HEAD MOUNTED APPARATUS FOR VIEWING AN IMAGE

Abstract: A head mountable apparatus is described for transmitting an image to the user's eye using switchable holographic optical elements. In one embodiment, an optical system is provided that is configured to receive an image provided by an image generator and which forms a light path along which light is transmitted from the image generator to an eye of the user. The optical system includes a first section disposed at an end of the optical system adjacent to a user's eye and a second section configured to transmit light received by the optical system to the first section. The first section of the optical system includes at least one switchable holographic optical element.

Description: TITLE: HEAD MOUNTED APPARATUS FOR VIEWING AN IMAGE BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention generally relates display systems, and more particularly to a head mounted display system. 2. Description of the Relevant Art Head mountable display devices are becoming more commonly used with the advent of faster computing systems and smaller display devices. Typically, a head mountable display device transmits an image from an image generator to the eye of a user. Because the device is mounted to the head of the user, the image is only projected to the user, and not to the surroundings. Such devices have become popular for military, industrial and entertainment uses. Head mountable display devices are particularly useful for the transfer of computer images and for 3D applications. For example, 3D applications typically require a distinct image to be sent to each eye of a user. Each of the images will represent a 2 dimensional image which, when combined with the other image by the user's eyes, produces the appearance of a 3D image in front of the user. Such images are difficult to generate for a larger audience, requiring the use of specialized glasses to create a 3D image. Many existing head mountable display devices include an image generating system which is positioned directly in front of the user's eye. Older head mountable display devices typically used an opaque image generating system. Such an image generating system would prevent the user from observing their surroundings while viewing the image. More recently the use of translucent or transparent image generating systems allows a user to view a portion of their surroundings while also viewing an image produced by the generator. Such systems typically require an image generating system to be placed in front of the user's eye. Such elements tend to make the display devices "front heavy." These front heavy display devices tend to be uncomfortable for a user of the device. The placement of the image generating system in front of the display device tends to place pressure on the user's head leading to increased fatigue. Many users may find it uncomfortable to wear such devices after a few hours. In an effort to avoid such problems, some head mountable display devices use an image generator that is offset from the direct field of view of a user. An optical system is then constructed to transfer the image from the image generator to the user's eye. In this manner the weight associated with the image generator and some components of the optical system may be better distributed through the display device and onto the user's head. However, in order to project the image at a user's eye, a number of optical elements must be placed around and in front of the user's eye. These optical elements not only are used to transfer the image to a user's eye, but also help to reduce chromatic aberrations and monochromatic aberrations and distortions, such as astigmatism, spherical aberration, coma, pincushion and barrel distortions, keystoning, etc. Many of the aberrations occur as the image is transferred through the various optical components of the system. While these display devices may have a better weight distribution than the previously described front mounted image generator display devices, there is still substantial weight distributed over the user's eye due to the presence of these optical elements. It would be desirable to prepare a head mountable display device that minimizes the weight distribution of the image generator and optical elements, especially in the front portion of the device. This would reduce the fatigue associated with such devices, allowing a user to use the device for longer periods of time. SUMMARY OF THE INVENTION The problems outlined above are in large part solved by a head mounted apparatus for projecting an image which uses switchable holographic optical elements. In one embodiment, an optical system is provided that is configured to receive an image provided by an image generator and which forms a light path along which light is transmitted from the image generator to an eye of the user. The optical system includes at least one switchable holographic optical element. The switchable holographic optical element is configured to operate in an active or inactive state. In the active state the switchable holographic optical element is configured to diffract incident light. In the inactive state the switchable holographic optical element is configured to allow the incident light to pass through the switchable holographic optical element without any substantial alteration. In one embodiment, the switchable holographic optical element includes a holographic recording medium. The holographic recording medium includes liquid crystal elements and a photo-polymer. In one embodiment, the holographic recording medium includes a monomer, dipentaerythritol hydroxypentaacrylate, a liquid crystal, a cross-linking monomer, a coinitiator, and a photoinitiator dye. In one embodiment, a hologram is recorded in the holographic recording medium. For a transmissive switchable holographic diffractive element, the hologram recorded is a transmissive diffraction grating. For a reflective switchable holographic diffractive element, the hologram recorded is a reflective diffraction grating. Either type of hologram is recorded by a process in which a polymer dispersed liquid crystal material undergoes a phase separation to create regions populated by liquid crystal droplets and to create regions of optically clear photopolymer interspersed by regions populated by liquid crystal droplets. In one embodiment, the optical system includes two holographic optical elements. The first holographic optical element is a reflective holographic diffractive element. The second holographic optical element is a transmissive holographic diffractive element. The reflective holographic diffractive element is positioned in front of the eye of a user. The transmissive holographic diffractive element is positioned to the side of the reflective holographic diffractive element. The transmissive holographic diffractive element is configured to receive and transmit light toward the reflective holographic diffractive element. At least one of the holographic optical elements is a switchable holographic optical element. In one embodiment, both the reflective holographic diffractive element and transmissive holographic diffractive element are switchable holographic optical elements. In one embodiment, the image is a color image. When a color image is to be transmitted the switchable holographic optical element includes a three holographic layers, each of which is operative to act upon a primary color (i.e., red, green or blue light). In an embodiment in which two holographic optical elements are used, both holographic optical elements include three holographic layers operative upon a primary color. When the holographic optical element is a switchable holographic optical element, each of the three holographic layers is independently switchable between an active and inactive state. The apparatus may further include a shutter disposed in front of the users eye. The shutter may be switchable between a light transmitting and a light obstructing condition. In the light transmitting condition, the shutter allows a user to see through the shutter. This may allow a user to simultaneously view the user's surroundings and the transmitted image. In the light obstructing condition the user is unable to see through the shutter and will only be able to view the transferred image. In one embodiment, the optical system is enclosed in a casing. The casing may be mountable upon a human head. The casing may be mounted along a side of the head or over the top of the head. Alternatively, the casing may be in the form of a helmet. BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the accompanying drawings in which: FIG. 1 is a cross-sectional view of an electrically switchable hologram made of an exposed polymer dispersed liquid crystal (PDLC) material made in accordance with the teachings of the description herein; FIG. 2 is a graph of the normalized net transmittance and normalized net diffraction efficiency of a hologram made in accordance with the teachings of the description herein (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 made in accordance with the teachings of the description herein 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 electric field for a PDLC material 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 showing the switching response time data for the diffracted beam in the surfactant- containing PDLC material 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 experimental arrangement for recording reflection gratings; FIGS. 8a and 8b are elevational views of a reflection grating, made in accordance with the teachings of the description herein, having periodic planes of polymer channels and PDLC channels disposed parallel to the front surface in the absence of a field (FIG. 8a) and with an electric field applied (FIG. 8b) wherein the liquid- crystal utilized in the formation of the grating has a positive dielectric anisotropy; FIGS. 9a and 9b are elevational views of a reflection grating, made in accordance with the teachings of the description herein, having periodic planes of polymer channels and PDLC channels disposed parallel to the front surface of the grating in the absence of an electric field (FIG. 9a) and with an electric field applied (FIG. 9b) wherein the liquid crystal utilized in the formation of the grating has a negative dielectric anisotropy; FIGS. 9c and 9d depict chemical formulas of various types of liquid crystal materials; FIG. 10a is an elevational view of a reflection grating, made in accordance with the teachings of the description herein, disposed within a magnetic field generated by Helmholtz coils; FIGS. 10b and 10c are elevational views of the reflection grating of FIG. 10a in the absence of an electric field (FIG. 10b) and with an electric field applied (FIG. 10c); FIGS. 11a and lib are representative side views of a slanted transmission grating (FIG. 11a) and a slanted reflection grating (FIG. l ib) 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 grating, made in accordance with the teachings of the description herein, when a shear stress field is applied thereto; FIG. 13 is an elevational view of a subwavelength grating, made in accordance with the teachings of the description herein, having periodic planes of polymer channels and PDLC channels disposed peφendicular to the front surface of the grating; FIG. 14a is an elevational view of a switchable subwavelength, made in accordance with the teachings of the description herein, wherein the subwavelength grating 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 in FIG. 14a disposed between crossed polarizers whereby the incident light is transmitted; FIGS. 14c and 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, made in accordance with the teachings of the description herein, wherein the subwavelength grating functions as a quarter wave plate whereby plane polarized light is transmitted through the subwavelength grating, retroreflected by a mirror and reflected by the beam splitter; FIG. 15b is a side view of the switchable