Patent Publication Number: US-6337706-B1

Title: Solid scanning optical writing device, light amount correction method therefor and light amount measuring device

Description:
This application is based on an application No. 11-29308 filed in Japan, the contents of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention pertains to a solid scanning optical writing device to write images using a PLZT light shutter array or LED array, light amount correction method therefor and light amount measuring device. 
     2. Description of the Related Art 
     Various optical writing devices have conventionally been provided that form images on photographic paper or film using a silver halide material, or on an electrophotographic photoreceptor, by controlling the light for each pixel using a light shutter array or LED array comprising PLZT or other elements. In such an optical writing device, line noise may occur in the output image due to a variation in the amount of light passing through or emitted from the optical elements of the light shutter array or LED array. In order to eliminate this noise, the light amount output from each optical element is measured, correction calculation is performed based on the measurement results, and correction of the light amount variation (shading correction) is performed. 
     In measuring the light amount, it is preferable from the standpoint of ensuring measurement accuracy and making the measuring device small in size that the light amount from each optical element of the light shutter array, etc., be directly measured. However, it is not easy to accurately measure the light amount from each of the optical elements, which have an extremely dense arrangement in order to form high resolution images, and correlate the measurement results to each optical element. 
     FIGS. 8 and 9 show how light amount measurement is conventionally performed. They show how the amount of light that passes through each light shutter  31  (optical element) of the PLZT light shutter array is measured by means of a measuring device. The multiple light shutters  31  are aligned in two lines. A light shutter  31  of one line is located between two light shutters  31  of the other line. When voltage is impressed, the light shutter  31  causes the plane of polarization of the light that entered it from behind to rotate, allows the light to pass through and outputs it from the front. When voltage is not impressed, the plane of polarization of the light that enters the light shutter  31  from behind does not rotate and the light is output from the front just as it struck the light shutter.  91  indicates a light receiving mask located in front of the sensor that receives the light from the light shutters  31 . In FIG. 8, the light receiving mask  91  has a slit  91   a  with the same configuration as the light shutter  31 , and regulates the light that strikes the sensor. The light receiving mask  92  shown in FIG. 9 has a slit  92   a , the length of which that runs along the secondary scanning direction perpendicular to the length of the lines of the light shutter is equal to the length of the light shutter  31 . When the amount of light passing through the light shutter  31  is measured using such a mask  91  or  92 , the light amount output from each light shutter  31  may be accurately measured. 
     However, when a light shutter array is actually used as an optical writing device, from the standpoint of efficiency of use of the light, and to ensure that the necessary distance from the exposure surface is maintained, an image forming lens (selfoc lens array) is often used together with the light shutter array. FIG. 10 is a view of the positional relationship between a selfoc lens array and the light shutters  31  of a PLZT light shutter array. Ordinarily, due to the effect of the selfoc lens array on the image forming resolution (hereinafter ‘MTF’), the output light from the light shutters  31  is slightly blurred when it forms an image on the exposure surface. 
     Because a selfoc lens array comprises multiple rod lenses  35   a  aligned in a zigzag fashion, as shown in FIG. 10, the MTF varies depending on the positional relationship between the light shutters  31  and the rod lenses  35   a . In other words, the image formed on the exposure surface by the output light from the light shutters  31  may be sharp or blurry depending on the area. Where light is output from both lines of the light shutters  31 , the light amount from the gap between light shutters  31  (trough light amount) does not become completely zero. As explained above, where the light shutters  31  are aligned in a zigzag fashion such that they form two lines, exposure is performed by means of the light from a particular light shutter  31 , to which some of the light from the neighboring light shutters  31  located in the other line is added. 
     In addition, in the manufacturing process by which the light shutters  31  are aligned in a zigzag fashion such that they form two lines, a groove  32  (see FIGS. 8 and 9) is formed between any two light shutters  31 , and a slight amount of light escapes of this groove  32 . This escaping light affects the actual exposure as well. 
