Young-Helmholtz theory
Encyclopedia
The Young–Helmholtz theory (proposed in the 19th century by Thomas Young
and Hermann von Helmholtz
) is a theory of trichromatic color vision
– the manner in which the photoreceptor cells in the eyes of humans
and other primates work to enable color vision. In 1802, Young postulated the existence of three types of photoreceptors (now known as cone cells) in the eye, each of which was sensitive to a particular range of visible light.
Hermann von Helmholtz developed the theory further in 1850: that the three types of cone photoreceptors could be classified as short-preferring (blue
), middle-preferring (green
), and long-preferring (red
), according to their response to the wavelengths of light striking the retina
. The relative strengths of the signals detected by the three types of cones are interpreted by the brain
as a visible color.
For instance, yellow light uses different proportions of red and green, but little blue, so any hue depends on a mix of all three cones, for example, a strong blue, medium green, and low red. Moreover, the intensity of colors can be changed without changing their hues, since intensity depends on the frequency of discharge to the brain, as a blue-green can be brightened but retain the same hue. The system is not perfect, as it does not distinguish yellow from a red-green mixture, but can powerfully detect subtle environmental changes.
The existence of cells sensitive to three different wavelength ranges was first shown in 1956 by Gunnar Svaetichin
. In 1983 it was validated in human retinas in an experiment by Dartnall, Bowmaker, and Mollon, who obtained microspectrophotopic readings of single eye cone cells. Earlier evidence for the theory had been obtained by looking at light reflected from the retinas of living humans, and absorption of light by retinal cells removed from corpses.
Thomas Young (scientist)
Thomas Young was an English polymath. He is famous for having partly deciphered Egyptian hieroglyphics before Jean-François Champollion eventually expanded on his work...
and Hermann von Helmholtz
Hermann von Helmholtz
Hermann Ludwig Ferdinand von Helmholtz was a German physician and physicist who made significant contributions to several widely varied areas of modern science...
) is a theory of trichromatic color vision
Color vision
Color vision is the capacity of an organism or machine to distinguish objects based on the wavelengths of the light they reflect, emit, or transmit...
– the manner in which the photoreceptor cells in the eyes of humans
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...
and other primates work to enable color vision. In 1802, Young postulated the existence of three types of photoreceptors (now known as cone cells) in the eye, each of which was sensitive to a particular range of visible light.
Hermann von Helmholtz developed the theory further in 1850: that the three types of cone photoreceptors could be classified as short-preferring (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...
), middle-preferring (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...
), and long-preferring (red
Red
Red is any of a number of similar colors evoked by light consisting predominantly of the longest wavelengths of light discernible by the human eye, in the wavelength range of roughly 630–740 nm. Longer wavelengths than this are called infrared , and cannot be seen by the naked eye...
), according to their response to the wavelengths of light striking 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...
. The relative strengths of the signals detected by the three types of cones are interpreted by the brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
as a visible color.
For instance, yellow light uses different proportions of red and green, but little blue, so any hue depends on a mix of all three cones, for example, a strong blue, medium green, and low red. Moreover, the intensity of colors can be changed without changing their hues, since intensity depends on the frequency of discharge to the brain, as a blue-green can be brightened but retain the same hue. The system is not perfect, as it does not distinguish yellow from a red-green mixture, but can powerfully detect subtle environmental changes.
The existence of cells sensitive to three different wavelength ranges was first shown in 1956 by Gunnar Svaetichin
Gunnar Svaetichin (scientist)
Gunnar Svaetichin was a Swedish-Finnish-Venezuelan physiologist who, in 1956, showed by examining the external layers of fish retinas that electroretinograms display particular sensitivity to three different groups of wavelengths in the areas of blue, green and red. This provided the first...
. In 1983 it was validated in human retinas in an experiment by Dartnall, Bowmaker, and Mollon, who obtained microspectrophotopic readings of single eye cone cells. Earlier evidence for the theory had been obtained by looking at light reflected from the retinas of living humans, and absorption of light by retinal cells removed from corpses.