ABSTRACT

Following some comments on the nature of stereo perception as it relates to stereo video displays, a number of areas of interest are briefly reviewed and accompanied by extensive citations from the patent and technical literature. These include single camera(70 refs.) and dual camera(100 refs) stereoscopy, compatible 3D recording and transmission(57 refs.), head mounted displays(85 refs.), field sequential stereo(285 refs.), and autostereoscopic systems including lenticular(64 refs.), parallax barrier(22 refs.), stereoptiplexer(17 refs.), integral imaging(24 refs.), direction selective mirrors, lenses or screens(26 refs.), volumetric displays(133 refs.), holovision(13 refs.), stereoendoscopy(14 refs.),and stereosculpting(15 refs.). Interfaces for stereo graphics and the interaction of stereo video and stereo graphics are also discussed.

HISTORY

Stereoscopic television was a goal of the earliest experimenters with this new medium. Electronics pioneers such as Hammond, Logie Baird, Lee DeForest, Zworykin and others described 3DTV devices in their early patents(U.S. 1,725,710, 2,107,464, 2,163,749, British 266564, 292365, 321441, 552582, 557837, 562168, 573008 ). Logie Baird seems to have been the first to actually build working devices. The first commercial device may have been Dumont's dual CRT system that appears to have been sold in the 50's. Experiments with anaglyph(colored eyeglasses) video were numerous and broadcasts were done at least as early as 1953. Anaglyph broadcasts continue to be made sporadically and anaglyph cassettes and videodiscs appear occasionally but this technique, like those employing Pulfrich or prism glasses is hopeless for high quality or comfortable viewing with video, but is better with computer displays. James Butterfield broadcasted side by side stereo images for viewing with prism glasses in Mexico in the 50's. He was one of many to make stereo systems with dual cameras viewed through a binocular stereoscope. He was also one of the first to use polarized glasses to view anaglyphs by placing dichroic polarizers on the face of the CRT, an idea later refined by Benton(U.S. 4,431,265). Polarized glasses for use with dual cross polarized CRTs as well as, interdigitated images on a single display covered with crossed polarizers were proposed many times in the patent literature but the problem of manufacture prevented commercialization until Faris applied lithography to create the micropolarizer arrays(Faris). This may be a viable alternative to field sequential techniques where flat screens or projection are involved, but for CRT's it has the same problem of aligning pixels with the optical elements through a thick glass surface as the lenticular technique. Field sequential devices were described in the patent literature many times without a commercially viable product appearing until the 1980's.

Shmakov, working in St. Petersburg, Russia, devoted much time to this field during the 40's and 50's and wrote the first text on the subject in 1953 but it only appeared in 1958(Shmakov). The proceedings of the SPIE(Merritt and Fisher) and several symposia(Hamasaki-1992) are the best recent sources. The literature on stereoscopic video is large and the patent literature vast. The present review will concentrate on the field sequential technique since it is currently dominant and is likely to remain so well into the 21st century.

STEREOVISION AND ELECTRONIC DISPLAYS

Stereo vision evolved hundreds of millions of years ago in invertebrates as a critical survival mechanism. The first definitive demonstration of stereovision in insects was recently accomplished by a Swiss researcher who glued tiny prisms to the eyes of a praying mantis, which then missed its prey by precisely the calculated amount. Humans have become so genetically degenerate that serious visual problems including loss of stereo perception are common. The vast majority have good depth perception but sophisticated tests show wide variations. The individual variations in stereovision should be of vital concern in the creation and use of stereo systems but are usually completely ignored. As with all other physiological systems, stereovision may improve rapidly with use, both short term and long term. Repeated use of a stereo display can lead to more rapid fusion and greater comfort. Except for a few persons who practice frequently with a wide variety of stereo displays and images, it is not possible to evaluate a stereo display system or image by casual examination. As with any other parameter, a randomly selected individual may be several standard deviations from the mean in either direction including perhaps 10% who have severe problems with stereo under any conditions and 10% who qualify as stereo prodigies due to their rapid, prolonged and comfortable fusion of images which may be unpleasant or impossible for the average person, or to their other abilities such as making very fine depth determinations. Variation with age is to be expected as is a circadian rhythm. Evaluation by a battery of users with known stereovision abilities using the hardware and software exactly as it will be employed by the end user is essential. This should include frequency and duration of use, similar imagery, ambient illumination, viewing distance and exactly the same monitor. The latter is necessary since in the dominant field sequential technique the exact hues and saturations of the images, contrast and brightness and the different persistences of various phosphors are very important. Also, the same hardware and software may yield dramatically different results if the color of figure and background are altered. Long persistence green phosphors are a common problem. Screen size and viewing distance, horizontal and vertical parallax, binocular asymmetries(illumination etc.) and nonstereo depth cues are critical. Most stereo displays and images are created and used with little attention to these factors even when highly skilled personnel are involved. A vital component of a stereo project should be a stereoscopist having extensive experience with many systems and images. This is seldom considered necessary, resulting in defects in hardware, software , viewing conditions and viewers and less than optimal images that are regarded as natural limitations of electronic stereoscopy or of field sequential input or head mounted displays.

