Low-pressure Sodium Lighting and Visual Performance

Brief summary and comments on
W. Adrian, "The Influence of the Spectral Power Distribution for Equal Visual performance in Roadway Lighting Levels," Proceedings: Vision at Low Light Levels, EPRI/LRO Fourth International Lighting Research Symposium, pg. 85, 1999

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Christian B. Luginbuhl
U.S. Naval Observatory Flagstaff Station
PO Box 1149
Flagstaff AZ 86002
cluginbuhl@nofs.navy.mil

Version 1.0
26 May 2004

The author shows that the available laboratory data from many workers measuring contrast sensitivity, visual acuity, pupil size, visibility of Landolt rings, and "visual performance" (the earlier experiments of Lewis), can all be understood by measuring lighting levels using the "equivalent luminance," rather than standard luminance. At higher lighting levels, the equivalent luminance is the same as the usual measure of luminance, while at lower lighting levels the equivalent luminance must be adjusted for the particular lighting level, and the correction factor will be different for different types of lighting sources. For professional recommendations for roadway lighting (~ 0.2 -- 1.0 cd/m²), the ratio of equivalent luminance to standard luminance ranges from approximately 0.6-0.9 for LPS, 0.9-1.0 for HPS, and 1.1-1.0 for metal halide. This means that to achieve equal performance at for example 0.2 cd/m² you must use 1/0.6=1.67 times as much LPS light, 1/0.9=1.11 times as much HPS light, and 1/1.1=0.91 times as much metal halide light. This is a much more modest effect than shown in the more recent work of Lewis (see here).

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