subwavelength grating of FIG. 15a and showing 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 the beam splitter; FIGS. 16a and 16b are elevational views of a transmission grating, made in accordance with the teachings of the description herein, having periodic planes of polymer channels and PDLC channels disposed peφendicular to the front face of the grating in the absence of an electric field (FIG. 16a) and with an electric field applied (FIG. 16b) wherein the liquid crystal utilized in formation of the grating has a positive dielectric anisotropy; FIG. 17 is a side view of five subwavelength gratings wherein the gratings are stacked and connected electrically in parallel thereby reducing the switching voltage of the subwavelength grating; FIG. 18 is a cross-sectional view of an apparatus for viewing an image; FIG. 19 is a schematic side view of an embodiment of an image generator; FIG. 20 is a perspective view of the switchable holographic optical elements of the apparatus; FIG. 21 is a perspective view of the casing of the apparatus; FIG. 22 is schematic view of the optical elements of an embodiment of the apparatus in which the ray traces through the optical elements are shown. FIG. 23 is a schematic view of an embodiment of an apparatus for viewing an image which includes a transmissive and a reflective optical elements; FIG. 24 depicts a schematic view of an embodiment of an apparatus for viewing an image which includes two reflective optical elements; FIG. 25 depicts a schematic view of an embodiment of an apparatus for viewing tiled images. While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawing and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 1. Switchable Hologram Materials And Devices The present invention employs holographic optical elements formed, in one embodiment, from a polymer dispersed liquid crystal (PDLC) material including a monomer, a dispersed liquid crystal, a cross- linking monomer, a coinitiator and a photoinitiator dye. These PDLC materials exhibit clear and orderly separation of the liquid crystal and cured polymer, whereby the PDLC material advantageously provides high quality optical elements. The PDLC materials used in the holographic optical elements may be formed in a single step. The holographic optical elements may also use a unique photopolymerizable prepolymer material that permits in situ control over characteristics of resulting gratings, such as domain size, shape, density, ordering and the like. Furthermore, methods and materials taught herein may be used to prepare PDLC materials for optical elements including switchable transmission or reflection type holographic gratings. Polymer dispersed liquid crystal materials, methods, and devices contemplated for use in the present invention are also described in R. L. Sutherland et al., "Bragg Gratings in an Acrylate Polymer Consisting of Periodic Polymer dispersed Liquid-Crystal Planes, " Chemistry of Materials, No. 5, pp. 1533-1538 (1993); in 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. Bunning et al., "The Moφhology and Performance of Holographic Transmission Gratings Recorded in Polymer dispersed Liquid Crystals/' Polymer, Vol. 36, No. 14, pp. 2699-2708 (1995), all of which are fully incoφorated by reference into this Detailed Description. U.S. Patent application Serial 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 Dosimetry Badge," respectively, are also incoφorated by reference and include background material on the formation of transmission gratings inside volume holograms. The process by which a hologram may be formed 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 incident light pattern. In one embodiment of polymer dispersed liquid crystal (PDLC) material may be used to create a switchable hologram in a single step. A feature of one embodiment of PDLC material is that illumination by an inhomogeneous, coherent light pattern initiates a patterned, anisotropic diffusion (or counter diffusion) of polymerizable monomer and second phase material, particularly liquid crystal (LC). Thus, alternating well- defined channels of second phase-rich material, separated by well-defined channels of a nearly pure polymer, may be produced in a single-stop process. The resulting PDLC material may have an anisotropic spatial distribution of phase-separated LC droplets within the photochemically cured polymer matrix. Prior art PDLC materials made by a single-step process may achieve at best only regions of larger LC bubbles and smaller LC bubbles in a polymer matrix. 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-rich material. Reasonably well- defined alternately LC-rich channels and nearly pure polymer channels in a PDLC material are possible by multi-step processes, but such processes do not achieve the precise moφhology control over LC droplet size and distribution of sizes and widths of the polymer and LC-rich channels made possible by one embodiment of PDLC material. The same may be prepared by coating the mixture between two indium-tin-oxide (ITO) coated glass slides separated by spacers of nominally 10-20 m thickness. The sample is placed in a conventional holographic recording setup. Gratings are typically recorded using the 488 nm line of an Argon ion laser with intensities of between about 0.1-100 mW/cm 2 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. Photopolymerization is induced by the optical intensity pattern. A more detailed discussion of exemplary recording apparatus may be found in R.L. Sutherland, et al., "Switchable holograms in new photopolymer-liquid crystal composite materials," Society of Photo-Optical Instrumentation Engineers (SPIE), Proceedings Reprint, Volume 2402, reprinted from Diffractive and Holographic Optics Technology II (1995), incoφorated 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 incident light pattern. In one embodiment, the prepolymer material comprises a mixture of a photopolymerizable monomer, a second phase material, a photoinitiator dye, a coinitiator, a chain extender (or cross-linker), and, optionally, a surfactant. In one embodiment, two major components of the prepolymer mixture are the polymerizable 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 material. As a result, the second phase material is moved anisotropically out of the polymer region and, thereby, separated into well-defined polymer-poor, second phase-rich regions or domains. Highly functionalized monomers may also be preferred because the extensive cross-linking associated with such monomers yields fast kinetics, allowing the hologram to form relatively quickly, whereby the second phase material will exist in domains of less than approximately 0.1 m. In one embodiment, a mixture of penta-acrylates in combination with di-, tri-, and/or tetra-acrylates may be used in order to optimize both the functionality and viscosity of the prepolymer material. Suitable acrylates, such as triethyleneglycol diacrylate, trimethylolpropane triacrylate, pentaerythritol triacrylate, pentaerythritol tetracrylate, pentaerythritol pentacrylate, and the like may be used. In one embodiment, it has been found that an approximately 1 :4 mixture of tri- to penta-acrylate facilitates homogeneous mixing while providing a favorable mixture for forming 10-20 m 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, reducing 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 increased 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-cyanobiphenyl, 4'-n-heptyl-4-cyanobiphenyl, 4'-octaoxy-4- cyanobiphenyl, 4'-pentyl-4-cyanoteφhenyl, -methoxybenzylidene-4'-butylaniline, and the like. Other second phase components are also possible. The polymer dispersed liquid crystal material employed may be formed from a prepolymer material that is a homogeneous mixture of a polymerizable monomer including dipentaerythritol hydroxypentacrylate (available, for example, from Polysciences, Inc., Warrington, 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 chain-extending monomer N-vinylpyrrolidinone ("NVP") (available from the Aldrich Chemical Company, Milwaukee, Wisconsin), coinitiator N-phenylglycine ("NPG") (also available from the Aldrich Chemical Company, Milwaukee, Wisconsin), and the photoinitiator dye rose bengal ester; (2,4,5,7-tetraiodo-3',4',5',6'-tetrachlorofluorescein-6-acetate ester) marketed as RBAX by Spectragraph, Ltd., Maumee, Ohio). Rose bengal is also available as rose bengal sodium salt (which must be esterified for solubility) from the Aldrich Chemical Company. This system has a very fast curing speed which results in the formation of small liquid crystal micro-droplets. The mixture of liquid crystal and prepolymer material are homogenized to a viscous solution by suitable means (e.g., ultrasonification) and spread between indium-tin-oxide (ITO) coated glass sides with spacers of nominally 15-100 m thickness and, preferably, 10-20 m thickness. The ITO is electrically conductive and serves as an optically transparent electrode. Preparation, mixing and transfer of the prepolymer material onto the glass slides are preferably done in the dark as the mixture is extremely sensitive to light. The sensitivity of the prepolymer materials to light intensity is dependent on the photoinitiator dye and its concentration. A higher dye concentration leads to a higher sensitivity. In most cases, however, the solubility of the photoinitiator dye limits the concentration of the dye and, thus, the sensitivity of the prepolymer material. Nevertheless, it has been found that for more general applications, photoinitiator dye concentrations in the range of 0.2-0.4% by weight are sufficient to achieve desirable sensitivities and allow for a complete bleaching of the dye in the recording process, resulting in colorless final samples. Photoinitiator dyes that may be useful in generating PDLC materials are rose bengal ester (2,4,5,7-tetraiodo-3',4',5',6'- tetrachlorofluorescein-6-acetate ester); rose bengal sodium salt; eosin; eosin sodium salt; 4,5-diiodosuccinyl fluorescein; camphorquinone; methylene blue, and the like. These dyes allow a sensitivity to recording wavelengths across the visible spectrum from nominally 400 nm to 700 nm. Suitable near-infrared dyes, such as cationic cyanine dyes with trialkylborate anions having absoφtion from 600-900 nm as well as merocyanine dyes derived from spiropyran may also find utility in the present invention. The coinitiator employed in the practice of the present invention controls the rate of curing in the free radical polymerization reaction of the prepolymer material. Optimum phase separation and, thus, optimum diffraction efficiency in the resulting PDLC material, are a function of curing rate. It has been found that favorable results may be achieved utilizing coinitiator in the range of 2-3% by weight. Suitable coinitiators include N-phenylglycine; triethyl amine; triethanolamine; N,N-dimethyl-2,6-diisopropyl aniline, and the like. Other suitable dyes and dye coinitiator combinations that may be suitable for use in the present invention, particularly for visible light, include eosin and triethanolamine; camphorquinone and N- phenylglycine; fluorescein and triethanolamine; methylene blue and triethanolamine or N-phenylglycine; erythrosin B and triethanolamine; indolinocarbocyanine and triphenyl borate; iodobenzospiropyran and triethylamine, and the like. The chain extender (or cross linker) employed in the practice of the present invention may help to increase the solubility of the components in 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 may react with the acrylate positions in the pentacrylate monomer, which are not easily accessible to neighboring pentaacrylate monomers due to steric hindrance. Thus, reaction of the chain 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 may be selected from the following: N-vinylpyrrolidinone; N-vinyl pyridine; acrylonitrile; N- vinyl carbazole, and the like. It has been found that the addition of a surfactant material, namely, octanoic acid, in the prepolymer material lowers the