Patent Publication Number: US-2006017801-A1

Title: Image recording apparatus

Description:
This application claims priority on Japanese patent application No. 2004-206941, the entire contents of which are hereby incorporated by reference. In addition, the entire contents of literatures cited in this specification are incorporated by reference.  
     BACKGROUND OF THE INVENTION  
      The present invention relates to an image recording apparatus which adopts a multi-beam scanning optical system and serves to record an image corresponding to image data on a recording medium using a plurality of laser light sources for emitting laser beams as an exposure light source.  
      In an image recording apparatus which adopts a multi-beam scanning optical system and serves to record an image using a plurality of laser light sources, chromatic aberrations such as a chromatic aberration of magnification occur due to a difference between oscillation wavelengths of laser beams (light beams) emitted from the respective laser light sources.  
      For this reason, in order to suppress the chromatic aberrations in the image recording apparatus adopting the multi-beam scanning optical system, it has been proposed to select glass materials of optical lenses, or as disclosed in JP 10-149430 A, JP 11-70698 A, and JP 11-88619 A, it has been proposed that a plurality of laser light sources are controlled using different scanning clocks to thereby carry out the correction.  
      A technique disclosed in JP 10-149430 A described above relates to a construction of an optical system in an optical scanning apparatus for executing a processing of, for example, applying a plurality of laser beams with different wavelengths to an object of the scanning to scan the object of the scanning with a plurality of laser beams, thereby recording information read through the scanning operation. In the technique disclosed in JP 10-149430 A, emission time and emission timing when respective laser beams are emitted from respective green, blue, and red semiconductor lasers are controlled, thereby compensating for the chromatic aberration of magnification.  
      In addition, a technique disclosed in JP 11-70698 A relates to a color printer using a plurality of laser light sources for emitting respective light beams with different wavelengths. In the technique disclosed in JP 11-70698 A, three light beams are modulated at different three data speeds, thereby correcting the transverse chromatic aberration of an f-θ lens.  
      In addition, a technique disclosed in JP 11-88619 A relates to an image exposing apparatus for applying three or more kinds of emitted light beams with different wavelengths to a photosensitive material based on image data to form a latent image on the photosensitive material. In the technique disclosed in JP 11-88619 A, the scanning lengths on an exposure surface of the two kinds of emitted light beams are made substantially equal to each other by a scanning lens permitting the characteristics of the chromatic aberrations of the two kinds of emitted light beams to be substantially equal to each other. Moreover, the frequency of a scanning clock for the two kinds of emitted light beams and the frequency of a scanning clock for the emitted light beam other than the two kinds of emitted light beams are determined so that the scanning length on the exposure surface of the two kinds of emitted light beams and the scanning length on the exposure surface of the emitted light beam other than the two kinds of emitted light beams become substantially equal to each other.  
      For example, in the application of the photographic print in which an image is recorded on a photographic paper, the above-mentioned multi-beam scanning optical system is used in the exposing unit. Taking the spectral sensitivity of the photographic paper into consideration, the exposure needs to be carried out using the red, green, and blue laser light sources as the exposure light source in the photographic print. However, since the laser beams emitted from the laser light sources have wide oscillation wavelengths in the visible range, it is very difficult to suppress the chromatic aberration of magnification up to about several tens of μm.  
      In addition, some laser light sources have large dispersion in oscillation wavelength. For example, there exists the laser light source having oscillation wavelength dispersion even in the range of about ±5 to 10 nm. However, when the selected laser light source is used, this leads to cost-up. On the other hand, when the laser light source having the large dispersion in oscillation wavelength is used, there occurs a problem that the optical design for correcting the chromatic aberration of magnification becomes very difficult, or the system becomes expensive since special glass materials need to be used for the optical lenses.  
     SUMMARY OF THE INVENTION  
      The present invention has been made in order to solve the above-mentioned problems in the prior art. Therefore, it is an object of the present invention to provide an image recording apparatus which is capable of suppressing chromatic aberrations irrespective of optical lenses, even when a laser light source having large dispersion in oscillation wavelength is used, to thereby reduce cost.  
