Visual performance was measured at four luminance levels (10.0, 3.0, 1.0, and 0.1 cd/m(2)) using five different light sources (incandescent, HPM, HPS, LPS, and metal halide). These luminances produced adaptation conditions that ranged from photopic to nearly scotopic. Visual tasks included contrast sensitivity measures and reaction times to simple sinusoidal gratings and to a simulated roadway hazard. At the higher two luminances, visual performances were equal under all light sources. However, at the two lower luminances, contrast sensitivity was greater and reaction times were faster with sources that had more of their energy at short wavelengths. These results are consistent with the notion that visual sensitivity shifts toward shorter wavelengths as we dark adapt and suggests that the use of the lumen, which is defined at photopic luminances, is inappropriate to characterize light source effectiveness under low light level conditions such as may exist in many common outdoor lighting situations. Furthermore, comparisons of lamp efficacy that are based on lumens per watt may be misleading at low levels of visual adaptation.
