HD2+DLP: The Next Wave A New HT Measurement: Contrast Ratio

A New HT Measurement: Contrast Ratio
Believe it or not, contrast ratio is not the only thing you need to know about a projector. Like any measurement, it shouldn't be taken alone; in many cases, it shouldn't be taken at all. The human eye can, at best, discern a contrast ratio of a few hundred to one. So, even a display with a claimed 800:1 contrast ratio has more than enough contrast to appease the eye. There's more to it than that, but that's a discussion for another article.

So, what's with a contrast ratio of 2,000:1 or even 3,000:1? Well, it's misleading at best and a lie at worst. Unlike most product stats, there's no regulation for how a company measures a display's contrast ratio. One company could measure a full-white field versus a 0-IRE signal, while another company could use a 100-IRE window versus no signal at all. The American National Standards Institute (ANSI) has defined one relatively reasonable method that many manufacturers use, but it is by no means mandatory.

From this point on in our display reviews, we're going to cut though all of that and offer you a series of measurements consisting of contrast ratio, lowest and highest light level, and ANSI contrast all in one section. Don't worry, we'll still include the color-temperature and color-gamut graphs we've given you all along.

Decoding the New Measurements
For our reviews, we will define contrast ratio as the ratio of the light level produced by a 100-IRE full field divided by the light level produced by a 0-IRE full field. The number inside the black box is the total light output (in foot-lamberts) that the display produces when supplied with a 0-IRE signal from a DVD (see the measurement numbers in this feature). This represents the darkest the display will get when it's active and fed a signal; it should be as close to zero as possible. With projectors, we use a Da-Lite Da-Mat 1.0-gain screen that measures 87 by 49 inches (93 inches diagonal). If a display has good DC restoration, the measured black level will be similar to the black level the display produces when there's other video material on the screen.

The white box (second number) is the display's total light output when displaying a full-white (100-IRE) field in the same mode that produces the listed black level. (The mode chosen will be the one with the best contrast ratio.) Now, I'll be the first to tell you that a full-white field almost never happens in actual source material (for that matter, neither does a full-black field); for this measurement, though, it's what we're going to use. Why? Because we have to use a full-black field (a black window just wouldn't work), and this is its true opposite.

Doing the measurement this way is like testing an amp's maximum power output: It may not reflect the most real-world scenario, but it does give an overall view of the device's output. Using a full-white field will affect plasmas the most, as they are built to have less light output on a full field compared with a 100-IRE window. If there's a difference between the windowed measurement and the full screen, we'll list it in the measurement text.

ANSI measures contrast ratio using a 16-box checkerboard pattern on the screen, so we'll do that, too. This is done by dividing the average level of the eight white boxes by the average level of the eight black boxes.

If a display has different settings that affect contrast ratio, we will test all of them and use the best one for both contrast-ratio measurement tests. Most importantly, all contrast-ratio measurements will be done after the display is fully calibrated to as close to D6500 as it can be.

To conduct these tests, we're using a Konica/Minolta LS-100 light meter, the most accurate tool for the job. We'll still be using our Photo Research PR-650 spectroradiometer for color temperature and color measurements, as it's the best tool for that. Questions? Send us an e-mail at htletters@primedia.com.

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