In spectroscopy, spectral flux density is the quantity that describes the rate at which energy is transferred by electromagnetic radiation through a real or virtual surface, per unit surface area and per unit wavelength. It is a radiometric measure, as distinct from measures that characterize light in terms of the electromagnetic field or photons. In SI units it is measured in W m^-3, although it can be more practical to use W m^-2 nm^-1 (1 W m^-2 nm^-1 = 1 GW m^-3 = 1 W mm^-3) or W m^-2 µm^-1 (1 W m^-2 µm^-1 = 1 MW m^-3), W·m^-2·Hz^-1, Jansky or solar flux units. The terms irradiance, radiant exitance, radiant emittance, and radiosity are closely related to spectral flux density.