In
radiometry,
radiant intensity is the
radiant flux emitted, reflected, transmitted or received, per unit
solid angle, and
spectral intensity is the radiant intensity 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 radiant intensity is the
watt per
steradian , while that of spectral intensity in frequency is the watt per steradian per
hertz and that of spectral intensity in wavelength is the watt per steradian per metre —commonly the watt per steradian per nanometre . Radiant intensity is distinct from
irradiance and
radiant exitance, which are often called
intensity in branches of
physics other than radiometry. In
radio-frequency engineering, radiant intensity is sometimes called
radiation intensity.