This effect introduces a difference in color contrast under different levels of illumination. For instance, in bright sunlight, geranium flowers appear bright red against the dull green of their leaves, but in the same scene viewed at dusk, the contrast is reversed, with the petals appearing a dull red and the leaves appearing bright green.
The Purkinje effect occurs at the transition between primary use of the photopic (cone-based) and scotopic (rod-based) systems: as intensity dims, the rods take over, and before color disappears completely, it shifts towards the rods' top sensitivity.
Under conditions where it is desirable to have both systems active, red lights provide a solution. Submarines are dimly lit to conserve energy, but the bridge must be lit to allow crew members to read instrument panels. By using red lights, the cones can receive enough light to provide photopic vision (namely the high-acuity vision required for reading; albeit under red light the photopic vision will be monochromatic). Because the rods are not saturated by bright light and are not sensitive to long-wavelength red light, however, the crew member remains dark adapted. If the crew member left the bridge for some dimly lit part of the ship, rather than being functionally blind (as would be the case had the bridge been illuminated by full spectrum light), the scotopic system is fully dark adapted and able to provide high-sensitivity vision.
Red lights are also often used in research settings. Many research animals (such as rats and mice) have only scotopic vision - they do not have cone photoreceptors. By using red lights, the animal subjects remain "in the dark" (the active period for nocturnal animals), but the human researchers, who have one kind of cone that is sensitive to long wavelengths, are able to read instruments or perform procedures that would be impractical even with a fully dark adapted (but low acuity) scotopic vision. For the same reason, zoo displays of nocturnal animals often are illuminated with red light.
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