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Lighting 102

LEDs, HIDs & advanced lighting techniques for advanced results
By Ben Owens

Once you understand how lighting is measured, what types of lighting exist, and how to evaluate the quality of your lighting; you can begin to understand your plant's lighting needs on a more advanced level and accommodate them using advanced techniques, setups, and cycles.


Watts (w) - A Watt is a unit of power. Traditionally, input wattage was equal to the output of the light. In LED systems, equivalent wattage ratings are often used, but the actual wattage is based on the input.

Photons - Light particles received by plants as energy used in photosynthesis.

Photosynthetic Active Radiation (PAR) - Refers to the visible spectrum of light (wavelengths within the 400-700 nanometer range) that power photosynthesis. PAR is not a unit of measure but rather a type of light on the spectrum that supports photosynthesis. The amount and quality of light are the most important factors.

Photosynthetic Photon Flux (PPF) - The total PAR in a system that is produced per second. The higher the number, the better.

µmol/micromol - A measurement of photons within the visible spectrum of a plant that trigger photosynthesis. The higher the measurement, the more photons present in the system.

Daily Light Integral (DLI) - A measurement of how much PPF is needed daily for your particular crop. This measure varies with different agricultural zones.

In recent years, there has been a significant amount of information produced on the nuances of lighting and the various ways to finesse the balance between spectrum and intensity (ppfd)—and how to deliver the proper ratio of both for perfect photosynthetic response in plants.

"Back in the day, it was more rare, but not unheard of, that people would increase the amount of blue light spectrum in a flowering room in effort to produce higher quality characteristics—or what they were thinking was producing more resin," explains Lee McCall, representative of Dutch Lighting Innovations and pHive.8 Distribution.

"In my opinion, what growers were possibly experiencing was just a more enhanced spectrum from pairing both the metal halide blue light with the HPS red light; that combination delivers a fuller spectrum and likely a better result. It's not just the blue light; it's the fact that people were using both, receiving advantages of both; in turn you were using two tools instead of just one."

Advanced growers like McCall gravitate towards the idea of providing as wide an arsenal as possible to their plants—from lighting to nutrient sources—which allows the plant to potentially achieve the best with the utilization of more assistants.

"The more you provide to the plant, the more likely you have of the effect being revealed because you are giving the plant more outlets to do what it needs to do," he explains. "With spectrum enhancement—when you are bringing in blues, greens, UVs, far reds—it's providing that full balance of light based on the photosynthetic curve that emulates the sun. Growers are trying to replicate that in their own way and find recipes that emulate that."

The biggest challenge: data.

As more legal markets come online, more research is being done, but the industry is still in its infancy, a point echoed by both McCall and Colin Gordon.

"Indoors, lights are formulated to deliver as much light as plants can handle," says Gordon. "That’s a good starting point, but we need to be more nuanced with what plants need; what is the highest efficacy of the plant?"

Studies on cannabis are limited by legal hindrances as well as intellectual property concerns; those that are doing the research often have a competitive advantage in the market.

"For example, I can’t find any studies that show reducing uMole mid-day can be beneficial," continues Gordon. "They talk about total uMole throughout the cycle [DLI] but there have not been specific tests on what happens if we raise normal levels but, in the middle of day, we give them clouds."

Experiments like this sound easy enough to run, but the time it takes to do so becomes the limiting factor.

Advanced Lighting Cycles: 'Enhanced' Natural Light

As indoor growers, we try to create the best environment for our plants. Often, this environment emulates natural outdoor conditions, and, more often, growers work to enhance these natural conditions for specific end games.

"We try to emulate the best of the natural environment," begins McCall, "but we want to enhance that natural environment. Everything about nature we are doing–water, wind, light, food–it's all there, but now, we are focused on what we can do to manipulate and make each one of those things better."

Whether its nutrition or lighting, enhancing what nature typically provides can result in a superior product.

As growers, our recipe for success hinges on a balance between emulation and enhancement. The following are a variety of advanced lighting cycles and techniques that can be used to enhance your efforts:

Sunrise & Sunset

One common technique is to emulate the natural brightening and dimming of the sun each day.

