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   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.