Spectrum

A measurement of light that plots its Wavelength content, i.e. the energy content as a function of Wavelength. Spectra are used in Spectroscopy, but are generally not considered in most domains of Color Theory, where the 3-D Color Models are used.

Illuminants, Reflectance, Transmissibility and Perceived Light can all be quantified as Spectra.

A spectrum of the standard Illuminant D65 is given below, which is the spectrum of typical Daylight. It is very close to Equal Energy White, which would be a straight horizontal line.