Color Perception and Deficiencies
A person with normal color perception equivalent to the CIE standard observer is called a normal trichromat, that is, a person with average red, green, and blue color perception. A person with defective or variant color vision is missing one of these three color sensitivities. Color deficient persons can be further subdivided into protans (red deficient), deutans (green deficient), and tritans (blue deficient). The more familiar term, color blind, is a misnomer, as few people perceive no color at all.
Dichromats are those persons with only two of the three color sensitivities. The most common dichromatic deficiency (found in roughly eight percent of Caucasian males and one percent of females)1 is an inability to discriminate between colors in the red-green region of the spectrum. The inability to discriminate between colors in the blue-green spectral region also occurs but is rare.
In addition to those persons with a deficient or missing color sensitivity, there are those people with anomalous color vision such that they have a red, green, and blue visual perception which is greater or less than that of the general population. For example, people with anomalous trichromatic vision may have a red color perception that is lower than normal. They still perceive red, but not to the degree of people with normal trichromatic vision. Anomalous trichromats are not easy to diagnose, as they often have developed adaptive visual techniques to compensate for their perceptual imbalance. In making judgments on fine color differences for some colors, their ratings may consistently differ from those with normal color vision.
Color discrimination is independent of color deficiency, so it is possible for some color normal persons to have poor color discrimination. Likewise, those with variant color vision can have good or poor color discrimination ability.
1 Jeremy Nathans, “The Genes for Color Vision,” Scientific American, (February 1989: 42-49).
2 Dean Farnsworth, “The Farnsworth-Munsell 100 Hue and Dichotomous Tests for Color Vision,” Opt. Soc. Am., 33 (1943: 568).
Color Vision Tests
Tests have been developed for diagnosing color deficiencies and for assessing color discrimination ability. Each of these tests was designed to examine a specific characteristic of color vision and the results are valid only if the test is conducted correctly. For a complete description of the color vision tests and their validation, there is a government document produced by National Research Council3 entitled Procedures for Testing Color Vision that is recommended reading.
Color Deficiency Tests
Pseudo-isochromatic plates, color vision lanterns, and anomaloscopes isolate certain factors indicative of color deficiency. The Dvorine Pseudo-Isochromatic Plates, the Farnsworth Dichotomous Test, the HRR Pseudoisochromatic Plates, and the Ishihara Color Plates Test are simple tests used for detecting color deficiencies based on differentiating a set of figure-ground relationships. For example, a red-green normal observer would see the number 48 in the picture below where the background is green and the number is red. A red-green deficient individual may not be able to discern the number.
The color deficiency tests described below are readily available.
1. Dvorine Pseudo-Isochromatic Plates
2. Farnsworth Dichotomous
3. HRR Pseudoisochromatic Plates and Ishihara Plates
Color Discrimination Tests
Color discrimination tests attempt to distinguish between normal and color anomalous observers, and assess the degree of the individual’s color discrimination ability through tests involving the ranking of color chips. The following color discrimination tests are readily available.
1. The Farnsworth-Munsell 100-Hue Test separates observers with normal color vision into classes of superior, average, and low color discrimination ability. It also detects and measures the color confusion zones of observers with defective color vision. The F-M 100-Hue Test was not intended to distinguish fine degrees of differences between persons of superior color vision or to divide those persons with color deficiency into pass and fail classes (see the Dvorine Pseudo-Isochromatic Plates Test).
Source 1: Munsell Color
Source 2: The Psychological Corporation
Discrimination of Color Difference
The HVC Color Vision Skill Test was designed to assess the ability of individuals to discriminate between samples having small color differences in hue, value and chroma. The HVC Color Vision Skill Test4 attempts to go beyond testing for the presence of color deficiency or color anomaly to focus on the natural ability of an individual to discriminate between samples having small differences in hue, value, and chroma. The discrimination of color differences is similar to the task of most color QA applications. The size of these color differences falls at or below the commonly accepted color difference tolerances for most industries. Older tests tended to use either larger differences in hue only (no variations in value or chroma) or smaller differences (similar to HVC) in chroma only. The test consists of thirty-six mounted chips (nine chips each for blue, red, green, and yellow). The thirty-six similar loose chips, presented in random order, must be compared in turn and the color matching decision made as to the nature of the difference.
Source: Lou Graham & Associates
(See attached pdf file for the complete article with addresses and illustrations)