OHP *without* the stage glass?


I just bought a used OHP and there is a significant scratch on the stage glass. This unfortunate situation got me thinking. I could just buy replacement stage glass, or...

Of course if you're projecting transparencies you need a surface to lay them down on - but if you're using an LCD panel could you run the OHP/Panel without the stage glass?

There is a significant benefit to this (if it is possible). Try this:

1. Remove screws, tabs, etc. that hold the stage glass down so that the glass can be removed - but don't remove it yet.

2. Focus and project the OHP onto your screen without using a panel, just to get plain bright light.

3. Now, remove the stage glass.... voila! The reflected light (at least with my Buhl 2900) is about 10%-15% brighter!

I can imagine some possible problems. Please feel free to add your own, or to agree/disagree with mine. Looking for useful information here:

1. Removing the stage glass essentially opens up a new air intake. It is possible - although airflow may be increased - that it will also be changed in an adverse way that would cause the lamp(s) to overheat. Possible solution: seal up the gap between the fresnel and the stage surface.

2. The glass acts as a heat shield for the LCD panel. Removing the glass causes the panel to be exposed to more radiated heat. (I suppose that the opposite may also be true - that by removing the hot glass from close proximity to the panel you are causing it to be exposed to *less* heat.)

I might just run an experiment with a thermometer concerning the 2 modes of operation. Would anyone be interested in the results? Come on, wouldn't anyone like their OHP to be noticeably brighter - for free?


How did you determine that it became too hot?

I couldn't do the temperature test inside the unit near the lamp fixture because temperatures there quickly get to over 120 degrees... I worried about the plastic probe on my thermometer melting.

I did however, run a test with the probe on top of the stage glass (114 degrees) and with the probe hanging over the corner of the glass with the glass sitting on top of and kind of to the side of the stage opening, covering only about 30% of the surface (109 degrees).

I think normally that hot stage glass is in such close proximity to the panel's glass that it's hard to believe things would be hotter with the glass removed. If things get too hot for my (halogen) lamp(s) in my buhl 2900, its not too big of a loss for me since those bulbs are pretty cheap.

I think what I'm going to do is seal up the gaps between the fresnel and the stage surface. That way the airflow should be about the same as with the glass on.
The glass temp is so high cause of the high amount of IR frequency in Halogens. Also the bulb itself makes alot of heat cause it is inefficient at making light. About 10% of watts used is being made into light. With incadescent house lights about 2-5%. And MH is about 40%, really close to flourescents. Halogens are a trade off for cheap bulbs. They are not ideal in the least. Some run off household current so no ballast is needed making them even cheaper to produce. So with that Halogen you get bulb heat and IR heat. The glass will stop the imdeiate heat from the bulb. And also filter out a small amount of IR too aswell as UV. My MH bulbs light beam when focused is extreemly cool. The halo. is very hot as a light beam and duller (not bright white) this is why the lcd is getting so hot. Used both and MH is the one I'm staying with...Panle stays cool to the to the touch. The reason lcd's dim when around heat isnt the bulb getting to hot. The bulb is not the prob. Its you pummeling the lcd polarizer with s and P polarized light. P is passed on BUT S is absorbed by the panel. This makes the polarizer work harder and overheat. When the polarized film overheats it causes a dimmer picture. So your problem is both heat and polarization. Try using a polarizer film with your lcd. trust me it helps temps ALOT. But get one that reflects S not absorbs it or else you will get the same thing. 3m's dbef fits the bill. I have some. Works very well. Also the more light you hit the panel with the more wrong polarity that turns to heat. Its a never ending battle. After the lcd's film weakens you get a perminatly dim pic. So the 3m film is also insurance to protect the inner film.:D
I knew it was too hot when black started to turn to white on my panel. It would begin at the edges, then slowly creep to the middle. Also, the glass got very hot, too hot to touch.

If someone were willing to replace my panel for free -- I'd leave it on the projector till it melted and tell you how long it lasted. :D
I wonder what can explain the 109 degree temp I observed from my probe dangling just above the fresnel (basically without any glass) and the 114 degree temp I observed with the probe sitting on the glass (with the glass fully in place)?

My guess is airflow. By removing the glass I was introducing airflow around the probe where originally there was none. Of course with the panel in place the airflow will be greatly reduced...
Well guys, you scared me enough. I don't want to take a chance of melting my panel, no matter what my thermometer reads. So I've ordered replacement stage glass for my Buhl 2900.

I noticed a couple of other Buhl 2900 owners on this forum. If you need or want replacement stage glass here is a company that supplies them:


Note that the glass is different depending on whether you have the metal stage frame or the plastic stage frame. (Mine is plastic.)

The cost (for me) was $22.95 + $9.10 s/h

Looks like they have other replacement parts (sockets, fresnels, etc.) for a variety of other OHPs as well.