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A local dimming implementation on LED LCDs allows them to dim parts of the screen that need to be darker – without dimming the parts that need to be bright. The result is a higher contrast ratio.
Because LED LCD displays rely on their backlights to create the picture, they cannot produce truly deep black shades; Some light will always get through, resulting in grayish blacks.
Of course, not everyone can afford a high-end OLED display. This is where local dimming comes into play to improve the contrast ratio of LED-backlit LCDs.
Local dimming is available on both direct-lit (full-array local dimming) and edge-lit LED displays. So, there are four different types of backlights in total:
- Direct-lit with Global Dimming (no local dimming)
- Direct-lit with full-array local dimming (FALD)
- Edge-lit (without local dimming)
- Edge-lit with local dimming
Direct-Lit Global Dimming
A standard LED LCD display with global dimming has a grid of LEDs at the back of the screen. These LEDs, however, cannot be individually dimmed. Instead, the entire screen gets brighter or darker according to the picture.
In other words, global dimming means that there’s no local dimming and that the black shades on a display won’t be very deep.
Full-Array Local Dimming
A display with a full-array local dimming solution consists of numerous zones of LEDs which are placed across the entire back of the screen.
These zones will dim the part of the screen that needs to be darker without affecting the areas of the screen that need to be bright.
FALD displays deliver the best image quality out of all LED-backlit types, but they are also more expensive and heftier in design.
They can’t deliver as high contrast ratio as OLED displays, but they also don’t have some disadvantages of OLEDs, such as image burn-in/retention. LED displays, especially those with quantum dot enhanced films, can also get a lot brighter than OLEDs.
In certain scenes where one zone of LEDs is lit, but the surrounding zones are dimmed, some light will bleed into the dimmed zones creating a halo/bloom effect, though this doesn’t occur often. This can also be further reduced by using Mini-LEDs instead of standard LEDs.
While there are many FALD TVs, there are only a few FALD monitors currently available, such as the ASUS ROG Swift PG27UQ which we reviewed. It has a 384-zone FALD and needless to say, it delivers an outstanding image quality.
Edge-Lit Local Dimming
Edge-lit displays have LEDs placed around the edges of the backlight facing the center of the screen. Due to this, they are cheaper to make and can have very thin designs.
The biggest disadvantage of edge-lit displays is uniformity and black level in dark scenes as the image is brighter around the edges of the screen and less bright in the center. We recommend avoiding edge-lit displays without local dimming.
Edge-lit displays with local dimming partially address the above-mention issue. In dark scenes, the LED zones will be dimmed which results in deeper blacks than those of standard direct-lit and edge-lit displays.
However, this also introduces additional issues such as clouding and blooming of moving objects, but it all depends on how well the local dimming is implemented.
Too aggressive local dimming produces more prominent blooming whereas too weak local dimming barely improves the contrast ratio. Depending on a display, there may be a few options available to adjust the intensity of local dimming. Some displays will even include the option to turn it off completely.
Overall, full-array local dimming is the most effective method of improving the contrast ratio on LED displays. Although it also significantly increases the price of the display, it’s mandatory for the optimal HDR (High Dynamic Range) viewing experience.
Local dimming on edge-lit displays can be beneficial, but it can also be pointless if badly implemented. A good direct-lit display without localized dimming is much better than an edge-lit display with poor local dimming.
If you want to be certain that you’re getting a good TV for your money, head over to our best TV buyer’s guide for more information.
Rob is a software engineer with a Bachelor’s degree from the University of Denver. He now works full-time managing DisplayNinja while coding his own projects on the side.