Humans can see over a light intensity range of several million to one. In order to achieve this extraordinary feat while maintaining good contrast sensitivity, the eye adjusts to the prevailing conditions and changes its mode of operation as light levels decline from day to night. The shift from photopic to scotopic represents a delicate balancing act. Lower light level changes the balance in contradictory demands of light sensitivity on the one hand and contrast sensitivity on the other. During the day when there is plenty of light, the visual system operates in ‘photopic’ mode which employs cone photoreceptors and which is optimized for seeing contrast. The real action occurs after the photoreceptors, where the visual system essentially throws light away by creating “lateral inhibition,” edge-sharpening that actually decreases neural activity in response to light (see Green, 1984, for example). The effect is similar to the edge-sharpening function on a computer graphics program. The cost of this edge-sharpening is lower light sensitivity. During the day when there is plenty of light, the visual system gladly pays the price. One effect of switching to rods, however, is the “Purkinje shift.” During photopic (cone) vision, viewers are most sensitive to light that appears greenish-yellow. In scotopic vision, they are most sensitive to light which would appear greenish-blue during the day. (Of course, viewers can’t actually see color in scotopic vision. It is incorrect to say that “people are most sensitive to blue light at night.”) One main result of switching to rods is loss of most sensitivity to long wavelength colors (red). Mesopic vision is more complicated than photopic or scotopic vision. Visual performance will depend greatly on whether objects lie in the sightline and cast images on the cone-dominated fovea or in rod-dominated peripheral vision.
The sun creates three main illumination components. The direct component is light that travels on a straight line from the sun to the surface. The amount of illumination falling on an outdoor surface is then related to sun position, which is specified by elevation (angle above the horizon) and azimuth (clockwise horizontal angle from true north), and the relative orientation of the surface. On cloudy days there is no direct component because the sun is hidden behind clouds. The sun still creates a second, diffuse component by means of atmospheric scatter. This lessens the importance of azimuth variation. Similarly, as the sun goes below the horizon, the effect of azimuth variation becomes vanishingly small. Lastly, the sun can illuminate objects indirectly through reflection off other surfaces, such as the ground.
IESNA report Recommended Practice For the Calculation of Daylight Availability (1983) provides a more detailed method for calculating both direct and ambient solar illumination as well as a model for sky luminance. The formula contains some errors that are corrected by supplements.
Twilight And Night
Several landmark astronomical events occur during the change from photopic to scotopic vision. One is sunrise/sunset, the time when the upper edge of the sun’s disk is on the horizon. The second is twilight, the time when the sun is below the horizon but still providing illumination. Finally night begins at the end of twilight in the evening and at the beginning of twilight in the morning.
Twilight is a common time for outdoor accidents. Contrast sensitivity declines rapidly as the eye moves to mesopic operation. The sun is still providing both foreground and background illumination, so artificial light sources, such as car headlamps are less effective (see Green, 2002c). In addition, visual skills, such as reaction time, fall rapidly (Campbell, Rothwell and Perry, 1987).
Crawford, B. (1947). Visual adaptation in relation to brief conditioning stimuli, Proceedings of the Royal Society of London B, 134, 283-302.
Green, M. (1981). Spatial Frequency Effects in Masking by Light, Vision Research, 21, 801-806.
Green, M., 1984, Masking by Light and the Sustained?Transient Dichotomy.” Perception and Psychophysics, 34, 617-635.
IESNA, 1983, Recommended Practice For the Calculation of Daylight Availability.