Scotopic/Photopic ratio (S/P ratio) for street, roadway and area lighting

Splendor

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There are three vision states for human beings: scotopic vision, mesopic vision, and photopic vision. Photopic vision is the vision of the eye under well-lit conditions where luminances are generally in excess of about 3.4 cd/m2. When the light level is below 0.034 cd/m3, the conditions are described as scotopic. Mesopic vision or twilight vision takes place in in the luminance range 0.034 cd/m2 and 3.4 cd/m2. The retina in our eye is made up of rod and cone photoreceptors. Cones enable photopic vision. Scotopic vision is primarily mediated by rods. The sensitivity of our cones peaks at approximately 555 nm, which is greenish-yellow color. The sensitivity of our rods peaks at approximately 505 nm, closer to the blue end of the visible spectrum, which means a light spectrum with high blue content allows better scotopic vision.

For street, roadway and area lighting, average light levels fall in the mesopic luminance range, with maximum luminance close to or within the photopic vision range. Therefore, it is important to reach high luminous efficacy of radiation (LER) in both photopic and mesopic visions. In mesopic vision, both the rods and the cones of the retina are active. The scotopic/photopic (S/P) ratio is defined as the ratio of LER in scotopic and photopic vision. White light with a high S/P ratio enables stimulation of both rod and cone photoreceptors, which results in good visibility at both mesopic and photopic vision states.

In general, the relative amount of blue light in a light spectrum is correlated with the color temperature of a light source. Warm white light which has a low amount of blue wavelengths has a low S/P ratio, whereas neutral and cool white light which contains a higher amount of blue has higher S/P ratios.
 
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