WO2000030066A1

WO2000030066A1 - Holographic display system

Info

Publication number: WO2000030066A1
Application number: PCT/US1999/027014
Authority: WO
Inventors: Milan M. Popovich
Original assignee: Digilens Inc.
Priority date: 1998-11-16
Filing date: 1999-11-15
Publication date: 2000-05-25
Also published as: AU1625000A

Abstract

A holographic display system (20) comprising an image generator (30) operable to generate a left image and a right image. The display system (20) includes a first holographic device (40) operable to transmit the left image to a left eye of a viewer when in an active state and a second holographic device (44) operable to transmit the right image to a right eye of a viewer when in an active state. The system further includes a projection system (32) operable to project the left and right images from the image generator to the first and second holographic devices.

Description

HOLOGRAPHIC DISPLAY SYSTEM

RELATED APPLICATION

The present application claims the benefit of U.S. Provisional Application Serial No. 60/108,672, filed November 16, 1998.

FIELD OF THE INVENTION

The present invention relates generally to electronic displays, and more particularly, to electronic displays with switchable holographic optical elements.

BACKGROUND OF THE INVENTION

Head mounted displays have received considerable attention as a technique for displaying high magnification, large field of view and high definition virtual images. The head mounted display generally includes a support member for mounting the display on a head of a user and various optical and display components. The components are arranged to magnify an image displayed on a compact image display panel such as a liquid crystal display (LCD) and to display the magnified image ahead of the user through the optical system. The user typically does not directly observe an image displayed on a monitor or screen, but instead observes a magnified virtual image converted from the image displayed on the image display panel.

Conventional head mounted displays typically require two separate light sources, display panels, and projection optics to provide left eye and right eye images for viewing with both eyes. The requirement for two sets of components increases the size and weight of the display.

There is, therefore, a need for a compact and lightweight holographic display system that provides two optical paths for left and right eye viewing with a single image generator.

SUMMARY OF THE INVENTION

A holographic display system of the present invention is operable to create two optical paths for left and right eye viewing with switchable holographic devices and a single image generator. According to one embodiment of the present invention, the holographic display system generally comprises an image generator operable to generate a left image and a right image. The system further includes a first holographic device operable to transmit the left image to a left eye of a viewer when in an active state and a second holographic device operable to transmit the right image to a right eye of a viewer when in an active state. A projection system operable to project the left and right images from the image generator to the first and second holographic devices is also included in the system.

In another aspect of the invention, a holographic display system generally comprises an image generator operable to generate a left image and a right image. The system further includes a left switchable holographic device operable to transmit the left image when in an active state and a right switchable holographic device operable to transmit the right image when in an active state. A left reflective device is positioned to receive the left image from the left holographic device and direct it to a left eye of a viewer and a right reflective device is positioned to receive the right image from the right holographic device and direct it to a right eye of the viewer.

The above is a brief description of some deficiencies in the prior art and advantages of the present invention. Other features, advantages, and embodiments of the invention will be apparent to those skilled in the art from the following description, drawings, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic of a holographic display system of the present invention.

Fig. 2 is a front view of the holographic display system of Fig. 1.

Fig. 3 is a side view of the holographic display system of Fig. 1.

Fig. 4 is a perspective view of a holographic optical element and light source of the display system of Fig. 1.

Fig. 5 is a partial front view of the holographic optical element of Fig. 4 illustrating an electrode and an electric circuit of the holographic optical element. Fig. 6 is a schematic of a holographic device of the holographic display system of Fig. 1 with three holographic optical elements each optimized to diffract red, green or blue light.

Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.

DETAILED DESCRIPTION OF THE INVENTION The following description is presented to enable one of ordinary skill in the art to make and use the invention. Descriptions of specific embodiments and applications are provided only as examples and various modifications will be readily apparent to those skilled in the art. The general principles described herein may be applied to other embodiments and applications without departing from the scope of the invention. Thus, the present invention is not to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features described herein. For purpose of clarity, details relating to technical material that is known in the technical fields related to the invention have not been described in detail.

