Abstract:
In an image recording apparatus  1  which records an image on a recording medium  9  by moving an optical head  10  while rotating the recording medium  9  held by a holding drum  7,  a photodetector  2  having a plurality of photodetector elements  21  is provided in a range of movement of the optical head  10 . In correcting beam intensity, the optical head  10  is positioned so precisely that the photodetector  2  can receive the signal light beams. This constitution makes it possible to detect signal light beams from the light modulator with high S/N ratio as compared with a conventional case where one photodetector element sequentially receives the signal light beams, and therefore possible to correct the beam intensities with high accuracy.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an image recording apparatus which uses a multi-channel light modulator, more specifically calibration of the beam intensity.  
           [0003]    2. Description of the Background Art  
           [0004]    Use of a multi-channel light modulator well known in the art of image recording. Such a light modulator is illuminated by light from a light source and creates multiple writing beams. To ensure high image quality, each writing beam is required to have a same target intensity. A conventional method for this purpose is to calibrate all writing beams in accordance with signals from a single-element photodetector.  
           [0005]    If a multi-channel light modulator has a low extinction rate(contrast), however, a signal from a single-element photodetector can be lost in the noise generated by light from unselected modulator elements. Provided a modulator has 500 elements with a contrast of 100:1, total beam intensity of the unselected 499 elements is about five times the intensity of a selected element. The purpose of this invention is to overcome this difficulty.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention is intended for an image recording apparatus for recording an image on a recording medium, and an object of the present invention is to provide a precise beam intensity calibration method for a multi-channel recording apparatus.  
           [0007]    According to an aspect of the present invention, the image recording apparatus comprises a light source; a light modulator for modulating light from the light source; a holding means for holding the r10g a plurality of photodetector elements; a detecting optical system for bringing the beams from the light modulator which are subject to intensity calibration; and a correction electronics for providing correction data to the light modulator.  
           [0008]    The calibration method of the present invention can correct the beam intensity more precisely compared with a conventional method.  
           [0009]    According to a preferred embodiment of the present invention, the image recording apparatus further contains a different optical path for detection from that for recording.  
           [0010]    According to another preferred embodiment of the present invention, the detection optics includes a beam splitter disposed after the light modulator. Preferably, the photodetector elements outnumber the light modulator elements.  
           [0011]    The present invention is also intended for a method for beam intensity calibration in a multi-channel recording apparatus. According to the present invention, the method comprises a step of selecting some of the light modulator elements which are spatially separate from one another; a step of creating correction data based on the measurement data on the selected elements.  
           [0012]    In accordance with the present invention, the beam intensity of a multi-channel light modulator is corrected rapidly and precisely.  
           [0013]    These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a view showing a constitution of an image recording apparatus in accordance with a preferred embodiment of the present invention;  
         [0015]    [0015]FIGS. 2 and 3 are views showing a constitution of the inside of an optical head;  
         [0016]    [0016]FIG. 4 is an enlarged view of light modulator elements;  
         [0017]    [0017]FIG. 5 is a block diagram showing prime components and peripheral components relevant to control of the image recording apparatus;  
         [0018]    [0018]FIGS. 6 and 7 are flow charts showing an operation of the image recording apparatus in light intensity correction;  
         [0019]    [0019]FIG. 8 is a view showing an example of measurement data on a group of separate elements;  
         [0020]    [0020]FIG. 9 is a view showing measurement data on another group of separate elements;  
         [0021]    [0021]FIG. 10 is a graph showing an intensity profile obtained after repeated measurements;  
         [0022]    [0022]FIG. 11 is a view showing a relation between the correction parameter and the beam intensity;  
         [0023]    [0023]FIG. 12 is a view showing another exemplary constitution of the image recording apparatus;  
         [0024]    [0024]FIG. 13 is a view showing still another exemplary constitution of the image recording apparatus;  
         [0025]    [0025]FIG. 14 is a view showing another exemplary constitution of the optical head; and  
         [0026]    [0026]FIG. 15 is a view showing further exemplary constitution of the image recording apparatus. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]    &lt;1. Constitution of Apparatus&gt; 
         [0028]    [0028]FIG. 1 is a view showing a constitution of an image recording apparatus  1  in accordance with a preferred embodiment of the present invention. In this figure, a light source is not shown. The image recording apparatus  1  has an optical head  10  for delivering writing beams and a holding drum  7  for holding a recording medium  9  on which an image is recorded by the writing beams. As the recording medium  9 , for example, used are a printing plate, a film for forming the printing plate and the like. A photosensitive drum for plateless printing may be used as the holding drum  7  and in this case, it is understood that the recording medium  9  corresponds to a surface of the photosensitive drum.  
         [0029]    The image recording apparatus  1  further has a photodetector  2  for correcting the writing beam intensity. In FIG. 1, the optical head  10  at a position for image recording is represented by a phantom line (two-dot chain line) and that at a position for correcting the beam intensity.  
