Glasses-free 3D technologies
There are several different approaches to glasses-free 3D display as summarized in the diagram below, which have been pursued by different research groups and companies. The ultimate goal of these systems is to provide the highest possible number of perspective views, while at the same time ensuring the narrowest possible angular width of an individual perspective and the widest possible total viewing angle, thus approaching to the natural perception of physical objects.

Holographic systems: although representing the most perfect rendering of physical 3D objects, this approach requires transfer of colossal data amounts, as well as imaging devices (displays) with extraordinary pixel resolution and high refresh rate, neither of which is attainable at the present level of technology or even in the foreseeable future.

Volumetric systems:
they naturally provide such important visual cues as motion parallax and toe-in, but are not able to create the occlusion effect (since images created in such systems are translucent) and cannot deliver sufficiently high 3D resolution. Because of their physical volumetric principle, these systems have large physical dimensions and are quite heavy.

Multiple-projection systems:
known largely by the name “Holographica” after the company marketing such systems, they rely on a large number of individual display devices (one for each perspective view, sometimes more than 50) and a special back-projection screen based on lenticular sheets or holographically recorded optics. These systems provide good-quality 3D images, but their very heavy weight, daunting dimensions, and excessive power requirements effectively preclude their adoption in all but very special niches.

The 3D display techniques currently considered the most mature and perfected are commonly referred to as multi-view auto-stereoscopic technologies. They also seem to strike a reasonable balance between complexity and image quality compared to holographic systems on the one hand (allowing a near-perfect 3D image perception, but technologically challenging) and simple stereoscopic (two-view) devices on the other. There exist two general methods for creation of multi-view auto-stereoscopic images called correspondingly space sharing (or multiplexing) and time sharing (or time multiplexing).

The idea of space multiplexing consists in separating the pixels of a display matrix into (approximately equal) groups forming interleaved sub-raster patterns over the display area and using each of these groups to show one of the perspective views. Such means as stationary periodic masks (also called parallax barriers) and micro-lens arrays installed in front of the display are normally used in order to project the output of different pixel groups at appropriate angles to the screen plane. Since the pixels of the display panel are shared between different views, the final resolution of the 3D image is inversely proportional to the number of projected views. Existing flat-panel 3D displays (e.g. from 3D Fusion, Fujitsu, Toshiba, etc.) are based on the space-multiplexing principle. More information about space-sharing display technology and its features can be found at the following link.

Time-multiplexed (time-sequenced) 3D displays rely on sequential projection of individual perspective views and an optical steering component that dynamically projects the image shown on the display panel at a given time and corresponding to a particular perspective into the appropriate angle of view. Because these perspectives are shown one at a time, the display device must be able to update its output at a multiple of standard frame rate (i.e. 60 Hz) proportional to the number of the displayed views. The resolution of such systems does not depend on the number of the perspective views. The patented Zecotek technology using a time-sequenced approach comes much closer than the competition to the ultimate goal of perfect 3D display in providing large number of narrow perspective views at high resolution and without visual discontinuities. An overview of the features and advantages of Zecotek 3D displays can be found at this link. More detailed information concerning the principle of operation of time-sequenced 3D displays and the Zecotek system is available in the section about time-sequenced auto-stereoscopic 3D displays.
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