Purkinje effect

Purkinje effect

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 sensitivity of the human eye to shift toward the blue end of the color spectrum at low illumination levels.cite book | author=Frisby JP | title=Seeing: Illusion, Brain and Mind | publisher=Oxford University Press : Oxford | year=1980] cite book | author=Purkinje JE | title=Neure Beitrage zur Kenntniss des Sehens in Subjectiver Hinsicht | publisher=Reimer : Berlin | year=1825] pn

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.

Physiology

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.cite book | author=Cornsweet TN | title=Visual Perception | publisher=Academic Press : New York | year=1970] pn 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.

Use of Red Lights

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.

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.

History

The effect was discovered by Jan 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 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.

ee also

* Kruithof curve

References


Wikimedia Foundation. 2010.

См. также в других словарях:

  • Purkinje effect — n PURKINJE PHENOMENON * * * see under phenomenon …   Medical dictionary

  • Purkinje effect —    Also known as Purkinje shift. Both eponyms refer to the Bohemian physiologist Johannes Evan gelista Purkyne (1787 1869). They are used to denote the physiological phenomenon that at dusk, when the light intensity gradually decreases, the… …   Dictionary of Hallucinations

  • purkinje effect — noun see purkinje phenomenon …   Useful english dictionary

  • Purkinje phenomenon — n a shift of the region of apparent maximal spectral luminosity from yellow with the light adapted eye toward violet with the dark adapted eye that is presumably associated with predominance of cone vision in bright and rod vision in dim… …   Medical dictionary

  • Purkinje — is a name attributed to several biological features, so named for their discovery by Czech anatomist Jan Evangelista Purkyně: *Purkinje cells, located in the cerebellum *Purkinje fibers, located in the heart *The visual Purkinje effect *Purkinje… …   Wikipedia

  • Purkinje, Jan Evangelista — born Dec. 17, 1787, Libochovice, Bohemia died July 28, 1869, Prague, Czech. Czech experimental physiologist. He discovered the Purkinje effect (as light decreases, red objects appear to fade faster than blue ones), Purkinje cells (large branching …   Universalium

  • Purkinje figure —    Also known as Purkyne figure. Both eponyms refer to the Bohemian physiologist Johannes Evangelista Purkyne (1787 1869). They are used to denote an entoptic phenomenon consisting of a black, tree like structure that can be made visible under… …   Dictionary of Hallucinations

  • Purkinje , Johannes Evangelista — (1787–1869) Czech physiologist Born at Libochovice (now in the Czech Republic), Purkinje began studying to be a priest but changed to medicine and graduated MD from Charles University, Prague, in 1819. He became professor of physiology and… …   Scientists

  • purkinje phenomenon — noun or purkinje shift also purkinje effect Usage: usually capitalized 1st P : a shift of the region of apparent maximal spectral luminosity from yellow with the light adapted eye toward violet with the dark adapted eye that is presumably… …   Useful english dictionary

  • Purkinje shift —    see Purkinje effect …   Dictionary of Hallucinations


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