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HID vs. LED in Cannabis Cultivation

By Eric Harrington. I.E.E.E., I.A.L.D., A.S.H.R.A.E.

Prev.  Chief Scientist, Commercial Sales Manager, and Technology Evangelist – California LightWorks, Principal Consultant, EDT

High Intensity Discharge (HID) refers to a class of gas discharge lamps which include High and Low Pressure Sodium (HPS), Mercury Vapor (MV), and Metal-halide (MH).  Their comparatively high efficiencies and long operating life have allowed them to dominate the commercial lighting and horticultural lighting industry for nearly a half century.   And although there have been significant improvements made in both HPS and MH in the last decade, the advent of high-power LED systems spells the end of their dominance for a number of reasons: 

  1. Fixture Inefficiencies

When we say a DE HPS system has 1.85 uMol/j efficiency, we are referring to a bare lamp, but a bare lamp in any grow room will waste a lot of its light on ceilings and walls, so they require a fixture to direct the light where it will be used.  The very best reflective materials run around 95% — that means every time light reflects off that surface it still absorbs 5%. It is almost impossible to design a fixture that emits every photon from an HID lamp in one reflection or less, and often it can be many reflections, and with each reflection you lose at least 5%.  So even the best HID fixtures typically have an overall fixture efficiency of no better than 90%.  That means a 1.8 mmol/joule rating is effectively 1.6 uMol/joule actually directed to the plant, and often much less. New Gavita lenseless, DE fixtures have made great improvements in this.

 The photosynthetic output of HID lamps can degrade as much as 10% after only one year of operation, and in the case of HPS, as they degrade the spectrum shifts toward the Green/Yellow range which is the range of spectrum most poorly utilized by plants. (An even greater spectral SHIFT occurs when dimming an HPS lamp, (the photosynthetic activity drops faster than the power) so dimming HPS in horticultural applications is not recommended.) So unlike commercial lighting applications,  most professional growers replace their HID lamps a minimum of once every year and many even replace them every 2 harvests, with significant on-going lamp ($50-$70) and labor (the most commonly overlooked factor)expense.

HID Lamps have an interior wall temperature from 300-400C, so they require significant cooling, and can be potential fire hazards. Many city safety codes are beginning to prohibit HID systems in residential indoor grows due to the dramatic increase in “closet fires”. Roughly 70% of all the energy consumed by an HID lamp is emitted as heat,and mostly in the form of Infra-Red (IR) radiation.  So not only does HID require significant air-conditioning,  but the high levels of IR heat the plants without raising the air temperature, and this differential can cause localized heat stress, fox-tailing, and other heat related problems even when the room air temperature is in the normal range. Also, by heating the AIR instead of the plant, the heat is distributed more evenly to the plant and thus generates more consistent flower maturity deeper in the canopy. For this reason LEDs require the room air temp to be 3-4 degrees higher than HPS for similar metabolic activity.

Plants absorb light and use the photons energy to strip the hydrogen out of water and combine it with the carbon from the Carbon Dioxide in the air (plus relatively small amounts of soil minerals) to create plant matter.  They have several pigments that can absorb light and fuel plant processes, but far and away the most prevalent and efficient pigment for photosynthesis is Chlorophyll A and B. 

Spectrum and Photosynthesis

As you can see from this graph, there are two forms of Chlorophyll called Chlorophyll A and Chlorophyll B.  Each has absorption peaks in both the Red and Blue spectrum’s, and both reflect yellow and green, (giving plants their green color,) so while the green/yellow bands can be absorbed by other pigments like the Carotinoids,  over 40% of this spectrum range is reflected away or passes through the leaf and is thus poorly utilized. 

This graph shows plant absorption throughout the PAR spectrum range. As you can see, absorption right around the green/yellow boundry is 30%and from the orange/red boundry to almost the Blue/Green boundry it is under50%.  This is because that is the range of light that Chlorophyll reflects. There are some pigments that can absorb and utilize this spectrum but they are less efficient and typically located deeper in the leaf because they get most of their light from light that is reflected off the leaves deeper into the canopy and absorbed through the BOTTOM of the leaf, or that which penetrates past the Chloroplast layer.

