Patent Publication Number: US-2009237685-A1

Title: Color image printer and gradation correcting method therefor

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a gradation adjustment technique for a color image printer forming an image on a photosensitive material by laser scanning exposure, the invention relating more particularly to a dot adjustment technique. 
     2. Description of the Related Art 
     According to the type of color printer mainly employed in the photographic printing industry in recent years, a photographic image formed on a photographic film is digitized by a film scanner into photographic image data, or a photographic image is directly digitized by a digital photographic instrument, such as a digital camera, into photographic image data. Then, this photographic image data is subjected to such image processing as density adjustment, color adjustment, etc. The resultant adjusted image data is converted into print data, based on which an exposure engine is driven to print the photographic image on a photosensitive material (print paper). Such color image printer usually employs a laser exposure engine configured to effect an image forming operation by scanning laser beam on the photosensitive material in a main scanning direction while moving this photosensitive material in a sub scanning direction. With such laser exposure engine, the laser beam is irradiated onto a polygon mirror while this polygon mirror is being rotated, thereby to polarize the laser beam, thus realizing scanning along the main scanning direction. 
     The art has made various proposals for overcoming the problem of image quality deterioration which tends to occur when such laser exposure engine is employed in the color image printer. For instance, according to the technique disclosed in Japanese Patent Application “Kokai” No. 2006-58677 (paragraphs 0008-0012, FIG. 12), in a scanning exposure apparatus for exposing/forming an image with using a plurality of kinds of laser beam, in order to reduce color bleeding due to difference between color beam modulation methods used, arrangement is provided such that the spot size of the second laser beam subjected to direct modulation by driving current control is set larger than the spot size of the first laser beam subjected to intensity modulation by AOM (Acousto-Optic Modulation). The arrangement changes the integrated light amount of the second laser beam, thereby reducing color bleeding. According to a further technique disclosed in Japanese Patent Application “Kokai” No. 2-160563, a clock employed for one-dot exposure is frequency-divided into four clocks. Then, when effecting exposure of one black dot, the laser beam exposure is effected in correspondence with three of the frequency-divided four clocks, thereby to reduce the exposure period for this dot, thus reducing the area of the dot formed by the exposure. With this, through the reduction in the exposure period for the dot based on the determination result, undesired enlargement of the dot area is restricted, thereby to enable exposure with the theoretically determined dot size. As a result, an image with high quality is obtained. 
     With the laser scanning exposure, when a horizontal line extending along the main scanning direction and a vertical line extending along the sub scanning direction are exposed, in the main scanning direction, a control error occurs due to such factors as delay in rising or falling, overshoot, ringing of the modulated laser; whereas, such control error does not occur in the sub scanning direction. Accordingly, between the vertical line greatly influenced by the control error and the horizontal line free from such control error, there occurs a difference between the density values thereof even if they are exposed with a same gradation value. This problem appears especially conspicuously when a high-definition laser exposure engine is employed for outputting high-quality photographic prints. The conventional apparatuses including those of the above-cited references have been unable to solve this problem satisfactorily. 
     SUMMARY OF THE INVENTION 
     The primary object of the present invention is to provide a gradation correcting technique capable of restricting the density value difference between a horizontal line and a vertical line, due to control error of modulated laser. 
     For accomplishing the above-noted object, in a method for correcting gradation for a color image printer configured for forming a color image on a photosensitive material by laser scanning exposure, according to the present invention, the gradation correction is effected based on measured density values of a test print including two kinds of test patterns, i.e. a horizontal stripe test pattern including a plurality of horizontal lines extending in a main scanning direction and a vertical stripe pattern including a plurality of vertical lines extending in a sub scanning direction, the method comprising the steps of: 
     calculating, based on said measured density values, a rising correction amount for main scanning rising correction of adding a rising correction component to one dot after rising of a modulated laser in order to reduce density difference between the horizontal line and the vertical line; and 
     calculating, based on said measured density values, a falling correction amount for main scanning falling correction of adding a falling correction component to one dot before falling of the modulated laser in order to reduce the density difference between the horizontal line and the vertical line. 
     With this gradation correcting method, the rising correction amount is calculated with using two kinds of test patterns, namely, the horizontal stripe test patterns having a plurality of horizontal lines hardly affected by bleeding due to control error of modulated laser and the vertical stripe test patterns having a plurality of vertical lines affected by the bleeding due to control error of modulated laser. Similarly, the falling correction amount is calculated with using the two kinds of test patterns. In doing these, the rising correction amount is calculated so as to reduce the density difference between the horizontal line and the vertical line by reducing bleeding due to rising control error of the modulated laser, and the falling correction amount is calculated so as to reduce the density difference between the horizontal line and the vertical line by reducing bleeding due to falling control error of the modulated laser. The print outputted after these main scanning rising correction (referred to simply as “rising correction” hereinafter) and the main scanning falling correction (referred to simply as “falling correction” hereinafter) is a high quality print with little conspicuous density difference, i.e. minimized density difference, between the horizontal lines and the vertical lines, even when this print includes, in a mixed state, Chinese characters using more vertical lines than horizontal lines and Chinese characters using more horizontal lines than vertical lines. Incidentally, in this rising correction, an overshoot component is pseudo-added to the rising portion, thereby reducing the rising delay in the actual modulated laser. Further, in the falling correction, an undershoot component is pseudo-added to the falling portion, thereby reducing the falling delay in the actual modulated laser. 
     According to one preferred embodiment of the present invention, the rising correction amount is calculated, based on measured density values of a plurality of horizontal stripe patterns comprised of horizontal stripe patterns having one-dot width formed with different gradation values; and measured density values of a plurality of vertical stripe patterns comprised of vertical stripe patterns having one-dot width formed with different gradation values. The rising characteristics is the characteristics when one dot is about to be formed. So that, the bleeding due to control error at the time of rising of the modulated laser appears clearly on the vertical stripe test pattern having one-dot width. Therefore, by calculating the rising correction amount through comparison between this vertical stripe test pattern having one-dot width and the horizontal stripe test pattern having one-dot width and hardly affected by the bleeding, it is possible to reduce the density difference between the horizontal lines and the vertical lines. Further, as the density difference between the horizontal lines and the vertical lines varies according to the gradation values given to the laser as drive signals, it is preferred that the vertical stripe test pattern and the horizontal stripe test pattern be formed with different gradation values and the rising correction amount be obtained for each gradation value, using each test pattern. 
