WO2000068715A1

WO2000068715A1 - Image generating system

Info

Publication number: WO2000068715A1
Application number: PCT/US2000/012430
Authority: WO
Inventors: Milan M. Popovich; David C. Nabors; Jonathan J. Storey; Jonathan David Waldern
Original assignee: Digilens Inc.
Priority date: 1999-05-07
Filing date: 2000-05-04
Publication date: 2000-11-16
Also published as: AU4503300A

Abstract

A system (10) for illuminating a display device (18) comprising a plurality of optical filters (30, 32, 34) each configured to reflect a light beam having a specified wavelength range and a plurality of holographic optical elements (42, 44, 46) each disposed on a common optical path with one of the filters. The holographic optical elements are switchable between an active state wherein light incident on the element is diffracted and a passive state wherein light incident on the element is transmitted without substantial alteration. Each of the holographic optical elements when in its active state in combination with the filter on the common optical path therewith are operable to receive polychromatic light and produce the light beam to illuminate the display device.

Description

IMAGE GENERATING SYSTEM

RELATED APPLICATIONS

The present application claims the benefit of U.S. Provisional Application Serial No. 60/133,173, filed May 7, 1999.

BACKGROUND OF THE INVENTION

The present invention relates generally to image generating systems, and more particularly, to an image generating system employing electrically switchable holograms.

Image display systems typically include a display screen configured to display monochrome images. Current microdisplays are typical of display panels that are configured to display monochrome images. When a multi-color display is required, a sequence of images is displayed and illuminated sequentially with red, green, and blue lights. The switching from one image to the next is performed rapidly (e.g., at a rate faster than the response time of a human eye) so that a color image is created in the viewer's eye due to the integration of red, green, and blue monochrome images. This allows a viewer to see a full color image generated from a display system having a display screen operable to produce only monochrome images. The display system typically includes a white light source and a rotating color wheel having red, green, and blue filters to provide color sequential illumination of a display device. However, these rotating filters are often susceptible to mechanical failures and tend to be large and noisy. The risk of mechanical failures, noise, and size may be reduced by using solid state techniques such as liquid crystal polarization switches. However, these switches work only with polarized light, thus, half of the light produced by the light source is never projected from the display screen, resulting in low illumination. Furthermore, the liquid crystal polarization switches and mechanical rotating wheels often do not provide sufficiently fast switching speeds between the different colors. If the switching speed is too slow, the image projected from the display screen will not appear as a multi-color image.

There is, therefore, a need for a reliable, efficient, and fast switching optical system operable to generate red, green, and blue light from a white light source.

SUMMARY OF THE INVENTION

A system for illuminating a display device is disclosed. The system generally comprises a plurality of optical filters each configured to reflect a light beam having a specified wavelength range and a plurality of holographic optical elements each disposed on a common optical path with one of the filters. The holographic elements are switchable between an active state wherein light incident on the element is diffracted and a passive state wherein light incident on the element is transmitted without substantial alteration. The holographic optical elements in their active state in combination with the filter on the common optical path are operable to receive polychromatic light and produce the light beam to illuminate the display device.

In another aspect of the invention a system for illuminating a display device comprises a plurality of optical filters positioned to receive light from a polychromatic light source and configured to reflect a light beam having a specified wavelength range and a plurality of holographic optical elements positioned to receive the light beams from the plurality of filters. The holographic optical elements are switchable between an active state wherein light incident on the element is diffracted and a passive state wherein light incident on the element is transmitted without substantial alteration. The system further includes a controller operable to switch the holographic optical elements between their active and passive states and a beam deflector operable to receive and deflect the light beams onto a display device to illuminate the display device.

An image generating system of the present invention generally comprises a display device operable to display monochrome images and a display image controller coupled to the display device and operable to switch the monochrome images displayed by the display device. The system further includes a plurality of optical filters each configured to reflect a light beam having a specified wavelength range and a plurality of holographic optical elements each disposed on a common optical path with one of the filters. The holographic optical elements are switchable between an active state wherein light incident on the element is diffracted and a passive state wherein light incident on the element is transmitted without substantial alteration. The system further includes a holographic element controller operable to sequentially switch the elements between their active and passive states generally in synchronism with the display image controller switching between the displayed monochrome images.

