Color as we humans perceive it, can be described in terms of hue (determined by the wavelength) and saturation (determined by the amount of white light mixed in it).

How do we perceive color?

The color of objects is a complex property that depends on the wavelengths of incidental light, its absorption (by the object) and its reflection (off the object’s surface). In the photosensitive part of the eye we have two types of specialized cells – the rods and the cones. The cones are responsible for perception of color. The rods are for the perception of intensity of light.

Are there different types of cones?

Cones are of only one type. Based on the trichromatic theory of light (Young 1802) cones contain 3 types of pigment - red-sensitive, green-sensitive and blue-sensitive. We can thus use a combination of these 3 primary hues to match any color.

A4L4 = A1L1 + A2L2 + A3L3 (where Arepresents the intensity and Lrepresents the wavelength – A4L4 is the color we want to derive from the 3 primary colors 1, 2, and 3)

How is color vision tested?

There are a variety of clinical tests of color vision. (Click here to take a rapid 6 plate Ishihara test)

The Anomaloscope based on the observation that, within a narrow range, any two normal observers will accept each other's formula for matching a color. For example the subject may be asked to view a yellow light in a split screen and asked to match it mixing red and green light. The amount of red-green light used to match this yellow light is characteristic for a normal person and a red-green deficient person.

Pseudo-isochromatic plates of Ishihara and Hardy-Rand-Rittler - depict colored numbers or figures that stand out from a background of dots. The colors of the test figure and background are purposefully pale and are carefully chosen from hues that are difficult for a color-deficient subject to distinguish.

Thus, individuals with defective color vision see either no pattern at all or an alternate pattern based on brightness rather than hue. These tests have the advantage of speed and ease at screening color-deficient people but are marginal at classifying a deficiency. Not only that it must be realized that they are only valid in controlled light (specified by the manufacturer) as incident light influences the reflected spectrum. Also if the plates are too old there is some amount of "fading".

The Farnsworth D-15 and 100-hue tests are more accurate in classifying color deficiency because the subject's errors can be plotted to define an "axis" of color deficiency. The 100-hue test actually consists of 85 tablets over a 4 part color spectrum to be arranged in a color sequence. The D-15 panel is a more rapid and convenient test for clinical use as it consists of only a single box of 15 colored tablets. The D-15 panel is used to evaluate most patients with retinal disease.

What is color-blindness?

The term color-blindness is unfortunate because it implies an inability to see colors. Most individuals with color-deficiency can distinguish a great many colors, but interpret them differently. Color-deficiencies may be of 2 types - Primary (or congenital - since birth) and Secondary (or acquired - due to a disease of the macula, retina or optic nerve).

How are color-deficiencies classified?

Primary Color deficiencies may be classified thus

Monochromatism and Achromatopsia	Monochromats have only one cone pigment instead of the 3 primary color based pigments. Achromats have only rods and no cones and are truly color-blind. This form is extremely rare. Such people are able to distinguish objects only by brightness and usually have very poor vision due to the lack of cones.
Anomalous Trichromacy (most common) - the affected person has all 3 (thus tri-chrom) cone pigments but one is abnormal.	Protanomaly. A Protanomal has abnormal red-sensitive cones and requires excess red to match the yellow standard. Deuteranomaly. A Deuteranomal has abnormal green-sensitive pigment and needs excess green to match the yellow standard. Tritanomaly. Very rare. Abnormal blue-sensitive cones. Has difficulty in distinguishing blues from yellows and my use excessive blue to match the yellow standard.
Anomalous Dichromacy - the affected person has only 2 cone pigments and thus cannot distinguish certain colors.	Protanopia - lack of red-sensitive cone pigment. Deuteranopia - lack of green-sensitive cone pigment. Tritanopia. Very-very rare. Lack of blue-sensitive pigment. Can't distinguish blue from yellow. Usually an inherited syndrome with optic atrophy.

How common is color-deficiency?

Absolute color blindness is extremely uncommon. However protanopia and deuteranopia are X-linked and affect about 1% of the male population.

How come females are never color-deficient?

Congenital (or inherited) color deficiency is X-linked, since females have 2 X chromosomes, they are spared. Females however can suffer from acquired color-deficiencies.

Is there any treatment/visual aid for the color deficient?

Unfortunately none yet. But the way science and micro-electronics are headed there may soon be in the form of maybe cone replacement by chips that can identify and transmit colors in a manner the brain can understand.

Does a color deficient person need to take any precautions?

There are certain professions in which color vision is critical for the safety of self and others. Fortunately such hopefuls are scanned for color vision deficiencies beforehand. A protanopic may miss a red signal completely (he will see it in a grey tint) and my endanger himself and others. Railways, airways and armies have their own rules of recruitment and you may find a job therein that does not require such perfect color perception.