WO2000007061A1

WO2000007061A1 - Three dimensional projection systems based on switchable holographic optics

Info

Publication number: WO2000007061A1
Application number: PCT/US1999/017343
Authority: WO
Inventors: Milan M. Popovich; Jonathan D. Waldern
Original assignee: Digilens, Inc.
Priority date: 1998-07-29
Filing date: 1999-07-28
Publication date: 2000-02-10
Also published as: AU5246999A

Abstract

Switchable holographic optical elements (HOEs) (320) that can be turned on and off are used in systems and methods for projecting 3-D images, or for projecting 2-D tiled images with increased size and/or resolution. In one embodiment, the method projects monochrome 3-D images through the steps of (a) displaying, at an object plane (310), a 2-D image (306; 307; 308) that is a cross section of a 3-D image, (b) activating an HOE (302) to focus the 2-D image onto an appropriate image plane (331; 332; 333; 334), and (c) repeating steps (a)-(b) for different cross sections of the 3-D image. Each of the cross sections is focussed onto a different image plane, so that the resulting images appear in an 'image volume' (330) as a 3-D image. Steps (a)-(c) are then rapidly repeated at a fast rate (such as 60 frames per second) to create a continuously displayed 3-D image (335). The 3-D image may be a static image or a moving image. In another embodiment, the method includes steps of sequentially displaying 2-D cross-sectional images, and, in synchronization, sequentially activating switchable HOEs that image the cross sections onto different image planes. Also described is a 3-D projection system that includes (a) a 2-D display configured to sequentially display a series of cross sections and (b) an HOE configured to focus the 2-D display onto a series of image planes at different distances from the 2-D display. The switchable HOEs are also used in an image projection system that projects an image composed of an array of 'tiles'.

Description

TITLE: THREE DIMENSIONAL PROJECTION SYSTEMS BASED ON SWITCHABLE HOLOGRAPHIC OPTICS

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates generally to image projection systems, and particularly to three-dimensional projection systems

Description of the Related Art

Video display and projection systems are ubiquitous devices with applications in many settings Simple TV's are a major tool m the communication of images for entertainment, news, and advertising

Computer monitors are an indispensable tool for displaying text, images, video, and other graphics From the simplest black and white television to the most advanced computer-graphics monitors, however, video displays have hitherto been inherently two-dimensional devices They display information on a flat surface, with any three-dimensional structure compressed into a single plane of view Efforts have been made on a number of fronts to present 3-D structures on 2-D media One class of techniques mvolves stereo imaging, which uses the binocular aspect of the human visual system to simulate 3-D imagery In these techniques, the viewer's eyes are presented with two separate 2-D views of a 3-D object Each eye is provided with the picture that would been seen from a point m space where that eye would be located while viewing the 3-D object When the viewer's eyes align to merge the separate images into a single image, parallax effects create the illusion of an image with depth

3-D motion pictures are a common application of stereo imaging In this application, two images are simultaneously projected onto a single movie screen using polarized light The polarizations used in generating the two images are orthogonal to each other When viewed directly, the screen shows a blurred combination of two separate images When viewed through polarized "3-D glasses," however, each of the viewer's eyes sees only one of the two images The viewer can then naturally combme the two images into a single fused 3-D image

Stereo imaging is also used in "random-dot stereograms " These are pictures that initially appear to be a random collection of dots When a viewer appropriately aligns his eyes, two separate collections of dots m the stereogram overlap, fusing into a single 3-D image Another technique used to present 3-D images is to combine several views of the same 3-D object into parallel stπps on a single flat picture, and then to cover the picture with a lenticular screen so that each of the views is only visible when the picture is seen from a particular angle While this technique does not rely on binocular vision, it is similar to stereo imaging in that the number of different views available to an observer is strongly limited The observer can only view the 3-D object from those angles corresponding to the views that were combmed into the single flat picture The lenticular screen technique has been used in cameras that take 3-

D photographs and in the production of "holographic" images on trading cards and other novelty items

A simpler approach to displaying 3-D objects has been to display them as flat images with visual cues that suggest the solid nature of the objects This technique generally starts with wire-grid drawings that illustrate the principal vertices, edges, and surfaces of a 3-D object The illusion of depth is created by providing visual cues such as perspective elements are drawn smaller when they are closer to a "vanishing point" in the image The objects may then be rendered into solid-looking structures by adding color and shading to the unhidden surfaces Because of the small amount of information required to render a wire-gπd drawing. this technique has found widespread use in computer animation Nonetheless, it is inherently a 2-D technique the position of the viewer's eye does not affect what is seen by the viewer

True holograms are lnterferometπc recordmgs of three-dimensional objects onto a holographic recording medium Unlike standard photographs, which record only the intensity of incident light, holograms record the phase as well as the intensity of the light that illuminates them When appropriately illuminated by coherent light scattered from a three-dimensional object, a hologram records the wave fronts created by the object During subsequent viewing, the hologram is appropπately illuminated by a light source It functions as a diffraction grating, scattering the light m various directions The hologram scatters the light in such a way as to recreate the originally recorded wavefronts The result is a real or virtual image of the original three- dimensional object Unlike the images generated by stereo images, lenticular stereographic images, wire-grid drawings, and other 2-D images, holograms provide a 3-D image that can be mspected from a number of different viewpoints selected by the observer

Holograms tend to be largely monochromatic, smce the interference effects used to record and display a hologram are highly wavelength-specific They are also limited to the recording of static objects Thus, holograms have not found widespread practical applications in video display technology

Instead of recording a particular three-dimensional object, a hologram may be used as a diffraction grating that reproduces the effects of a particular optical element, such as a lens or a mirror These "holographic optical elements" (HOEs) may be far easier and less expensive to produce than then glass counterparts, especially when the optical element is complicated or must meet stringent tolerances HOEs may generally be employed m any place where the corresponding glass optical element is used HOEs have found applications in video projection systems, where large-dimension (wide) optical elements are required They take the place of large lenses and other beam-shaping elements that can be expensive to produce in glass

SUMMARY OF THE INVENTION Described herein are systems and methods for projecting images that use switchable holographic optical elements (HOEs) These systems and methods can be used to project three-dimensional images or to project two-dimensional tiled images with increased size and/or resolution

In one embodiment, a method for projectmg three-dimensional images includes steps of (a) displaying, in an object plane, a two-dimensional image that is a cross-section of a three-dimensional image, (b) activating a switchable holographic optical element and focussmg the two dimensional image onto an image plane with the activated switchable HOE, (c) deactivating the switchable HOE, (d) displaying a second two-dimensional image (which is a different cross-section of the 3-D image) at the object plane, and (e) activatmg a second switchable HOE and focussmg the second two dimensional image onto a second image plane with the activated second switchable HOE The second image plane is adjacent to the first image plane In another embodiment, the method for projectmg three-dimensional images includes (a) displaying, at an object plane, a two-dimensional image that is a cross-section of a three-dimensional image, (b) activatmg a switchable holographic optical element (HOE) to focus one color component of the two dimensional image onto an appropπate image plane, and (c) repeatmg steps (a)-(b) for different color components of the two dimensional image (such as red, green, and blue color components) and for different cross-sections of the three- dimensional image Each of the cross sections is focussed onto a different image plane, so that the resultmg images are not limited to a flat image plane, but rather, appear in an "image volume" as a 3-D image Steps (a)- (c) are then repeated to create a continuously displayed 3-D image

Step (c), in which the various color components are projected onto the various image planes, is performed quickly, within a time less than the integration tune of the human eye This allows the different projected images to be perceived as a smgle 3-D image Each frame - a complete cycle through the various color components and image planes - is preferably completed in a tune interval short enough to allow frame rates of 25, 30, 50, 60, 70, 72, 75, or more frames per second

Each frame may reproduce the same 3-D image, resulting m a static 3-D image In a preferred embodiment, the images change from frame to frame, allowing motion in the projected image In one embodmient, the method projects monochrome 3-D images through the steps of (a) displaying, at an object plane, a two-dimensional image that is a cross-section of a three-dimensional image, (b) activatmg a switchable HOE to focus the two dimensional image onto an appropnate image plane, and (c) repeating steps (a)-(b) for different cross-sections of the three-dimensional image Each of the cross sections is focussed onto a different image plane Steps (a)-(c) are then repeated to create a continuously displayed 3-D image In another embodiment, the method includes steps of sequentially displaying two-dimensional images

(which are cross-sections of a three-dimensional image) at an object plane, and, m synchronization, sequentially activating switchable HOEs that image the object plane onto a series of spatially separated image planes

Also described is a three-dimensional projection system This system includes a two-dimensional display configured to sequentially display a seπes of cross-sections of a three-dimensional image, and a switchable HOE configured to focus the two-dimensional display onto a sequence of image planes at a plurality of distances from the two-dimensional display The two-dimensional display is preferably a reflective LCD display, and the switchable HOE is preferably configured to sequentially focus a seπes of three colors for each image plane In one embodiment, the system also includes a combmer lens configured to direct beams from the switchable HOE onto overlapping positions on the image planes The switchable HOE preferably includes a senes of switchable HOEs, each of which can be made to either diffract light (m an "on" state) or to transmit light (in an "off state) By appropπately switching the lenses on and off, the switchable HOE can be controlled to focus the two-dimensional display onto an appropnate object plane The switchable HOEs may be arranged m parallel on a common optic axis Alternatively, they may be arranged side-by-side on a smgle plane A preferred embodmient of the projection system uses a combmation of these aπangements three sheets of HOEs are arranged m a stack, with each sheet capable of diffracting a single color of light Each sheet includes an array of HOEs with varying focal lengths The sheets are aligned m the stack so that each lens element for a given focal length on one sheet is coaxially aligned with lens element for the same focal length on the other two sheets

