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3D film glasses

  1. May 1, 2010 #1
    Hi all,
    When viewing 3D films, you are given a pair of glasses without which, the pictures are bad and blur . So what is the principle of the technique? Is that polarization of the light?
    Thanks
     
  2. jcsd
  3. May 1, 2010 #2

    mgb_phys

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    If the glasses aren't colored then yes.
    The image filmed with the left and right cameras are projected through left and right polarizers and viewed through left and right polarized glasses.
    This means the left eye only sees the left camera's image and so on.
     
  4. May 1, 2010 #3
    Thanks for the answer.
    The glasses are not really colored, just a little redder (left) and greener (right), but through it, the color of the film does not change (or I just can't see).
    (Next time I will check by turning the glasses by 90 degrees!)
     
  5. May 1, 2010 #4
    They may do linear polarization. It would be easy to test that with two pair of glasses. The reflection from the screen must maintain the polarization. If i remember correctly, a standard screen won't work because the polarization is lost in the scattering. A screen with some sort of metallic surface is required.
     
  6. May 1, 2010 #5

    DaveC426913

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    Yes. Turn the left lens of one glasses 90 degrees to the left lens of the other glasses. It will be opaque; no light will get through.

    But what is really freakin' awesome, is to take a third pair of glasses and insert it between the first two at a 45 degree angle. You will now be able to see through all three lenses.
     
  7. May 2, 2010 #6

    mgb_phys

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    The cinema glasses are normally circular polarized so you don't see any other effects looking at other things that are polarized.
     
  8. May 2, 2010 #7

    DaveC426913

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  9. May 2, 2010 #8

    mgb_phys

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    One eye is left circular and the other is right - in a frankly amazing bit of industry common sense they even managed to agree on a standard where the left eye image is left polarized and the right eye image is right polarized!

    Circular polarizers are a bit of a min-nomer.
    They are really linearly polarized and then have a quarter wave plate to rotate the polarization.

    You use circular polarizers because two different linear polarizations would have different effects on reflection from the screen or reflective objects in the shot.
     
  10. May 2, 2010 #9

    DaveC426913

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    Well, except that the labelling of "left-and right-polarized" is not standardized...

    http://en.wikipedia.org/wiki/Circular_polarization#Left.2Fright_handedness_conventions

    :biggrin:
     
  11. May 2, 2010 #10
    The engineers seem to mess everything up. That's alot for keeping the US on the english system we we tried to switch to metric.
     
  12. May 3, 2010 #11

    Andy Resnick

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    Circular polarizers are also used in 3D movies to reduce effects caused by (for example) tilting and rotating your head.

    I agree- it's an impressive bit of engineering to fit movie theaters with this technology.
     
  13. May 3, 2010 #12

    HallsofIvy

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    Many years ago, my son had a "Johnny Quest" video game that came with non-colored 3d glasses. That was my first experience with non-colored 3D and I quickly came to the conclusion that they were using polarized light.

    But if this was only soft ware and could be used with any computer, how in the world were they producing that?

    (Now, that I think about it, this was a long time ago and it is possible that it did have red and green lenses so I may just be remembering the impossible!)
     
  14. May 3, 2010 #13

    mgb_phys

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    You can get 'sort of' 3d with one lens darker than the other and then move the object sideways - your eyes see different changes in the object and tricks your brain into thinking it's moving toward/away from you = like the famous animation of the spinning dancer silhouette

    It's called the Pulfrich effect.
     
  15. May 5, 2010 #14

    sophiecentaur

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    Circular polarisation is the perfect description - it is hardly a mis-nomer. The plane of the E field rotates, clockwise (or anticlockwise) as the wave passes. A better word could possibly be spiral polarised but it is definitely not the same as plane polarised. The quarter wave plate provides a 90degree phase shift for one component of E. The resultant of the two waves when they emerge produces this circular polarisation.

    The main advantage for stereo is that head tilting preserves the right and left images separately in each eye.

    btw, i wish they wouldn't call it 3D. It's only stereoscopic, which is a very limited version of true 3D and, unlike a hologram, can't be viewed differently by moving your head. You can't 'see round' things. A shocking extra expense for no real purpose except to make the film makers more money. Imagine "Brief Encounter" in 3D!!!
     
  16. May 5, 2010 #15

    sophiecentaur

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    Yep, I think you are.:smile:
     
  17. May 5, 2010 #16

    mgb_phys

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    Don't say it - they will remake it in 3D now ( with explosions and car chases and wild sex scenes)
     
  18. May 5, 2010 #17

    sophiecentaur

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    In the very shoddy Loren / Burton version there was a bit of a sex scene. Poor Celia would have turned in her grave.
     
  19. May 5, 2010 #18
    It makes you wonder, how do they keep the light from depolarizing when it reflects off the screen. The screen would need to be a conductive material and so it would look like a giant mirror, but it doesn't.

    Also, how do the Samsung at home 3d glasses work for their new led lcd 3d tv?
     
  20. May 5, 2010 #19

    Andy Resnick

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    My understanding is that a 'scrim' is placed in front of the regular screen, the scrim contains metallic threads that maintain the polarization.

    http://en.wikipedia.org/wiki/RealD_Cinema

    http://en.wikipedia.org/wiki/Silver_screen
     
  21. May 5, 2010 #20
  22. May 5, 2010 #21

    sophiecentaur

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    The depolarisation won't be much is the screen is a flat reflector and the reflection is near normal, will it?.

    In any case, the whole stereo thing is lost on a huge proportion of the population who either have poor acuity in one eye or whose vision is dominated by one particular eye.
    "But the King hasn't got any clothes on."


    Could the Samsung system be field sequential (left/right/left/right) with optical shutters on each of the spectacle lenses?. I can't imagine the individual pixels can be separately polarised. I saw a demo several years ago and it was tolerably good.
     
  23. May 5, 2010 #22
    Wouldn't those people have the same problem with their normal vision. When all is said and done the 3D technology is no different then how we see 3D all around us. We see two different images with different angles.
     
  24. May 5, 2010 #23

    mgb_phys

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    I thought the silver screen was more to boost the reflectivity - since you lose half the light with the polariser.

    With projected 3d you are limited to a narrow range of reflection angles (<5deg), the image has to be projected very close to normal on the screen so that both polarizations are reflected equally.
    It's a problem with IMAX 3D where the screen is much larger and you view the edges at a bigger angle

    http://www.the3drevolution.com/3dscreen.html
     
  25. May 5, 2010 #24

    mgb_phys

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    Two methods

    Active glasses switch to block left and right eye alternately.
    Works with regular TV and you get to use the full resolution of the TV - but you need twice the frame rate.
    Blueray and HDMI-2 already supports this.
    The glasses are heavy as they need electronics and batteries, and they need either a cable or an IR link to the player to sync the fields.

    Passive glasses have fixed left/right polarized filters, the TV has left/right polarized filters over alternate columns (or rows - but generally columns).
    Advantage is the glasses are cheap and light.
    But you need to buy a new TV and you lose half the resolution.

    Sanyo use passive glasses., LG+Panasonic are going for active. Don't know about Samsung
     
  26. May 5, 2010 #25
    How do they block? Some sort of electric double polarizer? Samsung glasses are wireless and battery powered but weigh no more than my sunglasses, they are really nice. They don't work with regular TV.
     
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