A subcommittee of the American Society of Agricultural and Biological Engineers (ASABE) recently defined Photosynthetic Photon Flux (PPF) as the main metric to measure lighting for plant growth.

Many organizations and utilities will likely adopt a PPF Standard of 1.6 µmol/joule as the minimum level for lighting systems at which occupancy permits may be issued and rebates may be offered to licensed growers. This new standard has many limitations and does not account for how spectral quality affects plant growth or what different qualities of light are needed for the vegetative and flowering stages.

The new ASABE Standard X640 will normalize the world of grow lighting for plants into a single common metric. Lighting, artificial or supplemental, will be categorized into one classification of “goodness.” That metric is micromoles (μmol) per second, or, in use of energy, μmol per joule. It doesn’t matter whether one grows leafy lettuce, watermelons, or cannabis; or if they are cloning, starting seeds, or flowering, the metric will be the same.

It’s like measuring the quality of a car based on fuel efficiency only.

Since the legalization of recreational marijuana in Colorado and Washington State in 2012, there’s been an eruption of companies developing products to help commercial growers cultivate, process, and dispense the controversial benefits of cannabis.

For those engaged the last couple of decades in the underground market, it is both an opportunity and a challenge to serve the greater good of their dedicated hobby-turned-commercial clientele.

The new wave of legalization has now put a burden on certain resources. The use of electricity and water, two of the main ingredients needed to grow successfully indoors, has come under scrutiny. Regulatory standards are being established in such a manner that a one-size-fits-all philosophy may stifle the innovation and creativity that once engaged this market.

Is history a good lesson to follow?

The debate on finding this one measurement has still not been settled. There are factions who want to follow in the footsteps of what has happened in the general lighting market where commercial LED lighting fixtures and residential lamps are categorized by lumens per watt (LPW).

The LPW metric is based upon the photopic visual acuity of humans. That is where the term “Lumens are for Humans” originated. More lumens for each watt of energy mean a greater ability for humans to see in the most energy-effective manner… sort of. But let’s not go there for now. Just take note that LPW does not define the quality of light, only an amount per unit of energy.

While trying not to bog you down with the science of lighting, there is something for you to understand. The LPW metric, while suitable for humans, does not carry the same suitability of visibility for other animals, like cats, dogs, horses, or even snakes.

LPW is the efficacy of either a bare lamp measured in an Integrating Sphere and reported in a certified Photometric Report, or an HID or LED Fixture measured by a Gonio photometer. HID fixtures are reported in relative terms. LED fixtures are characterized in absolute terms, as the system is power and temperature dependent.

But this article is about plants, not humans. So instead of using LPW as we do in the general lighting industry, let’s talk about terms with which growers are familiar.

To understand PPF you must first know where it comes from and why it was coined. The McCree Curve represents the average photosynthetic response of plants to light energy, known as PAR (Photosynthetic Active Radiation). The McCree Curve, also known as the Plant Sensitivity Curve, begins at 350 and extends to 750.

When it Comes to Cannabis, Not All Grow Lights are Created Equal

Above: V: relative luminous efficiency of equal incident radiant fluxes as a function of wavelength, for the C.I.E Standard Photometric Observer (6). P: relative photosynthetic efficiency of equal absorbed quantum fluxes, as a function of wavelength, for average green leaf (8). McCree KJ (1973) A rational approach to the light measurement in plant ecology.

PPF is the most commonly used method of measuring PAR. It gives equal value to all photons generated by a light source between the wavelengths of 400 and 700 nanometers (nm) and is measured as µmol/second. This happens to be the same range at which humans use light to see. PPF Efficacy, in either μmol/second/watt or μmol/joule, is the energy efficiency term. It is akin to LPW, but in plant talk.

While PPF or PPF Efficacy is a good overall metric for what plants use to see light, plants do more than just see. They feel light. That spectrum falls across a broader range. It’s called photomorphogenesis.