         [0030]    The holding drum  7  is driven by a motor  81  and rotates about a central axis of its cylindrical surface holding the recording medium  9  and the optical head  10  can be moved by a motor  82  and a ball screw  83  in parallel to a rotation axis of the holding drum  7  (in the X direction of FIG. 1). The position of the optical head  10  is monitored by an encoder  84 .  
         [0031]    The optical head  10  has a light modulator  12  having a plurality of light modulator elements aligned in the X direction and a recording optics  13  which brings the signal lights from the light modulator  12  to the recording medium  9 . The recording optics  13  also leads the beams from the light modulator  12  to the photodetector  2 .  
         [0032]    [0032]FIGS. 2 and 3 are views showing an example of the disposition of a light source  11 , the light modulator  12  and optics in the optical head  10 . The X, Y and Z directions in FIGS. 2 and 3 correspond to those in FIG. 1.  
         [0033]    The light source  11  is a semiconductor laser bar, having a plurality of light emitters which are aligned in the X direction. Light beams from the light source  11  are collimated in the Y direction by a cylindrical lens  14   a  and overlapped in the X direction and focused in the Y direction on the light modulator  12  by a lens  14   b.    
         [0034]    The light modulator  12  used in this embodiment is the Grating Light Valve(trademarked by Silicon Light Machines, Sunnyvale, Calif.) which has a plurality of modulator elements aligned in the X direction.  
         [0035]    [0035]FIG. 4 is an enlarged view of the aligned light modulator elements  121 . The light modulator elements  121  are manufactured by using a semiconductor manufacturing technique, and each of the light modulator elements  121  consists of a plurality of ribbon-like diffraction grating members  121   a  and  121   b . The diffraction grating members  121   a  go down in response to an electric potential applied between them and the substrate, while the diffraction grating members  121   b  are virtually stationary.  
         [0036]    When a potential is applied, a modulator element  121  produces a square-well diffraction grating which generates diffracted light beams(non-zeroth order diffracted light beams). In this embodiment, a normally reflected light beam(zeroth order diffracted light beam) is used as a signal beam; and diffracted light beams as non-signal beams.  
         [0037]    The signal beam is brought to the holding drum  7  through a lens  13   a  as shown in FIG. 3, and applied to the recording medium  9 . On the other hand, the non-signal beams are blocked by a pair of light shielding plates  15 . The light shielding plates  15  are so disposed at the front side and the rear side of the paper with an optical axis interposed therebetween as to receive non-zeroth order diffracted beams, and the signal beam passes through the pair of light shielding plates  15 .  
         [0038]    The optics  13  forms an image of the light modulator  12  with a variable magnification on the recording medium  9  (during recording) and the photodetector  2  (during intensity calibration), which has a plurality of photodetector elements  21 .  
         [0039]    The photodetector  2  should have at least as many elements as (preferably three times or more elements than) the light modulator  12 .  
         [0040]    [0040]FIG. 5 is a system block diagram of the recording apparatus  1 . An image signal processing means  31  processes image data to be recorded on the recording medium  9 . An image recording control means  32  controls the light source  11  and the light modulator  12  according to the image data. Calibration control means  33  controls the image recording apparatus  1  during beam intensity calibration. An A/D converter  43  converts an analog signal from the photodetector  2  to a digital signal, which is recorded in a memory  401  as intensity data  411 .  
         [0041]    A correction parameter calculation means  34  generates correction parameters  421  and stores the parameters in a memory  402 . The correction parameters  421  are used when the light modulator  12  is operated to record an image.  
         [0042]    &lt;2. Calibration Procedure&gt; 
         [0043]    [0043]FIGS. 6 and 7 are flowcharts showing a procedure in calibrating the beam intensity in the recording apparatus  1 .  
         [0044]    At the first step S 11 , the parameters for the modulator elements are all set to 100 and the light source  11  is turned on. Following that, three modulator elements at center and both ends are brought to “On-state” so as to bring the modulator elements into correspondence with the photodetector elements(S 12  and S 13 ). The term “On-state” means the state where the modulator element is operated to render a signal light beam with the intensity according to the parameter. After the initialization, the photodetector  2  measures the “Off-state” intensity distribution with all the modulator elements are operated not to render a signal light beam (S 14 ).  
         [0045]    Prior to the “On-state” intensity distribution, a group of modulator elements which are spatially separate from one another is selected (S 15 ). Then all the selected elements are brought to “On-state” and the beam intensities are measured by the photodetector  2  (S 16  and S 17 ).  
         [0046]    [0046]FIG. 8 shows an example of an intensity profile with a group of selected elements in “On-state.” Each element is selected to be separate from the adjacent one by a distance of n(integer) elements. The dashed line describes the “Off-state” distribution.  
