Seeing colour 

 

Colour is not actually inherent in any object. We only see colour because objects absorb some colour from light, and refl ect others. It is the reflected ones that we see and that give an object a set ‘colour’. Therefore, for example, grass is not green, it purely absorbs all other colours in light and refl ects back green. If an object refl ects all colours we will see it as white, if it absorbs all colours we see it as black. We use cones to perceive colour as rods are blind to colour.  

 

Ishihara based his test on pseudo-isochromaticism, but with the intention of delivering results that were more easily interpreted and thus more reliable. Almost nine decades on from its first edition, the Ishihara test remains widely used, able to quickly screen for colour vision defects that other, more exacting tests can then elucidate in detail. The Ishihara test can only detect the more common red-green

colour vision deficiencies (not the rarer blue ones), and then with only limited precision. A mild form of red-green deficiency occurs when either the red or green sensitive photopigment in the retina has an altered response to colour; this results in reduced discrimination between the colours red and green. A more severe deficiency occurs when either the red or green photopigment is missing entirely.

Normal colour vision uses all three types of light cones correctly and is known as trichromacy. People with normal colour vision are known as trichromats.

The different anomalous conditions are protanomaly, which is a reduced sensitivity to red light, deuteranomalywhich is a reduced sensitivity to green light and is the most common form of colour blindness and tritanomaly which is a reduced sensitivity to blue light and is extremely rare.

People with deuteranomaly and protanomaly are collectively known as red-green colour blind and they generally have difficulty distinguishing between reds, greens, browns and oranges. They also commonly confuse different types of blue and purple hues.

People with reduced blue sensitivity have difficulty identifying differences between blue and yellow, violet and red and blue and green. To these people the world appears as generally red, pink, black, white, grey and turquoise.

People with monochromatic vision can see no colour at all and their world consists of different shades of grey ranging from black to white, rather like only seeing the world on an old black and white television set. Achromatopsia is extremely rare, occuring only in approximately 1 person in 33,000 and its symptoms can make life very difficult. Usually someone with achromatopsia will need to wear dark glasses inside in normal light conditions.