Typically, the contrast ratio is limited by the maximum brightness emanating to the viewer’s eyes divided by the minimum light from the same screen surface with black content or even switched off. A screen can’t get any blacker then when it is switch off, but even when it is off, it can reflect ambient light in the viewing room. So in your “real world living room” your screen will never get darker then it is when it is switched off with the ambient light you will typically use to view your screen. There have been a number of innovations to help screens reflect less ambient light beginning with black surrounding the phosphor dots on the CRTs. The best I have ever seen is the rear projection consoles with a flat lens/screen. In that case, the ambient light falling on the screen surface is conducted through to the inside black box and absorbed, rather then reflected and dispersed to the viewer.

When considering the factors in the black contrast, you might consider the light from the screen image as it reflects off the surroundings in the room and back to the screen to illuminate it. If the viewing room has light colored walls, carpet, drapes, or furniture, then those will effectively reduce the screen contrast in all cases except the rear projection lens system. I designed a rear projection system in a church sanctuary using a special “screen” which was really a Fresnel lens on the inside surface and a lenticular lens on the front viewing surface. This has the feature to gather the projected light from the projector, columimate it as it passes through the screen and the lenticular structure disperses it into the seating/viewing area. This sanctuary not only has white walls, but a cove along the back with florescent lamps washing the wall in light! Only glass windows and outside light could have been worse. However, even with the wall-wash light on, the video image is very viewable from the seating area, including the main floor and the balcony. Because of this ‘expensive’ lens/screen system, super bright projectors were not required and the less expensive projectors in the 2000 lumen range have worked very well on this 8 ft x 6 ft projection screen.

Front projection screens also present variables in how they effect contrast ratio. Screen gain is an important factor in directing the projector’s light back to the viewer’s position. You might think at first, that you should select the highest gain available, but not so. Gain comes at the expense of viewing angle, and if the gain is high, the project needs to be placed very close to the viewer’s head position to get the maximum reflected light back to the viewer’s eyes. Still, this may produce a bright spot near the center and dimmer towards the outsides of the screen. If you don’t get the light from the projector to the viewer, you’ll loose contrast ratio as well.

Some of you have asked “what’s the bottom line in selecting a TV.” The reasons this is not so simple are that there are various factors and values that go into such a decision:
* how much can you afford,
* how is your room decorated,
* what time of day will you do your critical watching,
* how much outside light can you control,
* what kind of program material do you watch,
* do you care about how efficient your system is,
* how much resolution do you require,
* how big of viewing area do you wish to have,
* and how good are your eyes and mind at discerning these values???

Our eyes are very adaptive, especially when it comes to color and brightness, as well as other factors. We don’t do well at seeing something, like a TV in one viewing environment, and comparing it to another.
What you see in a store may be very different in your living room in the daylight.

Recently, I bought one of those DCR LED/LCD 26″ for my kitchen. The sharpness is very good, but I get very annoyed by the dynamic “breathing” of the brightness as the backlight changes during a program. Charley Rose, an interview format with a completely black set except for a table, is very noticeable as the camera is switch from close up to wide shots. I wish I could disable that feature on this set.

Most of what you buy a TV to view is entrainment, and you will very likely be well entertained as long as you can see the subject matter reasonable well. Control and select those factors you can at reasonable cost, and sit back and enjoy the program!

Good write-up, by the way Carlton