In the evolution of flat-panel HDR (High Dynamic Range) screens, MiniLED technology represents a massive leap in bringing OLED-like contrast ratios to LCD panels. However, this hybrid design suffers from a unique artifact known as Backlight Blooming (or the halo effect).
1. How LCD vs. OLED Backlighting Works
To understand blooming, we must compare screen structures:
- OLED (Self-emissive): Every individual pixel produces its own light. To display pure black, a pixel simply shuts off, resulting in an infinite contrast ratio.
- Standard LCD (Edge-lit): A global light source sits at the edges of the panel, illuminating the entire screen uniformly. Liquid crystals twist to block light to display dark colors, but they block it imperfectly, making dark gray appear instead of pure black.
- MiniLED (FALD): Replaces the edge backlight with thousands of miniature LEDs arranged directly behind the LCD layer. These LEDs are grouped into hundreds or thousands of independent grid segments called Local Dimming Zones.
2. The Cause of Backlight Blooming
Blooming occurs due to a mismatch in density resolution. A typical 4K screen has 8.3 million pixels, but a premium MiniLED monitor might only have 1,000 to 2,000 physical dimming zones.
This means a single local dimming zone is responsible for illuminating a block of roughly 4,000 to 8,000 individual screen pixels.
When a small, bright object (like a cursor, white subtitle, or a star in space) appears on a pitch-black background, the monitor's TCON dimming algorithm turns the backlight zones behind that object to 100% brightness while keeping neighboring zones turned off. Because the backlight zone is physically wider than the bright object, light leaks out from the edges, creating a noticeable hazy white glow (halo) surrounding the object.
3. Dimming Algorithm Trade-Offs
Display manufacturers tune local dimming algorithms differently:
- Aggressive Dimming: Limits blooming by capping the peak brightness of small elements. This makes subtitles look dimmer but keeps blacks deep.
- Aggressive Brightness: Prioritizes HDR highlights. Small objects pop with bright energy, but significant halo blooming artifacts appear.
To stress-test your panel's backlight zones, you can run the online MiniLED Blooming Test to observe how local dimming zones switch under motion.