WO2004072713A2

WO2004072713A2 - A high contrast liquid crystal dispersion system cell and transmissive device

Info

Publication number: WO2004072713A2
Application number: PCT/US2004/003550
Authority: WO
Inventors: James John Lupino; Homer Lee Webb
Original assignee: James John Lupino; Homer Lee Webb
Priority date: 2003-02-10
Filing date: 2004-02-10
Publication date: 2004-08-26
Also published as: WO2004072713A3

Abstract

A projection system. The system includes a first polarizer (102); and an electro-optical light scattering medium (103) disposed in a transmissive path between the first polarizer and a light source (108, 109, 110). A plane of polarization of the first polarizer is substantially parallel to a plane of polarization of the light source. The light scattering medium is switchable from a first state to a second state in response to an applied electrical field. The electro-optical light scattering medium in the first state forms a bright region in the projection display and in the second state forms a dark region in the projection display. The transmission path makes a single pass through the light scattering medium.

Description

A HIGH CONTRAST LIQUID CRYSTAL DISPERSION SYSTEM CELL AND TRANSMISSIVE

DEVICE FOR FIELD SEQUENTIAL COLOR PROJECTION DISPLAY

AND SYSTEMS USING SAME

CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to United States provisional application, Serial No. 60/446,304 (filed

February 10, 2003). This application is also related to International Application Serial No. PCT/US03/18762 (filed Junel3, 2003). Both application are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to the field of projection flat panel displays, and more particularly to a flat panel display projection systems comprising cells that include light scattering material between a light source and a viewing surface enabling a field sequential color display.

BACKGROUND INFORMATION

There has been recent interest in various approaches to making liquid crystal displays capable of displaying fiill-color images without the use of color filters. The conventional approach to making LCDs requires the inclusion of color-filters within the light modulation path of the LCD to create the necessary color effect. Normally, a color filter is a patterned filter of red, green and blue elements, which are related to sub- pixels within a display. Therefore, each pixel of a display requires three (3) subpixels to show full-color.

Field sequential color (FSC) displays enables the display of color without the use of color filters, but rather through the use of fast switching liquid crystal material (or other optical material) in combination with fast switching light sources comprised of different colors. Rather than sub-pixels for spatial modulation of color, FSC displays use time multiplexing of colored fight in one pixel to show color. It is recognized that FSC are possible with other non-LCD related technologies such as MEMS (such as TI's DLP projection display) which is outside the scope of this invention.

The transmissive LCD-based approaches to FSC include ferroelectric, optically controlled birefringence (OCB) or pi-cell, and modified drive techniques applied to twisted nematic (TN). Each of these approaches have their own benefits but also problems with respect to reproducibility or cost-performance vis a vis incumbent color LCDs.

Projection displays - which project a relatively small area display onto a much larger area display surface - have recently seen accelerated activity in development and commercialization in the area of large screen display applications such as TV and projectors for business presentation. The technical approaches to projection display include cathode ray tube (CRT), transmissive liquid crystal display (LCD), reflective liquid crystal on silicon (LCOS) display, and micro electro-mechanical systems (MEMS). Most projection displays require three (3) displays to combine and project each red, green and blue image to create the full-color image on the screen. The optics for collecting the colored light, and recombining the light for projection is complex and costly. Therefore, FSC projection displays are preferred for cost reduction.

