illuminants

An illuminant is a light source characterized by its spectral power distribution (SPD), the amount of light it emits at each wavelength. In color science, photography, film, and computer graphics, illuminants define the lighting condition under which colors are observed or computed. The SPD of an illuminant interacts with the reflectance properties of objects to determine the colors that are perceived.

Illuminants are often described by their chromatic properties and color temperature. The Planckian locus describes the approximate SPD shapes of blackbody radiators, which helps relate color temperature to perceived white balance. In practice, many standard illuminants are defined by the International Commission on Illumination (CIE) to represent common lighting conditions, enabling consistent color calculations. Common examples include A (approximately 2856 K, incandescent tungsten), D-series daylight (such as D65 at about 6500 K and D50 at about 5000 K), and E (an equal-energy white source). F-series denotes various fluorescent spectra. These standards provide reference SPDs for color rendering and colorimetric calculations.

Applications and implications include color management, white balance, and color rendering. When colors are measured or reproduced, the chosen illuminant affects perceived hue, saturation, and brightness. The color rendering index (CRI) and other metrics quantify how faithfully an illuminant renders colors compared with a reference source. In computer graphics and photography, illuminants are used to light scenes and to simulate realistic or stylized color appearances. Real-world lighting is often a mix of sources with different SPDs, leading to continual adjustments in color balancing and rendering.