The switchable HOEs are also used in image projection method and system that projects an image composed of an aπay of "tiles " Each tile is a smgle image displayed on a display device, such as a reflective

LCD display The display device is configured to sequentially display tile elements of the image The image projection system also includes a switchable HOE configured to focus the two-dimensional display onto a sequence of positions on an image plane The display device and the switchable HOE are synchronized so that tile elements are focussed onto their appropriate corresponding array positions m the image plane The system sequences through the plurality of tile elements rapidly, displaying an entire image in less than 100 ms Thus, the displayed image appears as a single, continuous image BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the mvention will become apparent upon reading the following detailed description and upon reference to the accompanying drawings m which FIG 1 is a cross-sectional view of an electrically switchable hologram made of an exposed polymer- dispersed liquid crystal (PDLC) matenal, FIG 2 is a graph of the normalized net transmittance and normalized net diffraction efficiency of a hologram (without the addition of a surfactant) versus the rms voltage applied across the hologram, FIG 3 is a graph of both the threshold and complete switching rms voltages needed for switching a hologram to minimum diffraction efficiency versus the frequency of the rms voltage, FIG 4 is a graph of the normalized diffraction efficiency as a function of the applied electπc field for a PDLC matenal formed with 34% by weight liquid crystal surfactant present and a PDLC material formed with 29% by weight liquid crystal and 4% by weight surfactant, FIG 5 is a graph showmg the switchmg response time data for the diffracted beam m the surfactant- containing PDLC matenal in FIG 4, FIG 6 is a graph of the normalized net transmittance and the normalized net diffraction efficiency of a hologram, FIG 7 is an elevational view of typical expenmental aπangement for recording reflection gratings, FIG 8a and FIG 8b are elevational views of a reflection gratmg having peπodic planes of polymer channels and PDLC channels disposed parallel to the front surface m the absence of a field

(FIG 8a) and with an electric field applied (FIG 8b) wherem the liquid-crystal utilized m the formation of the gratmg has a positive dielectric amsotropy,

FIG 9a and FIG 9b are elevational views of a reflection gratmg havmg peπodic planes of polymer channels and PDLC channels disposed parallel to the front surface of the gratmg m the absence of an electnc field (FIG 9a) and with an electric field applied (FIG 9b) wherem the liquid crystal utilized in the formation of the gratmg has a negative dielectric amsotropy, FIG 10a is an elevational view of a reflection grating disposed withm a magnetic field generated by Helmholtz coils, FIG 10b and FIG 10c are elevational views of the reflection gratmg of FIG 10a in the absence of an electnc field (FIG 10b) and with an electnc field applied (FIG 10c), FIG 11a and FIG l ib are representative side views of a slanted transmission gratmg (FIG 11a) and a slanted reflection grating (FIG 1 lb) showing the orientation of the grating vector G of the periodic planes of polymer channels and PDLC channels, FIG 12 is an elevational view of a reflection gratmg when a shear stress field is applied thereto,

FIG 13 is an elevational view of a subwavelength grating having periodic planes of polymer channels and PDLC channels disposed perpendicular to the front surface of the gratmg, FIG 14a is an elevational view of a switchable subwavelength wherem the subwavelength gratmg functions as a half wave plate whereby the polarization of the incident radiation is rotated by 90 : , FIG 14b is an elevational view of the switchable half wave plate shown m FIG 14a disposed between crossed polarizers whereby the mcident light is transmitted, FIG 14c and FIG 14d are side views of the switchable half wave plate and crossed polarizes shown in FIG 14b and showing the effect of the application of a voltage to the plate whereby the polarization of the light is no longer rotated and thus blocked by the second polarizer,

FIG 15a is a side view of a switchable subwavelength grating wherem the subwavelength grating functions as a quarter wave plate whereby plane polaπzed light is transmitted through the subwavelength gratmg, retroreflected by a minor and reflected by the beam splitter,

FIG 15b is a side view of the switchable subwavelength gratmg of FIG 15a and showmg the effect of the application of a voltage to the plate whereby the polarization of the light is no longer modified, thereby permitting the reflected light to pass through he beam splitter,

FIG 16a and FIG 16b are elevational views of a transmission grating havmg peπodic planes of polymer channels and PDLC channels disposed perpendicular to the front face of the gratmg m the absence of an electπc field (FIG 16a) and with an electric field applied (FIG 16b) wherem the liquid crystal utilized m formation of the gratmg has a positive dielectric amsotropy, and FIG 17 is a side view of five subwavelength gratings wherem the gratmgs are stacked and connected electrically m parallel thereby reducing the switchmg voltage of the subwavelength gratmg FIG 18 shows a system for projectmg 3-D objects that uses switchable holographic elements, FIG 19 shows an embodiment of the system from FIG 18 that uses a stack of holographic elements that can be selectively made transparent, FIG 20 shows another embodiment of the system that uses an anay of switchable holographic optical elements, FIG 21 shows a system for projecting a tiled-array image onto a screen, and FIG 22 shows a momtormg system for enhancing the portion of an image in the center of a user's field of view

While the mvention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example m the drawings and will herem be described in detail It should be understood, however, that the drawing and detailed description thereto are not intended to limit the mvention to the particular form disclosed, but on the contrary, the mtention is to cover all modifications, equivalents and alternatives falling within the spmt and scope of the present mvention as defined by the claims set forth below

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The introduction of switchable (or "reconfigurable") holographic optical elements (HOEs) enables the construction of several types of video projection systems These systems may be configured to project true 3-D images They may also be configured to project large 2-D images by projectmg a seπes of tiles that make up the large image In one embodiment, the switchable HOEs can be switched by an applied electric field from a state in which they diffract light to a state in which they merely transmit light without substantial alteration In other embodiments, an optical system of several switchable HOEs may be switched between a number of states m which the optical system operates as one of several distinct optical elements Figs. 1-17: Switchable Hologram Materials And Devices

Holographic optical elements are formed, m one embodiment, from a polymer dispersed liquid crystal (PDLC) matenal compnsmg a monomer, a dispersed liquid crystal, a cross-linking monomer, a coimtiator and a photoimtiator dye These PDLC mateπals exhibit clear and orderly separation of the liquid crystal and cured polymer, whereby the PDLC matenal advantageously provides high quality optical elements The PDLC matenals used in the holographic optical elements may be formed in a single step The holographic optical elements may also use a unique photopolymenzable prepolymer material that permits in situ control over characteristics of resulting gratmgs, such as domam size, shape, density, ordeπng and the like Furthermore, methods and materials taught herem can be used to prepare PDLC materials for optical elements compnsmg switchable transmission or reflection type holographic gratmgs

Polymer dispersed liquid crystal mateπals, methods, and devices contemplated for use in the present mvention are also descnbed m R L Sutherland et al., "Bragg Gratmgs m an Acrylate Polymer Consistmg of Penodic Polymer dispersed Liquid-Crystal Planes, " Chemistry of Materials, No 5, pp. 1533-1538 (1993), m R L Sutherland et al , "Electrically switchable volume gratings in polymer dispersed liquid crystals," Applied Physics Letters, Vol 64, No 9, pp 1074-1076 (1994), and T J Bunmng et al , "The Morphology and

Performance of Holographic Transmission Gratings Recorded m Polymer dispersed Liquid Crystals," Polymer, Vol. 36, No 14, pp 2699-2708 (1995), all of which are fully incorporated by reference mto this Detailed Descπption U.S. Patent application Seπal Nos. 08/273, 436 and U.S. Patent 5,698,343 to Sutherland et al, titled "Switchable Volume Hologram Materials and Devices," and "Laser Wavelength Detection and Energy Dosrmetry Badge," respectively, are also incorporated by reference and include background matenal on the formation of transmission gratmgs mside volume holograms

In one embodiment, the process of forming a hologram is controlled primarily by the choice of components used to prepare the homogeneous starting mixture, and to a lesser extent by the intensity of the mcident light pattern In one embodiment, the polymer dispersed liquid crystal (PDLC) matenal employed m the present mvention creates a switchable hologram in a smgle step A feature of one embodiment of PDLC matenal is that illumination by an inhomogeneous, coherent light pattern initiates a patterned, amsotropic diffusion (or counter diffusion) of polymenzable monomer and second phase matenal, particularly liquid crystal (LC) Thus, alternating well-defined channels of second phase-rich material, separated by well-defined channels of a nearly pure polymer, can be produced in a single-stop process The resultmg PDLC matenal may have an amsotropic spatial distribution of phase-separated LC droplets withm the photochemically cured polymer matrix Prior art PDLC materials made by a smgle-step process can achieve at best only regions of larger LC bubbles and smaller LC bubbles m a polymer matnx The large bubble sizes are highly scattering which produces a hazy appearance and multiple ordering diffractions, in contrast to the well-defined first order diffraction and zero order diffraction made possible by the small LC bubbles of one embodiment of PDLC material in well-defined channels of LC-πch matenal. Reasonably well- defined alternately LC-πch channels and nearly pure polymer channels in a PDLC matenal are possible by multi-step processes, but such processes do not achieve the precise morphology control over LC droplet size and distribution of sizes and widths of the polymer and LC-πch channels made possible by one embodmient of PDLC matenal The same may be prepared by coatmg the mixture between two indium-tin-oxide (ITO) coated glass slides separated by spacers of nominally 10-20 μm thickness The sample is placed in a conventional holographic recording setup Gratmgs are typically recorded using the 488 nm line of an argon ion laser with intensities of between about 0 1-100 mW/cm² and typical exposure times of 30-120 seconds The angle between the two beams is varied to vary the spacing of the intensity peaks, and hence the resulting grating spacing of the hologram Photopolymenzation is induced by the optical intensity pattern A more detailed discussion of exemplary recording apparatus can be found m R L Sutherland, et al , "Switchable holograms m new photopolymer-hquid crystal composite mateπals," Society of Photo-Optical Instrumentation Engmeers (SPIE), Proceedings Reprint, Volume 2402, reprinted from Diffractive and Holographic Optics Technology II (1995), incorporated herein by reference

