"Secret of Ishihara"
Hidden digit plates of Ishihara color blind test
Description
The hidden digit plates of Ishihara color blind test, that normal color vision observer don't see! Only individuals with color blindness could recognize the figure. The more severe color blindness, the more frequently read these plates.
The fourth type of plate design is the hidden-digit plate. Hidden-digit plates are designed so that dichromats, but not normal observers, can see the intended figure. Other plate’s colors for figure and background are separated by large color differences. This is not so in the hidden-digit plate, in which the use of three different colors and small variations in saturation prevent normal observers from seeing a figure but allow observers with red-green color defects to do so. The latter perceive two color groupings that are distinct enough from each other to follow two separate isochromatic lines: the more saturated orange, khaki, and yellow-green dots form the background, and the less saturated pinks, grays, and greens form the figure.
The ability to read the hidden-digit plates (itself an error) depends on the degree of red-green defect; those whose color blind defects are more severe read these plates more frequently. The ability of normal observers to read them seems to be a function of age. About half of the normal subjects between 20 and 30 years of age read hidden-digit plates easily, but these plates are hardly ever read by subjects over 50 or by young children.
(Source: Procedures for Testing Color Vision: Report of Working Group 41., National Research Council (US) Committee on Vision: National Academies Press (US); 1981.)
Colorimetric data for “hidden-digit” plates from Ishihara.
Hidden digit plates
The hidden digit plates were made visible by image processing.
The first image (number 2) seen by a protanop color blind person:
