Abstract:
A system and method to divert a reference beam from intersecting with a diffuser disposed adjacent to a holographic recording material for recording a hologram. The invention includes a thin holographic deflector designed to deflect the reference beam away from the diffuser and to prevent the reference beam&#39;s passage and impingement onto the diffuser surface. The holographic deflector is designed to deflect only light impinging on it from the particular angle that the reference beam strikes the holographic recording material, and to transmit nearly all other light striking it. The deflector eliminates artifacts from the resulting hologram introduced by the reflected reference beam, while allowing the diffuser to be placed very close to the holographic material.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/148,137, filed Aug. 10, 1999, under 35 U.S.C. § 119 (e). The above-referenced provisional application is hereby incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates in general to the field of hologram production and display and, more particularly, to a system and method for diverting at least a portion of a reference beam from impinging upon a diffuser disposed adjacent to holographic recording material. 
     BACKGROUND OF THE INVENTION 
     One-step hologram (including holographic stereogram) production technology has been used to satisfactorily record holograms without the traditional step of creating preliminary holograms. Both computer image holograms and non-computer image holograms may be produced by such one-step technology. In some one-step systems, computer processed images of objects or computer models of objects allow the respective system to build a hologram from a number of contiguous, small, elemental pieces known as elemental holograms or hogels. To record each hogel on holographic recording material, an object beam is conditioned through the rendered image and interfered with by a reference beam. Examples of techniques for one-step hologram production can be found in the U.S. Patent Application entitled “Method and Apparatus for Recording One-Step, Full-Color, Full-Parallax, Holographic Stereograms,” Ser. No. 09/098,581, naming Michael A. Klug, Mark E. Holzbach, and Alejandro J. Ferdman as inventors, and filed on Jun. 17, 1998, which is hereby incorporated by reference herein in its entirety. 
     In many holographic recording systems, and particularly in one-step reflection holographic recording systems, a diffuser is used to evenly distribute light in the object beam on to the holographic recording material. Typically, the diffuser is an anisotropic diffuser. To achieve a high quality hologram, the diffuser is placed as close as is possible to the holographic recording material. In recording a reflection hologram, the reference beam is directed at the holographic recording material from the opposite side as the object beam. Because of the closeness of the diffuser to the holographic recording material, the reference beam passes through the holographic recording material and impinges upon the surface of the diffuser. The surface of the diffuser usually reflects light from the reference beam back through the holographic recording material a second time. Moreover, because of the nature of the diffuser, the reflected light from the reference beam is typically reflected at a variety of angles. 
     The reflected light from the reference beam can be reflected such that it interferes with the reference beam as it traverses the holographic recording material. This problem is illustrated in FIG.  1 . Light from reference beam  25  passes through holographic recording material  70  and is reflected by diffuser  58  as reflected reference beam portions  125 . That interference pattern is recorded in the holographic recording material, resulting in an undesirable artifact that resembles a vertical line seemingly positioned infinitely deep with respect to the hologram plane. This results from the recording of a single beam hologram of the diffuser surface. This artifact is both distracting to the viewer and damaging to the diffraction efficiency, and thus the brightness, of the image. Additionally, reflected light from the reference beam can be reflected such that it interferes with the object beam, potentially creating interference patterns that are recorded in the holographic recording material. While in principle, those recorded interference patterns are similar to the interference patterns that are intended to be recorded (i.e., the interference pattern created by the original, un-reflected, reference beam and the object beam), the fact that the interference patterns were formed using light reflected from the reference beam means that additional distortion or unwanted artifacts might be present. 
     A number of strategies have been used to reduce and/or eliminate the problem of interaction between the reference beam and the diffuser. One solution is to place an anti reflection coating on the diffuser surface. However, anti-reflective coatings usually are effective only for particular bandwidths of wavelengths and certain angles of incidence of incoming light. Due to the extreme and varied angles at which a reference beam may strike a diffuser, and due to the fact that some diffusers are volumetric devices that have no surface relief, this technique has not proven successful. In practice, anti-reflective coatings have proven to eliminate only about 30% of reflected reference beam light, whereas to eliminate the artifacts described above, a greater percentage of the reflected reference beam light should be eliminated. Furthermore, anti-reflective coatings are difficult to uniformly apply over large areas such as the surface area of a diffuser. 