Therefore, what's needed is a cost-effective, FSC transmissive LCD, which is simple to manufacture, low in cost and provides sufficient performance for commercialization in projection display application. SUMMARY

The problems outlined above are addressed by the present invention. Accordingly, there is provided in one embodiment a projection display system. The system includes a first polarizer; and an elecro-optical light scattering medium disposed in a transmission path between the first polarizer and a light source. A plane of polarization of the first polarizer is substantially parallel to a plane of polarization of the light source. The fight scattering medium is switchable from a first state to a second state in response to an applied electrical field. The elecro-optical light scattering medium in the first state forms a bright region in the projection display and in the second state forms a dark region in the projection display. The transmission path makes a single pass through the light scattering medium. The foregoing has outlined rather generally the features and technical advantages of one or more embodiments of the present invention in order that the detailed description of the invention that follows maybe better understood. Additional features and advantages of the invention will be described hereinafter which may form the subject of the claims of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS A better understanding of the present invention can be obtained when the following detailed description is considered in conjunction with the following drawings, in which:

Figure 1 illustrates a field sequential color (FSC) projection display system in accordance with an embodiment of the present invention;

Figure 2 illustrates the FSC system of Figure 1 in further detail; and Figure 3 illustrates an alternative embodiment of a FSC projection display having an light source with integral polarizer.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth to provide a thorough understanding of the present invention. For example, particular cell dimensions and scatteriing medium compositions may be referred to. However, it will be apparent to those skilled in the art that the present invention may be practiced without such specific details. In other instances, well-known circuits have been shown in block diagram form in order not to obscure the present invention in unnecessary detail. For the most part, details considering timing considerations and the like have been omitted inasmuch as such details are not necessary to obtain a complete understanding of the present invention and are within the skills of persons of ordinary skill in the relevant art.

A field sequential flat panel display device and methods of manufacturing such devices have been described in the above-referenced co-pending International Application Serial No. PCT/US03/18762 entitled "A FIELD SEQUENTIAL DISPLAY DEVICE AND METHODS OF FABRICATING SAME," which is hereby incorporated herein by reference in its entirety. Field sequential color (FSC) displays enables the display of color without the use of color filters, but rather through the use of fast switching liquid crystal material (or other optical material) in combination with fast switching light sources comprised of different colors. Rather than sub-pixels for spatial modulation of color, FSC displays use temporal multiplexing of colored fight in one pixel to show color. In particular, described therein are scattering LCD systems incorporating crossed polarizers for FSC applications. Such scattering LCDs of the type made with localized volumes created either by the addition of polymer or other techniques, in combination with crossed-polarizers provide a direct view display device.

Heretofore, scattering LCDs, such as liquid crystal dispersion systems such as polymer dispersed liquid crystal (PDLC), have not been developed for transmissive FSC presumably due to the perceived lack of optical contrast with such systems. The primary advantage of PDLC is reportedly the lack of a need for polarizers; thus, uses of PDLC in display applications focuses on the reflective scattering mode - direct view and projection

- without use of polarizer films.

Displays using a scattering medium such as scattering LCDs, may in accordance with the present inventive principles include liquid crystal dispersion systems (LCDS) which represent one embodiment of a display device based on a light scattering medium to modulate the transmittance of the display to create a displayed image. Additionally, other embodiments of the present invention may use scattering media other than light scattering cells of the LCDS type. Each of theses classes of light scattering materials will be discussed further below. It would be appreciated by those of ordinary skill in the art that the present inventive principles may be practiced with any scattering medium exhibiting the required optical and switching characteristics imposed on display devices by the attributes of human perception, such as persistence of vision.

For the purposes herein, LCDS may be defined to encompass all light scattering liquid crystal systems whereby multiple surfaces are created in the cell; including as examples, but not limited to, the following systems: polymer dispersed liquid crystal (PDLC), reverse-mode PDLC (such as described in U.S. Patent Nos. 5,056,898 and 5,270,843, and Menial-Reflection Inverted-Scattering (IRIS) Mode of Seiko-Epson Corp.), holographic PDLC (H-PDLC), nematic c-irvilinear aligned phase (ΝCAP), polymer network liquid crystal (PΝLC), polymer encapsulated liquid crystal (PELC), polymer stabilized cholesteric texture (PSCT), phase separated composite film (PSCOF), colloidal templated liquid crystal composition such as the composition disclosed in U.S. Pub. No. 2001/0035918, which is hereby incorporated herein by reference, PMMA resin LC composition, and LC and macromolecular LC molecule compositions.

