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. When I was growing up, a projection screen—except for the one in the local movie house—was something you took out of the closet to watch home movies on. The screen surface material was beaded glass, which better reflected the light from the projector. It rolled up into a case, which attached to a rickety stand that seemed to open up differently every time you tried; everyone in the family could take turns figuring out the wrong way to do it.
Non-video home movies and slide shows are now quaint anachronisms. Today most projection screens are sold to businesses and schools (and to commercial movie theaters). For the small, but growing, number of customers looking for a screen to complement that expensive new video projector in a home theater, the variety of screens can be bewildering. It pays to keep in mind that only a few of the available screens are truly suitable for home-theater applications. To make sense of the situation, we must first look at the viewing environment and characteristics of the screen material itself.
The viewing environment
In the business/school market for screens, control of ambient lighting is not always possible or desirable. The speaker must see the audience and vice versa. The audience may want to take notes or refer to written materials. Some ambient light is usually required in such situations.
In the home-theater environment, especially when configured with a separate video projector and screen, you don’t want any ambient light. Let me say this as directly as I can. No screen can make up for the loss of picture quality that results from ambient light in the room—even stray ambient light. You may be able to create a watchable picture in modest room lighting using a highly reflective screen and a pricey, high-performance projector, but you will not get the quality picture that the expensive setup is capable of producing.
I recently heard of a buyer who insisted on putting a projection system in a room with a wall of curtainless windows—he refused to cover them, even with retractable blackout blinds. He tried to overcome the problem by throwing money at it, going so far as to install a $90,000 light-valve projector. (Light-valve projectors are normally used for stadium rock concerts and other large-scale events. In addition to being intensely bright, they are large, noisy, and run very hot; they really require a separate projection booth.) For all that money, he got a picture not much larger than that from a good one-piece rear-projection TV. And I have no doubt that the result was little more than barely watch- able. While I couldn’t confirm the accuracy of this story, it drives home the point.
The reason for light control is quite simple. A video projector does not project black. Black is simply the color that results when no light from the projector falls on the screen. Since the screen must be white to best reflect the output of the projector, it can only show a true black in a darkened room. The ability of the projector/ screen/room system to generate true black is critical to the reproduction of colors as well. This is a physical fact. Even huge expenditures of funds will not completely overcome it; the same funds spent in a darkened space will result in a better picture.
And what about those “black” screens that are so heavily promoted by manufacturers of direct-view televisions and PTVs?
They are simply a response to consumer demand to watch television in a lighted room and the dealer’s need to sell TVs in a brightly lit retail environment. Under such conditions, dark-tint screens create blacker blacks (and thus greater perceived contrast) more effectively than a lighter screen. But they are no panacea. The set must work harder to penetrate the tinted screen with picture information, making it more difficult—impossible, actually—to design and adjust the set to optimize shadow detail, black level, color, and contrast within the sees linear operating range.
Today, you can project a video image on a fiat, matte-white surface in a darkened room and get a pleasing picture. It still will not be searingly bright—even the light from a well-projected movie in a good theater is a lot dimmer than you might expect. But when video projectors were in their infancy, the same fiat screen would have produced an unwatchable picture. The solution was a screen with gain.
As a passive device, of course, a screen cannot have overall gain. But it can focus the light falling on it over a narrower angle. Limiting the angle over which the picture is watchable increases the relative brightness 0 the image in the center. Early high-gain screens were curved. Such screens are still available, but flat screens with slightly less, though still significant, gain are now more widely used. Two common types of higher-gain screens with which you are most likely familiar are beaded screens (as on the old slide-projector screen) and screens with a metallic silver surface.
With today’s projectors, however, using a high-gain screen in a home-theater environment is a lose-lose proposition. You lose in usable viewing angle, and you lose in image quality. The higher the gain, the harder it is to control image consistency across the full expanse of the screen. You end up with “hot spots” in the center and uneven color balance. Not a pretty picture.
Nevertheless, a little gain, given the inherently less-than-generous light out put of most video projectors, is desirable. Most experts recommend a gain of about 1.3 as a workable maximum that does not result in visible picture degradation (a flat mane-white surface has a gain of about 1.0).