     Due to the details explained above, correcting the variation in light amount among the optical elements simply based on the information on the light amount measured using the mask  91  or  92  shown in FIG. 8 or  9  is insufficient to correct the variation in light amount on the exposure surface where an image is actually formed, and a variation in light amount still occurs on the exposure surface. 
     OBJECTS AND SUMMARY 
     The object of the present invention is to provide an improved solid scanning optical writing device, light amount correction method therefor and light amount measuring device. 
     Another object of the present invention is to provide a solid scanning optical writing device, light amount correction method therefor and light amount measuring device that are capable of accurately measuring the output light amount of each optical element and performing good light amount correction. 
     Yet another object of the present invention is to provide a solid scanning optical writing device, light amount correction method therefor and light amount measuring device in which the variation in light amount among the optical elements is corrected on the exposure surface where an image is formed. 
     In order to attain these and other objects, one aspect of the present invention comprises a solid scanning optical writing device that, based on the image data, turns ON/OFF the multiple optical elements that are aligned in a zigzag fashion such that they form two lines in the main scanning directions, said optical writing device having a light amount measuring unit including a light amount sensor to measure the amount of light output from the optical elements, wherein the light receiving area of the light amount sensor has an open slit and the length of the opening of the slit in the secondary scanning direction is set to be as long as or longer than the length of the optical element in the secondary scanning direction. 
     Another aspect of the present invention comprises a light amount measuring device having a light amount measuring unit including a light amount sensor to measure the amount of light output from the multiple optical elements aligned in a zigzag fashion such that they form two lines, a means to move the light amount measuring unit forward and backward in the direction of the alignment of the optical elements, and a means to adjust the position of the light amount measuring unit, wherein the light receiving area of the light amount sensor of the light amount measuring unit has an open slit and the length of the opening of the slit in the direction perpendicular to the direction of the alignment of the optical elements is set to be as long as or longer than the length of the optical element. 
     Yet another aspect of the present invention comprises a light amount correction method for the solid scanning optical writing device, wherein this method turns ON/OFF the multiple optical elements aligned in a zigzag fashion such that they form two lines in the main scanning directions based on the image data, and has (i) a process in which only the optical elements in one of the two lines are caused to illuminate and the amount of light output from the optical elements of this line is measured while the light amount measuring unit is caused to be moving in the main scanning directions, (ii) a process in which only the optical elements in the other line are caused to illuminate and the amount of light output from the optical elements of this line is measured while the light amount measuring unit is caused to be moving in the main scanning directions, and (iii) a process in which the correction amount regarding the amount of light output from each optical element is calculated from the ridge light amount of the optical element and the trough light amount from the space located at the same position as the optical element but in the other line. 
     Using the construction described above, because not only the ridge light amount (the maximum light amount) of the optical element but also the trough light amount (the light amount from between two optical elements) of the space located at the same position as the optical element but in the other line is added into the calculation, the amount of light output from each optical element may be measured with accuracy, making it possible to obtain highly accurate shading correction (light amount variation correction). 
     In addition, by making the length of the opening of the slit of the light amount sensor light receiving area in the secondary scanning direction sufficient to cover the two lines of optical elements aligned in a zigzag fashion, the amount of light escaping from the groove between any two optical elements may be detected, making the light amount correction more accurate. 
     Further, in the process in which the amount of correction of the optical element output light amount is calculated, by calculating the optical resolution per unit length from the alignment pitch of the optical elements and calculating the amount of correction regarding the optical element output light amount using this optical resolution, the trough light amount (the light amount from between two optical elements) when the ridge amount (the maximum light amount) for the optical element changes is calculated, making better light amount correction possible. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and features of the present invention will become apparent from the following description of a preferred embodiments thereof taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a perspective view of the optical writing device; 
     FIG. 2 is a plan view showing the basic construction of the light amount measuring device; 
     FIG. 3 is a front elevation showing the configuration of the light receiving area of the light amount sensor; 
     FIG. 4 is a flow chart showing the correction algorithm for light amount variation during light amount measurement. 
     FIG. 5 is a light amount distribution chart showing the change in the light passing through the optical element when measurement is made while moving the light amount sensor for each line of optical elements. 
