Finding a suitable reflector

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I have a reflector idea...

Hello everyone. I have spent about a month now gathering parts to build a projector (reflector included).

It seems that one problem people are having here is finding a decent reflector to focus all of the light from giant metal halide bulbs to a small point (BT-28's and BT-37's (what I have)). People are resorting to sheet metal and glueing little mirrors ect..

All of our problems would be solved if we all had a reflector like Marklar's! I have checked with my lighting supplier, and yes, custom ordering this reflector alone would be expense, about $80.

I talked to a customer service representative from seattle lighting, and he said he had a reflector in his garage he would give me for free. So I drove to his house and picked it up. Here it is:

An externally hosted image should be here but it was not working when we last tested it.

The problem is that it is just too small for a BT-37 lamp (I have a 1000w). It can burn cardboard, but not all of the light is concentrated to a small point. I have tested the projected image, also this reflector leaves a dark spot in the middle of the screen due to the arc tube construction. (Yes, convex lens mounted to the front)

I was thinking... There are many mixing bowls out there. A lot of them have mirror like finishes. Some of them are deep and shaped like maklar's reflector.
An externally hosted image should be here but it was not working when we last tested it.

Of course, these bowls are probably not deep enough, this picture is just to give you an idea of the reflectiveness.

Yes, MIXING BOWLS might be the answer! We might have to look hard to find the right size and shape, but this might work.

robv60 originally brought up this idea in the IRC. I just wanted to make it known.

Please tell me what you guys think. PLEASE, I am desparate for a good reflector!
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I searched all open forums and found three posts regarding mixing bowls that you might find useful.

In the bar at the top click search.
In search page change the radio button from show as threads to show as posts. Type in mixing bowl

Undream I think found a very suitable bowl for $5

Hope this helps.
Hi Justinjay,

What a cool and interesting idea!!!

While the bowls don't look like pure parabolas, or ellipses, the MH bulbs are not point sources either...

I'm sure it would be worth it to many of us to try these cheap bowls out by taking a MH bulb, holding it inside the bowl, and seeing if it can be positioned to give the desired light pattern. As you found out, an even illumination (with no rings or hotspots) is a must, while a parallel, or focussed beam, or hybrid output can be manipulated one way or another... ;)

It can't be much worse than many of the reflectors being used successfully today, and is probably a lot better!!! (Of course, the insides will need to be polished...)

I must ask you, however: 1000W???

:eek: :eek: :eek:

Are you planning on getting a sun-tan while you watch your films/football???

:cool: :cool: :cool:

1000w baby

No. I read that the arc of a metal halide lamp produces little UV, and besides, ordinary glass (like the outer envelope on the bulb) are opaque to the ultra-violet.

BTW, I got much better results with the reflector now. I cut it so the bulb can fit deeper into the reflector. Also, I found out that my convex lens was not at the focal point.

Basically I am trying to copy Marklar's design, but make it a vertical projector.

1000w Metal Halide is VERY VERY bright and hotter too. The concetrated light burns carboard much like the sun does with a magnifying lens.

Sunglasses are a must have when working with these bulbs!

I would say it is about as hot as my original OHP bulb which was 600watts. Not bad for effieciency.
Hi Justin,

I was actually joking about the suntan thing - you are right about the UV.

My worry with the 1000W bulbs, though, is twofold:

1) The heat you mention needs to be blocked from the LCD. For something so powerful, look at cold mirror options, or an IR blocking filter - a film was identified by mycamel (I think) in the other thread that blocks/reflects 85% of the IR, while transmitting 85% of the visible light.

2) You might reach a point where your panel is operating outside of its design parameters, and literally cannot block out enough of the light to give you a good dark black and a high contrast ratio for the screen size you choose.

This is the reason why they talk about contrast ratios when they give projector parameters. Ideally, you want the blacks to be black, and the whites to be white. Poor brightness gives you black blacks, but grey, dim whites. To much brightness with poor contrast gives you white whites, but washed-out grey instead of black.

Nobody here has suffered from this problem yet, but if you couple a 1000W bulb with a really good reflector, you will definitely be outside of the envelope compared to the people here.

I DO hope it works for you though! You might have the answer we have all been waiting for!

Hmmm... Thinking about it, if your lamp was too bright, you might still be able to do something really cool: Xblocker identified a commercial projector that uses parallel beams of light and TWO LCD panels to achieve a high contrast ratio. If you could get your beams REALLY parallel, then the light lost through the second LCD (if properly aligned) would be much less than in the first panel, but your contrast ratio would be massive - comparable to the commercial units of today...

