighting scheme.
c. Consume no more energy than necessary.
d. Provide the stakeholders with a hybrid solution whereby the facade and street lights contribute to each other's function and achieve the desired lighting scheme in a sustainable way.
Designed street lighting typically aims at allowing a road user to achieve at least the following goals:
a. To see the road clearly,
b. To see stationary or moving hazards on the road,
c. To be able to see pedestrians and,
d. To be able to achieve all the above in a sustainable way.
The sustainability guidelines are based on energy efficiency, light trespass and light pollution. Both the IES and the CIE have developed guidelines on this subject. CIE 150:2003 [CIE, 2003] provides guidelines to minimize values of vertical illuminance on windows of adjacent properties based on environmental zones as defined in CIE 126-1997 [CIE, 1997]. Whereas, IESNA TM-11 [IESNA, 2000] contains limits on the maximum illuminance on a plane perpendicular to the line of sight to the luminaires as a function of environmental zones [DiLaura and others, 2011; Lewin, 2000]. As an example, for areas with an intrinsically dark landscape and strict limitation on light trespass (environmental zone E1), the maximum pre-curfew illuminance is set to be 1 Lux [IESNA, 2000]. The United Kingdom's Institution of Lighting Engineers have adopted CIE 150:2003 [CIE, 2003] in its note on the reduction of obtrusive light in which the maximum vertical illumination at the center of the window is set to be 2 Lux for environmental zone E1. This illuminance reaches as high as 25 lx for high district brightness areas. Pseudo color analysis can be used to assess obtrusive light [Pimenta and Speer, 2003].
Regulations for outdoor lighting that curtail light pollution, and minimize adverse off-site impact of lighting such as light trespass are described in the IDA/IES model lighting ordinance [IDA/IES, 2011]. Five lighting zones are specified in the ordinance (zone LZ0-LZ4) based on how the natural environment might be affected by the lighting with LZ0 being the most sensitive and LZ4 being the area that requires high ambient lighting. The ordinance outlines the allowable BUG rating for luminaires based on the lighting zone and the distance between the luminaire and the property line. Maximum vertical Illuminance at any point in the plane of the property line is also specified. Brons and others  presented an approach whereby a virtual calculation box surrounding an outdoor lighting installation was formed. The values generated are based on the Illuminance calculations representing light crossing defined areas. Trespass was analyzed in terms of how much light crosses the property line.
The effect on the illumination on building facades from an adjacent parking lot or street lighting luminaires was investigated by Saraiji . A comparison was made between various types of area and road light fixtures such as Type III and type II as well as medium-cutoff and semi-cutoff luminaires. The paper compared the effect of different classes of roadway luminaires such as medium, short, Type III and Type II on the maximum and average vertical facade Illuminance. Type II luminaires provided less light trespass than type III when both luminaires faced the facade. By orienting the poles so that the light was aimed away from the facade, poles could be placed closer to the facade. If we are interested in reducing the spill light onto the building facade, then we have to place the light poles away from the building as much as possible without affecting the parking lot illuminance. Polynomial equations were developed to find the maximum or average facade vertical illuminance as well as the height of the 1-lx contour line above ground as a function of the distance between the poles and the facades. Other studies have focused on sky-glow and discomfort glare [Brons and others, 2008; Bullough and others, 2008; Kosiorek and others, 2000; Schreuder, 2000].
The objective of this work was to develop a method to analyze the interaction between street lighting, facade lighting, and light pollution in an integrated manner and, to study the process of analyzing and controlling light pollution.
A virtual street was created that was 23 m wide with buildings that are 10 m high on both sides. The street was modeled without trees or other landscaping using Dialux software [DiaLux, 2012]. Horizontal and vertical calculation grids were added to get results as shown in Fig. 2.
Horizontal calculation surfaces (Street grids SA and SB) were placed on the main street and on the sidewalks to calculate the horizontal illuminance. Vertical calculation surfaces were placed on the two facades, namely FA and FB. Vertical calculation surfaces (PA, PB, PC, PD) at 1.5 m height from the ground were placed along the sidewalk and crossing the street to calculate vertical illuminance on pedestrians. A more elaborate study on vertical illuminance along cross walks was performed by Saraiji [2009b].