     Similarly, according to one preferred embodiment of the present invention, the falling correction amount is calculated, based on measured density values of a plurality of horizontal stripe test patterns comprised of horizontal test patterns having two-dot width and three-dot width in alternation and formed with differing gradation values; and measured density values of a plurality of vertical stripe test patterns comprised of vertical stripe test patterns having two-dot width and three-dot width in alternation and formed with differing gradation values. The falling characteristics is the characteristics when forming of one dot is about to end. Then, in order to distinguish from the control error at the time of rising of the modulated laser described above, the bleeding due to a control error at the time of falling appears on the downstream side in the main scanning direction of the vertical stripe test pattern having the plurality of dot width. The experiment showed that for this purpose, a vertical stripe test pattern which is substantially in the middle between a vertical stripe test pattern having two-dot width and a vertical stripe test pattern having three-dot width is appropriate. However, as it is impossible to form a line having such intermediate width as 2.5, the above-described construction employs vertical stripe test pattern and the horizontal stripe test patterns, the lines having two-dot width and three-dot width are used in combination. Then, as the vertical stripe test pattern and the horizontal stripe test pattern thus constructed are compared with each other to calculate the falling correction amount, it becomes possible to further reduce the density difference between the horizontal line and the vertical line. In this case too, as the density difference between the horizontal lines and the vertical lines varies according to the gradation values given to the laser as drive signals, it is preferred that the vertical stripe test pattern and the horizontal stripe test pattern be formed with different gradation values and the falling correction amount be obtained for each gradation value, using each test pattern. 
     According to a further preferred embodiment of the present invention, the method further comprises the step of calculating an undershoot correction amount for main scanning undershoot correction of adding an undershoot component to one dot after falling, in order to reduce the bleeding due to the falling control delay of the modulated laser. In order to form dots having stable shape, the rising correction and the falling correction alone can sometimes be insufficient. This is because at the time of falling of the modulated laser, there occurs a delay until reaching the white level, which delay results in occurrence of dot bleeding. As this problem cannot be solved by the falling correction alone, main scanning undershoot correction (to be referred to simply as “undershoot correction” hereinafter) is effected in order to overcome this problem. The undershoot correction amount for use in this undershoot correction can be determined from a table set in advance based on understood amplitude amounts. So, there is no use to employ any measured density values from a test print. 
     Incidentally, the undershoot correction, the rising correction and the falling correction are effected in this mentioned order And, with these three kinds of correction, the dot correction process will be completed. Notwithstanding, color irregularity can occur in black areas. This is because of occurrence of color development density difference in the respective color components (C, M, Y in the case of print paper) in the black areas. For this reason, according to a preferred embodiment of the present invention, the method further comprises the step of effecting black balance correction for rendering color development densities of respective color components in the black area uniform, thereby to adjust the black area in the gradation conversion curve (table). 
     The present invention is intended to encompass within its scope, not only the gradation adjusting method described above, but also a gradation conversion curve correcting program for implementing such method on a color image printer as well as a color image printer using such method. For instance, in a color image printer forming an image on a photosensitive material by laser scanning exposure, for accomplishing the above-noted object, the color printer comprises: 
     a test print managing section for outputting a test print including a horizontal stripe test pattern having a plurality of horizontal lines extending in a main scanning direction and a vertical stripe test pattern having a plurality of vertical lines extending in a sub scanning direction; 
     a density determining section for measuring densities of the test patterns of the test print and outputting measured density values; 
     a rising correcting section for calculating, based on said measured density values, a rising correction amount for main scanning rising correction of adding a rising correction component to one dot after rising of a modulated laser in order to reduce density difference between the horizontal line and the vertical line; and 
     a falling correction section for calculating, based on said measured density values, a falling correction amount for main scanning falling correction of adding a falling correction component to one dot before falling of the modulated laser in order to reduce the density difference between the horizontal line and the vertical line. 
     The color image printer having the above-described construction also has and achieves the functions/effects described above for the gradation conversion curve correcting method, and this printer too can include one or more of the above-described various additional characterizing features. 
     Further and other characterizing features and advantages of the present invention will become apparent upon reading the following detailed description of the preferred embodiments thereof with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an appearance view of a photographic print apparatus acting as a color image printer implementing a gradation correction method according to the present invention, 
         FIG. 2  is a diagram showing a print station of the photographic print apparatus shown in  FIG. 1 , 
         FIG. 3  is a diagram for explaining the construction of a print engine, 
         FIG. 4  is a diagrammatic cross section showing a discharging/conveying section and a colorimeter included in the photographic print apparatus, 
         FIG. 5  is a functional block diagram showing principal functions relating to the invention built in a personal computer included in the photographic print apparatus, 
         FIG. 6  is a diagram showing delays in exposure control, 
         FIG. 7  is a diagram showing a first test print, 
         FIG. 8  is a diagram showing a second test print, 
         FIG. 9  is a diagram illustrating undershoot correction, 
         FIG. 10  is a diagram illustrating rising correction, 
         FIG. 11  is a diagram illustrating falling correction, 
         FIG. 12  is a flow chart showing flow of correction value calculating method, 
         FIG. 13  is a diagram illustrating the correction value calculating method, 
         FIG. 14  is a diagram illustrating a method for obtaining an approximate curve of the correction value, 
         FIG. 15  is a flow chart of a gradation conversion curve correcting operation of the invention, 
         FIG. 16  is a graph showing input value-measured density value relationship, 
         FIG. 17  is a graph showing black area pseudo conversion curve, 
         FIG. 18  is an explanatory view illustrating a condition before combining the black area pseudo conversion curve with the base gradation conversion curve, 
         FIG. 19  is an explanatory view illustrating the combining process of the black area pseudo conversion curve and the base gradation conversion curve, 
         FIG. 20  is a graph showing the base gradation conversion curve after its black balance adjustment, and 
         FIG. 21  is a flow chart showing procedure of the overall adjustment operation of the color image printer. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Next, preferred embodiments of the present invention will be described. 
