Readers of this site know that a projector is the only way to have a truly giant image without taking out a second mortgage on your home. But that's only half the story—you also need a screen. Now, some of you might be thinking, "I don't need a screen, I've got white walls that will work just fine." Judges? BZZZZZ! Sorry, that's just wrong.
While it's tempting to project the image onto a white wall, the result will almost certainly be disappointing. For one thing, most walls are not entirely flat—the surface has some sort of non-uniform texture that might not be obvious with a cursory glance, but it can be painfully evident when you project a video image onto it. Also, a wall's reflective characteristics are not well controlled, leading to hot spots, dull areas, and unpredictable performance at different viewing angles. Finally, most white walls are not neutral white, so they will alter the color of the image.
For a projector to work its best—and to get the most out of your investment in said projector—you really need a projection screen. The next step is to choose the right type of screen for your circumstances from among the myriad choices available today.
If you're going to use a projector in a light-controlled, dark-walled, dedicated home theater, a matte-white screen is a good choice. To deepen the blacks, you can use a gray screen, though that can also reduce the brightness of the image.
These screens are examples of diffusive screens, which reflect light in all directions. That's fine for a dedicated, dark-walled home theater, but what if you want to see a good projected image in a room with light walls and ambient light from windows or lamps? If you were to use a diffusive screen in such a room, the image would look totally washed out. The best choice for that type of environment—say, a living room or rec room—is known as an ambient-light rejecting (ALR) screen.
In addition, you might be considering a new type of projector that's starting to gain widespread popularity. These so-called ultra-short-throw (UST) projectors sit just a few inches from the wall and shoot their light up at a steep angle onto the screen. In this case, the screen must be specially designed to reflect the projector's light straight out into the room while preventing ambient light from washing out the image.
First, let's define exactly what we mean by "ambient light," which poses a problem for any projection system. It's light in the room coming from sources other than the projector. That can include windows during the day, lights on the ceiling, lamps around the room, and light spilling in from openings to adjacent rooms.
To reduce the amount of ambient light in a room, you can cover the windows with curtains or shades and turn off any lights in the room. But that doesn't make for a convivial atmosphere when you want to watch the big game with family and friends. Fortunately, there's another solution: an ambient-light rejecting screen.
The term "ambient-light rejecting screen" is something of a misnomer. These screens actually absorb some of the ambient light in the room and/or redirect some of it away from the viewing area, depending on where it's coming from. But "ambient-light absorbing and/or redirecting screen" is a bit cumbersome, so everyone uses the term "ambient-light rejecting" or ALR.
There are two basic types of ALR screens: angular or specular reflective and retro reflective. Angular-reflective ALR screens rely on a basic tenet of optics: the angle of reflection equals the angle of incidence. In other words, the angle at which the light hits the screen is the same as the angle at which the light is reflected (see Fig. 1). Examples of this type of ALR screen include the Da-Lite Parallax Stratos, Elite Screens CineGrey 3D and CineGrey 5D, Screen Innovations Black Diamond and Slate, and Stewart Filmscreen Phantom HALR.
By contrast (pun intended!), a retro-reflective ALR screen reflects light back toward its source—or, more specifically, it reflects light from the projector back toward the projector and the viewers while absorbing light from other angles. This type of screen typically employs a layer of material with microscopic triangular ridges (often called a serriform or lenticular microstructure; see Fig. 2). Examples of this type of screen include the Da-Lite Parallax Pure, dnp Supernova and Supernova Infinity STD, and EVP/Elite DarkStar.
In most cases, the ridges run horizontally across the screen, which means they are most effective with overhead lights and less effective with light sources to the sides. To improve side-light response, the base layer can be made angular-reflective.
Not surprisingly, ALR screens can't do much about light sources near the projector itself or at a similar angle as the projector (see Fig. 3). If there's a window or lamp there, its light is reflected into the viewing area along with that of the projector, so be sure to turn off any lamps and cover any windows in those positions.
Both types of ALR screens typically have a gray base layer, which enhances the contrast of the image. In addition, many ALR screens incorporate multiple layers, possibly including a somewhat diffusive layer to increase the viewing angle so more viewers see a good picture as well as layers of microscopic optical beads and black beads to absorb ambient light (see Fig. 4). However, optical beads can also cause a speckling effect and reduce uniformity—that is, create hot spots—if they are not carefully designed. These layers also allow the manufacturer to control the gain of the screen, which is a measure of exactly how much light from the projector is reflected into the viewing area.
