How is measuring PPFD in your 8" sphere a more accurate representation of total light output than PPF or averaging PPFD readings over an area?
I like the way this pdf and Black Dog Led page explained it:
Understanding PPF and PPFD pdf
as µMol/S is the value that a grow lamp should specify for its total light output (Radiant Watts would also be appropriate), this would be similar to using Lumens for a regular area light.
is the primary measurement taken by a Quantum PAR meter and is expressed as a density over a unit of area, this would be similar to using Lux or Foot-Candles on a regular light meter
Black Dog Led PF vs PFD FAQ:
A shower head can provide a useful analogy to explain the difference. If you put a balloon around a shower head to capture all of the water coming out of it, you can measure the total amount of water the shower head is putting out per second- its total flow rate, regardless of the angle or concentration of the various water streams. This is analogous to measuring total photon flux (PPF) from a light fixture.
If you take a small cup and put it on the floor of the shower, it will only capture the portion of water coming out of the shower head aimed at the area of the cup. This is analogous to photon flux density (PPFD). Moving the cup around inside the shower will make it fill at different rates- if you put it right under the shower head it will fill faster than if it is farther away, because the water flow is more concentrated close to the shower head.
So, photon flux density (PPFD) will change depending on where you measure it. If you get closer to the light source, PPFD will increase, and if you get farther away it will decrease. Total photon flux (PPF) is not measured at a particular point in space- it is counting all the photons coming out of the light.
When measuring light intensity with a PAR meter it is possible to get a higher PPFD reading than the total PPF for the entire fixture. For example, the total PPF of the PhytoMAX-2 1000 is 1485 μmol/s, but if you put a PAR meter one foot under the center of the light it will read 4944 μmol/m2/s (PPFD).
This is because the PAR sensor is smaller than the one square meter it is reporting the measurement for (the cup in the shower)- if the sensor was actually one meter square, at one foot away from the light it would be capturing virtually all of the photons the PhytoMAX-2 1000 puts out, and averaged over the entire square meter this would read 1485 μmol/m2/s (PPFD).