a) Amounts of the three reference color stimuli in a given trichromatic system required to match the color of the stimulus considered.

Note: In the CIE standard colorimetric systems, the tristimulus values are represented by the symbols X, Y, Z and X~, Yn, Zn. b) The amounts of the three reference or matching stimuli required to give a match with the light considered in a given trichromatic system.