I've built a dual layer LCD proof of concept from some 1440p 5 inch LCDs from Topfoison:http://www.avsforum.com/forum/24-digita ... st50006521
It's quite easy.
1) Aperture ratio / transmissivity losses:
To make a mega contrast LCD, you need to incur the losses from both panels. Right from the start, the backlight having unpolarized light loses 50% to get beyond the back panel's polarizing grid. Ok, that's normal for all LCDs. Then you talk about aperture ratio which is around 60% for a grayscale panel, 80% for grayscale + low res (800 x 480). Low res + grayscale is good enough for the back panel, but good luck getting the exact same size + alignment with a colour LCD front panel.
Next: Between panels, you need to rotate the polarization angle by 90 degrees so see anything whatsoever. This is because coming out the front of the back panel, entering the rear of the front panel, the light needs to be rotated back to the direction of the polarizing grid. You can get around this by using differently aligned panels (harder) or placing them back to front (easier) + reverse the signal electronically or use a piece of cellophane aka Feynman's trick (easiest + trivial).
So, assuming two identital panels:
If your light source can do X lumens on its own (say, my 1600 lumen LED torch):
-50% loss from first polarizing filter.
-60% pass-through from each panel (aperture ratio). So 0.6 X 0.6 = 0.36.
You're already at 0.18 X => 288 lumens. Ouch.
Then you need to light up, say, a 120 inch screen. And get HDR too ideally.
If I use a relay lens and a grayscale LCD with low res and higher aperture ratio, I could get it to 0.8 X 0.6. And if I use an Epson 3LCD projector and double up each grayscale LCD (grayscale has much better transparency ratio when it's fully "white"), then it should be possible to do HDR10, assuming of course that you boost the back light to like 13,000 lumens.
The other thing is that if you use a 3LCD tech and RGB lasers, then you can have dynamic dimming (all the way to off) and have excellent dynamic contrast ratio per colour
. AND reach rec 2020 with the appropriate laser wavelength / primaries. AND potentially even avoid the 50% polarization losses just by aligning the lasers to the back panel's polarization grid.
To be continued...