Referring now to the drawings, and first to Fig. 1 , a holographic display system, generally indicated at 20, is shown. The holographic display system is

operable to create two output images (i.e., left eye image and right eye image) from a single input image display and transmit the images along two optical paths for viewing by left and right eyes of a viewer. As further described below, the system uses a single input image display and common left eye and right eye projection optics to provide a compact, low weight holographic display system for head mounted use.

The head mounted display includes a headpiece (not shown) designed to be worn by a viewer and the display system 20 for producing wide-angle, electronically generated virtual images to each eye of the viewer. The headpiece includes a frame configured to fit over a viewer's head and a mask which fits over the viewer's eye region, as is well known by those skilled in the art.

As shown in Fig. 1, the display system 20 of the present invention, includes a light source (not shown), an image generator 30, projection optics 32, a left optical system and a right optical system. The left optical system includes a left eye electrically switchable holographic lens stack 40, projection optics 110, and an electrically switchable holographic mirror stack 42. Similarly, the right optical system includes a right eye electrically switchable holographic lens stack 44, projection optics 110, and an electrically switchable holographic mirror stack 46. The left optical system creates a left eye optical path and the right optical system creates a right eye optical path. The left eye optical path is formed when the left eye holographic lens stack 40 is active and the right eye holographic lens stack 44 is inactive. The right eye optical path is formed when the right eye holographic lens stack 44 is active and the left eye holographic lens stack 40 is inactive. By alternately switching the left and right eye holographic lens stacks 40, 44 between their active and inactive states, a single image generator can be used to transmit both left and right eye images. It is to be understood that the arrangement and configuration of components may be different than shown in Fig. 1 without departing from the scope of the invention.

The left and right optical systems are preferably mirror images of each other and generally identical in all other aspects. Therefore, in the following description, only the left optical system will be described in detail. The following description begins with the light source and provides a description of the components within the display system going from the light source to the holographic mirror stacks 42, 46.

Figs. 2 and 3 show a front and side view, respectively, of the holographic display system 20 for use in a head mounted display. A light source 50 is positioned to project a light onto the image generator 30 which then projects an input image to the projection optics 32 which is operable to transmit light from the display panel to the left and right optical systems. The light source 50 may be any high intensity white light source (such as a conventional LCD backlight, for example) or alternatively it may be based on red, green, and blue high intensity LEDs (Light Emitting Diodes) or red, green, and blue solid state lasers. The light source 50 is positioned to direct light at an angle onto a front surface of the input image display device 30 with the use of illumination optics. The illumination optics may include, for example, a beam splitter cube 52, a quarter wave plate (not shown), and a trichromatic optical interference filter (not shown) which filters the light source 50 into three narrow band components centered on red, green, and blue peak wavelengths.

The image generator 30 is preferably operable to display stereoscopically- paired images and is switchable between a first mode in which the image generator displays one of the paired images and a second mode in which it displays the other of the paired images. The switching of the image generator 30 between its first and second modes is preferably performed in synchronism with switching of the left eye and right eye holographic lens stacks 40, 44, as described below.

The image generator 30 is driven by video or graphic information and may comprise a liquid crystal display (LCD) panel, or any other spatial light modulator (SLM) which reflects light produced externally. The image display panel 30 may be a miniature reflective LCD having either a nematic or ferroelectric material on a silicon backplane, for example. The reflective display panel 30 utilizes the external light source 50 to reflect and modulate light off the front of the microdisplay. The display panel 30 may also be based on transmissive display technologies. Preferably, the display panel 30 is color sequentially illuminated using separate red, green, and blue sources or, alternatively, a white source combined with a color sequential filter. The latter may be based on electro-mechanical techniques involving band pass filters which are rotated or displaced in some manner in front of the source, for example.

The image generator 30 may be a miniature reflective silicon backplane device, such as a SVGA (800x600 pixels) device available from Colorado MicroDisplay, of Boulder Colorado, for example.

A micro-electromechanical system, such as a Digital Light Processor (DLP) using a Digital Micromirror Device™ (DMD) available from Texas Instruments, may also be used as the input image display panel 30. The DMD is a micromechanical silicon chip having movable mirrors which reflect light to create high quality images. An image is formed on the reflective surface of the DMD by turning the mirrors on or off digitally at a high speed. Color is added to the image by filtering light through a color system. The color system may comprise a light source which directs white light through a condenser lens and a red, green, and blue color filter and then onto the surface of the DMD chip, for example. Mirrors are turned on or off for different times depending upon how much light of each color is needed per pixel.

