There are a number of tests that seamen have to undergo in order to verify whether they are fit to take up a position on ship. One of the many prescribed tests, include tests to see whether a person’s eyes function normally or not. The Ishihara colour blindness test is a very important eye test for seamen and sailors.
Developed by a Japanese doctor, Dr. Shinobu Ishihara in 1917, a professor in the University of Tokyo, the Ishihara colour blindness test, has over the years become a compulsory requirement in the marine world. The test enables the doctors to find out whether a person can recognise and differentiate between the various colours that exist.
Colour blindness is a very vast and an encompassing concept. Some people are fully colour blind – meaning that they cannot identify any colour whatsoever, while some people cannot identify only a few colours. The type of colour blindness where people cannot differentiate between colours is known as monochromacy and the type of colour blindness where only two-three colours cannot be identified is known dichromacy.
The Ishihara test is done in order to find out whether people interested in becoming seamen can identify and recognise colours like yellow, green and red. This type of colour blindness is known as Protanopia and Deuteranopia. They are very rare and occur only among 1% of the male population; however since seamen have to have a perfect visual ability, the occurrence of this disability will cause prospective seamen to lose out on their passion.
There are 38 plates which are numbered accordingly i.e. from 1-38. The numbers however are not visible clearly. They are scattered, random and are spread across coloured dots which cover the plate entirely. If a person is unable to spot the colour difference in the coloured plates and thereby the numbers, then it is verified that the person has a form of colour blindness.
The pattern of the dots in the circular plates can be explained as follows:
- The dots spread on the circular plate could be composed of colours like yellow and green. The numbers placed amongst these coloured dots will be either red or brown in colour. Protanopia can be found out through this colour combination of the eye test
- As an alternative option, the dots on the circular plate could be composed of colours like red and brown. The numbers placed amongst the coloured dots would be composed of green and yellow colours. This type of Ishihara colour blindness test helps to find out whether a person has Deuteranopia.
In order to verify completely and without any bias that a person is suffering from a form of colour blindness, around 24 plates are tested. The extent of the Ishihara colour blindness can be found out through the number of plates tested. It is only with the help of an eye test like this that many marine accidents and mishaps can be prevented successfully.
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References: toledo-bend, colour-blindness