Holographic 3-D Printer. Masahiro YAMAGUCHI, Nagaaki OHYAMA, Toshio HONDA

Transcription

1 (C) 1990 the Society of Photo Optical nstrumentation Engineers Box 10, Bellingham, Washington 98227, USA Holographic 3-D Printer Masahiro YAMAGUCH, Nagaaki OHYAMA, Toshio HONDA Tokyo nstitute of Technology, maging Science and Engineering Laboratory, 4259 Nagatsuta, Midori-ku, YOKOHAMA 227, JAPAN ABSTRACT This paper proposes a holographic printer, which produces 3-D hard copies of computer processed objects. For the purpose of automatic making of 3-D hard copies of distortion free, a new method to synthesize holographic stereogram is proposed. t is is flat format and lippmann type holographic stereogram which can be printed by one optical step. The proposed hologram has not only horizontal parallax but vertical parallax, so that the reconstructed image is completely free from distortions. Though a basic experiment, a holographic stereogram of 8.0x6.4cm 2 was synthesized and a 3-D image is correctly reconstructed. n this paper the principle and the method of the new technique are described, and the system constitution and the problems with the holographic 3-D printer are also discussed. 1.NTRODUCTON The technologies of three dimensional ( 3-D) image processings are currently progressed and their requirements are also arisen in many fields. The schematic diagram of 3-D image processing system is shown in fig.l. n medical field, X-ray CT( Computed Tomography ) and MR ( Magnetic Resonance maging ) produce the 3-D informations of human body and assist medical doctors to diagnose and to plan the surgical operations with this system. n industrial field, 3-D CAD ( Computer Aided Design ) system assists the designs of such as buildings. And in the field of science, 3-D structures of molecules or biological objects are analyzed in computers. CO~1PUTER CRT o 41:f11ff1~ 3-D CAD SYSTEM MAGNET! C DSK MAGNETO-OPTCAL DSK ----~? 3-D MAGE HARD COPY.., MAGNETC TAPE Fig.l Schematic diagram of 3-D image processing system. 84 / SPE Vol 1212 Practical Holography V(1990)

2 As shown in fig.1, these systems acquire 3-D data as inputs, process them, and display them on CRT. For 3-D soft copy like images on CRT, stereoscopic display systems are currently developed 1,2 and produce 3-D images with or without glasses. Nevertheless no device can produce a 3-D hard copy, yet. The purpose of this research is to make high quality 3-D hard copies of computer data. Holographic Stereogram( HS ) 3 is the most excellent method for the natural display of imaginary 3-D object. However, conventional methods of HS can be hardly applied to the 3-D hard copy of computer data. n this paper a new type of HS which can be automatically recorded is proposed and shown to be suitable for the 3-D hard copy. This paper describes a system constitution and the problems with holographic 3-D printer which automatically produces 3-D hard copy by means of the proposed technique synthesizing HS. Experimental result is also demonstrated. 2.EXSTNG TECHNOLOGES OF HOLOGRAPHC STEREOGRAMS Many types of HS has been proposed for the output of computer generated or processed 3-D data. Multiplex Hologram 4 is cylindrical format and rainbow type HS, and is used for the display of medical images S,6. Alcove hologram? which is concave semi-cylindrical format, is also invented for the 3-D hard copy of CAD and medical images. Moreover several methods were presented to make flat type HS, which are printed in two optical steps, and either rainbows or lippmann 9 type, but these HS cannot satisfy the following requirements as the handy 3-D hard copy; 1) Whole 3-D image should be accurately recorded and displayed in distortion free. Thus, the HS must have also the vertical parallax. 2) No complicated viewer system should not be required for easy handling. Then, the hologram shape should be flat format. 3) t should be automatically printed. The 2-step type HS is not applicable. These requirements are not satisfied in any type of existing technology of HS, especially no method satisfies 1). The principle of the proposed method satisfying all these conditions is described in the next section. 3.PRNCPLE OF THE ONE-STEP LPPMANN HOLOGRAPHC STEREOGRAM The principle of this one-step, lippmann type HS is described in this section. The basic theory of recording parallax information is similar to that of Multiplex hologram, on which only horizontal parallax is recorded, while both parallax informations are recorded on this type of HS. The recording methods of Multiplex hologram and this HS are shown in fig.2. n recording Multiplex hologram, the object beam is horizontally converged and the hologram film is moved to one direction at the each exposure. To record both horizontal and vertical parallaxes on this HS, the object beam is converged by spherical lens and the hologram film is moved both directions just as painting the hologram plane. This HS is recorded on thick recording material as reflection type hologram, then the both parallaxes are reconstructed under white light illumination because of the wavelength selectivity. The recording optics is shown in fig.3(a). An exposing image is calculated in computer and displayed on the liquid crystal( LC ) panel, through which light rays are passed to converge on the hologram recording plane by the spherical lens. The reference beam is illuminated from another side of the hologram. n this system~ one small hologram which we shall call elemental hologram is exposed. After each SPE Vol Practical Holography V (1990) / 85

