Current lighting technologies for plant growth

Summary

Dive deep into current lighting technologies for reach in plant growth chambers and walk in rooms including fluorescent, high pressure sodium, metal halide, ceramic metal halide, LEDs and more. Reg Quiring, Sr. Designer, Conviron will discuss current lighting technologies as it applies to plant growth in controlled environment conditions.

Reg is recognized as the most experienced controlled environment designer in the world, with many of today’s products being the outcome of his design. Reg is an active participant and contributor to the industry’s largest information exchange seminars. Significant design successes include development of a superior barrier light canopy and downward air flow chamber that is now available as a standard product offering. He is a Certified Engineering Technologist (CET) and past President of the Committee on Controlled Environment Technology and Use. Reg has over 33 years of design, management, sales and service experience in growth chambers, electronics and electro mechanical systems.

Video Transcript

Thank you all for your interest in this presentation about current lighting technologies for plant growth. Current technology includes the following main lighting types. Some of them, like T5HO fluorescent, high-pressure sodium, and metal halide lamps have been generally available for some time, but they are still useful and can be quite cost-effective.

They’re supported by a vast, worldwide general lighting market. The ceramic metal halide lamp type is rising in demand for plant growth lighting. In particular, the Phillips Elite 315W Agro lamp has a very complete spectrum, is more efficient than almost any other lamp type at delivering photosynthetically active radiation (or PAR), including the current pulse spectrum LED sources.

LED lighting is increasingly on the minds of researchers and plant facilities managers. Researchers are learning about the specialized photobiological investigation that is made possible with LED lighting, and facilities managers are tempted by claims of incredible energy savings that many LED vendors are promoting.

Several LED types, from simple fixed-spectrum to complex multi-band types will be discussed.

Current fluorescent lamps like T8 or T5HO have offered a safe lighting spectrum for cost-effective general plant growth for many years. For this brief webinar, only the most common 4100 kelvin color temperature type will be included. These are similar to the older ‘cool white’ designation.

Fluorescent lamps are widely available, economical, relatively efficient, reliable, and available in several spectrums including primary colors. Intensities from 20-800 micromoles per meter squared per second at 30 cm distance are practical. To control the intensity, one can have simple stepped level switching of lamp banks, or stepless dimming of all lamps is becoming more common. Many T5 electronics dimmable ballasts can go as low as 1% of the maximum intensity.

One drawback of all fluorescent lamps is their temperature dependency. Their light output decreases at lower temperatures and very warm temperatures. However, for the majority of research between 20C to 35C it’s not of great concern. Often incandescent halogen lamps, or 730 nanometer LEDs are added as supplementary light source for far red radiation. The supplementary lamps are controlled separately so they may either be included with the primary lamps, or on their own as an end-of-day treatment.

Here is a spectral power distribution of a common, 4100 kelvin T5HO lamp. The Y axis is relative intensity and the X axis is the wavelength in nanometers. All spectral distribution charts in this presentation are similar. The multiple data lines are the same lapse, but progressively dimmed as indicated in the legend. The spectrum is quite spikey, but some light of all wavelengths is included. Notice there is no obvious shift in the spectral distribution regardless of the amount of dimming. This is a good characteristic since changes in the program light intensity by dimming should not introduce a hidden variable.

Here is a simplified representation of the same T5HO spectral distribution. The radiation from each hundred nanometer band is summed and shown as a bar chart. Notice the color bands all remain in consistent relative amplitudes as the lamps are dimmed from 100% down to 10% in increments. Here is the blue. Here is the green. Here is the red. By the way, the dash lines are not extended to the lowest 10% value as the last dimming increment is smaller than the others.

Here’s a spectral chart of a chamber with fluorescent lamps alone, and then showing both 10% and 40% by wattage added of supplementary tungsten incandescent lamps. The blue line is the fluorescent lamps alone. The magenta line is the fluorescent lamps with 10% incandescent added. The yellow line is the fluorescent lamps with 40% incandescent added.

Incandescent lamps do not add any significant amount of PAR. They are only really useful to add radiation of the far red region around 730 nanometers.

Those older tungsten filament lamps are mostly obsolete. Chambers now ship standard with newer halogen incandescent supplementary lamps. This chart shows that the spectrum in intensity of a 43W halogen lamp is very close to that of a traditional 60W tungsten incandescent lamp. Halogen lamps therefore save approximately 30% energy.

