In
radiometry,
radiant exitance is the
radiant flux emitted by a
surface per unit area, and
spectral exitance is the radiant exitance of a
surface per unit
frequency or
wavelength, depending on whether the
spectrum is taken as a function of frequency or of wavelength. This is the emitted component of
radiosity. The
SI unit of radiant exitance is the
watt per square metre , while that of spectral exitance in frequency is the watt per square metre per
hertz (W·m
-2·Hz
-1) and that of spectral exitance in wavelength is the watt per square metre per metre (W·m
-3)—commonly the watt per square metre per nanometre . The
CGS unit erg per square centimeter per second is often used in
astronomy. "Radiant emittance" is an old term for this quantity. Radiant exitance is often called "intensity" in branches of physics other than radiometry, but in radiometry this usage leads to confusion with
radiant intensity.