Abstract:
The amount of exposure to a light beam is coarsely adjusted by a coarse adjusting unit which has a wider adjustment range, and finely adjusted by a fine adjusting unit which has a narrower adjustment range but a higher adjustment accuracy. With the coarse and fine adjusting units, the amount of light of the light beam for recording an image can be adjusted to a nicety by a relatively inexpensive arrangement.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of and an apparatus for adjusting an amount of light in an image exposure recording system which scans a recording medium with a light beam to record an image on the recording medium by exposure to the light beam. 
     2. Description of the Related Art 
     Heretofore, there have widely been used image exposure recording systems in which a light beam modulated by image information is deflected by a light deflector such as a galvanometer mirror or a resonant scanner, and applied to scan a recording medium such as a film that is being fed in one direction, in another direction substantially perpendicular to the direction in which the recording medium is fed, for thereby recording an image on the recording medium by exposure to the light beam. 
     The image recorded on the recording medium has its density depending on the amount of light of the light beam. Therefore, the amount of light of the light beam applied to the recording medium needs to be set highly accurately in order to obtain an image of desired density. When a laser beam whose amount of light has a Gaussian distribution is applied to record an image on a recording medium which produces a color upon being supplied with a light energy beyond a certain level, the amount of light of the laser beam needs to be set with high accuracy in order to achieve a desired coloring range. 
     It has been customary to adjust the amount of light of a light beam by inserting an optical filter such as an ND filter or the like which has its optical transmittance varying stepwise depending on the location on the optical filter, forming on a recording medium a test pattern which is area-modulated by varying the. position of the optical filter, and measuring the density of the test pattern to determine an amount of light that can achieve an optimum exposure state. 
     The accuracy of the adjustment of the amount of light according to the above process greatly depends on the resolution of the optical filter. Therefore, a highly expensive optical filter is required if the amount of light is to be adjusted to a nicety. However, it is very difficult to manufacture an optical filter which has a high resolution and a wide adjustment range. In addition, an expensive measuring unit is necessary to measure the density of an area-modulated screen-tint test pattern. 
     SUMMARY OF THE INVENTION 
     It is a general object of the present invention to provide a method of and an apparatus for adjusting the amount of light of a light beam to record a desired image in an image exposure recording system, highly accurately with an inexpensive arrangement without the need for a special measuring unit. 
     A major object of the present invention is to provide a method of and an apparatus for adjusting the amount of a continuously oscillating light beam effectively and highly accurately in an image exposure recording system. 
     Another object of the present invention is to provide an inexpensive amount-of-light adjusting device having a means for modulating a light beam depending on an image to be recorded, doubling as a means for finely adjusting an amount of light, without increasing the number of components in an image exposure recording system. 
     The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which preferred embodiments of the present invention are shown by way of illustrative example. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a printing plate producing system; 
     FIG. 2 is a schematic perspective view of a light beam scanning device, with a control circuit thereof shown in block form, in the printing plate producing system shown in FIG. 1; 
     FIG. 3 is a perspective view of a variable-transmittance ND filter; 
     FIG. 4 is a perspective view of another variable-transmittance ND filter; 
     FIG. 5 is a block diagram of an amount-of-light adjusting circuit incorporated in the printing plate producing system shown in FIG. 1; 
     FIG. 6 is a flowchart of a processing sequence of an amount-of-light adjusting process; 
     FIG. 7 is a flowchart of a subroutine of the processing sequence of the amount-of-light adjusting process shown in FIG. 6; 
     FIG. 8 is a diagram of amount-of-light control characteristic data for detected values of amounts of light with respect to step numbers of a variable-transmittance ND filter; and 
     FIG. 9 is a diagram showing a test pattern outputted to a recording medium. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows in perspective a printing plate producing system  30  which incorporates a method of and an apparatus for adjusting an amount of light in an image exposure recording system according to the present invention. 
     As shown in FIG. 1, the printing plate producing system  30  directly produces a printing plate  32  on which image information for producing a printed material is recorded, from digital image data. The printing plate producing system  30  basically comprises a plate supplying device  34  for supplying an unexposed printing plate  32 , a light beam scanning device  36  for scanning an unexposed printing plate  32  with a light beam modulated by image information to record an image on the printing plate  32 , and a developing device  38  for developing the image recorded on the printing plate  32 . 
