An Ishihara test image as seen by subjects with normal<unk>  vision and by those with a variety of<unk>  deficiencies The Ishihara<unk>  test, which consists of a series of pictures of<unk> ed spots, is the test most often used to diagnose red–green<unk>  deficiencies. A figure (usually one or more Arabic digits) is embedded in the picture as a number of spots in a slightly different<unk> , and can be seen with normal<unk>  vision, but not with a particular<unk>  defect. The full set of tests has a variety of figure/background<unk>  combinations, and enable diagnosis of which particular visual defect is present. The anomaloscope, described below, is also used in diagnosing anomalous trichromacy. Because the Ishihara<unk>  test contains only numerals, it may not be useful in diagnosing young children, who have not yet learned to use numbers. In the interest of identifying these problems early on in life, alternative<unk>  vision tests were developed using only symbols (square, circle, car). Besides the Ishihara<unk>  test, the US Navy and US Army also allow testing with the Farnsworth Lantern Test. This test allows 30% of<unk>  deficient individuals, whose deficiency is not too severe, to pass. Another test used by clinicians to measure chromatic discrimination is the Farnsworth–Munsell 100 hue test. The patient is asked to arrange a set of<unk> ed caps or chips to form a gradual transition of<unk>  between two anchor caps. The HRR<unk>  test (developed by Hardy, Rand, and Rittler) is a red–green<unk>  test that, unlike the Ishihara, also has plates for the detection of the tritan defects. Most clinical tests are designed to be fast, simple, and effective at identifying broad categories of<unk> . In academic studies of<unk> , on the other hand, there is more interest in developing flexible tests to collect thorough datasets, identify copunctal points, and measure just noticeable differences. Classification These<unk>  charts show how different<unk> blind people see compared to a person with normal<unk>  vision. Types of<unk>  and the terms used Cone system Red Green Blue N=normalA=anomalous N A N A N A 1 Normal vision Trichromacy Normal 2 Protanomaly Anomalous trichromacy Partialcolorblindness Red–green 3 Protanopia Dichromacy 4 Deuteranomaly Anomalous trichromacy 5 Deuteranopia Dichromacy 6 Tritanomaly Anomalous trichromacy Blue–yellow 7 Tritanopia Dichromacy 8 Achromatopsia Monochromacy Total<unk>  9 Tetrachromacy Tetrachromacy 10 Based on clinical appearance,<unk>  may be described as total or partial. Total<unk>  is much less common than partial<unk> . There are two major types of<unk> : difficulty distinguishing between red and green, and difficulty distinguishing between blue and yellow. Immunofluorescent imaging is a way to determine red–green<unk>  coding. Conventional<unk>  coding is difficult for individuals with red–green<unk>  (protanopia or deuteranopia) to discriminate. Replacing red with magenta or green with turquoise improves visibility for such individuals. The different kinds of inherited<unk>  result from partial or complete loss of function of one or more of the three different cone systems. When one cone system is compromised, dichromacy results. The most frequent forms of human<unk>  result from problems with either the middle (green) or long (red) wavelength sensitive cone systems, and make it hard to discriminate reds, yellows, and greens from one another. They are collectively referred to as "red–green<unk> ", though the term is an over-simplification and is somewhat misleading. Other forms of<unk>  are much more rare. They include problems in discriminating blues from greens and yellows from reds/pinks, and the rarest form of all, complete<unk>  or monochromacy, where one cannot distinguish any<unk>  from grey, as in a black-and-white movie or photograph. Protanopes, deuteranopes, and tritanopes are dichromats; that is, they can match any<unk>  they see with some mixture of just two primary<unk> s (in contrast to those with normal sight (trichromats) who can distinguish three primary<unk> s). Dichromats usually know they have a<unk>  vision problem, and it can affect their daily lives. Out of the male population, 2% have severe difficulties distinguishing between red, orange, yellow, and green. (Orange and yellow are different combinations of red and green light.)