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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 22, Iss. 15 — Jul. 28, 2014
  • pp: 18010–18019
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Practical design for compact image scanner with large depth of field by compound eye system

Hiroyuki Kawano, Tatsuki Okamoto, Hajime Nakajima, Shigeru Takushima, Yoshitaka Toyoda, Satoshi Yamanaka, Tetsuo Funakura, Kosaku Yamagata, Taku Matsuzawa, Tatsuya Kunieda, and Tadashi Minobe  »View Author Affiliations


Optics Express, Vol. 22, Issue 15, pp. 18010-18019 (2014)
http://dx.doi.org/10.1364/OE.22.018010


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Abstract

We designed a new image scanner using the reflective optics of a compound eye system that can easily assemble plural imaging optical units (called imaging cells) and is compact with a large depth of field (DOF). Our image scanner is constructed from 32 reflective imaging cells, each of which takes an image of approximately a 10-mm field of view (FOV) that slightly overlap the adjacent imaging cells. The total image is rebuilt by combining the 32 images in post processing. We studied how to fold the optical path in the imaging cells and simplified the structure, resulting in the following three advances of our previous work: 1) greater compactness (50 × 31 mm2 in the cross section), 2) less variable optical characteristics among the imaging cells, and 3) easy assembly thanks to small number of optical components constructing the imaging cell.

© 2014 Optical Society of America

1. Introduction

Image scanners take two-dimensional high resolution images by a linear image sensor when an object moves at a relatively constant speed, for example, observations from artificial satellites [1

1. R. E. Noll and R. A. Tracy, “Application of visible linear array technology to earth observation sensors,” Proc. of Scanners and Imagery Systems for Earth Observation, 0051, 124–131 (1974). [CrossRef]

] or product inspection in factories. Image scanners are also useful to take precise images without distortion or magnification error, for example, the digitalization of heritage art works [2

2. J. A. Toque, Y. Sakatoku, and A. Ide-Ektessabi, “Analytical imaging of cultural heritage paintings using digitally archived images,” Proc. of SPIE-IS&T Electronic Imaging, SPIE 7531, 75310N–1-9 (2010). [CrossRef]

] and copying documents [3

3. J. P. McNaul, “Scanners As Image Input Devices,” Proc. of Industrial Applications of Solid State Image Scanners, SPIE 0145, 58–64 (1989).

]. Such a reduction type image scanner in which a monocular lens makes a reduced-size image of an object on one chip image sensor is usually used for such applications. Since the scanner needs a large object distance to take an image of a wide field of view (FOV) and needs a driving mechanism to scan the mirrors in order to keep the object distance, it occupies a large space in the copier, for example, 400 × 80 mm2 in the cross section.

Over three decades, compound eye optical systems have been researched to reduce optical size by dividing a large FOV into many small areas so that plural optical units (called imaging cells) take images of the separated areas. A wafer-level camera [4

4. A. Brückner, J. Duparré, R. Leitel, P. Dannberg, A. Bräuer, and A. Tünnermann, “Thin wafer-level camera lenses inspired by insect compound eyes,” Opt. Express 18(24), 24379–24394 (2010). [CrossRef] [PubMed]

] and TOMBO [5

5. J. Tanida, T. Kumagai, K. Yamada, S. Miyatake, K. Ishida, T. Morimoto, N. Kondou, D. Miyazaki, and Y. Ichioka, “Thin observation module by bound optics (TOMBO) concept and experimental verification,” Appl. Opt. 40(11), 1806–1813 (2001). [CrossRef] [PubMed]

] are compound eye optical systems that target thin optics. Brady et al. developed a gigapixel camera with super high resolution [6

6. D. J. Brady, M. E. Gehm, R. A. Stack, D. L. Marks, D. S. Kittle, D. R. Golish, E. M. Vera, and S. D. Feller, “Multiscale gigapixel photography,” Nature 486(7403), 386–389 (2012). [CrossRef] [PubMed]

] using many cameras whose FOVs overlap each other. Anderson used two arrays of lenses to form an erect composite image [7

7. R. H. Anderson, “Close-up imaging of documents and displays with lens arrays,” Appl. Opt. 18(4), 477–484 (1979). [CrossRef] [PubMed]

