It isn’t too hard to appreciate how a camera with two lenses is able to record the two (left-eye and right-eye) images necessary for 3D. What’s much more difficult is designing a viewing system that ensures that each eye sees only one of those two images. Over the years countless types of stereoscopic display have been devised.
Active 3D: active shutter glasses
The most common 3D techonology is the active display, so-called because you have to wear glasses that contain active electronic circuitry. The left and right images are displayed one at a time, the hardware swapping between them in quick succession.
The active glasses have lenses which can be made transparent or opaque electronically. Circuitry in the glasses controls the lenses, swapping between left eye opaque / right eye transparent and left eye transparent / right eye opaque in rapid succession, synchronised to the swapping of the on-screen images so that each eye sees only the image intended for it.
This means the display has to show two separate images in the same time it would usually show one. Returning to our UK example, the display would have to show 50 images per second instead of 25.
Active displays work by displaying left and right images alternately and must be used with special glasses containing built in electronics.
Passive 3D: passive glasses
The next most common 3D technology is the passive display. Rather than interleaving the two images in time, they’re interleaved in space. In other words, a single image is composed of horizontal lines from both the left and right stereo images which are displayed alternately down the screen.
Immediately in front of the screen’s LCD panel is a polarising filter which is made up of alternate horizontal strips of right-hand and left-hand polarising material that line up with the horizontal stripes of the image. This means that the stripes of one of the images becomes left-polarised and those from the other are right-polarised. The passive 3D glasses (they don’t contain any electronic circuitry) used with this sort of display also contains left-hand and right-hand polarising filters, one over each eye.
A left-polarising filter will allow left-polarised light to pass but not right-polarised light and vice versa, which is how each eye will see just one of the images. The drawback with this method is that the horizontal resolution is halved: a full HD screen using passive 3D can show only 540 'lines' from each 1920 x 1080 pixel image.
It's possible that a better-equipped monitor with twice the vertical resolution could overcome this limitation, but none exist in the shops today. On the plus side, passive glasses are also a lot less expensive (a few pounds) than active ones (that cost upwards of £50 per pair) and are slimmer and lighter.
Passive displays work by displaying left and right images in alternate stripe and are used with glasses containing polarising filters.
Glasses-free 3D screens
The third technology, which is surely going to become more popular in future is the auto-stereoscopic screen, or 'glasses-free 3D'. This means that no viewing aid such as glasses has to be used. Nintendo's 3DS is probably the most common device used today with a 3D screen that doesn't require special glasses.
As with passive 3D displays, the two images are interleaved on the screen as thin stripes but in this case vertical ones, not horizontal. Instead of the polarising filter, though, there's something called a lenticular sheet in front of the screen which is comprised of thin vertical cylindrical lenses that cause the alternate stripes of the two images to be directed left and right towards the eye they are intended for.
Apart from the halving of the vertical resolution, the main disadvantage is that the 3D effect is visible only for a narrow range of viewing positions. For a TV that’s going to be viewed by several people this is a show-stopper. However, lenticular monitors and laptops are available to buy.