Which is more important- total photon flux (PF / PPF) or photon flux density (PFD / PPFD)?
Both numbers are important to consider. Measuring total photon flux is like looking at the total flow rate of a showerhead. A showerhead that puts out one drop per second won't get you clean- but at the same time, a showerhead that puts out one gallon per minute as a single stream of water doesn't work well either. However, a showerhead that puts out that same amount of water, but spreads it into multiple streams to cover a larger area is ideal.
Total photon flux gives an accurate reading of the total raw power of a grow light, but if all that light is focused into one small area of your grow, it won't grow well over the entire area. Similarly, if half of the light is aimed away from your plants, toward the ceiling, as with HID (HPS, MH, CMH / LEC) lights, it isn't doing much good either. You may think that reflectors can get 98% of this light back to your plants, but that isn't the case.
LED grow light companies using secondary lenses to focus their light can make their light statistics look good on paper simply because PPFD readings close to the center of the light's footprint will be high. However, this is at the expense of very low light levels everywhere except for the center of the footprint.
It may seem like as long as you're getting enough light levels in the center of your grow area you are going to get good results, but this is deceptive. For example, let's consider a 5x5 foot growing area. You might think that if you had great light levels in the center, 3x3 feet of this area, you are covering the "bulk" of your footprint. (in fact some of our LED grow light competitors make this claim; that the center 3x3 foot area is the "bulk of the footprint" of a 5x5 area). But a 5x5 foot area is 25 square feet, and the central 3x3 foot area is 9 square feet, only 36% of the total 5x5 growing area. This means that 64% of the growing area is not receiving adequate light, which is truly the bulk of the footprint!
For this reason, it is critical to evaluate the PPFD over the entire lighting footprint, including areas that may seem to be less important because they are on the perimeter. This is why we give the averaged PPFD levels, measured from edge-to-edge, corner-to-corner, over every square inch of our advertised footprints, and not just the average of a few light measurements cherry-picked from the center of the footprint, or from under specific portions of our fixtures.