The Ishihara test is a globally recognized screening tool designed to detect color vision deficiencies, most notably the common form of red-green color blindness. Japanese ophthalmologist Dr. Shinobu Ishihara first published this series of color plates in 1917, providing a simple yet highly effective method for assessment. The test quickly distinguishes between individuals who possess normal color perception and those who exhibit an inherited red-green color vision defect. It utilizes a unique visual mechanism that exploits the specific color confusions experienced by the color-deficient eye.
How the Plates Create Visual Illusions
The scientific principle behind the test relies on the use of “pseudo-isochromatic plates,” meaning the colors appear the same to some viewers but different to others. Each circular plate features a mosaic of dots varying in size, hue, and brightness, with a figure or number embedded within the pattern. For a person with normal color vision, the dots forming the figure have a chromaticity that clearly contrasts with the background dots, allowing the number to be seen easily.
The colors are specifically selected to fall along “confusion lines” in the color space. These are lines on a color diagram where colors appear identical to an individual with a specific color vision deficiency. Because the figure and background dots share a color axis confused by the deficient eye, the embedded number visually blends into the surrounding dots, creating an illusion of a uniform field. The plates also employ dots of varying luminosity to prevent the color-deficient person from distinguishing the number based solely on light intensity.
Identifying Specific Color Vision Deficiencies
The Ishihara test is engineered to target deficiencies related to the red and green photoreceptor cones, which are the most common inherited forms of color vision defects. These deficiencies fall into two main categories: protan defects, relating to long-wavelength (red-sensitive) cones, and deutan defects, involving medium-wavelength (green-sensitive) cones. Protanopia refers to total red blindness, while deuteranopia is total green blindness; milder forms are called protanomaly and deuteranomaly, respectively.
Certain diagnostic plates are included to differentiate between these two types of red-green defects. For example, a plate might be designed so that a person with a protan defect sees one number, while a person with a deutan defect sees a completely different number. The distinction is possible because the perception of brightness is also affected differently. Individuals with protanopia experience a darkening or dimming of the red end of the color spectrum because their red cones are less effective, a phenomenon not observed in those with deuteranopia.
Administering and Scoring the Test
To ensure reliable results, the Ishihara test must be administered under standardized viewing conditions. The test requires consistent, high-quality illumination, typically provided by a light source that simulates natural daylight (such as CIE Standard Illuminant C or D65). The plates are usually held approximately 75 centimeters (30 inches) from the viewer, who should not be wearing any colored lenses that could alter color perception.
The viewer is asked to identify the number or trace the path on each plate, with a strict time limit of three to five seconds to prevent memorization. A standardized scoring system determines the presence of a deficiency. For instance, in the 38-plate edition, reading 17 or more of the initial 21 plates correctly indicates normal color vision. If a person fails to correctly read the primary screening plates, a red-green deficiency is confirmed, and subsequent diagnostic plates determine the specific type and degree of the color vision defect.