Purkinje effect
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The Purkinje effect (sometimes called the Purkinje shift, or dark adaptation) 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 against paler green leaves.
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.
[edit] Physiology
The effect occurs because the colour-sensitive cones in the retina are most sensitive to yellow light, whereas the rods, which are more light-sensitive (and thus more important at dusk) but which do not distinguish colours, respond best to green-blue light. This is why we become virtually colour-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 colour disappears completely, it shifts towards the rods' top sensitivity.
Incidentally, the same "lack of sensitivity to red" is why red lights are used in instrumentation panels: the red light does not disrupt the night vision (because the rods do not respond to it), and the fovea is largely L (red-sensitive) cones, hence they are easy to read.
[edit] History
The effect was discovered by Johannes Evangelista Purkinje. Purkinje was a polymath who would often meditate at dawn during long walks in the blossomed Bohemian fields. Purkinje noticed that his favorite flowers appeared red on a sunny afternoon, while at dawn they looked bluish-red. He figured that the eye has not one but two systems adapted to see colours, one for bright overall light intensity, and the other for dusk and dawn.