     FIGS.  6 (A),  6 (B) and  6 (C) are graphs showing the distribution of light amount measured by the optical writing device; 
     FIG. 7 is a view of the basic construction of a color printer in which the optical writing device is mounted; 
     FIG. 8 is a front elevation showing the configuration of the light receiving area of a conventional light amount sensor; 
     FIG. 9 is a front elevation showing the configuration of the light receiving area of another conventional light amount sensor; 
     FIG. 10 is a view of the relationship between a selfoc lens array and the optical elements of a PLZT light shutter array. 
     In the following description, like parts are designated by like reference numbers throughout the several drawing. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the solid scanning optical writing device, light amount correction method therefor and light amount measuring device are explained below with reference to the accompanying drawings. 
     [Optical Writing Head] 
     FIG. 1 shows an optical writing head  20  to write color images on photographic paper using a silver halide material. This optical writing head  20  essentially comprises a halogen lamp  21 , a heat-absorbing filter  22 , a color correction filter  23 , a diffusion cylinder  24 , an RGB filter  25 , an optical fiber array  26 , a slit plate  27 , a light shutter module  30 , an image forming lens array (selfoc lens array)  35  and a dustproof glass panel  36 . 
     The light emitted from the halogen lamp  21  is deprived of heat by the heat-absorbing filter  22  and is adjusted by the color correction filter  23  so that the light quality can match the spectral-response characteristic of the photographic paper. The diffusion cylinder  24  improves the light use efficiency and reduces the unevenness in light amount. The RGB filter  25  is driven to rotate in synchronization with the writing performed by the light shutter module  30 , described below, and changes the color of the passing light for each line. 
     The optical fiber array  26  comprises a large number of optical fibers. The ends  26   a  are bound together and face the diffusion cylinder  24  via the RGB filter  25 . The other ends  26   b  are aligned in the main scanning directions indicated by the two-headed arrow X and emit light in a linear fashion. The slit surfaces  27   a  of the slit plate  27  have a smooth finish to efficiently lead the light emitted from the optical fiber array  26  to the light shutter module  30 . The slit plate  27  also has a heater (not shown in the drawing) to maintain the PLZT shutter chips at a constant temperature, and temperature control is performed based on the detection results from a temperature detecting element (not shown in the drawing) mounted on the module  30 . 
     The light shutter module  30  comprises multiple light shutter chips, which comprise PLZT shutters, aligned to form an array in a slit opening in a ceramic substrate or in a glass substrate, as well as driver ICs mounted along the array. The multiple light shutters  31  formed on each light shutter chip are aligned in two lines extending in the main scanning directions (X). Along these lines, one light shutter  31  in one line is located between two light shutters  31  in the other line (see FIG.  3 ). In other words, the light shutters  31  are aligned in a zigzag fashion such that they form two lines. Each light shutter  31  is independently driven by its driver IC based on the image data. A polarizer  33  and an analyzer  34  are mounted in the front and back of the module  30 , respectively. PLZT is a ceramic material having a light permeability with a large Kerr constant electro-optical effect, as is well known. The light that strikes the light shutters  31  is linearly polarized when passing through the polarizer  33  before striking the light shutters  31 . The light that strikes a light shutter  31  undergoes rotation of the plane of polarization when a voltage is impressed to the light shutter  31 , and is thereby emitted from the light shutter  31  toward the analyzer  34 . When no voltage is impressed to the light shutter  31 , the light that enters the light shutter  31  does not undergo rotation of the plane of polarization, and is emitted just as it struck the light shutter. The analyzer  34  located behind the light shutters  31  allows the light that has undergone rotation of the plane of polarization to pass by means of the light shutters  31  and blocks the light that has not undergone rotation of the plane of polarization. In other words, in the ON state in which a voltage is impressed to the light shutter  31 , the light is emitted through the analyzer  34 , and in the OFF state where no voltage is impressed, the light is not emitted through the analyzer  34 . 
     The light emitted through the analyzer  34  passes through the image forming lens array  35  and the dustproof glass panel  36  and forms an image on the photographic paper, forming a latent image. The photographic paper is conveyed at a constant speed in a direction perpendicular to the main scanning directions (X) (the secondary scanning direction). 
