The Space Station(TM) introduced by 3DTV Corp. in 1993 has both demultiplexing and field doubling and will give two fields out for each field input. Thus, each projector will have the full number of fields and will give completely flickerless 3D It also will shift either field horizontally or vertically to correct parallax errors or create real time stereo image manipulations. Various models will permit composite, YC, RGB or VGA input, and above/below or side by side compression and/or decompression of stereo pairs. It also performs many other unique stereo image manipulations.

The use of Kerr cells at the CRT with viewers wearing passive polarizing glasses likewise grew out of the early work with sequential color schemes and is mentioned in many of the above citations. Many other references specifically describe screen switching (GB-1448520, JAP-50-75720, 52-110516, 54-101689, 60-153694, 60-203095, 61-9618, 61-203794, 62-71394, 62-81620, 62-299932, 63-85717, 63-182991, 63-203088, 1-128039, EP-0136696, 0223636, 0264927, USSR-544183, 642884, 657673, 1166344, Neth. Appl. 7807206, US-3858001, 4719482, 4719507, 4792850,4870486, 4879603, 4877307). 3DTV Corp. sells a StereoPlate for polarized projection with a single videoprojector.

Much attention has also been directed to adapting existing tape and disc systems for high quality 3D recording and playback. An obvious route is use of a dual head VCR and/or double speed rotation (U.S. 5050010, JAP-62-102679, 62-165488, 62-166669, 62-245784, GER-3234846). David Burder and his colleagues in England have modified an old quad VCR for multichannel 3D. The new JVC digital VCR (1995) is capable of recording two full bandwidth composite signals. Work on 3D discs has included Sanyo's dual system with right and left images on separate discs, Hitachi's machine with the two images on opposite side of a disc (JAP-276393), Pioneer's optical disc recorders (JAP-63-266980), Alps' magnetic disc recording on two adjacent tracks (JAP-1-109892), and numerous others with field sequential or dual track systems usually with 2D compatibility (JAP-55-50638, 55-50639, 61-212192, 61-252778, 62-91095, 62-128294, 62-176394, 62-260496, 62-266995, 62-276989, 62-285595, 62-295594, 62-295595, 63-6992, 63-116590, 63-151293, 63-227296, 63-228895, 63-229994, 63-232789, 63-276393, 63-316981, 1-49396, 1-94794, 1-109989, 1-109990, 1-177294, 1-183993, 1-206798, US-4739418). A field sequential 2D-3D compatible system was offered for a brief period by several Japanese companies in the now defunct VHD system.

Much thought has gone into means of interlacing fields and/or doubling lines (US-3991266, 4736246, 4772943, JAP-61-212191, 61-212192, 61-280193, 62-145993, 62-154894, 62-210797, 62-230292, 63-94794, 63-164598, 1-24693, 1-55999, 1-225295) or otherwise processing video (JAP-61-24393, 1-272286, 63-84292, 63-84393, 63-84394, 63-84395, USSR-303736, 1188910, Woods et al., and many others referenced above) for improving the resolution in field sequential systems. Lowell Noble and Ed Sandberg of SOCS Research in Saratoga, Ca. have developed a stereo compatible board which line doubles and image enhances to give superb 2D or 3D on any VGA or other multisync monitor or projector. Two different groups applied ghost canceling techniques for eliminating crosstalk due to slow phosphor decay (JAP-55-85181, 56-106491). Some workers have described means to compensate for subject motion (JAP-1-165293, 1-171389) while many others have devices for parallax reduction for reduced ghosting and visual fatigue or to manipulate the stereo window (JAP-57-21194, 63-62485, 63-86691, 63-142794, 63-176081, 63-227193, 63-245090, 63-306795, 63-314990, 1-265798, US-4399456).

INTERFACES FOR STEREOSCOPIC COMPUTER GRAPHICS

Until recently, those who wished to work with stereo graphics have had to spend many thousands of dollars for cards, multisync monitors and LCD glasses and have then had to write their own software systems. Gloves or 6D mice have cost thousands more. Beginning in 1990, 3DTV Corp. began introducing low cost system including universal interfaces, several models of StereoVisors(LCD viewing glasses) and stereoscopic computer sofeware. Several of these interfaces have ports for gloves and related devices. In 1994 and 1995, many other companies began using this technology and complete systems for interactive stereo imaging became available for nearly any computer for less than a tenth the previous cost.

