Photons are quantum entites, shifting energy to or from stores in discrete amounts. So simply counting the number of photons second^{-1} sets the power in the beam, for a particular frequency of photon.

The activity (which is the number of photons second^{-1}) at the source and detector set the power absorbed and emitted.

Varying the frequency of the photon sets the energy shifted by each photon: the activity and the frequency together characterise what is seen

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So varying the activity and the frequency might be expected to determine the power.

And the activity and frequency are compensated quantities – a useful and repreating pattern to think of whenever considering power or energy.