Colour Vision

Human eye can recognize about 150 different colors in the visible spectrum. Discrimination and appreciation of colors depend upon the ability of receptors in retina.

Colour Vision


VISIBLE SPECTRUM

SPECTRAL COLORS

When sunlight or white light is passed through a glass prism, it is separated into different colors. Series

of colored light produced by the prism is called the visible spectrum. Colors that form the spectrum are

called spectral colors. Spectral colors are red, orange, yellow, green, blue, indigo and violet (ROYGBIV or

VIBGYOR). In the spectrum, colors occupy the position according to their wavelengths. Wavelength is the

distance between two identical points in the wave of light energy. Accordingly, red has got the maximum

wavelength of about 8,000 Å and the violet has got the minimum wavelength of about 3,000 Å.

Light rays longer than red are called infrared rays. Rays shorter than violet are called ultraviolet rays. But, these two extraordinary types of rays do not evoke the sensation of vision. Refraction of spectral colors by the prism also depends on wavelengths. Red is refracted less and violet is refracted more. So, longer the light rays, lesser is the refraction by the prism.

Purkinje Phenomenon

Purkinje phenomenon is the shift of brightest part of spectrum, when the intensity of illumination is changed. When white light is passed through a prism, it splits into spectral colors from red to violet and if the colors are viewed at high illumination, the brightest part of the spectrum is the yellow, i.e. the brightest part of the spectrum is shifted to left. But when the light intensity is reduced to that of twilight, the color of the spectrum fades. Now the brightest part of spectrum is green, i.e. the brightest part of spectrum is shifted to right. It is called Purkinje shift or effect. According to Purkinje, this effect is due to the

maximal stimulation of cones by yellow and the maximal stimulation of rods by green.

EXTRASPECTRAL COLORS

Extraspectral colors are the colors other than those present in visible spectrum. These colors are formed

by the combination of two or more spectral colors. For example, purple is the combination of violet and red. Pink is the combination of red and white.

PRIMARY COLORS

Primary colors are those, which when combined together produce the white. Primary colors are red, green and blue. These three colors in equal proportion give white.

COMPLEMENTARY COLORS

Complementary colors are the pair of two colors, which produce white when mixed or combined in

proper proportion. Examples of complementary colors are red and greenish blue; orange and cyan blue;

yellow and indigo blue; violet and greenish yellow; purple and green.

THEORIES OF COLOR VISION

Many theories are available to explain the mechanism of perception of color by eyes. However, most of the theories are not accepted universally. Following are the five theories, which are recognized:

1. THOMAS YOUNG TRICHROMATIC THEORY

According to Thomas Young, retina has three types of cones. Each one possesses its own photosensitive

substance. Each cone gives response to one of the primary colors – red, green and blue. Different color

sensations are produced by the stimulation of various combinations of these three types of cones. For

sensation of white light, all the three types of cones are stimulated equally.

2. HELMHOLTZ TRICHROMATIC THEORY

Helmholtz substituted the sensitive filaments of optic nerve for cones. The sensitive filaments of nerves give response selectively to one or other of the three primary colors. It is also called Young-Helmholtz theory.

3. GRANIT DOMINATORMODULATOR THEORY

Granit observed that the ganglionic cells of retina are stimulated by the whole of the visual spectrum. He

studied the action potentials in ganglionic cells, which are stimulated by light and obtained some sensitivity curves. Sensitivity curves were recorded by using different wavelengths of light both in light-adapted anddark-adapted eyes. On the basis of these sensitivity curves, Granit classified the ganglionic cells into two groups namely, dominators and modulators.

Dominators

Dominators are responsible for brightness of light. Dominators are further divided into two types:

i. Dominators for cones, which respond in lightadapted eye and a broad sensitivity curve is

produced with the maximum response around the wavelengths 55 Å

ii. Dominators for rods, which respond in darkadapted eye and in the sensitivity curve the

maximum response is given at the wavelengths of 500 Å.

Modulators

Modulators are responsible for different color sensations.

Modulators are of three types:

i. Modulators of blue, which are stimulated by light with wavelengths of 450 to 470 Å

ii. Modulators of green, which are stimulated by light with wavelengths of 520 to 540 Å

iii. Modulators of red-yellow, stimulated by light with wavelengths of 580 to 600 Å.

If green light falls on retina, modulators of green are stimulated and other two are less affected. Thus,

according to Granit, the dominators are responsible for brightness or intensity of light, both in dark-adapted (rods) and light-adapted (cones) eyes. The modulators are responsible for color vision in light-adapted eyes.

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