[FoRK] Circular polarization for Avatar3D and photon spin
sdw at lig.net
Fri Jan 22 11:16:45 PST 2010
I was intrigued by the 3D technology used to show Avatar: The glasses
seemed like linear polarized glasses, which I understand well. However
if you played with 2 of them you might have noticed that rotating a
right and left lens from 2 different glasses didn't produce the gradual
blacking out that you would expect.
Based on my meager, but I thought somewhat complete physics
understanding, I could only think of one solution: Each lens was
filtering out the 3 frequencies that the other eye was seeing as RGB.
By using 6 different frequencies that were roughly seen the same in sets
of 3 by each eye, you could get that effect. Turns out that that
exists, "Dolby3D", but it is not what they used.
Here is some discussion from a private email exchange. I see light in a
whole new way now. ;-)
The quarter-wave plate is supposed to only work at certain frequencies.
Perhaps visible light is all roughly the same frequency for these
purposes. I have a couple photographic circular polarizers, so I
suppose I already know it works.
I didn't know that photons have spin and can impart different momentum
on receptors depending on spin. There is still a lot of confusion over
"circular" polarization. It is used to describe several things:
* circularly polarized electromagnetic radiation, easy for me to
understand with radio waves, strange for me for light (better
after below with propagation through crystals)
* multi-polarization (i.e. amorphous light, sometimes also called
"circularly polarized", but not in the left / right sense as far
as I can tell. It also seems to be the same as "unpolarized" light.)
* Photon spin components -
* A spinnable linear polarizing glass filter for photography
o I didn't know they paired it with a quarter-wave
de-polarizer that produces light with "circular
polarization"! Required for SLR/DSLRs. And now I know why
cameras sometimes don't handle glare properly: the beam
splitter for metering / autofocus is a linear polarizer!
It was still a mystery how the RealID electro-optic right/left
polarizing unit and their passive glass right/left filter work. It
seems useful for more than 3D movie viewing, probably including for
bioluminescent or bioreflective detection. The Wikipedia page on
circular polarization mentions that the mantis shrimp sees circularly
polarized light. The linked articles indicate that it has a quarter
wave converting layer of cells in one part of its eye. There is a
beetle that has a body that reflects only left polarized light. This
page seems to describe how all of this works to some extent:
It appears that RealID have to use something like this:
Probably with an LCD shutter to change the polarization from horizontal
to vertical. How did they create a wave plate that responds to enough
frequencies? Most wave plates are only for a single frequency range
according to the article.
And how do they avoid chromatic aberation? Perhaps with thin wave
plates it isn't an issue, or perhaps the receiving wave plate reverses
It appears that the receiving glasses probably use a 1/4 wave plate with
opposing linear polarizing filters. So the stack is:
Unpolarized light projector -> alternating frames -> synchronized
electro-active circular polarizer -> screen that necessarily reverses
light polarization rotation (not linear, not photon spin) -> in glasses:
quarter wave plate -> linear polarizers horizontal / vertical.
The projector circular polarizer could be done a number of ways. The
original beam could be split, run through a shutter of some kind, then
through opposing linear polarizers, then combined and run through a
quarter wave plate or through dual quarter wave plates. The result
would be left (counter clockwise) and right (clockwise) circular
So, except for the questions above, I think it works this way:
The quarter wave plate has different light propagation speeds depending
on axis. When fed linear polarized light, it creates circular polarized
light. Not clear exactly how this works, however it seems like it is
probably based on crystal excitation where the light pulse echos through
the axii to produce the combined waveform.
When circularly polarized light travels through a quarter wave plate,
the phased light becomes aligned and ends up linearly polarized. I
believe that means that non-polarized light into a quarter wave plate
would probably cancel somewhat.
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