Abstract:
A system for illumination of a hologram includes a deflection mirror to direct light from a moving light source to illuminate the hologram, and a heliostat for tracking the moving light source and positioning the deflection mirror to direct light therefrom to illuminate the hologram. Such as system is optimized for illuminating outdoor holographic displays while still providing a desirable light source and illumination angle, and doing so efficiently and at low cost.

Description:
This application claims the benefit, under 35 U.S.C. §119 (e), of U.S. Provisional Application No. 60/166,627 Nov. 19, 1999, entitled “Light Source Following Optical System for Hologram Illumination”, and naming inventors Mark E. Holzbach and Michael A. Klug. 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 display and, more particularly, to a system and method for tracking a light source to illuminate a holographic display and to tracking sunlight to illuminate large scale holograms such as holographic billboard displays. 
     BACKGROUND OF THE INVENTION 
     Holograms of any size generally have fixed optical illumination angles for best displaying characteristics such as brightness, spatial image fidelity and color fidelity. Moreover, it is generally preferable to illuminate holograms with light sources that approximate point light sources in order to achieve optimal image resolution. This is because light sources having extended sizes (e.g., incandescent light sources) tend to cause the blurring of image points in the hologram proportional to the size of the light source. Similarly, it is generally preferable to reduce and/or minimize the amount of diffuse light that illuminates a hologram. Consequently, hologram illumination schemes typically use single (or small numbers of) well controlled light sources that approximate point light sources, and contribute little to diffuse lighting of the hologram. 
     The need to tailor the location of the illumination source with respect to the hologram in order to have optical illumination angles for best displaying characteristics can, for example, be accomplished when an individual viewer can generally hold a small hologram and quickly adjust the illumination angle to establish an optimal viewing position relative to the available light source. The production of relatively large scale holograms is becoming more practical. The holograms typically have fixed positions, potentially unlimited size, and are intended for observation by multiple viewers. For such holograms, handheld manipulation and positioning is no longer a practical solution. Examples of techniques for one-step hologram production of potentially unlimited size holograms 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. 
     The aforementioned problems associated with optimal hologram display are additionally complicated when holograms are used for outdoor displays, and particularly large outdoor display (e.g., billboards) which are now possible due to the previously discussed advances in hologram production technology. The sun is a logical light source for illumination of outdoor holographic displays because sunlight is generally an extremely bright, efficient and low cost illumination source. However, the location of the sun, and thus angle at which the sun illuminates an outdoor holographic displays, changes throughout the day. Additionally, diffuse sunlight (e.g., sunlight scattered by clouds) illuminates outdoor holographic displays in an undesirable manner. 
     Accordingly, it is desirable to have a hologram illumination system optimized for illuminating outdoor holographic displays while still providing a desirable light source and illumination angle, and doing so efficiently and at low cost. 
     SUMMARY OF THE INVENTION 
     In accordance with teachings of the present disclosure, a system is described for illuminating a hologram such as a holographic billboard type display using the sun or other moving light source. One aspect of the present invention includes a system for illuminating relatively large scale holograms with a controlled angle beam of sunlight. Another aspect of the present invention includes using a heliostat to provide illumination from a moving light source for illumination of a hologram. For some applications an artificial light source may be combined with an optical tracking system incorporating teachings of the present invention to provide illumination when a sufficient amount of light from the moving light source is no longer available. 
     Technical benefits of the present invention including providing bottom illumination of a large scale hologram used in an outdoor display to avoid blurring of the associated image or images on cloudy days due to the effect of diffused light from the clouds. Bottom illumination of a large hologram used in an outdoor display is often more compatible with installation and positioning of an artificial light source relative to the hologram. 
     Accordingly, one aspect of the present invention provides a system for illumination of a hologram. The system includes at least one deflection mirror to direct light from a moving light source to illuminate the hologram, and a heliostat for tracking the moving light source and positioning the at least one deflection mirror to direct light therefrom to illuminate the hologram. 
    
    
     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 is a schematic diagram showing a system for illumination of a hologram with light from a moving light source, such as the sun. 
