Get the Picture?

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I see it all the time: award-winning home theaters with front-projection video systems and light-colored wall surfaces. It really bugs because the client is not experiencing the full potential of the video system.

Light-colored rooms act as reflectors for the light from the screen. The light bounces off the walls/floor/ceiling back onto the screen, washing out the dark areas of the picture. In simple terms, that is a loss of contrast ratio. It is not caused by a decrease in peak white level, but by an increase in achievable black level. Unfortunately, the human eye is much more sensitive to black resolution than white resolution, so blacks are the worst place to lose contrast.

The ISF has long maintained that contrast rationot color saturation, color accuracy, or even resolutionis the most important element of a good picture. So, for the average person, you should concentrate on good contrast ratio, long before worrying about the number of pixels in the picture. The best projector in the world will still lose tons of contrast if it is handed a room with light-colored surfaces.

To determine whether or not the contrast ratio of a video system is adequate, you have to measure the actual projector/screen/room combination. There are two ways to do it: the right way and the misleading way. The misleading way is to measure a full white field followed by a full black field with a spotlight meter. What you get is the difference between the brightest white and blackest black that the projector and screen can send to a viewers eyes. Unfortunately, that is not what you want, because it doesnt match typical picture conditions. The right way is to measure the difference between white squares and black squares on a checkerboard test pattern. You get to see what the system can do when 50 percent of the image is white and 50 percent is black. This is a much closer match to actual picture content, which is a mix of light and dark areas. The white areas of the checkerboard light up the room surfaces around the screen, which reflect light back to wash out the blacks, simulating the actual contrast ratio with a real picture on the screen.

Dont get the idea that all of the home theaters that you design from now on must be flat black, although it would be the ideal color for a viewing environment. Instead of black, you can use dark neutral tones like dark blue-gray, dark wood tones, etc. Also, pay particular attention to the area directly above the screen. The common white ceiling is among the most deadly enemies of contrast ratio. Also consider the sidewalls close to the screen, and in some cases, the wall behind the screen. Acoustically transparent perforated or woven fabric screens, which are becoming more and more common, let light through as well as sound. While a light reflection from behind the screen is not quite as serious as a sidewall or ceiling reflection, it cannot be ignored. The area behind the screen should be as dark as possible.

What about existing rooms that suffer from poor contrast ratios? It is possible to alleviate some room-related contrast issues by using directional screens (screens with gain) combined with gray screen surfaces, but there are some trade-offs. Increasing the directionality of a screen inevitably reduces the uniformity and tends to show off hotspots. The gray surface also absorbs light, which could potentially drop the screen luminance too much. It seems almost inconceivable that modern lamp projectors could lack sufficient light output, but when youre dealing with a very large, acoustically transparent, gray screen, they just might run out of steam.

Next time you look through the listing of candidates for Best Home Theater awards pay attention to the colors in the rooms. Remember that darker is always better. It may take some work to convince the client or interior designer, but I trust that you have by now developed a keen sense of how to do a sales pitch.

Chase Walton contributed to this article.

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