Whiteness is defined as a measure of how closely a surface matches the properties of a perfect reflecting diffuser, i.e. an ideal reflecting surface that neither absorbs nor transmits light, but reflects it at equal intensities in all directions. For the purposes of this standard, the color of such a surface is known as preferred white.
So with that being said, what is the Whiteness index? It is the measure which correlates the visual ratings of whiteness for certain white and near-white surfaces. The two most commonly used methods for computing a whiteness index are: CIE Whiteness Index and Ganz-Griesser Whiteness Index.
The CIE Whiteness Index
To make the white weighting more informative, the CIE recommended in 1981 a formula that is today known as “CIE Whiteness.” These indices specified by the CIE for the D65 and illuminant C in combination with either 2° or 10° observer function. However, the equation is commonly used with other illuminants; therefore the value shown will depend on the primary illuminant you have chosen.
The Granz-Griesser Whiteness Index
Tint-CIE Two quantifying parameters are based on the white evaluation by Ganz/Griesser. It is about a simplified version of the Ganz/Griesser-formula. The difference is, that the Ganz/Griesser-formula contains variable parameters, which are calibrated to a white standard, while the CIE-formula is based on fixed factors.
Producers of white products decided that it would most beneficial dealing with whiteness as a single number index rather than three color scale parameters. As a result, various whiteness indices were developed. In order for the whiteness index to be valid, it must be used on the type of materials for which it was intended. This is most commonly used on textiles When items are being compared using a whiteness index, they must be similar in gloss, texture, translucency, and other physical attributes.
Setting the Whiteness Index (WI) displays XXX.X
When taking a measurement for Whiteness Index (WI) the instrument must be set to one of the following illuminant/observer combinations (per ASTM E313-98):
C/2°, C/10°, D50/2°, D50/10°, D65/2°, D65/10°
Setting the instrument to any other illuminant/observer will cause the output to be XXX.X