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Colour is an appearance attribute derived from the spectral composition of the viewed light, and is distinct from shape, size, position or gloss. An internationally-agreed method for describing colour has been developed by the CIE (Commission Internationale de l’Éclairage). This mathematically combines the spectral reflectance, transmittance or emission data for the material, object or light source with the spectral visual efficiency of a 'standard' human eye and, if appropriate, a defined light source (formally called an Illuminant), to generate a set of numbers that yield a unique numerical description of the colour. This is a more precise and unambiguous means of communicating colour than using names such as 'red', or 'duck egg blue'. The measurement of colour is called colorimetry.

NPL measures reflected colour using a calibrated spectrophotometer which may be used in 0°:45°a, specular included (8°:di) and specular excluded (8°:de) geometries and has a resolution down to 1 nm over the spectral range 380 nm to 780 nm. Measurements are made by substitution with respect to an NPL master white standard, calibrated on the National Reference Reflectometer at NPL^[1] . The results are traceable to the ‘NPL – 2007’ scale and are quoted for a temperature of 23 °C.

The spectrophotometer evaluates the spectral reflectance of the measured surface, which is then combined with the spectral power distribution of a defined illuminant and the internationally agreed CIE standard observer functions to give the required (perceptually-weighted) colorimetric values. Typically these colorimetric values are computed using the CIE 1931 2° Standard Colorimetric Observer and the CIE 1964 10° Supplementary Standard Colorimetric Observer for each geometry and for each of the CIE Standard Illuminants A, C and D65. Certificates quote CIE (x, y) chromaticity coordinates, luminous reflectance Y%, CIE 1976 Colour Space (u', v') coordinates and L*, a* and b* values; other colorimetric values can be calculated on request.

^[1] NPL scales for radiance factor and total diffuse reflectance, C. J. Chunnilall, A. J. Deadman, L. Crane, E. Usadi, Metrologia 40, S192-S195 (2003)

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