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 side view of a first embodiment of an image generating system of the present invention.

Fig. 2 is a side view an optical system of the image generating system of Fig. 1 illustrating additional detail.

Fig. 3 is a perspective of a holographic optical element and light source for use with the optical system of Fig. 2.

Fig. 4 is a partial front view of the holographic optical element of Fig. 3 illustrating an electrode and electric circuit of the holographic optical element.

Fig. 5 is a schematic of a holographic device having three holographic optical elements each optimized to diffract red, green, or blue light.

Fig. 6 is a side view of a second embodiment of an image generating system of the present invention.

Fig. 7 is a side view of a third embodiment of the image generating system of the present invention.

Fig. 8 is a side view of a fourth embodiment of the image generating system of the present invention.

Fig. 9 is a schematic of an optical system of the image generating system of Fig.

8.

Fig. 10 is a plan view of the optical system of Fig. 9. 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 first embodiment of an image generating system, generally indicated at 10, is shown. The system includes a light source 12, collimating optics 14, optical system, generally indicated at 16, and a display device 18 operable to display monochrome images. The optical system 16 includes a plurality of optical filters 30, 32, 34 and a light guide 40. The optical system 16 receives collimated white light and produces at least three distinct bandwidths of illumination light. In the preferred embodiment, the optical system 16 generates red, green, and blue lights which are used to sequentially illuminate the display 18 and produce red, green, and blue monochrome images, respectively. By switching between the resultant red, green, and blue images produced by the display 18 in a very rapid manner, the projected images appear to be displayed as a composite multicolor image so that a viewer will perceive what is effectively a full color image. The optical system 16 may also simultaneously illuminate three distinct areas of the display device with the red, green, and blue illumination lights, as described in U.S. Patent Application Serial No. 60/125,924, filed March 23, 2000 (Attorney Docket No. M-8433 US), which is incorporated herein by reference.

The display device 18 includes a display surface typically comprising an array of pixels for displaying monochromatic data or monochromatic images in accordance with signals generated by an image control circuit (display controller) 52. The display 18 may comprise a liquid crystal display (LCD) panel, or any other spatial light modulator

(SLM) which reflects or transmits light produced externally. The display 18 may be a miniature reflective LCD having either a nematic or ferroelectric material on a silicon backplane, for example. The display 18 may also be based on transmissive display technologies. 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 display 18. 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. An image is generated by color sequentially illuminating the display and turning individual mirrors on or off for durations which depend on the amount of each primary color required to generate the required color value at each pixel.

The display 18 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 vary how light is reflected from multiple ribbon structures which can move small distances to create a grating that selectively diffracts specified wavelengths of light. Picture elements (pixels) are formed on the surface of a silicon chip and become the image source for display projection. It is to be understood that the display devices described above are merely examples and different display devices may be used without departing from the scope of the invention.

The light source 12 is polychromatic and preferably provides incoherent white light 22. The white light includes red, green, and blue bandwidth light components (i.e., light beams having different wavelength ranges). The light 22 emitted from the light source 12 is collimated into a parallel beam 24 by collimating optics 14. The collimating optics 14 may include condenser lenses, mirrors, collimating lenses, and heat rejection filters as is well known by those skilled in the art. The parallel beam 24 is directed towards optical filters 30, 32, 34 which are each disposed at approximately a 45 degree angle with respect to a front surface of the light guide 40. The filters 30, 32, 34 are preferably dichroic mirrors (e.g., glass coated with multilayer dielectric and/or metallic coatings that reflect certain colors of light while allowing others to pass therethrough). Filter 30 is configured to reflect red light and allow green and blue light to pass therethrough. Similarly, dichroic filter 32 reflects green light and allows blue light to pass therethrough and filter 34 reflects blue light. The respective light beams enter the light guide 40 which produces a sequence of red, green, and blue output beams used to sequentially illuminate the display device 18, as further described below.