It is even said that these are unnatural ways to look at images(as though 2D CRT'S, photos, and books grew on trees). This brings to mind the classic experiments with prism glasses performed three generations ago. When one first puts on glasses which turn the visual world upside down, it is nearly impossible to function. After a few days subjects learn to navigate and the world gradually appears more or less normal. The key phrase in the evolution of most organic systems is "plasticity equals survival". There is even some recent evidence that many strabismic (cross eyed) subjects have some depth perception due to a type of field sequential activation of the optic pathways by the reticular activating system in the brain stem.

FLICKER AND ASYMMETRICAL ILLUMINATION

Another common scapegoat for inadequate hardware, software and lack of stereo training is flicker most noticeable in standard frequency(e.g., 60Hz) field sequential systems. Flicker has been the subject of a great deal of research, nearly all of it monoscopic. It varies with many factors, especially screen brightness and screen size. We must distinguish the flicker due to ambient illumination("room flicker") from the flicker due to the display("image flicker"). In addition, the image may flicker due to high luminosity areas or to low rates of update. The image may still flicker even at 120Hz screen refresh if the image is not updated in the proper way(Woods). Decreasing the level of ambient illumination in the room can reduce the room flicker to imperceptible levels while reducing screen luminosity with brightness and contrast controls will reduce image flicker to low or imperceptible levels. This may reduce the contrast excessively and some level of image flicker is usual in 60Hz displays. A white wall will have a noticeable flicker which is exacerbated if a man in a dark shirt is in front of it and even more noticeable if he has a lot of horizontal parallax. The same conditions will tend to give considerable crosstalk with passive glasses or even autostereoscopic systems. However, when the image lacks high luminosity areas flicker and crosstalk may be nearly imperceptible even at fairly high brightness. This condition occurs frequently in natural subjects and can be avoided much of the time if one has the field sequential display in mind.

It occurred to me a decade ago that one could eliminate flicker by color coding the two images in every field and viewing them with field sequential anaglyphic shutters. Field or frame one would contain the right image on the green phosphor and the left on the blue and red, field two the reverse and the viewer would wear dichroic LCD shutters which would filter out the other eyes image. Each eye would get 60 images a second. The coding could also be done with 3 color dichroic shutters and 3 field or frame encoding. A color flicker would then replace the brightness flicker but since the visual system is less sensitive to color flicker this should not be bothersome. British inventor Graham Street patented this approach and actually tested it with rotating color wheels. He maintains that the resulting image was entirely satisfactory and flickerless. The Liquid Crystal Color Shutter invented by Tektronix and now marketed by several companies would probably be suitable for this use.

A related approach would work with three tube video projectors. An LCD polarizing plate is placed over the each of the three tubes. Field one could have the right image on the red tube and the left on the blue and green and field two the reverse. The polarizing plates would switch their polarizing angle by 90 degrees each field so as to images so as to always permit the right eye image to pass the right eye polarizer of the viewer wearing passive standard polarizing glasses. Again, each eye would get 60 images a second. Both these systems would work best with RGB input from video or computer systems.

