GB2162356A - Improved liquid crystal optical switch with reduced cross talk - Google Patents

Abstract

A field sequential color display system (10) employs a liquid crystal device operating as a variable optical retarder (12) which is positioned in front of the screen (40) of a raster scanned cathode ray tube (38) to develop a high quality multicolored image. The liquid crystal device is divided into upper and lower cell segments (34 and 36) to decrease the time between successive color fields. Each cell segment is separately commanded by a switching circuit (46) to a predetermined optical retardation state when the electron beam scans the portion of the screen covered by the cell segment. Each cell segment has two electrode structures (22 and 30; 24 and 32). Electrode structures 22 and 24 are electrically isolated and are connected to the ground potential of the switching circuit to minimize the optical cross talk between cell segments and thereby minimize the spurious transmission of colored light from the display system. <IMAGE>

Description

SPECIFICATION Improved liquid crystal optical switch with reduced cross talk TECHNICAL FIELD The present invention relates to color display systems and, in particular, to a field sequential color display system that employs a liquid crystal device which is divided into a plurality of electrically isolated and separately electrically driven cell segments to decrease the time between successive color fields, and thereby improve the image color quality, of the display system.

BACKGROUND OF THE INVENTION High performance field sequential liquid crystal color display systems can develop high resolution multicolored images with strong contrast and good color purity. However, a wellknown problem associated with liquid crystal switching devices may cause display systems incorporating them to produce colored images of relatively poor quality. Specifically, the speed of response of a liquid crystal device is known to be relatively slow in switching from the fieldaligned ("ON") state to the relaxed ("OFF") state. This causes the information of one color field to be displayed in the color of the immediately preceding color field during the transition from the "ON" state to the "OFF" state of the device.

A solution to this problem is suggested in U.S. Patent No. 4,295,093 of Middleton who describes a spectrum analyzer which employs a liquid crystal switching device that is split into first and second contiguous regions of liquid crystal material, each of which is controlled separately by a voltage applied by a different output of a drive circuit. Both regions share a common electrode which is held at a fixed reference potential. In Column 4, lines 21-38, the Middleton patent describes with reference to Fig. 2 thereof the construction of the liquid crystal device as follows. "[The device] consists of a film 20 of liquid crystal material of the twisted nematic kind . .One side of the film 20 is provided with a single transparent electrode 21 which is held at a fixed potential, and the other side is provided with two transparent electrodes 22 and 23 which each cover a respective half of the film, and these are connected to the colour select circuit 15 via conductors 24 and 25." The above-described liquid crystal device is positioned in front of a cathode ray tube screen that is raster scanned by an electron beam. The split line separating the first and second contiguous regions of the liquid crystal device is oriented substantially parallel to the line scanning direction of the raster scanning pattern. In operation, the first region is scanned in its first color state while the second region of the liquid crystal material is switched from a second color state to a first color state.Upon completion of the scan of the first region, the second region is scanned in its first color state while the first region is switched from the first color state to the second color state.

Alternate switching between color states of the first and second regions of the liquid crystal device allows the previously scanned region to settle to its other color state prior to the return of the electron beam to scan that region of the display.

U.S. Patent No. 4,328,493 of Shanks et al. describes a color display which employs a liquid crystal device constructed with first and second contiguous regions of liquid crystal material as in the Middleton patent. The Shanks et al. patent describes a timing sequence for signals developed at the outputs of a drive circuit which applies AC pulses of one of two frequencies alternately to the first and second regions of liquid crystal material. Light rays of two colors alternately appearing at the output form a multicolored image in a manner similar to that described in the Middleton patent.

The liquid crystal devices described in the Middleton and Shanks et al. patents suffer from the disadvantage of introducing optical cross talk between the contiguous regions of the liquid crystal device. The optical cross talk is characterized in that the electric field applied to the region being scanned by the electron beam is coupled to the unscanned region and causes a spurious transmission of light from it. The two contiguous regions of liquid crystal material are, therefore, not electrically isolated from and optically independent of each other. The result is a color display system with degraded image quality.

One of the objects of this invention is to provide a high performance field sequential liquid crystal color display system that develops a high resolution multicolored image with strong contrast and good color purity.

Another object of this invention is to provide in such a display system a liquid crystal device which is divided into a plurality of segments that have a minimum amount of optical cross talk among them.

