The reason Illuminant D50 is mandatory is the need for a standardized common language across the entire supply chain. D50 is officially defined by the CIE and provides an accurate simulation of daylight with a 5000 Kelvin temperature. This light is a technical requirement based on the ISO 3664 standard, ensuring that color data and visual evaluation are interpreted identically throughout all production stages.

D50, with its high Color Rendering Index (CRI), is vital for activating the brain's Color Constancy mechanism during evaluation. Failure to use D50 causes color data to be misinterpreted, and this mismatch directly leads to color difference and the emergence of the Metamerism phenomenon.