Yellowness scales

In whites there are often degrees of yellowness for which numerical evaluation is desirable. Some colorimeters give direct readings. e.g. Hunterlab colorimeter yellowness coordinate = +b

The ASTM have derived an equation using direct colorimeter readings:

YI =[100(R-B)]/G Where YI = Yellowness index

When converting into CIE coordinates (ASTM E313-98) is calculated as follows:

YI E313 = 100(C_xX - C_zZ)/Y

where X, Y and Z are the CIE tristimulus values and the coefficients depend on the illuminant and observer as indicated in the table below:

These scales are for samples which visibly look yellow or blue and do not vary in redness/greenness.