To calculate light pollution, a calculation grid spanning the entire site and at a height higher than any light emitting or reflecting object was placed to obtain Illuminance on the grid (in this study it is at 10 m high). The product between the average Illuminance on the above mentioned grid and the area of the grid provides the lumens going into the sky, which includes the reflected component. The lighting pollution percentage (LPP) was then calculated using (1):
(1) LPP = 100 ([[phi].sub.up]/[[phi].sub.T])
[[phi].sub.up] = Total lumens going upward and is equal to (E)*(A)
E= Average illuminance (lux) on the pollution grid shown in Fig. 2
A= Area of the pollution calculation grid in square meter
[[phi].sub.T] Total lumen output from all luminaires
LPP was then compared to the light pollution index (LPI), which is developed in the next section.
Once the calculation grids were in place, luminaire selection and placement were done and lighting simulation was performed in an iterative process to evaluate average Illuminance and lighting uniformity based on the average to minimum illuminance ratio per the IES RP 8.00 [IESNA RP-8-00, 2005].
The independent variables matrix and the level of control are shown in Table 1. The following dependent variables were calculated:
Variable Control Level Street lighting Height Fixed at 9 m Lamp HPS LED BUG rating B1-U1-G2, B2-U0-G1 Watt 54, 70, 100, 103 Surface Asphalt 10% fixed reflection Sidewalk Concrete paved 35% reflectance Facade Red brick 34% materials reflectance Street Width 8.5 m each way configuration Median 2m Sidewalks 2m Facade lighting Type Linear washer up light Location Bottom of facade, top of facade Aiming Downward, upward Source LED narrow beam Metal halide narrow beam Fluorescent Beam type Symmetric narrow beam Symmetric medium beam Asymmetric wide beam TABLE 1. Independent variables matrix
1. Horizontal illuminance on street level
2. Vertical illuminance 1.5 m above floor level
3. Uniformity ratio (average horizontal illuminance/minimum horizontal illuminance)
4. Luminous flux (lumens) going upward into the sky
5. Light pollution percentage
6. Lighting power density (LPD). (The LPD values in this manuscript consider only the street and not the intersections)
7. Street light pole spacing was designed to meet the IESNA RP-8 - 00 standard then it was fixed.
4 LIGHT POLLUTION STUDY
The light pollution study was done in seven steps:
1. Initially, a pollution calculation was made when linear facade lighting was on.
2. The pollution values from using varying forms and sizes of the pollution calculation grids were compared to ascertain the optimum size of the grid.
3. A three dimensional graph of the illuminance on the pollution calculation grid was made.
4. The virtual building on one side of the street was removed and the pollution grid size was increased in increment of 5 m.
5. The virtual buildings on both sides of the street were removed and the pollution grid from only the street lighting was studied.
6. A series of calculations using light blocking ledges of various sizes on the building facades were performed.
7. Comparison between light pollution percentages resulting from different placement and aiming of facade lighting was made.
4.1 CALCULATION GRID CONFIGURATION
In an effort to find the best configuration of the pollution calculation grid, three different pollution grid configurations were used; PGa, PGb and PGc, as illustrated in Fig. 3. PGa is a horizontal grid at the edge of the building height extending between the facades of the buildings over the street. PGb is a horizontal grid extending 10 m from the outer edge of the building. PGc is a five sided box grid. In the pollution grid PGc, the vertical sides were positioned so that they were higher than any light reflecting/emitting element in the scene to avoid capturing any lumen going downward and only the flux going upward was captured. Table 2 shows that the horizontal pollution grid PGb gave similar pollution values to that of the box grid PGc.
