LCDs don’t emit light on their own and must instead have a backlight that shines through the LCD material to display an image. Today’s LCD-based displays use a number of LEDs for the backlighting. To achieve a greater contrast ratio than a standard dynamic range display, the LEDs in the backlight change their brightness level, allowing the display to dim the backlight for darker blacks and brighten it for brighter whites, which creates a wider contrast ratio. There are several different dimming designs that can be used in the backlight to accomplish this.
Global Dimming: The backlight, which consists of a string of LEDs on one edge of the LCD panel, is treated as a single “zone” and is dimmed for dark scenes and brightened for bright scenes. This is the least expensive type of dimming and can be accomplished with a standard LCD panel. This approach works well for scenes with a limited dynamic range. This type of dimming is typically found on notebooks as it has the lowest power consumption of any dimming technique and generates the least amount of heat. The disadvantage of this design is that the simultaneous contrast ratio is never greater than the contrast ratio of the LCD panel, usually around 1000:1.
Local Dimming: This represents a wide variety of different sub-designs, each detailed below. What differentiates all of the local-dimming designs from global-dimming is that global dimming has a single backlight zone, the entire screen’s backlight is adjusted as one control, in local dimming the screen’s backlight is split into segments which can be independently adjusted.
1D Local Dimming: This design also uses an “edge-lit” string of LEDs, but in this case groups of LEDs on the string can be independently controlled. For most displays, the string of LEDs is located at the bottom of the panel, resulting in a number of vertical zones, equally spaced across the horizontal edge of the display. An edge-lit LED string typically contains between eight and sixteen LED groups, resulting in eight to sixteen dimming zones. This design allows for simultaneous contrast ratios of 6,000:1 to 100,000:1. 1D local dimming is currently the most common design found in HDR televisions and displays.
1.5D Local Dimming: Similar to the 1D local dimming, using edge lighting. However in this design an LED lighting string exists on two sides of the panel, typically top and bottom although left and right designs also exists. The advantage of this design is that it typically has 2×16 zones, so twice as many zones as 1D, but more importantly the top and the bottom of the screen are independently controlled, versus the 1D design where each zone is typically the full vertical height of the screen.
2D or Full array local dimming (FALD): In this design the backlight LEDs are moved from the edge of the panel to the rear of the panel and are arranged in a two-dimensional matrix of LEDs. Each LED is independently controlled and adjusts the brightness of just one “square of a checkerboard” on the display, although typically they are rectangles rather than perfect squares. Today’s HDR displays and televisions typically have between 384 and 1152 zones. These designs are the most expensive, due to the complexity of the circuitry and the processing demands required. The design can also generate a large amount of heat, and often requires cooling fans and/or heat sinks to be placed behind the LCD panel to draw heat away from the display electronics. Full array local dimming produces the best image quality of all of these designs and can achieve simultaneous contrast ratios of 20,000:1 to 500,000:1. Due to the high cost of this design, these displays command the highest prices and typically cost thousands of dollars.
Active-dimming: is the term VESA adopted for one of the new tests in our Certification Test Spec v1.1 (CTS v1.1) where we added a new kind of validation procedure to ensure that displays were actually dimming the backlight based on real-time analysis of the video content, rather than merely only dimming when metadata changes occurred in the video stream. It would be typical that during a movie or game that the metadata for HDR10 would not change, however each frame may have a different peak luminance than the prior frame, and thus could adjust the backlight accordingly. This yields better power saving, and better HDR blacks. The new tests in CTS v1.1 ensure that we test, without changing the metadata of the signal, a dramatic reduction in peak luminance from a full-white checkerboard, to a checkerboard where the white boxes are only 5 cd/m2, this provides ample opportunity for the dimming algorithm to reduce the backlight power. When reducing the backlight power the black level of the black segments of the checkerboard will also reduce, and this is what is measured and used in our calculation of active-dimming stops, (for the more technical, “stops” originally used in photography are a power-of-2 logarithmic function).