Screw-in 730 nanometer far red LED globe-style lamps are slowly becoming available to directly replace line-voltage halogen lamps. There are few certified commercially-available options to choose from that distribute light as evenly as a traditional lamp.

730 nm LEDs are a very efficient secondary light source for far red radiation. It is the few commercially available options for approved far red lamps with wide light distribution that limit their adoption. White light screw-in LED lamps that now widely replace incandescent lamps for general illumination cannot replace incandescent lamps as a source of far red. They emit little far red radiation.

Here is the spectrum of a typical Phillips Warm Light LED lamp. This 6W lamp has the great advantage of providing as much general illumination for human vision as a 40W incandescent lamp, but it has very little far red light at 730 nanometers.

This chart further illustrates why general purpose white LED lamps are not suitable replacements for incandescent halogen lamps as a source for far red radiation. The blue line in this chart is a halogen lamp spectrum. The magenta and yellow lines are high-quality Switch brand LED lamps meant to replace incandescent lamps. Note with the halogen lamp spectrum there is far more far red radiation at 730 nanometers than red radiation at 660 nanometers. This is why a halogen lamp is useful as a source of far red.

Looking at the spectral distribution of a typical white LED replacement lamp, note that there is almost no far red radiation at 730 nanometers. White LED lamps will add visible PAR in a chamber but do not contribute far red. That energy would be wasted in standard LED lamps, which are intended to excel at delivering light per human vision without wasting energy producing far red, which we cannot see.

Here is the spectral curve of a typical 730 nanometer far red LED source. See how most of its energy goes directly in the far red sweet spot.

This is a photo of a small reach-in chamber with primary lighting by T5HO fluorescent lamps. Supplementary far red radiation is provided by Illumitex 730 nanometer LED bars.

Here is the spectral chart, which is mostly a typical fluorescent lamp. Look at the region from 700-750 nanometers to see the effect of adding LED far red at various drive powers. In this case, the user specification required a red to Far Red ratio of 1.1:1, defined by the ratio of red radiation from 655-665 nanometers versus the far red radiation from 725-735 nanometers.

The first column of the table shows the far red LED power set points, while the last column of this table shows the range of red to far red ratios that were achieved across the dimming range of the far red LEDs. While the primary fluorescent lamps were delivering 500 micromoles of PAR between 400-700 nanometers.

Dimmable 730 nanometer LED fixtures are a very efficient method at delivering adjustable far red radiation.

Moving on to high-intensity (or HID) lamps – high-intensity discharge lamps – those are typically high-pressure sodium (HPS), metal halide (MH), and ceramic metal halide (CMH). These lamps are capable of delivering very high PAR irradiance levels but usually at working differences at 0.75 to 1 meter due to the associated radiant heating.

Here’s a spectral power distribution of a common 400W high-pressure sodium lamp. The multiple data lines are the same lamps but progressively dimmed. The spectrum is quite spikey and is characterized by a lack of blue light from 400-450 nanometers. It is the lack of blue that makes HPS lamps appear so orange, not so much a surplus of red. There is no serious shift in the spectral distribution, regardless of the amount of dimming except where narrow inversion around 590 nanometers. They’re not quite as consistent as T5HO lamps when dimmed, but not bad either.

HPS lamps are often used for daylight extension in greenhouses, but the lack of blue lights makes them generally unsuitable as the only light source in a growth chamber.

Here is the simplified representation of the HPS spectral distribution. Notice the color bands all remain at fairly consistent relative amplitudes as the lamps are dimmed from 100% down to 30% in increments. Here’s the blue. Here’s the green. And here’s the red. Once again the dash lines are not extended to the lowest 30% value as that last dimming increment is only half the other steps.

By the way, HPS lamps cannot be dimmed any lower than 30%.

Looking at metal halide spectrum. Here is the spectral power distribution of a Pulse Start 400W metal halide lamp. As before, the multiple data lines are the same lamp but progressively dimmed. The spectrum is quite spikey and characterized by a sufficient amount of blue light from 400-450 nanometers. It is the balance of more blue to red that makes metal halide light appear whiter than HPS lamps. There is no obvious spectral distribution shift regardless of the amount of dimming, except at the narrow peaks of 435, 545, and 575 nanometers. Generally quite good across the majority of the 400 nanometer to 700 nanometer PAR band. Metal halide lamps are not as consistent as dim T5HO fluorescent lamps as was the case with HPS lamps. These lamps have sufficient blue light to provide a balanced spectrum for plant growth.

