The contrast problem in an LCD
An LCD cannot make light. It filters light from a backlight, and no filter is perfect, so a screen that is trying to show black still lets some light leak through. If the backlight is one uniform sheet of light at fixed strength, a dark scene and a bright scene both have the full backlight behind them, and the black level can look more gray than black in a dim room.
Local dimming addresses this by dimming parts of the backlight where the picture is dark. Mini LED is a way to make that approach finer.
What “mini LED” means
Mini LED does not describe the pixels of the screen. It describes the backlight. Instead of a few dozen larger LEDs, or a strip along an edge, a mini LED backlight uses a much larger number of small LEDs spread behind the panel and grouped into zones that can be dimmed independently. The number of LEDs and zones is often in the hundreds or thousands on higher-end products, though it varies a lot by model and is best checked in the manufacturer’s documentation. Smaller LEDs also allow a thinner backlight cavity for a given quality.
The screen itself is still an LCD, with liquid crystals and color filters in front. The light source behind it is just more finely controlled. A backlight can combine this with quantum dots to widen the color range, which the article on quantum dots covers.
How local dimming works
Here is the general process for each frame of video:
- The display’s processor analyzes the image and estimates how bright each zone of the screen needs to be.
- It sets the brightness of each backlight zone, lowering the dark ones and raising those behind bright regions.
- It adjusts the signal sent to the LCD pixels to compensate, so that the combination of backlight and pixel opening gives the intended brightness.
If it works well, a dark sky with a bright moon can have a nearly dark backlight in most zones and a bright one near the moon, which raises the effective contrast of the whole picture. This also helps with high dynamic range, where bright highlights next to dark areas are typical, though the final result depends on peak brightness, color and the quality of the processing.
Edge-lit, direct-lit and zone counts
Backlights come in a few layouts:
- Edge-lit: LEDs along the sides send light across the panel through a light guide. Dimming is usually limited to coarse bands or none.
- Direct-lit without dimming zones: LEDs behind the panel, but all at once.
- Full-array local dimming: LEDs behind the panel in independently controlled zones. Mini LED is a high-density form of this.
More zones generally allow tighter control, but the number alone does not decide quality. The lighting algorithm, the optics that spread light between zones, and the LCD’s own contrast all matter. Two screens with similar zone counts can behave differently, and a smaller number of well-managed zones can sometimes look better than a larger number managed poorly.
Where blooming comes from
Even with many zones, each zone is much larger than a single pixel. A zone covers a block of pixels, so a small bright object, like a star, a line of subtitle text or a mouse cursor on a black background, lights its whole zone. Light also spreads a little into neighboring zones through the optical layers. The glow around the object is called blooming or haloing.
The processor must balance three goals:
- keep dark areas dark,
- keep small bright details at full brightness, and
- avoid obvious halos.
Choices that reduce halos can dim small highlights or make the image look uneven, and choices that protect highlights can leave halos. Different manufacturers tune this in different ways, and some offer settings to control the strength of dimming.
Related effects can include:
- Zone fading or pumping: a visible change in brightness as zones adjust between frames, noticeable in slow fades or scrolling content.
- Crushed shadows: dark detail lost when a zone is dimmed too far.
- Viewing-angle effects: a VA panel behind the backlight tends to show less glow in a straight-on view, whereas an IPS panel, which leaks a bit more light, may show more. Off-axis, the effect changes.
Backlight bleed from poor construction is a separate issue from blooming, though the two can be confused in a dark room.
How it compares with OLED
An OLED display has no backlight; each pixel emits its own light and can turn fully off, so there is no blooming of this kind. Mini LED LCDs, in return, can often reach higher full-screen brightness, and they do not share the same concerns about image retention. Neither technology is better in every respect. Which is preferable depends on room lighting, content and budget, and the power consumption of each depends on the picture shown.
Practical tips
- Judge local dimming with the kinds of content you will watch. Starfields, credits and dark menus are tough tests.
- Look for settings that control dimming strength, and check whether they change brightness.
- In a bright room, blooming is less visible, and contrast benefits may matter less.
- For desktop work with small bright windows on dark backgrounds, local dimming can sometimes be distracting, and some people turn it off.
Mini LED is best understood as an evolution of the LCD that lowers the gap in black level, not a replacement for it. Its quality depends heavily on how well the whole system is designed and tuned.