Spectral flux density

<p>In <a href="/facts/Spectroscopy/mj71FuNK">spectroscopy</a>, spectral flux density is the quantity that describes the rate at which <a href="/facts/Energy/wd8PrQM7">energy</a> is transferred by <a href="/facts/Electromagnetic_radiation/Ynx7ujCW">electromagnetic radiation</a> through a real or virtual surface, per unit surface area and per unit wavelength (or, equivalently, per unit frequency). It is a <a href="/facts/Radiometry/w9j9PXSB">radiometric</a> rather than a <a href="/facts/Photometry_(optics)/eo6J7qj3">photometric</a> measure. In <a href="/facts/International_System_of_Units/S7YLglNz">SI units</a> 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), and respectively by W·m−2·Hz−1, <a href="/facts/Jansky/8m8DQU5F">Jansky</a> or <a href="/facts/Solar_flux_units/FHud9lGH">solar flux units</a>. The terms <a href="/facts/Irradiance/fgHwu7uh">irradiance</a>, <a href="/facts/Radiant_exitance/US1T54p8">radiant exitance</a>, <a href="/facts/Radiant_emittance/US1T54p8">radiant emittance</a>, and <a href="/facts/Radiosity_(heat_transfer)/hUyqEqIj">radiosity</a> are closely related to spectral flux density.
</p><p>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:
</p>
<ul><li>Characterizing remote telescopically unresolved sources such as <a href="/facts/Star/euBFuo07">stars</a>, observed from a specified observation point such as an observatory on earth.</li>
<li>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.</li>
<li>Characterizing an artificial collimated electromagnetic radiative beam.</li></ul>

Spectral flux density open-in-new

Spectral flux density