      In order to attain the above-mentioned object, the present invention provides an image recording apparatus including: an exposure control circuit for controlling exposure corresponding to image data; and an exposing unit for imaging a plurality of laser beams with different oscillation wavelengths which are modulated with respective modulation signals outputted from the exposure control circuit to be emitted from a plurality of laser light sources, respectively, on a recording medium through optical lenses, thereby recording an image corresponding to the image data on the recording medium, wherein  
      the exposure control circuit includes: a clock selecting circuit for outputting different scanning clocks corresponding to each of the plurality of laser light sources based on information on the oscillation wavelengths of the plurality of laser beams; and laser driving circuits operating synchronously with the scanning clocks to output the respective modulation signals corresponding to the image data for the respective laser light sources, and wherein  
      the clock selecting circuit selectively output one scanning clock among a plurality of scanning clocks with different oscillation frequencies which are previously prepared based on the information on the oscillation wavelengths of the plurality of laser beams with respect to at least one laser light source among the plurality of laser light sources.  
      Here, it is preferable that the clock selecting circuit of the exposure control circuit selectively outputs the one scanning clock based on information on oscillation wavelength of a laser beam supplied from outside.  
      In addition, it is preferable that the exposing unit includes an information recording circuit in which the information on the oscillation wavelengths of the plurality of laser beams is recorded, and the clock selecting circuit of the exposure control circuit selectively outputs the one scanning clock based on the information on the oscillation wavelengths of the plurality of laser beams supplied from the information recording circuit.  
      Here, it is preferable that the information recording circuit is singly provided corresponding to the plurality of laser light sources, or the information recording circuit is provided in correspondence with each of the plurality of laser light sources. In addition, it is preferable that the information recording circuit is a ROM in which the information on the oscillation wavelengths of the plurality of laser beams is recorded.  
      In addition, it is preferable that the exposing unit include a wavelength detector for detecting the oscillation wavelengths of the plurality of laser beams, and the clock selecting circuit of the exposure control circuit selectively outputs the one scanning clock based on the information on the oscillation wavelengths of the plurality of laser beams supplied from the wavelength detector.  
      Here, it is preferable that the wavelength detector is provided in correspondence with each of the plurality of laser light sources.  
      In addition, at least one of the plurality of laser light sources is preferably a semiconductor laser for outputting a blue laser beam having an oscillation wavelength of 480 nm or shorter.  
      Further, the image recording apparatus is preferably a digital photographic printer for recording an image corresponding to the image data onto a photographic paper as the recording medium.  
      Here, the image data is preferably image data which is obtained by photoelectrically reading an image captured on a photographic film, or image data of an image which is captured with a digital camera. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic view showing a construction of an exposing unit of an image recording apparatus according to an embodiment of the present invention;  
       FIG. 2  is a schematic view showing a positional relationship among optical elements disposed on a downstream side with respect to an fθ lens  42  in the exposing unit shown in  FIG. 1 ;  
       FIG. 3  is a block diagram showing a schematic configuration of the exposing unit and an exposure control circuit of the image recording apparatus according to an embodiment of the present invention;  
       FIG. 4  is a schematic view of an example of an image to be recorded; and  
       FIGS. 5A and 5B  are schematic views each showing a relationship between a scanning clock and recorded pixels when the image to be recorded shown in  FIG. 4  is recorded. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      An image recording apparatus according to the present invention will hereinafter be described in detail based on preferred embodiments shown in the accompanying drawings.  
      As shown in  FIG. 3 , an image recording apparatus according to the present invention includes an exposing unit  10  and an exposure control circuit  60 . Firstly, with reference to  FIGS. 1 and 2 , the details of the exposing unit  10  used in the image recording apparatus according to the present invention will be described.  FIG. 1  is a schematic view showing a construction of an exposing unit of the image recording apparatus according to the embodiment of the present invention, and  FIG. 2  is a schematic view showing a positional relationship among optical elements disposed on a downstream side with respect to an fθ lens  42  in the exposing unit shown in  FIG. 1 .  
