On your latest garden talk Q&A (#6) video someone asked about dimming your grow lights and you mentioned that you use a PAR meter to determine your light height but also that a PAR meter is super expensive. Next you suggested going by the manufacturers "height above the canopy" recommendation. Question: I know that Illuminance (lux/fc) is not exactly the correct metric to use since it is "photopically corrected" (i.e. weighted against the sensitivity of OUR eyes, not a plants needs) but certainly it is a better metric to use than rules of thumb distances above the canopy? Also, a decent lux meter is less than $20 on amazon. I am curious as to why I see more growers sticking by "X number of inches above canopy at Y stage of growth" rather than measuring the canopy illuminance and adjusting accordingly.
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Some people do it and get away with it. The problem is that not all PAR is included so you never really know how much you're giving the plant. Some would argue the PAR charts are more accurate. But if it's working for you then keep going with it.
Unfortunately I cant remember where I saw it but if your like me and can't afford an expensive par meter, you can opt for a decent lux meter and convert the measurements over. Like I said though I cant remember where. I have personally never used a par meter and really cant justify the money to buy one. I dont think it would help enough for the cost. It would be interesting to know for sure, but I have done fine without it.