       FIG. 1  shows an appearance view of a photographic print apparatus as a color image printer implementing the gradation correction method of the invention.  FIG. 2  is a diagrammatic view of the same. This photographic print apparatus comprises essentially of a print station  1 B as a photographic printer which effects an exposure operation and a development operation on a silver halide print paper P (“print paper P” hereinafter) as an example of recording medium, and a control station  1 A for effecting generation/transfer of print data used in the print station  1 B. This photographic print apparatus is referred to also as “digital minilab”. As may be clearly understood from  FIG. 2 , in the print station  1 B, one of print papers P in the form of rolls stored within two print paper magazines  11  is drawn and cut to a print size and on this cut print paper P, a print engine  13  effects an exposure. Then, this exposed print paper P is sent for its development to a developing section  14  having a plurality of developing tanks  14   a . Thereafter, the resultant print paper P is dried in a drying section  15  and then discharged onto a transverse feeding conveyer  16  provided at an upper portion of the apparatus and the conveyer  18  conveys the print paper P into a sorter  17 . A plurality of such print papers P, i.e. photographic prints P, sent to the sorter  17  are accumulated on a plurality of trays  17   a  as being sorted by the unit of customer&#39;s order. A print paper conveying mechanism  12  is provided for conveying the print paper P drawn from the print paper magazine  11  to the print engine  13 , the developing section  14  and the drying section  15 . This print paper conveying mechanism  12  is comprised substantially of a number of clamping conveying roller pairs, except for chucker type print paper conveying units arranged before and after the print engine  13 . 
     At a loading section where the print paper magazine  11  is loaded, there is provided an ID code reader  11   a  for reading a print paper ID code attached to the print paper magazine  11 . This print paper ID code is used for uniquely specifies the type of print paper P. With recognition of this print paper ID code, the type of the print paper P accommodated in the print paper magazine  11 , that is, the type of the print paper P to be used for printing is recognized. 
     The print engine  13  effects irradiation of beams of three primary colors, i.e. R (red), G (green), B (blue), on the print paper P, based on print data sent from the control station  1 A. In this exposure operation, the exposure is done linearly along the main scanning direction at a same conveying speed and in synchronism with conveying operation of the print paper A along the sub scanning direction. 
     The print engine  13 , as shown in  FIG. 3 , includes a laser beam generator  130  for generating three kinds of laser beams of the three primary colors, R (red), G (green), B (blue), a polygon mirror  135  for scanning the laser beams while the mirror being rotated at a high speed about a vertical axis, an fθ lens  136  for converting the scanning speed of the laser beam from the polygon mirror  135 , and a deflecting mirror  137  for changing the orientation of the laser beam from horizontal to downwards. The laser beam generator  130  includes, in correspondence respectively with the three primary colors of R (red), G (green) and B (blue), a laser beam source  131  and a laser beam source  132  which both comprise semiconductor lasers, and a laser beam source  133  which comprises a solid laser, the generator  130  further including a light amount modulating mechanism  134  comprised of an acousto-optic modulating element (AOM) and three beam mirrors Mr, Mg, Mb for reflecting the three kinds of laser beams respectively. 
     This photographic print apparatus  1  is constructed of a main body frame  2  having its outer side covered with a plurality of sheet metal panels. Some of these sheet metal panels are configured as doors which can be opened for inside inspection. Especially, the sheet metal that covers the upper side of the developing section  14  is constructed as an upper cover  10  which can be opened by approximately 90 degrees so as to allow maintenance from above of the developing tanks  14   a . The drying section  15  is disposed inside a protruding frame  2   a  which is disposed adjacent the developing section  14  and protrudes upwardly above this developing section  14 . The transverse feeding conveyer  16  is arranged to extend in the horizontal direction from the protruding frame  2   a  and forms a space relative to the upper cover  10 . In this space between the upper cover  10  and the transverse feeding conveyer  16 , as may be clearly seen in  FIG. 1  and  FIG. 2 , a colorimeter  20  as a density meter, is mounted via a rail type moving mechanism  30  to be movable toward and away from the protruding frame  2   a.    
     In the instant embodiment, the upper cover  10  is attached to the main body frame  2  to be pivotable about a horizontal axis extending in the right/left direction on the deeper side of the main body frame  2  in  FIG. 1 . Accordingly, the upper cover  10  is pivotable for 90 degrees approximately between a posture for closing the upper side of the developing section  14  and an opening posture for opening up the upper side of the developing section  14 . In order to avoid interference between the upper cover  10  and the transverse feeding conveyer  16  in the course of the pivotal movement of the upper cover  10  to its opening posture, the transverse feeding conveyer  16  too is configured to be pivotable for 90 degrees before its conveying surface comes into contact with the protruding frame  2   a.    
     As shown in  FIG. 4 , inside the protruding panel  2   a , as a pair of the print paper conveying mechanism  12 , there is provided a discharging conveying section  60  for discharging the print paper P past through the drying section  15  to the transverse feeding conveyer  16  and charging a test print TP past through the drying section  15  into the colorimeter  20 . A clamping conveying roller pair  61  as the principal component of the discharging conveying section  60  comprises combination of a drive roller and a driven roller. In operation, as the drive roller is rotated with clamping the print paper P or the test print TP at the gap formed between the two rollers, the print paper P or the test print TP is conveyed to the downstream. A turn roller pair  62  comprises a group of rollers configured for changing the conveying direction so that the print paper P or the test print TP having been conveyed upward from the upstream so far may now be changed in its direction to be conveyed downwards thereafter. This roller pair  62  is comprised of e.g. a drive roller and a plurality of driven rollers. A conveying switching guide  63  is disposed immediately downstream of the turn roller pair  62 . In case the print paper P is being conveyed, a photographic print paper discharging conveying passage (shown by solid line in  FIG. 4 ) is used for feeding the photographic print P to the transverse feeding conveyer  16 . Whereas, in case the test print TP is being conveyed, a test print discharging conveying passage (shown by broken line in  FIG. 4 ) is used for feeding the test print TP to the colorimeter  20 . The photographic print paper discharging conveying passage and the test print discharging conveying passage branch from each other, at the exit region of the turn roller pair  62 . A discharging roller pair  64  are rollers configured for discharging the print paper (photographic print) P guided by the conveying switching guide  63  to the transverse feeding conveyer  16 . In operation, as the drive roller is rotated with clamping the print paper (photographic print) P between the two rollers, the print paper (photographic print) P is conveyed. A turn guide  66  is a guide provided in the test print discharging conveying passage for guiding the test print TP being guided by the conveying switching guide  63  to the colorimeter  20 . By this turn guide  66 , the test print TP which has been conveyed from the vertically upper side as being fed from the turn roller pair  62  is changed in its conveying direction to the substantially horizontal direction, so that the test print TP is caused to pass the discharging opening  67  and fed into the introducing opening of the colorimeter  20 . 
