A patent application by Apple was published that shows a method for creating High Dynamic Range (HDR) images using a dual-LCD approach. While this will definitely work, it is not likely to be a very cost effective commercial approach, so it may never see the light of day.
As reported by Apple Insider, the “Devices and methods of image-splitting for dual-layer high dynamic range displays” application was published by the U.S. Patent and Trademark Office on June 16, 2016. Instead of having a array of LEDs direct backlight that can be modulated in zones, the Apple patent specifies two full resolution LCDs to be placed in series. The content for the back panel is processed to show the expanded parts of the image – i.e. the shadow details and the highlight details. This is a monochrome grayscale image that modulates the light just like a direct LED backlight does – but with much higher resolution.
The front panel is a conventional full color LCD that modulates the light again to produce very high contrast images. Double modulation schemes like this effectively multiple the contrast of each panel. The LCDs typically have a native contrast of around 1,000:1 so double modulation enables displays with a 1,000,000:1 contrast.
The backlight can now be an edge lit design or a full area LED array that may or may not be modulated as well. The brightness of the backlight would need to be much higher however, to deliver the same brightness level to the viewer. Getting to 1000 cd/m² of peak brightness would require a very bright backlight, which has thermal, cost and contrast ramifications.
Stacking two LCDs has been done before mostly to create a 3D like effect. But doing so is tricky. Not only do you need to have pixel-level alignment between the two panels, but the gap between them can create a parallax effect that limits the viewing angle of the display. As a result, such a device is much better aimed at mobile or single-person desktop applications – not TVs or signage.
To combat the introduction of unwanted effects, such as parallax artifacts, moiré, halo artifacts and clipping, Apple proposes an improved image-splitting algorithm capable of intelligently processing an original image for output on a per-panel basis. It says the algorithm analyzes an image and splits it into two separate images generated for display on the front and back LCD panels. Depending on image content, hardware limitations and other considerations, the system blurs and downsizes the image sent to drive the back LCD panel.
Apple’s system is able to prevent parallax without negatively impacting brightness or clarity. The algorithm also prevents clipping artifacts and light bloom, or halos, from appearing on the front panel. Apple says the technique can be used in both small and large format screens, from iPhone to Mac. – CC
Analyst Comment
Sharp showed a technology demo of a panel with 1 million to one contrast at CES several years ago and we speculated at the time that this was achieved by this kind of dual modulation technique, all the company would not confirm it. (BR)
