Why You Need To Know About Gamma on Your HDTV

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ID-100107006.jpgIf you've read a lot of TV reviews (or even just few, actually), you've likely seen mention of a TV's gamma, a performance characteristic that helps determine the overall accuracy of the grayscale. Unless it's an entry-level model, your TV probably includes multiple gamma options in the Picture setup menu, with numeric choices that generally range from 1.8 to 2.6. What is gamma, what do those numbers mean, and which one is the right choice for your system? We're here to answer those questions for you.

The gamma curve dates to back to the days of CRT TV. If you imagine a graph showing the relation of light output (vertical axis) to input signal level (horizontal axis), the ideal result would be a straight diagonal line at a 45-degree angle extending from zero - i.e., 20 percent brightness at a 20 percent input signal level, 30 percent brightness at a 30 percent input signal level, etc. However, that's not how CRT TVs behaved; instead, it produced a nonlinear curve. According to the Imaging Science Foundation, a 50 percent input signal level produced only about 18 percent light output (which corresponds to a numerical gamma of 2.5). Content creators decided to compensate for this by incorporating the exact opposite curve into the source to result in perfectly linear output. That's why you'll often see it referred to as gamma correction.

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In today's digital world, TVs can offer linear output, but the gamma correction existed in so much content that - like many early tricks of the trade - the system needed to carry over to the digital realm. Therefore, TV manufacturers were forced to add gamma correction to make the digital TV act like a CRT TV. For much of the history of digital displays, 2.2 has been the target gamma for the TV to perfectly offset the content and create a linear output. As always, though, the system has evolved, and 2.2 is no longer considered the optimal gamma setting for every situation. The ISF continues to recommend 2.2 for TV watching in a dim viewing environment, but it recommends 2.4 for a completely dark room and 2.0 for a bright environment. How do these numbers alter what's being shown on the TV? Good question.

The best way to describe how gamma affects picture quality is that gamma represents the level of brightness difference between each step in the grayscale, or how "fast" blacks get brighter. The human eye is much more sensitive to changes at the dark end than at the bright end, which is why a correct gamma setting is particularly important for darker film scenes. Picture a grayscale test pattern with reference black and white at each end, which are adjusted by the TV's brightness and contrast controls, respectively. Gamma affects the steps in between. A lower gamma number like 1.8 makes blacks get brighter faster, so the mid-blacks and grays will look lighter. A higher gamma number like 2.4 keeps blacks darker longer, so those same bars will look darker.

Click on to Page 2 to to learn about viewing in light and dark rooms and what BT.1886 is all about . . .

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