      In an exposing unit  10  shown in  FIG. 1 , three light beams (laser beams) L (Lr, Lb, and Lg) which correspond to R (Red) exposure, B (Blue) exposure, and G (Green) exposure and which are modulated in correspondence to an image to be recorded (image data) are deflected in a main scanning direction (a direction indicated by an arrow x in  FIG. 1 ) to be made incident to a predetermined recording position (exposure position), whereby a photosensitive material S (see  FIG. 2 ) which is conveyed in a sub scanning direction (a direction indicated by an arrow y in  FIG. 1 ) nearly perpendicularly intersecting the main scanning direction is two-dimensionally scanned and exposed to the three light beams L to record an image.  
      Such an exposing unit  10  is utilized, for example, in a printer (printing apparatus) of a digital photographing system which produces a photographic print from image data obtained by photoelectrically reading an image photographed on a photographic film, image data of an image which is photographed with a digital camera, or the like.  
      In the example shown in  FIG. 1 , the exposing unit  10  includes a frame  12  as a chassis having one open face, a cover  14  (represented by a dotted line in  FIG. 1 ) for covering the open face (upper face) of the frame  12 , and various kinds of optical elements which are disposed and fixed at predetermined positions in the frame  12 .  
      In the example shown in  FIG. 1 , the frame  12  is the chassis operating as an optical plate which is provided in a light beam scanning optical system and which serves to accommodate/fix the various optical elements constituting the light beam scanning optical system. In the example shown in  FIG. 1 , the frame  12  is made, for example, of an aluminum alloy, and its inside is roughly separated through partition walls  22  ( 22   a ,  22   b , and  22   c ) into a light source portion  16 , a light deflecting portion  18 , and an emission portion  20 .  
      A cutout is formed in a portion of the partition wall  22   a  corresponding in position to optical paths of the light beams L, and a transparent window member  28   a  is fixed to the cutout. Similarly, a cutout is formed in a portion of the partition wall  22   c  as well, corresponding in position to the optical paths of the light beams L, except for an upper portion of the partition wall  22   c . Also a transparent window member  28   b  is fixed to the cutout. The frame  12  is covered with the cover  14 , and the cover  14  is fixed to the frame  12  by screwing in a predetermined number of tapped holes  26  formed near an external wall and the partition walls  22 .  
      A light source  30 R for emitting the light beam Lr with which the R exposure is carried out, a light source  30 B for emitting the light beam Lb with which the B exposure is carried out, a light source  30 G for emitting the light beam Lg with which the G exposure is carried out, an acoustic-optical modulator (AOM)  32 B for modulating the light beam Lb, an AOM  32 G for modulating the light beam Lg, a mirror  34  for reflecting the light beams L (Lb, Lg, Lr), light amount/beam focus adjusting means  36 R for adjusting a light amount and a beam focus (beam diameter) of light beam Lr, light amount/beam focus adjusting means  36 B for adjusting a light amount and a beam focus of light beam Lb, and light amount/beam focus adjusting means  36 G for adjusting a light amount and a beam focus of light beam Lg are disposed in the light source portion  16  provided inside the frame  12  in the example shown in  FIG. 1 .  
      In the example shown in  FIG. 1 , each of the light source  30 R of the light beam Lr and the light source  30 B of the light beam Lb is a laser diode (LD, i.e., semiconductor laser). The light source  30 G of the light beam Lg is obtained by combining the LD and a second harmonics generation element (SHG element, i.e., wavelength conversion element) and emits the light beam Lg having a ½ wavelength (second harmonic) of a wavelength of the light beam emitted from the LD. In addition, the light beam Lr is modulated in correspondence to image data through direct modulation operation for modulating and driving the light source  30 R, and the light beams Lb and Lg are modulated in correspondence to the image data by the AOMs  32 B and  32 G, respectively.  
      In addition, a polygon mirror  40  and an fθ lens (scanning lens)  42  are disposed in the light deflecting portion  18 .  
      Moreover, a cylindrical lens  46 , a cylindrical mirror  48 , and a mirror  50  for downward reflecting a light beam are disposed in the emission portion  20  to show a positional relationship shown in  FIG. 2 . The light beams L are obliquely reflected slightly upward by the cylindrical mirror  48 , and are then reflected downward by the mirror  50  for downward reflecting a light beam. Note that the cylindrical lens  46  and the cylindrical mirror  48  constitute an optical face tangle error correcting system for the polygon mirror  40 .  