     FIG. 2 illustrates an example of a device used for deflecting a mirror to direct light from a moving light source on to a hologram. 
    
    
     DETAILED DESCRIPTION 
     The following sets forth a detailed description of at least the best contemplated mode for carrying out the one or more devices and/or processes described herein. The description is intended to be illustrative and should not be taken to be limiting. 
     FIG. 1 shows hologram  100  which can be illuminated by hologram illumination system  110  using light from a moving source  120 , such as the sun. Hologram illumination system  110  preferably includes a heliostat  130  for tracking the moving light source, computer  135 , deflection mirror  150 , and mirror deflection system  160 . Additionally, the system can include light shield  140 , as will be described below. 
     In general, heliostat  130  is an optical system designed to track a moving light source such as the sun. In so doing, heliostat  130  can itself produce either a static or controlled angle-output. That is, some heliostats are designed to track the movement of a moving light source, while ensuring that reflected light from that light source is always reflected to a specific location at a specific angle, or at least some controlled set of locations and angles. Thus, some heliostats include devices such as computer  135  and mirror deflection system  160  implicitly. In the broadest sense, heliostats include devices that track a moving light source, and provide some manner of output information (e.g., control signals) based upon the determined location of the light source. Some heliostats accomplish this task using some type of sensor to monitor the position of the moving light source. In other cases, the motion of the light source may be sufficiently known (or at least calculable from initial conditions) that no sensor is needed. In the example illustrated in FIG. 1, heliostat  130  provides such information to computer  135 , which in turn controls mirror deflection system  160 . Those having ordinary skill in the art will readily recognize that a variety of hologram illumination systems fall within the scope of the present invention, due in part to the expansive definition of the term “heliostat.” Moreover, examples of the various types of heliostats referred to above are well known to those having ordinary skill in the art. 
     For the embodiment of the present invention as shown in FIG. 1, hologram  100  can be a large outdoor holographic display such as a holographic type billboard designed for observation by multiple viewers. Hologram  100  is preferably designed and produced for bottom illumination such as shown in FIG.  1 . During cloudy days or at night when sufficient light is no longer available, an artificial light source (not expressly shown) can also be included as a component in hologram illumination system  110  to provide desired illumination of hologram  100 . 
     Bottom illumination of hologram  100  is often preferable for optimum positioning of hologram illumination system  110 , particularly when one or more relatively heavy artificial light sources are included as a component thereof. Powerful artificial light sources are often large and heavy and therefore more conveniently and safely placed below a large scale hologram. Even though hologram  100  is bottom illuminated, a light shield  140  or other device will preferably be provided to protect hologram  100  from direct overhead light. Moreover, light shields can also be mounted along the sides of hologram  100 , to further prevent unwanted light from illuminating the hologram. This is particularly useful when diffuse light sources are present, such as sunlight scattered by overhead clouds. Shielding the hologram this way can greatly increase the image contrast. 
     The hologram illumination system  110  as shown in FIG. 1 can include a variety of different types of mirror deflection systems  160 . In the example shown, mirror deflection system  160  can be generally described as a gimbal mounted mirror deflection system. A wide variety of gimbal mounted mirror systems incorporating teachings of the present invention may be satisfactorily used to illuminate a large scale hologram. For one embodiment of the present invention, deflection mirror  150  is preferably mounted on or attached to first axle  163  such that deflection mirror  150  can rotate about a corresponding first axis which passes through center point  155  of deflection mirror  150 . First axle  163  is preferably rotatably mounted by bushings and/or bearings to first gimbal mount  162 . First axle  163  is preferably rotated by first motor  164  which is controlled by a motor controller (not expressly shown) and/or computer  135 . First motor  164  and first axle  163  are attached to first gimbal mount  162 . 
     First gimbal mount  162  also includes second axle  168  attached thereto and extending therefrom. Second axle  168  defines in part a second axis which extends through center point  155  approximately perpendicular or normal to the first axis. Second axle  168  is preferably rotatably mounted by bearings and/or bushings within second gimbal mount  167 . Second motor  169  is preferably mounted on one end of second axle  168 , and, like first motor  164 , is controlled by a motor controller (not expressly shown) and/or computer  135 . 
