Build Your Own Metal Halide (Low Cost “HMI”) Fresnel – Part II

I got into this “Cool Lights” business of mine because I thought prices were outrageous on pro level fluorescent fixtures. You can see all over this site the manifestation of what I decided to do about it; starting with the Cool Lights video and ending with low cost fluorescent fixtures. Well, I’ve gotten the same feeling again on another lighting technology: HMI. I was searching for an energy efficient hard or point light source to complement our fluorescent Cool Lights Softlight Series product line and was learning about HMI as the natural solution. Crazy prices though and no good reason at this point to have them so high. Read on to find out more about what I’m talking about and how you can make an HMI fresnel for under $500 in Part Two of a multipart article…

Welcome Back

In the first part of this article last month, we showed how “HMI” (a trademark of Osram Corporation) has been way overused by many fixture manufacturers. In fact, the mystique surrounding what it is has helped conspire to keep prices high on a technology (ceramic metal halide or CMH for short) that’s really available under different names for quite a bit less money.

In this part, we’ll actually dig in and convert an old fresnel to see just how easy it is to make your own HMI. Our budget was $500 but I came in even under $200 on mine. Let’s look at the tools and parts we’ll need to complete this project. First though, let’s have a safety check before we go much further.

NOTE: We are encountering at least three different dangers in this project. 1). Working with electricity. 2). Metal Halide bulbs can explode. 3). Metal halide bulbs can emit harmful UV radiation. So what are we going to do about all these issues?

1). We will follow certain guidelines about working with electricity and if we are not confident, then we shouldn’t go forward. 2 and 3). No open face bulb experimentation. All the more reason to put the bulb in a fresnel housing with a lens on the front of it. I do not advocate using one of these bulbs in a softbox hood or in any open face or flood configuration without some protective housing and/or protective glass on the front. This provides protection from explosion and somewhat from UV radiation–we will go into this UV protection aspect more later. We can experiment but should do it safely!

Recommended Preparation Reading:

Electrical Safety Guidelines

Metal Halide Exploding (Metal Halide Safety Section)

UV Radiation from Metal Halide Posing Dangers to Eyesight

Parts List

Bulb. Pick your bulb first. It will dictate the ballast you use for the most part and also the size of fresnel you get to use. The garden variety (pun intended) that you’ll find when searching for these bulbs is for use in grow lamp fixtures. The Color Rendering Index (CRI) will usually be very low and color temperature unsuitable. Of course, the E26 or E40 types will be easier to use though as the screw-in socket bases are commonly available. The most convenient thing will be to find a bulb/ballast combination with suitable characteristics for our use (silent ballast, high CRI bulb of a color temperature as close to 5600K or 3200K as possible). Also pay close attention to the orientation that the bulb must have while in operation. We’ll talk more about that in a moment.

When we get to the smaller bulbs, we get the more exotic sockets like G12, G22, RX7S, etc.; then our work gets a bit more complex. You’ll have a choice of single or double-ended types. I’m going to show you a double-ended, common type 150w bulb in this article and how I mounted it in the fixture. The bulb at left is just a standard metal halide, made by Yaer Lighting Company in China. It’s one of the ones I obtained when I went on my sample run for the R&D for my Hardlight Series fixtures that are coming out later this year. 5200K color temperature and 82 CRI with RX7S socket. These types are about as cheap as they come and CRI of 82 is about as high as you can get in standard metal halide. This one can apparently burn in a vertical position without any problem. If you choose to use the screw-in base type, get a much bigger fresnel than I got as many of those bulbs are 9 inches in length and even longer for high wattages.

For the screw in Mogul base type you should try a bulb from the Iwasaki Electric Company which has many good choices. The MT250D and MT400D (also MT250DL and MT400DL) look very good with CRI ratings of 90 and color temperature of 6500K; which may be about as close to 5600K as you’ll get unless you are ready to make a special order of 5000 bulbs. Use CTO gels to bring it down to match other lighting. For instance, a 1/8 CTO should color correct from 6500K to 5500K.