It just keeps on getting more interesting...


In any of these bowls, the bottom would probably need to be removed with some kind of cutter for actual use so that the MH bulb could be fed in. For testing, it would probably suffice to position the bulb the "wrong end in," just to see of the beam profile would be useful.

The thing about the profile is that while a Parabolic profile would be ideal, the bulbs themselves are usually not. Thus, it becomes a question of compromise, and the biggest thing that seems to be compromised here at the moment is the brightness of the image due to inefficiencies in both the reflectors and the panels.

The panels are a topic on their own, but if we can get a reflector that, when working with the real-world MH bulbs, gives us an output beam that allows us, in some way or other, to use MOST of the light produced, and not just a small percentage, then we will have made much progress.

This is very much a "suck it and see" exercise in many cases, and for the prices on some of the bowls, it could turn out very worthwhile. Or, it could come to nothing. In which case a few of us will have some extra cheap aluminium mixing bowls with polished insides... ;)

You never know...

I'll skip the mixing bowl for now

I was chatting with Marklar in IRC last night, and I found out that he is using a condenser lens after his reflector to spread the light out onto the LCD.

I changed my mind. My reflector will probably work quite well. It concentrates all of the light to a small point and burns a hole in cardboard. It's just my convex lens is not the right type. I need a condenser lens instead of a projection lens from an OHP. I found one on surplus shed that looks exactly like the lens of Marklar's.

Anyway, yeah he showed me a picture of his lens. It looks exactly like this one:
An externally hosted image should be here but it was not working when we last tested it.

I liked the look of these kitchenaid bowls becuase they are the same shape as his reflector.

Acually you can find some on ebay. Type in "mixer bowl" instead of mixing bowl.

This one is a little expensive, but looks like it would work:

These bowls are advertised as having a polished interior and are mirror reflective. If you go this route make sure that you don't get a "brushed steel interior" becuase it is not shiny at all.

Some new bowls are pretty darn shiny!

I am basically trying to copy his design as far as the reflector and condenser lens goes.

BTW, I would rather have problems with the projector being too bright than being too dim. It seems everyone is having the latter problem
Hi gaff,

The only reason I bought it was because nobody else seemed interested, and they DO say that they have more...

Send them an email and see what they say. If you can't get one from them, then since I blocked you, you can have mine for what it costs me. Currently I'm at $10 plus shipping, and waiting for them to contact me with the shipping costs...

What really interested me is that they quote the normal retail price for these things at $22. At this price, if they work, it is not really worth using anything else.

The prime reason I ordered one was to see what they are made of, and if, for $22 they are worth pursueing as the new standard; maybe we can all have a beast like Marklar and Justin. From the protograph, it looks like they are made of the same cold reflectors that the OHP bulbs use...

I personally am looking for a really cool parabolic rather than elliptic, because I want to experiment with a more parallel beam, a 100W bulb, and two panels for the contrast...


Thanks for the offer but I think I will save you the trouble. I have seen Altman "Super" Reflector Kits that are supposed to work in the 360Q's also for around $30. My main concern about the reflector is the diameter. I have a 400W Metal Halide bulb and don't know if the reflector will be big enough. It looks like I will definitely have to make the base hole bigger to fit the bulb through though.

Did I show you this picture yet?

An externally hosted image should be here but it was not working when we last tested it.

I think my reflector will work quite well.

Also, the size of the BT-37 lamp I have does not really matter because the light is all focused to that small point.

*cough cough** If I hold the carboard up to the reflector long enough it will catch on fire!

I consructed a light-holder and a sheet metal cover for the front of it with an opening in it about half the size of the reflector's opening. (seems to help concetrate more of the light to the point)

Now I am waiting for that condenser lens I ordered from surplus shed to arrive, I hope it does a good job of spreading that light out onto the LCD.

So much for the mixing bowl idea for me. This reflector was a pretty good deal at $0.

In case you are wondering it comes from a HALO recessed HPS lighting fixture (not sure of the model #)

Anyway I can't wait to see the final results!
Low-E film?

I heard that low-e film is available at any hardware store such as Home-Depot or lowes.

Will this stuff work? It is designed to be put on windows.

Or will I have to buy special glass?

I'll have to pick some up on my next trip to the harware store when I pick up some black laminate for the project.

This project has been going very fast for me, thanks to the DIY projector thread!

Also I have a design like Marklar's to follow.

I would say my projector will be finished in less than a month
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