The light guide 40 comprises three holographic optical elements 42, 44, 46 mounted on a front surface of a transparent (e.g., glass) plate. The holographic elements 42, 44, 46 are preferably positioned such that front surfaces of the elements are generally in the same plane and aligned vertically adjacent to one another. The holographic elements 42, 44, 46 are each switchable between an active (diffracting) state and a passive (non-diffracting) state. It is to be understood that in the passive state (non- diffracting state), the incoming light may still be slightly diffracted, however, the light is not substantially altered. Switching of the holographic elements 42, 44, 46 is controlled by controller 50 which operates to switch each of the elements between their active and passive states such that the light guide 40 produces red, green, and blue output beams sequentially in a rapid cycle. The controller 50 is synchronized with the display controller 52 so that the red beam is emitted from the light guide 40 when the display device 18 displays the red image, the green beam is emitted from the light guide when the display device displays the green image, and the blue beam is emitted when the blue image is displayed. The light guide 40 further includes a beam deflector 56 mounted on the front surface of the light guide and laterally spaced below the holographic optical elements 42, 44, 46. A rear surface 52 of the light guide 40 comprises a reflective surface to reflect beams received from the holographic elements 42, 44, 46 onto the beam deflector 56. A mirror coating may be applied to the rear face 52 of the light guide 40 to maximize reflection of the red, green, and blue light beams.

The light guide 40 may be replaced with a mirror with an air gap separating the holographic elements and the mirror. However, the light guide 40 is preferred because it allows the use of extreme beam deviation angles without the reflection losses that would result from the use of a mirror at high incidence angles. The light guide is normally based on the principle of total internal reflection due to the beam angles at the back surface of the light guide exceeding the glass/air critical angle.

The dichroic filters 30, 32, 34 and holographic elements 42, 44, 46 create three separate optical paths, red, green, and blue, respectively. For example, filter 30 reflects a red beam 60 from the white light 24 incident thereon and directs this beam to the holographic optical element 42, which diffracts the beam when in its active state and directs it towards the reflective rear surface 52 of the light guide 40 where it is reflected onto the beam deflector 56. The red beam 60 is then deflected from the beam deflector 56 and exits the light guide 40 at an angle approximately 180 degrees from the direction that the red beam entered the light guide 40 and is directed to the display device 18.

After the red image is projected by the display device 18, the holographic device 42 is switched to its passive state and holographic device 44 is switched to its active state.

The green component of the white light 24 passes through dichroic filter 32 and is reflected at an angle of 45 degrees from the filter onto the green holographic diffraction element 44 which is in its active state (Figs. 1 and 2). The green beam 62 is diffracted so that it is incident on the rear face 52 of the light guide 40 and totally internally reflected thereby. The green beam 62 is reflected onto the beam deflector 56 which deflects the green beam towards the display device 18. The image control circuit 52 sends a signal to the display device 18 to display the green image at approximately the same time that the controller 50 switches holographic element 44 to its active state.

The holographic device 44 is then switched to its passive state and the holographic device 46 is switched to its active state. The blue light beam 64 passes through dichroic filters 30 and 32, and is reflected by filter 34 onto holographic device 46 which diffracts the blue beam onto the rear face 52 of the light guide 40. The blue light 64 is then reflected onto the beam deflector 56 which directs the beam towards the display device 18, which has switched to the blue image.

The spectral bandwidth profiles of the red, green, and blue output beams 60, 62, 64 are preferably determined by the characteristics of the respective dichroic filters 30, 32, 34, so that the holographic elements 42, 44, 46 are used only for beam switching. The diffraction efficiency spectral bandwidth of each holographic diffraction element 42, 44, 46 is usually greater than the bandwidth of the respective filters 30, 32, 34.

The holographic optical elements 42, 44, 46 each include a hologram interposed between two electrodes 70 (Figs. 3 and 4). The hologram may be a Bragg (thick or volume) hologram or Raman-Nath (thin) hologram. Raman-Nath holograms are thinner and require less voltage to switch light between various modes of the hologram, however, Raman-Nath holograms are not as efficient as Bragg holograms. The Bragg holograms provide high diffraction efficiencies for incident beams with wavelengths close to the theoretical wavelength satisfying the Bragg diffraction condition and within a few degrees of the theoretical angle which also satisfies the Bragg diffraction condition.

