Everything you need to know about 4K UHD TV backlighting and local dimming
Stephan Jukic – October 11, 2016
Backlighting in 4K TVs may not be one of those specs which many consumers pay a tremendous amount of attention to but its importance is crucial in affecting the overall display performance of any 4K TV. With this in mind, it’s important to note that 4K TV backlighting systems are quite varied and there are several different designs for the backlights in 4K LCD/LED TVs today. And among these several designs are a couple of core backlighting patterns which are the most common in today’s 4K TVs. All of them have their own distinct characteristics and way in which they affect how well your TV renders dimming and brightness on different sections of its display as you’re watching content or playing games
Then of course there are also other core technologies within the rubric of TV backlighting which you should also bear in mind. These mainly consist of local dimming and OLED display.
However, not to worry, the following guide to 4K TV local dimming and backlighting in both OLED and LCD/LED 4K TVs will cover everything you need to know about how these televisions manifest these technologies and what it means for your home entertainment quality and buying decisions. We’re also going to make some specific TV model and brand suggestions as we move through the following guide. Let’s get started.
First thing’s first, Local Dimming: what it means and how it works
First, before we move onto our descriptions of all the different 4K TV backlighting technologies and arrays, we have to make sure to give you a fundamental understanding of what local dimming is and how it works. This technology is a key aspect of the LED backlighting and OLED display configurations we’re about to cover, either by its presence or its absence in some TVs.
- LCD/LED 4K TV Local Dimming
For LCD/LED 4K TVs, the LEDs behind the LCD panel produce light and it passes through the LCD display to create brightness and allow pixel color to function. In the case of all LCD 4K TVs, the LCD panel itself blocks part of the LED backlight as needed via individual sections of the display’s pixel array. However, this light blocking capacity isn’t perfect and LED backlight can often bleed through the screen when and where it’s not supposed to if the TV is turned on.
This is where local dimming comes in. Some 4K TVs, especially better quality models of the premium type, come with the ability to turn selectively turn LEDs in the backlight array off to enhance darkness beyond the level allowed by the LCD panel’s own light dimming capacity. Local dimming is found in edge-lit LCD/LED TVs, direct-lit models and of course in full-array direct-lit 4K televisions as well. Obviously, the technology performs in radically different levels in each of these backlighting configurations.
Basically, however, the majority of local dimming found in today’s LCD 4K TVs only turns off some LEDs in large zones behind the screen, creating a sort of imprecise level of darkening which results in a “halo” effect around brightly lit objects on a dark background. Most viewers won’t notice this too much for a majority of content (unless they’re looking at a single bright point against a mostly black background in a high contrast piece of movie scenery) but the halo effect is always there because local dimming in LCD TVs is rarely perfectly precise down to the single LED level.
The 4K TVs which deliver the best local dimming performance are full-array backlit models with the technology. In the best of these, the array of LEDs which completely covers the space behind a TV’s LCD display is broken down into numerous zones of LEDs which can then be turned off or on as needed for contrast and lighting of content. This is called Full-array local dimming, or FALD. The more LED dimming zones a 4K TV has, the better (and more expensive) it usually is. The best and most expensive 4K LCD/LED TVs on sale today can even turn off and turn on each individual LED out of hundreds behind a 4K TVs screen but this is a rare and very expensive feature found in only the best models like Sony’s ZD9 4K HDR TVs or Vizio’s Reference Series television models.
In the better edge-lit 4K TVs which make up the majority of models sold on the market, “local dimming” is also found but since it only dims some LEDs along a single edge of a screen, the precision of the technology is usually really low, even in the best models. Basically, premium edge-lit 4K LCD TVs depend on high quality light blocking in their LCD panels much more than on local dimming for the quality of their black level performance.
- OLED 4K TV dimming
In OLED TVs, local dimming is found in its absolute best form though the phrase isn’t even really used to describe what you’ll see with one of these televisions since the very nature of an OLED TV means that it automatically comes with the most precise possible control of brightness and darkness down to the single pixel level. As we explain I better detail further below, OLED 4K TVs have no LED backlights and no LCD display panel covering them. Instead, their individual pixels create light and as a result “local dimming” in an OLED 4K TV can be controlled right down to the single pixel level. This creates a display and dimming performance for onscreen content which completely blows even the best LCD 4K TVs and their backlights out of the water in its precision.
Now lets see how 4K TV backlighting works in both OLED TVs and LCD/LED models. Here we’ll also explain more about how local dimming plays its part in the different types of backlighting systems in today’s 4K UHD TVs.
4K TV Backlighting types: LCD/LED vs. OLED
In the broadest possible sense, 4K TV display lighting breaks down into two types: LED/LCD and OLED. These two mechanisms for illuminating a 4K TV screen selectively are enormously different even though they both work to achieve the same purpose, which is to shine or cut off light passing through pixels on a television screen display surface for the sake of brightening or darken specific areas of the display as needed for the color and lighting patterns of content playing on its surface. We mention color as well here because the RGB (Red, Green, Blue) subpixels of each 4K TV pixel which are activated in different ways to make all the millions of colors your TV displays depend fundamentally on a source of light for their color saturation patterns. Both OLED and LCD/LED 4K lighting technologies perform the above basic tasks in their own varied ways.