switching voltage and also improves the diffraction efficiency. In particular, the switching voltage for PDLC materials containing a surfactant are significantly lower than those of a PDLC material 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 scattering in such materials is reduced due to the dominance of smaller droplets, thereby increasing the diffraction efficiency. Thus, it is believed that the shape of the droplets becomes more spherical in the presence of surfactant, thereby contributing to the decrease in switching 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 switching voltage. It has also been found that, when optimizing for low switching 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 in diffraction efficiency, as well as an increase in switching voltage (possibly due to a reduction in total phase separation of LC). Suitable surfactants include 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 owing to the presence of the free carboxyl (COOH) group in 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 holographic gratings may be formed from a polymerizable monomer, vinyl neononanoate ("VN") C 8 HπC0 2 CH=CH 2 , commercially available from the Aldrich Chemical Co. in Milwaukee, Wisconsin. Favorable results have also been obtained where the chain 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 obtained with about 70% diffraction efficiency, and the resulting gratings could be electrically switched by an applied field of 6V/ m. PDLC materials used in the present invention may also be formed using a liquid crystalline bifunctional acrylate as the monomer ("LC monomer"). LC monomers have an advantage over conventional acrylate monomers due to their high compatibility with the low molecular weight nematic LC materials, thereby facilitating formation of high concentrations of low molecular weight LC and yielding a sample with high optical quality. The presence of higher concentrations of low molecular weight LCs in the PDLC material greatly lowers the switching voltages (e.g., to ~2V/ m). Another advantage of using LC monomers is that it is possible to apply low AC or DC fields while recording holograms to pre-align the host LC monomers and low molecular weight LC so that a desired orientation and configuration of the nematic directors may be obtained in the LC droplets. The chemical formulate of several suitable LC monomers are as follows: CH 2 =CH-COO-(CH 2 ) 6 0-C 6 H 5 -C 6 H 5 -COO-CH=CH 2 CH 2 =CH-(CH 2 ) 8 -COO-C 6 H 5 -COO-(CH 2 ) 8 -CH=CH 2 H(CF 2 ) , 0 CH 2 O-CH 2 -C(=CH 2 )-COO-(CH 2 CH 2 O) 3 CH 2 CH 2 O-COO-CH 2 C(=CH 2 )-CH 2 O(CF 2 ) , 0 H Semifluorinated polymers are known to show weaker anchoring properties and also significantly reduced switching fields. Thus, it is believed that semifluorinated acrylate monomers which are bifunctional and liquid crystalline may find suitable application in the present invention. Referring now to FIG. 1, there is shown a cross-sectional view of an electrically switchable hologram 310 made of an exposed polymer dispersed liquid crystal material made according to the teachings of this description. A layer 312 of the polymer dispersed liquid crystal material is sandwiched between a pair of indium- tin-oxide coated glass slides 314 and spacers 316. The interior of hologram 310 shows Bragg transmission gratings 318 formed when layer 312 was exposed to an interference pattern from two intersecting beams of coherent laser light. The exposure times and intensities may be varied depending on the diffraction efficiency and liquid crystal domain size desired. Liquid crystal domain size may be controlled by varying the concentrations of photoinitiator, coinitiator and chain-extending (or cross-linking) agent. The orientation of the nematic directors may be controlled while the gratings are being recorded by application of an external electric field across the ITO electrodes. The scanning electron micrograph shown in FIG. 2 of the referenced Applied Physics Letters article and incoφorated herein by reference is of the surface of a grating which was recorded in a sample with a 36 wt% loading of liquid crystal using the 488 nm line of an argon ion laser at an intensity of 95 mW/cm 2 . The size of the liquid crystal domains is about 0.2 m and the grating spacing is about 0.54 m. This sample, which is approximately 20 m thick, diffracts light in the Bragg regime. FIG. 2 is a graph of the normalized net transmittance and normalized net diffraction efficiency of a hologram made according to the teachings of his disclosure versus the root mean square voltage ("Vrms") applied across the hologram. is the change in first order Bragg diffraction efficiency. T is the change in zero order transmittance. FIG. 2 shows that energy is transferred 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 may approach 100%, depending on the wavelength and polarization of the probe beam, by appropriate adjustment of the sample thickness. The minimum diffraction efficiency may 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 may be decreased significantly. This is illustrated in FIG. 3, which is a graph of both the threshold rms voltage 20 and the complete switching rms voltage 22 needed for switching a hologram made according to the teachings 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 in the previous paragraph, using alternating current switching voltages at high frequencies helps reduce the needed switching voltage. As demonstrated in FIG. 4, it has been found that adding a surfactant (e.g., octanoic acid) the prepolymer material in amounts of about 4%-6% by weight of the total mixture results in sample holograms with switching voltages near 50Vrms at lower frequencies of 1-2 kHz. As shown in FIG. 5, it has also been found that the use of the surfactant with the associated reduction in droplet size, reduces the switching time of the PDLC materials. Thus, samples made with surfactant may 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 anchoring of the liquid crystals at the interface between liquid crystal and cured polymer. Thermal control of diffraction efficiency is illustrated in FIG. 5. FIG. 5 is a graph of the normalized net transmittance and normalized net diffraction efficiency of a hologram made according to the teachings of this disclosure versus temperature. The polymer dispersed liquid crystal materials described herein successfully demonstrate the utility for recording volume holograms of a particular composition for such polymer dispersed liquid crystal systems. As shown in FIG. 7, a PDLC reflection grating is prepared by placing 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 against the first, thereby causing the prepolymer material 112 to fill the region between the slides 1 14a and 1 14b. Preferably, the separation of the slides is maintained at approximately 20 m 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 behind 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 material is preferably exposed to the 488 nm line of an argon-ion laser, expanded to fill the entire plane of the glass plate, with an intensity of approximately 0.1-100 mWatts/cm 2 with typical exposure times of 30-120 seconds. Constructive and destructive interference within the expanded beam establishes a periodic intensity profile through the thickness of the film. In one embodiment, the prepolymer material utilized to make a reflection grating comprises a monomer, a liquid crystal, a cross-linking monomer, a coinitiator, and a photoinitiator dye. The reflection grating may be formed from prepolymer material including by total weight of the monomer dipentaerythritol hydroxypentacrylate (DPHA), 35% by total weight of a liquid crystal including a mixture of cyano biphenyls (known commercially as "E7"), 10% by total weight of a cross-linking monomer including N- vinylpyrrolidinone ("NVP"), 2.5% by weight of the coinitiator N-phenylglycine ("NPG"),and 10 "5 to 10 '6 gram moles of a photoinitiator dye including rose bengal.ester. Further, as with transmission gratings, the addition of surfactants is expected to facilitate the same advantageous properties discussed above in 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 resulting material comprises a fine grating with a periodicity of 165 nm with the grating vector peφendicular to the plane of the surface. Thus, as shown schematically in FIG. 8a, grating 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 moφhology 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 in the material were significantly smaller having diameters between 50 and 100 nm. Furthermore, unlike transmission gratings where the liquid crystal-rich regions typically comprise less than 40% of the grating, the liquid crystal-rich component of a reflection grating is significantly larger. Due to the much smaller periodicity associated with reflection gratings, i.e., a narrower grating spacing (~0.2 microns), it is believed that the time difference between completion of curing 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 in the matrix during gelation and precludes any substantial growth of large droplets or diffusion of small droplets into larger domains. Analysis of the reflection notch in the absorbance spectrum supports the conclusion that a periodic refractive index modulation is disposed through the thickness of the film. In PDLC materials 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%) indicates that shrinkage of the film is not a significant problem. Moreover, it has been found that the performance of such gratings is stable over periods of many months. In addition to the materials utilized in the one embodiment described above, it is believed that suitable PDLC materials could be prepared utilizing monomers such as triethyleneglycol diacrylate, trimethylolpropanetriacrylate, pentaerythritol triacrylate, pentaerythritol tetracrylate, pentaerythritol pentacrylate, and the like. Similarly, other coinitiators such as triethylamine, triethanolamine, N,N-dimethyl- 2,6-diisopropylaniline, and the like could be used instead of N-phenylglycine. Where it is desirable to use the 458 nm, 476 nm, 488 nm or 514 nm lines of an Argon ion laser, the photoinitiator dyes rose bengal sodium salt, eosin, eosin sodium salt, fluorescein sodium salt and the like will give favorable results. Where the 633 nm line is utilized, methylene blue will find ready application. Finally, it is believed that other liquid crystals such as 4'- pentyl-4-cyanobiphenyl or 4'-heptyl-4-cyanobiphenyl, may be utilized. Referring again to FIG. 8a, there is shown an elevational view of a reflection grating 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 domains is formed in a direction peφendicular to the periodic channels 130a and 130b of the grating 130 and peφendicular to the front surface 134 of the grating 130. Thus, when an electric field E is applied, as shown in FIG. 8b, the symmetry axis 136 is already in a low energy state in alignment with the field E and will reorient. Thus, reflection gratings 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 may be used as a reflection filter. In one embodiment, however, the reflection grating is formed so that it will be switchable. More particularly, switchable holographic reflection gratings may be made utilizing negative dielectric anisotropy 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 anisotropy ( ) will rotate in a direction peφendicular 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 will also be disposed in a direction peφendicular to the periodic channels 130a and 130b of the grating 130 and to the front surface 135 of the grating. However, when an electric field E is applied across such gratings, as shown in FIG. 9b, the symmetry axis of the negative liquid crystal will distort and reorient in a direction peφendicular to the field E, which is peφendicular to the film and the periodic planes of the grating. As a result, the reflection grating may be switched between a state where it is reflective and a state where it is transmissive. FIG. 9c