     The plate supplying device  34  holds a plurality of unexposed printing plates  32  and supplies one at a time of the unexposed printing plates  32  to the light beam scanning device  36 . The light beam scanning device  36  feeds the unexposed printing plate  32  by the exposure stage  40  supplied from the plate supplying device  34  in an auxiliary scanning direction indicated by the arrow Y, and at the same time scans the unexposed printing plate  32  with a laser beam L, which has been modulated by image information supplied from an image recording unit  42 , in a main scanning direction indicated by the arrow X that is perpendicular to the auxiliary scanning direction, thereby recording a two-dimensional image on the printing plate  32 . The developing device  38  develops the image recorded on the printing plate  32  that is supplied from the light beam scanning device  36 . 
     FIG. 2 shows in perspective the light beam scanning device  36  together with its control circuit shown in block form. 
     As shown in FIG. 2, the light beam scanning device  36  has a recording light source  46  energizable by a laser driver  44  for outputting a continuously oscillating laser beam L for recording an image on a printing plate  32 , and a synchronizing light source  50  energizable by a laser driver  48  for outputting a synchronizing laser beam S for generating a synchronizing clock signal used when the laser beam L scans the printing plate  32  in the main scanning direction. 
     The light beam scanning device  36  includes a mechanical shutter  52 , a variable-transmittance ND filter  54  as a means for coarsely adjusting an amount of light, an acousto-optic modulator (AOM)  56  as a means for finely adjusting an amount of light or a modulating means, a resonant scanner  58 , a scanning lens  59 , reflecting mirrors  60 ,  62 , and a mechanical shutter  63  which are successively disposed in the light path of the laser beam L that is outputted from the recording light source  46 . 
     The mechanical shutter  52  is movable into and out of the light path of the laser beam L by a displacing unit  64  for selectively supplying and blocking the laser beam L to the printing plate  32 . 
     As shown in FIG. 3, the variable-transmittance ND filter  54  has an arcuate stepped edge  55  whose transmittance varies stepwise in an area thereof which transmits the laser beam L therethrough. The arcuate stepped edge  55  can be changed in position with respect to the light path of the laser beam L by an ND filter drive motor  66  that can be energized by an ND filter driver  57 . The arcuate stepped edge  55  coarsely adjusts the amount of light of the laser beam L depending on its position with respect to the light path of the laser beam L. 
     As shown in FIG. 4, the means for coarsely adjusting an amount of light may alternatively comprise a variable-transmittance ND filter  69  having a linear stepped edge  67  whose transmittance varies stepwise in an area thereof which transmits the laser beam L therethrough, the variable-transmittance ND filter  69  being movable by a displacing unit  71 . If the laser beam L outputted from the recording light source  46  is a linearly polarized beam, then the means for coarsely adjusting an amount of light may comprise a rotatable polarizing filter for coarsely adjusting the amount of light of the laser beam L depending on its angular displacement. 
     The AOM  56  turns on and off the laser beam L depending on image information to be recorded, and finely adjusts the amount of light of the laser beam L that passes through the AOM  56  according to a controlled variable determined by an amount-of-light adjusting process to be described later on. The image information is read from an image memory  68 , and converted into an on/off modulation signal by an image signal controller  70 . The on/off modulation signal is supplied to an AOM driver  72 . The AOM driver  72  supplies the AOM  56  with a drive signal whose intensity is finely adjusted by the controlled variable and which is turned on and off depending on the image information. 
     The means for finely adjusting an amount of light or modulating means may comprise, rather than the AOM  56 , an electro-optic modulator (EOM) or a magneto-optic modulator (MOM). It is also possible to finely adjust the amount of light of the laser beam L by adjusting a drive signal supplied from the laser driver  44  to the recording light source  46 , directly with a light source control circuit. 
     The resonant scanner  58  oscillates a mirror at a high speed with a drive signal supplied from a scanner driver  74 , and deflects the laser beam L from the AOM  56  in the main scanning direction indicated by the arrow X and supplies the deflected laser beam L to the scanning lens  59 . The laser beam L that has passed through the scanning lens  59  is adjusted in its scanning speed with respect to the main scanning direction, and is then reflected by the reflecting mirrors  60 ,  62  toward the printing plate  32 . 
