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Screening the Screens Page 2

Smaller direct-view televisions and rear-projection sets, particularly the new digital sets, often produce many times this brightness level. But on a large screen in a darkened room, an excessively bright image can be uncomfortable to watch, particularly with program material that shifts suddenly from bright to dark scenes and back.

This discussion is important because it will help you choose the optimum screen size and screen gain for your installation with a few basic facts about your projector.

Putting It Together
A projector's light output is usually specified in ANSI lumens. If you divide the ANSI lumens number by the screen area in square feet, you should, in theory, get the light output from the screen in foot-lamberts (for a screen gain of 1.0). In practice, however, lumen specs are often wildly unrealistic.

You'll get more realistic results using the foot-lambert readings we provide in our reviews at UAV. Unlike most of the projector reviews you'll see elsewhere, we tell you the size and type of screen we use in our evaluations. Of course, there may be unit-to-unit variations from different samples of the same projector, and possibly even some variations in gain with different production runs from a screen manufacturer. But if you adjust our measured numbers to account for the difference in screen size and gain between our screen and yours, you should get useful results.

Crunching the Numbers
So how, exactly, do you determine the light output you can expect from your chosen projector in your setup? The following examples will show you.

Example 1: The projector produces a maximum light output, measured at the main viewing seat, of 9fL on an 80-inch-wide, 1.0 gain screen. The measured black level is 0.005fL, for a peak contrast ratio of 1800. (Contrast ratios are often given by their first number only; the ":1" is implied.)

While 9fL is noticeably dimmer than my goal, it can produce a pleasing image. But I'd like a little more punch in the whites, and a little reserve for the inevitable dimming of the projection lamp. If we go to a screen with a gain of 1.3, we'll get a peak white level of 9x1.3=11.7fL at the main viewing seat. The screen gain will also increase the black level by the same percentage, to 0.0065. Of course, we want the lowest black level possible, but this is still a respectable number. Since we have increased both the peak whites and video black by the same amount, the peak contrast ratio will remain about the same.

Suppose we wish to use the benefit of the higher-gain screen to increase the screen size instead of the brightness. We can accomplish this by increasing the screen area by 30%, which means increasing the screen width to approximately 90 inches. Backing off to 87 inches wide (a relatively common screen size) will buy us a little more light output than the original setup along with a bigger picture. But in both cases we lose some or all of our lamp-wear margin.

Do you want a larger but somewhat dimmer image? You get to choose, but you can only get larger and brighter by going to an even higher gain and accepting the compromises this will bring. (There's also the issue of how much you can blow up an image of a given resolution without losing other image qualities besides brightness. More on that further on).

Example 2: On that same 80-inch, 1.0-gain screen, the projector produces a maximum light output of 30fL, with a black level of 0.015fL, for a peak contrast ratio of 2000.

This is an exceptionally bright projector. And the contrast ratio is actually pretty good, all things considered. But for a rich, three-dimensional image and filmlike dark scenes, you need not only a high contrast ratio but deep blacks as well. The best blacks we have ever measured from a digital projector on a screen of this size have been under 0.005fL—one third of the number here. And the results from a good CRT projector would be unmeasurable with our test equipment (a Minolta LS-100 light meter, specified down to 0.001fL).

This Vutec Vision X2 actually puts two separate screens in a single case˜another solution to the multi-aspect ratio problem.We can improve the situation here, though at some sacrifice in peak white output, by adjusting the screen size, the screen gain, or both. Let's change both.

If we use a screen with a gain of 0.9 (many so-called gray screens fall in this range), we'll reduce the peak white/video black numbers to about 27fL/0.0135fL. Again, the peak contrast ratio will remain nearly the same. We still have plenty of light output, so let's enlarge the screen to a size that will give us a black level of 0.008fL—not great, but definitely good for a digital projector. We have plenty of peak light output available, so we'll let that fall where it may with the larger screen.

To do this, we need to increase the screen area to about 1.7 times its present size. We can come very close to this with a width of 104 inches (8.7 feet). Now we'll have a peak brightness of about 16fL and a video black level of 0.008fL. Not bad at all, and you'll still get an adequately bright image when that new projection lamp looses 30% of its light output in a few hundred hours.