LCDS may also include LC mixtures including dispersed nanoparticles (such as silica made by Nanotechnology Inc., Austin, TX or Altair Nanotechnology, Reno, NV) which creates the necessary effect to enable light scattering by the LC molecules. The particles themselves are small and transparent.

LCDS may also include those LCDS made with channels, pockets or other cavities within the cell which have the same effect as polymer dispersion for scattering light. Examples of such techniques may be

Plastic Pixels™ a product and process of Viztec, Inc., Cleveland, OH, Microcup LCD, a product and process by SiPix Imaging, Milpitas, CA, (described in U.S. Pub. No. 2002/0126249 Al, which is hereby incorporated herein by reference) and PoLiCryst, as described by L. Vicari, I. Opt. Soc. Am. B, Vol. 16 pp. 1135-1137 (1999), which is hereby incorporated herein by reference. Other techniques include filling open or connected micropores of a plastic sheet with a nematic or other type of liquid crystal (as disclosed in U.S. Patent No.

4,048,358, which is hereby incorporated herein by reference). Such pores could be fabricated today for example with microreplication technologies employed by such companies as 3M, Minneapolis, MΝ and Avery Dennison, Pasadena, CA or for example utilizing the a pixilated foil platform such as that developed by Papyron B. V., The Netherlands.

Each of these systems would be recognized as being an LCDS by those of ordinary skill in the relevant art. As noted above, embodiments of the present invention are not only limited to light scattering cells of the LCDS type, but also may include other light scattering liquid crystal materials such as chiral nematic liquid crystal or cholesteric liquid crystal which exhibits a light scattering mode in the focal conic state and a transparent state in the planar state. Also, smectic A liquid crystal is also known to scatter light in one state and change to a transparent state in another state. Cholesteric and smectic A liquid crystal do not require a polymer network or dispersion within a polymer matrix to create the scattering effect, but may be created with a polymer network.

Further, this invention is also applicable to non-liquid crystal materials which may be optically switched from a light scattering state to a substantially light transparent state. For example, small particulate matter may be suspended in a medium and behave in the same manner (scattering and non-scattering) as described herein. One such example of particulate matter suspended in a medium is Suspended Particle Device

(SPD) light control technology developed by Research Frontiers, Inc., Woodbury, NY. This is only one of several types of non-liquid crystal electro-optical (switchable) light scattering materials which could be used in conjunction with the present inventive principles.

Referring now to Figure 1, there is illustrated therein an embodiment 100 of a transmissive FSC projection system in accordance with the present inventive principles. FSC projection system 100 provides for

FSC displays using an electro-optical scattering medium 103, such as PDLC is disposed between two polarizers.

These include analyzer 102 and polarizer 104. Polarizer 104 and analyzer 102 are substantially parallel. When the medium 103 is in the optically transmissive clear mode, light from light sources 108-110 passes through polarizer 104, medium 103 in a single pass, analyzer 102, and through projection optics system 101 to the projection screen 111, where it may be perceived by an observer (not shown) in the usual way. Thus, the transmission path from the light source to the projection optics makes a single pass through the polarizer, the electro-optical scattering medium and the analyzer. Screen 111 may, in alternative embodiments be a conventional reflective screen with the viewer (not shown) positioned in front of, that is on the same side of the screen as the illumination, or a conventional translucent rear projection screen with the viewer (not shown) positioned behind the screen, that is, on the side opposite the illumination. In yet a third embodiment, screen

111 may be a transparent reflective surface as may be employed in a "heads-up" display (HUD).

