How do 3D films work?

Avatar. © Twentieth Century Fox3D films like Avatar trick your brain, bringing images projected onto a flat cinema screen to life in full three dimensional glory.

If you look at an object near you and close your left and right eyes in turn, you’ll see that each has a slightly different view of the world. Your left eye sees a bit more of the left side of the object, and your right eye sees a bit more of its right side. Your brain fuses the two images together allowing you to see in three dimensions. This is known as stereoscopic vision.

To create a similar effect, 3D films are captured using two lenses placed side by side, just like your eyes (or by producing computer generated images to replicate the same effect).

In old fashioned 3D films, footage for the left eye would be filmed using a red lens filter, producing a red image, and footage for the right eye would be shot using a blue filter, resulting in a blue image. Two projectors then superimposed the images on the cinema screen.

3D glasses with blue and red filters ensured viewers’ left and right eyes saw the correct image: the red filter would only let red light through to your left eye, and the blue filter would only let blue light through to your right eye. Your brain would then combine these two slightly different images to create the illusion of 3D. Unfortunately, this meant that old fashioned 3D films couldn’t make full use of colour.

To get around this problem, modern 3D films use polarised light instead of red and blue light.

What is polarised light?

A polarised light wave vibrates on only one plane. The light produced by the sun is unpolarised, meaning it is made up of light waves vibrating on many different planes. It can however be transformed into polarised light using a polarising filter.

A polarising filter has tiny parallel lines etched into it, a bit like the slats on a set of venetian blinds. This means it will only let light vibrating on a particular plane through.

£D glasses. © HowStuffWorks.com
As with old fashioned 3D, the film is recorded using two camera lenses sat side by side. But in the cinema, the two reels of film are projected through different polarised filters. So images destined for viewers' left eyes are polarised on a horizontal plane, whereas images destined for their right eyes are polarised on a vertical plane.

Cinema goers’ glasses use the same polarising filters to separate out the two images again, giving each eye sees a slightly different perspective and fooling the brain into 'seeing' Avatar's planet Pandora as though they were actually there.

Find our more about polarised light with physics.org

How IMAX 3D works

Embedded youtube video

 

07 January 2010

 


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