Pollution Values Scene Lighting Pollution Pollution Upwards Description Grid [E.sub.avg] Lumens[[phi].sub.up] (lux) PB Only facade PGa 44 61,016 linear fluorescent wash up PGb 30 64,968 PGc Various 63,689 PJ Only facade PGa 9 12,924 linear fluorescent wash down PGb 7 14,683 PGc Various 14,609 PK Only facade PGa 31 42,988 linear LED wash up PGb 22 47,643 PGc Various 45,366 PL Only facade PGa 7 9,721 linear LED wash down PGb 5 11,348 PGc Various 11,261 Scene Lighting Pollution Total Lumens LPP % LPI Description Grid Output [[phi].sub.T] PB Only facade PGa 139,374 43.78% 26.712 linear fluorescent wash up PGb 139,374 46.61% 30.284 PGc 139,374 45.70% 29.104 PJ Only facade PGa 139,374 9.27% 1.198 linear fluorescent wash down PGb 139,374 10.53% 1.547 PGc 139,374 10.48% 1.531 PK Only facade PGa 92,715 46.37% 19.932 linear LED wash up PGb 92,715 51.39% 24.482 PGc 92,715 48.93% 22.198 PL Only facade PGa 92,715 10.48% 1.019 linear LED wash down PGb 92,715 12.24% 1.389 PGc 92,715 12.15% 1.368 TABLE 2. Various Pollution Percentages Using Only the Linear Facade Washers. PGa--Pollution grid between buildings, PGb--Pollution grid extending 10 m along the street from the building edge, PGc--Pollution grid box starting 10 m above building height and dropping down to top of the building
4.2 LIGHT POLLUTION INDEX
To reduce light pollution, different facade lighting schemes were used. Both LED and fluorescent linear wall washers were used and they were placed either at the bottom of the facade aiming upward or at the top of the facade aiming downward. When we compare scene PB and PK in Table 2, we notice that the upward lumens using LED facade lighting is 42,988 lumens, whereas, it is 61,016 lumens when using fluorescent lamps. The light pollution percentage (LPP), however, was higher when the LED fixtures were used. The led fixtures (when faced upwards) yielded a higher LPP than their fluorescent counterparts as shown in Table 2, despite the fact that the luminous flux going upwards decreased. This is because the LED fixtures are more efficient in illuminating the facade and used fewer lumens to do so. When the pollution percentage was calculated, the ratio of lumens going upwards divided by the total lumens emitted by the fixtures made the comparison between the LED and fluorescent facade light misleading. This result highlights the need for a better light pollution index whereby different design alternatives are evaluated in a way that combines design efficiency as well as the total amount of flux going upward. The Light Pollution Index (LPI) developed below as (2) gives an added emphasis to the upward flux;
(2) LPI = ([[phi].sub.up]/[[phi].sub.T]) [[phi].sub.up]/1000
From sky-glow point of view, the LPI values shown in Table 2 indicate that using the LED facade lighting solution is a better than using the fluorescent lamps. This suggests that the LPI index does not have the same shortcoming as the LPP index.
4.3 CALCULATION GRID SIZE
To study the impact of the size of the pollution grid on the light pollution computation, a horizontal calculation grid was placed. The extension length of the calculation grids on both sides was increased in 5 m increments from 5 m to 50 m. The calculation grid was higher than the highest reflecting/emitting element on the site. Facade lighting using LED linear wash light with up-light and LED street lights were used. Figure 4 shows the changes in light pollution values as a function of the changes in grid size. The values increased initially then stabilized indicating that any further increase in the size of the light pollution grid would not add any incremental benefit. This indicated that, in the case of our site, a calculation grid extending 20 m beyond the site borderline would be sufficient to capture all the lumens going upward. Figure 4 shows that the LPI and LPP exhibit similar trend which reinforces the fact that LPI could be used to gauge light pollution without the shortcomings of LPP that were explained earlier.
In another simulation, one building and its facade lights were removed and the size of the pollution grid along the lateral direction was changed in increments of 10 m as shown in Fig. 5. Figure 6 indicates that an extension of 110 m beyond the site borderline was sufficient to capture all the lumens going upward. At this distance the illuminance on the pollution grid was near zero.
The three dimensional graph of illuminance on the calculation grid provides additional insight on upward luminous flux. As there is a large range of illuminance levels, a logarithmic scale was used, which illustrates lower and higher values of illuminance. Figures 7 and 8 show the 3-D graph of illuminance values for the scene that has one building. Figure 7 shows the area towards the front side of the building and Fig. 8 shows the area towards the rear side of the building. From these two figures, the dimensions needed for the pollution grid to capture all the flux going upward can be determined. The optimal calculation grid is shown in Fig. 5. The dimensions and configurations of the pollution calculation grid is site specific which is a function of the reflecting elements as well as photometry, quantity and, location of light fixtures used. Therefore, the three dimensional graph of the illuminance on the pollution grid should be used to make sure that the configuration of the pollution calculation grid is optimal.
