Colour blindness (or colour vision deficiency) is the name given to the condition where people find it difficult to identify or distinguish different colours. An Ishihara test is a common way to test for certain types of colour blindness, and here we’ll talk you through how this simple test works.
What is the Ishihara colour blindness test?
The Ishihara test was developed in 1917 by Japanese ophthalmologist, Shinobu Ishihara. It’s become one of the most common ways to test for red and green colour deficiencies, particularly in children. The test involves identifying coloured numbers or shapes contained within a series of differently coloured dots.
Other common colour vision tests involve colour arrangement in which you’re asked to arrange colours in order of their shade or to identify matching colours. The City University Test (developed by City University London) and the Farnsworth D-15 test are examples. These can be more useful in detecting blue light deficiencies, which the Ishihara test doesn’t cover.
How does the Ishihara test work?
You’ll easily recognise an Ishihara test – it’s quite a common test in children’s eye tests where these types of colour deficiencies can be picked up.
Your optician will have a book made up of 38 different plates, known as pseudoisochromatic plates. Each plate features a circle formed by a range of dots in different colours and sizes. Within each circle, there are a series of dots in a different colour that usually make up the shape of a number (for small children these might be other shapes that they can easily identify).
They will show you a range of these plates and ask you what number you can see for each one. Colours that may at first seem equal (or ‘iso’) to your brain, but are actually different, are shown to you together – it’s up to your brain and eyes to detect the difference in order to see the hidden number. People with normal colour vision are able to work out the false (or ‘pseudo’) similarity between the two colours and see the hidden number with no problem. If you find it difficult to tell what the number is on certain plates, then it can indicate a colour vision deficiency.
If you have difficulty seeing the numbers on a certain amount of plates, your optician will be able to make a diagnosis of colour blindness.
What are the most common forms of colour blindness?
The Ishihara test is used to detect the most common types of colour blindness, which are categorised as red-green colour deficiencies (known as protanomaly and deuteranomaly).
This is a general term that covers a range of types and severities, but in general means that people are unable to see or differentiate colours that have red or green as part of the whole colour. So for example, people with a red deficiency will find it difficult to see the difference between blue and purple because they are unable to detect the red properties of purple.
Your optician might carry out a colour arrangement test too, to confirm the results from the Ishihara test, as well as testing for the blue-light deficiency (tritanomaly), which an Ishihara test doesn’t test for.
Who should have an Ishihara test?
It's likely that red-green colour deficiency would be picked up during childhood, and opticians may include an Ishihara test during a children’s eye test to spot this early. But colour blindness can happen at any age and can often develop as a result of certain health conditions, like diabetes and multiple sclerosis, or as a consequence of the eye disease glaucoma. Medications or exposure to certain chemicals can also cause colour deficiencies. So if you notice any changes in your colour vision, you should see your optician who can check this for you.
Colour vision tests aren’t part of our usual eye tests, so just let us know if you’d like one.
Are online colour blindness tests reliable?
There are some online tests around that can give an indication of a possible problem with your colour vision, but they’re not always very reliable because the display settings on your screen can differ quite a bit. So it’s best to get this tested properly with an optician.