In particular light sources 108-110 may be a red/green/blue FSC light source. These may include, for example, conventional light emitting diodes (LED) or organic light emitting diodes (OLED) in embodiments of the present invention. It would be appreciated by those of ordinary skill in the art that the particular type of FSC light source does not implicate the present inventive principles, and that any type of light source commonly used to generate field sequential color may be used in conjunction with the present invention. A lens system composed of elements 101 and 107 may be used to collimate the light from sources 108-110 and project the light emanating from the cell components (102-106) onto the viewing surface, such as screen 111. Optical element 101 maybe a Schlieren optical system, having a projection lens 113 and diaphragm 115 in accordance with known optical principles. (Diaphragm 115 may also be referred to as a Schlieren stop.) Collimating optics 107 focuses the light source onto the cell composed of elements 102-106 to avoid artifact obstructions such as TFT transistors (not shown) which may be employed to drive medium 103 (Driver systems for LCDS which may be used in conjunction with the present invention have been described in detail in the aforementioned co- pending International Application Serial No. PCT/US03/18762 incorporated herein by reference.)

When medium 103 is in the transmissive clear mode, the light, albeit attenuated by polarizer 104, passes through medium 103 in a single pass and then through analyzer 102 where little light is lost, giving high efficiency to the projected light. Conversely, when the medium 103 is in the optically scattering mode, the polarized light from polarizer 104 is scattered by medium 103 and is diffused and is not projected by projection optics 101 and the region imaged on the screen appears dark. Approximately 50% of this light impinging on the medium in the scattering mode is diffused and depolarized and is blocked by the analyzer 102. As such, the light reaching the projection optics is further reduced by 50% thereby doubling the contrast ratio. Further, in an embodiment employing a Schlieren projection optical system 101, scattered light may be intercepted by the diaphragm 115 included in such systems, thereby reducing any scattered light that might otherwise reach the projection screen.

Medium 103 may be driven via electrodes 105 and 106. These may be a transparent conductive coating, such as Indium-Tin oxide (ITO). Pixels maybe formed by patterning electrode 105. Electrode 105 is illustrated as applied to analyzer 102, however in alternative embodiments, maybe applied to medium 103 or to a substrate (not shown). Such alternate structures have been described in the aforementioned co-pending PCT

Application, Serial No. PCT/US03/18762. Likewise electrode 106 is illustrated as applied to polarizer 104, although in an alternative embodiment may be applied to a substrate (not shown). Medium 103 may be switched between the scattering mode and transmissive clear mode by the application of an electrical potential to the electrodes. Mechanisms for driving LCDS media, such as medium 103, have been described in aforementioned co-pending International Application, Serial No. PCT/US03/18762, entitled "A FIELD

SEQUENTIAL DISPLAY DEVICE AND METHODS OF FABRICATING SAME". Such mechanisms may be used in conjunction with the present invention. These may include multiplexing, direct drive, active matrix TFT, active matrix diodes, in-plane switching, fringe field switching, field oriented addressing, etc. The drive voltage required depends on the thickness of the cell formed by medium 103 and electrodes 105, 106. For voltages compatible with existing active matrix backplane processes, it is preferable to construct devices with this invention with cell gaps no larger than 10 microns. A thinner cell gap minimizes scattering in the clear state, thereby providing high transmission of collimated light for high brightness and enables drive voltages less than 18V, suitable for active matrix backplanes commercially produced for other displays. For high speed switching, a high percentage of polymer is preferable (more than 25% by weight) which normally enables smaller droplet size formation during fabrication. It would be appreciated by persons of ordinary skill in the art that these compositions and voltages are exemplary of particular embodiments of the present invention, and other compositions and voltages would fall within the spirit and scope of the present inventive principles as set forth in the claims. The operation of an FSC projection system in accordance with the present invention may be farther understood by referring to Figure 2, which illustrates the transmission of light through such a. system in additional detail. System 200a illustrates an FSC system, such as system 100, Figure 1, in the "ON" state. That is, system 200a represents a bright or iUuminated, region in a displayed image. Conversely, system 200b illustrates an FSC system in accordance with the present invention in an "OFF" state, corresponding to a dark region in the displayed image.