We’ve already covered the other numerous differences between OLED and LCD/LED 4K TVs in our guide to both display types, here, so in this guide we’ll focus only on their screen illumination differences.
Between OLEDs and LCD/LED 4K TVs, only LCD/LED models use backlight technology with illumination behind an LCD panel and they are what we’re going to focus on first in this guide since LED/LCD 4K TV display is much more common than OLED on today’s market and deserves the broader explanation.
LCD TV backlighting: Edge-lit, Direct-lit and Full-array
Speaking specifically about LCD/LED TVs, their backlighting systems can be broken down into three main types, which are edge-lit, direct-lit and full-array with local dimming. One or another of these backlighting types will be found in all LCD/LED 4K TVs on sale today depending on TV price and quality. This may be a bit confusing at first glance so let’s look at each type individually.
The vast majority of 4K TVs sold today are edge-lit models and generally speaking, this type of backlighting design is the cheapest and easiest to produce in a 4K LCD TV. However, while edge-lit 4K TV backlighting is fairly affordable per-se, it also comes with a few disadvantages that we’ll get to shortly.
Basically in an edge-lit television model, the LEDs used to illuminate content across a screen are found along the edges of the TV behind its LCD panel. In the very cheapest edge-lit LCD/LED TVs, only one or two of the bottom, top or possibly side edges will have an LED array running along their length for all the backlighting needs of that particular 4K TV. Better 4K TVs with edge-lit displays will on the other hand have all four of their edges covered in LED array strips (all-sides configuration). Generally, the major 4K TV makers are pretty leery of letting the public know which of their edge-lit 4K TVs come with all-sides LED arrays along all four edges and which models only offer LEDs along one or two edges but as a general rule, the all-sides configuration is the best by far at producing local dimming and better black uniformity in a TV display.
The cheaper edge-lit 4K TVs on the market, like Samsung’s KU7000 or Sony’s X800D and X700D 2016 4K TV models offer edge-lit or even direct-lit backlighting without local dimming technology of any kind. They also possibly offer only partial LED coverage along only some of their edges (though we’re not sure) or limited LED spread behind the LCD panel in the case of direct-lit models like the X700D without local dimming. More importantly still, because these TV’s lack local dimming, the precision of their specific illumination or dimming of segments of the screen is not nearly as good as it is in a 4K TV with local dimming.
On the other hand, many edge-lit 4K TVs can still be high-end premium models. They offer excellent dimming quality and even if their local dimming isn’t the best, the high brightness and strong LCD light-blocking performance of their screens still produces both rich precise black levels and bright but specific bright highlights for content. Their black levels and black uniformity can also be excellent thanks to much better LCD panel design. Samsung’s 2016 SUHD HDR TVs are all edge-lit except for the KS9800 flagship model and they are without a doubt superb 4K TV models. The same applies for Sony’s X930D HDR 4K TV for 2016 and the company’s premium 2015 XBR-C Series 4K TVs.
One problem of edge-lit LCD 4K TVs is that the light bleed common to most LCD panels can show itself more strongly where the LED arrays are along the edges. This can ruin black uniformity on the TV screen by creating an ugly sort of edge-glow even in fully black scenes when you’re watching darker scenes in a movie.
Direct-Lit (with and without local dimming)
After edge-lit LCD TVs, direct-lit models are the next most common type found among the more affordable LCD 4K televisions sold today. With a direct-lit backlight panel, the LEDs behind the LCD surface of a TV are arrayed right behind the panel, facing the viewer instead of along the TV’s edges. This design can mean a lot of things but one particular feature it offers is reduced light bleed along a TV’s edges. Also, if a 4K TV has both direct-lit backlighting and local dimming, the precision of the local dimming can be much better than it is in edge-lit 4K television models.
Now, to clarify, in the most basic terms, “direct-lit” is also a phrase used to describe the full-array LED array backlighting we’re going to describe the next segment below. So to distinguish the two, we need to make clear that in cheaper 4K TVs, the direct-lit backlight panels used are a lot different from full-array direct-lit premium backlight panels in that they contain far fewer individual LEDs and in many cases, they also lack local dimming of any kind.
In other words, without local dimming technology, the array of LEDs always stays fully active and only the LCD panel in front of it blocks out light selectively as needed for rendering dimming patterns in content. This is a design often found in cheaper 4K TVs and it can also often mean that a TV display offers even worse contrast than an edge-lit model.
Even in direct-lit 4K TV models with local dimming, the quantity of LEDs arrayed directly behind the LCD panel is really limited and offers only a few really big local dimming zones. This means weak local dimming precision for content and plenty of halo effect on bright points over a dark background.
