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Light pollution index (LPI): an integrated approach to study light pollution with street lighting and facade lighting.

1 INTRODUCTION

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 [2008] 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 [2009]. 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].

2 OBJECTIVES

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.

3 METHODOLOGY

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])

Where,

[[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.

6 CONCLUSIONS

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.

ACKNOWLEDGMENTS

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: riad.saraiji@gmail.com.

[C]2012 The Illuminating Engineering Society of North America

doi: 10.1582/LEUKOS.2012.09.02.004
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Author:Saraiji, Riad; Oommen, M. Saju
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Date:Oct 1, 2012
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