Passive technology

Today, two types of passive technologies exist.

1/ Polarization:

This technology uses circular polarizers (thin plastic film with specific characteristics) with different properties for the left and the right eyes. This difference allows filtering the good image for each eye. We talk about left handed and right handed polarization. For instance, the glasses transmit the left handed polarization and block the right handed polarization for the left eye and inversely for the right eye. The use of these glasses implies to polarize the image. As the projector doesn’t polarize the image naturally, it is necessary to add a special liquid crystal filter (Z-screen) in front of the projector. This filter is synchronized with the projector to polarize differently the left and the right images.

Nevertheless this system requires a special metallic screen (silver screen) to ensure an optimal reflection of each polarization. Without this screen the degree of polarization of the reflected images will decrease drastically generating the appearance of a strong ghosting. Then it will be impossible to watch the 3D content.

It is also possible to work with linear polarization. The principle is the same as the circular polarization except that the extinction quality depends on the direction of the glasses. Indeed, the angle of the head leads to the appearance of ghosting because the polarization axe on the glasses is no more orthogonal to the polarization axe on the filter in front of the projector.

3D principle: Polarization

2/ Color filters

The oldest technique of this filtering method is the anaglyph one where there is a red filter for the left eye and a blue filter for the right eye. Today the technology has been improved but the principle is the same. It consists in selecting a narrow band light for each eye.

In a single projector, a rotating wheel synchronized with the images is placed between the light source and the imaging system. This wheel is divided into two parts: a half which selects the spectrum of light for the left eye and the other half for the right eye. In order to filter the good image for each eye, the glasses are constituted of thin layers deposit technologies. Their production process includes more than a dozen layers.

This technology doesn’t need to use a metallic screen. It works very well with a white screen.

3D principle: Color filter