Goodness, doesn't it get very complex. I really don't understand the nuances yet, but this post helped me to understand the basics, so thanks for that!

You're back! OK *rubs hands together* here we go...

"why aren’t both beams affecting each other? Don’t the waves add together? Wouldn’t two spiraling lights moving together add up as it is?"

The left and right frames happen one after the other remember. So they are separated in time and would not interact with each other (as it happens they wouldn't interact with each other anyway cos that's not how light works - waves belonging to different photons - different packets of light - ignore each other).

What's the point of circularly polarising? First a little bit more clearing up on what happens. Think about only the left hand lens for now - The clockwise and anticlockwise lights hits the quarter wave plate first. They are both tuned into linear polarised light by this plate. The difference is that the clockwise light becomes linearly polarised in the up-down direction whereas the anticlockwise light becomes linearly polarised in the left-right direction. You can now use a linear filter to block out the one you don't want to reach the viewers eye.

The benefit really is all to do with head tilting. The image doesn't darken when tilting your head in the circular set up. With the linear-only set up tilting your head causes a bit of the left image to get into your right eye and vice-versa. It really spoils the 3D I think.

I think it's GREAT that you reverse engineered 3D for your self before reading my post. Excellent.

I did suggest a segment on 3D for Blue Peter. It could happen!

Hello! Long time no 3D!

My buddy built me ye olde Spectrum computer console out of the blue yesterday (I mean who does random acts of kindness like that? Just fab!) and it got me to thinking.... "waaaait.... speaking of home projects! THE THREEDEE!" So I have returned once again to perplex and confuse!

If both glasses' lenses are the same crystalline material with no variation, then both light sources probably go through the same crystal inside the projector to start with... now what I don't understand about that is if both lights travel through the same crystal then why aren't both beams affecting each other? Don't the waves add together? Wouldn't two spiraling lights moving together add up as it is?

I still don't get the POINT of polarising circularly. Maybe if I explain to you my problem with what you are telling me you will see where I'm slipping up.

Let me copy and paste this sentence you wrote about the glasses: "without the second layer blocking one or the other you’d be getting both images in both eyes."

If both quarter wave plate layers on each eye are converting the exact same information as you suggest, and without the horizontal and vertical filters behind them you see the same thing with both eyes, to me it sounds utterly pointless. It may as well be just a plain horizontal or linear filter if the corkscrewing light is interpreted the same by both wave plates on the eyes. If however each corkscrew were interpreted differently by different eye lenses I see no need for the linear conversion as using that alone would have differentiated the eyes from each other. Do you follow my confusion here? I'm not sure how well I am articulating my thoughts (a bit sleepy).

From what you are saying I conclude that the use of circularly polarised light is purely for the glasses to work better when tilting your head. However, if one image no longer creeps into the other eye when tilting, then the alternative can only be that you start losing vision in your eyes as you tilt your head, thus gradually getting darker, am I right? Now is the complete loss of vision that much better than ghosting and worth all the extra cost of these quarter plate projectors in the cinemas?

Ah! Am a bit depressed reading your other post. You see, one night my friend sent me an e-mail asking: "how do you make animation into 3D", I was just typing back to him something along the lines of "why in the world do you think I would know that???" when I stopped and suddenly had a thought, DO I know? Maybe I can work it out... I used to make those Magic Eye pictures for fun so started my theories from there and stayed up all night experimenting with my camera and overlaying displaced pictures in two different colours, in effect reverse engineering it for myself. I was SO excited when I worked out how to make animation into 3D on my own you have no idea! I felt how the original creators must have felt, EUREKA! But now, I see, I could have just read it all on your site in 5 mins boooo....... :(

Oh yeah I will absolutely upload pics of my creation if I build my own set up! Just need to find the time.. and space... but there is no way I won't at least attempt it at some point!

By the way, I've just taken a better look around your site and had a *headdesk* moment when I saw that you are Blue Peter's "Science Guy". Figures, of course I would need someone to explain things to me in a child friendly simple way ha ha! *Sigh* I think you should definitely recommend this as a "make" by the way! "Hey kids! Build your own 3D cinema projector!" Well if that particular pitch flies with the producers I'll certainly tune in! (Special props if we can make one cheaply out of household items!)

PS: Not really related but oh so cool: http://www.moillusions.com/2010/02/neil-dawsons-horizons.html

I know! Glad you liked it. Everyone else I know started backing away from me slowly when I tried to tell them about it. Philistines.

