The Bright Stuff: Setting The Record Straight On Grow Room Lighting

Lighting options for cannabis production will continue to evolve as the industry marches forward — but, barring a profound breakthrough, the principles and science behind lighting will remain constant.

As we match light fixtures with specific grow situations, it’s imperative to lay to rest once and for all some of the myths and misconceptions that have been propagated by well-meaning growers and manufacturers. Part of that involves understanding and agreeing upon key terms that are relevant to lighting.

“I’m big on terms because, how do you compare one grow light to another if we’re not all speaking the same language?” says Christopher Sloper, chief horticultural officer at lighting manufacturer FOHSE, and author of The LED Grow Book.

Another problem for growers, whether large-scale licensed producers or at-home hobby growers, is that manufacturers often don’t provide the details needed to make an informed decision on proper lighting setups. They will mention wattage or lumens, but those terms simply tell us how bright a light is to humans and how much electricity will be used, not how they may affect plants.

“It’s a serious, serious problem,” Sloper says. “Any manufacturer you talk to is going to sell around a product’s limitations.”

The answer, of course, is to be familiar with cannabis lighting terminology and lighting science so you know what to look for in a light fixture and know the right questions to ask manufacturers.

Lighting Terms You Need To Know

(Source: The LED Grow Book: The Way to Grow, by Christopher Sloper.)

DLI (daily light interval): The measurement of PAR photons a plant receives in a 24-hour period, expressed as moles of light per square meter per day (mol/m2/day).

HID (high-intensity discharge): A collective term for high-pressure sodium (HPS), metal halide (MH) and ceramic metal halide (CMH) lamps.

Integrating sphere: This device counts the number of individual photons emitted by a lamp or fixture. Total photon output is detected and reported as PPF. It’s the most accurate device for measuring PPF.

PAR (photosynthetic active radiation): The qualitative description of the radiation between 400nm and 700nm, similar to the visible light spectrum for humans.

Photomorphogenesis: How a plant uses light for various stages of development, including germination, rooting, stem elongation, leaf unrolling, flowering and dormant bud development.

Photon: A single packet of light energy at a specific wavelength measured in nanometers (nm).

PPF (photosynthetic photon flux): The amount of PAR the lamp can produce per second (umol/s).

PPFD (photosynthetic photon flux density): The quantitative measurement of how much PPF is delivered to a particular target or, in the case of cannabis, the plant canopy.

Quantum meter: This device measures the number of photons reaching the surface of the sensor (measuring PPFD). Note: A meter designed for HID lighting sources may not give accurate measurements for LEDs, and vice versa.

Red light vs. blue light: Red light has a longer wavelength and its low energy is responsible for stem elongation and flowering. Blue light has a shorter wavelength and its high energy will prevent plants from stretching toward a light source. Light in the red spectrum is an important component in the veg room, while light in the blue spectrum is essential in the flowering room.


Cannabis growing indoors under LED lights, cultivated by TKO Reserve.

Clearing Up The Myths And Misconceptions Of Cannabis Lighting

These myths and misconceptions about lighting are more prevalent than mites in an unsecured grow room. Some growers may not agree with all the opinions presented, and they may perform additional research and discover an even better way to light a grow room for cultivating bigger and better buds.

