Only Half Of The Equation
Are gray screens the key to better contrast and grayscale rendering?
Pete Putman, CTS, ISF
The problem is that the projector is only half of the system. Without a reflective (or transmissive) surface, a projector is simply a flashlight, a heat generator, or an elaborate lighting instrument. Although the projection screen industry hasn't had any distribution wars or image quality issues to write about, it's hardly been sitting on its hands over the past decade — a time during which the first low-resolution microdisplay projectors that struggled to pump out more than a couple hundred lumens fully came of age.
Back in the good old days, a good projection screen was as essential to the performance of those first front projectors as your parents were to you when you first learned how to ride a bicycle. Now that you're all grown up and zooming around in a car, they're content to stand on the porch and wave goodbye as you head off down the road.
Not so with screens! Just because you can purchase a 2,000-lumen, 6-pound XGA projector for less than $1,500 doesn't mean you can project its images on any surface and expect to get good results. Indeed, the projection screen industry has already recognized the shortcomings of these high-brightness “mighty mite” projectors, and adjusted their product lines accordingly.
A decade ago, the talk was of high-gain screens to capture and reflect every one of those precious lumens toward the viewer. No one worried about color temperature or hot spots. For that matter, no one worried about black levels — the projectors were so dim that just getting a decent white level was the main problem.
Today, we have an abundance of light spilling from projection lenses, and more customers are focused on image quality issues than ever before. Look through sales literature for business and consumer projectors, and you'll see a fixation on contrast ratios. For projector manufacturers, the higher the contrast ratio, the better.
It stands to reason that controlling the levels of “black” on LCD, DLP, and LCoS projectors is the best way to improve contrast ratio, but there's only so much a projector manufacturer can do. For example, light scattering within liquid crystal layers is a fact of life, and leads to refraction and phase cancellation. That, in turn, adversely impacts black levels.
Enhancements such as auto iris controls have made some improvements, but an even bigger helping hand is available from screen manufacturers: gray surface projection screens. The science behind a gray surface screen is quite simple. Instead of using a 100 percent white reflective surface, the reflective properties and screen gain are actually slightly reduced to take advantage of all that light coming from the projector.
It's a yin-yang design. If the lowest black level the projector can produce is actually 5 percent above black (typically, it's higher), then compensate in the screen by making it 95 percent reflective. To the naked eye, the screen still appears to be white. But the images projected on the screen appear to have richer blacks and low-level shades of gray.
Gray screens have proven to be particularly effective with DLP projectors, which are currently the champs at low black levels. But they also work well with the new breed of high-performance LCD projectors from Sony, Panasonic, and Sanyo — all of which incorporate manual or dynamic iris systems.
However, there's one big caveat: grayscale rendering. Just because a projector can create low black levels by selective irising doesn't mean it can somehow magically improve grayscales and image dynamic range.
Imagine a CRT projector and LCD projector mounted side-by-side. Both have been adjusted for best grayscale, meaning the lowest possible black levels and highest possible white levels without crushing (compressing) the grayscale at either end. After calibration, the LCD projector measures 500 lumens, while the CRT projector is clocked at 150 lumens. There is a 3.3X advantage in brightness to the LCD projector. Next, a multistep grayscale pattern is projected on both machines, and a light meter shows 0.5 lux for the lowest level of black on the CRT, resulting in a contrast ratio of 300:1.
However, while viewing the grayscale ramp on the LCD projector, that same black bar —which is almost indistinguishable from the next higher step on the grayscale pattern — produces a reading of 5 lux or 10 times as high as the CRT reading. That results in a contrast ratio of 100:1 for the LCD projector, or one third of the CRT projector's performance.
If a movie with lots of shadow detail and a high contrast ratio is played through both projectors, more of that shadow detail will be evident on the CRT projector, while dark scenes will appear to have a muddy, washed-out look on the LCD projector. While switching to a gray screen will decrease overall reflectivity and drop the level of that washed-out gray, it won't do a thing to improve the contrast ratio (white levels are reduced, too) or grayscale rendering (shadow detail is still lost).
The only way to improve the dynamic range of the grayscale and get better black levels is to somehow dynamically modulate the light from the projector, using the instantaneous dynamic range of the original movie at any given instant. If the dynamic range of a scene falls within the limits of the LCD projector, then the iris/gray screen combo can work wonders.
If, however, the dynamic range of a given scene exceeds the capabilities of the projector, then detail is going to be lost at either the high end or low end, depending on the settings of the brightness, contrast, and Gamma controls. Granted, the pictures may look better with deeper blacks, but the projector isn't showing everything it could be.
After all is said and done, is a gray screen worth the investment? Certainly — particularly if you have a bright LCD, DLP, or LCoS projector and don't enjoy watching those washed-out black levels, or if your projector doesn't offer an auto iris system. But you could probably accomplish the same thing by reducing the overall brightness of the image by 5 to 10 percent, which in many projectors can often improve their dynamic range.
Pete Putman is a contributing editor for Pro AV and president of ROAM Consulting, Doylestown, PA. Especially well known for the product testing/development services he provides manufacturers of projectors, monitors, integrated TVs, and display interfaces, he has also authored hundreds of technical articles, reviews, and columns for industry trade and consumer magazines over the last two decades. You can reach him at firstname.lastname@example.org.