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When you sit for a medical check-up, especially for military or aviation careers, a doctor might ask you to read a number hidden in a circle of coloured dots. This is the Ishihara test, the most common way to check for red-green colour blindness.
The test was designed by Dr Shinobu Ishihara, a Japanese ophthalmologist. He published the test in 1917, and it has remained the global standard for more than a century (Wikipedia).
Who made the Ishihara colour blindness test?
Shinobu Ishihara (1879–1963) was a Japanese ophthalmologist and army surgeon (Wikipedia). Born in Tokyo, he studied medicine on a military scholarship. By 1910, he had become an instructor at the Army Medical College, focusing on what was then called battlefield ophthalmology.
In 1917, the military tasked him with finding a fast, reliable way to screen new recruits for colour vision problems. A soldier who cannot tell red from green might struggle with signal lights, maps, or identifying uniforms in the field.
To solve this, Ishihara developed a set of pseudoisochromatic plates. He was assisted by a colour-blind physician, who helped him refine the painted designs until they worked perfectly. Interestingly, his very first hand-painted prototypes used Japanese Hiragana characters rather than the numerals we know today.
After retiring from the Imperial University of Tokyo, Ishihara moved to the Izu Peninsula. He worked as a country doctor, often refusing payment, and used whatever patients gave him to build a library for local children.
How do the Ishihara plates work?
Each plate is a circle filled with dots of random sizes and slightly different shades (Wikipedia). Within the circle, dots of a specific colour form a number or shape. To someone with normal colour vision, the number stands out clearly. To someone with a red-green deficiency, the dots blend together.
If you are studying biology, understanding how traits like colour blindness pass from parent to child is a standard syllabus topic. The Fermi Concept Library covers the biology of X-linked recessive inheritance.
The full Ishihara test contains 38 plates. The test uses different visual tricks to check your vision, not just hiding numbers in the background.
Here is what the different plates do:
Demonstration plates: The very first plate typically shows the number 12. It is designed so that almost everyone can read it, even those with severe colour blindness. This proves the person understands the test.
Transformation plates: People with normal vision see one number, but people with red-green colour blindness see a completely different number.
Vanishing plates: The number is clear to normal eyes, but someone with colour blindness sees nothing but random dots.
Hidden digit plates: In a clever twist, these plates contain numbers that only colour-blind people can see. The colours are chosen to confuse normal eyes but stand out to someone with a red-green deficiency.
Diagnostic plates: These help the doctor tell whether the person has protanopia (a red deficiency) or deuteranopia (a green deficiency), and how severe it is.
What should you expect in a medical screening?
If you are planning a career in medicine, the armed forces, or aviation, your colour vision will be tested. Here is a brief guide on what to expect during an exam.
It is genetic: Red-green colour blindness is usually inherited on the X chromosome, making it a classic example of genetic inheritance alongside Mendelian disorders like sickle-cell anaemia and PKU. Because of this, it is much more common in men than in women.
The test is quick: You usually only get three seconds to look at each plate (Wikipedia). The doctor wants to see your immediate reaction, not a studied guess.
What a pass means: If you read the required number of plates correctly (allowing for a few normal errors depending on the length of the test used), you have normal colour vision.
Because colour blindness is a standard genetics topic, you may even need to use a test cross or back cross to trace a gene on an exam. If you need a step-by-step breakdown of a genetics problem, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning.
