Mesopic Vision

Abstract

There are three categories of vision: scotopic, mesopic and photopic. The activity of the two types of light-sensitive cells in the eye, the cones and the rods, is different in each category. At very low adaptation levels only rods are active and vision is scotopic. With rising adaptation levels, from 0.005 cd/m2, the cones become gradually active and the rods less active. Vision is then mesopic. At adaptation levels higher than 5 cd/m2 only the cones are active and vision is photopic. Most road lighting installations have lighting levels corresponding to mesopic vision. The cones and rods have differing spectral sensitivities: the rods are more sensitive to short wavelengths while the cones have their maximum sensitivity in the middle of the visible spectrum. Cones are concentrated mainly at that part of the retina of the eye that is used for on-line vision while the rods are located only at locations used for peripheral vision. As a consequence of the above, in mesopic vision, peripheral task performance becomes better with light sources that have a relatively large short wavelength component. The same holds true for the subjective impression of brightness. At lower adaptation levels more rods become active and therefore these advantages become greater at lower lighting levels. Since all photometric units are based on photopic vision, these advantages are not apparent from these units. Correction factors have therefore been determined according to the spectrum of the light source, characterized by its S/P ratio, and of the lighting level obtained from the road lighting installation.