Contrast Ratio: How Important Is It When Selecting an HDTV?
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By Adrienne Maxwell
As you shop for an HDTV or projector, prepare to be bombarded by a litany of technical specs. Should you give more weight to one particular spec over all others? Some will insist that a display device's contrast ratio is the de facto performance indicator. Contrast ratio is an especially big selling tool for marketing and sales types who like to have solid numbers to throw around. The question is, just how much importance should you give to this number when selecting the right display for you? Our answer: not very much.
It's not that contrast ratio isn't an important performance parameter for a display device. Contrast ratio is defined as the difference in brightness between an image's brightest white and its darkest black; for instance, a contrast ratio of 5,000:1 suggests that the device's brightest white is 5,000 times brighter than its darkest dark. When you're looking specifically at one TV, its contrast ratio can be a key indication of how rich and three-dimensional the picture will look. The problem is that the hard numbers quoted by display manufacturers are not based on any form of objective measurement criteria, so they're basically meaningless as a tool of comparison between products. Different manufacturers use different techniques, test patterns and room environments to obtain their results and, in all likelihood, will choose a methodology that most benefits their product or technology.
Probably the most common method of measuring contrast ratio is called full-on/full-off: First, you measure black level with a full-screen 0-IRE black test pattern; then, you measure white level with a full-screen 100-IRE white test pattern and divide that number by the black-level number to get the contrast ratio. In the case of flat panels, this method can give LCD an edge over plasma. Plasmas generally have a peak white limiter that limits the panel's brightness with an all-white screen to reduce power consumption; LCDs do not have this feature and therefore can achieve better brightness numbers, which may result in a better contrast ratio (depending on how good the black level is). Plasma manufacturers will likely use a white window for their brightness measurement, as this will be much brighter than a full white screen. Regardless, it's safe to say that most people don't spend their days watching a primarily black or white screen, so this number really doesn't quantify the difference in contrast you'll see with real-world sources.
Full-on/full-off numbers are further called into question when you introduce the new crop of TVs and projectors that manipulate image brightness with an auto iris or lamp and then quote a "dynamic" contrast ratio. By manipulate, I mean that the device automatically alters the image's brightness based on the average light level of the incoming signal. For instance, if a scene is quite dark, the device will cut brightness to the backlight or lamp to produce a deeper black. In a brighter scene, it will ramp up the backlight or lamp. Some devices perform this more effectively than others, but the technique has an inherent problem: you can't forcefully alter one parameter without affecting the other. In a scene where someone is looking at a star-filled sky, for instance, the display will drive down the brightness to make the black sky look darker but, in doing so, will also cut brightness to the white stars, causing them to lose their pop. Also, you might see the distracting fluctuations in picture brightness as you move from scene to scene. Even when auto irises and dynamic lamp modes are well executed, you can't legitimately compare the resulting contrast ratio numbers with those of a display with a non-dynamic light source.
Arguably the best indication of a display's real-world contrast is gleaned from its ANSI contrast. This measurement uses a test pattern with eight black and eight white boxes in a checkerboard pattern that fills the screen. Through measuring and averaging all of the white and black boxes, you obtain a final ANSI contrast ratio. (This pattern is also good for checking brightness uniformity around the screen.) In most cases, ANSI contrast will be lower (sometimes significantly so) than full-on/full-off contrast, which explains why manufacturers seldom quote it on their spec sheets.
Even if all manufacturers used the same testing procedures, they can't all use the same room, and room conditions can have a huge impact on contrast ratio measurements. Factors like ambient light and, in the projection realm, wall color, screen gain and screen size will affect the end result. Manufacturers and reviewers often try to take contrast ratio measurements in a completely dark room; however, introduce some ambient light (which is how most people watch TV) and the viewable contrast can change drastically.
Ultimately, instead of considering a display's final contrast ratio number, we recommend you evaluate the individual black-level and brightness numbers based on your room conditions and how you plan to watch TV. When shopping for a projector, consider your room's lighting. If you plan to watch content primarily in a completely dark room, look for a model with a better black level. How big is your screen? Larger screens require more light output, as does viewing in a room with ambient light. The same is true of TVs. If you're in the market for a TV that you plan to watch primarily during the day in a very bright room, a model with higher light output may look better than a model that has a higher contrast ratio but isn't as bright.
In the end, contrast ratio numbers are only one piece of the whole performance puzzle. Color accuracy, detail, video processing and gray scale (or the display's ability to render all the steps between black and white) are all important, even if they don't fit neatly into a column on a spec sheet. If a display's performance is your primary consideration when making a purchase, then you can't simply look a list of technical specs and draw conclusions. Your best bet is to check out hands-on reviews (if available) of the models you're considering and find a store that will let you demo the products under conditions that suit your needs. Trust your eyes; they will tell you more than any spec ever could.
Keywords
Contrast ratio, brightness, white level, black level, full on/full off, 0 IRE, 100 IRE, plasma, LCD, auto iris, dynamic contrast, ANSI contrast, ambient light, color accuracy, detail, video processing, gray scale
