Patent Publication Number: US-7715071-B2

Title: Apparatus for measuring color values of prints

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to an apparatus for measuring color values of prints for performing a shading correction of image data obtained by imaging reference plates with an imaging device, and to a shading correcting method. 
   2. Description of the Related Art 
   A known such apparatus is disclosed in Japanese Unexamined Patent Publication No. 8-98023. This apparatus includes an imaging device for imaging objects, and a sensitivity adjusting device for adjusting sensitivity of the imaging device based on reference values obtained by imaging a white reference plate and a black reference plate. The apparatus can thereby obtain proper color values from the objects. 
   In order to reduce spoilage, an actual printing operation may be carried out such that printing is performed at low speed after starting the printing until the printing stabilizes, and at high speed after the printing attains stability. Further, printing speed may be adjusted as appropriate according to the thickness of printing paper and the like. When, with the printing speed varied as above, densities or colorimetric values (hereinafter collectively called “color values”) of prints are measured while the prints are transported, a device is used for varying intervals for imaging the prints (hereinafter called sampling intervals) in order to equalize resolution. As a result, when print transport speed is changed, the amount of color value data obtained by imaging the prints at predetermined sampling intervals varies even if the prints are identical. Therefore, the apparatus described in Japanese Unexamined Patent Publication No. 8-98023 cannot obtain proper color values of the prints, despite the sensitivity adjustment, when reference paper and the prints are transported at different speeds. 
   On the other hand, Japanese Patent Publication No. 2-57772 discloses a technique for correcting color values to proper values by multiplying actually detected color values by a ratio between a transport speed in time of reference measurement of prints and a transport speed in time of inspection. 
   However, the color value and transport speed are not necessarily in exact proportionality. It is therefore impossible to correct color values to proper values only by multiplying actually detected color values by a ratio between a transport speed in time of reference measurement of prints and a transport speed in time of inspection. 
   SUMMARY OF THE INVENTION 
   The object of this invention, therefore, is to provide an apparatus for measuring color values of prints and a shading correcting method, capable of correcting the color values of the prints accurately even when print transport speed changes. 
   The above object is fulfilled, according to this invention, by an apparatus for measuring color values of prints, having an imaging device with a plurality of imaging elements arranged transversely of a print transport direction for imaging, at predetermined intervals, the prints transported by a transport device, the apparatus performing a shading correction of image data from the plurality of imaging elements by using reference values obtained by imaging reference plates with the imaging device, the apparatus comprising a control device for variably controlling a print transport speed of the transport device; a storage device for storing a relationship between the print transport speed and reference values corresponding to the print transport speed; and a correcting device for correcting color values obtained by imaging the prints with the imaging device, by using reference values corresponding to an actual print transport speed obtained from a print transport speed in time of actual printing and the relationship, stored in the storage device, between the print transport speed and the reference values corresponding to the print transport speed. 
   With the shading correction noted above, the color values of prints may be corrected accurately even when a print transport speed changes. 
   In a preferred embodiment, the relationship, stored in the storage device, between the print transport speed and the reference values corresponding to the print transport speed is determined by using preliminary reference values obtained by imaging the reference plates transported at a plurality of different speeds as a preliminary step. 
   In another preferred embodiment, the reference values are a white reference value obtained by imaging a white reference plate, and a black reference value obtained by imaging a black reference plate. 
   In a different aspect of the invention, a shading correcting method for performing a shading correction of image data from the plurality of imaging elements by using reference values obtained by imaging reference plates with the imaging device, the method comprising a reference plate imaging step for imaging the reference plates with the imaging device; a storing step for storing a relationship between the print transport speed and reference values corresponding to the print transport speed; a print imaging step for imaging, with the imaging device, the prints transported by the transport device; and a correcting step for correcting color values obtained by imaging the prints with the imaging device, by using reference values corresponding to an actual print transport speed obtained from a print transport speed in time of an actual printing operation and the relationship, stored in the storing step, between the print transport speed and the reference values corresponding to the print transport speed. 
   Other features and advantages of the invention will be apparent from the following detailed description of the embodiments of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For the purpose of illustrating the invention, there are shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangement and instrumentalities shown. 
