BY THEO TEKSTRA – MARKETING MANAGER, GAVITA INTERNATIONAL BV

PART

6

Is this the new step

in plant lighting?

A WHITE

REVOLUTIONGa

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In the series “Light Matters,” Theo Tekstra discusses the different aspects to lighting, such

as quantity, quality, efficacy, special applications, new developments, and the science behind

it. In Part VI, Theo explains the applications of the Ceramic Metal Halide.

Image 1:

Philips CDM Elite Agro lamp

dated March 2012

LIGHT MATTERS

Why Evolve from MHMetal Halide (MH) lamps have always been popular in the

grow light industry because of their wide spectrum, including

much more blue light than HPS lamps. Instead of sodium,

MH lamps contain, as the name implies, metal halides, or

metal salts. When excited in a plasma these components

start emitting a bright light. The spectrum depends on the

composition of the salts.

To understand why CMH lamps are a step forward we need

to know a few distinct disadvantages of MH first:

1. Efficacy

Though they look quite bright to us, in terms of plant

light (PAR) MH lamps are not so efficient. Think 25-

40% less efficient than HPS.

2. Color stability

MH lamps shift in colour during their lifetime.

3. Light maintenance

MH lamps lose up to 20% over the first 2000 hours

and an average loss of more than 8% per 1000 hours

over their lifetime is no exception. Compare this to

high-quality DE HPS lights which lose less than 3% per

5000 hours of operation.

4. UV emittance

They emit a lot of UltraViolet radiation, including

UVC. According to the FDA’s latest report, “UV

Burns From MH Lighting Remains a Public Health

Concern,” one broken bulb can emit an entire day’s

exposure to the sun in just eight minutes. In most

cases, regulations require MH lamps to be used

behind a glass filter, to filter out the UV radiation but

also for safety. See point 5.

What’s in a NameCMH stands for Ceramic Metal Halide, a relatively

new type of High-Intensity Discharge (HID) lamp, like HPS

and Metal Halide. It’s a little bit of both, but we will touch

on this later. Developed in the early 80’s, Philips made it

a commercially successful product in the 90’s, when it

launched a line of CMH lamps under their product family

name “CDM.” So, that’s the first ”alternative name” you will

see pop-up.

Sunlight Supply, a hydroponics supplier and lighting

manufacturer, introduced a fixture with that lamp,

registering the trade name “LEC” - Light Emitting Ceramic.

It has a very similar structure as LED (Light Emitting Diode)

and LEP (Light Emitting Plasma), and it aims to compete

with these. The only flaw in the name is that the ceramic

material doesn’t actually emit any light, but that’s a detail.

So, there is the second brand name of CMH.

I’m sure you have heard about a new type of lamp that is creating waves in the grow community:

CMH, CDM or LEC!!?? So what is this and how relevant is it? Is this the new step in plant

lighting? Where did it originate from and what are the benefits?

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Image: high-speed camera image of an exploding MH lamp

(source: YouTube)

Image: ceramic arc tube (GE CMH 250 lamp)

LIGHT MATTERS

57

Images: /O versus /E MH lamps (source: Venture Lighting)

- A rupture of the outer glass balloon will not automatically

extinguish the lamp. As the glass balloon is responsible for

filtering out all dangerous UV light, a dangerous situation

will result. Accidents causing broken outer balloons in

public spaces, such as stadiums and large halls, have caused

regulations to become more strict. Lamps suitable for open

fixtures, therefore, have protection to prevent this from

happening, usually through a double glass balloon.

So, What is Ceramic Metal Halide?The only real difference between MH and CMH is the

C which stands for “Ceramic.” It refers to the arc tube

material used in a CMH lamp, which is a ceramic material,

polycrystalline alumina, the same material used for HPS

lamps. More stable than quartz glass (resistive to the

aggressive salts), it can withstand much higher temperatures,

ones at which quartz glass would almost start to melt. By

using a higher temperature inside the arc tube the efficiency,

colour stability, and light maintenance can be greatly

improved. I do say “can,” because not all CMH are much

more efficient or have a much better light maintenance.

5. Not always suitable for open fixture

Most MH lamps are not suitable for installation in a

fixture without a glass filter. There are two reasons

for this - UV emittance, as seen in point 4, and the

behaviour of the arc tube, if it fails, is a safety issue.

Over time the quartz glass arc tube of an MH lamp

corrodes from the extremely aggressive MH salts

inside the lamp, and cause discoloration of the arc

tube, diminishing the output. In some cases, ignition of

a warm lamp or a deteriorated arc tube can lead to an

explosion of the arc tube. The pressure inside an MH

lamp is much higher than the pressure inside an HPS

lamp and the temperature inside can reach over 6000°

Kelvin. An explosion creating super hot particles can

even be a cause of fire.

6. There are few MH lamps suitable for

open fixtures, ones that have special preventive

measures to contain an arc tube explosion, such as a

double outer jacket or an extra glass shroud around

the arc tube, are suitable for open reflector. Look for

the ANSI designation /O (suitable for open fixtures).

The National Electrical Code (NEC) and UL prohibit

other MH lamps to be used in open fixtures - you

are in violation of the installation code if you do so,

and damages as a result of such an installation are not

covered by your insurance.

