The application of mesopic vision models to street lighting

摘要

A number of key conclusions have been drawn from the literature review: 1. Published values for peak sensitivity of the eye for the three types of colour-sensitive receptors (cones) vary at least from 440nm to 498nm for the S-cones (short), 535 to 545nm for the M-cones (medium) and 564nm to 580nm for the L-cones (long). In the conventional model of vision, luminance information is derived from the summation of the signals from all three of these channels, while chrominance information is derived from the differences between them. The brain uses these chrominance signals to compute the sensation of colour. 2. Other published work gives evidence of four types of receptor in the human eye, which have peak sensitivities at wavelengths of 342nm, 437nm, 532nm and 625nm (although the shortest of these wavelengths is suppressed by absorption in the lens in the human eye). The conventional monochromatic rods and colour cones model is shown to be poorly supported, by a large body of published work. The Commission International d'Eclairage (CIE) curves for the overall spectral response of the eye are shown to be greatly smoothed versions of the true response, based on inadequate measurement capabilities in the 1930's. 3. In the mesopic zone, light that stimulates the short and medium wavelength receptors is likely to be more effective than light of longer wavelengths. 4. The shape of the visual response curve changes with the adaptation level throughout the mesopic zone, in a complex and poorly defined manner. 5. As luminance decreases through the mesopic zone, there is a loss of colour sensitivity. 6. Photopic measurements are not a reliable guide to the performance of the visual system particularly at the levels used in street lighting. Photopic luminance does not correlate well with perceived brightness. 7. Visual reaction time is considered to have a more direct influence on the hazard avoidance performance of road users than visual acuity. 8. Detection times or detection distances of objects are considered the most appropriate parameters to the road user's task. Performance of the peripheral field is also important to object detection speed. 9. Brightness matching has not been successful in defining the mesopic vision model and is not necessarily the most important to road users under street lighting. Evidence suggests that visual acuity and target detection speeds are more appropriate measures. 10. Reaction times are not significantly affected by opposing headlamp glare. As headlamp luminance levels increase glare is increased. Older drivers take longer to recover from exposure to glare. Discomfort glare is task dependent and may be more important overall than disability glare, as discomfort glare is more difficult to prevent at street lighting levels. 11. The road user task depends on both central and peripheral vision. Several experiments have shown that central vision is little affected by changes in the spectral power distribution (SPD) of the lighting. However, the peripheral field is very sensitive to the SPD of the lighting, which is important for the detection of hazards. 12. None of the British and European standards attempt to account for spectral effects and all are based on photopic lumen measurements.