PERRET OPTICIANS COLOR VISION

COLOR VISION

Three primary colours:

Of course, the use of three primary colours alone remains a good learning exercise. In this case, it is necessary to choose the red, blue and yellow which are the purest, eg. the red which is as far as possible mid way between a blue shade and yellow shade. This ensures the cleanest violets and the cleanest oranges when using only one red.

Theoretically, the three primaries are magenta, cyan and yellow. But, remember that each artists’ colour has a masstone and an undertone; that artists require a package of handling properties and that permanence is also important. The recommended primaries therefore offer the best practical mixing properties combined with permanence wherever possible.

ADDITIVE COLOUR MIXING: The mixing of coloured light is ADDITIVE, secondary colours are purer, ie. away from black. This is the opposite to what happens when artists’ colours are mixed and is the reason for much of the confusion regarding
colour mixing.

SUBTRACTIVE COLOUR MIXING:
The mixing of pigments is SUBTRACTIVE, secondary colours become less pure,
ie. towards black. This is the opposite to what happens when coloured light is mixed.

The Spectral Basis for Color

Visible light, ultraviolet light, x-rays, TV and radio waves, etc are all forms of electromagnetic energy which travels in waves. The wavelength of these waves is measured in a tiny unit called the Angstrom, equal to 1 ten billionth of a meter. Another unit sometimes used to measure wavelength of light waves is nanometers (nm) which are equal to 1 billionth of a meter.

The electromagnetic spectrum

There is a narrow range of this electromagnetic energy from the sun and other light sources which creates energy of wavelengths visible to humans. Each of these wavelengths, from approximately 4000 Angstroms to 7000 Angstroms, is associated with a particular color response. For example, the wavelengths near 4000 Angstroms (400 nm) are violet in color while those near 7000 (700 nm) are red.

The colors of the wavelengths of visible light

The CIE Color Model

Though some colors can be created by a single, pure wavelength, most colors are the result of a mixture of wavelengths. A French organization, the Commission International de L'Eclairage (CIE), worked in the first half of the 20th century developing a method for systematically measuring color in relation to the wavelengths they contain. This system became known as the CIE color model (or system). The model was originally developed based on the tristimulus theory of color perception. The theory is based on the fact that our eyes contain three different types of color receptors called cones. These three receptors respond differently to different wavelengths of visible light. This differential response of the three cones is measured in three variables X, Y, and Z in the CIE color model. This gives a three dimensional model which is then projected onto one plane to give a 2 dimensional graphic (See figure 3a). XYand Z are mapped to X and Y coordinates. This variation of the CIE model is seen below in Figure 3b.

In the CIE model - Z coordinates are projected onto the XY plane

The CIE color model mapped to X and Y coordinates

Notice, that the perimeter edge marks the wavelengths of visible light. Along this edge will be the 'pure' spectral light colors. Other colors are developed by mixing varying amounts of different wavelengths. Notice the purples at the bottom do not have a wavelength associated with them. These purples are non-spectral colors, that is they can only be seen by mixing wavelengths from the two ends of the spectrum. White light is perceived when all three cones are stimulated, like purple it is only seen when light from many different wavelengths is mixed.

Basic color perception theory

Although many people know that colors are related to wavelength, few people seem aware that what we call color is not a fundamental aspect of reality, but rather a description of the particular way our visual system works. There are two ways one could define color strictly in terms of physics. One way is to say that a color is light of a single wavelength. Another is to say that a color is a light spectrum, consisting of a combination of many different wavelengths in different intensities. What we call color is neither of these two things.

Color is not equal to single wavelengths, since that would only include the colors of the rainbow: violet, blue, green, yellow, orange and red (sometimes indigo is added between violet and blue and cyan between blue and green). But we see many more colors which do not exist in the rainbow, such as brown, pink, olive, black, grey and white. These non-rainbow colors are "fantasy colors" created by our visual system and arise from certain combinations of wavelengths.

Color is also not equal to a light spectrum, since we have only a very poor ability to discriminate different spectra. What we call a single color can be created by many different spectra. A spectrum is an infinite dimensional space, while color is only a three dimensional space. This is due to the fact that our color vision system works with three different types of light detectors (called "cones") each of which gives a single signal based on its wavelength sensitivity curve:

The curves in the first figure are normalized so that each has a peak of 1. Note the large degree of overlap between the curves. The rods, which are not show in the above figure are most sensitive to green and are primarily active during night vision. They produce a black and white image sensation. The curves in the second curve are based on imaginary primaries with negative light components each of which would stimulate only a single cone. This transformation is performed for purposes of linearity.

