As we all know, the (transmissive) liquid crystal panel itself does not emit light, but the backlight module behind it hits the liquid crystal panel, and the liquid crystal lattice (pixel) in the liquid crystal panel rotates at a certain angle under the action of voltage to block or expose the backlight for display (structures such as color filters and polarizers are omitted here).
Generally speaking, there are two kinds of backlight structures: side-entry backlight and direct backlight. Common notebook computers and mobile phones use side-entry backlight. The light-emitting element (CCFL or LED) injects light from the side of the LCD panel, and uses the light guide plate and the reflective film to turn the light to an angle perpendicular to the panel. The advantage of this structure is that it can be made very thin, but the uniformity of light emission is relatively poor. Moreover, with the increase of the size of the LCD panel, every time the size is doubled, the luminous area will be expanded to four times, and the brightness will be greatly affected. The direct type is directly illuminated by a light source (currently LED) from the back of the LCD panel. Because LED is a point light source, it needs to keep a certain distance (for example, 2cm) from the panel itself to enlarge the light spot. This structure is almost not limited by size, but the thickness will be relatively large.
Then, when the traditional LCD screen is displayed as "black", the backlight of the LCD itself is on, and only liquid crystal molecules block the light, and in this process, there will inevitably be light leakage, which is what we often see (the all-black display still has a little hazy luminous feeling).
Hisense's ULED adopts a direct-type structure, which can control the backlight by zones (thanks for mentioning 240 zones), and adjust the brightness of the corresponding area of the backlight module by analyzing the brightness of different areas of the screen (thanks)
It is mentioned that the brightness can be actually controlled rather than simply switched on and off, and the demonstration video is here: the magical performance of Hisense ULED dynamic backlight partition technology), which can darken the black area and enhance the contrast.