In the "ON" state, 200a, typically voltage is applied across electrodes 205 and 206. Light from light sources which may be unpolarized (201), passes through optical system 207 which focuses the light through polarizer 204, and electrode 206, which may be formed from ITO, onto the active portion of the electro-optical scattering medium 203. The light is polarized by polarizer 204 wherein only light in vibrating in one plane (212) is transmitted through the polarizer 204. This polarized light is passed through the medium 203 in a single pass, which is in a transparent, or equivalently transmissive clear, state and though transparent electrode 205, which may also be of ITO, and then through analyzing polarizer, or simply analyzer, 202. Therefore, in the transmissive clear mode 203 a, the light is attenuated by approximately 50% by polarizer 204 (an alternative polarization mechanism which is significantly less lossy is discussed below) but little light is lost in analyzer

202 giving very high efficiency to the projected light. That is, medium 203a is in the optically "ON" transmissive clear mode. As polarizer 204 and analyzer 202 are aligned, the polarized light passes through the cell formed by electrode 206, medium 203, electrode 205, and analyzer 202 with little attenuation. The light the passes through optical system 208, which maybe a Schlieren system and is projected onto screen 210. (Note that, for simplicity, optical system 208 is schematically represented as a single element in Figure 2 to not unnecessarily complicate the illustration, however, those of ordinary skill in the art would appreciate that a Schlieren system includes a projection lens and Schlieren stop, as illustrated in Figure 1.)

However, in the scattering mode, medium 203b, the polarized light from polarizer 204 is scattered by medium 203b. That is, medium 203b is in the optically "OFF" scattering mode. The light passing through medium 203b is diffused and depolarized (214) by the medium and is substantially not projected by optics 208, thereby casting a shadow 209b on screen 210. This results in an observer (not shown) viewing screen 210 seeing a darkened portion of an image. Approximately 50% of the light 212 is diffused and depolarized and is blocked by the analyzer 202. As such, the light reaching the projection optics is further reduced by approximately 50% enhancing the contrast ratio. Note that one of ordinary skill in the art would appreciate that any degree of polarization, from approximately 1% to the theoretical maximum of 50%, at analyzer 202

(likewise analyzer 102, Figure 1, and analyzer 302, Figure 3, below) would increase the contrast of the display.

Figure 3 illustrates an alternative embodiment 300 of an FSC projection device in accordance with the present invention. FSC projection device 300 includes collimating optical system 307 and projection optical system 301 in similar fashion to device 100 Figure 1. Optical system 301 may also be a Schlieren system including a projection lens 313 and Schlieren stop 315. Similarly, analyzer 302, electrode 305 and electrode

306 operate as previously discussed in conjunction with the corresponding elements in Figure 1. Device 300 includes light sources 308-310 with an integrated polarizer 304. Such an embodiment may reduce the amount of polarizer required, thereby reducing cost. Additionally, polarizer 304 may be a recovery polarizer, or, equivalently, recovery polarizer system. (Likewise, polarizers 104 and 204 in Figures 1 and 2, respectively, may be embodied using recovery polarizer systems.) Although a recovery polarizer system may be more costly than a conventional polarizer, the 50% reduction of intensity experienced when an unpolarized source passes through a conventional polarizer may be reduced to 30% or less, thereby improving the light efficiency of FSC projection devices in accordance with the present inventive principles. Several methods in the optical art are known to enhance the light transmission in polarizer-based optical systems by incorporating techniques to

"recycle" the non-transmitted light and convert it to substantially the polarization state intended for transmission. These techniques may be used to embody recovery polarizer systems. One method of polarization recovery uses an arrangement of prisms to split an unpolarized light source into two orthogonally- polarized beams. The plane of polarization of one of the two is rotated by ninety degrees (90°) to bring the polarization into alignment with that of the other beam. The two beams are recombined to produce a linearly polarized source. Polarization recovery films and light pipes are commercially available from several sources including Wavien, Inc., Santa Clarita, CA, 3M Optical Systems Division, St. Paul, MM, and Optical Coating Laboratories, Inc. (OCLI), Santa Rosa, CA.