     The colorimeter  20  is configured to measure densities of test patterns formed on the test print TP while advancing this test print TP conveyed therein, by frame-by-frame. This colorimeter  20  comprises a spectrometer, which is a standard commercially available device having an effective measurement density range of about 2.2 D. The colorimeter  20  includes a plurality of crimping type conveying roller pairs  23  driven by a stepping motor and a sensor section  22 . The sensor section  22  incorporates therein sensor elements configured to irradiate white light onto the test print TP and receive its reflection light and obtains from this reflection light, density data for each one of the three primary colors of R (red), G (green), B (blue), or C (cyan), M (magenta) and Y (yellow). The obtained density data are transferred to the controller  4 . A lid  21  for the colorimeter  20  is provided for rendering the temperature and brightness inside the calorimeter  20  constant, the lid  21  being formed of e.g. a synthetic resin member or the like. The conveying roller pairs  23  are configured to convey the test print TP in such a manner as to allow the sensor section  22  to measure the densities of test patterns in the test print TP. After the densities of all the test patterns are obtained by the sensor section  22 , the test print TP is conveyed by the conveying roller pairs  23  to a discharging section  24 . 
     The photographic print apparatus  1  effects measurement on the respective test patterns (density patterns) in the test print TP by using the sensor portion  22  mounted in the colorimeter  20 . The test print TP is printed for setup at each time of startup of the apparatus which is effected daily. And, based on the result of this measurement, the controller  4  effects setup operation for adjusting output conditions of the print engine  13 . Normally, e.g. the quality of print (density of print) outputted from the photographic print apparatus  1  undergoes some change due to the illumination condition of the print engine  13 , the conditions of the respective developing tanks  14   a  (developing liquid temperature, oxidization condition, activation degree condition, etc.). For this reason, the respective densities of a plurality of test patterns having densities differing from each other and formed in the test print TP are measured by the sensor section  22 , and based on the result of this measurement, the adjustment of the output condition of the print engine  13 , so-called “setup”, is effected. By effecting such setup, photographic prints of high reproducibility can be outputted. 
     The control station  1 A is constructed as a control table disposed adjacent the main body frame  2 . In this embodiment, the control station  1 A comprises essentially of a film scanner  3  for obtaining image data from a photographic film, a monitor  5  for displaying various information, a general-purpose computer acting as the controller  4  for effecting processing of photographic image read from the photographic film by the film scanner  3 , or image data directly read from a semiconductor memory, generation of print data, control of the print station  1 B. To the controller  4 , there are connected also such peripheral devices as a keyboard and a mouse functioning as control input sections, a media reader for obtaining photographic image from a semiconductor memory used as a photographic image memory for a digital camera. 
     The controller  4  is comprised of a computer as described above and its functional sections for effecting various operations of the photographic print apparatus are comprised of hardware and/or software. Referring to principal functional sections relating to the present invention, an image processing section  41  is provided for effecting various kinds of photo-retouch operations such as color correction or filtering (softening, sharpening, etc.), trimming. A gradation conversion LUT  42  is provided for storing a group of data (gradation conversion characteristics) used for covering gradation values (input gradation values) of image data to be printed/outputted to output gradation values for the print engine  13  for the respective color components, with the group of data being stored for respective types of print paper in the form of gradation conversion curves. A print data generating section  43  includes a gradation converting section  43   a  for converting final image data to output gradation values for the print engine  13  with using the gradation conversion LUT  42  and generates print data for driving the print engine  13  from the image data converted into appropriate gradation values. A print paper type recognizing section  44  is provided for specifying the type of print paper P held in a currently loaded print paper magazine  11 , based on a print paper ID code read by an ID code reader  11   a . Further, a correcting module  50  is provided for effecting dot adjustment, correction of the LUT  42 , etc. Incidentally, in the instant embodiment, the gradation conversion curves representing the gradation conversion characteristics of the print papers P are stored in the form of a lookup table in the gradation conversion LUT  42 , as described above. 
     The correcting module  50 , as sections thereof relating to the present invention, a test print managing section  51 , a density value relationship deriving section  52 , a density value relationship analyzing section  53 , an LUT managing section  54 , an undershoot correcting section  55 , a rising correcting section  56 , and a falling correcting section  57 . 
     The test print managing section  51  manages the output of the test print TP which includes horizontal stripe test patterns having horizontal lines extending in the main scanning direction and vertical stripe test patterns having vertical lines extending in the sub scanning direction. The density value relationship deriving section  52  obtains measured density values of the test patterns formed in the test print TP and processes these into a format which can be easily processed by the functional sections downstream. 
     The density value relationship analyzing section  53  functions for a black balance correction which will be detailed later. That is, this density value relationship analyzing section  53  receives, from the density value relationship deriving section  52 , a test input gradation value-density value relationship derived based on the measured density values of a predetermined test pattern in the test print TP and test input gradation values corresponding to that particular test pattern; and then specifies, in the respective test input gradation values, a minimum measured density value of a color component which has the minimum measured density value of them all; and also calculates, for each color component, a correction input gradation value that has a substantially same density value as this minimum measured density value. 
     The LUT managing section  54  is provided for managing the gradation conversion LUT  42 . Especially, as its functions provided for the black balance correction, the LUT managing section  54  includes a black area pseudo conversion relationship creating section  54   a  for creating a black area pseudo conversion curve from an output gradation value obtained based on a gradation conversion curve from a correction input gradation value calculated by the density value relationship analyzing section  53  for each color component and from the test input gradation value. The LUT managing section  54  further includes a combining adjusting section  54   b  for creating a black area correction gradation conversion curve by combining the black area pseudo conversion curve with a gradation conversion curve of a non-black area and a correction executing section  54   c  for correcting the gradation conversion curve based upon the black area correction gradation conversion curve created by the combining adjusting section  54   b.    
