New Technology for Plant Lighting—Part I.

By Bill Lermer

The factors of growing plants are interdependent and synergistic. The most important of these is light. Growers should start by getting the best possible light their budget can afford. What light is best, depends on the space and situation available. The following techniques are valid for just about any vegetable or herb, such as tomatoes and basil.

LIGHTING AND SPACE.

Light Spectrum.

The average color temperature of the sun is 5800 Kelvin throughout the year. What we see on earth has barely measurable spectrum differences between spring and fall.

The sun produces energy in pulses, like a carrier wave. Let us consider the photosynthetically active portion of this pulse, from the lowest frequency--red (675 nm)--to the highest frequency--blue (400 nm). (“nm” means “nanometer”. A lightwave that is one nanometer is one billionth of a meter in length.)

This pulse is like a train, with the blue in front and the red in the rear. The Red light acts like an engine placed at the end of the train, pushing the orange, causing a chain reaction, the whole spectrum working together synergistically. The best light for plant growth is full spectrum, like the sun, with a slight increase in red spectrum due to evolution and the fact that red travels through forest canopy better than blue.

Red light is the most efficient monochromatic spectrum for plant growth. However, all the colors have functions. If a person were especially good at hearing bass sound, that means the other pitches should be emphasized, so that one could hear the whole melody. For light to be efficient for plant growth, it must be full spectrum with close to equal linearity and amplitude through the production of 400-700 nm, with a slight bump in the red.

The blue spectrum has the highest energy and shortest wavelength [see: Light Measurement Handbook (1997), by Alex Ryer, page 8].

Blue is in the front of the light train, acting like a spearhead to penetrate the leaf, carrying the other colors with it. The far blue range includes UV-B, similar to what is found at high altitudes, and increases the prized phenolic compounds. This increases the flavor of peppermint, licorice, pepper, etc.

Conventional wisdom dictates that the growth phase be illuminated by a quartz metal halide (for blue light), the bloom phase by a high pressure sodium (for yellow/

orange light). However, a full spectrum is needed for all phases for a variety of functions. Blue induces growth hormones and rooting, and reduces stem elongation. Red induces flowering hormones, and provides energy for growth of flowers and leaves. Using only quartz halide during growth phase results in slow growth due to less red light. Using only HPS light during flowering stage often causes leaf yellowing, due to lack of growth hormones; and tends to cause stem elongation.

An artificial light which reproduces a red-enhanced full spectrum is the “Ceramic Metal Halide” or “High Pressure Metal Halide”. This has more blue than a metal halide with a conventional quartz arc tube, and more red than an HPS, so it’s spectrum is optimum for all stages. The Ceramic Metal Halide (CMH) features a ceramic arc tube like an HPS, and uses an HPS magnetic ballast. Philips has recently come out with a horizontal version of this bulb, resulting in longer bulb life and 1000 lumens more compared to a vertical cmh bulb operating horizontally. Generally, horizontal works best.

For those limited to magnetic ballasts, CMH is probably the best plant light available. For the Life Light electronic ballast, Life Lights also produces full-spectrum pulse-start metal halide bulbs. These are more efficient than normal halides, with better spectrum. They are designed to accommodate the 100,000 pulse rate of the Life Light electronic ballast. These come in four spectrums:

*6K for rooting and early veg

*4K for general growth/bloom

*3K for late bloom

*10K for finishing (last 2 weeks)

If you can’t afford all four bulbs, the 4K is fine for all stages.

The CMH has a better spectrum and is more efficient electrically than the retro-fit HPS that operates on a MH ballast. The retro-HPS is more expensive, lasts only half as long, and has reduced output, compared to a regular HPS.

Light Distribution.