An entirely different approach has been taken recently by Sadig Faris of VREX Inc. who has used lithographlic techniques to interdigitate orthogonal polarizers. Alternate strips of such a polarizing sheet can be aligned with LCD projection panels, LCD projectors or LCD, electroluminescent, or plasma screens to give flickerless stereo viewable with standard passive polarizing glasses. In some cases, such as the common LCD projection panels, the NTSC to VGA conversion gives perfect stereo from interlaced field sequential stereo with standard equipment retrofitted with a VREX polarizer. Other advantages are low cost, nonintrusiveness and retrofitability.

LCD displays could be engineered from the beginning to give cross polarized stereo pairs but except for a pair of $100,000 custom LCD projectors demonstrated by Sanyo in 1995, no such product has appeared, in spite of its description by various Japanese researchers.

Asymmetrical illumination of all or part of the image in stereo or autostereo systems will exacerbate the flicker even if the difference is only in a small area and even though it may be only a few percent off. Beldie and Kost, studying an autostereo display, found that asymmetries in the range of 3 to 6dB were noticeable and that for moving objects, even a small area of the image with 0.2dB difference was perceived. In a field sequential system Diner found that he had to take special measures to match the camera illuminations and when they were brought to within a few percent, there was a dramatic reduction in flicker. Perhaps as little as 3% difference in transmission of the right and left lenses of LCD glasses may be too much and none of the glasses manufacturers to date seem to have controlled for this. Once again, one is reminded that a great many stereo projects yield modest results which are blamed on hardware, software, or difficulties with stereo perception, but are really due to poor technique. A high degree of stereoscopic literacy is still a rare commodity.

A useful device to have would be an intelligent white gamma reducer which monitored the video pixel by pixel and automatically turned down the brightness of high luminosity areas. Such devices have been discussed in various contexts, but only Stephens(U.S. 4979033) seems to have specifically addressed stereo. I did an experiment with an expensive digital video device called a DA Vinci and found that turning down whites about 10 IRE units and turning up greys and blacks reduced flicker while retaining contrast.

With stereo graphics, it is even easier to avoid serious flicker by avoiding high luminosity areas. A black wireframe figure on a white field at 60Hz will have a serious flicker while the reverse can have no perceptible flicker provided ambient illumination is modest. This does not mean the room has to be movie theater dark, but just lacking in direct outdoor or nearby overhead lights. It is usually easy to turn up the frequency of PC video cards to decrease flicker and 3DTV Corp. was the first to include an automatic FlickerFixer™ in its software. Most television sets can be driven at higher frequencies than the normal 50 Hz (PAL) or 60 Hz (NTSC). One of the many capabilities of the SpaceStation™ (marketed by 3DTV Corp.) is the production of variable frequency field sequential stereoscopic NTSC or VGA out put from 60Hz NTSC input. Of course, as frequency increases, the number of lines per field eventually decreases. At 72Hz on a Sony TV, there was about 1/4 inch of black at the top and bottom of the screen. Experiments show that most TV’s can run at 66 to 70 Hz NTSC and flicker drops off noticeably even at the lower rate (33 Hz/eye). A variable frequency external FlickerFixer box could be built for about $200 as a consumer item, but a custom LSI chip could reduce cost to $20. Broadcast of higher field frequency signal for stereoscopic programs is also a possibility.

BANDWIDTH, INFORMATION AND STEREO

It is frequently stated that stereo images will be of inferior quality to mono images on the same system since each eye is getting half the bandwidth. With graphics, it is often hard to tell since there is usually no clear reference but with standard video camera imagery, the subjective resolution is often strikingly superior. Ordinary consumer NTSC tv's with well done VHS stereoscopic tapes look equal or superior to any HDTV I've seen. One reason is that stereoscopic acuity(resolving ability) is superior to monoscopic acuity. This is due to the fact that that stereo will usually have a greater information content than mono and the highly sophisticated image processing systems in the brain have been evolved to take advantage of this.

In the extreme case, two views having a million pixels each, taken respectively from the right and left sides of the head will present a richer image processing potential than a single two million pixel image taken from directly in front. Of course, there are a wide variety of possibilities and the relative 2D vs 3D vividness, usefulness and information content will depend on precisely how the images are captured, processed, stored, displayed and used.

