A Step Towards the Classification of 3-D Displays
A recent article in a prestigious journal has inspired this blog. In it was included a foldout chart classifying stereoscopic moving image systems. The chart was obscure and confusing. I prefer to have people understand stereoscopic imaging, and the chart is of no help. It isn’t as if the classification of stereoscopic imaging systems is at the same level of complexity as that of the Periodic Table. But classifying a technology family, a system created by the human mind, can also be a challenging.
I divide the field into two parts: projection systems and other displays such as flat panels. There are only three ways that a plano-stereoscopic image (a two-view system) can be selected. You can select the image based on color (or wavelength), time, and polarization. One interesting thing about this is that the time selection technique can work with the other two. In fact, for the present generation of stereoscopic projection systems using the Texas Instruments DMD engine, time is combined with the other two methods to provide a shuttering system; so you can’t classify any of the present projection systems under one category, except for shuttering eyewear that use time for selection.
You can have a train of left-right images, variously described as field-sequential or time-multiplexed, in which shuttering eyewear like CrystalEyes are used. XpanD eyewear are used in some theatrical installations and their eyewear shutters open and close in synchrony with the video field rate and, by a well-known principle, when the left image is on the screen the left eye is seeing an image (and the right is blocked), and vice versa. Provided that the repetition rate is high enough, you see left images with the left eye, right images with the right eye, and a good-quality stereoscopic image (if everything else is done correctly) results. This is time-division multiplexing or temporal multiplexing, and it uses a shutter. There are two other systems that I alluded to. One uses color and the other uses polarization. As noted, both are combined with the temporal multiplexing or shuttering approach for projection.
Color selection has been called the “anaglyph,” and I am not going to depart from that terminology. Generally anaglyphs use broad filtering for two halves of the visual spectrum–one towards the reddish end and one towards the blue end. Such a technique can use two projectors, or the images can be combined on a single file or print and projected using a single projector.
The Dolby system is the modern version of the anaglyph, and they license the technology from INFITEC GmbH. It is a wavelength selection system but it uses very narrow spikes of filtration at three parts of the visual spectrum which are at different locations for the left and right eye. In this way you can get good color images, unlike the traditional anaglyph (which I find to be an abomination for 3-D projection). Dolby uses a spinning filter incorporated into the projector so that the output is a sequence of field-sequential color-encoded images. When combined with proper selection device eyewear, the left eye sees only the left train of images and the right eye sees the right train of images. One advantage is that you don’t need shuttering glasses, which are electronically driven devices that presumably would be more costly than passive devices that employ filtration. Unfortunately, this isn’t the case with regard to the Dolby system in which the selection devices’ lenses, using retarder stacks, cost about as much as the shuttering eyewear.
The next system I’ll describe uses polarization. There are two kinds of polarization: linear and circular. Circular polarization is outputted by my invention, the ZScreen. The ZScreen, when combined with the single DMD projector, produces a selection technique that can be classified as both polarization-selection and temporal-selection. When you’re designing a system like this you have to pay attention to both the polarization and shuttering aspects of the design. That means you need a polarization-conserving screen, which both the Dolby and the XpanD shuttering system don’t require. The ZScreen alternates the characteristic of polarized light at the frame rate to produce alternate trains of left and right images with polarization encoding. When you put on polarizing glasses using this system you’re actually looking through a shutter and the parts of the shutter are distributed among the eyewear, the screen, and the ZScreen. You can say the same thing about the Dolby system. The Dolby system is a shuttering system as well as a color-selection system, with the parts of the shutter distributed among the eyewear, the screen, and the spinning color wheel incorporated in the projector.
For polarization, what has been done since the late ‘30s (and perhaps even earlier) is to put polarizing filters over the left and right projectors. You need a polarization-conserving screen and everybody wears 3-D glasses with polarization filters which could be circular or linear.
For projection systems the wavelength and polarization techniques are combined with the temporal technique for a single-display-device projector (in other words, a single projector coming out of a single optical path). For flat panels or electronic displays of any type we can do the same thing, and we can also use a dot- or line-sequential (spatial) approach. If you have a single display, somehow or other on the surface of the screen, either in time or in space, you need to share the image. This sharing is then combined with the other selection techniques by a means that is analogous to what has been described with regard to single projector systems.
Let’s take the case of a liquid crystal display, because that is the dominant display and will be the dominant display for years to come. If you could make liquid crystal displays run fast enough, you could view the display stereoscopically using shuttering eyewear, for example. The only viable means art this time is line-sequential selection combined with microscopic polarizer. The dominant player in this field is Arisawa Corporation, and they manufacture a micro-retarding device that is applied to the surface of a liquid crystal display. Because the display already uses a linear polarizer for image formation the combination produces areas of left and right handed circularly polarized light. This produces a line-sequential display that alternates, in the case of their embodiment, left- and right-handed circular polarization states in horizontal lines.
The CRT display is history for the most part but for years it dominated desktop stereo displays using the field-sequential technique. Flat panel displays can, without equivocation, locate each pixel, but you can’t do that with a CRT. However, CRTs are fast enough to use either shuttering eyewear or a polarization modulator placed in front of the screen. CRTs have been supplanted almost entirely by liquid crystal displays. That’s too bad, because they work very well for stereoscopic desktop applications for scientific imaging and visualization.
The other type of display that has characteristics similar to a CRT display is the rear-projection television (RPTV) sets made by a number of companies who license the technology from Texas Instruments. To get a high resolution of 1920 pixels, and because of the physics of the DMD engine, they use the diagonal interlace technique, also called the checkerboard technique. Because of the rapid refresh capability of the DMD they are able to use a time-sequential technique, so shuttering eyewear can be used with these kinds of monitors and produce an excellent image (albeit half resolution in each eye).
After I completed this blog article I hit upon the classification system I will present in the next blog (how I hate the sound of that word) article.
Source: Lenny Lipton
