Inner Workings: Inside a Movie Theater Screen

The screen at your local movie theater is obviously a lot larger than the specialty screens used in home theaters, but they actually have a lot in common. The main difference is perforation. The screens in almost every movie theater have the front left, center, and right speakers behind them, along with a few subwoofers. Tens of thousands of tiny holes allow the sound from the speakers to come through the screen.

A typical cinema screen has about 5,500 perforations per square foot, which is enough to allow the high frequencies from the powerful horn-driven speakers used in movie theaters to reach the audience. Harkness Hall, a major supplier of cinema screens, makes three levels of perforation, but the standard screen has holes 0.047 inches in diameter. Since the audience sits relatively far from the screen (at least compared to a home theater), the holes are invisible to them.

"Gain" is a measure of a screen's reflectivity, and 1.0 gain means that the screen reflects 100% of the light projected onto its surface. Screens with gains of less than 1.0, such as Stewart's Greyhawk and Da-Lite's Da-Mat, are popular in home theaters because they make shadows and black areas appear darker. But in movie theaters, screens almost always have gains greater than 1.0 - typically, 1.4 to 1.8. These high gains focus the light into a concentrated pattern, making the image bright even on a very large screen. That's important in movie theaters, where screens lit by one projector can measure up to 70 feet wide.

Most screens are made of vinyl coated with magnesium carbonate, which looks like white chalk and has the unique property of being perfectly reflective - it absorbs no light, scatters light equally in all directions, and has a gain of 1.0. Higher gain can be achieved by coating the matte white surface with a diffuser that focuses the light selectively, making it brighter to viewers who see the screen straight on, as most of the audience does in a movie theater.

Da-Lite uses platelets of mica as a diffuser; under magnification, they look like steppingstones. Some higher-gain screens use glass beads or a silver or aluminum coating, which provide gains higher than 2.0. Silver is the only surface for movie theater screens that can be used in 3-D presentations.

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