In
radiometry,
radiant flux or
radiant power is the
radiant energy emitted, reflected, transmitted or received, per unit time, and
spectral flux or
spectral power is the radiant flux per unit
frequency or
wavelength, depending on whether the
spectrum is taken as a function of frequency or of wavelength. The
SI unit of radiant flux is the
watt (W), that is the
joule per second in SI base units, while that of spectral flux in frequency is the watt per
hertz and that of spectral flux in wavelength is the watt per metre —commonly the watt per nanometre .