     The mechanical shutter  63  is positioned between the reflecting mirror  62  and the printing plate  32  and is elongate in the main scanning direction indicated by the arrow X. The mechanical shutter  63  is movable into and out of the light path of the laser beam L by a displacing unit  76 . The mechanical shutter  63  has a reflecting mirror  78  disposed centrally therein. When the mechanical shutter  63  is in the light path of the laser beam L, the reflecting mirror  78  reflects the laser beam L toward a photodiode  80  for monitoring an amount of light. The photodiode  80 , which serves as a means for detecting an amount of light, may be replaced with a phototransistor. 
     FIG. 5 shows in block form an amount-of-light adjusting circuit  96  for adjusting the amount of light of the laser beam L outputted from the recording light source  46 . As shown in FIG. 5, the amount-of-light adjusting circuit  96  includes the variable-transmittance ND filter  54  (means for coarsely adjusting an amount of light), the AOM  56  (means for finely adjusting an amount of light), the photodiode  80  (means for detecting an amount of light), an I/V converter (gain control circuit)  81  for converting a current into a voltage, and an A/D converter  82  for converting an analog voltage signal from the I/V converter  81  into a digital voltage signal. The laser driver  44 , the ND filter driver  57 , the AOM driver  72 , the I/V converter  81 , and the A/D converter  82  are connected to a CPU  84  which serves as a means for calculating detected values of a target amount of light and a means for setting an adjustment quantity. To the CPU  84 , there is connected a data storage unit  89  for storing various data for adjusting an amount of light for exposure. The data storage unit  89  serves as a first storage means and a second storage means. 
     The variable-transmittance ND filter  54 , which serves as the means for coarsely adjusting an amount of light, may be replaced with an optical filter such as a polarizing filter. Further, the acousto-optic modulator (AOM)  56 , which serves as the means for finely adjusting an amount of light, may be replaced with an optical filter such as an electro-optic modulator and a magneto-optic modulator. Alternatively, it is possible to adjust the amount of light directly by the laser driver  44 . 
     The I/V converter  81  comprises a pair of series-connected front- and rear-stage amplifiers  83 ,  85 , a plurality of resistors R 1 , R 2 , R 3  connected parallel to and between input and output terminals of the front-stage amplifier  83 , and a plurality of switches SW 1 , SW 2 , SW 3  connected in series to the respective resistors R 1 , R 2 , R 3 . The switches SW 1 , SW 2 , SW 3  can be controlled by the CPU  84  for controlling a gain, i.e., an amplification factor, of the I/V converter  81 . 
     As shown in FIG. 2, the resonant scanner  58 , the scanning lens  59 , the reflecting mirror  60 , a reflecting mirror  87 , a reference grating  86 , a light guide rod  88 , and photodiodes  90   a ,  90   b  for generating a synchronizing signal are successively disposed in the light path of the synchronizing laser beam S that is outputted from the synchronizing light source  50 . 
     The synchronizing light source  50  is positioned to apply the synchronizing laser beam S to the resonant scanner  58  at an angle different from the laser beam L. The synchronizing laser beam S is reflected and deflected in main scanning direction indicated by the arrow X by the resonant scanner  58 . The synchronizing laser beam S deflected by the resonant scanner  58  travels through the scanning lens  59  to the reflecting mirror  60 . The synchronizing laser beam S is reflected by the reflecting mirror  60  toward the reflecting mirror  87 , which reflects the synchronizing laser beam S toward the reference grating  86 . The synchronizing laser beam S passes through the reference grating  86 . 
     The reference grating  86  is elongate in main scanning direction indicated by the arrow X, and has a linear succession of slits  92  along its longitudinal direction, the number of the slits depending on the resolution. 
     The light guide rod  88 , which is substantially cylindrical in shape, is disposed behind the reference grating  86  to receive the synchronizing laser beam S that has passed through the reference grating  86 . The light guide rod  88  is made of a material capable of transmitting light therethrough. The synchronizing laser beam S that has entered the light guide rod  88  is repeatedly reflected therein and travels therethrough to the photodiodes  90   a ,  90   b  which are disposed on the respective ends of the light guide rod  88 . 
     To the photodiodes  90   a ,  90   b , there is connected a synchronizing clock generator  94  for generating a synchronizing clock signal from the synchronizing laser beam S. The synchronizing clock signal generated by the synchronizing clock generator  94  is supplied, as a recording timing signal for the image information to be recorded with respect to the main scanning direction indicated by the arrow X, to the image signal control circuit  70 . 
