Tristimulus colour values

LMS tristimulus colour values underpin the system of measurement and representation of colour perceptions that constitute the LMS colour space. The system works on the assumption that any colour can be described in physiological terms by measuring the response of  L, M and S cone cells in the retina of the human eye to different wavelengths of light.

XYZ tristimulus colour values are equivalent to LMS colour values. The CIE (1931) XYZ colour space uses XYZ tristimulus colour values as the foundation of the CIE colour system which has become the universal standard for communicating precise colour information around the world.

  • LMS tristimulus colour values have a real correspondence with the range of colours that fall within the observable visible spectrum of a typical human observer.
  • XYZ tristimulus colour values have a virtual correspondence with observable colours meaning that some colours are hypothetical, an anomaly resulting from adjusting colour values to account for variations in brightness. The adjustments allow for the fact that a fully saturated yellow, green or cyan is much lighter than red or blue for example.
    • Tristimulus values are the backbone of colour measurement whether in terms of the physiological response of the human eye to light or within the world of colour matching or colour management.
    • LMS tristimulus colour values have three components corresponding with the response of the L, M and S cone types. Each response is measured against a scale with values between 0 and 1.
    • XYZ tristimulus colour values also correspond with the response of the L, M and S cone types.
    • Tristimulus colour values are colour-matching functions in so far as, if you know a tristimulus value then you can predict the corresponding colour experience.
  • The human eye with normal vision has three kinds of cone cells that sense light, having peaks of spectral sensitivity in short (“S”, 420 nm – 440 nm), middle (“M”, 530 nm – 540 nm), and long (“L”, 560 nm – 580 nm) wavelengths.
  • These cone cells underlie human colour perception in conditions of medium and high brightness; in very dim light colour vision diminishes, and the low-brightness, monochromatic “night vision” receptors, denominated “rod cells”, become effective.
  • Thus, three parameters corresponding to levels of stimulus of the three kinds of cone cells, in principle describe any human colour sensation. Weighting a total light power spectrum by the individual spectral sensitivities of the three kinds of cone cells renders three effective values of stimulus; these three values compose a tristimulus specification of the objective colour of the light spectrum.
  • The three parameters denoted “S”, “M”, and “L”, are indicated using a 3-dimensional space denominated the “LMS colour space”, which is one of many colour spaces devised to quantify human colour vision.
  • Stimuli that account for colour perception: can be specified by a set of tristimulus values, defined as the “amounts of the 3 reference colour stimuli, in a given trichromatic system, required to match the colour of the stimulus considered”.
  • A human eye with normal vision has three kinds of cone cells and each senses light of different wavelengths. They are identified as follows:
    • L – Long (560 nm – 580 nm)
    • M = Medium ( 530 nm – 540 nm)
    • S = Short (420 nm – 440 nm)
  • Every human colour sensation can be explained in terms of the stimulus each cone type receives.
  • If the stimulus received by the three cone type is measured then any colour perception can be identified by the three components of a tristimulus colour value which is the basis for the LMS colour space.
  • The LMS colour space was the subject of intense scientific study during the 1920s because it established a direct link between the subjective human experience of colour and wavelengths of the visible spectrum.
  • There were technical problems interpreting the LMS colour space however that led to the development of the CIE 1931 colour space in which LMS tristimulus values are denoted by X, Y, and Z tristimulus values.
  • One of the most important innovations associated with the CIE 1931 colour space is the CIE xy chromaticity diagram.