In a third simulation, both buildings were removed, but the sidewalks, street surface and, street lights were kept. Figure 9 shows the light pollution percentages LPP and LPI as a function of grid extension in the lateral and the longitudinal directions. The figure shows a steady increase in light pollution percentages until the extension is greater than 30 m at which point light pollution values reached stability.
4.4 METHODS TO REDUCE LIGHT POLLUTION
To reduce light pollution, two methods were used. First, the facade lights were placed on the upper part of the facade and aimed downward instead of upwards. Second, a ledge was added to the parapet of the building. The ledge acted as a light pollution blocker.
The illuminance on the pollution grids was plotted onto a 3-D chart. This chart proved to be a useful method to spot the areas that were causing the largest amount of sky-glow. In this exercise, the streetlights were off and only LED linear wall washers were used. A significant reduction of sky-glow was obtained when the facade lights were placed high on the facade and aimed downward, as can be seen in Figs. 10 and 11.
As far as adding a ledge to the parapet of the building, four different ledge sizes were used; 0.5 m, 1 m, 1.5 m and 2 m. In all of these simulations only upward-aimed linear LED facade lighting was used. Figure 12 shows the effect of changing the ledge size on the light pollution values, with a 1.5 m ledge providing values that are close to the ones obtained when the facade lighting was aimed downward.
5 THE INTERACTION BETWEEN FACADE LIGHTING AND STREET LIGHTING
To study the interaction between facade lighting and street lighting, several generic scenes were made in Dialux Software. The scenes are listed in Table 3. The renderings generated by Dialux, as well as the pseudo colors, are shown in Figs. 13 and 14.
Scene Luminaire Types Mounting BUG Rating Wattage Quantity Used in the Height Simulations A Asymmetric 0m NA 70 W 7 narrow beam metal halide uplight Asymmetric wide 0m NA 70 W 2 beam metal halide uplight B Ground level 0m NA 54 W 20 fluorescent wall washer Wall mounted 6 m NA 54 W 34 fluorescent wall washer C Asymmetric 0m NA 70 W 7 narrow beam metal halide uplight Asymmetric wide 0m NA 70 W 2 beam metal halide uplight Ground recessed 0m NA 54 W 20 fluorescent wall washer Wall mounted 6m NA 54 W 34 fluorescent wall washer D Only LED street 9m B1-U1-G2 103 W 6 lights on either side of the road (Type III, short, full cutoff) E LED street 9m B1-U1-G2 103 W 6 lights on median (Type III, short, full cutoff) F Asymmetric 0m NA 70 W 7 narrow beam metal halide uplight Asymmetric wide 0m NA 70 W 2 beam metal halide uplight Ground recessed 0m NA 54 W 20 fluorescent wall washer Wall mounted 6m NA 54 W 34 fluorescent wall washer Only LED street 9m B1-U1-G2 103 W 6 (Type III, short, full cutoff) G Only HPS street 9m B2-U0-G1 100 W 6 (Type II, medium, full cutoff) H Asymmetric 0m NA 70 W 7 narrow beam metal halide uplight Asymmetric wide 0m NA 70 W 2 beam metal halide uplight Ground recessed 0m NA 54 W 20 fluorescent wall washer Wall mounted 6m NA 54 W 34 fluorescent wall washer Only HPS street 9m B2-U0-G1 100 W 6 (Type II, medium, full cutoff) J Wall mounted 4.5 m NA 54 W 20 fluorescent wall washer aiming down Wall mounted 9m NA 54 W 34 fluorescent wall washer aiming down Wall mounted 9m NA 54 W 34 fluorescent wall washer aiming down TABLE 3. Luminaire types used in the simulations. Building height (10 m), street width (8.5 m), median and sidewalks are each (2 m) wide. Reflectance values: red brick facade (34%), asphalt street (10%), and concrete paved sidewalks (35%)
We found that a moderately lit facade contributes to an average illuminance [(E.sub.avg]) of 5 lx or more to street illumination, as shown in Table 4. Therefore, in areas of a city center with street width less than 15 m, facade lighting alone may be able to illuminate the street with the required light levels per IES RP-8 - 00 [IESNA, 2005] and there would be no need for extra street lighting. A hybrid approach with light poles on either side of the street and accentuation of the facade will provide the needed light levels. When both the street lighting and the facade lighting were used, the [E.sub.avg] was 30 lx, which is around twice of what is required for a major road with medium pedestrian conflict and R3 pavement per RP-8 - 00 [IESNA, 2005]. When the facade is lit by linear washers all aimed downward (scene J), the horizontal average Illuminance increased to 30 lx from the 5.37 lx that was achieved when the linear washers were illuminating the facade upward (scene B). The light pollution percentage (LPP), meanwhile, was reduced to 9.54 percent (LPI=1.27) in scene J; down from 43.42 percent (LPI=21.07) in scene B. Furthermore, the pseudo color renderings in Fig. 14 suggest that it is possible to space the street lighting based on the illuminance received from the facade lighting and not necessarily in regular intervals as would be the normal approach.
