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Many animals are able to see colour but we’re just going to stick with human ability here! How we see and interpret colour is a very complicated, but intriguing subject involving physics, chemistry, biology and psychology and this is only a very quick peek and doesn't even scratch the surface!
The human eye does not have a linear response to colours and this is why we perceive some colours to be brighter than others. The light coming from a green laser will appear to us to be very much brighter than the light coming from a red laser with the same output power.
Green is roughly at the centre of the human eye's visible spectrum, as we approach the upper and lower limits of the spectrum sensitivity falls off sharply. As a result there is a big difference between how we perceive the colours to which the eye is most sensitive and to which the eye is least sensitive.
The way the brain responds to visual stimuli is also a major factor in colour perception and of course this is a very individual experience; which is why one person may judge an object to be red and another person judge the same object to be orange even when they are in identical lighting conditions.
Rod, Cones and Colour Perception
What, and where, are rods and cones?
The photoreceptor cells of the human eye, known as rods and cones due to the shape of them, are found in the retina
Rods are situated on the periphery of the retina and are very sensitive to light - even a single photon of light will cause a response! But they have only one achromatic pigment which does not allow for colour perception.
Cones are less sensitive to light than rods but contain chromatic photo-pigments allowing for colour perception. Cones are found in the central area of the retina - the fovea - where they receive images from the visual field centre.
How do they work?
There are normally 3 types of cone cells, each with a different pigment:
- S-Cones: sensitive to short wavelength light (400 - 500 nm) at the blue end of the visible spectrum
- M-Cones: sensitive to medium wavelength light (450 – 630 nm) peaking at green
- L-Cones: sensitive to long wavelength light (500 – 700 nm) peaking at red
Signals received by the brain from the different cone types as they are stimulated allow us to perceive a range of colours. The reason the eye is more sensitive to green/yellow light than other colours is that those wavelengths stimulate the more abundant M and L cones to almost the same level.
Rods, Cones and Ambient Lighting
Rods and cones react differently according to ambient lighting conditions resulting in changes in colour perception between the conditions - known as the Purkinje effect or Purkinje shift.
There are states of vision named for 3 conditions of ambient lighting:
- Photopic vision is the state of vision in normal or well-lit conditions; here the visual acuity and colour perception of the human eye are at their best as cone cells dominate for light sensing.
- Scotopic vision is the state of vision under low light conditions where cone cells cannot function, so the rod cells dominate. Scotopic vision in humans is poor in respect of both spatial acuity and colour perception.
- Mesopic vision lies between photopic and scotopic vision so it is the state of vision in intermediate light levels and in this situation both the rods and cones are active.
Perception of colour not only depends on vision and light, it also depends on a individual’s interpretation of what they see. So, the science of light, the physiology and chemistry of the human eye and brain, the psychology of how the brain reacts to visual stimuli and many more factors are involved.
The science around colour, light and how the eye and brain work is much more involved and complicated than our very brief snippet shows and as always we encourage you to read further.
- Find out more about light, wavelengths, the visible and non-visible spectrums
- Research the biology of the eye and how the eye and the brain ‘see’ and ‘interpret’ images
- Look into the psychology of colour, how different cultures perceive and give meaning to colours.
- The Encyclopædia Britannica has provided a wealth information since it’s foundation in 1768 in Edinburgh, Scotland; and continues to do so on the Britannica website.
- The BBC Bitesize website has a wealth of information about many subjects which is tailored to various age groups.
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