The hologram is used to control transmitted light beams based on the principles of diffraction. The hologram selectively directs an incoming light beam from light source 12 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 70. Light passing through the hologram in the same direction as the light is received from the light source 12 is referred to as the zeroth (0th) order mode 74 (Fig. 3). When no voltage is applied to the electrodes 70, liquid crystal droplets within the holographic optical element 42, 44, 46 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 76 of the hologram. When a voltage is applied to the holographic optical element 42, 44, 46, 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 74. It is to be understood that the holographic optical elements 42, 44, 46 may also be reflective rather than transmissive as shown in Fig. 3 and described above. In the case of a reflective holographic optical element, the arrangement of the holographic devices and beam deflector 56 within the light guide 40 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 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. 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 in 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 diffractive properties of the holographic optical elements 42, 44, 46 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.

The electrodes (electrode layers) 70 are positioned on opposite sides of the emulsion and are preferably transparent so that they do not interfere with light passing through the hologram (Fig. 4). The electrodes 70 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. An anti-reflection coating (not shown) may be applied to selected surfaces of the switchable holographic optical element, including surfaces of the ITO and the electrically nonconductive layers, to improve the overall transmissive efficiency of the optical element and to reduce stray light. The electrodes 70 are connected to an electric circuit 78 operable to apply a voltage to the electrodes, to generate an electric field (Fig. 4). Initially, with no voltage applied to the electrodes 70, the hologram is in the diffractive (active) state and the holographic optical element 42, 44, 46 diffracts propagating light in a predefined manner. When an electrical field is generated in the hologram by applying a voltage to the electrodes 70 of the holographic optical element 42, 44, 46, the operating state of the hologram switches from the diffractive state to the passive 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 without departing from the scope of the invention. For example, a plurality of smaller electrodes may be used rather than two large electrodes which substantially cover surfaces of the holograms.

Each holographic optical element 42, 44, 46 is holographically configured such that only a particular monochromatic light is diffracted by the hologram. The red optical element 42 has a hologram which is optimized to diffract red light, the green optical element 44 has a hologram which is optimized to diffract green light, and the blue optical element 46 has a hologram which is optimized to diffract blue light. The holographic device controller 50 drives switching circuitry 84 associated with the electrodes 70 on each of the optical elements 42, 44, 46 to apply a voltage to the electrodes (Figs. 4 and 5). The electrodes 70 are individually coupled to the device controller 50 through a voltage controller 86 which selectively provides an excitation signal to the electrodes 70 of a selected holographic optical element 42, 44, 46, switching the hologram to the passive state. The voltage controller 86 also determines the specific voltage level to be applied to each electrode 70.

Preferably, only one pair of the electrodes 70 associated with one of the three holographic optical elements 42, 44, 46 is energized at one time. In order to display a color image, the voltage controller 86 operates to sequentially display three monochromatic images of the color input image. The electrodes 70 attached to each of the holograms 42, 44, 46 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 86 switches the green and blue holograms 44, 46 to the passive state by applying voltages to their respective electrodes 70. The supplied voltages to the electrodes 40 of the green and blue holograms 44, 46 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 44, 46 to the passive state. With the green and blue holograms 44, 46 in the passive state and the red hologram 42 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 44 is next changed to the diffractive state by deenergizing the corresponding electrodes 70 and the electrodes of the red hologram 42 are energized to change the red hologram to the passive state so that only green light is diffracted. The blue hologram 46 is then changed to the diffractive state by deenergizing its electrodes 70 and the electrodes of the green hologram 44 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 (e.g., less than 150 microseconds) 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 display device 18 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 42, 44, 46 may be cycled on and off in any order. The holographic diffraction elements 40, 42, 42 may be stacked on top of one another or spaced laterally from one another as illustrated in the first embodiment 10 shown in Fig. 1. It is also possible (with respect to the embodiments shown in Figs. 8, 9, and 10) to record the red, green, and blue interference patterns separately in a single holographic optical element.

