Yeah, thats why I say to calculate both the "theoretical" average irradiance, meaning the absolute photon emission divided by the surface area of the grow (footage) and or/ volume area.

And aditionaly take measurements using a standard light meter (a 20-30$ one) and convert it to photons/m2 using an Excel sheet. This is a true, actual measurement of the light reaching leaves (the main measurement is taken at top of canopy level).

There is no differential absortion of different wavelenghts in air (when talking about 1, 2 or 3 meters as most, and mostly distance is way below 1m). Another thing is how light quality changes below canopy due the differential transmision of leaves, but I always claim to use undercanopy lighting as the most efficient one, that apart of it, largely avoids this problem. So irradiance measurement using LEDs is usefull both at top canopy and undercanopy.

It would be nice people has spectrometer or spectrorradiometers, but I wouldnt expect it. Its necessary to find ways of measurent that any grower interested may do it if he has the will, having a limited equiptment, as a standard light meter. My purpose is to build a database of the spectrums of LEDs most used, so all people has to do is measure with a light meter and then convert lux or fc into uE/m2.