     Another technique is the use of a light control or “louver screen” film between the diffuser and the associated holographic recording material. As illustrated in FIG. 2, light from reference beam  25  passes through holographic recording material  70  and impinges upon louver screen film  59 , where the light is absorbed, and/or generally prevented from reflecting back toward reference beam  25  by louvers  159 . Object beam  20  passes through diffuser  58  and, because of the structure of louvers  159 , generally passes through louver screen film  59 . The result is diffused object beam  120 . Louver screen film is a commercially available volumetric substrate that typically contains microscopic opaque strips or louvers, arranged in a parallel formation at a selected variable angle analogous to a venetian blind arrangement. Louver screen film is chosen with a particular blind spacing and angle that allows passage of the object beam light, for example, at angles of zero to plus or minus thirty degrees (±30°), while absorbing reference beam light impinging at higher angles of, for example, approximately forty five degrees (45°). Such louver screen film successfully prevents reference beam light from impinging and reflecting off the surface of the diffuser, and thus eliminates the unwanted artifacts. 
     One problem associated with using louver screen film is the film&#39;s requisite thickness (on the order of 1 mm) which necessarily further separates the diffuser from the surface of the holographic recording material. Because the louver screen film separates the diffuser and holographic recording material, the diffuser plane and the hologram plane are not as close together as is possible, which leads to poorer quality recorded holograms. Louver screen film may also introduce other artifacts into the hologram, due to the film&#39;s periodicity and diffractive effects associated with the passage of light through the narrow louvers of the film. Finally, louver film often absorbs a significant percentage of the object beam light, again due to the existence of louvers within the film material, along with intrinsic substrate and surface absorption and reflection. 
     Accordingly, it is desirable to have a deflector that overcomes the deficiencies of the prior art, including for example, the thickness, efficiency, and ease of construction and use. 
     SUMMARY OF THE INVENTION 
     In accordance with teachings of the present invention, a system and method are provided to prevent at least a portion of a reference beam from intersecting with an associated diffuser. A holographic deflector incorporating teaching of the present invention may be placed between a diffuser and a holographic film or emulsion, similar to the placement of louver screen film. However, the holographic deflector preferably has a much thinner profile, requiring approximately 100-200 microns, as compared to the 1 mm needed to accommodate the louver film. 
     The holographic deflector is designed to deflect only light impinging on it from the particular angle that the reference beam strikes the holographic emulsion, and transmits nearly all other light strking it. These deflection characteristics are due to the Bragg selectivity of the holographic structure and its recording geometry. 
     A holographic deflector incorporating teaching of the present invention may have nearly 100% diffraction efficiency, ensuring the passage of little or no reference beam light to the diffuser. The holographic deflector allows the diffuser to be placed close to the holographic emulsion, enabling a closer approach to the ideal case of coincident vertical diffusion and hologram planes. Close placement of the diffuser to the emulsion also reduces another image artifact that is produced when rays from different but neighboring points in the object beam intersect prior to propagation through the holographic emulsion. Such a situation gives rise to a coarse fringe pattern, which manifests itself as dark horizontal lines in the image. Finally, the holographic deflector does not impart any visible artifacts of its own into the fine structure of the image as does louver film. This advantage is due to the microscopic nature of the fringes in a holographic optical element, as compared to the macroscopic structure of the louver film. 
     Accordingly, one aspect of the present invention provides a system for recording a hologram in a holographic recording material. The holographic recording material has at least a portion including a first surface and a second surface. The system includes a diffuser and a holographic deflector. The diffuser is disposed adjacent to the second surface whereby an object beam directed at the second surface can pass through the diffuser prior to contacting the holographic recording material. The holographic deflector is disposed between the second surface and the diffuser to prevent at least a portion of a reference beam directed at the first surface from impinging on the diffuser. 
     In another aspect of the present invention, a method for recording a hologram in holographic recording material is disclosed. The holographic recording material has at least a portion disposed in a plane having a first surface and a second surface. A reference beam is directed at the first surface. An object beam is directed at the second surface. A diffuser is disposed adjacent to the second surface whereby the object beam will pass through the diffuser prior to contacting the holographic recording material. At least a portion of a reference beam is deflected with a holographic deflector disposed between the second surface and the diffuser to prevent the reference beam from impinging on the diffuser. An interference pattern formed by the reference beam and the object beam is recorded in the holographic recording material. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the present invention and advantages thereof may be acquired by referring to the following description and the accompanying drawings, in which like reference numbers indicate like features. 
     FIG. 1 illustrates the problem of reference beam reflection by a diffuser. 
     FIG. 2 shows a prior art reference beam deflection solution using louver screen film. 