The features of the PDLC material are influenced by the components used in the preparation of the homogeneous starting mixture and, to a lesser extent, by the intensity of the mcident light pattern In one embodiment, the prepolymer matenal comprises a mixture of a photopolymenzable monomer, a second phase matenal, a photoinitiator dye, a co nitiator, a cham extender (or cross-linker), and, optionally, a surfactant

In one embodiment, two major components of the prepolymer mixture are the polymeπzable monomer and the second phase material, which are preferably completely miscible Highly functionalized monomers may be preferred because they form densely cross-linked networks which shrink to some extent and to tend to squeeze out the second phase matenal As a result, the second phase matenal is moved amsotropically out of the polymer region and, thereby, separated mto well-defined polymer-poor, second phase-rich regions or domams Highly functionalized monomers may also be prefened because the extensive cross-linking associated with such monomers yields fast kmetics, allowing the hologram to form relatively quickly, whereby the second phase matenal will exist in domains of less than approximately 0 1 μm

Highly functionalized monomers, however, are relatively viscous As a result, these monomers do not tend to mix well with other materials, and they are difficult to spread into thm films Accordmgly, it is preferable to utilize a mixture of penta-acrylates m combination with di-, tn-, and/or tetra-acrylates in order to optimize both the functionality and viscosity of the prepolymer matenal Suitable acrylates, such as tnethyleneglycol diacrylate, tπmethylolpropane tπacrylate, pentaerythntol tnacrylate, pentaerythntol tetracrylate, pentaerythntol pentacrylate, and the like can be used In one embodmient, it has been found that an approximately 1 4 mixture of tn- to penta-acrylate facilitates homogeneous mixing while providmg a favorable mixture for forming 10-20 μm films on the optical plates

The second phase material of choice is a liquid crystal (LC) This also allows an electro-optical response for the resulting hologram The concentration of LC employed should be large enough to allow a significant phase separation to occur in the cured sample, but not so large as to make the sample opaque or very hazy Below about 20% by weight very little phase separation occurs and diffraction efficiencies are low Above about 35% by weight, the sample becomes highly scattering, reducmg both diffraction efficiency and transmission Samples fabricated with approximately 25% by weight typically yield good diffraction efficiency and optical clarity In prepolymer mixtures utilizing a surfactant, the concentration of LC may be mcreased to

35% by weight without loss in optical performance by adjusting the quantity of surfactant Suitable liquid crystals contemplated for use in the practice of the present invention may include the mixture of cyanobiphenyls marketed as E7 by Merck, 4'-n-pentyl-4-cyanobιphenyl, 4'-n-heptyl-4-cyanobιphenyl, 4'-octaoxy-4- cyanobiphenyl, 4'-pentyl-4-cyanoterphenyl, cc-methoxybenzylιdene-4'-butylanιhne, and the like Other second phase components are also possible The polymer dispersed liquid crystal material employed in the practice of the present invention may be formed from a prepolymer material that is a homogeneous mixture of a polymenzable monomer compnsmg dipentaerythntol hydroxypentacrylate (available, for example, from Polysciences, Inc , Warnngton, Pennsylvania), approximately 10-40 wt% of the liquid crystal E7 (which is a mixture of cyanobiphenyls marketed as E7 by Merck and also available from BDH Chemicals, Ltd , London, England), the cham-extendmg monomer N-vmylp-yrrohdinone ("NVP") (available from the Aldnch Chemical Company, Milwaukee, Wisconsm), coinitiator N-phenylglycine ("NPG") (also available from the Aldrich Chemical Company, Milwaukee, Wisconsin), and the photomitiator dye rose bengal ester, (2,4,5,7-tetraιodo-3',4',5',6'- tetrachlorofluoresceιn-6-acetate ester) marketed as RBAX by Spectragraph, Ltd , Maumee, Ohio) Rose bengal is also available as rose bengal sodium salt (which must be esteπfied for solubility) from the Aldrich Chemical

Company This system has a very fast curmg speed which results in the formation of small liquid crystal micro- droplets

The mixture of liquid crystal and prepolymer matenal are homogenized to a viscous solution by suitable means (e g , ultrasomfication) and spread between indium-tin-oxide (ITO) coated glass sides with spacers of nominally 15-100 μm thickness and, preferably, 10-20 μm thickness The ITO is electrically conductive and serves as an optically transparent electrode Preparation, mixing and transfer of the prepolymer matenal onto the glass slides are preferably done m the dark as the mixture is extremely sensitive to light

The sensitivity of the prepolymer materials to light intensity is dependent on the photomitiator dye and its concentration A higher dye concentration leads to a higher sensitivity In most cases, however, the solubility of the photomitiator dye limits the concentration of the dye and, thus, the sensitivity of the prepolymer matenal Nevertheless, it has been found that for more general applications, photomitiator dye concentrations in the range of 0 2-0 4% by weight are sufficient to achieve desirable sensitivities and allow for a complete bleachmg of the dye m the recordmg process, resultmg in colorless final samples Photomitiator dyes that may be useful m generatmg PDLC matenals are rose bengal ester (2,4,5,7-tetraιodo-3',4',5',6'- tetrachlorofluoresceιn-6-acetate ester), rose bengal sodium salt, eosm, eosm sodium salt, 4,5-dnodosuccinyl fluorescein, camphorqumone, methylene blue, and the like These dyes allow a sensitivity to recordmg wavelengths across the visible spectrum from nominally 400 nm to 700 nm Suitable near-infrared dyes, such as cationic cyanme dyes with tnalkylborate amons havmg absorption from 600-900 nm as well as merocyanme dyes derived from spiropyran may also find utility in the present mvention The coinitiator employed m the formulation of the hologram controls the rate of curing m the free radical polymerization reaction of the prepolymer matenal Optimum phase separation and, thus, optimum diffraction efficiency m the resulting PDLC material, are a function of curmg rate It has been found that favorable results can be achieved utilizing coinitiator in the range of 2-3% by weight Suitable comitiators include N-phenylglycine, tnefhyl amme, tπefhanolamine, N,N-dιmefhyl-2,6-dnsopropyl aniline, and the like Other suitable dyes and dye coinitiator combmations that may be suitable for use m producmg holographic optical elements, particularly for visible light, mclude eosm and tnethanolamme, camphorqumone and N-phenylglycme, fluorescein and tnethanolamme, methylene blue and tnethanolamme or N-phenylglycme, erythrosin B and tnethanolamme, mdolinocarbocyanine and tnphenyl borate, lodobenzospiropyran and triethylamine, and the like The chain extender (or cross linker) employed m creatmg holographic optical elements may help to mcrease the solubility of the components m the prepolymer material as well as increase the speed of polymerization The chain extender is preferably a smaller vinyl monomer as compared with the pentacrylate whereby it can react with the acrylate positions in the pentacrylate monomer, which are not easily accessible to neighbormg pentaacrylate monomers due to steπc hindrance Thus, reaction of the cham extender monomer with the polymer increases the propagation length of the growing polymer and results in high molecular weights It has been found that chain extender in general applications in the range of 10-18% by weight maximizes the performance in terms of diffraction efficiency In the one embodiment, it is expected that suitable chain extenders can be selected from the following N-vmylpyrrohdinone, N-vmyl pyridine, acrylomtnle, N-vinyl carbazole, and the like

It has been found that the addition of a surfactant material, namely, octanoic acid, in the prepolymer matenal lowers the switchmg voltage and also improves the diffraction efficiency In particular, the switchmg voltage for PDLC materials containmg a surfactant are significantly lower than those of a PDLC matenal made without the surfactant While not wishing to be bound by any particular theory, it is believed that these results may be attributed to the weakening of the anchoring forces between the polymer and the phase-separated LC droplets SEM studies have shown that droplet sizes in PDLC materials including surfactants are reduced to the range of 30-50nm and the distribution is more homogeneous Random scatteπng m such matenals is reduced due to the dommance of smaller droplets, thereby increasing the diffraction efficiency Thus, it is believed that the shape of the droplets becomes more sphencal in the presence of surfactant, thereby contnbunng to the decrease m switchmg voltage

For more general applications, it has been found that samples with as low as 5% by weight of surfactant exhibit a significant reduction in switchmg voltage It has also been found that, when optimizing for low switchmg voltages, the concentration of surfactant may vary up to about 10% by weight (mostly dependent on LC concentration) after which there is a large decrease m diffraction efficiency, as well as an mcrease m switchmg voltage (possibly due to a reduction m total phase separation of LC) Suitable surfactants mclude octanoic acid, heptanoic acid, hexanoic acid, dodecanoic acid, decanoic acid, and the like In samples utilizing octanoic acid as the surfactant, it has been observed that the conductivity of the sample is high, presumably owmg to the presence of the free carboxyl (COOH) group m the octanoic acid As a result, the sample increases in temperature when a high frequency (~2 KHz) electrical field is applied for prolonged periods of time Thus, it is desirable to reduce the high conductivity introduced by the surfactant, without sacrificing the high diffraction efficiency and the low switching voltages It has been found that suitable electrically switchable gratmgs can be formed from a polymenzable monomer, vmyl neononanoate

("VN")

commercially available from the Aldπch Chemical Co m Milwaukee, Wisconsin Favorable results have also been obtained where the cham extender N-vinylpyrrolidinone ("NVP") and the surfactant octanoic acid are replaced by 6 5% by weight VN VN also acts as a chain extender due to the presence of the reactive acrylate monomer group In these variations, high optical quality samples were obtamed with about 70% diffraction efficiency, and the resulting gratings could be electrically switched by an applied field of 6V/ m

