Because this type of calculation is very time intensive if done by hand, a
computer is almost always used to quickly run the many calculations required.
This software calculation uses the Inverse Square Law to calculate the direct
component of illuminance at a point and luminance.
The inputs required include Initial Lamp Lumens, Light Loss Factor, roadway or parking lot classifications, area or roadway dimensions, detailed Luminaire Photometric Data, and precise x,y,z fixture location coordinates. The program output usually includes an Iso-Footcandle plot for the area or roadway and the average illumination found by taking the average of all the points calculated. Uniformity ratios are calculated, both average/minimum and maximum/minimum. Some software programs have the advanced capability to determine lighting blockage due to obstructions, showing shadowing and even reflections off of nearby buildings.
IES recently included recommendations for roadway luminances and luminance uniformity ratios. Luminance is the amount of light reflecting off the pavement into the driver's line of sight and the amount of light leaving the luminaire that enters the driver's line of sight directly. Luminance is a better measure of a driver's visibility under a particular lighting system. Luminance calculations involve complex geometry and many analysis points, so computers are employed.
