Seated against the broad shouldered tree
I face the shades sheltered from the sun
across the expanse soft clouds do flee
trying to catch the brilliance in their run
their playful dance modulates the light
gracefully sculpting faded shadows at play
providing such intricate nuances of sight
that by far exceed fifty shades of grey
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Humans can see in the order of 10 million colours[1], while they are only able to perceive about 30-60 different shades of grey[2]. This is despite the receptor cells in the retina of our eyes accommodating some 120 million rods for seeing whites, greys and blacks and some 6 million cones for seeing colour. We can only see colour in well lit conditions, at lower light intensities we see only greys. Humans are most susceptible to the colour green for which we can distinguish more shades than for any other colour. We can also see green at lower light intensities than any other colour. The reason for this is that green is in the middle of the visible spectrum and therefore optimum for our receptor cells[3]. Also, as historically our foraging environment was mostly green, we have adapted to seeing shades of green (and the colouring of ripening fruits) better than any other colour. Interestingly, as a painter I have more shades of green in my paint box than any other colour as optically I sense this is the colour with most variations in hue.
Tjen Photography - Oslo Slottsparken, 2015
Electromagnetic radiation exists across a broad spectrum of wavelengths such as radio waves, microwaves, infrared (700 nm - 1mm), visible light (400-700 nm), ultraviolet (10 - 400 nm) and x-rays. Visible light therefore only represents a narrow sliver in this large spectrum. It is the way we process information in the visible light range that we see this radiation as colours. Radiation outside of the visible range we do not perceive optically, neither as colours, nor as light.
In terms of energy, sunlight that reaches the Earth's surface[4] is around 52 to 55 percent infrared, 42 to 43 percent visible , and 3 to 5 percent ultraviolet. The peak of the spectrum is the middle of our visible range. It is not surprising therefore that the visible spectrum specifically covers almost half of the radiation energy emitted by the sun. The reason that we do not see a broader spectrum of radiation is that we do not need to[5][6]. There are animals that can ‘see’ (perceive) into the infrared range, but these are night predators such as rattlesnakes that can sense radiation from emitted body heat. Animals that can see ultraviolet radiation include fish, amphibian, reptilian, avian species and some mammals. This is useful for recognising specific UV reflections for food or flashy mating targets.
Notably, we actually do not see colour directly. When light falls on our retina, the trichromatic cone receptor cells split light information into either red, green or blue colours. The individual electrical impulses (with brightness, colour and spatial information) are sent through the optical nerve to our brain that integrates these signals into information patterns that we interpret as colours and shapes. Therefore, my colour red may not be the same as your colour red. The perception of colour is a private sensation[7]. There is research that indicates that men see colours differently from women. So hopefully that should end misunderstandings in the debates on bathroom colour selection and clothes coordination. Blame it on men’s testosterone[8].
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[1] Number Of Colors Distinguishable By The Human Eye
[2] Shades of Gray
[3] Visible Light and the Eye's Response
[4] Spectrum of sunlight on earth’s surface
[5] Why is human vision restricted to 400-700 nm?
[6] Research with infrared lasers has shown that under certain circumstances humans can see infrared light
[7] Your Color Red Really Could Be My Blue
[8] When it Comes to Color, Men & Women Aren't Seeing Eye to Eye