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
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.