One of the most significant changes in this system, in comparison to an ImmersaDesk, is the use of passive, polarized stereo. To achieve the stereo effect, two projectors are used for the single screen, one for each eye's view. Differently polarizing filters are placed in front of each projector lens, and then users wear polarizing glasses where each lens only admits the light from the corresponding projector.
There are a few reasons for the switch to passive stereo. First, it allows us to use LCD projectors. The traditional active stereo approach requires high video frequencies -- 96Hz or higher in order to guarantee that each eye sees a flicker-free image. This in turn requires the use of CRT projectors, as LCD and DLP projectors cannot handle these high frequencies (except for new, very expensive DLPs). Good CRT projectors are very expensive, whereas LCD projectors are very common and much more affordable. In addition, LCD projectors can be much brighter than CRT ones; one major flaw of the CAVE has always been its dimness.
In addition to the cheaper projectors, polarized stereo glasses cost much less than the LCD shutter glasses used in active stereo. The polarized glasses are also less fragile, which is important in a museum or other public environment, where they will be subject to rougher handling than in a research lab.
The polarization for passive stereo can either be linear (where the light for one eye is polarized at right angles to light for the other eye) or circular (where the light for one eye has left-hand polarization and that for the other has right-hand polarization). Linear polarizers are more common; however, when linear polarization is used, viewers must keep their heads level or they will lose the stereo effect as the polarization of their glasses no longer matches that of the projector filters. Because we want users to be able to interact freely with the system without affecting the stereo, we chose circular polarizing filters.