There are probably neural hardware functions for edge enhancement, shadow detail, perspective, texture, glitter, sparkle, feature extraction etc. which work only (or best) when the stereo systems in the optic cortex are activated. It is to be expected that these will interact in complex ways. The effects of training, fatigue, motivation, drugs and other factors on perception suggest that these functions are programmable to varying degrees(again, this will vary greatly with the individual). This is fertile ground for research, especially with the recent availability of low cost means for field sequential generation and presentation of stereo images. The SpaceStation™ from 3DTV Corp. and the Tiga Stereoscope from Vision Research Graphics are unique devices for such studies.

SINGLE CAMERA STEREOSCOPIC VIDEO

There are several approaches for creating stereo images with a single camera. One of the simplest and most frequently used has been to place an optical adapter in front of the existing lens. A lens of this type employing liquid crystal shutters was briefly marketed by Azden Corporation in 1990. These lenses have many limitations such as the need to operate at telephoto, ghosting, and lack of control over interaxial, though a recent design minimizes some of these (JAP-1-147444).

Alternatively, various types of mechanical or electrooptic devices can block the light through parts of the optical path to create field sequential stereo pairs (USSR-138273, 369732, 568220, 1125783, 1109959, US-2508920, 4486076, 4943852, 4281341, 5028994, JAP-57-5490 to 5493, 57-14268 and 14269, 57-25783, 59-225692, 62-98895, 63-227192, 1-22187, 1-55998, 1-41397, 1-41398, 1-47192, 1-47193, 1-132294, 57-72134, 63-237687, 57-14268, 57-14269, 57-75089, 57-62686, 56-158590, 56-83193, 83194, 83195, 83196, EP-269075, GER, 3214021, 2032977). The fact that a small interaxial (stereo base) results from dividing the lens into right and left halves means this technology is only good for close-ups. Stereoendoscopes using internal liquid crystal shutters have recently been created by several companies (SOCS, International Telepresence, OLYMPUS).

An interesting variation is offered by cameras which translate in the Z-axis or have elements which cyclically change their index of refraction to give depth information (JAP-61-80992).Limitations of sensors have led to somewhat complicated line scanning arrangements for single sensor infrared stereocameras (4574197, 4682029), but recent advances in sensors and image intensification may make these obsolete. Alternatively, mechanical, optical or electrooptic barriers can divide up the frame or interdigitate the stereo pair on the image surface every field (USSR-510,812,1107344, JAP-51-958, Masters, US-2317875, GER-3205483). Palmer devised a method for getting an over/under wide aspect ratio stereo pair with one or two cameras in 1951 (US-2786096-cf US-4583117, 5049988). Anamorphic fiber optics which could be useful in this application are now feasible(U.S. 5015065). Many of these approaches using single sensors have had as their object the input for an autostereoscopic display (GB-1401003, EP-335282, 4943860, FRE-1362617, US-4945407, 3932699).

If the subject or camera are moving, stereo pairs can be created by various optical, electrooptic, mechanical or electronic means (JAP-1114293, GB-2180719, US-4231642, 5014126). This approach has been the subject of a great deal of interest in recent research in robotics, stereophotogrammetry and pattern recognition. Light can be scanned over the surface of an object from one or more locations and its spatial location, frequency, time of flight or polarization can be analyzed by the multiple elements of a single sensor to yield positional information (US-4945408, 5018854, 5024529, JAP-56-34289). In some cases this technique can replace the lens and camera with photodiodes. Also, two images can be passed through colored filters, completely overlapped every field and separated at a subsequent stage with colored filters or electronically (USSR-873464, 291376). An inexpensive lens of this type is available from Spondon Film Services in Derby, England. Phillips' method of underscanning the raster on tube cameras could give field sequential stereo suitable for closeup work (US-4740839).

Finally, much effort is being expended in pattern recognition on extracting depth information from a single point of view combined with other information about the scene (US-4754327, Lippert, Alvertos). Any of these imaging techniques with one or more cameras can be combined with a wide variety of display modalities including stereoscopes, polarized, prismatic, anaglyphic, mechanical or electrooptic spectacles, or autostereoscopic (no spectacles) means including lenticular, louvered, or parallax barrier screens as well as large diameter mirrors or lenses or a wide variety of volumetric displays.