You can’t buy the Iwasaki bulbs direct from them but you can find them on Ebay, and at selected aqaurium supply stores and grow light stores. for example carries some of those bulbs. Just do a search on Ebay for MT250D or MT400D or “iwasaki“. They also carry a double ended 150w 6500K similar to the Yaer bulb but with CRI of 96 with part number of MTD150D.

Finally you might look at the Iwasaki MT150D/G12 single ended bulb for G12 socket (or equivalent from another supplier) if the horizontal burning aspect troubles you. The picture at the right shows a generic 150w bulb single ended G12 bulb in a G12 socket. This one has a CRI of 85/Color Temperature is 5200K. The G12 socket will of course be a bit harder to find and also to use but yield a very professional result with a vertically mounted bulb (see below in the “sockets” section). That bulb is only about 4.2″ tall whereas the double ended MTD150D is about 5.2″ long contact-to-contact but must be horizontally mounted. The screw-in Iwasaki models are about 8.7″ long for the 250w and 9.8″ long for the 400w. Hence, you can see how this will dictate the diameter and size of your fresnel.

NOTE 1: I need to say it again; one drawback of many of these bulbs is that they must burn in the horizontal position. Pay close attention to the way the bulb must be oriented. Horizontal position will be awkard with most fresnels. The bulb manufacturer orientation codes are as follows:

Universal or “U” means any position

Base Up or “BU” means the base of the bulb must be up and the globe down

Horizontal or “HOR” bulb must be in horizontal position

Vertical or “V” means bulb can be in vertical position with base down

You will also notice that they quote a specification of degrees of angle in addition to the above. It will look like HOR+/-45 or BU+/-15 which means that the bulb can be horizontal plus or minus 45 degrees or base up plus or minus 15 degrees respectively.

NOTE 2: These bulbs are somewhat like halogen bulbs in that they have a quartz envelope around them and you must take care with touching and allowing skin oils to get on them. The oils and salts in your skin can reduce bulb life, therefore only handle them with cloth gloves or at least a cloth. If you do handle them, clean the bulb with a gentle solvent such as 90% rubbing alcohol.

NOTE 3: You may even be able to use an HMI Osram bulb in your fixture if you chose your ballast carefully. In particular, make sure the voltage specs of your HMI bulb match those of the output voltage of the ballast and of course, make sure the wattages match for ballast and bulb. None of this has anything to do with the voltage type of your country. That’s the input voltage to the ballast and it is the ballast that takes care of supplying the correct output voltage that the bulbs are expecting, regardless of country input voltage type.  Please be sure about the ballast working with your bulb!  As I said before, these bulbs can have violent failures.

Bulb Sources:

Iwasaki Electric Company

Venture Lighting Company


Ballast. Hopefully, as I said above, you can find a bulb and ballast kit for sale somewhere, in which case your task of finding the most important parts is much easier. Another advantage of a set that comes together is that you know they are matched then to work with each other.

There can be some issues with one ballast not working with certain bulbs. One major thing to watch out for is to always match the wattage of the bulb you pick with the ballast output wattage. 250w bulbs go with 250w ballasts. 400 with 400, etc. As I said earlier, output voltage is another spec to watch. Also, it is unlikely that a high pressure sodium ballast (HPS or sodium vapor is a high intensity discharge or HID technology also) will work with a CMH bulb but it is possible though. Iwasaki has some high CRI metal halide bulbs that are designed to swap out in a sodium vapor fixture and use the same ballast that the sodium bulb did. I would be most concerned with checking flicker at that point though, which I think is very likely in a “street lamp” grade ballast. Talk with the dealer or manufacturer and get recommendations for the best ballast to go with the bulb you picked.

