Abstract:
An apparatus for reading an image of the obverse side of a thin sheet includes a light source for emitting a light for illuminating the obverse side of the thin sheet. The light contains two fractions having different wave lengths from each other. When the light has been reflected on the thin sheet, first and second image pickup means receive one and the other of the aforesaid two fractions respectively. In order to be free of the adverse effect of characters, etc. printed on the reverse side of the thin sheet and seen therethrough on the obverse side, an signal processing system is operable such that image information on the obverse side of the thin sheet alone is extracted from image data outputted from the first and second image pickup means respectively.

Description:
BACKGROUND OF THE INVENTION 
     This application is based on Japanese Patent Application No. 09-064360 filed on Mar. 18, 1997, the contents of which are incorporated herein by reference. 
     1. Field of the invention 
     This invention relates to an apparatus for reading an image, and has particular reference to an apparatus capable of reproducing the obverse side of paper such that a copy taken is free of the adverse effect of characters, etc. printed on the reverse side of the paper and seen therethrough. 
     2. Description of the prior art 
     Books and magazines commonly in print consist of leaves, each of which has characters and/or charts printed on both sides of each leaf. In a known apparatus of the kind indicated above, it has been found that, when a book or magazine consists of thin leaves, the apparatus reads not only characters, etc. printed on the obverse side of a leaf but also characters, etc. printed on the reverse side of the leaf and seen therethrough. This will inevitably cause an impairment of picture quality. 
     Two methods of image information processing have been previously proposed to reduce the effects of characters, etc. printed on the reverse side of thin paper. One of these two methods is described in Japanese Laid Open Patent Application No. 7-87295, which discloses a copying machine adapted to copy both sides of an original. In this prior art method, a specific picture element contained in an image of the obverse side of the original is regarded as representing a portion of an image of characters, etc. printed on the reverse side of the original if ( 1 ) the photographic density of the specific picture element is less than a prescribed value and ( 2 ) if the photographic density of a picture element contained in an image of the reverse side of the original and corresponding to the specific picture element is greater than another prescribed value. The other of the two methods involves preparing a histogram for the distribution of the luminances of picture elements. For the purpose of correction, the luminances of picture elements read from the upper surface are calculated from the shape of the histogram. 
     A drawback of the first of the two methods is that this method can be implemented only in a copying machine adapted to copy both sides of an original. On the case of a copying machine in which an original has to be laid prone on an original glass plate and a copy of only one side of the original can be taken at one time, it is difficult to ensure the correct mating of the position of the original after turning-up with that before turning-up. Especially when one double-page spread after another of a book or magazine is an object to be read, difficulty is encountered all the more because, every time a leaf is to be turned over, the book or magazine has to be turned-up and positional again to lie prone on the original glass plate. In the case of a book scanner wherein a book or magazine is mounted on a baseboard so as to lie face up, difficulty is likewise encountered because, every time a leaf has been turned over, the positional relationship between the characters, etc. and the baseboard changes. Furthermore, the first of the two methods requires two image memories for storing data read from both sides of paper respectively. Obviously, this requirement has imposed a serious economic disadvantage to the first of the two methods. 
     With the second of the two methods, it has been found that a lightly printed or written image or photograph such as characters, etc. written in pencil or in cinnabar is frequently confused with that darkly printed or written on the reverse side of thin paper and is erased by mistake. 
     SUMMARY OF THE INVENTION 
     A principal object of this invention is to provide an apparatus of the kind indicated above, which eliminates the necessity of providing two image memories and which does not read characters, etc. printed on the reverse side of a thin leaf and seen therethrough. 
