Despite improvements in technology, an improper screen choice limits the performance of even the best projectors.

Stewart Filmscreens’ Grayhawk line of front screens offers a gain figure of .95 as opposed to the more common 1.3 to 1.5 gain figure for standard matte finish screens.

This rapid growth in sales of projectors and monitors, coupled with the development of digital interfaces and content servers, has pushed traditional broadcast and video production tools to lesser stages for many Asian manufacturers. And it's easy to see why: 3lb., 1500-lumen projectors and 50in. HDTV-ready plasma monitors don't need much explanation. Their pictures are already worth a thousand words.

Indeed, it has been said that projectors and monitors are the “tail wagging the dog” these days, particularly when it comes to media production. Companies like Sony have built entire sales and marketing divisions around display products, and many booths you'll visit at NAB will have plasma/LCD monitors and projectors displayed prominently next to video servers, editing workstations, cameras, and VTRs.

But that tail is long, and we need to travel all the way to its tip to revisit an often neglected part of the display chain — the projection screen. Screens aren't very sexy, and they don't change much from year to year. Although there are innovations in screen coatings and storage/transport mechanisms, those announcements are often lost in the blitz of new projector rollouts.

Just as the best HD cameras can be hamstrung by poor optics, the best projectors are limited by an inappropriate choice of projection screens (or, in many cases, no screen at all). In media production environments, it is worth the effort to select the right screen to match a given projector. More important, the viewing environment must also be tweaked.

Today's front LCD and DLP projectors are a blessing — and a curse. They bless us with amazing light output from high-efficiency light engines, and they provide sufficient resolution to watch everything from interlaced analog video to workstation graphics. Many of today's projectors are light enough to easily truck from room to room (or even across the country) and set up quickly to project on off-white walls, whiteboards, or whatever surface happens to be handy.

That's fine for folks making quickie business presentations on the fly, but it's not the right way to look at multimedia and video content. The curse of portable projectors is that they can illuminate a wide variety of surfaces, no matter how inappropriate those surfaces may be. Next time you attend a trade show, look to see how many exhibitors take the time to set up a projector and screen correctly for optimum image quality. Is the ambient light managed? Are contrast and brightness set correctly? What about color temperature?

Projection screens exist for a reason, and you should think about having one room in your facility that combines a quality front screen with a controlled, optimal projection environment to present and evaluate your work. This is particularly true if you are working in compositing, special effects, and timing and transfers. While much of this work can be performed with smaller calibrated monitors, a projector/screen combination can be easier on the eyes and provides an immersive environment for judging image quality.

Projection screens have been around for nearly a century. Over that time, they have evolved into some sophisticated designs, but the basics of screen illumination haven't changed much. (Physics is physics, after all!)

The moment a photon is emitted from a projector's lens, it begins a mad dash to escape and travel as far as it can before running out of energy. It typically takes a random, straight-line path — one that can be best described as diffused.

Multiple photons diffuse over wider and wider areas as they flee their light source, a phenomenon predicted by the “inverse square” law. This principle tells us that image brightness (produced by photons reflecting from a surface back towards our eyes) is inversely proportional by a square root function to the distance traveled.

Here's an example: You place a projector 10ft. from a screen and, using a calibrated light meter, measure 500 lumens using a 100% white test image. But the image size isn't big enough to fill the desired screen area, so you move the projector back another 10ft., doubling the projection distance.

An educated guess tells you the brightness will probably decrease by 50%. Surprise! Your meter now shows only 125 lumens, a quarter of what you measured previously. To recover those lost 375 lumens, you'll need a projector that's four times as bright. Either that, or choose a screen that has gain to reflect more of the light back to you.

How can a screen — a passive device — possibly have gain? Easy. By using small lenses embedded into the screen surface to capture and “collimate” the photons — or light rays — back into a narrower angle of travel. Just as a magnifying glass can collimate the rays of the sun, so can a projection screen round up those wild, free-ranging photons and make 'em behave more orderly.

But there's a drawback to this trick. For one thing, the angle at which collimated light rays travel back toward your eyes is made progressively smaller as screen gain increases. The result is smaller viewing angles and (often) picture hot spots.

The answer is, of course, to use a screen with low gain. Such screens usually have a matte finish and do little to redirect light rays into a particular path as they are reflected. The result is a wide viewing angle both in the horizontal and vertical planes, and no hot spotting.

For many years, video and data projectors were challenged when it came to producing enough light. The first commercially available “portable” LCD projectors from companies like Hitachi, Sharp, and Epson were breathing hard to push out 400 to 500 lumens. These boxes benefited from gain screens, although the projectors themselves often had uneven condenser illumination and internal hot spots.

With time, projectors became more efficient and powerful. Those 30lb. models from 10 years ago have been replaced by projectors one-tenth as light, about one-fifth as small, and two to three times as bright. (Still want that 30lb. projector? Today's models can generate from 3000 to 5000 lumens!)

With a much larger supply of photons to work with, we can forget about viewing angles and hot spots and instead select the best matte-finish front-projection screen that fits our size and budget. But there are other issues to consider, such as color temperature and contrast.

