Trichromatic colour theory

The starting point for trichromatic colour theory is an understanding of the physiological basis for the subjective experience of colour.

  • Contemporary versions of trichromatic colour theory developed out of two parallel lines of research:
    • Research got underway in the early part of the 19th century into the structure of the human eye and went on to reveal the function of rod and cone cells along with the other types of neurons found within the eyeball.
    • Systematic research into the relationship between stimulation of the retina by different wavelengths of light and the corresponding subjective experience of colour reached maturity during the 1920s.
  • The outcome of this century of enquiry into trichromacy was the LMS colour model and the CIE (1931) XYZ colour space among others.
  • Trichromatic colour theory established that there are three types of cone cells in the human eye that carry out the initial stage of colour processing that ultimately produces the world of colours we see around us.
  • Cone cells are daylight photoreceptors which means they are able to convert light into electrical charges through a process called photo-transduction.
  • The sensitivity of cone cells was established using spectroscopy which measures which wavelengths are absorbed and which are reflected.
  • The three types of cone cells were identified along with the range of wavelength they absorbed:
    • L = Long (500–700 nm)
    • M = Medium (440 – 670 nm)
    • S = Short (380 – 540 nm)
  • Each of the three cone types was found to absorb with a bias towards a favoured range of wavelengths of light within the visible spectrum.
    • L = sensitive to the red region of the visible spectrum (biased towards 560 nm).
    • M =  sensitive to the green region (biased towards 530 nm).
    • S = sensitive to the blue region (biased towards 420 nm).
  • It also became clear that the three types of cone cells work in combination with one another to enable the human eye to respond to all wavelengths of the visible spectrum and produce the fine gradation of colours we see across the visible spectrum.
  • Some research suggested that the sensitivity of these biological processes enables us to distinguish between as many as seven million different colours.
  • A closer look at the biases detailed above reveals a complicated picture. There is a certain amount of overlap in the range of wavelengths that rods and three types of cones are receptive to:
    • L cones: Respond to long wavelengths so to a region that includes red, orange, green and yellow but with a peak bias between red and yellow.
    • M cones: Respond to medium wavelengths so to a region of sensitivity that includes orange, green, yellow and cyan but with a peak bias between yellow and green.
    • S cones: Respond to short wavelengths so to a region of sensitivity that includes cyan, blue and violet but with a peak bias between blue and violet.
    • Rods: Rod cells which come into their own in low-level lighting, are most sensitive to wavelengths around 498 nanometres, with a peak sensitivity towards green-blue, and are insensitive to wavelengths longer than about 640 nanometres.
  • Another crucially important strand of research produced experimental evidence around 1850 that a test subject could produce a match for various different colour swatches by adjusting the intensity of three monochromatic light sources, one in the red, one in the green, and one in the blue part of the spectrum. This research concluded that if the correct wavelength was selected for each of the three lights then any colour within the visible spectrum could be produced.
  • The fact that mixtures of red green and blue light at different levels of intensity could be used to stimulate the L, M and S cones types to produce any human observable colour was a discovery that provides the underpinning for almost every form of colour management in practice today.
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