CP and LP switching by multilayer LCD plates was specifically patented for 3D by many companies including StereoGraphics, the company I started in 1979, and marketed by them under the name “Z-Screen”, sometimes called “Z-Filter”. LCD shutter glasses and CP switching screens were originally developed and marketed by Tektronix in the 70’s and, after poor management destroyed their LCD division-- the USA’s finest LCD R&D facility--it was licensed to NuVision of USA and Delta of Taiwan. Independently, various companies worked on this, including the Japanese petrochemical company Idemitsu, who released an all plastic version (i.e., no glass whatsoever) that I used for some time in the late 1990’s (EP 0892563 A2 (1999)). A few years ago ColorLink began marketing one and Real D (the new name of StereoGraphics after some Hollywood hotshots bought controlling interest in 2004) solved the problem of competition from a superior product by buying ColorLink ( http://www.reald-corporate.com/story030807.asp ).

A little known aspect of this history is that Tektronix was sued by LC pioneer James Ferguson over pi-cell patents, and, despite assurances from their patent and tech staff that they would win easily, they paid him off rather than pursuing it, since they had a lucrative business selling high end devices such as time domain reflectometers and they did not want to interrupt the cash flow. Possibly this enabled StereoGraphics Corp. to survive since Tek might have sued them for patent violations.

The CP switching method has the same problem as other active or passive (e.g., dual projector) CP methods—more ghosting or crosstalk than LP (Linear Polarizer) methods. Another problem is that the volume accumulation of ions may quickly decrease image quality during the movie, and some of the patents describe quenching techniques for amelioration. In fact there are so many problems that Real D says it will not work for screens wider than 40 feet and has filed a whole series of patents trying to correct them ( e.g., US 2008/0206155 and above). This necessitates the preprocessing of all 3D films by Real D to decrease ghosting (US 2008/0268104, US 2007/188602, EP001883835), though they say they will put the algorithms in a chip soon and do it realtime on the projector. Ghost reduction, realtime or not, is a very good idea for every 3D program, regardless of viewing method-provided of course that other aspects of image quality do not take a hit. There is a long history of ghost reduction going back to the days of ghosting in 2D television broadcasting and there have been a number of stereoscopic implementations in the patent and technical literature (e.g, see the patents by Street US 6075555 etc and others in my SPIE article and Konrad et al. --Cancellation of image crosstalk in time sequential displays of stereoscopic video. IEEE Transactions on Image Processing 9:897-908(2000)) and also in the common educational 3D system from Neotek www.neotek.com.

The angle from the projection lens to the edges of the screen for an active polarization switch should not be wider than about 12 degrees as the crosstalk will begin to exceed acceptable limits at the edges of the screen. This is a problem with all polarization methods including passive dual projection but is naturally worse with multilayer active devices and worst of all with the CP switcher favored by Real D. Going to an LP switcher ameliorates the problem somewhat and ColorLink has developed their linear ALPS device for this reason (US 2006/0291053 and Sharp and Robinson—Enabling stereoscopic 3D technology. SPIE vol. 6490(2007). In addition, since the retardation is tuned to the green (which always has highest luminosity) red and blue objects will show greater ghosting in all parts of the screen.

Crosstalk is always present in any polarized system and it gets worse the further off the axis from projector to screen. Thus one should always sit in the middle of the theater and to the back if possible and particularly avoid seats to the extreme right and left close to the screen. Sitting in the rear of the theater is always a good idea to minimize stereoscopic errors (including the horizontal parallax so beloved by stereo cinematographers). To calculate acceptable screen width in a Real D theater, just measure the throw from the front of the projection lens to the screen and use trigonometry to determine width for 12 degrees.

From the earliest days of LC’s in the 60’s to the present, there is a massive body of literature (tens of thousands of patents and papers) relevant to polarization switching and there is no possibility that anyone has a fundamental blocking patent on LCD shutter glasses, or CP or LP switching. Countless companies worked on this in the 70’s and 80’s and you can get a good sampling in the SPIE review paper I published over a decade ago, which is also on my page www.3dtv.jp as the Stereoscopic Imaging Tech article, but this only relates to certain areas of 3D and barely touches on the much larger literature relevant to polarization switching and related issues.

Consequently, it is clear that active CP (or LP) switching for active glasses or projector StereoPlates (the name I have long used for these devices when placed in front of a projector or CRT) is a public domain technique, though possibly some companies have protectable refinements. Real D claims they will release a new XL version of the ColorLink CP switch in late 2008 with double the brightness (which can be achieved e.g., with sufficiently rapid switching, by eliminating the polarizers and using a cholesteric LC layer that can theoretically convert to CP 100% of the unpolarized light).

This technology is well understood by thousands of engineers in the LC industry and new products from other companies are already appearing, but it is possible that one of the mechanical LP or CP alternating systems (see below) will obsolete them all. The simple rotating CP disk system works, but has problems which will be obvious to any EO engineer, but modern tech provides other options and they are being pursued by many.

IBM 2008 patent application on magnetically controlled frame sequential polarized 3D projection with the polarizers, sensors and dampers in Fig 8A and the timing diagram in 8B. E.G., one promising improvement in frame sequential polarized technology prototyped over the last 3 years and patented by IBM (US 2008/555402, 2008/555401, 2008/0055546) uses small pieces of magnetically oscillated polarized filters placed at the internal focal point of the projector with magnetic bearings and magnetic or air core solenoid damping. I estimate a parts cost of about $20 and it can be modularized for quick install by unskilled personnel. However, the same IBM researchers are hedging their bets with a conventional rotating polarized wheel (WO 2007/071614 ). All methods which place optical components internally near the focal point have to dissipate heat very rapidly. This is less scary than it seems as it is normal for videogamers to cool their overclocked processors with special thermal units including some with liquid coolants and advanced cooling tech is readily available.

With these devices, internal or external, it should be easy to retrofit theaters currently using a CP switcher or other means, thus eliminating the need for preprocessing, expensive glasses and licensing fees. Sony’s single lamp, dual LCD projector: Sony’s modular, single lamp, dual LCD projector uses polarizers, two half wave plate. Like most of the other methods, it should also work with GLV (Gated Light Valve) projectors (http://www.siliconlight.com/brochure1.pdf), a laser addressed MEMS technique that has been exclusively licensed to SONY for display applications. Perhaps SONY will finally recover its investment on the PlayStation 3 this year and be able to afford developing GLV, which, blindsided by the 3D revolution, they sorely need, as their high end projector is LCOS, incompatible with all frame sequential methods. Of course this is a problem with all types of LCD’s and much research has gone into attempts to increase the speed (e.g., WO 2007/021456, WO 2007/021457) but there are no commercial FS compatible panels or projectors (though NVida/ViewSonic have shown a prototype). SONY has however not been idle with other approaches and what appears to be a very nice patent ( WO 2005/101821) shows how to use dual LCD’s in a modular projector with native cross polarization using only one lamp and a reflective electronic color filter (RECS) of unspecified nature; the wasted light from one image being used to illuminate the other. With the 3D market booming, Thomson’s dual LCOS projector also looks feasible (US7192139, US 7204592, WIPO2004/051994). However the Lagrange invariant limits light flux for high brightness projectors using small imaging chips so Kodak has described methods using larger TFT LCD’s with such amenities as wire grid polarizers, glass fresnel lenses and dichroic filters to solve this problem (WO 2007/070245).