The color vision is made possible by our so-called color sense. We have this because our retina has sensory cells that can perceive colors. These sensory cells are called “cones”.
Color vision is made up of various characteristics of vision. The eye has the ability to perceive the hue, saturation and brightness of light. The normal eye can distinguish up to 150 different hues of light.
In darkness, however, our eye is virtually “color blind” and can only distinguish between lightness. Now only the second type of sensory cells of our retina are working, the rods, which are responsible for black and white vision. This is where the saying “At night all cats are grey. “. Even if you suspect night blindness, the examination of color vision makes sense.
Examination of the sense of color
Checking color vision is very easy. Everybody has seen the color charts of Ishihara. They are circular and consist of many small color dots.
In the center of the circle there is a number that differs in the color tone. While normal sighted people can recognize the number without any problems, the color-weak reads either a wrong number or no number at all. The principle of these charts is that all color dots on the Ishihara chart have the same saturation and brightness.
So they can only be distinguished by their color. For the color-impaired person, these dots all look more or less the same gray. The best way to determine the red – green – weaknesses of the population is to use these tables.
The extent, however, i.e. whether there is a red-green weakness or a red-green blindness, cannot be determined in this way. A more exact analysis of the color weakness can be done with the so-called anomaloscope. This is a device in which a certain color is given, which must be restored by the patient by mixing red and green.
Depending on how much of each color has been mixed in – which can be read off a scale on the device – there is either a red or green weakness. For example, if there is a green weakness, the person concerned mixes in too much green. From the mixing ratio the so-called anomaly quotient is determined, which is important for certain professions (policeman, pilot, etc.).
The red – green – weakness or blindness is a congenital disease. Men are more often affected than women. This is because the disease is inherited on the X chromosome.
Since men have only one of these types (the other one is the Y chromosome), the disease strikes as soon as they have this gene on their X chromosome. In women, a defective gene is compensated by an intact gene on the second X chromosome. About 8 percent of men suffer from such a color weakness.
In most cases of color weakness it is a disturbance of the red – green – sensation. Very rarely, yellow-blue weaknesses also occur. Rarely it also comes to complete color blindness.
The function of the cones has failed here. At dusk and dawn, however, the latter patients see exactly the same as the normal-sighted person, who can then no longer distinguish colors. Besides the congenital color weaknesses there are also acquired species.
As soon as the center of the retina or the region around the optic nerve is affected, color perception is impaired. This happens, for example, when the pressure in the eye is too high (glaucoma), and thus the optic nerve is squeezed. See also our topic “Glaucoma“.