Road lighting and mesopic vision

Human beings perceive information from the environment by using their sense organs. Vision is believed to provide the most information. There are three categories of vision: scotopic, mesopic, and photopic vision. The activity of two types of light-sensitive receptors in the eye, cones and rods, is different for each category. Cones and rods have a unique ability to adapt to different levels of illumination. Adaptation is the ability of the human eye to adapt to changing lighting conditions. Thanks to this mechanism, the human visual system has the ability to operate in a very wide range of pupil illuminations, that is, at different adaptive levels. At very low adaptive levels, only the rods are active, and the vision is called scotopic. As adaptive levels increase, starting at 0.005 cd/m2, the rods become less active, meanwhile the cones become active, and then the vision is called mesopic. At adaptive levels above 5 cd/m2, only cones are active, and the vision becomes photopic. Cones and rods have different spectral sensitivities: rods are more sensitive to short wavelengths, while cones are most sensitive within the visible spectral range. Cones are concentrated mainly in the part of the retina of the eye that is used for direct (on-line) vision, while rods are localized only in the areas used for peripheral vision. As a result, in case of mesopic vision, the peripheral vision becomes better with light sources that have a relatively large component of short wavelengths. The same is true for the subjective perception of brightness. At low adaptation levels, more rods become active, and thus these benefits are greater at low illumination levels. Since photometric units are based on photopic vision, these advantages are not obvious. Thus, the correction factors should be determined according to the spectrum of light sources, which is characterized by the S/P ratio, and the illumination levels of road lighting. The International Commission on Illumination has prepared a standard (CIE 115:2010 Lighting of roads for motor and pedestrian traffic). The main goals of road lighting are to ensure visual performance, visual comfort and to keep drivers alert.