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.
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Notes
- 1.
On an asphalt road surface full moon results in an average of 0.02–0.03 cd/m2, which is clearly too “bright” for scotopic vision. Dimly-lit residential streets have an average luminance of some 0.2 cd/m2 to 0.5 cd/ m2, which is far too bright for scotopic vision.
- 2.
Because of this shift, our perception of colour is changed somewhat towards blue. This colour-change effect is called the “Purkinje effect ”.
- 3.
Visual performance tests under foveal mesopic viewing conditions give conflicting results. Some show a positive effect for cool-white light (Fotios et al. 2005; Fotios and Cheal 2007; Szabó et al. 2010), while some others show a positive effect for warm-white light (Bisketzis et al. 2009; Fotios and Cheal 2011). Others show no specific spectral effect (Kebomaki et al. 2003; Alferdinck and Hogervorst 2013). It may be expected that there exists some interaction between rods and the three different types of cones that influences foveal mesopic vision (Ripamonti et al. 2009).
- 4.
The eye has 126 million rods and cones but only 1 million nerve fibres . In the far periphery of the retina some hundreds of rods are connected to a single nerve fibre (Tovée 1996).
- 5.
Sometimes we see that the S/P ratio itself is used as correction factor. In this example this would lead to correction factors of 1.45/0.65 and 2.25/0.65 = 3.46 and 2.23 respectively. This is misleading and wrong: the correction factors are, as shown, only 1.18 and 1.35 respectively for an adaptation luminance of 0.3 cd/m2. That already at an adaptation luminance of 3 cd/m2 the correction factors are more or less neglectable, illustrates how wrong it is to claim corrections for light sources used in high light level applications as for example outdoor sports lighting.
- 6.
The photopic and scotopic luminance together with the chromaticity coordinates x and y (as also used to position lights in the colour triangle) are the quantities used.
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van Bommel, W. (2015). Mesopic Vision. In: Road Lighting. Springer, Cham. https://doi.org/10.1007/978-3-319-11466-8_6
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DOI: https://doi.org/10.1007/978-3-319-11466-8_6
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