The retina, which lines the back of the eye, is made up of a variety of cells. The main cells are called photoreceptors and they respond to light. There are two main types of photoreceptor, rods and cones. There is one type of rod, they are very sensitive and allow us to pick up even the faintest light in the dark. Rods therefore are principally used for night vision. There are three types of cone cell, red, blue and green, and each responds particularly to those colours.
Imagine that there is one cone of each colour feeding information to the brain. Imagine that a red light is shone onto those cells. The red cone responds strongly as it detects red. Blue and green won't respond as red light doesn't contain those colours. The brain receives the information, and as only the red cone responded the brain knows the light is red.
If a purple light was shone onto the three cones, the combination of red and blue that makes up purple would make those cones respond. The green would not respond at all. The brain receives these signals, and sees that as the red and blue have both responded the light must be a mix of the two, so it sees purple.
Because there is only type of rod cell, we are not able to see different colours with them. They can only tell us how much light there is, not how much of a particular colour. We use the rods at night, as the cones are not sensitive enough. If you wake up at night and the lights are off everything is in black and white.
If someone is colour deficient there is a problem with the cones, which are usually present in the retina but don't respond as fully as they should - think of it in terms of faulty wiring! Generally it is the red and green cones which are affected, and some colour deficient people can make out strong reds and greens, but not others. It is possible for the cones not to be present at all, and this gives a much more intense deficiency in colour vision, colour blindness. You would only see in black and white, and as you would have just the sensitive rods, you would be very light sensitive too.
Causes of Colour Deficiency
It can be related to DNA and genetic makeup, so if there is family history of it it can be passed onto children. It generally passes on the mother's side but can be completely random with no family history. Very few girls are colour deficient (around 0.5%) and so it is more common in boys (around 8%)
It is possible to become colour deficient in later life, due to a pathology rather than a genetic condition, and as such is often reversible. Simple conditions like cataract can affect colour perception, and other conditions, including side effects of medication can cause changes.
Colour Deficiency and Career
As you can be born with colour deficiency and it will stay that way forever, awareness early on is important as it can restrict some careers. We use a simple technique of asking the patient to tell us what they can see on colour test charts. Colour deficiency can be a problem in the emergency services and armed forces. Obviously it can also affect subjects where colour is important, such as design, chemistry, geography or if you want to be an electrician! Your optometrist can test you and advise of any career restrictions.