Zones are the whole story
A Mini LED backlight is a grid of small LEDs grouped into zones, and the screen dims or brightens each zone on its own. Dark parts of the picture get a dim backlight, bright parts get a bright one. Real screens have hundreds to thousands of zones, and the zone count decides how well it works.
Picture a 64 by 36 grid. That's 2,304 zones. On a 4K panel, each zone sits behind 3600 pixels, while an OLED screen dims each of its 8,294,400 pixels on its own.
Blooming
A zone is far bigger than a pixel, so a bright object on a dark background lights its whole zone. The result is a faint halo around the object, called blooming. More zones make the halo smaller, and no zone count removes it.
It's easiest to see in a dark room with white text or a cursor on black, and in HDR scenes with bright highlights next to shadow. Reviews that show a dark test pattern with a small bright shape tell you more than a zone count does.
Compared with OLED and a plain backlight
Against OLED, Mini LED reaches higher sustained brightness and has no burn-in, but blooming stays. Against a plain LCD backlight it gives deeper blacks and brighter highlights. DisplayHDR's 1000 and 1400 tiers require 2D local dimming.
The LCD in front of the backlight is still an IPS or VA panel, so that panel's strengths and weaknesses carry over: IPS viewing angles and IPS glow, or VA contrast and slow dark transitions.
What to look for
Before you buy a Mini LED monitor, check:
- The zone count, and whether the local dimming is 2D or only 1D.
- Brightness at full screen as well as the peak, since the peak only holds on a small patch.
- Reviews that show blooming on a dark pattern, and the local dimming modes the screen offers.