] for a close-up imaging system. Although the above examples are area cameras, there are examples of image scanners. A contact image sensor based on a gradient index (GRIN) lens array [8

8. M. Kawazu and Y. Ogura, “Application of gradient-index fiber arrays to copying machines,” Appl. Opt. 19(7), 1105–1112 (1980). [CrossRef] [PubMed]

], which can also be classified in a compound eye optical system, is widely used as a scanner in automated teller machines or the automatic document feeders of copiers. Other constitutions for contact image scanners use refractive lens arrays [9

9. K. Nagatani, K. Morita, H. Okushiba, S. Kojima, and R. Sakaguchi, US patent 5399850 (1995).

] or reflective concave mirror arrays [10

10. I. Maeda, T. Inokuchi, and T. Miyashita, US patent 4776683 (1988).

]. These scanners using compound eye systems have a DOF limit, typically under 1 mm, which is insufficient for flatbed scanners that must read the floating inner margins of a book. The limit is caused by combining images taken by non-telecentric imaging cells that change the magnification ratio with the object distance.

We previously developed image scanners with compact size and large DOFs by a new concept of compound eye optical systems that overcame the above difficulties [11

11. H. Kawano, T. Okamoto, T. Matsuzawa, H. Nakajima, J. Makita, N. Fujiyama, E. Niikura, T. Kunieda, and T. Minobe, “Compact image scanner with large depth of field by compound eye system,” Opt. Express 20(12), 13532–13538 (2012). [CrossRef] [PubMed]

,12

12. H. Kawano, T. Okamoto, T. Matsuzawa, H. Nakajima, J. Makita, Y. Toyoda, T. Funakura, T. Nakanishi, T. Kunieda, and T. Minobe, “Compact and large depth of field image scanner for auto document feeder with compound eye system,” Opt. Rev. 20(2), 254–258 (2013). [CrossRef]

]. The basic constitution is shown in Fig. 1
Fig. 1 Conceptual structure of our compound eye scanner. Each cylinder in (a) expresses an imaging cell constructed from two lenses and one aperture stop. Imaging cells are telecentric in object space and aligned in a zigzag alignment of two lines of A and B along X direction [11].
. Imaging cells shown as cylinders in Fig. 1(a) are aligned in a zigzag alignment made of two lines of A and B. A large FOV of 312 mm along the X direction is divided into 32 areas, and each imaging cell takes an image of approximately 10-mm FOV for each divided area. The divided FOVs slightly overlap as shown in Fig. 1(b), and the 32 images are electrically combined in the signal processing. The optical axes are inclined in the Y direction as shown in Fig. 1(c) to read the same position in the Y direction. One of the features of our optics is the telecentricity in the object space that enables a large DOF in the compound eye optical systems. Since the magnification ratio is constant regardless of the object distance change, the images are easily combined without magnifying or shrinking the image size. The aperture size of the objective lens must be bigger than the FOV in telecentric optics as drawn in Fig. 1(b), and therefore it is impossible to arrange lenses in a row without a clearance gap of the FOV among adjacent imaging cells. That is the reason why we arranged the imaging cells in a zigzag alignment in two rows to keep the overlapping of the adjacent FOVs while avoiding the mechanical interference of the lenses.

2. Optical design

Aberrations in an imaging system can be easily corrected when rays are gradually deflected at plural surfaces. The easiest aberration correction is probably done when the maximum deflection angle of the marginal ray is minimum. When the deflection angles of the marginal ray at L1 and L2 are defined as φ1 and φ2, the above condition equals φ1 = φ2 under thin lens approximation.

Here we consider an imaging system with reduced magnification ratio of f1 > f2. Distance a satisfying φ1 = φ2 condition is expressed as
a=lf1lf1+f2,
(1)
under thin lens approximation. If we suppose a double-sided telecentric imaging system, then l = f1 + f2, resulting in

a=12(f1+f12f2).
(2)

Figure 2(e) shows such an optical layout. If the object plane can be above L2 as shown in Fig. 2(f) after replacing the two lenses by two concave mirrors, distance a must become larger than (f1 + f2), resulting in the following condition:
f2<12f1,
(3)
derived from Eq. (2) and assuming f1 > 0 and f2 > 0. To release the strong restriction of f2 expressed in Eq. (3), a flat mirror, M1, is inserted between L1 and the aperture stop as shown in Fig. 2(g). The total height represented by distance a becomes smaller than in Fig. 2(f).

