Is there a direct relationship between Lumen and PAR? I am not an expert, but I thought they are two different units, and things like spectrum play a role? Like having two LEDs with the same luminous flow, one with peaks in the yellow/green area, another one more in the red/blue photosynthetically speaking more significant area ?
Yes on all the questions. On Apogee's site there is a lumens to par conversion equation. There is a relationship but it's an approximation for white sun light. A par sensor will have a peak toward the blue.