PPFD...

DLI is what matters most... Read the wiki most people using these terms don't even use them correctly. e.g. PPFD is about photons/s PAR relative to 1 m^2. Most often it's referred to or inferred to be a single point measurement.. The benefit of PPFD is you can compare this value in an apples to apples way with any sized garden. "600" PPFD means the same intensity of light applied across the canopy whether it's 4sq ft or 20 sq ft. DLI takes that a step fruther and makes differeing hours of operations irrelevant when comparing... 35dli is the same regardless of garden size (same as above) and hours of operation. 35 dli over 12 hours is the same as 35 dli over 18 hours and garden size is irrelevant to that comparison, too. The math is precise, but your local "max" target will vary due to local variables. Temp, RH and atmospheric CO2 will dictate the max DLI you can give... so it can vary by garden.. People blindly choosing some random value of DLI are doing it wrong.. You start with an initial target that makes sense (35-40dli in ambient cow), then you still have to observe and adjust to the plant's growth. Even genetics of the plant at hand may cause some variability. You may find variability due to seasonal weather. It's most likely a moving target... So, you simply find a happy medium relative to observed plant growth. The math is a starting point.. it is not "the answer" -- though, techinically, with the proper information and knowing the exact variables, you could precisely predict very accurately... but even then you'd still have some variation due to genetics. Average hygropmeter is +/- 5% .. thermometer +/- 3-5F etc... most people don't know what their atmospheric co2 ppm averages out to.. but that's okay.. you can work backward from plant morphology. it will make your life easier.. it will shorten a learning curve to maximizing your light. But, even without it, you could get to the same outcome simply through a little extra trial and error, too. Even a lux meter can be used to help dial your light in... it may inform you as accurately about photosynthetically active radiation (PAR), but it's readings are consistent per light characteristics and you can use proportional intensity (central values vs corner etc) to help even out the canopy... and determing upper threshold of light before you cause damage... you just may not be able to translate those readings from one light to the next with 100% accuracy. Different cct or spectrum flux will result in a different flux:PAR conversion needed.... but proportional intensity is very useful, regardless.... most peopledon't have 20 differentlights, so keeping notes on targets relative to a couple is not much effort... .and no need fora 400 dollar PAR meter (quantum meter)... they cheap ones are garbage.

Just a number that let's you know how much photons pass through any given metre squared space any given 1 second. Ppfd is historically meant for 400-700nm. New ePar framework pushes this upward of 380nm-780nm. ePPFD. The Emerson Effect shows when you add extra 660 along with extra 730 it creates a synergy that boosts yield around 20% from increased efficiency of ΦPSI and ΦPSII. Luckily all doubt can be cast aside as its based in hard science, removing one's ability to think or doubt.