It is to be understood that the holographic diffraction elements may be different than described herein without departing from the scope of the invention. For example, the optical system 16 may include additional holographic elements that perform optical functions other than color filtering. Also, the optical system 16 may include more than one holographic optical element configured to diffract each color wavelength band.

The light diffracted by the holographic optical elements 42, 44, 46 is reflected onto the beam deflector 56 which deflects the red, green, and blue output beams 60, 62, 64 in a common direction towards the display device 18. The beam deflector 56 preferably comprises a holographic diffraction device having three separate holographic diffraction elements, each optimized for red, green, or blue light. The elements may be switchable as previously described and switched between their active and passive states in synchronism with the switching of the holographic diffraction elements 42, 44, 46. However, since the red, green, and blue beams 60, 62, 64 are incident on the beam deflector 56 from different angles, and the holograms in the deflector are relatively insensitive to light of a given wavelength incident at a non-Bragg angle, the holographic diffraction elements of the beam deflector do not need to be switchable. Further, since the angular separation between the red, green, and blue beams 60, 62, 64 is relatively large (i.e., larger than the angular bandwidth of the Bragg holograms), the Bragg angular and wavelength selectivity will be sufficient to ensure that there is no appreciable crosstalk between the red, green, and blue wavelengths.

The beam deflector 56 is preferably also configured to correct dispersion, chromatic, and geometric aberrations created due to the holographic diffraction elements operating off-axis and over large spectral bandwidths. More particularly, the characteristics of the holographic optical elements 42, 44, 46 and the red, green, and blue holograms of the beam deflector 56 are optimized so that the dispersion introduced by the elements are compensated for by the beam deflector holograms, respectively.

Additional optical components (not shown) may also be provided at the input or output of the light guide 40 to generate desired optical characteristics in the red, green, and blue output beams 60, 62, 64.

A second embodiment of the image generating system is shown in Fig. 6 and generally indicated at 100. The image generating system 100 is similar to the image generating system 10 shown in Figs. 1 and 2 except that the reflective rear surface 52 of the light guide 40 and beam deflector 56 are replaced with dichroic filters 102, 104, 106. When holographic element 42 is in its active state, the element diffracts light 60 received from dichroic filter 30 and directs it at an angle towards filter 102. The red light beam 60 then passes straight through filters 104 and 106 and is incident on the display device 18. When the holographic diffraction element 42 is in its passive state, the light received from filter 30 passes straight through the element and misses filter 102. Similarly, when holographic diffraction element 44 is in its active state, it diffracts light received from filter 32 towards filter 104. The green light beam 62 is then reflected by filter 104 and passes through filter 106 onto display device 18. When holographic diffraction element 46 is in its active state, it diffracts light received from filter 34 towards filter 106. The blue light beam 64 is reflected by filter 106 onto the display device 18.

Fig. 7 shows a third embodiment of the image generating system of the present invention, generally indicated at 110. The third embodiment 110 is similar to the second embodiment 100 shown in Fig. 6 except that the light is diffracted in a different direction as illustrated by the insert of Fig. 7. In the second embodiment 100 the holographic diffraction elements 42, 44, 46 are positioned to diffract the light received from the respective filters 30, 32, 34 in a plane containing the drawing. In the third embodiment 110 the light is diffracted out of the plane containing the drawing (see insert of Fig. 7). The dichroic filters 112, 114, 116 are inclined in the plane containing the drawing and at an angle to that plane, such that light reflected thereby is directed towards the display device 18 in a direction generally parallel to the direction in which the light is emitted from the light source 12 and collimation optics 14 onto the filters 30, 32, 34.

The beam deflector 56 is not required for the second and third embodiments 100, 110 since the output beams 60, 62, 64 are directed towards the display device 18 by the filters 102, 104, 106 and 112, 114, 116, respectively. The output beams 60, 62, 64 are preferably supplied to light diffusion or light integration devices (not shown), to eliminate the need for correction of dispersion aberrations introduced by the holographic optical elements 42, 44, 46. In the previous embodiments, the beam deflector 56 corrected for dispersion abberations introduced by the elements 42, 44, 46.

