Whiteness is a property important in many industries. Producers of white
products decided that it would be easier to deal with whiteness as a single
number index rather than three color scale parameters. As a result, various
whiteness indices were developed. In order for a whiteness index to be
valid, it must be used on the type of materials for which it was intended.
When items are being compared using a whiteness index, they must be similar
in gloss, texture, translucency, and other physical attributes. They must
also be very near white. Whiteness values assigned to colored items are
meaningless.
A perfect white is a Perfect Reflecting Diffuser (PRD), which has a
value of 100. Pressed magnesium oxide (MgO) and pressed barium sulfate
(BaSO4) are high-reflectance materials that closely approximate a PRD.
Various whiteness indices exist and are used for numerous applications.
The whiteness index of the CIE is described in
CIE Publication 15.2 (1986), pages 36-38.
WI
is the whiteness index, Y the
tristimulus value and xn, yn the chromaticity
coordinates.
WI
increases the more white an object appears to be. It takes the value 100
for a perfect reflecting diffuser (PRD). The WI
value also depends on tint as well as lightness, e.g. if WI
increases with increasing blueness of the object. For details, please
refer to the tinting index calculation.
The whiteness of an object does not only depend
on the whiteness index, but also on the tint and lightness. The whiteness
must be interpreted as relative distance from a defined white point. Thus
a sample cannot be called white, if tint increases excessively. The combination
of all parameters let a sample appear white with a certain white temperature.
To set a quantitative limit on it, the CIE tinting index TI
is used.
The tinting index is calculated from the following
equations:
