Abstract:
A three dimensional display provides a true 3-D display of an image defined by luminous voxels within a virtually transparent volume. The voxels may be generated by illuminating a virtually transparent fluorescent gas or dye with beams of excitation energy that cause the gas or dye to emit light at voxels where the energy beams intersect. The voxels may be refreshed in a manner to create either fixed or animated images.

Description:
The invention described below is assigned to the United States Government and is available for licensing commercially. Technical and licensing inquiries may be directed to Harvey Fendelman, Legal Counsel For Patents, SPAWARSYSCEN SAN DIEGO CODE D0012 Room 103, 53510 Silvergate Avenue, San Diego, Calif. 92152; telephone no. (619)553-3001; fax no. (619)553-3821. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to generating a three dimensional representation of an object that may vary in position, orientation, and shape. More specifically, but without limitation thereto, the present invention relates to illuminating a substance with excitation energy to generate a visible representation of a three dimensional object within a spatial volume. 
     3-D display technologies such as holography, stereoscopic displays, and advanced 3-D graphics engines generally render three dimensional (3-D) images on a two dimensional (2-D) display by mapping the coordinates of the 3-D images into a 2-D perspective. However, these technologies lack the physiological depth cues needed for true 3-D display imaging, such as motion parallax, accommodation, convergence, and binocular disparity. A 3-D volumetric display provides the physiological depth cues needed for such applications as air traffic control, submarine undersea navigation, automotive design, architecture, and medical imaging. 
     One method for displaying a true three-dimensional image is to illuminate a series of points on a two-dimensional surface as it moves through a display volume. However, these volumetric displays suffer the disadvantages of mechanical constraints on the voxel refresh rate, because a voxel may only be refreshed when the display surface is in the correct position within the display volume. A voxel is defined as a graphic unit of information representative of a point in a three dimensional image space, analogous to a pixel as a graphic unit of information representative of a point in a two dimensional image space. This type of display also suffers from the disadvantages of instability in the moving two dimensional surface that may cause the image to jitter, added weight and mechanical complexity that may make shipboard and aircraft applications impractical, and the display surface motion itself that may distract attention from the image being displayed. 
     Another approach to a 3-D display is described in U.S. Pat. No. 5,684,621 issued on Nov. 4, 1997 to Elizabeth Downing and incorporated herein by reference thereto. This display suffers from the disadvantages associated with voxels that require expensive rare earth crystalline materials and the impracticality of scaling to large sizes. 
     A need therefore exists for a three dimensional volumetric display that does not require a two dimensional surface within the display volume and which may readily be scaled in size. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to overcoming the problems described above, and may provide further related advantages. No embodiment of the present invention described herein shall preclude other embodiments or advantages that may exist or become obvious to those skilled in the art. 
     A three dimensional display of the present invention provides a true 3-D display of an image defined by luminous voxels within a virtually transparent display volume. The voxels may be generated by illuminating a fluorescent gas with multiple beams of excitation energy that cause the gas to emit light at voxels where the energy beams intersect. The voxels may be refreshed in a manner to create either fixed or animated images within the display volume. 
     An advantage of the three dimensional display of the present invention is that a true 3-D image may be generated within a virtually transparent volume without the refractive distortions of solid media. 
     Another advantage is that the weight and cost of the display materials is substantially reduced over that of solid state displays that require a glass doped with rare earth ions. 
     Still another advantage is that highly visible displays may be generated with low power lasers. 
     Yet another advantage is that the three dimensional display of the present invention may readily be scaled in size. 
     The features and advantages summarized above in addition to other aspects of the present invention will become more apparent from the description, presented in conjunction with the following drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of a three dimensional display of the present invention. 
     FIG. 2 is an alternative embodiment of a three dimensional display of the present invention with a curved mirror. 
     FIG. 3 is an alternative embodiment of a three dimensional display of the present invention with an infrared-absorbing phosphor. 
     FIG. 4 is an exemplary source of excitation energy for both infrared and ultraviolet wavelengths. 
    
    
     DESCRIPTION OF THE INVENTION 
     The following description is presented solely for the purpose of disclosing how the present invention may be made and used. The scope of the invention is defined by the claims. 
     FIG. 1 is a diagram of a three dimensional display  10  of the present invention comprising a voxel (a three dimensional graphic coordinate)  102  inside a display volume  104  filled with a substantially transparent fluorescent gas. Instead of a gas, display volume  104  may also comprise a fluorescent dye in a solid or liquid matrix. Voxel  102  is illuminated by the intersection of two laser beams  106  and  108 . Laser beams  106  and  108  may be, according to well known techniques, continuously generated and deflected to generate images of solid shapes or pulsed to generate wire frame images as well as images defined by a series of illuminated points. 
