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As an amateur astronomer for 60 years and always as interested in the science as in the observing, I thought I had a pretty good understanding of the fundamentals. But I'm distance-auditing a third-year course in the fundamentals of applied astronomy, and I found that I had a misconception about the color index.
I had assumed the CI was derived by measuring the star's own magnitude in each of, say, the B and V passbands, then subtracting to get the CI (e.g., by taking the ratio of flux intensities, expressed as a magnitude).
Not so! It's derived by measuring the star's magnitude relative to standard zero points (the magnitude of Vega at each of those wavelengths) that are defined to be magnitude 0 for those wavelength, then subtracting to get the CI.
My question is: Why measure two fluxes relative to a standard and then compare those two values, rather than simply measuring them relative to each other? Why not just measure the flux at each wavelength, take the ratio, express this as a magnitude, and use that as the CI? The relationships are the same either way, just with a different zero point (think 0K and -273°C). It seems like finding how far apart Cleveland and Chicago are by measuring the distance of each from NYC and subtracting, rather than just measuring the distance between them. I don't see that the convention accomplishes anything other than assigning a CI of 0 to "neutral" white stars. Or am I missing something?
What you may be missing is that no two telescope, filter and detector combinations are the same. A colour index defined by your method would be measured differently by different observers.
The solution is to define standards that have defined magnitudes and colours that all observers can use to calibrate their observations.
In actual fact, the basic measurement is $$ B-V = alpha (b-v) + z + eta X,$$ where $b$ and $v$ are native measured magnitudes (-2.5 log the detected adu with a correction for exposure time) and $X$ is the airmass. The constants $alpha$, zeropoint $z$ and extinction coefficient $eta$ are determined by observing a network of secondary standards with a range of colours at a range of airmasses.
The Definition of Color in Astronomy
In the last section, you looked at the colors of some stars in the SDSS database. You may have classified them as red, blue, yellow, or white. But you may have had trouble figuring out exactly what color some of the stars were. Was it red or orange? Yellow or white? Color is a subjective judgment what one person calls "blue" may be a different shade than another person's "blue."
If astronomers are going to learn anything from star color, they first need to have a definition of color that everyone can agree on a measurement that everyone can make to compare the colors of different stars. The measurement they chose is the one you found in the last section: color is the difference in magnitude between two filters.
Color Wheel Definition:
A color wheel is an illustrative set of color hues around an abstract circle, that shows the relationships between three categories of colors, including primary colors, secondary colors, and tertiary colors.
This is just a basic and simple explanation about the color wheel, while it shows more than just relationships, which we will cover throughout this article.
Color Wheel poster | Dopely Digital Goods
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Colour index, in igneous petrology, the sum of the volume percentages of the coloured, or dark, minerals contained by the rock. Volume percentages, accurate to within 1 percent, can be estimated under the microscope by using a point-counting technique over a plane section of the rock volumes also can be approximated visually in hand specimens in the field.
As originally presented, the terms felsic and mafic were used in a broadly descriptive sense to indicate the relative abundances of light-coloured and dark-coloured minerals, respectively, in an igneous rock. The most common light-coloured minerals are the feldspars, feldspathoids, and silica or quartz, giving the term felsic other felsic minerals are corundum, zircon, muscovite, lepidolite, and calcite. The abundant dark-coloured minerals include olivine, pyroxene, amphibole, biotite, garnet, tourmaline, iron oxides, sulfides, and metals. Most minerals fall within these two broad groups.
Broadly speaking, mineral colour indicates the specific gravity of the mineral minerals that are lighter in colour are also lighter in weight. Darker minerals typically contain more of the relatively heavy elements, notably iron, magnesium, and calcium.
In almost all cases, color blind people retain blue–yellow discrimination, and most color blind individuals are anomalous trichromats rather than complete dichromats. In practice, this means that they often retain a limited discrimination along the red–green axis of color space, although their ability to separate colors in this dimension is reduced. Color blindness very rarely refers to complete monochromatism. 
Dichromats often confuse red and green items. For example, they may find it difficult to distinguish a Braeburn apple from a Granny Smith or red from green of traffic lights without other clues—for example, shape or position. Dichromats tend to learn to use texture and shape clues and so may be able to penetrate camouflage that has been designed to deceive individuals with normal color vision. 
Colors of traffic lights are confusing to some dichromats as there is insufficient apparent difference between the red/amber traffic lights and sodium street lamps also, the green can be confused with a dirty white lamp. This is a risk on high-speed undulating roads where angular cues cannot be used. British Rail color lamp signals use more easily identifiable colors: The red is blood red, the amber is yellow and the green is a bluish color. Most British road traffic lights are mounted vertically on a black rectangle with a white border (forming a "sighting board") and so dichromats can more easily look for the position of the light within the rectangle—top, middle or bottom. In the eastern provinces of Canada horizontally mounted traffic lights are generally differentiated by shape to facilitate identification for those with color blindness. [ citation needed ] In the United States, this is not done by shape but by position, as the red light is always on the left if the light is horizontal, or on top if the light is vertical. However, a lone flashing light (e.g. red for stop, yellow for caution) is still problematic.
Color vision deficiencies can be classified as acquired or inherited.
- Acquired: Diseases, drugs (e.g., hydroxychloroquine ), and chemicals such as styrene  or organic solvents  may cause color blindness. 
- Inherited: There are three types of inherited or congenital color vision deficiencies: monochromacy, dichromacy, and anomalous trichromacy.
Color blindness is typically an inherited genetic disorder. It is most commonly inherited from mutations on the X chromosome, but the mapping of the human genome has shown there are many causative mutations – mutations capable of causing color blindness originate from at least 19 different chromosomes and 56 different genes (as shown online at the Online Mendelian Inheritance in Man (OMIM)).
