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Computer simulation of reconstructed image for Computer-Generated Holograms Tomoki Yasuda *a, Mitsuru Kitamura a, Masachika Watanabe **a, Masato Tsumuta b, Takeshi Yamaguchi b and Hiroshi Yoshikawa b a

Research and Development Center, Dai Nippon Printing Co., Ltd. 250-1 Wakashiba, Kashiwa, Chiba 277-0871, Japan b Dept. Electronics and Computer Science, Nihon University 7-24-1 Narashinodai, Funabashi, Chiba 274-8501, Japan ABSTRACT

This report presents the results of computer simulation images for image-type Computer-Generated Holograms (CGHs) observable under white light fabricated with an electron beam lithography system. The simulated image is obtained by calculating wavelength and intensity of diffracted light traveling toward the viewing point from the CGH. Wavelength and intensity of the diffracted light are calculated using FFT image generated from interference fringe data. Parallax image of CGH corresponding to the viewing point can be easily obtained using this simulation method. Simulated image from interference fringe data was compared with reconstructed image of real CGH with an Electron Beam (EB) lithography system. According to the result, the simulated image resembled the reconstructed image of the CGH closely in shape, parallax, coloring and shade. And, in accordance with the shape of the light sources the simulated images which were changed in chroma saturation and blur by using two kinds of simulations: the several light sources method and smoothing method. In addition, as the applications of the CGH, full-color CGH and CGH with multiple images were simulated. The result was that the simulated images of those CGHs closely resembled the reconstructed image of real CGHs. Keywords: Computer-Generated Hologram (CGH), Electron Beam (EB) lithography, relief-type hologram, reconstructed image, simulation, Fast Fourier Transformation (FFT)

1. INTRODUCTION Computer-generated hologram (CGH) which is fabricated based on the three dimensional (3D) polygon data for Computer Graphics (CG) has been manufactured at Dai Nippon Printing Co., Ltd. (DNP). The CGH has been named "Virtuagram,®" which is a high-quality image-type relief hologram observable under white light. A master plate of the CGH is fabricated by calculating the interference fringe data between object light and reference light on the computer, and forming the fringe data as the minute relief structure with electron-beam (EB) lithography system1-3. Previously, the reconstructed image of the CGH was only able to confirm after the EB recording process. But, it takes long time to record the interference fringe pattern with EB lithography system and results in high cost. Therefore, a method of confirming the reconstructed image of CGH before the EB recording process was the hope. Meanwhile the method of simulating the reconstructed CGH image using Fourier transform of an interference fringe was developed at Nihon University4-5. However, because it was difficult to record the interference fringe data accurately without an EB lithography system, it was not clear how an accurate reconstracted image could be obtained by the simulation method. In this report, Chapter 2 describes the CGH manufacturing process. Chapter 3 describes the computer simulation method for obtaining a reconstructed CGH image from interference fringe data. Chapter 4 presents reconstructed image under a light source which has finite area. Finally Chapter 5 reports the results of simulated images of full color CGH and multi-image CGH. * [email protected] ** [email protected]

Practical Holography XXIII: Materials and Applications, edited by Hans I. Bjelkhagen, Raymond K. Kostuk, Proc. of SPIE Vol. 7233, 72330H · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.809774

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2. COMPUTER-GENERATED HOLOGRAM 2.1 Features of the Computer-Generated Hologram Hologram is a medium able to reconstruct 3D images. In the mastering process for ordinary holograms, object light scattered by an object and reference light interferes with each other, and recording interference fringes on holographic material produces the holograms. Alternatively, in the mastering process for the Computer-Generated Hologram (CGH), the distribution of the interference fringes of virtual objects constructed by polygon-data of 3D-CG is calculated by optical numerical simulation and the CGH is obtained by forming the interference fringes on the substrate. The interference fringe data is written accurately on the substrate with EB lithography for semiconductor manufacturing that is able to fabricate with sub-micron accuracy. Since the CGHs are produced by faithfully simulating the principle of the hologram recording process, they have characteristics of both CG and holograms and are able to precisely place objects in a 3D space. They are also able to reconstruct 3D images. The image-type hologram observable under white light "Virtuagram" made using the technology of this CGH is used as a design usage and a security label. Three manufacturing CGH processes, interference fringe data calculation, interference fringe recording, and mass-production and finishing, are explained. 2.2 Interference fringe data calculation process In the first step of the interference fringe data calculation process, the distribution of the interference fringes to be recorded is calculated numerically using 3D-CG technology and optical simulation. First, the 3D shape of the object to be recorded as a CGH is defined as digital data. For this polygon modeling tools that are commercially available can be used. Second, in preparation for the calculation of interference fringes, the geometrical and optical constants of the object are defined. The constants to be set include the size of the objects, the spatial relationship between the objects and the recording plane, the divergence angle of the object light, the incident angle and wavelength of the reference light, as well as the intervals of points where the intensity of the interference fringes is calculated. The method of calculating the interference fringe data is described below. As shown in Figure 1, the arrangement of 3D objects and the recording plane is defined in 3D space. Recording plane (CGH) Slicing plane

