For monochromatic light, you  have the relation I_E = I_N h nu, where I_E is the energy flux of the light, measured in energy/(area*sec), and I_N is the photon number flux, measured in (number of photons)/(area*sec). For general light you do the same in each frequency range.

If resolution of individual photons is possible, or desirable, then what's relevant is the photon shot noise of the source. Cf. for instance, http://ocw.mit.edu/courses/biological-engineering/20-309-biological-engineering-ii-instrumentation-and-measurement-fall-2006/readings/lec14.pdf

for ways of calculating its characteristics.

Dear All,

Thanks very much for your helpful input, in particular references suggested were very useful.

Thanks,

Amir