 Photon emission rate from monochromatic power

A monochromatic beam of frequency $f$ and mean radiated power $P$ carries mean photon rate $\dot{N}=P/(hf)$. It follows by dividing energy per unit time by energy per photon. A broadband source instead requires integrating spectral power divided by the frequency-dependent photon energy; electrical input power must first be converted to radiated power.