The type of color vision deficiency depends on which cone pigment is missing, and the severity depends on whether or not it has been replaced by another type of photopigment.

Dichromacy is the most severe form of inherited CVD, and it occurs when a person only has two types of cone pigment present in their eye. These conditions are described by the terms Protanopia, Deuteranopia, and Tritanopia, referring to the missing L, M, or S cones, respectively.

Protan color vision deficiency occurs when the long-wavelength sensitive (L) photopigment, sometimes called the “red” pigment, is missing, leaving affected individuals with a diminished capacity to make distinctions in the red-green region of the rainbow.

The most serious Protan defect is called Protanopia, which occurs when a person only has two types of cones: S cones and M cones. They see the world entirely in shades of blue and yellow. For them, certain shades of red and blue-green appear gray, and many shades of red, orange and green appear yellow.  Also, since the sensations of red and green are absent for protanopes, they cannot distinguish purple from blue. This is because they cannot see the red component in the color purple, which is produced by the simultaneous sensation of red and blue.

Protanopic individuals have a particularly hard time seeing red colors and red colored lights, which appear as different shades of gray with very little associated color. Additionally, red lights will appear much dimmer. This can be a particular hazard for people with Protan deficiencies, because they can fail to detect red signal lights, including red flashing stop lights and flashing red railroad signs.

Milder forms of Protan deficiency are called Protanomaly, which occurs when the missing L cones have been replaced by another subtype of M cone. These individuals have color vision based on three cone types, but instead of one S, one M, and one L, they have one type of S cone and two types of M cone that are slightly different from one another.

Because of this second type of M cone, Protanomalous individuals have a small-but-significant amount of red-green vision, which will vary depending on their genetics:

Deutan color vision deficiency occurs when the middle-wavelength sensitive (M) photopigment, sometimes called the “green” pigment, is missing, leaving affected individuals with a diminished capacity to make distinctions in the red-green region of the rainbow.

The most serious Deutan defect is called Deuteranopia, which occurs when a person only has two types of cones: S cones and L cones. They see the world entirely in shades of blue and yellow. For them, certain shades of pink and green will appear gray, and many shades of red, orange and green will appear yellow. Also, since the sensations of red and green are absent for Deuteranopes, they cannot distinguish purple from blue. This is because they cannot see the red component in the color purple, which is produced by the simultaneous sensation of red and blue.

Milder forms of Deutan deficiency are called Deuteranomaly, which occurs when the missing M cones have been replaced by another subtype of L cone. These individuals have color vision that is based on three cone types, but instead of one S, one M, and one L, their vision is based on one type of S cone and two types of L cone that are slightly different from one another.

Because of this second type of L cone, Deuteranomalous individuals have a small-but-significant amount of red-green vision, which will vary depending on their genetics:

Congenital Deutan deficiencies are X-linked, making them much more common in males than females. Approximately 1% of males will be Deuteranopes and about 5% of males will be Deuteranomalous.

A “mild Deutan” diagnosis on the HRR usually means that the patient is Deuteranomalous. They will still be able to distinguish red and green in many circumstances, but they need these hues to be more saturated in order to accurately tell them apart. A “medium Deutan” diagnosis could indicate either Deuteranopia or Deuteranomaly, and may require further testing. A “strong Deutan” diagnosis usually means that the patient has Deuteranopia.

Tritan color vision deficiency occurs when the short-wavelength sensitive (S) photopigment, sometimes called the “blue” pigment, is missing, leaving them with a diminished capacity to make distinctions in the blue-yellow region of the rainbow.

Unlike Protan and Deutan deficiency, Tritan color vision deficiency is rarely congenital and often progresses to full Tritanopia, or “blue blindness,” where the individual is entirely missing the blue-yellow dimension of color vision by middle age. If the diagnosis is made early, the Tritan deficiency may be milder, but their vision will eventually progress until it looks like the image on the right:

Tritan color vision deficiency is extremely rare, occurring in less than 0.01% of individuals and appearing equally in both males and females. Although congenital Tritan deficiency is rare, Tritan color vision deficiencies can be “acquired” because of exposure to toxic chemicals or adverse reactions to certain drugs.  Tritan deficiency can also be an early symptom of some more severe vision disorder.

A “mild” or “medium” diagnosis on the HRR indicates a Tritan deficiency that has not yet progressed to full Tritanopia. A “strong” diagnosis means the person has entirely lost the function of their S cones and can no longer distinguish blue from green and red from yellow.