WO2000016135A1 - Electrically switchable holographic display - Google Patents

Electrically switchable holographic display Download PDF

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Publication number
WO2000016135A1
WO2000016135A1 PCT/US1999/021005 US9921005W WO0016135A1 WO 2000016135 A1 WO2000016135 A1 WO 2000016135A1 US 9921005 W US9921005 W US 9921005W WO 0016135 A1 WO0016135 A1 WO 0016135A1
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WO
WIPO (PCT)
Prior art keywords
emulsion
electrodes
hologram
green
red
Prior art date
Application number
PCT/US1999/021005
Other languages
French (fr)
Other versions
WO2000016135A9 (en
Inventor
John J. Storey
Milan M. Popovich
Jonathan D. Waldern
Michael D. Walshe
Original Assignee
Digilens, Inc.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Digilens, Inc. filed Critical Digilens, Inc.
Priority to AU61430/99A priority Critical patent/AU6143099A/en
Publication of WO2000016135A1 publication Critical patent/WO2000016135A1/en
Publication of WO2000016135A9 publication Critical patent/WO2000016135A9/en

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Classifications

    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
    • G03H1/02—Details of features involved during the holographic process; Replication of holograms without interference recording
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00—Optical elements other than lenses
    • G02B5/32—Holograms used as optical elements
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
    • G02F1/1333—Constructional arrangements; Manufacturing methods
    • G02F1/1334—Constructional arrangements; Manufacturing methods based on polymer dispersed liquid crystals, e.g. microencapsulated liquid crystals
    • G02F1/13342—Holographic polymer dispersed liquid crystals
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
    • G02F1/1333—Constructional arrangements; Manufacturing methods
    • G02F1/1335—Structural association of cells with optical devices, e.g. polarisers or reflectors
    • G02F1/1336—Illuminating devices
    • G02F1/133621—Illuminating devices providing coloured light
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
    • G02F1/1333—Constructional arrangements; Manufacturing methods
    • G02F1/1347—Arrangement of liquid crystal layers or cells in which the final condition of one light beam is achieved by the addition of the effects of two or more layers or cells
    • G02F1/13476—Arrangement of liquid crystal layers or cells in which the final condition of one light beam is achieved by the addition of the effects of two or more layers or cells in which at least one liquid crystal cell or layer assumes a scattering state
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
    • G02F1/1333—Constructional arrangements; Manufacturing methods
    • G02F1/1347—Arrangement of liquid crystal layers or cells in which the final condition of one light beam is achieved by the addition of the effects of two or more layers or cells
    • G02F1/13475—Arrangement of liquid crystal layers or cells in which the final condition of one light beam is achieved by the addition of the effects of two or more layers or cells in which at least one liquid crystal cell or layer is doped with a pleochroic dye, e.g. GH-LC cell
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
    • G03H1/02—Details of features involved during the holographic process; Replication of holograms without interference recording
    • G03H2001/026—Recording materials or recording processes
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
    • G03H1/02—Details of features involved during the holographic process; Replication of holograms without interference recording
    • G03H2001/026—Recording materials or recording processes
    • G03H2001/0264—Organic recording material
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H2250/00—Laminate comprising a hologram layer
    • G03H2250/38—Liquid crystal
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2310/00—Command of the display device
    • G09G2310/02—Addressing, scanning or driving the display screen or processing steps related thereto
    • G09G2310/0235—Field-sequential colour display
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
    • G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
    • G09G3/34—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
    • G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
    • G09G3/34—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source
    • G09G3/3406—Control of illumination source

Definitions

  • TITLE ELECTRICALLY SWITCHABLE HOLOGRAPHIC DISPLAY
  • the present invention relates generally to electronic displays and more particularly to electronically switchable holographic displays.
  • the most common type of graphics display for low-cost consumer items are liquid crystal devices that rely on the bulk properties of the liquid crystals to achieve a desired optical effect.
  • the problems with liquid crystals include low brightness, which makes them difficult to view in daylight, and relatively slow refresh rates.
  • Active matrix liquid crystal displays have improved brightness and refresh rates but are limited in resolution.
  • One new technology that appears to overcome the problems associated with liquid crystal displays utilizes an electrically switchable hologram that selectively directs light towards or away from a viewer in order to create a display. While these displays are capable of producing brighter displays with better resolution, they have not yet become commercially useful because of the problem of simultaneously reducing the switching voltage and the switching time, while providing high diffraction efficiency. In addition, the generation of color images with this technology has proven difficult.
  • the present invention is an improvement in electronic displays that include an electrically switchable hologram recorded in an emulsion.
  • the emulsion in one embodiment comprises a monomer dipentaerythritol hydroxypentaacrylate, a liquid crystal, a cross-linking monomer, a coinitiator and a photoinitiator dye.
  • the polymer-dispersed liquid crystal (PDLC) material may optionally further comprise a surfactant.
  • the PDLC material may be approximately 10-40 wt% of the liquid crystal.
  • the PDLC material may be approximately 5-15 wt% of the cross-linking monomer.
  • the amount of the co-initiator may be 10 "3 to 10 "4 gram moles and the amount of the photoinitiator dye may be 10 '5 to 10 "6 gram moles.
  • the surfactant when present, may be up to approximately 6 wt% of the PDLC material.
  • a number of electrodes adjacent the emulsion cause light to be selectively diffracted between modes of the hologram in proportion to an applied voltage.
  • the electrodes are disposed over the surface of the emulsion in a pattern of pixels. A voltage is applied to each of the electrodes so as to selectively direct the light that illuminates the emulsion at that pixel towards or away from a viewer.
  • a color display may be created by aligning three individual holograms that are optimized to diffract red, green, or blue light such that when simultaneously illuminated from a white light source, unwanted interference is minimized.
  • Each hologram has a set of electrodes arranged to form a pattern of pixels. A voltage is applied to the electrodes for each of the holograms sequentially while no voltage is applied to the electrodes of the other two holograms. In this manner, a monochrome red, green and blue image may be sequentially directed towards a viewer and integrated in the viewer's eye to create a color image.
  • a color display is created with a single emulsion in which three superimposed holograms are recorded.
  • Each hologram is optimized to diffract red, green or blue light and to minimize interference from the other two colors.
  • a set of electrodes is positioned adjacent the emulsion to form a pattern of pixels. If a single electrode is used for each pixel, the emulsion is sequentially illuminated with red, green and blue light.
  • a voltage is applied to an electrode for a particular pixel in the display so as to selectively diffract a portion of the red, green or blue light towards a viewer and create a monochromatic image.
  • a color image is created by integrating the sequential monochrome images in the eye of the viewer.
  • each electrode may be divided into a number of sub-electrodes.
  • the emulsion is then continuously illuminated with white light and a voltage is applied to each sub-electrode to diffract a desired quantity of red, green and blue light toward a viewer.
  • a color display is created with a single emulsion having a set of tiled red, green and blue holograms.
  • a set of electrodes adjacent the emulsion form a pattern of pixels. If a single electrode is used for each pixel, the emulsion is sequentially illuminated with red, green and blue light while a voltage is applied to the electrodes to diffract a selected amount of red, green or blue light toward a viewer to create a monochrome image.
  • a color image is created by integrating the sequence of monochrome images in the eye of the viewer.
  • the electrodes may be divided into sub-electrodes, each of which is positioned over a tiled red, green or blue hologram.
  • the emulsion may then be continuously illuminated with a white light and a voltage is applied to each of the sub-electrodes to diffract a portion of red, green and blue light towards a viewer for each pixel in order to create a color image.
  • FIGURE 1 illustrates a display including a transmissive, electrically switchable hologram in accordance with the present invention
  • FIGURE 2 illustrates a display including a reflective, electrically switchable hologram according to the present invention
  • FIGURE 3 illustrates a color display including separate emulsions, each of which has a hologram that is optimized to diffract red, green or blue light according to the present invention
  • FIGURE 4 illustrates a constantly illuminated color display having a single emulsion which includes superimposed holograms that are optimized to diffract red, green and blue light according to another aspect of the present invention
  • FIGURE 5 illustrates a sequentially illuminated color display having a single emulsion which includes superimposed holograms that are optimized to diffract red, green and blue light according to another aspect of the present invention
  • FIGURE 6 illustrates the geometrical parameters that define superimposed holograms recorded in a single emulsion to mmimize mutual interference
  • FIGURE 7 illustrates a constantly illuminated color display having a single emulsion with tiled holograms that are optimized to diffract red, green or blue light according to another aspect of the present invention
  • FIGURE 8 illustrates a sequentially illuminated color display having a single emulsion which includes tiled holograms that are optimized to diffract red, green and blue light according to another aspect of the present invention.
  • FIGURE 9 illustrates how a set of tiled holograms are recorded in a single emulsion.
  • the present invention relates to a electrically switchable holographic display having a reduced switching voltage and switching time as well as a high diffraction efficiency.
  • the present invention produces color images using electrically switchable holograms.
  • FIGURE 1 illustrates a first embodiment of a display made in accordance with the present invention.
  • the display 20 includes a hologram that is formed within holographic recording medium (hereinafter referred to as an
  • emulsion material or "emulsion” 22).
  • the emulsion comprises a polymer material having microdroplets of liquid crystal dispersed therein.
  • Recorded in the emulsion is a hologram that can selectively direct an incoming light beam either towards or away from a viewer.
  • a light source 24 such as a laser or substantially monochromatic light-emitting diode (LED) directs an incoming light beam toward the hologram.
  • the hologram recorded in the emulsion has the ability to selectively diffract light at certain wavelengths into different modes in response to a voltage applied to either side of the emulsion.
  • Light passing through the hologram in the same direction as the light is received from the light source 24 is referred to as the zero (0th) order mode 26.
  • the liquid crystals droplets are oriented such that the hologram is present in the emulsion and light is diffracted from the 0th order mode 26 to a first order mode 28 of the hologram.