One of the most useful of these devices is the Model 3000 StereoDriver from 3DTV Corp.. It has a cable which replaces or adds on to the end of the VGA cable between the monitor and the PC. It is a high density(15 pin) cable with a D9 size plug. It has an extra wire which takes vertical sync from the PC . This wire terminates in an RCA plug which is attached to either RCA jack of the Model 3000 StereoDriver. The StereoVisors(LCD glasses) plugged into the Driver will now cycle in sync with the right and left eye images. On starting up, there is a 50% chance that the right eye image will go to the left eye. To be certain when this happens, it is advisable to put an R on the right frame and an L on the left frame when creating the software so one can tell immediately which field is being viewed by closing one eye. If the left eye field is being seen by the right eye(pseudoscopic image), flipping the polarity reversal will correct the polarity. It will follow sync to at least 120Hz and can also be used for viewing stereoscopic videotapes, discs, CD ROMS etc. in any format(NTSC, PAL, SECAM, etc.). If the right eye image is always recorded on field one, the field recognition circuit will automatically route the right eye image to the right eye. It has two standard stereo headphone mini jacks, can be used with all commonly available wired LCD glasses, and with use of stereo splitters will drive up to 8 pairs of Visors. Similar cables can be supplied to adapt the StereoDriver to any computer having external access to video sync(usually on the green pin for RGB monitors).

A second stereoscopic interface marketed briefly was the Model RF StereoVisor and Model RF StereoDriver. This driver contains a magnetic pickup which obtains sync for the glasses from the magnetic flux of the monitor. The driver is plugged into AC power and laid on top of the monitor . It will drive the Model RF wireless glasses or most models of wired LCD glasses via two jacks on the rear of the driver. Wired and wireless Visors may be used simultaneously. As with the 3D Cable, there is a 50% chance of a pseudoscopic image on startup. Switching the driver power on and off or moving it a few times will result in a stereoscopic image. Again, the optimal situation will be to have the R and L frames identified in software and with driver will work with monitors, tv's and videoprojectors with the exception of some which are too well shielded. The RF Driver should work will all video and computer systems to at least 80 Hz and perhaps higher and can drive any number of RF Visors and at least 4 pairs of wired Visors. Since it depends only on the monitor flux, the RF system should work with nearly any platform without the need for any connection. At least one other company marketed a magnetic pickup but this method is unreliable and has been discontinued.

A third device is the PCVR. This will interface with parallel or serial ports by flipping a dip switch to the appropriate position. Another dip switch permits line selection when changing from one type of serial port to another and a switch to select polarity of LCD glasses. It will drive the glasses and the Power Glove (marketed as a low cost game device by Mattel in 1990), or other interface devices and has an additional port for the printer which is put on or off-line by turning a knob. Pseudoscopic images are not a problem since the computer now has complete control over the right and left lenses of the LCD glasses, but a switch is provided to reverse the lens polarity. It is probably still a good idea to put R and L indicia in the lower right of the right and left frames respectively, at least when programs are being written. The indicia or any other graphics should not be put at the very top of the screen because hardware and/or software problems producing distortions at the top have arisen in many systems from various sources.

Model O driver uses an optical pickup on the CRT which is triggered by alternate white and black screen indicia created in software.

The Model IR Driver takes sync from VGA like the Model 3000 but transmits the sync via infrared to wireless glasses.

Another device similar to the PCVR is the PGSI, produced as a class project by a group of college students in 1993. This interface plugs into serial ports and drives the glasses and the Power Glove. It contains a microprocessor and has software which allows more sophisticated control of the Power Glove.

The PCS and PCP are the most compact and least expensive of the interfaces, being small enough to fit inside a gender changer and taking power from the serial port or parallel port respectively.

Flicker with these low cost computer systems can be small or even imperceptible if the problem is understood and all parameters are controlled, as discussed above. Most of the LCD glasses that have been marketed have incorporated a layer of black plastic in front of the LCD to reduce room brightness. If the room lighting is reduced and monitor brightness decreased, even field sequential video displayed in the European 50Hz PAL system can be quite acceptable. Most PC graphics cards run at 72 Hz in VGA which much reduces flicker even in bright environments. Any card can have its frequency increased by using its menu. Some 3DTV Corp. software contains an automatic flicker fixer which works with most cards in 320 x 200 mode.

STEREO EYE AND HEADTRACKING

Starks(91) surveys the available eye and head tracking techniques. Though most of the work to date has been monoscopic, most of the hardware can be used stereoscopically and applications are appearing (G.B.-2,201069, Yamada et al). An obvious use is to couple such a device to a stereocamera for totally automated socs and 3D videotaping (JAP-62-115989). An interesting device from NEC anticipated virtual reality research by using eye movements to alter images in a helmet mounted stereo display (JAP-61-198892). Deering (1992) notes that stereo eyetracking will be necessary for highly accurate interactive stereo.