PDLC mateπals may also be formed usmg a liquid crystalline bifunctional acrylate as the monomer ("LC monomer") LC monomers have an advantage over conventional acrylate monomers due to then high compatibility with the low molecular weight nematic LC materials, thereby facilitating formation of high concentrations of low molecular weight LC and yieldmg a sample with high optical quality The presence of higher concentrations of low molecular weight LCD in the PDLC material greatly lowers the switching voltages (e g , to ~2V//jm) Another advantage of using LC monomers is that it is possible to apply low AC or DC fields while recording holograms to pre-ahgn the host LC monomers and low molecular weight LC so that a desired oπentation and configuration of the nematic directors can be obtamed m the LC droplets The chemical formulate of several suitable LC monomers are as follows

• CH₂=CH-COO-(CH₂)₆0-C₆H₅-C₆H₅-COO-CH=CH₂

• CH₂=CH-(CH₂)₈-COO-C₆H₅-COO-(CH₂)₈-CH=CH₂

• H(CF₂) ₁₀CH₂O-CH₂-C(=CH₂)-COO-(CH₂CH₂O)₃CH₂CH₂O-COO-CH₂C(=CH₂)-CH₂O(CF₂) ₁₀H

Semifluormated polymers are known to show weaker anchormg properties and also significantly reduced switchmg fields Thus, it is believed that semifluormated acrylate monomers which are bifimctional and liquid crystalline may find suitable application in the formulation of holograms

Referring now to FIG 1, there is shown a cross-sectional view of an electncally switchable hologram 10 made of an exposed polymer dispersed liquid crystal matenal made accordmg to the teachmgs of this descnption A layer 12 of the polymer dispersed liquid crystal material is sandwiched between a pair of lndium- tin-oxide coated glass slides 14 and spacers 16 The mtenor of hologram 10 shows Bragg transmission gratmgs

18 formed when layer 12 was exposed to an interference pattern from two intersecting beams of coherent laser light The exposure times and intensities can be vaπed depending on the diffraction efficiency and liquid crystal domam size desired Liquid crystal domam size can be controlled by varymg the concentrations of photomitiator, coinitiator and chain-extending (or cross-linking) agent The onentahon of the nematic directors can be controlled while the gratmgs are bemg recorded by application of an external electnc field across the ITO electrodes

The scanning electron micrograph shown in FIG 2 of the referenced Applied Physics Letters article and incorporated herem by reference is of the surface of a gratmg which was recorded m a sample with a 36 wt% loading of liquid crystal using the 488 nm lme of an argon ion laser at an intensity of 95 mW/cm^" The size of the liquid crystal domains is about 0 2 μm and the gratmg spacmg is about 0 54 μm This sample, which is approximately 20 μm thick, diffracts light m the Bragg regime

FIG 2 is a graph of the normalized net transmittance and normalized net diffraction efficiency of a hologram made accordmg to the teachmgs of his disclosure versus the root mean square voltage ("Vrms") applied across the hologram Δη is the change m first order Bragg diffraction efficiency ΔT is the change in zero order transmittance FIG 2 shows that energy is transfened from the first order beam to the zero-order beam as the voltage is increased There is a true minimum of the diffraction efficiency at approximately 225 Vrms The peak diffraction efficiency can approach 100%, dependmg on the wavelength and polarization of the probe beam, by appropπate adjustment of the sample thickness The minimum diffraction efficiency can be made to approach 0% by slight adjustment of the parameters of the PDLC material to force the refractive index of the cured polymer to be equal to the ordinary refractive index of the liquid crystal

By increasing the frequency of the applied voltage, the switching voltage for minimum diffraction efficiency can be decreased significantly This is illustrated in FIG 3, which is a graph of both the threshold rms voltage 20 and the complete switchmg rms voltage 22 needed for switchmg a hologram made accordmg to the teachmgs of this disclosure to minimum diffraction efficiency versus the frequency of the rms voltage The threshold and complete switching rms voltages are reduced to 20 Vrms and 60 Vrms, respectively, at 10 kHz Lower values are expected at even higher frequencies

Smaller liquid crystal droplet sizes have the problem that it takes high switching voltages to switch their orientation As described m the previous paragraph, using alternating current switchmg voltages at high frequencies helps reduce the needed switchmg voltage As demonstrated in FIG 4, it has been found that addmg a surfactant (e g , octanoic acid) the prepolymer material m amounts of about 4%-6% by weight of the total mixture results in sample holograms with switchmg voltages near 50Vrms at lower frequencies of 1-2 kHz As shown m FIG 5, it has also been found that the use of the surfactant with the associated reduction m droplet size, reduces the switching time of the PDLC materials Thus, samples made with surfactant can be switched on the order of 25-44 microseconds Without wishing to be bound by any theory, the surfactant is believed to reduce switching voltages by reducing the anchoπng of the liquid crystals at the interface between liquid crystal and cured polymer

Thermal control of diffraction efficiency is illustrated m FIG 5 FIG 5 is a graph of the normalized net transmittance and normalized net diffraction efficiency of a hologram made according to the teachmgs of this disclosure versus temperature

The polymer dispersed liquid crystal materials descnbed herein successfully demonstrate the utility for recordmg volume holograms of a particular composition for such polymer dispersed liquid crystal systems

As shown in FIG 7, a PDLC reflection gratmg is prepared by placmg several drops of the mixture of prepolymer material 112 on an indium- tin oxide coated glass slide 114a A second indium- tin oxide coated slide 114b is then pressed agamst the first, thereby causmg the prepolymer matenal 112 to fill the region between the slides 114a and 114b Preferably, the separation of the slides is mamtamed at approximately 20 μm by utilizing uniform spacers 118 Preparation, mixing and transfer of the prepolymer material is preferably done in the dark Once assembled, a mirror 116 may be placed directly behmd the glass plate 114b The distance of the mirror from the sample is preferably substantially shorter than the coherence length of the laser The PDLC matenal is preferably exposed to the 488 nm lme of an argon-ion laser, expanded to fill the entire plane of the glass plate, with an intensity of approximately 0 1-100 mWatts/cm² with typical exposure times of 30-120 seconds Constructive and destructive interference within the expanded beam establishes a penodic intensity profile through the thickness of the film

In one embodiment, the prepolymer material utilized to make a reflection gratmg comprises a monomer, a liquid crystal, a cross-linking monomer, a coinitiator, and a photomitiator dye The reflection gratmg may be formed from prepolymer matenal compnsmg by total weight of the monomer dipentaerythntol hydroxypentacrylate (DPHA), 35% by total weight of a liquid crystal compnsmg a mixture of cyano biphenyls (known commercially as "E7"), 10% by total weight of a cross-linking monomer compnsmg N- vmylpyrrohdmone ("NVP"), 2 5% by weight of the coinitiator N-phenylglycme ("NPG"),and 10⁵ to 10^"6 gram moles of a photomitiator dye compnsmg rose bengal ester Further, as with transmission gratmgs, the addition of surfactants is expected to facilitate the same advantageous properties discussed above m connection with transmission gratings It is also expected that similar ranges and variation of prepolymer starting material will find ready application in the formation of suitable reflection gratings

It has been determined by low voltage, high resolution scanning electron microscopy ("LVHRSEM") that the resultmg material comprises a fine gratmg with a periodicity of 165 nm with the gratmg vector perpendicular to the plane of the surface Thus as shown schematically m FIG 8a, gratmg 130 includes periodic planes of polymer channels 130a and PDLC channels 130b which run parallel to the front surface 134 The grating spacing associated with these periodic planes remains relatively constant throughout the full thickness of the sample from the air/film to the film/substrate interface

Although interference is used to prepare both transmission and reflection gratings, the morphology of the reflection grating differs significantly In particular, it has been determined that, unlike transmission gratings with similar liquid crystal concentrations, very little coalescence of individual droplets was evident Further more, the droplets that were present m the matenal were significantly smaller havmg diameters between 50 and 100 nm Furthermore, unlike transmission gratmgs where the liquid crystal-nch regions typically comprise less than 40% of the grating, the liquid crystal-nch component of a reflection gratmg is significantly larger Due to the much smaller periodicity associated with reflection gratings, I e , a narrower grating spacmg

(~0 2 microns), it is believed that the time difference between completion of curmg in high intensity versus low intensity regions is much smaller It is also believed that the fast polymerization, as evidenced by small droplet diameters, traps a significant percentage of the liquid crystal m the matπx duπng gelation and precludes any substantial growth of large droplets or diffusion of small droplets mto larger domams Analysis of the reflection notch m the absorbance spectrum supports the conclusion that a penodic refractive mdex modulation is disposed through the thickness of the film In PDLC matenals that are formed with the 488 nm line of an argon ion laser, the reflection notch typically has a reflection wavelength at approximately 472 nm for normal incidence and a relatively narrow bandwidth The small difference between the writing wavelength and the reflection wavelength (approximately 5%) mdicates that shrinkage of the film is not a significant problem Moreover, it has been found that the performance of such gratmgs is stable over peπods of many months

In addition to the materials utilized in the one embodmient descnbed above, it is believed that suitable PDLC matenals could be prepared utilizing monomers such as tnethyleneglycol diacrylate, tnmethylolpropanetπacrylate, pentaerythntol tnacrylate, pentaerythntol tetracrylate, pentaerythntol pentacrylate, and the like Similarly, other coinitiators such as tnethylamine, tnethanolamme, N.N-dunethyl-

2,6-dnsoρropylanιlιne, and the like could be used instead of N-phenylglycme Where it is desirable to use the 458 nm, 476 nm, 488 nm or 514 nm lines of an argon ion laser, that the photomitiator dyes rose bengal sodium salt, eosm, eosm sodium salt, fluorescein sodium salt and the like will give favorable results Where the 633 nm lme is utilized, methylene blue will find ready application Finally, it is believed that other liquid crystals such as 4'-pentyl-4-cyanobιphenyl or 4'-heptyl-4-cyanobιphenyl, can be utilized

