Radiant flux is the power carried by electromagnetic radiation: radiation energy transferred per unit time, measured in watts, . Specify what is counted: all radiation emitted by a source, radiation within one beam, or radiation intercepted by a photodetector. Writing for the cumulative energy crossing the specified boundary givesThe first expression is an instantaneous derivative; the second is an average over the stated time interval. If arrives during , the mean radiant flux is . The photodetector's absorbed power equals this incident radiant flux only if it absorbs all of the incident radiation.
Keeping radiant flux fixed while widening a beam reduces its irradiance, the incident power per receiving area. Radiance additionally resolves the propagation direction and has units ; an optical camera can collect different radiant fluxes from sources with the same radiance but different apparent solid angles. Luminous flux, measured in lumens, weights the optical spectrum by the human eye's standardized response instead of counting energy equally at every wavelength. Equal radiant fluxes need not give equal luminous fluxes.[ref]
Radiant power also differs from the optical power of an optical lens, which is reciprocal focal length in dioptres. Likewise, astronomical radiative flux is power per area. Stating the units prevents these shared words from hiding different physical quantities.
Radiant flux, also known as radiant power, is the measure of the total optical power of electromagnetic radiation emitted, transmitted, or received per unit time. It is expressed in watts (W) and accounts for all wavelengths across the electromagnetic spectrum, not just those in the visible range.
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