The reflection from a surface can be diffuse (“body”), specular (“interface” or “regular”) or a mix of the two (Wyszecki & Stiles 2000). In the diffuse case, the incoming light is scattered by the surface without any regularities. Mirror like interaction with light is called specular reflection. The mixed reflection can be either gloss or retro-reflection.
Because an ordinary visual system describes the spectra only with a few descriptors, different reflectances can obtain the same descriptor values. If two colour samples with different reflectance functions have the same colour appearance (= the same descriptor values) under one viewing condition whereas under another they are discriminated to be separate colours, they are called metameric samples. A common factor causing metamerism is illumination change.
Illumination can be described accurately using spectral power distribution SPD which is its radiant output over a wavelength range. A more rough descriptor of illumination is colour temperature. Colour temperature (Wyszecki & Stiles 2000) relates a light source or an illuminant to an ideal model called a Planckian radiator (also called a blackbody radiator and a full radiator) and illustrates the relationship between the red and blue wavelength areas of the SPD. The Planckian radiator is a thermal radiator (hot body) with a continuous SPD depending only on the temperature of the body material. Colour temperature gives a reasonable good sensation of the “colour” of light: a high colour temperature refers to a more bluish light, while a low colour temperature means a light with more reddish components. It defines uniquely the SPD of a Planckian radiator which presents a light emitted by an ideal blackbody source when heated at this certain temperature.
The Planckian locus is the curve formed by the chromaticities of different Planckian radiators in a colour space. The CIE colour spaces model the colour processing of the human vision system. The basic human colour space is CIE XYZ tristimulus values which can be obtained by an illumination dependent transformation from the linear RGB values of the camera. The CIE xy chromaticity coordinates are obtained from the normalization of X and Y tristimulus values by the sum of all three tristimulus values. The CIE Lab and CIE Luv spaces were developed to obtain more perceptually uniform space for colour presentation like Farnsworth’s uniform-chromaticity-scale UCS. The CIE Luv values can be processed further to obtain CIE SH values which correspond to the saturation and hue of the colour.
Device colour spaces like RGB, HSV, YIQ and NCC rgb describe the colour responses for a device which on the other hand can be very different from those of human space. Their and other colour spaces’ formulae can be found in Appendix 1.