     FIG. 3 illustrates an example of a system for producing one-step, monochromatic, holographic-stereograms. 
     FIGS. 4A-B show the construction and operation of one example of a holographic deflector. 
     FIGS. 5A-B show the construction and operation of another example of a holographic defector. 
     FIG. 6 illustrates the use of the holographic deflector of FIGS. 5A-B in a holographic printer. 
     FIG. 7 shows still another example of a holographic deflector. 
    
    
     DETAILED DESCRIPTION 
     FIG. 3 illustrates a simplified example of a system (e.g., a holographic printer) for producing one-step, monochromatic, holographic-stereograms. Typically, holographic printers like that depicted in FIG. 3 include a monochromatic coherent light source such as laser  1 , lenses  42 , mirrors  40 , and optical system  29 , a shutter  10 , a mechanism for translating holographic recording material  69 , holographic recording material  70 , usually in the form of film, a computer  85  for controlling the timing of an exposure sequence, and a separate high-speed computer  87  for image calculations. 
     The holographic printer of FIG. 3 is typically supported by a vibration isolation table  80 . Shutter  10  is located at the output of laser  1 , and beam-splitter  15  splits beam  5  into an object beam  20  and a reference beam  25 . The polarizations of the object and reference beams are typically adjusted by a pair of half-wave plates  30  and a pair of polarizers  35 . The half-wave plates  30  and the polarizers  35  can also be adjusted to control the ratio of the intensity of the two beams  20  and  25 . A number of mirrors  40  are used to steer beams  20  and  25  as necessary, while lens  42  serves to expand the object beam prior to introduction into optical system  29 . 
     Optical system  29  includes a diffuser  45 , typically a band-limited diffuser, or an isotropic diffuser, a liquid crystal display (LCD) panel  50 , and a converging lens  55 . LCD panel  50  receives image data calculated by a high-speed computer  87  via an analog or digital signal. LCD panel  50  serves as a spatial light modulator for light passing through the panel. Converging lens  55  focuses images from LCD panel  50  to the holographic recording material  70 , through diffuser  58  and holographic deflector  60 . Holographic deflector  60  is a holographic optical element designed to “deflect” light from the reference beam. Specifically, at least one of a variety of particular interference patterns is recorded in holographic deflector  60  so that light from the reference beam  25  is diffracted in a preferred direction. Thus, holographic deflector  60  includes one or more holograms that are constructed so that when they are illuminated by a light source such as reference beam  25 , light is preferentially deflected. To prevent the exposure of parts of the holographic recording material  70  that are not part of the elemental hologram meant to be exposed, an object beam masking plate (not shown) can be used. Similarly, reference beam masking plate  65  serves to prevent unwanted exposure of parts of the holographic recording material. 
     Although the present invention will be discussed in the context of simple monochromatic hologram production systems, those having ordinary skill in the art will readily recognize that the principles disclosed herein can be extended to multi-color hologram production systems, such as those disclosed in the aforementioned U.S. patent application Ser. No. 09/098,581. 
     FIGS. 4A-B (collectively FIG. 4) show the construction and operation of one example of a holographic deflector which deflects at least a portion of a reference beam, thereby preventing that portion of the reference beam from impinging upon a diffuser. The holographic deflector of FIG. 4 is made by attaching (e.g., laminating) a holographic recording material (e.g., a photopolymer holographic film)  420  to the front surface of a mirror  410 . A beam of light from a coherent light source  430  is oriented with respect to holographic recording material  420  and mirror  410  at the same angle as the reference beam that will be used by to produce a hologram in a holographic printer, for example at the angle between reference beam  25  and holographic recording material  70  in FIG.  3 . When light beam  430  is reflected by mirror  410 , there is a portion of the holographic recording material  420  within which an interference pattern  440  is recorded. Interference pattern  440  is formed by the incoming beam, and its reflection. Holographic recording material  420  is appropriately processed (e.g., cured) to produce holographic deflector  460 . 
     The resulting hologram “remembers” the angle of incidence and the deflection angle from which it was created. Subsequent light impinging from that angle of incidence will be deflected from the hologram at the deflection angle. This occurs, for example, when holographic deflector  460  is used in the printer of FIG.  3 . FIG. 4B illustrates this process. As reference beam  450  impinges upon holographic deflector  460 , interference pattern  440  deflects (more specifically, it diffracts) reference beam  450  as deflected beam  470 . The deflected beam  470  is deflected at the angle of reflection of reference beam  430  of FIG. 4A (which equals the angle of incidence of the reference beam  430 ). 