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Image: CDM Elite 315 lamp for open fixtures

Image: CDM Elite 315 lamp for closed fixtures –

no extra outer balloon

CMD Elite 930

CMD Elite Agro

CMD Elite 942

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LIGHT MATTERS

Image: MasterColor Ceramic Metal Halide HPS-Retro White

Renewed InterestThe CMH lamp that created all this new interest is the

CDM 315 lamp from Philips, in particular, the CDM Elite 315

Agro. The CDM 315 already was around for a while when

Philips created a new spectrum, adding more red light than

the standard 3000 Kelvin lamp, producing a sort of purple

glow. It also added efficiency, reaching 1.9 µmol s-1 per Watt

initially with a light maintenance of about 94% per 5000

hours. There are two more versions of this lamp, developed

and suitable for general lighting: The regular 942, with a

4200K colour temperature, and the 930 at 3000K, but at a

lower efficiency in the PAR spectrum. Make sure you get the

special Agro lamp if it is efficiency you want

The standard CDM lamps are also available in versions for

a closed fixture. Remember, these are NOT to be used in

open reflectors:

It was only in the early 2000s that manufacturers were able

to produce a lamp with a higher wattage than 150W. The first

commercial “medium wattage” lamps (250W and 400W),

were launched by Philips under the brand MasterColor

Ceramic Metal Halide HPS-Retro White. They were meant to

replace 400W HPS lamps in magnetic low-frequency ballasts in

general lighting, and provide a much better quality light. Hence

the HPS-Retro (HPS retrofit). The wire around the ceramic

arc tube was to prevent fatal explosions, and so it passed the

/O certification for open fixtures. With the introduction of

the new generation, these versions have been phased out.

A Closer Look at SpectrumNote the difference in the red spectrum between the 930

and the Agro:

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Image: Chinese CMH 315 lamp

61

LIGHT MATTERS

The CDM Elite 315 Agro is the most suitable for plant production, as it produces the most photons in the PAR spectrum. However, if you are looking for the best colour reproduction and most sun-like spectrum, the 942 would be the best choice.

Limitations• These lamps require a specific low-frequency ballast.

Most are around 200 Hz and provide the lamp with a square wave output. The ballasts have a warm restart protection, which automatically times out re-ignition for about 10 minutes.

• The lamp holders only receive the /O type CDM lamps. • The output power of 315W. There are higher wattage

CDM lamps available, but these have not been optimised for plant growth, are not as efficient, nor do they provide the same light maintenance. Perfect for general lighting, not for plant lighting. The 315W being the only real available option means that in order to get the same output as a 1000W DE HPS running at 100%, you need 3.5 – 4 CDM fixtures. This translates into a higher overall investment and more maintenance. The CDM lamps are slightly less efficient than DE HPS (1.9 µmol s-1 per Watt versus 2.1 µmol s-1 per Watt) and have a lower light maintenance (3-4% less light maintenance per year), so the average light output is actually a little bit lower.

The spectrum of the CDM, however, is superior over the HPS lamp: It contains much more blue than the HPS lamp. By no means does the CDM 315 Agro mimic the sun, but you can expect a healthier plant. Most advantages will be seen in the morphogenesis of the plant and the production of terpenes and essential oils. To really make a difference in that latter department though you will need to start adding UV light in safe dosages. It will not, however, result in an extra-spectacular yield.

So Where Would I Use the CDM 315 Lamp?A big advantage of a lower wattage is that you can come closer to the crop. Using more, low wattage lights instead of one large fixture, will give you a better light uniformity and horizontal penetration of the crop. If you have a small room, and the money to invest in more fixtures, a few CDM 315W lamps really do an outstanding job. In vegetative rooms where you need less intensity, and more blue light is an advantage, CDMs can also be a solution. Do remember that in a large room, the initial investment can be quite high.

China! China!Of course, there are other CMH lamps out there, many coming from China. There is one manufacturer who makes a decent lamp with a good spectrum, but with a lower efficiency (10% less light – 1.7 µmol s-1 per Watt!) and a

Retrofit WarningThere are many (Chinese) double ended MH lamps sold on the market today for use in open, double ended fixtures. A REALLY bad plan, for many reasons:• These lamps do not have a /O rating (with one exception, which

uses an extra glass cylinder around the lamp).• Many discharge very high UV levels, making it dangerous to work

under without proper protection. One manufacturer proudly announced that they emitted “the good UV – UVC.”

• Most double ended fixtures have specifically been developed to drive HPS lamps, which do NOT need a warm re-start protection. Running MH lamps on these ballasts is dangerous, as re-starting a warm MH lamp can lead to an explosive failure of the lamp. We have been able to replicate this in our labs. Do NOT try this at home, boys and girls!

• You will lose the warranty on your ballast if it has been specifically designed for HPS lamps. 3

lower light maintenance. You can recognise it by the arc tube shape, which is less round than the original. Most of the private label CDM lamps all come from that same factory.

To counter the lower wattage limitation, there are even double ended lamps which contain 2 or 3 CMH arc tubes in series. This is not a good plan, as it can lead to ignition failures and premature failures of one of the arc tubes. Also, remember that MH and CMH lamps need to have a double jacket to be suitable for use in an open reflector, and use a LOW-frequency ballast with a time-out for re-igniting warm lamps. They are NOT suitable for a retrofit to a high-frequency HPS lamp!

So, there you have it: A primer in CDM and the application of it. A great lamp, with some limitations. Is it a full HPS replacement? For small grows it can be, due to the high investment costs compared to HPS. Does it replace full spectrum plasma lights? Though it is more efficient, the spectrum is still a bit too peaky to mimic sunlight, and the lamp lacks UVA and UVB. CDM is an expensive lamp to replace every year, but for a small cash crop that should not be a limitation.

Gard

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