     In order to be free of the image of characters, etc. printed on the reverse side of a thin leaf and seen therethrough, this invention takes advantage of the fact that the reflection characteristics of a light incident on a manuscript or the like depend on the wave length. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view illustrating the appearance of a book scanner embodying this invention; 
     FIG. 2 shows the spectral sensitivity characteristics of line sensors; 
     FIG. 3 is a block diagrammatic representation of the book scanner; 
     FIG. 4 is a schematic representation to help explain how a known apparatus reads an image printed on the reverse side of a leaf; 
     FIG. 5 is a flow diagram representing successive steps in a typical operation of the book scanner; and 
     FIG. 6 is a view showing the component parts of an auxiliary scanning mechanism incorporated in a second preferred embodiment of the inventive apparatus. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 1, a book scanner  1  embodying this invention will be found advantageous when used in reading one double-page spread after another of a book BD. The book scanner  1  comprises a housing  10  within which an electrical circuit is disposed, a supporting member  12  extending upwardly from the upper surface of the housing  10 , a baseboard  20  projecting forwardly from the housing  10  such that an object to be read is mounted thereon so as to lie face up, an image pickup unit  30  supported by the supporting member  12  so as to overhang the baseboard  20  and adapted to convert an image into electric signals, and a lamp unit  40  mounted on the underside of the image pickup unit  30  at the rearward end of the lamp unit  40  and adapted to illuminate the object by means of radiation emitted by a tungsten halogen lamp with a wave length of 400 to several thousands of nanometers falling within the visible and infrared regions. There is a relatively greater amount of space  80  between the baseboard  20  and the image pickup unit  30  so as to facilitate the work of turning over the leaves of the book BD on the baseboard  20 . 
     A control panel OP having a liquid crystal display is mounted on the front wall of the housing  10  in the upper end portion of the housing  10 . A profile projector  18  for measuring the height of page S 1  at which the book BD is opened is mounted on the front wall of the housing  10  in the lower end portion of the housing  10 . The end face Se of the book BD is reflected in the profile projector  18  and is photographed together with characters, etc. printed on page S 1 . The height of page S 1  is calculated from the shape of the photographed end face Se. 
     The housing  10  is provided on its flank with a main switch  51 . Start keys  52  and  53  are spaced apart transversely to the baseboard  20  on the upper surface thereof. An arm rest  25  is mounted on the front edge of the baseboard  20 . 
     The image pickup unit  30  includes line sensors  31  and  32  consisting of CCD arrays and having an identical photoelectric transfer characteristic. The image pickup unit  30  further includes an optical system OS consisting of a mirror  33 , image-forming lens  34 , and filter mirror  35 . By virtue of the optical system OS, characters, etc. printed on page S 1  are allowed to cast their reflections on the light-receiving surfaces of the line sensors  31  and  32 . The distribution of spectral sensitivity characteristics of the line sensors  31  and  32  is normal if the light beam reflected back thereto has a wave length ranging substantially from 400 to 1,000 nm. The image-forming lens  34  can be moved forwardly and rearwardly to a new position determined by an automatic focusing mechanism (not shown). The line sensors  31  and  32  are mounted on the moving parts of auxiliary scanning mechanisms (not shown) so as to be horizontally displaceable. Main scanning is carried out in forward and rearward directions on the upper surface of the baseboard  20 . These directions correspond to the upward and downward directions respectively on the image pickup surfaces of the line sensors  31  and  32 . Area sensors may be used in place of the line sensors  31  and  32 . After passage through the image-forming lens  34 , the radiation is divided by the filter mirror  35  into two fractions. Infrared rays are reflected on the surface of the filter mirror  35  and directed to the line sensor  32  as infrared rays Li. Visible rays with wave lengths less than about 650 nm are allowed to pass through the filter mirror  35  and directed to the line sensor  31  as visible rays Lv. Properly selected characteristics of the line sensor  31  and the filter mirror  35  will permit the spectral sensitivity characteristic of the line sensor  31  to approximate the human visual sensitivity, with wave lengths centering around 550 nm as shown in FIG.  2 . The spectral sensitivity characteristic of the line sensor  32  is represented by wave lengths centering around 850 nm. 