We specifically designed optics of an imaging cell based on the structure of Fig. 2(g). Since the target resolution is 600 dots per inch (dpi) and the resolution of the image sensor we chose is 1200 dpi, the magnification ratio is 0.5. A space is necessary over the imaging cell for an illumination unit and a top glass on which manuscripts are put. Therefore, distance from the object plane to M1 (c as shown in Fig. 2(g)) must be large (c = 23 mm). If we suppose the system is double-sided telecentric with the magnification ratio of 0.5, f2 equals to 0.5f1. The aberration correction is the easiest when distance a satisfies the condition of a = 1.5f1 according to Eq. (2). However, distance c cannot be as long as 23 mm in this condition. Therefore, the condition of telecentricity in the image space is released, forming one-sided telecentric in the object space.

Figure 3
Fig. 3 Optical design layout. (a) Perspective view of four imaging cells. (b) Cross-sectional view. The height of 31 mm includes the illumination unit and the base plate. Total width is 50 mm.
shows the optical layout designed with a commercially available optical design software to reduce aberrations. Figure 3(a) is a perspective view in which four imaging cells are aligned in zigzag alignment, and Fig. 3(b) is the cross-sectional view. The total height is 31 mm including the illumination unit and the base plate, and the width is 50 mm including image sensors. In compound eye optical systems, it is important that optical characteristics of each imaging cell are uniform within the FOV because we easily recognize image characteristics that are shown repeatedly in the combined total image. Therefore, we applied free-form surfaces to L1 and L2 mirrors to suppress aberrations like distortion and to uniform values of modulation transfer function (MTF) within the FOV.

Figure 4
Fig. 4 Simulation result of optical characteristics. (a) MTF vs object height in X. Spatial frequencies in legend are translated values in object space. 12 lp/mm corresponds to Nyquist spatial frequency of 600 dpi. (b) Distortion in Y vs object height in X. Distortion is expressed in position translated in object space. (c) MTF vs defocus amount in object space. Spatial frequency of MTF is 6 lp/mm in object space. Legends are object heights h in X and Y directions.
shows the simulation result of some optical characteristics. In Fig. 4(a), MTF is plotted as the function of the object height in the X direction, where graph (1) and (2) are calculated at the spatial frequency of 6 line-pairs/mm (lp/mm) in the X and Y directions, respectively, and graph (3) and (4) are that of 12 lp/mm corresponding to Nyquist spatial frequency of 600 dpi. Every graph is almost horizontal, which shows that image quality is even within the FOV. Figure 4(b) shows the image distortion against the object height in the X direction, that is to say how distorted the straight line object along the X axis is in the image. The vertical axis is the value of 2Yimg, where Yimg is the Y coordinate value of the arrived point of the chief ray on the image plane and Yimg is multiplied by the coefficient two to translate in the object space value. The distortion value is 1 μm at most, showing that the distortion is negligible for the pixel size of 42 μm of 600 dpi. Figure 4(c) shows MTF at 6 lp/mm against the defocus amount in the object space, calculated at two object heights, h = 0 and 5 mm, in the X and Y directions. The four graphs almost overlap each other, showing that the image quality is uniform even at defocused positions.

3. Assembly

Aperture stops are made on holders, which are placed behind the coating of the M1 mirror in Fig. 5(b). The holders also have roles to hold the image sensor substrates and prevent stray light between the imaging cells. Four neighboring holders are combined as one piece made by plastic molding. The structure is simple with a low number of components.

Figure 5(c) shows the finished prototype of our new image scanner after an illumination unit was placed on the imaging cells. It is compact; its width and height are 50 and 31 mm, respectively.

