Reflective liquid crystal light valve with hybrid field effect mode

• US 4,019,807 A
• Filed: 03/08/1976
• Issued: 04/26/1977
• Est. Priority Date: 03/08/1976
• Status: Expired due to Term

• Current Assignee: JVC Americas Corporation (JVC Kenwood Corporation)
• Original Assignee: Hughes Aircraft Company (Rtx Corporation)
• Inventors: Grinberg, Jan, Jacobson, Alexander D., Boswell, Donald D., Myer, Gary D.
• Primary Examiner(s): Bauer, Edward S.
• Application Number:
  US05/664,776
• Time in Patent Office: 414 Days
• Field of Search: 350/150, 350/160 LC, 250/213 R
• US Class Current: 349/25
• CPC Class Codes: G02F 1/135 Liquid crystal cells structurally associated with a photoconducting or a ferro-electric layer, the properties of which can be optically or electrically varied G02F1/133348 takes precedence
• CPC Class Codes: G02F 1/139 based on orientation effects in which the liquid crystal remains transparent

Abstract

There is disclosed a high performance reflective mode liquid crystal light valve suitable for general image processing and projection and particularly suited for application to real-time coherent optical data processing. A preferred example of the device uses a CdS photoconductor, a CdTe light absorbing layer, a dielectric mirror, and a liquid crystal layer sandwiched between indium-tin-oxide transparent electrodes deposited on optical quality glass flats. The non-coherent light image is directed onto the photoconductor; this reduces the impedance of the photoconductor, thereby switching the AC voltage that is impressed across the electrodes onto the liquid crystal to activate the device. The liquid crystal is operated in a hybrid field effect mode. It utilizes the twisted nematic effect to create a dark off-state (voltage off the liquid crystal) and the optical birefringence effect to create the bright on-state. The liquid crystal thus modulates the polarization of the coherent read-out or projection light responsively to the non-coherent image. An analyzer is used to create an intensity modulated output beam.

Claims

1. In a reflective mode alternating current driven liquid crystal light valve of the type having first and second parallel opposed transparent electrode means for application of an alternating electric field across first and second parallel opposed transparent insulating layers which are positioned between and parallel to said first and second electrode means to define in cooperation with an annular spacer member separating said first and second insulating layers an enclosed space containing nematic liquid crystal material, said light valve further having photoconductor means and mirror means positioned between said second transparent electrode and said second transparent insulating layer, said photoconductor means being positioned to receive input writing signal light through said second transparent electrode to spatially and temporally vary the magnitude of said electrode field applied across said liquid crystal material as a function of the intensity of said writing light, the hybrid filed effect mode improvement comprising:

a. means operatively associated with the first of said transparent insulating layers in the absence of an electric field for aligning the axes of the molecules of a first portion of said liquid crystal material adjacent to said first layer in a first direction in a plane which is parallel to said first layer;

b. means operatively associated with the second of said transparent insulating layers in the absence of an electric field for aligning the axes of the molecules of a second portion of said liquid crystal material which is adjacent to said second layer in a second direction in a plane which is parallel to said second layer, the respective orthogonal projections of said first and second directions in any plane which is between and parallel to both said first and second planes intersecting at an acute angle for producing a twisted alignment of the axes of said nematic liquid crystal molecules in the absence of said electric field and a tilted orientation of said axes in the presence of said electric field;

c. means for providing polarized projection light through said first transparent electrode and said first transparent insulating layer to traverse said liquid crystal material contained between said first and second insulating layers and be reflected by said mirror means to re-traverse said liquid crystal material and exit through said first transparent electrode of said light valve, said polarized projection light being subject to changes in polarization state as it traverses said layer of liquid crystal material in both directions, said polarization sate being rotated by said twisted nematic alignment of said molecules of said liquid crystal material in the off state of said field and by the birefringence of said tilted oreintation of said molecules of said liquid crystal material in the on-state of said field; and

,

d. means positioned to intercept said light exiting from said first transparent electrode means for blocking all of said projection light exiting from said light valve except that which is in a predetermined polarization state.

2. A device as in claim 1 wherein said acute angle between said first and second alignment directions is substantially 45°

.

3. A device as in claim 1 wherein said projection light passes through said first transparent insulating layer at an acute incidence angle to the normal thereto, said normal being the optic axis of said light valve.

4. A device as in claim 3 wherein said acute angle between said first and second alignment directions is substantially 45°

, and wherein said acute incidence angle is less than 35°

.

5. A device as in claim 1 wherein said means for blocking said projection light exiting from said light valve comprises a linear polarizer, the polarization axis of said polarizer making an angle with the axis of polarization of said polarized projection light entering said light valve which is equal to ninety degrees with respect to the polarizing direction of said polarized projection light.

6. A device as in claim 1 wherein said liquid crystal material has a thickness substantially equal to two micrometers.

7. A device as in claim 1 wherein the thickness of said liquid crystal material has a variation of less than 0.25 micrometers across its surface.

8. A device as in claim 1 wherein both said means for aligning the axes of the molecules of said liquid crystal material are formed at least in part by grazing angle deposition of said insulating layers respectively in the direction in whih molecules adjacent to said layer are to be aligned.

9. A device as in claim 1 wherein said liquid crystal material is of positive dielectric anisotrophy.

10. In a high performance reflective mode liquid crystal light valve of the type having means for confining a layer of positive dielectric anisotropy nematic liquid crystal material between opposed plane parallel surfaces, means for establishing a spatially and temporally modulated electric field across said liquid crystal layer orthogonally to said surfaces in response to input signals, means for providing polarized projection light to one side of said layer of liquid crystal material and means for reflecting said polarized projection light from the other side thereof back through said liquid crystal material, the improvement comprising:

a. means operative in the absence of said electric field for aligning the axes of the molecules of a first portion of said liquid crystal material adjacent to one of said surfaces in a first direction in a first plane which is parallel to said one surface;

b. means operative in the absence of an electric field for aligning the axes of the molecules of a second portion of said liquid crystal material which is adjacent to the other of said surfaces in a second direction in a second plane which is parallel to said other surface, the respective orthogonal projections of said first and second directions in any plane which is between and parallel to both said first and second planes intersecting at an acute angle for producing a twisted alignment of the axes of said nematic liquid crystal molecules about an axis of rotation which is orthogonal to said parallel surfaces in the absence of said electric field and a tilted orientation of said axes with respect to said opposed plane parallel surfaces in the presence of said electric field;

c. said twisted and tilted orientations of said axes of the molecules in said layer of liquid crystal material subjecting said polarized projection light as it traverses through said layer to said reflection means and thence back through said layer to changes in polarization state in both directions, said polarization state being rotated by said twisted nematic alignment of said molecules of said liquid crystal material in the OFF state of said field and by the birefringence of said tilted orientation of said molecules in said liquid crystal material in the ON state of said field; and

d. means positioned to intercept said light exiting from said first transparent electrode means for blocking all of said projection light exiting from said light valve except that which is in a predetermined polarization state.