Referring again to FIG 8a, there is shown an elevational view of a reflection gratmg 130 made in accordance with this disclosure having periodic planes of polymer channels 130a and PDLC channels 130b disposed parallel to the front surface 134 of the grating 130 The symmetry axis 136 of the liquid crystal domams is formed in a direction perpendicular to the penodic channels 130a and 130b of the grating 130 and perpendicular to the front surface 134 of the gratmg 130 Thus, when an electnc field E is applied, as shown in

FIG 8b, the symmetry axis 136 is already in a low energy state m alignment with the field E and will reonent Thus, reflection gratmgs formed in accordance with the procedure described above will not normally be switchable

In general, a reflection grating tends to reflect a narrow wavelength band, such that the grating can be used as a reflection filter In one embodiment, however, the reflection gratmg is formed so that it will be switchable More particularly, switchable reflection gratmgs can be made utilizing negative dielectπc amsotropy LCs (or LCs with a low cross-over frequency), an applied magnetic field, an applied shear stress field, or slanted gratings

It is known that liquid crystals having a negative dielectric amsotropy (Δε) will rotate in a direction perpendicular to an applied field As shown in FIG 9a, the symmetry axis 136 of the liquid crystal domains formed with a liquid crystal having a negative Δε will also be disposed in a direction perpendicular to the penodic channels 130a and 130b of the grating 130 and to the front surface 135 of the gratmg However, when an electric field E is applied across such gratings, as shown m FIG 9b, the symmetry axis of the negative Δε liquid crystal will distort and reorient m a direction perpendicular to the field E, which is perpendicular to the film and the periodic planes of the grating As a result, the reflection grating can be switched between a state where it is reflective and a state where it is transmissive The following negative Δε liquid crystals and others are expected to find ready applications in the methods and devises of the present invention

1

Liquid crystals can be found m nature (or synthesized) with either positive or negative Δε Thus, it is possible to use a LC which has a positive Δε at low frequencies, but becomes negative at high frequencies The frequency (of the applied voltage) at which Δε changes sign is called the cross-over frequency The cross-over frequency will vary with LC composition, and typical values range from 1-10 kHz Thus, by operating at the proper frequency, the reflection gratmg may be switched It is expected that low crossover frequency matenals can be prepared from a combination of positive and negative dielectric amsotropy liquid crystals A suitable positive dielectric liquid crystal for use m such a combination contains four nng esters as shown below

A strongly negative dielectπc liquid crystal suitable for use such a combmation is made up of pyπdazines as shown below

Both liquid crystal mateπals are available from LaRoche & Co , Switzerland By varying the proportion of the positive and negative liquid crystals m the combination, crossover frequencies form 1 4-2 3 kHz are obtamed at room temperature Another combmation suitable for use m the present embodmient is a combmation of the following p-pentylphenyl-2-chloro-4-(p-pentylbenzoyloxy) benzoate and benzoate These materials are available from Kodak Company

In still more detailed aspects, switchable reflection gratmgs can be formed usmg positive Δε liquid crystals As shown m FIG 10a, such gratmgs are formed by exposmg the PDLC starting material to a magnetic field during the curing process The magnetic field can be generated by the use of Helmholtz coils (as shown in FIG 10a), the use of a permanent magnet, or other suitable means Preferably, the magnetic field M is oriented parallel to the front surface of the glass plates (not shown) that are used to form the grating 140 As a result, the symmetry axis 146 of the liquid crystals will orient along the field while the mixture is fluid When polymerization is complete, the field may be removed and the alignment of the symmetry axis of the liquid crystals will remam unchanged (See FIG 10b ) When an electric field is applied, as shown m FIG 10c the positive Δε liquid crystal will reorient m the direction of the field, which is perpendicular to the front surface of gratmg and to the periodic channels of the grating

FIG 11a depicts a slanted transmission gratmg 148 and FIG 1 lb depicts a slanted reflection gratmg 150 A holographic transmission gratmg is considered slanted if the direction of the grating vector G is not parallel to the grating surface In a holographic reflection gratmg, the gratmg is said to be slanted if the gratmg vector G is not perpendicular to the gratmg surface Slanted gratings have many of the same uses as nonslanted gratmg such as visual displays, minors, lme filters, optical switches, and the like

Primarily, slanted holographic gratings are used to control the direction of a diffracted beam For example, in reflection holograms a slanted gratmg is used to separate the specular reflection of the film from the diffracted beam In a PDLC holographic grating, a slanted grating has an even more useful advantage The slant allows the modulation depth of the grating to be controlled by an electric field when using either tangential or homeotropic aligned liquid crystals This is because the slant provides components of the electric field in the directions both tangent and perpendicular to the gratmg vector In particular, for the reflection gratmg, the LC domain symmetry axis will be oriented along the grating vector G and can be switched to a direction perpendicular to the film plane by a longitudinally applied field E This is the typical geometry for switching of the diffraction efficiency of the slanted reflection grating

When recordmg slanted reflection gratings, it is desirable to place the sample between the hypotenuses of two nght-angle glass prisms Neutral density filters can then be placed m optical contact with the back faces of the prisms usmg index matchmg fluids so as to frustrate back reflections which would cause spuπous gratmgs to also be recorded The mcident laser beam is split by a conventional beam splitter mto two beams which are then directed to the front faces of the pnsms, and then overlapped in the sample at the desired angle The beams thus enter the sample from opposite sides This prism couplmg technique permits the light to enter the sample at greater angles The slant of the resulting gratmg is determined by the angle which the prism assembly is rotated (l e , the angle between the direction of one incident beam an the normal to the prism front face at which that beam enters the prism)

As shown in FIG 12, switchable reflection gratmgs may be formed in the presence of an applied shear stress field In this method, a shear stress would be applied along the direction of a magnetic field M This could be accomplished, for example, by applymg equal and opposite tensions to the two ITO coated glass plates which sandwich the prepolymer mixture while the polymer is still soft This shear stress would distort the LC domains m the direction of the stress, and the resultant LC domam symmetry axis will be preferentially along the direction of the stress, parallel to the PDLC planes and perpendicular to the direction of the applied electπc field for switchmg

Reflection gratmg prepared in accordance with this descnption may find application m color reflective displays, switchable wavelength filters for laser protection, reflective optical elements and the like

In one embodiment, PDLC materials can be made that exhibit a property known as form birefringence whereby polarized light that is transmitted through the gratmg will have its polaπzation modified Such gratmgs are known as subwavelength gratings, and they behave like a negative umaxial crystal, such as calcite, potassium dihydrogen phosphate, or lithium niobate, with an optic axis perpendicular to the PDLC planes Refemng now to FIG 13, there is shown an elevational view of a transmission gratmg 200 made in accordance with this descnption having periodic planes of polymer planes 200a and PDLC planes 200b disposed perpendicular to the front surface 204 of the gratmg 200 The optic axis 206 is disposed perpendicular to polymer planes 200a and the PDLC planes 200b Each polymer plane 200a has a thickness t_p and refractive index n_p, and each PDLC plane 200b has a thickness t_{PD C} and refractive mdex n_PDI__c Where the combined thickness of the PDLC plane and the polymer plane is substantially less than an optical wavelength (i e (t_PDLc + t_p) « λ), the grating will exhibit form birefringence As discussed below, the magnitude of the shift m polarization is proportional to the length of the gratmg Thus, by carefully selecting the length, L, of the subwavelength gratmg for a given wavelength of light, one can rotate the plane of polarization or create circularly polanzed light Consequently, such subwavelength gratings can be designed to act as a half-wave or quarter-wave plate, respectively Thus, an advantage of this process is that the birefringence of the material may be controlled by simple design parameters and optimized to a particular wavelength, rather than relying on the given birefringence of any material at that wavelength

To form a half- wave plate, the retardance of the subwavelength gratmg must be equal to one-half of a wavelength, 1 e retardance = λ/2, and to form a quarter-wave plate, the retardance must be equal to one-quarter of a wavelength, 1 e retardance = λ/4 It is known that the retardance is related to the net birefringence, I Δn | , which is the difference between the ordinary index of refraction, n„, and the extraordinary index of refraction n., of the sub-wavelength grating by the following relation

Retardance = | Δn I L = | n. - n_<, | L Thus, for a half- wave plate, 1 e a retardation equal to one-half of a wavelength, the length of the subwavelength gratmg should be selected so that

L = λ / (2 I Δn I ) Similarly, for a quarter-wave plate, l e a retardance equal to one-quarter of a wavelength, the length of the subwavelength gratmg should be selected so that

L = λ / (4 I Δn I ) If, for example, the polarization of the mcident light is at an angle of 45° with respect to the optic axis

210 of a half- wave plate 212, as shown m FIG 14a, the plane polarization will be preserved, but the polarization of the wave exiting the plate will be shifted by 90° Thus, referring now to FIG 14b and 14c, where the half- wave plate 212 is placed between cross polarizers 214 and 216, the incident light will be transmitted If an appropriate switchmg voltage is applied, as shown in FIG 14d, the polarization of the light is not rotated and the light will be blocked by the second polarizer

For a quarter wave plate plane polarized light is converted to circularly polarized light Thus, referring now to FIG 15a, where quarter wave plate 217 is placed between a polarizing beam splitter 218 and a mirror 219, the reflected light will be reflected by the beam splitter 218 If an appropriate switchmg voltage is applied, as shown m FIG 15b, the reflected light will pass through the beam splitter and be retroreflected on the mcident beam

