In 1931, the International Commission on Illumination (Commission Internationale de l'Eclairage, or CIE) developed a standard observer model [13] based on work by Wright [14] and Guild [15]. This model assumes a brightness sensitivity function which is a linear combination of trichromatic stimuli [16]. Guild's determination of a standard observer measured the spectral sensitivity of three additive primaries in relation to each other by use of a small field (2-degree) bipartite matching to sample experiment where the reference stimulus was a white tungsten incandescent light of color--temperature of about 
kelvin and the primaries (650nm, 530nm, and 460nm) were obtained by appropriate filters. Guild and Wright obtained similar results for their chromaticity coordinates and the means of the data from these two experiments will be used as the empirical color matching data for the analyses reported here.