Lighting is weak and diagonal around the edge of a cluster (i.e. group) of horizontal lights; this can be compensated for by 100w side lights. If a single horizontal light were used, non-uniform growth would result from the non-uniform distribution of light. For a single vertical light, a paraboloid reflector would give circular distribution of lighting, uniform except for the dark spot under the bulb. However, a cluster of full spectrum lights, like a spinning chandelier, is the best:

* The lower wattage enables bulb placement closer to the plants

* Full spectrum light sources are in phase with each other, and similar to natural sunlight

* Light directly from the bulb has more energy than reflected light

* The motion from spinning changes the angles from the lights to the plants, thus simulating diffusion

* Overlapping of light patterns:

--increases total light due to the array effect (the total amount of light is increased when light rays intersect)

--allows for sufficient light in fringe areas, at edges of most light footprints

--increases diffusion because light comes from different angles; this increases reception of light by the leaf surface.

A 1000w HPS light, mounted horizontally, would need at least 1.5’ (.4572 meter) distance to the top leaves, to avoid burning. A 400w light needs only 1’ (.3048 meter), much closer if spinning.

Even if a mixture of halide and HPS were used, their light spectrum can still be mixed by spinning the reflector. The best way to distribute light would be the spinning method, with multiple horizontal lights.

A benefit of horizontal lights is that almost half the light goes down to the plant directly from the bulb, without reflective losses. This direct light has more energy and efficiency than reflected light.

Not only do multiple lights increase the number of angles from the bulb to the leaf surface, but the spinning of the lights changes the angles. This increases absorption by the leaves, whose surfaces are not smooth, because multiple angles of light casts fewer shadows on the leaf surface.

The array effect enhances light output by 20% when two or more lights are separated (but close) and whose light rays intersect with each other. This is because fringe light tends to drift toward dark areas of the room, whereas strong light tends to be more cohesive. (You may verify this by constructing two Newton Boxes, each one cubic foot, painted black on the inside, with a 12.57” circular hole in between the boxes. Your light meter in the center of one box will read 1 footcandle if there is one candle in the center of the other box; but 2.4 footcandles if the other box has two candles, each separated by a distance equal to the width of the flame.)

A horizontal 400w CMH full spectrum light, with a horizontal paraboloid diffuse reflector, covers about a 3’ x 4’ oval area by itself. When combined in a cluster, each such light covers about a 4’ x 5’ area. This is because outside the 3’ x 4’ area of high intensity, there is a fringe area of lesser intensity. When the fringe areas overlap, they become high intensity areas. A similar effect occurs with lights of different wattages.

The most reflective surface is 98% reflective, and this is specular or mirror-like. The specularity reflects light at exact angles, like the edges of a pool table. This has the disadvantage of creating hotspots. Spinning would disperse the hotspots. The lack of diffusion (i.e.multiple angles) of light rays can create shadows on the leaf surface. So, the leaf absorbs light better when light is coming from different angles. When horizontal lights are in a cluster, the different angles of light increase diffusion.

White diffuses light. The best (affordable) white paint is about 91% reflective. Glossy white is easier to clean, but is only 68% reflective. Flat white can’t always be cleaned, but is more reflective and can be repainted. The real solution for durable white is the manufactured kind, with 91% reflectance and a protective coating. Diffusion decreases the penetration of light, so having short plants is especially important with white.

Silver reflective surfaces are normally specular, but there are brands of 97% reflective silver that have indentations for diffusion.

Light Intensity.

The plant doesn’t want more than 5500-6000 foot candles. More than that usually slows down growth. To measure this, I recommend getting a light meter. The Chinese-made Giros light meter is at the minimum-quality end. Indoors with artificial lighting:

--for every foot distance from the bulb, about ¾ of the light energy is lost. (For a discussion of the inverse square law, see Light Measurement Handbook, page 25 (Ryer, 1997). That’s a good reason to have short plants--and lower wattage, spinning bulbs (which can be placed closer to the plants).

--the plants will stretch toward the bulb if light intensity is highly concentrated as in a 1000w bulb.

This high concentration of light intensity is associated with high concentration of harmful heat.

CONTINUOUS vs. SIMULTANEOUS METHODS.

The main idea of the continuous method is to provide plants for one bloom light at a time, in a rotation. After plants are moved to under a bloom light, place rooted cuttings under the now-vacant veg light. And so on, until all the bloom lights are occupied. When the oldest bloom plants are harvested, that space is ready for a new batch of grown plants.