       FIG. 1  is a schematic view of a printing machine according to this invention; 
       FIG. 2  is a schematic view showing, along with a print discharge cylinder, an image pickup station for reading images from prints; 
       FIG. 3  is a block diagram showing a principal electrical structure of the printing machine; 
       FIG. 4  is a flow chart of a shading correcting method according to this invention; and 
       FIG. 5  is a graph showing a relationship between white preliminary reference value Wc and black preliminary reference value Bc, and storage time t. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An embodiment of this invention will be described hereinafter with reference to the drawings. The construction of a printing machine according to this invention will be described first.  FIG. 1  is a schematic view of the printing machine according to this invention. 
   This printing machine records images on blank plates mounted on first and second plate cylinders  11  and  12  in a prepress process, feeds inks to the plates having the images recorded thereon, and transfers the inks from the plates through first and second blanket cylinders  13  and  14  to printing paper held on first and second impression cylinders  15  and  16 , thereby printing the images in four colors on the printing paper. 
   The printing machine has the first plate cylinder  11 , the second plate cylinder  12 , the first blanket cylinder  13  contactable with the first plate cylinder  11 , the second blanket cylinder  14  contactable with the second plate cylinder  12 , the first impression cylinder  15  contactable with the first blanket cylinder  13 , and the second impression cylinder  16  contactable with the second blanket cylinder  14 . The printing machine further includes a feed cylinder  17  for transferring printing paper supplied from a paper storage station  31  to the first impression cylinder  15 , a transfer cylinder  18  for transferring the printing paper from the first impression cylinder  15  to the second impression cylinder  16 , a discharge cylinder  19  with chains  23  wound thereon and extending to and wound on sprockets  22  for discharging prints from the second impression cylinder  16  to a discharge station  32 , a device for measuring color values of prints which includes an image pickup station  60  for reading images from the prints and measuring densities of detecting patches printed on the prints, and a control panel  100  of the touch panel type. 
   Each of the first and second plate cylinders  11  and  12  is what is called a two-segmented cylinder for holding two printing plates peripherally thereof for printing in two different colors. The first and second blanket cylinders  13  and  14  have the same diameter as the first and second plate cylinders  11  and  12 , and each has blanket surfaces for transferring images in two colors. 
   The first and second impression cylinders  15  and  16  movable into contact with the first and second blanket cylinders  13  and  14 , respectively, have half the diameter of the first and second plate cylinders  11  and  12  and the first and second blanket cylinders  13  and  14 . The first and second impression cylinders  15  and  16  have grippers, not shown, for holding and transporting the forward end of printing paper. 
   The feed cylinder  17  disposed adjacent the first impression cylinder  15  has the same diameter as the first and second impression cylinders  15  and  16 . The feed cylinder  17  has a gripper, not shown, for holding and transporting, with each intermittent rotation of the feed cylinder  17 , the forward end of each sheet of printing paper fed from the paper storage station  31 . When the printing paper is transferred from the feed cylinder  17  to the first impression cylinder  15 , the gripper of the first impression cylinder  15  holds the forward end of the printing paper which has been held by the gripper of the feed cylinder  17 . 
   The transfer cylinder  18  disposed between the first impression cylinder  15  and second impression cylinder  16  has the same diameter as the first and second plate cylinders  11  and  12  and the first and second blanket cylinders  13  and  14 . The transfer cylinder  18  has a gripper, not shown, for holding and transporting the forward end of the printing paper received from the first impression cylinder  15 , and transferring the forward end of the printing paper to the gripper of the second impression cylinder  16 . 
   The discharge cylinder  19  disposed adjacent the second impression cylinder  16  has the same diameter as the first and second plate cylinders  11  and  12  and the first and second blanket cylinders  13  and  14 . The discharge cylinder  19  has a pair of chains  23  wound around opposite ends thereof. The chains  23  are interconnected by coupling members, not shown, having a plurality of grippers  30  ( FIG. 2 ). When the second impression cylinder  16  transfers prints to the discharge cylinder  19 , one of the grippers  30  on the discharge cylinder  17  holds the forward end of the prints having been held by the gripper of the second impression cylinder  16 . With movement of the chains  23 , the prints are transported to the discharge station  32  to be discharged thereon. 