DOUBLE CAMERA STEREOSCOPIC VIDEO

Hundreds of researchers have created mechanisms for controlling various parameters of a stereocamera pair. Though much of the work on stereophotography and stereo motion pictures is relevant, we will limit the discussion to some of the more recent efforts with video. The two cameras need to be kept aligned within close tolerances in all three axes. Most recent work has taken this for granted and Toshiba's patent on it's three axis adjustment means for the two lenses of it's stereocamcorder is one of the few to describe this mechanical setup in detail (JAP-177530, cf. JAP-63-164596, 63-164597, 1-89796). There is a need to control the zoom, focus, interaxial (distance between the cameras) and the convergence point of the two optical axes. Since there are fairly precise relationships between these parameters, much work has been directed at interlocking several functions. The older literature described mechanisms for manual interlock of focus and convergence (USSR-506,953, 506954, 527030, 803128, 902323, 918926, 849547, 720819,506954, 228069, 471689, 1053329, 445175 , JAP-51-142218, 60-216205, 62-100095, 63-228141, 1-11254, FRE-1251830), or of zoom and focus (JAP-57-62687), or for manual adjustment of one parameter at a time for both cameras (JAP-59-192239, 1-225936, 1-212079, 1-11490). Some altered convergence by changing the scanning position on the image pickup surface (JAP-57-109492, cf. US-4740839,5049988, 5063441).

More recent efforts have usually attempted to automate these functions with application specific circuits or with programs written into a dedicated microprocessor or general purpose computer (JAP-56-106490,61-101882, 61-101883, 62-21396, 62-122493,62-266534, 62-266535, 63-228141, 63-164594, 63-153987, 1-212976, 1-93983, 1-93984, 1-251990, GB-2168565, USSR-873458, 552729, 1,148128, 1095454, EP-332403, 146476, US-4819064, 4818858, 4881122, cf. 5020878). Some have relied on digital storage and image processing to compensate for binocular asymmetries from zooming (JAP-1-231590), to reduce excessive horizontal parallax (US-4677468, 4723159 and many others), to effect simultaneous image capture (JAP-1-86692, 1-68192, 1-93977, 1-93978, US 4772944) or to eliminate camera shake (JAP-1-228392. Morishita of NEC has suggested (US-4677468) increasing aperture to blur objects with excessive parallax and automatic locking of the video levels of the two cameras-the latter also described in Japanese patents 63-158993 and 1-177795. Kinoshita also dealt with luminance matching and convergence (JAP-63-7094). A clever Japanese patent shows how to automatically adjust image size during zooms to size of the display to avoid image cutoff and miniaturization (63-296489). We are clearly entering the era of the "smart" stereocamera. Several companies have offered prototypes for sale including Ikegami's system with broadcast cameras and 120Hz scan converter for about $140,000, and one from 3DTV Corp. using for $10,000 which has microprocessor controlled synced zooms. Stereoscopic video is most conveniently and inexpensively created with a pair of genlocked cameras and the Model 100(composite) or Model 200(component) StereoMultiplexer available from 3DTV Corp. These units are battery powered and about the size of a VHS cassette.(Starks, 1990) Stereo video can be genlocked to stereo graphics easily, but one has to be alert to match up the right eye pairs. The same comments on flicker apply as for graphics with the addition that cameras should be very closely matched for luminance(Starks, 1992). The multiplexers give field sequential stereo for recording and for aligning cameras and viewing stereo with any CRT. Hardware for converting 50 or 60Hz stereo to higher frequencies is available from 3DTV Corp.

Demultiplexing of the field sequential image can be done by the Model A StereoDemultiplexer from 3DTV Corp. which separates out the R and L images for dual videoprojection viewed with passive polarized glasses. Flicker is a problem with tube projectors but LCD projectors give little flicker. LCD projectors may require orientation of polarizers different from the movie standard, but this is easily corrected with half wave plates. The Model A takes in composite field sequential video and puts out 30Hz right eye fields alternating with 30Hz black from one BNC and 30 Hz left eye alternating with 30Hz black from the other. The Model B does the same thing with composite or two or three component video. The SpaceStation marketed by 3DTV Corp. in 1994 adds back the missing fields to give the 60Hz right and left fields. To eliminate the trouble of dual VCR record and playback systems, the SpaceStation also permits the two fields to be record on one tape in a side by side or above/below compressed format. This will again give dual 60Hz output on playback.

