Photosynthetically active radiation
number of Photons in mol that are in the photosynthetically active range from 400nm to 700nm
Calculation:
at 500 nm each photon has hc/λ = 3.9756e-19 J energy
So 1 mol (6.022e23) of photons have 2,394e5 J energy
So to have 1J you need 4,177e-6 mol of photons.
1 W/m² = 1 J/s/m² corresponds to 4,177e-6 mol/s/m²
=> at 500 nm, 1 W/m² = 4,177 µmol/s/m²