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Posted by Frank Angel (Member # 248) on 08-01-2018, 06:26 PM:
 
The booth I am working in now has 3 ports made of wood which consists of two panes of glass in each -- one pane closet to the screen is slanted and the one closest to the lens is parallel to the lens. One major problem that the glass is held in place by a locally constructed wooden box and designed in such a way that it is set into the wall with no way to remove the glass for cleaning. So after many years, because the box evidently isn't airtight and the inside panes are now filthy.

The wooden box in the wall opening is impossible to get out and the only way to get the glass out is to use a reciprocal blade tool to cut away the wood which is not screwed together but cut as a single piece with slots that the glass was fitting into -- i.e., one single piece. I have begun extracting the parallel pane, cutting away the wood -- a long and arduous task. We only need the slant pane -- two are not needed and the parallel pane is a no-no anyway.

Here's the problem. When we extracted the parallel pane in the first port, I discovered that it was plane old window glass with the tell-tale deep green edge. When we removed the second port's parallel pane, it broke and looking at the edge of the pieces, there is no green tint. Could this be water-white optical glass that broke? Does the glass need to be a large size for the green tint to show up or will it look greenish even on smaller pieces? I need to know if this is indeed optical glass.

Next question -- the second piece of glass (slanted) in each of the ports have the same problem -- they are in "for life" unless the entire wooden structure is cut to pieces. They simply were not designed to release the port glass with a few screws. Thing is, if any of them ARE window glass like the first one we extracted, they need to be changed, and the only way to do that is saw away the wooden structure. I would rather not do that unless absolutely necessary. I need be able to accurately determine which are optical glass and which are plate glass. There is now what I can see the edges of the slant pieces until they are extracted from the wooden frame.
Is there anyway to know if the glass is optical water-white glass without seeing the edge? Is there a test of some kind?

On any optical glass I install, I always stick a very small label at the very top edge of the pane and runs the width of the glass that says "This is special Coated Optical Glass -- DO NOT CLEAN. The projectionist is responsible for cleaning this glass. Using regular glass cleaner will destroy the properties of this glass." This to prevent the conscientious cleaning staff from dousing it with come industrial chemical cleaner and rubbing it will Brillo.

In this case it sure would help me to know which of these panes is actually optical glass and which is not.

Would appreciate any thoughts.
 

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Posted by Monte L Fullmer (Member # 2797) on 08-01-2018, 06:47 PM:
 
Is the water glass optically coated - see a purplish tint when light reflects off of it?

Optical coating can be wet cleaned, but there is a lot of buffing afterwards with a microfibre cloth.
 

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Posted by Stephan Shelley (Member # 8587) on 08-01-2018, 07:54 PM:
 
Also no cleaning solutions that have ammonia in them. It can cause the coating to cloud. A lot of glass cleaners do contain ammonia so check the label first. Alcohol is okay.
 

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Posted by Frank Angel (Member # 248) on 08-01-2018, 08:28 PM:
 
Right Monte -- I see no coating tint; it just looks clear. It's the coated optical that is the expensive stuff. If it isn't coated, I want to replace it with a piece of the coated; the Barco lenses are something like $600 lenses -- I don't want the picture to then go thru any piece of glass that is of lesser transmission quality than the lens glass.

The slated pieces can only be extracted from the audience side of the wall and that will create a hug damaging mess. We may just use a diamond glass cutter and cut a rectangle in the
existing glass still in the frame and just epoxy glue a piece of real coated optical right on the existing glass to avoid all the damage extracting the it would cause.

I did this once in a theatre that had this problem -- the entire front of the booth was plate glass -- someone thought it would look sexy if the audience could see all the tech.

And Stephan, the only thing I ever use on port glass is lens cleaner...why take chances?
 

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Posted by Terry Monohan (Member # 8285) on 08-02-2018, 09:18 AM:
 
One of the first things I do when going into a movie theatre is turn around and look up at the projection booth glass ports both for digital or film. About 75% of the cinemas I visit from independent or circuit cinemas are filthy and full of dust coated filth. The projection beam ray light just goes thru the grime to the screen. I am sure If the glass is not cleaned the focus and amount of light will be downgraded when it reaches the screen. Many of these projection managers are probably just using old Windex® with ammonia to clean the glass when and if they ever get around to cleaning it. I do complain when I see dusty glass. Most are too lazy to get a ladder out and dust the outside part of the booth glass. When I sit up high in the last few rows It is shocking to see also all the white dust on top of the side and back surround speakers that never get cleaned in most cinemas. The floor on most theatres get cleaned after each showing but the lens projection glass and surround speakers just gather dust year after year.
 

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Posted by Steve Guttag (Member # 268) on 08-02-2018, 11:02 AM:
 
As for dust on the speakers, you'd be hard pressed to show it having any negative effects on their reproduction. You'd also be hard pressed to show any significant number of patrons that have noticed and those that do that would change their viewing habits accordingly. Tell me this, if the speakers were located behind the wall fabric so you couldn't see them, and yet were dusty behind that fabric, would you still care as much?

Onto the dust on the ports. With blown air systems, it is a losing battle and your perceived dust with light shining through it versus the level of dust that would impact the actual image are not going to be uniform.

some have advocated mounting the port so that the angle of the glass points down on the theatre side so that the dust collecting side is in the booth. The downside is that you just created an audio reflector to catch sounds and point them to the audience in a delayed fashion.

A more worthwhile way to go about it is to determine at what point does image degradation occur (resolution, contrast and light level) and stay ahead of it. When I operated, I cleaned the ports weekly. I know some that did it daily. When I've measured, light wise, you really don't have much of an impact nor does your resolution unless you have something like grease on the port. Contrast, however, does go down with dust and the more dust you have the light level does go down.

BTW...any reasonably designed port system allows for the operator to pull the glass from the booth side so nobody is getting on a ladder on the theatre side to clean it.
 

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Posted by Marcel Birgelen (Member # 6801) on 08-02-2018, 11:03 AM:
 
I guess a clean porthole conveys a certain message to the patron, like yes we do care about the details.

Then again, the effect of a totally clean porthole might be somewhat overrated. Some dust on it will most definitely not visibly affect the focus and will hardly visibly impact brightness, but buildup of dust and grease will eventually impact the brightness and contrast of the picture and possibly even impact the color of the image.

Edit: Steve answered essentially the same with more detail roughly parallel with me.

quote: Frank Angel

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Is there anyway to know if the glass is optical water-white glass without seeing the edge? Is there a test of some kind?

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