With reference to "Intro to LP I - Sky glow" 's Purkinje effectThe Purkinje effect is the tendency for the peak sensitivity of the human eye to shift toward the blue end of the color spectrum at low illumination levels.
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.
In visual astronomy, the Purkinje shift can affect visual estimates of variable stars when using comparison stars of different colors, especially if one of the stars is red. The effect occurs because the color-sensitive cones in the retina are most sensitive to yellow light, whereas the rods, which are more light-sensitive (and thus more important in low light) but which do not distinguish colours, respond best to green-blue light. This is why we become virtually color-blind under low levels of illumination, for instance moonlight.
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. The insensitivity of rods to long-wavelength light is related to the use of red lights under certain special circumstances - for example, on the bridges of submarines, in research laboratories, or during naked-eye astronomy. Under most circumstances, either the photopic system or scotopic system is active, not both. Under low light levels, the cones are insensitive and do not function. Under high light levels, the rods are saturated, and do not function. (Eg. Cats have great night vision but they are not sensitive to colours; because they have more rods than cones in their eyes)
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. Red lights are also used as safelights in darkrooms: many photographic papers are engineered to be insensitive to red light.
