How does a stereo 3D projection work?

Our two eyes have a particular distance from each other. If we look at an object, each eye sees the object from a slightly different angle. Therefore, each eye delivers a different image of the object to the brain. The visual center in our brain processes the information from both eyes to a three-dimensional image. This is how we see height, width and depth.

Conventional visualizations are two-dimensional and we see the same image with both eyes. We receive height and width information. Our brain must add the depth. 3D stereo visualizations work with two different images that are either simulated on a computer or taken by two different cameras positioned next to each other at a distance equal to that between our eyes. In order to produce a 3D stereo vision, we simply must make sure that we see the left image only with the left eye and the right image only with the right eye.

In passive 3D projections, two projectors reproduce the left and right image simultaneously on the screen. The required separation of the channels is achieved with the help of polarization filters that are fitted in front of the projector lenses and are used in the special 3D glasses. Polarization filters in combination with 3D glasses make sure that the viewer sees the left image only with his left eye and the right image only with his right eye. 

A drawback is the loss of brightness, however, which is always involved when using polarization filters. The underlying principle is: the higher the transmission factor respectively the transmission rate of the filter, the brighter the projection. Traditional polarizers typically have a transmission rate of 38 %. TrueColor polarizers achieve a rate of 46 %. 

Passive 3D stereo can be used for front and rear projection. In both cases, the selection of the material is essential for the quality of the results. A front projection requires a special silver screen. For a rear projection, special rear projections plates or rear projections films are needed. Traditional projection material is unsuitable, because it does not allow a separation of the channels. This drawback can produce undesirable echo images, which are referred to as "ghosting".

The benefits of passive 3D stereo:

• Simple and inexpensive system
• Excellent channel separation
• Even longer watching is not tiring for the eyes 



The drawbacks of passive 3D stereo:

• Loss of brightness caused by the polarization filters
• Front projections require a non depolarizing silver screen
• Rear projections require non depolarizing rear projection plates or rear projection films