Despite my interest in dual layer LCD technology, it looks as though I missed an interesting article about a development of a version with even more contrast, as much as 2,000,000:1, at last year’s Display Week. I spotted it on the 8K Association’s website – thanks to them for spotting it.
The panel was developed by Sharp and combined a 31.5″ 8K dual cell (with a 4K modulating panel) with an FALD backlight that can also be modulated. Up to now, the dual panel designs that I have seen have used a fixed backlight with all the modulation controlled by the back panel. The front imaging panel was built on an 8K IGZO backplane. Sharp pointed out in its paper that one of the challenges of dual panel LCDs is that efficiency tends to be low, so the firm worked hard to improve the transmittance of the rear panel.
To maximise the transmission ratio of the back monochrome modulation panel, the company removed the black mask that is normally put between the RGB pixels, as well as removing the filters themselves. It also reduced the thickness of the gate line and as a result, the source line went from 8?m to 4?m and the gate line went from 43?m to 5?m.
The use of IGZO also reduced the size of the TFTs. Normally, to create bigger pixel areas, you would need bigger TFTs, but instead Sharp used IGZO transistors and the result of the changes mean an aperture ratio of 30% larger than a regular LCD. The firm also used an optimised LC material so that the overall transmission was 40% higher than before. Despite the better aperture ratio, the backlight still needs to be very bright to achieve the 1,000 cd/m² that the group was aiming for to allow the display of HDR images.
The backlight was designed to have 2048 backlight zones. The team compared the contrast of several different configurations. First, they tried the front panel with the backlight and without the modulating LCD. That gave contrast of just over 300,000:1. Substituting a flat backlight and the modulating layer raised the result to over 1,292,000:1. However, combining the dimming backlight with the modulating layer took the level of contrast to 2,184,000:1, seven times the level of the backlight alone. In each test, the white level was over 1,000 cd/m². The black level of the combined system was at the same level as the minimum level measurable by the research instrument. (0.0005 cd/m²).
(as an aside, when makers first started claiming 1,000,000:1 contrast ratio, we didn’t understand how companies were confirming that as accurate instruments at the time could not reliably measure the low levels of brightness needed!).
Although, the idea of dual cell LCDs has been losing favour recently, because of the failure of the concept in the TV market, the performance and stability of the concept is still attractive in applications such as broadcasting where good displays are needed that can achieve at least 1,000 cd/m² for grading HDR content but that avoid the haloing of miniLEDs and with more long term stability and output than current OLEDs. OLEDs can achieve peak levels of that level, but only small parts of the display, whereas dual cell LCDs are not so limited. I was also quite impressed with the potential for dual cell LCDs for automotive applications and made on plastic substrates. (Flexenable Boosts Dual LCD for Automotive (and….))
Panasonic has had a business making dual cell panels for broadcast applications in recent years, but that company is pulling out of making LCDs altogether. I suspect that the Sharp development is aimed at helping the firm to pick up the business that Panasonic had developed.
One of the challenges is calculating the correct signals for controlling the backlight, modulating layer and imaging cell. In broadcast and other sensitive applications such as medicine, it’s really critical that if the viewer sees any artefacts, they are in the signal and not in the processing of the display. That is quite a challenge with two layers, but with three, it is even more complex.
The History….
By coincidence, I was looking at the history of dual cell LCDs. The Sharp paper led me to a paper (in Japanese) by Panasonic in 2018. That, in turn, led me to an article by Philips Fimi that was presented at EuroDisplay in 2008. In that paper there was a reference to a patent:
P. A. Penz, “Stacked electro-optic display,” U.S. Patent No. 4,364,039 granted in 1982.
So, as so often in the display world, ideas can take a long time to come to fruition in practical products! (BR)