     The undershoot correcting section  55  calculates an undershoot correction amount for a main scanning undershoot correction in which an undershoot component is added to one dot after a fall in order to reduce bleeding due to a delay in fall control of a modulated laser. The rising correcting section  56  calculates a rising correction amount based on a measured density value of the predetermined test pattern of the test print TP for a main scanning rising correction in which a rising correction component is added to one dot after a rise in order to reduce density difference between a horizontal line and a vertical line. The falling correcting section  57  calculates based on a measured density value of the predetermined test pattern of the test print TP for a main scanning falling correction in which a falling correction amount is added to one dot after a fall in order to reduce density difference between a horizontal line and a vertical line. 
     Before explaining the test print TP employed in the gradation correction according to the present invention, with reference to  FIG. 6 , there will be explained the reason why this test print TP includes horizontal stripe test patterns having horizontal lines extending along the main scanning direction and vertical stripe test patterns having vertical lines extending along the sub scanning direction. 
       FIG. 6  ( a ) is a pattern diagram showing an ideal light amount (solid line) of the exposure head in the main scanning direction and a light amount (dotted line) actually outputted from the exposure head. On the other hand,  FIG. 6  ( b ) shows an ideal light amount in the sub scanning direction and an actually outputted light amount (dotted line) in the sub scanning direction. As may be clearly seem from these figures, in the main scanning direction there occurs a displacement between the ideal light amount and the actual light amount, whereas such displacement does not occur in the sub scanning direction. The reason for this is as follows. When a single horizontal line extending along the main scanning direction is formed, the control signal for the exposure engine rises at its start end and falls at is terminal end, simply. So, there occurs no deformation of the dot due to a delay in the rising or falling of the control signal for the print engine, hence, resultant bleeding, if any, on the print will not be conspicuous. Whereas, when a single vertical line extending along the sub scanning direction is formed, the rising and the falling of the print engine control signal occur before and after the dot each time a main scanning is effected past a sub scanning. So, there occurs conspicuous deformation in the dot which appears as bleeding in the print, due to a delay in the rising and/or falling of the print engine control signal. Because of this, there occurs a density difference in the exposed dot between the main scanning direction and the sub scanning direction, which leads to image quality deterioration. Therefore, there is a need for effecting a correction for rendering uniform for the measured density of the horizontal stripe pattern and the measured density of the vertical stripe pattern. To this end, the test print TP having the horizontal stripe test patterns and the vertical stripe test patterns is useful. 
     Under the management by the test print managing section  51 , two kinds of test prints TP are outputted. The first test print TP is shown in  FIG. 7 . As may be understood from the partially enlarged figure in  FIG. 7 , with irradiation of laser beam, the is formed a line as a row of dots on the print paper P (test print TP). The dot row extending along the main scanning direction is referred to as a horizontal line. Whereas, when laser beam irradiation is repeated with movement of the print paper P along the sub scanning direction, there is formed a row (or a column) of dots extending along the sub scanning direction. This is referred to as a vertical line. In this invention, a horizontal stripe test pattern is an area having a plurality of such horizontal lines, and a vertical stripe test pattern is an area having a plurality of such vertical lines. And, a horizontal stripe is formed by continuous juxtaposition of horizontal lines of differing densities, e.g. juxtaposition of black (gradation value 0) horizontal lines and white (gradation value: 255) horizontal lines. Similarly, a vertical stripe is formed by continuous juxtaposition of vertical lines of differing densities, e.g. juxtaposition of black (gradation value 0) horizontal lines and white (gradation value: 255) horizontal lines. 
     This first test print TP contains 21 (twenty-one) kinds of test patterns from No. 1•1 to No. 1•21. The No. 1•21 pattern is a solid white pattern, and all the others are stripe patterns. Of the stripe patterns, the odd-numbered patterns (counted from the top) are horizontal stripe patterns and the even-numbered patterns are vertical stripe patterns, respectively. Also, the stripe patterns include those formed by repetition with lines 1-dot width, those formed by repetition with 2-dot width and those formed by repetition with 3-dot width. In  FIG. 7 , the 1-dot width patterns are denoted with “1ON1OFF”, and the patterns formed by repeated alternation of 2-dot width and 3-dot width are denoted with 2ON2OFF&amp;3ON3OFF”. More particularly, the patterns No. 1•1 through No. 1•10 are the 1ON1OFF patterns. Of the patterns No. 1•11 and thereafter, the odd-numbered patterns are 2ON2OFF&#39;3ON3OFF patterns, and the even-numbered patterns are 1ON1OFF patterns. The input gradation values for these patterns are also varied respectively. Input gradation values (R, G, B)=(0, 0, 0) are represented by gradation [0]. Input gradation values (R, G, B)=(25, 25, 25) are represented by gradation [25] and so on. The gradation values are represented as 8-bit gradation and the gradation [0] corresponds to black. That is, the first and second test patterns from the top (No. 1•1 and No. 1•2) have gradation [0]. The next two patterns have gradation [25]. The next two have gradation [50]. The next two have gradation [100]. The next two have gradation [150], and so on. Subsequent thereto, from the patterns No. 1•11 and No. 1•12, the gradation returns to gradation [0], followed by gradation [25], gradation [50], gradation [100]. And, the last two patterns No. 1•19 and No. 1•20 have gradation [150]. It should be noted, however, that while the odd-numbered (from the top) test patterns are laser-exposed based on a reference output gradation value obtained from the input gradation value based on a base correction curve, the even-numbered test patterns are laser-exposed based on a corrected output gradation value obtained by changing the input gradation value shown by α % from the base. The purpose of creating such α % changed test pattern is to obtain a comparison test pattern which is offset to the opposite sides from the base for the base test pattern. The values of α are: −5 for No. 1•2, No. 1•4 and No. 1•6, −10 for No. 1•8 and No. 1•10, +5 for No. 1•12, No. 1•14, No. 1•16, and +10 for No. 1•18 and No. 1•120. The test patterns No. 1•11, No. 1•13, No. 1•15, No. 1•17 and No. 1•19 are utilized for the falling correction, and the other patterns are utilized for the rising correction. 