One of the most promising kits is at Lumenlab is a company modeled in a very similar way to Cool Lights DIY ideas but in another area: DIY Video Projectors. It’s a very interesting site for people inclined to DIY. They supply lots of components for those that want to build their own video projector. One major component they carry is an electronic metal halide ballast and CMH bulb kit which looks great, but I have not personally tried it yet. The ballast is universal voltage input from 90 to 240v 50/60 hertz. Since I have not acquired one, I cannot attest to how silent it is nor it’s flicker level. Since it’s being used for a video projector we can surmise the quality should be good. The company claims it is completely silent but you never know until you actually try it–different people have different definitions for silent from what I’ve seen.

The CMH bulb that comes with it is a 400w, 6500K and the CRI is 91. It appears to be the 400w Iwasaki Electric Company “Eye” MT400DL. Very good value! The main disadvantage is the bulb’s length which will practically dictate a much larger fresnel than the one I chose for this project: 9.75″ long (or 248mm). Another disadvantage is the 6500K color temperature but you can always use a CTO gel to filter it down to 5600K (or even 3200K for that matter).

The main advantages are the use of the E40 mogul screw type socket and the high CRI. The current price of the kit is $199 and also includes a ceramic mogul base. A really great value at that price. This brings our fresnel in way under the $500 target if you buy the used fresnel right too. At some point in the future, I will acquire this Lumenlab kit to review it or we’ll sell something similar to make this type of project much easier.

The next best source for acquiring a reasonably-priced bulb and/or ballast will be Ebay. You’ll want to search for “electronic metal halide ballast” to see what you come up with. Another search is “electronic HID ballast.”

NOTE: For now, I don’t recommend trying to build a unit with wattage much larger than 400w until you get confidence in this area and a track record of success. I started with a 150w for many reasons, including size. I wanted a small fresnel that would be the equivalent of about a 500w tungsten. At around 80 lumens per watt (conservative) for this solution, I obtained my goal.

The ballast I used in this project was a 150w and was obtained in China (as any ballast you ultimately pick will probably have come from there too–by way of a reseller). It’s about as much of a plain “black box” as you can get. I tried two or three before I found one that was suitable. I never had one that flickered, but I did have one that had a slight hum and the other had a really objectionable high frequency pitch. I rejected them instantly even though the hum on the one was not loud by any means. These ballasts all come in a metal box of varying size and weight with 4 or 5 wires protruding. 2 to 3 will be dedicated to the power input which can be anything from a 120v model to 240v and even some with universal voltage input like the Lumenlab one. Color coding could be just about anything. If there is only 2 wires for input power, then you must make your own chassis ground from the power cord.  Simply attach the power cord ground to the metal parts of your fresnel somewhere in the most convenient spot.

For an American power cord, white is neutral, black is hot and green is ground. Hot is also known as “Line” sometimes. You will have to use the ballast wiring diagram to figure out which color codes are hot, neutral and ground on your chosen ballast. For power cords in other countries, please check your local electrical codes for the color code standards.

The other two wires coming from the ballast will be for the bulb socket. There’s no polarization on the socket or lamp so nothing to worry about in wiring it. If you are really lucky, you might find a ballast which just has a power cord (and maybe even a power switch) already attached and strain-reliefed to the case. This will save you a step later if you can get it and will also save you the color coding / matching problems for different ballasts and different country’s power cords. It may also be the case that your chosen ballast won’t have any wires at all but just connection terminals. In this case you’ll have to wire it yourself. Use only 600V-rated 18 gauge wire. A high heat teflon jacket is best for any wiring which goes into the fresnel and connects to the sockets. If you pick a ballast that already has wires you won’t have to hunt a source for wire! Most online sources are expensive for wire and will want to sell you 100 feet or more. You only need a few inches for this project.

Ballast Sources:

selected aquarium and grow light supply stores

Socket. Since I chose the double ended Yaer bulb with RX7S base (similar to the kinds halogen double ended lamps use), I needed to fashion a holder for the two ends that would not, in any way, block the parabolic reflector. When I found out the sockets in a halogen worklight are similar to what I needed, I tried one out just as it was. The metal bracket used to mount the two sockets was just a bit short in the distance between the two ends to allow this bulb to go in. You can see the oversize in the picture on the left: I put the bulb next to the sockets for reference. I would need to modify the bracket to allow the bulb to work with these sockets.