  • the liquid crystal droplets become realigned effectively erasing the hologram and the incoming light passes through the emulsion in the 0th order mode.
  • the percentage of light diffracted into the various modes of the hologram varies in proportion to the strength of the electric field or voltage applied to the emulsion.
  • the different modes of the hologram are recorded in the hologram such that light in one mode is seen by a viewer of the display while light in another mode is not. Therefore, by changing the amount of light that is diffracted into each mode, the display appears brighter as more light is directed toward the viewer and darker as less light is directed towards the viewer.
  • a pattern of electrodes 30 are positioned on one side of the emulsion 22.
  • the electrodes are substantially transparent so as not to substantially interfere with the light passing through the hologram.
  • the electrodes 30 are preferably made from a vapor deposition of indium tin oxide (ITO) or other substantially transparent metal that is deposited on glass plate that is positioned in front of the emulsion or directly onto the emulsion itself.
  • ITO indium tin oxide
  • On the rear surface of the emulsion is a common electrode that covers the entire area of the hologram used to create the display.
  • the light passing through the hologram at that pixel location is selectively diffracted between the 0th order mode 26 and the first order mode 28 (and higher order modes to a lesser extent).
  • the light at the particular pixel location will appear brighter or dimmer in order to change the appearance of the display.
  • the voltage on the electrodes for each of the pixels in the display can be individually controlled such that a two- dimensional image is created by controlling the amount of light or brightness at each pixel location.
  • Controlling the voltage that is supplied to the electrode for each pixel is an electronic control circuit (not shown).
  • the electronic circuit may include a digital-to-analog converter that allows a processor to write a digital value to each electrode location and to have that digital value converted to a corresponding analog voltage that selectively controls the amount of light transferred between the modes of the hologram at that pixel location.
  • the electronic circuit may be designed to simultaneously address all the electrodes or may write to the electrodes in a raster fashion.
  • the present invention utilizes an emulsion that comprises the monomer dipentaerythritol hydroxypentaacrylate, a liquid crystal, a cross-linking monomer, a coinitiator and a photoinitiator dye.
  • the polymer-dispersed liquid crystal material may optionally further comprise a surfactant.
  • the PDLC material may be approximately 10-40 wt% of the liquid crystal.
  • the PDLC material may be approximately 5-15 wt% of the cross-linking monomer.
  • the amount of the co-initiator may be 10 "3 to 10 "4 gram moles and the amount of the photoinitiator dye may be 10 "5 to 10 "6 gram moles.
  • the surfactant when present, may be up to approximately 6 wt% of the PDLC material.
  • the composition of the emulsion is fully described in published PCT application serial No. PCT/US97/12577. This emulsion material has been found to produce acceptable switching efficiencies at a few volts per micron of emulsion thickness, potentially as low as 1 volt per micron thickness.
  • FIGURE 2 illustrates another embodiment of the display according to the present invention that utilizes an emulsion 32 containing a reflective hologram.
  • a light source 34 produces light which is directed through a light guide (not shown) and into the hologram at a relatively steep angle ⁇ .
  • a reflective backplane 38 contains the electronic control circuit that supplies the voltage on each of the electrodes adjacent the emulsion.
  • Non-diffracted light i.e., in the 0th order mode of the hologram, will exit the hologram at the angle ⁇ substantially equal to the angle at which the light entered the hologram.
  • the input light is selectively divided between the 0th order mode 40 and the first order mode 42. If the first order mode is aligned with the viewer, then the pixels in the display will appear to grow brighter as more and more light is diverted from the 0th order mode into the first order mode.
  • FIGURES 1 and 2 allow a grey scale, monochrome image to be created by selectively dividing a portion of the light between 0th and first order modes of the hologram at each pixel location.
  • a selective amount of red, green and blue light must be directed towards a viewer for each pixel location.
  • FIGURE 3 illustrates a first embodiment of the present invention that creates a color display using three emulsions 52, 54, and 56.
  • Emulsion 52 has a hologram recorded in it that is optimized to diffract red light.
  • the emulsion 54 has a hologram recorded in it that is optimized to diffract green light and the emulsion 56 has a hologram recorded in it that is optimized to diffract blue light.
  • Each of the emulsions 52, 54, and 56 have a set of electrodes associated with each of the pixels in the display.
  • a control circuit 58 drives associated switching circuitry 60 associated with the electrodes on each of the emulsions to apply a variable voltage to the electrodes. The particular voltage level applied to each electrode is determined by a display processor 62.
  • a white light source 64 produces a continuous beam of white light that is directed toward the emulsions 52, 54, and 56.
  • only one set of electrodes associated with the emulsions 52, 54, and 56 is activated at any one time. With the electrodes enabled, a selected amount of light is diffracted into the 1st order mode of the hologram and towards a user, while light in the 0th order mode is directed such that it cannot be seen by the user.
  • the electrodes on each of the three holograms are sequentially enabled such that a selected amount of red, green and blue light is directed towards a user for each pixel location on the display.
  • the rate at which the holograms are sequentially enabled is faster than the response time of a human eye.
  • a color image will be created in the viewer's eye due to the integration of the red, green and blue monochrome images created from each of the emulsions 52, 54, and 56.
  • the holograms recorded in each of the emulsions 52, 54, and 56 are oriented such that the 1st order mode is in the direction of the viewer while the 0th order mode diffracts light in a direction that cannot be seen by the viewer.
  • the display illustrated in FIGURE 3 will produce color images, three emulsions are required as well as three separate sets of electrodes and driving circuitry.
  • the display shown in FIGURE 4 utilizes a single emulsion 70 that has recorded in it three superimposed holograms. Each hologram is optimized to diffract red, green and blue light respectively.
  • the electrodes 72 associated with each pixel are divided into sub-electrodes 72A, 72B, and 72C. Each of the sub-electrodes is selectively addressable by a control circuit 80 and driving circuitry 82 such that a variable voltage can be applied to each of the sub-electrodes.
  • Controlling the voltage applied to each of the sub-electrodes is a display processor 84.
  • a white light source 86 provides a source of red, green and blue light to illuminate the emulsion 70.
  • a variable amount of red, green and blue light can be directed towards a viewer for each pixel.
  • a voltage is applied to the sub-electrode 72A to diffract a given amount of red light towards a viewer while a different voltage may be applied to the sub-electrodes 72B and 72C in order to diffract a different amount of green and blue light towards the viewer for a particular pixel location.
  • the holograms recorded in the emulsion 70 are arranged such that interference for the red, green and blue light is minimized due to the holograms for the other primary colors.