i=N i=N-1

Object

x

y y

Reference light

x

z

i=1 i = 2 Sample point i=3 ・ ・ ・ i Object Ai(x, y) r i (x, y)

θ z

P(x, y)

Figure 1. Arrangement of the object, the recording plane and the slicing plane

Recording plane

Calculation point

Figure 2. The cross section of the object and the recording plane

To reduce the amount of interference fringes to be calculated, the divergence of the object wave is limited to one horizontal plane and the reference light is set to enter from the vertical direction. Therefore, the vertical distribution can be omitted in calculating the object light1. Consequently, the interference fringe data is calculated within each horizontal line slicing the recording plane. The resultant CGH is constructed as an aggregation of slit-like elements of CGH and observed as a 3D image with only horizontal parallax and vertically changing colors as in a rainbow: the so-called rainbow hologram6. Figure 2 shows the cross section of the object and recording plane in which the interference pattern data are calculated within the horizontal plane (the x-z plane shown with y being constant). The object is assumed to be an aggregation of a number of point light sources. They are arranged on the object surface as a sample point close enough to be visually inseparable. Interference fringe intensity I (x, y) at calculation point P (x, y) on the recording plane, is

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obtained from the following calculation process. First, object wave O (x, y) at calculation point P (x, y) is given by Equation (1), N

O( x, y ) = ∑ [Ai ( x, y ) / ri ( x, y ) exp{kri ( x, y ) + φi )}]

(1)

i =1

where, N is the number of sample points of the object at each calculating plane, Ai (x, y) is the amplitude of the object wave at the sample point i, ri (x, y) is the distance between the sample point i and the calculation point P (x, y), k is the wave number (= 2π / λ, λ is wave length) and φi is the initial phase of the object wave at the sample point i. A (x, y) is influenced by several parameters, such as the surface property of the object, direction of the light source and direction of the object surface. However, these parameters vary with the change of observation angle. Therefore A (x, y) is calculated for all sample points by applying CG technology. Secondly, reference wave R (x, y) at calculation point P (x, y) is given by Equation (2), R( x, y ) = R0 exp{ j (ky sin θ + φ r )} (2) where, R0 is the amplitude of the reference wave,θ is the incident angle of the reference wave and φr is the initial phase of the reference wave. Finally, Interference fringe intensity I (x, y) at calculation point P (x, y) is given by Equation (3).

I ( x , y ) = O ( x, y ) + R ( x, y )

2

(3)

The distribution of the interference fringes is obtained by calculating at all calculation points. 2.3 Interference fringe recording process The second step for producing CGHs is the interference fringe recording process. CGH can be produced by recording the distribution of the interference fringes obtained from Equation (3). It is necessary to minutely process the interference fringe data by sub-micron accuracy to make CGH able to observe the reconstructed image of the image-type hologram with enough viewing angles. To manufacture CGH, the EB lithography system for semiconductor manufacturing able to fabricate with sub-micron accuracy is used. However, the EB lithography system that is used for semiconductor manufacturing can only control On-off switching. Therefore, it is impossible to fabricate the interference fringe data consisting of multilevel data. Moreover, to facilitate the reconstruction of realistic 3D images with shades and shadows, the interference fringes need to be recorded with gradation. This is particularly the case with image type holograms in which the holographic image is reconstructed near the recording plane. Consequently, as shown in Figure 3, the EB lithography system realizes gradation representation of the interference fringes by utilizing pulse-width modulation (PWM) that varies the width of the minute rectangles according to the intensity of the interference fringes2. Using the EB lithography system, the master for the CGH is completed by forming the data of the interference fringes converted to an aggregation of minute rectangles by PWM on the substrate.