Here is a simplified bar chart representing the metal halide’s spectral distribution. The color bands remain at consistent relative amplitude as the lamps are dimmed from 100% down to 30% in increments. Once again here is the blue, here is the green, and here is the red.

As with HPS lamps, the dash lines are not extended to the lowest 30% value as the last dimming increment is not the same as the others. Metal halide lamps can be dimmed down to about 25%, but we used 30% to be consistent with HPS lamps.

Now this is a growth room – a very common solution especially in North America – which contains 50% high-pressure sodium, and 50% metal halide lamps to provide a more complete spectrum. This blended solution takes advantage of the blue in the metal halide spectrum, and the higher efficiency of HPS lamps. An obvious disadvantage of using two different primary lamp types is that there is – if the plants grow too close to the lamps, they see different light. Using wide diffusion reflectors helps mitigate this.

So this is the spectral power distribution of those lamps combined – metal halide and high-pressure sodium – at full and at dimmed powers. The blended spectrum is still quite spikey, but has a better overall balance for general purpose plant growth. Generally quite good across the majority of the 400-700 nanometer PAR band. Quite useful and used successfully for many years.

Here is the simplified bar chart representing the blended high-pressure sodium and metal halide light source spectral distribution from the last chart. The color bands all remain at relatively consistent, relative amplitudes as the lamps are dimmed from 100% down to 30% in increments. Here is the blue, here is the green, and here is the red, and as before the dash lines are not extended to the minimum 30% value, and that last dimming increment is different than the others.

Phillips, and others, have developed newer, ceramic arc-tube metal halide lamps for efficient, high-quality general illumination, having a very complete spectral distribution. Any of these lamps has a more complete and balanced spectrum than the 50/50 blended HPS and metal halide solution just discussed. Therefore, a single lamp type can be used, ensuring a consistent spectrum anywhere in the growth area. These ceramic metal halide lamps are certainly more efficient than standard metal halide lamps, and more efficient than HPS lamps for plant growth.

This complex chart shows all three Phillips 315W color types at full power. The 930 type, 3000 kelvin ‘warm white’ is the yellow line. The 942 type, 4200 kelvin ‘cool white’ is the black line. And the special Agro version, with its unique large deep red peak at 660 nanometers, is the magenta line. All of these lamps have a broader spectrum, including a bit more UV and considerably more Far Red radiation than either metal halide or high-pressure sodium lamps, or those in combination. Ceramic lamps are more efficient than standard metal halide and high-pressure sodium lamps for plant growth. Actual test results in a Conviron walk-in chamber show as much PAR radiation with the 315W Agro ceramic lamps as with the same number of standard 400W metal halide and high-pressure sodium lamps.

These are the simplified spectral power distributions of the 3 different Phillips 315W ceramic lamp types at full power. The 930 type on the far left has less blue and more green than the Agro lamp on the right. So it’s not as desirable for general plant growth. For now, it’s removed from further discussion.

The 942 lamp in the middle is of interest primarily because of its abundance of blue light which encourages vegetative growth, however when dimmed, surprisingly the ratio of red to blue light actually inverts. It is questionable for this reason.

The Agro type has a full broad and balanced spectrum with plenty of blue for good morphology, and lots of deep red to drive photosynthesis. And happily, it has the most consistent spectral distribution across its full dimmable range of any HID lamp we have ever measured.

The Agro lamp specifications state efficiency at 1.9 micromoles per watt. This is higher PAR efficiency than any other commonly available lamp and also higher than any broad-spectrum LED fixture we have investigated. It’s nearly twice the micromole per watt efficiency of typical plasma lamps, which also feature a broad spectrum.

The focus on the Agro lamp’s spectral distribution in the simplified version, and across its dimmable range. Notice the color bands all remain very consistent in relative amplitude as the lamps are dimmed from 100% down to 40% in equal increments of 20%. Here’s the blue. Here’s the green. Here’s the red. And we’ve also included far red as there’s quite an abundance of it in this lamp type.

This Agro lamp is the best HID lamp we have ever tested for efficiency and dimming consistency.

Here’s an example of a 50 square meter growth room using ceramic metal halide lamps in broad dispersion reflectors. This density of fixtures delivers 600 micromoles at 1 meter below the lamps.

