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Tests for color vision deficiency
Views:
Vision test examples.
Normal people should see A=8, B=5, C=29, and D=74. Color deficient people may see A=3, B=2, C=70, or D=21.
The so-called pseudo-isochromatic plates use printed dots of different chromaticity to produce patterns that can be identified as numbers, letters or shapes. By choosing chromaticities that are confused by red-green dichromats, patterns can be produced that are visible to color normals, but invisible to dichromats, or that appear different to color normals and dichromats. The first examples of such plates were those by Stilling (1st edition, 1875), but the more popular set of plates for color vision testing are those by Ishihara (1st edition, 1917).
Note the colors of your screen are probably different from those in calibrated test books. If you suspect that you are color deficient, we suggest that you have your color vision professionally evaluated.
How color vision testing works
Step 1: A color vision test pattern for testing red/green deficiency starts from a random pattern of gray level dots.
Views:
Step 2: We now add a digit pattern which is defined by yellow/blue variation only. This pattern can be detected by any observer which is able to do yellow/blue discrimination. Since most people with red/green deficiency will be able to do yellow/blue discrimination they will be able to see the digit 5 in this test pattern.
Step 3: Now we add another digit pattern which is defined by red/green variation and is easier to see than the pattern defined by yellow/blue variation. Here is the pattern composed of the random brightness pattern and the red/green pattern:
Step 4: Finally we add all three components: the random brightness pattern, the yellow/blue pattern, and the red/green pattern. Observers with red/green deficiency will not be able to see the red/green pattern and base their response on the yellow/blue pattern only. Normal observers will see both the yellow/blue and the red/green pattern. Since the red/green pattern is stronger than the yellow/blue pattern the normal subject will base his or her judgment on the red/green pattern and see the digit 6.