I also got some better quality RX7S sockets but they would have been much harder to fashion a bracket for so I went with the cheaper halogen versions. In retrospect, I highly recommend using the highest quality sockets you can find–no matter which type you are using. You don’t want to have to go back later and redo your work.

Note: You’ll also need to make sure your sockets are rated for really high wattage and the (as much as) 4KV startup pulse of a CMH bulb. I noticed some sockets have ratings of 5KV–maybe that’s even better to be the safest. Also, use nothing but ceramic-based sockets to put up with the high heat involved with these bulbs! Try to find sockets with wires already attached so you can avoid trying to find a source of suitable wire for this project.

Socket Sources:

A Used Fresnel. The size of the fresnel will mostly dictate the size of bulb we use (or vice verse, whichever way you prefer). With your choices already made you can now figure the size fresnel you’ll need. To use some of the more commonly available bulbs with screw in bases, we’ll need a pretty large fresnel because those bulbs are long. It doesn’t matter if the fresnel is working or not. The important parts are the housing, mounting yoke, the lens, a working focus mechanism and a parabolic mirror. You’ll throw away all the working (or non-working) socket and electrical parts later anyway.

I decided for my first one to use a smaller bulb because I wanted a smaller fresnel. The fixture I used was a very old Kliegl Brothers 1K model I got on Ebay for less than $40 about 2 years ago. I didn’t use the fixture much and it was gathering dust (that was about the time I made the decision to go totally fluorescent).

Project Tools

Useful tools for this project will include flat head and philips screwdrivers, wire stripper/cutter, electric drill and a 9/64″ bit.


First, take out any socket in your fresnel and all wiring. None of it is useful unless for some reason you have a fresnel with the same type socket as your chosen CMH bulb–but this is not likely. In my Kliegl Brothers model, the focusing mechanism is very simple and the entire assembly was easy to remove. Just totally untighten the focus knob bolt and remove it and the focus mechanism was loose and could be taken out. Your chosen model may have a more complex, screw drive mechanism which may make it more difficult to get to the socket. Choose a very simple model frensel for this exercise to make your work easier!

Here’s another view of the bottom of the sled focusing mechanism. Now, it’s time to take out this old socket. You can see that I already cut the wires leading to it. Like I said before, we won’t need any of this old wiring at all. In this fresnel, removing the socket was just a matter of unscrewing two screws and it popped right out. You can see the sheet metal heat protector on the sled is pretty rusted and nasty looking.

I could take the time to clean it up but people won’t see this part anyway and it won’t affect the operation. The parabolic mirror was still in great shape and totally scratch free as you can see in this photo where I put the sled back in so you can get a better view of how it works and looks before we put the sockets in.

Once you’ve gutted the fresnel, you are ready to put in your new socket. As I mentioned earlier, I had to modify the RX7S socket assembly from a halogen worklight to use in our fixture and not block the parabolic reflector at all. So the first stop was the metal shop here in China where I do a lot of my R&D. I made a longer bracket (really a shame because it wasn’t very much longer at all than the standard 500w halogen double ended bulb) and reattached the sockets to that bracket. After that, it was just a question of attaching the new assembly to the focusing assembly of the fresnel. This is a tricky operation and requires your lining up the lighting “point” of the bulb with the middle of the parabolic reflector (as close as possible).

Actually, this is not as difficult an operation as it would be in a “par” type fixture as fresnels are not nearly as precision in the interaction between bulb and mirror. In a fresnel, you have mirror and then socket/bulb in front of that, all fastened to the focusing mechanism like a”sled.” As you move the focusing sled back and forth, the light focuses into a more narrow beam on one end and becomes a wider pattern at the other end. It’s just a question of moving the entire assembly (bulb/mirror distance is fixed and stays the same though) closer or farther away from the lens.

