Contrast ratio has been mentioned by LCD manufacturers as a feature determining screen quality. The truth is far from this--contrast ratios are controversial, and like LCD statistics such as response time, remain highly individual and subjective.
Contrast ratio means the difference between the brightest and darkest values a screen can display. Therefore, the format used by most manufacturers is "XXXXX:1", the 1 being the single darkest point on the screen, and the bigger number representing "units" of brightness. This formula has gained popularity since the mainstream introduction of LCDs, whereas during the tube era it was less known.
This is where the controversy begins--manufacturers claim the higher the contrast ratio, the sharper and clearer the image displayed. The logic is simple: a screen capable of better bright/dark definition is also able to do better justice to images displayed on it. However, none of this has been established with any credibility as of this writing.
Just like response time, there's more than one way of figuring contrast ratio, but the Video Electronics Standards Association method seems to be the most popular. This means the numbers shown by manufacturers don't correspond to a common standard, rendering them mostly useless.
Also, the human eye is only comfortable with so much brightness before taking damage. Some screens claim contrast ratios so high, if true they'd burn a person's retinas. Ditto for darkness--our eyes simply can't tell the difference beyond a certain point.
Static and dynamic
Some manufacturers trick consumers by measuring the difference between extreme brightness and extreme darkness at different times and based on completely separate image sources.This contrast ratio is based on the absolute maximums achieved by the screen, but not at once and not as part of one image--making the dynamic ratio very deceiving, as it doesn't exist in actual usage.
Static contrast ratios are more honest, reporting the difference between bright and dark spots within the same image at the same time.
Regardless of what manufacturers claim, contrast ratio has no impact on color quality, resolution, response time or any other aspect of an LCD's operation. The only reason to even consider contrast ratio is dark areas of an image. If the contrast ratio is low, the overall impression of the screen will be that it's too dark and murky, making it hard to make out detail.
So what's a good number?
Based on everything said in this article, there's no single number to keep in mind. However, from experience with older LCDs, anything under 500:1 is too dark and not pleasant to look at, while the 800:1 or above range is fine. Screens claiming 20,000:1 and such should not be taken seriously, as these numbers don't represent something a consumer's eye can even discern.
The truth test
Any screen made after 2006 or so can be trusted to have sufficient contrast, as all of them use essentially the same technology. This means contrast ratio is largely moot and can be written off as marketing fluff. The only way to get an impression of a screen is to test it in person. Fiddle with image settings, watch video from different sources and do so over at least a few minutes, to allow your eyes to adjust.
Don't take contrast ratio numbers at face value. They are nowhere near as authoritative as figures for resolution, and have very little practical value.
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