local dimming (local dimming) is a feature on some LCD TVs and monitors that dims areas of the screen to reproduce deep blacks. This improves colors and contrast, but the effect depends on the arrangement of LEDs in the backlight.
How does local dimming work?
LCD displays do not generate their own light, so they need backlighting. In old TVs or monitors, the lighting comes from CCFL lamps. In modern equipment, lamps have been replaced by LEDs or MiniLEDs.
In all cases, the backlight illuminates the entire screen. This makes images that should be black look gray. But, with local dimming, it is possible to make LEDs corresponding to dark areas of the image not to be activated. The result varies according to the distribution of these LEDs.
understanding the structure
LCD screens that use LEDs as a backlight can be classified into three groups, depending on the backlight arrangement:
- Edge-lit: the LEDs are distributed along the edges of the screen, being able to occupy only the upper and lower edges, only the sides or the entire contour;
- Direct lit: the LEDs are evenly distributed throughout the screen, even occupying the central area;
- full-array: it also distributes the LEDs across the entire screen, but in larger numbers and dividing them into zones. Of the three, it is the most sophisticated arrangement, which is why it is usually present in high-end equipment.
Local dimming was created to reduce the illumination of dark scenes, without affecting areas that should be bright. For this purpose, the backlight LEDs are grouped into dimming zones. Each covers a specific area of the screen. Thus, in a night sky scene, only zones covering the Moon should be activated.
The more dimming zones a screen has, the more accurate the lighting. This is because, in these circumstances, each zone controls a small number of LEDs, so that it is possible to delimit with more precision the points on the screen that must be darkened.
Cheaper TVs or monitors employ local dimming in an edge-lit arrangement. But the most accurate results, with the best contrast ratio, appear on screens with the arrangement Full Array Local Dimming (FALD)existing in more advanced equipment.
Models with Direct-lit arrangement usually don’t have local dimming because they simply distribute the LEDs behind the screen, without grouping them into zones. Full-array mode is an evolution of Direct-lit, therefore.
FALD vs edge-lit: advantages and disadvantages
Screens with Full-array Local Dimming are preferred in the market because they allow more precise control over the backlight. The FALD also offers the following advantages and disadvantages over the edge-lit arrangement:
- Uniform lighting: FALD screens have a higher amount of LEDs, which are evenly distributed in the backlight. This allows for more accurate and targeted local dimming;
- HDR support: FALD screens improve color gamut accuracy and increase contrast, favoring HDR. That’s because the dimming zones allow for deeper black tones, albeit lower than OLED;
- “Blooming” effect: FALD displays are subject to bloombing or haloing, which occurs when light from a shiny object leaks into surrounding areas. The smaller the number of zones, the greater the risk of the problem arising;
- Edge-lit is cheaper: equipment with Edge-lit arrangement reach the market with a lower price compared to FALD panels because they are less complex to manufacture. Also, they tend to be less thick.
micro dimming and the name that Samsung uses to existing local dimming technology in their LED backlit TVs. With it, contrast and colors are improved, says the company. SEMP TCL adopts the same name on its screens.
global dimming it’s just a way of expressing that the backlight reaches the entire screen at once, with no zones that can be turned on or off independently. Direct-lit screens are of the global dimming type.
OLED screens don’t need local dimming because they are made from an organic compound that emits light when energized, eliminating backlighting. Thus, each pixel can be individually turned on or off to form images with high contrast and deep blacks.