      In addition, in order to determine a start-of-scan (SOS) position for the photosensitive material S, an optical sensor  54  for detecting the light beam Lr corresponding to the R exposure is disposed in the emission portion  20  in the frame  12 .  
      The light beam Lr corresponding to the R exposure is modulated in correspondence to the image to be recorded (the image data of R) to be emitted from the light source  30 R, reflected by the mirror  34 , adjusted with its light amount and beam focus by the light amount/beam focus adjusting means  36 R, and then transmitted through the window member  28   a , thereby being made incident to the polygon mirror  40 .  
      In addition, the light beam Lb corresponding to the B exposure is emitted from the light source  30 B, modulated in correspondence to the image to be recorded (the image data of B) by the AOM  32 B, reflected by the mirror  34 , adjusted with its light amount and beam focus by the light amount/beam focus adjusting means  36 B, and then transmitted through the window member  28   a , thereby being made incident to the polygon mirror  40 . Similarly, the light beam Lg corresponding to the G exposure is emitted from the light source  32 G, modulated in correspondence to the image to be recorded (the image data of G) by the AOM  30 G, reflected by the mirror  34 , adjusted with its light amount and beam focus by the light amount/beam focus adjusting means  36 G, and then transmitted through the window member  28   a , thereby being made incident to the polygon mirror  40 .  
      The light beams L (Lr, Lb, and Lg) are deflected in the main scanning direction by the polygon mirror  40  and are further adjusted by the fθ lens  42  so that the scanning speed is uniform. The light beams L which have passed through the fθ lens  42  are transmitted through the window portion  28   b , pass through the cylindrical lens  46 , and then are reflected by the cylindrical mirror  48 , i.e., adjusted with their optical paths to correct the optical face tangle error, and are further reflected downward by the mirror  50  for downward reflecting a light beam to be made incident to the recording position (on the photosensitive material S).  
      In the exposing unit  10  in the example shown in  FIG. 1 , the three light beams L emitted from the light sources  30 R,  30 B, and  30 G are made incident to the same point on the polygon mirror  40  to be deflected by the polygon mirror  40 , and are then made incident to a predetermined recording position to form one and the same scanning line. Consequently, the light beams L travel through the optical paths which differ from each other in main scanning direction, but are approximately identical to each other in the sub scanning direction, to be made incident to the recording position (non-beam-synthesizing light beam scanning optics, more specifically, three-laser beam different-angle incidence optics or three-light source non-beam-synthesizing optics).  
      When an image is recorded, the light beam Lr corresponding to the R exposure is detected by the optical sensor  54 , and the SOS recording position for the photosensitive material S is determined. In addition, the photosensitive material S (photographic printing paper) is conveyed in the sub scanning direction at a predetermined speed in the recording position. Thus, the photosensitive material S is two-dimensionally scanned and exposed with the light beams L deflected in the main scanning direction to record a latent image. The photosensitive material S having the latent image formed thereon is supplied to a processor (developing processor) (not shown). Then, the various processings such as color development, bleach fixing, washing, drying, and classification are executed in the processor.  
      Next, with reference to  FIGS. 3, 4 , and  5 A and  5 B, an outline of the exposure control circuit  60  used in the image recording apparatus according to the present invention will be described.  FIG. 3  is a block diagram showing a schematic configuration of the exposing unit and the exposure control circuit of the image recording apparatus according to the present invention. In addition,  FIG. 4  is a schematic view of an example of an image to be recorded, and  FIGS. 5A and 5B  are schematic views each showing a relationship between a scanning clock and recorded pixels when the image to be recorded shown in  FIG. 4  is recorded.  
      In the exposing unit  10  shown in  FIG. 3 , light sources  56 R,  56 B, and  56 G conceptually represent the light sources  30 R,  30 B, and  30 G, and the AOMs  32 G and  32 B shown in  FIG. 1 . That is, the light source  56 R corresponds to the light source  30 R. In addition, the light source  56 B corresponds to the light source  30 B and the AOM  32 B, and the light source  56 G corresponds to the light source  30 G and the AOM  32 G.  