You can see by the picture that I lined mine up pretty well and also chose the profile of the mounting socket such that it didn’t block the mirror at all. I mounted the assembly to the sheet metal which was surrounding the parabolic mirror.

I don’t know for sure the function of this metal piece but I would speculate that it is a bit extra heat protection to keep the heat focused upwards into the venting at the top of the fixture –in this way, the sides of the fixture would be somewhat insulated by the airspace between the metal protector and the side of the fixture. The bulb blocks the mirror a bit but it always does anyway. It did in the original fresnel just as I found it. Since most of the bulb is transparent though it doesn’t really matter that much. With the socket mounting complete, it was time to wire the sockets to the ballast.

First though, let’s attach the ballast to the mounting yoke so it’s not just dangling around somewhere. I used a metal plate attached to the yoke first and then attached the ballast to the metal plate. If the mounting holes on your ballast are not too far apart, you may just be able to attach your’s directly to the yoke. My chosen electronic ballast was too big so hence the need for a metal plate first.

After attaching the ballast mounting points to the metal mounting plate it was time to wire everything. Now we’re getting close to an exciting test!

I just wired the power cord black and white wires to the ballast black and white wires with wire nuts. There was no ground on this ballast so I attached the power plug green ground to the metal mounting plate under the ballast with a spade lug connector. This is just as good for safety purposes as a green wire being provided from the ballast but not as convenient of course. You can see in the picture that I routed the blue and red lamp wires into one of the existing holes on the back of the fresnel.

These are teflon, high heat wires coming from the ballast so they are good to use in the hot environment inside the fresnel. It’s hard to imagine that the ballast you order wouldn’t have this feature but if the wire is the normal plastic coating, I would attach some high heat solid core 18 gauge to those wires outside of the fresnel and then route the high heat wire into the fresnel fixture itself for attaching to your socket. My sockets also had high heat wires attached to them but the wires weren’t long enough to route to the ballast wires outside of the case so I attached them to each other inside with wire nuts.

If you are unsure of what you have as far as wire goes, ask the reseller where you got the ballast about what type of wire it is and whether it is high heat rated. Ask for the reply in writing. The socket you use may already have high heat wires attached as well which will solve your problem if they are long enough.

At this point, put your bulb in (being careful about handling the bulb with bare hands and getting finger oils and salts on the quartz), close the fixture up, face the lens away from you (in case the bulb explodes because of a failure somewhere) and power up! Mine worked first time. It was quite exciting to see it light. It took about 1 minute to lose the green tint which most of these bulbs have during startup. In addition, don’t be surprised if there is a bit of a “squealing” sound coming from the bulb or ballast during startup. This is not uncommon with new bulbs in HMI’s or any metal halide / HID fixture and bulb. On the subject of turning the bulb off and then immediately on again (this is an issue with some metal halide bulbs and ballasts that don’t provide a “hot restrike” capability), a restrike test verified it took about 5 minutes before we could relight with this particular bulb/ballast combination after turning it off. This will be the case with most of these less expensive ballasts and bulbs.

Our bulb did excellently in photographic tests afterwards. No green cast whatsoever. If you still have a green cast after the fixture has been lit for at least 5 minutes and the CRI of your bulb was claimed to be 80 or higher then I would say you either have a defective bulb or the manufacturer misrepresented the specifications for CRI. It’s not uncommon to find a CRI of 60 in some of the grow light kinds and that’s completely unsuitable for video production. I generally am suspicious of manufacturers that make me write to them for CRI specs and in some cases I don’t even bother if they don’t list CRI on the website right up front. There’s no excuse for ignorance on this subject from a lighting manufacturer these days. My general rule now is: if they don’t report it at all, then they either don’t know what the value is, what CRI is at all or they are not proud of the specification and hope people won’t ask. If you find a bulb between 80 and 90 and it’s really in that range, you should find the results to be excellent with your new metal halide fresnel.