I could go on, but I hope the point is clear. You need to consider screen size, screen gain, and the capabilities of the projector when choosing the best screen for the job. Often, some compromise will be necessary. That's where your preferences come in. Do you want maximum brightness or a bigger image? A brighter image or deeper blacks? Rarely will you find just the perfect combination.

Dimming the Projector
In addition to choosing the right screen size and gain, some projectors also offer other means of adjusting their light output. The most obvious are the contrast and brightness controls found on all projectors. But there is really only one correct setting for these with any given projector; a setup DVD like Digital Video Essentials can guide you in this. Once these controls are properly set, they should not be used in an attempt to produce a brighter image or darker blacks (any such effort will just crush out details at the black and white ends of the brightness spectrum.

Other features, like multi-level lamp settings and adjustable irises, can help produce a better picture. And some enthusiasts have used neutral-density filters in front of the projection lens to reduce the light output. (I did this myself in a review of the InFocus ScreenPlay 777 3-chip DLP projector.) But none of these things reduce the importance of choosing the proper screen in the first place.

Just How Big?
There are other factors to consider when deciding on a screen size apart from how it affects the brightness of the image. As an image fills a wider angle in your field of vision, the eye becomes more sensitive to flaws in that image and the elements that comprise it: the pixels in a digital display, the scan lines of a CRT. SMPTE recommends that the screen in a commercial movie theater cover a minimum of 30° of your field of vision.

In my experience, the image quality in a home theater, using a good DVD as a source, begins to deteriorate significantly (that is, its flaws become more visible) when the screen occupies more of your field of vision than that minimum SMPTE recommendation. That 30° angle is reached at about 12 feet back from an 80-inch-wide screen.

The optimum distance can be a little closer with HDTV, but do you really want to reduce the subjective improvement you get with HDTV to DVD level or worse just to get a bigger image? You might, but when you think about it that way, a really big screen doesn't look quite so appealing.

The best advice I can give is the same advice I consistently gave for CRT projectors, even after being chided by readers who know that bigger is always better: avoid the temptation to go for an oversized screen. In addition to the image-quality issue, you should also remember something I addressed earlier: You can count on a significant light reduction from most digital projectors in their first 200-300 hours of use due to the rapid wear typical of a fresh lamp. You'll want a reserve to allow for this. If the screen is so large that the image is barely bright enough with a new lamp, you'll soon be opening a special savings account for your lamp replacement expenses!

The Options
Now that I've covered the basics, there are other options unrelated to image quality that you need to consider when selecting a screen.

I discussed the issue of acoustically transparent screens thoroughly in a recent review of the Screen Research ClearPix2. I'll only point out here that such screens can be useful if you're willing to accept certain limitations. When used with digital projectors, they must be chosen with care to insure that the pixel structure does not interract with the holes in the screen (or the screen weave in the case of a woven design like the ClearPix2) to produce moiré patterns and similar artifacts. Generally, such artifacts are a function of the pixel size—which depends on the size of the image. There are enough variables that you might want to seek the advice of a good custom installer experienced with such screens combined with pixel-based projectors—even better, with the exact projector you plan to use.

There are also screen materials that let you create a custom rear-projection setup, though this tends to consume a lot of space. In this configuration, the projector is often mounted in an adjoining room behind the screen. There are also special mirrors and racks that let you build a rear-projection set that looks like a jumbo version of a commercial rear-projection television. Rear-projection screens are usually a little more forgiving of room light than front-projection designs of similar gain, but otherwise most of the principles discussed here still apply.

Finally, there's the physical configuration of the screen to consider. The most common—and affordable—screen type is the fixed-frame, non-retractible style designed to be mounted on a wall or a stand. It can also be hung from the ceiling near a wall, or even part way out into a room.

A fixed-frame design is the least expensive type of screen, and it performs as well as any other configuration using the same screen material. But unless it's covered in some way when not in use, it will be constantly exposed to the risks of everyday family activities. Screens are delicate and difficult to clean. They can seldom be repaired if damaged and must instead be replaced.