For those with dedicated home theaters, it's important to note that both types of ALR screens scatter much less of the projector's light to the sides than fully diffusive screens. As a result, they add less light pollution to darkened, dedicated theaters.
Retro-reflective ALR screens tend to have a relatively narrow viewing angle, which means that viewers must be close to the center of the screen to see the best image quality. Also, they can be prone to hot spots and speckling if they include optical beads, and they're not as good as angular-reflective screens for ceiling-mounted projectors. (Elite Screens says that its DarkStar 9 retro-reflective material has a wide viewing angle and no hotspotting.) On the other hand, retro-reflective screens tend to have better black levels than angular-reflective designs.
In general, angular-reflective screens are easier to manufacture and thus less expensive. Also, they can often be installed in a retractable mechanism. Retro-reflective screens are more difficult to make and therefore typically more expensive. In addition, they are usually more limited in size than angular-reflective screens, and because of their multilayer microstructure, they are often available only as fixed screens. (The dnp Supernova Infinity consists of seamlessly assembled modular tiles, so it can be virtually any size.)
Ultra-short throw projectors are quickly becoming an attractive alternative to large flat-panel TVs. As I mentioned earlier, a UST projector sits a few inches from the wall on a credenza or other surface and shoots its light up at a steep angle onto the screen. If such a system is to compete with flat-panel TVs, the screen must reflect the projector's light into the viewing area as well as divert and absorb ambient light.
To accomplish this, virtually all UST screens use lenticular ridges, though they are not retro-reflective; otherwise, light from the projector below the screen would be reflected down toward the projector! In this case, the cross section of the ridges looks like a series of right triangles with the angled side facing downward (see Fig. 5). Examples include the Da-Lite Parallax Pure UST 0.45, dnp Supernova STW and Supernova Infinity UST, Elite Aeon/StarBright CLR and CLR 2, Screen Innovations Short Throw, and Stewart Filmscreen Balón Edge UST.
XY Screens offers two straight-lenticular UST ALR screens. The PET Crystal has horizontal lenticular ridges, while the PET Grid has crossed ridges (see Fig. 6).
Another variation of lenticular ridges is called a Fresnel microstructure, named after French physicist Augustin-Jean Fresnel. His original invention was a lens consisting of concentric discs of glass, but the basic idea has since been applied to projection screens. In this case, the lenticular ridges form concentric semicircles; the downward-facing sides reflect light from the projector out toward the viewing area, while ambient light is reflected away from the viewing area or absorbed (see Fig. 7). Examples of Fresnel-based UST ALR screens include the dnp Supernova STS and XY Screens INP-FNE.
Like long-throw ALR screens, UST screens are generally gray in color to improve contrast, though some have a white substrate, and the lower reflective surface of the ridges might be white while the upper absorptive surface is black. Also, many include multiple layers, including layers of angular-reflective material as well as optical and absorptive microscopic beads, to improve the viewing angle, redirect and/or absorb ambient light from the sides, and adjust the screen's gain.
Because of their lenticular, layered structure, most UST screen materials are not generally retractable; some are rolled up for delivery, but could suffer delamination and deformation as they are rolled and unrolled over time. However, a few companies have been working on this—Screen Innovations currently offers its ST material in a rollable motorized casing, and Elite Screens plans to have a rollable version of its CLR material available later this year.
UST projectors represent one of the fastest-growing segments in the projector market, and for good reason. They offer huge, high-contrast images in brightly lit rooms for a whole lot less than a comparably sized flat-panel TV—as long as you pair them with a UST-specific screen. Such a screen works like magic to direct the projector's light from below out into the viewing area while absorbing and redirecting much of the ambient light away from the audience, leaving a bright, punchy image for all to enjoy.
Many thanks to Rhen Taylor of Da-Lite, Johnny Jensen of dnp, Jaime Abrego of Elite Screens, Ryan Gustafson of Screen Innovations, and Mark Robinson of Stewart Filmscreen for their help with this article.