3 HOLOGRAM SPHERCAL LENS (A) CYLNDRCAL LENS (B) Fig.2 Recordings of (A) 1-D and (B) 2-D parallax. EXPOSNG MAGE FROM VEWER OBJECT HOLOGRAM OBJEC BEAM qa...~,/ ' VEWER RAYS (c) Fig.3 (A) Recording optical system. A point is exposed in each exposure to paint the whole surface with the movements of the hologram plate. (B) mage calculation and recording geometry. The intensities of all light rays passing to every point on the hologram are calculated and recorded. (C) Reconstruction geometry. The recorded rays are correctly reconstructed. exposure, the hologram film is moved at a pitch to horizontal or vertical direction so that the whole hologram surface is exposed. mages to be exposed are calculated by perspective projection according to the geometry shown in fig.3(b). The center of projection is located at the position of the elemental hologram to be exposed, while the viewing position is not there but at the right part in fig.3(b). Thus the hidden surfaces are removed based on the viewer's position. The relationships between the object coordinates ( x, y, z ) and the projected image coordinates ( xi' Yi ) are represented by ( xi' Yi ) d ( Yo - Yh ) }, (1) where d is the distance between the converging lens and the hologram, (x h ' Yh) is the position of the exposing elemental hologram. The range of the light rays to be calculated is determined by the F-number of the converging lens in fig.3(a). n this way, the intensities of all light rays passing through a point on the hologram are calculated at the each point of the hologram. 86 / SPE Vol Practical Holography V (1990)

4 LC Fig.4 The field of view( the shaded region ) is determined by F number of the converging lens. At the time of recording HS, the light rays are converged on the hologram surface in the same geometry of the calculation as shown in fig.3(b). The converged light forms a small lippmann hologram and whole surface of the hologram is exposed by scanning over the hologram film. The syntnesized HS is reconstructed as shown in fig.3(c). All light rays are properly reconstructed from every elemental hologram so that the 3-D image is correctly observed. Because the all light rays including the vertical parallax are reconstructed, the 3-D image is free from distortion and no distortion compensation is needed. f the hologram is compared to a window, all light rays passing through the window are recorded and reconstructed by this method. The viewing angle of this hologram is limited by the power of the converging lens as shown in fig.4. Since the light rays are extended from the hologram surface within the range between the marginal rays, only the viewers in the shaded region in fig.4 can observe the entire image. For the wide field of view, the converging lens should be low F-number. The relationships between the F-number of the lens (F) and the angle ~ in fig.4 is represented as F = 1 2 x tan( fj/2 ) (2) For example, F-number must be smaller than 0.90 for the viewing field of the angle ~ in fig.4 larger than 45 degree. 4.FEATURES OF TBB HEW METHOD The proposed method is one-step and lippmann type HS with both horizontal and vertical parallaxes, and can be said suitable for holographic 3-D printer, because of the advantages listed below; 1) As already mentioned, both parallax informations are recorded and full 3-D image is accurately recorded and displayed. 2) This HS is reflection type of volume grating, and the image is reconstructed by the light of narrow wavelength. Thus real color 3-D hard copy is achieved by means of the use of three different wavelength 10 3) The optics is very simple with only a spherical lens without any special optical SPE Vol 1212 Practical Holography V (1990) / 87