depicts some examples of negative liquid crystals which may be in the methods and devices described herein. Liquid crystals may be found in nature (or synthesized) with either positive or negative . ThusJt is possible to use a LC which has a positive at low frequencies, but becomes negative at high frequencies. The frequency (of the applied voltage) at which 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 grating may be switched. It is expected that low crossover frequency materials may be prepared from a combination of positive and negative dielectric anisotropy liquid crystals. A suitable positive dielectric liquid crystal for use in such a combination contains four ring esters as shown in FIG. 9D. A strongly negative dielectric liquid crystal suitable for use in such a combination is made up of pyridazines as shown in FIG. 9D. Both liquid crystal materials are available from LaRoche & Co., Switzerland. By varying the proportion of the positive and negative liquid crystals in the combination, crossover frequencies form 1.4-2.3 kHz are obtained at room temperature. Another combination suitable for use in the present embodiment is a combination of the following: p-pentylphenyl-2-chloro-4-(p-ρentylbenzoyloxy) benzoate and benzoate. These materials are available from Kodak Company. In still more detailed aspects, switchable holographic reflection gratings may be formed using positive liquid crystals. As shown in FIG. 10a, such gratings are formed by exposing the PDLC starting material to a magnetic field during the curing process. The magnetic field may 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 remain unchanged. (See FIG. 10b.) When an electric field is applied, as shown in FIG. 10c the positive liquid crystal will reorient in the direction of the field, which is peφendicular to the front surface of grating and to the periodic channels of the grating. FIG. 11a depicts a slanted transmission grating 148 and FIG. l ib depicts a slanted reflection grating 150. A holographic transmission grating is considered slanted if the direction of the grating vector G is not parallel to the grating surface. In a holographic reflection grating, the grating is said to be slanted if the grating vector G is not peφendicular to the grating surface. Slanted gratings have many of the same uses as nonslanted grating such as visual displays, mirrors, line 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 grating 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 peφendicular to the grating vector. In particular, for the reflection grating, the LC domain symmetry axis will be oriented along the grating vector G and may be switched to a direction peφendicular 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 recording slanted reflection gratings, it is desirable to place the sample between the hypotenuses of two right-angle glass prisms. Neutral density filters may then be placed in optical contact with the back faces of the prisms using index matching fluids so as to frustrate back reflections which would cause spurious gratings to also be recorded. The incident laser beam is split by a conventional beam splitter into two beams which are then directed to the front faces of the prisms, and then overlapped in the sample at the desired angle. The beams thus enter the sample from opposite sides. This prism coupling technique permits the light to enter the sample at greater angles. The slant of the resulting grating is determined by the angle which the prism assembly is rotated (i.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 holographic reflection gratings 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 applying 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 in the direction of the stress, and the resultant LC domain symmetry axis will be preferentially along the direction of the stress, parallel to the PDLC planes and peφendicular to the direction of the applied electric field for switching. Reflection grating prepared in accordance with this description may find application in color reflective displays, switchable wavelength filters for laser protection, reflective optical elements and the like. In one embodiment, PDLC materials may be made that exhibit a property known as form birefringence whereby polarized light that is transmitted through the grating will have its polarization modified. Such gratings are known as subwavelength gratings, and they behave like a negative uniaxial crystal, such as calcite, potassium dihydrogen phosphate, or lithium niobate, with an optic axis peφendicular to the PDLC planes. Referring now to FIG. 13, there is shown an elevational view of a transmission grating 200 made in accordance with this description having periodic planes of polymer planes 200a and PDLC planes 200b disposed peφendicular to the front surface 204 of the grating 200. The optic axis 206 is disposed peφendicular 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 PDLC and refractive index n PD c. Where the combined thickness of the PDLC plane and the polymer plane is substantially less than an optical wavelength (i.e. (t PD Lc + tp) « ), the grating will exhibit form birefringence. As discussed below, the magnitude of the shift in polarization is proportional to the length of the grating. Thus, by carefully selecting the length, L, of the subwavelength grating for a given wavelength of light, one may rotate the plane of polarization or create circularly polarized light. Consequently, such subwavelength gratings may 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 grating must be equal to one-half of a wavelength, i.e. retardance = 12, and to form a quarter-wave plate, the retardance must be equal to one-quarter of a wavelength, i.e. retardance = 14. It is known that the retardance is related to the net birefringence, n , 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 = n L = | n e - n 0 L Thus, for a half-wave plate, i.e. a retardation equal to one-half of a wavelength, the length of the subwavelength grating should be selected so that: L = 1 (2 n ) Similarly, for a quarter-wave plate, i.e. a retardance equal to one-quarter of a wavelength, the length of the subwavelength grating should be selected so that: L = 1 (4 n ) If, for example, the polarization of the incident light is at an angle of 45 with respect to the optic axis 210 of a half- wave plate 212, as shown in 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 switching 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 switching voltage is applied, as shown in FIG. 15b, the reflected light will pass through the beam splitter and be retroreflected on the incident beam. Referring now to FIG. 16a, there is shown an elevational view of a subwavelength grating 230 recorded in accordance with the above-described methods and having periodic planes of polymer channels 230a and PDLC channels 230b disposed peφendicular to the front surface 234 of grating 230. As shown in FIG. 16a, the symmetry axis 232 of the liquid crystal domains is disposed in a direction parallel to the front surface 234 of the grating and peφendicular 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 reorients in a direction along the field E, which is peφendicular to the front surface 234 of the grating and parallel to the periodic channels 230a and 230b of the grating 230. As a result, subwavelength grating 230 may be switched between a state where it changes the polarization of the incident radiation and a state in which it does not. Without wishing to be bound by any theory, it is currently 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 anisotropic diffusion of monomer and liquid crystal during recording of the grating and that this gradient causes the liquid crystal domain symmetry to orient in a direction peφendicular to the periodic planes. As discussed in Born and Wolf, Principles of Optics, 5 th Ed., New York (1975) and incoφorated herein by reference, the birefringence of a subwavelength grating is given by the following relation: n, 2 - n 2 = -[(fpoLc) (f p ) (n PDLC 2 - n p 2 )] / [f PDLC n PDLC 2 + f P n p 2 ] Where ri o = the ordinary index of refraction of the subwavelength grating; li e = the extraordinary index of refraction; np DLC = the refractive index 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; fpDLC = tpDLC / (tpDLC + tp) f P = t P / (tpDLC + 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 c, 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 χ. Thus, by application of an electric field, the refractive index of the liquid crystal, n C , and, hence, the refractive index of the PDLC plane may be altered. Using the relationship set forth above, the net birefringence of a subwavelength grating will be a minimum when n PDL c is equal to n P , i.e. when n L = n P . Therefore, if the refractive index of the PDLC plane may be matched to the refractive index of the polymer plane, i.e. n PDL c = n P , by the application of an electric field, the birefringence of the subwavelength grating may be switched off. The following equation for net birefringence, i.e. n = n^ - n-, , follows from the equation given in Born and Wolf (reproduced above): n = -[(fpDLc) (fp) (npoLC 2 - n p 2 )] / [2n AVG (fpDi-c n PD ι-c 2 + fpn p 2 )] where n A vG = fa + n <> ) 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 index of the surrounding polymer plane, n P , by the following relation: N PDLC = n P + f LC [n LC - n P ] Where f C is the volume fraction of liquid crystal dispersed in the polymer within the PDLC plane, f C = [V C / (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 ΠL C = 1.7, and for the polymer layer n P , = 1.5. For the grating where the thickness of the PDLC planes and the polymer planes are equal (i.e. t PD C = t P , fpoi-c = 0-5 = fp) and f L c = 0.35, the net birefringence, n, of the subwavelength grating is approximately 0.008. Thus, where the incident light has a wavelength of 0.8 m, the length of the subwavelength grating should be 50 m for a half-wave plate and a 25 m for a quarter-wave plate. Furthermore, by application of an electric field of approximately 5 V/ m, the refractive index of the liquid crystal may be matched to the refractive index of the polymer and the birefringence of the subwavelength grating turned off. Thus, the switching voltage, V n , 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 may be switched between the on and off (zero retardance) states on the order of microseconds. As a means of comparison, current Pockels cell technology may be switched in nanoseconds with voltages of approximately 1000-2000 volts, and bulk nematic liquid crystals may 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 grating may be reduced by stacking several subwavelength gratings 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 m yields the thickness required for a half- wave plate. It should be noted that the length of the sample is somewhat greater than 50 m, because each grating includes an indium-tin-oxide coating which acts as a transparent electrode. The switching voltage for such a stack of plates, however, is only 50 volts. Subwavelength gratings in accordance with the this description 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 holographic 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 computing, beam steering for laser surgery, beam steering for laser radar, holographic image storage and retrieval, digital zoom optics (switchable holographic lenses), graphic arts and entertainment, and the like. A switchable hologram is one for which the diffraction efficiency of the hologram may be modulated by the application of an electric field, and may 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 static hologram may also be created. In this variation of this description, the holograms are recorded as described previously. The cured polymer film is then soaked in 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 index modulation in the film ( n~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 may also be formed. Due to the fact that the refractive index for air is significantly lower than for most liquid crystals, the corresponding thickness of the half-wave plate would be reduced accordingly. Synthesized wave- plates in accordance with this description may be used in many applications employing 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 includes, as may be appropriate, solutions in which none of the monomers have yet polymerized or cured, solutions in which some polymerization has occurred, 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 in 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 correct prepolymer dispersed liquid crystal material or a more grammatically correct starting material for a polymer dispersed liquid crystal material. 