Color is not a physical properly of object, but rather a human perception enabled by light. Nevertheless the color of light sources is described by the industry primarily in terms of two metrics, correlated color temperature (CCT) and color rendering index (CRI), that are only indirectly related to human perception. CCT is intended to characterize the appearance of the illumination generated by source, and CRI is intended to characterize the appearance of objects illuminated be the source. There two color metrics developed nearly of half-century ago, are increasingly being challenged because new source are being developed with increasingly exotic spectral power distribution.
The new color metric applicable to the color appearance of the light emitted by at light source and quantified by the CCT and D.
The chromaticity is one of the critical parameters for light sources for general lighting and normally specified with chromaticity coordinates CIE (x, y) or (', '). However these two numbers do not provide the color information intuitively. For practical purposes, collated color temperature (CCT) is commonly used to provide the chromaticity information of general illumination source CCT, however, provides only one dimension of the chromaticity and there is another dimension, which is the position of chromaticity with respect to Plancian locus. For this purpose «D» or similar terms as distance from Plancian locus have been need used in some part at the industry but these had not beer officially defined is any standard.
Color rendering is general term for describing the ability of a light source to provide color information to human observer when objects are illuminated by that source like CCT color rendering index (CRI), the most accepted measure of color rendering.
CRI was developed, through the system of colorimetry, simply to be an indication of how «natural» or «undistorted» the light source makes the color of objects appear when illuminated by the source when used as the sole measure of color rendering for a light source, CRI simply cannot meet expectations. With the advent of SSL, these limitations have become more widely recognized.
It was proposed a two-metric system combining CRI, a measure of color consistency with respect to a reference source, with gamut area index (GAI), a measure of color saturation. When used to gather, the two metrics appear to optimize the color appearance of natural objects like fruits and vegetable enhancing their vividness with making them appear unnatural.
CCT is shown in commercial instruments but D is often available. D needs to be calculated using the method of triangular solution:
Create a table CCT is distance di to Plancian locus on () coordinate;
Find the closest point in the table;
Solve the triangle for the neighboring 2 points.
Gamut area of light source is commonly calculated as the area of the polygon defined by the chromaticities in CIE 1977 (', ') color space of the light CIE TCSs. Gamut area of EES is sealed to 100 and defined as gamut area index. The gamut area of any other light source is scaled accondingle.
Assessing skin tone heterogeneity under various light sources