Two of the most common inherited forms of color blindness are protanomaly (and, more rarely, protanopia – the two together often known as "protans") and deuteranomaly (or, more rarely, deuteranopia – the two together often referred to as "deutans").  Both "protans" and "deutans" (of which the deutans are by far the most common) are known as "red–green color-blind". They comprise about 8% of human males and 0.6% of females of Northern European ancestry. 
Some of the inherited diseases known to cause color blindness are:
Inherited color blindness can be congenital (from birth), or it can commence in childhood or adulthood. Depending on the mutation, it can be stationary, that is, remain the same throughout a person's lifetime, or progressive. As progressive phenotypes involve deterioration of the retina and other parts of the eye, certain forms of color blindness can progress to legal blindness, i.e. an acuity of 6/60 (20/200) or worse, and often leave a person with complete blindness.
Color blindness always pertains to the cone photoreceptors in retinas, as it is the cones that detect the color frequencies of light.
About 8% of males, and 0.4% of females, are red–green color blind in some way or another, whether it is one color, a color combination, or another mutation.  Males are at a greater risk of inheriting an X‑linked mutation because males only have one X chromosome (XY, with the Y chromosome carrying altogether different genes from the X chromosome), and females have two (XX) if a woman inherits a normal X chromosome in addition to the one that carries the mutation, she will not display the mutation. Men do not have a second X chromosome to override the chromosome that carries the mutation. If 8% of variants of a given gene are defective, the probability of a single copy being defective is 8%, but the probability that two copies are both defective is (0.08)² = 0.0064 = 0.64% .
Other causes Edit
Other causes of color blindness include brain or retinal damage caused by accidents and other traumas which produce swelling of the brain in the occipital lobe, and damage to the retina caused by exposure to ultraviolet light (wavelengths 10 to 300 nm). Damage often presents itself later in life.
Color blindness may also present itself in the range of degenerative diseases of the eye, such as cataract and age-related macular degeneration, and as part of the retinal damage caused by diabetes. Vitamin A deficiency may also cause color blindness. 
Some subtle forms of color blindness may be associated with chronic solvent-induced encephalopathy (CSE), caused by long-time exposure to solvent vapors. 
Red–green color blindness can be caused by ethambutol, a drug used in the treatment of tuberculosis. 
The typical human retina contains two kinds of light cells: the rod cells (active in low light) and the cone cells (active in normal daylight). Normally, there are three kinds of cone cells, each containing a different pigment, which are activated when the pigments absorb light. The spectral sensitivities of the cones differ one is most sensitive to short wavelengths, one to medium wavelengths, and the third to medium-to-long wavelengths within the visible spectrum, with their peak sensitivities in the blue, green, and yellow–green regions of the spectrum, respectively. The absorption spectra of the three systems overlap, and combine to cover the visible spectrum. These receptors are known as short (S), medium (M), and long (L) wavelength cones, but are also often referred to as blue, green, and red cones, although this terminology is inaccurate. 
The receptors are each responsive to a wide range of wavelengths. For example, the long wavelength "red" receptor has its peak sensitivity in the yellow–green, some way from the red end (longest wavelength) of the visible spectrum. The sensitivity of normal color vision actually depends on the overlap between the absorption ranges of the three systems: different colors are recognized when the different types of cone are stimulated to different degrees. Red light, for example, stimulates the long wavelength cones much more than either of the others, and reducing the wavelength causes the other two cone systems to be increasingly stimulated, causing a gradual change in hue.
Many of the genes involved in color vision are on the X chromosome, making color blindness much more common in males than in females because males only have one X chromosome, while females have two. An estimated 2–3% of women have two slightly different red color cones  and can be considered tetrachromats. One such woman has been reported to be a true or functional tetrachromat, as she can discriminate colors most other people can't.  
The Ishihara color test, which consists of a series of pictures of colored spots, is the test most often used to diagnose red–green color deficiencies.  A figure (usually one or more Arabic digits) is embedded in the picture as a number of spots in a slightly different color, and can be seen with normal color vision, but not with a particular color defect. The full set of tests has a variety of figure/background color combinations, and enable diagnosis of which particular visual defect is present. The anomaloscope, described above, is also used in diagnosing anomalous trichromacy.
Position yourself about 75cm from your monitor so that the colour test image you are looking at is at eye level, read the description of the image and see what you can see!! It is not necessary in all cases to use the entire set of images. In a large scale examination the test can be simplified to six tests test, one of tests 2 or 3, one of tests 4, 5, 6, or 7, one of tests 8 or 9, one of tests 10, 11, 12, or 13 and one of tests 14 or 15. [ This quote needs a citation ]
Because the Ishihara color 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 color vision tests were developed using only symbols (square, circle, car).
Besides the Ishihara color test, the US Navy and US Army also allow testing with the Farnsworth Lantern Test. This test allows 30% of color 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 colored caps or chips to form a gradual transition of color between two anchor caps. 
The HRR color test (developed by Hardy, Rand, and Rittler) is a red–green color 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 color blindness. In academic studies of color blindness, on the other hand, there is more interest in developing flexible tests to collect thorough datasets, identify copunctal points, and measure just noticeable differences. 
Types of color blindness and the terms used
|2||Protanomaly||Anomalous trichromat||Partially |
|8||Achromatopsia||Monochromat||Totally color blind|
Based on clinical appearance, color blindness may be described as total or partial. Total color blindness is much less common than partial color blindness.  There are two major types of color blindness: difficulty distinguishing between red and green, and difficulty distinguishing between blue and yellow.   [ dubious – discuss ]
Immunofluorescent imaging is a way to determine red–green color coding. Conventional color coding is difficult for individuals with red–green color blindness (protanopia or deuteranopia) to discriminate. Replacing red with magenta or green with turquoise improves visibility for such individuals. 