2. Head Mountable Apparatus for Viewing an Image FIG. 18 depicts an embodiment of an head mountable apparatus for viewing an image. The apparatus includes a casing 10 configured to be mounted on the head of a user (shown schematically as 11 in FIG. 18). The casing, in one embodiment, is composed of a generally straight portion 12 which extends along the user's head 11, and a curved front portion 13 which extends from a front end of the straight portion 12 across the adjacent eye 14 of the user. An image generator 15 may be disposed within the straight portion 12 adjacent its rear, and includes a display screen 16 on which an image is displayed. An optical system is disposed within the remainder of the casing 10 and acts to transmit light along a path from the image generator to the user's eye. The optical system, in one embodiment, includes a first section 18, a portion of which is disposed in front of the user's eye 14, and a second section 17 which transmits light from the display screen 16 to the first section 18. The first section 18 is composed of at least one switchable holographic optical element. Examples of switchable holographic optical elements have been described in detail in the previous section. In general, switchable holographic optical elements include a holographic recording medium. Within the holographic recording medium a thick or thin phase hologram is recorded. The holographic recording medium is formed from a photopolymer-dispersed liquid crystal mixture. The photopolymer-dispersed liquid crystal mixture undergoes phase separation during a hologram recording process, creating fringes composed of regions densely populated by liquid crystal microdroplets interspersed within regions of clear photopolymer. The resultant phase volume hologram exhibits a very high diffraction efficiency. However, when an electric field is applied, by way of electrodes coupled to the holographic recording medium, the natural orientation of the liquid crystal droplets changes, causing a reduction in the fringe modulation. As a result, the efficiency of the hologram diffraction pattern drops to a very low level, thereby effectively erasing the hologram. Thus, a switchable holographic optical element may exist in two states. The active state is defined as the state in which the hologram is apparent in the holographic recording medium. The inactive state is the state when the hologram is effectively erased, due to the application of an electric field to the holographic recording medium. In one embodiment, the front section includes a diffractive element 20 and a reflective element 19. Light from the second section 17 of the optical system is transmitted through the element 20 and is then reflected by the element 19 toward the user's eye A. The element 19 is positioned in front of a window 21 (See FIGS. 18 and 21) in the front casing portion 13, with a shutter 22 being disposed behind the element 19 with respect to the user's eye. Either of these elements, the reflective element 19 and the diffractive element 20 may be formed from a switchable holographic optical element. The other components of optical system may be formed from standard optical components. Examples of standard optical components include, but are not limited to, non-holographic diffraction gratings, lenses, mirrors, Fresnel lenses, and non-switchable holographic diffraction gratings or lenses. Thus, in one embodiment, the diffractive element 20 may be formed using a standard optical component while the reflective element 19 is formed from a switchable holographic optical element. Alternatively, the diffractive element 20 may be formed from a switchable holographic optical element while the reflective element 19 may be formed from a standard optical component. It is noted that the optical components of the optical system other than diffractive element 19 and reflective element 20, may be formed from switchable holographic optical elements. It should be understood, that while the holographic optical elements are depicted as planar elements, curved holographic optical elements may be used. Curved optical elements may facilitate the correction of aberrations and improve the optical efficiency of the system. The formation and use of curved switchable holographic optical elements is described in detail in U.S. patent application no. 09/416,076 which is incoφorated by reference as if set forth herein. The reflective element 19 may be a reflective switchable holographic diffractive element. A reflective switchable holographic diffractive element includes a holographic recording medium in which a hologram is recorded. For a reflective switchable holographic diffractive element the hologram is of a reflective diffraction grating. The reflective switchable holographic diffractive element as element 19 may mimic the function of a mirror, that is, the reflection of incident light toward the eye of the user. A reflective switchable holographic diffractive element has the ability to operate in both an active and inactive state. In the active state the reflective switchable holographic diffractive element will reflect incident light. In the inactive state the reflective switchable holographic diffractive element will change to a transmissive state, allowing incident light to pass through the element without any substantial reflection. The inactive state may be induced by application of an electric field by electrodes attached to the holographic recording medium. FIG. 20 depicts a reflective switchable holographic diffractive element 19 to which an electrode is attached. The electrode is coupled to a controller 35. The controller is configured to control the application of an electric field to the reflective switchable holographic diffractive element. The diffractive element 20 may be a transmissive switchable holographic diffractive element. A transmissive switchable holographic diffractive element includes a holographic recording medium in which a hologram is recorded. For a transmissive switchable holographic diffractive element the hologram is of a transmissive diffraction grating. A transmissive switchable holographic diffractive element has the ability to operate in both an active and inactive state. In the active state the transmissive switchable holographic diffractive element will diffract incident light as it passes through the element. In the inactive state, the hologram recorded within the transmissive switchable holographic diffractive element will be effectively erased, allowing incident light to pass through the element without any substantial diffraction. The inactive state may be induced by application of an electric field by electrodes attached to the holographic recording medium, as described above. In one embodiment, both the reflective element 19 and the diffractive element 20 are composed of switchable holographic optical elements. Reflective element 19 is a reflective switchable holographic diffractive element. Diffractive element 20 is a transmissive switchable holographic diffractive element. The combination of two or more diffractive elements (switchable or non-switchable) allows the high chromatic dispersions and off-axis aberrations generated by each of the diffractive elements to be balanced. In one embodiment, the image generator is configured to generate color images. Typically, color display devices emit red, blue and green light to produce a color image. In many cases a pixel of a color display device may be composed of three sub-pixels a red sub-pixel, a blue sub-pixel, and a green sub-pixel. Alternatively, a pixel may be configured to sequentially emit red, blue and green colors. The image generator may be based on any transmissive, reflective, diffractive, or self-emissive technology. For example the input image display could be based on an emissive technology such as an electroluminescent panel or a miniature cathode ray tube. It could be also be based on diffractive technology such as the Grating Light Valve manufactured by Silicon Light Machines, CA. In one embodiment, the image generator includes an array of light emitting diodes (LEDs) 30 disposed above a polarizing beamsplitter cube 31 with an array of Fresnel lenses 32 inteφosed between the LEDs and the beamsplitter cube, as depicted in FIG. 19. Light from the LEDs 30 is initially collimated by the Fresnel lens array 32, and is then reflected by an interface 33 of the cube 31 towards the display screen 16. The screen 16, in one embodiment, displays a monochromatic image that is illuminated by light from the LEDs 30, and the resultant image is transmitted through the cube interface 33 towards the second section 17 of the optical system. The display screen 16 may take any suitable form, such as a miniature reflective silicon backplane device or an LCD panel. Although not shown, the image generator 15 also includes a quarter wave plate and a trichromatic interference filter which filters the light from the LEDs 30 into three narrow bandwidths centered respectively on red, green and blue peak wavelengths. In alternative arrangements, the image generator 15 may include integrated optics and/or holographic optical elements. As a further alternative, the image generator may utilize solid state lasers as the light source, which have inherently narrow wavelength emissions and which avoid the need for bandwidth filtering. FIG. 19 shows the use of a reflective LCD panel in the image generator. I another embodiment, the LCD panel may be illuminated incident off-axis at an incident angle that is sufficiently large for the reflected light beams from the LCD panel to avoid the incident light. Thus the use of a beam splitter cube may no longer be necessary. In another embodiment, a rear illuminated transmissive LCD panel may be used. Thus the image is generator on an LCD panel and illuminated by a light source positioned behind the LCD panel. In one embodiment, the light source may be provided by remote lasers via a fiber optic cable. For the transmittal of color images, each of the switchable holographic optical elements 19 and 20 are formed by a stack of three holographic layers, 19a, 19b, and 19c for element 19, 20a, 20b, and 20c for element 20. The three holographic layers may be formed as discrete layers separated by a glass plates. Alternatively, the three holographic layers may be formed within a single holographic recording medium. The following discussion will be applied to only element 19 and holographic layers 19a, 19b, and 19c. However, it should be understood that the holographic layers 20a, 20b, and 20c are configured in an analogous fashion to the holographic layers of element 19, differing only in the holographic images recorded in the layers. Switchable holographic optical element 19a has a hologram recorded in it that is optimized to diffract red light. Switchable holographic optical element 19b has a hologram recorded in it that is optimized to diffract green light. Switchable holographic optical element 19c has a hologram recorded in it that is optimized to diffract blue light. Each of the switchable holographic optical elements 19a, 19b, and 19c have a set of electrodes configured to apply a variable voltage to each of the switchable holographic optical elements. Since element 19 is a reflective switchable holographic diffraction