A further object of this invention is to provide a display system which incorpoates a liquid crystal switching device that decreases the time between successive color fields to improve the image color quality of the display system.

Still another object of this invention is to provide a display system which incorporates a liquid crystal switching device that is divided into a plurality of segments, each having a separate reference voltage electrode to minimize the optical cross talk among the cell segments.

SUMMARY OF THE INVENTION The present invention relates to a liquid crystal device operating as a high-speed optical switch in a color display system. The liquid crystal display device receives from a light source light rays in a defined polarization state. The liquid crystal display device comprises first and second contiguous cell segments, the first cell segment including first and second spaced-apart electrode structures having a liquid crystal material captured between them, and the second cell segment including third and fourth spaced-apart electrode structures having a liqujd crystal material captured between them. The first and third electrode structures are electrically connected to the electrical reference of a switching means or circuit.

A first output of the switching circuit is in communication with the second electrode structure.

The first output applies a first voltage across and introduces a first electric field within the first cell segment. A second output of the switching circuit is in communication with the fourth electrode structure. The second output applies a second voltage across and introduces a second electric field within the second cell segment. The switching circuit is operable to iridependently change the character of the polarization state of the light rays passing through each one of the first and second cell segments.

Electrically isolating the first and third electrode structures of the liquid crystal device isolates the first and second electric fields within the cell segments and thereby minimizes any change in character of the polarization state of the light rays passing through one of the cell segments due to a change in the character of the polarization state of the light rays passing through the other cell segment.

In a preferred field sequential color display system, the liquid crystal device of the present invention is positioned in front of and receives light rays from the screen of a raster scanned cathode ray tube. The liquid crystal device operates as a variable optical retarder and is disposed between color selective and neutral light polarizing filters. The polarizing filter receiving the light rays from the cathode ray tube defines the polarization state of the light rays incident to the variable optical retarder. The variable optical retarder changes the character of the polarization state of the light rays passing therethrough in response to a voltage applied by the switching means or circuit.The variable optical retarder produces a first amount of optical retardation to develop a display system light output in one color and a second amount of optical retardation to develop a display system light output in a second color. The first amount is essentially zero retardation of light rays of all wavelengths, and the second amount is substantially half-wave retardation of light rays of the wavelength of one of the colors developed at the display system output.

The liquid crystal variable optical retarder is divided into upper and lower cell segments. Each cell segment is separately driven by a different output of the switching circuit in a predetermined time sequence to cause in each color field the display of one-half of the light image information developed by the raster scanned cathode ray tube.

The liquid crystal devices described in the Middleton and Shanks et al. patents utilize for both cell segments a common electrode structure which is connected to an electrical reference. The liquid crystal device of the present invention, however, utilizes separate, electrically isolated electrode structures, which are connected to the electrical reference of the switching circuit. The use of separate reference electrode structures minimizes the electrical cross talk between cell segments by isolating from one cell segment the effect of noise voltages induced on the reference electrode of the other cell segment. Minimizing the electrical cross talk minimizes the optical cross talk between the two cell segments.The result is a display system which develops a high quality colored image by changing color fields sufficiently fast and by minimizing the spurious transmission of colored light.

Additional objects and advantages of the present invention will be apparent from the following detailed description of a preferred embodiment thereof, which proceeds with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a simplified block diagram of a field sequential color display system incorporating a liquid crystal variable optical retarder designed in accordance with the present invention.

Figure 2 is an enlarged plan view of the liquid crystal device incorporated in the display system of Fig. 1.

Figure 3 is a diagrammatic elevation view of the left or right side of the liquid crystal device of Fig. 2.

Figures 4A and 4B are schematic diagrams of the director alignment configuration of one of the cell segments of the liquid crystal device of the present invention in, respectively, the fieldaligned ("ON") state and the partly relaxed ("OFF") state.

Figure 5 is a timing diagram illustrating the timing relationship of the switching signals applied to the cell segments of the liquid crystal device of Fig. 2.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT General Arrangement and Operation of the Color Display System With reference to Fig. 1, a preferred embodiment of a field sequential color display system 10 designed in accordance with the present invention includes liquid crystal variable optical retarder 1 2 which is in optical communication with and is disposed between color selective polarizing filter 14 and neutral polarizing filter 16. The input surface 18 of variable optical retarder 1 2 is divided along a horizontal nonconductive split line 20 into two separate electrode structures 22 and 24, which are connected to ground. The output surface 26 of the variable optical retarder 1 2 is divided along a horizontal nonconductive split line 28 into two separate electrode structures 30 and 32.Electrode structures 22 and 30 define a first or upper cell segment 34, and electrode structures 24 and 32 define a second or lower cell segment 36 as shown.

