Quote Originally Posted by SnSstealth
Lumens are a way of measuring light yes but the way it is calculated is based on the human eye not a plant leaf. Each wavelength in the spectrum is given a multiplier based on the eyes sensitivity to that color, therefore greens and yellows are given an exponentially larger number to multiply with than will reds and blues which are much darker to our eyes.
To measure the light we see, measure luminous flux.
To measure the light a plant sees, measure radiant flux.