There may be special circumstances justifying a little more gain than that. Some projectors—particularly economy LCD models—could use a little boost. Brightness isn’t everything, but up to a point, it is. If you do choose a higher-gain screen, be aware that there are definite tradeoffs involved and be sure the installer knows how to deal with such screens. There are particular projector vertical-placement requirements that must be followed with higher-gain screens to ensure that the light from the projector is reflected back to the viewers where it is needed, and not toward the floor or ceiling.
Incidentally, when you see a dealer demonstrate a projector that you are considering buying, always ask what the screen gain is. Differences in screen gain will significantly alter the picture characteristics. If the projector being demoed is a very high-end model, and the gain is greater than 1.3, find out why.
Size definitely counts
Now we’re getting down to the nitty-gritty. You want a BIG screen. That’s why you paid big bucks for a video projector. Ten feet, minimum. Twelve feet, even better. Gimme gimme GIMME. Whoa! Time for a reality check.
If you’re considering a screen bigger than seven feet wide (not diagonal), you have some serious thinking—and budgeting—to do. And if you’re considering a modestly priced projector without a line doubler (and in video projectors, modestly priced means under about $8000), you might want to limit the size to 6 feet. Why? It all comes down to light output.
There is no single video projector that can put adequate light onto anything larger than a 10-foot-wide screen, short of the very expensive—and not very home-theater practical—light-valve type. And to get decent performance on even a 10-foot- wide screen, you should plan on either a projector with 9-inch tubes, or two projectors with 7-inch tubes, stacked (for more on stacked projectors, see “On the BIG Screen” elsewhere in this issue). Both solutions are very expensive.
A top-quality projector with 7-inch tubes will probably provide satisfactory performance, when new, on a 10-foot- wide screen. But projection tubes wear out, with a noticeable loss of output in the first year of operation. If you “over- screen” from the start, you will leave yourself with no reserve when the tubes start to age. Don’t complain to your dealer if the projector with 7-inch tubes you insisted on using with a 10-foot screen starts to look washed-out less than a year after you bought it. And don’t think you can compensate by cranking up the contrast control. The picture will get brighter all right—and frizzier as you drive the tubes into blooming.
When considering screen size, you obviously need to consider seating distance, as well. An often quoted rule of thumb suggests a minimum seating distance of 5 to 8 times the screen height. However, for a large picture, this can get impractical very quickly. I have viewed a high-quality, line-doubled, 48-inch-high picture very comfortably from a distance of about 12 feet (three times screen height), and a good, non-doubled, 50-inch-high image from roughly the same distance. In the latter case, the scanning lines were visible but not annoying. But just as in a commercial cinema, everyone has their own limits on how close to sit. I would not recommend sitting closer than 3 times image height, and perhaps even as much as 5 times if you are using a projector without line doubling capability and you are particularly annoyed by visible scanning lines.
Most projectors are best used on a screen with a conventional TV aspect ratio of 4:3 (aspect ratio is the ratio between screen width and height). Letterboxed films will then occupy the center of the screen with black bars at the top and bottom—as with a normal television set.
Some video projectors, however, have the ability to display a widescreen picture the same height as a conventional 4:3 image, only wider. With such a projector, your 4:3 programming will now occupy the center of the screen, with black bars at the sides (one-piece, widescreen TV sets also operate in this fashion). For such a projector, you need a screen with an aspect ratio of 16:9. The advantage: widescreen material becomes an event, with a larger, wider picture than conventional programming. It’s the way things were meant to be. Letterboxing on a 4:3 screen allows you to see the film shaped as the director intended, but with a smaller picture area than a 4:3 image on the same screen.
If your projector lets you opt for the 16:9 screen, you should still keep the total screen width down to the size recommended earlier. Keep in mind that conventional programming will be narrower, with black bars on the sides. If you primarily watch 4:3 programming (sports or broadcast TV shows, for example) with only an occasional widescreen film, you might want to consider a 4:3 screen of the same width. Yes, you’ll have unused screen area at the top and bottom when watching widescreen films, but the taller screen will provide greater impact with 4:3 images.
The most economical type of screen is fixed, with a rigid frame to keep it stretched flat. It hangs on the wall or stands on the floor on optional legs. If you want to hide it when it is not in use, you could devise some sort of movable cover—anything from simple drapes to art or photographs that slide out of the way by hand or motor.
