The detailed dependence on wavelength of the intensity of radiation absorbed by a given medium. Absorption spectra of gases are typically composed of discrete spectral lines and bands of overlapping lines that depend on the molecular or atomic composition of the absorbing substance and that may be used to identify it uniquely. When measuring the absorption spectrum, the medium should be considerably colder than the source of incident radiation (which should also be continuous in wavelength), in order to clearly distinguish the absorption spectrum from the analogous emission spectrum.