CMH lights were actually introduced in the 1980s, but with recent discoveries in horticulture research, it makes sense why many growers are utilizing this technology.
Philips Lighting invented ceramic metal halide, introducing the first 400w CMh in the year 2000. This was a 400w “Retro-white”, designed to operate on a 400w HPS magnetic ballast. While at the time this was a revolutionary improvement in horticultural lighting spectrum, it lacked a digital ballast. The “Farmer in the Sky” urged the foremost hortilcultural lighting expert to ask Philips if they would further develop this CMH light. As a result, Philips assigned Dr. Gelten to develop the 315w CMH “Elite Agro” light bulb, and accompanying digital ballast by Advance.
Philips owns Advance. This is important, because the bulb and ballast should be designed together as a pair. Why?
No ballast is perfect and all generate harmonic distortions. This THD (total harmonic distortion) should be measured at the output, not the input. Yet most manufacturers list only their input THD and all measure at less than 10% input THD. That is like measuring the pollution of an engine by sampling the air going into the engine instead of the air going out of an engine. The Advance 315w CMH ballast has less than 14% output THD, one of the lowest in the industry; and is very rugged with virtually no burnout.
Each type of bulb has frequencies at which it will resonate. If the resonance nodes of the bulb coincide with the output harmonic distortions of the ballast, the result is “acoustic resonance” which shortens bulb life and reduces it’s efficiency. This acoustic resonance can even be heard. So, the output THD of the ballast should not be at frequencies which are the same as the frequencies which will resonate with the bulb.
Light Spectrum and Quality
The Philips “Elite Agro” cmh bulb has a CRI (color rendering index) of 92% of natural sunlight. Plants have evolved for millenia under natural sunlight and every slice of the spectrum has beneficial functions. While HPS has a CRI of only 20, and regular Metal Halide only 60. Many LEDs claim to be “full spectrum” but in reality are mostly red and blue.
Blue light helps induce growth hormones, reduce excessive stem elongation, and help rooting. Red light induces bloom hormones, and helps speed of plant growth. Green light is most abundant in natural sunlight so plants don’t have to be very efficient at absorbing green (which is why most leaves are green); but still is needed, particularly for adult plants. UV-B light below 300nm helps increase potency or phenolic compounds, but is dangerous and virtually no grow-lights have significant amounts of UV-B.
Infra-red is present in natural sunlight, and is beneficial where additional heat is needed. However for indoor grows, infra-red tends to be net harmful because it is reflected into the plant zone and increases plant temperatures. This can induce excessive stem elongation, and thinner leaves with more water content.
Single-ended HPS has twice the infra-red of CMH, and DE HPS has even more (which is why they must be 3′ away from the top leaves).
HPS is mostly green-yellow-orange, with little red or blue. While regular MH is about the same, with a little blue added. CMH is the best spectrum of all HID lights, at less cost per watt than LED. CMH has a nice continuous spectrum curve, unlike almost all LED (which have very low intensity in certain parts of the spectrum). Some LED have solved this problem, but are more expensive.
Standard MH bulbs use a quartz arc tube, while HPS and CMH use the more efficient and longer-lasting ceramic arc tube. Instead of sodium inside the arc tube, CMH has metal halides which give a full spectrum, with enhanced photosynthetic photon flux levels compared to HPS or regular MH.
Full spectrum lighting is more efficient than partial spectrum, thus requiring fewer watts. In addition, the lower infra-red levels of CMH (compared to HPS) means less A/C cooling costs and less excessive stem elongation and water content of leaves.
Every watt produces about 3.41 of heat, measured in BTU. Which means, a 315w CMH produces 3.412142 x 315 = 1075 BTU; while a 600w HPS produces 2047 BTU. This understates the effective heat produced by HPS in the plant zone, because the additional infra-red is reflected into the plant zone, increasing plant temperatures more than the ambient temperature in the room.
About 5% additional heat is created by the digital ballast. More on this later.
Generally, temperature in the plant zone should be approximately 76*F. Though some people claim that new research indicates, temperatures can go higher if everything else were optimized.
Light Placement
The 315w CMH arc tube should be placed 12″-18″ above the top leaves. Much higher, and more light will tend to hit the walls, but this is less important in large rooms. There is an advantage to placing the lights higher, the plants will effectively perceive more points of light, and hence more angles of light which can more effectively penetrate the light canopy. Intensity does drop off by the inverse square law, but when all the lights are raised, they each cover a larger area and the footprints overlap (with intensities adding up and more angles of light).
At about one foot, each 315w CMH will cover about a 3′ circle, or about four medium size plants yielding between 2-4 oz per plant or 1/2 and 1 pound per light. Plant height should be about 2′ or less, and 6″ at the bottom of the stem stripped bare to minimize risk of fungal infection from the wet soil.
More plants can be grown for greater efficiency, but height should be reduced to about one foot. This is called “Sea of Green” and was invented by an associate of Hydro Tech. A newer method is “Screen of Green”, with fewer plants, and plant height reduced by tying the branches to a horizontal screen.
Reflector Design
Generally, diffuse reflective surfaces work best, because they produce more angles of light which can penetrate the canopy, through more pathways from the arc tube to leaf surfaces. Diffuse reflectors should be shallow, coming about 1/4 inch below the bulb (to project the light at nearly 180*, and protect the bulb from breakage when the fixture is set down).
Diffuse reflective surfaces do increase number of angles of light, which is good once the light is outside the reflector. But they do tend to increase reflective losses inside the reflector. Keeping the reflector shallow does reduce these losses.
The greatest reflective losses occur from the highest point of the reflector. So at this point, Life Light has a flat specular reflective surface, which reflects light at exact angles, thus avoiding reflection of light back through the bulb.
The Life Light Quantum reflector is the only composite hood, with both diffuse and specular reflective surfaces.
For technical data sheets (including light intensity patterns from a testing lab, please contact Kevin at 206 547 2202.
Shape of the CMH Arc Tube and why that matters
Notice from this photo of the 315w CMH bulb, the arc tube is nearly spherical. Unlike the long, cylindrical arc tube of the HPS (and nearly as long arc tube of conventional MH). Most traditional horizontal reflectors take this into account, and are themselve long and cylindrical.
Typically they are multi-faceted, with many panels that join together in creases, which increases reflective losses as light is bounced back and forth from one panel to the other.
The Life Light Quantum hood has a revolutionary circular design, which does not have this problem (because no creases). Because the nearly spherical shape of the arc tube enables a circular hood design.