element, the holograms are optimized for the reflection of the appropriate bandwidth of light. As described above, an image generator may be configured to generate, sequentially, the red, green, blue components of a color image. In one embodiment, one set of electrodes associated with the emulsions 19a, 19b and 19c is activated at any one time. With the electrodes activated, a selected amount of light is diffracted into the 1st order mode of the hologram and towards a user, while light in the 0th order mode is directed such that the user cannot see the light. The electrodes on each of the three holograms are sequentially activated such that a selected amount of red, green and blue light is directed towards a user. Provided that the rate at which the holograms are sequentially activated is faster than the response time of a human eye, a color image will be created in the viewer's eye due to the integration of the red, green and blue monochrome images created from each of the holograms 19a, 19b, and 19c. The switching of the holographic optical elements 19a, 19b, and 19c is coordinated with the colors emitted by image generator. When the image generator emits red light, for example, the holographic optical elements associated with green light and blue light (19b and 19c) are inactivated such that the they are substantially transparent to the incident light. The holographic optical element 19a, however, is left in an active state so that the incident red light is diffracted toward the user. Similarly, when green light is emitted by the image generator, holographic optical elements 19a and 19c are inactivated while holographic optical element 19b is in an active state. Finally, when blue light is emitted by the image generator, holographic optical elements 19a and 19b are inactivated while holographic optical elements 19c is in an active state As noted before, the combination of two or more diffractive elements allows the high chromatic dispersions and off-axis aberrations generated by each of the diffractive elements to be balanced. The use of separate red, green and blue elements is particularly advantageous in this regard because the optical system may be separately optimized for red, green, and blue light. In a conventional color display system which does not include separate diffractive elements for each color, it would be necessary to optimize the optical system for the full visible bandwidth. Such an optimization may be difficult to perform for system which include holographic/diffractive elements The second portion 17 of the optical system, in one embodiment, includes (in order along the optical path away from the image generator 15) four lens elements 23, 24, 25, and 26, a reflective element (mirror) 27, and two further lens elements 28 and 29. For each of the lens elements the surface facing towards the image generator is designated by the suffix a, while the surface facing away from the image generator is designated by the suffix b. The surface of the mirror 27 is designated by 27a. These optical elements, together, form an optical subsystem for transferring the light produced by the image to the first section. The optical subassembly is also configured to combat aberrations and reduce dispersion of the light as it travels through the second section. It should be understood that, while depicted in FIGS. 18 and 22 as including a specific number of discrete optical elements, the optical subassembly may include more or less optical elements depending on design factors required for a particular application. Also, while many of the components are depicted as standard lenses and mirrors, it should be noted that holographic optical elements (either static or switchable) may be used in the optical subassembly. Additional, other types of standard optical components such as Fresnel lenses may be used. In the depicted embodiment, the optical subassembly may be divided into three portions, a first condenser system (which includes elements 23, 24, 25, and 26), a reflective element (element 27), and a second condenser system (which includes elements 28 and 29). The first and second condenser systems are optimized using standard optical design techniques to transmit the image light from the input image display source to the reflective element or from the reflective element to the first section, respectively. Both condenser systems incoφorate optical elements that help reduce the dispersion of light as the light passes through the system. The optical elements are also designed to reduce chromatic and monochromatic aberrations as the light passes through the second section. Monochromatic aberration include spherical aberrations, coma, astigmatism, field curvature, and geometric distortions. The above described optical subassembly is configured such that a viewable image will only exist at the input image panel 16 and at the final output of the display. However, in other embodiments an intermediate image may be formed at a diffusing screen positioned at some point along the optical train. The intermediate image may effectively act as a new input image for the elements 19 and 20. This may allow a larger exit pupil to be used. The combination of holographic elements 19 and 20 is configured to reduce both dispersion of the light and aberrations. Elements 19 and 20 are optimized such that their chromatic and monochromatic aberrations and distortions are compensated. In particular, element 20 has the primary function of "focusing" the light in such a way as to avoid chromatic aberration, while element 19 serves the primary puφose of achieving a desired field of view. However, the high incidence angles involved give rise to off-axis aberrations (particularly astigmatism, geometric distortion and keystoning), the main purpose of the components in the section 17 of the optical system is to correct these aberrations. One advantage of the currently described system, is that the use of switchable holographic optical elements allows the use of low weight optical elements in the vicinity of the eye. A typical head mounted display system will require a number of optical components in the vicinity of the eye to correct the aberrations caused by transmitting the image from an off-axis position to the eye. Typically, large aperture images are required in the vicinity of the eye to correct aberrations. By using the switchable holographic optical elements, the weight of the apparatus, especially in the vicinity of the eye, may be minimized. The apparatus may also include a stop to define the limiting aperture. This stop is preferably located at or near the lens element surface 26a (i.e., the centered aspheric surface that is nearest to the mirror 27) and is of elliptical form. The stop may be formed as a separate component added to the system (e.g., a plastic or metal plate having an aperture of the appropriate dimensions) or may be "painted" on the back surface of the element. The above-described apparatus has several advantages some of which includes compact construction and the reduction of structure located in front of the user's eye, the bulk of its weight being positioned instead to the side of the user's head or, in the case of a top mounted design, upon the upper surface of a user's head. Although this means that the projection optical system is highly off-axis, dispersion and chromatic aberration are minimized by the use of switchable holographic diffraction elements. If conventional optical components were to be used in place of the switchable holographic optical elements, it would be necessary to have additional conventional optical elements such as tilted off-axis aspherical lenses, prismatic elements and cylindrical elements. The additional optical elements which perform the functions of the reflective eye pieces would need to be bigger and therefore heavier. The apparatus has been described above with reference to one of the user's eyes. In practice, however, a similar apparatus may be provided for the other eye as well, with the respective display screens showing either identical or stereoscopically-paired images. In this case, the casings 10 of both apparatuses may be combined into a unified headset. The unified headset may take on the appearence of a helmet. Alternatively, the unified headset may resemble a pair of glasses. In addition to viewing images as produced by the image generator 15, the apparatus can also be employed for viewing the ambient surroundings, either with or without the generated image superimposed thereon. A shutter, element 22, is placed behind the reflective element 19, in front of the users eye. To view the surroundings, a shutter 22 is switched so that it becomes light-transmitting rather than light-obstructing. In the case where the generated image is not to be viewed at the same time, the holographic diffraction elements 19 and 20 are turned off. Alternatively, the shutter may be opened, while an image is being projected to the user to create an effect in which the image produced by the image generator appears to be superimposed upon the surroundings. FIG. 23 depicts an embodiment of the optical system of a display apparatus. The optical system includes an image generator 15 an optical subassembly 17 and two diffractive elements 19 and 20. Element 20 is a transmissive element while element 19 is a reflective element. At least one the elements, 19 or 20, is a switchable holographic element. The other element may be any of a variety of standard optical components such as a non-switchable holographic/diffractive, Fresnel, refracting, or reflecting optical element. The transmissive element 20 may be configured such that a virtual image is only produced at the final output of the display. In another embodiment, the element 20 may be a transmissive diffusing screen. The optical subassembly 17 is configured such that a real intermediate is formed at element 20. This real image is transmitted through the screen to the reflective element 19 which forms a final virtual image for the user. Alternatively, the system of FIG. 23 may be configured to produce a directly viewable image. In this alternate embodiment, the reflective element 19 may be a reflective diffusing screen. The final image is then formed on the screen element 19, as opposed to being transmitted to the user as a virtual image. In contrast to the system depicted in FIG. 23, the system of FIG. 24 may include two reflective diffractive elements. Both element 19 and element 20 may be reflective diffractive elements. At least on of the elements, 19 or 20, is a switchable holographic optical element. The other element may be any of a variety of standard optical components such as a non-switchable holographic/diffractive, Fresnel, refracting, or reflecting optical element. The reflective element 20 may be configured such that a virtual image is only produced at the final output of the display. In another embodiment, the element 20 may be a reflective diffusing screen. The optical subassembly 17 is configured such that a real intermediate is formed at element 20. This real image is reflected from the screen to the reflective element 19 which forms a final virtual image for the user. Alternatively, element 19 may be a reflective diffusing screen while element 20 is a reflective switchable holographic diffractive element. The final image is then formed on the screen element 19, as opposed to being transmitted to the user as a virtual image. In another embodiment, depicted in FIG. 25, switchable holographic optical elements may be used to generate a tiled image by having additional layers in the switchable element 20 to create separate fields of view which can be tiled to give a composite view. To accomplish this the transmissive element may be formed from two stacked transmissive elements 20a and 20b. The reflective element is also formed from two reflective elements 19a and 19b. The reflective elements are configured to direct the incident light toward the user's eye. The transmissive elements are configured to diffract the incident