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Whiteness scales
Since whiteness is a subjective quality where preferences vary there
is difficulty in agreement as to what is the preferred white. However,
there are several 'whiteness' scales:
The whiteness index per CIE is equivalent to the whiteness index
published in ASTM Method E313 in 1998
WI CIE = Y + 800(xn - x) + 1700(yn
- y) = WI ASTM
where Y, x, y are the luminance factor and chromaticity coordinates of
the specimen, and xn and yn are the chromaticity
coordinates for the standard illuminant and source used. These are
given in the table below based on the illuminant and observer used.
| Value |
C/2º |
D50/2º |
D65/2º |
C/10º |
D50/10º |
D65/10º |
| xn |
0.3101 |
0.3457 |
0.3127 |
0.3104 |
0.3477 |
0.3138 |
| yn |
0.3161 |
0.3585 |
0.3290 |
0.3191 |
0.3595 |
0.3310 |
Another way to measure whiteness is to calculate the distance from a
'perfect' white (i.e. Y=100, a=0, b=0) by using a colour difference
equation such as CIELab as follows:
 where
p = CIE purity. Chemstrand scale
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