Understand 3D Films: Learn About the Principle of Polarization

[pixel01]

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

 

[mgb_phys]

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.

 

[pixel01]

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.

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!)

 

[flatmaster]

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.

 

[DaveC426913]

They may do linear polarization. It would be easy to test that with two pair of glasses.

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.

 

[mgb_phys]

The cinema glasses are normally circular polarized so you don't see any other effects looking at other things that are polarized.

 

[DaveC426913]

The cinema glasses are normally circular polarized so you don't see any other effects looking at other things that are polarized.

I have wondered about this. If they are circular polarized, how can each lens be different?

[EDIT: Ah. http://en.wikipedia.org/wiki/Circular_polarization#Left.2Fright_handedness_conventions ]

 

[mgb_phys]

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.

 

[DaveC426913]

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!

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

Unfortunately, two opposing historical conventions exist. In physics, astronomy, and optics, polarization is defined as seen from the receiver, such as a optical or radio telescope...

In electrical engineering however, it is more common to define polarization as seen from the source, such as from a transmitting antenna.

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

 

[flatmaster]

The engineers seem to mess everything up. That's a lot for keeping the US on the english system we we tried to switch to metric.

 

[Andy Resnick]

<snip>

You use circular polarizers because two different linear polarizations would have different effects on reflection from the screen or reflective objects in the shot.

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.

 

[HallsofIvy]

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!)

 

[mgb_phys]

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.

 

[sophiecentaur]

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.

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!

 

[sophiecentaur]

(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!)

Yep, I think you are.

 

[mgb_phys]

Imagine "Brief Encounter" in 3D!

Don't say it - they will remake it in 3D now ( with explosions and car chases and wild sex scenes)

 

[sophiecentaur]

Don't say it - they will remake it in 3D now ( with explosions and car chases and wild sex scenes)

In the very shoddy Loren / Burton version there was a bit of a sex scene. Poor Celia would have turned in her grave.

 

[LostConjugate]

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?

 

[Andy Resnick]

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.

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

 

[LostConjugate]

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

Sweet, that was the only part of the theater technology I was not able to find an article on. So it is a partially conducting screen.

 

[sophiecentaur]

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.

 

[LostConjugate]

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."

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.

 

[mgb_phys]

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

 

[mgb_phys]

Also, how do the Samsung at home 3d glasses work for their new led lcd 3d tv?

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

 

[LostConjugate]

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.
g

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.

 

[mgb_phys]

How do they block?

LCDs

Samsung glasses are wireless and battery powered but weigh no more than my sunglasses, they are really nice.

I'm guessing you're under 30?
One problem the makers are facing is that young people are happier with large glasses / bluetooth headsets etc, while older people object to too much 'bling' on their faces.
That's a problem if you are trying to sell $2K TVs to people who already have flat screen TVs.

They don't work with regular TV.

They can with double rate blue-ray and a sync source.
Of course whether the makers enable this (standalone syncs etc) has an obvious answer!

 

[LostConjugate]

I'm guessing you're under 30?
One problem the makers are facing is that young people are happier with large glasses / bluetooth headsets etc, while older people object to too much 'bling' on their faces.
That's a problem if you are trying to sell $2K TVs to people who already have flat screen TVs.

!

Lol, I am under 30, almost there. I thought sunglasses were heavier in the old days. Anyways, its not like your wearing these things to the beach :P

 

[mgb_phys]

Lol, I am under 30, almost there. I thought sunglasses were heavier in the old days. Anyways, its not like your wearing these things to the beach :P

It sounds like a silly thing but if you are Sony or Panasonic and you present the active option to the board who are all over 60 and have never worn spectacles and they make the wrong decision it could cost $Bns.

 

[LostConjugate]

Well the technology is selling great at the theaters. I went to go see a 3D movie on a workday at 11am and not only was it sold out but there was a ton of people outside yelling about how they bought their tickets yesterday and demanding a seat. And it wasn't Avatar.

The theater glasses are heavier in my opinion, or the same weight but bulkier

 

[mgb_phys]

Movie theaters are hoping it will put bums on seats (like sound, color and cinemascope before it)
TV makers are hoping you are all going to go out and buy new TVs
TV companies are hoping it will be big for sport (and porn but they aren't saying that)
And the studios are hoping they can sell you all the movies you already bought on DVD and Blueray again in 3D!

 

[matonski]

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.

A quarter wave plate turns the incoming circularly polarized light into linearly polarized light. Left-hand circular and right-hand circular will end up oriented 90 degrees from each other. Then you can selectively block one with a linear polarizer. Thus, the 3D glasses consist of a quarter wave plate followed by a linear polarizer.

The main reason for using circular polarization is so that each eye still gets the correct image even if you tilt your head.

 

[sophiecentaur]

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.

Yes, many people just don't have (or use) binocular vision. Giving them two stereo images won't portray a 3D effect. Stereo is not 3D - it's stereo.

But all of us get 3D information from other clues than binocular vision, like parallax when moving your viewpoint (moving the head) and contrast; distant objects are subject to scattering effects more than near objects (I think it was called 'perspective of light' by the old painters). Stereo only carries two images and it can be very striking at times but very much gives a feeling of 'layers', rather than natural 'depth'. You can see this effect even through large binoculars, which produce an exaggerated / artificial stereo effect due to the effective wider spacing of you eyes and the magnification of the image.

The stereo system cost is around twice that of a single image. Also, to get a 'good' pictorial effect for every scene in Stereo, you have to make significant compromises in the composition, depth of focus and layout - which actually restricts you quite severely at times.
Yes, it's a potentially very dramatic medium in certain appropriate circumstances but it certainly isn't what it's been cracked up to be. Big, sharp, bright pictures with good colourimetry are what is needed most of the time and, of course, Good Programme Material.

 

[Andy Resnick]

[...]colourimetry [...] Programme [...]

Ah... now I understand your 'plug-and-chug' comments. cheers!