TABLE 4. Results Brief, Generic Street--Facade and Street Lighting. Building height (10 m), street width (8.5 m), median and sidewalks are each (2 m) wide. Reflectance values: red brick facade (34%), asphalt street (10%), and concrete paved sidewalks (35%) Scene Description Street Values Total LPD W/ [E.sub.avg] [E.sub.avg] Power sqm Lux [E.sub.mtn] W A Facade 765 0.68 5 4.5 uplights only B Facade 3,294 2.91 5.37 1.7 linear wash only C Facade 4,059 3.59 9.98 1.9 uplights and linear washers D LED street 618 0.55 20 1.8 lights only E LED street 618 0.55 20 2.5 lights on median F Facade + LED 4,677 4.14 30 1.8 street light G Only HPS 600 0.53 20 3.6 street H All facade + 4,659 4.12 30 2.7 HPS street J Only facade 3,294 2.91 30 7.8 linear wash aiming down Scene Description Facade Sidewalk ([E.sub.avg]] ([E.sub.av lux) g] lux) FA FB SA SB A Facade 31 21 10 21 uplights only B Facade 45 43 20 5 linear wash only C Facade 75 64 30 25 uplights and linear washers D LED street 5 5 18 18 lights only E LED street 7 7 16 16 lights on median F Facade + LED 80 68 48 42 street light G Only HPS 4 4 14 13 street H All facade + 79 67 43 37 HPS street J Only facade 60 48 137 86 linear wash aiming down Scene Description Pedestrian ([E.sub. avg] lux at 1.5 m) PA PB PC PD A Facade 4 4 1 2 uplights only B Facade 5 4 9 7 linear wash only C Facade 8 8 10 9 uplights and linear washers D LED street 5 5 12 12 lights only E LED street 18 18 13 12 lights on median F Facade + LED 13 13 22 21 street light G Only HPS 4 4 13 14 street H All facade + 12 12 23 23 HPS street J Only facade 4 4 19 11 linear wash aiming down Scene Description Pollution Values Lumens Up[ Total [phi].sub.up] Lumens [[phi ].sub.TT] [E.sub.avg ]Lux A Facade 9 11,801 41,208 uplights only B Facade 35 48,535 111,784 linear wash only C Facade 43 59,629 152,992 uplights and linear washers D LED street 2 3,231 33,759 lights only E LED street 3 3,564 33,759 lights on median F Facade + LED 45 62,402 186,751 street light G Only HPS 2 2,968 42,180 street H All facade + 45 62,402 195,172 HPS street J Only facade 10 13,299 139,374 linear wash aiming down Scene Description LPP LPI (%) A Facade 28.64% 3.37 uplights only B Facade 43.42% 21.07 linear wash only C Facade 38.98% 23.24 uplights and linear washers D LED street 9.57% 0.31 lights only E LED street 10.56% 0.37 lights on median F Facade + LED 33.41% 20.85 street light G Only HPS 7.04% 0.21 street H All facade + 31.97% 19.95 HPS street J Only facade 9.54% 1.27 linear wash aiming down
If the objective is to illuminate the street only, then the most efficient way is to use street lighting since this is what provides the least lighting power density (LPD). This is illustrated in scene D, where 20 lx was provided with an LPD of 0.55 w/[m.sup.2]. The least light pollution was found when only streetlights were used (scenes D and G). In contrast, scenes C, F, H, which combine streetlights and facade light, caused the highest light pollution percentage. When using street-lights only, an average of 7 lux is achieved on the building facades.