HEAD MOUNTED DISPLAYS

Head mounted displays have a long history. McCollum described a field sequential, head mounted system with dual CRT's and wireless transmission in 1943 (U.S. 2388170). Science fiction pioneer Hugo Gernsbach modeled a prototype of unknown origin in the early 1960's. They have been the subject of extensive R&D in many countries, mostly for avionics but more recently for tanks (Brooks, Rallison and Schicker), foot soldiers (Varo Inc.), vision aids(U.S. 5060062), surgery (GER-3532730, U.S.-4737972, 5039198, Pieper et al.), computer workstations (Teitel) and entertainment (U.S.4982278, 5034809). Varo Inc. has a series of intriguing patents describing wireless infrared transmission of video from gunsight to helmet and to other soldiers (U.S.-4884137, 4970589). Another patent uses a head mounted camera for simulator purposes (U.S.-4398799) and Thompson-CSF inputs stereocameras through fiber optics (FRE-2517916). The SPIE volumes series Helmet Mounted Displays, Display System Optics, Large Screen Projection- Avionic and Helmet Mounted Displays, Cockpit Displays and Visual Simulation, etc., the SID Digests of Technical Papers and the NTIS searches on HMD's (PB89-872105/CBY), etc., provide good surveys. Most of these devices have aimed to provide a head up display of flight information or other data with the pilot having his normal view of the cockpit with the data displayed on a semisilvered mirror or holographic optical element (Amitai et al., U.S. 5035474, 5040058). For many other purposes, it is unnecessary or even undesirable to see the real world and the helmet displays all the information. Telepresence and robotics have been mainly concerned with displaying video, while virtual reality research has so far used such systems for computer graphics. One Air Force project developed "hands off binoculars" (U.S.-4465347).

Earlier work used miniature CRT's (U.S.-3614314, 3059519, 2955156) but LCD's and lasers are now frequently used. Most devices have required complicated optical trains to get the CRT image in front of the eyes (U.S.-4859030, 4269476, 4322135, 4761056, 4969724, 4902116, 4468101, 4775217, 4968123, 4961626, 4969714), but recently fiber optics has been employed (Thomas et al., Webster, FRE-2517916, CAE Electronics). Most of the recent systems incorporate head and/or eyetracking (U.S.-4897715, 4028725, East Ger-224691, Arbak). The Eyephone (TM) from VPL Research was marketed in the late 1980's followed closely by the Cyberface(TM) of Pop Optics Labs and the elegantly designed Virtual Research Flight Helmet(TM) in 1991. An ultracompact design by William Johnson of England used his GRIN optics. Some of the more expensive avionics devices such as the Agile Eye from Kaiser Electronics (Arbak) are available to defense contractors and possibly others. Dual LCD systems from three companies entered the personal computer and toy markets in 1995.

There have been many descriptions of dual LCD devices intended to display video for low cost applications (JAP-63-82192, 63-177689,59-117889, 59-219092, 62-272698, 1-61723WO 84/01680, GER-3628458, 3532730, U.S.-4571628, 4982278, 4933755, 4952024, 4805988). The Litton magnetooptic chip has also been used, but it cannot display blue, so full color is not possible (U.S.-4575722).

Another group of lightweight displays intended for helmet or eyeglass mounting has recently appeared (U.S.-4311999, 4902083, 4934773, 4753514, 4867551, Upton and Goodman, Pausch et al.). These involve vibrating optical elements such as mirrors or fiber optics to scan the image onto a mirror. Though these have been monochrome data displays, color and full video would be possible. A device of this kind called The Private Eye has been marketed by Reflection Technology of Waltham, Ma. and Peli has published a careful evaluation of it. British stereographer David Burder has created a stereo HMD using two of these devices which gave a reasonable stereo effect. Nintendo licensed this technology and introduced the Virtual Boy Game System in 1995.

Much of the recent work with HMD's has emphasized wide angle viewing (Howlett, Howlett et al, Fisher, Robinett, Robinett and Rolland, Teitel). Wide angle stereo has a long history in photography and Harvey Ratliff deserves mention as a pioneer in this area and as the father of wide angle stereoscopic video. He built several devices and proposed others in a series of patents in the 1960's (U.S.-3511928, 3504122, 3376381, 3293358,3291204). Ratliff used conventional lenses, while more recent patents on panoramic HMD's have proposed more exotic optical techniques (U.S.-4385803, 4853764, 4874235). It is not clear that wide angle optics give sufficient advantage to justify the trouble and expense nor do there appear to be enough data to tell whether most people will find them comfortable with prolonged or repeated use.