  • FIGURE 5 illustrates another embodiment of the invention wherein a color image is created with a single emulsion 70 having separate red, green and blue holograms superimposed therein.
  • the electrodes 74 associated with a particular pixel are not divided into sub-electrodes but cover the entire area defined by the pixel.
  • the emulsion 70 is illuminated sequentially by separate red, green and blue light sources 85, 86, and 88.
  • the voltage applied to the electrode 74 is selected such that a given amount of red, green or blue light will be diffracted towards the viewer.
  • the rate at which the hologram is illuminated with the red, green and blue light is faster than the response time of the human eye, a color image will be created in the viewer's eye due to the integration of the individual monochrome images created by the illumination of the emulsion with the red, green and blue lights.
  • FIGURE 6 illustrates the geometrical parameters that define the recording configuration for superimposing red, green and blue holograms in a single emulsion shown in FIGURES 4 and 5.
  • the aim of recording a set of superimposed holograms is to generate three separate fringe patterns corresponding to red, green and blue wavelengths which have angular acceptance characteristics, such that a ray of light which is diffracted by one set of fringes does not also satisfy the diffraction condition for the other two fringe patterns.
  • n is the average refractive index of the holographic medium
  • d is the separation of the hologram fringes formed at the intersection of the recording beams
  • ⁇ is the wavelength.
  • the slant angle of their fringes, >, the separation, d and the Bragg angle, 1, are all determined by the angles made by the two recording beams with respect to the holograms surface.
  • the recording beam directions are represented by the rays, Rl and R2.
  • the ray, Rl is incident at an angle u. Since in most cases, the preferred viewing direction is normal to the hologram's surface, the emerging ray angle of the ray, R2 will be fixed at 90 degrees to the front surface of the emulsion.
  • the refractive index of the substrate will be made as close as possible to the average refractive index of the holographic material.
  • anti-reflection codings will be applied to the ITO surfaces since the reflection losses will be high due to the large refractive index mismatch that exists between ITO (typical refractive index is 1.8) and the holographic emulsion (typical refractive index is 1.52).
  • FIGURE 6 applies to a transmission hologram, similar mathematical arguments apply to the specification of the recording angles for a reflection hologram.
  • Equation 1 high diffraction efficiency will be achieved when Equation 1 is satisfied to a reasonable degree of accuracy.
  • the range of angles and wavelengths that will approximately satisfy the Bragg equation is defined by the angular bandwidth -1 and the spectral bandwidth - ⁇ respectively. These parameters depend upon the recording geometry and on the characteristics of the recording material. By selecting recording parameters ⁇ , 1 and > (subject to the constraints imposed by - ⁇ and -1) and by keeping the output direction beam fixed, it is possible to define recording conditions that give rise to three superimposed gratings which will exhibit minimal interference.
  • tiled it is meant that the emulsion contains a hologram that at any particular location in the hologram is optimized to diffract red, green or blue light. At each pixel location in the emulsion, there will be a separate hologram that is optimized to diffract red, green and blue light.
  • another embodiment of the present invention for creating a color display includes a single emulsion 90 having a set of tiled holograms recorded therein that are optimized to diffract red, green and blue light.
  • a set of sub-electrodes 92A, 92B, and 92C Associated with each pixel in the display are a set of sub-electrodes 92A, 92B, and 92C.
  • a variable voltage is applied to each of the sub-electrodes by a control circuit 94 and driving circuitry 96.
  • the particular voltage supplied to each of the sub-electrodes is selected by an image processor 98.
  • Illuminating the emulsion 90 is a constant white light source 100 that provides continuous red, green and blue light.
  • a variable voltage is applied to a sub-electrode 92A in order to diffract a known amount of red light towards a user while other voltages may be applied to the sub-electrodes 92B and 92C in order to diffract a known amount of green and blue light towards the user for a particular pixel.
  • the holograms recorded in the emulsion 90 are recorded to cause udinimal interference to the other primary colors, the red light is only affected by the tiled red hologram is unaffected by the tiled blue and green holograms. The same is true for the green and blue light applied to the emulsion.
  • FIGURE 8 shows an alternative embodiment of the color display shown in FIGURE 7.
  • a single electrode 95 is used for each pixel in the display.
  • the emulsion is sequentially illuminated with light from a red light source 101, a green light source 102, and a blue light source 104.
  • a red light source 101, 102, 104 illuminates the emulsion.
  • a color image will be created in the eye of the viewer due to the integration of the red, green and blue monochrome images.
  • FIGURE 9 illustrates how tiled holograms that are optimized to diffract red, green, and blue light are recorded in a single emulsion.
  • a metallic mask 110 including a plurality of slots corresponding to each tile in the hologram is applied by vapor deposition or etching techniques to a glass substrate.
  • a first mask may contain a transparent slot 112 where it is desired to pass red light into the emulsion and create a hologram that is optimized to diffract red light.
  • a second mask includes a number of slots 114 where it is desired to pass a green light and create a hologram that is optimized to diffract green light.
  • a third mask includes a number of slots 116 where it is desired to pass blue light and create a hologram that is optimized to diffract blue light. Together the slots 112, 114, 116 define the areas occupied by pixels in the display.
  • a mask for that particular color is placed on either side of an emulsion 118 and illuminated with beams of a corresponding color of laser light. Each color laser is used to expose the emulsion through its corresponding mask.
  • the result is a set of tiled holograms 120 that are optimized to diffract red, green, and blue laser light with minimal interference.
  • the particular angles used in the recording beams as well as wavelengths of light used can be selected as described above for recording the superimposed holograms.
  • a diffuser in the recording beams.
  • the diffuser will create a hologram having a wider range of viewing angles.
  • a diffusing plate can be placed in front of the hologram during use in order to extend the range of viewing angles.
  • the present invention relates to a display that uses the selective light directing ability of an electrically switchable hologram to create a two-dimensional graphic display. Because the hologram is relatively efficient, the display can be made sufficiently bright by increasing the power of the lasers or LED's that illuminate the hologram. In addition, because the electrodes can be created using photolithographic or other etching techniques, the display can have a high resolution. Finally, the emulsion material described above, in which the hologram is recorded allows light to be switched at high speed with high diffraction efficiency in the hologram using relatively low voltages that are easily produced by integrated circuits.
  • Raman-Nath holograms have the advantage of being thinner and therefore require less voltage to switch light between various modes of the hologram.
  • light is generally directed into symmetrical modes (+/-1, +1-2 etc.). Therefore, light from the symmetric mode -1, -2 etc. should be directed towards the user with external optics in order to maintain efficiency.

