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Today's liquid crystal shutter glasses, when in the "transparent" state, exhibit only 40% light transmission.

They work using two polarizer layers, one which is liquid crystal and goes {vertical <-> horizontal} under electronic control, and the other which is static horizontal. Thus, even when the polarizers are aligned, the total system is still a horizontal polarizer, which necessarily discards half the light, setting a theoretical upper bound of 50% transmission for unpolarized scenes.

However, a friend of a friend working at a commercial lab has offhandedly remarked that a liquid crystal shutter with > 90% transmission in the transparent state is now possible. Unfortunately, he is under NDA and cannot explain the technique.

Given that > 90% transmission is possible, how can this be achieved? What would be the layers in the stack-up, and which kinds of liquid crystals and/or polymers would be used? Try to stick to materials and techniques that today's manufacturers already use.

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up vote 3 down vote accepted

These guys seem to be doing just what you asked about. Have a look here:

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Here's a paper that achieved 80% transmission:

And another that hit 84%:

But I don't know enough physics yet to understand these systems, so if anyone with optics expertise can please understand these and explain (including both theory of operation and possible manufacturing strategies), that would be great!

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The second paper describes an optical filter, not a shutter. The first paper works in a different way than you describe above - instead of blocking the light with crossed polarizers and transmitting it with parallel polarizers, it doesn't block any light at all. Instead of blocking, it diverts it to the side, outside of an arbitrarily defined "viewing angle". Figure 2 of that paper is a fairly clear illustration of this. – ptomato Oct 8 '12 at 14:13
A suggestion: as I mentioned in my answer, so much engineering goes into these things that nobody here is going to be able to help you with manufacturing strategies. Instead, why not partner with that research group at NCSU? That would benefit you both (science funding always seems to flow more readily if the funding agencies see that you are partnering with industry.) – ptomato Oct 8 '12 at 14:17

Your friend is not under NDA for nothing. If the technique is a layer stack, then a lot of engineering work has probably gone into it and nobody here will "just know" a good answer to give you here about the specific materials and thicknesses without replicating that engineering work.

I realized (after originally writing this answer) that liquid crystal shutters don't quite work as you originally described, though. The polarizer layers are fixed at vertical and horizontal. The liquid crystal cell in between the two will rotate the polarization from vertical to horizontal (when voltage is applied) or not (when no voltage is applied.)

However, I am skeptical of the claim that 90% transmission is possible using the generally known technique, which, as you point out, has a 50% theoretical upper bound whenever a polarizer occurs in the stack and the input light is unpolarized. My guess is that they have 90% transmission by restricting the polarization state of the light that goes in. If the input state is a known polarization state instead of unpolarized light, then you don't have to throw away 50% of it and you could theoretically get up to 100% transmission.

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Agree. The 50% limit is sacrosanct as long as there is unpolarized light to start, and at least one polarizer in the system. If I had to guess, the "90%" quote is actually shorthand for "90% of the theoretical limit" -- i.e. 45%. Especially if it was a brief offhand remark, it may have been misunderstood! – Steve B Oct 3 '12 at 17:44
Thanks for the answer, ptomato. Steve B, the comment was along the lines of "full transparency is now possible, over 90% transmission". – AlcubierreDrive Oct 3 '12 at 18:46
My bet would be a polarized source. If you can make a diode that doesn't waste energy emitting unwanted polarizations, you can reasonably make the 90% claim. – Colin K Oct 4 '12 at 3:28

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