Ultraviolet Protection

I promised earlier I would say something about this subject. I am planning to market some fixtures based on CMH technology later in 2007. We are looking at providing a separate glass UV filter to put behind the regular borosilicate glass lens of our pars and fresnel models. This will offer protection from UVA, UVB and UVC. It’s beyond our scope here to talk about what each of these is other than to say that they are different classifications of UV reaching into the part of the light spectrum that is harmful. Much of this radiation, especially UVC is normally filtered out by our atmosphere but we are not talking about natural sun as our source here–we have a man made source in our lighting. The atmosphere won’t help us at all in this case so we need extra help.

These type filters which would be suitable for use in a fresnel (permanent and heat resistant) are really a hard product to find. We could just depend upon buying the right bulbs with protection built-in but we can’t depend upon users buying our bulb later. We’re designing the fixtures to work well with quite a range of bulbs from different manufacturers within the limitations of whatever wattage the fixture and ballast are supposed to be.

For your purposes of building your own metal halide fresnel you’ll need to find something to guarantee that your fixture is safe to be shining into the eyes of your talent. If you only planned to shine it on the wall for background use, none of this would be an issue. But sooner or later you’ll want it out front as a key light. In this case you have to think about this problem of protecting all those that are looking into the fixture. So the main question is, does the borosilicate glass found on most fresnel lenses protect from UV radiation?

In doing research on this subject I came up with at least one source talking about the transmittance levels of various materials in protecting against UV for eyeglasses. It was a paper by an undergraduate at the University of Florida by the name of Cross Reardon. The conclusions seemed to point to plastic materials providing the most protection and glass the least. For anyone interested in the paper you can find it here.

What I take away from this research is that the borosilicate glass lens on the front of most fresnels and pars is not enough to guarantee protection from the entire harmful spectrum of UV radiation. The paper shows a specific test with borosilicate eyewear and it clearly is less effective than a soda-lime silicate type lens (common window glass!) which still allows UVA radiation through. This is, no doubt, one reason why borosilicate is chosen as lens material because it is more efficient at light transmission than regular window glass.

Until we find a heat resistant permanent UV filter solution, UV correction gels such as this one from Rosco (Rosco Cinegel #3114 Tough UV Filter) should work fine. It’s available at Amazon for instance. The Rosco product is listed as filtering 90% of UV below 390nm which is the upper limit of UVA–therefore this covers most all of the UVA, UVB and UVC spectrum protecting us from the “man-made” radiation source in our bulb.

Wrap Up

That’s it for this segment. Take your time during the conversion process. Be careful at every step. Think about what you are doing and do each step right particularly paying attention to the power cord wiring and fixture grounding. Follow all the electrical safety suggestions and finally get some of that UV protection gel. Never take the process of building an electrical device for granted.

Let’s measure our success for this project now. Did we obtain our goal of building an HMI type fresnel for under $500? Absolutely. Here’s the cost breakdown for mine. $40 for fresnel. $35 for bulb. $100 for ballast. $5 for incidental metal pieces. That’s $180. We came in way under. Even if you use the lumenlab kit, you can still do it. $199 for the kit which has the socket, bulb and ballast, $250 for a larger fresnel you will need. Thats $450.

Next time we’ll cover some more interesting aspects of these CMH / HMI fixtures.


HeraldNewsOnline: “Light bulbs blamed for eye damage”

Architectural Lighting Magazine Article Reprint: “Advances and Improvements in Metal Halide (Safety Section)”

Cross Reardon’s Paper on UV Protection Levels in Eyewear: “Ultraviolet Transmittance in Eyeglass Materials”

(c) Copyright 2007 All rights reserved. May not be used in part or total without the express permission of the author in writing. Waiver of damages: This information and all included material are provided as is, and Cool Lights USA can not under any circumstances be made responsible for any damage, injury or losses caused directly or indirectly by implementation of the information in this article.

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