Refemng now to FIG 16a, there is shown an elevational view of a subwavelength gratmg 230 recorded in accordance with the above-described methods and having peπodic planes of polymer channels 230a and PDLC channels 230b disposed perpendicular to the front surface 234 of gratmg 230 As shown m FIG 16a, the symmetry axis 232 of the liquid crystal domams is disposed m a direction parallel to the front surface 234 of the gratmg and perpendicular to the periodic channels 230a and 230b of the grating 230 Thus, when an electric field E is applied across the grating, as shown in FIG 15b, the symmetry axis 232 distorts and reoπents m a direction along the field E, which is perpendicular to the front surface 234 of the gratmg and parallel to the penodic channels 230a and 230b of the gratmg 230 As a result, subwavelength gratmg 230 can be switched between a state where it changes the polarization of the incident radiation and a state in which it does not Without wishmg to be bound by any theory, it is cunently believed that the direction of the liquid crystal domain symmetry 232 is due to a surface tension gradient which occurs as a result of the amsotropic diffusion of monomer and liquid crystal during recording of the gratmg and that this gradient causes the liquid crystal domain symmetry to orient in a direction perpendicular to the periodic planes As discussed in Born and Wolf, Principles of Optics. 5^th Ed , New York ( 1975) and incorporated herein by reference, the birefringence of a subwavelength gratmg is given by the following relation

i - i = -[(fpo c) (f_P) (npoLC² - )] / [fpDLC n_PDLC ² + f_pn_p ²] Where n_o = the ordinary index of refraction of the subwavelength grating, n,. = the extraordinary mdex of refraction, n_PDLC ⁼ the refractive mdex of the PDLC plane, n_p = the refractive index of the polymer plane n_Lc ⁼ the effective refractive index of the liquid crystal seen by an incident optical wave. fpD_LC ⁼ tpDLC / (t_PD c + tp) fp ⁼ tp/ (t_PD c + tp)

Thus, the net birefringence of the subwavelength grating will be zero if n_PDLC = n_P

It is known that the effective refractive index of the liquid crystal, n_LC, is a function of the applied electric field, having a maximum when the field is zero and value equal to that of the polymer, n_P, at some value of the electric field, E_MA\ Thus, by application of an electric field, the refractive mdex of the liquid crystal, n_LC, and, hence, the refractive mdex of the PDLC plane can be altered Usmg the relationship set forth above, the net birefringence of a subwavelength grating will be a minimum when n_PDLC is equal to n_P, I e when n_LC = n_P Therefore, if the refractive mdex of the PDLC plane can be matched to the refractive index of the polymer plane, 1 e n_PDLc = n_P, by the application of an electric field, the birefringence of the subwavelength gratmg can be switched off

The following equation for net birefringence, l e | Δn | = | n,. - n-, | , follows from the equation given in Born and Wolf (reproduced above)

Δn = -[(fpoLc) (f_P) (npoLC² - n_p ²)] / [2n_AVG (fpDi n_PDLC ² + f_pn_p ²)] where n_AVc ⁼ (t + rio) /2 Furthermore, it is known that the refractive index of the PDLC plane n_PDLc is related to the effective refractive index of the liquid crystal seen by an incident optical wave, n_LC, and the refractive mdex of the surrounding polymer plane, n_P, by the following relation

NPDLC = n_P + f_LC [n_LC - n_P] Where f _C is the volume fraction of liquid crystal dispersed in the polymer withm the PDLC plane, f_LC = [V_LC/ (V_LC + V_P)]

By way of example, a typical value for the effective refractive index for the liquid crystal in the absence of an electric field is n _C = 1 7, and for the polymer layer n_P, = 1 5 For the gratmg where the thickness of the PDLC planes and the polymer planes are equal (I e t_PDLc = t_P, f_PDLc = 0 5 = f_P) and f_LC = 0 35, the net birefringence, Δn, of the subwavelength grating is approximately 0 008 Thus, where the mcident light has a wavelength of 0 8 μm, the length of the subwavelength grating should be 50 μm for a half-wave plate and a 25 μm for a quarter-wave plate Furthermore, by application of an electric field of approximately 5 V/μm, the refractive index of the liquid crystal can be matched to the refractive mdex of the polymer and the birefringence of the subwavelength gratmg turned off Thus, the switching voltage, V_π, for a half- wave plate is on the order of 250 volts, and for a quarter- wave plate approximately 125 volts By applying such voltages, the plates can be switched between the on and off (zero retardance) states on the order of microseconds As a means of comparison, current Pockels cell technology can be switched in nanoseconds with voltages of approximately 1000-2000 volts, and bulk nematic liquid crystals can be switched on the order of milliseconds with voltages of approximately 5 volts In an alternative embodiment, as shown in FIG 17, the switching voltage of the subwavelength gratmg can be reduced by stacking several subwavelength gratmgs 220a-220e together, and connecting them electrically in parallel By way of example, it has been found that a stack of five gratings each with a length of 10 μm yields the thickness required for a half- wave plate It should be noted that the length of the sample is somewhat greater than 50 μm, because each gratmg mcludes an indium-tin-oxide coatmg which acts as a transparent electrode The switching voltage for such a stack of plates, however, is only 50 volts

Subwavelength gratmgs in accordance with the this descnption are expected to find suitable application in the areas of polarization optics and optical switches for displays and laser optics, as well as tunable filters for telecommunications, colorimetry, spectroscopy, laser protection, and the like Similarly, electrically switchable transmission gratings have many applications for which beams of light must be deflected or holographic images switched Among these applications are Fiber optic switches, reprogrammable NxN optical interconnects for optical computmg, beam steermg for laser surgery, beam steermg for laser radar, holographic image storage and retneval, digital zoom optics (switchable holographic lenses), graphic arts and entertainment, and the like

In a prefened embodiment, a switchable hologram is one for which the diffraction efficiency of the hologram may be modulated by the application of an electπc field, and can be switched from a fully on state (high diffraction efficiency) to a fully off state (low or zero diffraction efficiency) A static hologram is one whose properties remain fixed independent of an applied field In accordance with this description, a high contrast status hologram can also be created In this variation of this descnption, the holograms are recorded as descnbed previously The cured polymer film is then soaked m a suitable solvent at room temperature for a short duration and finally dried For the liquid crystal E7, methanol has shown satisfactory application Other potential solvents include alcohols such as ethanol, hydrocarbons such as hexane and heptane, and the like

When the material is dried, a high contrast status hologram with high diffraction efficiency results The high diffraction efficiency is a consequence of the large mdex modulation in the film (Δn~0 5) because the second phase domains are replaced with empty (air) voids (n~l)

Similarly, in accordance with this description a high birefringence static sub-wavelength wave-plate can also be formed Due to the fact that the refractive index for air is significantly lower than for most liquid crystals, the conespondmg thickness of the half-wave plate would be reduced accordingly Synthesized wave- plates m accordance with this description can be used in many applications employmg polarization optics, particularly where a material of the appropriate birefringence that the appropriate wavelength is unavailable, too costly, or too bulky The term polymer dispersed liquid crystals and polymer dispersed liquid crystal material mcludes, as may be appropriate, solutions m which none of the monomers have yet polymerized or cured, solutions in which some polymerization has occuned, and solutions which have undergone complete polymerization Those of skill in the art will clearly understand that the use herein of the standard term used m the art, polymer dispersed liquid crystals (which grammatically refers to liquid crystals dispersed in a fully polymerized matrix) is meant to include all or part of a more grammatically conect prepolymer dispersed liquid crystal material or a more grammatically correct starting material for a polymer dispersed liquid crvstal material Figs. 18 and 19: Three-dimensional projection with switchable holographic optical elements

FIG. 18 illustrates the operation of one embodiment of a 3-D projection system. The system comprises a flat display unit 305, such as an LCD display that can display a sequence of images, and a switchable holographic optical system 320. In this embodiment, a viewer may observe a projected 3-D image by looking toward switchable holographic optical system 320; the 3-D image appears in an image volume 330, between the viewer and switchable holographic optical system 320.

Display unit 305 displays a series of images 306-308 in sequence. These images are cross-sectional views through a solid 3-D object. Display unit 305 is preferably an LCD display, such as a reflective display mounted on a silicon substrate. In another embodiment, the LCD display is a transmissive display. Other types of displays may also be used for display unit 305, such as standard cathode ray tubes. Display unit 305 is mounted at the object plane 310 of the projection system.

Among the optical elements of the projection system is a switchable holographic optical system 320. Switchable holographic optical system 320 can be switched among several operating modes. In each operating mode, system 320 behaves as one of several conventional optical elements, including lenses with various focal lengths. As the successive cross-sections are displayed on display unit 305, switchable holographic optical system 320 is successively switched between different modes of operation so that it focuses object plane 310 onto a series of image planes 331-334. This switching of focal lengths is synchronized with the switching of cross-sections displayed on display unit 305, and is performed at a rate that is fast in comparison with the integration time of the human eye (approximately 100 milliseconds). The result is that the several cross-sections displayed on display unit 305 are imaged onto an "image volume" 330 that is made up of image planes 331-334.

The image 335 cast onto image volume 330 by this system is a real image. Thus, if the light from object plane 310 has sufficient intensity, a viewer may directly observe image 335 as a reconstructed three-dimensional image that appear to float in the image volume 330. With lower intensities of light, a "viewing screen" can be placed at image volume 330 to facilitate observation of 3-D image 335. This viewing screen may be composed of a volumetric semi-transparent diffuser. In other embodiments, the diffuser is a stack of planar diffusers. In yet another embodiment, the 3-D diffuser is made of one or a few vibrating planar diffusers.