     In one instance of use of this invention, the book BD is allowed to lie face up on the baseboard  20  in such a manner that a border line between two pages constituting the double-page spread S 1  is brought into line with a center mark put on the upper surface of the baseboard  20  medially of the flanks of the baseboard  20  and that the upper edges of the front and back covers of the book BD engage the lower edge of the profile projector  18 . 
     Referring now to FIG. 3, a block diagrammatic representation of the book scanner  1  is shown. A CPU  101  incorporating a microcomputer effects control over the book scanner  1  and carries out data processing for measuring the height of page S 1  and the luminance of the white ground of page S 1 . A RAM  105  is provided as a work area for a program to be executed by the CPU  101 . A CCD driving circuit  130  for supplying clock pulses to the line sensors  31  and  32 , auxiliary scanning mechanisms  131 A and  131 B for displacing the line sensors  31  and  32  respectively in horizontal directions, an automatic focusing mechanism  132  for moving the image-forming lens  34 , and a lamp control circuit  140  for effecting on-off control over the lamp unit  40  are connected to the CPU  101 , in addition to the main switch  51 , start keys  52  and  53 , and control panel OP. 
     The book scanner  1  includes a signal processing system  100  comprising A/D converters  121  and  122 , illuminance correcting circuits  123  and  124 , line memories  125  and  126 , arithmetic unit  127  and image information processing circuit  128 . A photoelectric transfer signal outputted from the line sensor  31  is converted by the A/D converter  121  into, e.g., an 8-bit image data, which is subjected to shading correction in the illuminance correcting circuit  123 . An image data Dv outputted from the illuminance correcting circuit  123  is temporarily stored in the line memory  125  having a capacity of storing data from a line. A photoelectric transfer signal outputted from the line sensor  32  is quantized by the A/D converter  122 , from which the signal is transmitted to the illuminance correcting circuit  124  and then to the line memory  126  for being temporarily stored in the line memory  126 . The line memories  125  and  126  serve as data buffers that hold data being transferred from the illuminance correcting circuits  123  and  124  to the arithmetic unit  127 . 
     The arithmetic unit  127  performs arithmetic operations as will appear hereinafter on the basis of image data Dv and Di outputted from the line memories  125  and  126  respectively and produces an image data D 1  free of characters, etc. printed on the reverse side of the leaf. The image data D 1  is fed to the image information processing circuit  128 , in which luminance is corrected so that the image may be reproduced with a designated photographic density. Size is also corrected in accordance with paper size or designated expansion ratio. The distortion of the image caused by the curved portions of leaves in the immediate vicinity of the border line between two pages constituting the double-page spread S 1  is also corrected by variable power. An image data D 2  outputted from the image information processing circuit  128  is fed to an external device such as a printer, display, image memory or image editing device. 
     The function of the arithmetic unit  127  will now be explained in connection with FIG. 4, in which the uppermost leaf of the book BD on the baseboard  20  is designated as P 1 , while the second leaf from the top is designated as P 2 . However, the following description is applicable also to a single sheet of paper. The upper surface of the leaf P 1  is designated as S 1 , while the underside of the leaf P 1  is designated as S 2 . 
     Calculation of Reflectivity 
     Specific reference is now made to how the radiation L 0  emitted by the lamp unit  40  is reflected on the surfaces of the leaf P 1 . As is known, the quantity I′ of light reflected on the surface of an object is Proportional to the reflectivity R as shown by the formula: 
     