The different kinds of inherited color blindness 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 color blindness 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 color blindness", though the term is an over-simplification and is somewhat misleading. Other forms of color blindness are much more rare. They include problems in discriminating blues from greens and yellows from reds/pinks, and the rarest form of all, complete color blindness or monochromacy, where one cannot distinguish any color from grey, as in a black-and-white movie or photograph.
Protanopes, deuteranopes, and tritanopes are dichromats that is, they can match any color they see with some mixture of just two primary colors (in contrast to those with normal sight (trichromats) who can distinguish three primary colors). Dichromats usually know they have a color 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.) Colors in this range, which appear very different to a normal viewer, appear to a dichromat to be the same or a similar color. 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 color deficiency.  People with protanomaly, deuteranomaly, or tritanomaly are trichromats, but the color 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 color vision. Some may not even be aware that their color 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 color blindness Edit
Protanopia, deuteranopia, protanomaly, and deuteranomaly are commonly inherited forms of red–green color blindness 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 color blindness affects males much more often than females, because the genes for the red and green color receptors are located on the X chromosome, of which males have only one and females have two. Females (XX) are red–green color blind only if both their X chromosomes are defective with a similar deficiency, whereas males (XY) are color blind if their single X chromosome is defective. 
The gene for red–green color blindness is transmitted from a color 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 colorblind woman, their daughters may be colorblind 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 color 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 color blindness (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 blindness among men of northwestern European descent (with an incidence of 8%) it follows that the proportion of carriers (and of potential deuteranomalous tetrachromats) among the females of that genetic stock is 14.7% (i.e. 92% × 8% × 2), based on the Hardy–Weinberg principle. 
- Protanopia (1% of males): Lacking the red cones for long-wavelength sensitive retinal cones, those with this condition are unable to distinguish between colors in the green–yellow–red section of the spectrum. They have a neutral point at a cyan-like wavelength around 492 nm (see spectral color for comparison)—that is, they cannot discriminate light of this wavelength from white. For a protanope, the brightness of red, orange, and yellow are much reduced compared to normal. This dimming can be so pronounced that reds may be confused with black or dark gray, and red traffic lights may appear to be extinguished. They may learn to distinguish reds from yellows primarily on the basis of their apparent brightness or lightness, not on any perceptible hue difference. Violet, lavender, and purple are indistinguishable from various shades of blue because their reddish components are so dimmed as to be invisible. For example, pink flowers, reflecting both red light and blue light, may appear just blue to the protanope. A very few people have been found who have one normal eye and one protanopic eye. These unilateral dichromats report that with only their protanopic eye open, they see wavelengths shorter than neutral point as blue and those longer than it as yellow. This is a rare form of color blindness.
- Deuteranopia (1% of males): Lacking the green cones for medium-wavelength cones, those affected are again unable to distinguish between colors in the green–yellow–red section of the spectrum. Their neutral point is at a slightly longer wavelength, 498 nm, a more greenish hue of cyan. A deuteranope suffers the same hue discrimination problems as protanopes, but without the abnormal dimming. Purple colors are not perceived as something opposite to spectral colors all these appear similarly. This form of colorblindness is also known as Daltonism after John Dalton (his diagnosis was confirmed as deuteranopia in 1995, some 150 years after his death, by DNA analysis of his preserved eyeball). Equivalent terms for Daltonism in Romance languages such as daltonismo (Spanish, Portuguese and Italian), daltonisme (French), daltonism (Romanian) and in Slavic languages such as daltonizam (Croatian), daltonizam/далтонизам (Serbian), далтонизъм (Bulgarian), далтонизам (Macedonian),дальтонизм (Russian), дальтонізм (Ukrainian) and дальтанізм (Belarusian) are still used to describe color blindess in a broad sense or deuteranopia in a more restricted sense. Deuteranopic unilateral dichromats report that with only their deuteranopic eye open, they see wavelengths shorter than neutral point as blue and longer than it as yellow. 
- Protanomaly (1% of males, 0.01% of females):  Having a mutated form of the long-wavelength (red) pigment, whose peak sensitivity is at a shorter wavelength than in the normal retina, protanomalous individuals are less sensitive to red light than normal. This means that they are less able to discriminate colors, and they do not see mixed lights as having the same colors as normal observers. They also suffer from a darkening of the red end of the spectrum. This causes reds to reduce in intensity to the point where they can be mistaken for black. Protanomaly is a fairly rare form of color blindness, making up about 1% of the male population. Both protanomaly and deuteranomaly are carried on the X chromosome.
- Deuteranomaly (most common—6% of males, 0.4% of females):  These individuals have a mutated form of the medium-wavelength (green) pigment. The medium-wavelength pigment is shifted towards the red end of the spectrum resulting in a reduction in sensitivity to the green area of the spectrum. Unlike in protanomaly, the intensity of colors is unchanged. The deuteranomalous person is considered "green weak". For example, in the evening, dark green cars appear to be black to deuteranomalous people. As with protanomates, deuteranomates are poor at discriminating small differences in hues in the red, orange, yellow, green region of the spectrum. They make errors in the naming of hues in this region because the hues appear somewhat shifted towards green. However, unlike protanomates, deuteranomalous people do not have the loss of "brightness" problem.
Blue–yellow color blindness Edit
Those with tritanopia and tritanomaly have difficulty discerning between bluish and greenish hues, as well as yellowish and reddish hues.
Color blindness involving the inactivation of the short-wavelength sensitive cone system (whose absorption spectrum peaks in the bluish-violet) is called tritanopia or, loosely, blue–yellow color blindness. The tritanope's neutral point occurs near a yellowish 570 nm green is perceived at shorter wavelengths and red at longer wavelengths.  Mutation of the short-wavelength sensitive cones is called tritanomaly. Tritanopia is equally distributed among males and females. Jeremy H. Nathans (with the Howard Hughes Medical Institute) demonstrated that the gene coding for the blue receptor lies on chromosome 7, which is shared equally by males and females. Therefore, it is not sex-linked. This gene does not have any neighbor whose DNA sequence is similar. Blue color blindness is caused by a simple mutation in this gene.