The input image display panel 30 may also be a diffractive display device such as a Grating Light Valve™ (GLV) available from Silicon Light Machines (formerly Echelle, Inc.). The GLV uses micro-electromechanical systems to create multiple ribbon structures which can move small distances to create a grating which selectively diffracts specified wavelengths of light. Each grating defines a picture element (pixel) formed on the surface of a silicon chip and the array of pixels formed become the image source for display projection. A white light source is filtered sequentially through red, green, and blue filters. By synchronizing the image data stream's red, green, and blue pixel data with the appropriate filtered source light, combinations of red, green, and blue light are diffracted to the lens group for projection of the image into the holographic device. It is to be understood that display panels other than those described herein may be used without departing from the scope of the invention.

The common projection optics 32 provide an optical interface between the image generator 30 and the holographic lens stacks 40, 44. As shown in Fig. 3, the projection optics 32 include a plurality of optical lenses. The lenses magnify the input image and are configured and positioned to provide appropriate focal length and other optical characteristics. It is to be understood that the number and configuration of lenses may be different than shown herein. Additional optical elements may be provided to correct for aberrations, as is well known by those skilled in the art. For example, the lenses may include cylinders, prisms, and off- axis aspheric elements to correct for aberrations due to the off-axis, non-symmetric nature of the display system. Since the holographic display system 20 is preferably designed to be as compact as possible, the projection optical system will be highly off-axis. Thus, the projection optics 32 are preferably designed to minimize dispersion and chromatic aberrations contributed by the holograms. In order to minimize the weight of the display system 20, as well as the manufacturing cost, the projection optics 32 are preferably fabricated from optical plastics, e.g., optical acrylics. The projection optics 32 may also include a beam folding mirror which may be interposed between the holographic lens stacks 40, 44 and the holographic mirror stacks 42, 46 or, alternatively, between the projection optics 32 and the holographic lens stacks (40, 44), for example.

After passing through the projection optics 32, the image passes through the holographic lens stacks 40, 44, only one of which is active. The left eye holographic lens stack 40 is operable to direct light from the image generator 30 to the left holographic mirror stack 42 and the right eye holographic lens stack 44 is operable to direct light from the image generator 30 to the right holographic mirror stack 46 (Figs. 1 and 3). Preferably, the left holographic lens stack 40 is switched to an inoperative state when the right eye holographic lens stack 44 is switched to an operative state, and vice versa. The frequency at which the holographic lens stacks 40, 44 are switched is dependent on the type of image displayed. For example, with a stereoscopic display, if video fields are alternately encoded with left and right eye data in a conventional 60 Hz input image display, the resulting reduction in the number of fields displayed per second will be below the eye critical fusion frequency. Therefore, the input image display is preferably updated at a frequency of at least 120 Hz. In a non-stereoscopic display with the same input image used for both the left and right eyes, it is possible to run the input image display at 60 Hz with the holographic lens stacks 40, 44 being switched at a much higher frequency.

Each holographic lens stack 40, 44 is formed from a switchable holographic device comprising one or more holographic optical elements 56 which are selectively activated and deactivated to transmit the image which is formed by sequentially manipulating different colors (Figs. 1 and 4). The holographic optical element 56 includes a hologram interposed between two electrodes 62 (Fig. 5). The hologram is used to control transmitted light beams based on the principles of diffraction. The hologram selectively directs an incoming light beam from the light source 50 either towards or away from a viewer and selectively diffracts light at certain wavelengths into different modes in response to a voltage applied to the electrodes 62. Light passing through the hologram in the same direction that the light is received from the light source 50 is referred to as the zeroth (0th) order mode

68 (Fig. 4). When no voltage is applied to the electrodes 62, liquid crystal droplets within the holographic optical element 56 are oriented such that the hologram is present in the element and light is diffracted from the zeroth order mode to a first (1st) order mode 70 of the hologram. When a voltage is applied to the holographic optical element 56, the liquid crystal droplets become realigned effectively erasing the hologram, and the incoming light passes through the holographic optical element in the zeroth order mode 68.

