Friday, December 29, 2006

Contrast Ratios, Pt. I

If you follow HDTV reviews in magazines and online, you come across terms like black level and contrast ratio. They affect how dark the dark parts of the picture seem, and how bright the bright parts seem. But what do these terms really mean?

The eye-affecting power of light coming from a TV screen (technically, it's called luminance, which is the luminous intensity per unit area of light being radiated in one particular direction) can be measured in either foot-Lamberts (ft-L) or candelas per square meter cd/m2, also called nits. One ft-L equals 3.4262591 cd/m2.

Ideally, when a TV displays a full-field black input signal, it ought to have zero luminance. No TV can do that, though, not even a studio-monitor CRT. When its brightness control is properly adjusted, a TV will exhibit the lowest black level it can, consistent with retaining the visibility of near-black shadow detail. That minimum luminance produced by a properly adjusted TV when it receives a reference black input signal and displays it in a darkened room is its measured black level.

Once you know a TV's black level measured in ft-L or cd/m2, you theoretically know how inky its blacks can get. But you also need to know how bright its whites can be. Peak white luminance is measured in the same way as reference black luminance, except that a full-field white test signal is used instead.

You can then derive the TV's contrast ratio from those two numbers. It's the peak white luminance divided by the reference black luminance, expressed in the form of a ratio. For instance, if peak white luminance is 40 ft-L and the black level is 0.02 ft-L, the is 2000:1. A TV's contrast ratio is sometimes referred to as its dynamic range.

The full on/full off contrast ratio is a misleading figure, though, for at least two reasons. One, most HDTVs can't match such a best-case contrast ratio in any single program scene. Two, the human eye at any given instant can't handle anything like that nominal 2000:1 contrast ratio — much less the 10000:1-and-higher figures being claimed by some HDTV makers.

What's really important, scene by scene, is not the full on/full off contrast ratio that is obtained when peak white and reference black signals are input one at a time and then compared. It's the ratio between peak white and reference black as reproduced in a single scene that counts.

The standard way to measure this is to input a four-by-four checkerboard pattern with alternating white and black rectangles. The ratio between the luminances of the white and black rectangles gives you the so-called ANSI contrast ratio.

This ANSI measurement measures the "simultaneous" contrast ratio of the TV, where the full on/full off measurement tells you its "sequential" contrast ratio. The simultaneous contrast ratio is sometimes called the "static" contrast of the TV, and the sequential contrast ratio is its "dynamic" contrast.

For various reasons, ANSI contrast ratios are typically much lower than full on/full off contrast ratios. A TV that has a nominal 2000:1 full on/full off contrast ratio may have only, say, a 144:1 ANSI ratio.

Or, to take a real-world example, the Sony Bravia KDL-40XBR2 HDTV that I recently purchased has, according to Sony, a best-case contrast ratio 7000:1. That figure represents its dynamic or sequential contrast ratio. Yet, says Sony, "a more stringent method that measures the amount of black and white levels that can appear on the screen at the same time" — i.e., the ANSI static or simultaneous method — measures the contrast ratio at 1300:1.

PC Magazine's review of the Sony
KDL-40XBR2 measured the latter figure at 1205:1. When the backlight of the TV was dropped from its maximum to the minimum level and the TV's peak white levels were "reduced for dark-room viewing," that figure became "a still impressive 550:1."

Why is the ANSI contrast so much lower than the full on/full off contrast? Unavoidably, light from bright areas of the screen will pollute dark areas. That can happen in any display technology, due to light "flare" within the image-forming device or reflections in the optics between it and the viewing screen — not to mention light bouncing back to the display screen from the room itself.

In the next part of this series on contrast ratios, I'll discuss why they're so important.

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