If you take notes of the lux values relative to each "type" of light.. it's very useful. If the CCT and other properties are the same, then the values can be re-used under different equipment with those same specs. Otherwise, you'd want to take new measurements and similar trial and error of finding what gives the resulting "optimal" growth pattern for your environement. No matter how you measure this, you need to observe the plant and adjust to what you see and not some theoretical target DLI that is only meant as a ballpark idea and not an exact answer.
It's the same exact systematic process you'd need to take with a "quantum meter" that accurately measures photons/s of PAR. The initial starting point will be more informed with a quantum meter, but you still have to go through a trial and error process to find optimal hours of operate (a commonly overlooked tool of manipulation) x intensity of light (PPFD and resulting DLI) with good distribution across your canopy. But even this isn't a huge difference because some common sense range of hanging distance is going to put you in a good starting spot anyway.
Consider impace of how light spreads out very quickly, therefore it increasingly loses intensity in any 1 spot the further it is away from the light source. So, a power/hangingdistance/hours of operation combo that results in the lights being very close to the plants is find for smaller, younger plants. Except in extreme cases of diodes perfectly distiributed relative to growing area, A combo of those facters that require a bit more distance from the canopy will give better light penetration -- a complete waste early on with young plants. But once you figure it out for one context, the same lux values at canopy can be used in later life stage contexts - even if going from 18hours to 12hours.
usually with a qb you need 18-24" to properly cover the footprint that 100% power and 12hours opeartion (for photoperiods, or longer for autos). "Bar" style lights can often get much closer. Working backward from your ending canopy size and distributing light to edges and corners is more about geometry - take measurements all over the place and make sure the deviation from central area to edges is reduced to acceptable levels. Start here for obvious reason. Then find the resulting power from light x hours of operation that give you the best growth.
the proportions will remain the same.. if the corner is 60% of middle, it'll be 60% of middle whether you have the light at 100% or 50%. This is why the lux readings have the integrity of a 500 usd quantum meter's readings of umol/s of PAR. You'd also see a 60% reading in corner compared to central area with the quantum meter. it is 1:1 proportional in this way.
the drawback to lux readings is that light with different properties will read different values for the same umol/s of PAR. umol/s of PAR is what the plant cares about and nothing else. Resulting DLI is what matters underneath. So, if you use different equipment, you may notice slight differences as to max lux you can give to canopy, but underneath it'll still be a similar umol/s of PAR (PPF) and DLI (ppfd x hours of operation, simlpy).
Read wiki on Daily Light Integral (DLI). Get the gist of it. You dont need to memorize the math.
going back to above how once you figure out a good luz value at canopy and hours of operation in vege phase can be translated to a 12 hour cycle without re-measuring anything. inversely proportional to the change in hours of operation you need a boost to power. so 18 down to 12 needs 150% more power, or lux. ( x 18/12 or 3/2 or 150%) In the end the same DLI is the goal, and it is proportional in this way to hours of operation.
Light's dimming power is usually also mostly 1:1 but not something you can assume, but if the plants react the same, that is confirmation. (see examples of this in cocoforcannabis light model reviews.. you see the umol/s output is mostly proportional to the percent power of the light dimmer, though not perfect.
learning the math from the wiki and using your light's spec sheet in conjunction with a lux meter is going to be pound for pound just as good and just as easy to optimize as a quantum meter. it jsut takes a bit more effort on your part. And half assing it is still very useful along with trial and error, as stated above.
this looks like a lot.. but it's quite simple and should be common sense deductive processes to reach a goal vs anything that needs to be re-read. learn the definitions and how to perceive light and plants absorption of it. then it's just common sense from there.
i'd defnitely trust a lux meter over a phone app, but the important thing to this process is that the readings are consistent, which they are. It doesn't matter if your trial and error leds you to a reading of 25k or 60klux or 900 PPF over 12 hours. What matters is that specific level of light is always "60klux", which it will be under that same exact model of light (or same exact light properties). The apps and camera lenses give inflated values by comparison, but it'll be inflated in the same exact way each time. the cheap lux meters are more directional in the light it captures - i believe is the cause for deviation. it is mechanical in nature.
