The Ishihara Test Explained
The Ishihara test is the most widely used colour vision screening test in the world. Named after its creator, Dr. Shinobu Ishihara, it has been the standard tool for detecting red-green colour blindness since 1917. Despite being over a century old, it remains highly effective, inexpensive, and fast to administer.
History
Dr. Shinobu Ishihara (1879–1963) was a Japanese ophthalmologist who developed the test during his time at the Tokyo Army Medical School. The original purpose was to screen Japanese military recruits for colour vision deficiency. The test was first published in 1917 and has since been updated in several editions, with the modern version comprising 38 plates.
The test has seen remarkably little fundamental change since its creation — a testament to its effectiveness. It is now used globally in ophthalmology clinics, military screening, occupational health assessments, and school health checks.
How Does It Work?
The Ishihara test uses pseudo-isochromatic plates — a design principle based on the science of colour confusion in colour vision deficiency.
The Science Behind the Plates
Each plate consists of a large circle filled with hundreds of small dots varying in:
- Size — varied randomly to prevent edge detection as a cue
- Colour — from carefully chosen hue sets that exploit colour confusion
- Brightness (luminance) — carefully matched between figure and background
The dots form a pattern — typically a number or path — that is visible only when the viewer can distinguish the specific colours used. For a person with red-green CVD:
- The figure dots and background dots appear to have the same colour and brightness
- The number becomes invisible (or a different number becomes visible instead)
Types of Plates
| Plate Type | What Normal Vision Sees | What CVD Sees |
|---|---|---|
| Transformation | Number A | Number B |
| Vanishing digit | A number | Nothing (blank) |
| Hidden digit | Nothing | A number |
| Classification | One set of digits | A different set (distinguishes protan from deutan) |
Control plates — which everyone should be able to read — are included at the start to verify the test is being administered and interpreted correctly.
The 38-Plate vs. 24-Plate vs. 14-Plate Editions
The Ishihara test exists in several editions:
- 38-plate edition — the full diagnostic set; includes all plate types and classification plates
- 24-plate edition — the most commonly used clinical version
- 14-plate edition — a quick screening version for time-limited settings
Most online Ishihara tests, including ours, use a representative selection of plates covering all four plate types. The ColorVision Ishihara Test uses 24 carefully selected plates that cover all diagnostic categories.
What Does the Ishihara Test Detect?
The Ishihara test is specifically designed to detect red-green colour vision deficiency (protanopia, protanomaly, deuteranopia, and deuteranomaly). It is not designed to detect:
- Tritanopia (blue-yellow deficiency)
- Achromatopsia (complete colour blindness)
- Acquired colour vision changes from disease or medication
For a comprehensive assessment of all colour vision types, the Farnsworth-Munsell 100 Hue Test provides a broader evaluation.
How to Interpret Results
Results are scored by counting the number of plates correctly identified:
- 17–24 correct (out of 24 non-control plates): Normal colour vision
- 13–16 correct: Possible mild colour vision deficiency
- Under 13 correct: Colour vision deficiency likely
For online tests, these thresholds are approximate. Factors that affect accuracy include screen calibration, ambient lighting, and the user’s distance from the screen.
Limitations of Online Ishihara Tests
Online Ishihara tests provide useful screening information but have inherent limitations:
- Screen calibration — different monitors display colours differently; an uncalibrated monitor may affect results
- Ambient lighting — bright room light, sun glare, or blue-light filters distort colour perception
- Screen type — OLED vs LCD vs CRT displays render colours differently
- No colour management — printed plates use precisely controlled Munsell colour inks; digital screens cannot perfectly replicate this
For these reasons, online tests are screening tools, not clinical diagnostic tools. A positive result (indicating possible CVD) should be followed up with a formal clinical test.
Frequently Asked Questions
Who invented the Ishihara test? Dr. Shinobu Ishihara, a Japanese Army ophthalmologist, who published the test in 1917 while at the Tokyo Army Medical School.
Is the Ishihara test accurate? For red-green CVD, sensitivity is approximately 97% — meaning it correctly identifies the condition in 97% of affected individuals. For clinical use with printed plates, it is highly reliable.
How long does the Ishihara test take? In clinical settings with printed plates, approximately 5 minutes. Our online version with 24 plates typically takes 3–5 minutes.
Can the Ishihara test detect the severity of colour blindness? Only partially. The standard Ishihara test can suggest whether CVD is mild or severe based on the number of transformation plates failed, but the Farnsworth-Munsell 100 Hue Test is better suited for measuring severity.
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