This is a graph of a typical HPS spectrum. You can see from these graphs that the majority of the HPS spectrum actually falls in the region of lowest plant absorption.  New Double-Ended DE lamps run at higher pressures and thus produce a little more in the near-red region, but their overall photosynthetic utilization has improved only 10-15% over the traditional SE lamp

Metal-Halide (MH, CMH) Spectrum

Metal-halide (MH) differs from HPS in the fact that the spectrum bands are generated by individual metal halide’s, for example a green halide, blue halide and red halide, Which turn to gas and emit light when heated. So the MH spectrum can be blended much like Phosphors in Fluorescent lamps or LEDs.  But the efficiencies of the individual halide’s vary and the red halide’s are the least efficient, most expensive, and most reactive, i.e. shortest lived.  So MH lamps have traditionally favored the blue end of the spectrum, for as the lamp light gets “warmer” in color i.e. generating more red, they get less efficient, more expensive,  and degrade faster.  And as you can see from the graphs they still produce minimal amounts of far red at the critical 675nm Chlorophyll A peak.  So historically, MH lamps have been used for Vegetation phases, where the red spectrum is less important.

LED Performance

Individual LED chips produce very narrow spectrum bands, so LED grow-lights can be precisely mixed to deliver a spectrum optimized for maximum plant absorption and utilization.

 And it is this ability to deliver exactly those frequencies the plant uses best, and all at very high efficiencies, that allow the most recent generation of top-tier LED grow-lights to deliver comparable yield sand often superior quality to that produced by HID lamps with 30-40% less input power.

White LEDs

 There is no such thing as a “white” LED.  Typical white LEDs are a hybrid system consisting of a blue LED chip with a primary lens coated with phosphors much like a Fluorescent lamp. So they are in reality a miniature fluorescent lamp. The Blue light from the LED stimulates the phosphors to produce the other spectrum’s, where in a Fluorescent lamp, the blue to UV light produced by the mercury vapor gas does the same thing.. 

 White LED’s are quite efficient, because even though there are conversion losses when using Phosphors, the Blue LEDs are the most efficient LEDs.  But they suffer the same limitation as Fluorescents, namely that the red phosphors are less efficient so White LED’s tend to be more efficient the “cooler white” or more blue dominant they are.  And to be truly “white” they still need to produce a significant amount of green spectrum—and far more green than is optimal for plant photosynthesis.  So in general, White LEDs are far more suited for Vegetation phases of growth than for Flowering, which favors Red spectrum’s for optimal yields. But more on that later. 

Hybrid Spectrum LED’s

The most versatile and efficient LED designs for Flowering use an approach called Hybrid Spectrum.  In this design, the majority of the light output is generated in 4 spectrum bands aligning with the4 Chlorophyll peaks, and then a smaller percentage, usually around 10-20% is provided by White LED’s (the kind with the phosphors).  The White LEDs provide both the middle spectrum of green / yellow to supply photosynthetic activity to the other pigments such as the carotinoid system, to support hormones and other less understood plant systems, as well as providing full spectrum light in view mode to make plant and inspection easier.  The Osram SSL LED platform is the state of the art LED for this Hybrid approach with 5 extremely high efficiency 3w LEDs  (Deep Blue, Blue,White, Red and Far Red). All the Osram SSL family of LEDs have state-of-the-art ceramic substrates for better heat management and mechanical stability.  The heat transfer of the substrate is the key to LED efficiency and life and this new ceramic approach is far superior to the older plastic substrates still in common use in most of the Horti LED systems.

Designing a Plant with Light -Photomorphogenetics

The previously discussed process of light absorption and utilization in plants is very well understood in relation to overall plant energy production.  But the effects different spectrum have on specific plant morphology (changes in plant characteristics such as leaf and stem size, plant shape, etc..) is far less clear, but some basic principles are understood, and the effect of light on plant morphology is referred to as “Photomorphogenetics.”

One aspect of Photomorphogenetics that’s well understood is that the ratio of deep- red to red light triggers the shade-stretch response. Shade has a higher ratio of deep-red to red than direct sun, so when a plant sees a higher ratio, it stretches to get above the surrounding canopies shading it. It is also well understood that the hours of red absorption vs. hours of darkness is what triggers flowering in “short day” plants like cannabis. This flowering trigger is performed by the same Red and far red phytochromes as the shade stretch response, that’s one reason why cannabis stretches so much in pre-flower.

But what is also well known but not fully understood is that red spectrum’s tend to promote stem and flower production in Cannabis and many plants. By contrast, blue light promotes leaf, resin, and terpene (fragrance) production, as well as stimulating other pigments which can bring out more pronounced colors in the flowers. Again, this is known from the results of various studies with other plants and from experience in professional cannabis circles, but no formal university tests have been done with cannabis in this regard to this author’s knowledge.   