     The construction process of FIG. 4A produces a holographic deflector that, for a given band of wavelengths, deflects light in a similar fashion as a mirror (where the angle of incidence of the beam equals the angle of deflection) for only a narrow set of angles surrounding the reference beam. 
     A holographic deflector similar to holographic deflector  460  can be made by intersecting two separate coherent beams within a holographic recording material (eliminating the front-surface mirror). FIGS. 5A-B (collectively FIG. 5) show the construction and operation of such a holographic deflector. The holographic deflector of FIG. 5 is made by intersecting two separate beams from a coherent light source, beam  510  and beam  520 , each having a specified angle of incidence with respect holographic recording material  530 . As beam  510  and beam  520  interfere, an interference pattern  540  is formed and recorded in respect holographic recording material  530 . Holographic recording material  530  is appropriately processed to produce holographic deflector  560 . By varying the angle between the two beams, and the orientation of the holographic recording material with respect to the beams, a holographic deflector can be made for which the angle of deflection differs from the angle of incidence of the reference beam that it is designed to deflect. As shown in FIG. 5B, when an incoming beam  550  impinges on holographic deflector  560  from the angle of incidence of beam  510 , incoming beam  550  will be deflected at an angle determined by the angle of incidence of beam  520 . 
     The use of holographic deflector  560  in a printer such as the printer of FIG. 1 is further illustrated in FIG.  6 . Holographic deflector  560  is interposed between diffuser  58  and holographic recording material  70  to prevent reference beam  25  from impinging on diffuser  58  by deflecting reference beam  25  away from diffuser  58 , and in a direction that does not cause an interference pattern to form between reference beam  25  and the deflected beam  625 . Since a holographic optical element is a hologram that is specially designed to diffract light a particular way, a holographic deflector that is a holographic optical element can be designed to operate as a lens, as a diffuser, a mirror (concave, convex, or planar), or a variety of other optical elements. 
     FIG. 7 illustrates a specialized case of a holographic deflector constructed according to the description of FIGS. 5 and 6. In this example, holographic deflector  760  is constructed with one beam that is oriented at an angle of incidence with respect to holographic recording material that is comparable to that of the reference beam in the printer with which the deflector will be used, and a second beam that is parallel to the holographic recording material. For example, the second beam could be introduced through a substrate laminated to the holographic recording material, and having a similar index of refraction. This would produce an edge-lit, or “trapped beam” holographic deflector, in which the impinging reference beam in the printer system would produce a diffracted beam that would travel through the holographic recording material and substrate itself, as if it were a light guide. Thus, holographic deflector  760  prevents the diffracted light from being reflected back into the holographic recording material  70 . 
     One important aspect of the holographic deflector that makes it viable for producing reflection holograms in a one-step production process is that the holographic deflector has no appreciable effect on the object beam impinging from the opposite side. Moreover, unlike a standard mirror, which reflects broadband radiation nearly equally regardless of the input angle, the holographic deflector only deflects light in the desired bands impinging on it from the designed input angle. Thus, the holographic deflector can be designed to deflect a significant percentage of the light from the reference beam (e.g., impinging at 45 degrees from normal), while transmitting a maximum amount of the light from the object beam (e.g., impinging at 0 degrees). 
     Using a holographic deflector to prevent artifacts in a recorded hologram is readily adaptable to recording full color holograms. In a three color system, such as those disclosed in the aforementioned U.S. patent application Ser. No. 09/098,581, a separate holographic deflector is preferably provided for each color wavelength. The three deflectors can be laminated together in a sandwich to provide protection in all three wavelength bands. Also, if the holographic deflector has adequate efficiency, all three wavelength mirrors could be recorded in a single holographic recording material layer. If it were desirable, the three holographic deflectors could be made to operate at different impingement angles, and with different impingement wavefront curvatures. Finally, if the object beam diffuser  58  is a volumetric device, or a device with one planar side (as is the case for a lenticular diffuser), the holographic deflector can be applied directly to the planar surface, provided that the planar surface is facing the holographic emulsion during exposure of the hologram. This configuration would ensure the thinnest possible separation between diffuser  58  and the holographic recording material  70 . 
     Although the disclosed embodiments have been described in detail, it should be understood that various changes, substitutions and alterations can be made to the embodiments without departing from their spirit and scope. In particular, those having ordinary skill in the art will readily recognize additional types of holographic optical elements, and techniques for constructing same, which can be used as holographic deflectors.