Colour blindness (Dyschromatopsia)

Colour blindness, which affects between 8 and 10% of the male population, is a visual disorder that alters the perception of certain colours. It is most often genetically transmitted, and it is much more common in men. This male predisposition is due to chromosomes. Colour blindness only develops in women if both their X chromosomes are affected. Men have only one X chromosome, so the risk is greater. The most common form of colour blindness is a difficulty in distinguishing between shades of green and red.       

To understand what colour-blind people perceive, we must first understand the link between the retina and colour perception. Certain retinal cells, called “cones”, are there to help us read and distinguish colours from each other. There are three types of cones (S, M and L*), which react to the colours blue, green, and red respectively. Colour blindness is the result of a functional abnormality affecting some of these cones. 

This dysfunction prevents colour-blind people from distinguishing between a number of colours, in particular red, green and sometimes even blue. The colour deficiency depends on the type of colour blindness: deuteranomaly, protanomaly or tritanomaly.   

*For Short, Medium, and Long

There are many tests available nowadays to detect possible colour blindness. The Ishihara test, with its pseudo-isochromatic plates and coloured dots, is the most famous and most common of all. The aim in that case is to try to distinguish a number from a mosaic of red and green dots. Other tests, in particular those designed for classifying shades, allow for a more precise detection of different deficiencies. Simplified versions of some of these screening tests are also available online, but an appointment with an optician is necessary in case of doubt.