Film-Tech Forum ARCHIVE: Port Hole Question

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Posted by Brad Morris (Member # 1175) on 05-29-2002, 07:11 PM:

While I'm no stranger to a projection booth, I've recently been in a few built in the days of nitrate film. In addition to the 2 projector and 2 viewing ports, I've noticed a 5th, located at the machine level. They are rectangular with the long side horizontal. Does anyone know what these might have been used for?

Posted by John Anastasio (Member # 607) on 05-29-2002, 07:20 PM:

It was most likely used for a spotlight. The port was wide so that you could swing the light to cover the whole stage.

Posted by Kenneth Wuepper (Member # 1174) on 05-29-2002, 09:01 PM:

Oddly shaped ports were common in the booths of the early teens and twenties. The large rectangular opening was, and still is, the spotlight port. Many booths had two spotlights, carbon arcs which were hand fed. The steel dowser and fire links were to automatically close the port in case of a Nitrate fire.

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.

Machines for dissolving slides were made in Detroit, Michigan under the Brenkert name.

Posted by Josh Jones (Member # 447) on 05-29-2002, 10:22 PM:

Kenneth, is the machine you speak of a Brenkert "Master Benograph?"

Josh

Posted by Kenneth Wuepper (Member # 1174) on 05-30-2002, 05:06 AM:

Yes!

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

Posted by John Anastasio (Member # 607) on 05-30-2002, 06:46 AM:

We've got TWO Brenographs at the Loews here in NJ. Neither one is in operational condition, but they're still up in the booth.

Posted by Stephen Furley (Member # 1277) on 05-30-2002, 07:16 AM:

The Byrd is the only theatre I know of still having a Brenograph in regular use. There is one in working order in the collection of the Projected Picture Trust at Bletchley Park, near London, but this has been converted to xenon. Many of the large 1930s super cinemas in the uk had them at one time. I have operated one a couple of times. Over here, Ross made a similar machine, but these were much less common even over here than the Brenograph; The Odeon Leicester Square had one, I don't know if it's still there.

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.

Last month I visited the Loew's Jersey Theater, they still have two Brenographs installed, sadly not in working order at the moment, but they look restorable. There is something odd about their machines, it was nommal practice to install a hood over the machine for fume extraction, the Loew's is different, there is ducting attached directly to the vents on each lamp, the two pipes join above the machine, and then go out through the roof of the box (booth). I see a problem with this arrangement; you do not focus a Brenograph by moving the lens, the lens stays still, and everything else, including the lamp moves, I am not sure how the lamp moves, when it has ducting attached to it.

Posted by Frank Angel (Member # 248) on 05-30-2002, 06:49 PM:

Second porthole question: We all know about the evils of plate glass and best to use water glass, and best of all coated glass, but what about Lexan? The theatre supplier who did our installation many years ago swore that plexi was every bit as good as coated glass and a lot cheaper. We put it in our smaller theatre and it seemed to work just fine. I did as many tests as I could, metered with and without -- moved it around while projecting an RP40 and no distortion was apparent. It's been there ever since. However, even with using only lens cleaner to clean it occasionally, I have noticed very slight scratch marks. It's time to replace it.

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?

Frank

Posted by John Pytlak (Member # 331) on 05-30-2002, 08:30 PM:

It's likely that the Lexan or Plexiglas will introduce much more optical flare than optical glass with anti-reflection coatings. Plastics also seem to "light pipe" more. And as you note, they scratch more readily.

------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Research Labs, Building 69, Room 7525A
Rochester, New York, 14650-1922 USA
Tel: +1 585 477 5325 Cell: +1 585 781 4036 Fax: +1 585 722 7243
e-mail: john.pytlak@kodak.com
Web site: http://www.kodak.com/go/motion

Posted by Kenneth Wuepper (Member # 1174) on 05-30-2002, 08:47 PM:

Hello, Frank,

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

Posted by Matthew Bailey (Member # 601) on 05-30-2002, 11:02 PM:

Coul it be possible to use Pyrex or laboratory grade glass?

Posted by Kenneth Wuepper (Member # 1174) on 05-31-2002, 07:06 AM:

Hello, Matthew,
Pyrex (R) has one great physical property, it does not shatter with thermal shock. There is no real problem with heat in the port application so the trade offs would be in the other qualities.
Do you know of a supplier of optical grade Pyrex?

KEN

Posted by Josh Jones (Member # 447) on 05-31-2002, 02:28 PM:

I know edmund scientific sells mirror blanks in Pyrex(R), but I dont know if they sell sheets of it.

Edmund scientifics optics division

Josh

Posted by Frank Angel (Member # 248) on 05-31-2002, 07:15 PM:

OK, I get the picture -- optical grade, coated glass is the best. Thanks for the feedback Ken. John said the two words that will make me resist the cheap way out -- optical flare. It is the one factor that I couldn't measure.

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

Posted by Josh Jones (Member # 447) on 05-31-2002, 08:46 PM:

I would venture to say that the thickness of the glass alone would create a double image. Because of the way lenses are made, they are not a flat piece of glass, so uniform relection across the frame would be impossible. Then add to this fact that *most* lenses are coated, you could not get a strong enough reflection off the last element to reach the screen.

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.

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

Posted by Gordon McLeod (Member # 33) on 05-31-2002, 10:05 PM:

that is why glass is coated with an anti reflective coating it works both ways

Posted by Frank Angel (Member # 248) on 06-03-2002, 01:22 AM:

Hmmm...but if the double reflection was caused by the thickness of the glass, wouldn't we get the same double image with the 35mm & 16mm projectors? The double image only happened with the slide projectors when they were placed close to the window.

My understanding is that the need for angling the windows is to prevent parallel glass surfaces, but that is only hearsay (and I have no idea where I heard this). I never heard about reflections into other theatres; that's something we single booth operators never have to worry about.

Posted by Gordon McLeod (Member # 33) on 06-03-2002, 05:56 AM:

When the projector was close to the port the image on it was smaller so the double image effect was in fact representing a larger percentage of the total image
Angled glass was to minimize the internal reflection within the glass itself where it starts to act like a prism

Posted by Steve Guttag (Member # 268) on 06-03-2002, 06:33 AM:

Try and experiment using a flashlight....

Stand in front of your glass and point the flashlight (with it on) at the glass and stand right behind the flashlight (with your eyes at flashlight level). See the reflection nice and bright? Now angle the flashlight (since angling the port glass will be more difficult at this point)...notice it isn't so bad? If it was coated glass, about all you would see is a faint violet reflection at that point. Therefore, the key is coated, angled glass.

Steve

------------------
"Old projectionists never die, they just changeover!"

Posted by Bill Enos (Member # 440) on 06-04-2002, 12:47 AM:

In response to an earler post about Brenographs. We still have our 1928 Brenograph Model F7 in use every night at the Byrd. We do our screen ads on it. The slides are two pieces of 8 x 9.5 ince window glass with an ink jet printed transparency sandwiched between and taped around the edges. Since the two projectors are about 4 feet apart vertically and about 12 feet off center of the screen the transparencies are rotated considerably from horizontal and are not interchangable between the two machines. We still have 10 of the original glass effects slides and the clockwork operated colorwheel. A 30 amp carbon arc thru a 4.5 aperture blows away 35 mm slide projectors in brightness. Is anybody else still using one of these monsters every day?

Posted by Fred Georges (Member # 505) on 06-04-2002, 12:22 PM:

The Fox in Atlanta uses their ancient Brenagraph anytime they have a film show. They show ads & singalong slides with the Pipe Organ.