Desktop and installation projectors often come with RGB bias and drive adjustments, which allow us to dial-in a specific color temperature (or get close to it). This is invaluable when judging images created for a specific viewing environment, such as D5400 for film or D6500 for video.

But some of that work can be undone if the front screen adds color bias or creates a color shift to the image. The compounds used in projection screen coatings can have greater response (reflectivity) at high color temperatures than they do at lower color temperatures. The result will be a shift towards a blue tint and not a neutral gray, as red/yellow components of the image are absorbed and not reflected. The screen is, in effect, acting like a high-pass optical filter.

To compensate, such companies as Stewart Filmscreens have introduced coatings that are optimized for high color uniformity, such as the StudioTek material. Screens made from this material typically have a low gain number (1.3 to 1.5) and have even spectral red, green, and blue response with a D6500 source image. Now, the screen is acting more like a bandpass filter, leaving color bias to be controlled from the projector.

Let's see. We've picked out a xenon-lamped front projector, and we've chosen a neutral D6500 screen with low gain for best color quality to go with it. But we can't help noticing the high black levels when no image is present on the screen. Any way to deal with that?

Fixed-pixel projectors using DLP, LCD, and LCoS imagers all suffer from the problem of light leakage and higher black levels than a CRT projector or 16mm/35mm film can provide. In a previous column, I explained how gray-scales are easily compressed on these projectors because their higher black levels limit their dynamic range (See Video Systems February 2003).

There is an answer, and that is to use a screen surface with less than 100% white coatings. By reducing the reflectance of the screen to offset the proportionately higher black levels of the projector, we can improve contrast and create what appears to be a wider grayscale (although it really isn't).

These gray screens appear to be mostly white to our eyes, but indeed have a lower level of reflectance. Stewart Filmscreens has also taken the lead here with its Grayhawk line of front screens, providing a gain figure of .95 as opposed to the more conventional 1.3 to 1.5 gain figure for standard matte finish screens.

TI’s Mustang/HD2 DLP chipset is an example of the improvements projector manufacturers are making to reduce low levels of luminance and expanding grayscales.

As mentioned in last month's column, this is a somewhat elegant fix for the higher black levels of DLP, LCD, and LCoS projectors, but it does nothing to preserve shadow detail that may be lost in the projected image. Assuming a 1:1 transformation between luminance levels of a video signal and corresponding grayscale ramps on the projector, the grayscale will be compressed below about 7% to 10% gray.

As a compromise, you can increase black levels slightly to compensate for this loss of detail (TVs do this with a “black stretch” circuit), then correct back in the other direction by projecting onto a gray-surface screen. Assuming your brightness adjustments don't compact the upper end of the grayscale too much, the resulting images may be satisfactory.

Projector manufacturers are making their own improvements to optical systems with the goal of bringing down low levels of luminance and expanding grayscales. TI's Mustang/HD2 DLP chipset for home theater projectors and rear-projection TVs is one example of this research. The “dark” chips used in DLP Cinema products are another.

LCD and LCoS projectors have to deal with light scattering throughout the individual pixels, which can be tricky. Watch how much light still leaks from behind closed window blinds, and you can get an idea of the problem. Other improvements can be made with multiple light integrators (condensers), re-polarizing components to capture refracted and scattered light, and dark coatings on all non-imaging surfaces. Even better-quality optics will make a difference.

The last consideration in any screening area is control of ambient light. Ideally, there shouldn't be any — the only light should come from the screen. However, for a variety of reasons having to do with safety, etc., it's often necessary to have very low-level lights in the screening room. If so, these lights ought to have as neutral a color temperature as possible. Small fluorescent lamps close to D6500 are available for a reasonable cost.

Under no circumstances should any ambient light spill onto the screen surface. This has the effect of raising black levels and compresses the visual grayscale. (Another good reason not to project onto a wall!) Some DLP front projectors have a small frame area around the active mirrors on the DMD that shows up as brighter than the darkest gray possible, and that can be very annoying to see. A good way to deal with it is to use a screen with a large black frame/border to trap this area and also provide a nice clean edge to the image.

If you wish to use a projector with native 4:3 imaging devices to show widescreen content (and many folks do; there are some really nice 4:3 front projectors out there!), your only options are to (1) use an anamorphic lens, which is usually a clumsy and expensive solution, or (2) overshoot the top and bottom of the image to fit the width of the screen. This means you'll have to mask off the unused image area because it will show up as dark gray, not black.

The surface behind the screen should be painted a matte-finish neutral gray color. A matte finish paint job or drapes will work nicely. Remember, your eyes are very sensitive to slight changes in white balance, which can affect your ability to judge whether flesh tones and pastel colors are rendered correctly. You should also control any light leaking from the projector, although many manufacturers now incorporate internal light baffles that still allow heated air to circulate out of the projector.

Resources

It could take several articles to cover the topics of screens and screen environments in detail. If you want to purse this topic further, why not check out the free literature at these websites: www.da-lite.com educational_materials/(Da-Lite Corporation), www.stewartfilm.com/index2.htm (Stewart Film-screens), and http://www.draperinc.com/Front_projection_screens.htm (Draper Inc.)?