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[Liquid Crystal on Silicon (LCoS) mode]

An alternative way to achieve high-resolution LCD is to use the Liquid Crystal on Silicon (LCOS) devices. LCOS devices use only one glass substrate, and employ a silicon wafer for the back substrate. The pixels are then generally coated with a reflective aluminum layer, and then a polyimide alignment layer. Thus, the liquid crystal industry can take advantage of the existing silicon technology to produce high resolution microdisplays that is easy and inexpensive to manufacture. LCoS is widely used in projection display and projector. A simple picture of the optics geometry for an LCOS system is shown in the figure 1:


Figure 1. A simple Liquid Crystal on Silicon cell configuration (the TFT structures on the silicon wafer is not shown).

Microdisplays are likely to be used in a wide range of applications. The two most likely uses involve virtual displays, in which a series of passive optical elements is used to project the image from the display into your eye, and in projectors. In all of these cases, color may be obtained by one of three realizations:

  • Color Pixellization
  • Multiple LCOS panels
  • Field-Sequential color

The first realization is generally not adopted by the industry because of the expense of patterning color filters onto such small displays. The second one is common in large projectors and projection monitors. The third one is commonly used with virtual displays. Field sequential color (FSC) consists of separating color temporally rather than spatially. This requires extremely fast switching of the LC cell, at least 3 times as fast as a video frame C several hundred Hz. FSC is easily achieved with ferroelectric devices and MEM devices, but these are more difficult and expensive to manufacture than nematic LCDs.

There are a few typical optical architectures which have been using in the LCoS projection system:

  • 1). Philips color prism

    This set of three coupled prisms is used in many 3 CCD video cameras to separate and direct the three primary colors to the three CCDs. In the first application to LCoS projectors IBM developed an prototype LCoS display based on the Philips prism coupled with a PBS. It required double pass control of both the polarization and color and at the same time skew ray compensation for low F number systems. Although extensive design and prototype work has been done at several companies and in academic labs, this approach has the difficulty to maintain the polarization state in the dichroic prism, which results in poor contrast ratio.

    Figure 2. Philips-Color-Prism LCoS projection system


  • 2). Three PBS plus x-cube

    The first commercial LCoS front projectors were designed by IBM, Nikon and JVC. They used dichroic mirrors to separate the three colors, each of which is directed to the corresponding LCoS panel with a separate PBS.

    Skew rays are corrected for by QWPs between the PBS and the panel. The reflected images are combined into a full color image in an x-cube prior to the projection lens. This design is conceptually simple and allows the optimization of each color channel separately. A recent LCoS RPTV product has been announced by a major Japanese consumer electronics company using this design.

    Figure 3. 3-PBS/x-cube LCoS projection system

    Other versions of this basic design are under development to eliminate or reduce the skew ray depolarization. Some of these target lower F number designs and hence higher optical through put. Others target elimination of the QWPs altogether.

  • 3). Three PBS plus three lenses

    In an RPTV application the 3 PBS approach has been modified by Prokia by replacing the x-cube with three separate projection lenses. In this case the recombination of the three color images occurs on the rear projection screen (this is similar to the standard three CRT projection TV). The advantage of this system is elimination of the x-cube prism and the simpler projection lenses (albeit there are three rather than one, each with more limited spectral requirements and shorter back focal lengths). An LCoS RPTV has been announced with this design.

  • 4). ColorQuad and related architectures

    A compact and effective color management system is the ColorQuad developed by ColorLink. This design utilizes four PBSs and five polarization selective color filters (Color Select Filters) assembled as a single unit. The filters and PBSs separate the colors, direct the colors to the appropriate LCoS panels and recombine the images before they enter the projection lens. This system is used in the RCA L50000 50 ̄ HDTV. Because the color separation occurs in the quad this method provides a very compact system not requiring additional dichroic mirrors. However, it is interesting to note that the quad and the 3 PBS plus x-cube designs both use four optical prisms.

    Figure 4. ColorQuadTM LCoS projection system

    Several related quad-like architectures are under development by many companies around the world. These include different quad geometries, replacement of one or more of the PBSs with dichroic cubes or mirrors, different polarization selective filters, etc. The Hitachi CPSX5500W and JVC DLA-SX21 are projectors based on versions of the quad-related LCoS architectures.

    5). Off Axis Illumination

    PBSs are used in all of the above architectures in order to separate the incident illumination from the reflected image. S-Vision and now Aurora have developed a design in which the LCoS panel is obliquely illuminated. Consequently the incident illumination and reflected image can be separated without a PBS. This apparently simplified design has its own challenges. These include more oblique angles at the dichroic color separation and recombination and a more sophisticated projection lens. Off-axis projectors have been announced by Aurora and China Display.

    Figure 5. Off-axis LCoS projection system

However, LCoS systems still have problems, for instance, the life-time is affected because of the high temperature inside, and the LC director alignment will change as polyimide morphologies change. This issue has been solved by using inorganic alignment layer-SiOx. Another issue is the color break-up in the sequential color LCoS system, which is also called "rainbow effect", as appearing in single panel DLP system. Moreover, the optical engine design is complex, in order to accurately control the state of polarization.

Further Readings and References:

IBM, "Special session for high-resolution displays", IBM J. Res. Dev., 42, no. 3/4 (1998).

M. R. Greenberg and B. J. Bryars, "Skew ray compensated color-separation prism for projection display applications", SID¨00, Digest, pp.88C91 (2000).

M. F. Bone, M. Francis, P. Menard, M. E. Stefanov, and Y. Ji, "Novel optical system design for reflective CMOS technology", Proceedings of 5th Annual Flat Panel Display Strategic and Technical Symposium, p.81 (1998).

R. L. Melcher, M. Ohhata, and K. Enami, "High-information-content projection display based on reflective LC on silicon light valves", SID¨98, Digest, pp.25C28 (1998).

C. L. Bruzzone, J. J. Ma, D. J. W. Aastuen, and S. K. Eckhardt, "High-performance LCOS optical engine using Cartesian polarizer technology", SID¨03, Digest, pp.126C129 (2003).

C. L. Bruzzone, J. J. Schneider, and S. K. Eckhardt, "Photostability of polymeric Cartesian polarizing beam splitters", SID¨04, Digest, pp.60C63 (2004).

M. Robinson, J. Korah, G. Sharp, and J. Birge, SID¨00, Digest, pp.92C95 (2000).

G. Sharp, M. Robinson, J. Chen, and J. Birge, "LCOS projection color management using retarder stack technology", Displays, 23, pp.139C144 (2002).


Last update: April, 2006
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