I’ll now move on to the popular topic of LED – or light emitting diode – lighting technology.

LED lamps are the latest form of producing light efficiently for plant growth. LEDs are useable for both primary and supplemental lighting applications. They are available with fixed and adjustable spectrums. There are many suppliers of LEDs for integrations in chambers, but there is little industry standardization yet. This leads to challenges incorporating LED fixtures into new and retrofit chamber applications, and many LED lamps and fixtures on offer are not properly tested and certified.

However, approximately 40% energy saving may be possible over T5HO lamps especially at the higher intensity specifications.

Here’s an example of a large, 5-level Conviron growth room with Phillips fixed output and fixed spectrum LED bars. After their own growing trials, the client selected a mixture of deep red with blue bars, and deep red with white bars. This is a simple fixed LED spectrum. Non-dimming production lighting chosen for efficiency and repeatable results for validating seed quality and seedling growth consistency. As such, it is not really a research chamber, but a production chamber.

Here is a 40 square meter growth room fitted with Illumitex F3 fixed spectrum LED bars. The spectrum is predominantly red with a bit of blue, and a very small amount of white light in between in order to claim it is a full spectrum LED source. This light spectrum may be efficient at driving photosynthesis, but more and more research is revealing that a lack of sufficient radiation across the entire PAR spectrum may compromise many subtle plant responses. This is the same F3 spectrum and it shows that temperature dependent shift in the color of the direct LED output – that is, LEDs without secondary phosphorous. It is particularly noticeable at the large red peak.

Across a 30°C chamber temperature span from 10-40°C, the spectroradiometer data reveals a 7 nanometer shift of the red peak to the right with increasing temperature. This seems it would be a liability in research since the narrow peak of the predominant source of radiation can move about with chamber temperature and the dimming power of the LEDs that they’re driven at. This is not just an Illumitex characteristic. All direct radiating red LEDs behave similarly as far as we have seen.

Valoya is a Finnish company specializing in LED fixtures for plant growth. Valoya uses custom secondary phosphor blends to flatten out the narrow peaks of discrete colour LEDs, which then provide a broad spread of radiation. The black line in this chart shows an early Valoya wide spectrum LED having far more light distributed across the spectrum than direct radiating narrow band LEDs.

Here’s an early test of a Valoya broad spectrum LED fixture in a small growth chamber. Note the light still has a pinkish colour cast, but much less so than the magenta of the Phillips deep red-blue and white bars, or the Illumitex F3 LED fixtures shown earlier. Under this spectrum, one can accurately assess general plant health as the leaves show as green instead of almost black.

Here is a 60 square meter growth room fitted with Valoya wide spectrum LED fixtures. The fixtures and the power drivers are mounted on height-adjustable frames that are motorized for various crop heights from corn, to cotton, and cereals.

This is a newer Valoya LED spectrum type NS1 with very natural color balance, but also including a small amount of UV and some far red as well.

This is a spectral power chart shows the excellent color stability of Valoya LED plus phosphor AP673 wide spectrum fixtures. In a chamber set at 5C, 25C, and 40C at both maximum and minimum drive power. Notice the absence of any noticeable shift in the spectral peaks and valleys. This is the data from 5C to 40C at 100% LED drive power. The data from 5C to 40C at minimum LED power is just as favorable and stable.

As further confirmation of the stability of wide spectrum LEDs with phosphorous, the same fixtures were operated at 10 incremental drive powers from 10% to 100% at 25°C. See that there’s no perceptible spectral shift across the entire dimming range. This interesting project specification for a 12 square meter growth room included a requirement to deliver 550 micromoles of PAR at 1 meter below the lamps, with a stable 1.2:1 red / far red ratio across a 15C-35C chamber temperature range. To accomplish this, we required the temperature stability of Valoya broad spectrum LED fixtures, which already have a significant far red component. Interspersed were 730 nanometer LED fixtures to add sufficient far red to achieve the required red / far red ratio.

The left chart in this slide shows a separate blue spectrum line for the Valoya AP67 main fixtures and a red line for the supplementary 730 nanometer far red LED fixtures. By varying the 730 nanometer LED fixture drive power, the red / far red ratio could be adjusted from 3.0:1 all the way to 0.89:1. The right chart shows the aggregate spectrum with red / far red at 1.2:1, while the main light provided 550 micromoles of PAR for 400-700 nanometers.