Color selective polarizing filter 1 4 has orthogonally related polarization states, one polarization state passing light of only a first color and the other polarization state passing light of only a second color. Neutral polarizing filter 1 6 has two orthogonally related polarization states, one polarization state passing light of all colors and the other plarization state absorbing light of all colors. In a preferred embodiment of the color display system, polarizing filters 1 4 and 1 6 have mutually orthogonally aligned linear polarization axes.

The optical assembly formed by variable optical retarder 1 2 and polarizing filters 1 4 and 1 6 comprises a high-speed color switch that is positioned in front of a source of light or image source 38 which emits light from phosphor screen 40 to produce a light image in two primary colors, e.g., red and green. In a preferred embodiment of the display system, image source 38 constitutes a cathode ray tube which by means of television type raster scan a signal produced by raster generator 42 in response to the output of frame synchronization circuit 44 presents sequential frames of image information. Each frame includes two fields of image color information in alternating first and second color field time intervals.

In the first color field time interval, information pertaining to both the form of any image which is to appear in a first color, such as red, and the form of any image which is to appear in a color that is a combination of the color red and a second color, such as green, is written on phosphor screen 40. In the second color field time interval, information pertaining to both the form of any image which is to appear in the color green and the form of any image which is to appear in a color that is a combination of the colors red and green is written on phosphor screen 40. Color selective polarizing filter 1 4 receives the light transmitted from phosphor screen 40 and orthogonally and linearly polarizes it in the red and green colors. The polarized light is then transmitted to input surface 1 8 of variable optical retarder 12.

Switching means or circuit 46 receives at its input a signal from the output of frame synchronization circuit 44 to drive variable optical retarder 1 2 synchronously with the sequential frame rate of image information produced by image source 38. During the first color field time interval, therefore, switching circuit 46 commands in a time sequence that is described below the cell segments 34 and 36 of variable optical retarder 1 2 to the partly relaxed or "OFF" state which causes an orientation of its optic axis that accomplishes substantially half-wave retardation of light of the color green passing through polarizing filter 1 4. No light of the color green passes through the linear polarizer 1 6 during this time interval, and the undesired image components in the color green which appeared on phosphor screen 40 during the first time interval are thereby eliminated. During the second color field time interval, switching circuit 46 commands in a time sequence that is described below the cell segments 34 and 36 of variable optical retarder 1 2 to the field-aligned or "ON" state which causes an orientation of its optic axis that accomplishes essentially zero retardation of light of both of the colors red and green passing through polarizing filter 1 4. The absorption axis of neutral polarizing filter 1 6 absorbs light only of the color red as will be further hereinafter described.

Fig. 5 is a timing diagram which shows the timing relationship between the switching signals applied to cell segments 34 and 36 to develop the two color fields of an image frame. The first color field time interval spans the time from T1 to T5, and the second color field time interval spans the time from T5 to Tg. Line A of Fig. 5 represents the voltage signals applied to electrode structure 30 of cell segment 34 at output 48 of switching circuit 46, and line B of Fig. 5 represents the voltage signals applied to electrode structure 32 of cell segment 36 at output 50 of switching circuit 46. Electrode structures 22 and 24 of cell segments 34 and 36, respectively, are connected to the electrical reference of switching circuit 46, which reference is preferably ground. The voltage signal applied to comman each cell segment to the "ON" state is preferably a 54 volts peak-to-peak or 20 Vrms AC pulse of between 1 kHz and 4 kHz, which is depicted in Fig. 5 as a rectangular envelope marked with diagonal lines. The voltage signal applied to command each cell segment to the "OFF" state is preferably zero volts.

To decrease the time between successive color fields, each of the first and second color field time intervals is subdivided into first and second subintervals. As shown in Fig. 5, the first subinterval of the first color field spans the time from T1 to T3 and the second subinterval of the first color field spans the time from T3 to T5.

