It is computed as:: formula_9where formula_10 is a coefficient that depends upon the illuminant (for D65, Ka is 172.30; see approximate formula below) and formula_11 is the "X" tristimulus value of the specified white object.
a represents, roughly, Redness (positive) versus Greenness (negative), and is computed:formula_64where formula_65 is a coefficient which depends upon the illuminant (for D65, Ka is 172.30; see approximate formula below) and Xn is the X tristimulus value of the specified white object.