Spectral power distribution

A function specifying how much radiant power a light source emits at each wavelength across the spectrum, typically 380–780 nm.

Radiometry: The science of measuring electromagnetic radiation's power, independent of human vision. The SPD is a radiometric quantity; radiometry supplies its units and measurement rules.
Color rendering index: A CIE metric from 0 to 100 grading how faithfully a source reveals object colors versus a reference. CRI is computed by simulating test colors illuminated by the source's measured SPD.
Visible spectrum: The band of electromagnetic radiation between roughly 380 and 780 nm that human vision detects. The SPD is conventionally tabulated only over this wavelength range.
Standard illuminant D65: The CIE's tabulated average midday daylight spectrum, roughly 6500 K, used as a colorimetric reference. The most-cited SPD in color science, the default white point for sRGB.
Spectral reflectance: The fraction of incident light a surface reflects at each wavelength. Reflected SPD equals source SPD times spectral reflectance, linking the two functions.
Irradiance: Radiant power arriving per unit area of a surface, in watts per square meter. Spectral irradiance is the SPD per unit area and wavelength interval at a receiving surface.
Correlated color temperature: The temperature of the Planckian radiator whose chromaticity is closest to a light source's. CCT summarizes an SPD's warm-or-cool appearance with a single Kelvin number.
Wavelength: The distance between successive crests of a light wave, usually measured in nanometers. The SPD's independent variable is wavelength; the abscissa of every SPD plot.
Incandescent lamp: A tungsten-filament lamp emitting a smooth continuous spectrum dominated by long wavelengths. Its smooth warm SPD is the textbook example of a thermal continuum.
Color matching functions: The CIE curves describing how standard observers match any spectrum with three primaries. These functions are integrated against an SPD to compute tristimulus values.

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