5 component such as cylindrical lens, and the system can be compact. 4) Large size 3-D hard copy can be produced by the same optics by the increase of the number of elemental holograms. The size of the optical system is not concerned with the size of the HS. 5) The light rays are converged by a lens, quite small area is exposed at a time, and the light is effectively used. This means that only small power laser is needed and the constraint about the stability of the optics becomes relaxed. 6) Hologram can be synthesized by one optical printing step, and can be applied for the automatic 3-D printer. However, several problems have to be overcome. 1) The quantity of information treated is enormous, and the calculation and synthesis takes long time. This problem concerning time will be solved by the progress of the technologies of computer, besides the hologram generation process is fully automated. 2) Because the sampling is on the hologram plane, the dots of elemental holograms can be seen if the sampling pitch is rough. For the dots to be unnoticeable, the pitch should be reduced and the number of exposures is increased. After the settlement of the problem about time this problem can be overcome. n spite of these disadvantages, this method still seems to be most promising for holographic 3-D printer system. 5.BOLOGRAPBC 3-D PRNTER The proposed method leads to the practical holographic 3-D printer, which automatically makes 3-D hard copies of computer generated or processed objects. The concept of the system is shown in fig.5. This can be said to be the 3-D version of an existing video printer or dot printer. This system can be utilized as a peripheral system of a 3-D image processing system. Once an operator depides a viewing direction of the object, this system provides a 3-D hard copy, which can be carried and observed everywhere. AUTOMATC DEVELOPER OUTPUT Fig.5 An example of the schematic diagram of holographic 3-D printer system. 88 / SPE Vol 1212 Practical Holography V (1990)

6 When the 3-D data is sent from the host computer to this system, the image for exposure on each point on the hologram is calculated by the graphic processor. The calculated image is displayed on the LC panel and the laser light modulated by the image is converged on the hologram and exposed. After the exposure of an elemental hologram, the hologram film is moved at a pitch to horizontal or vertical direction and the next image is exposed. n this sequence the whole surface is exposed. The exposed hologram must be developed by the automatic developer if the film is silver halide material. Once this system is constructed, realistic 3-D hard copies of imaginary objects can be automatically produced and holographic display will be widely used for many applications. 6.EXPERMENT A hologram of the proposed type is experimentally synthesized as a test of this method. The experimental optical setup is shown in fig.6. The object beam is modulated by the image calculated and di.splayed on the LC panel, and after the removal of higher-order diffracted light of the matrix structure by spatial filtering, converged on the hologram plane. The converging lens used in this experiment is the aspherical condenser lens of F-number=O.63, for the wide field of view. The reference beam is slender and parallel, and incident on the hologram from the opposite side of the object beam. The hologram plate is moved after each exposure like a raster scanning by X-Y pulse stage controlled by personal computer, which also controls the shutter and the frame memory for displaying images on the LC panel. Because at present the LC panel cannot display enough gray levels, the wire frame object shown in fig.7(a) is synthesized in the HS. The object contains a base frame, and a cube and a quadrangular pyramid on the frame. The images for exposure are calculated in the workstation and transferred to personal computer via ethernet interface. The four examples of 160x128 calculated images are shown in fig. 7(B). These patterns show the intensities of a point on the hologram viewing from various directions. The number of image for exposure is 160x128 and the pixel number contained in each image is 64x64 pixels. The exposed hologram is developed with CWC2 and bleached with PBQ2 according to the method proposed by Cook and Wald 11 and the recording wavelength is remained in reconstruction. The reconstructed image from the synthesized HS is shown in fig.7(c,d). The HS has both horizontal and vertical parallaxes and the reconstructed image is distortion free. The pitch of each elemental hologram is O.5mm, but the discontinuity of the dots is not so obviously observed viewing from over SOcm front of the hologram. The whole image is observed in the width of 110cm and the height range of 99cm, viewing from 96cm front of the HS. The wide field of view is obtained, for the angle ~ in fig.4 is about 63.4 degree. SPE Vol 1212 Practical Holography V (1990) / 89

7 cr: UJ (f) <l:..j UJ z: UJ :c C OBJ.BEA1 Fig.6 Experimental optical setup. REF.BEAM Fig.? (A) Four examples of the projected images of the object recorded on hologram. (B) Four examples of the calculated images for exposure. (C,D) The reconstructed images from the HS viewing from (C) upper right and (D) lower left directions. 90 / SPE Vol Practical Holography V (1990)