     [Light Amount Measuring Device and Measuring Method] 
     FIG. 2 shows a measuring device  70  that measures the amount of light from each light shutter of the optical writing head  20 . 
     This measuring device  70  comprises a measuring unit  71  having a light amount sensor  72  and a tool microscope  77 , which is mounted to a guide rod  76 , such that the measuring unit can slide along the guide rod. The guide rod  76  is located parallel to the main scanning directions of the light shutter module  30  (the directions of the two-headed arrow X), so that the measuring unit  71  moves forward and backward at a constant speed in the directions of the two-headed arrow X while the light amount sensor  72  is positioned immediately above the light shutters  31 . The light amount sensor  72  measures the amount of light output from the light shutters  31 . 
     A light receiving mask  73  and a light diffusion plate  74  are located on the light entry side of the light sensor  72 . The light receiving mask  73  has an open slit  73   a . This open slit  73   a  is designed such that its opening width in the main scanning directions (X) is essentially the same as the width of the light shutter  31  in the main scanning directions (X) and the opening length in the secondary scanning direction is as long as or longer than the length of the light shutter  31  in the secondary scanning direction. In this embodiment, the opening length of the open slit  73   a  in the secondary scanning direction is long enough to cover light shutters  31  in the two lines (see FIG.  3 ). By having this length in the open slit  73   a , the leak light amount from the groove between two light shutters  31  may be detected, enabling more accurate light amount correction. The light receiving mask  73  is located on the focusing plane F of the image forming lens array  35 . For the light amount sensor  72 , a sensor having a spectral-response characteristic range that is equal to or larger than that of the photographic paper, the recording medium, is used. 
     The tool microscope  77  is mounted as an integrated unit with the CCD camera  78 . The image of the light shutters is taken by means of the CCD camera  78  via the tool microscope  77  and displayed on a monitor  79 . The operator performs fine adjustment (focusing and position adjustment) of the position of the optical writing head  20  at both ends of the light shutters while viewing the image on the monitor  79 . In other words, the optical writing head  20  is mounted by means of a mounting platform not shown in the drawing such that it may be adjusted in terms of height, angle and distance from the light amount sensor  72 . When the device is shipped from the factory, the tool microscope  77 , CCD camera  78  and monitor  79  are usually removed. 
     The light amount measuring device  70  and optical writing head  20  having the construction described above are controlled by a microcomputer CPU, so that the timing for the forward and backward movement of the measuring unit  71  and light amount measurement is controlled. The optical writing head  20  is driven using pre-programmed drive parameters, which include drive frequency and ON duty. Drive frequency is the frequency of the voltage that is impressed to the light shutters  31 , and ON duty is the period in which the voltage is impressed in one cycle. The measuring device  70  samples the light amount of each light shutter in synchronization with this driving. Normally, sampling of the light amount is performed multiple times per element due to the relationship between the drive frequency and the drive speed of the light amount sensor  72 . The output from the light amount sensor  72  undergoes A/D conversion, and the converted data is transferred to the microcomputer CPU for necessary processing. 
     The light amount correction method will now be explained with reference with FIG.  4 . 
     Control regarding light amount correction is carried out by the microcomputer CPU in response to the turning ON of power to the printer. In step S 1 , the RGB filter  25  is adjusted to bring up the prescribed color, and the ON duty is set to the prescribed value. In step S 2 , the light shutters  31  of one line only are driven using the ON duty thus set. In this embodiment, voltage is impressed for the maximum time per cycle, i.e., at all times, and light is emitted from the entire optical writing head  20 . With the optical writing head  20  in this state, the light amount sensor  72  is moved forward from the initial position outside the scanning range of the light shutters  31 . When this occurs, as shown in FIG. 3, the light receiving mask  73  moves in a straight line in the direction of the arrow (a) as the light amount sensor  72  moves forward. An area of the first line of light shutters  31  and an area of the second line appear simultaneously inside the open slit  73   a . After moving the light amount sensor  72  over a distance slightly longer than the main scanning length, sampling of the light amount data for the light shutters  31  of the first line is stopped and the light amount sensor  72  is moved backward to the initial position. 