Heliospectra, a Swedish company, have developed several multi band LED fixtures for photobiological research. This slide shows multiple photos of the same Heliospectra 600W L4A fixture in a small Conviron growth chamber set to various colors out of a multitude that are possible using 9 independently dimmed LED color channels.

Here’s a 1.5 square meter chamber fitted with three independently controlled Heliospectra L4A LED fixtures. The three fixtures can be operated in unison or independently to provide three different light treatments in the same environment with the help of roller blinds to separate them into three zones.

This spectral distribution chart shows the nine individual LED colour channels of the Heliospectra fixture at their full power. The range also includes UV and far red channels.

This spectral distribution chart shows all the Heliospectra’s nine LED colours combined at their full power.

An emerging LED lighting and control system development by Valoya is called ‘light DNA’. It is a combination of individual LED narrow-band and wide-band color channels and a fixture, and sophisticated Cloud-based data on global lighting conditions. This opens the possibility to closely mimic natural dynamic outdoor lighting editions such as: different seasons, different locations, morning and evening, canopy shade, and even sun flex.

This left image in this spectral data shows daytime clear sky radiation of 1784 micromoles PAR with the expected solar spectrum. The right image shows when clouds appear, reducing the radiation to 205 micromoles PAR and relatively less red and far red in the spectrum. Quite a substantial shift in the spectrum with clouds in the sky.

This left image illustrates sunset clear sky radiation of 109 micromoles PAR with a spectrum greatly different from daytime, having almost no blue and proportionally greater deep and far red. The right image shows when clouds appear, massively reducing the radiation to three micromoles of PAR and significantly altering the spectrum.

This actual chart of the Valoya light DNA output shows the ability of a system to simulate daytime clear sky spectrum to better than 90% from 380-750 nanometers. For replicating natural light qualities through the day at any point in the globe, this system is poised to enable sophisticated photobiological research flexibility in controlled environments.

This chart shows the ability of Valoya light DNA system to simulate dusk clear sky spectrum to better than 90% from 380-750 nanometers.

Moving on, here is a current lighting system of interest that has been recently delivered to a client. It is a hybrid system with 4 stages of primary ceramic metal halide lamps combined with a small stage of broadband LED to help smooth the intensity transitions as the ceramic lamp stages sequence on or off. This system is capable of ramping up from less than 50 micromoles to over 1700 micromoles at 1 meter and back down. Also included are a set of 730 nanometer far red flood lamps for independent, end-of-day treatment.

This chart shows light intensity versus time, showing the ability of the hybrid ceramic metal halide and LED system to follow a lighting program wrap up to 1300 micromoles and back down. Two different tests are shown to illustrate adjustments to the control sequence software to improve the rising wrap. The rising wrap is the most challenging because the ceramic lamp stages must operate at full power for 15 minutes before they are allowed to dim by their electronic ballast firmware.

I’m gonna end with a very special purpose lighting chamber system. Xenon Lighting. To simulate the solar spectrum – in particular, the UV portion, the only light source that can reliably do this is a Xenon arc lamp with some adjustment filtering. The purpose of this chamber is to predetermine the environmental fate of new pesticides and herbicides as part of the approval process for licensing their use in the field. Accurate solar UV radiation on demand is required to verify any new chemical’s efficacy, and more importantly, ensure that any new compounds that may form when the original breaks down under UV exposure are not highly toxic.

The three Xenon lamps can be operated in unison or separately, with reflected partition panels between them to allow three different light treatments. The three 6000W water-cooled Xenon lamps and the auxiliary lighting power control and cooling cabinet were themselves more than 2/3 the cost of the chamber. Very expensive, very unique, and a very inefficient light source, but a critical piece of equipment in this case where accurate replication of solar UV radiation was of paramount importance.

With that, I just want to remind you of some considerations when deciding on lighting technology. And that, really, there is no one optimal light technology for all cases. Some considerations you might take into account of course are research objectives, the plant type, what plant responses may be being investigated. And the lighting characteristics of the source, the intensity required, spectral properties, and whether or not a fixed or adjustable spectrum will be of use in the research that’s being considered. Then there are the physical facility requirements, the life span of the lights, energy conservation, and budgetary restrictions.

I’m going to add that, where project budget allows for LED lighting, if at all possible, make sure to trial the specific LED lighting that’s being considered. It can be a very wise investment, but one that is hard to change if chosen incorrectly for the intended use.