       
         I′=I 0 ·R  (1) 
       
     
     where I 0  is the quantity of radiation emitted by a light source. On the other hand, the reflectivity R is given by 
     
       
         R={fraction (1/10)} D   (2) 
       
     
     where D is the photographic density of characters, etc. printed on the surface of the object. 
     When the book BD is irradiated by the radiation L 0 , a major portion L 1  of the radiation L 0  is reflected on the upper surface S 1  of the leaf P 1 , while the remainder L 1 ′ of the radiation L 0  enters the leaf P 1  and is either reflected on a coloring material attached to the underside S 2  of the leaf P 1  as a result of printing characters, etc. thereon or reflected on the upper surface S 3  of the leaf P 2 . The reflected light beam from the coloring material is designated as L 2 . This phenomenon presents itself irrespective as to whether or not there are characters, etc. printed on the upper surface S 1  of the leaf P 1 , and an adverse effect on the picture quality results from the reflected light beam L 2 . 
     From the foregoing, it will be apparent that a light L incident on the line sensors  31  and  32  is the sum of the reflected light beams L 1  and L 2 . Therefore, the quantity I′ of the light L is given by 
     
       
         I′=I 0 ·(1−T)·Rs+I 0 ·T·Rb  (3) 
       
     
     where Rs=reflectivity on the upper surface S 1   
     Rb=reflectivity on the underside S 2   
     T=transmittance by the leaf P 1   
     Ordinarily, a book consists of porous leaves, the surfaces of which are microscopically uneven to the extent of scattering the radiation L 0 . The ratio of scattered light to the incident light is inversely proportional to the biquadrate of the wave length. Consequently, the quantity of infrared rays scattered on the upper surface S 1  of the leaf P 1  is smaller than the quantity of visible rays scattered thereon, because the wave lengths of infrared rays are greater than those of visible rays. This means that the quantity of infrared rays entering the leaf P 1  is greater than the quantity of visible rays entering the leaf P 1 . Consequently, the quantity Iv′ of visible rays incident on the line sensor  31  differs from the quantity Ii′ of infrared rays incident on the line sensor  32 . The quantities Iv′ and Ii′ are given by 
     
       
         Iv′=I 0 ·(1−Tv)·Rs+I 0 ·Tv·Rb  (4) 
       
     
     
       
         Ii′=I 0 ·(1−Ti)·Rs+I 0 ·Ti·Rb  (5) 
       
     
     where Tv=transmittance of visible rays 
     Ti=transmittance of infrared rays 
     Accordingly, general expressions for finding the reflectivities Rv and Ri of visible and infrared rays on the upper surface S 1  may be derived from equations (4) and (5) respectively as: 
     
       
         Rv=Iv′/I 0 =(1−Tv)·Rs+Tv·Rb  (6) 
       
     
     
       
         Ri=Iv′/I 0 =(1−Ti)·Rs+Ti·Rb  (7) 
       
     
     Extraction of Characters, etc. Printed on the Upper Surface S 1   
     As stated above, a scattering loss is inversely proportional to the biquadrate of a wave length. This means that the ratio α of the transmittance Ti of infrared rays to the transmittance Tv of visible rays is equal to the biquadrate of the ratio of the wave length λi of infrared rays to the wave length λv of visible rays as shown by the formula a: 
     
       
         α=Ti/Tv=(λi/λv) 4   (8) 
       
     
     As will be appreciated, the most favorable result will of course be obtained from arithmetic operations performed on the basis of the actual distribution of wave lengths found at the time of image formation. For the sake of simplicity, however, 550 and 850 nm, around which the wave lengths representing the spectral sensitivity characteristics of the line sensors  31  and  32  respectively are assumed to be centering in this embodiment, are used for calculating the ratio α. Then, we obtain 
     
       
         α=(850/550) 4 ≈5.7 
       
     
     Putting equation (8) into equation (7), we have 
     
       
         Ri=(1−Ti)·Rs+Ti·Rb 
       
     
     
       
          =(1−α·Tv)·Rs+α·Tv·Rb  (9) 
       
