The purity discrimination threshold of monochromatic light and mixed light was investigated in the mesopic condition. Monochromatic light and mixed light were first resolved into their components of pure color light of dominant wavelength and white light. By Using the definition of colorimetric purity, the luminance ratios of the pure color light to white light were obtained. The pure color light component in mixed light was calculated Using the luminance ratios. The purity discrimination threshold of monochromatic and mixed light is expressed by the ratio of the pure color component to the white light of a reference field. Therefore, this purity discrimination threshold can be expressed in a unified manner.