Nice guest appearance from Marcus du Sautoy :)

That's the book you recommended I get! A jolly good read.

Whooooooo! Interesting. So each frame of the film is played left-right-left-right-left-right? That's pretty crazy! So when I am looking at the film without glasses and I see the image ghosting, it is not two overlaid images but the frame rate moving faster than my eye can differentiate? Well what if I were to pause it? Would I only be looking at one image or would it still ghost?

Am ashamed to admit I didn't know the Vue 3D was RealD and that Wiki article is so useful!

Makes total sense that the same lens reversed will put the wave back in sync with itself. However I'm confused again. I don't now see the point in converting the left or right corkscrewing waves back into linear or vertical polarisation. As one side lets in only anti-clockwise light and the other clockwise, why can't that just differentiate it? Why the need for converting it once more? Also I don't see why the circling would prevent it messing up when you tilt your head to the side as it still converts to horizontal or vertical polarisation and when you tilt your head, you are still not looking at the screen correctly.

Crystals are fascinating. I'm going to have to research the prisms and this quarter plate a bit more before I make any more comments on it.

You know the more I discuss it the more I fancy having a go at building makeshift 3D viewing at home. Two projectors, an opposing linear filter on each, a reflective fabric, cereal cardboard box glasses and opposing filters on each eye. Then possibly, if I researched it more, stick a crystal somewhere in there. Can't be that hard right? (Famous last words. :P )

Topic of conversation with my (long suffering) friends today were 3D TVs, the type without glasses to convert it. Now there's something interesting! My theory is it must mix this 3D technology with hologram technology somehow. Did you ever own one of those budget rulers when you were a kid that had holograms with ridges sticking out of it? I'm guessing it works by something along those lines, it's a toughie but after I get a grasp on this type of 3D I'm going to look into the TVs next. (The start of this road to discovery began at stereograms...) If you ever take a look at it yourself and make a post on the subject then do send me a link.

That's right (first paragraph). And if you paused it you would only see one of the frames. And if you were wearing the glasses one eye wouldn't see anything!

As for paragraph starting "Makes total sense", the trick is to realise that the first layer of the filter is the SAME for both eyes. It's a quarter wave plate. But remember, a quarter wave plate doesn't filter anything. it only converts. It converts light that's circularly polarised in a clockwise direction into horizontally polarised light and converts light that's circularly polarised in a ANTIclockwise direction into VERTICALLY polarised light (or the other way round depending on how it's set up). So without the second layer blocking one or the other you'd be getting both images in both eyes.

And the reason it makes head tilting better is because it eliminates 1 of 2 problems with head tilting. So head tilting is still not optimal viewing but it's better than before! The 1 problem it eliminates is ghosting where part of one image sneaks into the wrong eye. With a purely linear filter system (like at the IMAX) when you present vertically polarised light to a horizontal filter none of it gets through. But if you tilt it slightly then there will be a small component of the light's oscillation in the direction of the filter, and that little bit will get through. The other problem with tilting is that the images won't exactly line up. Here's another post about 3D by the way http://blog.stevemould.com/3d-pictures-of-me-and-ben-miller/. Some stereograms in there but not autostereograms sadly. I was obsessed with them when they came out!

If you ever build your own 3D set up let me know!

I think you're right about 3D TVs that don't require glasses. They use hologram technology. The problem with them is that there is 1 sweet spot. One place to sit where it works so it's not a communal affair. Though I think they're working on ones that have more than one spot.

YEEEESSSS! That makes perfect sense! The total sum of an out of phase wave will make the wave move in a circle!!! Ah thank you so much I knew you'd be able to explain it to me. So to set the light wave to move anti clockwise am I right in thinking that the horizontally polarised wave would have to initially have an amplitude in the positive of the x axis from the source (where y is the direction of the amplitude of the vertically polarised light, z is the direction of the light that both waves oscillate along and x is the direction of the amplitude of the horizontally polarised light), then to set the light to move clockwise the horizontally polarised wave would have to initially have an amplitude in the negative of the x axis from the source?

So wait, does this mean that for these type of cameras there are actually 4 light sources? Two that have horizontal polarisation and two that have vertical polarisation, one of each feeding into either a clockwise or anti-clockwise direction? (Though I suppose theoretically with some mirrors you could feed the horizontal and vertical through from only two light sources.)

