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Often there is also a narrow and tall port with a viewing port beside it. These ports were used for the lantern slide projectors. There were usually two lamps and projectors stacked above each other. The glass 4 x 5 inch slides were projected alternately by use of a dissolver. Light for these projectors was usually hand fed carbon arc without any reflectors.

There was one in the Temple Theatre in Saginaw as well as one in the Regent Theatre in Bay City.

I think there is a picture of one in the photo gallery. The Byrd Theatre in Richmond VA might still have one.

KEN

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Film-Tech has a 1929 Brenkert available in the manuals section which has pictures of the Brenograph and some of its accessories. Note the prices of this equipment, it was not cheap, over $800 for the basic machine in 1929. I like the 'teapot spout' style smoke vents from the lamps.

Question is, should I replace it with more Lexan, or would it be better to spring for the more expensive (MUCH more expensive) coated glass?

There is no question that the best port substance is air! The Fire Marshal has some idea that it should be a fire restraining material. The audience thinks it should be a silencing material. (They don't like hearing the operators stale jokes)(And the intermittent is silent!)
Truth is-the closer to the quality of the lens that port material becomes, the better the image will be.
Have you taken the Lexan out and examined it carefully? There will be an area which is very discolored where the image was projected. The hardness of the material will be very high at this area also.
Plate glass is a possible choice since the surfaces are very parallel and do not doistort the transmitted light rays. The color of the glass (usually green) does effect the color rendering of the image. Water Clear glass has parallel surfaces and does not effect the color rendering of transmitted images. Whenever there is a change of density in the light path, some diffusion occurs and also, the light is partially reflected back to the source which reduces the efficiency of the material. Optical coatings minimize this problem.
As you can see... Parallel surfaces, colorless material and optical coatings are all players in the port covering issue. The final point is the angle of the glass through which the light passes. This angle of reflection equals the angle of incidence and so if the glass is parallel to the film plane, light will be reflected back into the lens. It may be reflected across the booth into the adjacent theatre or perhaps, when angled too much, even back into the sound head where it will sound like the reader is seeing shutter pulsed light. (Because it is!)
Enough of this, I need to get some sleep.
KEN

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Edmund scientifics optics division

Josh

In our big theatre we have such a steep throw to the screen that mounting the port glass paralled to the wall, i.e., no angle, works just fine because the projector is angled -- saves us the need to buy those expensive port boxes.

Funny thing about glass parallel to the lens.... in the museum theatre, they installed a window the width of the booth made of optical glass (imported at immense expense from Germany no less, but then didn't have enough money to install digital sound....say-
la-vee). There is precious little angle in this booth so the glass is virtually parallel to the lenses. One of the tech guys was testing out a slide projector with a test grid slide and claimed that he couldn't focus the image. I checked it out and sure enough, the image was out of focus evenly across the entire frame and couldn't be brought into focus. I told him to switch lenses. A new lens didn't fix it. Ok, it's not the lens. Put in another test slide. Any number of slides were tried -- problem didn't go away. Finally I went down to the screen to see what this looked like from close up. The test grid went back in and I stood at the screen. From close up I could see that the image wasn't really out of focus; it was just a double image, one slightly off from the other. I yelled up for the techie to slide the glass panel out of the way. He did; the image cleared up. Evidently the projector was positioned in such a way that the port glass reflected the image directly back at the lens which reflected it back to the screen. We corrected the problem by moving the projector so that it was further away from the window and slightly angled. I never had that problem with the 35mm in this booth, so it must have been just weird random optics geometry coming together to produce this effect. But there is no doubt parallel glass surfaces CAN impact on the image.

Frank

Just a thought or two

Josh

P.S. I have been told by more than one person that the only reason to tilt the glass is to keep the reflection out of the adjacent auditorium.

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"Film is made of silver, video is made of rust"
'nuf said