Vision information > Human eye and color vision

The color vision is a very complicated psychophysical process during which human eye distinguishes different colors. From the physical point of view the color does not exist and it is only a visual perception which is conditioned by the wavelength of the light. The certain part of the spectrum reflects from different objects, then it falls on the eye where it causes the irritation of the light sensitive elements which react to colur – the cones. For its realization, it is necessary to have a correct and undisturbed function of the eyesight and the human eye as the receiving organ, the optical path as the transmitting system and the centers of the brain thanks to which we distinguish and thus they are the analyzer of the perception.

The color vision of all people depends on the external and internal conditions. On one side there is the external environment which creates the color input and on the other side there is our eyesight which evaluates the same input. From the group of external conditions, the illumination is really important since we stop to perceive colors from the short-wave end of the spectrum during excessive illumination. When there is a lack of light we stop to perceive colors as well only in reverse order. The perception of colors becomes impossible after the crossing of certain liminal value and physiological monochromacy occurs and consequently all the colors seem to be gray.

The color vision does not vary only from person to person. It is physiological that the ability to distinguish colors changes even in an individual as the time flows. Gradual dying of the retinal cells adds mostly to this process. Also the lens loses its transparency and becomes yellow which causes that much less light with the impression of yellow filter falls on the retina of elder people. In all the respects the vision is less sharp and the colors are less distinctive. Elder people perceive the colors of short-wave end of the spectrum as much darker than younger people.

We distinguish also the so-called scotopic zone of vision which is a zone with the subliminal value of illumination. In this case only the rods function and the human eye does not perceive colors. It is called scotopic because the cones do not function in subliminal values and the central scotoma is formed. On the other hand the function of rods predominates in the photopic zone. The narrow part between scotopic and photopic zone is called mezopic zone where both types of receptors function and in this zone we can distinguish only blue and yellow colour.

The sensitivity of the human eye

The human eye is sensitive to the rays of light in the range of 380-780 nm. The wavelengths, which are lower than 380 nm, are absorbed by the eye lens. The distinguishable spectrum represents only a certain and lesser part of the whole field of electromagnetic waves. Its boundaries are not precisely defined since the sensitivity of the human eye is asymptotically approaching zero on both sides of the spectrum. If we consider the boundaries to be the waves in which the sensitivity decreases to 1% of its maximal value we will get the range between 430 nm to 690 nm. However, the human eyesight is able to perceive the wavelengths even behind this range if the intensity is sufficiently high.

We can distinguish up to 160 colors and some sources speak of more than 600 thousands shades (the colors of various richness and brightness). In the case of undisturbed color vision we distinguish the wavelengths with the difference of 1 nm. We have three types of cones whose photopigment differs in its maximal absorption in the region of blue 440-450 nm, green 535-555 nm and red 570-590 nm. By irritating only one type of the cones we perceive the color shades and white up to gray color. Then the black color is distinguished if none of the three types of cones is irritated. The sensitivity of the human eye is decreased for violet and red color therefore the retina contains higher amount of the cones with the absorption maximum in the red part of the spectrum.

Our eyes are not equally sensitive to all colors. During the photopic conditions the sensitivity is on maximum for the color which has the wavelength 555 nm and that is yellow-green color and it represents roughly the middle of the visible spectrum. It is very interesting that the sun radiates the most energy right on the wavelength around 550 nm. However, the shift in the sensitivity occurs in the scotopic zone. At that moment the colors differ only in their brightness and the wavelengths around 500 nm (blue-green color) appear to our eye as the brightest ones. This phenomenon proves the double function of the retina: the vision during higher and lower levels of illumination.

Color characteristics

The color characteristics, which can be used to evaluate various colous, are independent on each other and by their transformation we can create a countless number of color variations. The color is determined by several different factors.

Color tone

The color tone or the color shade is the subjective perception of the human sight. It is determined by the wavelength of the rays falling on the human eye. The wavelength is a physical characteristic of the light radiation and it precisely determines the color.

Color richness

The richness gives the content to white color or it is also possible to say that it determines the amount of white light mixed with the given color. With the decreasing richness the color of one tone turns pale and becomes brighter and in the end it vanishes in white. From the physical point of view the richness represents the cleanness of the color.

Brightness

The brightness depends on the intensity of the source of light or on the contrary on the absorption admixture. The more powerful is the source of light the brighter is the color. The color shade also changes when the light intensity increases. After crossing the maximal intensity the perception of yellow-white color is created for all the wavelengths. The brightness can be defined as the mixing of colors with the colorless range (white-gray-black). Physically, this characteristic is described by the luminance of stimulus. And besides that the brightness depends also on the wavelength of the source of light.

As it was mentioned before, the wave characteristics are independent on each other. This means that for example two color shades can have the same wavelength with the same richness and can differ only in the brightness. By gradual transformation of all three physical characteristics (the shade, richness and brightness) it is possible to get an enormous number of colors.

Besides the basic characteristics the color perception of people with normal color vision is influenced also by other factors. The quality of color perception depends for example on the size of stimulus. If its size is smaller than 15%, the color response is considerably lower. The intensity of stimulus is important as well. It has to be above liminal value to allow us to perceive different colors at all and on the contrary the dazzling must not occur because it would lead to the decrease in the ability to distinguish colors. The position of stimulus on the retina that corresponds with the position of light sensitive elements plays also important role. We must not forget also on the actual level of adaptation of the retina which relates to the duration of the color stimulus. As long as the eye is adapted to the white light the sensitivity for individual color components is equally suppressed.