I'm still unsure as how the camera would be able to change the phases between the two. Could it be a distance thing? Like one light source is half a wavelength more forward? Would that work? Seems a bit too miniscule of a distance to do that....I found a waveplate on Wiki ( http://en.wikipedia.org/wiki/Wave_plate ), I'm guessing it is to do with this.. though how that works... ehhhh.....

I don't actually understand birefringence, I mean as far as the basics go I do, I know that some materials hold this property (are there crystals in the movie cameras?) but I don't understand what the 'birefringence' property relies on in the structure of the material, or how one would actually artificially be able to create something of the like to the specific polarisations they want. Besides... how would you be able to get something out of phase from within a crystal..... Any ideas? I found these prisms by the way: http://en.wikipedia.org/wiki/Wollaston_prism , http://en.wikipedia.org/wiki/Nicol_prism so it seems the trick is in cutting the crystal and gluing back together, why would that work to separate it? And how would one put it back together to cause the circular polarisation?

As for the circular filters on the glasses... does the blocking of light entering have to do with the angle of incident? Like if the light corkscrews towards the glasses from one way it will hit it from a different direction and angle than from the light rotating in the other direction (I guess). I know I should probably accept it as "the glasses have a special filter" and leave it at that, but I'm a curious cat and am wondering what these crystal lattices actually are?

I'm holding up a pair of the Vue glasses in front of me and I see no difference between the lenses (is it wrong that I thought I would?), they just seem like transparent tinted (tinted due to not letting all light through I guess) plastic.... how are these lenses actually made and what from?

If a projector didn't have two lights and two lenses how would a single reel of film (I'm just going to say reel for ease even if it is a digital file) hold both images in one image (in the way a red and blue stereoscopic image could) and manage to polarise the light differently? Surely they will need to come out of separate places to be polarised differently? I know you said that there is the quick alternating version but I suppose that is in the cinemas using special glasses that alternate quickly between which eyes you see through. I'm really pondering over the kind of 3D they have in Vue cinemas because as I understand it they are using the most advanced 3D technology available to the public. (Or not?)

It was exactly because of the glare thing that I asked if the light would lose polarisation when bouncing back because when glare happens usually the polarised light is shone directly into your eyes not on to something else that you look at. I never realised they would have to change their screens to a more reflective material, I wonder how that effects the flat films that they play.

I think you're right about the clockwise/anti clockwise. But to be clear(ish!)... to be spiralling clockwise you'd need the vertical component to be at maximum positive when the horizontal is at zero and increasing. For anti clockwise you'd need the vertical component to be at maximum positive when the horizontal is at zero and DECREASING.

All this stuff about polarisation is to do with the projector not the camera. You can basically forget about the camera setup safe in the knowledge that it's just two regular cameras strapped together about the same distance apart as a pair of human eyes.

The polarisation comes into play when you've got these two reels of images and you need to get one into one eye and the other into the other.

The way a 3D projector can have just one lens is to do with frame rate. A regular film shows 24 frames per second. That's 24 still images that change so fast our brains are tricked into thinking they are watching something moving. What if, for each of those individual frames, you flicked back and forth between the left and right eye images loads of times (and switched the filter in front of the lens at the same time)? That's exactly what they do at RealD at VUE (according to wikipedia). They show left and right 3 times each adding up to 144 frames per second.

The wave plate thing is exactly right. The first stage (or layer) of the circular filter is a quarter wave plate. That means that it will shift one axis by a quarter of a wave relative to the other axis. That means light going in that's circularly polarised will be linearly polarised when it comes out. The details of how a crystal can do this I'm not sure about. But it is to do with the thickness. If you make it thinker it becomes a half wave plate and so on.

So the first layer, the quarter wave plate, doesn't filter anything, it's just a converter. The second layer is a linear filter that filters what comes out of the quarter wave plate.

You wouldn't expect the two filters to look different because your eye can't tell the difference between types of polarised light.

The filters on the projector are the same as the ones on the glasses but flipped over. So the light is filtered first by the linear filter layer. Then that linear light passes through the quarter wave plate putting one axis out of phase with the other and turning it circular.

Interestingly, these crystals that treat the different axes differently aren't quite perfect because they also treat different wavelength slightly differently meaning that red light goes slightly less that a quarter out of phase and blue goes slightly more. If you put the two lenses in front of each other and rotate them you can see this effect.

Phew!

Hi there, sorry to stop by and harass you, but someone who explains polarisation in terms of "up and down light" and "side to side light" is the kind of person I'd love to help explain a few things to me.