"We do sunrise and sunset every day in our rooms, both VEG & BLOOM, always ramping up and curving down," McCall says.

"We never start at 100% on; we graduate our intensity just like it would every morning where it starts dark and then eventually brightens with morning. We ramp lighting up from about 60% to 100% over the course of an hour, automated through our internal lighting controller. At the end of day, we ramp down intensity like a sunset for the last hour."

Cloud Patterns

Similar to growing outdoors, cloud patterns can give your plants a break in intensity throughout their daily cycle. This allows plants to still receive multidirectional light, while also giving them a chance to replenish nutrients and amino acids.

"We've been turning off 20% of the room in stages throughout the day in Veg, with a different 20% of the room going off each 2-4 hours," explains Gordon.

"We were having a 20 hour day and the off cycle was only four hours, but, for them to get the full 20 hours, it seemed that they liked the “shade” or cloud cover we were replicating. If their section got low light for a few hours in the middle of that, the plants recovered and were more lush and green and better able to enjoy the 20 hour on-cycle."

Everyone still gets light, just not directly above them.

"Very rarely do plants get all day bright light [in nature]," Gordon reminds. "We used to be in a big room that we had to split in half for watering. Each day, one half was off for a couple of hours (something we had to do for electric purposes to run A/Cs and everything at same time) and it was something I noticed then: they were healthier when we turned half the room off for two hours. It makes sense as far as thinking about how plants receive light in nature; there’s an arc, and variations within."

"Our opinion is absolutely that [clouds] in your crop do have an effect on the plant and I would not be surprised if it is positive," adds McCall.

Mid-Vegetative Interval Lighting

Similar to cloud patterns, shorter on and off intervals can be used to intermittently give the plants a break while maintaining a short enough dark period as to avoid triggering flowering.

"In the middle of veg, we are 16/8 (they like a little extra dark)," explains Gordon. "We use a 6/2/5/2/5/4 schedule; on for six, off for two, on for five, and so on.

"You can't have more than 10 hours of extended darkness in veg, but the darkness that you have doesn't have to be consecutive. Four hours off is plenty long enough for them to lose their turgor and excrete sugars."

Late flower, Light-Specific Dimming

If you are running multiple lights, especially a combination of LEDs and HIDs, your customization options are greater, allowing you to diminish or cut off certain light spectrums while leaving others.

"When we get to the end of our cycle—usually our winter frost week [Week 8]—we like to reduce our LED footprint," explains McCall.

"By turning off our LEDs, we are reducing most of the enhanced spectrum of blue and white, leaving more red intensity. The concept there, in thought, is that with the seasonal shift from summer to autumn, the sun gets redder, the hues get redder, and perhaps that is what's invoking the ripening of fruit, flower and flora that grows naturally. Whether it's vegetables or flowers,  that's when your fruits and produce start to ripen, with redder shorter days, and a lot redder light."

McCall leaves their HPS on at full intensity for Week 8 but turns the LEDs off, reducing the overall intensity that the plants receive, but shifting more red spectrum for last moment ripening effects.

"We are reducing light intensity because we turned off all supplementation from our LED; so we are reducing our overall PPF, but our HPS lights solely operate during [Week 8] with that higher red supplement."

Flush Week Dimming

The final week before harvest is also the time that the plant receives the least natural light intensity. For this reason, lowering the intensity of your lights may help to produce more desirable results.

"For our flush (water only) week, we will reduce the intensity of our HPS fixtures to 60%," notes McCall.

During this time, sunrise and sunset cycles can still be utilized, but at a reduced level, ranging from 50-60% rather than 50-100% as you would earlier on.

"We don't do total darkness," McCall adds.

"I'm not against it; some people get great results. I just don't like doing it. I feel it could invoke stress right at the very last minute that isn't wanted. I mean, where does it happen in the world where it stays dark for 2 days and plants thrive during that time?"

Advanced DLI Adjustments

The more you get to know your garden and your plants, the better equipped you are to adjust to specific lighting needs; some plants have a higher DLI requirement than others.

If you are running at a higher intensity, you may not have to run your lights as long, suggests McCall.