Level 1 Level 2 Level 3 Level 4 Level 5

400 nanomete Figure 3. Gradation of interference fringes by pulse-width modulation (PWM)

2.4 Mass-production process In the third step, the mass-production and finishing process, the CGH is replicated on resin film in large quantities using the master produced in the interference fringe recording process. Relief holograms can be mass-produced by molding the groove pattern representing the interference fringes on thermoplastic resin or ultraviolet curable resin. Finally, depending on the purpose of the final product, the formation of the reflection layer and adhesive layer, as well as the fabrication of slits and die-cuts are performed to produce the final CGH. Figure 4 shows microscope images of the CGH and relief hologram. The shape of interference fringes of CGH is similar to that of relief holograms recorded by laser exposure. As shown in Figure 4, the CGH is formed as sets of slit-like interference fringes. This is because the interference fringe data is calculated under the setting of the object waves which are spreading to only the horizontal direction and not to the vertical direction.

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Figure 4. Photographs of CGH and embossed hologram 2.5 Problem of the CGH manufacturing process Previously, the qualities of CGH were only able to confirm after the interference fringe recording process was done. In the case of the quality of CGH is lower than that of the demand, it is necessary to modify the CG data and record modified interference fringe pattern again with the EB lithography system. It takes a long time to record the interference fringe pattern with EB lithography system, resulting in high cost. Therefore, a method for confirming the reconstructed image of CGH before the interference fringe recording process was the hope. Chapter 3 presents computer simulation of reconstructed image of CGH from interference fringe data.

3. COMPUTER SIMULATION OF RECONSTRUCTED IMAGE 3.1 Type of computer simulation One of the computer simulation methods to calculate the reconstructed image of CGH from the interference fringe is the Fresnell-Kirchhoff diffraction integral7. Using this method, an intensity of light coming from interference fringe is obtained on any point. Another method of obtaining the reconstructed image is using Fourier transform4-5. Two dimensional Fourier transform of the small segment of an interference fringe gives the direction and intensity of a diffraction light from the small segment. Therefore, the reconstructed image of the CGH can be simulated by calculating the diffraction light travels toward viewing point from all segments of the interference fringe. In this report, the Fourier tarnsform method, which is able to obtain reconstructed image for image-type hologram under white light at any viewing point, was adopted to simulate reconstructed image of CGH. The simulation method is explained as follows. 3.2 Computer simulation method using Fourier transfourm

2n

The computer simulation of reconstructed image of CGH by Fourier transform has two steps. In the first step, the interference fringe data is processed by Fourier transform. In the second step, the reconstructed image is simulated from Fourier transform image. 2n In the first step, the interference fringe data is divided into small segments, e.g., 64 pixels x 64 pixels. Then each segmented hologram is processed by Fourier transform. As the size of the interference fringe data is huge, fast Fourier transform (FFT) is used for Figure 5. FFT image of a shortening the calculation time. Therefore, the segment size needs to be n n segmented hologram set to 2 pixels x 2 pixels. The FFT image of a segmented hologram is shown in Figure 5. The FFT image shows the spatial frequency spectrum of the segmented hologram, That is, the image shows the intensity of each spatial frequency of interference fringe. In the second step, intensity of the diffracted light of each wavelength is calculated from the FFT image in the direction of a viewing point. The arrangement of viewing point, light source and segmented hologram is shown in Figure 6. The illumination light enters into the segmented hologram by the angle of θin. Then the diffraction light travels

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toward viewing point by the angle of θout. The intensity of the diffracted light in the direction toward the viewing point is calculated from the spatial frequency spectrum on the intersection line between the plane composed of the light source, viewing point and the segmented hologram and the plane of the segmented hologram as shown in dotted line in Figure 7. Slope ‘a’ of the dotted line in Figure 7 can be calculated by setting the coordinates of light source and viewing point. Light source

y

y x

θin

Viewing point θout

x

z

Hologram

y = ax Figure 6. Arrangement of viewing point, light source and hologram

Figure 7. Calculation of a wavelength from a FFT image

Then intensity of the diffracted light in the direction toward the viewing point in each wavelength is calculated by the spatial frequency spectrum on the dotted line in the FFT image shown in Figure 7. The equation for spatial frequancy f is written as

f =

sin θ out − sin θ in mλ

(4)

where θin is an angle of the illumination light, θout is an angle of diffracted light, m is diffraction order and λ is wavelength. The cordinate (x, y) corresponding to the spacial frequancy f is calculated from equation (5) and (6).

