Film-Tech Forum ARCHIVE: "Reflective square" on screen?

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Posted by Jen Pelto (Member # 6447) on 08-29-2011, 05:41 PM:

In one auditorium, there seems to be a bright, reflective square-ish image on the screen while showing movies. I believe it happens on both scope and flat (ruling out the lenses). I have noticed this at other theaters before, too. People have never complained about it at my theater or appear to be bothered by it in any way -- however, I (obviously) want it gone!

Is this a problem within my projector, or on my screen, or something else entirely?

Thanks guys!

Posted by John Wilson (Member # 269) on 08-29-2011, 06:11 PM:

Is there a screw missing in the lamphouse perhaps? I had this once where a screw had come out and shot a light straight through the port to the screen.

Posted by Jen Pelto (Member # 6447) on 08-29-2011, 07:18 PM:

Hmmm, interesting question. Will have to check after the movies tonight!

Posted by Jeremy Weigel (Member # 4208) on 08-29-2011, 07:28 PM:

Look behind the screen for any square electrical junction boxes that haven't been painted flat black or at the least covered with a non-reflective material.

Posted by Gordon McLeod (Member # 33) on 08-29-2011, 07:34 PM:

Thread up and project some solid black film and see if it is there then block the porthole and see if it is there
probably something behind the screen reflecting though

Posted by Paul H. Rayton (Member # 1817) on 08-29-2011, 07:40 PM:

Jen - I'm not aware of the layout of your venue, but another item to check for, bearing in mind that you said the light pattern was "square", would be a reflection bounceback from your projection port. The port glass almost always reflects some light back (toward the projector) -- from which some shiny spot on the front of the projector (or lens barrel) could be doing a secondary reflection back out to your screen.

Or, as I've seen at least once, from another projector completely, with light bouncing across a projection room, to occasionally cause light to show up (again, in a squarish pattern) on a screen facing the opposite direction. (In other words, a portion of the light intended for screen #1, a "west facing" screen will be bounced off the port glass of that theatre, returns back through the projection room area (going westward), goes out through the observation port that is used for screen #2, to an "east facing" screen. So, some images, esp. light scenes from screen #1 can occasionally show up as a ghostly, secondary image on screen #2. This latter situation is, admittedly, a somewhat remote possibility, but I've actually encountered it before, and we "solved" it by keeping a cardboard in the port at this one place!

Posted by Aaron Stroop (Member # 2874) on 08-29-2011, 08:28 PM:

Hello Jen
I have the same type of problem here. Stray light from the box refects out onto the screen and in one cinema onto the seats.
You can test this buy turning off all the lights in the projection room and seeing if it is still there. Then turn them back on 1 at a time and see if it comes back. We had to block the dead side of a couple of lights to stop the problem. I also have a polished black floor which bounces light around abit. Good luck if its a projection room work light you should be able to see it with the auditorium lights off and the projector not running.

Posted by Robert D Hall (Member # 5674) on 08-29-2011, 08:37 PM:

could it be your stage speakers? in my theater the installer of the screens cut holes in the black screen backing where the horns of the speakers are. i guess not the muffle the highs. any way if your sitting in the right spot and its a bright scene you can see a square reflection from the shiny plastic of the horns.

Posted by Monte L Fullmer (Member # 2797) on 08-29-2011, 10:24 PM:

..I call it an unpainted "J" box if it's square.. or, just something reflective that's behind the screen...

Posted by Ian Parfrey (Member # 5122) on 09-02-2011, 09:05 PM:

Hi Jen

East things first.

Move to the square showing on your screen and look directly at that spot with your eyes as close as possible - about 2-3" is good. You may need a ladder depending on the height of this spot.

Does the mark go away?
If so, turn around keeping your eyes at the same level as before and look for stray light. This may be from any direction in the auditorium.

If the spot does not go away, look THROUGH the screen and search for reflective surfaces such as speaker horns, driver surrounds, back stage exit lights &tc.

Cheers.

Posted by Manny Knowles (Member # 1171) on 09-03-2011, 05:31 PM:

There's a theatre in town here with a similar problem. For them, it's light reflecting off of a metallic wall sconce.

Posted by Louis Bornwasser (Member # 3063) on 09-03-2011, 05:56 PM:

I'm with the group that says "unpainted box" behind screen.

Looks like the quality control people missed one. (I carry black spray paint with me for every screen instrallation just to deal with this.) Getting harder and harder with the "squeeze" that excellent installers now have to deal with. Louis

Posted by Manny Knowles (Member # 1171) on 09-04-2011, 11:48 PM:

I'm with that group, too. But rather than repeat "unpainted j-box" I thought it more helpful to suggest something else to look for in case it isn't that (for the original poster, or anyone in the future with a similar symptom)