Both active and passive 3D glasses are used with display devices that show a movie as two offset streams of imagery. For a viewer, essentially this means that each eye receives slightly differing information, which – without conscious thought – the brain combines to form a sequence of three-dimensional images.
However, the method that the glasses employ to achieve this differs somewhat. First, with active shutter 3D the streams are displayed as an alternating sequence of matching frames so rapidly that it looks like they are transmitted together. The glasses respond to wireless signals from the display device by applying voltage to a liquid crystal layer in each lens successively. This basically makes the lenses ‘wink’ at the speed at which frames are swapped.
Passive 3D glasses, on the other hand, have no circuitry and essentially just filter light. Passive displays show two streams of images simultaneously as light waves aligned at different angles. The glasses act as polarising filters that reflect back the light wave matching the alignment of their polarisation. If working correctly, both types of glasses ideally ensure that each eye only ever sees one stream of footage and the brain does the rest of the work, fusing the two streams into a 3D scene.
Written by Michael Simpson.