The Young–Helmholtz theory (based on the
work of Thomas Young and Hermann von Helmholtz
in the 19th century) 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 (most sensitive
to yellowish green, cyanish-green, and blue
– not red, green and blue) 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