The psychophysics of depth perception in head mounted displays has been the subject of many recent investigations. Uchida and Miyashita were particularly concerned with eyeglass mounted LCD's, Gibson with head up displays and Kruk and Longridge with a fiber optic design. Rebo's thesis is the most extensive published work to date and includes a very detailed analysis of the Polhemus headtracker. The study by Setterholm et al. is also very useful. Many systems have been investigated by the German aerospace company MBB (Bohm et al.). Other workers have been especially concerned with determining the optimal amount of binocular overlap and related parameters (Moffitt, Warren et al., Melzer and Moffitt, Self). Numerous studies have been done in the last three years.

My experiences with a wide variety of stereo displays has been that the greatest problems are usually with inadequate software (Starks). In examining some of these HMD's, it has become obvious that the stereo images need much improvement and with the computer generated images, it is often difficult to tell whether one is seeing stereo or pseudostereo (right and left eye images reversed). This is evident on some of the images presented in the stereoscopic virtual reality tape sold by 3DTV Corp. (Cyberthon in 3D) which are direct video feed from the computer and are not subject to any of the limitations of the HMD. More recently, wider experience with stereo has resulted in much excellent work.

COMPATIBLE TRANSMISSION OF STEREOSCOPIC VIDEO

One of the most sought after goals in 3D video has been a means for recording and /or transmission compatible with 2D reception by ordinary receivers. The 3D receiver would decode the signal to display a stereoscopic picture and in some schemes 2D receivers could be retrofitted with decoders to display 3D. Most of these subtract the two channels to obtain a difference signal which modulates some component of one channel for transmission. The great advances in video bandwidth compression in recent years should render many of these schemes more feasible. Such schemes have been described for many years (Brit- 706182) but it is getting more serious since some recent contributors to this field have been IBM (U.S.-4884131, E.P.-306448), CBS (US-4027333), the BBC (U.S.-4905081, E.P.-267000), NHK (U.S.-4704627, 4743965, WO-86-03924, 86-06914, JAP-59-265798, 60-46402, 60-96285,61-253993, 1-202093), Hitachi (63-256091, 63-100898, 63-217793,63-217794, 63-164593, 62-236294, 62-235896, 62-272697, 63-56089), NTT (Gomi et al.), Sony (52-9317), NEC (1-5291, 1-5292, 1-67094) Seiko (61-251396), Sharp (63-82191, 62-283792, 62-283793), Ricoh (62-150991), Thomson (U.S. 5055927), Telediffusion (U.S. 5043806), Toshiba (63-294090, 1-179593, 63-74292), Matsushita (63-1192),Canon (1-54992), Clarion (63-38386, 61-253992, 61-293094), ATR (1-114294, 1-64489) and others (JAP-51-142212, 59-86383, W0-84-00866, 83-00975, 88-01464, US-4266240,4287528, 4517592, Hudgins, Tamtoui and Labit, Chaissang et al). There has also been considerable work in the USSR by Dzhakoniya and others (USSR-128049). For an introduction to the vast amount of related work on video compression see JAP-63-294087, 52-72117, 62-245784, 62-165488, 62-166669, 63-201878.

Recent advances in hardware and software probably obsolete most of the above work.

AUTOSTEREOSCOPIC DISPLAYS USING LENTICULAR SCREENS

Autostereoscopic displays are those which do not require the user to wear viewing aids. Displays using lenticular screens have been the subject of intensive research for nearly 80 years. The two Ives laid the foundations (US 666424, 725567, 771824, 1262954, 1814701, 1882424, 1883290, 1883291, 1905469, 1905716, 1916320, 1918705, 1937118, 1960011, 1970311, 1987443, 2002090, 2011932, 2012995, 2039648). Hundreds of researchers followed and there are perhaps 2000 patents and several hundred technical papers on the use of lenticular screens and related means for photography, motion pictures and television. It is impossible to cover more than a few of the more prominent or recent which relate most directly to video. Photographic systems have become common with both professional and consumer lenticular cameras. Motion picture applications have been rare with only the Russian lenticular glass screen being publicly shown in the USSR and at the Osaka Expo in 1970. Eight years before his invention of Holography in 1948, Gabor filed three patents on lenticular methods for movie projection(US 2351032,2351033,2351034) and in 1953 he filed what is probably the longest and most detailed patent ever granted on autostereo projection(GB 750911). Remarkably, later researchers seem to have almost completely ignored this work and even Gabor in his 1969 patent on holographic movie projection(US 3479111) fails to reference his last and most complete patent on this topic.

