Light entering the eye consists of many different wavelengths. The light is absorbed by tiny cells at the back of the eye called cones. In a normal eye there are three different types of cones. Some cones are best at capturing long wavelength (red) light. Other cones catch medium wavelength (green) light while other cones respond best to short wavelength (blue) light. The signals from these cones are sent to the brain where they are perceived as colour. Some people are born with one or more cone types absent or “different”. This can lead to difficulties differentiating between certain combinations of colours. Colour vision deficiencies can be detected and graded using various colour vision tests.