One type of cone is primarily sensitive to short wavelengths (blue), another to medium wavelengths (green) and one to long wavelengths (yellow). The yellow cone is usually referred to as the red cone. While its sensitivity peak lies in the yellow wavelength band, it is also quite sensitive to red. The center of vision, the fovea, contains no rods and no blue cones. A single cone cannot detect color, as it provides only a scalar number indicating the total light energy it absorbs. For example, the red cone by itself cannot distinguish red from yellow, green or orange. Red is detected by a combination of high activation of the red cone, low activation of the green cone and no activation of the blue cone. That is why a color monitor can generate most colors by a combination of red, green and blue light. All humans can see in terms of color is a relative activation strength of its three types of light sensitive cones. This means that many complex spectra of light can only be differentiated by a spectrometer, such as a prism, while we cannot distinguish them with the eye. For example, a human sees no difference between a flat spectrum (such as sun light) and a graphic on a computer monitor whose spectrum consists of three spikes in the colors blue, green and red. Both are called white. Similarly, we cannot distinguish spectral yellow from a particularly balanced combination of red and green light. Additive color mixing, such as with a computer monitor, is something different than subtractive color mixing, as with paints. A computer monitor makes yellow by adding green and red light. A painter gets green, for example, by mixing cyan (blue-green) with yellow, because the cyan absorbs the red part of the spectrum, while the yellow absorbs the blue part of the spectrum, leaving green when both are absorbed by a mixture of both paints.

Colorblindness

The term colorblindness suggests that "colorblind" individuals cannot see certain colors. A more accurate description would be that they cannot distinguish certain colors. All "colorblind" individuals see every color, except for some people who cannot see red in the sense that red appears black to them. Blindness only refers to the fact that two colors can look the same which appear as different colors to a person with normal color vision.

The following table shows the prevalence of different types of inherited color vision:

Main type	Sub type	Defective color system	Prevalence caucasian men	Prevalence women
Trichromatic	Normal	-	92% (88% European men, 94% Asian men, 96% African men)	99.6%
	Protanomalous	Red	1%	0.01%
	Deuteranomalous	green	5%	0.25%
	Tritanomalous	Blue-yellow	Trace	Trace
Dichromatic	Protanopia	Red	1%	0.01%
	Deuteranopia	green	1%	0.01%
	Tritanopia	Blue	0.002%	0.002%
Monochromatic	Atypical (incomplete) achromasy (single cone vision)	Red-green, blue-yellow	0.000001%	0.000001%
Monochromatic	Typical achromasy (limited rod-based vision)	Red-green, blue-yellow	0.003%	0.003%

The cause of the disproportionately high numbers of males affected compared to females is that colour blindness is a X chromosome sex linked recessive disorder. This means that the loci for the most frequent types of colour blindness are on the X chromosome. The normal ability to see colours depends on several genes, X linked and autosomal (Rothwell 1993). Being a recessive trait, the presence of an other X chromosome that is not carrying the recessive i.e. the dominant allele for colour vision, will mask the effect of the recessive. The presence of the second X chromosome is the normal homozygous genotype of females therefore explaining the lower incidence, as two recessives must be present for the phenotype to be exhibited.

Normal color Vision

Monochromate Vision

Anomalous Trichromatic :

Protanomalous Vision (Anomalous Trichromatic) Red system defectif

Deuteranomalous Vision (Anomalous Trichromatic) Green system defectif

Triteranomalie Vision (Anomalous Trichromatic) Green system defectif

Bichromatic:

Protanopia Vision entire defectivity of the Red system

Deuteranopia Vision entire defectivity of the Green system

Tritanopia Vision entire defectivity of the Blue system

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What numbers do you see revealed in the patterns of dots below?

This is an Ishihara plate commonly used to check for red/green color blindness

Lenses for learning

A few opinions and statments:

"To my knowledge, congenital (you're born with it) colorblindness is permanent and there is no cure or treatment. Tinted lenses, reportedly for colorblindness, have been recently introduced. For further information on two of these new lenses, you can visit their promotional websites at ColorMax Lenses and Solarchromic Lenses. The X-chrome Contact Lens (a red tinted monocular contact lens for colorblindness) has been on the market for decades. Yes, these lenses will help you pass some color vision tests, but so would looking through "any" red glass lens or piece of red tinted cellophane. It is unlikely that an examiner (e.g. Federal Aviation Administration) would let you use any of these devices to take their color vision test. Would these new lenses let you identify the red and green lights on an airplane at night or help you correctly identify different colored wires? I do not know. I have not read any credible scientific studies which validate these devices from a practical stand point. Personally, for me the jury is still out on the true benefits of these new devises."

"An exciting part of developing the Solaz color enhancing lenses has been the discovery they can aid color deficient subjects in the discrimination of red and green. The Ishihara and Farnsworth test results suggest that the laser dye lenses can help three out of four of colorblind persons. Additionally, the subjective responses and observations by this group in outdoor settings have been quite positive. It is very important to use a quartz halogen light, when taking one or both of the above mentioned tests, so as to best duplicate the spectral qualities of sunlight. A cool white type of fluorescent light used for illumination will not produce beneficial results in most cases, and therefore should not be used. "

"ColorMax® Color Vision Enhancement Lenses are the first and only lenses clinically proven to improve color vision discrimination in colorblind individuals. This unique technology alters the spectral energy composition of the retinal stimulus and adjusts the deficiencies of brightness, hue and saturation to balance color vision. ColorMax® Lenses are now available in 10 different types depending upon the degree of your red-green color vision deficiency."

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http://directorylasik.com/articles/understanding-color-blindness.html

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http://imagers.gsfc.nasa.gov/ems/ems.html

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http://imagine.gsfc.nasa.gov/docs/ask_astro/xrays.html
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http://www.acm.org/sigchi/chi96/proceedings/papers/Douglas/sad_txt.htm