From T, to T2, cell segment 34 is in the "OFF" state, receives the light rays produced by the scanning electron beam, and causes red light to exit the display system output covered by cell segment 34. From T3 to T5, cell segment 36 is in the "OFF" state, receives the light rays produced by the scanning electron beam, and causes red light to exit the display system output covered by cell segment 36. Cell segment 36 is commanded to remain in the "ON" state from T1 to T2 to delay the change to the "OFF" state of the first color field of the image frame shown until the light emitted from the phosphors on screen 40 during the second color field of the immediately preceding image frame sufficiently decay.This delay ensures that continuity of color in the composite image exists for each color field.

As shown in Fig. 5, line A, cell segment 34 is commanded to the "ON" state at T4 prior to the time the electron beam begins to scan into the region of screen 40 covered by cell segment 34. Switching cell segment 34 at T4 provides sufficient time for the liquid crystal molecules to stabilize in the "ON" state prior to the start at T5 of the first subinterval of the second color field time interval. As shown in Fig. 5, line B, cell segment 36 is commanded to the "ON" state at T6 prior to the time the electron beam begins to scan into the region of screen 40 covered by cell segment 36.Switching cell segment 36 at T6 provides sufficient time for the liquid crystal molecules to stabilize in the "ON" state prior to the start at T7 of the second subinterval of the second color field time interval.

As shown in Fig. 5, the first subinterval of the second color field spans the time from T5 to T7 and the second subinterval of the second color field spans the time from T7 to T9. From T5 to T7, cell segment 34 is in the "ON" state, receives the light rays produced by the scanning electron beam, and causes green light to exit the display system output covered by cell segment 34.

From T7 to T9, cell segment 36 is in the "ON" state, receives the light rays produced by the scanning electron beam, and causes green light to exit the display system output covered by cell segment 36. Cell segments 34 and 36 are commanded to remain in the "ON" state from, respectively, T7 to T8 and T9 to T10 to delay the change to the "OFF" state of the first color field of the next succeeding image frame for the same reason described hereinabove for time T, to T2.

As shown in Fig. 5, line A, cell segment 34 is commanded to the "OFF" state at T8 after the electron beam has begun to scan into the region of screen 40 covered by cell segment 36.

Switching cell segment 34 at T8 provides sufficient time for the liquid crystal molecules to stabilize in the "OFF" state prior to the start at T9 of the first color field time interval of the next succeeding image frame. The above-described process is repeated for successive image frames.

The alternating image information fields provided in the first and second color field time intervals are transmitted through color selective polarizing filter 1 4 and are synchronously transmitted by variable optical retarder 12 and neutral polarizing filter 16. The retinal persistence of an observer's eye integrates the information presented at the output of polarizing filter 1 6 during the two intervals of time corresponding to alternate fields of the television raster signal to create the impression of a single, multicolored image. Intensity modulation of the light image source will produce a color gamut within a spectral range of between the red and green colors.

Optical Component Orientation With reference to Fig. 1, polarizing filter 1 4 has a color selective vertical polarization axis 52 which passes light of a color C1, such as red, and a color selective horizontal polarization axis 54 which passes light of a color C2, such as green. Polarizing filter 1 6 has a light absorbing vertical polarization axis 56 which passes no light and a light transmitting horizontal polarization axis 58 which passes light of all wavelengths.

Variable optical retarder 1 2 includes nematic liquid crystal cell segments 34 and 36, each comprising a zero to substantially half-wave optical retarder which selectively provides in the "ON" state essentially zero retardation for normally incident light of all colors and in the "OFF" state substantially half-wave retardation of normally incident green light. The projection 60 of the optic axis of cell segment 34 on its light communicating surface 62 and the projection 64 of the optic axis of cell segment 36 on its light communicating surface 66 are disposed substantially at 45" angles with respect to each one of the polarization axes of polarizing filters 1 4 and 16, which axes are depicted in phantom lines on surfaces 62 and 66 of variable optical retarder 12.

Each cell segment 34 and 36 of variable optical retarder 1 2 is independently switched between the two optical retardation states. The two optical retardation states provide display system 10 with two selectable optical transmission states, each of which developing a display system output of red or green light.

Whenever either one of the cell segments of variable optical retarder 1 2 is commanded to the "OFF" optical retardation state by a voltage signal applied to an output conductor of switching circuit 46, the direction of polarization of normally incident light rays of the color green passing through horizontal polarization axis 54 of polarizing filter 14 is rotated at a 90" angle.Light rays of the color green are projected onto and absorbed by vertical polarization axis 56 of polarizing filter 1 6. The direction of polarization of normally incident light rays of the color red passing through vertical polarization axis 52 of polarizing filter 1 4 is rotated at an angle which is slightly different from 90 by variable optical retarder 1 2 in the "OFF" state. Light rays of the color red, therefore, are separated into components which lie along the vertical and horizontal polarization axes of polarizing filter 16.Light rays of the color red include major components of light projected onto and transmitted by horizontal polarization axis 58 and minor components of light projected onto and absorbed by vertical polarization axis 56 of polarizing filter 1 6.

