3D Display Technologies Overview

 

By Andy from Digital Tiger, 1st, Dec. 2008, all rights reserved

Separating Left and Right Eye Views
When viewing stereo pairs, a mechanism is required so that the left eye sees only the left eye view and the right eye sees only the right eye view. There are many mechanisms which have been proposed to accomplish this. The View Master uses two images each directed to the appropriate eye by lenses. The images are shown in parallel and there is no way one eye can see any part of the other eye view.
It is common in display technologies to use a single screen to reflect or display both images either simultaneously (time parallel) or in sequence (time multiplexed or field sequential). The technologies used to direct the appropriate image to each eye while avoiding mixing the left and right eye images require sophisticated electro-optics or shuttering. Some of the more common methods are described below.

Time-Parallel Techniques
Time-parallel methods present both eye views to the viewer simultaneously and use optical techniques to direct each view to the appropriate eye.
3D movies often used the anaglyph method, which requires the user to wear glasses with red and green lenses or filters. Both images were presented on a screen simultaneously; hence, it is a time-parallel method. Many observers suffered headaches and nausea when leaving the theater, which gave 3D, and stereo in particular, a bad reputation. (A phenomenon called ghosting or cross talk was a significant problem. Colors were not adjusted correctly and the filters did not completely eliminate the opposite-eye view, so that the left eye saw not only its image but sometimes part of the right-eye image as well. Other problems included poor registration of the left and right eye images causing vertical parallax and projectors out of synch.) The View Master is another example of a time-parallel method.
An early technique for viewing stereo images on a CRT was the half-silvered mirror originally made for viewing microfiche [4]. The device had polarizing sheets, and the user wore polarized glasses that distributed the correct view to each eye.
Polarizing filters can also be attached to glass-mounted slides. Incorrect positioning of the projectors relative to the screen can cause keystoning, in which the image is trapezoidal shaped caused by foreshortening resulting in vertical parallax.
If more than one projector is used, as is often the case when projecting 35 mm. stereo slides, for example, orthogonal polarizing filters are placed in front of each projector and both left and right eye images are projected onto a non-depolarizing screen simultaneously. Hence, the technique is time-parallel. The audience wears passive glasses in this case. Using more than one projector always brings with it the difficulties of adjusting the images. L/R views should be correctly registered; there must be minimal luminosity differences, minimal size differences, minimal keystoning, minimal vertical parallax, minimal ghosting, and so forth.

Cross talk
Stereo cross talk occurs when a portion of one eye view is visible in the other eye. In this case the image can appear blurred or a second or double image appears in regions of the scene being viewed creating a phenomenon called ghosting. Cross talk can create difficulty in fusing L/R views.
When using the same display surface to project both eye views, cross talk can be a problem. When stereo displays are evaluated, the cross talk issue should be addressed.

Field Sequential Techniques
A popular method for viewing stereo on a single display device is the field-sequential or time multiplexed technique. The L/R views are alternated on the display device, and a blocking mechanism to prevent the left eye from seeing the right eye view and vice versa is required. The technology for field-sequential presentation has progressed rapidly. Historically, mechanical devices were used to occlude the appropriate eye view during display refresh. A comparison of many of these older devices can be found in [4]. Newer technologies use electro-optical methods such as liquid-crystal plates. These techniques fall into two groups: those that use active vs. passive viewing glasses.
In a passive system, a polarizing shutter is attached to the display device as in the case of a CRT or the screen produces polarized light automatically as in the case of an LCD panel. The system polarizes the left and right eye images in orthogonal directions (linear or circular), and the user wears passive polarized glasses where the polarization axes are also orthogonal. The polarizing lenses of the glasses combine with the polarized light from the display device to act as blocking shutters to each eye. When the left eye view is displayed, the light is polarized along an axis parallel to the axis of the left eye lens and the left eye sees the image on the display. Since the axis is orthogonal to the polarizer of the right eye, the image is blocked to the right eye.
The passive system permits several people to view the display simultaneously and allows a user to easily switch viewing from one display device to another since no synchronization with the display device is required. It also permits a larger field of view (FOV). The drawback is that the display device must produce a polarized image. Projector mechanisms must have polarizing lenses and CRT or panel displays must have a polarizing plate attached to or hanging in front of the screen or the projector. When projecting an image on a screen, the screen must be coated with a material (vapor deposited aluminum) that does not depolarize the light (the commercially available“silver” screen). Polarization has the added disadvantage that the efficiency or transmission is poor; the intensity of the light to reach the viewer compared to the light emitted from the display device is very low, often in the range of 30%. Hence, images appear dark.
LCD’s can also be used as blocking lenses. An electronic pulse provided by batteries or a cable cause the lens to “open” or admit light from the display device. When no electronic pulse is present, the lens is opaque, blocking the eye from seeing the display device. The pulses are alternated for each eye while the display device alternates the image produced. The glasses must be synchronized to the refresh of the display device normally using an infrared signal or a cable connection. For CRT based systems this communication is accomplished using the stereo-sync or Left/ Right (L/R) signal. In 1997 the Video Equipment Standards Association (VESA) called for the addition of a standard jack that incorporates the L/R signal along with a + 5 volt power supply output. With this new standard, stereo equipment can be plugged directly into a stereo-ready video card that has this jack.
Active glasses have an advantage that the display device does not have to polarize the light before it reaches the viewer. Hence, efficiency is higher and back projection can be used effectively. The disadvantage is obviously the synchronization requirement.
While the initial cost of the passive system is higher the cost to add another user is inexpensive. This makes the passive system a good choice for theaters and trade shows, for example, where one does not want to expose expensive eyewear to abuse.
In both systems, if the images are delivered at a sufficiently fast frame rate (120 Hz) to avoid flicker, the visual system will fuse the images into a three-dimensional image. Most mid- to high end monitors can do this. A minimum of 100 Hz is acceptable with active eyewear systems. One may be able to use 90 Hz with a passive system without perceiving flicker even in a well-lit room.

From http://research.csc.ncsu.edu/stereographics/wiley.pdf
David F. McAllister
Department of Computer Science
North Carolina State University
 
Tags:stereo,3D,image,passive system