     The printing plate  32  is positioned on and held by an exposure stage  40 , which can be fed in the auxiliary scanning direction indicated by the arrow Y by a ball screw  100  that is rotatable about its own axis by an auxiliary scanning motor  98 . The auxiliary scanning motor  98  is energizable by an auxiliary scanning motor driver  104  based on a motor driving reference clock signal that is supplied from an auxiliary scanning motor driving clock generator  102 . The motor driving reference clock signal is generated by the auxiliary scanning motor driving clock generator  102  based on a scanning clock signal which is a main scanning start timing signal supplied from the scanner driver  74 . 
     The printing plate producing system  30  is basically constructed as described above. Operation of the printing plate producing system  30  will be described below. 
     First, an image recording process carried out by the printing plate producing system  30  will be described below with reference to FIGS. 1 and 2. 
     When the printing plate producing system  30  is turned on, the plate supplying device  34  supplies an unexposed printing plate  32  to the exposure stage  40  of the light beam scanning device  36 . The exposure stage  40  which has been supplied with the unexposed printing plate  32  is displaced in the auxiliary scanning direction indicated by the arrow Y by the ball screw  100  that is rotated by the auxiliary scanning motor  98 , thus feeding the printing plate  32  to a given position in the image recording unit  42 . 
     In the light beam scanning device  36 , the scanner driver  74  supplies a drive signal to the resonant scanner  58 , whose mirror starts to oscillate at a high speed. At this time, the scanner driver  74  also generates a scanning clock pulse each time the mirror of the resonant scanner  58  oscillates in one main scanning cycle, and supplies the scanning clock pulse to the image signal controller  70 . 
     Then, the laser driver  48  supplies a drive signal to the synchronizing light source  50 , which outputs a synchronizing laser beam S. The synchronizing laser beam S outputted from the synchronizing light source  50  is reflected and deflected by the resonant scanner  58 , and guided by the scanning lens  59  and the reflecting mirrors  60 ,  87  to the reference grating  86 . 
     The synchronizing laser beam S applied to the reference grating  86  successively passes through the slits  92  as the synchronizing laser beam S moves along the reference grating  86  in the main scanning direction indicated by the arrow X, and enters as a pulsed light signal into the light guide rod  88 . The pulsed synchronizing laser beam S is repeatedly reflected in the light guide rod  88  and travels therethrough to the photodiodes  90   a ,  90   b  on the respective ends of the light guide rod  88 . The photodiodes  90   a ,  90   b  convert the pulsed synchronizing laser beam S into an electric signal and supplies the electric signal to the synchronizing clock generator  94 . The synchronizing clock generator  94  shapes the waveform of the electric signal and multiplies its frequency thereby to generate a synchronizing clock signal. The synchronizing clock signal generated by the synchronizing clock generator  94  is supplied to the image signal control circuit  70 . 
     Based on the scanning clock pulse from the scanner driver  74  and the synchronizing clock signal from the synchronizing clock generator  94 , the image signal control circuit  70  converts image information read from the image memory  68  into an on/off modulation signal, which is supplied to the AOM driver  72 . Based on the on/off modulation signal, the AOM driver  72  supplies a drive signal, which has been finely adjusted by a controlled variable determined by an amount-of-light adjusting process to be described later on, to the AOM  56 . 
     The recording light source  46  energized by the laser driver  44  outputs a continuously oscillating laser beam L for recording an image. The laser beam L is guided to the AOM  56  via the variable-transmittance ND filter  54  which is angularly moved by the ND filter drive motor  66  to coarsely adjust the amount of light of the laser beam L. In this image recording mode, the mechanical shutter  52  that is positioned in front of the variable-transmittance ND filter  54  is retracted out of the light path of the laser beam L by the displacing unit  64 . 
     Further, the mechanical shutter  63  positioned between the reflecting mirror  62  and the printing plate  32  is retracted out of the light path of the laser beam L by the displacing unit  76 . 
     The laser beam L that is applied to the AOM  56  is turned on and off by the AOM  56  depending on the image information, and the amount of light of the laser beam L is finely adjusted by the AOM  56 . The laser beam L is then supplied from the AOM  56  to the resonant scanner  58 . The resonant scanner  58  reflects and deflects the laser beam L, which is guided by the scanning lens  59  and the reflecting mirrors  60 ,  62  to the printing plate  32 . 