Abstract

A display (20) includes an emulsion (22) containing an electrically switchable hologram that is illuminated from a light source (24). A pattern of electrodes (30) placed adjacent the emulsion directs light into different modes (26, 28) of the hologram. The emulsion material is made of the monomer dipentaerythritol hydroxypentaacrylate, a liquid crystal, a cross-linking monomer, a coinitiator and a photoinitiator dye. The polymer-dispersed liquid crystal material may optionally further comprise a surfactant. A color display can be created by aligning separate holograms that are optimized for red, green, or blue light, and simultaneously illuminating the hologram with red, green, and blue light. Alternatively, a color display can be created using a single emulsion (70) containing superimposed on tiled holograms (120) that are optimized to diffract red, green, and blue light in the same direction towards a user.

Description

TITLE: ELECTRICALLY SWITCHABLE HOLOGRAPHIC DISPLAY
Field of the Invention
The present invention relates generally to electronic displays and more particularly to electronically switchable holographic displays.
Background of the Invention
As electronic technology advances, there is an ever-increasing desire to incorporate color graphics displays into a variety of products. For example, many telephones, digital cameras, hand-held personal digital assistants, etc., now include graphical displays to provide an interface with which the user operates the device.
The most common type of graphics display for low-cost consumer items are liquid crystal devices that rely on the bulk properties of the liquid crystals to achieve a desired optical effect. The problems with liquid crystals include low brightness, which makes them difficult to view in daylight, and relatively slow refresh rates. Active matrix liquid crystal displays have improved brightness and refresh rates but are limited in resolution. One new technology that appears to overcome the problems associated with liquid crystal displays utilizes an electrically switchable hologram that selectively directs light towards or away from a viewer in order to create a display. While these displays are capable of producing brighter displays with better resolution, they have not yet become commercially useful because of the problem of simultaneously reducing the switching voltage and the switching time, while providing high diffraction efficiency. In addition, the generation of color images with this technology has proven difficult.
Given the potential of electrically switchable holographic displays, there is a need to develop a display that can be switched with low voltage integrated circuits. In addition, there is a need to develop a practical color display with this technology.
Summary of the Invention
The present invention is an improvement in electronic displays that include an electrically switchable hologram recorded in an emulsion. To reduce the switching voltage and switching time and maximize the diffraction efficiency, the emulsion in one embodiment comprises a monomer dipentaerythritol hydroxypentaacrylate, a liquid crystal, a cross-linking monomer, a coinitiator and a photoinitiator dye. The polymer-dispersed liquid crystal (PDLC) material may optionally further comprise a surfactant. The PDLC material may be approximately 10-40 wt% of the liquid crystal. The PDLC material may be approximately 5-15 wt% of the cross-linking monomer. The amount of the co-initiator may be 10"3 to 10"4 gram moles and the amount of the photoinitiator dye may be 10'5 to 10"6 gram moles. The surfactant, when present, may be up to approximately 6 wt% of the PDLC material. A number of electrodes adjacent the emulsion cause light to be selectively diffracted between modes of the hologram in proportion to an applied voltage. The electrodes are disposed over the surface of the emulsion in a pattern of pixels. A voltage is applied to each of the electrodes so as to selectively direct the light that illuminates the emulsion at that pixel towards or away from a viewer.
A color display may be created by aligning three individual holograms that are optimized to diffract red, green, or blue light such that when simultaneously illuminated from a white light source, unwanted interference is minimized. Each hologram has a set of electrodes arranged to form a pattern of pixels. A voltage is applied to the electrodes for each of the holograms sequentially while no voltage is applied to the electrodes of the other two holograms. In this manner, a monochrome red, green and blue image may be sequentially directed towards a viewer and integrated in the viewer's eye to create a color image. In an alternative embodiment, a color display is created with a single emulsion in which three superimposed holograms are recorded. Each hologram is optimized to diffract red, green or blue light and to minimize interference from the other two colors. A set of electrodes is positioned adjacent the emulsion to form a pattern of pixels. If a single electrode is used for each pixel, the emulsion is sequentially illuminated with red, green and blue light. A voltage is applied to an electrode for a particular pixel in the display so as to selectively diffract a portion of the red, green or blue light towards a viewer and create a monochromatic image. A color image is created by integrating the sequential monochrome images in the eye of the viewer.
Alternatively, each electrode may be divided into a number of sub-electrodes. The emulsion is then continuously illuminated with white light and a voltage is applied to each sub-electrode to diffract a desired quantity of red, green and blue light toward a viewer. In yet another embodiment of the invention, a color display is created with a single emulsion having a set of tiled red, green and blue holograms. A set of electrodes adjacent the emulsion form a pattern of pixels. If a single electrode is used for each pixel, the emulsion is sequentially illuminated with red, green and blue light while a voltage is applied to the electrodes to diffract a selected amount of red, green or blue light toward a viewer to create a monochrome image. A color image is created by integrating the sequence of monochrome images in the eye of the viewer.
Alternatively, the electrodes may be divided into sub-electrodes, each of which is positioned over a tiled red, green or blue hologram. The emulsion may then be continuously illuminated with a white light and a voltage is applied to each of the sub-electrodes to diffract a portion of red, green and blue light towards a viewer for each pixel in order to create a color image.
Brief Description of the Drawings FIGURE 1 illustrates a display including a transmissive, electrically switchable hologram in accordance with the present invention;
FIGURE 2 illustrates a display including a reflective, electrically switchable hologram according to the present invention;
FIGURE 3 illustrates a color display including separate emulsions, each of which has a hologram that is optimized to diffract red, green or blue light according to the present invention;
FIGURE 4 illustrates a constantly illuminated color display having a single emulsion which includes superimposed holograms that are optimized to diffract red, green and blue light according to another aspect of the present invention;
FIGURE 5 illustrates a sequentially illuminated color display having a single emulsion which includes superimposed holograms that are optimized to diffract red, green and blue light according to another aspect of the present invention;
FIGURE 6 illustrates the geometrical parameters that define superimposed holograms recorded in a single emulsion to mmimize mutual interference; FIGURE 7 illustrates a constantly illuminated color display having a single emulsion with tiled holograms that are optimized to diffract red, green or blue light according to another aspect of the present invention;
FIGURE 8 illustrates a sequentially illuminated color display having a single emulsion which includes tiled holograms that are optimized to diffract red, green and blue light according to another aspect of the present invention; and
FIGURE 9 illustrates how a set of tiled holograms are recorded in a single emulsion.
Detailed Description of the Preferred Embodiment As indicated above, the present invention relates to a electrically switchable holographic display having a reduced switching voltage and switching time as well as a high diffraction efficiency. In addition, the present invention produces color images using electrically switchable holograms.
FIGURE 1 illustrates a first embodiment of a display made in accordance with the present invention. The display 20 includes a hologram that is formed within holographic recording medium (hereinafter referred to as an
"emulsion material" or "emulsion" 22). The emulsion comprises a polymer material having microdroplets of liquid crystal dispersed therein. Recorded in the emulsion is a hologram that can selectively direct an incoming light beam either towards or away from a viewer.
A light source 24 such as a laser or substantially monochromatic light-emitting diode (LED) directs an incoming light beam toward the hologram. The hologram recorded in the emulsion has the ability to selectively diffract light at certain wavelengths into different modes in response to a voltage applied to either side of the emulsion. Light passing through the hologram in the same direction as the light is received from the light source 24 is referred to as the zero (0th) order mode 26. When no voltage is applied on either side of the emulsion, the liquid crystals droplets are oriented such that the hologram is present in the emulsion and light is diffracted from the 0th order mode 26 to a first order mode 28 of the hologram. Alternatively, when a voltage is applied to the emulsion, the liquid crystal droplets become realigned effectively erasing the hologram and the incoming light passes through the emulsion in the 0th order mode. The percentage of light diffracted into the various modes of the hologram varies in proportion to the strength of the electric field or voltage applied to the emulsion.
