The basic functionality of passive 3D technologies is based on light polarization. Light polarization is a property of light (actually, 'light orientation'), that even though we are not aware of it in every day life, is still there.
When we look around, we see light coming from all sources and bouncing on all surfaces around us. We care about properties of light such as wavelength (color), but we can not see the orientation of the light, amongst other things, because is of no relevance for us.
Nonetheless, light orientation offers some interesting behaviors, such as polarization. When we polarize light, we filter the light not by its color, but by its orientation.
Polarization of light works more or less like in a house moving; the movers try to pass a large table through a door, and if they orient the table in the adequate position, they may be able to move the table through the door, but if they rotate the table in the wrong way, the table will be stuck.
At light levels, this may look something like this:
The light that is emitted from the source comes in all kinds of orientations, but after passing through a polarizing filter, only one orientation (or a small range of orientations) goes through. If a light beam that is oriented some other way tries to cross through the filter, it will be blocked.
In a stereoscopic system used to display 3D content, what we want to accomplish is to display two images, each one for each eye, and that each eye receives only the image intended for it, not the image intended for the other eye. The passive 3D systems based on light orientation emit from the TV set both images at the same time, but each image with different orientation.
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You may get an idea of the functionality of these systems with the next interactive explanation. The 'slits' in real polarized filters are of microscope magnitude, and can not be seen with the naked eye (or represented in the screen in their real size, unlike the slits in the interactive presentation below). In real passive 3D sets you do NOT see lines across the screen (as you don't see them in the theater either).
You require Adobe Flash player to view the next interactive presentation.
What the TV set normally does is, it divides the screen in 'even' and 'odd' lines. The screen is made by lines of light dots, or pixels. Alternately, each line of pixels may polarize the image in some direction, and the light of that line of pixels may be blocked with a polarizing filter oriented in the perpendicular direction.
You may see an interesting effect when you place two polarized lenses in perpendicular directions: the result is that the light can not cross through them (but in the glasses used for TV this effect may not be as explicit as in the image below).
As only a part of the light emitted from the screen reaches the eye (because the lenses and the TV set itself block a great deal of light which is not oriented in the same way as each filter), the image received by the eye may be darker than the image of conventional TV sets, but this may get compensated with adequate brightness and contrast by the device.
Compared to other systems, passive technologies offer some advantages: as they require more or less generic glasses (which are normally cheaper than glasses for active systems, for example; these are the same glasses used in theatres), more users can watch the movie in the same screen without scaling costs. TVs that use passive technologies are commonly more expensive than active sets, but with the purchase of additional glasses in an active set, the difference may go the other way.