It is observed that when the poles are all on the median (scene E), the street is lit uniformly and there are higher Illuminance levels on the facade than when the street light poles are on the sidewalks (scene D). This indicates that positioning the poles on the sidewalk reduces light spill onto the facade. However, the placement of the poles on the sidewalks will cause undesirable shadows onto the facade (scenes D and G). When street lights are on the median, we were able to receive ample lighting onto the lower part of the facades. This may be acceptable if the intended facade lighting is a uniform wash. In this case, only the higher elements of the facade need to be treated with facade lighting.
As for sidewalk horizontal illuminance, the lux levels were close to the recommended values in all scenarios studied. Table 4 shows the vertical illuminance at 1.5 m above ground, for grids along the two sidewalks (PA, PB) and for two grids across the street facing either directions of incoming traffic (PC, PD). Vertical illuminance of 5 lx or more was achieved in most cases. However, for scene A the vertical light levels were small.
For an optimum hybrid solution, keeping the lighting power densities low and combining the street lights with up lights is an ideal possibility. Even though the results indicate that adding any kind of facade up light contributes to light pollution, the facade lighting adds to the reflected light which is good to raise the vertical illuminance for pedestrian visibility and safety. Down-lighting from the facade walls can be an option to light the facades which increases the vertical illuminance for pedestrians and gets some light onto the streets, as shown in Fig. 15.
A method for studying street lighting, facade lighting and light pollution in an integrated way was shown. In this method, six types of calculation grids were made;
1. Street horizontal calculation grid,
2. Sidewalk horizontal calculation grid,
3. Sidewalk vertical calculation grid; 1.5 m high and facing the street,
4. Two cross walk vertical calculation grids 1.5 m high facing both ways of a two way street,
5. Vertical calculation grid on the facades,
6. Pollution grid to calculate the effect on sky-glow.
It was found that for some street types, facade lighting could provide the required light levels needed for the street as well as pedestrians. It was also found that streetlights can spill over onto the building facade, especially the lower part of the facade. The illumination of the facade can use this spill over and the facade lighting can complement the parts of the facade that is unlit. Positioning the street lighting poles on the sidewalk will reduce the spillover. However, undesirable shadows may result.
To make sure that all flux going upward was captured by the calculation grid, a three dimensional plot of the illuminance on the pollution grid was made. The plot used a logarithmic scale to highlight the low illuminance values as well as the high illuminance values. If near zero illuminance levels are not reached within the boundary of the pollution calculation grid, the grid should be extended to capture the flux that was not captured. Without this, light pollution calculation can be fluctuating and misleading results could be obtained. The three dimensional illuminance graph on the pollution grid proved to be a valuable method to pinpoint the areas causing the largest contribution of sky-glow.
In some cases, the light pollution percentage (LPP) was found not to be a viable index to compare alternative design options. This is because the pollution percentage could increase in one design alternative despite the fact that the actual lumens going upwards had decreased. To overcome this shortcoming a light pollution index LPI was developed. This index gives added weight to the flux going upward that actually causes sky-glow. The developed index was found to exhibit similar trends as LPP but without the shortcomings of LPP that can result when different lamp types or fixture types are used to evaluate different design alternatives.
Two methods to reduce sky-glow were demonstrated. One method would be to place the facade wall washers toward the upper part of the building and aimed downward. The other method is to have a parapet that sticks out as a ledge to block the light going into the sky.
This work was supported by United Arab Emirates University grant number NRF 3260.
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Riad Saraiji (1) * PhD, and M. Saju Oommen (1)
(1.) United Arab Emirates University, El Ain, United Arab Emirates
Corresponding author: Riad Saraiji, E-mail: email@example.com.
[C]2012 The Illuminating Engineering Society of North America
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|Author:||Saraiji, Riad; Oommen, M. Saju|
|Date:||Oct 1, 2012|
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