In one embodiment of the projection system, the cross-sections used in images 306-308 are line drawings of the outline of 3-D object. When these cross sections are projected onto image volume 330, the result is an image 335 showing the surfaces of the original 3-D object. In another embodiment, the line drawings in images 306-308 are filled, resulting in an apparently solid image 335. In another embodiment, the cross sections in images 306-308 are created with hidden-line / hidden-surface removal to eliminate some of the extraneous lines and surfaces from the displayed image 335.

Switchable holographic optical system 320 is an optical element whose optical properties are controlled by an applied control signal. In one embodiment, switchable optical system 320 is a single switchable holographic optical element whose optical properties can be rapidly switched among several operating modes.

In another embodiment, switchable optical system 320 comprises several switchable holographic optical elements, each of which can be switched between a diffracting state and a substantially transparent state.

FIG. 19 illustrates one embodiment of the 3-D projection system comprising several switchable holographic optical elements (HOEs). In this embodiment, the switchable holographic optical system 320A is made out of switchable HOEs 321-324, each of which can be switched between a diffracting state and a substantially transparent state. One or more of the switchable HOEs 321 -324 may include an exposed PDLC material such as, for example, the material presented in FIG 1 The PDLC material undergoes phase separation dunng the exposure process (1 e , during the hologram recording process), creatmg regions densely populated by liquid crystal droplets, interspersed by regions of clear photopolymer In the substantially transparent state, an electric field is applied to the exposed PDLC and changes the natural orientation of the liquid crystal droplets therein which, m turn, causes the refractive index modulation of the frmges to reduce and the hologram diffraction efficiency to drop to very low levels, effectively erasing the hologram recorded therem No electnc field is applied m the diffracting state, in which the exposed PDLC material exhibits its very high diffraction efficiency The exposed PDLC switches between the diffracting state and the substantially transparent state very quickly (e g , the exposed material can be switched in tens of microseconds, which is very fast when compared with conventional liquid crystal display materials

The switchable HOEs 321-324 are, m one embodiment, Bragg-type elements that provide a high diffraction efficiency. However, switchable thm-phase HOEs (sometimes referred to as Raman-Nath type) may also be employed, although thin phase HOEs may not provide a high level of diffraction efficiency when compared to Bragg type HOEs Moreover, the switchable HOEs descnbed herein are transmissive type, it bemg understood that reflective type switchable HOEs may be employed in addition or instead In a projection system employing switchable reflective HOEs 321-324 m the optical system 320, the image planes would be on the same side of the optical system 320 as the object plane

A display unit (such as 305 from FIG. 18) is placed at object plane 310 m FIG. 19 and is imaged onto an image volume 330 composed of several image planes In this embodiment, however, switchable HOEs 321- 324 are used to focus object plane 310 onto the image planes 331-334, respectively, m image volume 330

Switchable HOEs 321-324 are placed next to each other in a holographic optical system 320A In one embodiment, the number of switchable HOEs in holographic optical system 320A is equal to the number of desired image planes in image volume 330 By way of example, FIG 19 shows holographic optical system 320A with four switchable HOEs, 321-324, each of which is configured to focus object plane 310 onto one of four image planes 331-334 The switchable HOEs preferably operate m sequence: at any given time, only one of them is active The active HOE diffracts light so that object plane 310 is focused onto the conespondmg image plane Meanwhile, the remainder of the switchable HOEs are inactive, that is, they are substantially transparent and do not further modify the light transmitted from object plane 310 to image volume 330

Three-dimensional color objects may be displayed in volume 330 by using additional switchable HOEs in holographic optical system 320A Instead of using one switchable HOE for each of the image planes, each of the image planes is preferably associated with three switchable HOEs, one HOE for each of three primary color components Thus, holographic optical system 320A, in one embodiment, compnses four groups of HOEs 321a-c, 322a-c, 323a-c, and 324a-c, where HOEs 321a, 322a, 323a, and 324a diffract a first color component of image light when active, HOEs 321b, 322b, 323b, and 324b diffract a second color component of image light when active, and HOEs 321c, 322c, 323c, and 324c diffract a third color component of image light when active

The three color components are preferably red, green, and blue (RGB) components In other embodiments, the color image may be composed of cyan, yellow, and magenta (CYM) components or other sets of three basis colors appropriate for spanning the range of sensitivity of the human eye

In this embodiment, the projection system rapidly cycles through a series of display modes In each mode, one color component for one cross-section of three-dimensional object 335 is imaged at one image plane of image volume 330 via one activated HOE element 321a-c, 322a-c, 323a-c, or 324a-c The projection system sequentially images all three color components of a cross section at an image plane before sequentially imaging the three color components of the next cross section at the next image plane When all image planes are sequentially imaged, the cycle is restarted The cycle time of imaging all image planes w ith the three color components is smaller than an eye integration time Instead of cyclmg through the three color components for a smgle image plane before switching to another image plane, it is noted that the color components and image planes may be addressed in some other order For example, holographic optical system 320A may be controlled so that one color component is projected onto all of the image planes before holographic optical system 320A switches to another color component Each cycle effectively comprises one frame in a continuous display It is well known that to achieve a smoothly flowing image, the frames must be updated at a rate equal to or greater than 22 frames per second Thus, the projection system cycles through imaging the several colors onto the several planes m a time less than or equal to 45 milliseconds The cyclmg is preferably performed m substantially shorter time, thereby achievmg frame rates of 25, 30, 50, 60, or 72 frames per second In one embodiment, optical system 320A includes further HOEs for that enable a variable-focus function Image volume 330 is located at a particular distance from switchable holographic optical system 320A, this distance depends on the effective focal lengths of HOES in optical system 320A, and on the distance between optical system 320A and display unit 310 In this embodiment, image volume can be set at a chosen location from by selecting HOEs with appropriate focal lengths in optical system 320A Note that this "zoom" function is thus preferably achieved without any physical movement (translation along an optical axis) of optical system 320 A

Fig. 20: Three-dimensional projection with a switchable holographic array

Instead of having the switchable HOEs ananged in a stack as shown in FIG 19, the switchable HOEs may be ananged m a planar aπay A system usmg such a switchable holographic lens anay 420 is shown in

FIG 20 The system also includes an LCD display 405, a planar anay of conventional lenses 450, shutter anay 460, and a combiner lens system 470

Switchable holographic lens anay 420 comprises, m one embodiment, a series of switchable HOEs arranged side-by-side m a plane In a prefened embodiment, lens anay 420 comprises a stack of three switchable HOEs, one for transmitting each of three basis colors (such as RGB or CYM) It is noted that the lens anay 420 may comprise a stack of conventional static HOEs (to achieve a monochrome display) However, FIG 20 will be descnbed with lens anay 420 comprising switchable HOEs formed from PDLC material described above

In this embodiment, light is transmitted through a series of only three switchable HOEs mstead of through a larger number required in the embodiments depicted m FIG 19 and FIG 20 Thus, the beam path in this embodiment is less susceptible to attenuation and abenations that may be introduced by the inactive holographic optical elements

LCD display 405 is preferably a reflective LCD display illuminated by a high-intensity light source

(not shown) LCD display 405 is preferably an active display that uses TFT (thin-film transistor) elements to mamtam the on/off status of the pixels between each refresh of the screen LCD display 405 is preferably configured for fast refresh rates, high reflection efficiency, and high contrast As an alternative to LCD technology, it is possible to use micro-mirror anays such as the devices manufactured by Texaas Instruments Inc

LCD display 405 is placed at the object plane of the display system, and is preferably illuminated by one or more bnght light sources, such as high-power mcoherent sources (incandescent lamps, fluorescent lamps, halogen lamps, induction lamps, or LEDs, among others) or lasers In a prefened embodiment, LCD display

405 reflects light over a broad range of angles, so the light reflected from LCD display 405 is cast upon the entire lens anay 450 Each of the elements or series of three elements m switchable holographic lens array 420 is configured to focus light from LCD display 405 onto one of several planes 431-434 m an image volume 430

As described below, combiner lens system 470 directs the light from different elements in lens anay 420 onto the compact image volume 430 Light from LCD display 405 is selectively passed to anay 420 via shutter array

460 At any given time, preferably only one beam path, such as that indicated by 481, is open for transmitting light from LCD display 405 to image volume 430 The shutter can rapidly switch so that a different element is unblocked, thereby allowing the light from LCD display 405 to reach image volume 430 by another beam path, such as the one mdicated by 482 The HOEs m holographic lens anay 420 are constructed with different focal lengths conespondmg to the different focal planes m image volume 430 Thus, when shutter anay 460 unblocks beam path 481, light from LCD display 405 is focussed onto image plane 431 in image volume 430 When beam path 482 is open, light from LCD display 405 is imaged onto image plane 433

Combmer lens 470 redirects the different beam paths from lens anay 420 so that they overlap m image volume 430 Each beam path selected by shutter anay 460 goes through a different lens element in lens array 420 Thus the vaπous beams emergmg from lens anay 420 do not share a common optical axis Combmer lens

470 redirects the light commg from different locations on lens anay by different amounts so that they emerge from combiner lens 470 on substantially the same optical axis

In one embodiment, combiner lens 470 is a system of conventional optical elements Alternatively, combmer lens 470 may be constructed from switchable holographic optical elements, or from a combmation of conventional optics and switchable HOEs Switchable HOEs m combiner lens 470 would preferably be used for conectmg chromatic abenations introduced by holographic lens anay 420

Holographic lens anay 420 preferably mcludes, for example, a stack of separated red-, green-, and blue-sensitive hologram anays that are switched sequentially at a rate synchronized with the refresh rate of LCD display 405 Likewise, although the HOEs of holographic lens anay 420 m FIG 20 are represented as smgle lenses, m practice more HOEs may be added m order to optimize the beam characteπstics and to conect abenations Therefore, prefened embodmients of holographic lens anay 420 would have multi-layer configurations m many situations

