Selecting a Projector Screen

When choosing a projection screen for a home theater, a key decision is the selection of the screen aspect ratio. Nearly all home theater projectors have a native image aspect ratio of 1.78:1—this is the shape of the chips within the projector that actually create the image. So, the first and perhaps most obvious choice for a screen aspect ratio is 1.78:1, to match the projector’s native ratio. When projecting a wide-format movie in the popular “CinemaScope”-type format, these screens are commonly masked off on the top and bottom to hide the unused letterboxed area. This is commonly called a “Constant-Width” projection system—the projected image always fills the width of the screen, but the top and bottom may be masked off to varying degrees. This is the simplest system to install.

Another popular choice for a home cinema is to use a screen with an aspect ratio of 2.37:1. This ratio is a good fit for popular widescreen “CinemaScope”-type formats, which range from 2.35 to 2.40:1. To fill a wide screen like this one, the projector is often outfitted with an additional anamorphic lens, which stretches the native 1.78 image horizontally by a factor of 33%, filling the 2.37 screen. This lens works in concert with image processing in the projector or source device to allow letterboxed widescreen movies to fill the screen without the black bars on the top and bottom. Similarly to the 1.78 screen, when projecting a film at an aspect ratio that doesn’t fit the full width of the screen, masking is employed to hide the unused sides of the image. Systems like these are commonly called “Constant-Height” systems—the image always fills the screen from top to bottom, but portions of the side may be masked off. These systems are more complex to install because of the need for the anamorphic lens and the image processing.

A disadvantage of both of these screen formats is that, when projecting a movie where the aspect ratio doesn’t match that of the screen, you end up seeing a smaller image—one format will always seem smaller than the other.

A third option is to use a screen with a 2.0:1 aspect ratio, with a screen masking system that can mask all four sides of the image, and a projector that can zoom and shift the image to multiple memorized positions. A system like this is configured so that a wide-format movie fills the width of the screen, with small black bars on the top and bottom that are masked off. A standard-format movie fills the height of the screen, with small black bars on the side that are again masked off. These systems are called “Constant-Area” systems, because the common film formats are displayed with approximately the same image area (in square inches or square feet), so although they are different in shape, they do not substantially differ in perceived size. Systems like this are the most complex to install.

One of the keys to enjoying a movie is to be able to successfully suspend disbelief—to lose yourself in the story, forgetting that you’re watching an image on a screen. One thing that facilitates suspension of disbelief is to have the image floating in space—free from distractions surrounding the image that can draw attention away from the story.

Some distractions are obvious—you wouldn’t want to have shiny objects to the sides of the screen that would produce distracting reflections, for example. But motion pictures are filmed in different aspect ratios depending upon the artistic intent of the director, and this means that it’s common for a portion of the screen to be unused for a given movie.

With the exception of OLED displays, which can be perfectly dark when displaying black, other flat panel and projection systems can only approximate black—they end up displaying a very dark gray. Having your movie surrounded by a dark gray halo presents a significant distraction to the enjoyment of the film.

Fortunately, a solution exists for this problem—a screen masking system. A screen masking system is just a set of movable, black panels that can cover the portions of the screen that are not in use. These are typically employed with projection screens, and the panels are made of material that will absorb the small amount of light that remains where the projector is displaying black.

For projection screens that are acoustically transparent (with the speakers behind the screen), acoustically transparent screen masking material is also available. This means that even if the speakers happen to be behind an area that is masked off for a given movie, the audio is unaffected.

Depending upon the screen aspect ratio (see the chapter on screen aspect ratios for more detail), a masking system may include one, two, or four movable panels. Systems with two panels are most common. Four panels provide maximum flexibility and adaptability to essentially all possible aspect ratios.

Masking systems can be fully manual, where the viewer has to actually move them by hand. More commonly, they are motorized, with a set of presets to move the masking panels to accommodate common image aspect ratios. The viewer just presses the button for the correct ratio, and the masks move into position.

With a Kaleidescape system, every movie’s aspect ratio is included with the proprietary metadata that accompanies the movie. When used with a home control system, this aspect ratio metadata can be used to automatically mask the screen to the correct aspect ratio as soon as the movie starts, without the viewer’s explicit intervention. They just press Play, and the rest is taken care of.

Projection screens can be made with different types of materials, and their surfaces can be treated with coatings that affect the way in which they reflect light. By choosing the right combination of materials and treatments, it is possible to optimize the projected image for the screen size, the seating location, and the ambient light characteristics of the room.

One of the benefits of projection screens is that speakers can be mounted behind the screen without degrading the sound quality that the viewer experiences. This allows the use of larger screens, since space doesn’t need to be left to the sides for speakers, and it produces a cleaner aesthetic in the theater. More importantly, having the speakers behind the screen can significantly improve the perceived realism of the movie soundtrack, since the audio is perceived as coming directly from the people and objects on the screen. This is especially important with dialog, since humans are highly attuned to the nuances of human speech.

Acoustically transparent screens fall into two major categories: perforated screens and woven screens. There are, of course, pluses and minuses to each type of screen.

Perforated screens are made of a sheet of material (often vinyl) with tens of thousands of tiny holes distributed uniformly across the surface. The holes allow the sound from the speakers to pass through, but they are small enough to not be noticeable under most conditions. While these screens are designed for audio transmission, since most of the screen surface is still solid material, the screens will usually attenuate the sound, often unequally across the frequency spectrum. For this reason, perforated screens will sometimes come with an audio processing component that helps to equalize the sound going to the behind-the-screen speakers to compensate for the attenuation caused by the screen material.

Perforated screens can have the same gain materials and ambient light rejection features described in the previous section. Since some part of the screen surface is “missing” (where the perforations are), perforated screens will have slightly lower gain than a non-perforated screen made from the same material.

The second major class of acoustically transparent screen is a woven screen. These screens are woven from synthetic fibers, using a very slightly loose weave that allows sound to pass with minimal obstruction. Woven screens generally have less of an effect on the sound from the speakers, principally limited to slight high-frequency attenuation that can be addressed with audio equalization. The tradeoff is that these screens do not typically have gain, and do not have ambient light rejection properties. As a result, a brighter projector may be required, and careful attention must be paid to room finishes and lighting to prevent washing out of the image.