Why LCD needs local dimming
LCD panels cannot turn individual pixels off. They are lit by a backlight - usually an array of LEDs - and the liquid crystal layer modulates how much of that light passes through. Even a "black" pixel has some backlight shining through it, creating the glow you see in a darkened room.
In HDR content, a scene may have a bright star against a deep black sky. A standard LCD will light the entire panel at whatever brightness the star requires - washing out the blacks.
Local dimming solves this by controlling the backlight in zones independently. The zone behind the star is bright; the zone behind the black sky is dimmed or off.
Edge-lit vs. full-array local dimming
Edge-lit local dimming: LEDs are placed on the edges (top/bottom or all four sides). Light is directed inward with a light guide. Local dimming zones are very coarse - typically only 8–32 zones across a 27-inch panel. The effect is limited. You may see light blooming around a bright object into adjacent dark areas.
Full-array local dimming (FALD): LEDs are mounted directly behind the panel in a dense array. Each zone diameter is smaller. High-end monitors use 512, 1024, or even 2000+ zones. The more zones, the more precise the dimming - less halo, better black levels.
Mini-LED: Uses thousands of tiny LEDs behind the panel (Samsung, LG, ASUS), enabling hundreds to thousands of dimming zones. This is the technology that makes LCD competitive with OLED for contrast.
The blooming artifact
No zone-based local dimming is perfect. The zone around a bright object is lit, illuminating nearby pixels. On a 32-zone display, a white subtitle in a black letterbox can create a visible rectangle of light around it. This is called blooming or haloing.
The solution is more zones. At 2000+ zones, blooming becomes nearly invisible in practice.
OLED: pixel-level dimming
OLED sidesteps this entirely. Each OLED pixel is its own light source and can switch completely off. Contrast ratio is effectively infinite - a lit pixel next to a black pixel has no crosstalk. This is why OLED black levels look better than even the best local dimming LCD.
What to look for when buying
Check how many local dimming zones the monitor has. A "Full Array" monitor with 8 zones is barely better than edge-lit. Look for 500+ zones for quality HDR on LCD. For truly excellent dark scene performance, Mini-LED (1000+ zones) or OLED are the standards to compare against.