Timecoding of tapes and playback with dual computer controlled VTR’s permits cheap flickerless high quality stereo. It is also useful to have separate R and L tapes when doing standards conversion since standards converters will destroy field sequential stereo. The R and L tapes can be separately converted and then mixed into stereo in the new standard with the StereoMultiplexers. However this is likely to produce serious artefacts. 3DTV Corp. markets a unique standard converter, that is compatible with field sequential stereo.

Other techniques have been proposed and occasionally marketed, but they involve use of expensive, bulky, nonstandard equipment for recording and display. A sensible approach is to begin with the StereoMultiplexer at 60Hz and move to the dual 60 Hz if desired.

Others have devised new techniques to improve camera performance. Karibe of Sharp Corp. described an automatic camera tilt detector (JAP-62-276987, 62-266533). Many have described camera switching, digital storage and/or processing or novel display techniques to improve the actual or apparent vertical resolution since there is often a decrement in this parameter (JAP-63-164598 and other cited later). Shimada of SONY mixes arbitrary numbers of left and right eye fields (JAP-1-202985). Osawa uses two cameras with electrooptic shutters and a single common optical element to facilitate synchronous zooming (JAP-1-54438). It has occurred to several researchers that one or more high resolution black and white cameras can be combined with a low resolution color camera to give a high resolution stereo image that would be otherwise unobtainable or very expensive (JAP-62-73896, 63-177690, 1-177292). A Mitsubishi patent employs an ultrasonic sensor on the monitor to automatically adjust the camera parallax to a viewers position (JAP-60-236391). Yatagai shows how to transfer charges between two CCD cameras to obtain low light stereo (JAP-1-93982). One of Maezawa's many stereo patents for Sharp describes a simple optical device for matching stereo camera pairs (JAP-63-143524).

Many designs have been directed at robotics, photogrammetry or pattern recognition applications (JAP-60-140113, 60-27085, 60-119191, 60-119192, Schenk and Toth). Hitachi engineers have created sophisticated automatic stereocamera controls for incorporation in a robot used in nuclear facilities (JAP-62-115989, 62-21396, 62-122493). The Harwell nuclear plant has an elegant system (Dumbreck et al., Scheiwiller et al.) which uses computer control to couple focus and convergence but they note that cases arise when the operator should be able to decouple these parameters. This system has also been installed in plants in Korea and elsewhere. Suzuki's stereocamera automatically tracks objects and adjusts the zoom to keep them centered (JAP-60-152193). Multiple fiber optic bundles coupled to sensors have been used as stereo pickups (JAP-60-58789). It is also feasible to use three or more cameras with rapid updating to obtain the best stereo pair or to extract depth information with algorithms that combine all viewpoints (JAP-61-125685, EP-0199269, Cheung, and Brown, Dhond and Aggarwal, Stewart, Wilcox et al.). Copeland suggests using wing mounted cameras as a navigational aid to increase interaxial from the normal 65mm to 65m (U.S.-4805015). Simulator experiments on terrain following with stereo video were carried out in the 1970's (Bruns).

FIELD SEQUENTIAL STEREOSCOPIC VIDEO

Much of the early research on color television involved field sequential color systems and many of these workers described means for using their devices in a stereo mode. Baird's efforts (GB-321441) are well known but others were even earlier. Hammond's patent, filed in 1923, described sequential color and stereo (US-1725710). Interestingly, a toy company briefly marketed a field sequential stereo, field sequential color vector graphics system sixty years later. Many subsequent efforts used mechanical shutters for projection and or viewing of stereo slides, motion pictures or television (US-2362030, 2,384259, 2384260, 2408115, 2825,263) and patents on such devices continue to appear (GER-3303739, W0 79/01035) but very few resulted in a commercial product. Knauf's "rotating beer can" (US-3464766) is now obsolete as is the Matsushita viewer for the Sega Subroc 3D arcade game (JAP-56-69985, 56-155917, 56-156079, 57-5490, 57-5491, 57-5492, 57-5493, 57-14269, 57-25783, 59-171392, 60-7291).

