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The method I knew is use the density of photon multiplied by the velocity of photon. Considering the solid angle distribution, for a certain direction only a part of π/4π can be delivery, therefore the actually we need to multiply c/4 where c is the speed of light in the media.

But the problem is the density. In the conventional situation, such as when we discuss the photon flux from the black body radiation, the density of photon can be calculated by the photonic density of states in the media multiplied by the distribution function, the BE distribution with a temperature equals to the source's. This is the typical method we can find in many book, especially in those about solar cell and thermal radiation.

But how about the light is monochromatic here? If the source is still spontaneous emission here, dose it possible to emit a monochromatic light without external filter? If it cannot, then we can still use BE distribution to calculate the photon flux on the passing wavelength. If it can, how can we calculate the photon flux here? And what distribution function we can use here?

Because the distribution function is defined on a thermal equilibrium environment. Is there any other more general method to calculate the photon flux(photon density) without using distribution function?