Colour Physiology
A. Cone Photoreceptors
When light enters our eye, it is focused by the lens onto the retina. The light-sensitive cells in the retina are called rods and cones. There are approximately 6 million cones in our visual system, and 120 million rods. Cones are where photosensitive pigments initiate the encoding of the wavelength composition of light the eye receives. There are three different types which are sensitive to short, medium, and long wavelengths. Thus, the Young-Helmholtz theory was proven to be true, as there are actually three cone types, and humans do, in fact, have trichromatic vision.
The following graph demonstrates how each cone’s response varies dependent on the wavelength.
B. Photopigment Sensitivity
Each cone is more sensitive to light of a certain wavelength, as seen in the graph above. Distributed coding shows that the ratio of responses is what specifies a colour.
Approximately 430 nanometres = Blue
Approximately 530 nanometres = Green
Approximately 560 nanometres = Red
C. LGN: Colour opponent cells
The Goethe-Hering Theory says that there are pairs of primary colours when looked at together, will strengthen the visual effect of colour contrast. This colour opponency is seen in the visual system, in our colour opponent ganglion cells on our retinas. Ganglion cells are what sends information to the lateral geniculate nucleus, or LGN, which informs our brain about the colours we see. The ganglion cells add and subtract signals from many cones because they are excited in the middle of their receptive fields, and inhibited in the area surrounding the middle. Therefore, by comparing responses from different wavelengths of cones, a ganglion cell can determine the amount of red or green, or blue or yellow.