Three-dimensional films have been around for decades, although James Cameron’s recent blockbuster, Avatar, has thrown 3D back into the spotlight. Beyond the hype, however, the phenomenon raises the fundamental question: how is it that 3D glasses actually work?
3D glasses rely on the fact that humans, like many other animals, have binocular vision. Your eyes view the world from slightly different perspectives, which enables your brain to accurately calculate distance, in addition to providing you with a wider field of view. Other animals, such as rabbits and buffaloes, have their eyes positioned on opposite sides of their heads. This maximises their field of view, but does not provide them with the same depth perception.
The ability to make fine depth discriminations using the slightly differing images from both eyes is called stereopsis. Thus, three-dimensional imaging is often alternatively called stereoscopic imaging, because of the illusion of depth created in the image.
3D glasses take advantage of the fact that your brain is trained to use the two slightly different images coming from your eyes to create one comprehensive image. The glasses essentially feed your eyes two different images, with differences in the images emulating the differences normally perceived by your eyes as a result of their spacing a few centimetres apart.
This double-image phenomenon occurs naturally in the real world, but does not occur naturally when viewing a single, two-dimensional image on a television or movie screen. In a three-dimensional programme, the screen displays two images, instead of one, and your glasses discriminate and filter these two images, so that each of your eyes is seeing one. The result is three-dimensional vision.
The original system for 3D vision relied on different colours, such as red and blue, or red and green. One of the images is displayed on-screen in each of the two colours, and coloured glasses filter the images so that each eye only receives one. The result is 3D vision, but the use of colour to separate the images limits the spectrum of colours that can effectively be used in the film.
More recently, polarised glasses have been used to achieve the same 3D effect without compromising colour. Each of the lenses on the glasses has a different polarisation, which separates the two images being displayed on-screen.