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 (or, equivalently, per unit frequency). It is a radiometric rather than a photometric measure. 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), and respectively by W·m·Hz, Jansky or solar flux units. The terms irradiance, radiant exitance, radiant emittance, and radiosity are closely related to spectral flux density.
The terms used to describe spectral flux density vary between fields, sometimes including adjectives such as "electromagnetic" or "radiative", and sometimes dropping the word "density".  Applications include:
*Characterizing remote telescopically unresolved sources such as stars, observed from a specified observation point such as an observatory on earth.
*Characterizing a natural electromagnetic radiative field at a point, measured there with an instrument that collects radiation from a whole sphere or hemisphere of remote sources.
*Characterizing an artificial collimated electromagnetic radiative beam.