         [0047]    After one measurement is done, another group of modulator elements are selected. FIG. 9 shows an example of an intensity profile with a group of the elements next to the previous ones in “On-state.” Such measurements are repeated until all the modulator elements are selected. The intensity data are stored in the memory  401 .  
         [0048]    In this way, the beam intensity of each modulator element is obtained with no effect of light from the neighboring elements.  
         [0049]    [0049]FIG. 10 shows “On-state” and “Off-state” intensity profiles obtained after the repeated measurements. The poor uniformity of the profiles is mainly due to the non-uniform illumination on the light modulator  12 .  
         [0050]    The correction parameter calculation means  34  finds out a correction parameter for each modulator element which makes the beam intensity the lowest value among the data(indicated as Imin in FIG. 10.) A typical relationship between the correction parameter and the beam intensity is shown in FIG. 11. The best correction parameter, P, is calculated from the following equation:  
                   I                   s        (   P   )         -     I                   s        (   0   )               I                   s        (   100   )         -     I                   s        (   0   )             =         Im      in     -     Ioff        (   x   )             Ion        (   x   )       -     Ioff        (   x   )                   (   1   )                               
 
         [0051]    After such calculations, all the best parameters are stored in memory  421 .  
         [0052]    If any of the beam intensities corresponding to the modulator elements  121  is within acceptable limits, then the beam intensity calibration comes to an end (S 21 ). Otherwise, the light power of the laser source  11  is adjusted to make the minimum intensity equal to a predetermined target intensity (S 23 ) and the correction data are recalculated (S 14  and S 15 ).  
         [0053]    After the method mentioned above, it is possible to find out the best correction parameter for each modulation element, even if the extinction rate is not so high. Furthermore, it takes very short time to finish the calibration because of the simultaneous measurement on a group of modulator elements.  
         [0054]    &lt;3. Another Constitution of Apparatus&gt; 
         [0055]    Several optical elements can be either added to or used in lieu of the optics  13  to facilitate the calibration.  
         [0056]    The recording apparatus shown FIG. 12 includes a prism  221  and a relay optics  22 . This constitution can be applied if the space facing the optics  13  is not big enough to dispose the photodetector  2 . The relay optics  22  can have the function of (de)magnifying the image on the photodetector  2 .  
         [0057]    Any of the embodiments shown in FIGS. 13, 14, and  15  has a optical head  10  which contains a photodetector  2 . The optical head  10  shown in FIG. 13 further contains a turning mirror (prism) actuated by a motor  232  for switching between the recording path and the calibration path. FIG. 14 shows a optical head which has a relay optics  22  and a beam splitter  24  disposed between the light modulator  12  and the optics  13  for bringing means of the light beams to the relay optics  22 . The optical head  10  shown in FIG. 15 contains a beam splitter  24  disposed between the optics  13  and the holding drum  7 . Any of the opticalf heads in FIGS. 13, 14, and  15  does not need moving to a predetermined calibration position.  
         [0058]    &lt;4. Variations&gt; 
         [0059]    Though the preferred embodiments of the present invention have been discussed above, the present invention is not limited to the above-discussed preferred embodiments, but allows various variations.  
         [0060]    The light source  11  of the above preferred embodiments is not limited to a semiconductor laser bar, but may be a semiconductor laser having a single emitter or a two-dimensionally arranged semiconductor laser array. Light sources other than the semiconductor laser may be also used.  
         [0061]    The light modulator  12  may have a constitution in which a plurality of two-dimensionally arranged light modulator elements  121 . In this case, it is preferable that the photodetector elements  21  of the photodetector  2  should be also two-dimensionally arranged.  
         [0062]    The light modulator element  121  is not limited to the Grating Light Valve™, but may be an electro-optic shutter array such a PLZT (lead lanthanum zirconium titanate) modulator and a TIR (total internal reflection) type of device. The technique of using a plurality of photodetector elements allows a proper light intensity correction in a case where various light modulator elements which do not have high contrast of intensity between a light from a light modulator element in “ON state” and that from a light modulator element in “OFF state” are used.  
         [0063]    Though the recording medium  9  is held by the holding drum  7  and the main scanning is performed by rotation of the holding drum  7  and the sub-scanning is performed by movement of the optical head  10  in the image recording apparatus  1 , there may be a case where the recording medium  9  is held on a plane base and the main and sub-scanning are both performed by relative movement between the optical head  10  and the recording medium  9 .  
         [0064]    Thus, the light beams from a plurality of light modulator elements  121  of the light modulator  12  which are subject to calibration are received by a plurality of photodetector elements  21  and if the photodetector elements outnumbers the light modulator elements  121  which are subject to calibration, the calibration can be achieved with higher accuracy.  
         [0065]    While the invention has been shown and described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is therefore understood that numerous modifications and variations can be devised without departing from the scope of the invention.