Spectral distribution of a light source

Spectral distribution represents the distribution of the energy radiated at different wavelengths in the visible part of the spectrum.

It can be defined for every type of light source.
For example, as illustrated below, the spectral distribution of a high-pressure sodium vapour lamp has the main part of energy radiated in yellow-orange colours while the spectral distribution of a metal halide lamp has its energy radiated in all parts of the spectrum resulting in white light emission.

Spectrum-Metal-Halide Spectrum-High-Pressure-Sodium
High-pressure sodium lamp Metal halide lamp