     The scanner driver  74  also supplies a scanning clock signal generated in each main scanning cycle to the auxiliary scanning motor driving clock generator  102 . Based on the supplied scanning clock signal, the auxiliary scanning motor driving clock generator  102  generates and supplies a motor driving reference clock signal to the auxiliary scanning motor driver  104 . Based on the supplied motor driving reference clock signal, the auxiliary scanning motor driver  104  generates a drive signal and applies the drive signal to energize the auxiliary scanning motor  98 , which rotates the ball screw  100  about its own axis. The exposure stage  40  is now displaced in the auxiliary scanning direction indicated by the arrow Y in synchronism with the scanning clock signal. 
     Therefore, the laser beam L modulated with the image information is applied to the printing plate  32  in the main scanning direction indicated by the arrow X while the printing plate  32  is being fed in the auxiliary scanning direction indicated by the arrow Y, thereby forming a two-dimensional image on the printing plate  32 . 
     The printing plate  32  with the two-dimensional image formed thereon is delivered to the developing device  38 , which develops the image recorded on the printing plate  32 . Thereafter, the printing plate  32  is fed to a printing process. 
     A process of adjusting the amount of light of the laser beam L in the printing plate producing system  30  will be described below with reference to FIGS. 6 and 7. 
     First, the displacing unit  76  is actuated to displace the mechanical shutter  63  to a closed position to allow the laser beam L outputted from the recording light source  46  to be reflected by the reflecting mirror  78  toward the photodiode  80  in step S 1 . 
     Then, the switch SW 1  of the I/V converter  81  is turned on to set the amplification factor thereof to a minimum level, and an offset value ofs of the A/D converter  82  is measured in step S 2 . If no ambient light is applied to the photodiode  80  at this time, then the processing in step S 1  may be dispensed with. The measured offset value ofs is stored in the data storage unit  89 . 
     Then, amount-of-light control characteristic data of the variable-transmittance ND filter  54  is measured in step S 3 . Specifically, the recording light source  46  outputs a laser beam L having a constant amount of light, and the laser beam L is guided by the variable-transmittance ND filter  54 , the AOM  56 , the resonant scanner  58 , the scanning lens  59 , and the reflecting mirror  78  to the photodiode  80 . The amount of light of the laser beam L is detected by the A/D converter  82 . At this time, the ND filter drive motor  66  is energized to displace the stepped edge  55  of the variable-transmittance ND filter  54  stepwise, and amounts of light ad (n,g) at respective step numbers n of the stepped edge  55  are detected with respect to respective gains g (=1, 2, 3) of the I/V converter  81 . The detected amounts of light ad (n,g) are stored as amount-of-light control characteristic data (see FIG. 8) in the data storage unit  89 . The gains g represent a parameter for determining an amplification factor when the switches SW 1 , SW 2 , SW 3  of the I/V converter  81  are successively turned on. 
     The CPU  84  calculates detected values adt (i,g) (i=1, 2, . . . ) of a target amount of light for respective set amounts of light p (i) for exposure in step S 4 . Specifically, if the reflecting mirror  78  has a reflectance r, the photodiode  80  has a sensitivity s, and the I/V converter  81  has a signal amplification factor m (g), then the detected value adt (i,g) of a target amount of light is calculated as follows: 
     
       
           adt (i,g)= p (i)· r·s·m (g)+ ofs   (1) 
       
     
     where ofs is the offset value determined in step S 2 . 
     FIG. 7 shows a detailed process of calculating detected values adt (i,g) of a target amount of light in step S 4 . 
     First, i=g=1 in steps S 4   a , S 4   b , S 4   c , and a detected value adt (1,1) of a target amount of light is calculated in step S 4   d . If the calculated detected value adt (1,1) of a target amount of light is smaller than 300 in step S 4   e , then the gain g is set to g=2 to increase the amplification factor m (g) in step S 4   c . Then, a detected value adt (1,2) of a target amount of light is calculated again in step S 4   d . The loop is repeated until the calculated detected value adt (i,g) of a target amount of light becomes equal to or greater than 300, whereupon the detected value adt (i,g) of a target amount of light and the gain g are stored in the data storage unit  89  in step S 4   f.    
