Trichromatic vision in Humans

The human eye is the visual pathway to the world around us, and enables most of us to see thousands upon thousands of different colours. As shown in figure 1, the human eye is a very complex organ, with many different structures help us get the best vision in different situations- dim or bright etc. - and when all of these parts perform in harmony, we are able to see clearly.

In order to understand how we see different colours, it is first imperative to understand how the eye works as a unit: the starting point of vision is when light rays reflect off an object and enter the eyes through the cornea-the outermost, transparent layer of the eye. The rays are then refracted by the cornea and pass through the pupil- a whole created by the iris to control the amount of light passing through it. After that the rays pass through the lens- which can bulge or shrink to further refract the rays in order to focus them on the retina at the back of the eye.

The retina is an extremely thin layer of cells at the back of the eye which contains millions of light-sensitive cells called rods and cones- and are also known as photoreceptors. Cones are concentrated in the center of the retina (the macula) and in bright light conditions, they provide precise vision and detect colours. Rods however, are located outside the macula and extend all the way to the outer edge of the retina. They provide peripheral vision and allow the eyes to detect motion and help us see in dim light and at night. These photoreceptors then convert the light into electrical impulses which are sent to the brain via the optic nerve at the back of the eye, and create an image in our head.

As previously mentioned, we are able to see colours due to cone photoreceptors, of which there are 6-7 million of in the retina of each eye. Most of them are located in a 0.3mm spot on the retina called the fovea centralis, and over the last few centuries experiments have given evidence that amoung these cones there are three different types of colour reception: red (64%), blue (2%) and green (34%). This was proved by two different groups of scientists: Wald and Brown at Harvard, and Marks, Dobelle and MacNichol at Hopkins in 1959. However, the first original theory of there being three different light sensitive ‘particles’ was put forward by Thomas Young in 1802, 136 years after Sir Isaac Newton’s famous discovery that white light contained thousands of different colours (due to their different wavelengths on the electromagnetic spectrum), and so enabled us to understand where colour ‘comes from’.

"Colour is the visual effect that is caused by the spectral composition of the light emitted, transmitted, or reflected by objects” and when a light ray of a certain wavelength hits the fovea centralis, it activates the three different types of cone to varying degrees, and with an infinite amount of varying combinations, we are able to see thousands of different colours. This is shown in a simplified diagram in figure 3, but in reality this diagram would be a lot more complex because of the ranging number of shades which belong to each colour. To prove that any colour visible to humans can be created from this trichromatic system, we can use the example of TV sets; if you look at a normal television up close when it is switched on, the tiny pixels contain just 3 colours: red, blue and green (as shown in figure 4).

The human eye can perceive many more variations in warmer colours than cooler ones due to the fact that almost 2/3 of the cones process longer light wavelengths and so we are able to see more yellows, oranges and reds. Additionally, the reason we can’t see colours in the dark is because the rods ‘take over’ to control the amount of light that we see, and so the cones aren’t in control anymore. Furthermore, about 8% of men and 1% of women have some type of colour impairment; the most common of which is red and green dichromatism, which causes the colours red and green to appear indistinguishable.

In conclusion the reason why humans can see thousands upon thousands of different colours is that, despite only having 3 types of cone photoreceptor, the cones send-off varying amounts of blue, red and green to the optic nerve to be carried to the brain, and by changing these amounts of light, all of the colours in the visible spectrum can be produced.