Trichromatic colour model

Trichromatic colour model

Trichromatic colour models (and the trichromatic colour theory that underpins them) provide methods for visually matching and mixing colours from combinations of three primary colours – red, green and blue ( or cyan, magenta and yellow). The information about how much of each primary colour is needed to produce a target colour is stored as tristimulus values. Tristimulus values are simply codes that can be used to record and pass on colour information.

The LMS colour model (long, medium, short), is a trichromatic colour model that represents the response of the three types of cones of the human eye, named for their responsivity (sensitivity) peaks at long, medium, and short wavelengths. It is used to systematize the response of the three types of cones of the human eye to different visual stimuli, that is, different wavelengths of light. The strength of the LMS colour model is its concern for the connection between the physiological aspects of vision and the everyday visual experience of an observer. L M and S refer to the bands of wavelengths that each cone type within the retina responds to.

Let’s look more carefully at this connection between trichromacy and tristimulus systems of which the RGB colour model provides a good example.

We start with the premise that trichromatic processing within the retina reduces all colours an observer sees to responses corresponding with the spectral biases of L, M and S cone cells.

This premise can be demonstrated experimentally by positioning an observer in front of three different light torches, each covered with a red, green or blue filter, that project light onto the same area of a neutrally coloured surface. The effect of each filter is to block all wavelengths except one. If the three torches project light at equal intensities the surface appears white. If the intensity of light or the colours of the filters are not exactly matched a colour cast will be apparent. If one light is turned off, then a secondary colour appears. Depending on which colour is absent the result will be cyan, yellow or magenta.

The reason the surface appears white to the observer in this experiment when all three torches are turned on is that each of the three cone types in their retina is being triggered evenly. This means that each of these types of photosensitive neurons are registering the presence of the wavelengths of light they are tuned to.

In the second part of the demonstration, a calibrated dial is used to alter the intensity of each torch. By setting each dial to a component of a tristimulus value for a known colour, it is possible to test whether the resulting stimulus causes the observer to see the intended colour.

This experiment corresponds directly with the way all RGB devices such as TVs, computer monitors, phone screens and projectors work in so far as tristimulus RGB values are used to stimulate the L, M and S cone cells on the retina to produce the intended experience of colour.

Opponent processing does not play a determining role in this experiment. We know from opponent-processing theory that after trichromatic processing takes place, the signals will be processed based on whether the cone responses indicate that the stimulus is bright or dull, more red-or-green, and at the same time, more blue-or-yellow. The output of this process will be fed into the million-or-so fibres of each optic nerve encoded into two channels of chromatic information and one dealing with the perception of brightness.

Experiments by several generations of scientists and artists have confirmed the connection between trichromacy and tristimulus systems. Opponent-processing cannot be demonstrated quite so directly but visual illusions and unexpected consequences of different attributes of colour perception have been used experimentally to unravel what is going on with extraordinary success.

One of the outcomes of research into tristimulus systems is the requirement, when choosing primary colours, that two of them cannot be combined to produce the third. Each must be unique so far as the human eye is concerned.
Research into the opponent-process has established that there are in fact four unique colours, red, green, blue and yellow, each of which shows no perceptual similarity to any of the others.

The implications of the fact that human vision can be stimulated by three distinct colour inputs are:

  • In normal conditions, any particular colour seen by an observer is produced by complex patterns of different wavelengths and intensities of light from across the visible spectrum as they enter the eye and are absorbed by cone cells within the retina in real-time.
  • The complex pattern of wavelengths and intensities of light being emitted by a light source at any moment is called its spectral power distribution. A spectral power distribution can be plotted on a graph and always appears as a wavy line with red at one end and violet at the other. The profile of the plot rises for high and falls for low intensities of light.
  • The colour notation used to record tristimulus values can, in principle, describe any human colour sensation.
  • If tristimulus values corresponding with the full range of human observable colours are plotted on a graph, with three axes drawn perpendicular to one another, they can produce an inclusive representation of colour perception in the form of a 3-dimensional colour solid.
  • The three axes correspond with the range of responses of the three cone types and so can be labelled L, M and S. A scale along each axis can be added to correspond with a minimum cone response at one end and a maximum at the other. This is the basis of the LMS colour model, which is one of a number of colour models devised to quantify human colour vision.
    Colour models such as RGB colour and the Munsell colour system also use tristimulus notation to record colour information. The implications are that LMS, RGB and Munsell are all grounded in the trichromatic nature of human vision and take advantage of the resulting opportunities in terms of systems that use additive colour.
    Other colour models such as HSB colour, HSV colour and HSL colour which are all variants of RGB colour do not use forms of notation that correspond directly with tristimulus value.