Research on different cannabis strains has determined that the optimal PPFD for cannabis is 1500 umol/m2/s-1 with supplemental CO2.
Now the key word is WITH SUPPLEMENTAL CO2.
Photosynthesis rate is a factor of 3 main measurements… available co2, PPFD and VPD (vapor pressure deficit) If all of these 3 things are at optimal levels, then cannabis will be at optimal photosynthesis. If one of these factors is limited, then the max rate of photosynthesis is much less, therefore….
IF YOU ARE NOT using CO2, then the most PPFD you really want to go is around 1000 umol/m2/s-1. Since CO2 are broken apart in the process, if you don’t have enough co2, then its quite obvious increasing light will not make a faster rate.
Most home growers and LED enthusiasts shoot for around 800-1000 umol/m2/s-1.
Ive found other light loving plants such as tomatoes have a minimum optimal DLI of around 22, which is the minimum accepted DLI for optimal conditions, therefore 22 DLI translates into roughly 510 Umol/m2/s-1 at 12/12 schedule.. .
So ideally if not using CO2, you want to stay between 500-1000 umol at the top of your canopy, across the ENTIRE canopy, not just directly centered under the panel.
Now with that being said, any light lower than 500 umol, still will flower cannabis, but it wont be at optimal levels, density and may create airy buds. Also with potency being related to intensity of light, a higher intensity will produce a more potent product.
Also, with more intensity, you will get more yield, but your gram/watt will go down. The lower the intensity, the higher the gram per watt generally.