light from one reflective element or the other. The transmissive diffractive elements 20 may be switchable, such that only one element at a time transmits the incident light. By rapidly alternating the two elements between an active and inactive state two distinct images may appear to be superimposed to a user. This method of generating a tiled image is described in U.S. patent application no. 09/388,944 which is incoφorated by reference as if set forth herein. Alternatively, the apparatus may be used as a combined imaging and display system. Such a system is described in U.S. patent application no. 09/313,431 which is incoφorated by reference as if set forth herein. The apparatus may also include an eye tracker device which includes a plurality of emitters 2 disposed around the outer periphery of the element 19. The emitters 2 are configured to project radiation in a broad wash onto the eye 6. The projected radiation is reflected back from the eye and directed to a detector 4. Signals from the detector 4 are processed by a processing system 20 in order to measure changes in the attitude of the eye 6, and data corresponding to those changes is fed back to the image generator 15. This in turn causes the image generator 15 to alter the image displayed by the apparatus, so that the view seen by the observer move with his or her direction of gaze. The detector 4 may be a miniature two-dimensional detector array, crossed one-dimensional detector array, or a peak intensity detection device (such as a position sensing detector). Moreover, the various components of the eye tracker device and the wavelength of the radiation used, are chosen such that their characteristics may be optimized to allow particular features of the eye 4 to be easily recognized and tracked. OPTICAL SYSTEM COMPONENTS The following described optical components were used to form a viewing apparatus as depicted in FIGS. 18-22. While these optical components represent a practical example of the components for an head mountable apparatus for viewing an image, it is to be understood that the invention is not to be limited to the use of the described components, but rather us untended to cover various modifications and equivalent constructions included within the spirit and scope of the invention. It should also be noted that the elements of the optical system, as depicted in FIG. 22, may be truncated such that the unused portion of the optical elements is removed when the element is disposed in the casing. FIG. 18 depicts the same optical components as depicted in FIG. 18, however the unused portions of the lenses have been removed to allow a more streamlined appearence for the casing. Optical Component 23 Optical component 23 is a spherical/aspherical lens made from an acrylic material. The lens includes two surfaces, surface 23a is oriented towards the image generator, and 23b which is the surface oriented away from the image generator (See FIG. 18). The acrylic material used to form the lens has the following refractive indices at the listed wavelengths: n(656.27 nm) = 1.488394 ± 0.0006 n(587.56 nm) = 1.491002 ± 0.0006 n(486.13 nm) = 1.496978 ± 0.0006 The surface 23b is a spherical surface having a concave radius of curvature of 204.375 mm. The surface 23a is a polynomial asphere surface. The surface 23a has a convex radius of curvature of 16.927 mm. The deviation of the surface 23a from a spherical surface along the optical axis (defined as the z axis) of the lens ("Sag (z)"), is defined by the following equation: Sag (z) = [(1/R)*h 2 ] / [1 + sqrt(l-(h R) 2 )] + Ah 4 + Bh 6 + Ch 8 where sqrt() represents the square root of the value enclosed within the parenthesis; h 2 = x 2 + y 2 , where x and y equal the Cartesian coordinates along the x and y axis of the lens element; R = -16.92694 A = 0.551681 X 10 _4 B = 0.170580 X 10 "6 C = 0.310160 X 10 "9 The lens element 23 has a central thickness of 4.624 mm. The edge to edge diameter is 19.800 mm. When mounted within the casing the clear aperture diameter of the mounted lens is 17.4 mm. Optical Component 24 Optical component 24 is a planar/aspherical lens made from an acrylic material. The lens includes two surfaces, surface 24a is oriented towards the image generator, and 24b which is the surface oriented away from the image generator (See FIG. 18). The acrylic material used to form the lens has the following refractive indices at the listed wavelengths: n(656.27 nm) = 1.488394 ± 0.0006 n(587.56 nm) = 1.491002 ± 0.0006 n(486.13 nm) = 1.496978 ± 0.0006 The surface 24b cylindrical along the x axis having a convex radius of curvature of 25.63731 mm. The surface 24a is a polynomial asphere surface. The surface 24a has a convex radius of curvature of 68.952 mm. The deviation of the surface 24a from a spherical surface along the optical axis (defined as the z axis) of the lens ("Sag (z)"), is defined by the following equation: Sag (z) = [(1/R)*h 2 ] / [1 + sqrt(l-(h/R) 2 )] + Ah 4 + Bh 6 where sqrt() represents the square root of the value enclosed within the parenthesis; h 2 = x 2 + y 2 , where x and y equal the Cartesian coordinates along the x and y axis of the lens element; R = -68.95221 A = 0J56537 X 10- 4 B = -0.167323 X I 0 "6 The lens element 24 has a central thickness of 4.461 mm. The edge to edge diameter is 23.000 mm. When mounted within the casing the clear aperture diameter of the mounted lens is 20.600 mm. Optical Component 25 Optical component 25 is a spherical/aspherical lens made from an acrylic material. The lens includes two surfaces, surface 25a is oriented towards the image generator, and 25b which is the surface oriented away from the image generator (See FIG. 18). The acrylic material used to form the lens has the following refractive indices at the listed wavelengths: n(656.27 nm) = 1.488394 ± 0.0006 n(587.56 nm) = 1.491002 ± 0.0006 n(486.13 nm) = 1.496978 ± 0.0006 The surface 25b is a spherical surface having a convex radius of curvature of 138.955 mm. The surface 25a is a polynomial asphere surface. The surface 25a has a convex radius of curvature of 11.813 mm. The deviation of the surface 25a from a spherical surface along the optical axis (defined as the z axis) of the lens ("Sag (z)"), is defined by the following equation: Sag (z) = [(1/R)*h 2 ] / [1 + sqrt(l-(l+K)*(h/R) 2 )] + Ah 4 + Bh 6 + Ch 8 + Dh 10 where sqrt() represents the square root of the value enclosed within the parenthesis; h 2 = x 2 + y 2 , where x and y equal the Cartesian coordinates along the x and y axis of the lens element; R = -11.81344 K = -1.807381 A = -0.285278 X 10 "4 B = 0.209903 X I 0 '6 C = -0.502354 X 10 "9 D = 0.425282 X 10 '12 The lens element 25 has a central thickness of 14.000 mm. The edge to edge diameter is 36.800 mm. When mounted within the casing the clear aperture diameter of the mounted lens is 34.400 mm. Optical Component 26 Optical component 26 is a spherical/aspherical lens made from an acrylic material. The lens includes two surfaces, surface 26a is oriented towards the image generator, and 26b which is the surface oriented away from the image generator (See FIG. 18). The acrylic material used to form the lens has the following refractive indices at the listed wavelengths: n(656.27 nm) = 1.488394 ± 0.0006 n(587.56 nm) = 1.491002 ± 0.0006 n(486.13 nm) = 1.496978 ± 0.0006 The surface 26b is a spherical surface having a convex radius of curvature of 101.398 mm. The surface 26a is a polynomial asphere surface. The surface 26a has a convex radius of curvature of 145.335 mm. The deviation of the surface 26a from a spherical surface along the optical axis (defined as the z axis) of the lens ("Sag (z)"), is defined by the following equation: Sag (z) = f(l/R)*h 2 ] / [1 + sqrt(l-(h/R) 2 )] + Ah 4 + Bh 6 + Ch 8 where sqrt() represents the square root of the value enclosed within the parenthesis; h 2 = x 2 + y 2 , where x and y equal the Cartesian coordinates along the x and y axis of the lens element; R = 101.39766 A = -0.351519 X 10- 4 B = -0.501521 X 10- 6 C = 0.363217 X 10 "8 The lens element 26 has a central thickness of 3.000 mm. The edge to edge diameter is 13.800 mm. When mounted within the casing the clear aperture diameter of the mounted lens is 11.4 mm. Optical Component 27 Optical component 27 is a piano/cylindrical mirror made from glass. The mirror includes two surfaces, surface 27a is oriented towards the image generator, and 27b which is the surface oriented away from the image generator (See FIG. 18). The surface 27a is a planar surface. Surface 27a is coated with a high-reflection coating having a maximum reflectance over 460 - 628 nm. The surface 27b is cylindrical along the x axis having a convex radius of curvature of 69.000 mm. The mirror 27 has a central thickness of 4.000 mm. The edge to edge diameter is 26.000 mm. When mounted within the casing the clear aperture diameter of the mounted mirror is 23.600 mm. Optical Component 28 Optical component 28 is a spherical/aspherical lens made from an acrylic material. The lens includes two surfaces, surface 28a is oriented towards the image generator, and 28b which is the surface oriented away from the image generator (See FIG. 18). The acrylic material used to form the lens has the following refractive indices at the listed wavelengths: n(656.27 nm) = 1.488394 ± 0.0006 n(587.56 nm) = 1.491002 ± 0.0006 n(486.13 nm) = 1.496978 ± 0.0006 The surface 28b is a spherical surface having a convex radius of curvature of 60.612 mm. The surface 28a is a polynomial asphere surface. The surface 28a has a convex radius of curvature of 25.510 mm. The deviation of the surface 28a from a spherical surface along the optical axis (defined as the z axis) of the lens ("Sag (z)"), is defined by the following equation: Sag (z) = [(l/R)*h 2 J / [1 + sqrt(lJh/R) 2 )] + Ah 4 + Bh 6 + Ch 8 where sqrt() represents the square root of the value enclosed within the parenthesis; h 2 = x 2 + y 2 , where x and y equal the Cartesian coordinates along the x and y axis of the lens element; R = 25.51037 A = -0.155134 X 10 "4 B = 0.288638 X I 0 -6 C = -0.569516 X 10 -8 The lens element 28 has a central thickness of 13.365 mm. The edge to edge diameter is 43.000 mm. When mounted within the casing the clear aperture diameter of the mounted lens is 40.600 mm. Optical Component 29 Optical component 29 is a cylindrical asphere lens made from an acrylic material. The lens includes two surfaces, surface 29a is oriented towards the image generator, and 29b which is the surface oriented away from the image generator (See FIG. 18). The acrylic material used to form the lens has the following refractive indices at the listed wavelengths: n(656.27 nm) = 1.488394 ± 0.0006 n(587.56 nm) = 1.491002 ± 0.0006 n(486.13 nm) = 1.496978 ± 0.0006 The surface 29b is cylindrical along the x axis having a convex radius of curvature of 47.13109 mm. The surface 29a is a polynomial asphere surface. The surface 29a has a concave radius of curvature of 54.966 mm. The deviation of the surface 29a from a spherical surface along the optical axis (defined as the z axis) of the lens ("Sag (z)"), is defined by the following equation: Sag (z) = [(1/R)*h 2 ] / [1 + sqrt(l-flι/R) 2 )] + Ah 4 + Bh 6 + Ch 8 where sqrt() represents the square root of the value enclosed within the parenthesis; h 2 = x 2 + y 2 , where x and y equal the Cartesian coordinates along the x and y axis of the lens element; R = -54.96615 A = 0.215568 X I 0 "4 B = -0.108402 X I 0 '7 C = 0.280821 X 10 "10 The lens element 29 has a central thickness of 3.000 mm. The edge to edge diameter is 31.600 mm. When mounted within the casing the clear aperture diameter of the mounted lens is 29.2 mm. While the present invention has been described with reference to particular embodiments, it will be understood that the embodiments are illustrated and that the invention scope is not so limited. Any variations, modifications, additions and improvements to the embodiments described are possible. These variations, modifications, additions and improvements may fall within the scope of the invention as detailed within the following claims.

