Understanding REALD 3D Cinema Glasses: Experiments and Confusions Explained

[dgwsoft]

When my kids asked me how these work, I said "that's easy - the lenses are circularly polarized filters, right for the right eye, left for left eye (or possibly vice-versa)"

Indeed that is what Wikipedia says: http://en.wikipedia.org/wiki/3-D_film#Polarization_systems

But some simple experiments at home with the glasses leave me confused.

e.g. I would have expected that if I put on one pair, and hold the left side of another pair in front of my right eye, I would see nothing, the first filter having let through only the L-component and the second R-filter blocking it.

In fact this is what I find:

A) Looking at a street sign lit by sunlight outside my window.

1) wearing one pair and holding another pair in front, forwards facing, I see no dimming, whether I hold R in front of R, L in front of R, and whatever angle I rotate the front pair by. (There are some slight colour effects on rotation but nothing dramatic).

2) on flipping the second pair over so the filters are reversed,

a) R in front of R has no effect, at any angle

b) L in front of R shows compete extinction of the image at rotations of 90 and 270 degrees. The image is considerably dimmed and discoloured at the 0 and 180 points.

B) viewing my LCD monitor screen, there is no change from the above, except that R in front of R now gives extinction at 45 and 225 degrees, with a bright image at the intermediate points.

Now, can anyone either explain why circularly polarized filters should give these effects, or suggest what other fiendish kind of polarization these devices are using?

Also, if anyone else has two pairs of these glasses, could they perhaps repeat the experiment and confirm my observations?

(NB these are cheap unpowered glasses with the logo REALD. The lenses are thin plastic which seems to block out about half the brightness of a natural light source. They were obtained at Cineworld, Gloucester, UK).

 

[mgb_phys]

Now, can anyone either explain why circularly polarized filters should give these effects, or suggest what other fiendish kind of polarization these devices are using?

Circular polarizers are a linear polarizer followed by a 1/4wave plate to generate circular (effectively unpolarized) light.

So if you put a normal polarizer on the input you would see the normal polarizer filter effects, that's what the linear part is for - it transmits only one view of the film.
But the output of a circular filter is effectively unpolarized so you can't use another circular filter to demonstrate this.

 

[dgwsoft]

Circular polarizers are a linear polarizer followed by a 1/4wave plate to generate circular (effectively unpolarized) light.

So if you put a normal polarizer on the input you would see the normal polarizer filter effects, that's what the linear part is for - it transmits only one view of the film.
But the output of a circular filter is effectively unpolarized so you can't use another circular filter to demonstrate this.

Thanks mgb_phys, your answer took me to the right page on Wikipedia: http://en.wikipedia.org/wiki/Jones_calculus

What confused me was that a circular polarizer in reverse is also a circular polariser, in the same sense, so I did not see how a right eyepiece and a reversed right eyepiece could block the light from my LCD screen. But of course the phase shift is different in reverse.

I think what happens is the quarter wave plates cancel, out leaving a linear polarizer, which can then block the linearly polarized light from the lcd screen, at the correct orientation. When the right eyepiece and the reversed right eyepiece are at an angle, the quarter-wave plates must add to give a phase shift, because I get a linear polarizer at a different orientation!

What happens with Right + Reversed Left is a bit different. I guess it would all drop out from the Jones calculus.

Anyway the effects are far more subtle than I would have thought. Hey, ho back to work ...

 

[daveb49]

Follow on question: OK, so with just one pair of RealD 3D (Avatar) glasses, look at your LCD laptop monitor screen. Tilt the glasses 45 degrees (lift right side) and the colour goes to a distinct blue shade. Now tilt the other way - ie 45 degrees lifting left side - and things go to a distinct yellow shade. Why?

I am guessing it's something to do with the 1/4 wavelength coating which can only be 1/4 wavelength at one place (probably the middle) of the visible spectrum, and yellow and blue are at the two ends more or less. But how does this tie into the behaviour described?

 

[Manchot]

I wonder how they managed to make a decent broadband quarter-wave plate so cheaply, especially when achromatic wave plates cost you about $800 at http://www.thorlabs.com/NewGroupPage9.cfm?ObjectGroup_ID=854".

 

[mgb_phys]

They don't have to be very good, you only need them do remove any polarization on the output so people don't see any effects (especially if they are also wearing other specs)

A 1um layer of MgF2 is probably good enough

 

[daveb49]

Tilt the glasses 45 degrees (lift right side) and the colour goes to a distinct blue shade. Now tilt the other way - ie 45 degrees lifting left side - and things go to a distinct yellow shade. Why?

So how does the quarter wave plate give this behaviour?
And also why is there no difference in the left and right lens? I would have thought that since they have opposite cicular polarization, the yellow/blue effect would be flipped between them??

 

[nickelrock]

Yea,,i don't know if any of you guys/gals have tried it,but put on a pair of real d 3d glasses and look in a mirror close one eye only!.the eye that's open will look dark but the eye that's closed is clear...its stupid but i found it crazy...

 

[JDługosz]

Circular polarizers are a linear polarizer followed by a 1/4wave plate to generate circular (effectively unpolarized) light.

That's what CPL means for photo lenses, working around the problems with auto-focus but getting the filter effect (removing glare, etc)

But, the Read3D lenses are more like what you expect from the term Circularly Polarized. That is, you can tilt your head and not mess up the registration of the polarization with the lenses. Your eye doesn't care about the result being polarized or un-polarized.

The basic design criteria are:
1) the orientation of each lens doesn't matter
2) the passage is mutually exclusive

However, criteria 2 might be true only for their prepared content, not for normal light! Perhaps the Left lens totally rejects Right light, but that doesn't mean that the Right lens passes nice Right light -- it might pass everything except Left. A lightbulb source would have all sorts of polarizations at random, perhaps including some that pass fairly well through either lens.

 

[evimarni]

I have removed the glass lens pieces from the frame and when I look at my lcd monitor only through one glass lens rotated at angle to totally block vision, and inserting the second glass lens between the lcd and the first glass lens and also rotating it at a particular angle vision returns for the area covered by the second lens.
A good explanation is needed. Thanks.

 

[smedni]

evimarni,
This is actually a really cool phenomenon.
So, the intensity of the light coming out of a linear polarizer is given by Malus' Law:
I = I_o * cos^2(a) ... eq1)
Where I_o = intensity of incident light; a = polar angle between incident waves' polarization and the orientation of the polarizer; I = Intensity of light coming out of polarizer

Now, the light coming out of the LCD TV is linearly polarized (because of how those TVs work). So when you place a linear polarizer oriented 90* (polar angle) with respect to the TV's rays, then no light will go through because cos^2(pi/2)=0.
But, if you place a polarizer 45* between the light's polar angle and the first polarizer, then cos^2(pi/4)=1/2 the light will make it through. Now, take a second polarizer and place it after the first one, and rotate it such that 2nd polarizer is 90* with respect to the TV's light, but 45* w.r.t. to 1st polarizer. So, the light comes through the first polarizer, it has a polar angle of 45*, then it goes through the second polarizer and gets reduced by half again, because as it goes through the second polarizer, "a" in eq1) is pi/4 (NOT ZERO), so some of it gets through.

So, you have unpolarized light, and three linear filters, you can do this neat experiment:
Hold two of the polarizers perpendicular to each other, and the light is totally blocked. Hold the third polarizer in the middle angled diagonal to the other two, and light will get through.

 

[evimarni]

Cheers that is a good explanation, it makes things clearer now.
Thanks again
evimarni