Autostereoscopic displays ensure that each eye sees the correct image automatically, that is without the user having to wear glasses of any kind. To achieve this the display needs to incorporate an optical element whose role is to direct different images in different directions to the two eyes. There are four main categories.
Two-view displays These simply generate the individual left and right views and direct them to each of the viewer's two eyes. Typically the eyes must be in a sweet-spot to see the views although some designs can track the viewer's head movements so that the sweet-spot can be moved over a limited range to follow the eyes.
The directional optics in autostereoscopic displays are most often lenticular elements or parallax slit barriers. In the former case the lenses direct light from the left image to the left eye and from the right image to the right eye. In the latter case the slit barrier blocks the left image from the right eye and vice-versa.
Multi-view displays extend the horizontal viewing freedom by simultaneously generating more than than two views. This results in a range of horizontal viewing positions at which the viewer can see a different but valid stereo pair. Additionally it allows a number of viewers to watch the same 3D display at the same time.
The optics used to do this can be lenticular elements or a parallax slit barrier designed to direct the multiple views appropriately in different directions. The number of simultaneous views generated by these displays is frequently of the order of nine or ten, although there are designs that can generate one hundred or more views. Recent research suggests that the number of views does not affect task performance and can be chosen to optimise other factors, for example to extend the horizontal viewing freedom.
These extend viewing freedom further by generating changing views both horizontally and vertically. This becomes closer to the real life experience of being able to look-around and above/below the scene. However the cost to do this is significant and vertical parallax may not in many applications be an import visual cue.
Volumetric displays Volumetric displays optically generate images at different positions in space, unlike all the displays above where the image optically exists in the plane of the display. For example, one design uses a spinning projection screen to produce a spherical image volume, others use vari-focal lenses to position multiple slices at different optical depths from the viewer.