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Guidelines for Nighttime Illumination

Mesopic characterization of nighttime lighting described in new ASSIST guidelines

Lighting Research Center Press Release - 2/27/2009

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Outdoor Lighting

Outdoor Lighting

TROY, N.Y. - 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.

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 the 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. 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.

The volume is available for free download from the ASSIST Web site:

Unified system of photometry explained

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.

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, Ph.D., one of the authors of the ASSIST recommends volume.

"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 Dr. 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.

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.


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.

About ASSIST recommends

ASSIST has developed a publication program called ASSIST recommends to provide a set of formal recommendations to the LED and lighting communities about issues important for the reliable performance of LED lighting and its comparison to other light source technologies. The publications include recommendations for LED life definition, testing and measurement, best practice guides for different lighting applications, and recommendations for selecting LED lighting.

Unlike traditional test procedures that require products to be tested under standardized, ideal conditions, ASSIST recommends methods call for testing products under conditions similar to those found in the application environment, where the light source could experience many different temperatures and may perform poorly as a result. Testing products by intended application also allows for apples-to-apples comparisons of product performance because test methods have been developed from a technology-neutral standpoint.

ASSIST recommends publications are developed under the guidance of ASSIST sponsors using research conducted by the Lighting Research Center (LRC). Each publication undergoes internal review, first by LRC researchers and then by ASSIST sponsors. Industry input also is gathered during the writing process through one or more roundtable sessions hosted by ASSIST and the LRC. Based upon this industry input, the publications are revised and then published online for free download.

As warranted, the publications are updated from time to time to reflect new research, technologies, methods, and equipment.

About the Lighting Research Center

The Lighting Research Center (LRC) is part of Rensselaer Polytechnic Institute of Troy, N.Y., and is the leading university-based research center devoted to lighting. The LRC offers the world's premier graduate education in lighting, including one- and two-year master's programs and a Ph.D. program. Since 1988 the LRC has built an international reputation as a reliable source for objective information about lighting technologies, applications, and products. The LRC also provides training programs for government agencies, utilities, contractors, lighting designers, and other lighting professionals. Visit

About Rensselaer Polytechnic Institute

Rensselaer Polytechnic Institute, founded in 1824, is the nation's oldest technological university. The university offers bachelor's, master's, and doctoral degrees in engineering, the sciences, information technology, architecture, management, and the humanities and social sciences. Institute programs serve undergraduates, graduate students, and working professionals around the world. Rensselaer faculty are known for pre-eminence in research conducted in a wide range of fields, with particular emphasis in biotechnology, nanotechnology, information technology, and the media arts and technology. The Institute is well known for its success in the transfer of technology from the laboratory to the marketplace so that new discoveries and inventions benefit human life, protect the environment, and strengthen economic development.

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