About Color Vision Deficiency (Color Blindness)

Overview

1. Definition

The colors we perceive are not the inherent colors of objects themselves, but the colors of light that objects partially absorb and partially reflect. For example, violet appears at wavelengths of 430 nm, green at 520 nm, yellow at 575 nm, and red at 650 nm. Visible light is part of the electromagnetic spectrum, and its wavelengths range from 350 to 750 nm. In other words, color can be said to be a phenomenon created by combinations of light wavelengths in the visible spectrum. However, a person's color vision is a subjective sense, and it is not the physical property inherent to the object itself, but the result created by each individual through the eyes and the brain's nervous system.

Therefore, even when looking at the same object, perception of color may differ slightly from person to person. However, while such differences do not cause problems in communication or information exchange in daily life, when the difference in color perception is large enough that a person has color vision different from that of a person with normal vision, this is called color vision deficiency. The characteristic of color vision deficiency is being unable to perceive certain colors at all or being unable to distinguish them from other colors.

Color perception is specifically determined by the function of cone cells, one type of photoreceptor in the retina. Cone cells detect light and convert that stimulus into neural signals. These cone cells differ in the range of light wavelengths to which they respond sensitively (spectral sensitivity), and this is an inherent property of each cone cell. Based on spectral sensitivity, they can be divided into three types: cone cells sensitive to long wavelengths, cone cells sensitive to medium wavelengths, and cone cells sensitive to short wavelengths.

These are also called red cone cells, green cone cells, and blue cone cells according to the color corresponding to those wavelengths. Ultimately, people can distinguish all colors according to the relative stimulation of these three cone cells. This is called the trichromatic theory of color vision, which is the concept that all colors can be expressed by combining the three primary colors of red, green, and blue. Therefore, when there is an abnormality in the function of these cone cells, color vision deficiency occurs.

2. Frequency

Color vision deficiency is a common condition, occurring in about 5 to 8% of the total male population. In Western countries, 8% of men and 0.5% of women are said to have color vision deficiency, and in Korea it is known that 5.9% of all men and 0.4% of all women have color vision deficiency.

Among color vision deficiencies, green weakness is generally the most common, accounting for 25 to 45% of all cases. This is followed by green blindness, red blindness, and red weakness. However, the frequencies of these three are similar, each accounting for about 1% of the total male population. Dichromatic color vision deficiency and complete color blindness are very rare, with a frequency of about 0.005%.

Definition

Color vision deficiency is classified according to the degree and type of abnormality in cone cells. This is a more specific classification based on pathology than the older terms color blindness, color weakness, and color defect.

1. Classification by the degree of cone cell abnormality

1. Trichromatic color vision

This refers to a person with normal color vision who has all three types of cone cells in the retina: red, green, and blue cone cells with normal spectral sensitivity, and therefore normal color recognition and discrimination ability.

2. Anomalous trichromatic color vision

This refers to a condition in which all three cone cells—red, green, and blue—are present, but the spectral sensitivity of one type of cone cell is abnormal, so the perception of the corresponding color differs from that of a person with normal vision.

3. Dichromatic color vision

This refers to a condition in which only two of the three cone cell types are present, resulting in a significant impairment in color perception.

4. Monochromatic color vision

This refers to a condition in which only one type of cone cell is present, or there are no cone cells at all. This is rare, and unlike other forms of color vision deficiency, most people also have very poor visual acuity and nystagmus, a condition in which the eyes move back and forth involuntarily.

2. Classification by type of abnormal cone cell

1. Protan color vision deficiency

This is when red cone cell function is abnormal or absent. Because red is the first of the three primary colors of light, it is called protan color vision deficiency. Among these, dichromats who have no red cone cells are called protanopes. If red cone cells are present but their photopigment is different and therefore their spectral sensitivity differs from normal, this is called protanomaly.

2. Deutan color vision deficiency

This is when green cone cell function is abnormal or absent. Dichromats with no green cone cells are called deuteranopes, and cases in which green cone cells are present but their photopigment is abnormal are called deuteranomaly.

3. Tritan color vision deficiency

This is when blue cone cell function is abnormal or absent, and it is a very rare form seen in less than 0.005% of the total population. Ninety-nine percent of congenital color vision deficiency is protan or deutan deficiency. Dichromats with no blue cone cells are called tritanopes, and cases in which they are present but their photopigment is abnormal are called tritanomaly.

3. Problems with existing terms for color vision deficiency

The previously used terms such as color blindness, color weakness, and red-green color blindness are inappropriate by current standards. The term color blindness has been used to refer to all people with color vision deficiency, but the word "blindness" literally means being completely unable to see, so color blindness can be understood as meaning that a person cannot see colors at all. In reality, however, people with color vision deficiency have reduced color discrimination ability; they are not completely unable to see colors. Therefore, the term color blindness is inappropriate to represent all people with color vision deficiency and may also carry negative connotations. Thus, except in the very rare case of monochromatic color vision, it is best to avoid using it whenever possible. The terms red-green color blindness or red-green color weakness are also less appropriate than naming the condition according to the cause of the color vision deficiency: red blindness or red weakness when there is an abnormality in red cone cells, and green blindness or green weakness when there is an abnormality in green cone cells.

So far, we have explained color vision deficiency (color blindness).

In the next part, we will look at the causes and symptoms of color vision deficiency (color blindness).

Source: Korea Centers for Disease Control and Prevention, National Health Information Portal