[FoRK] Circular polarization for Avatar3D and photon spin

Stephen Williams 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 -
         o  
http://74.125.155.132/search?q=cache:TG_eKaXvpaQJ:people.bu.edu/birubio/ch203/pdfs/l12.pdf+clockwise+polarization+filter+rotating+photon&cd=6&hl=en&ct=clnk&gl=us 

   * 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!
               + http://www.camerapedia.org/wiki/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:
http://www.polarization.com/beetle/beetle.html

It appears that RealID have to use something like this:
http://en.wikipedia.org/wiki/Wave_plate
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 
the shift.
http://en.wikipedia.org/wiki/Chromatic_aberration

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 
polarized light.

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.

sdw



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