      In this embodiment, for the sake of simplicity of description, the description will be here given on the assumption that the laser beams Lr, Lb, and Lg emitted from the respective light sources  56 R,  56 B, and  56 G are modulated with respective modulation signals LDRR, LDRB, and LDRG, which are supplied from the exposure control circuit  60 .  
      In the case of this embodiment, the light source  56 R emits the red laser beam Lr with an oscillation wavelength of 655 nm to 665 nm which has been modulated with the modulation signal LDRR. In addition, the light source  56 B emits the blue laser beam Lb with an oscillation wavelength of 435 nm to 445 nm which has been modulated with the modulation signal LDRB, and the light source  56 G emits the green laser beam Lg with an oscillation wavelength of  532  nm which has been modulated with the modulation signal LDRG.  
      In addition, the exposing unit  10  includes an information recording circuit  58  in which the information on the oscillation wavelength of the laser beam is recorded. Any kind of read only memories (ROMs) or the like can be used as the information recording circuit  58 . Further, the information on the oscillation wavelength can be obtained by actually measuring the oscillation wavelength of the laser beam, for example, in the same environment as that when an image is actually recorded in the image recording apparatus.  
      In the case of this embodiment, the information on the oscillation wavelength of the laser beam Lb emitted from the light source  56 B is stored in the information recording circuit  58 . Note that the information recording circuit  58  may be provided to the light sources  56 R,  56 B, and  56 G, respectively.  
      On the other hand, the exposure control circuit  60  shown in  FIG. 3  serves to control the exposure corresponding to the image data. The exposure control circuit  60  includes a clock selecting circuit  62 , laser driving circuits  64 R,  64 B, and  64 G, and a frame memory  66 .  
      The frame memory  66  is a buffer in which the image data of the image to be recorded in the exposing unit  10  is temporarily stored. Any kinds of random access memories (RAMs) or the like can be used as the frame memory  66 .  
      Then, the clock selecting circuit  62  outputs scanning clocks CLKR, CLKB, and CLKG for controlling the operations of the light sources  56 R,  56 B, and  56 G (i.e., the laser driving circuits  64 R,  64 B, and  64 G), respectively. The oscillation frequencies of the scanning clocks CLKR, CLKB, and CLKG are determined corresponding to the oscillation wavelengths of the laser beams Lr, Lb, and Lg so that the chromatic aberration of magnification due to the optical lenses used in the exposing unit  10  can be suppressed.  
      In the case of this embodiment, the scanning clocks having the respective oscillation frequencies which are previously determined corresponding to the respective oscillation wavelengths of the laser beams Lr and Lg are used as the scanning clocks CLKR and CLKG used in the laser driving circuits  64 R and  64 G, respectively. This reason is that even when the oscillation wavelengths of the laser beams Lr and Lg disperse, the actual occurrence situation of the chromatic aberration of magnification due to the optical lenses used in the exposing unit  10  hardly changes.  
      On the other hand, with respect to the oscillation frequency of the scanning clock CLKB used in the laser driving circuit  64 B, the scanning clock CLKB with the oscillation frequency which is determined based on the information on the oscillation wavelength of the laser beam Lb supplied from the information recording circuit  58  is selectively outputted among a plurality of previously prepared scanning clocks CLKB with different oscillation frequencies. That is, the oscillation frequency of the scanning clock CLKB is changed corresponding to the oscillation wavelength of the laser light source Lb.  
      In the case of this embodiment, the oscillation wavelengths of the laser beam Lb are classified into five groups consisting of the oscillation wavelength of 435 to 437 nm, the oscillation wavelength of 437 to 439 nm, the oscillation wavelength of 439 to 441 nm, the oscillation wavelength of 441 to 443 nm, and the oscillation wavelength of 443 to 445 nm at intervals of 2 nm. The scanning clocks CLKB 1  to CLKB 5  having predetermined oscillation frequencies are prepared for the respective groups. Then, there is outputted one scanning clock which is selected among the scanning clocks CLKB 1  to CLKB 5  based on the information of the oscillation wavelength of the laser beam Lb.  
      Note that the oscillation wavelengths of the laser beam Lb may be classified into two or more groups. In addition, the scanning clock is not limited only to the scanning clock CLKB. Thus, a constitution may be adopted in which one scanning clock which is selected among a plurality of previously prepared scanning clocks with different oscillation frequencies is outputted based on the information of the oscillation wavelength of the laser beam with respect to the scanning clock of at least one laser beam having large dispersion in oscillation wavelength.  