       FIG. 8  shows a second test print TP similar to the first test pattern TP. This second test print TP contains twenty-two kinds of test patterns from No. 2•1 to No. 2•22. The pattern No. 2•22 is a solid white pattern, all the others being stripe patterns. Of the stripe patterns, the odd-numbered (counted from the top) test patterns are horizontal stripe patterns and 4ON4OFF patterns. The even-numbered test patterns are vertical stripe patterns and 2ON2OFF&amp;3ON3OFF patterns. Also, the odd-numbered test patterns are utilized for the black balance correction, whereas the even-numbered test patters are utilized for falling correction. The test patterns used for the black balance correction have gradation values which are varied from gradation [0] to gradation [60] by multiple of 6 (six). All of the test patterns included in this second test print TP and used for the falling correction are comparison test patterns with α % changes from the base. And, the value of α are −5 for the three patterns from the top, −10 for the next two, +5 for the next three and +10 for the last two, respectively. 
     What is important for the layouts of the test patterns in these two test prints is that the base test patterns and the comparison test patterns with α % changes from the base are arranged in alternation. For laser-exposing a comparison test pattern, it is necessary to set, in a register, a correction value for changing α % from the base. The alternate layout allows this operation for setting, in a register, a correction value for changing α % from the base, to be effected while the preceding base test pattern is being laser-exposed. 
     [Undershoot Correction] 
       FIG. 9  is a diagram illustrating the concept underlying the undershoot correction. Undershoot correction is a correction effected for reducing bleeding due to falling control delay for directly modulated R and B laser beams in the main scanning direction. Incidentally, an LD bias set value O 2  in  FIG. 9  is the limit point of light amount where color development is possible on the print paper. Namely, with an amount of light generated by a control current corresponding to a D/A conversion value below this LD bias set value, the color cannot be developed on the print paper. Normally, when a white dot is to be formed, in consideration of subsequent rise in the current, the D/A conversion value is not set to 0, but set to the LD bias set value. 
     However, with such control described above, bleeding will occur from the black dot immediately before the white dot, thus resulting in image quality deterioration. The undershoot correction is the correction for reducing this bleeding. Specifically, this is a process for adding a correction value calculated by the following method from an output gradation value immediately after a rise in the main scanning direction. 
     Namely, a correction value B for the undershoot correction is a product obtained by multiplying a falling amount A and an undershoot correction coefficient: X[i]≦0 which is set in advance for the falling amount A. That is, the correction value B for the undershoot correction can be obtained from: B=A×X [f (A)], where i is a register number, f( ) is a function for converting a falling amount to a register number. Incidentally, the undershoot correction coefficient is obtained by the predetermined method and then stored in the LUT  42 . 
     The solid line in  FIG. 9  shows the variation in the D/A conversion value (output gradation value) in the main scanning direction. In this example, the line includes two falling portions. For the respective falling portions, correction values B and B′ are obtained and the undershoot correction is effected based on these correction values (the dot line portions in  FIG. 9 ). 
     [Rising Correction] 
       FIG. 10  is a diagram illustrating the concept underlying the rising correction. The solid line shows the variation in the D/A conversion value (output gradation value) in the main scanning direction. In the case of the example shown in  FIG. 10 , the line includes two rising portions. One rises from O 1  to O 2 , and the other rises from O 3  to O 4 , where, A=O 2 −O 1 , A′=O 4 −O 3 . 
     In the above, rising correction values C and C′ are determined, based on rising amounts A and A′ and rising correction coefficients calculated by the above-described process and stored in the LUT  42 . Supposing the rising correction coefficient stored in the LUT  42  is: −1≦Y [i]≦1, then, the rising correction value C can be obtained from: C=A×Y[f(A)], where i is a register number, f( ) is a function for covering an output gradation to a register number. Similarly, C′=A′×Y[f(A′)]. 
     A rising correction value thus obtained is added to an output gradation value immediately after a rise, whereby a new output gradation value is obtained (dotted line portions in  FIG. 10 ). Incidentally, in this example, the rising correction value has a positive value. In actuality, however, the correction value can be negative also. Further, sometimes, the corrected output gradation value may exceed the maximum output gradation value or fall below the minimum output gradation value. In such cases, the value will be rounded off to the maximum output gradation value or the minimum output gradation value as needed. 
     [Falling Correction] 
       FIG. 11  illustrates the concept underlying falling correction. The solid line shows variation in the D/A conversion value (output gradation value) in the main scanning direction. In this example, the line includes two falling portions, one falling from O 1  to O 2 , the other falling from O 3  to O 4 . The falling amounts of these portions are: A=O 1 −O 2  and A′=O 3 −O 4 , respectively. 
     In the above, like the rising correction values described above, falling correction values D and D′ are determined, based on falling amounts B and B′ and rising correction coefficients stored in the LUT  42 . Supposing the falling correction coefficient stored in the LUT  42  is: −1≦Z[i]≦1, then, the falling correction values are D=A×Z[f(B)], D′=A′×Z[f(B′)]. Here, it should be noted, however that the falling correction is done only when such condition continues for 2 or more dotes after the falling. Namely, no falling correction is done when a rise occurs immediately after that fall. 
     A falling correction value thus obtained is added to an output gradation value immediately before the fall, whereby a new output gradation value is obtained (dotted line portions in  FIG. 11 ). Incidentally, in this example, the falling correction value has a positive value. In actuality, however, the correction value can be negative also. Further, sometimes, the corrected output gradation value may exceed the maximum output gradation value or falls below the minimum output gradation value. In such cases, the value will be rounded off to the maximum output gradation value or the minimum output gradation value, as needed. 
     Next, there will be explained a procedure for obtaining the rising correction coefficients or falling correction coefficients described above. As the procedures take place similarly in the two cases, here only the procedure for obtaining rising correction coefficients will be explained with reference to the flowchart in  FIG. 12 . 
     The test print TP used here is the first test print shown in  FIG. 7 . and, measured density values of its test patterns other than No. 1•11 and No. 1•13 and No. 1•15, No. 1•17 and No. 1•19 are utilized. 
     First, from the base test pattern (No. 1•1) having gradation [0], the procedure obtains a measured density value as a base value (# 01 ). Next, from the comparison patterns (No. 1•2 and No. 1•12) for the gradation [0], the procedure obtains measured density values as two comparison values (“first comparison value” and “second comparison value” hereinafter) (# 02 ). 