<unk> s in this range, which appear very different to a normal viewer, appear to a dichromat to be the same or a similar<unk> . The terms protanopia, deuteranopia, and tritanopia come from Greek, and respectively mean "inability to see (anopia) with the first (prot-), second (deuter-), or third (trit-) cone". Anomalous trichromacy is the least serious type of<unk>  deficiency. People with protanomaly, deuteranomaly, or tritanomaly are trichromats, but the<unk>  matches they make differ from the normal. They are called anomalous trichromats. In order to match a given spectral yellow light, protanomalous observers need more red light in a red/green mixture than a normal observer, and deuteranomalous observers need more green. From a practical standpoint though, many protanomalous and deuteranomalous people have very little difficulty carrying out tasks that require normal<unk>  vision. Some may not even be aware that their<unk>  perception is in any way different from normal. Protanomaly and deuteranomaly can be diagnosed using an instrument called an anomaloscope, which mixes spectral red and green lights in variable proportions, for comparison with a fixed spectral yellow. If this is done in front of a large audience of males, as the proportion of red is increased from a low value, first a small proportion of the audience will declare a match, while most will see the mixed light as greenish; these are the deuteranomalous observers. Next, as more red is added the majority will say that a match has been achieved. Finally, as yet more red is added, the remaining, protanomalous, observers will declare a match at a point where normal observers will see the mixed light as definitely reddish. Red–green<unk>  Protanopia, deuteranopia, protanomaly, and deuteranomaly are commonly inherited forms of red–green<unk>  which affect a substantial portion of the human population. Those affected have difficulty with discriminating red and green hues due to the absence or mutation of the red or green retinal photoreceptors. It is sex-linked: genetic red–green<unk>  affects males much more often than females, because the genes for the red and green<unk>  receptors are located on the X chromosome, of which males have only one and females have two. Females (XX) are red–green<unk>  blind only if both their X chromosomes are defective with a similar deficiency, whereas males (XY) are<unk>  blind if their single X chromosome is defective. The gene for red–green<unk>  is transmitted from a<unk>  blind male to all his daughters, who are usually heterozygote carriers and are thus unaffected. In turn, a carrier woman has a 50% chance of passing on a mutated X chromosome region to each of her male offspring. The sons of an affected male will not inherit the trait from him, since they receive his Y chromosome and not his (defective) X chromosome. Should an affected male have children with a carrier or<unk> blind woman, their daughters may be<unk> blind by inheriting an affected X chromosome from each parent. Because one X chromosome is inactivated at random in each cell during a woman's development, deuteranomalous heterozygotes (i.e. female carriers of deuteranomaly) may be tetrachromats, because they will have the normal long wave (red) receptors, the normal medium wave (green) receptors, the abnormal medium wave (deuteranomalous) receptors and the normal autosomal short wave (blue) receptors in their retinas. The same applies to the carriers of protanomaly (who have two types of long wave receptors, normal medium wave receptors, and normal autosomal short wave receptors in their retinas). If, by rare chance, a woman is heterozygous for both protanomaly and deuteranomaly, she could be pentachromatic. This situation could arise if, for instance, she inherited the X chromosome with the abnormal long wave gene (but normal medium wave gene) from her mother who is a carrier of protanomaly, and her other X chromosome from a deuteranomalous father. Such a woman would have a normal and an abnormal long wave receptor, a normal and abnormal medium wave receptor, and a normal autosomal short wave receptor—5 different types of<unk>  receptors in all. The degree to which women who are carriers of either protanomaly or deuteranomaly are demonstrably tetrachromatic and require a mixture of four spectral lights to match an arbitrary light is very variable. In many cases it is almost unnoticeable, but in a minority the tetrachromacy is very pronounced. However, Jameson et al. have shown that with appropriate and sufficiently sensitive equipment it can be demonstrated that any female carrier of red–green<unk>  (i.e. heterozygous protanomaly, or heterozygous deuteranomaly) is a tetrachromat to a greater or lesser extent. People in whom deuteranopia or deuteranomaly manifest are sometimes referred to as deutans, those with protanopia or protanomaly as protans. Since deuteranomaly is by far the most common form of red–green<unk>  among men of northwestern European descent (with an incidence of 8%) it follows that the proportion of
Color blindness
Flash blindness
Tunnel vision
Myopia
Cortical blindness
Amblyopia
Red eye 
Retinopathy