It is to be understood that the holographic optical element 56 may also be reflective rather than transmissive. In the case of a reflective holographic optical element, the arrangement of the holographic device and optical components would be modified to utilize reflective properties of the hologram rather than the transmissive properties described herein.

The light that passes through the hologram is diffracted to form an image by interference fringes recorded in the hologram. Depending on the recording, the hologram is able to perform various optical functions which are associated with traditional optical elements, such as lenses and prisms, as well as more sophisticated optical operations which would normally require very complex systems of conventional components. The hologram may be configured to perform operations such as deflection, focusing, or color filtering of the light, for example.

The holograms are preferably recorded on a photopolymer/liquid crystal composite material (emulsion) such as a holographic photopolymeric film which has been combined with liquid crystal, for example. The presence of the liquid crystal allows the hologram to exhibit optical characteristics which are dependent on an applied electrical field. The photopolymeric film may be composed of a polymerizable monomer having dipentaerythritol hydroxypentacrylate, as described in PCT Publication, Application Serial No. PCT US97/12577, by Sutherland et al., which is incorporated herein by reference. The liquid crystal may be suffused into the pores of the photopolymeric film and may include a surfactant.

The refractive properties of the holographic optical element 56 depend primarily on the recorded holographic fringes in the photopolymeric film. The interference fringes may be created by applying beams of light to the photopolymeric film. Alternatively, the interference fringes may be artificially created by using highly accurate laser writing devices or other replication techniques, as is well known by those skilled in the art. The holographic fringes may be recorded in the photopolymeric film either prior to or after the photopolymeric film is combined with the liquid crystal. In the preferred embodiment, the photopolymeric material is combined with the liquid crystal prior to the recording. In this preferred embodiment, the liquid crystal and the polymer material are pre-mixed and the phase separation takes place during the recording of the hologram, such that the holographic fringes become populated with a high concentration of liquid crystal droplets. This process can be regarded as a "dry" process, which is advantageous in terms of mass production of the switchable holographic optical elements. As further described below, the optical properties of the holographic optical element primarily depend on the recorded holographic fringes in the photopolymeric film.

The electrodes (electrode layers) 62 are positioned on opposite sides of the emulsion and are preferably transparent so that they do not interfere with light passing through the hologram. The electrodes 62 may be formed from a vapor deposition of Indium Tin Oxide (ITO) which typically has a transmission efficiency of greater than 80%, or any other suitable substantially transparent conducting material. The electrodes 62 are connected to an electric circuit 78 operable to apply a voltage to the electrodes, to generate an electric field (Fig. 5). Initially, with no voltage applied to the electrodes 62, the hologram is in the diffractive (active) state and the holographic optical element 56 diffracts propagating light in a predefined manner. When an electrical field is generated in the hologram by applying a voltage to the electrodes 62 of the holographic optical element 56, the operating state of the hologram switches from the diffractive state to a passive (inactive) state and the holographic optical element does not optically alter the propagating light. It is to be understood that the electrodes may be different than described herein. For example, a plurality of smaller electrodes may be used rather than two large electrodes which substantially cover surfaces of the holograms.

Each holographic device preferably includes three holographic optical elements (red 80, green 82, and blue 84) for projecting a color image to the viewer (Fig. 6). Each holographic optical element 80, 82, 84 is holographically configured such that only a particular monochromatic light is diffracted by the hologram. The red optical element 80 has a hologram which is optimized to diffract red light, the green optical element 82 has a hologram which is optimized to diffract green light, and the blue optical element 84 has a hologram which is optimized to diffract blue light. A holographic device controller 90 drives switching circuitry 94 associated with the electrodes 62 on each of the optical elements 80, 82, 84 to apply a voltage to the electrodes. The electrodes 62 are individually coupled to the device controller through a voltage controller 102 which selectively provides an excitation signal to the electrodes 62 of a selected holographic optical element, switching the hologram to the passive state. The voltage controller 102 also determines the specific voltage level to be applied to each electrode 62.

