Electromagnetic radiation's spectral energy distribution (SED) is a function, i.e., plot of brightness or flux density versus the frequency or wavelength (the choice of which produce different results: see below). When measured on Earth, the effects of Earth atmosphere are relevant. Capturing a wide distribution requires multiple instruments, which causes data-calibration challenges. A distribution over a limited bandwidth often provides signatures for specific types sources.
The alternate term, spectral power distribution (SPD) is used in some other (non-astronomy) fields.
Plotting by wavelength versus plotting by frequency has consequences: the peak wavelength differs in the two cases if each is plotted linearly, a consequence of plotting density against values that are reciprocals. Taking black-body spectrum as an example, plotting wavelengths by millimeter (0mm, 1mm, 2mm, 3mm, ...) crowds all the infrared, visible light, ultraviolet, etc., between 0 and 1 and spreads radio over a wide range. Plotting frequency by 100GHz (0GHz, 100GHz, 200GHz, 300GHz, ..) crowds the radio into 0-300GHz and spreads the others over the rest. Taking the CMB as an example, its peak is cited at different wavelengths and frequencies even though they are speaking of same spectrum, for a temperature of 2.72548 K:
|plotted by||peak frequency||peak wavelength||peak photon energy|
|linear wavelength||282 GHz||1.063 mm||1.168 × 10-3 eV|
|linear frequency||160.23 GHz||1.871 mm||6.26 × 10-4 eV|
|log of either||222.6 GHz||1.347 mm||9.2 × 10-4 eV|
Plots against the log of either produce an equivalent plot: 1mm≡300GHz, 10mm≡30GHz, 100mm≡3GHz, etc.
SED fitting (aka spectral energy distribution fitting) is a method of classifying distant galaxies, to help determine their age and degree of star formation. A challenge is that a young galaxy with dust has a SED across ultraviolet, visible light, and near infrared as an old galaxy. A fitting method is Markov chain Monte Carlo (MCMC), using random walks around a parameter-space to determine the degree of unique fit between an SED and a known type of object. GalMC is an example of software that takes this approach.