What is the difference between photon flux (PF / PPF) and photon flux density (PFD / PPFD)?
Total Photon Flux or Photon Flux (PF) is a measurement of the total number of photons coming out of a light source per second. It counts all photons, no matter which direction they are aiming or how they are concentrated, and is usually expressed in micromoles of light per second, or μmol/s.
When you only count photons in the 400-700 nanometer Photosynthetic Active Radiation (PAR) range, this is called Photosynthetic Photon Flux, or PPF, but is still measured in μmol/s.
Photon Flux Density is a measurement of the number of photons passing through a particular area per second. It only counts photons going through the specified area, and is usually expressed in micromoles of light per square meter per second, or μmol/m2/s.
When you only count photons in the 400-700 nanometer PAR range, it is called Photosynthetic Photon Flux Density, or PPFD, but is still measured in μmol/m2/s, and is still a measurement of flux per area.
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 (PF) 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 (PFD). 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 (PFD) will change depending on where you measure it. If you get closer to the light source, PFD will increase, and if you get farther away it will decrease. Total photon flux (PF) is not measured at a particular point in space- it is counting all the photons coming out of the light.
Relatively inexpensive "PAR meters" exist to measure photosynthetic photon flux density (PPFD)- although be warned that some PAR meters should not be used with LED grow lights as they will give incorrect readings. To measure total photon flux, the entire light fixture has to be placed inside a measuring device called an integration sphere, which captures and measures all photons coming out of the fixture. Unfortunately, integration spheres are very expensive and must be much larger than the fixture they are testing to give accurate results- to measure our smallest light, the PhytoMAX-2 200, requires an integrating sphere 3 meters (almost 10 feet) across. Here's a picture of the PhytoMAX-2 200 in a slightly-open 3-meter integration sphere for testing:
Note that when measuring light intensity with a PAR meter it is possible to get a higher PPFD reading than the total photosynthetic photon flux for the entire fixture. For example, the total photosynthetic photon flux 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. This is because the PAR sensor is smaller than the one square meter it is reporting the measurement for- 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. The small size of the sensor allows it to see that dead-center under the light the photosynthetic photon flux is 4944 μmol/m2/s averaged out over the area of the sensor, but if you move the sensor 20 inches left or right (still 1 foot under the light), the PPFD level can drop to 210 μmol/m2/s.
This example is for illustrative purposes only- the recommended hanging height for the PhytoMAX-2 1000 is much more than one foot away from your plants to allow the light to spread out and more evenly cover the entire area- plus, PPFD levels of 4944 μmol/m2/s will kill plants! Also note that the total photon flux measured from 300-800nm of the PhytoMAX-2 1000 is 1602 μmol/s- but of this, 73 μmol/s is of ultraviolet light (300-400nm) and 47 μmol/s is of infrared light (700-800nm), leaving 1485 μmol/s in the 400-700nm PAR region of the spectrum.