   The feed cylinder  17  has a gear attached to an end thereof and connected to a gear  26  disposed coaxially with a driven pulley  25 . A belt  29  is wound around and extends between the driven pulley  25  and a drive pulley  28  rotatable by a motor  27 . Thus, the feed cylinder  17  is rotatable by drive of the motor  27 . The first and second plate cylinders  11  and  12 , first and second blanket cylinders  13  and  14 , first and second impression cylinders  15  and  16 , feed cylinder  17 , transfer cylinder  18  and discharge cylinder  19  are coupled to one another by gears attached to ends thereof, respectively. Thus, by the drive of motor  27 , the feed cylinder  17 , first and second impression cylinders  15  and  16 , discharge cylinder  19 , first and second blanket cylinders  13  and  14 , first and second plate cylinders  11  and  12  and transfer cylinder  18  are rotatable synchronously with one another. The rotating speeds of these cylinders are variable under control of a control unit  80  described hereinafter. 
   The first plate cylinder  11  is surrounded by an ink feeder  20   a  for feeding an ink of black (K), for example, to a plate, an ink feeder  20   b  for feeding an ink of cyan (C), for example, to a plate, and dampening water feeders  21   a  and  21   b  for feeding dampening water to the plates. The second plate cylinder  12  is surrounded by an ink feeder  20   c  for feeding an ink of magenta (M), for example, to a plate, an ink feeder  20   d  for feeding an ink of yellow (Y), for example, to a plate, and dampening water feeders  21   c  and  21   d  for feeding dampening water to the plates. 
   Further, arranged around the first and second plate cylinders  11  and  12  are a plate feeder  33  for feeding plates to the peripheral surface of the first plate cylinder  11 , a plate feeder  34  for feeding plates to the peripheral surface of the second plate cylinder  12 , an image recorder  35  for recording images on the plates mounted peripherally of the first plate cylinder  1 , and an image recorder  36  for recording images on the plates mounted peripherally of the second plate cylinder  12 . 
     FIG. 2  is a schematic side view showing the image pickup station  60  for reading images from the prints, along with the discharge cylinder  19 . 
   The pair of chains  23  are endlessly wound around the opposite ends of the discharge cylinder  19  and the pair of sprockets  22 . The chains  23  are interconnected by coupling members, not shown, having a plurality of grippers  30  arranged thereon each for gripping the forward end of prints transported.  FIG. 2  shows only two grippers  30 , with the other grippers  30  omitted. 
   The pair of chains  23  have a length corresponding to a multiple of the circumference of first and second impression cylinders  15  and  16 . The grippers  30  are arranged on the chains  23  at intervals each corresponding to the circumference of first and second impression cylinders  15  and  16 . Each gripper  30  is opened and closed by a cam mechanism, not shown, synchronously with the gripper on the discharge cylinder  19 . Thus, each gripper  30  receives the prints from the discharge cylinder  19 , transports the prints with rotation of the chains  23 , and is then opened by the cam mechanism, not shown, to discharge the prints on the discharge station  32 . 
   Each print is transported with only the forward end thereof held by one of the grippers  30 , the rear end of print not being fixed. Consequently, the print could flap during transport, which impairs operations, to be described hereinafter, of the image pickup station  60  to read images and measure densities of the detecting patches. To avoid such an inconvenience, this printing machine provides a suction roller  70  disposed upstream of the discharge station  32  for stabilizing the prints transported. 
   The suction roller  70  is in the form of a hollow roller having a surface defining minute suction bores, with the hollow interior thereof connected to a vacuum pump not shown. The suction roller  70  has a gear  71  attached to an end thereof. The gear  71  is connected through idler gears  72  and  73  to the gear attached to an end of the discharge cylinder  19 . Consequently, the suction roller  43  is driven to rotate in a matching relationship with a moving speed of the grippers  30 . Thus, the prints are sucked to the surface of the suction roller  70 , thereby being held against flapping when passing over the suction roller  70 . In place of the suction roller  70 , a suction plate may be used to suck the prints two-dimensionally. 
   The device for measuring color values of the prints includes the image pickup station  60 , and a print transport device having the discharge cylinder  19  and chains  23  noted above. The image pickup station  60  includes a pair of linear light sources  61  extending parallel to the suction roller  70  for illuminating the prints on the suction roller  70 , a pair of condensing plates  62 , reflecting mirrors  63  and  64 , a condenser lens  65 , a CCD line sensor  66 , and a shutter  69  acting as a shield for stopping light traveling to the condenser lens  65 . The prints transported by the print discharge mechanism including the discharge cylinder  19  and chains  23  are illuminated by the pair of linear light sources  61 , and photographed by the CCD line sensor  66 . The images of the prints and density data derived from image data are displayed on the control panel  100  of the touch panel type. 
