An Ishihara colour test, also known as the colour vision test, measures a person’s ability to tell the difference between colours. Ishihara test checks for red-green colour blindness.
In an Ishihara test, a person looks at a series of circles (known as Ishihara plates) with dots of different colours and sizes. A person who has trouble seeing red and green will find the shapes and numbers hard to see, or may not see them at all. Depend on the degree of difficulty in identifying the colours on an Ishihara plate, one can either be diagnosed with poor colour vision or colour blind blindness. However, complete colour blindness is a very rare condition in which a person is only able to see shades of gray.
Causes of poor colour vision
The most common type of colour vision deficiency is the inability to distinguish between the shades of green and red. Causes of poor colour vision can be by:
- Certain medications and diseases.
- Exposure to certain chemicals.
- Diseases: such as diabetes, alcoholism, macular degeneration, leukaemia, Alzheimer’s disease, Parkinson’s disease and sickle cell anaemia.
- Colour-sensitive photoreceptors (an inherited problem with the cones).
Ishihara colour test procedure
The test is conducted in a well lit room. The patient covers one eye, and then, using the uncovered eye, looks at a series of test cards. Each of these card contains a multi-coloured dot pattern.
There’s a number, symbol or shape in each colour plate. If you are able to identify the number or symbol, you’ll tell the doctor conducting the test. For those with normal colour vision, numbers, shapes and symbols should be easy to distinguish from their surrounding dots. For those with colour vision impairment, they might not be able to see the symbols or find it difficult to distinguish the patterns among the dots.
When through with the first eye, the second eye is used to look at the test cards again. The results are compared.
Also, you may be asked to describe a particular colour’s intensity as perceived by one eye against the other.
It’s likely to have a normal result on an Ishihara colour test but still experience loss of colour intensity in one eye or the other.
Interpretation of Ishihara colour test results
An Ishihara test can help pinpoint several colour vision difficulties, including:
- Protanopia: Difficulty differentiating between blue and green, and red and green.
- Tritanopia: Difficulty differentiating between yellow and green, and blue and green.
- Deuteranopia: Difficulty differentiating between red and purple, and green and purple.
- Achromatopsia: Complete colour blindness, which is a rare condition in which only shades of grey are seen.
Aftermath of an Ishihara test
There’s no precise treatment that directly addresses colour vision problems. However, if the colour vision deficiency is the result of an underlying illness (like diabetes or glaucoma), addressing the illness may improve the quality of colour vision.
Also, using coloured filters on eyeglasses or coloured contact lenses might make colour contrasts easier to identify. However, neither of these will improve the innate ability to tell colours apart.