Red light of pure quality exits display system 10, therefore, in its first optical transmission state. The slight amount of red light absorbed by vertical polarization axis 56 of polarizing filter 1 6 results in a virtually imperceptible diminution in red light intensity in the first optical transmission state.

Whenever either one of the cell segments of variable optical retarder 1 2 is commanded to the "ON" optical retardation state by a voltage signal applied to an output conductor of switching circuit 46, the directions of polarization of light rays of the color red passing through vertical polarization axis 52 and of the color green passing through horiziontal polarization axis 54 of polarizing filter 14 are unchanged as the light rays pass through variable optical retarder 1 2 and strike polarizing filter 1 6. Normally incident light rays of the color red are absorbed by vertical polarization axis 56, and normally incident light rays of the color green exit display system 10 through horizontal polarization axis 58 of polarizing filter 1 6. Green light of pure quality exits display system 10, therefore, in its second optical transmission state.

Table I summarizes the color of light exiting display system 10 in each one of the two abovedescribed optical transmission states.

TABLE I Optical Variable Optical Color of Transmission State Retarder 1 2 Light Output 1 OFF Red 2 ON Green Construction of Liquid Crystal Device The preferred embodiment of the display system incorporates a liquid crystal device which is constructed as described hereinbelow.

With reference to Figs. 2 and 3, a liquid crystal device 100 comprises spaced-apart and generally parallel first and second electrode structures 102 and 104 and spaced-apart and generally parallel third and fourth electrode structures 106 and 1 08. A continuous region of nematic liquid crystal material 110 is captured within the space between electrode structures 102 and 106 and electrode structures 104 and 1 08. Electrode structures 102 and 106 have a common glass dielectric substrate 11 2 which is substantially flat and has on its inner surface two contiguous layers or regions 11 4 and 11 6 of electrically conducting, optically transparent material, such as indium tin oxide.Conductive region 11 4 forms a portion of the first electrode structure 102, and conductive region 116 forms a portion of the third electrode structure 106.

Conductive regions 11 4 and 11 6 are separated by a straight, nonconductive gap or split line 11 8. Split line 11 8 is about 0.025 mm in width and is preferably formed by using a laser beam to divide a single layer of conductive material on substrate 11 2 into regions 114 and 11 6 of substantially equal size.

A director alignment film layer 1 20 is applied to conductive regions 114 and 11 6 and forms a boundary between liquid crystal material 110 and electrode structures 102 and 106. The material constituting and the corresponding method of conditioning the director alignment film 1 20 are described in detail- hereinbelow. Electrode structures 104 and 108 are of a construction similar to that of electrode structures 102 and 106, and the components corresponding to those of electrode structures 102 and 106 are shown with identical reference numerals followed by primes.

Spacers 1 22 may be comprised of any suitable material, such as polyester film or fused glass frit, and serve to maintain the desired separation between glass substrates 11 2 and 112'. Glass substrates 11 2 and 112' are preferably aligned so that split lines 11 8 and 118' are oriented in a place substantially perpendicular to the surfaces of the substrates. Electrode structures 102 and 104 define a first cell segment 124, and electrode structures 106 and 108 define a second cell segment 1 26. Cell segments 1 24 and 1 26 are included within the broken line outlines shown in Figs. 2 and 3.

When liquid crystal device 100 is incorporated in a system such as display system 10, the separate voltage signals independently developed at the outputs of switching circuit 46 (Fig. 1) are applied to cell segments 1 24 and 1 26 at terminals 1 28 and 130, respectively. A common reference potential for cell segments 1 24 and 1 26 is applied to terminals 1 32 and 134, respectively. Terminals 1 32 and 1 34 are preferably connected to the ground potential of switching circuit 46 (Fig. 1).

