Contrast Ratio: More Than Meets The Eye
We talk a lot about contrast—but what is it really, and why does it matter?
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There are many debatable facets of display technology: How close do I sit? Do I calibrate for color accuracy, or luminance level? Does color temperature really matter? Is this thing even on?
In truth, whether you want hyper-saturated reds like an eyeful of Martian dust, or a bottomless trench of a black level that no light can escape, is—remember to breathe—a matter of subjective preference. Each individual human being has slightly different cones and rods in his or her eyes, and thus, what looks like a perfect picture to me in X room, at Y time of day, with Z prescription glasses, might not be perfect for you. While there are guidelines for achieving a perfect picture, these guidelines must be adapted to the display's capabilities and the constants of its environment—not the other way around.
There are objective aspects related to a display's efficacy, however, and contrast ratio—despite being initially easy to understand—is also one of the most heavily abused spec points throughout the industry. For the last few years, really, display manufacturers have been touting ridiculously larger and larger ratios that their products achieve: 10,000,000:1, 20,000,000:1, ad infinitum. The truth is a little softer, a little more subtle, and—ironically—a little less black and white.
What is a contrast ratio?
While used primarily to describe an aspect of light-emitting electronics like televisions and computer monitors, "contrast ratio" is, simply, the ratio of an element to a structurally different element. If you counted all the books in a library, you could create a ratio of dog-eared pages to flat pages within the books (please don't do that). The word contrast here simply denotes that one element's existence both implies and relies on the existence of the opposing element, and the contrast that they create when conceptualized simultaneously.
For televisions, contrast ratio is measured by dividing a display's peak luminance by its minimum luminance—the most light it can "create," divided by the least. Believe it or not, televisions are much better at pumping out bright light than they are at creating a very small amount; the goal remains to have a large amount of differentiation between peak and minimum.
The problem with measuring contrast ratio lies in the lack of standards by which it is measured. There can be static contrast ratio, which is a measurement of minimum luminance and maximum luminance taken together. Marketing will also often boast of a huge dynamic contrast ratio, where the peak output and minimum luminance are taken in unfairly advantageous settings to boost them. So what is the real contrast ratio? It's the one you actually see.
Natural contrast ratio
Here at TelevisionInfo, our crafty scientists and reviewers have penned our own test to determine a television's "natural contrast ratio"—in short, the contrast ratio that it most consistently creates. All content displayed on a TV's screen, except for something still like a photograph, creates a naturally changing contrast ratio, depending on how much of the screen is at peak brightness, at minimum luminance, and everything in between. Our "natural contrast ratio" measures a set percentage of average picture level (APL) that is inversely proportionate to the black level measured. On-screen content with an APL of 20% refers to the amount of the picture that is "peaked"—meaning the other 80% of the screen will be at a lesser luminance. It just so happens that this number also corresponds to "most content" well—rarely is APL at 50% or more, unless you're watching a documentary of Antarctica.
This method differs heavily from the ANSI checkerboard—probably the most common industry standard for measuring static contrast ratio—in both the APL measured and the number of backlights triggered while the TV displays the image. Due to plasma panel technology, a contrast test measuring 20% APL (against 80% black level) is a more realistic measure of a plasma TV's practical performance. As confusing as it may sound, 20% average picture level is an "average average picture level" (we promise never to write that again).
Why does it matter?
Contrast ratio is perhaps the most interesting aspect of the displays industry—at least, to me. Just as the concept of warm denotes the concept of cool, the concept of bright denotes a necessary darkness, an unavoidable "end of bright," that gives luminance a purpose. All of the research and technology that has gone into the display industry hangs pendulous upon a single idea: That a human eye will be there to perceive the product, a human eye that has evolved to perceive objects as bright as the sun and as dark as the far side of the moon. At their most noble, televisions are a way to approximate real life, to illustrate what something really looks like.
Contrast ratio becomes important when we consider not only the peak and minimum luminance, but the levels of detail between them. The larger a television's contrast ratio, the more detail it can afford its entire 256-step intensity scale (called the greyscale) between absolute black and peak white. This added detail allows for smoother gradient between neighboring steps—which is severely important when attempting to capture and approximate real life. After parallax, image gradient is what helps us see depth. The fade of a shadow beginning at the visual edge of a basketball and seamlessly darkening into black is what tells us that the object is round.
No one measures contrast ratio of non-digital light sources—at least, not to my knowledge—but to do so would reveal a lot about the nature of the numbers we come up with after testing a TV. For example, the difference in perceived luminance between a basketball and its darkening edge would be a consistently increasing number: 3:1, 10:1, 27.5:1, 100:1, until a "maximum" was hit, wherein the brightest part of the ball's orange hue (a naturally luminous color, for containing yellow) was compared to the area of the object shrouded most deeply in shadow. However, if only this full contrast were present (peak against minimum), the image or object would appear two-dimensional. It is all of the contrast ratios from 1:1 to 13,000:1 that, for example, make a plasma TV's image look so incredible and realistic—because that is closer to what we see in the real world.
In truth, the human eye is more sensitive to fluctuations in luminance than we are to differences in color or hue. At very low light levels, our night-vision takes over, and we actually lose the ability to perceive color at all. Fortunately, televisions lend all of their light-producing abilities to the colors they create, meaning all of these little details about contrast ratio also apply to red, green, blue, and all of the colors they make when combined. For this reason, contrast ratio is perhaps the most important aspect of any display, be it tablet, smartphone, or TV.
Bigger is better
The conclusive evidence as to why TV contrast ratio is important comes down to the amount of "breathing room" a television can give its on-screen content. You could watch the same movie side-by-side on two different TVs, and have entirely different aesthetic experiences, because one TV has twice the contrast ratio as the other. A star sitting high in a cloudless sky is most impressive when the star's imminent brightness is framed by the inversely positioned darkness; because contrast ratio handles both ends of the light-dark spectrum, the larger contrast ratio is always going to look more realistic.
Regardless of actual luminance levels, a very bright TV with a narrow contrast ratio will also have a bright minimum luminance, and be unable to create a truly black backdrop for the star to shine against. Conversely, a very dark TV with a narrow contrast ratio will have dim, greyish peak luminance, and be unable to realistically create the massive luminance of the star. Only by achieving both realistic darkness and realistic brightness is a television poised to properly recreate real life.
For this reason, we test the contrast ratio of every TV we come across, and we test luminance differentiation in the most realistic, natural way possible. It is a very important score to the final rating a TV receives after testing is complete, and you'll always find our contrast results on the Science page of any TV review.
Hero image: Bonito Club, Flickr [CC-BY-3.0]