In my explanation of how we perceive colours, I will try and keep things as simple as possible because one can get lost in the physiological and anatomical analysis of the eye!
As described in my previous post on ‘What is Colour’, we now know that light is waves of energy travelling at different wavelengths.
Light travels from the main light source, strikes the objects we are viewing, absorbing some of the colours in the light and reflecting others. The light, or wavelength of light pertaining to a particular colour, that is reflected off the object then travels to our eyes which collaborate with our brain in a complex process to interpret what we are viewing.
What happens when light enters our eyes?
Now, when wavelengths of light pass into the eye they are refracted by the cornea as they enter the eye chamber through the pupil. The iris surrounding the pupil dilates in bright light and constricts in low light altering the size of the pupil to allow the right amount of light to enter. Passing through the lens, the light focuses a sharp, inverted image on the retina, which is made of photoreceptor cells called rods and cones. According to their particular attributes, these photoreceptors absorb the light and translate it into patterns of electrical signals which are transmitted through a complex process to the head of the optic nerve which then transmits them to the visual cortex at the back of the brain.
What are rods and cones and how do they work?
The human eye has approximately 120,000,000 rods throughout the retina. Rods do not distinguish colour and do not need much light to be activated. They contain a visual pigment called rhodopsin and are most sensitive to blue/green light of 505 nanometers. For this reason, they enable us to ‘see’ in low light conditions helping us, for instance, to distinguish objects at night.
Cones, however, of which the human eye has approximately 6,000,000, help us to distinguish colour. There are three types of cones which are sensitive to red, green and blue, with a predominance of red and green cones. These are concentrated in the fovea. When the red and green cones are equally stimulated, the impression of yellow is generated. The blue cones are found sparsely scattered among the rods with maybe a few contained in the fovea.
When the brain receives these impulses of red, blue-violet and green it mixes them into a single message that informs us of what colour we are viewing. For example, when we see re it is because the red-sensitive cones are being activated while the green and blue-violet cones are relatively dormant.
Influencing factors in colour perception
The human eye can see 7,000,000 colours but our perception of colour is greatly affected by a myriad of factors amongst which are the ambient lighting conditions influencing the quality (value and intensity) of colours that we are viewing -in other words, whether it is dark or light or if we are indoors or outdoors- the surrounding conditions and light that will be reflected off other coloured surfaces, the materials used and their reflective and absorbent attributes will further affect our perception of them.
Lastly, certain colours are more easily perceived than others. Yellows and greens are seen before other hues, while red and violet are the most difficult to perceive.