During display system operation, noise voltages appear on the grounded electrode structure of a cell segment when it is switched between optical retardation states. Nonconductive split line 11 8 isolates conductive layers 114 and 11 6 and thereby minimizes the effect of the noise voltages of one grounded electrode on the character of the electric field within the adjacent cell segment. Such isolation of the electric fields within the cell segments minimizes the optical cross talk between cell segments. The spurious transmission of colored light which is caused by optical cross talk is thereby eliminated from the display system.

It will be appreciated by those skilled in the art that a liquid crystal device can be divided into more than two cell segments, each having separate reference electrode structures to accomplish the above performance objective. Additionally, when designed with separate reference electrode structures, other liquid crystal devices, such as, for example, a twisted nematic device, can provide reduced optical cross talk in color display systems incorporating them.

Liquid Crystal Variable Optical Retarder The preferred embodiment of display system 10 of the present invention incorporates variable optical retarder 1 2 that comprises a liquid crystal device which is divided into two cell segments, each operating as a zero to substantially half-wave optical retarder. The molecular alignment configurations of the liquid crystal material captured within cell segment 1 24 of liquid crystal device 100 are described herein by way of example to explain the operation of the cell segment as a variable optical retarder. For purposes of clarity, reference is sometimes made to the operation of switching circuit 46 of Fig. 1 to describe the switching between optical retardation states.

With reference to Figs. 4A and 4B, the nematic director alignment configuration of layers 1 20 and 120' in liquid crystal cell segment 124 is described in Column 7, lines 48-55, of U.S.

Patent No. 4,333,708 of Boyd et al. It will be understood, however, that the liquid crystal cell described in the Boyd et al. patent differs from that of the present invention in that the former includes an alternating-tilt geometry type configuration of which the director alignment of liquid crystal device 100 comprises only a portion. The cell of the Boyd et al. patent is constructed to promote disclination movement within the cell in an attempt to provide a bistable switching device.

The film layer 1 20 of electrode structure 102 is conditioned so that the electrode structure surface contacting directors 140 are aligned parallel to each other at a tilt bias angle + H, which is measured in the counterclockwise sense with reference to the surface of film layer 1 20. The film layer 1 20' of electrode structure 104 is conditioned so that the electrode structure surface contacting directors 1 42 are aligned parallel to each other at a tilt bias angle #, which is measured in the clockwise sense with reference to the surface of film layer 120'.Thus, cell segment 1 24 is fabricated so that the surface contacting directors 1 40 and 1 42 of the opposed surfaces of director alignment layers 1 20 and 120' of electrode structures 102 and 104, respectively, are tilt-biased in opposite directions.

A first preferred method of effecting the desired alignment of the surface contacting directors entails the use of polyimide as the material which comprises the alignment film layers 1 20 and 120' on electrode structures 102 and 104, respectively. Each alignment film layer is rubbed to produce a tilt bias angle 101, with 2" to 5" being the preferred range. A second preferred method of effecting the desired alignment of the surface contacting directors entails the use of silicon monoxide as the material which comprises the alignment film layers 1 20 and 120' of electrode structures 102 and 104, respectively.The silicon monoxide layer is evaporated and vapor deposited preferably at a 5" angle measured from the electrode structure surface in an amount sufficient to produce a tilt bias angle 101 of between 10" to 30", with 1 5" to 25" being the preferred range.

It will be appreciated that methods-for depositing silicon monoxide or other alignment materials to align liquid crystal molecules in a predetermined direction have been disclosed previously by others and are known to those having ordinary skill in the art. One such method, for example, is disclosed in U.S. Patent No. 4,165,923 of Janning.

Fig. 4A depicts the orientation of surface noncontacting directors 1 44 when an AC signal V, of approximately 2 kHz and 20 Vrms is applied to conductive layers 114 and 11 4' of electrode structures 102 and 104, respectively. The signal V, on conductive layer 11 4' with conductive layer 114 grounded constitutes a first switching state produced at the output 48 of control circuit 46 and produces an alternating electric field, E, between electrode structures 102 and 104 within the cell segment 1 24 to force the cell segment into its "ON" optical retardation state.A substantial number of the surface noncontacting directors 144 of a liquid crystal material 110 which has a positive anisotropy value align essentially end-to-end along direction 146 of the electric field flux lines within the cell segment, which direction is normal to the conditioned surfaces of the electrode structures. Thus, when cell segment 1 24 is excited into its "ON" optical retardation state, the surface noncontacting directors 1 44 are aligned perpendicularly to the surfaces of the cell segment.

