February 25, 2009--Outdoor lighting supports many types of nighttime activities, from transportation to recreation to business, and has become a quickly growing market for LED lighting systems. Much attention has been paid to increasing the luminous efficacy of outdoor lighting systems, and particularly to LED systems looking to replace traditional light sources used in this application. Yet, traditional means of finding the most efficacious light source for outdoor applications are not always appropriate, as research by Rensselaer's Lighting Research Center (Troy, NY) shows.
Unified system of photometry
The human visual system uses two types of photoreceptors, cones and rods, found in the retina. Cones are used to process visual information under daytime or "photopic" light levels, while rods work under completely dark "scotopic" conditions. There is, however, a range of light levels called "mesopic," where both cones and rods together provide input to the visual system. Mesopic light levels are typically found outdoors at night, where streetlights, cars, and buildings all contribute to the total light level.
(Note: a World News story in the upcoming March issue of Laser Focus World covers this very topic of mesopic light levels from a slightly different angle--that of phosphor design for dim lighting conditions. The story, "Persistent phosphors are seen in a new light," will appear March 1 both online and in print.)
Commercial photometry is based entirely upon the photopic luminous efficiency function, which considers how the eye "sees" during daylight hours. As a result, conventional photometry may misestimate the effectiveness of some light sources used in nighttime applications in terms of energy efficiency and visual safety, according to LRC director Mark Rea.
"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 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.
"In effect, it is a system for choosing among commercially available light sources to deliver the same unified, rather than photopic, photometric quantity," says Rea.
The Alliance for Solid-State Illumination Systems and Technologies (ASSIST; established in 2002 by the LRC) has published a new volume in its ASSIST recommends series, "Outdoor Lighting: Visual Efficacy." The volume describes the unified system of photometry, developed through previous research by the LRC, which can better characterize the photometric performance of light sources under nighttime applications. This system can help lighting specifiers and decision-makers to better optimize, and thereby reduce the cost, of operating lighting systems at night, including LED lighting systems.
Calculating unified luminance
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.
About ASSIST
ASSIST is a collaboration between researchers, manufacturers, and government organizations. Its goal is to identify and reduce major technical hurdles currently facing solid-state lighting. The Lighting Research Center conducts research, demonstration, and educational activities on behalf of ASSIST.
ASSIST is sponsored by Acuity Brands Lighting; Bridgelux; China Solid State Lighting Alliance; Cree; Everlight Electronics Co., Ltd.; Federal Aviation Administration; GE Lumination; ITRI, Industrial Technology Research Institute; Lighting Science Group; Lite-On; NeoPac Lighting; New York State Energy Research and Development Authority (NYSERDA); OSRAM SYLVANIA/OSRAM Opto Semiconductors; Permlight; Philips Color Kinetics; Photonics Cluster (UK)/The Lighting Association; Seoul Semiconductor; United States Environmental Protection Agency; USG; WAC Lighting.