   The CCD line sensor  66  has a plurality of CCDs arranged transversely of the print transport direction. The sensor  66  detects light from a print in each sampling interval L in the print transport direction and stores a resulting amount of electric charges in a RAM  82 . At the same time, the electric charges accumulated in one sampling interval L are initialized to detect light from the print and accumulate electric charges in a next sampling interval L. 
     FIG. 3  is a block diagram showing a principal electrical structure of the printing machine. The machine includes the control unit  80  having a ROM  81  for storing operating programs necessary for controlling the machine, a RAM  82  for temporarily storing data and the like during a control operation, and a CPU  83  for performing logic operations. The control unit  80  has a driving circuit  85  connected thereto through an interface  84 , for generating driving signals for driving the ink feeders  20 , dampening water feeders  21 , and image recorders  35  and  36 . The control unit  80  has also a driving circuit  85  connected thereto through the interface  84 , for generating driving signals for the motor  27  for controlling the rotating speeds of the first and second plate cylinders  11  and  12 , first and second blanket cylinders  13  and  14 , first and second impression cylinders  15  and  16 , feed cylinder  17 , transfer cylinder  18  and discharge cylinder  19 . The control unit  80  is connected through the interface  84  to the image pickup station  60 , touch panel type control panel  100  and an image data source  86  storing images for use in platemaking and printing. Further, the control unit  80  is electrically connected to the shutter  69  for opening and closing of the shutter  69 . The printing machine is controlled by this control unit  80  to perform a shading correcting process described hereinafter. 
   In the printing machine having the above construction, a printing plate stock drawn from a supply cassette  41  of the plate feeder  33  is cut to a predetermined size by a cutter  42 . The forward end of each plate in cut sheet form is guided by guide rollers and guide members, not shown, and is clamped by clamps of the first plate cylinder  11 . Then, the plate is wrapped around the peripheral surface of the first plate cylinder  11 , and the rear end of the plate is clamped by other clamps of the first plate cylinder  11 . While, in this state, the first plate cylinder  11  is rotated at low speed, the image recorder  35  irradiates the surface of the plate mounted peripherally of the first plate cylinder  11  with a modulated laser beam for recording an image thereon. 
   Similarly, a printing plate stock drawn from a supply cassette  43  of the plate feeder  34  is cut to the predetermined size by a cutter  44 . The forward end of each plate in cut sheet form is guided by guide rollers and guide members, not shown, and is clamped by clamps of the second plate cylinder  12 . Then, the second plate cylinder  12  is driven by a motor, not shown, to rotate at low speed, whereby the plate is wrapped around the peripheral surface of the second plate cylinder  12 . The rear end of the plate is clamped by other clamps of the second plate cylinder  12 . While in this state, the second plate cylinder  12  is rotated at low speed, the image recorder  36  irradiates the surface of the plate mounted peripherally of the second plate cylinder  12  with a modulated laser beam for recording an image thereon. 
   The first plate cylinder  11  has, mounted peripherally thereof, a plate for printing in black ink and a plate for printing in cyan ink. The two plates are arranged in evenly separated positions (i.e. in positions separated from each other by 180 degrees). The image recorder  35  records images on these plates. Similarly, the second plate cylinder  12  has, mounted peripherally thereof, a plate for printing in magenta ink and a plate for printing in yellow ink. The two plates also are arranged in evenly separated positions, and the image recorder  36  records images on these plates, to complete a prepress process. 
   The prepress process is followed by a printing process for printing the printing paper with the plates mounted on the first and second plate cylinders  11  and  12 . This printing process is carried out as follows. 
   First, each dampening water feeder  21  and each ink feeder  20  are placed in contact with only a corresponding one of the plates mounted on the first and second plate cylinders  11  and  12 . Consequently, dampening water and inks are fed to the plates from the corresponding water feeders  21  and ink feeders  20 , respectively. These inks are transferred from the plates to the corresponding regions of the first and second blanket cylinders  13  and  14 , respectively. 
