Growing cannabis is complicated enough. Whether it’s finding the right balance of soil nutrients, the perfect CO2 level, providing just the right amount of water to keep the plants fed while still staving off root rot, fungus, and mold… and then there’s the lights. With it’s own glossary of terminology, lighting a grow house can be a pain to figure out, especially without a degree in physics or biology. Some manufacturers are even banking on you not knowing the lingo, so that they can throw acronyms and data points at you to make you think you’re getting a good deal. 

Thankfully, we here at Fohse are shedding some light on the tricky business to keep growers from getting lost in the dark. It is our hope that by familiarizing new growers with the jargon, they can start to see what makes our selection of high performing, intelligently designed grow lights so effective. 

PAR 

You may be familiar with the basics of light. It’s essentially radiation that travels in particles which also act as waves. These waves are measured by their wavelengths, the distance between two crests of a wave. The human eye perceives only a portion of the full spectrum of light, from a wavelength of about 380 nanometers (violet) to 800 nanometers (red), which we call the visible light spectrum. Outside of that spectrum are ultraviolet light (100 – 400 nm) and infrared light (780 nm – 1mm). While they’re invisible, they aren’t unimportant. Ultraviolet helps us absorb vitamin D and get a suntan, while infrared is basically heat. 

Unlike humans, plants don’t perceive light so much as they absorb it and turn it into energy, and the light that makes plants produce energy the best and most efficiently exists within the visible light spectrum. This range, between 400 and 700 nanometers, is the PAR zone.

Photosynthetically Active Radiation, or PAR, is the part of the electromagnetic radiation spectrum that is useful to plants and algae to activate photosynthesis. Any instrument used to produce light for plants will be measured in how effectively it is able to produce light within this range. Therefore, any figures on a grow light’s effectiveness will be completely useless to someone who doesn’t understand what PAR is. 

In PAR, different colors produce different effects within the plant. Blue photons inhibit cell expansion, which may sound bad at first, but at low levels it can help the plant grow thicker before it flowers. Green photons penetrate leaves, bringing light through the canopy to lower parts of the plant, and play a key factor in how our eyes are able to perceive plants. Red photons are great for photosynthesis, but lack the green photons’ penetrative ability. For plants, a combination of all three is necessary to achieve an optimal yield. Much like humans, a plant needs a diverse range of food to be healthy. Just one type of photon, and the plants suffer. 

PPF 

The PPF, or Photosynthetic Photon Flux, measures the amount of photons within the PAR zone emitted per second by a light. It is measured in the micromoles of photons, millions of particles of light, expressed over time, written as μmol/seconds. In layperson’s terms, it’s how much light (photons) is coming out of a light (light). 

It’s important to note that PPF isn’t a measurement of how much light reaches a plant, just how much is being emitted by the lamp. Not every lamp with the same PPF is as effective in bringing that light down to the crops. Some lamps may have less focus, allowing light particles to spread out wide and be wasted on your walls, the same way old street lamps create more light pollution. However, with the PPF of a grow light, you can estimate the number of lamps needed to reach your required light level on the plants. 

Photons emitting at wavelengths above 700 nm do not contribute to the PPF and are not measured, and neither do ultraviolet wavelengths, or anything below 400 nm. A lamp can be producing inordinate amounts of ultraviolet light and it would contribute nothing to the PPF. 

PPFD 

A manufacturer may boast that their lamps have an incredibly high PPF, pumping out photons like a firehose, but if the water from the hose isn’t hitting the fire, what good is it? Photons that aren’t making it to your plants are useless, so while knowing the PPF of a lamp may be helpful, It’s the PPFD that really matters. 

Photosynthetic Photon Flux Density, or PPFD, is what you really need to pay attention to, because it’s PFFD that measures the photons that make it to the plant. It is the number of photons within the PAR zone that fall on a given surface each second. 

Depending on many variables, light can be lost, blocked, or diffused. The PPFD is a measurement of the light that actually makes it to a specific spot every second, expressed in micromoles of photons per square meter per second or μmol/m²/s. 

It’s measured with a radiospectrometer, or PAR meter, a small device that you place on your plant under a lamp. The device will tell exactly how many photons are hitting its sensor, which is tiny. Because of the sensor’s size, for grow lamps, it’s important to get measurements in many areas on a plant, and average them together to get a real idea of the overall PPFD of a grow light. 

Many things can be adjusted to achieve a desired PPFD, from the height of the lamp to the spacing of the plants. Some manufacturers incorporate lenses or reflectors to achieve more focused PPFD. This can result in a lower PPF, so growers will have to weigh whether it’s better for their grows to have more photons hitting the plant directly, or more photons being emitted in general. 

DLI 

The Daily Light Integral (DLI) measures the total amount of light received in one day by a plant. While PPFD shows how much light arrives in a particular spot per second, DLI measures how much light was received in the designated area in total for the whole day, or the number of moles (not micromoles) of photons per square meter per day, expressed as mol/m²/d. 

What’s fascinating is that the limit to the amount of photons a cannabis plant can take per day is unknown. This means that there are few downsides to higher and higher DLIs. It is important to note that increasing the amount of photons a plant receives will also mean that its CO2 must be increased as well as its water levels, and the nutrition in its soil. A high amount of light is like a strong foot on the gas pedal. The plant will grow faster, but it will need a lot more fuel, meaning the real question is not how much light can the plant take, but how much can a grower afford to give it. 

PPE 

PPE, or photosynthetic photon efficacy,  measures a light fixture’s ability to convert electrical energy into PAR light. This is expressed as micromoles of photons per Joule. The better a light’s PPE, the more energy efficient it is, meaning less money spent on energy for the grower. This is an excellent point to look at for growers who want to test the limits of how much light a plant can take. 

The formula for PPE is PPF/watts of power used, meaning PPE does not take into consideration the PPFD of a grow light. A grow light can claim a high PPE, meaning that its PPF is higher using less energy, but from what we know about PPFD, this doesn’t necessarily mean you’re seeing a benefit from that higher efficiency. In fact, there are several ways to manipulate a light to have a higher PPE that could have negative effects. For example, a manufacturer could make LED’s with only red diodes, which use less energy and still emit photons in the PAR zone. However, only red diodes means that the lamp is only emitting red light, so the plant isn’t getting the full range of photons it needs in order to grow, flower, and propagate. 

And that is why it’s important for new growers to arm themselves with as much knowledge as possible. New discoveries are changing the way we think about horticultural science and engineering, and learning the language is the first step towards entering a field that is constantly evolving. Because the sooner you know where the industry’s been, the sooner you’ll be able to join in on where it’s going.

The Future of Horticultural Science and Engineering.

FOHSE