4. Experimental result

Figure 6
Fig. 6 Experimental setup.
shows our experimental setup where a test chart is placed on a moving stage shown on the left and the prototype of our image scanner is facing downward in the center. The stage moves back and forth to the right and the image scanner reads the test chart’s image, which is a sterically-bulky object of accessories on a piece of rough cloth. The image information taken by the 32 image sensors is sent to a personal computer and the images are combined. The monitor on the right in Fig. 6 displays the processed image. Figure 7
Fig. 7 Image of sterically-bulky object of accessories on a piece of rough cloth. Picture width is 130 mm.
shows the part of it that indicates a clear image from the top of the accessory to the cloth under it. Figure 8(b)
Fig. 8 Image of five test charts pasted on a step-like object. Arial is a registered trademark of The Monotype Corporation. (a) Cross section of test chart of step-like object. (b) Image. Depths are 4.0 mm to 0 from left to right.
is an image of a step-like object, which shows the images from the depth of the reference to 4.0 mm. Figure 8(a) is the cross section, and five pieces of the character charts are pasted on top of each step surface. 6-point characters are clearly imaged at all the depths.

Figure 9
Fig. 9 Contrast at defocused position. Chart pattern is ronchi-ruling of 7.1 lp/mm spatial frequency (358 dpi).
compares the measured data and the designed value of MTF at 7.1 lp/mm through Z, which is the floating distance from a reference plane to the object. The test chart’s pattern is a Ronchi ruling, where black lines and spaces are equally spaced at 7.1 lp/mm, equivalent to 358 dpi, in the two orthogonal directions of X and Y. The images were taken at different distances of Z and contrast C is calculated by Eq. (4):
c=ImaxIminImax+Imin,
(4)
where Imax and Imin are the maximum and the minimum values of the intensity. The contrast value of C is plotted as MTF in Fig. 9, where the numerical aperture in the object space is NA = 0.021, and the calculated MTF data are shown in solid lines. Our prototype’s DOF exceeds 4.0 mm when we define DOF as a range exceeding 30% of MTF at 7.1 lp/mm.

5. Comparison

6. Summary

References and links

1.

R. E. Noll and R. A. Tracy, “Application of visible linear array technology to earth observation sensors,” Proc. of Scanners and Imagery Systems for Earth Observation, 0051, 124–131 (1974). [CrossRef]

2.

J. A. Toque, Y. Sakatoku, and A. Ide-Ektessabi, “Analytical imaging of cultural heritage paintings using digitally archived images,” Proc. of SPIE-IS&T Electronic Imaging, SPIE 7531, 75310N–1-9 (2010). [CrossRef]

3.

J. P. McNaul, “Scanners As Image Input Devices,” Proc. of Industrial Applications of Solid State Image Scanners, SPIE 0145, 58–64 (1989).

4.

A. Brückner, J. Duparré, R. Leitel, P. Dannberg, A. Bräuer, and A. Tünnermann, “Thin wafer-level camera lenses inspired by insect compound eyes,” Opt. Express 18(24), 24379–24394 (2010). [CrossRef] [PubMed]

5.

J. Tanida, T. Kumagai, K. Yamada, S. Miyatake, K. Ishida, T. Morimoto, N. Kondou, D. Miyazaki, and Y. Ichioka, “Thin observation module by bound optics (TOMBO) concept and experimental verification,” Appl. Opt. 40(11), 1806–1813 (2001). [CrossRef] [PubMed]

6.

D. J. Brady, M. E. Gehm, R. A. Stack, D. L. Marks, D. S. Kittle, D. R. Golish, E. M. Vera, and S. D. Feller, “Multiscale gigapixel photography,” Nature 486(7403), 386–389 (2012). [CrossRef] [PubMed]

7.

R. H. Anderson, “Close-up imaging of documents and displays with lens arrays,” Appl. Opt. 18(4), 477–484 (1979). [CrossRef] [PubMed]

8.

M. Kawazu and Y. Ogura, “Application of gradient-index fiber arrays to copying machines,” Appl. Opt. 19(7), 1105–1112 (1980). [CrossRef] [PubMed]

9.

K. Nagatani, K. Morita, H. Okushiba, S. Kojima, and R. Sakaguchi, US patent 5399850 (1995).

10.

I. Maeda, T. Inokuchi, and T. Miyashita, US patent 4776683 (1988).

11.

H. Kawano, T. Okamoto, T. Matsuzawa, H. Nakajima, J. Makita, N. Fujiyama, E. Niikura, T. Kunieda, and T. Minobe, “Compact image scanner with large depth of field by compound eye system,” Opt. Express 20(12), 13532–13538 (2012). [CrossRef] [PubMed]

12.

