The Purkinje effect (sometimes called the Purkinje shift, or dark adaptation and named after the Czech anatomist Jan Evangelista Purkyně) is the tendency for the peak luminance sensitivity of the human eye to shift toward the blue end of the color spectrum at low illumination levels.[1][2]
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, or adjacent blue flowers, but in the same scene viewed at dusk, the contrast is reversed, with the red petals appearing a dark red or black, and the leaves and blue petals appearing relatively bright.
The sensitivity to light in scotopic vision varies with wavelength, though the perception is essentially black-and-white. The Purkinje shift is the relation between the absorption maximum of rhodopsin, reaching a maximum at about 500 nm, and that of the opsins in the long-wavelength and medium-wavelength cones that dominate in photopic vision, about 555 nm.[3]
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.
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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 colors, respond best to green-blue light.[4] 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, that is, in the mesopic state: as intensity dims, the rods take over, and before color disappears completely, it shifts towards the rods' top sensitivity.[5]
The insensitivity of rods to long-wavelength light is related to the use of red lights under certain special circumstances – for example, in the control rooms of submarines, in research laboratories, or during naked-eye astronomy.[6]
Under conditions where it is desirable to have both the photopic and scotopic systems active, red lights provide a solution. Submarines are dimly lit to preserve the night vision of the crew members working there, but the control room must be lit to allow crew members to read instrument panels. By using red lights, or wearing red goggles, the cones can receive enough light to provide photopic vision (namely the high-acuity vision required for reading). 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, in case they need to look out the periscope at night, for example.[7]
Red lights are also often used in research settings. Many research animals (such as rats and mice) have limited photopic vision - as they have far fewer cone photoreceptors.[8] 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 fully dark adapted (but low acuity) scotopic vision.[9] For the same reason, zoo displays of nocturnal animals often are illuminated with red light.
The effect was discovered in 1819 by Jan Evangelista Purkyně. Purkyně was a polymath[10] who would often meditate at dawn during long walks in the blossomed Bohemian fields. Purkinje noticed that his favorite flowers appeared bright red on a sunny afternoon, while at dawn they looked very dark. He reasoned that the eye has not one but two systems adapted to see colors, one for bright overall light intensity, and the other for dusk and dawn.
Purkinje wrote in his Neue Beiträge:[10][11]
Objectiv hat der Grad der Beleuchtung grossen Einfluss auf die Intensität der Farbenqualität. Um sich davon recht lebendig zu überzeugen, nehme man vor Anbruch des Tages, wo es eben schwach zu dämmern beginnt, die Farben vor sich. Anfangs sieht man nur schwarz und grau. Gerade die lebhaftesten Farben, das Roth und das Grün, erscheinen am schwärzesten. Das Gelb kann man von Rosenroth lange nicht Unterscheiden. Das Blau war mir zuerst bemerkbar. Die rothen Nüancen, die sonst beim Tageslichte am hellsten brennen, nämlich carmen, zinnobar und orange zeigen sich lange am dunkelsten, durchaus nicht in Verhältnisse ihrer mittleren Helligkeit. Das Grün erscheint mehr bläulich, und seine gelbe Tinte entwickelt sich erst mit zunehmenden Tage.
Objectively, the degree of illumination has a great influence on the intensity of color quality. In order to prove this most vividly, take some colors before daybreak, when it begins slowly to get lighter. Initially one sees only black and grey. Particularly the brightest colors, red and green, appear darkest. Yellow cannot be distinguished from a rosy red. Blue became noticeable to me first. Nuances of red, which otherwise burn brightest in daylight, namely carmine, cinnabar and orange, show themselves as darkest for quite a while, in contrast to their average brightness. Green appears more bluish to me, and its yellow tint develops with increasing daylight only.
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