There are two types of 3D glasses; passive and active. Passive glasses are designed with polarized lenses and the left lens is polarized differently than the right. With this method, the shutter is created in the projector, either with a third-party polarizer placed in front of the lens (similar to a filter), or a third-party color modulator placed inside the projector – electronically upstream of the projector's DLP (Digital Light Processing) chip. Active lenses, on the other hand, have a built-in electronic shutter, which makes them more expensive. These glasses are synchronized to the projector via an infra-red beam emanating from the screen. No additional hardware is required in the projector. Both processes are driven by frame pulses in the movie's digital file (as stored on the server). The pulses identify the left and right frames. For the "passive" method, the pulses tell the third-party polarizer which frames to turn on and off, in the proper sequence, thus creating the shutter. When this "pre-polarized" light reaches the passive glasses, the left eye only sees the left frame, and the right eye is blanked. In the next frame the right eye sees the right frame, and the left eye is blanked – and so on, throughout the movie. In the case of "active" glasses, the pulses drive an infra-red beam which synchronizes the frame sequence to the glasses. When a "left" frame is projected, the beam tells the glasses to open the left eye and shutter the right. When a "right" frame is projected, the glasses' open the right eye and shutter the left – and so on.
From "Video World," by Paul Berliner, PLSN, Dec. 2009
