Standard illuminants are defined light sources used to provide a consistent reference for color measurement and comparison. They are essential in fields like photography, printing, and lighting design, ensuring that colors appear the same under different lighting conditions. These illuminants are based on the spectral power distributions of various light sources, such as daylight or incandescent bulbs. The most commonly referenced standard illuminants are part of the CIE (International Commission on Illumination) system, which includes types like Illuminant A and Illuminant D65, each representing different lighting conditions.