  1. Measuring PAR instead of PPFD. A PAR reading tells you the color spectrum of light that is being delivered to a plant and measures the quality of that light. Sloper says that some people incorrectly use it in place of PPFD, which is a quantitative measurement.
  2. Confusing grow lamp efficiency with the amount of electricity used. “Most new indoor growers conflate grow lamp efficiency with the amount of electricity it uses,” says Leslie Halleck, horticulture industry consultant and author of Gardening Under Lights. For example, some may think a 45-watt LED must be more efficient than a 600-watt HPS lamp because it uses less electricity and looks pretty bright. “When it comes to grow lamp efficiency, our goal isn’t brightness, it’s driving photosynthesis,” she says. According to Halleck, efficient grow lamps take more of the energy they pull and convert it to the type of light that plants can use for photosynthesis (PAR) as opposed to heat, concluding that some new growers erroneously assume that low-wattage LEDs are the most efficient choice.
  3. Using metrics like lumens, lux, Kelvin and watts to calculate your lighting needs. These terms are only meaningful to humans (“lumens are for humans,” Sloper asserts), referring to the brightness of a bulb or the energy being used and having nothing to do with plants. This misconception is in part due to manufacturers not providing information such as the PPFD on the packaging. Sloper says that growers are catching on to these terms as understanding of light continues to evolve.
  4. Lux light meters versus cheap hobby meters. If you want an accurate PPFD reading for your grow, you need to cough up the dough for a quality quantum sensor meter.
  5. How high do I hang my lights? This is a common question, but Sloper advises that a better way to determine light height is to find your desired light levels and work backward. He says CAD technology is available that can determine a profile of your grow situation and can be used when making a major investment in grow lights. There are many variables that will determine light height, including the light itself, environment in the grow room or greenhouse, and even the strain of cannabis you’re growing (indicas don’t like a lot of heat, a factor that could come into play with HID lighting, according to Sloper).
  6. Lamp for life. “I think another surprise for indoor growers is that lamps don’t last forever,” Halleck says. And although an LED lamp might be rated to last for 30,000 hours, you’re not going to get the same light output for that duration — light output is going to degrade as the lamp ages, and the spectral output will also change. Grow lamps do need to be replaced, even if they are still working — some types faster than others. For example, fluorescent lamps degrade more slowly than HID lamps such as HPS, MH or CMH, according to Halleck.
  7. Mislabeled LEDs. “Another thing I see a lot is dual-band LEDs that emit only red and blue light labeled as ‘full spectrum,’” Halleck says. “I consider that misleading marketing, as true full-spectrum grow lighting will emit some volume of each light color within the PAR spectrum.”
  8. Finding the perfect light spectrum. Sloper says that doesn’t exist. He says it’s a sales pitch and the number of wavelengths is irrelevant. “All this really comes down to is ratios between red light and blue light and far read ratios,” Sloper says.
  9. The more light the better? Some growers seem to think that if 18 hours of light is good, then 24 hours must be even better. This is debatable, but like us humans, plants need to rest, too. Not to mention the high cost of keeping those lights on 24 hours a day…
  10. High-pressure sodium lamps outperform LED lights. A separate article could be written for this debate — you can make this call. Sloper and Halleck present pros and cons of each in their respective books.
  11. LEDs and PPFD to lux reference. This isn’t really a myth but a reminder that standard conversions of lux to PPFD don’t apply to LEDs because there is a wide spectral difference between all various types of LEDs.
  12. Power equals penetration. Sloper says he hears all the time that a 1,000-watt HID will penetrate deeper into the crop canopy than a 600-watt HID, or that an LED grow light with a five-watt emitter will penetrate deeper than one with a one-watt emitter. Rather, he says, light design will better determine penetration.
  13. Not believing in Murphy’s Law. Things can fail at the most inopportune times, such as the night before an early flight for a vacation or trade show. Sloper recommends keeping spare bulbs on hand for these times.
  14. The idea that large light fixtures block light in a greenhouse where supplemental light is being used. Sloper says this isn’t as big a problem as some might think. With the sun low in the sky during the winter months, sun can still reach the leaves of your plants, and in the summer, it isn’t so critical if plants are slightly shaded by light fixtures.

So, When It Comes To Lighting, What Is A Cannabis Grower To Do?

Halleck says that we’re starting to see better information included with newer lamps, but adds that there’s still lux/lumen information too often attached to grow lamps, and those metrics are not meaningful for photosynthesis and plant growth.

“The information you want to see included with your lamp are its PPF and a few PPFD measurements at specific distances from the lamp,” Halleck says. “This will give you important information about how efficient the lamp is, and what crops it is suitable for and at what distance.”

Sloper adds that it’s important to read up on lighting and how plants use light. University studies and research can be beneficial and provide some of the latest innovations on lighting and scientific thought on the subject. “My advice,” he says, “is to read everything and stay off the growing forums. Find authors that have been peer reviewed.”

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