When it Comes to Cannabis, Not All Grow Lights are Created Equal

Above: Absorption spectra for 2 forms of phytochrome. Adapted from Biochemistry and Molecular Biology of Plants by the American Society of Plant Biologists.

Photomorphogenesis “The control exerted by light over growth, development, and differentiation of plants that is independent of photosynthesis.” Different receptors, such as phytochromes, cryptochromes, and phototropins in plants react differently to those parts of the spectrum that are missing from PPF. For example, UVA and UVB (<400nm), as well as far reds (>700nm) have a significant effect on plant growth. So they shouldn’t be ignored.

How is lighting for plants measured and compared?

As previously stated, PPF is based upon the wavelengths emitted between 400 and 700 nanometers. That region is where humans see, though human sight is weighted heavily around the 560nm range.

While most humans do not see below 400nm or above 700nm level, they do “feel” light radiation at those levels; one in the form of tanning by means of UVA 350nm~400nm (and some UVB), the other in the form of infrared or heat-warmth beginning at 700nm~800nm and extending higher.

Humans need both to feel good. Plants do, too. The shortfall of using PPF as a metric for plant growth is that it “normalizes” the spectrum of light, treating each nanometer the same.

But if humans need a weighting curve to help characterize their ability to see, shouldn’t plants also have a weighting curve to help them see and feel the light to grow? Yes, they should.

In numerous casual but controlled experiments of different plants, a higher PPF did not always produce the better results. That’s because the spectrum to produce higher PPF does not necessarily contain the best components of light that allow all plants to flourish, especially in cannabis. In other words, not all μmol are created equal.

In other highly-controlled third-party evaluations of different spectra on cannabis plant growth, there were some interesting outcomes. Over a six-week “veg” period using different “spectra” but at the same PPF, cannabis sativa clones exhibited measurably different results.

At the end of that period, experienced growers reviewed what they deemed important for best plant yields: plant height, plant width, the number of leaves, leaf size, the number of nodes, and the average spacing between nodes. They also observed each plant qualitatively.

In the final outcome, there were definitely some “best spectrum” winners among the test specimens, even though the PPF was the same. Spectrum mattered.

How else should lighting be measured and compared?

Plant Biologically Active Radiation (PBAR, also known as YPF-Yield Photon Flux) uses the McCree curve to weight spectrum. It encompasses UVA (350~400nm) and the far red spectra (700~800nm), making it a better choice for evaluating light sources used for growing plants. There have been numerous studies that plants need some level of UV. Too much, and they can overdose on it, wilt, or even die off.

While PBAR, also in μmol/second, appears to be much better, the fear is that it is being passed over as the measurement by which lighting for plant growth will be measured and compared. PPF seems to be the preferred metric.

We anticipate state governments and utilities will likely adopt the PPF recommendation of 1.6 μmol/joule as the minimum level of acceptable light for indoor cultivation of plants. No different than a 100 LPW streetlight. In fact, most 100 LPW streetlights will meet the 1.6 μmol/joule threshold. That means growers can purchase a dozen LED street lights or parking lot lights, get a rebate, and then try to grow their crop.

So where does that put the industry if it goes the way of PPF?

No one is absolutely sure how the standards and subsequent regulations will all materialize. There has been much discussion and deliberation on the subject.

There are those that believe “intensity,” “spectrum,” and “uniformity” should be considered in this standard. Using just an energy-efficient metric like PPF would not be optimum for plant growth. Just like you wouldn’t base your entire car purchase on miles per gallon.

Products that feature “intensity, spectrum, and uniformity” will likely succeed in the long run, especially in hobby markets where energy efficiency rebates from utilities may not drive the buying decision. Those who choose intensity, spectrum, and uniformity will be giving their plants and themselves the best opportunity to maximize their crop output in terms of quality and quantity.

And while PBAR is a much better term to characterize a broader spectrum than PPF, it’s not the end of the story for what plants need to grow vibrantly and vigorously.