"I know people who compensate [for increased intensity] by running 10 on and 14 off. Because their 10 on is so intense; that makes sense. DLI is micromols; it's PPF. It's the amount of photons hitting a surface. The more sources you have, the more photons you get, the PPF goes up.

"The more photons you deliver to a surface, the faster you achieve your DLI, and, once you achieve your DLI, subjectively, you're done; at least in theory."

Growers that hit their DLI in a shorter window of time can turn their lights off earlier, saving up to two hours of energy on lighting as well as environmental conditioning (A/C, humidifiers, etc.), with similar results.

"A shorter time ON with higher intensity is the same as longer with less."

In terms of DLI, every plant has a certain threshold. With advances in the technology of grow lights, it has become easier to manipulate DLI and the nuances used to manipulate it.

A Note on Dimming and Light Height:

LEDs and HID lights do not dim the same way.

When you dim an LED fixture, the spectrum doesn't change. When you dim an HPS (or any HID fixture), you are affecting the gas chemistry, so your spectrum changes.

Distance from your light source is directly related to the concentration of light your plants are receiving.

The closer you are to the source, the more intense the light, similar to being on a mountain compared to living at sea level. Intensity and PPF are much more intense at elevation than at sea level.

If you are dimming your lights to 80%, but running them four inches from your canopy, that's still a high intensity of light.


Traditionally, high intensity discharge (HID) lights like high pressure sodium (HPS) were used in greenhouse cultivation as a supplement.

These bulbs offered an enhanced red spectrum in addition to the full spectrum of blues and reds provided by the sun.

"What you can't get with the sun alone is high red, so that's why they influence greenhouses with red lighting," explains McCall. "This used to be done traditionally with HPS; that's where high micromol red lights came from. Then, people took them and put them indoors."

"HPS is a bulb that ignites with gas chemistry, so you have radiation (how sunlight works), but with LEDs, the way chips and diodes work is different. You have to put specific chips together that are only capable of certain bandwidths, so there is a recipe."

The advantage to LED lighting is that there is an increasing amount of research and you can dial in a high quality spectrum with good intensity, using less energy than an HID light.

"There is a lot of sensitivity around plant response to LED light vs. HID light," continues McCall. "I believe a lot of it has to do with heat; light is heat. LEDs adjust that concept of light and heat by giving plants light but not the same heat."

Due to the change in heat, growers often must make other environmental changes.

"I've had growers that we have consulted and helped with that didn't get a good initial result [with LEDs] or they had to fine tune their situations because when they put in LED lighting their temperatures dropped substantially. Everything, including VPD, has to do with temperature and relative humidity."

With LEDs, your lights are on but you're getting a different temperature reading because of the difference in heat output.

Overall, this is great, because it leads to a reduction in energy consumption overall. Less heat means your cooling units (HVAC) don't have to run as much, which means your air is less conditioned, which, in McCall's opinion, is why people who include LEDs in their grow often get better results than those that don't.

"It's not just the light, it's the fact that your environment is now actually applicable to your garden," McCall explains. "Most people have a situation that isn't ideal, so, when they put in LED lights, it makes their situation more ideal and therefore they get a better result."

Choose Lighting Based on End Game

Generally speaking, LED's lean towards quality, while HID's lean towards yield.

If you're looking to win a cup, LED's with a dialed-in spectrum and moderate intensity would be a great option. Newer model LED systems are beginning to offer the ability to tweak spectrum, but this is only recommended for advanced users who are experienced with their phenos.

If you're looking for yields above all else, HID's will get you there.

If you're looking for the best mix of spectrum quality and light intensity, a combination will be your best bet.

"The nuance that goes along with that theory is that this is because LEDs have more blue-white light incorporated into them," explains McCall.

"HPS are known for having a higher micromol output threshold for higher production; that makes sense because HPS have a higher range of red light which influences yield. LEDs have a  higher density and variety of white, green, blue, everything else...when you combine them, you have this certain recipe from one thing and a certain recipe from another, and you get the little nuances from both so you are getting a fuller spectrum."

