Visual Performance as a Function of Spectral Power Distribution of Light Sources at Luminances Used for General Outdoor Lighting

The need to provide outdoor lighting for orientation, identification, and safety at a cost that is reasonable in terms of both dollars and energy has resulted in the use of light sources that differ significantly in spectral power distribution from those used in interiors and for which almost all data on visual performance have been accumulated. Furthermore, the illuminances used in most outdoor lighting applications are vastly lower than those under which the greatest amount of visual data is available. Under such conditions, the visual system is, at best, a mesopic state for which the standard definition of "light," the lumen, is almost certainly inappropriate.

The results of this study strongly suggest that there are real and significant differences in performance under sources with different spectral power distributions, but that those differences occur primarily at levels of adaptation where the spectral sensitivity of the visual system is well known to vary from that under which "light" is commonly defined. The luminances used in this work were defined according to the photopic sensitivity function - a definition that is suitable only for moderate to high levels of adaptation. As the adaptation level decreases, the sensitivity of the eye becomes progressively greater at shorter wavelengths until at very low luminances that peak sensitivity has shifted downward about 45 nm.