A fourth embodiment of the image generating system is shown in Fig. 8 and generally indicated at 130. In this embodiment 130, the dichroic filters 30, 32, 34 are optically disposed between the holographic optical elements 42, 44, 46 and the beam deflector 56, such that the filters deflect light received from the elements onto the beam deflector. This allows the holographic optical elements 42, 44, 46 to be stacked upon one another such that either a front or back surface of each of the elements is adjacent to a front or back surface of another element. The dichroic filters 30, 32, 34 may similarly be stacked upon one another and positioned along the rear surface 52 of the light guide 140. As previously described for the first three embodiments 10, 100, 110, controller 50 switches the holographic optical elements 42, 44, 46 into their active states sequentially and in synchronism with the displaying of the monochrome images on the display device. When the elements 42, 44, 46 are in their passive states, the respective color components of the white light incident thereon passes through undeflected and is lost through the rear face 52 of the light guide 140. Holographic optical element 42 is switched to its active state while elements 44 and 46 are in their passive states. The red component of the light source is diffracted by the hologram of element 42 at an angle and directed towards the stack of dichroic filters 30, 32, 34. The light is filtered by reflection from the filter 30 and directed towards the beam deflector 56 which directs the red light beam towards the display device 18. The holographic optical elements 44, 46 are then sequentially switched to their active states to deflect the green and blue light beams, respectively, onto the display device 18. The spectral bandwidth of each of the dichroic filters 30, 32, 34 is preferably significantly less than that of the light diffracted by the respective holographic diffraction elements 42, 44, 46.

The transmission holograms described above are sensitive to the polarization state of incident light and exhibit maximum diffraction efficiency for p-polarized light, with the response to s-polarized light being around 1% for that of p-polarized light. In order to make use of the full output of the light source 12, the optical system 16 may include optical filters which make use of both the p-polarized light and s-polarized light, such as disclosed in U.S. Patent Application Serial Number 09/478,150, filed January 5, 2000, (Attorney Reference No. 5454-00700/RDP029) which is incorporated herein by reference in its entirety. For example, pairs of holographic diffraction elements may be used with one element in the pair acting on the p-polarized component and the other acting on the s-polarized components. This may be achieved either by interposing a polarization rotator between the elements in the pair or by arranging for the interference fringes in the elements of each pair to be mutually crossed. If reflection holograms are used, these additional provisions are not required since reflection holograms only start to become polarization sensitive at large angles of incidence, typically much greater than 45 degrees.

Figs. 9 and 10 illustrate optical system 116 of the fourth embodiment 130 modified to enable both p-polarized components and s-polarized components of the incident light to be utilized. Fig. 10 is a plan view of the optical filter system 116 shown in Figs. 8 and 9. The light from light source 12 and collimation optics 14 is incident upon a polarizing beam splitter 152 (Figs. 8, 9, and 10). The s-polarized component of this incident light passes straight through an inclined polarizing beam splitter surface 154 and is incident upon the holographic optical elements 42, 44, 46 after passing through a plane light transmitting plate 156. The s-polarized component of the incident light is deflected by the beam splitter surface 154 to a second beam splitter 160 and is reflected by an inclined beam splitting surface 162 thereof. The second beam splitter 160 does not need to be polarizing. The s-polarized light then passes through a polarization rotor 164 (e.g., half wave plate) and is incident as p-polarized light upon the holographic optical elements 42, 44, 46 at a location that is laterally spaced from the p-polarized light emitted from the first beam splitter 152. The two beams of p-polarized light are then processed by the optical system 116 as described above. The refractive index of the plate 156 is preferably matched to that of the polarization rotator 164. A diffuser (not shown) may be provided to mask the line where these two components meet.

Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations made to the embodiments without departing from the scope of the present invention. Accordingly, 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

CLAIMSWHAT IS CLAIMED IS:

1. A system for illuminating a display device, the system comprising:

a plurality of optical filters each configured to reflect a light beam having a specified wavelength range; and

a plurality of holographic optical elements each disposed on a common optical path with one of the filters and switchable between an active state wherein light incident on the element is diffracted and a passive state wherein light incident on the element is transmitted without substantial alteration;

wherein each of the holographic optical elements when in its active state, in combination with the filter on the common optical path therewith are operable to receive polychromatic light and produce the light beams to illuminate the display device.