- Tritanopia (less than 1% of males and females): Lacking the short-wavelength cones, those affected see short-wavelength colors (blue, indigo and spectral violet) as greenish and drastically dimmed, some of these colors even as black. Yellow and orange are indistinguishable from white and pink respectively, and purple colors are perceived as various shades of red. This form of color blindness is not sex-linked.
- Tritanomaly (equally rare for males and females [0.01% for both]):  Having a mutated form of the short-wavelength (blue) pigment. The short-wavelength pigment is shifted towards the green area of the spectrum. This is the rarest form of anomalous trichromacy color blindness. Unlike the other anomalous trichromacy color deficiencies, the mutation for this color blindness is carried on chromosome 7. Therefore, it is equally prevalent in both male and female populations. The OMIM gene code for this mutation is 304000 "Colorblindness, Partial Tritanomaly". 
Total color blindness Edit
Total color blindness is defined as the inability to see color. Although the term may refer to acquired disorders such as cerebral achromatopsia also known as color agnosia, it typically refers to congenital color vision disorders (i.e. more frequently rod monochromacy and less frequently cone monochromacy).  
In cerebral achromatopsia, a person cannot perceive colors even though the eyes are capable of distinguishing them. Some sources do not consider these to be true color blindness, because the failure is of perception, not of vision. They are forms of visual agnosia. 
Monochromacy is the condition of possessing only a single channel for conveying information about color. Monochromats possess a complete inability to distinguish any colors and perceive only variations in brightness. It occurs in two primary forms:
- , frequently called achromatopsia, where the retina contains no cone cells, so that in addition to the absence of color discrimination, vision in lights of normal intensity is difficult. While normally rare, achromatopsia is very common on the island of Pingelap, a part of the Pohnpei state, Federated States of Micronesia, where it is called maskun: about 10% of the population there has it, and 30% are unaffected carriers. The island was devastated by a storm in the 18th century (an example of a genetic bottleneck) and one of the few male survivors carried a gene for achromatopsia. The population grew to several thousand before the 1940s. is the condition of having both rods and cones, but only a single kind of cone. A cone monochromat can have good pattern vision at normal daylight levels, but will not be able to distinguish hues. Blue cone monochromacy (X chromosome) is caused by lack of functionality of L and M cones (red and green). It is encoded at the same place as red–green color blindness on the X chromosome. Peak spectral sensitivities are in the blue region of the visible spectrum (near 440 nm). People with this condition generally show nystagmus ("jiggling eyes"), photophobia (light sensitivity), reduced visual acuity, and myopia (nearsightedness).  Visual acuity usually falls to the 20/50 to 20/400 range.
There is no cure for color deficiencies. ″The American Optometric Association reports a contact lens on one eye can increase the ability to differentiate between colors, though nothing can make you truly see the deficient color.″ 
Optometrists can supply colored spectacle lenses or a single red-tint contact lens to wear on the non-dominant eye, but although this may improve discrimination of some colors, it can make other colors more difficult to distinguish. A 1981 review of various studies to evaluate the effect of the X-chrom contact lens concluded that, while the lens may allow the wearer to achieve a better score on certain color vision tests, it did not correct color vision in the natural environment.  A case history using the X-Chrom lens for a rod monochromat is reported  and an X-Chrom manual is online. 
Lenses that filter certain wavelengths of light can allow people with a cone anomaly, but not dichromacy, to see better separation of colors, especially those with classic "red/green" color blindness. They work by notching out wavelengths that strongly stimulate both red and green cones in a deuter- or protanomalous person, improving the distinction between the two cones' signals. As of 2012, sunglasses that notch out color wavelengths are available commercially. 
Many mobile and computer applications have been developed to help color blind individual to view better differentiate between colors. Some applications launch a simulation of colorblindness to allow people with typical vision to understand how people with color blindness see the world, which can improve inclusive design for both groups. This is achieved using an LMS color space. 
After analyzing what colors are confusing, daltonization algorithms can be used to create a color filter for people with color blindness to notice some color differences more easily. 
|Protanopia (red deficient: L cone absent)||1.3%||0.02%|
|Deuteranopia (green deficient: M cone absent)||1.2%||0.01%|
|Tritanopia (blue deficient: S cone absent)||0.001%||0.03%|
|Protanomaly (red deficient: L cone defect)||1.3%||0.02%|
|Deuteranomaly (green deficient: M cone defect)||5.0%||0.35%|
|Tritanomaly (blue deficient: S cone defect)||0.0001%||0.0001%|
Color blindness affects a large number of individuals, with protans and deutans being the most common types.  In individuals with Northern European ancestry, as many as 8 percent of men and 0.4 percent of women experience congenital color deficiency. 
The number affected varies among groups. Isolated communities with a restricted gene pool sometimes produce high proportions of color blindness, including the less usual types. Examples include rural Finland, Hungary, and some of the Scottish islands. [ citation needed ] In the United States, about 7 percent of the male population—or about 10.5 million men—and 0.4 percent of the female population either cannot distinguish red from green, or see red and green differently from how others do (Howard Hughes Medical Institute, 2006 [ clarification needed ] ). More than 95 percent of all variations in human color vision involve the red and green receptors in male eyes. It is very rare for males or females to be "blind" to the blue end of the spectrum. 
|Indians (Andhra Pradesh)||292||7.5|
The first scientific paper on the subject of color blindness, Extraordinary facts relating to the vision of colours, was published by the English chemist John Dalton in 1798  after the realization of his own color blindness. Because of Dalton's work, the general condition has been called daltonism, although it usually refers specifically to red–green color blindness.