     After the light shutters  31  of the first line are turned OFF, the light shutters  31  of the second line are driven using the same parameters as for the first line to measure the light amount of the light shutters  31  of the second line and to incorporate the data. After this process is finished, light amount measurement regarding all light shutters  31  is completed. Naturally, it is acceptable if light amount measurement regarding the light shutters  31  of the second line is performed when the light amount sensor  72  is moved backward, which is more efficient. 
     FIG. 5 shows the change in sampled light amount for each line thus obtained. Since the slit  73   a  having a width that is essentially the same as the width of the light shutter  31  in the main scanning directions (X) is moved in the main scanning directions (X) during light measurement, the maximum light amount (ridge light amount) is reached when the light amount sensor  72  faces a light shutter  31 , and the minimum light amount (trough light amount) is reached when the light amount sensor  72  is between two light shutters  31 . Consequently, by detecting the peaks of the output light amount, the positions (addresses) of the light shutters  31  may be identified. The positions of the minimum light amount points between light shutters  31  may be identified by detecting the opposite peaks in the same manner as for the maximum light amount points, but instead, the point in the middle between two maximum light amount points may be deemed a minimum light amount point. 
     In step S 3 , the nth trough coefficients for the first line and for the second line (Kbn, Kan, respectively) defined by the following equations (1) and (2) are then calculated. 
     The nth trough coefficient for the first line: 
     
       
           Kbn=Gbn /( Pbn+Pbn +1)  (1) 
       
     
     The nth trough coefficient for the second line: 
     
       
           Kan=Gan /( Pan+Pan +1)  (2) 
       
     
     Where 
     Gbn: the nth trough light amount in the first line 
     Gan: the nth trough light amount in the second line 
     Pbn: the ridge light amount of the nth light shutter in the first line 
     Pan: the ridge light amount of the nth light shutter in the second line 
     These trough coefficients Kbn and Kan are calculated in order to calculate the trough light amounts Gbn and Gan when the ridge light amounts Pbn and Pbn+1, Pan and Pan+1 of the neighboring light shutters  31  change, and are equivalent to the optical resolution (MTF) per unit length derived from the alignment pitch of the light shutters  31 . 
     In step S 4 , preliminary correction is performed so that the ridge light amounts Pbn and Pan of the light shutters  31  in the first and second lines, respectively, may become located on standard lines and the ridge light amounts Pbn and Pan may become the same. In step S 5 , the trough light amounts Gbn and Gan are calculated from the ridge light amounts Pbn and Pan of the aligned light shutters  31  and the trough coefficients Kbn and Kan, respectively. In step S 6 , for each light shutter  31  in the first and second lines, the ridge light amount Pbn (Pan) of the light shutter  31  is added to the trough light amount Gan (Gbn) from the space that located is at the same position as the light shutter  31  but in the other line. Specifically, the following equations (3) and (4) are calculated. 
     Light amount of the nth light shutter in the first line: 
     
       
         ( Pbn+Gan ×α)×β  (3) 
       
     
     Light amount of the nth light shutter in the second line: 
     
       
         ( Pan+Gbn ×α)×β  (4) 
       
     
     Here, α and β are parameters that are dependent on the image forming lens array  35 , α being a trough addition coefficient and β being a dark light amount threshold value. 
     The average value of the light amount regarding all light shutters  31  thus obtained is calculated and is deemed the target light amount T. 
     In step S 7 , while the trough light amount Gan for the nth light shutter  31  in the second line is left alone, the ridge light amount Pbn for the nth light shutter  31  in the first line is corrected. In other words, Pbn is adjusted such that T=Pbn+Gan is achieved. After this adjustment, the trough light amount Gbn in the first line is re-calculated from the trough coefficient Kbn and the corrected ridge light amount Pbn for the first line. Further in step S 9 , while the trough light amount Gbn for the nth light shutter  31  in the first line is left alone, the ridge light amount Pan for the nth light shutter  31  in the second line is corrected. In other words, Pan is adjusted such that T=Pan+Gbn is achieved. After this adjustment, the trough light amount Gan in the second line is re-calculated from the trough coefficient Kan and the corrected ridge light amount Pan for the second line in step S 10 . 