2. The system of claim 1 further comprising a holographic optical element controller operable to sequentially switch the elements between their active and passive states.

3. The system of claim 2 further comprising the display device operable to display monochrome images and a display controller operable to switch between different monochrome images in synchronization with switching of the holographic optical elements between their active and passive states.

4. The system of claim 1 wherein the optical filters are dichroic filters.

5. The system of claim 1 further comprising a polychromatic light source and a collimating lens operable to collimate light generated by the light source.

6. The system of claim 1 wherein the light source is operable to generate red, green, and blue light beams.

7. The system of claim 1 wherein the holographic optical elements comprises a hologram interposed between two electrode layers operable to apply an electrical field to the hologram.

8. The system of claim 7 wherein the hologram is formed from a polymer and liquid crystal material.

9. The system of claim 1 wherein said plurality of holographic optical elements comprises three holographic optical elements.

10. The system of claim 9 wherein the three holographic optical elements each have a hologram recorded therein which is optimized to diffract red, green, or blue light.

1 1. The system of claim 10 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.

12. The system of claim 1 1 further comprising the display device.

13. The system of claim 11 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.

14. The system of claim 1 further comprising a beam deflector operable to receive and deflect the light beams in a common direction.

15. The system of claim 1 wherein the beam deflector comprises a plurality of holographic optical elements.

16. The system of claim 15 wherein said plurality of holographic optical elements comprises three holographic optical elements each having a hologram recorded therein that is optimized to diffract red, green, or blue light.

17. The system of claim 1 further comprising a plurality of filters each positioned to receive light beams from one of the holographic optical elements and deflect the beams in a common direction.

18. The system of claim 17 wherein the filters are dichroic filters.

19. The system of claim 1 wherein the holographic optical elements are aligned such that a front face of each of the elements is located generally in a common plane with front faces of the other elements.

20. The system of claim 1 wherein the holographic optical elements are stacked together such that at least one of a front surface and a back surface of each of the elements is adjacent another one of the elements.

21. The system of claim 1 wherein each of the filters is positioned to receive light from a light source and reflect the light beam onto holographic optical element located on the common optical path.

22. The system of claim 1 wherein each of the holographic optical elements is positioned to receive light from the light source and reflect the light beam onto the corresponding filter located on the common optical path.

23. The system of claim 22 wherein the holographic optical elements are stacked together such that at least one of a front surface and a back surface of the element is adjacent to the front or back surface of another one of the elements.

24. The system of claim 22 wherein the filters are stacked together such that at least one of a front surface and a back surface of the filter is adjacent to the front or back surface of one of the other filters.

25. The system of claim 1 further comprising:

a beam splitter operable to split light incident thereon into first and second beams of light having orthogonal planes of polarization, the beam splitter configured to deflect the first beam onto a front surface of one of the holographic optical elements and pass the second beam without substantial deflection; and

a polarization rotator positioned to receive the second beam and operable to change the plane of polarization of the beam and deflect the beam onto one of the holographic optical elements.

26. A system for illuminating a display device, comprising:

a plurality of optical filters positioned to receive light from a polychromatic light source and configured to reflect a light beam having a specified wavelength range;

a plurality of holographic optical elements positioned to receive said light beams from said plurality of filters and switchable between an active state wherein light incident on the element is diffracted and a passive state wherein light incident on the element is transmitted without substantial alteration;

a controller operable to switch the holographic optical elements between their active and passive states; and

a beam deflector operable to receive and deflect the light beams onto a display device to illuminate the display device.

27. An image generating system comprising:

a display device operable to display monochrome images;

a display image controller coupled to the display device and operable to switch the monochrome images displayed by the display device;

a plurality of optical filters each configured to reflect a light beam having a specified wavelength range;

a plurality of holographic optical elements each disposed on a common optical path with one of the filters and switchable between an active state wherein light incident on the element is diffracted and a passive state wherein light incident on the element is transmitted without substantial alteration; and

a holographic element controller operable to sequentially switch the elements between their active and passive states generally in synchronism with the display image controller switching between the monochrome images.