x2 + y2 pM y = ax f =

(5)

(6) where p is an pixel size of the interference fringe data and M is the number of the pixel of the segmented hologram. The pixel value of the FFT image at calculated cordinate (x, y) shows the intensity of the diffracted light at the wavelength λ. The color and the brightness of the diffracted light from the segmented hologram can be obtained as RGB pixel values by accumlating intensity of diffracted light of each wavelength calculated from the spatial frequency spectrum on the dotted line shown in Figure 7, and converting it into RGB pixel value. The computer simulated reconstructed image of the CGH is obtained by applying these calculations to all segmented holograms. 3.3 Simulation results The optical reconstructed images of a CGH fabricated by EB lithgraphy system are shown in Figure 8 and the numerical reconstructed images generated from the interference fringe data is shown in Figure 9. Figure 8 shows a reconstructed image of the CGH illuminated by white LED light source. This is almost the same setting as the reconstructed image simulation setting point light source as an illuminated light. The numerical reconstructed images shown in Figure 9 are similar to the optical reconstructed images shown in Figure 8 in view of shape, shade and parallax change.

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*

(a) from the left (b) from the front (c) from the right Figure 8. Optical reconstructed images of the CGH under white LED

(a) from the left (b) from the front (c) from the right Figure 9. Numerical reconstructed images simulated from FFT images generated from interference fringe data

4. RECONSTRUCTED IMAGE SIMULATION UNDER AREA LIGHT SOURCE In reality, CGH is observed under a light source which has finite area and which emits light beams at various angles. That is the reason the observed CGH image has been different from the simulation image under one point light source. Actually, the 3-D hologram image under a real light source has shown inevitable quality degradation. To estimate the quality loss, it might be useful to obtain simulation image considering the shape of light source. The followings are two actual examples of quality degradation. The first is when the diffused light source extends vertically to the plane of the hologram. In this case, the chromatic dispersion is vertically averaged; then chroma saturation of the image degrades. The second is when the light source extends horizontally. In this case, the right and left parallaxes of the hologram are reconstructed at the same time. The more the reconstructed image was displaced from the plane of hologram, the more the CGH image defocused. As shown in Figure 10 (a), (b) and (c), the photograph images of CGH have been taken under a point light source, under a diffused light source which extended vertically to the hologram plane and under a diffused light source which extended horizontally, respectively.

(c) light source extended to (b) light source extended the horizontal direction. to the vertical direction. Figure 10. Optical reconstructed images of the CGH

(a) point light source.

Each light source was arranged at the position of (x,y,z)=(0,667,-1000) at the system of coordinates with origin at the center of the hologram. For Figure 10(b) the light source was 200mm in width and for Figure 10(c) the light source was 100mm in width. Compared with Figure 10(a), the chroma saturation of (b) is inferior. Furthermore, in the case of (c) when compared with (a), the farther reconstructed the images sited from the hologram plane, the less the images were focused.

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To obtain more similar image to the actual, two methodologies have been studied. In the first method, the array of several point light sources has been representative of the light source which has finite area. One simulated image has been obtained with the condition under one point light source. The same number of images as point light sources have been similarly simulated. All simulated images have been combined into one with use of the image-editing software to compose the image in which the light source with finite area has been considered. Specifically, five point light sources have been presupposed as shown in Figure 11(a) and (b) to be representative of the condition of Figure 10(b) and (c), respectively. The resultant images simulated with combination of the five simulated images are shown in Figure 12. 100mm