For the reasons above, cheaper direct-lit backlight designs with few LEDs are actually less common than edge-lit backlighting in many premium edge-lit 4K TVs. Models which use this technology include Samsung’s KU6300 and KU6500 2016 4K TVs and Vizio’s cheaper E-Series and D-Series 2016 4K models.
Full-array direct-lit LED backlighting with FALD
Full-array direct-lit LED Backlighting, or full-array backlighting as it’s more commonly and simply called, is the absolute star of 4K LCD/LED TV backlight design. As we said above, direct-lit and full-array direct-lit backlight arrays use the same basic design strategy of putting LEDs right behind an LCD panel, but the full-array version is basically direct-lit backlighting on steroids and it’s usually found in the very best flagship 4K TV models from all of the major brands.
With full-array backlighting, hundreds or even thousands of LEDs are arrayed behind a 4K TV LCD panel and in all full-array LED TV designs, local dimming is pretty much a guaranteed feature. Even more importantly, all of these thousands or hundreds of LEDs are divided into numerous local dimming zones in which small groups of individuals diodes can be turned on or off as needed for onscreen content. In fact, the very best 4K LCD TVs with full-array local dimming (or FALD, as it’s also called) can even activate and deactivate individual LEDs as needed for the best possible levels of black performance, contrast, dimming/brightness precision and overall picture performance possible in an LCD/LED 4K TV today.
To give you some examples of how many local dimming zones today’s best premium 4K LCD TVs offer: Vizio’s Reference Series models with HDR come with an estimated 320 local dimming zones across their displays. Samsung’s flagship KS9800 full-array backlit HDR 4K LCD TV reportedly offers about 120 local dimming zones and Sony’s X940D 2016 X-Series TV comes with an estimated 96 local dimming zones. Each of these dimming zones is made up of a group of LEDs which can be turned on or off together as needed for content on the screen and the technology as a whole offers a dramatic improvement over edge-lit local dimming or even basic direct-lit local dimming when it comes to picture quality and picture depth.
Taking things even further, Sony has now unveiled their new Z-Series 2016 4K ultra-premium LCD TVs, which supposedly allow each individual LED out of thousands behind their LCD displays to be activated or deactivated individually and pointed more narrowly at the LCD panel in front of it for a much precise level of light control. This is something we’re going to see more of in the coming years with LCD 4K TV displays because it almost (but not quite) manages to effectively compete with OLED.
OLED 4K TV display lighting
Finally, we come down to OLED TV display lighting. Maybe you noticed that we said “lighting” instead of “backlighting”. This is for a good reason, namely because OLED 4K TVs have no backlighting built into them at all.
With OLED display, each individual pixel on the TVs screen (there are 8.29 million of them on a 4K UHD OLED TV display) comes with its own tiny organic light emitting diode (OLED) and each of these OLEDs behind each individual pixel on an OLED TV display can be turned on or off individually to produce light or darkness. This means no backlight panels filled with LEDs, no LCD display surface and no local dimming zones. Instead, an OLED 4K ultra HD TV’s display lighting comes with what would technically be 8.29 million local dimming zones. Obviously this means that an OLED television can create levels of local dimming and brightness precision which no LCD 4K TV can even come close to matching in sheer precision.
Furthermore, because the OLEDs inside each pixel themselves produce their own light and can be turned off completely and individually, an OLED TV can produce perfect total black levels, with absolutely no backlight bleeding through an LCD panel as is the case in even the very best 4K LCD TVs on the market today.
Finally, because they come with no bulky row of LEDs arrayed behind and LCD display panel, OLED TVs can offer ridiculously thin displays, with the LG 2016 G6 and E6 flagship models being a little over 1/8 of an inch thick.
So what should I buy in a 4K TV?
Full-array LED backlighting with FALD (full-array local dimming) is the absolute best 4K LCD/LED TV display technology you can get your hands on if you want top notch display performance, contrast, black levels and realism. Unfortunately, most of the major brands offer this only in their flagship 4K TV models, so full-array with FALD will cost at least a few thousand dollars in most name brand 4K TVs.
However, one brand, Vizio, has developed full-array LED backlighting with FALD in its 2016 P-Series and M-Series 4K TV models and both of these are priced much more affordably than their FALD-enabled rivals from Sony, LG, Samsung or other major brands. So if you’re on a budget but still want top-notch local dimming, and a complete LED array, go for a Vizio M-Series or P-Series TV.
Edge-lit 4K TVs with local dimming aren’t nearly as good at delivering high quality picture performance as their full-array cousins but if they deliver better picture quality than edge-lit or direct-lit TVs without local dimming, so always go for a TV with this spec if you can afford to.
Finally, if you want the absolute unbeatable best in local dimming and precision display lighting technology, pretty much nothing beats 4K OLED TVs and especially the newest models on today’s market due to their exceptionally high peak brightness. LG’s C6 and B6 TVs are particularly “affordable” by OLED standards and thy will blow most LCD televisions out of the water with their display illumination performance.
Story by 4k.com