The different modes of the hologram are recorded in the hologram such that light in one mode is seen by a viewer of the display while light in another mode is not. Therefore, by changing the amount of light that is diffracted into each mode, the display appears brighter as more light is directed toward the viewer and darker as less light is directed towards the viewer.
To create individual pixels of the display, a pattern of electrodes 30 are positioned on one side of the emulsion 22. The electrodes are substantially transparent so as not to substantially interfere with the light passing through the hologram. The electrodes 30 are preferably made from a vapor deposition of indium tin oxide (ITO) or other substantially transparent metal that is deposited on glass plate that is positioned in front of the emulsion or directly onto the emulsion itself. On the rear surface of the emulsion is a common electrode that covers the entire area of the hologram used to create the display.
By applying a variable voltage to an electrode associated with a particular pixel, the light passing through the hologram at that pixel location is selectively diffracted between the 0th order mode 26 and the first order mode 28 (and higher order modes to a lesser extent). Depending on which mode is in line with the viewer, the light at the particular pixel location will appear brighter or dimmer in order to change the appearance of the display. The voltage on the electrodes for each of the pixels in the display can be individually controlled such that a two- dimensional image is created by controlling the amount of light or brightness at each pixel location.
Controlling the voltage that is supplied to the electrode for each pixel is an electronic control circuit (not shown). The electronic circuit may include a digital-to-analog converter that allows a processor to write a digital value to each electrode location and to have that digital value converted to a corresponding analog voltage that selectively controls the amount of light transferred between the modes of the hologram at that pixel location. Depending upon the number of pixels in the display, the electronic circuit may be designed to simultaneously address all the electrodes or may write to the electrodes in a raster fashion.
As discussed above, one of the problems associated with prior art switchable holographic displays is the fact that the required voltage required to cause the liquid crystals to affect the hologram is too high to be produced by conventional low power integrated circuits. To reduce these high voltages, while keeping the switching time low and the diffractor efficiency high, the present invention, in one embodiment, utilizes an emulsion that comprises the monomer dipentaerythritol hydroxypentaacrylate, a liquid crystal, a cross-linking monomer, a coinitiator and a photoinitiator dye. The polymer-dispersed liquid crystal material may optionally further comprise a surfactant. The PDLC material may be approximately 10-40 wt% of the liquid crystal. The PDLC material may be approximately 5-15 wt% of the cross-linking monomer. The amount of the co-initiator may be 10"3 to 10"4 gram moles and the amount of the photoinitiator dye may be 10"5 to 10"6 gram moles. The surfactant, when present, may be up to approximately 6 wt% of the PDLC material. The composition of the emulsion is fully described in published PCT application serial No. PCT/US97/12577. This emulsion material has been found to produce acceptable switching efficiencies at a few volts per micron of emulsion thickness, potentially as low as 1 volt per micron thickness. At this voltage, switching times have been reduced to less than 20 microseconds, which is considerably faster than that achieved with previous switchable holographic displays and an order of magnitude faster than the switching time of liquid crystal displays, which may take as long as 20 milliseconds to switch. FIGURE 2 illustrates another embodiment of the display according to the present invention that utilizes an emulsion 32 containing a reflective hologram. In this case, a light source 34 produces light which is directed through a light guide (not shown) and into the hologram at a relatively steep angle Θ. A reflective backplane 38 contains the electronic control circuit that supplies the voltage on each of the electrodes adjacent the emulsion. Non-diffracted light, i.e., in the 0th order mode of the hologram, will exit the hologram at the angle Θ substantially equal to the angle at which the light entered the hologram. By varying the voltage applied to the electrodes adjacent the emulsion, the input light is selectively divided between the 0th order mode 40 and the first order mode 42. If the first order mode is aligned with the viewer, then the pixels in the display will appear to grow brighter as more and more light is diverted from the 0th order mode into the first order mode.
As will be appreciated by those skilled in the art, the embodiments shown in FIGURES 1 and 2, allow a grey scale, monochrome image to be created by selectively dividing a portion of the light between 0th and first order modes of the hologram at each pixel location. To create a color display, a selective amount of red, green and blue light must be directed towards a viewer for each pixel location.
FIGURE 3 illustrates a first embodiment of the present invention that creates a color display using three emulsions 52, 54, and 56. Emulsion 52 has a hologram recorded in it that is optimized to diffract red light. The emulsion 54 has a hologram recorded in it that is optimized to diffract green light and the emulsion 56 has a hologram recorded in it that is optimized to diffract blue light. Each of the emulsions 52, 54, and 56 have a set of electrodes associated with each of the pixels in the display. A control circuit 58 drives associated switching circuitry 60 associated with the electrodes on each of the emulsions to apply a variable voltage to the electrodes. The particular voltage level applied to each electrode is determined by a display processor 62. To illuminate the display, a white light source 64 produces a continuous beam of white light that is directed toward the emulsions 52, 54, and 56. In this embodiment of the invention, only one set of electrodes associated with the emulsions 52, 54, and 56 is activated at any one time. With the electrodes enabled, a selected amount of light is diffracted into the 1st order mode of the hologram and towards a user, while light in the 0th order mode is directed such that it cannot be seen by the user. The electrodes on each of the three holograms are sequentially enabled such that a selected amount of red, green and blue light is directed towards a user for each pixel location on the display. The rate at which the holograms are sequentially enabled is faster than the response time of a human eye. A color image will be created in the viewer's eye due to the integration of the red, green and blue monochrome images created from each of the emulsions 52, 54, and 56.
As will be appreciated from viewing FIGURE 3, light passing through the holograms with no voltage applied on the electrodes must be directed away from the user. Therefore, the holograms recorded in each of the emulsions 52, 54, and 56 are oriented such that the 1st order mode is in the direction of the viewer while the 0th order mode diffracts light in a direction that cannot be seen by the viewer.
While the display illustrated in FIGURE 3 will produce color images, three emulsions are required as well as three separate sets of electrodes and driving circuitry. To reduce the number of emulsions and driving circuitry required, the display shown in FIGURE 4 utilizes a single emulsion 70 that has recorded in it three superimposed holograms. Each hologram is optimized to diffract red, green and blue light respectively. The electrodes 72 associated with each pixel are divided into sub-electrodes 72A, 72B, and 72C. Each of the sub-electrodes is selectively addressable by a control circuit 80 and driving circuitry 82 such that a variable voltage can be applied to each of the sub-electrodes. Controlling the voltage applied to each of the sub-electrodes is a display processor 84. A white light source 86 provides a source of red, green and blue light to illuminate the emulsion 70. By applying a selected voltage to each of the sub-electrodes 72A, 72B, and 72C, a variable amount of red, green and blue light can be directed towards a viewer for each pixel. For example, a voltage is applied to the sub-electrode 72A to diffract a given amount of red light towards a viewer while a different voltage may be applied to the sub-electrodes 72B and 72C in order to diffract a different amount of green and blue light towards the viewer for a particular pixel location. As will be described in further detail below, the holograms recorded in the emulsion 70 are arranged such that interference for the red, green and blue light is minimized due to the holograms for the other primary colors.