The HOEs in holographic lens anay 420 are preferably configured so that light reflected from each LCD element of display 405 fills the entire aperture of each HOE Design considerations for lens anay 420 preferably include the efficient collection of light from display 405

In one embodiment, stacks of anays are used in holographic lens anay 420, further increasing the number of image planes can be generated

It is also contemplated that shutter anay 460 can be replaced by Bragg-type holograms whose angular selectivity perform the same function as the shutter anay Additional functionality can also be added by incorporating additional hologram layers to perform specific optical operations For example, in one embodiment of the projection system holographic lens anav 420 mcludes a hologram stack with elements having different optical powers This configuration of the lens anay provides the system with rapidly switchable variable magnification (zoom)

Figs. 21 and 22: Multiple-tiled images from a single projector with switchable holographic elements. Instead of (or in addition to) usmg switchable HOEs to direct an image onto several different focal planes, the switchable HOEs may be used to direct an image onto several different regions of a smgle focal plane FIG 21 shows a system that projects a 2-D image onto a flat screen 510 The image is made of an anay of image tiles, each of which is projected onto screen 510 by a pro_jector 501

Projector 501 time-multiplexes its projection angle It has a display that sequentially generates the images intended for display on the different tiles The display is preferably a smgle reflective LCD display, although other types and numbers of displays may also be used At any smgle point in tune, one or more switchable HOEs in projector 501 focus the display onto a particular image tile 520 The switchable HOEs work in synchrony with the display so that when the display switches to displaying an image for a new tile 525, the switchable HOEs switch to a mode in which they direct the image onto the new tile By sequencing through the images for all the tiles m a time smaller than the eye integration tune, the pro_jector generates an apparently continuous image over the entire screen 510 As discussed earlier, the display and the switchable HOEs must have a sufficiently fast update rate to provide a smooth flicker-free image The update rate is preferably 25, 30, 50, 60, or 72 frames per second

The tiling technique may be used to increase the size or the resolution of a display, or both, as shown by the following examples

In one embodiment of the tiled display, an image from a 1024 x 768 LCD display is projected onto a 10 x 10 anay of tiles The focussmg optics are configured so that each tile has the dimensions of a 12" video monitor Thus, the pixel size of the display is comparable to that of a standard video monitor, but the overall size of the display is much larger a factor of 100 greater m area This configuration allows the generation of a large viewmg area with adequate resolution for viewing at close range

In another embodiment, a 3 x 3 anay of tiles is generated from an LCD display havmg a resolution of 800 x 600 pixels The 9 tiles are projected onto a 24" monitor, thereby providmg a high resolution monitor (2400 x 1800 pixels) readily usable in a graphics workstation

Other numbers of tiles and other numbers of pixels m each tile may also be used, as appropriate for a given application The details of the configuration would depend on the intended viewing distance (which determines the size of the displayed pixels, accordmg to the Rayleigh two-point discrimination cntena) the desired size of the overall screen 510, and the costs and availability of the LCD display, video processmg hardware, and projection optics

In some applications, it may be desirable to use a reduced resolution and/or a reduced update rate for certain portions of the overall screen For example, when displaying a scene m which most of the motion is restncted to one portion of the screen, one embodiment of the projection system would update the data used for the other tiles less frequently than the data used for the central tiles While each tile is preferably re-projected once in each frame period, the data used to make up that tile may not be updated for several frame penods if little no motion occurs in that tile Thus, the motion in the active portion of the screen would appear fluid, and digital video-processing power would not be expended on unnecessarily updating other portions of the screen In one embodiment of the projection svstem, the video-processing power may be reduced by using decreased resolutions in some portions of the screen One embodiment of such a system is depicted m FIG 22

FIG 22 shows a user 550 who is looking at tiled video screen 510 Video screen 510 is preferably sufficiently large that only a portion 511 of video screen 510 is in the center of the viewer's field of view A gaze-tracking system, such as one incorporating cameras 551 and 552, monitors the user's head position and/or eye position to determine which part of screen 510 is centered in the user's field of view This central portion 511 is then projected with high resolution by projector 501, smce it is this portion of the screen that is viewed by the fovea, the most sensitive portion of the user's eye Other regions of screen 510 may be projected with lower resolution to reduce the amount of video-processmg power required by the system Thus, the projection system considers the foveal characteristics of user 550 m determining how to allocate video data processmg resources

Instead of cameras 551 and 552, the gaze-tracking system may use additional layers of HOE optics m projector 501 to perform the optical functions necessary for head tracking For example, projector 501 may mclude elements for projectmg infrared (IR) radiation (or suitable visible-band light) onto the feature to be tracked and additional elements for imaging the back-scattered IR onto some imagmg sensor inside the projector

While the present mvention has been described with reference to particular embodiments, it will be understood that the embodiments are illustrative and that the invention scope is not limited to these particular embodiments Variations, modifications, additions, and improvements to the embodiments descnbed are possible These variations, modifications, additions, and improvements may fall withm the scope of the mvention as detailed by the following claims

Claims

What is claimed is

1 A method comprising (a) displaying a first two-dimensional image at an object plane, wherem the first two dimensional image represents a first cross-section of a three-dimensional image,

(b) activating a first switchable holographic optical element (HOE) and focussmg the first two dimensional image onto a first image plane usmg the activated first switchable HOE,

(c) deactivating the first switchable HOE, (d) displaying a second two-dimensional image at the object plane, wherein the second two dimensional image represents a second cross-section of the three-dimensional image, and,

(e) activating a second switchable HOE and focussmg the second two dimensional image onto a second image plane using the activated second switchable HOE, wherem the second image plane is adjacent to the first image plane

2 The method of Claim 1, wherein said steps (a)-(e) are performed within 100 microseconds

3 The method of Claim 1, wherem the first switchable HOE, when activated, focuses a first color component of the first two-dimensional image, and wherem the second switchable HOE, when activated, focuses a second color component of the second two-dimensional image

4 The method of Claim 1 , wherem the first switchable HOE, when activated, focuses a first color component of the first two-dimensional image, the method further comprising

(e) activatmg a third switchable HOE after said deactivating the first switchable HOE, and focussing a second color component of the first two dimensional image onto the first image plane usmg the activated third switchable HOE

5 The method of Claim 1, wherem the first optical element passes the second two-dimensional image without substantial alteration when operating in the deactive state

6 The method of Claim 1 , wherem the first HOE comprises an exposed polymer dispersed liquid crystal layer.

7 A method comprising

(a) displaying a first two-dimensional image at an object plane, wherein the first two-dimensional image represents a first cross-section of a three-dimensional image,

(b) focussmg the first two-dimensional image onto a first image plane using a switchable holographic optical system,

(c) displaying a second two-dimensional image at the object plane, wherein the second two- dimensional image represents a second cross-section of die three-dimensional image, and (d) focussing the second two-dimensional image onto a second image plane using the switchable holographic optical system wherein the second image plane is positioned adjacent to the first image plane

8 A method for projecting three-dimensional images, the method comprising

(a) displaying a two-dimensional image at an object plane, wherem the two dimensional image is one cross-section of a three-dimensional image,

(b) focussing one color component of the two dimensional image onto an image plane associated with the cross-section of the three-dimensional image, wherein said focussing comprises activating a switchable holographic optical element (HOE),

(c) repeating said steps (a)-(b) for different color components of the two dimensional image and for different cross sections of the three-dimensional image, wherem said steps (a)-(c) are performed in a time less than or equal to an mtegration time of the human visual system,

(d) repeating said steps (a)-(c)

9 The method of Claim 8, wherein said repeating said steps (a)-(b) for different color components of the two dimensional image comprises performing steps (a)-(b) for three different color components of the two dimensional image

10 A method for projecting three-dimensional images, the method compnsmg sequentially displaying two-dimensional images compnsmg cross-sections of a three-dimensional image at an object plane, and in synchronization with said sequentially displaying the two-dimensional images, sequentially activating switchable holographic optical elements (HOEs) that image the object plane onto a series of spatially separated image planes

11 A three-dimensional projection system comprising a two-dimensional display configured to sequentially display a series of cross-sections of a three- dimensional image, a switchable holographic optical system configured to focus said two-dimensional display onto a sequence of image planes at a plurality of distances from said two-dimensional display

12 The three-dimensional projection system of Claim 11, wherein said switchable holographic optical system is further configured to sequentially focus a series of three colors for each image plane m the seπes of image planes

13 The three-dimensional projection system of Claim 11, wherem said switchable holographic optical system comprises a plurality of switchable holographic optical elements (HOEs), wherem each switchable HOE focuses said two-dimensional display at one of the image planes, and wherem said switchable HOE is configured so that said switchable HOEs are activated sequentially in time 14 The three-dimensional projection system of Claim 13, wherein each of said switchable HOEs comprises a lens and a switchable HOE shutter

15 The three-dimensional projection system of Claim 11, wherein said switchable holographic optical system comprises a stack of sequentially-switched color-specific switchable holographic optical elements (HOEs), a shutter anay, and a lens anay configured to collect light from said two-dimensional display

16 A method comprising

(a) displaying a first section of a two-dimensional image at an object plane, (b) focussing the first section of the two-dimensional image onto a first position m an image plane using a switchable holographic optical system,

(c) displaying a second section of the two-dimensional image at the object plane, and

(d) focussing the second section of the two-dimensional image onto a second position m the image plane usmg the switchable holographic optical system

17 An image projection system for projecting an image comprised of an anay of image tiles, the projection system compnsmg a display device configured to sequentially display tile elements of the image, and a switchable holographic optical system configured to focus said two-dimensional display onto a sequence of positions on an image plane, wherem said display device and said switchable holographic optical system are synchronized so that tile elements are focussed onto conespondmg positions in the image plane, and wherein said display device and said switchable holographic optical system are configured to display an entire image in less than 100 ms