For reasons probably rooted in evolution, the preferred color domain for most artificial light sources is also on or near the blackbody locus x, y points above the locus are perceived as too green, those below as gloomy and purple. The color points for daylight also fall on or near the blackbody locus, toward the high-temperature end. The curved line extending through the center of the diagram is the locus of colors of blackbody sources of the indicated temperatures. The x, y value for the sum of two sources of color x′, y′, and x″, y″ are found along a line joining the two x, y coordinates, which is a useful additive property. Color points x, y for nonmonochromatic radiation sources lie in the space enclosed by the locus of spectrum colors. Two criteria, the whiteness values of the samples and their tinting factors, generally had a strong effect on the decisions of the observers.įigure 5 shows the locus of monochromatic colors on the x − y plane, forming the 1931 chromaticity diagram ( x − y diagram).
However, in the case of white samples with high CIE whiteness indices, the increase of the range of ranks assigned to samples and the distribution of their frequency showed that, for these samples, observers were often confused and their decisions varied significantly. For samples with low values of CIE whiteness indices, observers judged white samples much more easily and decisively than those with higher values of whiteness. The conformity between the CIE whiteness formula and the visual estimations were verified. In a study by Jafari and Amirshahi (2008), white fabrics that satisfied the CIE WI limitations were visually ranked by unskilled observers (rank ordering method) and the variations of the orders by different referees studied. Again, the measuring instruments must have illumination resembling daylight ( Ganz, 1979). The formulae produce relative, not absolute, white assessments seemingly adequate for commercial uses in many cases. Whiteness and tint formulae proposed by the CIE are restricted to samples differing not too broadly in tint and fluorescence. The tint deviation or hue value can still not be adequately matched. This also improves the matching of different measuring instruments for whiteness. Evaluations with the formulae are significantly improved if the sample illumination is stabilised and fitted as closely as possible to a desired illuminant. The CIE suggests that the formula should be used only for relative evaluations and these are valid only for measurements with a single instrument at a given time and without reference to a white scale. One clear disadvantage of CIE WI is that the system does not clarify whether the whiteness has any component of blueness or yellowness.