The Ishihara test is a widely recognized screening tool used to assess color vision. Developed by Japanese ophthalmologist Shinobu Ishihara in 1917, this test provides a quick and straightforward method for identifying certain color vision deficiencies. Its primary function is to serve as an initial assessment, helping to determine if an individual perceives colors normally or has a common type of color vision impairment.
What the Ishihara Test Identifies
The Ishihara test identifies red-green color vision deficiencies, the most common forms of color blindness. These conditions arise from issues with cone cells in the retina, responsible for color perception. Red-green deficiencies include protanomaly and protanopia (difficulty with red hues), and deuteranomaly and deuteranopia (issues with green hues). For instance, protanopia may cause red to appear black or dark gray, making it hard to differentiate reds, oranges, yellows, and greens. Deuteranopia can make red colors look brownish-yellow and green appear dark yellow.
The test does not detect blue-yellow color vision deficiencies, such as tritanomaly or tritanopia, which are much rarer. It also cannot identify total color blindness, known as achromatopsia, a condition where individuals see only in shades of gray.
How the Test Works
The Ishihara test uses pseudoisochromatic plates, circular patterns of colored dots. These plates embed numbers or pathways within a mosaic of dots varying in color and size. Normal vision allows clear distinction of the figure from the background. However, those with red-green color deficiencies find these figures difficult or impossible to discern, as colors blend together.
A standard Ishihara test typically consists of 38 plates, though shorter versions (10, 14, or 24 plates) are available. The first plate is a demonstration, showing a number like “12” visible to everyone, explaining the process. Other plate types include:
Transformation plates: Individuals with color vision deficiency see a different figure than those with normal vision.
Vanishing plates: Only those with normal vision can recognize the figure.
Hidden digit plates: Visible only to individuals with color vision deficiencies.
During the test, plates are held about 75 centimeters (30 inches) from the subject, at a right angle to their line of sight. Proper lighting is important for accurate results; a “daylight” bulb (6000-7000 Kelvin, ideally 6500K, with a CRI greater than 90) is recommended. Each plate should be viewed for no more than three seconds to prevent guessing or memorization. Subjects should not be coached or allowed to touch the plates.
Understanding Your Results
Interpreting Ishihara test results involves assessing how many and which plates an individual correctly identifies. A “normal” reading means the person correctly identifies all or most numbers or paths. For instance, in some versions, correctly reading 10 or more out of the first 11 plates indicates normal color vision.
An “abnormal” reading signifies a red-green color deficiency, indicated by difficulty or inability to see certain numbers, or by seeing different numbers than those intended for normal vision. For example, on a plate where a person with normal vision sees “74,” someone with red-green color blindness might see “21” or nothing. While the test indicates the presence and type of red-green deficiency, it does not quantify its severity. A comprehensive diagnosis and understanding of the deficiency’s extent should always be sought from a qualified eye care professional.
Beyond the Ishihara Test
While the Ishihara test is an effective initial screening tool for red-green color vision deficiencies, it is often supplemented by other tests for a more comprehensive diagnosis or to assess the severity of the condition. One such test is the Farnsworth D-15, which requires individuals to arrange colored caps in a specific order based on hue. This test can help differentiate between protan, deutan, and tritan defects and provides insights into the nature of color confusions.
Another accurate diagnostic tool is the anomaloscope. This instrument involves matching a mixture of red and green light to a yellow light, allowing an eye care professional to evaluate color perception more precisely. Further testing beyond the Ishihara may be necessary for individuals pursuing occupations where accurate color discrimination is important, such as pilots, electricians, or graphic designers. Additionally, if acquired color vision loss is suspected, perhaps due to an underlying eye disease, more in-depth evaluations would be conducted. The Ishihara test is a valuable first step, but it is part of a broader array of diagnostic tools.