H. Kawano, T. Okamoto, T. Matsuzawa, H. Nakajima, J. Makita, Y. Toyoda, T. Funakura, T. Nakanishi, T. Kunieda, and T. Minobe, “Compact and large depth of field image scanner for auto document feeder with compound eye system,” Opt. Rev. 20(2), 254–258 (2013). [CrossRef]

OCIS Codes
(040.1240) Detectors : Arrays
(110.0110) Imaging systems : Imaging systems
(120.5800) Instrumentation, measurement, and metrology : Scanners
(220.0220) Optical design and fabrication : Optical design and fabrication

ToC Category:
Imaging Systems

History
Original Manuscript: May 16, 2014
Revised Manuscript: June 20, 2014
Manuscript Accepted: June 20, 2014
Published: July 17, 2014

Virtual Issues
Vol. 9, Iss. 9 Virtual Journal for Biomedical Optics

Citation
Hiroyuki Kawano, Tatsuki Okamoto, Hajime Nakajima, Shigeru Takushima, Yoshitaka Toyoda, Satoshi Yamanaka, Tetsuo Funakura, Kosaku Yamagata, Taku Matsuzawa, Tatsuya Kunieda, and Tadashi Minobe, "Practical design for compact image scanner with large depth of field by compound eye system," Opt. Express 22, 18010-18019 (2014)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-15-18010


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References

  1. R. E. Noll and R. A. Tracy, “Application of visible linear array technology to earth observation sensors,” Proc. of Scanners and Imagery Systems for Earth Observation,0051, 124–131 (1974). [CrossRef]
  2. J. A. Toque, Y. Sakatoku, and A. Ide-Ektessabi, “Analytical imaging of cultural heritage paintings using digitally archived images,” Proc. of SPIE-IS&T Electronic Imaging, SPIE 7531, 75310N–1-9 (2010). [CrossRef]
  3. J. P. McNaul, “Scanners As Image Input Devices,” Proc. of Industrial Applications of Solid State Image Scanners, SPIE0145, 58–64 (1989).
  4. A. Brückner, J. Duparré, R. Leitel, P. Dannberg, A. Bräuer, and A. Tünnermann, “Thin wafer-level camera lenses inspired by insect compound eyes,” Opt. Express18(24), 24379–24394 (2010). [CrossRef] [PubMed]
  5. J. Tanida, T. Kumagai, K. Yamada, S. Miyatake, K. Ishida, T. Morimoto, N. Kondou, D. Miyazaki, and Y. Ichioka, “Thin observation module by bound optics (TOMBO) concept and experimental verification,” Appl. Opt.40(11), 1806–1813 (2001). [CrossRef] [PubMed]
  6. D. J. Brady, M. E. Gehm, R. A. Stack, D. L. Marks, D. S. Kittle, D. R. Golish, E. M. Vera, and S. D. Feller, “Multiscale gigapixel photography,” Nature486(7403), 386–389 (2012). [CrossRef] [PubMed]
  7. R. H. Anderson, “Close-up imaging of documents and displays with lens arrays,” Appl. Opt.18(4), 477–484 (1979). [CrossRef] [PubMed]
  8. M. Kawazu and Y. Ogura, “Application of gradient-index fiber arrays to copying machines,” Appl. Opt.19(7), 1105–1112 (1980). [CrossRef] [PubMed]
  9. K. Nagatani, K. Morita, H. Okushiba, S. Kojima, and R. Sakaguchi, US patent 5399850 (1995).
  10. I. Maeda, T. Inokuchi, and T. Miyashita, US patent 4776683 (1988).
  11. H. Kawano, T. Okamoto, T. Matsuzawa, H. Nakajima, J. Makita, N. Fujiyama, E. Niikura, T. Kunieda, and T. Minobe, “Compact image scanner with large depth of field by compound eye system,” Opt. Express20(12), 13532–13538 (2012). [CrossRef] [PubMed]
  12. H. Kawano, T. Okamoto, T. Matsuzawa, H. Nakajima, J. Makita, Y. Toyoda, T. Funakura, T. Nakanishi, T. Kunieda, and T. Minobe, “Compact and large depth of field image scanner for auto document feeder with compound eye system,” Opt. Rev.20(2), 254–258 (2013). [CrossRef]

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