Local dimming explained: how LCD TVs get deep blacks, and why bright objects glow

An LCD panel doesn’t make light; it blocks it. A backlight shines through liquid-crystal shutters, and even when the shutters close for black, some light leaks through. In a dark room, that leak is what makes a cheap TV’s black bars look gray. Local dimming attacks the problem from behind: if part of the picture is dark, turn the backlight down there.

How it works

The backlight is divided into zones, each with its own LEDs, that the TV’s processor dims or brightens frame by frame according to the picture in front of it. Behind a starfield, most zones go nearly dark and only those behind the stars light up. The LCD layer then draws the fine detail within each zone.

What decides how well it works:

1. Number of zones: how small a patch the backlight can control.
2. Where the LEDs are: behind the screen (full-array) or along the edges (edge-lit).
3. Processing: how the TV decides each zone’s level and how quickly it changes, which governs flicker, crushed shadows and dimmed highlights.

Backlight types, from fewest to most zones

Brands use their own names: Samsung’s Neo QLED, TCL’s QD-Mini LED, Hisense’s Mini-LED, LG’s QNED MiniLED and Sony’s BRAVIA 5 and 7 are Mini LED; Sony’s BRAVIA 9 II uses an RGB LED backlight. Plain QLED says nothing about the backlight; see QLED vs Mini LED.

Zones vs pixels: why blooming happens

A zone is always much bigger than a pixel. Here’s how big, on a 65-inch screen (1,805 sq in), for a range of zone counts:

Our calculation, assuming evenly sized zones on a 16:9 screen. Zone counts are illustrative; manufacturers rarely publish them on US spec sheets.

When a small bright object (a white subtitle, a star, a cursor) sits on black, its zone has to light up for the object to be bright, and every dark pixel in that zone gets the same light from behind. The LCD shutters block most of it, but not all, so a soft glow surrounds the object: blooming, or haloing. The TV can hide it by dimming the zone, which makes the small highlight duller instead. Every local-dimming TV trades between those two.

More zones make each halo smaller. Better processing decides the trade well and spreads changes smoothly. RTINGS, which tests both “lighting zone precision” and “zone transitions”, notes that issues like blooming and flickering as objects move between zones remain on every local-dimming TV, even as the best now approach OLED contrast.

Local dimming settings: what to choose

Most LCD TVs offer Off / Low / Medium / High, usually called Local Dimming in the picture or brightness menu (some brands use their own name):

• High: deepest blacks and brightest HDR highlights. Best for films in a dark room. Halos and occasional zone flicker are most visible.
• Medium / Low: fewer halos, slightly grayer blacks. Good for mixed TV watching, desktops and games with small bright elements on dark backgrounds.
• Off: uniform, grayish picture. Only worth it to diagnose a problem.

RTINGS’ advice runs the same way: higher settings raise contrast but can be more distracting, so step down if what you see bothers you. Picture modes interact with this too; see TV picture settings.

Which current TVs have it

Every Mini LED line in our catalogue has a full-array backlight with local dimming. Of the plain LED lines, Sony’s BRAVIA 3 is direct-lit, and Samsung’s Q8F and The Frame are QLEDs with conventional backlights.

Local dimming and durability

Full-array backlights also hold up better. In RTINGS’ 100-TV longevity test, 35% of full-array LCDs failed partly or completely over about 18,000 hours, against nearly 60% of edge-lit and basic direct-lit sets without local dimming. Edge-lit TVs concentrate heat along the edges, which warps and delaminates the layers inside. Details in how long TVs last.

Mini LED or OLED?

If blooming bothers you, OLED is the only display with none at all; if you watch in daylight, Mini LED’s brightness usually wins. The full comparison, with each line’s specs, is in Mini LED vs OLED.