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Types of Colour Blindness

Colour blindness or colour vision deficiency (CVD) includes a wide range of causes and conditions and is actually quite complex. Usually when people talk about colour blindness, they are referring to the most common forms of red-green colour blindness, which are genetic conditions caused by a recessive gene on the X-chromosome, but there are other types as well.

Red-green colour blindness can be broken down into two main types: Protan-type (“pro-tan”), which is a disorder of the first “prot-” type of retinal cones also called the L-cones, and Deutan-type (“do-tan”) which is a disorder of the second type of retinal cone also called the M-cones.

Protan Colour Blindness

Protan Colour Blindness (“pro-tan”) is an anomaly of the “L” cones. The “L” stands for Long Wavelength Light, which is generally seen as red light, mainly responsible for seeing red colours. In Protan-type CVD, the spectral sensitivity of the L-cone is shifted toward shorter wavelengths, so that it does not receive enough red light, and receives too much green light compared to a normal L-cone. Protan-type CVD includes protanomaly, which is a partial shift of the L-cone, and protanopia, which is a complete shift of the L-cone. It is estimated that about 25% of cases of red-green colour blindness are of the protan type.

A person with protan type colour blindness tends to see greens, yellows, oranges, reds, and browns as being more similar shades of colour than normal, especially in low light. A very common problem is that purple colours look more like blue. Another common issue is that pink colours appear to be gray, especially if the pink is a more reddish pink or salmon colour. Another symptom specific to protan colour vision deficiency is that red colours look darker than normal. For example, if red text is printed on a black background, it can be very hard to read because the red appears to be very dark.

Those with Deuteranomaly colour blindness may mistake purple for blue.

Deutan Colour Blindness

Deutan Colour Blindness (“do-tan”) is an anomaly of the “M” cone. The “M” stands for Medium Wavelength Light, which is generally seen as green light. In Deutan-type CVD, the spectral sensitivity of the M-cone is shifted toward longer wavelengths so that it effectively receives too much red light and not enough green light.

A person with deutan colour vision deficiency may experience confusions between colours such as green and yellow, or blue and purple. Another common symptom is that green traffic signals appear to be a very pale green or sometimes white. Common colour confusion also occurs between pink and gray or white, especially if the pink is similar to a light purple.

Tritan Colour Blindness / Tritanomaly

Tritan Colour Blindness (“try-tan”) includes tritanomaly and tritanopia. It is also sometimes called blue-yellow colour blindness. Tritan colour blindness most commonly acquired later in life due to aging of the eye or a medical condition such as glaucoma and is only very rarely inherited from birth. Tritan colour vision is generally characterized by a reduced sensitivity in the blue-sensitive “S” cone cells. “S” stands for Short Wavelength Light. The retinal S-cone cells make up only about 1% of the approximately 6 million retinal cone cells, so when they are damaged or not functioning properly, it can easily cause a degradation to colour vision. Typically a person with a tritan-type colour vision deficiency does not see blue colours well, and may have difficulty seeing the difference between blue and green. Cataracts, glaucoma, and age-related macular degeneration can cause symptoms of tritan colour blindness. Another factor that causes reduced sensitivity to blue is the yellowing of the crystalline lens within the eye: these cells do not regenerate and over a lifetime of exposure to light, especially UV light, the lens tends to become yellow in appearance and block the transmission of blue light, interfering with colour vision. Eventually this yellowing also leads to cataracts that must be treated surgically.

Monochromacy and Achromatopsia

Monochromacy and Achromatopsia describes a range of conditions that include rod-Monochromacy, S-cone Monochromacy and Achromatopsia. Sometimes these are collectively referred to as types of achromatopsia, as the word “achromat” meaning “no colour.” However, not all cases of achromatopsia have “no colour” vision. Similar to other forms of colour blindness, achromatopsia can be graded as incomplete (partial) achromatopsia or complete achromatopsia (total colour blindness). Achromatopsia is often associated with light sensitivity, photophobia, and glare sensitivity. In some cases, low vision disorders such as progressive cone dystrophy or retinitis pigmentosa can cause a gradual deterioration of colour vision that eventually turns into complete achromatopsia.

Trichromats, Dichromats, Monochromats are terms used in the vision science community to refer to different possible configurations of the human visual system having three (tri-), di (two) or one (mono) channel of colour information. However, these terms are simplified to a great extent, because the true capability of a colour vision system also depends on the degree of overlap between the channels, “perceptual noise” within the channels, and the cognitive processing capability for deciphering these signals in the visual cortex of the brain. Most cases of colour blindness are considered anomalous trichromacy which means they are effectively operating at somewhere between trichromat (normal colour vision with 3 channels) and dichromat (2 channels).

The EnChroma Colour Blind Test is specifically designed to determine your type of red-green colour blindness (deutan or protan) and level: mild deutan or protan, moderate deutan or protan, or strong deutan or protan. However there are limits to what can be tested with a self-administered online test. If you believe that you may have a colour vision deficiency, EnChroma recommends getting a complete eye exam by a qualified eye care professional.

For more information about colour blindness and the science behind it, go to our What is Colour Blindness page.