Kerr cells and related electroopic polarization rotating devices were employed from the earliest days of television, mostly as a means for obtaining color in field sequential or line sequential schemes and stereo means were often described (US-2002515, 2118160, 2417446, 2616962, 2638816, 2665,335, 3358079, GER-736457, 2055935, 2140944). When the transparent PLZT ceramics became available in the 1970's, they were quickly put into service but were soon supplanted by liquid crystals. The amount of research as evidenced in the technical literature has become staggering. Japanese patent applications on field sequential stereo have exceeded 400 in the last decade alone. A few of the earlier non-Japanese patents to specifically mention LC shutters are those of Varga (Romania-58504), Schieckel (GER-2111067, Hossmann (Swiss-534365), Kratomi (US-3737567), Roese (US-4021846) and Mears (GB-1523436).

The availability of low cost LC shutters greatly stimulated research and means were described to permit video field recognition to ensure the right eye image getting to the right eye (US-4145713, 4387396, JAP-63-164788,1-245693,1-86693), to sync the glasses via a photodiode on the monitor screen (JAP-62-209994, 63-214095, 63-294096, 1-248796,1-68191), via a magnetic pickup on the monitor (JAP-63-248294), or without wires via infrared, radio or ultrasonic transmission (JAP-58-62995, 62-91095, 62-239784, 63-1286, 63-64016, 63-59089, 63-117596, 63-64016, 1-67095, 1-68191, 1-17590, 1-206798, US-2388170, 3621127, 4286286, 4424529, 4562463, 4732457, 4979033, 4967268, FRE-2334255, 2399173, GER 3214021). Many patents contained variations on LC driving circuitry, often with the aim of decreasing the flicker of 60Hz systems (JAP-61-227498, 61-277918, 62-166314, 62-204226, 62-242914, 62-254118, 62-266996, 63-31393, 63-31394, 63-158994, 63-43621, 63-205641, 63-213821, 63-290485, 63-314991, 1-44421, 1-51789, 1-51790, 1-86694, 1-103394, 1-149590, GER-3413211), others were concerned with keeping the shutters transparent when the viewer looked away from the display (JAP-63-212290, 62-231578), when the viewer was looking at the camera of a videoconferencing system (JAP-63-194497), or when viewing a 2D part of the display (JAP-63-215195). One NTT researcher even devised means to remove the glasses entirely by using stored images of the viewers (JAP-1-251989). Some work has been directed at improving performance by novel methods of constructing the shutters (JAP-62-89925, 62-71395, 62-156619, 62-166314, US-4884876). Only a few of these designs ever were marketed. Four types were available from 3DTV Corp. in 1990 for prices ranging from $50 to $200 with a variety of drivers able to accept video or TTL input. These all work well at 60Hz and several perform well at 120Hz, particularly if the background and foreground hue and saturation are adjusted to minimize flicker and crosstalk. By 1992, four different companies had marketed wireless LCD shuttering glasses.

All the above work applied to twisted nematic LC shutters (and in a few cases to PLZT ceramics) incorporating crossed polarizers. Many have suggested using ferroelectric LC shutters (JAP-63-30088, 63-64016, US-4772943) because of their fast switching times. Vision Research Graphics introduced a commercial product in 1992. When used in conjunction with a special amber-green monochrome phosphor, there is virtually no crosstalk(ghosting). Some effort was made to develop cholesteric LC shutters for stereo viewing by scattering without polarizers by various Japanese scientists, and by Milgram in Canada (US-4698668), and Noble and McSherry in California. They seem to offer no advantage since they appear not to decrease flicker and give a milky look to the image, but Noble suggests using a black matrix to reduce scattering. Milgram markets them for use by perceptual psychologists.

Tapes in the field sequential format are compatible with all standard NTSC and monitors except some of the IDTV products and LCD TV's or LCD projectors which mix fields. Some VCR’s by Instant Replay (Miami, Fl. U.S.A.) or the Akai(now Mitsubishi) VSR19EMb, will play NTSC at 60Hz on PAL TV's, but none appear to be 3D compatible. This works with most PAL and SECAM monitors and receivers because they lack vertical countdown circuits and will sync to 60Hz. In some PAL countries(e.g., Sweden) nearly all the TV's accept 443MHz 60Hz NTSC. NTSC and PAL-M (Brazil) VCR's should play 3D NTSC tapes on PAL and SECAM TV's but without color. This trick of driving consumer televisions at higher frequencies should also work for 3D videogame systems and computer graphics and is employed in 3DTV Corp’s FlickerFixer device (SpaceCard).

