The Alliance for Solid-State Illumination Systems and Technologies (ASSIST) has published a new volume in its ASSIST recommends series, "Outdoor Lighting: Visual Efficacy." The volume describes a unified system of photometry, developed through previous research by the Lighting Research Center (LRC), which can better characterize the photometric performance of light sources under nighttime applications, including LEDs.
"A unified system of photometry would help to more accurately characterize different light sources at any light level, facilitating the specification of effective lighting systems for different applications, including those used outdoors at night," says LRC director Mark Rea.
The proposed unified system of photometry integrates both the scotopic and photopic luminous efficiency functions into a complete system that can be utilized across the entire range of light levels available to the human visual system. The system differentially weights the scotopic and photopic luminous efficiency functions depending upon light level.
The ASSIST publication provides step-by-step instructions for calculating the unified luminance of a given light source based on light level and the scotopic-to-photopic ratio of the light source. Different combinations of light sources and light levels may produce the same unified luminance, which indicates photometric equivalency. Therefore, the system can serve as a simple method for trading off light sources and light levels under mesopic conditions, and thereby aid in the selection of light sources for a given application.
The volume is available for free download from the ASSIST Web site: http://www.lrc.rpi.edu/programs/solidstate/assist/outdoorlighting.asp