Advice on LCD panel cooling

Hello all, I'm new to these forums and have a heat-related question about a projector I'm building.

I decided to go the cheap/easy route with this project and use an overhead projector and a computer monitor (some off-brand 14" diagonal 1024x768 thing, I don't remember who manufactured the actual panel) as my base. So far I'm fairly pleased with the results, but I am concerned about heating issues. Rather than trying to describe how this is constructed, I'll just link a few pictures.

Top (2016x1512, 1.5 MiB)
Side (2016x1512, 1.4 MiB)

For a rough reference, here (1280x960, 430 KiB) is a terrible shot of the screen it projects. Ignore just about everything but brightness since the wall in that room is yellow and it was too late for me to hold the camera steady. I'll take some better pictures later, but just be imaginative and envision that the color balance is correct and the image is very sharp. (The screen is a bit shy of 88" wide in that shot; the screen I'll actually be projecting onto will be closer to 72" wide).

Okay, as you can see from the images, the TFT panel is sandwiched between two pieces of thermoplastic (GE Lexan on the top, some PMMA on the bottom), mostly for protection and rigidity. The problem, as indicated by the thread title, is that I am worried about heat. With a 4,300 ANSI lumen projector, obviously this little TFT is absorbing quite a bit of light, and we all know where a good deal of that energy goes. You'll notice that the assembly has rubber feet that keep it around a half inch above the projection stage. I have rigged a fan and duct system (duct isn't pictured since it's not quite finished) that will blow air across the assembly, and I'm hoping this will do a lot in the way of cooling. However, in my initial testing (sans fans), after 10 minutes or so the bottom of the assembly is quite warm to the touch (not burning hot, but quite enough to make me worried). I don't have a thermometer that is suitable for checking the LCD temperature, but considering that thermoplastic is a terrible conductor of heat, I suspect most of the heat due to absorbed light in the LCD is more or less trapped there.

I've thought of a few designs for building a heat spreader, but most of them involve some pretty precise metal work and I'm unsure of the feasibility of this. Does anybody have any advice for me? Right now I'm unsure if I even HAVE a problem since I really haven't run the thing long enough to watch for the characteristic image discolouration that indicates overheating. Though I'm tempted to run it with a white image for a while to see if any problems form, I'd rather not play Russian Roulette with a ~ $170 US panel... The specs on the monitor claim a 35° C operational and 60° C storage temperature tolerence. These numbers seem a bit low to me, since I've seen plenty of panels operating under higher temperatures, but I'd rather like to avoid ruining this panel.

Thank you very much for any assistance you can provide!

-uberpenguin
 
Addendum

I forgot to mention that PMMA does not reflect UV or short IR wavelengths of light, so that is passed through to the TFT. (actually, I'm not 100% sure the bottom sheet is in fact PMMA, I'll have to check on that today) Should I be considering some sort of IR filter? The top Lexan sheet should reflect a good bit of UV, but that doesn't do much good since the light source is under the panel.

-uberpenguin
 
Perhaps I'm not asking the right questions, eh?

Allright then, let me put it this way. I've redesigned it a little bit to allow air to flow around the panel (I'll take some pictures of the new construction later; it's been a busy day), but I know that I assuredly have a heat problem. After 5-10 minutes of operation, spots of discolouration appear in the centre of the panel and only disappear after the panel has cooled some. I'm going to try and mount some much more powerful fans, but I have my doubts that this will be adequate.

So does anyone know of an effective IR filter that can be obtained in the size I need (about 12" x 12") and preferably fairly cheaply? I've heard a rumor that "low emissivity" glass does a decent job, but I can't find any straightforward information on how it affects the visible spectrum. If this stuff would work, I can probably cut it to replace the piece of glass that forms my projector's stage.

Don't hold back now, I could REALLY use some advice. Thanks!

-uberpenguin
 

Me2!

Member
2004-12-27 10:11 pm
BC
If your panel is being affected put something in it before its damaged. You can put lexan or a heat film inside. Lexan sheet between the bulb and mirror or heat film on the mirror. Heat film on the mirror will be 2x as effective because it blocks to and from the mirror but make sure its really transparent or it will reduce light twice!
 
Okay, that sounds somewhat promising. Any indications of a good place to obtain this heat film? Sorry for the naive question, but this is my first project that has involved any optics.

I should also clarify that the projector bulb is positioned directly under the convex (collection?) lens, Fresnel lens, and glass projection stage. There is no mirror between the glass top and the bulb, only lenses. The only mirror in the thing is a hemispherical reflector under the bulb. I'll take a picture of the interior shortly.

I have some extra Lexan, but I was under the impression that it does nothing to reflect IR. I'll have to look up the spectral response of the stuff again. Thanks!

-uberpenguin
 

Me2!

Member
2004-12-27 10:11 pm
BC
Ok,

I will put in for those trying with a mirror in their OHP that they need an absorbing filter - reflecting will pass on the heat.

For your setup then, you need a reflecting film that will send the heat to the bottom of your ohp (sheet metal i assume). You can apply the film to the bottom of your stage glass but your fresnel will take the heat twice and may warp. You can buy heat glass at home depot and install it below your fresnel or use regular glass with a heat film applied.

Look for window films in your yellow pages. Several companies make it and local sourcing is easy. Local window glass companies may have it.
 
Well, I found a local place that does window tinting where the folks were really helpful and applied a spectrally selective film (they say it blocks about 80% IR) to my projection stage. Now that I have the projector powered up, I'm inclined to believe their claim of 80% blockage; the difference is amazing. Unfortunately this film is also a heavy grade and blocks out a good bit of visible light, but initial results (with a good bit of ambient light) are promising. Tonight or tomorrow I'll test it out on my screen in a dark room. After running the projector and panel for about 10 minutes, so far there are no signs of overheating... I think this may solve my problem if the visible light blockage isn't too much...

Thanks for your help so far!

-uberpenguin
 
Well, the results seem pretty good after a bit of testing. This is definitely a projector for very low ambient light, but I expected as much in the first place. Perhaps in the future I'll investigate the possibilities of replacing the halogen lamp with a MH, but right now this will probably take more time and cash than I have to spare.

Thanks for the advice Me2! Whenever I can borrow a better camera than the one I have, I'll take some shots of the screen...

-uberpenguin