This calculator is designed to estimate the System Application Efficacy and overall cost of a given parking lot luminaire within a given task area, based upon the luminaire's IES distribution file and your other inputs. This calculator is based on ASSIST’s recommendation for testing and evaluating the photometric performance of parking lot luminaires for all light source technologies. The recommendation was developed by the Lighting Research Center (LRC) at Rensselaer Polytechnic Institute in collaboration with members of the Alliance for Solid-State Illumination Systems and Technologies (ASSIST).

ASSIST's recommendation to determine the System Application Efficacy of luminaires for parking lot applications is based on the premise that luminaires should meet the target photometric criteria for parking lots with minimum energy usage. The two main design criteria for parking lots are a minimum light level and a certain uniformity ratio. Because luminaires are designed to meet the needs of different parking lot geometries, manufacturers assign distribution types that describe the extent to which the luminaires are intended. Thus, the System Application Efficacy is based on a calculation area corresponding to the intended coverage for a given luminaire's distribution. The method accounts for the light (in lumens) that falls inside this target area, but only if the illuminance and uniformity criteria are met.

http://director.lightingresearch.org/parkinglot/default.aspx