Those of ordinary skill in the art would appreciate that the present inventive principles, although described in conjunction with embodiments representative of field sequential color display systems, may also be applied to improve the contrast ratio of conventional three-color transmissive projection systems, such as those described in United States Patent No. 6,088,075 of Nakao et al, and United States Patent No. 5,760,875 of

Daijogo et al.

Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

CLAIMS:

1. A projection display system comprising: a first polarizer; and an elecro-optical light scattering medium disposed in a transmission path between said first polarizer and a light source wherein a plane of polarization of said first polarizer is substantially parallel to a plane of polarization light from said light source, wherein said light scattering medium is switchable from a first state to a second state in response to an applied electrical field, wherein said elecro-optical light scattering medium in said first state forms a bright region in a projection display and in said second state forms a dark region in said projection display, and wherein said transmission path makes a single pass through said elecro-optical light scattering medium.

2. The display system as recited in claim 1, wherein said light scattering medium comprises a Polymer Dispersed Liquid Crystal (PDLC).

3. The display system of claim 1 wherein said first state comprises a transmissive clear state of said elecro-optical light scattering medium and said second state comprises a scattering state of said elecro-optical light scattering medium.

4. The display system of claim 1 wherein said light source comprises an unpolarized light source, the system including a second polarizer disposed between said unpolarized light source and said electro-optical light scattering medium, said second polarizer having a plane of polarization substantially parallel to the plane of polarization of the first polarizer, whereby said first polarizer has a plane of polarization substantially parallel to the plane of polarization light from said light source.

5. The display system of claim 1 further comprising: a substantially transparent conductive layer disposed between said first polarizer and said electro- optical light scattering material.

6. The display system of claim 5, wherein said substantially transparent conductive layer is disposed on said first polarizer.

7. The display device of claim 6, wherein said substantially transparent conductive layer is an Indium-Tin

Oxide (ITO) layer.

8. The display system of claim 1 further comprising collimating optics disposed in a transmission path between said light source and said electro-optical light scattering medium.

9. The display system of claim 1 further comprising projection optics for receiving light transmitted through said first polarizer and a forming an projection image for display.

10. The display system of claim 9 wherein said projection optics comprises a Schlieren lens system.

11. The display system of claim 4 wherein said second polarizer comprises a recovery polarizer.

12. The display system of claim 1 wherein said light source comprises a plurality of independently controllable colors.

13. A display device comprising: a first polarizer; and an elecro-optical light scattering medium disposed in a transmission path between said first polarizer and a light source wherein a plane of polarization of said first polarizer is substantially parallel to a plane of polarization of said light source, wherein said light scattering medium is switchable from a first state to a second state in response to an applied electrical field, and wherein said transmission path makes a single pass through said elecro-optical light scattering medium.

14. The display device of claim 13 further comprising a second polarizer operable for polarizing an unpolarized fight source, light transmitted by said second polarizer for illuminating said electro-optical scattering medium, wherein a plane of polarization of said second polarizer is substantially parallel to the plane of polarization of said first polarizer.

15. The display device of claim 13 wherein said second polarizer comprises a recovery polarizer.

16. The display device of claim 13 wherein said first state comprises a transmissive clear state of said elecro-optical light scattering medium and said second state comprises a scattering state of said elecro-optical light scattering medium.

17. The display device of claim 13 wherein The display system as recited in claim 1, wherein said light scattering medium comprises a Polymer Dispersed Liquid Crystal (PDLC).

18. The display device of claim 13 further comprising: a substantially transparent conductive layer disposed between said first polarizer and said electro- optical light scattering material.

19. The display device of claim 18 wherein said substantially transparent conductive layer is an Indium-Tin Oxide (ITO) layer.

20. The display device of claim 13 wherein said light source comprises a plurality of independently controllable colors.