8 1.DSCUSSON n future holographic 3-D printer system will be constructed by this new method for the 3-D hard copy. However the progresses of some other technologies are essential for the realization of the holographic printer. For example, the performance of the spatial light modulators used in the optical system is not sufficient with respect to the spatial resolution, signal to noise ratio, and the number of the gray levels. n our recent research, a LC panel was taken up and Multiplex hologram was synthesized to examine its ability for HS printing 12 According to the results we can say that the LC panel will be applicable to this object in near future. n this experiment, however, we use the LC panel, which is not sufficient especially for the display of the gray level images. The spatial light modulator has to be able to display 8 bit grey levels in low noise level under the coherent light illumination. Moreover a new photographic material that does not required any wet development process is also desired. f the silver halide material is used, the wet development process is indispensable, and the performances of the cost, maintenance, and the processing speed are low. Recently new ~hotopolyrner materials that can be record lippmann type hologram are developed 1, and it is a promising material for this purpose. Furthermore for real color printer, three lasers of red, green and blue are required in this system. The development of the diode lasers of three colors may help the realization of the system. Presently the system cannot be conventionally applied for every 3-D processing system, but it will be applied as the form like the service at the development and printing laboratory of photographies. This may be the first application step of the 3-D hard copy. After the developments of the diode lasers of three colors and the new photographic material that requires no wet process, such as photopolyrner, this system will be reasonable to be applied as the peripheral device of every 3-D processing system. a.concluson n this paper, a new type of HS that satisfies the requirements as the 3-D hard copy of computer processed objects is proposed. t is lippmann type HS, which can be recorded in one optical printing step so that 3-D hard copy of distortion free is obtained in completely automatic process. And the system of holographic 3-D printer with this type of HS is described. After the solution of the problems, the holographic 3-D printer will receive wide application in many fields. The authors wish to thank SEKO EPSON Co. for providing the liquid crystal panel for the hologram printing. 9.REFERENCES 1. G. T.Herman; "Dynamic stereo display and interaction with surfaces of medical objects", Proc.SPE, vol (1986) 2. J.S.Kollin; "Collimated View Multiplexing: A New Approach to 3-D", Proc.SPE, Vol (1988) 3. M.C.King, A.M.Noll, and O.H.BerrYi "A New Approach to Computer-Generated Holography", Appl.Opt., Vol.9 No (1970) 4. L.Huff and R.L.Fuseki "Cylindrical holographic stereograms", Proceedings of nternational Symposium on Display Holography, Vol.1 91 (1982) SPE Vol Practical Holography V (1990) / 91

9 5. J.Tsujiuchi, T.Honda, M.Suzuki, T.Saito, and F.lwata; "Synthesis of Multiplex Holograms and Their Application to Medical Objects", Proc.SPE, Vol (1985) 6. N.Ohyama, Y.Minami, A.Watanabe, J.Tsujiuchi, and T.Honda; "Multiplex holograms of a skull made of CT images", Opt.Commun., Vol.61 No.2 96 (1987) 7. S.A.Benton; "Alcove Holograms for Computer-Aided Design", Proc.SPE, Vol (1987) 8. Q.Zhimin and L.Jingsheng; "New Methods of Making Flat Holographic Stereograms" Proc.SPE, Vol (1986) 9. T.Kubota and T.Ose; "New technique for recording a lippmann hologram", Opt. Commun., Vol.28 No (1979) 10. L.H.Lin and C.V.LoBianco; "Experimental Techniques in Making Multicolor White Light Reconstructed Holograms", Appl.Opt., Vol.6 No (1967) 11. D.J.Cooke ans A.A.Wald; "Reflection-hologram processing for high efficiency in sliver halide emulsions", Appl.Opt., Vol.23 No (1984) 12. T.Honda, M.Yamaguchi, D.K.Kang, K.Shimura, J.Tsujiuchi, and N.Ohyama; "Printing of holographic stereogram using liquid-crystal TV", Proc.SPE, Vol (1989) 13. T.lngwall and H.L.Fieldingi "Hologram recording with a new photopolymer system", Opt.Eng. Vol.24 No (1985) 92 / SPE Vol Practical Holography V (1990)