What is TV contrast ratio? (original) (raw)

Contrast ratio, put simply, is the difference between the brightest a TV can be, and the darkest.

More so than color ( quantum dots or otherwise), resolution ( 4K or otherwise) or other aspects of picture quality, contrast makes the difference between a good- and a bad-looking image.

TVs with the best contrast look best to casual viewers and usually get the best reviews. And beyond the capabilities of the TV, room lighting, screen reflectance, picture settings and the content itself all affect the actual contrast ratio you see on screen.

Since contrast ratio is one of the most important and easily visible aspects of picture quality, it's vital to understand. So here's everything you need to know.

Before we begin, however, I'd like to mention an article I wrote four years ago called " Contrast ratio (or how every TV manufacturer lies to you)." That's one of my favorite articles I've ever written, but it's rather out of date, in part because TV manufacturers rarely report numeric contrast ratios anymore (and that's a good thing). So feel free to check it out for nostalgia, but I'll cover everything from that article, and a lot more, right here.

The basics

Like I said in the intro, contrast ratio is the difference between the brightest a TV can be, and the dimmest. There are myriad ways to measure light output, but for now we'll go with footlamberts because CNET is mostly based in the US and I know how much it annoys the metric pedants (fun fact, The Metric Pedants is the name of my next band).

Let's say a TV, while showing an entirely white image, puts out a bright 100 footlamberts (fL, or 342.6 cd/m2 -- see, just kidding, I love metric). When showing an entirely black image, it glows a dim-but-still-visible 1 fL (or 3.426 cd/m2). This would mean the TV has a contrast ratio of 100:1.

This TV would look terrible. Like, easily the worst you've ever seen. Something like this:

Geoffrey Morrison/CNET

The most common word used to describe a TV that looks like that is "washed out." That seems pretty apt.

Most TVs these days have a contrast ratio of around 5,000:1. Some are a lot more, but we'll get to that later. Let's use that as a baseline, and say that TV looks like this:

Geoffrey Morrison/CNET

Not bad, right? The dark parts are dark, the bright parts are bright. Well, sort of. Obviously I can't magically manipulate your screen to display what different contrast ratio TVs look like, so I'll have to use images that represent them.

In most comparisons I've done, and that others have done, with TV reviewers and enthusiasts alike, the TV with the best contrast ratio (as long as it doesn't mess up anything else) almost always wins. That's especially true in side-by-side, dark room comparisons like those used in CNET's TV reviews.

It's also the most obvious aspect of picture quality. If the colors are a little bit off, or if there's extra resolution, it isn't as instantly obvious without another TV nearby to compare to. A TV with a great contrast ratio "wows" regardless.

Dynamic vs. native

In the discussion of contrast ratio, a distinction has to be made between native contrast ratio and dynamic contrast ratio. Native contrast ratio is what the panel alone can do, whether OLED or LCD (or plasma, may it RIP). Native contrast ratios for LCD have improved quite a bit in the last few years, and rarely is it a serious issue (the exception is IPS panels; see below). Plasma's biggest advantage over LCD was its better native contrast ratio. That crown is now on OLED.

Dynamic refers to the practice of varying the LED backlights behind the LCD screen to achieve better contrast. The intensity of the LEDs behind the LCD screen are dimmed in dark scenes and brighten when showing bright scenes. With local dimming, the dark parts of the screen can get darker, while the bright parts stay bright. Good local dimming (more on that in a second) can make LCD TVs perform much better.

Local dimming vs. 'local dimming'

There are two types of local dimming, too. The first is "full-array" or "direct," which means the LEDs are arranged behind the screen, and the best can dim relatively small sections. How small depends on the total number of "zones" (the more the better), though most companies don't release this info. Roughly, it might look something like this:

Geoffrey Morrison/CNET

Notice how the sky is still dark, but the areas around the fireworks and Bolte Bridge aren't quite as dark. That's because the LEDs behind those sections have to remain lit, so you can see the bright parts if the image.

The other type of local dimming utilizes edge-lit LED backlights, where the LEDs are arranged along the edge of the panel rather than directly behind it. It can be effective too, but typically edge-lit dimming delivers worse performance than the full-array method.

Pretty much all local dimming will improve contrast ratio over non-local-dimming models, only the best full-array models can compete visually with OLED (contrast ratio-wise).

There's a lot to this, hence two full articles on it: " LED LCD backlights explained" and " LED local dimming explained."

IPS

Many TVs, primarily those with LCD panels manufactured by LG Display, use in-plane switching. This type off LCD has good color characteristics for off-axis viewing, but far worse contrast ratios than other types. IPS TVs with local dimming can look good, but not always. IPS is a notable exception to the "all TVs have gotten way higher contrast ratios" comment from earlier.

HDR

All of this is building toward the next step in contrast ratio: high dynamic range. HDR is like local dimming taken to the extreme. Ideally it delivers exceptionally bright highlights, paired with deep blacks.

The brightest parts of the HDR image get brighter than on a typical TV, but they're limited to only the bright areas of the image (the whole TV isn't just "brighter," it won't blow you out of the room). So in the example image above, the dark parts would be dark, and the fireworks would actually be really bright, like they would be in real life.

HDR promises to be even more realistic looking than TVs with high native contrast, at least in theory. We'll see, as models will ship this year. Part of their performance boost will be when HDR-enabled content ships, probably with 4K Blu-ray (also later this year).

For more info, check out high-dynamic-range-arrives and high-dynamic-range-dolby-vision-x-tended-dynamic-range-pro-and-beyond

LG showed off an OLED HDR prototype at CES, but it's not a real product just yet (and we don't have much info on it). OLED's brightness is directly related to power consumption, so that's a hurdle that will be interesting to see LG leap.

Bottom line

With all the hoopla about higher resolutions, and even more realistic color, contrast ratio is still king. Thankfully, it has gotten pretty good, and only the cheapest budget models still look washed out.

A TV with higher contrast ratio will always draw the eye as more realistic, presuming the image isn't horribly wrong in some other way. In other words, a 1080p OLED looks better than a 4K LCD pretty much every time.

Will a 4K HDR LCD beat an OLED TV? We'll know soon enough.

Got a question for Geoff? First, check out all the other articles he's written on topics like why all HDMI cables are the same, LED LCD vs. OLED, why 4K TVs aren't worth it and more. Still have a question?Send him an email! He won't tell you what TV to buy, but he might use your letter in a future article. You can also send him a message on Twitter @TechWriterGeoff or Google+.