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 (0.01 cd/m2.10 cd/rn") are vastly lower than those under which the greatest amount of visual data is available. Under such conditions, the visual system is, at best, in a mesopic state for which the standard definition of "light," the lumen, is almost certainly inappropriate. The lumen is defined as visually evaluated electromagnetic radiation. Its magnitude was initially derived using viewing conditions that limited the visual response to one that was mediated exclusively by the cone receptor systems of the human eye. Most critically, a high level of retinal illuminance and a central 2 degree visual field was used, which effectively eliminated any significant contribution by the rod system which is largely absent from that area of the retina. The relative spectral sensitivity of the eye under those conditions is nominally described by the 1924 erE Standard Observer for Photometry (VI). 1 For larger fields, or when retinal illuminances are insufficient to adequately stimulate the cone systems, the use of the lumen to predict the human response to light is questionable. As a result, additional spectral sensitivity functions have been described for larger fields (1964 erE Observer for 10 degree fields-VlO),)2 and for very low retinal illuminances (1951 eIE Scotopic Observer=-V'j ). " Although spectral sensitivity functions are adequate to predict the amount of energy required to stimulate vision, they are very poor at describing the perceived effect of suprathreshold lights. For example, it is well known that perceived brightness differs significantly from photometric luminance under all but very restricted viewing conditions. " The discrepancy between perceived effects and photometric quantities is especially sensitive to conditions where lights are of different color or where viewing conditions are markedly different from those under which the light units were defined. Author's Affiliation: Michigan College of Optometry, Ferris State University, Big Rapids, Michigan. The interest in the role of spectral power distribution on visual performance, originally of concern only to theoreticians trying to understand visual physiology, became of practical significance with the introduction of gaseous discharge sources for general illumination. Because of the obvious advantages of using such energy efficient and long-lived lamps for general illumination, studies were performed to evaluate their performance relative to the more conventional incandescent and fluorescent illuminants. Mercury, sodium (both low and high pressure), and metal halide lamps have all been studied in some detail at high illuminances with the result that suprathreshold visual performance, measured in terms of speed and accuracy on achromatic tasks, is relatively independent of light source color or type as long as contrasts are equated. ' Although the color characteristics of HID lamps have reduced their use in interior applications, they are widely employed for outdoor lighting. In the last few years two separate, but related, groups of investigations have raised new questions about the effects of both spectral power distribution (SPD) and light source color on performance. Berman et al., at the Lawrence Berkeley Labs, have shown that both pupil size