     In step S 11 , it is determined whether or not the steps of S 7  through S 10  have been repeated prescribed times, and if they have not been repeated prescribed times, step S 7  is returned to. Conversely, if they have been repeated a prescribed number of times, actual correction data to realize the light amount distribution narrowed down by repeating the calculation in steps S 7  through S 10  is obtained in step S 12  by referring to a basic light amount correction table. Good light amount correction is made possible in this fashion. 
     FIG.  6 (C) is a light amount distribution chart regarding the optical writing head  20  after light amount correction is performed in accordance with the present invention. For comparison purposes, FIG.  6 (A) is a light amount distribution chart regarding the optical writing head  20  before the correction, and FIG.  6 (B) is a light amount distribution chart regarding the optical writing head  20  after light amount correction is performed using the conventional method. 
     [Color Printer] 
     FIG. 7 shows the basic construction of a color printer to produce photo prints. This color printer comprises a photographic paper storage unit  1 , an image forming unit  2  and a processing unit  3 . The photographic paper  4  is stored rolled up in the storage unit  1 . In the image forming unit  2  are mounted an optical writing head  20  shown in FIG. 1 and a measuring unit  71  shown in FIG. 2 (the tool microscope  77 , CCD camera  78  and monitor  79  are removed, however). Further, in the image forming unit  2  are also mounted conveyance roller pairs  5 ,  6  and  7  for the photographic paper  4 , a cutter  8  and conveyance guide plates  11  and  12 . 
     The photographic paper  4  is guided into the image forming unit  2  from the conveyance roller pair  5  with the photosensitive surface facing downward. When a certain length of the photographic paper  4  has been introduced, the rotation of the roller pair  5  is stopped and the cutter  8  is operated to cut the paper. The cut-off photographic paper segment  4  is conveyed at a constant speed by the roller pairs  6  and  7 . When the photographic paper segment  4  passes the optical writing head  20 , it is exposed through the opening formed in the guide plate  11 , and an image (latent image) is formed on it. The photographic paper segment  4  is developed and dried in the processing unit  3  after exposure, and is then ejected onto the tray  15 . 
     In this color printer, R, B and G images are written by rapidly changing the light source color through the rotation of the RGB filter  25  of the optical writing head  20  and by turning ON/OFF the PLZT light shutters for each line. In this printer, power is turned ON by means of a timer and temperature control for the developer is performed. During this warm-up period, light amount measurement regarding the light shutters and light amount correction (calibration) is performed. Calibration is the process, as described above, in which the optical writing head  20  is driven using essentially the same parameters as for the exposure to perform light amount correction based on the output light amount results. Through this process, images with a good quality gradation exhibiting no unevenness may be obtained. Light amount measurement and correction may be performed at any given time other than during warm-up of the printer. 
     OTHER EMBODIMENTS 
     The solid scanning optical writing device, light amount correction method therefor and light amount measuring device pertaining to the present invention are not limited to the embodiments described above, and may be modified in various ways within their essential scope. 
     In particular, for the optical elements used in the solid scanning optical writing device, LED (light emitting diode), LCS (liquid crystal shutters), DMD (deformable mirror devices) or FLD (fluorescent devices) may be used in addition to PLZT shutters. 
     In addition, while this embodiment is explained using a situation in which the unevenness in light amount is corrected based on measurement of mono-color single gradation output, in order to further increase the accuracy of correction, measurement data may be obtained based on multiple different light amount outputs (multiple gradation) by changing the illumination duty for the light shutters, or measurement may be performed for all light source colors (R, G and B). 
     In addition, the present invention may be applied in an image writing device that writes images onto silver halide film or an electrophotographic photoreceptor, or in an image projector that projects images onto a display, as well as in an image writing device that writes images onto photographic paper using a silver halide material. 
     Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.