Lighting specifiers selecting outdoor luminaires typically consider the luminous efficacy of a fixture in order to gain maximum energy efficiency. However, the problem, LRC researchers say, is that the lumens per watt value is based on a photometry system that does not consider how the human visual system works at light levels typically found outdoors at night. To correct this problem, LRC researchers have proposed a new system of photometry designed to characterize light at any light level.

The current photometry system, says LRC Director Mark Rea, uses one of two luminous efficacy functions, each of which represent the sensitivity of the human eye at different light levels. The first and most common method, the photopic luminous efficacy function, represents the spectral sensitivity of the eye during the daytime. The second, known as the scotopic luminous efficacy function, characterizes sensitivity in near-complete darkness. Between photopic and scotopic is the mesopic region, which describes nighttime light levels experienced outdoors that are neither complete darkness nor complete light.

“The problem with the current photometry system is that it remains unclear which luminous efficacy function should be used for nighttime applications where electric lighting is used, such as driving down the road at night,” says Rea, who directed the LRC team’s research into the new system. Without a system that works at all light levels, he says, some light sources used at night draw excess energy to put out the necessary light level.

The proposed unified system characterizes light at all light levels using the parameter X, which describes the proportion of photopic light at any luminance. At high light levels, X equals 1; at scotopic levels, X equals 0. At mesopic levels, X falls in between. LRC researchers express the value of X as a function of the photopic light level (candelas per square meter) and the ratio of the photopic and scotopic efficacy for the light source.

Rea notes that with this system, lamp manufacturers and lighting practitioners can continue to use conventional photometric measurements using the photopic luminous efficacy function. “The system preserves existing measurements, which can be extended with values of X to compare the effectiveness of different light sources to produce a required luminance.” Potential results for this system, he says, include increased energy efficiency, reduced light pollution, and added safety and security at night.

A detailed explanation of the new photometry system can be found in the journal article “A proposed unified system of photometry” by Rea, Bullough, Freyssinier-Nova, and Bierman, published in Lighting Research and Technology, volume 36, issue 2. If you would like a copy of the paper from the LRC library, please email Julia Ceriotti at cerioj@rpi.edu.

Originally published in Lighting Research Center News, October 13, 2004

Published October 13, 2004