Spectral flux density

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, although it can be more practical to use W m nm (1 W m nm = 1 GW m = 1 W mm) or W m μm (1 W m μm = 1 MW m), W·m·Hz, Jansky or solar flux units. The terms irradiance, radiant exitance, radiant emittance, and radiosity are closely related to spectral flux density.