Fig. 4B depicts the orientation of surface noncontacting directors 144 after the signal V, is removed and electrode structure 104 is held at ground potential so that the alignment of surface noncontacting directors is influenced not by an electric field produced between electrode structures 102 and 104 within the cell segment, but by the intermolecular elastic forces which cause relaxation of the surface noncontacting directors from the end-to-end alignment of the "ON" optical retardation state. The grounding of electrode structure 104 constitutes a second switching state produced at output 48 of control circuit 46. The director orientation shown in Fig. 4B corresponds to that of the "OFF" optical retardation state of the cell segment.

Switching cell segment 1 24 to the "OFF" optical retardation state can also be accomplished by removing signal V1, momentarily grounding electrode structure 104 to allow the liquid crystal material within the cell segment to quickly relax to the "OFF" state, and applying to layer 114' of the cell segment an AC signal V2 produced at output 48 of control circuit 46 having a voltage level which is less than that of signal V, and generally about 1.0 volt. This value of signal V2 maintains the orientation of the liquid crystal molecules of the cell segment to keep it in the ''OFF" state. The frequency of signal V2 is generally the same as that of signal V,.

During the transition from the "ON" optical retardation state to the "OFF" optical retardation state of the cell segment, the surface noncontacting directors recede from the end-to-end alignment normal to the electrode structure surfaces and attempt to assume a generally parallel relation with the adjacent directors. Thus, surface noncontacting directors 1 44a and 1 44b rotate in a clockwise sense as shown by direction arrows 148a in order to achieve a near-parallel relation as respects directors 1 40 and 144a, respectively; and surface noncontacting directors 144c and 144d rotate in a counterclockwise sense as shown by direction arrows 148b to achieve a near-parallel relation as respects directors 142 and 144c, respectively.Thus, when cell segment 1 24 relaxes to its "OFF" optical retardation state, each one of a substantial number of the surface noncontacting directors is aligned so that it projects a director component onto the surfaces of the cell segment. The surface noncontacting directors, however, lie approximately in a plane which is perpendicular to the surfaces of the cell segment.

The method of operating the liquid crystal cell segment 1 24 as a zero to substantially halfwave optical retarder is directed to the disclination-free surface noncontacting director relaxation from the electric field-aligned or "ON" optical retardation state depicted by Fig. 4A to the planar configuration or "OFF" optical retardation state depicted by Fig. 4B.

In the present invention, liquid crystal cell segment 1 24 is operated as a zero to substantially half-wave optical retarder whose optic axis corresponds to the alignment direction of the nonsurface contacting directors 1 44.

Linearly polarized light which propagates in direction 1 50 normal to the surfaces of electrode structures 102 and 104 is coincident with the direction of surface noncontacting directors 1 44 when the liquid crystal cell segment is in the "ON" optical retardation state. Directors 144 are oriented in such "ON" optical retardation state so that there is a negligible projection of the optic axis on the electrode structure surfaces of the cell segment. Under these conditions, liquid crystal cell segment 1 24 produces substantially reduced optical retardation for incident light propagating in the direction 1 50.

Linearly polarized light which propagates in direction 1 50 normal to the surfaces of electrode structures 102 and 104 is noncoincident with the alignment direction of surface noncontacting directors when the liquid crystal cell segment is in the "OFF" optical retardation state. Directors 1 44 are oriented in such "OFF" optical retardation state so that each one of a substantial number of them projects a component on the electrode structure surfaces of the cell segment.

Under these conditions, liquid crystal cell segment 1 24 has an effective birefringence for generally normally incident light. The orientation of surface noncontacting directors 144 provides substantially half-wave optical retardation for light of the wavelength which satisfies the mathematical expression: And 1 2 2 where d represents the thickness 1 52 and An represents the effective birefringence of the cell segment.

It will be obvious to those having skill in the art that many changes may be made in the above-described details of the preferred embodiment of the present invention. The scope of the present invention, therefore, should be determined only by the following claims.

Claims (16)

1. An optical switching system including a liquid crystal device which, in use, is positioned to receive from a light source light rays in a defined polarization state, wherein the device comprises: first and second contiguous cell segments, the first cell segment including first and second spaced-apart electrode structures having a liquid crystal material captured therebetween and the second cell segment including third and fourth spaced-apart electrode structures having a liquid crystal material captured therebetween, the first and third electrode structures being electrically connected to a common reference potential; and wherein the switching system comprises: switching means in communication with the second electrode structure for introducing a first electric field within the first cell segment and with the fourth electrode structure for introducing a second electric field within the second cell segment, the switching means being operable to separately change the character of the polarization state of the light rays passing through each one of the first and second cell segments; the first and second electric fields being isolated to minimize any change in character of the polarization state of the light rays passing through one of the cell segments due to a change in the character of the polarization state of the light rays passing through the other cell segment.