FIGURE 5 illustrates another embodiment of the invention wherein a color image is created with a single emulsion 70 having separate red, green and blue holograms superimposed therein. In this example, the electrodes 74 associated with a particular pixel are not divided into sub-electrodes but cover the entire area defined by the pixel. In this embodiment, the emulsion 70 is illuminated sequentially by separate red, green and blue light sources 85, 86, and 88. The voltage applied to the electrode 74 is selected such that a given amount of red, green or blue light will be diffracted towards the viewer. Provided that the rate at which the hologram is illuminated with the red, green and blue light is faster than the response time of the human eye, a color image will be created in the viewer's eye due to the integration of the individual monochrome images created by the illumination of the emulsion with the red, green and blue lights.
FIGURE 6 illustrates the geometrical parameters that define the recording configuration for superimposing red, green and blue holograms in a single emulsion shown in FIGURES 4 and 5. The aim of recording a set of superimposed holograms is to generate three separate fringe patterns corresponding to red, green and blue wavelengths which have angular acceptance characteristics, such that a ray of light which is diffracted by one set of fringes does not also satisfy the diffraction condition for the other two fringe patterns.
In order to understand how the recording beam geometry for such a set of non-interfering holograms can be specified, we first consider the basic theory of a Bragg hologram. Referring to FIGURE 6, the geometry of the fringe surfaces is given by the Bragg equation:
2ndsinΘ = (1)
Where n is the average refractive index of the holographic medium, d is the separation of the hologram fringes formed at the intersection of the recording beams and Σ is the wavelength. A description of the basic principles of interference and holographic recordings are given in many standard optical textbooks (see for example, Born and Wolf, Principles of Optics; Hecht and Zajac, Optics and Hariharan, Optical Holography). The slant angle of their fringes, >, the separation, d and the Bragg angle, 1, are all determined by the angles made by the two recording beams with respect to the holograms surface. In FIGURE 6, the recording beam directions are represented by the rays, Rl and R2. The ray, Rl, is incident at an angle u. Since in most cases, the preferred viewing direction is normal to the hologram's surface, the emerging ray angle of the ray, R2 will be fixed at 90 degrees to the front surface of the emulsion.
Note that for purposes of illustrating the concept, the refraction of the rays at the air/substrate/ITO/hologram media interfaces are not shown. For purposes of specifying the angles of the recording beams (external to the hologram) it is sufficient to understand that according to Snell's laws, the incidence angle u in the holographic medium corresponds to the incidence angle u0, in air given by the equation
sin(u0) = nsin(u) (2)
Likewise, the propagation of the ray through the hologram substrates and the ITO electrodes is not shown.
In practice, the refractive index of the substrate will be made as close as possible to the average refractive index of the holographic material. In addition, anti-reflection codings will be applied to the ITO surfaces since the reflection losses will be high due to the large refractive index mismatch that exists between ITO (typical refractive index is 1.8) and the holographic emulsion (typical refractive index is 1.52). Although the geometry shown in
FIGURE 6 applies to a transmission hologram, similar mathematical arguments apply to the specification of the recording angles for a reflection hologram.
According to Bragg theory, high diffraction efficiency will be achieved when Equation 1 is satisfied to a reasonable degree of accuracy. The range of angles and wavelengths that will approximately satisfy the Bragg equation is defined by the angular bandwidth -1 and the spectral bandwidth -∑respectively. These parameters depend upon the recording geometry and on the characteristics of the recording material. By selecting recording parameters Σ, 1 and > (subject to the constraints imposed by -Σ and -1) and by keeping the output direction beam fixed, it is possible to define recording conditions that give rise to three superimposed gratings which will exhibit minimal interference.
As an alternative to recording superimposed holograms in the emulsion material, it is possible to record "tiled" holograms that are optimized to diffract red, green and blue light. By the term tiled, it is meant that the emulsion contains a hologram that at any particular location in the hologram is optimized to diffract red, green or blue light. At each pixel location in the emulsion, there will be a separate hologram that is optimized to diffract red, green and blue light.
As shown in FIGURE 7, another embodiment of the present invention for creating a color display includes a single emulsion 90 having a set of tiled holograms recorded therein that are optimized to diffract red, green and blue light. Associated with each pixel in the display are a set of sub-electrodes 92A, 92B, and 92C. A variable voltage is applied to each of the sub-electrodes by a control circuit 94 and driving circuitry 96. The particular voltage supplied to each of the sub-electrodes is selected by an image processor 98. Illuminating the emulsion 90 is a constant white light source 100 that provides continuous red, green and blue light. In operation, a variable voltage is applied to a sub-electrode 92A in order to diffract a known amount of red light towards a user while other voltages may be applied to the sub-electrodes 92B and 92C in order to diffract a known amount of green and blue light towards the user for a particular pixel. Because the holograms recorded in the emulsion 90 are recorded to cause iriinimal interference to the other primary colors, the red light is only affected by the tiled red hologram is unaffected by the tiled blue and green holograms. The same is true for the green and blue light applied to the emulsion.
FIGURE 8 shows an alternative embodiment of the color display shown in FIGURE 7. In this example, a single electrode 95 is used for each pixel in the display. When a single electrode is used, the emulsion is sequentially illuminated with light from a red light source 101, a green light source 102, and a blue light source 104. Provided that the rate at which the light sources 101, 102, 104 illuminate the emulsion, a color image will be created in the eye of the viewer due to the integration of the red, green and blue monochrome images.
FIGURE 9 illustrates how tiled holograms that are optimized to diffract red, green, and blue light are recorded in a single emulsion. A metallic mask 110 including a plurality of slots corresponding to each tile in the hologram is applied by vapor deposition or etching techniques to a glass substrate. For example, a first mask may contain a transparent slot 112 where it is desired to pass red light into the emulsion and create a hologram that is optimized to diffract red light. A second mask includes a number of slots 114 where it is desired to pass a green light and create a hologram that is optimized to diffract green light. Similarly, a third mask includes a number of slots 116 where it is desired to pass blue light and create a hologram that is optimized to diffract blue light. Together the slots 112, 114, 116 define the areas occupied by pixels in the display. To create the set of tiled holograms for any particular color, a mask for that particular color is placed on either side of an emulsion 118 and illuminated with beams of a corresponding color of laser light. Each color laser is used to expose the emulsion through its corresponding mask. The result is a set of tiled holograms 120 that are optimized to diffract red, green, and blue laser light with minimal interference. The particular angles used in the recording beams as well as wavelengths of light used can be selected as described above for recording the superimposed holograms.
In addition to defining the direction of the 0th and 1st order modes, is may be beneficial to include a diffuser in the recording beams. The diffuser will create a hologram having a wider range of viewing angles. Alternatively, a diffusing plate can be placed in front of the hologram during use in order to extend the range of viewing angles.
As can be seen, the present invention relates to a display that uses the selective light directing ability of an electrically switchable hologram to create a two-dimensional graphic display. Because the hologram is relatively efficient, the display can be made sufficiently bright by increasing the power of the lasers or LED's that illuminate the hologram. In addition, because the electrodes can be created using photolithographic or other etching techniques, the display can have a high resolution. Finally, the emulsion material described above, in which the hologram is recorded allows light to be switched at high speed with high diffraction efficiency in the hologram using relatively low voltages that are easily produced by integrated circuits. While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention. For example, although the currently preferred embodiments of the invention use Bragg or thick holograms, the invention can also be used with Raman-Nath holograms. Raman-Nath holograms have the advantage of being thinner and therefore require less voltage to switch light between various modes of the hologram. However, in a Raman-Nath hologram, light is generally directed into symmetrical modes (+/-1, +1-2 etc.). Therefore, light from the symmetric mode -1, -2 etc. should be directed towards the user with external optics in order to maintain efficiency.

