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Reference.org
Radiant flux
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<p>In <a href="/facts/Radiometry/w9j9PXSB">radiometry</a>, radiant flux or radiant power is the <a href="/facts/Radiant_energy/8QHpm6KY">radiant energy</a> emitted, reflected, transmitted, or received per unit time, and spectral flux or spectral power is the radiant flux per unit <a href="/facts/Frequency/QUSbPaW8">frequency</a> or <a href="/facts/Wavelength/kcJU0ymz">wavelength</a>, depending on whether the <a href="/facts/Spectral_radiometric_quantity/w9j9PXSB">spectrum</a> is taken as a function of frequency or of wavelength. The <a href="/facts/International_System_of_Units/S7YLglNz">SI unit</a> of radiant flux is the <a href="/facts/Watt/VK8oqRrm">watt</a> (W), one <a href="/facts/Joule/9ndq9jD2">joule</a> per second (J/s), while that of spectral flux in frequency is the watt per <a href="/facts/Hertz/6Wshj5qm">hertz</a> (W/Hz) and that of spectral flux in wavelength is the watt per metre (W/m)—commonly the watt per nanometre (W/nm). </p>