Lenticular television devices have been prototyped many times but whether the screens were inside the CRT( Wallman, JAP 58-38093) or on the front of the faceplate or projection screen(e.g., Yanagisawa), alignment of pixels with lenslets was a major problem. Makoto Kikuchi of Sony pursued this approach vigorously during the 80's(JAP 53-20347, 56-126235, 56-126236, 56-128085, 56-128086, 56-132752, 56-134895, 56-168326, 57-3487, 57-11592, 57-13886, 57-14270, 57-17546, 57-18189, 57-26983, 57-27544, 57-27545, 57-27546, 57-67393, 57-72250, 57-75090, 57-83990, 57-83991, 57-87291, 57-106291, 57-123787, 58-29283, 58-31692, 58-103285, 58-115737). Tripp(US 3932699) was probably the first to build an adequate system, solving the alignment problem with a 13 inch diagonal fiber optic faceplate with a vertical lenticular screen. The input was a one inch camera tube covered with a specially made lenticular screen having 525 lenticulations per inch. This was made from a metal master hand engraved with the aid of a microscope(the same technique used to engrave dollar bills). This was perhaps the best autostereo CRT based system to date, but it was never duplicated and was soon cannibalized for the expensive fiber optics. Tripp however is a very flexible and ingenious man(one of his early inventions was the escalator) and he claims to have recently invented an extremely high resolution(2000 line pairs/mm) "spatial hologram without lasers" intended for use with his high resolution low dose x-ray system.

The advent of flat panel displays, which do not have the problem of aligning pixels and lenticules through an intervening layer of glass is resulting in renewed interest in this approach(Ichinose). Work is ongoing in France(US4584604), England(Sheat) and Japan(Tetsutani et al.) on a system for a 3D picturephone.

Another problem is that it is desirable to have a large number of these laterally multiplexed stereo pairs to minimize image "flipping" and give a "look around" capability(Schwartz). However, with most of these autostereo techniques, resolution and number of views are inversely related. With a 0.5mm lens size and 50 views, one needs a resolution of 10 microns, near that used for holographic plates and certainly beyond that of any available video display(with the possible exception of some of Tripp's prototypes). When it becomes possible to interpolate many views from a stereo pair, it will stimulate the whole field of autostereoscopy. Scene interpolation has been the subject of much research for robotics and pattern recognition but only a few workers have attempted to apply this directly to autostereoscopic display(Oshima and Okoshi).

Front or rear projection of stereo with lenticular screens has been investigated by many but has rarely resulted in commercial product. Sanyo Corp. has shown large rear projection lenticular systems in 1994 and offered a 50 inch diagonal model for $50,000. Image quality was modest and restriction on head position severe. Joji Hamasaki in Tokyo has been one of the most persistent and successful in this work with multiple video projection and large diameter screens as well as with the Sony beam index CRT(JAP 61-77839, Hamasaki). NHK and other has an active program with multiple LCD rear projection on a Toppan Corp. plastic lenticular screen(Isono). Viewing distance is limited to about 3 meters plus or minus 10cm and careful head positioning is necessary to avoid pseudoscopic zones(problems for all lenticular systems). Hamasaki's efforts and those of NHK are shown on the 3D videotapes "3D TV Technology Vols 1 and 2" marketed by 3D TV Corp. A vigorous program was conducted at the Heinrich Herz Institute in Berlin with front and rear projection on lenticular screens custom made by Philips in Eindhoven(Borner). Philips has extremely high precision computer controlled diamond milling equipment for making lenticular screens for their videoprojectors. A 1500 line screen can be milled in a plastic master in about 2 hours and the poured acrylic screen rapidly cured with uv . Minute corrections in the screen can be reliably programmed, engraved, cured and ready to test in one day for a one time set up fee of about $25,000 and a cost of about $20,000 for a 1M by 1M pair of screens in prototype quantities. This process used to take months and was not very accurate. The final screen is accurate and repeatable to one micron. Dr. Schmitz demonstrated this by making a screen which copied the eye of a bee and proved its extreme accuracy with electron microscope photos. This may result in commercial lenticular systems in the next few years.

AUTOSTEREOSCOPIC DISPLAYS USING PARALLAX BARRIERS

Optically analagous to lenticular screens, parallax barriers consisting of thin vertical opaque strips seem to have been invented in the 17th century by G.A. Bois-Clair. The Frenchman Berthier revived it in 1896 and it has been the subject of hundreds of patents(GB 514426). 3D movies for viewing without glasses were shown commercially in Moscow in the 40's with a conical screen constructed by Ivanov from over 30,000 white enameled wires weighing six tons. The floor was slanted to accommodate an audience of about 250. For a period, the films were also projected with cross polarization so that those not located in the right viewing zones could see the films with polarized glasses. Autostereoscopic projection was apparently discontinued by the early 70's due to customer preference for the glasses.