It is common knowledge in the Cannabis industry that high ratios of red spectrum produce larger denser flowers. Most just don’t realize it. But it is clearly illustrated to all by the difference in price between outdoor and indoor flower. No matter what soil or nutrients you use, they still never come out the same. And the reason is that sunlight has roughly an even ratio of Red and Blue (actually slightly higher blue) where as HPS has VERY little Blue (~5%). And actually,whenever sunlight is less than full, direct sun, it’s even MORE blue because skylight is VERY blue. This higher ratio of blue to red light promotes higher ratios of leaf to flower in the actual flowers, i.e they become “fluffy.”  So 20 years of pro-level cannabis horticulture has demonstrated if you want BIG Dense flowers, you need to provide Cannabis predominantly RED light from week 2 to week 6.  (assuming an 8 week flower cycle)

Master growers from California to Canada have also discovered that switching back to blue-rich MH lamps during the last 2 weeks of flower (referred to as the “Ripening”or ”Finishing” period) significantly stimulates resin and terpene production in Cannabis. It is not a common practice however because it is a major hassle to change HID lamps in fixtures (especially ventilated ones) over the tops of a nearly finished flowering canopy. , But with the variable spectrum LED systems, such as the California LightWorks SolarSystem 550, it is as easy as just pressing a button. Customized “Veg, Pre-flower, Flower”, and “Ripen” spectrum mixes are programmable with just a few keystrokes. This is especially convenient for growers who Veg and Flower in the same room.

So the Hybrid LED systems with independent spectrum control allow you to tailor the spectrum mix not only to increase efficiency through optimal absorption but also to effectively control the plants growth (morphology), and consequently the yield and quality of the end product. In effect you are telling the plant where to focus it’s energy.

Variable Power Density

The other advantage to variable spectrum / variable power LEDs is based on the fact that Cannabis has 2 distinct genomes—equatorial Sativas, and 30thparallel Indicas. The light intensity is brighter at the equator than at the 30th parallel, so Sativas evolved with ability to utilize higher levels of light, and they formed narrower leaves in response.

However,the Cannabis industry has settled on the light of a 1000w HPS over a 4×4 tray(~1000 uMols/C2) as a maximum or ideal, and the industry is fixated on using LEDs to reduce power consumption with equal yields to HPS. But this strategy is flawed, because real-estate is a fixed and significant cost, and if you want to maximize profit, one needs to maximize their yield per sq ft and there is NO science that suggests 900uMols/sq ft will do that for every Cannabis strain.

 Ina 2008 study at the University of Mississippi, researchers took “Mexican Sativa”plants (clearly Sativa dominant hybrids) and tested them under 4 light levels and 4 temperatures with all combinations logged. And they found the highest photosynthetic activity occurred at 1500 uMols, (almost 50% higher than the1000 uMols industry norm) at 86 degrees F. This was not late flowering and probably reflects the plants utilization during pre-flower where maximum growth rates occurs.

So it is clear, that depending on the strain there is significant opportunity for flowering with higher power densities than traditionally used. Thus varying spectrum AND power density system can help a grower find the ideal for each strain by raising the power density until the yield per WATT begins to drop. That is the threshold whereby you would know you have found the upper limit for that strain.

Finally,a study by the University of Maryland in 1983 measured the THC levels of Cannabis Sativa plants grown indoors under HPS, and supplemented with 4different levels of UVB light (supplied by mercury vapor based “reptile lights”) from zero UVB on the low end, to equatorial sun levels UVB on the high end, and a couple increments between. 

The results were quite remarkable. The plants with no UVB measured 25% THC. The plants at Equatorial sun level UVB measured 33% THC. That’s over a 30% increase in THC levels by just supplementing UVB light throughout the growth cycle. This study lead one top LED company California LightWorks to integrate UVB lamps in their SolarStorm LED grow light line with great success. Currently they sell a25w UVB fixture that with 2 over a 4×4 tray generate California Sun level UVB (~300 mWatts/C2) with an associated THC increase of 20+%

So it appears the potential benefits from varying spectrum and power may be greater in Cannabis than virtually any other cash crop, and as more data is coming in on this fascinating topic all the time, it is clear there are far more benefits to LED’s than just energy savings.

LED Safety, Simplicity, and Ease of Use

The last really compelling advantage of LED’s is simplicity and ease of use. LED shave NO lamps to replace, ever, have40-50% less cooling requirements than unventilated HID systems, create absolutely NO fire hazard in the close quarters of grow tents, and generate very little growth stratifying infra-red (IR) radiation.

So while skepticism regarding LED’s for Flowering still exists in the smaller pro-sumer segment of the cannabis market, there is growing and widespread acceptance for the newest generation of the top LED products among the bigger commercial players. When business people consider the energy savings, the reliability, the lack of lamp maintenance, independent spectrum control and advanced capabilities such as the ability to dim levels up and down using photocells for varying sun levels in greenhouses,  and all with higher yields per watt and higher quality end product, the decision is fairly straight forward.

 And it is for all these reasons that the current generation of LEDs grow-lights are finally poised to permanently replace HID as the first choice for Horticultural lighting.


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