Design implications Edit
Color codes present particular problems for those with color deficiencies as they are often difficult or impossible for them to perceive.
Good graphic design avoids using color coding or using color contrasts alone to express information  this not only helps color blind people, but also aids understanding by normally sighted people by providing them with multiple reinforcing cues.  [ citation needed ]
Designers need to take into account that color-blindness is highly sensitive to differences in material. For example, a red–green colorblind person who is incapable of distinguishing colors on a map printed on paper may have no such difficulty when viewing the map on a computer screen or television. In addition, some color blind people find it easier to distinguish problem colors on artificial materials, such as plastic or in acrylic paints, than on natural materials, such as paper or wood. Third, for some color blind people, color can only be distinguished if there is a sufficient "mass" of color: thin lines might appear black, while a thicker line of the same color can be perceived as having color. [ citation needed ]
Designers should also note that red–blue and yellow–blue color combinations are generally safe. So instead of the ever-popular "red means bad and green means good" system, using these combinations can lead to a much higher ability to use color coding effectively. This will still cause problems for those with monochromatic color blindness, but it is still something worth considering.
When the need to process visual information as rapidly as possible arises, for example in an emergency situation, the visual system may operate only in shades of gray, with the extra information load in adding color being dropped. [ citation needed ] This is an important possibility to consider when designing, for example, emergency brake handles or emergency phones.
Color blindness may make it difficult or impossible for a person to engage in certain occupations. Persons with color blindness may be legally or practically barred from occupations in which color perception is an essential part of the job (e.g., mixing paint colors), or in which color perception is important for safety (e.g., operating vehicles in response to color-coded signals). This occupational safety principle originates from the Lagerlunda train crash of 1875 in Sweden. Following the crash, Professor Alarik Frithiof Holmgren, a physiologist, investigated and concluded that the color blindness of the engineer (who had died) had caused the crash. Professor Holmgren then created the first test using different-colored skeins to exclude people from jobs in the transportation industry on the basis of color blindness.  However, there is a claim that there is no firm evidence that color deficiency did cause the collision, or that it might have not been the sole cause. 
Color vision is important for occupations using telephone or computer networking cabling, as the individual wires inside the cables are color-coded using green, orange, brown, blue and white colors.  Electronic wiring, transformers, resistors, and capacitors are color-coded as well, using black, brown, red, orange, yellow, green, blue, violet, gray, white, silver, gold. 
Some countries have refused to grant driving licenses to individuals with color blindness. In Romania, there is an ongoing campaign to remove the legal restrictions that prohibit colorblind citizens from getting drivers' licenses. [ citation needed ]
The usual justification for such restrictions is that drivers of motor vehicles must be able to recognize color-coded signals, such as traffic lights or warning lights.
Piloting aircraft Edit
While many aspects of aviation depend on color coding, only a few of them are critical enough to be interfered with by some milder types of color blindness. Some examples include color-gun signaling of aircraft that have lost radio communication, color-coded glide-path indications on runways, and the like. Some jurisdictions restrict the issuance of pilot credentials to persons who suffer from color blindness for this reason. Restrictions may be partial, allowing color-blind persons to obtain certification but with restrictions, or total, in which case color-blind persons are not permitted to obtain piloting credentials at all. 
In the United States, the Federal Aviation Administration requires that pilots be tested for normal color vision as part of their medical clearance in order to obtain the required medical certificate, a prerequisite to obtaining a pilot's certification. If testing reveals color blindness, the applicant may be issued a license with restrictions, such as no night flying and no flying by color signals—such a restriction effectively prevents a pilot from holding certain flying occupations, such as that of an airline pilot, although commercial pilot certification is still possible, and there are a few flying occupations that do not require night flight and thus are still available to those with restrictions due to color blindness (e.g., agricultural aviation). The government allows several types of tests, including medical standard tests (e.g., the Ishihara, Dvorine, and others) and specialized tests oriented specifically to the needs of aviation. If an applicant fails the standard tests, they will receive a restriction on their medical certificate that states: "Not valid for night flying or by color signal control". They may apply to the FAA to take a specialized test, administered by the FAA. Typically, this test is the "color vision light gun test". For this test an FAA inspector will meet the pilot at an airport with an operating control tower. The color signal light gun will be shone at the pilot from the tower, and they must identify the color. If they pass they may be issued a waiver, which states that the color vision test is no longer required during medical examinations. They will then receive a new medical certificate with the restriction removed. This was once a Statement of Demonstrated Ability (SODA), but the SODA was dropped, and converted to a simple waiver (letter) early in the 2000s. 
Research published in 2009 carried out by the City University of London's Applied Vision Research Centre, sponsored by the UK's Civil Aviation Authority and the U.S. Federal Aviation Administration, has established a more accurate assessment of color deficiencies in pilot applicants' red/green and yellow–blue color range which could lead to a 35% reduction in the number of prospective pilots who fail to meet the minimum medical threshold. 
Inability to distinguish color does not necessarily preclude the ability to become a celebrated artist. The 20th century expressionist painter Clifton Pugh, three-time winner of Australia's Archibald Prize, on biographical, gene inheritance and other grounds has been identified as a protanope.  19th century French artist Charles Méryon became successful by concentrating on etching rather than painting after he was diagnosed as having a red–green deficiency.  Jin Kim's red–green color blindness did not stop him from becoming first an animator and later a character designer with Walt Disney Animation Studios. 
Rights of the color blind Edit
A Brazilian court ruled that people with color blindness are protected by the Inter-American Convention on the Elimination of All Forms of Discrimination against Person with Disabilities.   
At trial, it was decided that the carriers of color blindness have a right of access to wider knowledge, or the full enjoyment of their human condition.
