If you scratch under the surface of color blindness you sooner or later will come across many different terms related to color blindness which are not really self-explanatory. To get a better understanding of the terminology of color blindness I try to lift the curtain at least a bit.

General terms
Color blindness – a term which is misleading – is also known as color vision deficiency or daltonism. Color vision deficiency is not very well known but describes the phenomenon more precisely. Daltonism is named after the first scientist who wrote about color blindness. More details about these terms can be read in my recent article about Color Blindness is not ‘Color Blindness’.

Types of color blindness
There are four different types of color blindness which can be distinguished. This relates to the fact, that humans have three different color receptors in the eye (red, green and blue sensitive cones) and each of them can either be absent or working not properly. The fourth type describes the real color blindness.

• Protan: The first type of color blindness relates to the red cones. Protanopia describes the fact that these cones are missing at all whereas protanomaly describes a displacement of them. Better terms would be red-blind as a synonym for protanopic and red-weak as a synonym for protanomalous.
• Deutan: This term describes all green cone related conditions. If the green cones are missing it is called deuteranopia and a displacement is called deuteranomaly. Again some better terms to describe theses deficiencies in common speech would be green-blind (deuteranopic) and green-weak (deuteranomalous).
• Tritan: Blue cone deficiencies are either called tritanopia if the blue sensitive cones are absent or tritanomaly if they are displaced. According to red and green cone deficiencies tritanopic is also called blue-blind and tritanomalous blue-weak.
• Achromatopsia: This is the real color blindness. An other term for achromatopsia is rod monochromacy, because the cones are almost completely missing and since cones see colors whereas rods only see lightness this relates to complete color blindness and even a strong sensitiveness to bright light.

The very well known term red-green color blindness is an umbrella term including protan and deutan vision deficiencies. More information can be found in my earlier articles about protanopia and tritanopia.

Types of color vision
Color vision can be different in animals than in humans. Some animals have more types of cone cells and some have less. Even humans can not only have less but also more than three color receptors (see the article about tetrachromats).

• Tetrachromatism: Four different color receptors. This is very unusual in humans but can be found in some animals.
• Trichromatism: Three different color receptors related to red, green and blue. This is what we call normal vision.
• Anomalous trichromatism: Three different color receptors whereas one of them is more or less good working. This relates to protanomaly, deuteranomaly and tritanomaly.
• Dichromatism: Two different color receptors which describes the three different types of color blindness protanopia, deuteranopia and tritanopia.
• Monochromatism: Either no color receptors at all or only one type of color receptors. This is also called rod monochromacy or achromatopsia.

These most commonly used terms in color blindness are not really common speech and can hardly be remembered. I like the terms red-blind, red-weak, green-blind, green-weak, blue-blind and blue-weak definitely the most. But unfortunately they are not very well known.

At least the term red-green color blindness is very accurate, describes the most common form of color vision deficiency and is even very well known.