Tests for colour vision deficiency
A plate from the Ishihara Test for colour blindness. Can you see the number 74? However, whether you see the number or not, don’t take this as a final indication: it is only one plate of many plates in the full test and the colours on your computer screen might not be exactly right.
You can have your colour vision tested by your local optometrist or your general medical practitioner. The standard test is the Ishihara Test but other tests of the deficiency are needed for a full assessment. These other tests, however, are not often provided by the general practitioner, at least not in the UK.
The standard test is the Ishihara Test for Colour Blindness. It has 38, 24 or 14 colour plates, depending on the edition. Each plate is made of a background of coloured dots on which there are two numbers also made of coloured dots. The numbers can be seen because they have different colours from the background dots. You will be asked to state the numbers on each page: if you have abnormal colour vision you will not be able to see the numbers because their colours will look the same as the colours of the background dots. Dr Ishihara designed this test very cleverly because for some of the plates those with abnormal colour vision will see different numbers from those seen by someone with normal colour vision; and on other plates those with normal colour vision do not see any numbers but those with abnormal colour vision do.
If you make errors in more than 3 plates you are very likely to have abnormal colour vision and if you make more than 6 errors it is almost certain. Face up to this – the test is very reliable. A lot of people with abnormal colour vision refuse to believe the test (ref 11), but the test is rarely wrong (refs 1, 60).
OK, you have failed the test and have abnormal colour vision. What next? You need to know how severe your colour vision deficiency is. The Ishihara test does not test for severity: the number of errors made does not indicate whether your colour vision deficiency is mild or severe.
There are two tests that are commonly used to test for the severity of a colour vision deficiency. One is the Farnsworth D15 test, which your local optometrist may have. The other is the Anomaloscope, which gives a full diagnosis of the type of colour vision deficiency you have. You will have to go to a special colour vision clinic at a university optometry school to be tested with an anomaloscope. A few eye hospitals also have a colour vision clinic and can make a full colour vision diagnosis.
The Farnsworth D15 test for the severity of abnormal colour vision. The colour discs are given to you in random order and you have to arrange them in systematic order of changing colour, as they are arranged in the picture.
This is the Nagel anomaloscope, The insert is what you see when viewing through the eyepiece of the Nagel anomaloscope. One knob adjusts the brightness of upper yellow field and a second knob changes the colour of the lower half. Here it’s orange, so it has to be made less red. The task is to make the upper and lower halves the same brightness and the same colour. How you do this diagnoses the type of your colour vision deficiency and its severity.
The Farnsworth D15 test is a very simple one: it takes only a few minutes to do. You will be asked to arrange 15 coloured discs in a wooden box in order of changing colour. If you make no errors or only a few minor ones you have a mild colour vision deficiency. If you make major errors you have a moderate to severe deficiency.
The anomaloscope test is regarded as the definitive diagnostic test for the red-green colour vision deficiencies. You will be asked to view through the eyepiece to see a circle of colour, one half of which is yellow light and the other half is a mixture of red and green light. You will be asked to match the colour and brightness of the two halves using two knobs. The way you do this gives a diagnosis of the particular kind of colour vision deficiency you have and how severe it is.
Other colour vision tests. There are many. There are a number of other tests that use the same principles as the Ishihara test. Collectively they are called pseudo-isochromatic tests. The Ishihara is the one most often used and is probably the best of them, but the others work pretty well – so don’t be surprised if you are not given the Ishihara test but a similar looking test.
Likewise there are a number of other colour sorting tests, in addirion to the Farnsworth D15 test (refs 1, 60).
There are also lantern tests, which simulate colour coloured signal lights. These are used for testing in transport and defence occupations and not for colour vision diagnosis.