United States Edit
In the United States, under federal anti-discrimination laws such as the Americans with Disabilities Act, color vision deficiencies have not been found to constitute a disability that triggers protection from workplace discrimination. 
A famous traffic light on Tipperary Hill in Syracuse, New York, is upside-down due to the sentiments of its Irish American community,  but has been criticized due to the potential hazard it poses for color-blind persons. 
Some tentative evidence finds that color blind people are better at penetrating certain color camouflages. Such findings may give an evolutionary reason for the high rate of red–green color blindness.  There is also a study suggesting that people with some types of color blindness can distinguish colors that people with normal color vision are not able to distinguish.  In World War II, color blind observers were used to penetrate camouflage. 
In September 2009, the journal Nature reported that researchers at the University of Washington and University of Florida were able to give trichromatic vision to squirrel monkeys, which normally have only dichromatic vision, using gene therapy. 
In 2003, a cybernetic device called eyeborg was developed to allow the wearer to hear sounds representing different colors.  Achromatopsic artist Neil Harbisson was the first to use such a device in early 2004 the eyeborg allowed him to start painting in color by memorizing the sound corresponding to each color. In 2012, at a TED Conference, Harbisson explained how he could now perceive colors outside the ability of human vision. 
Researchers measure the basis of color vision
Dr. Wolf M. Harmening from University Eye Hospital Bonn, together with American colleagues, studied color vision by probing individual sensory cells -- photoreceptors -- in the human eye. The results confirm that the photoreceptor cells of the retina are especially sensitive to colors corresponding to their visual pigments, even when stimulated in isolation. A new observation is that proximity effects play a key role: sensitivity of tested photoreceptors varied depending on which cell classes were located in their immediate neighborhood. The results have now been published in advance online and will soon be published in the Journal of Neuroscience.
It is a constant 'aha' effect: when the light is switched on in a dark room, color vision sets in. "This not only makes the world more colorful," says Dr. Wolf M. Harmening, who heads an Emmy Noether research group at Bonn University Eye Hospital. "Color also allows spatial detail to become apparent that has proven vital for survival over the course of evolution." Some predator camouflage can only be identified through color. Poisonous animals and plants also provide warning signals through color. That human color vision emerges from three independent channels within the retina is well established in the vision science literature. By stimulating individual photoreceptor cells in living subjects, the lead authors Dr. Wolf M. Harmening from University Eye Hospital Bonn and Dr. William S. Tuten from the University of California, Berkeley, together with colleagues from the US universities in Seattle, Washington and Birmingham, Alabama, have now shown on a cellular scale how the human retina conveys color signals.
To do this, the researchers used an ophthalmoscope that can examine and stimulate the human retina non-invasively. The novel method -- Adaptive Optics Scanning Laser Ophthalmoscopy -- employs a combination of a laser and a very high-resolution microscope, which can even map individual sensory cells in the retina. The research team has now used this ophthalmoscope to study vision in the retinas of two human subjects. According to common theory, all color stimuli can be formed by mixing the primary colors red, green, and blue. While rod photoreceptors are specialized for seeing in the dark, cone photoreceptors convey color vision. They carry light sensitive pigments specialized to absorb wavelengths near the primary colors, the basis of trichromatic vision.
Mapping of the retina
The researchers initially mapped the cone mosaic on the subjects' retinas by measuring light absorption for certain wavelengths in each photoreceptor. In this way, they were able to determine the sensory cells' identity, or class, within the framework of trichromacy. By reducing the intensity of the stimulation light, the researchers were then able to determine a detection threshold in each cone, at which light was just barely seen by the subjects. "This is important because we could use the sensitivity of each cell to determine how overall perception is governed by the contribution of individual cones," reports Harmening.
Most notably, the sensitivity of single cells also depended on the immediate neighboring cells. "If a cone sensitive to red light is surrounded by cells that are more sensitive to green, this cone is more likely to behave like a green cone," summarizes Harmening. Studying visual processing of color is complex, in part because the brain does not receive raw data from individual photoreceptors but rather an already preprocessed retinal signal. Harmening: "Spatial and color information of individual cones is modulated in the complex network of the retina, with lateral information spreading through what are known as horizontal cells."
Their finding supports previous assumptions about color vision. "What's new is that we can now study vision on the most elementary level, cell-by-cell," says the scientist. Conventional tests of vision use stimuli that necessarily activate hundreds to thousands photoreceptor cells at the same time. Harmening emphasizes that cellular-scale retinal computation such as the proximity effect has important implications, for basic and clinical research. "When the basis of vision is understood better, we open avenues for new diagnoses and treatments in case of retinal disease," says Harmening. The novel single cell approach offers access to new findings in ophthalmology.
The C.I.E. Chromaticity Diagram
The diagram at left represents the the mapping of human color perception in terms of two CIE parameters x and y. The spectral colors are distributed around the edge of the "color space" as shown, and that outline includes all of the perceived hues and provides a framework for investigating color.
The diagram given here is associated with the 1931 CIE standard. Revisions were made in 1960 and 1976, but the 1931 version remains the most widely used version. The diagram at lower left is a rough rendering of the 1931 CIE colors on the chromaticity diagram.
We have shown that the Gesnerioideae flowers show remarkable color diversity, which is due to the use of the ABP up to its full capacity, including the deoxyanthocyanin branch, which is rarely employed in angiosperms. Utilizing both branches in the ABP provides Gesnerioideae with alternative options for floral color production without forcing an evolutionary dead-end, resulting in extraordinary floral color diversity and different signaling strategies adapted for bees and hummingbirds. By improving our understanding the biochemical basis of floral color in the Gesnerioideae, we provide clear perspectives to identify the genetic changes responsible for floral color shifts and test the effects of these transitions on plant-pollinator interactions and angiosperm diversification.