For the first crop, use the simultaneous method--the entire crop would be started at the same time. The first time through, there are no bloom plants at the beginning, so the bloom room will be available for the vegetative stage. The full spectrum lights are the best for both stages.

Limit the growth phase to 10" [25.4 centimeters] in height, and use the continuous (as opposed to simultaneous) method. The continuous method typically employs one veg light for every 4 bloom lights, on rotation. This assumes a two-week vegetative, and a two-month bloom stage. A longer bloom stage relative to grow would allow for more bloom lights with the same amount of veg light.

The short height will reduce the time spent on growth, and increase light levels on the lower leaves. If the plant is only 10”-12” [25.4-30.48 cent] when it goes into bloom, it will grow another 1’-1.5’ [.3048-.4572 meter] in the bloom phase--the maximum height (2’-2.5' ) [.6096-.762 meter] that can be adequately covered with efficient lighting.

If the container (e.g. Classic 1400) or hydro bucket is 10.5" [26.67 centimeters] wide, 12 plants can be placed one next to the other under a 400w CMH, the same number that can be placed in the bloom room with 4” [10.16 centimeters] spacing in between buckets. The 4” spacing isn’t needed in the growth phase if vertical height is limited to about 10” [25.4 cent], since the plant is usually not wider than it is tall.

Once they reach 10" [25.4 cent] place them under the bloom lights. These should be of higher intensity, because the plant is taller and will need the intensity to cover the lower leaves. This extra intensity can be gained by the greater efficiency of a cluster of horizontal lights; and also by topping the plant. In early veg, once you have 4 good branches, slice off the top apical shoot, even before it is big enough to be a cutting. This will slow growth for a few days, but results in a much better formation, with four main stems instead of one. The plant will be lower, resulting in greater light intensity on lower leaves.

ELECTRONIC BALLASTS.

Electronic ballasts are more efficient than the magnetic, saving 20% of electricity usage with new bulbs, up to 30% with older bulbs (because bulbs last longer with e-ballasts). E-ballasts improve bulb efficiency because they can respond intelligently in real time to changing conditions in the arc tube and line conditions. The spectrum is improved, because the spikes are smoothed out.

The Life Lights e-ballasts power the lights at a high frequency of 100,000 pulses per second. This is more like natural sunlight than the 50-60 hertz (cycles per second) of magnetic ballasts.

A common problem with magnetic (non-electronic) hps ballasts is short-range ignitors, which are rated for only 5’ (1.524 meter) from ballast to bulb. If the lamp wire is significantly longer than that, and even if thicker wire is used, the short-term failure rate can be 25% and ignitor life is lessened. Long-range ignitors are made for some of the wattages of magnetic ballasts, but due to lack of thought on the part of bean-counters at corporate HQ, are almost impossible to buy and are more expensive when available.

Without protection, electronic ballasts produce harmonics which cause electromagnetic interference (emi). Be sure to get an e-ballast that protects against this emi. Make sure you get an e-ballast that is US FCC compliant and German VDE listed. Europe and Canada don’t require e-ballasts used there to have emi protection.

The Life Lights e-ballast:

*has long-range ignitors (up to 90’) and thermal protection

*works on HPS, MH, and PSMH with no switches

*has 6KV pulse-rated wire and quick disconnect connectors, including a ground wire. Others may have only two bare non-pulse-rated wires sticking out of the ballast enclosure (with no strain relief), or have connectors that could melt.

*had only 2% failure rate in VDE destructive halt tests, and a 0% failure rate in tests of normal operation done by BC Hydro Power.

*have US patents for advanced technology

*are sealed, and cooling fans that pull air by the heat sink instead of the delicate circuits. If not sealed, the circuit would accumulate dust and moisture<

*have a high frequency pulse rate, not detected by power company detectors. This simulates natural sunlight, leading to greater absorption of light by the leaf

The best temperature for most plants is about 65-76 degrees F(18.3333333 C to 24.4444444 C).

Generally, for every 2 degrees F (1.111111 C) above 76F, growth of the entire plant is reduced 10%. You should have a temperature probe by the top leaves.

