Dich

7.1 THE CIE WHITENESS EQUATION
WHITENESS
7-1 The CIE whiteness equation.
It is not so simple that a material which reflects all light, that is the perfect reflecting diffuser, is the whitest white. Whiteness is a subjectively perceived property. Most people consider that it increases when the material has a slightly blue tone. It is thus clear that whiteness in some way is an aspect of colour perceiption, and that any measurement of whiteness must be based on colour measurement. A now generally accepted equation has been presented by CIE, where the whiteness W is calculated as: (74)
Where
Y10 = the V-value
X10, y10= the chromaticity co-ordinates
Xn.10, yn.10= the chromaticity co-ordinates for the D65-illuminant.
Y, x, y are calculated for an observer angle of 100 and the D65-illuminant. The weighting function for the calculation of X,Y,Z, 100 observer and the CIE illuminant D65, see Appendix.
The equation says that whiteness is composed of the luminance Y and a clour term 800 (xn.10 – x10) + 1700 (yn.10 – y10) which increases the whiteness when the material approaches blue and reduces the whiteness when the material is yellowish.
The whiteness is complemented with a calculation of a green/red tint value T according to (75)
When T is positive, the material is greenish, and when T is negative, the material is reddish.
Fig.7.1 The whiteness (CIE 1983) increases most along the dominant wavelength 425nm, whereas the neutral exis which separates a red line from a green tint lies along the dominant wavelength 465nm.