Color blindness

Color blindness, also called color vision deficiency (CVD), is when someone has a reduced ability to see color, tell colors apart, or notice differences between shades. How severe it is can range from barely noticeable to a complete lack of color perception. The condition is usually an inherited trait linked to sex, caused by a variation in how one or more of the three types of cone cells in the retina work—these cells are what allow us to see color.

The most common form is congenital red–green color blindness (which includes protan and deutan types). This affects up to 1 in 12 males (8%) and 1 in 200 females (0.5%). It's more common in males because the opsin genes responsible are on the X chromosome. Rarer genetic forms include congenital blue–yellow color blindness (tritan type), blue cone monochromacy, and achromatopsia. Color blindness can also come from physical or chemical damage to the eye, the optic nerve, parts of the brain, or from medication toxicity. Normal color vision also naturally declines with age.

Diagnosis is usually done with a color vision test, like the Ishihara test. There is no cure for most causes, though research into gene therapy for some severe forms is ongoing. Mild cases don't usually affect daily life much, and people with color blindness often develop their own adaptations and coping strategies. However, a diagnosis can help individuals, parents, or teachers make adjustments. Color blind glasses can help with some color tasks for red–green color blindness, but they don't give "normal color vision" or let someone see "new" colors. Some mobile apps use a device's camera to identify colors.

Depending on the country, color blind people may be barred from certain jobs, such as aircraft pilots, train drivers, police officers, firefighters, and armed forces members. The effect on artistic ability is debated, but several famous artists are thought to have been color blind.

**Classification**

There is a lot of terminology for classifying color blindness, but the typical system follows the von Kries classifications, which uses severity and which cone is affected to name the type.

**Based on severity**

Clinically, color blindness can be described as total or partial. Total color blindness (monochromacy) is much rarer than partial. Partial color blindness includes dichromacy and anomalous trichromacy, and is often defined as mild, moderate, or strong.

**Monochromacy**

Monochromacy is often called total color blindness because there is no ability to see color. The term can refer to acquired disorders like cerebral achromatopsia, but usually refers to congenital conditions: rod monochromacy and blue cone monochromacy. In cerebral achromatopsia, a person cannot perceive colors even though their eyes can distinguish them. Some sources don't consider this true color blindness, since the failure is in perception, not vision—it's a form of visual agnosia. Monochromacy means having only one channel for color information. Monochromats cannot tell any colors apart and see only differences in brightness. There are two main congenital forms:

- **Rod monochromacy** (complete achromatopsia): The retina has no cone cells, so besides no color discrimination, vision in normal light is difficult. - **Cone monochromacy**: Only one class of cone remains. A cone monochromat can see patterns well in daylight but cannot distinguish hues. This is divided by which cone class remains, though red and green cone monochromats haven't been clearly described. Blue cone monochromacy is caused by a lack of function in L (red) and M (green) cones, linked to the same genes as red–green color blindness (on the X chromosome). Peak sensitivity is in the blue region (near 440 nm). People with this often have nystagmus (jiggling eyes), photophobia (light sensitivity), reduced visual acuity, and myopia (nearsightedness). Visual acuity usually ranges from 20/50 to 20/400.

**Dichromacy**

Dichromats can match any color they see using a mix of just two primary colors (normal sight, or trichromats, need three). Dichromats usually know they have a color vision problem, and it can affect daily life. Human dichromacy includes protanopia, deuteranopia, and tritanopia. About 2% of males have severe trouble telling apart red, orange, yellow, and green (orange and yellow are different mixes of red and green light). Colors that look very different to a normal viewer appear the same or similar to a dichromat. The terms protanopia, deuteranopia, and tritanopia come from Greek, meaning "inability to see (anopia) with the first (prot-), second (deuter-), or third (trit-) [cone]."

**Anomalous trichromacy**

Anomalous trichromacy is the mildest type of color deficiency. It usually means not being able to tell apart colors that someone with normal vision can, but severity ranges from almost dichromacy (strong) to almost normal trichromacy (mild). Many mild anomalous trichromats have little trouble with tasks requiring normal color vision, and some may not even know they have it.