200mm 667mm

667mm y

y

x

x

-1000mm

-1000mm z

(a) Extended to vertical direction

z (b) Extended to horizontal direction

Figure 11. Arrangement of point light sources

(a) Extended to vertical direction

(b) Extended to horizontal direction

Figure 12. Simulated image from several light sources The chroma saturation of Figure 12(a) has shown as same sa figure 10(b). The looks more sickly greenish than the image in figure 10(a). In Figure 12(b), the object images located at a distance from the hologram plane have shown indistinct outline similar to the images in Figure 10(c). As the second simulation method, the smoothing process of FFT images has been examined. Smoothed pixel values have been defined as the mean values of series of the pixels in the specified smoothing area. Considering the light source of a certain size, there must be a certain range of incident angle in Equation (4). According to Equations (5) and (6), the correspondent FFT image coordinates (x, y) must be distributed in a certain area. To simulate the condition under the light source with finite area, all points in the correspondent FFT image area should be referred in the process of calculating the diffracted light intensity. To obtain similar effect to reference several points in the FFT image area, representative mean values of the pixel in a certain smooth area have been used. To determine the reasonable width of the smoothing area, the variation width in the FFT image coordinates (x, y) corresponding to the longitude of the light source should be estimated. The FFT image coordinates (x, y) could be calculated from Equations (4), (5) and (6). Specifically, in the case of wavelength 555nm, the variation width in the FFT image coordinates (x, y) has been estimated as smoothing area.

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(a) Extended to vertical direction

(b) Extended to horizontal direction

Figure 13. Simulated image by smoothing For example, under the 200mm wide light source extending in the vertical direction as shown in Figure 11(a), the variation width in the FFT image has been three pixels in y-axis direction. In the case of Figure 11(b) in which the width of light source has been 100mm in the horizontal direction, the correspondent FFT image area has been five pixels in the x-axis direction. The resultant simulation image using the smoothing process on the FFT image in the vertical direction is shown in Figure 13 (a) and in the horizontal direction is shown in Figure 13(b). Two kinds of simulation were done and compared. These simulations show one case of arranging the several light sources and combining several reconstructed images and another of smoothing the FFT image. These simulation results were compared with the real hologram reconstructed image. Here, the average pixel values of RGB were calculated using image-editing software in one area shown in Figure 14 and are listed in Table 1. Table 1. Average pixel value of the area (normarized by G value)

Real CGH

Several light source

Smoothing Figure 14. Area of obtaining pixel value

Reconstructing Light

R

G

B

Point Light Source

0.02

1

0.14

Light source to Vertical Direction

0.54

1

0.64

Light source to Horizontal Direction

0.01

1

0.31

Point Source

0.38

1

0.21

Light source to Vertical Direction

0.57

1

0.30

Light source to Horizontal Direction

0.38

1

0.20

Point Light Source

0.38

1

0.21

Light source to Vertical Direction

0.61

1

0.36

Light source to Horizontal Direction

0.35

1

0.21

As shown in Table 1, for both cases of several light sources and smoothing, the chroma saturation of the reconstructed image with the light source extended to the vertical direction decreases compared with the reconstructed image with a point light source or the light source extended to the horizontal direction. The partially magnified figures of the images displaced from hologram plane are shown in Figure 15.

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Point light source

Light source extended to vertical direction

Light source extended to horizontal direction

(a) Real CGH (b) Several light source (c)Smoothing Figure 15. The partially magnified figures of reconstructed images and simulated images Image from real hologram, simulation image from several light source and simulation image by smoothing the FFT image are shown in Figure 15(a), (b) and (c), respectively. For the three images in Figure 15, the only image from the light source that extends 100mm to the horizontal direction is blurred because these images are composed of many horizontal parallax images. As a result, these two simulations could reconstruct the image from different light sources as compared with the reconstructed image of real CGH on the point of chroma saturation and blur. The difference between these simulations is the timing of the image synthesis of images reconstructed from the different angles of illumination light. In the case of several light sources, image synthesis is done after the simulation of diffracted lights. This method has an advantage that the fine and complex light source can be assumed because it needs only to change the positions of the light sources. However, a long time is needed to simulate with so many light sources for simulating. Alternatively, in the case of smoothing, it is done before the simulation of diffracted lights. This simulation method has an advantage to simulate the light source parameter at one time and leads to the time reduction of simulation. As the sampling is done on the FFT image, it makes difficult reflecting the fine and complex shape of light source to the calculation results. These simulation methods should be used as the situation demands.

