How do I measure whiteness?

Whiteness is a complex perceptual phenomenon that depends not only the luminance of a sample but also on the chromaticity. To promote uniformity of practice in the evaluation of whiteness the CIE has recommended that the formula for whiteness, W or W₁₀, be used for comparisons of the whiteness of samples evaluated for CIE Standard Illuminant D65:

W = Y + 800(x_n - x) + 1700(y_n - y),

or

W₁₀ = Y + 800(x_n,10 - x₁₀) + 1700(y_n,10 - y₁₀),

where x_n and y_n refer to the chromaticities of the illuminant (D65), and the subscript 10 distinguishes the 10 degree observer data from the 2 degree data.

The higher the value of W or W₁₀ the greater the whiteness: the formulae, however, are only valid for samples that would commercially considered white and within certain other constraints.

If the measurement of whiteness is important, and if samples may be fluorescent, then it is very important that the light source in the spectrophotometer be a close approximation to the D65 illuminant.