Autostereo implies that the perception of 3D is in some manner automatic, and does not require devices such as glasses—either filtered or shuttered. Autostereoscopic displays use additional optical elements aligned on the surface of the screen, to ensure that the observer sees different images with each eye. 3D autostereoscopic displays (where no headgear needed) are still in the research phase at this time. We describe here displays based only on lenticular or parallax barrier binocular mechanisms (and not, for example holographic approaches).
Lenticular lenses are curved optics that allow both eyes to see a different image of the same object at exactly the same time. Lenticules are tiny plastic lenses pasted in an array on a transparent sheet that is then applied onto the display surface of the LCD screen (Fig. 2.10). A typical multi-view 3D display device shows nine views simultaneously and allows a limited free-viewing angle (some prototype products support a larger number of views). When looking at the cylindrical image on the TV, the left and right eye see two different 2D images that the brain combines to form one 3D image. The lenslet or lenticular
elements are arranged to make parts of an underlying composite image visible only from certain viewing directions. Typically, a lenticular display multiplexes separate images in cycling columns beneath its elements making them take on
the color of selected pixels beneath when viewed from different directions. LCDs or projection sources can provide the pixels for such display . A drawback of the technology is that it requires a very specific “optimal sitting spot” for getting the 3D effect, and shifting a small distance to either side will make the TV’s images seem distorted.
A parallax barrier is a device used on the surface of non–glasses based 3DTV system with slits that allow the viewer to see only certain vertical columns of pixels at any one time. The parallax barrier is the more consumer-friendly technology of the two and the only one that allows for regular 2D viewing. The parallax barrier is a fine grating of liquid crystal placed in front of the screen, with slits in it that correspond to certain columns of pixels of the TFT (Thin- Film Transistor) screen (Fig. 2.11). These positions are carved so as to transmit
alternating images to each eye of the viewer, who is again sitting in an optimal “sweet spot.” When a slight voltage is applied to the parallax barrier, its slits direct light from each image slightly differently to the left and right eyes, again
creating an illusion of depth and thus a 3D image in the brain . The parallax barrier can be switched on and off allowing the screen to be used for 2D or 3D viewing. However, the need still exists to sit in the precise “sweet spots,”
limiting the usage of this technology.
Autostereoscopic technology will likely not be part of early 3DTV deployments. For example, Philips reportedly folded an effort to define an autostereoscopic technology that does not require glasses because it had a narrow viewing range and a relatively high loss of resolution and brightness .