I'm really ripped now so take this as a disclaimer. I'm only sharing here to help the discussion and possibly some other thoughts. Much of this are my own GG theories that I would not go trying to publish, so don't take it as dictionary...but I have developed a good understanding of light, intensity, and plants over the years...so you might have an apiphany or two if you can hang on for the ride.
Lab measurements and calculations vs field measurements and reality...I've barked up this tree a few times.
I don't even know where to begin...get ready for a long one, I'm super irie...
We need to actually understand what they are expressing in order to use them in an effective manner. And also where/how/why they were developed.
They were developed based on field or outdoor situations where the sun, for all intents and purposes, is like a blanket of even light that isn't affected by source distance. You get 2000µmols on the ground or 100ft in the air...move it left 5 miles or right 5 miles...still 2000µmols. And the algorithms used in PAR measurements are based off this. And so is "avg PPFD". This is also similar to what a big facility with many lights creates...even spread is created, and variations are minimized over a majority of the crop. For single lights and/or smaller grows, you need to think deeper about what is going on.
What is avg PPFD, the calculation?
It's the PPF(total possible output) of a lamp in a square meter. And is very easy when working with a m^2. But, what if the area is not a m^2?? Then you would confine(divide) the light into the area you are working with(3x3=.836m)...then projects/predicts the rest of the square meter with imaginary light...because it's based off being outside where no fall of would happen.
Ex.
So 720 ppf...confined to a 3x3...is 720ppf÷
.836meter^2=~861ppfd. So that means that if that lights' intensity within the 3x3 were to continue out to a full m^2, as it would outdoors, it would be putting out 861µmols over that m^2. But it doesn't...no matter what, there is only 720µmols in the actual grow area for the plants to use...so remember and think about that. It is just LIKE growing in a m^2 area with 861ppf over it.
At first seems off...but when you have that, and can see the next step/style, you really can know the possibilities of a lamp.
What is a PAR meter showing?
A PAR meter is an instantaneous field measurement of µmols of photons per second in a m^2(square meter). It is a photodiode that creates a current. An algorithm is applied to come up with a figure for a m^2. But's it's based on the intensity at the sensor ...in that single point in space where the sensor is held...then extrapolated based on outside light behavior using the algorithm, to a figure expressing how many µmols of photons per second are falling in a m^2 based on that sensor as if the whole m^2 was covered in that intensity.
Ex.
So an 1000µmols dead center reading is saying...that if the sensor(1sqin in size) was the center of a m2, and the sensor is getting .645µmols...it assumes every 1550sqin of the m2 is roughly the same, and would give you 1000µmols/sec/m2. (.645 x 1550=~1000)
Lets keep the whole sensor is the center of a m^2 thing in our heads...
Now we move to the outer edges ofthe canopy. We get a low reading with our PAR meters...say 400µmols. Well that means you imagine the m2 around the sensor...half the m2 is now off of the canopy by almost 20" and outside the reach of many of the photons that it was sensing while in the center.
And then the corners are even worse...
Now tangent to the concept of 700-1000µmols per m2 is ideal. This is super easy to relate and cross check with "avg ppfd"...it is either in there or it isn't. But I believe this gets misunderstood when using instantaneous meters(PAR meters) because of what I just showed above. And what I am about to say almost the reverse of confining ppf to make "avg ppfd". Take pic#2...400µmols on the edge...but half of the expressed meter is outside the light...well what if we confine it to the lighted/canopy area...what would it show.? 800µmols is my theory and what I have seen worth of growth in my gardens over the years.
Time to recharge