Similar to comparing an isolate to a full spectrum distillate, with a fuller spectrum, you are getting all of the minute constituent differences that influence the effect of the product.

McCall and his team have seen these results firsthand, running experiments at their licensed R&D facilities with LED-only, HPS-only, and combination setups. Each time, the combination offered a higher quality final product that boasted higher total active cannabinoids (TAC), larger yields, and greater terpene concentration than product grown under a single type of lighting.

With that said, the comparison of LED to HPS is hard because the fixtures do not run on equal wattage.

"The thing that makes 1000w fixtures the heavy-hitting fixtures is that they run at 1000w; they are a high output fixture," reminds McCall. "The common LED fixture is a 600-750W fixture. If you put the equivalent 1000w of LED to 1000w of HPS, I believe you would have a higher chance of getting more yields with the 1000w of  LED because you are providing more attributes of full spectrum light from an LED than you are from an HPS at this time. That's not to say that HPS lights don't have characteristics that LED lights do not.

"Again, you're getting things from HPS lights that LED lights just can't emulate, and they are small nuances, and you would have to be extremely dialed-in in order to pick those up, but if you are trying to win a cup, that's what you do."

Advanced Lighting Combinations

If you're looking to get ahead of the curve with your lighting setup, then a combination of technologies could offer you the best blend of spectrum and intensity of light.

McCall and his team run LED + HPS, and further supplement with UV to provide a higher quality spectrum to the same canopy.

"The way we look at it is you get certain traits from an HPS, and you get certain traits from an LED. If you are doing a combo, you are ahead of the curve because you're a person who sees the benefit of what an HPS does that an LED cannot—which in my opinion is yields. UV is even more advanced."

Ceramic metal halide (CMH) is another technology that is often underutilized or misused, but can offer something that most others cannot: full bandwidths of UV.

"One thing that is unique about CMH that most other lights don't produce is full bandwidths of UV light," explains McCall. "UV light stimulates resin production, as well as cannabinoid production in plants. It makes sense because the resin glands on a cannabis plant are a form of sunscreen. The trichome head filters UV light when light passes through it, filtering out the UV and protecting against sunburn radiation from the lights; it's a defense mechanism. Resin protects the bract that is holding the seed...the more UV light, the bigger the trich head, potentially."

Pheno Hunt Lighting

Depending on how you look at breeding and selection, the best type of light will vary from grower to grower and style to style. 

“I know breeders who like to put as much stress on the offspring as possible in efforts to look for resilience,” begins McCall. “This is a brilliant way to determine differing factors for breeding and hybridization in my opinion.”

Again, the more response you can get from your plant, the more data you have in determining how to consult a project.

“If I have to nominate one type of light [for pheno hunts], it would be an LED, currently,” McCall offers. “Like I said, the controllability and efficiency in both output and energy consumption is a no-brainer in my opinion. LED tech will only get better and cleaner soon rendering HID truly to be useless, but currently we are not there yet.”

Is there such a thing as too much light?

McCall and Gordon agree that there is such a thing as too much light, largely based on the DLI of your plant.

"I do think there's a threshold of too much light," cautions McCall. "We are very adamant about focusing on delivering enough light but not an overabundance of light, and then making sure the spectral quality of the light is of high caliber."

“Plant health matters above everything, always," adds Gordon. "Nuance is irrelevant if the plant isn’t at maximum health."

"Healthy plants want more light, the same with unhealthy plants and less," confirms McCall. “The healthier the plant is the more it will reciprocate what you give it. If you want to give me more light and I am capable, I'm going to give you an enhanced result."

The healthier the plant, the more apt it will be to accept a more intensive environment. The opposite also holds true.

"A healthier plant is going to do better in a shittier environment and a better environment," explains McCall. "A shittier plant is not going to do as good even in a better environment; it's still going to be shitty.

"What we have definitely seen in our R&D is that better spectrums with higher intensities will produce better results," McCall says.

"Even at a higher intensity, higher quality light is not as detrimental as high intensity with low light quality. If you have the same measurable amount of intensity, but one of them has a better spectrum, you are going to get a better result from the combination of the spectrum with the intensity."

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