I've noticed on several occasions that perforated screens with a rectangular array (or any other regular array) of holes tends to create the appearance of a pixel structure in the image if one gets too close. On 3 or 4 occasions, when sitting close to a screen (4th row or closer), when the focus was perfect, the array of holes have caused the projected image to have a "digital, pixelized" appearance.

Couple this with a projected image that already has a rigid pixel structure of a given resolution. The pixel structure of the projected image and the structure of the perforations of the screen will create "beat patterns". Suppose the projected image has a resolution of 1280 by 1024 and the screen has an array of holes that is 1300 by 1050 (I just made those up). This will show up as a beat pattern that is made up of 20 vertical waves going across the width of the screen and 26 horizontal waves going up and down the screen. This may or may not be noticeable, but at certain combinations of resolutions, coupled with other irregularities caused by keystoning, this could be distracting if the wrong combination of screen perforation pitch and digital projection resolution were combined.

Has this possibility ever been investigated or mathematically analyzed? I'll bet that unless a screen is very small, the existing screens have a small enough "performation pitch" for this not to be a problem with current digital projectors, but as resolutions increase, could it be more of a problem?

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Evans A Criswell
Huntsville-Decatur Movie Theatre Information Site

Now with a small enough screen and a very high-resolution projector, this may become a problem.

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Having been involved with projecting video on to screens at the Telluride Film Festival these past 4 years and attending many digital shootouts at various trade shows, I can tell you this. So far, with any resolution of video, there are always fewer pixels than holes in the screen. Also the pixels have always been larger than the holes. So if you are close enough to the screen to see the holes, you are way too close to enjoy the video image.

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I'd bet that this phenomenon I'm describing would only be a problem if the ratio of the screen performation pitch to the projection material "pixel pitch" were in the range 0.5 to 2.0 , probably being most bothersome with values closer to 1.0 .

You're right that if you're close enough to the screen to see the holes, you're too close, but artifacts created by the style of interference I'm describing could be visible even from the auditorium rear.

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Evans A Criswell
Huntsville-Decatur Movie Theatre Information Site

In one regard they were successful. The screens were brighter and more evenly illuminated. But the downside was that the speakers were positioned above and below the screen and usually sounded very shill. The other problem was the parabolic reflector that worked so well for light also worked very well for sound. The result was that you could hear people chewing their popcorn five rows behind you with perfect clarity and also could completely understand conversations happening 20 rows back. I also think that the sound reflective screen was responsible for the sound being shrill.

Ok, now back to the discussion about Pixels and Holes. I am sure that if the right convergence of pixels in perfect focus and the right quantity of holes happened, that there might be an interesting phenomenon. But I'm sure that it's probability of this happening is about the same as the 34 pound terrier theory. (Its thought that the natural frequency of the Golden Gate bridge is such that on the right day, no wind, certain temperature and humidity, that a 34 pound terrier walking at a precise gate, would exactly match the natural frequency of the bridge thus bringing it down.) Nice theory but the parameters would never be matched.

Home theater screens are another story. There has been discussion of this on the AV Science forums in the case of very high-end home theater installations using perf screens with HD DLP or LCD displays. Since these screens are much smaller, the ratio of perfs to pixels is much smaller, getting into the range where moire becomes noticeable. Solutions have included changing to a solid screen or a micro-perf screen, changing the screen size, critical leveling of the projector to the screen, slight de-focusing, or decreasing the image resolution. Anything that increases the pixel to perf ratio will work.

BTW much more noticeable than perfs vs. pixels is the use of electronic picture geometry adjustments to eliminate keystoning and the like. All but the most high-end of HD image scalers produce quite visible moire as soon as the video raster no longer maps to the pixel array.

Paul
Feeling just duckie!
Hey! Some feel chipper, I feel duckie!
-26

All that Ian pointed out about the Torus screen is accurate, however, I feel that the worst characteristic of it is the geometric distortion it imposes on the picture. Sit anywhere outside of the "sweet spot" in a Torus auditorium and you will notice severe visual distortion of the image, especially noticeable during credits and up-down camera pans.

From a maintenance standpoint, the screen "sucks" too.

I reacall once when an HVAC problem created a pressure imbalance in the auditoriums. The poor little "sucky" motors just couldn't handle the stress and gave out. This left us with a billowing mass of fabric to pass for a screen. As much as they suck when they do suck, they REALLY suck when they don't suck.

There is a little sensor in the middle of the screen that senses the distance between the fabric and the frame. This is used to control the shape of the screen by telling the motor "to suck or not to suck." (Didn't Hamlet say that?) This sensor has been known to go bad from time to time, and, as you may imagine, is not very fun to replace.

You must dust a Torus screen very regularly or else dust will collect disproportionately on the bottom of the screen (the part that faces upward) and become suprisingly noticeable. Not that screens should not be kept dust-free, but it is just another strage quirk about it.

AMC has not installed the screen for years now, and they are not without advantage; try getting 19-20-19 on a conventional flat screen!
You can do it quite easily with a Torus.

For the most part though, they really do suck.

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do any of you have experience with HARKNESS HALL PM180+ microperforated screens (0.5mm diameter) ? what is the holes arrangement ( delta or square ) ?

The HH microperf is a bit odd in it's arrangement...is it dense in one dimesion be regular spacing in the other (looks kinda like stripes). The perf size is tiny though, especially as compared to conventional perforations.

The next smaller version I believe you mentioned was the plus as in MP plus...now that has a standard delta formation but it looks more like a scrim. The perfs are very tiny and they disappear when one is only but a few feet away. However, you better not have anything even remotely reflective behind the screen because it would be almost translucent.

Next week, I'm going to be doing some experiments with the mini-perf screens so I might be able to provide some more information.

I can say watch out for the MDI mini-perf...while the holes are indeed smaller than the standard perf, they tend to stand out and have a very odd pattern (not likely to morie) but are way too visible for close viewing...I can see the perfs at 20 feet away...it is almost like they high-lighted the perfs despite them being about 50% of the size of normal screen perforations.

Steve

MDI mini perf ?

yeah, I meant MP.

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