Colour Perception

The Ishihara Plate Test

• The Ishihara plates are used as a screening test for red/green color deficiency  throughout the world  & they have been shown to be efficient for that purpose.

• The plates do  not diagnose the type or severity of color deficiency, but simply identify a subject as normal or as having red / green deficiency.

• Ishihara plates consist of a series of color-defined numbers embedded within different colored dots. The plates are designed so that grouping of dots by color causes a number to emerge from the background that can be recognized correctly by people with normal color vision, but in the absence of normal color signals all the dots appear ’falsely of the same color’ (pseudoisochromatic).

• Therefore, color-deficient observers either fail to see the number altogether or make mistakes in recognizing it correctly

• The Ishihara plate test consists of either 24 or 38 colour plates. It is the most widely used screening test for red-green deficiency and has been shown to be the most efficient test for this purpose. A very general indication of protan and deutan defects is given in this test, but it is not, as such, a diagnostic test. The test does not screen for blue tritan defects and is unsuitable for testing acquired defects.

• Colour vision may be tested with any of the standard pseudoisochromatic test plate sets. Appropriate lighting must be provided for testing.

• If a special colour balanced light source is not used, daylight is best for screening. Fluorescent or incandescent lights may cause inaccurate readings.

• The type of plates (Pseudo-isochromatic, Ishihara etc.), the number of plates in the set (versus the number that should be used for testing) and the number of errors should be noted. Each plate should be held approximately 75 cm. in front of the applicant with the plate perpendicular to the visual line.

• A delay of up to three seconds is allowed for the answer to each plate and it is permissible to repeat a plate if the patient has a negative response. If two responses are given, the second should be recorded. The plates should be given in a random order so they cannot be memorized.

The result interpretation is as follows

Colour Perception Normal

• The numbers on all plates from no.  1/17 and 22-25 should be read correctly and any number on plates 18-21 should not be misread as they do not have any number.

Colour Perception Defective Safe

• Plates 22 to 25 are read correctly (one figure may be clear than the other) and some of the plates are misread as follows:

Colour Perception Defective Unsafe

• The individual is unable to read even plates 2-9 and 22-25. 

• Individuals who need confirmation of the colour blindness status need to undergo the Lantern Test to identify signal colours, Red, Green & White colour light.

Martin Lantern Test

• This test is very sensitive when properly carried out it detects anomalous trichromates who are likely to pass even on Ishihara at times. Such cases appear normal at most times and are rarely aware of their deficiency, the defect becoming manifest under unfavorable conditions. The room is to be completely darkened.

• It is not necessary to dark adapt the individual at this stage, but should it become apparent in the preliminary stages that there is a defect of colour perception; he is to be dark adapted for 15 minutes.

• The candidate should wear his glasses if his “form or basic vision” is defective. Care should be taken that the correcting glasses are not coloured because red coloured glasses are known to give a decided advantage to the individual in the test.

•  Using the large aperture, a single white light will be shown to the candidate who will be asked to name the colour.

• If he replies correctly the examiner need not speak to the candidate throughout the remainder of the examination.

• Should the candidate’s answer be incorrect, he will be told that the light is white (or clear). 

• Thereafter the examiner need not speak to the candidate until the examination is completed. 

• The candidate will be seated at a distance of 1.5 meters from the Martin Lantern and will be asked to name colours, presented singly with the large aperture, to satisfy the examiner that he can recognize correctly without guessing, signal red, signal green and white. 

• A candidate who passes the test will be assessed Colour Perception Defective Safe and, if he fails, Colour Perception Defective Unsafe. 

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