     
     Reflectivity Rs on the characters, etc. printed on the upper surface S 1  is found from simultaneous equations (6) and (9) as follows:                      α                 Rv     =                    α        (     1   -   Tv     )       ·   Rs     +     α   ·   Tv   ·   Rb                       α                 Rv     -   Ri     =                    α        (     1   -   Tv     )       ·   Rs     +     α   ·   Tv   ·   Rb     -       (     1   -     α   ·   Tv       )     ·   Rs     -     α   ·   Tv   ·   Rb                   =                  (     α   -   1     )     ·   Rs                   ∴   Rs     =                  (       α   ·   Rv     -   Ri     )     /     (     α   -   1     )                     (   10   )                                
     This reflectivity Rs is multiplied by the quantity I 0  of radiation L 0 . Image data obtained from this multiplication do not contain image information on the characters, etc. printed on the underside S 2  but contain image information on those printed on the upper surface S 1  alone, as shown by the formula:                        I   o     ·   Rs     =                  I   o     ·       (       α   ·   Rv     -   Ri     )     /     (     α   -   1     )                     =                  (       α                   Iv   ′       -     Ii   ′       )     /     (     α   -   1     )                     (   11   )                                
     The arithmetic unit  127  fetches image data Dv and Di on an identical picture element from the line memories  125  and  126  respectively. The values of the ratio α and the quantity I 0 , which are stored beforehand, are applied to equation (11), and the image data D 1  on the characters, etc. printed on the upper surface S 1  are calculated. The image data Dv represents the quantity Iv′ of visible rays incident on the line sensor  31 , while the image data Di represents the quantity Ii′ of infrared rays incident on the line sensor  32 . These arithmetic operations are performed in order of picture elements provided on the surfaces of the line sensors  31  and  32 . The image data D 1  free of characters, etc. printed on the underside S 2  is fed to the image information processing circuit  128 . 
     For a more complete understanding of the successive steps in a typical operation of the book scanner  1 , reference FIG.  5 . 
     An illuminating lamp is lighted when the start key  52  or  53  is actuated. Scanning is commenced with the line sensors  31  and  32  moved in horizontal directions (steps  1  to  3 ). Every time a line has been scanned, image data Dv and Di are temporarily stored in the line memories  125  and  126  respectively. Then, image data on the characters, etc. printed on the upper surface S 1  are extracted, subjected to image information processing, and fed to an external device (steps  4  to  7 ). When images have been outputted from all lines, the illuminating lamp is allowed to go out so as to provide standby conditions (steps  8  and  9 ). 
     As has been above explained, an objectionable feature of the prior art method is that, even when a copy of only the upper surface S 1  is desired, both sides of the leaf P 1  have to be scanned and two image memories have to be provided for storing data read from both sides of the leaf P 1  respectively. By contrast, an important facet of the invention is that the underside S 2  need not be scanned when a copy of only the upper surface S 1  is desired, because the image of characters, etc. printed on the underside S 2  are erased from image information obtained from the upper surface S 1 . This important facet of the invention obviates the necessity of providing two image memories and thereby serves to make the apparatus compact and relatively inexpensive in cost. 
     Referring now to FIG. 6, an auxiliary scanning mechanism  131  is shown and differs from that of FIG. 1 by the fact that the line sensors  31  and  32  are provided side by side on a single scanner  313 . The light beams are incident on the line sensors  31  and  32  after passage through the image-forming lens  34 . An interference filter located in front of the line sensor  31  passes only visible rays through to the line sensor  31 . Another interference filter located in front of the line sensor  32  passes only infrared rays through to the line sensor  32 . These two filters obviate the necessity of providing the filter mirror  35 . To cause the desired horizontal displacement in directions M 2 , the scanner  313  is carried by an adjustment control rod  312  having at one end a driven pulley that is connected by a belt to a driving pulley carried by a shaft from a motor  311 . The auxiliary scanning mechanism  131  further includes a pair of guide rails  314  for horizontal guiding of the scanner  313 . Photointerrupters S 311  and S 312  for detecting the position of the scanner  313  are provided one at each end of the path of travel of the scanner  313  so as to cooperate with a douser  315  provided on the scanner  313 . Since the line sensors  31  and  32  are carried along in the horizontal movement of the scanner  313 , an instant when a reflected light beam from a portion of the object is incident on the line sensor  31  differs from an instant when a reflected light beam from the same portion of the object is incident on the line sensor  32 . In order to compensate for such time difference established between the line sensors  31  and  32 , the line memory for the line sensor  31 , which is exposed to the reflected light beam earlier than the line sensor  32 , should be a buffer that holds data for such a length of time as to be required for storing data on n lines. The value of n is determined by a period of image formation and a space between two line sensors. By virtue of this buffer, image data obtained by the line sensors  31  and  32  from the same portion of the object can be fed to the arithmetic unit  127  at a time. 
     This invention may of course be applied also to an apparatus in which an original has to be laid prone on an original glass plate.