The 60Hz flicker can be virtually undetectable if the ambient light is low, monitor brightness is adjusted and images avoid large light areas. Acceptance by consumers and professionals has been excellent. Sega sold over 100,000 of their 60Hz home 3D game systems, mostly in the U.S. and Japan and perhaps 40,000 50Hz systems in Europe and elsewhere and Nintendo sold some 80,000 of their 60Hz units in Japan in the late 1980’s. Systems operating at 60Hz have been successfully marketed for the Atari, Amiga, and recently for PC's. Nevertheless, there has been much effort directed at methods of reducing flicker. Some have processed the video to reduce areas of high luminosity (U.S. 4979033). Many workers have suggested eliminating flicker entirely by doubling the field rate to 120Hz. Some have created a four fold interlace by inserting extra vertical sync pulses with standard monitors (US-2696523, 4523226, 4583117, 4517592, cf. US-2389646) while many others used field stores and broad bandwidth monitors to eliminate flicker and perform other image manipulations (JAP-54-30716, 56-168484, 57-87290, 57-119584, 57-138285, 58-139589, 60-100894, 60-203095, 60-223282, 60-263594, 61-113389, 61-273094, 61-293093, 62-86997, 62-132491, 62-133891, 62-136194, 62-145993, 62-150591, 62-265886, 62-278889, 63-30088, 63-31295, 63-46091, 63-88994, 63-95795, 63-116593, 63-131685, 63-131686, 63-133791, 63-164598, 63-181593, 63-219293, 63-224495, 63-231590, 63-232790, 63-245091, 63-258187, 63-266980, 1-27390, 1-39187, 1-47194, 1-47195, 1-47196, 1-54886, 1-61192, 1-61193, 1-69196, 1-93988, 1-93989, 1-93993, 1-93994, 1-212091, 1-252093, US-4,393400, 4672434, 4772944, USSR-1166344) and many others. Siemens, Philips, Sony, Metz and Grundig have marketed limited numbers of tv sets with field doublers, at least some of which can be modified to be stereo compatible(Woods et al.). The 3DTV Spacecard is unique in its ability to give continuously variable frame rates for NTSC or VGA, stereo output from field sequential NTSC 60Hz input. Ikegami Sony-Tek and 3DTV Corp. have introduced units to double the field rate of standard field sequential 3D video. Cahen in his French application of 1948 (US-2665335) and many subsequent researchers (US-3358079, 4772943, JAP-63-46410, 63-116591, 63-116592, 63-245091), noted that one can switch at line rate. Like 120Hz switching, this will eliminate flicker of ambient light, but will not eliminate image flicker unless each eye is given about 45 or more new images each second.

The use of two videoprojectors with crossed polarizers and a front or rear projection polarization preserving screen gives a large screen and allows the use of cheap, standard polarized glasses. In general, it will also give less ghosting than with a single field sequential display whether projected or direct view with active glasses or with passive glasses and screen polarization modulators. This is due to phosphor persistence in the active glasses case and phosphor persistence combined with scattering by LCD modulator in the passive glasses case, since these problems are absent with the cross polarized dual projector system. Two genlocked computers can generate the images or two video players can be run in sync with right eye and left eye time coded tapes and a suitable edit controller. A single field sequential source input to the StereoDemultiplexers. will output two separate signals of about 30Hz(depending on input frequency) alternating with video black. This will flicker most with 50Hz PAL input and CRT projectors and least with 60Hz NTSC input and LCD projectors. The LCD projectors are slower and flicker may be almost undetectable even when each projector is input with 30Hz NTSC. Some LCD projectors(e.g. Eiki, GE models available in 1992) require the use of polarizers at a nonstandard angle(45 degrees to right and left is standard for polarized glasses) but others such as some from Sharp work at the standard angle. A half wave plate will rotate the polarization place if needed.