Purkinje effect
Encyclopedia
The Purkinje effect is the tendency for the peak luminance
Luminance
Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square...

 sensitivity of the human eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...

 to shift toward the blue
Blue
Blue is a colour, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 440–490 nm. It is considered one of the additive primary colours. On the HSV Colour Wheel, the complement of blue is yellow; that is, a colour corresponding to an equal...

 end of the color spectrum at low illumination levels.

This effect introduces a difference in color contrast
Contrast (vision)
Contrast is the difference in visual properties that makes an object distinguishable from other objects and the background. In visual perception of the real world, contrast is determined by the difference in the color and brightness of the object and other objects within the same field of view...

 under different levels of illumination. For instance, in bright sunlight
Sunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...

, geranium
Pelargonium
Pelargonium is a genus of flowering plants which includes about 200 species of perennials, succulents, and shrubs, commonly known as scented geraniums or storksbills. Confusingly, Geranium is the correct botanical name of a separate genus of related plants often called Cranesbills. Both Geranium...

 flower
Flower
A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants . The biological function of a flower is to effect reproduction, usually by providing a mechanism for the union of sperm with eggs...

s appear bright red against the dull green
Green
Green is a color, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 520–570 nanometres. In the subtractive color system, it is not a primary color, but is created out of a mixture of yellow and blue, or yellow and cyan; it is considered...

 of their leaves
Leaf
A leaf is an organ of a vascular plant, as defined in botanical terms, and in particular in plant morphology. Foliage is a mass noun that refers to leaves as a feature of plants....

, or adjacent blue flowers, but in the same scene viewed at dusk
Dusk
Dusk is the beginning of darkness in the evening, and occurs after twilight, when the sky generally remains bright and blue. Civil dusk is when the earth has rotated enough that the center of the sun is at 6° below the local horizon...

, the contrast is reversed, with the red petal
Petal
Petals are modified leaves that surround the reproductive parts of flowers. They often are brightly colored or unusually shaped to attract pollinators. Together, all of the petals of a flower are called a corolla. Petals are usually accompanied by another set of special leaves called sepals lying...

s appearing a dark red or black, and the leaves and blue petals appearing relatively bright.

The sensitivity to light in scotopic vision
Scotopic vision
Scotopic vision is the vision of the eye under low light conditions. The term comes from Greek skotos meaning darkness and -opia meaning a condition of sight...

 varies with wavelength, though the perception is essentially black-and-white
Black-and-white
Black-and-white, often abbreviated B/W or B&W, is a term referring to a number of monochrome forms in visual arts.Black-and-white as a description is also something of a misnomer, for in addition to black and white, most of these media included varying shades of gray...

. The Purkinje shift is the relation between the absorption maximum of rhodopsin
Rhodopsin
Rhodopsin, also known as visual purple, is a biological pigment of the retina that is responsible for both the formation of the photoreceptor cells and the first events in the perception of light. Rhodopsins belong to the G-protein coupled receptor family and are extremely sensitive to light,...

, reaching a maximum at about 500 nm, and that of the opsin
Opsin
Opsins are a group of light-sensitive 35–55 kDa membrane-bound G protein-coupled receptors of the retinylidene protein family found in photoreceptor cells of the retina. Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical...

s in the long-wavelength and medium-wavelength cones that dominate in photopic vision
Photopic vision
Photopic vision is the vision of the eye under well-lit conditions. In humans and many other animals, photopic vision allows color perception, mediated by cone cells, and a significantly higher visual acuity and temporal resolution than available with scotopic vision.The human eye uses three types...

, about 555 nm.

In visual astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

, the Purkinje shift can affect visual estimates of variable star
Variable star
A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...

s when using comparison stars of different colors, especially if one of the star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s is red.

Physiology

The effect occurs because the color-sensitive cones
Cone cell
Cone cells, or cones, are photoreceptor cells in the retina of the eye that are responsible for color vision; they function best in relatively bright light, as opposed to rod cells that work better in dim light. If the retina is exposed to an intense visual stimulus, a negative afterimage will be...

 in the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...

 are most sensitive to yellow
Yellow
Yellow is the color evoked by light that stimulates both the L and M cone cells of the retina about equally, with no significant stimulation of the S cone cells. Light with a wavelength of 570–590 nm is yellow, as is light with a suitable mixture of red and green...

 light, whereas the rods
Rod cell
Rod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of visual photoreceptor, cone cells. Named for their cylindrical shape, rods are concentrated at the outer edges of the retina and are used in peripheral vision. On...

, which are more light-sensitive (and thus more important in low light) but which do not distinguish colors, respond best to green-blue light. This is why we become virtually color-blind under low levels of illumination, for instance moonlight
Moonlight
Moonlight is the light that reaches Earth from the Moon. This light does not originate from the Moon, but from sunlight. The Moon does not, however, reflect sunlight like a mirror, but it reflects light from those portions of its surface which the Sun's light strikes. See diffuse reflection.In...

.

The Purkinje effect occurs at the transition between primary use of the photopic
Photopic vision
Photopic vision is the vision of the eye under well-lit conditions. In humans and many other animals, photopic vision allows color perception, mediated by cone cells, and a significantly higher visual acuity and temporal resolution than available with scotopic vision.The human eye uses three types...

 (cone-based) and scotopic
Scotopic vision
Scotopic vision is the vision of the eye under low light conditions. The term comes from Greek skotos meaning darkness and -opia meaning a condition of sight...

 (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.

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, in the control rooms of submarines, in research laboratories, or during naked-eye astronomy.

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.

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.
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.
For the same reason, zoo displays of nocturnal animals often are illuminated with red light.

History

The effect was discovered in 1819 by Jan Evangelista Purkyně
Jan Evangelista Purkyne
Jan Evangelista Purkyně was a Czech anatomist and physiologist. He was one of the best known scientists of his time. His son was the painter Karel Purkyně...

. Purkyně was a polymath
Polymath
A polymath is a person whose expertise spans a significant number of different subject areas. In less formal terms, a polymath may simply be someone who is very knowledgeable...

 who would often meditate at dawn during long walks in the blossomed Bohemia
Bohemia
Bohemia is a historical region in central Europe, occupying the western two-thirds of the traditional Czech Lands. It is located in the contemporary Czech Republic with its capital in Prague...

n 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:
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.

External links

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