     The rising correcting section  56  calculates a difference value (“first difference value d 1 ” hereinafter) between the first comparison value and the second comparison value obtained from the density value relationship deriving section  52  and a difference value (“second difference value d 2 ” hereinafter) between the second comparison value and the base value (# 03 ). 
     Further, based on the first difference value d 1  and the second difference value d 2 , an approximate straight line is calculated (# 04 ). This operation will be specifically explained with reference to  FIG. 13 . First, there is assumed a plane defined by the horizontal line representing the difference between the comparison values and the base values and the vertical line representing the correction values. Then, in this plane, points based on the first difference value and the second difference value are plotted (“point P 1 ” and “point P 2 ” hereinafter). Here, a comparison pattern  62   a  providing the basis for the calculation of the first difference value d 1  represents the exposure corrected with the +α % correction coefficient. Therefore, the point P 1  has coordinates of (d 1 , α). Similarly, the point P 2  has coordinates (d 2 , —α). Then, by obtaining a segment interconnecting the point P 1  and the point P 2  by the well-known method, there is obtained an approximate straight line L. In the instant embodiment, as described above, two points are determined based on two comparison values and an approximate straight line is obtained therefrom. The invention, however, is not limited thereto. Instead, an approximate straight line can be obtained, based on three or more comparison values. In such case, the approximate straight line can be obtained by other well-known methods such as the least square method. 
     Then, based on the approximate straight line thus calculated, a correction coefficient is calculated (# 05 ). Specifically, the ordinate intercept of the calculated straight line will be obtained as the correction coefficient. In the case of the example shown in  FIG. 13 , the ordinate intercept of the straight line L is the point Pd (0, V), hence, the calculated correction coefficient is V. 
     With the above-described procedure, there is obtained a correction coefficient for the input gradation [0]. And, this procedure is repeated for all of the input gradations from gradation [0] to gradation [150] (NO branching at # 06 ). 
     With the above-described procedure, correction coefficients V 0 , V 25 , V 50 , V 100 , V 150  for input gradations [0], [25], [50], [100] and [150] are obtained. Then, correction curve calculating means  85  calculates and obtains a correction curve from these correction coefficients (# 07 ).  FIG. 14  is a diagram showing this. The upper section in  FIG. 14  shows an input/output characteristics curve which defines the relationship of the output gradations relative to input gradations. Whereas, the lower section thereof shows a plane with a horizontal axis representing output gradations and a vertical axis representing correction coefficients, in which points P 0 , P 25 , P 100 , P 150  for the correction coefficients V 0 , V 25 , V 50 , V 100 , V 150  calculated by the above-described procedure are plotted. For instance, the horizontal axis element for P 0  is determined based on the input/output characteristics curve. Namely, the output gradation for the input gradation [0] becomes the horizontal axis element for P 0 . Also, the vertical axis element for P 0  is the correction coefficient V 0  for the input gradation [0]. In this way, the coordinates for the point P 0  are determined and those of the other points P 25 , P 50 , P 100 , P 150  will be determined similarly. 
     Then, based on the points thus obtained, a correction curve is obtained by a well-known method. In the instant embodiment, there is effected approximation by a cubic curve, so the solid line shown in the lower section in  FIG. 14  is obtained as the correction curve. 
     If a value on the approximate correction curve is employed as a correction coefficient for an output gradation outside the range employed for the approximation of the correction curve, there can occur image quality deterioration due to low reliability. For this reason, for obtaining a correction coefficient for a gradation lower than the output gradation corresponding to the input gradation [150], the correction coefficient V 150  for the output gradation B will be employed. That is, the correction coefficient for the output gradation interval [0, B] will be V 150 . On the other hand, for correction coefficients greater than the output gradation A corresponding to the input gradation [0], the values of the correction curve will be employed as the correction coefficients, until the output gradation 1.1 A. However, as correction coefficients for over the output gradation 1.1 A, the correction coefficient V 0 ′ corresponding to the output gradation 1.1 A is employed. That is, the correction coefficient for the output gradation interval [1.1 A, maximum gradation]: is V 0 ′. In this way, correction coefficients for all the output gradations are obtained and stored in the LUT  42  (# 08 ). 
     [Black Balance Correction] 
     The undershoot correction, the rising correction and the falling correction described above are provided for eliminating density difference between a horizontal line and a vertical line due to control error for modulated laser. However, even if these corrections are done, a color development density difference can occur in the respective color components (C, M, Y in the case of print paper) in the black area, so there occurs color irregularity in this black area. The black balance correction is the correction for restricting such color irregularity in the black area. Through this correction, the black areas on the gradation conversion curves (group of gradation conversion data) stored in the LUT  42  are corrected. Incidentally, in this black balance correction, the second test print TP shown in  FIG. 8  is utilized. 
     Next, the procedure of the black balance correction will be explained with reference to the flowchart in  FIG. 15 . First, from the test print managing section  51 , image data of the test print TP are read out (# 11 ), so that gradation conversion is effected by the gradation converting section  43   a  of the print data generating section  43 , with using data presently set in the gradation conversion LUT  42 . Then, based on the generated print data, the exposure engine  13  is driven and controlled to expose the print paper P. The exposed print paper P is developed at the developing section  14  and then dried and outputted as the test print TP (# 12 ). 
     The outputted test print TP is sent into the colorimeter  20  through a test print discharging/conveying passage for measurement of density values of the respective test patterns herein (# 13 ). Upon completion of the density measurement on the test print TP, the result of the density measurement is transferred to the density value relationship deriving section  52 . The measured densities used in the black balance correction are shown in  FIG. 8 . As shown, of the twenty-two kinds of test patterns from No. 2•1 to No. 2•22, the measured densities of the eleven test patterns (all odd-numbered from the top) are used. The density value relationship deriving section  52  links the measured density values of the above-described kinds of test patterns with the test gradation values (input values) corresponding to these contrasting density test patterns, thereby to derive the test input gradation value-density value relationships shown in  FIG. 16  (# 14 ). Incidentally, when the exposure amount of red is decreased, this results in decrease in the color development of cyan on the print paper P. When the exposure amount of green is decreased, this results in reduction in the color development of magenta on the print paper P. When the exposure amount of blue is decreased, this results in reduction in the color development of yellow on the print paper P. Therefore, the vertical axis in  FIG. 16  shows the measured density value of cyan (shown as “C density value” in  FIG. 16 ) in response to change in the input value of red and shows the measured density value of magenta (shown as “M density value” in  FIG. 16 ) in response to change in the input value of green and shows the measured density value of yellow (shown as “Y density value in  FIG. 16 ) in response to change in the input value of blue. 
