What is LED Local Dimming Exactly?
Types of Local Dimming Techniques
Challenges and Limitations of LED Local Dimming
In the world of Ultra High Definition and High Dynamic Range (HDR), the concept of local dimming – specifically with regard to LCD LED display systems – is an important element when it comes to picture quality, and in this blog we’re going to explain why. The problem begins with a phenomenon known as “blooming,” a quite off-putting side effect of LED television technology that anyone who has attempted to watch a letterboxed film in a dark room via a poorly-implemented local dimming system has experienced. The blooming effect occurs when a flat-panel doesn’t have enough dimming zones and a highly-demanding scene, perhaps on an Ultra HD Blu-ray disc with extreme HDR highlights, causes the light near the edge of the letterbox bars to “bleed,” yielding quite the ugly, washed-out image.
To combat this, LCD LED display manufacturers implemented “local dimming,” which was originally designed to clamp down on the light output of the device where super-dark areas were needed – either reducing the blooming significantly or cutting it out almost completely. Of course, competing technologies such as OLED and QD-OLED emerged in the wake of LED blooming issues to address the concerns of hardcore home theater and gaming enthusiasts, but LED display technology continues to soldier on with always-evolving local dimming algorithms.
The biggest difference between display technologies such as LED/LCD and OLED is the way in which the panels are illuminated – on LCD screens, the pixels are illuminated via tiny LEDs, which is why they need local dimming systems in order to react to changes in content. OLEDs, on the other hand, are known as “self-emissive” displays, which basically boils down to this: these screens don’t require a backlighting system to illuminate, and as such don’t need local dimming to look their best.
We’ll get into this in more detail shortly.
As we alluded to above, local dimming is a technology used in displays, such as computer monitors and televisions, which enables more precise control over the brightness levels of varying parts of the screen. This results in areas that need to be black remaining dark without the distracting and aforementioned blooming, in addition to a host of other immediately noticeable visual benefits such as contrast improvement.
Unfortunately, all is not created equal on this playing field; some display types, such as edge-lit, don’t benefit from local dimming systems built into them because there aren’t enough dimming zones to control the light.
According to RTINGS.com, one of the most respected online resources in the area of display technology reviewing, local dimming can be defined as “a way for LED TVs to improve the contrast ratio in dark scenes by dimming backlight zones, making blacks appear deeper than they normally are without the feature and thus improving overall picture quality.”
Compared to traditional backlighting methods – as well as “global dimming,” which we’ll cover in a bit – local dimming approaches in LED displays enable the lights behind the LCD layer to adapt to the picture displayed, improving the contrast ratio. In essence, local dimming eliminates the single massive backlight in an LED through the use of multiple backlights, turning them on and off in response to what’s in an image.
Today’s TVs use a number of backlighting methods, but the biggest change has been the introduction of discrete backlighting zones – rather than illuminating the entire screen, the LED backlights of a TV can be addressed individually, resulting in what we covered a moment ago: they can be turned on or off and dimmed/brightened on demand to yield brighter or darker portions of the picture.
Since we mentioned it in the beginning of this section, let’s take a moment to look at global dimming and how it compares to local dimming.
What differentiates all of the local dimming designs from global dimming types is that global dimming utilizes a single backlight zone, meaning the entire screen’s backlight is adjusted as one control. In comparison, local dimming systems split the screen’s backlight into segments which can be independently adjusted.
There are three primary types of local dimming techniques: full-array (FALD), edge-lit and direct-lit. As everything else in life, there are pros and cons to each of these methods, but we can tell you from experience that if you’re setting up a system to take advantage of HDR in a dark room, you want to steer clear of edge-lit designs.
This approach encompasses a grid of LEDs behind the screen, separated into multiple dimming zones that can be adjusted independently. In short, if you’re buying an LCD display, look for one with FALD, as it allows for greater precision in local dimming to provide deeper blacks and boosted contrast.
Here, LEDs are placed along the edges of the screen to direct the light towards the center of the display; the LEDs can be dimmed in specific areas to create localized dimming zones, but this is nowhere near as effective as a FALD approach. The upside to edge-lit panels is that they’re less expensive to make, and as such to buy, compared to premium FALD models.
This method is similar to FALD, but rather than a grid of LEDs behind the screen, there are fewer, larger LEDs directly behind it; these larger LEDs can be turned on or off individually to create localized dimming zones, as well, but while direct-lit local dimming is more effective than edge-lit, it’s also more expensive to implement.
In direct comparison to self-emissive technology like OLED, there’s just no getting around it: the primary issue that continues to plague light-emitting diodes (LED) is the challenge of dimming. Although some LED light sources dim just as well as incandescent lamps, others exhibit very odd dimming behavior (see the aforementioned references to blooming).
Beyond the blooming, these LED light sources may:
• Produce no light, on the extreme end of the spectrum.
• Exhibit flicker or noise.
• Show a limited dynamic range.
• Change color as they dim.
• Dim erratically.
• Refuse to dim at all, in other extreme cases.
At the root of the problem is incompatibility – LED’s dimmable nature involves how well they dim, with considerations including smoothness, color, the time it takes to dim and how evenly light is dispersed being at the forefront of the argument.
Which backlight technology is best for your application or project? When choosing the right backlighting tech, you should be guided by the desired effect (i.e. the key parameters of the final device). As such, there is, unfortunately, no one-size-fits-all answer for comparing full-array local dimming and edge-lit local dimming, as the backlight achieved by these two solutions may or may not fit perfectly with the designed end device.
Knowing these dependencies, the designer is able to accurately select a solution to best suit the intended use of the application. So, if you’re wondering what to choose – edge LED local dimming, full-array local dimming – we recommend that you contact us today to discuss the target application in which the designed device will be placed.
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