Remember, this thread is four years old.
Old thread, but that doesn't mean it won't help anyone
I am always wondering about this. It's one of those questions you can ask and you get 20 different perspectives on it. I think the best way to go about it, seeing as people have different reflectors, bulbs, wattage, area that needs coverage, etc, is using a lux meter. Most, if not all smart phones can give you a rudimentary idea of how many lux's you're getting around your canopy. I plan on buying an actual lux meter though. Right now I use my phone with an app called LuxMeter installed on it. It uses the same sensor that determines how bright the phone screen should be for the light level you're in.
There's a lot of info about this here:
https://www.growweedeasy.com/lux-meter/ Here's a copy and paste from that link:
< 15,000 lux – sparse or "stretchy" growth – plant isn't getting enough light
15,000 – 50,000 lux – good amount of light for healthy vegetative growth
45,000 – 65,000 lux – optimal amount of light for cannabis plants in the flowering (budding) stage
70,000 – 85,000 lux – a
lot of light, some strains do okay at this light level, but some plants lose their top leaves early under this light intensity, especially plants that are not resistant to heat/light (like many indicas)
> 85,000 lux – at this light intensity, you've hit the plant's "saturation point" which means your plant can't use all the light (be careful of
light bleaching!)
One of the issues I have, is with an ever growing canopy area, what is the optimal height to have to give the best all around coverage to the total area. I'm on my first indoor grow though, so I'm sure with experience I won't need a lux meter after a while. Don't need one now. Just another tool to make it a little easier...