Rear projection usually requires a space behind the screen equal to the projector to screen throw distance. But mirrored racks are available that shorten the depth required. This Vutec XRP uses two mirrors between the projector and screen.

A retractable screen gets out of the way when not in use, protecting it from stray missiles, children, and pets. Most such screens are powered, more expensive than their fixed-screen cousins, and relatively heavy. They can also be hidden in a ceiling soffit, or even in the ceiling itself, though that adds considerably to the cost and complexity of the installation. Professional help is recommended when mounting any screen, but particularly this type. Among other concerns, a retractable screen will wrinkle if not properly levelled.

Apart from the aesthetic improvement and convenience offered by a retractable screen, audiophiles appreciate that it also gets that big, reflective thing out of the way, acoustically, for the times they just want to listen to music.

Then there are screens with adjustable masking. Want to watch 4:3 programming on a 16:9 screen without seeing black bars on the sides? How about a 2.35:1 film without black bars on the top and bottom? You can do either—or both—and more. But bring lots of money. Such screens can often run into five figures. They also tend to be very heavy. Again, leave the installation to the experts.

Just be certain that when you pop for a retractable screen, the amount of black "drop" material at the top of the screen is sufficient to bring the active image area down to an appropriate height for viewing. Many retractable screens are made to order, and the manufacturer can provide the amount of drop you require. If the case is mounted on or near the ceiling, the amount of drop depends on the ceiling height and how high you want the screen. I don't like to look up at a screen, either at home or in a movie theater, so I prefer a fairly low screen that puts the center of the image directly in front of my viewing seat.

But don't forget to allow sufficient space under the screen for your center-channel speaker. If it's a conventional, horizontal center-channel design, you should keep it at least a foot off the floor. Most retractable screens can be adjusted in how far they extend, so you are better off with a little too much drop than too little.

Wrapping it Up
If some of this discussion—particularly the numbers games I had to play—proved heavy sledding for you, here are a few shortcut tips. I still recommend you pay attention to the specifics addressed earlier, but these guidelines should keep you out of serious trouble.

Most digital projectors, particularly single-chip DLPs from most manufacturers, don't have brightness to spare. (The most obvious exceptions are the home theater projectors from InFocus, a company that appears to value high light output above all else.) With these modest-output designs, stick with a screen width of 80 to 87 inches. If the projector has good blacks, you'll definitely want a screen with a small gain (1.3 or so) at the higher end of that size range. I've found such a screen to work well even at 78 inches wide if the projector has very good to excellent blacks. The smaller screen will also give you a reserve to allow for lamp wear.

Four very different projectors that have worked beautifully on my 78-inch-wide Stewart Studiotek 130 screen (gain 1.3): the Sony Cineza VPL-HS51 (LCD), Yamaha DPX-1200 (DLP), Fujitsu LPF-D711W (LCD), and Sony Qualia 004 (SXRD, a variation of LCoS). The Sony Cineza and Yamaha, in particular, have startlingly good blacks for digital projectors even on this small, modest-gain screen.

Very bright projectors, used on screens in this size range, work best with a screen gain of 1.0 or lower.

Don't go too big with the screen or sit too close to it. Don't assume that because a projector is expensive it will automatically work better on a larger screen. Maybe it will, but maybe it won't.

The optimum seating distance differs with screen size. You can get the same impact and picture quality from a smaller screen as you would seated further away from a larger one. The only advantage to the larger screen in this context is that it provides more impact to multiple rows of seating. The smaller screen also requires a less bright—and possibly less expensive—projector.

The projected image will be a little dimmer after a few months of normal use than it is fresh out of the box. Fortunately, our eyes are fairly forgiving of this. But if the brightness is marginal initially (because you've pushed screen size to the max), you will notice the falloff and may find yourself replacing the projection lamp earlier than expected.

If all this sounds just a little too complicated, you can always hire a custom installer to do the heavy lifting for you (literally and figuratively). But it will all be worth it in the end, when you get the best picture your projector is capable of providing.

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