In
radiometry,
radiance is the
radiant flux emitted, reflected, transmitted or received by a
surface, per unit
solid angle per unit
projected area, and
spectral radiance is the radiance of a
surface per unit
frequency or
wavelength, depending on whether the
spectrum is taken as a function of frequency or of wavelength. These are
directional quantities. The
SI unit of radiance is the
watt per
steradian per
square metre , while that of spectral radiance in frequency is the watt per steradian per square metre per
hertz and that of spectral radiance in wavelength is the watt per steradian per square metre, per metre —commonly the watt per steradian per square metre per nanometre . The
microflick is also used to measure spectral radiance in some fields. Radiance is used to characterize diffuse emission and
reflection of
electromagnetic radiation, or to quantify emission of
neutrinos and other particles. Historically, radiance is called "intensity" and spectral radiance is called "specific intensity". Many fields still use this nomenclature. It is especially dominant in
heat transfer,
astrophysics and
astronomy. "Intensity" has many other meanings in
physics, with the most common being
power per unit area.