These days lighting manufacturers and commercial growers regularly cite PPFD values (a metric of plant-usable light intensity) as unquestionable, absolute measures of the potential productivity of their grow lights or indoor gardens.
However, while PPFD (or photosynthetic photon flux density) is a useful parameter when it comes to assessing the general viability and efficiency of a horticultural light source, it is far from the last word when it comes to actual plant response in real-world conditions.
How PPFD Falls Short
Firstly, there are the limitations of the measurement model itself. The most common unit used to measure PPFD is the micromole (µmol) which is essentially a count of photons arriving over a square meter during one second travelling within a specific wavelength band (400 – 700 nm)—this range is commonly referred to as PAR (photosynthetically active radiation).
It’s fair to say that a light source that can send an average of 700 µmol/M2/second to a given area is delivering twice as many photons as one with an average of 350 µmol/M2/second. But these statistics tell us nothing about the spectral quality of the light produced. For instance, the first light source may be low pressure sodium which produces the vast majority of its photons in the yellow / orange / red bands of the PAR spectrum, whereas the 350 µmol/M2/second light source may be a full spectrum light source, much better suited to overall plant growth and healthy morphology. In other words, PPFD numbers tell us nothing about the spectral quality of the light source.
PAR Meters are Not Plants!
There’s another, more subtle, part to this story however—and it’s to do with the PAR meters themselves. The sensors are very good at detecting light when it arrives from a light source directly above it, but are notoriously inaccurate when it comes to detecting angular light. In fact, many PAR (aka quantum) meters—even high-end models—underestimate angular light levels by 60 percent or more.