Claims: WHAT IS CLAIMED IS: 1. An apparatus comprising: an optical system configured to transmit image light to an eye of a user, the optical system comprising a first switchable holographic optical element configured to operate in an active state or an inactive state, wherein the first switchable holographic optical element is configured to diffract incident light if the first switchable holographic optical element operates in the active state, and wherein the first switchable holographic optical element transmits incident light without substantial alteration if the first switchable holographic optical element operates in the inactive state; and a casing mountable upon a head of the user, wherein at least a portion of the optical system is disposed within the casing. 2. The apparatus of claim 1 wherein the optical system further comprises a second switchable holographic optical element configured to operate in an active state or an inactive state, wherein the second switchable holographic optical element is configured to diffract incident light if the second switchable holographic optical element operates in the active state, and wherein the second switchable holographic optical element transmits incident light without substantial alteration if the second switchable holographic optical element operates in the inactive state. 3. The apparatus of claim 2 wherein the first switchable holographic optical element comprises a first surface, wherein the first surface is configured to receive incident light, and wherein incident light diffracted by the first switchable holographic optical element emerges from the first surface thereof. 4. The apparatus of claim 1 wherein the optical system further comprises: a second switchable holographic optical element configured to operate in an active state or an inactive state, wherein the second switchable holographic optical element is configured to diffract second bandwidth incident light if the second switchable holographic optical element operates in the active state, and wherein the second switchable holographic optical element transmits second bandwidth incident light without substantial alteration if the second switchable holographic optical element operates in the inactive state; a third switchable holographic optical element configured to operate in an active state or an inactive state, wherein the third switchable holographic optical element is configured to diffract third incident light if the third switchable holographic optical element operates in the active state, and wherein the third switchable holographic optical element transmits third bandwidth incident light without substantial alteration if the third switchable holographic optical element operates in the inactive state; wherein the first switchable holographic optical element is configured to diffract first bandwidth incident light if the first switchable holographic optical element operates in the active state, and wherein the first switchable holographic optical element transmits third bandwidth incident light without substantial alteration if the first switchable holographic optical element operates in the inactive state. 5. The apparatus of claim 4 wherein the optical system further comprises: fourth switchable holographic optical element is configured to diffract first bandwidth incident light if the fourth switchable holographic optical element operates in the active state, and wherein the fourth switchable holographic optical element transmits first bandwidth incident light without substantial alteration if the fourth switchable holographic optical element operates in the inactive state a fifth switchable holographic optical element configured to operate in an active state or an inactive state, wherein the fifth switchable holographic optical element is configured to diffract second bandwidth incident light if the fifth switchable holographic optical element operates in the active state, and wherein the fifth switchable holographic optical element transmits second bandwidth incident light without substantial alteration if the fifth switchable holographic optical element operates in the inactive state; a sixth switchable holographic optical element configured to operate in an active state or an inactive state, wherein the sixth switchable holographic optical element is configured to diffract third incident light if the sixth switchable holographic optical element operates in the active state, and wherein the sixth switchable holographic optical element transmits third bandwidth incident light without substantial alteration if the sixth switchable holographic optical element operates in the inactive state. 6. The apparatus of claim 5 wherein each of the first, second and third switchable holographic optical elements comprise a first surface, wherein each of the first, second and third switchable holographic optical elements is configured to diffract incident light received on the first surface thereof, wherein incident light diffracted by the first switchable holographic optical element emerges from the first surface thereof, wherein incident light diffracted by the second switchable holographic optical element emerges from the first surface thereof, and wherein incident light diffracted by the third switchable holographic optical element emerges from the first surface thereof. 7. The apparatus of claim 1 wherein the first holographic optical element comprises a holographic recording medium that records the hologram, wherein the holographic recording medium comprises: a monomer dipentaerythritol hydroxypentaacrylate; a liquid crystal; a cross-linking monomer; a coinitiator; and a photoinitiator dye. 8. The apparatus of claim 1 wherein the first switchable holographic optical element comprises a thick phase hologram recorded in a holographic recording medium. 9. The apparatus of claim 1 wherein the first switchable holographic optical element comprises a thin phase hologram recorded in a holographic recording medium. 10. The apparatus of claim 1 wherein the first switchable holographic optical element comprises a holographic recording medium, wherein a hologram is recorded in the holographic recording medium, wherein the hologram is formed during a hologram recording process whereby a polymer dispersed liquid crystal material undergoes phase separation to create regions populated by liquid crystal droplets and to create regions of clear photopolymer interspersed by regions populated by liquid crystal droplets. 11. The apparatus of claim 1, further comprising an image generator configured to generate the image light, wherein the image generator. 12. The apparatus of claim 11 wherein the image generator generates a color image. 13. The apparatus of claim 1, further comprising an image generator configured to generate image light, wherein the image generator comprises a light source and a display screen, and wherein the light source is positioned such that light generated from the light source illuminates the image formed on the display screen. 14. The apparatus of claim 13 wherein the light source comprises an array of lasers. 15. The apparatus of claim 13 wherein the display screen comprises a liquid crystal display panel. 16. The apparatus of claim 13 wherein the display screen comprises a reflective silicon backplane device. 17. The apparatus of claim 1 wherein optical system comprises an optical subassembly configured to receive the image light, wherein the optical subassembly is configured to reduce dispersion of image light, wherein the optical subassembly is configured to reduce optical aberrations in the image light. 18. The apparatus of claim 17 wherein the optical system comprises a first condenser system, a second condenser system, and a reflective optical element, wherein the first condenser system is configured to receive image light and transmit the image light to the reflective optical element, wherein the reflective optical element is configured to reflect the image light received from the first condenser system towards the second condenser system, wherein the second condenser system is configured to receive and transmit image light from the reflective optical element. 19. The apparatus of claim 1 wherein the casing is configured to be mounted along the side of the user's head. 20. The apparatus of claim 1 wherein the casing is configured to be mounted upon the top of the user's head. 21. The apparatus of claim 3 wherein the second switchable holographic optical element comprises a second surface, wherein the second surface is configured to receive incident light, and wherein incident light diffracted by the second switchable holographic optical element emerges from the second surface thereof. 22. The apparatus of claim 3 wherein the second switchable holographic optical element comprises first and second oppositely facing surfaces, wherein the first surface of the second switchable holographic optical element is configured to receive incident light, and wherein incident light diffracted by the second switchable holographic optical element emerges from the second surface thereof. 23. The apparatus of claim 1 wherein the optical system further comprises a first non-switchable holographic optical element configured to operate in an active state only, wherein the first non-switchable holographic optical element is configured to diffract incident light. 24. An apparatus comprising: an optical system configured to transmit light emitted from an image to an eye of a user, the optical system comprising a first section and a second section, wherein the first section is disposed at an end of the optical system proximate to the eye of the user such that light emitted from the first section is transmitted to the eye of the user during use, wherein the second section is coupled to the first section, wherein the second section is configured to receive the light emitted from the image and transmit the light to the first section, wherein the first section comprises a first switchable holographic optical element and a second switchable holographic optical element, both the first and second switchable holographic optical elements being configured to operate in an active state or an inactive state, wherein the first switchable holographic optical element is configured to diffract incident light if the first switchable holographic optical element operates in the active state, wherein the first switchable holographic optical element transmits incident light without substantial diffraction if the first switchable holographic optical element operates in the inactive state; wherein the second switchable holographic optical element is configured to reflect incident light if the second switchable holographic optical element operates in the active state, and wherein the second switchable holographic optical element transmits incident light without substantial reflection if the switchable holographic optical element operates in the inactive state, wherein the first switchable holographic optical element is positioned to receive light from the second section and direct the light toward the second switchable holographic element, wherein the second switchable holographic element is positioned to receive light from the first switchable holographic optical element and reflect the light toward the eye of the user, wherein the second section comprises an optical subassembly, wherein the optical subassembly is configured to transmit light through the second section, wherein the optical subassembly is configured to reduce dispersion of light as it passes through the second section, wherein the optical sub assembly is configured to reduce optical aberrations in the transmitted image; and a casing mountable upon the user's head, wherein the optical system is disposed within the casing. 25. The apparatus of claim 24, further comprising an image generator configured to generate the image, wherein the image generator is coupled to the second section, wherein the image generator comprises a light source and a display screen, and wherein the light source is positioned such that light generated from the light source illuminates the image formed on the display screen and is reflected toward the optical system.

Inventors: POPOVICH MILAN M, SAGAN STEPHEN FRANK, STOREY JOHN J, WALDERN JONATHAN D, POPOVICH, MILAN, M., SAGAN, STEPHEN, FRANK, STOREY, JOHN, J., WALDERN, JONATHAN, D.
Applicants: DIGILENS INC, POPOVICH MILAN M, SAGAN STEPHEN FRANK, STOREY JOHN J, WALDERN JONATHAN D, DIGILENS, INC., POPOVICH, MILAN, M., SAGAN, STEPHEN, FRANK, STOREY, JOHN, J., WALDERN, JONATHAN, D.
Application year: 1999
Main classification: G02B27
IPC codes: G02B5, G02B13, G02B17, G02B25, G02B27, G02F1, G03H1, H04N5