     From the test input gradation value-density value relationship derived as above, the density value relationship deriving section  53  specifies the respective test input gradation values, namely, the minimum measured density vales: Min [1] through Min [11] in the eleven test patterns (# 15 ). In the example shown in  FIG. 16 , in all of the test patterns, the C density value shows the minimum measured density value, hence, the C density value is the minimum measured density value. Next, for each test pattern, the process calculates correction input gradation values: (in R [Min [1]], in G [Min [1]], in B [Min [1]] through in R [Min [11]], in G [Min [11]], in B [Min [11]]) which are R, G, B representation of the input values that are to give substantially same density value as this minimum measured density value (# 16 ). Of these,  FIG. 16  shows only in R [Min [1]], in G [Min [1]], in B [Min [1]] and in R [Min [6]], in G [Min [6]], in B [Min [6]]. 
     Upon completion of the calculation of the correction input gradation values shown in R, G, B components, the black area pseudo conversion relationship creating section  54   a  calculates an output gradation value for each correction input gradation value, with reference to the currently set gradation conversion LUT  42  (# 17 ) and links this output gradation value with the test input gradation value (input value), thereby to create the input gradation value-output gradation value relationship shown in  FIG. 17  and establishes this relationship as a black area pseudo conversion curve (# 18 ). 
     Next, the combining adjusting section  54   b  combines the black area pseudo conversion curve with the gradation conversion curve (the data contents of the LUT  42 ) for the non-black areas to create a black area correction gradation conversion curve as a black-area-corrected, i.e. black-balance-adjusted gradation conversion curve (# 19 ). For the creation of this black area correction conversion curve, in this embodiment, first, as shown in  FIG. 18 , of the black area pseudo conversion curve, there is used a black area pseudo conversion curve portion where the input gradation value varies from [0] to [30]. On the other hand, of the gradation conversion curve, there is used a portion thereof which starts from a point where the input gradation value drops by 20% from the input gradation value of 30 on the black area pseudo conversion curve, namely, at the point where the input gradation value has 80% of the output gradation value for the input gradation value of 30. Then, the gap therebetween is interpolated by some appropriate quadratic mathematical formula to be bridged thereby. Next, this bridging or interpolating process will be described with reference to  FIG. 19 . In the following section, the process will be explained with using the R component curve as an example. It should be understood, however, that the other color components can be processed similarly. In  FIG. 19 , the black area pseudo conversion curve has a terminal end P 1  which has coordinate values: (x 1 , y 1 ), whereas the gradation conversion curve has a starting end P 2  which has coordinate values (x 2 , y 2 ). Then, an interpolation point P 3  is set therebetween. The coordinate values: (x 3 , y 3 ) of this interpolation point P 3  are defined, in this embodiment as under: 
         x 3=( x 1+ x 2)/2 
         y 3=(2 y 1+3 y 2)/5 
     Then, by using the quadratic expression calculated from the respective coordinate values of the points P 1 , P 2  and P 3 , the gap (blank) portion is interpolated (bridged). The resultant continuous black area correction gradation conversion curve is shown in  FIG. 20 . 
     Upon obtaining the black-balance adjusted black area correction gradation conversion curve through the above-described process, the gradation conversion curve, that is, the LUT  42 , is corrected by the gradation correction executing section  54   c , based on the data of this black area correction gradation conversion curve (# 20 ). 
     Next, the overall adjusting operation of the color image printer having the above-described construction will be explained with reference to the flowchart of  FIG. 2 . This adjusting operation is effected normally, when the kind of print paper used has been changed, for example. 
     First, the laser bias adjustment of the laser source (R laser source  131 , B laser source  132 ) is effected (# 51 ). In this laser bias adjustment, a laser bias values is obtained from result of measurement of laser current monitor. Further, the process executes a setup operation, which per se is well-known, and comprises general correction of the gradation conversion curve, including shading correction and so on (# 52 ). The values set by this stet-up operation are referred to as set-up values. 
     Next, the process initiates the plurality of corrections relating to the present invention. First, the process effects the undershoot correction in which an undershoot correction value is obtained by calculation from the laser bias value and the set-up values (# 53 ). Further, the process reads the base correction value from the base correction curve lookup table in the LUT  42  and then calculates correction values offset by +α % and −α % from this base correction value (# 54 ). First, the process creates image data for the first test print and outputs this first test print TP (# 55 ). The process obtains measured density values for all of the test patterns included in the outputted first test print TP (# 56 ). In this, first, the rising correcting section  56  obtains a rising correction value with using the measured density value of a predetermined test pattern and establishes a rising correction curve (rising correction table) (# 57 ). That is, in this rising correction, there is set a correction value for matching the density of the 1ON1OFF vertical stripe test pattern with the density of the 1ON1OFF horizontal test pattern. After setting of the rising correction, the process further creates image data for the second test print and outputs this second test print TP (# 58 ). The process obtains measured density values for all of the test patterns included in the outputted second test print TP (# 59 ). 
     Next, the falling correcting section  56  obtains a falling correction value, with using the measured density values of the base test pattern formed in the first test print TP and the comparison test pattern formed in the second test print TP and establishes a falling correction curve (falling correction table) (# 60 ). That is, in this falling correction, there is set a correction value for matching the density of the 2ON2OFF &amp;3ON3OFF vertical stripe test pattern with the density (corresponding to the 2.5 dots as a whole) of the 2ON2OFF &amp; 3ON3OFF horizontal test pattern. Then, the LUT managing section  54  executes a black balance adjustment for correcting black area in the base gradation curve, with using a measured density of a predetermined test pattern in the second test print TP (# 61 ). 
     Through the above-described series of correction operations, in this color image printer, occurrence of density difference between a horizontal line and a vertical line due to control error of modulated laser is restricted and also color irregularity in a black area between the horizontal line and the vertical line is restricted also. Incidentally, if satisfactory result is not obtained with one time adjustment operation, the operation from step # 53  through step # 63  will be repeated