Without knowing the true wattage and being able to measure the light levels nobody can give you a good answer to that. Your light's manufacturer should provide you with the recommended hanging height based on a PAR map. Getting a cheap LUX meter wouldn't help as they don't read blurple spectrum lights, which I'm guessing is what you have. Do some searching for reviews of the model you have and you might pick up some tips on hanging height.
"don't read blurple spectrum lights" - interesting point.
LUX meters don't read
any grow lights but what you wrote made me wonder if you can get a valid conversion factor despite the fact that the spectrum of a blurple is, speaking in technical terms, wonky.
LUX meters have a sensitivity similar to the human eye and, to your point, that means that they are far more sensitive to green than either red or blue. That doesn't mean, however, that a LUX meter will be any less
useful than a PAR meter when used with a blurple. Sure, blurples have no/very little green light but that's not an issue for a couple of reasons, one of which is that
ballpark numbers are good enough.Lemme explain…
Here's a copy and paste from an Excel document I put together when I did some testing of a Uni-T light meter vs my Apogee:
The majority of the data is either from HLG's website or Apogee; the factors for 0.015 and 0.016 are just an Excel formula; the value for the Vipar XS-1500 is from my testing.
(I thought I had tested values from my Growcraft X2 veg light and am surprised/disappointed that I didn't. The Rapid LED Royal Blue Puck is a valid proxy).
Per the table above, the conversion factor for a red-heavy light is 0.016. That's the approximate value for the Growcraft "Full Cycle" (flower) light and the HLG Diablo.
At the other end of the spectrum is the Rapid LED light that has a conversion factor of 0.013. I cannot come up with a reason why it's not logically valid to use the average of those values. And averaging those two values works out to ((0.013+0.016)/2) = 0.0145 ≈ 0.015.
Interestingly, 0.015 is the conversion factor that is when converting LUX from a standard "white" LED to PAR.
That's the short version*.
My take - get a light meter and feed your plants well ("nutrients" are not food to a plant; light is food to a plant).
Also, I would
never recommend a light from a manufacturer that doesn't provide hang height recommendations. A decent small grow light is about $100 delivered to your door so spending $50 on a blurple seems penny wise and pound foolish.
*Pieces from "the longer version" (a document I need to write since this issue comes up so often):
- Light readings do not need to be highly accurate. We just need to get the light levels in the ballpark and then we read the plants and change the light to suit (a canopy is an area target so we can adjust fire).
- "All
models meters are wrong, some are useful". An Apogee is calibrated to 5%± so when it reads 1000µmols, it's somewhere between 950 and 1050. Yup, $600 and it's still wrong.
- Given that PPFD changes so significantly with even tiny changes in distance from the source, even the scraggiest scrog will have only a tiny amount of the canopy getting the average PPFD. How many locations in a "regular" grow will be getting the "average" amount of light?
- If you shut the flaps to your tent, unless you take PPFD readings with the tent door shut, your readings are way off. In my grow, the "front row" gets >100µmols extra by closing the flaps.
