PAR Meters can be handy tools to measure light intensity at discrete points, but how much can growers really infer about their overall plant response?

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.

This is important for two reasons. Plants are naturally well-adapted to exploiting angular light. They have evolved for millions of years with the Sun moving throughout the sky, both on a daily and a seasonal basis. They exhibit a phenomenon known as “positive phototropism” where plants will actively orientate their leaf planes so as to exploit as much direct solar radiation as possible. Secondly, angular light is far more effective at penetrating beyond canopy level foliage so that leaves deeper into the plant can also photosynthesize.

Are Multi-Point LED Fixtures Fooling Growers?

When conducting PPFD tests over a given area, the sensor is held at different positions, typically across a two dimensional matrix beneath the fixture. Multi-point LED fixtures produce focused “beams” of light from multiple sources. Typically a PAR meter sensor will be directly beneath the light source when the measurement is taken. Unsurprisingly this excites the quantum sensor and a relatively high reading is produced.

When similar PPFD tests are conducted with single-point light sources (such as HPS and DE-HPS [for argument’s sake we’re referring to the cylindrical arc-tube as a ‘single-point’]) the quantum meter sensor is often positioned at an acute angle relative to the light source which produces much lower readings relative to the real-world plant response. Also, top-down LED light sources are not good at canopy penetration due to the acute shadowing effect, meaning actual photosynthetic response is limited to canopy level foliage.

So—what’s the answer? It’s quite simple really. Sure, wield your PAR meters around all you want. Take your measurements. Make a note of them—they’re useful! But don’t forget to actually grow plants and observe the real-world plant response. Angular light, intense and penetrating, can deliver much higher yields overall—and isn’t that what really counts?