Preferably, only one pair of the electrodes 62 associated with one of the three holographic optical elements 80, 82, 84 is energized at one time. In order to display a color image, the voltage controller 102 operates to sequentially display three monochromatic images of the color input image. The electrodes 62 attached to each of the holograms 80, 82, 84 are sequentially enabled such that a selected amount of red, green, and blue light is directed towards the viewer. For example, when a red monochromatic image is projected, the voltage controller 102 switches the green and blue holograms 82, 84 to the passive state by applying voltages to their respective electrodes 62. The supplied voltages to the electrodes 62 of the green and blue holograms 82, 84 create a potential difference between the electrodes, thereby generating an electrical field within the green and blue holograms. The presence of the generated electrical field switches the optical characteristic of the holograms 82,

84 to the passive state. With the green and blue holograms 82, 84 in the passive state and the red hologram 80 in the diffractive state, only the red hologram optically diffracts the projected red image. Thus, only the portion of the visible light spectrum corresponding to the red light is diffracted to the viewer. The green hologram 82 is next changed to the diffractive state by deenergizing the corresponding electrodes 62 and the electrodes of the red hologram 80 are energized to change the red hologram to the passive state so that only green light is diffracted. The blue hologram 84 is then changed to the diffractive state by deenergizing its electrodes 62 and the electrodes of the green hologram 82 are energized to change the green hologram to the passive state so that only blue light is diffracted.

The holograms are sequentially enabled with a refresh rate which is faster than the response time of a human eye so that 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 red, green, and blue holograms. Consequently, the holographic devices will be illuminated sequentially by red, green, and blue lights so that the final viewable image will appear to be displayed as a composite color. The red, green, and blue holographic elements 80, 82, 84 may be cycled on and off in any order.

The left and right holographic mirror stacks 42, 46 direct the image received from the left and light holographic lens stacks, respectively, and direct the images to the viewer. The mirror stacks 42, 46 are preferably formed from holographic devices comprising three holographic optical elements as described above. The elements are preferably reflective and comprise a plurality of holographic layers, each of which is operable to act upon a respective one of the wavelength bands produced by the image generator 30. The left and right mirror stacks 42, 46 preferably both remain active while the holographic lens stacks 40, 44 are switched. The mirror stacks 42, 46 may also be conventional mirror devices, rather than holographic devices.

In order to accommodate a large range of interpupillary distances, the exit pupil is preferably as large as possible. For example, the exit pupil may be 15 mm horizontal x 7 mm vertical. In order to provide a large pupil without imposing a large eye-relief and large eyepiece dimensions, it is advantageous for the holographic mirror stacks 42, 46 to be implemented on curved substrates. The curved holographic optical devices may be formed as described in U.S. Patent Application Serial No. 09/416.076. by M. Popovich, filed October 12, 1999, for example.

Additional projection optics 110 may also be positioned adjacent to the left and right mirror stacks 42, 46 to assist in directing the light from the lens stacks 40, 44 and correcting optical aberrations (Fig. 1).

The display system 20 may also be capable of operating in a see through mode by switching all of the holograms to their transparent state. During normal use, external ambient light may be excluded by a liquid crystal shutter.

While different embodiments of the invention have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the scope of the invention. For example, the system 20 can use Raman-Nath holograms rather than Bragg (thick holograms). Raman-Nath holograms are thinner and require less voltage to switch light between various modes of the hologram, however, the Raman-Nath holograms are not as efficient as the Bragg holograms. Also, since the Bragg holograms included in the lens stacks 40, 44 and mirror stacks 42, 46 are highly wavelength specific, each of the layers in these elements will act only on the light of the appropriate color. Therefore, it is possible for the image generator 30 to display red, green, and blue images simultaneously rather than in a cyclic succession, so that the layers in each element act simultaneously on the transmitted image. In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above constructions and methods without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Claims

WHAT IS CLAIMED IS:

1. A holographic display system comprising:

an image generator operable to generate a left image and a right image;

a first holographic device operable to transmit the left image to a left eye of a viewer when in an active state;

a second holographic device operable to transmit the right image to a right eye of a viewer when in an active state; and

a projection system operable to project the left and right images from the image generator to the first and second holographic devices.