     First motor  164  and second motor  169  can be, for example, stepper motors or DC servomotors. However, a wide variety of electrical motors can be satisfactorily used as first motor  164  and/or second motor  169 . When motor controllers are utilized in the system first motor  164  and second motor  169  can be operated by the same motor controller or by two separate motor controllers. First motor  164 , second motor  169  and other components associated with mirror deflection system  160  can be powered by solar panels (not expressly shown), or some other power source. Also, one or more computers  135  can be provided to operate first motor  164  and second motor  169  to position first gimbal mount  162  and second gimbal mount  167  at the desired orientation relative to each other to direct light from deflection mirror  150  at the optimum angle for illumination of hologram  100 . Additionally, computer control can be used to direct the deflected light to different locations on hologram  100  (and at different angles of illumination) as desired. For example, if hologram  100  includes multiple elemental holograms (i.e., “hogels”) it may be desirable to illuminate different hogels in different ways so as to display different images at different times. Such a system could, for example, present an apparently animated scene. 
     The computer  135  preferably cooperates with heliostat  130  to track the moving light source and direct light from deflection mirror  150  in generally constant direction and at an optimum angle for illumination of hologram. As with the components associated with mirror deflection system  160 , one or more solar panels can be provided for electrical power to operate the computer  135 . Additional information concerning gimbal mounts and mirror control systems satisfactory for use with the present invention may be found in copending U.S. patent application Ser. No. 09/098,581 filed on Jun. 17,1998 entitled “Method and Apparatus for Recording One-Step Full-Color, Full-Parallax Holographic Stereograms.” 
     FIG. 2 illustrates another example of a device used for deflecting a mirror to direct light from a moving light source on to a hologram. Mirror deflection system  260  includes a deflection mirror  250  that is fixedly mounted to a first rotatable mount  262 , such that when the first rotatable mount  262  rotates, the deflection mirror  250  rotates about a first axis  251  which passes through center point  255  of deflection mirror  250 . A motor  264  for the first rotatable mount is controlled by a motor controller  237  and rotates the first rotatable mount  262 . Motor  264  for the first rotatable mount is fixedly attached to a support  270 . First rotatable mount  262  is rotatably mounted to a support  270  with bearings or bushings to allow the first rotatable mount  262  to rotate about the first axis  251 . Support  270  is fixedly mounted by an attaching device on a second rotatable mount  267  such that when the second rotatable mount  267  rotates, the deflection mirror  250  rotates about a second axis  252  which passes through the center point  255  of the deflection mirror  250  and which is orthogonal to the first axis  251 . A motor  269  for the second rotatable mount is controlled by a motor controller  237  and rotates the second rotatable mount  267 . The motors for the first and second rotatable mounts can be, but are not limited to stepper motors or DC servomotors. The same or separate motor controller  237  controlled by computer  235  can control the motors  264  and  269  for the first and second rotatable mounts. 
     Referring again to FIG. 1, for some applications, multiple mirrors and/or lenses (not expressly shown) can also be included as additional components of hologram illumination system  110  for use in illuminating hologram  100 . For example light from deflection mirror  150  can be directed towards a curved reflective surface (not expressly shown) which then illuminates hologram  100 . The curved reflective surface may be used to control or limit any divergence of light rays traveling from deflection mirror  150  to hologram  100 . Examples of a curved reflective surface and other components which may be used to illuminate hologram  100  in accordance with teachings of the present invention are shown and described in pending U.S. patent application Ser. No. 09/151,330 filed Sep. 11, 1998, and entitled “System and Method for Hologram Illumination,” which is hereby incorporated by reference herein in its entirety. 
     Generally, it is most desirable to motorize the mount to automatically track the sun like a modem heliostat. Computer controlled motors may be the most desirable because they can be conveniently automated and the same computer can control a switch for the optional artificial light used at night, or on cloudy days. 
     Although the present invention has been described with respect to a specific preferred embodiment thereof, various changes and modifications may be suggested to one skilled in the art and it is intended that the present invention encompass such changes and modifications fall within the scope of the appended claims.