Cone cells detect colours

Spread over an extensive area in the human retina, a large number of non-colour sensitive rod cells, 120 million in all, respond to variations in the amount of light with great accuracy. These rod cells enlarge the field of vision, particularly in the lateral direction. Human beings have always found it necessary to detect any threats or prey in their lateral line of vision, and these cells have developed an extraordinary ability to detect motion through variation in luminance. Moreover, these cells require less light than the other light-sensitive cells known as cone cells, which are capable of distinguishing colours. Only the rod cells remain functional when there is little light available, and so colours are not perceived. In twilight, all cats appear to be grey, or at least black-and-white.

Numbering much fewer than rod cells, cones cells (6 million) are concentrated in a single spot in the retina known as the fovea, which is the area of sharp central vision. There is only a very small spot in the retina with a sufficiently large number of photoreceptors for accurate vision. People are only able to see a fairly limited area in sharp focus at any given time: when reading this text, you will see only 8 to 12 characters clearly and will have to constantly move your eyes. The perception of colour is also at its sharpest in this same small area.

There are three types of cone cells for distinguishing colours. One is particularly sensitive to red light wavelengths, another to green light, and a third to blue light. It is the combination of data received from these three sources that creates the impression of an extensive and varied world of colour. From this structure and function of the eye, it follows that the primary colours are red, green and blue.

The sensitivity curve of all three cone cells combined.

The curves represent the distribution of light wavelengths and the sensitivity of the individual cone cells. Cone cells sensitive to blue are represented by curve B, those sensitive to green by G, and those sensitive to red by R. Together, these three types cover the entire wavelength range of visible light, although the eye is the most sensitive to blue-green wavelengths that are most common in sunlight.

The standard RGB colour system is based on the fact that all colours of light can be represented as combinations of these three colours. Additionally, different colours can be produced by combining the three colours in the right proportions – here we are referring to light as perceived by the eye. As a result, the same colour sensation of, say, yellow can be produced by a markedly different range of  wavelength distribution. For example, monochromatic radiation containing a single wavelength at 580 nm is perceived as yellow, but light consisting of equal amounts of red and green, two adjacent colours in colour distribution, also appears to be yellow.