2. The holographic display system of claim 1 wherein the first and second holographic devices are each switchable between the active state in which the left and right images are diffracted by the holographic device and a passive state in which the images are not diffracted by the holographic device.

3. The holographic display system of claim 2 wherein the first and second holographic devices are configured such that only one holographic device is in its active state at a time.

4. The holographic display system of claim 1 wherein the holographic device comprises a hologram interposed between two electrode layers operable to apply an electrical field to the hologram.

5. The holographic display system of claim 4 wherein the hologram is formed from a polymer and liquid crystal material.

6. The holographic display system of claim 1 wherein the first and second holographic devices each comprise three holographic optical elements.

7. The holographic display system of claim 6 wherein the three holographic optical elements each have a hologram recorded therein, the hologram being optimized to diffract red, green, or blue light.

8. The holographic display system of claim 7 wherein each hologram is interposed between two electrode layers operable to apply an electrical field to the hologram to diffract the red, green, or blue light.

9. The holographic display system of claim 8 further comprising a controller operable to sequentially supply voltage to and remove voltage from the electrode layers of each holographic optical element to create a sequence of monochrome images which are combined to form a color image.

10. The holographic display system of claim 1 wherein the display system is configured for use as a head mounted display.

11. The holographic display system of claim 1 wherein the image generator comprises a reflective display panel.

12. The holographic display system of claim 1 further comprising a left reflective device positioned to receive the left image from the first holographic device and transmit it to the left eye of the viewer, and a right reflective device positioned to receive the right image from the second holographic device and transmit it to the right eye of the viewer.

13. The holographic display system of claim 12 wherein each of the reflective devices are holographic diffractive devices.

14. The holographic display system of claim 13 wherein each of the holographic diffractive devices comprise three holographic optical elements each having a hologram recorded therein and optimized to diffract red, green, or blue light.

15. The holographic display system of claim 13 wherein the holographic diffractive devices are formed on curved substrates.

16. The holographic display system of claim 1 further comprising left projection optics positioned adjacent to the first holographic device and right projection optics positioned adjacent to the second holographic device.

17. The holographic display system of claim 1 further comprising a light source for illuminating the image generator.

18. The holographic display system of claim 17 further comprising a beamsplitter interposed between the light source and image generator.

19. The holographic display system of claim 17 further comprising a trichromatic optical interference filter operable to filter the light transmitted from the light source into three narrow band components centered on red, green, and blue peak wavelengths.

20. The holographic display system of claim 1 wherein the display is a stereoscopic display.

21. The holographic display system of claim 20 wherein the image generator is operable to update the image at a minimum frequency of 120 Hz.

22. The holographic display system of claim 1 wherein the first and second holographic devices are positioned generally adjacent one another along a central longitudinal axis extending generally perpendicular to the devices.

23. The holographic display system of claim 1 wherein the left and right images are generally the same image.

24. A holographic display system comprising:

an image generator operable to generate a left image and a right image;

a left switchable holographic device operable to transmit the left image when in an active state;

a right switchable holographic device operable to transmit the right image when in an active state; a left reflective device positioned to receive the left image from the left holographic device and direct it to a left eye of a viewer; and

a right reflective device positioned to receive the right image from the right holographic device and direct it to a right eye of the viewer.

25. The holographic display system of claim 24 wherein the left and right holographic devices are each switchable between the active state in which the left and right images are diffracted by the holographic device and a passive state in which the images are not diffracted by the holographic device.

26. The holographic display system of claim 25 wherein the left and right holographic devices are configured such that only one holographic device is in its active state at a time.

27. The holographic display system of claim 24 wherein the holographic device comprises a hologram interposed between two electrode layers operable to apply an electrical field to the hologram.

28. The holographic display system of claim 24 wherein the image generator comprises a reflective display panel.

29. The holographic display system of claim 24 wherein each of the reflective devices are holographic diffractive devices.

30. The holographic display system of claim 24 wherein the reflective devices are formed on curved substrates.

31. The holographic display system of claim 24 wherein the display is a stereoscopic display.

32. The holographic display system of claim 31 wherein the image generator is operable to update the image at a minimum frequency of 120 Hz.