The simplicity of barrier systems has continued to create interest both for still(US 4927238, Sandin, Myers et al) and moving images(Sexton, Johnson et al.). Eichenlaub has marketed an autostereoscopic workstation employing an LCD in a manner analagous to the barrier(US 5036385, 4717949,4829365, Eichenlaub). In 1995, Sanyo showed several small LCD based systems like this with an 8 inch diagonal model priced at $3000. Image quality was good but head position was critical.

DYNAMIC PARALLAX BARRIERS

In this technique, one or more vertical slits are rapidly scanned in the horizontal direction and the appearance of the image points on the screen behind are timed precisely so that a viewer at any position will see a stereo image. Noaillon(US1772782, 2198678) created a device composed of a conical arrangement of slats which was rotated rapidly between the viewers and the screen on which were projected 3D movies. Subsequent improvements were made by Savoye(US 2421393) and others(Jennings and Vanet) and Savoye's version was shown to audiences of 90 persons at Luna Park in Paris after WWII. A smaller system was sold by A. Mattey of Paris for home use. Versions of this "cyclostereoscope" were recently reconstructed by Australian enthusiast R. Blum(Blum) and by French stereo equipment designer Claude Tailleur.

For many years, Homer Tilton has promoted a unique version of this technique using a single mechanically scanned slit called the "Parallactiscope" and has even written a book about it(Tilton). Meacham has built a vibrating multislit device(EP 0114406 A1, US 4740073, Meacham)and Noble has made a version with an LCD slit replacing Tilton's mechanical one. Travis has suggested a laser addressed LCD with a large lens to overcome the low light emission common to most of these approaches(Travis ). Hattori has a system with multiple crt's, and a large diameter lens with an LCD slit scanning inside the lens(Hattori). Kollin's rotating louvers are another approach(Kollin), but any device that uses mechanical moving parts appears unlikely of success. The devices of Tilton, Noble and Meacham are shown and commented on in the 3D videotape "3D TV Technology Vol.

THE STEREOPTIPLEXER

In the early 60's, Robert Collender invented a dynamic parallax barrier system which had many intriguing features(US 3178720, 3324760), but his most interesting insight was embodied in his next application in 1973(US 3815979). He realized that if one used a screen that was very direction selective horizontally(i.e., diffused normally in the vertical direction but was retroreflective horizontally) he could replace the physical slit between the observer and the screen with a virtual slit. Along with his mechanisms for scanning multiple images on the screen, this made it possible to create a practical system for any size audience with no pseudoscopic or bad viewing zones. In subsequent patents he has extended his ideas considerably to flat screen video displays with few or no(US 4676613)moving parts(US 4089597, 4158487, 4176923, 4231642, 4290083, 4323920, 4349252, 4547050, GB 2076611, GB 2095068, JAP 56-31579, 57-11591, 57-162898). I have seen his simple prototype working with 16mm film and it is exactly as expected-one sees a nice 3D image without glasses from anywhere in the room. Collender thinks it would require about $10 million to build a video prototype of his invention. As an engineer with 30 years experience in high tech design, he is probably not too far off.

INTEGRAL PHOTOGRAPHY

Invented by Gabriel Lippmann in 1908(Lippmann) this autostereo technique is often called "fly's eye lens photography" because of it's use of an array of tiny lenses for taking and displaying the image. As a result it possesses both horizontal and vertical parallax as do most types of holograms. Leslie Dudley was one of the more zealous researchers(US 3613539, 3683773, 3734618, 3675553) followed by Roger de Montebello(US 4732453). De Montebello's recent passing left his work in the hands of panoramic camera inventor Ron Globus of New York City. For video or computer graphics the images may be created by other means and displayed integrally. In spite of substantial problems in fabricating the multiple lens array and in reversing the pseudoscopic image, integral photography has continued to attract attention both for still photography(Shang, McCormick et al., Okoshi(71), JAP 1-154437, WO 89/06818, US 5040871, Ueda and Nakayama, Chutjian and Collier, Burckhardt et al. Burckhardt, McCrickerd) and video(US 3852524, 3878329, 5036385, FR 2014676, WO 88/05554, Igarashi et al,). W. Hickox of Airometric Systems Corp. in Glen Cove, N.Y. and Dave Roberts of Robert Engineering have also made integrams. Recent work with the fabrication of integral lenses holographically may further stimulate research(Hutley). Many other companies including Rank Pneumo, United Technologies, Adapative Optics Associates and Corning have begun fabricating microlens arrays.

