In order to make films and images appear in 3D (stereoscopy), two separately polarized images, which have been taken from different perspectives (angles), are shown on the same display or screen at the same time. To benefit from this effect, the viewer needs to wear low-cost polarized 3D glasses with 2 separate polarizing filters. Each of these filters conveys only one of the images and at the same time blocks the other image; that way, each of the 2 images can be viewed by only 1 eye, the designated eye. This method, together with the help of our brain, creates the desired 3D effect. To enable the best effect in 3D, the viewer should be seated right in front towards the center of the screen, mimicking the position of the original cameras which filmed the two separate images from different angles, creating 3D content.
During the projection of a 3D polarized system, the polarized light deflected from a normal movie picture screen usually loses much of its polarization, but this loss is minimal once a silver screen in 3D is being used.
The most common way to project polarized 3D is by setting up a dual projector system, which would transmit the 3D movie through the use of 2 projectors simultaneously to filter the light originating from them, destined for each eye, in a different way. This is done by positioning separate polarizing filters at the front of the projectors, like as in the case of linear polarized systems adopted by IMAX.
When using DepthQ or RealD modulators, however, a liquid crystal filter which is circularly polarized is put in front of the two projectors, which can change polarity multiple times per second, hence, polarizing each frame differently for each eye. In this case, only 1 projector will suffice, as the images destined for the right and left eye are polarized differently.
Linearly Polarized 3D Glasses
When using the linear technique, the 2 images are shown on the same screen after being filtered in an orthogonally polarizing way (normally set at 45 and 135 degrees or alternatively at 0 and 90 degrees). In this case the viewer would wear 3D glasses which are linearly polarized. The two polarizing filters installed on the projectors, along with the glasses, filter only the designated image to the corresponding eye, thus enabling the 3D effect. Glasses which are linearly polarized compel the viewer to maintain his head straight, as any tilting of the head would make the images of the right and left channels to spill over to the adjacent channel, thus increasing the “cross talk” effect (ghosting) and losing the 3D effect. This fact makes this type of polarization not recommended when viewing 3D for extended periods of time, like a full 3D movie.
Circularly Polarized 3D Glasses
In this 3D system, the 2 images are projected on one screen using polarizing filters which are circular and have opposing handedness. As with with the previous technique, the viewer has to wear 3D glasses, but this time circular polarized glasses which have 2 filters with opposing handedness. Polarized light deriving from left-circular is prevented from entering by the filter on the right, whilst polarized light deriving from right-circular is obliterated by the filter on the left. The end result is quite alike stereoscopic viewing using glasses which are linearly polarized, with the exception that the viewer is able to tilt their head and continue to keep right and left separation.
Circular polarizers, also known as circular filters, are made of two layers: a filter which is linearly polarized and a quarter-wave plate (QWP). This QWP always converts light which is circularly polarized into light which is linearly polarized and vice-versa. The angle of the light which is linearly polarized and created through QWP depends greatly on the handedness of the circular polarized light that enters the QWP. The light originating from the circularly polarized left side which reaches the circular filter is converted into linear light through the QWP, and which follows the course of polarization through the projection axis of the linear filter.
It is also possible to construct a circular filter to block left-handed, rather than right-handed light which is circularly polarized through the rotation of one of the LPF or QWP by up to 90 degrees over a perpendicular axis to the top and parallel to the propagation direction of the light wave.