Autostereoscopic displays are able to provide binocular depth perception without the hindrance of specialised headgear or filter/shutter glasses. The technology has existed for many years, and has been used to provide stereoscopic vision in research environments since the 1980’s.\n \nAutostereoscopic displays fool the brain so that a 2D medium can display a 3D image by providing a stereo parallax view for the user. This means that each eye sees a different image, having been calculated to appear from two eye positions. These stereoscopic displays lack several other cues that are normally used to build up a 3D image:\n[LIST]\n[*][B]Movement Parallax[/B] – the infinite number of images that can be seen as viewing position is moved around the object. An autostereoscopic display only has a single 3D view which has been calculated by the software.\n[*][B]Convergence[/B] – the natural way eyes will converge on an object. On an autostereoscopic monitor they are converging in front or behind the monitor on a virtual object.\n[*][B]Accommodation[/B] – the focusing of each eye on the object. This is at a different distance on an autostereoscopic display as the display and virtual object will be at different distances from the user.\n[/LIST]\nThe two main methods for providing autostereoscopic vision are the parallax barrier and Lenticular lens:\n \n[CENTER][ATTACH=full]6[/ATTACH][/CENTER]\n[CENTER][B]Method of operation for an autostereoscopic parallax barrier[/B][/CENTER]\n \nIn the parallax barrier a mask is placed over the LCD display which directs light from alternate pixel columns to each eye. Parallax barrier displays allow instant switching between 2D and 3D modes as the light barrier is constructed from a layer of liquid crystal which can become completely transparent when current is passed across, allowing the LCD to function as a conventional 2D display.\n \n[CENTER][ATTACH=full]8[/ATTACH][/CENTER]\n[CENTER][B]Method of operation for an autostereoscopic lenticular lens[/B][/CENTER]\n \nIn the lenticular lens, an array of cylindrical lenses directs light from alternate pixel columns to a defined viewing zone, allowing each eye to receive a different image at an optimum distance.\n \nBoth of these methods provide a restrictive view but it would be possible to view an image continuously across the viewing zones if head/eye tracking technology is used. Once the users eye passes from one image band into another the image would usually invert, however if the images shown to each side of the zone are flipped once the eye passes the barrier it is possible to create a continuous image. This will not provide motion parallax as the image view would remain the same but it would make it easier to hold focus of the image.\n \n[CENTER][ATTACH=full]7[/ATTACH][/CENTER]\n[CENTER][B]Correct viewing position of an autostereoscopic display[/B][/CENTER]\n \nIn the example shown above, three users are seen to be viewing an autostereoscopic monitor. The user to the right (“Inverse Image”) is aligned incorrectly and the image formed at each eye is the wrong way round. The “blended zone” user is standing too far away from the optimal distance datum and is seeing both images forming in each eye, causing a blurred and confusing image. The user in the centre is standing in the correct position, with each eye located within the correct viewing zone. Note that if this was a head tracking monitor and the viewers were in an inverse image zone, the monitor could switch the images being projected in each direction. However, for practical implementation of this switching process, it would need to be demonstrated that repeated traverses into and out the inverse image zone would not lead to flicker.\n \nIn order to simulate motion parallax a multiview autostereoscopic monitor must be used, which is a much more complex and costly device. It does not provide full motion parallax but a number of viewing angles can be provided (9 and 16 are common values). Multiview screens do not require head tracking, but do require a lens array to project all the views at once. As the user moves from left to right the viewing zones will transition through each viewing angle, however the processing behind the monitor will still have to project each image stream. This will not only require much more computing power, but may also reduce the resolution if monitor is used as the projecting device as the horizontal resolution is shared across each viewing angle.