Mobile, tablet, laptop, or larger electronic displays ideally have a large contrast between dark and light parts of the image. Since a human eye has a very high dynamic range (HDR) of 14 orders of magnitude, displays that support high contrast range allow for more faithful images to be rendered. High dynamic range displays should have high contrast, a very black state and high peak brightness, and allow for precise control of light levels. For example, an organic LED (OLED) based display can typically have perfect black (no light), offering a high contrast that supports HDR viewing. In comparison, liquid crystal displays (LCD) are not able to support perfect black due to light leakage from the backlight through the optical system. However, maximum light output for LCDs is generally higher than for OLED displays, allowing for extension of the dynamic range at the brighter end. If black values can be significantly reduced, LCD displays can match or exceed the dynamic range of OLEDS.
One recognized way of reducing black values in LCD displays is to switch off the backlight (locally) to enhance the contrast of an image. The smaller the area of the backlight that can be switched off, the better the contrast resolution. This technique is often applied in LCD based TV sets and is commonly called full array local dimming (FALD).
Unfortunately, conventional FALD techniques do not work on smaller displays provided for laptops, tablets, and mobile devices, primarily due to thinness (typically less than 0.4 mm) of the supporting backlight. LEDs supporting FALD are embedded in a light guide and normally emit light from the top. With a very thin backlight, the LED would be clearly visible through the light guide layer and the rest of the optical system (brightness enhancement and diffuser foils). Replacement of top-lighting LED sources with sidelight-emitting LEDs having virtually no emission from the top of the package is not feasible, since this would create a “black hole” right above LED, rather than a uniform illuminance.