What Is a Good Contrast Ratio on an LCD Television?by Steve Lander
One of the most heavily advertised statistics for LCD flat-screen televisions is the contrast ratio. Usually made up of improbable numbers, they purportedly reflect the difference between the brightest white that a set can display and the darkest black, with a 1,000,000-to-1 set showing whites that are a million times brighter than blacks, and a 10,000-to-1 set showing whites that are 10,000 times brighter than blacks. However, between the fact that contrast ratios aren't measured consistently across brands and that the eye perceives a 1,000-to-1 contrast ratio, which is less than any new LCD TV, the statistic is meaningless, so there's no magic, good number.
While television sets can, through various measures, have very high contrast ratios, the way that your eye works limits the contrast ratio that you can perceive. While the human eye supports a 1,000,000-to-1 contrast ratio, it can't see one million shades at once. At any given instant with a given pupil size, your eye's contrast ratio is approximately 1,000-to-1. The eye gets its 1,000,000-to-1 ratio by adjusting its sensitivity for daytime or nighttime conditions, which has some similarity to how LCD TVs increase their contrast. Another factor is that your eye responds to changes in brightness differently from display devices. Your TV is a linear device, so if you increase the light by a given amount, the image gets brighter by an equal amount. Your eye is logarithmic, so the brighter that an image gets, the more light it will take for your eyes to perceive a brightness change. For example, when you read a book in bed under a small light, you see the black text on the white paper almost exactly the same way you see it when you read on a beach on a sunny day, even though the light on the sunny day is hundreds of times brighter.
Components of LCD Images
LCD TVs are made up of two parts. They have a lighting element that's typically a cold-cathode florescent, or CCFL, tube or an array of light-emitting diodes. This backlight shines through the LCD panel, which is a clear piece of glass that gets tinted based on the picture that gets sent to it. You can think of it as being like, in the case of 1080p LCD TV, a stained glass window made up of around 2,000,000 pieces of glass that are always changing colors. With this in mind, there are two ways that the TV's brightness gets controlled -- through changing the color of the panel or through changing the intensity of the light.
Native Vs. Dynamic Contrast
Imagine a stained glass window on a sunny day. The clear portions shine brightly, while the pieces of glass stained black still pass some light through. This is the equivalent of a TV set's native contrast. Most TV manufacturers measure and advertise their sets' dynamic contrast, which is the brightest white it can produce with the backlight turned up compared to the darkest black it can produce with the backlight turned all of the way down or off. In other words, it's like comparing the brightness of a clear segment of a stained glass window at noon to the brightness of a black segment of a stained glass windows at midnight on a moonless night. When you compare it that way, the contrast ratio is much greater, even though it has nothing to do with real-world conditions; that is, you'll never be seeing a stained glass window at mid day and midnight at the same time.
ANSI Contrast and Real-World Conditions
Image scientists who really want to understand a display's contrast will measure an American National Standards Institute contrast rating. ANSI contrast gets measured by putting a black and white checkerboard image on the screen and comparing the average brightness of the black and white squares as they're perceived in the room. This not only takes into account a set's native contrast, but also the amount of light that's in the room and that gets reflected by the screen. In the real world, you usually can't see a set's blackest black since its screen picks up light from the surrounding room. As such, contrast ratios are even less important unless you're be watching the set in a perfectly dark environment like a dedicated home theater.
Technologies That Matter
There are a couple of technologies that can make a big difference on an LCD set's actual perceived contrast. Sets with local dimming LED backlights can actually dim a portion of the backlight to correspond to dark areas of the image. For instance, if you were showing an image of a freshly-plowed field of soil on a sunny day, the LED backlight behind the bottom of the screen could darken to increase the difference between it and the bright sky. These sets tend to be more expensive. In addition, LED backlights frequently offer better real-world contrast performance than the CCFL backlights in older and less expensive sets. Finally, although it's not a technology issue per se, higher quality sets typically have better panels that do a better job of blocking light when they're showing dark colors. They also have better contrast.
Zebras and TVs
One practical way to test a TV's real-world contrast ratio is to look at a good photograph of a zebra or some other extremely contrasty image that has a roughly equal mix of light and dark areas. TV's with good real-world contrast performance will show you details in both the dark areas and in the light areas. If you can see the grain of the zebra's hair in both colors of stripes, the TV has excellent real-world contrast.
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