   Then, the printing paper is fed to the feed cylinder  17 . The printing paper is subsequently passed from the feed cylinder  17  to the first impression cylinder  15 . The impression cylinder  15  having received the printing paper continues to rotate. Since the first impression cylinder  15  has half the diameter of the first plate cylinder  11  and the first blanket cylinder  13 , the black ink is transferred to the printing paper wrapped around the first impression cylinder  15  in its first rotation, and the cyan ink in its second rotation. 
   After the first impression cylinder  15  makes two rotations, the printing paper is passed from the first impression cylinder  15  to the second impression cylinder  16  through the transfer cylinder  18 . The second impression cylinder  16  having received the printing paper continues to rotate. Since the second impression cylinder  16  has half the diameter of the second plate cylinder  12  and the second blanket cylinder  14 , the magenta ink is transferred to the printing paper wrapped around the second impression cylinder  16  in its first rotation, and the yellow ink in its second rotation. 
   The forward end of the print printed in the four colors in this way is passed from the second impression cylinder  16  to the discharge cylinder  19 . The print is transported by the pair of chains  23  toward the discharge station  32  to be discharged thereon. At this time, the print being transported is illuminated by the pair of linear light sources  61 , and is photographed by the CCD line sensor  66 . The photographed image is displayed on the control panel  100 . 
   This printing machine has a reference reading mode for imaging blank paper as white reference plate and imaging the shutter  69  as black reference plate. 
   In time of white reference imaging in the reference reading mode, the first and second blanket cylinders  13  and  14  are placed in throw-off state separated from the first and second plate cylinders  11  and  12  and the first and second impression cylinders  15  and  16 . Blank paper is fed from the paper storage station  31 . The blank paper is passed to the discharge cylinder  19  as are the prints described hereinbefore. Then, the blank paper is transported by the chains  23  toward the discharge station  28 . At this time, the blank paper transported is illuminated by the pair of linear light sources  61  in the image pickup station  60 , and is imaged by the CCD line sensor  66 . 
   In time of black reference imaging in the reference reading mode, the shutter  69  is set to a shielding position for shielding the front of the condenser lens  65 . This stops light traveling to the condenser lens  65 , and the CCD line sensor  66  detects hardly any light as when imaging black paper. In this state, the CCD line sensor  66  images the shutter  69  at every accumulating time t needed for imaging blank paper in every sampling interval L. 
   The shading correcting method according to this invention will be described hereinafter. 
   In a conventional shading correcting method, color values Pa of prints are corrected by using white reference value Wa obtained by imaging a white reference plate and black reference value Ba obtained by imaging a black reference plate. The shading correcting method according to this invention measures white preliminary reference values Wc and black preliminary reference values Bc obtained by imaging the white reference plate and black reference plate at a plurality of different transport speeds as a preliminary step, and determines a relationship (equations (1) and (2) described hereinafter) between print transport speed Vp, and white reference value Wb and black reference value Bb corresponding to the print transport speed Vp. Then, color values Pa of prints are corrected by using values obtained by correcting white reference value Wa and black reference value Ba. 
     FIG. 4  is a flow chart of the shading correcting method according to this invention.  FIG. 5  is a graph showing white preliminary reference value Wc and black preliminary reference value Bc, and accumulating time t. 
   When performing a shading correction according to this invention, the printing machine is first set to the reference imaging mode for the image pickup station  60  to image blank paper transported at a plurality of different speeds as preliminary step (step S 1 ). CCD imaging time, i.e. accumulating time t, is determined by transport speed, and therefore the following description is based on the CCD accumulating time t. White preliminary reference values Wc, which are output values of the respective CCDs in the CCD line sensor  66  at this time, and the corresponding accumulating times t are stored in RAM  82  (step S 2 ). Similarly, the shutter  69  is set to the shielding position, and is imaged by the image pickup station  60  for a plurality of different accumulating times t (step S 3 ). The black preliminary reference values Bc, which are output values of the respective CCDs in the CCD line sensor  66  at this time, and the corresponding accumulating times t are stored in RAM  82  (step S 4 ). 