2. The switching system of claim 1 in which each one of the first and second cell segments comprises a variable optical retarder whose retardation changes in response to a change in the intensity of the electric field introduced within the cell segment by the switching means.

3. The switching system of claim 2 in which the liquid crystal material of each cell segment has directors and each electrode structure of the cell segment includes a layer of optically transparent and electrically conductive material and has the inner surface thereof conditioned so that the directors of the liquid crystal material in contact therewith become substantially uniformly aligned to form tilt bias angles with the conditioned surfaces, the tilt bias angles of the directors in contact with one conditioned surface being defined in a rotational sense opposite to the tilt bias angles of the directors in contact with the other conditioned surface.

4. The switching system of claim 2 in which the variable optical retarder provides zero retardation of light rays passing therethrough in response to a first intensity of the electric field introduced by the switching means and provides half-wave retardation of light rays of a predetermined wavelength passing therethrough in response to a second intensity of the electric field introduced by the switching means.

5. The switching system of claim 1 jn which the switching means delivers sequentially a first voltage pulse to the first cell segment to develop the first electric field and a second voltage pulse to the second cell segment to develop the second electric field, the trailing edge of the first voltage pulse occurring after the completion of the transition of the leading edge of the second voltage pulse, thereby to change in a continuous sequence the retardation of light passing through the cell segments.

6. The switching system of claim 1 in which the first and third electrode structures comprises a pair of coplanar conductive regions separated by a first nonconductive split line on a surface of a first dielectric substrate, and the second and fourth electrode structures comprise a pair of coplanar conductive regions separated by a second nonconductive split line on a surface of a second dielectric substrate.

7. The switching system of claim 6 in which the first and second dielectric substrates are substantially flat and aligned so that the first and second split lines between the conductive regions of the dielectric substrates are of substantially equal width and are oriented in a plane generally perpendicular to the surfaces of the dielectric substrates.

8. The switching system of claim 6 in which a continuous region of liquid crystal material is captured between the first and second dielectric substrates.

9. A high-speed color switch which, in use, is positioned to receive light of a plurality of wavelengths emitted from a light source, comprising: first and second light polarizing means in optical communication with the light source, the first light polarizing means including a color selective polarizing filter;; variable optical retarding means disposed between the first and second light polarizing means, the variable optical retarding means including a liquid crystal cell comprising first and second contiguous cell segments, the first cell segment including first and second spaced-apart electrode structures having a liquid crystal material captured therebetween and the second cell segment including third and fourth spaced-apart electrode structures having a liquid crystal material captured therebetween, the first and third electrode structures being electrically connected to a common reference potential; and switching means in communication with the second and fourth electrode structures for applying a separate potential difference to each cell segment, thereby to selectably transmit a light output of one of two colors through the first cell segment during a first time interval and a light output of one of two colors through the second cell segment during a second time interval.

10. The color switch of claim 9 which also includes means in communication with the light source for generating image color information and means for synchronizing to the switching means the image color information carried by the light rays emitted from the light source, thereby to develop during the first and second time intervals a composite image having a multicolored appearance.

11. The color switch of claim 9 in which the color selective polarizing filter of the first light polarizing means comprises first and second linear polarizers having orthogonally aligned polarization axes, the polarization axis of the first linear polarizer passing light of a first color and the polarization axis of the second linear polarizer passing light of a second color.

12. The color switch of claim 11 in which the second light polarizing means comprises a neutral linear polarizing filter and the switching means provides first and second switching states to each one of the cell segments of the variable optical retarding means to produce first and second amounts of optical retardation for transmitting from each cell segment light rays which exit the color switch in the first and second colors.

1 3. A color display system comprising a color switch or switching system as claimed in any preceding claim, and a light source emitting light having a plurality of component colors.

1 4. A color display system as claimed in Claim 13, in which the light source iS a raster scanned cathode ray tube.

1 5. A color switch or switching system substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

16. A color display system substantially as hereinbefore described with reference to the accompanying drawings and as shown in Fig. 1.