Alliance for Lighting Information

Unit Uplight Density (UUD)

by David M. Keith

Unit Uplight Density (UPD) is the ratio of the upward flux produced by a roadway lighting system to the corresponding area of the roadway. The model assumes that the reflectance of the roadway and surroundings are perfectly diffuse, and treats the direct uplight from the luminaires in the same manner. The model assumes that the roadway is a repeating cycle, so any flux from one cycle that goes beyond either end of that cycle is exactly matched by flux from the adjacent cycles.

The UUD calculation sums direct uplight and reflected downlight and divides by the roadway area, using values corresponding to the "average LLF". The downward lumens "LumsDn" and the upward lumens "LumsUp" are calculated from the photometric file and adjusted for any difference between the rated lumens of the photometry and the lamp lumens in the roadway calculation, then multiplied by the average Light Loss Factor (LLFavg). The average LLF is taken as the average of the maintained LLF (LLFmaint) and 1.0, and applied to the upward and downward lumens and also to adjust the illuminance onto the roadway (Eavg) from the maintained value to the average value. The reflected light was calculated as the sum of the roadway reflected light and the off-roadway reflected light. The roadway reflected light was calculated as the roadway reflectance times the roadway lumens, which was calculated as the average illuminance value times the roadway area. The off-roadway reflected light was calculated as the off-roadway reflectance times the off-roadway lumens, equal to the total downlight lumens minus the roadway lumens. The total downlight lumens were calculated as the number of luminaires per cycle times the downlight lumens per luminaire. The resulting value, when lengths are measured in meters, has units of lumens per square meter of roadway.

Eq. 1: UUD = [(#Lum * LumsUp * LLFavg) + (rhoRoad * Eavg * (LLFavg/LLFmaint) * Area) + (rhoOffRoad * {#Lum * LumsDn * LLFavg - Eavg * (LLFavg/LLFmaint) * Area})] / [Area]

Roadway reflectance (rhoRoad) is set at 0.07, the default Q0 value for R3 pavement, which is the default roadway surface. Off-roadway reflectance (rhoOffRoad) is set to 0.18, the "Kodak average reflectance value”. Area is the overall roadway width (the number of lanes times the width of each lane) times the LumCycle distance for the staggered layout. If the roadways have staggered layouts, then two luminaires are used when calculating lumens within each luminaire cycle.

For MH pulse start, a typical maintained LLF is 0.50, in which case the average LLF (LLFavg) would be 0.75 for MH pulse start systems. For HPS, a typical maintained LLF is 0.70, in which case the average LLF (LLFavg) would be 0.85 for HPS systems. The UUD values are all calculated using the average LLF, because typical roadway conditions - over both space and time - are midway between initial and maintained, so uplight calculations should be made at that level of maintenance.

The number of luminaires per luminaire cycle is two for staggered and opposite patterns and one for single-sided patterns. The area of the roadway is the extent of the traveled way, equal to the length of the luminaire cycle times the number of lanes times the width of each lane. Shoulders are not included in the area of the roadway.

UUD describes the amount of light that the system sends into the space immediately surrounding it. Of course the radiative transfer characteristics of that space significantly effect the amount, direction and even spectral profile of the uplight radiation that reaches the atmosphere and potentially becomes redirected and appears as skyglow. The number of assumptions necessary to make such a model useful is very high, and most of the assumptions would need to vary depending on locality and even the specific project. For UUD, the significant variables can be reliably predicted by lighting designers.

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