Printed pseudoisochromatic plates are the most widely used type of color vision test to screen for color vision deficiency. The principle is that the color of a target (digit or letter) embedded in a background of another color appears “falsely of the same color” to color-deficient people. Target and background chromacities are chosen carefully to be the ones confused by people with color vision deficiency.
Design of Pseudoisochromatic Plates
Pseudoisochromatic plates must be carefully designed in order to be effective. As a result of the printing process, minor misalignments between a figure of one chromaticity and the uniform background of another undistinguishable chromaticity may reveal the shape of the digits or letters. In addition, the relative luminous efficiency of the eye varies considerably within normal trichromats and even more so in color-deficient observers. Hence, a figure and background that are equally light for one color defective are not necessarily so for another. The first pseudoisochromatic plates, introduced in Germany by Stilling in 1877, addressed these issues by breaking up the test target and background into a number of discrete patches each with its own shape and contour, and the luminance of the individual patches was varied randomly rather than being equated. This ensured (as much as possible) that neither edge artifacts nor luminance differences could be used to discriminate the target from the background; thus, the target can be detected only by color discrimination.
Stilling’s pseudoisochromatic plates are all of “vanishing” design, so called because the target is read correctly by the normal trichromatic observer and not seen by the color defective (i.e., the “vanishing” figure is pseudoisochromatic to the color defective). A later variant was the “transformation” plate, where two figures are embedded in the background, one that is read by the normal observer and another that has the appropriate chromatic and lightness contrast to be read by the color-deficient observer. The Japanese ophthalmologist Ishihara published in 1917 the first edition of a set of plates that included vanishing and transformation plates, as well as hidden plates, in which the color defective can see a digit that is camouflaged for the normal by random color variation. Another design is the diagnostic or classification plate which is a more sophisticated version of the vanishing design plate that allows for differentiating between the type of red-green color deficiency, i.e., protan or deutan. The Ishihara plates are not designed for examining yellow-blue deficiencies, i.e., tritan deficiency. In 1954, the AO-HRR (American Optical – Hardy, Rand, and Rittler) plates were printed and comprise both screening and diagnostic plates for tritan defects, as well as diagnostic and grading plates for protan and deutan defects. The plates have vanishing designs containing geometric shapes (circle, cross, and triangle) that are printed in neutral colors on a background matrix of gray dots. The saturation of the neutral colors increases in successive plates to produce designs with progressively larger color difference steps identifying different levels of deficiency.
The Ishihara and the AO-HRR pseudoisochromatic plates are often used together because their functions are complementary. The Ishihara plates are used for screening for red-green color deficiency and the AO-HRR plates to confirm protan/deutan classification, estimate the severity of red-green deficiency, and identify tritans.
Some pseudoisochromatic tests have designs containing shapes or “pathways” for the examination of nonverbal subjects and young children.
Administration of Pseudoisochromatic Plates
The majority of pseudoisochromatic tests have been designed to be viewed at 60–70 cms and standardized for natural daylight illumination (or for CIE standard illuminant C). The examiner turns the pages and controls the viewing time. An introductory plate is often included to demonstrate the visual task.
Examples of Pseudoisochromatic Plates
A number of other pseudoisochromatic tests have been produced, but none have been used as widely as the Ishihara and the AO-HRR plates. Tests have been published in Japan, e.g., the Ohkuma plates (1973), Tokyo Medical College test (1957), and Standard Pseudoisochromatic Plates (1st and 2nd editions) and in the USA, e.g., the Dvorine plates; in Germany, e.g., the Velhagen-Broschmann plates (29th edition published in 1992); in Sweden, e.g., the Bostrom-Kugelberg plates (1972); and in France, e.g., the Lanthony Tritan Album, among many others. Despite many contenders, the pseudoisochromatic plates of Ishihara remain today the dominant instrument for routine screening of color vision.