Film-Tech Forum ARCHIVE: Strange Port Glass Reflections

This is topic Strange Port Glass Reflections in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Richard May (Member # 2627) on 10-06-2005, 07:06 PM:

I have noticed once in a while that we get a strange reflection on our screen. As the credits roll up, there is a dim but definitely noticeable mirror image rolling down. Also, if there are still words somewhere on the screen, that also gives a mirror image. It does not happen on all the screens and I am not sure that it happens with every movie. We are using optical glass in the ports. It seems somehow that the image is bouncing off the glass, back to the projector or lens, and back out again. Any ideas? [Confused]

Posted by Peter Mork (Member # 1336) on 10-06-2005, 07:17 PM:

You may have to pay attention to what's behind your screen. Those itty bitty holes in the surface let light through and create a dimmer version of the image on the wall behind the screen which is visible, especially from more direct viewing angles. Putting in black baffling, or just painting everything behind the screen flat black, is the cure for this.

Posted by Jesse Skeen (Member # 586) on 10-06-2005, 08:29 PM:

It's usually the speakers behind the screen.

Posted by Brad Miller (Member # 2) on 10-06-2005, 09:45 PM:

From your description, the image is hitting the port glass, bouncing back to the projector, and then something reflective on the front of your projector...usually the turret...is reflecting the image back.

I've seen this in many Century projector installations, and frequently people have taken a black marker and inked out the entire silver portion of the turret to aleviate the problem.

Posted by Phillip Grace (Member # 2262) on 10-06-2005, 10:16 PM:

The same thing can happen with Victoria 5's in front of large ports. The makers plate on the motor is an effective reflector. The ghost image always tracks a bright object in a dark scene. Same cure. Paint it black.

Posted by Rick Long Jr (Member # 541) on 10-06-2005, 10:32 PM:

I've seen this many times with many different projectors, but never when the glass is angled. Except for one theatre, where the theatre had the ports installed with the top edge instead of the bottom toward the auditorium against our wishes - his reasoning was that less dust could settle on the outside. Problem was that in one cinema it turned out that the projection angle was almost identical to the glass tilt. This phenomenon of the inverted image was very noticeable.

Posted by Steve Guttag (Member # 268) on 10-07-2005, 06:45 AM:

Rick hit the nail on the head...odds are, your port glass is not angled or not angled opposite to the projector. That is, if your projector is pointing straight or down to the screen, the port should be angled such that any reflected image points to the ground (with coated glass it may only be a blue or puple image but that should be hitting the ground, not the back wall of the booth or worse, the theatre on the other side.

If you have double ports (something I think is essential to keep noise down), then they should be angled in opposite directions so they don't bounce off each other. Ideally, any reflection on the theatre side glass should hit the inside of the port and not get back into the booth.

Posted by Richard May (Member # 2627) on 10-09-2005, 07:44 PM:

That sounds like the answer Steve and Rick. The projectors are not angled at all. They shoot straight on. The glass is also not angled. Next time I am there, I am going to tilt the glass and see if it changes. Thanks guys. [thumbsup]