Thanks,

 |  IP: Logged

Kelmar Systems Inc., 284 Broadway, Huntington Station, New York, 11746, telephone: 516-421-1230, fax: 516-421-1274, http://www.kelmarsystems.com also carries a line of porthole wall assemblies and optical quality port glass. Contact is Andy Marglin.

Goldberg Brothers, Inc., 8000 East 40th Avenue, Denver, Colorado, 80207, telephone: 303-321-1099, fax: 303-388-0749, carries a line of port glass frames and optical anti-reflection glass.

You should also check with your theatre equipment dealer to see what they carry.

My next article in Film Notes for Reel People discusses the importance of reducing stray light and optical flare to improve screen contrast. Having clean, properly-installed, optical quality, anti-reflection port glass is critical.

------------------
John Pytlak, Senior Technical Specialist
Worldwide Technical Services, Professional Motion Imaging
Research Labs, Building 69, Room 7419
Eastman Kodak Company
Rochester, NY 14650-1922 USA
Tel: 716-477-5325 Fax: 716-722-7243

Watch out on the various glass listed in John's post. There are MANY MANY various grades of glass having to do with their purity and actual manufacture.

One quick check is to look at the edge of the glass, it shouldn't be very green (preferrably not green at all) and compare with the same thicknesses. While looking at the edge, check how consistant it is...if you see waves in it...you picture will suffer.

I have used Goldberg's glass with very good results. Even the non-coated glass is much more clear than glass-shop plate.

 |  IP: Logged

I talked to the THX people about optical glass. They suggested several people, and one of those were Abrisa Industrial Glass (909) 698-8469.

I talked to a guy there who was very helpful, and sent me some samples. I held the samples in front of the image and it was pretty amazing how little reflection there was compaired to the regular glass. Not only good for the image but, if you have a problem where the light reflects off of one port window and shines out where you don't want it to, this would cure that also.

They offer two thickness'; 1/4" and 3/8." (THX suggests two panes of glass, one 1/4" and the other 3/8.") Also, Abrisa offers two different grades ("Clearview" and Clearview Plus".)

The price breakdown was this:

Clearview 1/4" $0.27 per sq inch
Clearview 3/8" $0.33 per sq inch
Clearview Plus 1/4" $0.84 per sq inch
Clearview Plus 3/8" $0.86 per sq inch

 |  IP: Logged

Can you go into a little more detail about "properly installed" port glass?

Meaning, what are the criteria for proper installation?

THX suggests that the two peices of glass be mounted on an angle for the projector port. If you are in the booth looking out, the top of the inside peice should tilt out (towards the aud) 7 deg. The top of the outside peice should tilt in (away from the aud) 7 deg. So if the projector is at a zero angle, the glass would be as described. There is always a 14 deg angle between the two peices of glass.

If the projector is tilted down 5 deg, then the inside glass tilts forward 12 deg, and the outside glass tilts back 2 deg.

If you go any less than the 14 deg difference, you'll get weird reflections that bounce back and forth, shining on peoples head below. Don't ask how I know.

The fellow I talked to (at THX) said somthing a bit odd; He explained that the two peices of glass be of different thickenss. He suggested that one peice be 1/4 inch thick,
and the other 3/8 inch. He said that if both peices were the same thickness, one peice might vibrate, and transmit that to the other peice. At certain harmonic frequencies, one of the peices of glass could crack. Two different thickness of glass would help keep "resonating" from occuring.

THX will fax you a picture showing the port and the wall insulation.

The viewing port does not have to be on angles, but they do want two peices of glass for sound insulation.

 |  IP: Logged

 |  IP: Logged

 |  IP: Logged

Ah, I didn't know that. I asked for a price on several peices, and went beyond the minimum.

My next article for "Film Notes for Reel People" is about improving screen contrast, and proper selection and use of port glass is a major part of reducing stray light. Dirty port glass is one of the biggest projection "sins" I see when visiting theatres. One way of looking at it is that the port glass is really the front-most element of the projection lens, and needs to be kept absolutely clean and scratch-free. Ideally, the port glass should be so invisible, that you should be barely able to see the image reflected by it.

------------------
John Pytlak, Senior Technical Specialist
Worldwide Technical Services, Professional Motion Imaging
Research Labs, Building 69, Room 7419
Eastman Kodak Company
Rochester, NY 14650-1922 USA
Tel: 716-477-5325 Fax: 716-722-7243