An object which emits light or radiant energy to which the human eye is sensitive. The emission of a light source can be described by the relative amount of energy emitted at each wavelength in the visible spectrum, thus defining the source as an illuminant, or the emission may be described in terms of its correlated color temperature. (Johnson SF (2001) History of light and colour measurement: A science in the shadows. Taylor & Francis, UK; Saleh BEA, Teich MC (1991) Fundamentals of photonics. Wiley, New York) See Illuminant and Correlated Color Temperature.