Light emitting devices can be categorized as either illumination devices or display devices. Illumination devices, known as luminaires, are primarily intended to make objects and surfaces visible while display devices are intended to graphically convey information. Both emit light by means of illuminants of different types, having different colors. Additive color mixing combines these illuminant types in appropriate proportions to achieve a selected color. Selectable colors include the entire human perceptual gamut from deeply saturated hues to every shade of white. While three illuminant types were traditionally used, four or more illuminant types are useful in providing a broader gamut for both luminaires and display devices. Four or more illuminant types are also useful for luminaires incorporating light sources such as LEDs (Light-Emitting Diodes) that emit a relatively narrow wavelength bandwidth because broader spectrum coverage improves color rendering.
When three or fewer illuminant types are used there is exactly one mixture of the brightness values for each of the illuminants for each possible color, but with four or more illuminant types there are infinite mixtures for each color, known as metamers of that color. Different metamers have varying qualities, among them brightness and color rendering (e.g., affecting the color appearance of surfaces being illuminated), so the best metamer for a selected criterion is preferred. However, finding this best metamer has been a difficult problem. Prior art methods applicable to four or more illuminant types include linear programming, calculus of variations, ad-hoc measurement with lookup tables, and probabilistic algorithms using Monte Carlo simulation or simulated annealing. Many of these methods generate metamers that imprecisely match the selected color. Most of these methods require significant computational resources, especially for large numbers of illuminants. All of these methods fail to achieve a best metamer under a selected criterion (e.g., brightness, color rendering, etc.) for at least some parts of the illuminant gamut, and most cause visual artifacts for at least some color changes. Visual artifacts result from sudden changes in the composition of the computed metamers, so while an aggregate original color and an aggregate new color may differ almost imperceptibly, constituent illuminant brightness can change utterly between them. Such sudden transitions trigger a strong reaction in an observer's visual system that is perceived as an irritating flicker.
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