     An example of a suitable gas for use with laser beams  106  and  108  inside display volume  104  is nitrogen dioxide. In this example, laser beams  106  and  108  preferably each have a wavelength of from about 900 nm to 1000 nm (the wavelengths may be equal if desired). Nitrogen dioxide absorbs within this range and fluoresces in the infrared with a decay time of about 44 usec. At the voxel located at the intersection of laser beams  106  and  108 , however, excitation to a higher energy state by the absorption of energy from both laser beams  106  and  108  results in fluorescence in the form of visible green light that emanates from the voxel. A glass vessel  110  may be used to contain the nitrogen dioxide gas within display volume  104 . 
     Another example of a suitable gas for use with laser beams  106  and  108  inside display volume  104  is iodine vapor. In this example, laser beam  106  preferably has a wavelength of about 841 nm and laser beam  108  preferably has a wavelength of about 2660 nm. At the intersection of laser beams  106  and  108 , excitation to a higher energy state by the absorption of energy from both laser beams  106  and  108  results in fluorescence in the form of visible red light emanating from the voxel. A glass vessel  110  may be used to contain the iodine vapor within display volume  104 , and transparent electrode  112  may be deposited on the glass vessel as a heating element to prevent condensation of iodine on the vessel walls. 
     In addition to a gas, a dye may be suspended in a liquid or solid solution within display volume  104  that preferably has a high quantum efficiency, i.e., a suitable ratio of visible fluorescence photons to excitation photons. An example of a suitable fluorescent dye is rhodamine 6G, which fluoresces with an orange-yellow color. A typical dye concentration for rhodamine 6G is 10 −3  to 10 −5   molar. Suitable solid state matrix materials for suspending fluorescent dyes include polymethyl methacrylate (PMMA or plexiglass), plastics, porous glass, and epoxies. Dye matrix materials should be transparent to both the excitation and fluorescence wavelengths and should not require high manufacturing temperatures that would destroy the dye or contain substances that would quench the fluorescence. 
     Suitable liquid matrix materials for suspending fluorescent dyes in display volume  104  include the solvents methanol and ethanol as well as a wide range of organic and inorganic fluids including water, and dimethyl sulfoxide (DMSO). Gels, or highly viscous materials such as gelatin and heavy transparent grease or oil, may also be used for the dye matrix. 
     Fluorescent dyes may also be suspended in display volume  104  as a gas vapor by heating the solid form of the dye in a vacuum. Other means may be used for suspending fluorescent dyes as have been demonstrated in the art of dye lasers. An advantage of dyes is that they do not require doping with rare earth elements. 
     For excitation of rhodamine 6G dye at the intersection of laser beams  106  and  108 , each of laser beams  106  and  108  may have a wavelength of, for example, 1064 nm. A suitable laser for this wavelength is an Nd:YAG laser. While the dye matrix is transparent at the excitation wavelength, the dye has broad absorption at half the laser wavelength. At the intersection of laser beams  106  and  108 , two-photon absorption is sufficient to generate fluorescence at voxel  102 . 
     In FIG. 2, laser beams  206  and  208  may be collimated and focused by, for example, a curved mirror  210  for concentrating laser energy at voxel  202  to generate visible fluorescence. In this embodiment, a single focused laser may be used to concentrate sufficient laser energy at voxel  202  to generate fluorescence. Curved mirror  210  may also be used with laser beams  106  and  108  in the embodiments of FIGS. 1 and 3. The difficulty of changing the focal point of curved mirror  210  at voxel  202  to accommodate a fast refresh rate may be relieved by sequencing multiple laser beams in ping-pong fashion so that each laser beam has sufficient time to refocus before its turn in the sequence repeats. 
     In the embodiment of a three dimensional volumetric display  30  shown in FIG. 3, an infrared phosphor  310  is suspended in display volume  304  in a matrix similar to that described above for fluorescing dyes. Because infrared phosphors are typically inorganic compounds that are resistant to heat, glasses may also be used as matrix materials. Infrared phosphor  310  may be, for example, zinc cadmium sulfide. In operation, infrared phosphor is first excited at voxel  302  by laser beam  306  having a wavelength in the ultraviolet, and then further excited by intersecting laser beam  308  having a wavelength in the infrared. The phosphor absorbs energy from each of laser beams  306  and  308  to generate visible fluorescence. 
     An example of a laser  40  for generating the wavelengths for both laser beams  306  and  308  in FIG. 3 is shown in FIG. 4. A YAG laser  420  generates a laser beam  422  having a wavelength of 1064 nm that is frequency-tripled by tripler  424 . An example of a tripler would be a KTP (potassium titanyl phosphate) crystal doubler coupled to a BBO (beta barium borate) sum frequency generating crystal. Laser beam  422  is tripled by frequency tripler  424  to a wavelength of 355 nm. A portion of laser beam  422  that is not converted to ultraviolet by tripler  424  is included in combined output laser beam  426 . A beamsplitter  428  may be used to separate the converted ultraviolet energy at 355 nm from the infrared energy at 1064 nm to generate the respective wavelengths for laser beams  306  and  308  in FIG.  3 . Beamsplitter  428  may be, for example, a dichroic beamsplitter. 
     Other modifications, variations, and applications of the present invention may be made in accordance with the above teachings other than as specifically described to practice the invention within the scope of he following claims.