Types of color blindness: protan, deutan and tritan explained

Three cones, three families of defects

Typical human colour vision is trichromatic. The retina has three kinds of cone cells, each most sensitive to a different part of the spectrum: long-wavelength (L, often called "red"), medium-wavelength (M, "green") and short-wavelength (S, "blue") cones. The brain compares their signals to build every colour you see.

When one cone type is missing or its sensitivity is shifted, some colours that look different to most people start to look alike. Vision scientists call these sets of confusable colours confusion lines. Each type of color blindness has its own set, which is why a well-designed test can tell the types apart.

The names follow the Greek numbering of the cones: protan (first, L cones), deutan (second, M cones) and tritan (third, S cones).

"-anopia" and "-anomaly": missing versus shifted

Within each family there are two degrees:

• Dichromacy (-anopia): one cone type does not work at all. A protanope, deuteranope or tritanope sees the world with two cone types.
• Anomalous trichromacy (-anomaly): all three cone types are there, but one has its sensitivity shifted. Protanomaly and deuteranomaly range from barely noticeable to almost as strong as dichromacy.

So "deutan" is an umbrella for both deuteranopia and deuteranomaly. That is also why many tests report a type plus a degree, such as "deutan, moderate".

How common is each type?

About 1 in 12 men has some colour vision deficiency, according to the US National Eye Institute. Commonly cited figures for people of Northern European descent look like this:

Type	Men	Women
Deuteranomaly	about 5%	about 0.35%
Deuteranopia	about 1.2%	about 0.01%
Protanomaly	about 1.3%	about 0.02%
Protanopia	about 1.3%	about 0.02%
All red-green types	about 8%	about 0.4-0.5%
Tritan types	rare, roughly 1 in 10,000, men and women equally

Rates are lower in many other populations. The big difference between men and women comes from genetics: the genes for the L and M cone pigments sit on the X chromosome. A man has one X, so one altered copy is enough. A woman needs two altered copies, which is much less likely. The S-cone gene is on a different chromosome, so inherited tritan defects affect both sexes about equally.

Protan: weak or missing red cones

People with protan defects confuse reds, oranges, browns and greens, and some purples with blues. A typical sign of protan vision is that red looks darker than it does to others: a red traffic light or a red laser pointer can look dim, and dark red can look almost black. That darkening of reds is one clue tests use to separate protan from deutan.

Deutan: weak or missing green cones

Deutan is the most common type. The confusions are similar to protan: red, orange, yellow-green and green, brown and dark green, pink and grey. Unlike protan, reds keep their brightness. Many people with mild deuteranomaly only find out at a school or job screening, because daily life gives them few hints: they have learnt the names that go with familiar objects.

Tritan: weak or missing blue cones

Tritan defects affect the blue-yellow dimension. Blues and greens can be confused, as well as yellow with violet or light grey, and pink with orange. Inherited tritan defects are rare. Tritan-type changes can also be acquired: cataract, which yellows the lens with age, diabetic eye disease, glaucoma and some medicines and toxins can all reduce blue-yellow discrimination. A new blue-yellow problem in an adult is a reason to see an eye doctor, not just to take a test.

Monochromacy

Rarely, a person has only one working cone type or none at all (achromatopsia). This is very different from common color blindness: it usually comes with poor visual acuity, strong sensitivity to light and involuntary eye movements, and it is diagnosed early in life.

Inherited versus acquired

Inherited colour vision deficiency is present from birth, affects both eyes equally and does not change over a lifetime. There is no cure for it, though most people adapt well. Acquired deficiency can appear at any age, may differ between the eyes, may change over time and can be a sign of eye or general disease. Typical acquired causes listed by the National Eye Institute include retinal and optic nerve damage, some brain conditions, certain medicines and cataract.

How the type is tested

• Pseudo-isochromatic plates (Ishihara and similar): quick red-green screening. The classic Ishihara book is not designed for tritan defects.
• HRR plates: symbols at several strengths, including blue-yellow plates, so they give type and degree.
• Arrangement tests (Farnsworth D-15, Lanthony D-15d, Farnsworth-Munsell 100 Hue): the pattern of mistakes runs along a protan, deutan or tritan axis.
• Computerised threshold tests such as the Cambridge Colour Test: they measure how much colour difference you need along each confusion line.
• Anomaloscope: the clinical reference instrument for red-green classification, found mainly in specialist clinics.

We compare these methods in detail in Ishihara vs D-15 vs Cambridge.

Screening at home

If you want an idea of your type and degree before an appointment, Trichrome puts nine screening tests modelled on these methods into one iPhone app, including blue-yellow plates and a Cambridge-style gap ring with separate protan, deutan and tritan thresholds. Its "See as" mode also shows a photo or the camera view through a simulation of each type, which helps when explaining your vision to others. It is a screening tool: a screen is not a calibrated test chart, and only an eye-care professional can diagnose a colour vision deficiency.