Purity, which is equivalent to saturation when talking about xy chromaticity, describes how “pure,” or monochromatic, a color is at a given lightness. For example, consider all spectra with a lightness of 50. The spectrum could be composed of equal intensities of all colors, in which case, the color would be grey. This is low purity or saturation. It could also be composed of one intense peak with a small FWHM. This is high purity or saturation.
Purity is similar to chroma, so much so that the terms are often interchangeable. The difference is that chroma involves a constant amount of light hitting a surface, while purity involves a constant amount of light being transmitted or reflected from a surface. In the SpectraWiz software, the 1932 CIE chromaticity diagram calculates the purity.