     The gain g is adjusted to make the detected value adt (i,g) of a target amount of light equal to or greater than 300for the following reason: If the A/D converter  82  has a resolution of 12 bits, then the detected value adt (i,g) of a target amount of light is of a value in the range from 0 to 4095. By setting the detected value adt (i,g) of a target amount of light to a value in the range from 300 to 3000for example, good linearity is obtained for increased detection accuracy. 
     The above process is carried out for each of the set amounts of light p (i) for exposure in step S 4   g . The set amounts of light p (i) for exposure may be spaced at intervals of 2 ⅓ , i.e., may be 10 mW, 10·2 ⅓ mW, 10·2 ⅔ mW, . . . , for example. 
     Then, using the amount-of-light control characteristic data (see FIG. 8) determined in step S 3 , step numbers nd (i) of the variable-transmittance ND filter  54  capable of obtaining detected values ad (n,g) of an amount of light closet to the detected values adt (i,g) of a target amount of light are determined in step S 5  (see FIG.  6 ). 
     Then, the ND filter driver  57  actuates the ND filter drive motor  66  to insert the stepped edge  55  at the respective step numbers nd (i) of the variable-transmittance ND filter  54  into the light path of the laser beam L, and the switches SW 1 , SW 2 , SW 3  are set to equalize the gain g of the I/V converter  81  to the gains g of the detected values adt (i,g) of a target amount of light. Thereafter, the photodiode  80  measures detected values ad (nd (i), g) of an amount of light of the laser beam L at the respective settings in step S 6 . 
     Since the amount of light of the laser beam L has been coarsely adjusted by the variable-transmittance ND filter  54 , the detected values ad (nd (i), g) of an amount of light obtained in step S 6  are close to the detected values adt (i,g) of a target amount of light. 
     In order to equalize the detected values ad (nd (i), g) an amount of light to the detected values adt (i,g) of a target amount of light, the intensity of the drive signal supplied from the AOM driver  72  to the AOM  56  is adjusted to finely adjust the amount of light in step S 7 . 
     A controlled variable aom (i) for the drive signal of the AOM driver  72  adjusted to satisfy the equation: 
     
       
           ad ( nd (i),g)= adt (i,g)  (2) 
       
     
     is stored in the data storage unit  89 . 
     After coarse adjustment quantities (step numbers nd (i)) and fine adjustment quantities (controlled variables (aom (i)) for the laser beam L with respect to the respective detected values adt (i,g) of a target amount of light have been determined, the displacing unit  76  is actuated to retract the mechanical shutter  63  to an open position out of the light path of the laser beam L in step S 8 , and a test pattern is recorded on the printing plate  32  by exposure to the laser beam L in step S 9 . 
     Specifically, the stepped edge  55  of the variable-transmittance ND filter  54  is set to a step number nd (i), and the drive signal supplied from the AOM driver  72  to the AOM  56  is set to a controlled variable aom (i). Then, the laser beam L is applied to the printing plate  32  for thereby producing a patch  91  at each of the set amounts of light p (i) for exposure, as shown in FIG.  9 . Below each of the patches  91 , there are simultaneously printed a patch number i, a set amount of light p (i) for exposure, a detected value adt (i,g) of a target amount of light, a step number nd (i), and a controlled variable aom (i). 
     The operator then visually observes the test pattern thus formed as shown in FIG. 9, selects a patch  91  that is considered to have an optimum density, and determines the step number nd (i) and the controlled variable aom (i) relative to the selected patch  91  as a coarse adjustment quantity for the variable-transmittance ND filter  54  and a fine adjustment quantity for the AOM  56  in step S 10 . Alternatively, such a coarse adjustment quantity and a fine adjustment quantity can automatically be determined by the CPU  84  when the patch number i of the selected patch  91  is entered into the printing plate producing system  30 . 
     According to another process of selecting a coarse adjustment quantity and a fine adjustment quantity, a patch  91  positioned at a boundary where a density starts to be applied is selected, and the step number nd (i) and the controlled variable aom (i) relative to a patch  91  with a density which is spaced from the selected patch  91  by a certain number of patches are determined as adjustment quantities. This process is effective to avoid a selection mistake and ensure more reliable adjustments. 
     Although certain preferred embodiments of the present invention have been shown and described in detail, it should be understood that various changes and modifications may be made therein without departing from the scope of the appended claims.