Some great responses, pardon us jumping in late here!
PPFD measures the number of photons in the 400-700nm range of the visible light spectrum that fall on a square meter of target area per second, and certainly this is the primary metric that every grower should be focusing on in regards to the quality and efficiency of grow lighting.
However, the PPF metric does not factor in the quantity of photons lost to aisles, walls, and spaces between the plants in your facility. That being said, beware of the footnotes—or the lack of them. More specifically, the distance between the grow-light itself and the plant needs to be factored into the equation, as you want to know not only how much light is released from the fixture but how much of it is actually landing on and being consumed by the plant. So, when evaluating your grow light options look at the distance at which this metric is derived…it should be the same.
We’ve been doing a lot of research and development with advanced optics to control the beam spread of our lights, getting a more directional focus on the plants themselves and avoiding light loss, something we detail on our website if you’re interested.