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
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
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.