On the Basis of Color
In her book, Abyssinian Nomad, Maskaram Haile writes about her 9-month journey from Cape to Cairo. As I was reading the book, I shared some of her stories with my parents. I jokingly suggested I would embark on the same journey. My mom immediately responded, “They will kill you thinking you are Middle Eastern.”
It was not the first time someone compared me to a middle eastern or a foreigner in general. I have a very light skin tone which has led some people to believe I am an outlander. I am Ethiopian, and to the best of my knowledge, I don’t have a foreign ancestor. Yet, my African curly hair is not enough proof to convince some people of that fact. Children have stopped me on streets, service providers have attempted to talk to me in English and occasionally, I have been nudged and verbally harassed.
The most memorable moment for me, however, came a few months ago. I tutor a 9-year-old girl and her 13-year-old brother. One day the girl said to me, “You are very pretty because you are white. I wish I was like you.” She even went further and compared me with another lady with a light skin tone. Based on her 9-year-old logic, I was “prettier” because I was “whiter.” It shocked me to my core to be faced with the fact that this beautiful and smart dark-skinned, young lady deemed herself inferior on the basis of color. Luckily, the 2019 winner of the Miss Universe beauty pageant was a black woman. So I was able to show my student one can be black and beautiful. It was also a chance for me to make her aware of how beauty pageants nowadays shed some light on the knowledge of contenders on global issues, diverting the contests from being entirely about external appearance.
Where does colorism come from?
It is believed colorism—discrimination based on skin color and not race—is the collateral damage of European colonialism and slavery. White was the suppressor and black was the suppressed. And long after that time has passed, we still associate light skin color with power and knowledge. In the western world, darker skin color is connected with crime, being uneducated and being scary. People are more fearful of black men than they are of fair skinned ones. And within the black community, dark-skinned people may dismiss light-skinned people thinking the latter “are not entirely black.” Colorism exists here in Africa, too. Light-skinned individuals are generally more likely to be trusted and given responsibility.
Is there a logical base to colorism?
The truth is there is no scientific justification that skin tone has a causal relationship with a person’s intelligence. Skin color is controlled by many genes in our DNA and by the amount of Melanin (a group of pigments that control UV radiation penetration) in the skin. Some researchers have attempted showing a connection between light skin color and intelligence, concluding intelligence is controlled by the Caucasian traits individuals possess. They made the common mistake in data analysis which is to confuse correlation with causation. The actual causation lies in the simple fact that light-skinned individuals are less likely to be discriminated against and more likely to have access to quality education.
But Ethiopia was not colonized. Why are we affected by colorism?
We pride ourselves with this part of our history. We celebrate it and show it off to the world telling every foreigner we meet, “We are the only African country that was never colonized!” While we are busy feeding our egos and living in our ancestors’ reality, the younger generation is being brainwashed to believe the rule of thumb is, “Go white or go home.” No, we were not colonized. Yes, we have a diverse and rich history. But instead of re-living the same story come Adwa memorial day, how about we write a new one?
What are the effects of colorism?
Multiple studies have been conducted showing the effects of skin color on employment. There is also the risk of being questioned and harassed by police. Some people go as far as bleaching their skins to be lighter, which usually has a negative consequence on their health. More close to home, colorism affects emotional health of individuals. Women, being shaped by society to give high consideration for physical appearance, tend to have lower self-esteem and self-worth simply because they have dark skin.
How can we mitigate this?
Like every other global issue, colorism calls for an effective leadership strategy—a strategy that starts with the self and builds up. Most of us are confused about who we are. We are angry at the world for the times we were hurt based on our color. We are divided among ourselves based on the tones of our skins when our ancestral roots are the same. We have to start by first loving who we are—internally and externally. We should be proud of our African scientists, painters, writers, the list goes on, that come in all shades of color. We have to focus on the media and the stories our siblings and kids are exposed to. If all they see is white scientists and white media personnel, they are going to believe white is the default. But while social media activism and niche discussions are important in creating a conducive environment for change, we need unity. We need a “clear and shared vision.” We need agile leadership.
Last week, former Starbucks CEO and 2020 presidential hopeful Howard Schultz drew outrage—not over policy positions or campaign slogans or even his company’s chronically burnt French roast, but word choice. In a clip from a January CNBC Q&A that surfaced on Twitter, Schultz was asked whether he thought that billionaires had too much influence on American public life. He responded, “The moniker billionaire now has become the catchphrase. I would rephrase that, and I would say that people of means have been able to leverage their wealth and their interest in ways that are unfair, and I think that … directly speaks to the special interests that are paid for by people of wealth and corporations who are looking for influence.”
To me, this quote and Schultz’s larger statement show him relatively clearly, if weakly, responding that rich people do have too much influence and that fixating only on billionaires would be too narrow of a focus. But that was not the consensus of the progressive internet. Instead, many came away with the impression that Schultz thinks that billionaire is a pejorative, and that we should all be nicer to folks like him by using the softer people of means.
Regardless of what Schultz really meant, it’s worth considering why we are so ready to hear people of means as a slimy obfuscation rather than as a neutral rephrasing or expansion of the category of person in question. Because that readiness speaks to a larger linguistic problem that has implications far beyond the primaries.
The “people of/with x” formulation—wherein people who have some quality, like size or disability, are condensed into a solid noun—has become increasingly common (particularly on the left) since the 1990s. The sentiment behind that semantic shift is the same one that underlies the move from terms like “victims of HIV” and “homeless” to ones like “people living with HIV” and “living unhoused,” respectively. Or the move from “disabled people” or “handicapped people” to “people with disabilities.” It is a euphemistic linguistic model that intends to center humanity separate from situation or identity, and it is a model that creates new terms that are supposed to, as John McWhorter wrote for Slate in 2016, “rise above pejorative connotations that society has linked to the thing in question.” Its biggest success story might be the phrase people of color.