According to Photosynthesis pp.13-14 (by Hal and Rao, 1974) the optimum temperature is about 76F, assuming 10,000 lux light intensity and 400 ppm of CO2. (The ambiant CO2 level is about 350 ppm, depending on location.) Some plant crops increase short- term production linearly up to 500 ppm CO2, but continued exposure to this high concentration injured the leaves. Concerning optimum temperature and CO2 levels for yield, your results may vary depending on genetics and other factors.

The best relative humidity is 48% to 57%. That’s because the lower humidity tends to close the leaf pores to counteract increased transpiration, and this also limits carbon dioxide absorption. Humidity too high can cause mold.

Within these optimum temperature and humidity ranges, adding CO2 brings only marginal benefits, and more than 500ppm can be harmful.

Higher levels of CO2 can compensate for high temperature or low humidity, and in extreme conditions, 5000 ppm of CO2 should be added. However, more growth and quality is lost through above optimum temperatures than is gained from extremely elevated levels of CO2.

The best way to reduce excessive heat is prevention. Electronic ballasts produce less heat. The ballast and bulb can be cooled by fans or by spinning. Spinning the bulbs and e-ballasts at a fast rate is more effective than fans, because of greater air movement over the heat sources.

Heat produces resistance, and more resistance produces more heat, and etc. This “cascade effect” also works in reverse; i.e. less heat leads to less resistance, and less resistance leads to less heat, etc.

Generally, the best way to lower excessive temperature and humidity is through venilation.

An air conditioner would lower temperature, but to avoid venting out the cooled air, the room should be sealed, which would require that CO2 be added. Venting would also vent out the added CO2. If the vent fan is on a thermostat, and the CO2 is added when the vent fan shuts off, CO2 would be needed frequently and the tank would be emptied quickly. Propane bottles last longer, but are unsafe in inhabited structures due to the danger of a fuel/air explosion of great force. Venting provides enough CO2 from ambient air, and also provides electrolytes (which float in free air, are catalysts and energy sources). Electrolytes can be generated artificially by splashing water over rocks. If still too hot, add a swamp cooler, which will also increase humidity.

Sometimes the air around the plants is sealed from both the outside air and the air around the plants, by a glass or lexan barrier attached to the bottom of the reflector. The idea is to vent out the hot air around the bulb, but not the air around the plants. However, about 90% of the people who use this system also vent out the air around the plants, running the air first through a deodorizing charcoal filter. This is because most people don’t want to buy an air conditioner, de-humidifier, electrolyte generator, and CO2 system. The A/C and de-humidifier use some electricity because the air around the plants would still get too hot (or humid), albeit less hot with the air around the bulb vented. That’s because the light that makes it through the glass shield eventually converts to heat. The venting tubes radiate heat, so some of the heat from around the bulb leaks to the air around the plants. Without venting the air around the plants, water evaporation from the plants would tend to increase humidity enough to require de-humidification.

The electricity used by the A/C and de-humidifier would usually be better used for more lights. So for most people using the sealed reflectors, the main benefit is the air movement past the bulb. However, for that benefit, the glass shield and isn’t necessary, and is a drawback because it filters out 6-15% of the light. The light that’s filtered out is converted to heat, lessening the benefit from the air movement around the bulb. The blue spectrum is the most important, and this is especially filtered out by the glass/lexan barrier (meaning that more than 6-15% of the blue spectrum is filtered out). See this chart from the Light Measurement Handbook:

There is a relatively new method that can supplement (and perhaps replace) atmospheric CO2. When a type of carbon is used as an emulsifier (i.e. transfection agent), complex organic molecules like sugars and hormones can be absorbed in a liquid form, through the roots. This liquid carbon should be added to either vented or non-vented systems, with or without CO2 gas supplementation. Liquid carbon isn’t affected by ventilation, and has 80 times the density of carbon atoms compared to carbon dioxide gas. With liquid carbon, the plants can thrive longer, enabling partial/double harvests. Theoretically, this liquid carbon could transport bloom hormones into the plant during the vegetative stage, enabling long-night blooming plants to bloom with long days.

Bill Lermer is a horticultural consultant, and may be reached at:

euroindoorgrow@yahoo.com

for questions.