So here is how I understand the principal of how the 3D projectors work. I'm guessing that there are two projections occurring simultaneously, so two lenses? I know you can get projectors with prisms in them that separate RGB but for it to show two images it would need light from two separate lenses polarised differently.. does this mean that for a 3D film there are two reels of film running simultaneously showing slightly displaced camera angles taken during filming? So would it be fair to say that for every movie, there are actually two movie reels, with one shot from a slightly different position?

I'm curious about the polarisation. Actually fascinated by this whole spiraling light thing you talk about! So if a projector in a 3D cinema is as I imagine and has two light sources projecting two images, one light is covered with I guess tiny horizontal slats and the other light covered with tiny vertical slats. Now, how in the world would one get light to actually sprial? I'm trying to imagine the filter covering the light that could do that..... but I can't. Also how would each glasses' lens be able to pick up a different spiral of light? If light is moving in a circular way surely it would get through both lenses the same? What difference could the filters on each possibly have? Please let me know how! I feel my mind melting contemplating it....

So if now I know how the filters make one spiral one way and one the other way, I'm wondering how the glasses' lenses transform the spiraling light into horizontal or vertical light? I mean the point of the glasses is to cut out different types of light waves. So really if there is a dual layer of filters on an eye it should block out all of the incoming light (the way it does when you cover the left eye lense with the right eye lense), yet as I am imagining it as one layer with a spiral hole on it and one layer with either horizontal or vertical gaps, I could see how if you overlay that there will be points where the light will get through... but then both sides of the glasses would let some of the same points of light through though no?

Also another thing I was thinking... which probably sounds a bit dumb. If the light is polarised when projected onto the screen, when the light bounces off the screen and back to our eyes in the cinema why doesn't the light change polarisation?

Sorry for the massive thought outburst here, but if you do fancy enlightening me a bit then I'd be very grateful!

Hi Zoe. Thanks for the questions. Here we go...

As I understand it, the projectors can work in different ways. The simplest to understand is how you describe it - 2 lenses, one shining light polarised vertically and one shining light polarised horizontally. A filter in front of each lens would do the trick. In this set up there would be two reels playing simultaneously, showing slightly displaced camera angles as you say. In reality, all modern 3D systems are digital so there's no physical reel of film.

Lenses and bulbs are some of the most expensive components of a projector so having 2 really puts up the price. So some projectors just have one lens and alternate very quickly between projecting the left and right images. At the same time, the filter in front of the lens is alternating between the horizontal polarising filter and the vertical polarising filter.

As for your question about spiralling light, that gets to the heart of what it means to polarise light. Light travels as a wave. But this isn't any normal wave like you could easily imagine because it's quantum mechanical. A lot of stuff in quantum mechanics defies understanding in a nice visual way in your head. I've had the same problem trying to understand this topic. But I'll give it a shot!

Suppose I had some light that was polarised at a 45 degree angle. I could look at it differently and say, this light have 2 components - a vertical component of oscillation and a horizontal component of oscillation that add up to an oscillation at 45 degrees. But what if I was able to shift the vertical component so it was out of phase with the horizontal component? Now instead of the two components being at a maximum at the same time and a minimum at the same time, you have a situation when when one component is at a maximum the other is at zero. The other won't be at a maximum until the wave as moved along a bit. The overall effect is that the angle where the total of the components is at a maximum goes round in a circle.

The circular filters work in two stages. Two layers. The first layer turns the circularly polarised light back into linearly polarised light by bringing the two components back into phase. The second layer is just a linear polarising filter.

Your mental picture of tiny slats is helpful. But in reality it's a crystal latices that has special optical properties in one direction that allows light oscillating in that direction to go through.

The first layer of the circular filter is able to bring the two components back into phase because in this crystal light travels at slightly different speeds depending on which direction it's oscillating. So you just have to make it the right thickness so that bay the time the light leaves, one component has gone through an extra quarter of a wave compared to the other.

The last question about bouncing off the screen is a good point. I think the cinemas have to get new screens that are more reflective and that preserve the polarisation of the light. A simple matt white screen for destroy any polarisation (I think) where as a mirror would preserve it all. You need a compromise between the two.

Interestingly, when sunlight reflects of a puddle for example, the light is polarised. That's why polarising sunglasses are good for driving in wet condition because the reduce glare. If you were to turn the lenses in those sunglasses though 45 degrees they would loose that property.