The fact that the vertical parallax provided by the integram is usually unnecessary, coupled with the need to reverse the pseudoscopic image and the problems of lens manufacture, make it likely that the integram will see only very limited application in the foreseeable future.

LARGE MIRRORS, LENSES and RETROREFLECTIVE SCREENS

It is common knowledge that when stereopairs are properly projected on suitably curved mirrors, lenses or screens, an observer in the appropriate viewing zones will see a stereo image. Hundreds of patents and dozens of prototypes attest to the simplicity and popularity of this approach to autostereo(US 3096389, 4062045,4315240, 4315241, 4509835, 4623223). A new type of retroreflector from Precision Lapping and Optical Co. of Valley Stream, N.Y. may make some new designs possible.

Ketchpel has proposed electronic modulation of a large diameter LCD for autostereo projection(Ketchpel, Williams et al.). Though large diameter glass lenses are impractical, the advent of high quality plastic fresnel lenses led to much many attempts to create autostereo systems, usually with rear projection of a stereo pair. A few such systems were created by Northrup for the military about 1980. The images came from a pair of high resolution black and white crt's projected through a custom fresnel about 30cm wide. I saw an excellent stereo image as long as I kept my head within an approximately basketball sized viewing zone. A similar system was built by Martin Marietta Corp. twenty years ago(Tewell et al.). Zehnpfennig has provided a detailed report on the construction of a smaller version (Zehnpfennig et al., US 3711188) and a very small version is commercially available in microscopes marketed by Vision Engineering and other companies.

It has long been recognized that a large diameter curved mirror will project a 3D image of an object suitably placed and illuminated. With appropriate masking of the mirror and object, the viewer sees an image floating in space. George Ploetz invented a clamshell arrangement of two mirrors that has been marketed in the US by Edmund Scientific and others(Ploetz, Coffey). Recently, Steve Welck of Grand Mirage Corp. in California has made large size plastic mirrors(US 4802750) which have begun appearing in advertising displays and even a video game from Sega. Though they could easily be in true 3D, so far all the devices using Welck's mirrors have used a single CRT to project a flat 2D image. Paul Kempf of Metron Optics in California has created a similar but smaller system with input from a stereo pair of cameras(US 4623223, 4840455). An interesting variant was created by Michiel Kassies of Amsterdam who encased millions of tiny mirrored balls between two sheets of plastic.

OTHER TYPES OF AUTOSTEREO SCREENS

A number of investigators have realized that a properly designed holographic screen would be able to fulfill the functions of a lenticular or parallax barrier screen(US 3479111, Umstatter and Trollinger) or even of the camera pickup for an autostereo system(Kopylov). In his US patent(3479111) Dennis Gabor described the design of a holographic screen for the projection of stereo movies by two or more projectors. The book by Hutley has several papers on the fabrication of holographic integral lens arrays. The screen would direct multiple images, projected by conventional means or by laser, to multiple viewing zones. A few experiments have been done for autostereo projection, but the largest such screens are 24 inch by 30 inch created by Komar and his colleagues at NIKFI in Moscow for projection of their holographic movies with four viewing zones for one person each(Komar, Komar and Serov.

Various proposals have been made to use the birefringent properties of liquid crystals to create stereo screens(e.g. Sirat and Charlot).

Okoshi has championed a type of direction device called the curved triple mirror screen(Okoshi et al, Okoshi). Efforts to make such a screen were abandoned due to cost and complexity but with advances in manufacturing since the early 70's, it is undoubtedly worth another look. Many have proposed variants on the standard lenticular screen(Dultz, JAP 59-33449, 60-244943, US 4871233).

VOLUMETRIC DISPLAYS

Volumetric displays are those in which the image points originate in a three dimensional space. There have been an amazing variety of volumetric display devices proposed and built(Balasubramonian, Williams) including dynamic ones having rotating or oscillating screens or lenses or mirrors(Withey, Muirhead, Jansson, Harris and Mathisen, Lazik, Yamada et al., Gregory, Szilard, Fajans{cf.Naegele},US 2330225, 2637023, 2967905, 2979561, 3077816, 3079585, 3097261, 3123711 3140415, 3154636, 3177486, 3202985, 3204238 3212084, 3258766, 3300779, 3316803, 3323126, 3335217, 3371155, 3428393, 3462213, 3493290, 3555505, 3604780, 3682553, 3976837, 4160973, 4294523, 4435053, 4315281, JAP 52-11533, 56-69612, 56-71387, 56-72595, 5674219,56-102822, 56-104316, 56-106215, 56-161788, 56-161789, 57-62020, 57-171313, WO80/02218, 82/01259) which may be light emitting(Matsumoto) or upon which the images are projected by CRTs, light valves or lasers(Pressel, Ketchpel, Brinkmann, Matsushita, Tamura and Tanaka, Soltan). A number of these displays make clever use of fiber optics(US 4173391, Kapany, Martin).