      Further, the laser driving circuits  64 R,  64 B, and  64 G serve to output the modulation signals LDRR, LDRB, and LDRG corresponding to the image data supplied from the frame memory  66  to thereby drive the laser light sources  56 R,  56 B, and  56 G, respectively. The laser driving circuits  64 R,  64 B, and  64 G operate synchronously with the scanning clocks CLKR, CLKB, and CLKG supplied from the clock selecting circuits  62 R,  62 B, and  62 G, respectively.  
      While an image is recorded, the laser driving circuits  64 R,  64 B, and  64 G operate synchronously with the scanning clocks CLKR, CLKB, and CLKG supplied from the clock selecting circuit  62  to thereby output the modulation signals LDRR, LDRB, and LDRG corresponding to the image data, respectively. Then, the laser beams Lr, Lb, and Lg modulated with the modulation signals LDRR, LDRB, and LDRG are outputted from the light sources  56 R,  56 B, and  56 G, respectively.  
      Here, when an image having a vertical-striped pattern as shown in  FIG. 4  is recorded, as shown in  FIGS. 5A and 5B , for example, a black pixel and a white pixel are recorded every two pixels.  FIGS. 5A and 5B  each show states of the recorded pixels in cases where the vertical-stripped pattern shown in  FIG. 4  is recorded using the modulated laser beams L when the widths of the scanning clocks CLK are set as T 1  and T 2  (T 1 &lt;T 2 ). As shown in  FIGS. 5A and 5B , the oscillation frequency of the scanning clock CLK is changed, thereby making it possible to adjust a transverse size (position) of the pixel to be recorded.  
      That is, the oscillation frequency of the scanning clock CLK is suitably set corresponding to the occurrence state of the chromatic aberration of magnification due to the optical lenses used in the exposing unit  10 , thereby making it possible to suppress the chromatic aberration of magnification. Consequently, the use of the optical lenses made of special glass materials can be suppressed to a minimum. Moreover, it is unnecessary to select the laser light source to be used. That is, since even the laser light source having the large dispersion in the oscillation wavelength can be used, it is possible to reduce the cost of the system.  
      In addition, the information recording circuit  58  in which the information on the oscillation wavelength of the laser beam Lb is previously stored is provided in the exposing unit  10 . Hence, even when the exposing unit  10  itself is exchanged for new one due to a failure or the like, the exposure control circuit  60  can automatically set the scanning clock suitable for the light source  56 B by reading out the information on the oscillation wavelength of the laser beam Lb from the information recording circuit  58  provided in the newly exchanged exposing unit  10 .  
      Note that the information recording circuit  58  in which the information on the oscillation wavelength of the laser beam Lb is previously stored is not a constituent element essential to the present invention. That is, a construction may also be adopted in which no information recording circuit  58  is provided in the exposing unit  10  and the information on the oscillation wavelength of the laser beam is supplied from the outside to the clock selecting circuit  62 . Alternatively, a constitution may also be adopted in which a wavelength detector for detecting the oscillation wavelength of the laser beam is installed in the exposing unit  10  and the information on the oscillation wavelength of the laser beam detected by the wavelength detector is supplied to the clock selecting circuit  62 .  
      In addition, while the above-mentioned embodiment has been described by giving an example of the oscillation wavelengths of the laser beams Lr, Lb, and Lg, the present invention is not intended to be limited thereto. For example, a laser beam with a wavelength of 470 nm to 480 nm may be used as the laser beam Lb in some cases. Further, as regards the image recording apparatus of the present invention, a digital photographic printer for recording an image on a photographic paper can be given as a preferred example. However, the present invention is not intended to be limited thereto. That is, the present invention can also be applied to the various image recording apparatuses using a plurality of laser light sources.  
      The present invention is basically described above.  
      While the image recording apparatus of the present invention has been described in detail hereinbefore, it is to be understood that the present invention is not intended to be limited to the above-mentioned embodiment, and thus the various improvements or modifications may be made without departing from the gist of the present invention.