Claims

What is claimed:
1. A color switchable hologram display, comprising: a set of emulsions, each of which has a hologram recorded therein that is optimized to diffract red, green or blue light into one or more modes, wherein the emulsions are aligned such that the modes of the holograms diffract light in substantially the same direction towards a viewer; a set of electrodes adjacent each of the emulsions that are arranged as a pattern of pixels for the display; a control circuit that applies a voltage to the electrodes of each emulsion in a sequence such that a series of monochrome images are directed towards the viewer and combined to produce a color image in the eye of a viewer.
2. A color switchable hologram display, comprising: an emulsion having recorded therein holograms that are optimized to diffract red, green and blue light into one or more modes, wherein one or more of the modes is aligned with a viewer; a set of electrodes adjacent the emulsion that are arranged as a pattern of pixels for the display; a light source that directs red, green and blue light into the emulsion; and a control circuit that applies a variable voltage to each of the electrodes to diffract a portion of the red, green and blue light at each pixel location into the mode that is aligned with the viewer.
3. The color switchable hologram display of Claim 2, wherein the holograms are superimposed.
4. The color switchable hologram display of Claim 3, wherein the light source produces constant red, green and blue light and the electrodes are divided into sub-electrodes for each pixel such that a voltage is applied by the control circuit to each sub-electrode simultaneously.
5. The color switchable hologram of Claim 2, wherein the light source illuminates the emulsion with sequential red, green and blue light and a voltage is applied to the electrodes to create a sequence of monochrome images that are combined as a color image in the eye of the viewer.
6. The color switchable hologram of Claim 2, wherein the holograms are tiled in the emulsion.
7. The color switchable hologram display of Claim 6, wherein the light source produces constant red, green and blue light and the electrodes are divided into sub-electrodes for each pixel such that a voltage is applied by the control circuit to each sub-electrode simultaneously.
8. The color switchable hologram of Claim 6, wherein the light source illuminates the emulsion with sequential red, green and blue light and a voltage is applied to the electrodes to create a sequence of monochrome images that are combined as a color image in the eye of the viewer.
9. A display, comprising: a hologram formed in an emulsion of the type that contains a dispersion of liquid crystal droplets, the hologram selectively diffracting light into one or more modes at least one of which is generally aligned with a viewer; wherein the emulsion comprises: a monomer dipentaerythritol hydroxypentaacrylate; a liquid crystal; a cross-linking monomer; a coinitiator; and a photoinitiator dye. a light source that illuminates the hologram; a plurality of electrodes positioned adjacent the emulsion that are arranged as a pattern of pixels for the display, the electrodes causing the liquid crystal droplets in the emulsion to change the mode in which light is diffracted in the hologram at a corresponding pixel location; and a circuit that applies a voltage to each of the plurality of electrodes in order to selectively change the amount of light that is diffracted towards a viewer.
10. A color switchable hologram display, comprising: a set of emulsions, each of which has a hologram recorded therein that is optimized to diffract red, green or blue light, wherein each hologram is configured to diffract red, green or blue light in substantially the same direction towards or away from a viewer; a set of electrodes adjacent each of the emulsions, wherein the set of electrodes that are arranged as a pattern of display pixels; a control circuit that applies a voltage to the set of electrodes.
11. A color switchable hologram display, comprising: an emulsion having recorded therein holograms that are optimized to diffract red, green and blue light; a set of electrodes adjacent the emulsion, wherein the set of electrodes are arranged as a pattern of display pixels; a light source that directs red, green and blue light into the emulsion; and a control circuit that applies a variable voltage to each of the electrodes to diffract a portion of the red, green and blue light at each pixel location toward or away from a viewer.
12. A display, comprising: a hologram formed in an emulsion of the type that contains a dispersion of liquid crystal droplets, the hologram selectively diffracting light; wherein the emulsion comprises: a monomer dipentaerythritol hydroxypentaacrylate; a liquid crystal; a cross-linking monomer; a coinitiator; and a photoinitiator dye. a light source that illuminates the hologram; a plurality of electrodes positioned adjacent the emulsion that are arranged as a pattern of pixels for the display; and a circuit that applies a voltage to each of the plurality of electrodes.
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EP1876490A1 (en) * 2006-07-05 2008-01-09 Nikon Corporation Display element, display device, and camera

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WO1998004650A1 (en) * 1996-07-12 1998-02-05 Science Applications International Corporation Switchable volume hologram materials and devices
EP0825462A1 (en) * 1996-08-21 1998-02-25 Sony Corporation Holographic optical element process for its production and projection liquid crystal display apparatus using the element
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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1876490A1 (en) * 2006-07-05 2008-01-09 Nikon Corporation Display element, display device, and camera
US8040316B2 (en) 2006-07-05 2011-10-18 Nikon Corporation Display element, display device, and camera

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AU6143099A (en) 2000-04-03

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