Other approaches have used 3D arrays of components which emit or valve light when addressed electronically(Alburger, Nithiyanandam, Hattori, US 2361390, 2543793, 2749480, 2762031, 2806216, 3005196, 3138796, 3501220, 3536921, 3555349, 3605594, 3636551, 3682553,3891305, 3912856, 4134104, 4173391, 4294516, 4333715, 4391499, 4472737, 4629288, GB 1513719, 1601607, JAP 52-68310, 54-32224, 54-143041,56-125720, 56-162714) or by electron or laser beams. In recent years, many have suggested stacking LCD's(Alampiev et al.) and one of the earliest of these remains the most detailed published account(Reiche et al., cf. Cole et al.).

Another common technique addresses a volume of gas, liquid or solid with one or more laser or electron beam to give potentially very high resolution displays(US 1813559, 2604607, 3474248, 3609706,3609707,3829838, 4023158, 4063233, JAP 55-92090, FRE 461600, 733118, GB 1245783). The reports by Hassfurther et al. and Flackbert et al. are the most detailed published studies on a laser addressed display(rare earth doped calcium fluoride crystals).

The only volumetric display that has had any commercial success is the varifocal mirror(Harris, Huggins and Getty, Harris et al., Sher, Fuchs, Mills, Stover, US 3493290, 3632184,3632866). A large diameter mylar mirror is vibrated with a loudspeaker while addressed with a crt or other light source. This display was developed by Bolt, Beroneck and Newman and marketed briefly by Genisco but the dozen or so units sold seem to see little use and given it's size, cost, image distortions, and the need for a high speed computer for processing, this technology is probably a dead end. Nevertheless, a few companies continue to research it as a display for medical images and graphics (US 4,462044, 4607255, 4639081,4674837, 4743748, Sher).

HOLOVISION

Okoshi has reviewed much of the work relevant to holographic television(Okoshi) It has been looked into by many since the earliest days of holography but progress has been slow. Until recently, most of the effort was by Kopylov and others in Russia(Shmakov and Kopylov). Progress in electronics, electrooptics and computers has recently renewed interest(Honda, Hashimoto, Katsuma, Sato, Fukushima, Benton, GER 3140418, Boudreaux and Lettieri, Hashimoto and Morokawa, Cheng and Lin, Shyyrap et al.) but a real time full color high resolution system still appears quite remote. The annual volumes on Holography published by the SPIE always have several papers on holomovies.

STEREOENDOSCOPY

There has been sporadic interest in stereoendoscopy, with Olympus Optical Co. most prominent(JAP 1-19319, 63-271493, 1-38811,1-38812,1-38813, US 4924853,4935810, 5039198). Recently, several other groups have developed prototypes with the idea of putting this instrument into clinical use(Jones, McLaurin, F. Oertmann of Aesculap A. G. in Tutlinger, Germany). One group developed techniques for digital image correlation of endoscopic stereopairs(Badique et al.). McKinley Optics of Southampton, Ma. has developed a stereoendoscope which was briefly marketed by American Surgical Technologies in 1993-1994. Lowell Noble of SOCS Research Inc. of Saratoga, California has applied the techniques for getting stereo with a single camera to create such instruments from single standard endoscopes with an internal LCD shutter, the image is superb and this product will be marketed by Smith and Nephew. The Canadian company International Telepresence has done similar work. The recent development of extremely high resolution quartz fiber endoscopes by Ultrafine Technology of North Brentford, England should further stimulate this research.

There has also been a steady trickle of papers on holographic endoscopy(Podbielska and Friesem, Friedman et al., Von Bally).

STEREOSCULPTING

Automatic creation of three dimensional objects from stereo information gathered by stereo cameras, lasers etc. or created in computers(stereolithography) has been researched for many years (Kelly Swainson of Omtec Replication in Berkeley, Calif. in the mid 70's) but is only recently coming into practical application(US 4752964,4931817,4935774, JAP 61-88106). A number of companies with devices for automatic acquisition of digital 3D information used for input to CAD-CAM systems now feed this information into computer controlled milling machines for rapid solid modelin.

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