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Aniseikonia

Color vision defects

Color vision testing

About color vision testing^1,2

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Introduction

The purpose of color vision testing is to identify, classify, or grade a color vision deficiency.  The reasons for looking into someone’s color vision capabilities may be to evaluate fitness for a certain occupation (congenital and acquired color deficiencies) or as a diagnostic aid (acquired color deficiency).

 

Different tests are usually designed to perform a different function.  The general notion is that no one color vision test is all-fulfilling.  If a complete and accurate diagnosis is required, it is necessary to use a battery of tests.

 

The gold standard in color vision testing are the anomaloscopes.  These devices are based on the principle of color matching.  For example, a mixture of red and green is matched to a yellow color.  Depending on the mixture ratio of  the red and green, a patient can be diagnosed in type and severity.  Also a distinction between anomalous trichromats and dichromat can be made.  Anomaloscopes are relatively costly and require a skilled and knowledgeable operator.  Therefore. they are rarely found outside research and teaching institutes.
 

Screening (identification)

In the screening process the question is simply if there is a color deficiency present or not.  Since the prevalence of protan and deutan defects is by far the highest in congenital color deficiencies, most screening color vision tests only identify these red-green deficiencies.  A screening test for acquired color vision deficiencies would have to be designed to detect both red-green as well as blue-yellow deficiencies.

 

Screening of color vision deficiencies is usually done with so called pseudoisochromatic plates of which the Ishihara test probably is the most well-known.  In these kind of tests, the subject has to identify an object (e.g. numeral, letter, or symbol) of a certain color(s) in a background of another color(s).  A color deficient subject will fail the test if the colors of the object and background lie on a corresponding confusion line and the colors are close enough together.  Contour and luminance cues are usually suppressed by using a (random) dot diagram and an equal average object and background luminance.

 

 

Type diagnosis (classification)

The aim of type diagnosis is to distinguish between protan, deutan, and tritan defects.  For occupational fitness purposes, it might be especially important to detect the difference between protan and deutan defects.  That is, protans have the additional handicap that red colors are not detected as well because of the reduced sensitivity on the red end of the spectrum.  In acquired color vision deficiencies it might be important to differentiate between the red-green and ‘tritan’ defects because of the possible underlying condition (see Table 4). 
            Besides using an anomaloscope, diagnosing the type of color deficiency can be done with specially designed pseudoisochromatic plates or with arrangement tests (e.g. Farnsworth panel D15 and the Lanthony desaturated D15).  In arrangement tests, the subject is offered a series of colors that need to be sorted either into a sequence (usually based on hue) or into groups (most often grays versus colors).  The advantage of arrangement tests is that they are less designed for specific confusion lines, so they may classify all congenital types (protan, deutan, and tritan).  Furthermore they are particularly useful for the examination of acquired color deficiency because non-specific defects can be identified and changes with time recorded.

 

 

Severity diagnosis (grading)

The severity of an anomalous trichromat may be graded as mild, medium, and strong.  Doing a severity diagnosis may be especially important in congenital color vision deficiencies.  That is, in certain professions a mild defect may be completely acceptable, while a strong defect would be unacceptable.
            Grading can again be done with an anomaloscope, specially designed pseudoisochromatic plates, or arrangement tests.  With an arrangement test like the Farnsworth panel D15, a distinction between medium and strong defects may be made based on the number of crossings.  If a subject passes the Farnsworth panel D15 test, but fails the Lanthony desaturated (which has smaller color differences), this subject is likely to have a mild defect.
 

 

References

1.  Birch J, (1993). Diagnosis of Defective Colour Vision. Oxford University Press. Oxford.
2.  Dain SJ, (2004).  Clinical colour vision tests.  Clinical and Experimental Optometry, 87, 276-293

 

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