   In this embodiment, as shown in  FIG. 5 , the white preliminary reference values Wc and black preliminary reference values Bc are obtained by measuring, as preliminary step, each of the white reference plate and black reference plate at intervals of 0.2 between accumulating time t=1 and accumulating time t=2. Specifically, each white preliminary reference value Wc is an average of electric charges accumulated by detecting light from the white reference plate W in every predetermined interval L in the transport direction of blank paper. Each black preliminary reference value Bc is an average of electric charges accumulated by detecting light from the shutter  65  at every predetermined time. The accumulating time t is a relative time unit, and accumulating time t=2 is twice as long as accumulating time t=1. The preliminary reference values are numerical values 0 to 16000 expressing amounts of light received by the CCDs in digital gradation. The white reference value and black reference value show different variations with the accumulating time as shown in  FIG. 5 , and should be corrected separately. 
   Next, the values obtained in steps S 1  through S 4  are calculated by multiple regression analysis, to determine equations (1), (2) and (3) set out hereunder for correcting the color value of prints (step S 5 ). Equations (1) and (2) express relations between the print transport speed Vp derived from the white preliminary reference value Wc and black preliminary reference value Bc measured as preliminary step, and the white reference value Wb and black reference value Bb corresponding to the print transport speed Vp. Equation (3) shows the white reference value Wb and black reference value Bb corresponding to the actual print transport speed Vp substituted for the white reference value Wa and black reference value Ba used in the conventional shading correcting method. Specifically, constants A through I in equations (1) and (2) are determined. Accumulating time t is obtained by dividing the predetermined interval L of imaging by the CCD line sensor, by transport speed Vw. Since the predetermined interval L is determined by reading resolution, accumulating time t and transport speed Vw are in one-to-one relationship. 
   
     
       
         
           
             
               
                 
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   In this embodiment, since the shutter  69  is set to the shielding position for shielding the front of the condenser lens  65  in time of the black reference imaging in the reference reading mode, L/Vb in equation (2) is regarded as accumulating time t. 
   After equations (1), (2) and (3) are determined as described above, blank paper is transported and imaged at the image pickup station  60  (step S 6 ). White reference value Wa and corresponding transport speed Vw are stored in RAM  82  (step S 7 ). The shutter  69  is set to the shielding position, and is imaged at every time needed to image the blank paper in one sampling interval L (step S 8 ). Black reference value Ba and corresponding imaging speed Vb are stored in RAM  82  (step S 9 ). The black reference value Ba may be obtained by transporting black paper and imaging it at the image pickup station  60 , as in the case of blank paper, instead of setting the shutter  69  to the shielding position. In this case, the predetermined interval L for the CCD line sensor to image the black paper and the transport speed Vb of the black paper are substituted for L/Vb in equation (2). 
   Subsequently, a printing operation is started as described hereinbefore (step S 10 ). For example, this printing operation is performed at low speed until printing stabilizes, and at high speed after printing attains stability. As a result, prints are transported at low speed until printing stabilizes, and at high speed after printing attains stability. 
   The prints are imaged at the image pickup station  60  (step S 11 ). The CCDs obtain color values Pa from the prints. The color values Pa obtained and a corresponding transport speed Vp are stored in RAM  82  (step S 12 ). 
   The white reference value Wa, transport speed Vw, black reference value Ba, imaging speed Vb, color values Pa, and transport speed Vp in time of actual printing, stored in RAM  82  are substituted into equations (1), (2) and (3) to calculate corrected color values Pb (step S 13 ). This calculation provides accurately corrected color values Pb. 
   Proper printing is carried out by comparing image data obtained from the color values Pb corrected as described above, and the reference image data set beforehand. 
   When the printing is completed (step S 14 ), the shading correcting process will also be ended. 
   In the embodiment described above, the corrected black reference value Bb corresponding to the print transport speed Vp is derived from the black reference value Ba obtained by actually measuring the black reference plate B. However, as seen from  FIG. 5 , the black reference value Ba, compared with the white reference value Wa, is hardly influenced by the variations in accumulating time t. Thus, although accuracy lowers somewhat, the calculation may be carried out by substituting the black reference value Ba, instead of the corrected black reference value Bb, into equation (3) above. That is, in this case, the black reference value is not adjusted according to the transport speed. 
   This invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of the invention. 
   This application claims priority benefit under 35 U.S.C. Section 119 of Japanese Patent Application No. 2004-188240 filed in the Japanese Patent Office on Jun. 25, 2004, the entire disclosure of which is incorporated herein by reference.