Y (in colour chemistry): (a) One of the three CIE tristimulus values, equal to the luminous reflectance or transmittance, the green primary; (b) Spectral colour matching function of the CIE standard observer used for calculating Y tristimulus value; (c) One of the CIEchromaticity coordinates calculated as the fraction of the sum of the three tristimulus values, attributable to the Y value.