15 May 2015

Colour perception confusion

My (shallow) research into visual perception, arising from the need to identify some "elements of visual perception" for the Elements quilt, is causing me great confusion. Yes we have three types of cones, for seeing three colours (blue, green, red) - but how do these three colours "combine" to let us perceive the 256 million (or however many there are) colours that can be represented on a computer monitor, never mind help us find just the right colour on a paint chart?

Wikipedia gives this succinct explanation, the nub of which is in the highlighted sentence: "Humans normally have three kinds of cones. The first responds the most to light of long wavelengths, peaking at a reddish colour; this type is sometimes designated L for long. The second type responds the most to light of medium-wavelength, peaking at a green colour, and is abbreviated M for medium. The third type responds the most to short-wavelength light, of a bluish colour, and is designated S for short. The three types have peak wavelengths near 564–580 nm, 534–545 nm, and 420–440 nm, respectively, depending on the individual. The difference in the signals received from the three cone types allows the brain to perceive a continuous range of colours, through the opponent process of colour vision. (Rod cells have a peak sensitivity at 498 nm, roughly halfway between the peak sensitivities of the S and M cones.)
All of the receptors contain the protein photopsin, with variations in its conformation causing differences in the optimum wavelengths absorbed.
The colour yellow, for example, is perceived when the L cones are stimulated slightly more than the M cones, and the colour red is perceived when the L cones are stimulated significantly more than the M cones. Similarly, blue and violet hues are perceived when the S receptor is stimulated more than the other two.
The S cones are most sensitive to light at wavelengths around 420 nm. However, the lens and cornea of the human eye are increasingly absorptive to shorter wavelengths, and this sets the short wavelength limit of human-visible light to approximately 380 nm, which is therefore called 'ultraviolet' light. People with aphakia, a condition where the eye lacks a lens, sometimes report the ability to see into the ultraviolet range.[12] At moderate to bright light levels where the cones function, the eye is more sensitive to yellowish-green light than other colors because this stimulates the two most common (M and L) of the three kinds of cones almost equally. At lower light levels, where only the rod cells function, the sensitivity is greatest at a blueish-green wavelength."

So, to see yellow ... because of the opponent process of colour vision, the blue cones are not firing, so yellow is made up of a combination of the red and green cones (which would show green and red, respectively, if they were not firing). Huh? a slight difference between red and green signals makes us see yellow??

As for red ... the blue cones are stimulated significantly more than the green cones. That sort of makes sense, but in a weird way - why is seeing red not due simply to stimulation of the red cones?

Blue and violet tones are produced when the blue receptor is stimulated more than the other two - that, at least, makes sense.

More research - and thought!! - is obviously needed. Meanwhile, every time I ponder these mysteries, a new approach to some detail of the Elements quilt appears. Which is why we do research....

This morning I've been playing with squares of fabric - black and white and grey, and red, green, and blue. Unsurprisingly.

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