5. EXAMPLES OF APPLICATION 5.1 Full-color CGHs Since CGHs are constructed from multiple horizontal slit-like CGH elements, CGHs function as a rainbow hologram. When the viewing point is vertically moved, the reconstructed image is observed with colors varying like a rainbow. This feature, which is caused by the chromatic dispersion of the reconstructed light, can be applied to the full-color reconstruction of 3D images3. Three types of interference fringes are calculated such that in a particular observation direction, the color of each reconstructed light, reconstructed by each interference fringe, becomes red (R), green (G) and blue (B). As shown in Figure 16, these three fringes are arranged adjacently on the recording plane at intervals close enough to be visibly inseparable. The horizontal slit-like interference fringes of RGB colors are placed alternately. Since the reconstructed red, green and blue images are observed simultaneously, due to additive color mixing when they are viewed from a designated direction, a full-color 3D image is observed. Figure 17 shows the reconstructed image of a full-color CGH. The color of the image changes as the viewing point moves in the vertical direction according to the principle of the rainbow hologram, but when it is observed from a specific observation direction, a 3D full-color image can be observed.

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White light

Elemental CGH for Red Elemental CGH for Green Elemental CGH for Blue

Reconstructed image

Figure 16. Schematic diagram for reconstructing color CGH The computer simulation of reconstructed image for full-color CGH is done. Figure 18 shows the simulation result. The simulation result and the real reconstruction image made by the EB lithography were almost the same image from the perspective of color, shape, contrasting density and parallax.

Figure 17. Reconstructed image of full color CGH

Figure 18. Simulated image of full color CGH

5.2 CGHs with multiple images The direction of the reconstructed image with respect to the direction of illumination light can be controlled in holograms. This feature enables the recording of multiple 3D images on one hologram. First, the incident angles of reference light for two objects are selected so that two reconstructed images can be observed from each direction. Then the interference fringes for each object are independently calculated. Finally, these two fringes are recorded in separate recording regions. Figure 19 shows an example of a slit-like arrangement of interference fringes for the CGH with multiple images. Illuminate light Reconstructed image A

Elemental CGH for Object A Elemental CGH for Object B Reconstructed image B Figure 19. Schematic diagram of reconstructing CGH with multiple images

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V.

Figure 20 shows the switching picture reconstructed from the real CGH. The 3D image is changing from a star to a sphere and ring by changing the angle of incident light from the upper side. The computer simulation result of the reconstructed image is shown in Figure 21. In a simulation for this kind of switching image, each image is simulated as a horizontal slit-like interference fringe and each fringe from two images is placed alternately. From the simulation results, it is observed that the image is switching the same as the real CGH when the viewing point is varticaly moved.

Figure 20. Reconstructed images of CGH with multiple images

Figure 21. Simulated images of CGH with multiple images

6. CONCLUSION It became easy to confirm that the image does not have any problem in the simulation of diffraction light from the image by Fourier transformation of interference fringes data. This simulation makes the risk of reproducing with EB lithography reduce when there are problems in the quality of the final CGH image. In this simulation, the real condition of observing the real CGH is realized combining several point light sources or smoothing the FFT image. Especially, although the incident light source is a finite area like a fluorescent light, the simulation can be accommodated. Also in this simulation, full-color CGH and multiple images CGH which interference fringes are placed as the horizontal slit-like interference fringes can be reconstructed the same as real CGH from the perspective of color, shape, contrasting density and parallax.

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T. Hamano and H. Yoshikawa, “Image-type CGH by means of e-beam printing," Proc. SPIE, 3293, pp. 2-14, 1998. T. Hamano, M. Kitamura and H. Yoshikawa, “Computer-generated holograms with pluse-width modulation for multi-level 3-D image,” Proc. SPIE, 3637, pp. 244-251, 1999. 3 T. Hamano and M. Kitamura, “Computer-generated Holograms for reconstructing multi-3D images by space-division recording method,” Proc. SPIE, 3956, pp. 23-32, 2000. 4 H. Fujita and H. Yoshikawa, “Computer simulation of reconstructed image from white light hologram(in Japanese),” HODIC Circular, Vol. 27, No. 3, pp. 6-9, 2005. 5 H. Yoshikawa, T. Yamaguchi and H. Fujita, “Computer Simulation of Reconstructed Image from Rainbow Hologram,” OSA Digital Holography and Three-Dimensional Imaging, paper DTuD3, 2007. 6 S. A. Benton, “Hologram reconstructed with extended incoherent sources,” J. Opt. Soc. Am., 59, pp. 1545-1546, 1969. 7 J. Sato, A. Wada, T. Takahashi and Y. Ishii, “Color digital holography using wavelength-shifting laser,” Optics & Photonics Japan 2006, 9pP43, pp. 382-383, 2006. 2

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