Forgive me if this is the incorrect section of the forum for a question like this. If I already have used the CIE Chromaticity diagram to plot a color with dominant wavelength of 600 nm and maximal purity, how should I proceed to describe a color with half the intensity of said color -- i.e. a color with half the brightness of a dominant wavelength of 600 nm with maximal purity? What model, and how? Obviously I can't use the CIE Chromaticity diagram as it doesn't describe luminance, what type of diagram should be used?