Colour Vision primer: How does colour vision work?
Light is a form of electromagnetic energy that we're able to see because the energy is of the size that our eyes are sensitive to.
This size, referred to as its 'wavelength' determines how the energy behaves, and how we use it, for example we listen to radio waves with our radio, heat our food with microwaves, see things with visible light waves and see through some solid objects with x-ray waves.
The electromagnetic waves we can see are between 380nm and 780nm long, which are detected by a combination of 5 types of light sensitive cells in our eyes.
Three of these types of cells are dedicated to our colour vision.
THE VISIBLE SPECTRUM OF LIGHT
RETINAL CONE ARRAY
Colour vision is based on the signal from 3 of these types of cells called "cones".
They are the:
L cone -sensitive to long wavelength light.
M cone -sensitive to medium wavelength light.
S cone -sensitive to short wavelength light.
The "retinal cone array" pictured is a visualisation of how cone cells of each type are jumbled together on the retinal surface at the back of the eye.
The number of each cone type every person has varies greatly, but they usually total around 7 million in each eye.
For each point of light we see, we are comparing the activity of a collection of these cells.
We see the range of visible light as a spectrum of colour because our brain compares the signal coming from each of these 3 cone cells. The curves you see in the diagram describe how much the cell will respond given stimulation of light of each wavelength. All our brain sees, though, is the amount of activity from each cell as a whole.
If the S cone is responding a lot and the M and L cone only respond a little, we get the visual sensation of blue. Alternatively, if the S cone is not active, and the M cone is responding more than the L cone, we experience green. If the M and L cones are responding equally we experience yellow.
L
M
S
CONE CELL SENSITIVITIES
There are many variations in the portion of the spectrum, the particular wavelengths, our cones respond to. Sometimes a person doesn't inherit the genes defining the usual group of S, M and L cones, and instead inherit the gene for an anomalous cone cell. These anomalous cones have a sensitivity that it much more similar to another of the cone cells, creating a colour vision deficiency (also known as "colourblindness").
When the sensitivities (the range of wavelengths the cells respond to) are too similar, the difference between the signal of the two cone types is very small. The size of the signal difference is what makes the appearance different, so to a person with colour vision deficiency, the smaller difference between their cone cell signals means colours will look more similar.