Motorized, retractable screens are also available from all the major screen manufacturers. Because a screen must be kept flat for best results (no curling edges like on mom and pop’s old slide screen), elaborate tensioning schemes have been devised. While more expensive than fixed screens, retractable screens have the undeniable advantage of getting out of their own way when you don’t need them. You might still want to camouflage the screen housing, but that’s a relatively simple task. And if you have the space, a retractable screen might allow you to put a conventional television set just behind it for casual viewing in normal room lighting.
Manufacturers have devised clever schemes for masking a screen to accommodate different aspect ratios. Both Stewart Filmscreen and Draper market frames in which motorized masking curtains drop down to cover the black bars. They are available in either a horizontal masking version (a 4:3 screen, masked at the top and bottom when used with letterboxed programming), or a vertical masking version (a 16:9 screen for use with widescreen-capable projectors, masked at the sides when used with 4:3 material). Although the masking material is motorized, the screen itself is fixed.
In retractable masked screens, Stewart markets the Electrimask, a retractable 16:9 screen with separately retractable side-masking curtains for use with 4:3 material. Da-Lite’s similar Dual Mask Electrol has an interesting wrinkle; after the masking material is lowered, it moves closer to the screen to prevent shadows that might occur with the masking material displaced from the screen. All of these opinions are expensive—the Electrimask and Dual Mask Electrol are several times the price of a fixed screen of the same size. They won’t provide a better picture, but they will provide a more dramatic, professional-looking show. Run a 4:3 cartoon on the masked portion of a big, masked screen, then retract the masking for the feature and hear your first- time guests gasp.
There is another category of screen type—actually material—which must be mentioned. Conventional screens are acoustically opaque. Loudspeakers must be set up below them, above them, or to the sides. Movie-theater screens are perforated, so the speakers can be mounted behind them. But theater- screen perforations are acoustically compromised; they are not totally transparent to sound, and there are losses involved when the sound passes through them. The perforations arc also rather large in size and would be visible from the seating distances found in home theaters.
Encouraged by THX specifications for home-theater screens, manufacturers have devised screens with much smaller, more numerous perforations than those in commercial theaters. Loudspeakers may be placed behind them with little acoustic compromise. The disadvantages: they are still not absolutely transparent acoustically (even the best loudspeaker grille cloth is not totally transparent to sound), and, more significantly, a measurable amount of precious projector light passes through such a screen instead of being reflected back at the audience. Furthermore you shouldn’t place reflective, metallic objects behind a perforated screen (i.e., exposed metal-dome tweeters); the light passing through them could reflect off the shiny metal surface, back through the screen, and be visible to viewers. (For an eyes and ears-on look at a THX-rated, perforated projection screen, see the “Holey Screens” sidebar in my review of the Runco 980 video projector elsewhere in this issue.)
Up to this point, we have been concerned with front projection, that is, the projector positioned on the same side of the screen as the audience. There are screen materials, however, that allow the projector to be set up behind the screen, functioning as a sort of Godzilla-sized PTV, except that it’s not all in one box.
The advantages: higher-gain screens are available, though with the same disadvantages as high-gain front-projector screens; ambient light is less of a problem, though for best results you still want a darkened room; the projection equipment is hidden; and you can walk in front of the screen without disrupting the picture. The disadvantages: a lot of space is required behind the screen (expensive, mirrored racks are also available which reduce but do not eliminate the required space); projector setup and adjustment are often more difficult; picture quality may be slightly degraded (though not everyone would agree on this last point); and there is no way to place the loudspeakers behind the screen.
You now know enough about video projection screens to be truly dangerous. Enough to discuss the subject intelligently with the installer—and you will need an installer when acquiring a separate video projector and screen. If an attempt is made to sell you a 12-foot wide, high-gain screen, red flags should go up. These are certainly available options but your installer should be aware of the tradeoffs involved and be willing to discuss them with you frankly. The screen is a vital part of the video-projection setup, and only when you make an intelligent choice of screen will the overall system provide the quality performance you paid for.
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[adapted from 1996 Stereophile Guide to Home Theater article]