While people of color may not have risen to the same level of prominence as minorities just yet, there is a growing sense that the former should replace the latter when specifically referring to people who aren’t white. Minority, along with nonwhite, necessarily defines people by a negative, as lacking some quality that would place them in the majority category. (And as American demographics continue to change, there’s a possibility that minorities will become as inaccurate nationally as it has always been globally.) Similarly, the now-passé colored people held associations with the state-sanctioned apartheid of the Jim Crow South, where roles in public life were defined by whether one was colored or not. Enter people of color. While the phrase has existed long before its current heyday (appearing as far back as 1807 in legal records), it seems to have begun its modern ascent in the late 1980s. A 1988 New York Times piece on the phrase describes a comic strip that suggests people of color as a “new-age” replacement for colored people. “Politically, [people of color] expresses solidarity with other nonwhites, and subtly reminds whites that they are a minority,” wrote columnist William Safire.
These are noble origins. But for all the good intentions behind it, the success of people of color has brought with it a strong potential for misuse. In our modern discourse, the phrase has come to be thought of as both the most courteous way to refer to a nonwhite person and a signal that its user is down for the cause of racial justice. It has become depressingly common for a well-meaning white person to, despite my fairly conspicuous self-identification as black, refer to me as a woman or writer of color. In that choice lies an uneasiness, either with referring to me as black—despite its accuracy—or with the potential of misidentification of my race. In either case, person of color on some level serves to make the (typically white) speaker feel better, rather than me, the person whom the terminology is theoretically for.
In many spaces, the term functions now as performative fauxgressive politeness—as one of the many buzzwords such as intersectionality or systemic that one can drop, with little understanding, to display her wokeness. In its presence, more accurate terminology is forgone because it feels easier and safer (mainly for white people) to just say people of color.
Take, for example, Rolling Stone describing Sen. Tim Scott, in a recent article on Republican support of noted racist Rep. Steve King, as a person of color rather than as a black man. This is a choice that, at the very least, creates a lack of journalistic clarity since, as the only black Republican senator, Scott’s toeing of the party line with regard to King’s racism is particularly newsworthy. What’s needed here is specificity, not genteel ease—and that’s not the only case where people of color elides crucial detail. For example, using people of color when discussing the history of chattel slavery or police brutality flattens the specificities of anti-black racism in America. Using people of color when referring to the genocide of native and indigenous people in America obfuscates particular histories of colonial violence. Suggesting that newsrooms or corporate boards need to hire more people of color when there are specifically no Latino people or Southeast Asians on the payroll suggests that any nonwhite person will do, that we are all the same and bring the same experience to the table.
The swift and intense reaction to Schultz’s “people of means” suggests, to me at least, that we have become sensitive to the misuse of this formulation. We know, on some level, that people of color and its cousins have evolved from a compassionate shift in our linguistic paradigms to tools that people in positions of racial or other kinds of power can use to appear politically sensitive while doing little, if any, of the actual work of social justice. In POC’s case, what was partially meant to support a sense of radical solidarity between different marginalized communities has been so watered down as to be comfortable in the mouth of someone who either wore blackface or a Klan hood, or thought either was worthy of appearing in his yearbook page. It’s a term that, in many ways, still centers whiteness and suggests that anti-blackness doesn’t exist in Latino communities or that anti-immigrant sentiments don’t exist in black American ones. A term that has happily been co-opted by vice presidents of diversity who think there is a way to make a space welcome to nebulous “people of color” without addressing issues specific to different communities.
This is not to say that people of color has no place in our lexicon anymore. There are times and places where it is the most accurate term—when discussing the need for diversity in the largely white publishing world, for example. But we cannot allow people of color to erase specificity for the sake of ease, to suggest that calling someone black is somehow impolite or to allow those uncomfortable with blackness to obscure their discomfort behind “progressivism.” There’s no question that the move toward the “people of/with x” formulation was meant to confer humanity onto those who have been dehumanized. But now, it’s increasingly apparent that the communities this linguistic shift was supposed to dignify might no longer be the primary beneficiaries of it. The pendulum of sensitivity feels like it’s swinging away from marginalized communities and toward the comfort of the powerful. That’s partially why Schultz’s word choice, whatever he actually meant, rankled so many. The only way to bring it back in the proper direction is to get rid of the hiding places in the language we use and to use the words we really mean.
Rachelle Hampton is a culture writer and reporter at Slate. Her work has appeared in the New Republic, Pacific Standard, Smithsonian Magazine, and In These Times. She still hasn't recovered from Tumblr's demise.
It is common practice to define pure colors in terms of the wavelengths of light as shown. This works well for spectral colors but it is found that many different combinations of light wavelengths can produce the same perception of color.
This progression from left to right is from long wavelength to short wavelength, and from low frequency to high frequency light. The wavelengths are commonly expressed in nanometers (1 nm = 10 -9 m). The visible spectrum is roughly from 700 nm (red end) to 400 nm (violet end). The letter I in the sequence above is for indigo - no longer commonly used as a color name. It is included above strictly for the reason of making the sequence easier to say as a mnemonic, like a person's name: Roy G. Biv - a tradition in the discussion of color. The University of Walkato posts a suggested set of characteristic wavelengths to associate with the colors.
The inherently distinguishable characteristics of color are hue, saturation, and brightness. Color measurement systems characterize colors in various parameters which relate to hue, saturation, and brightness. They include the subjective Munsell and Ostwald systems and the quantitative CIE color system.
White light, or nearly white light from the Sun, contains a continuous distribution of wavelengths. The light from the Sun is essentially that of a blackbody radiator at 5780 K. The wavelengths (spectral colors) of white light can be separated by a dispersive medium like a prism. Even more effective separation can be achieved with a diffraction grating.