"quality" - in the Frontiers paper I attached, the definition of quality is the ratio of the mass of the inflorescence to mass of the above vegetative matter (everything but the roots). The higher the better and, plants the get a lot of light tend to have a higher ratio.
From what I have read THC and cannabinoid content tend not to vary with PPFD/DLI and there's no concept of "dense nugs" when light levels are discussed. The only time I've seen "dense nugs" discussed was in a video by Mr Growit while interviewing Bugbee and, to Bugbee, it was a non-issue because that's usually a function of genetics, assuming there are no significant issues with the grow.
When plants receive very little light they have very distinct characteristics - large internodal distance, not a lot of inflorescence, and they tend to be quite tall. The word "scraggly" is apt.
I've attached two pictures of plants that were grown in "high light" levels and that were vegged using a veg light. They're the exact opposite of plants that have had very little light - short, compact, and very dense foliage.
This is a 29 day old Glookie (photo) that's running at 800µmol @ 24/0. It's 800 at dead center but I need to raise the light to get things to even out. The plant has been topped but I haven't started LST (lazy) yet. The spectrum for this light is the one on the left. The high percentage of blue light helps keep the plant short and compact, the high light levels result in a lot of photosynthesis/growth activity.
800 - great!
Two questions - why did you stop at 800 and how did you measure your PPFD?
RE. 800 - I've run as high as 1050µmol and my grow did fine. For one grow, I left it at 900, IIRC, because there was so much weed that I wasn't interested in taking more light readings. That grow ended up at 750gm± m2. If your grow can support, say, 1kµmol you might increase yield by ≈ 8% vs having an average PPFD of 800.
If your plants are getting too much light, the first sign is that leaves with "taco" or "canoe" or, perhaps, rotate the leaf around the petiole in the same way that a venetian blind turns. The plant does this because it can't process the incoming photons. If light levels remain high, plant tissue will be destroyed. I've also had a cola bend at 1200. The leaves recovered very quickly - they were back to their normal shape in a little over an hour. The cola that I bent had only a slight bend and didn't straighten out.
Re. measuring PPDF - per my other postings, I consider all Photone readings as suspect unless it's been calibrated against a known good source. For my $32, I'd go with a Uni-T Bluetooth light meter. For one, there's some level of accuracy out of the box (the manufacturer states ±5%) but I also like the Uni-T Bluetooth because it makes it easy to sample multiple points across the canopy. Also, I'd rather put a $32 light meter in a tent than my my $1k iPhone.
Another option for calibrating is to use Apogee's "ClearSky" app, iOS only. It's available on the App Store and Apogee put up a YouTube video on it recently. You might want to check that out.
Full disclosure - I use an Apogee as well as a Uni-T (not Bluetooth). I wrote the attached paper when I was testing my Uni-T against my Apogee.
