JP4661375B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus that forms an image on a recording medium by electrophotography.

  An image formed on a recording medium by an electrophotographic method is inferior to other recording methods such as an ink jet recording method in terms of color uniformity in the recording surface due to a change in characteristics of the photoreceptor. . For example, when the same color is recorded on the entire surface, a color difference in which the hue is different between the left side and the right side of the image is likely to occur, and fine stripes are likely to occur. It is difficult to solve these problems only by improving the electrophotographic material or improving the mechanical accuracy.

Therefore, based on the position information of the photoconductor, a technique for adjusting and correcting the charge amount and the development amount (for example, refer to Patent Document 1), and a technique for correcting the exposure amount (laser power) by changing it in the scanning direction (for example, , See Patent Document 2).
JP-A-9-197316 JP-A-8-30145

  However, the above correction method was used to correct temporary streaks and unevenness that were not reproducible, temporarily generated streaks and unevenness, falsely detected streaks and unevenness, or streaks and unevenness that could not be corrected by the correction method. In this case, not only the correction effect is not exhibited, but also the image quality may be deteriorated by correction (streaks and unevenness that did not originally exist).

  The present invention has been made in consideration of the above facts, and an object of the present invention is to obtain an image forming apparatus capable of reliably correcting various unevenness such as left and right color differences and fine stripes in the recording surface.

To achieve the above object, an image forming apparatus according to claim 1 is formed by an image forming unit that forms an image based on input image data and the image forming unit based on image data of a test pattern. Based on the read result obtained by reading the test pattern, a correction value for correcting the image data supplied to the image forming unit so as to improve the image quality of the image formed by the image forming unit is previously set. An arithmetic means for calculating each predetermined pixel, a reading result of the test pattern formed by the image forming means based on the image data of the test pattern corrected by the correction value, and before correction by the correction value Comparing the test pattern read result formed by the image forming means based on the test pattern image data, A determination unit configured Motogoto determining whether the image quality has been improved, without correcting the image data corresponding to the pixels determined to image quality is degraded by the determining means, the image quality is improved by the determining means and And a correction unit that corrects image data corresponding to the determined pixel and the pixel determined to have no change in image quality with the correction value.
According to a second aspect of the present invention, there is provided an image forming apparatus for forming an image based on input image data, and reading a test pattern formed by the image forming means based on the image data of the test pattern. A correction value for correcting the image data supplied to the image forming unit based on the read result obtained so as to improve the image quality of the image formed by the image forming unit is determined for each predetermined pixel. An arithmetic means for calculating, a reading result of the test pattern formed by the image forming means based on the image data of the test pattern corrected by the correction value, and the image data of the test pattern before being corrected by the correction value The image quality is improved for each pixel by comparing the result of reading the test pattern formed by the image forming unit based on Without correcting image data corresponding to a pixel determined by the determining unit, a pixel determined to have no change in image quality by the determining unit, and a pixel determined to have deteriorated in image quality by the determining unit, And correction means for correcting the image data corresponding to the pixel determined to have improved image quality by the determination means with the correction value.

According to the first and second aspects of the present invention, the image data to be supplied to the image forming unit is formed by the image forming unit based on the reading result obtained by reading the test pattern formed by the image forming unit. A correction value for correcting so as to improve the image quality is calculated for each predetermined pixel.

  Even if the correction value is calculated in this way, there is a temporary streak or unevenness that occurs temporarily without reproducibility, a falsely detected streak or unevenness, or a streak or unevenness that cannot be corrected by correcting the image data. In the calculated correction value, not only the correction effect is not exhibited, but also correction by the calculated correction value may deteriorate the image quality (create streaks and unevenness that did not originally exist).

Therefore, in the first and second aspects of the invention, the reading result of the test pattern formed based on the image data of the test pattern corrected by the correction value and the image of the test pattern before being corrected by the correction value Image data corresponding to the pixel for which the image quality is determined to be improved by comparing the result of reading the test pattern formed based on the data to determine whether the image quality has been improved for each predetermined pixel Is corrected with the correction value.

  Thereby, since the image data is corrected using the calculated correction value for the pixel determined to improve the image quality, the image quality can be reliably improved.

In the image forming apparatus of the invention of claim 1 and claim 2, wherein the correction means, the image data corresponding to the pixels determined to image quality is degraded by the determining means can be prevented positively complement .

  Thereby, unnecessary correction is not performed, and deterioration of image quality can be prevented.

According to a third aspect of the present invention, there is provided an image forming apparatus for forming an image based on input image data, and reading a test pattern formed by the image forming means based on the image data of the test pattern. On the basis of the read result obtained in this way, a correction value for correcting the image forming condition of the image forming unit so as to improve the image quality of the image formed by the image forming unit is calculated for each predetermined pixel. Based on the result of reading the test pattern formed by the image forming means based on the image forming condition corrected by the correction value and the image forming condition before being corrected by the correction value. Judgment means for comparing the result of reading the test pattern formed by the image forming means to determine whether the image quality has been improved for each pixel. When the image forming conditions for forming an image based on image data corresponding to the pixels determined to image quality is degraded by the determining means without correcting, it is determined that the image quality is improved by the determining means It is configured to include a correction means for correcting the previous Kiho positive value image forming conditions for the image formed based on image data corresponding to the pixels determined to no change in quality by the pixel and said determination means ing.
According to a fourth aspect of the present invention, there is provided an image forming apparatus for forming an image based on input image data, and a test pattern formed by the image forming means based on the image data of the test pattern. On the basis of the read result obtained in this way, a correction value for correcting the image forming condition of the image forming unit so as to improve the image quality of the image formed by the image forming unit is calculated for each predetermined pixel. Based on the result of reading the test pattern formed by the image forming means based on the image forming condition corrected by the correction value and the image forming condition before being corrected by the correction value. Judgment means for comparing the result of reading the test pattern formed by the image forming means to determine whether the image quality has been improved for each pixel. The image forming conditions when forming an image based on the image data corresponding to the pixel determined to have no change in image quality by the determining means and the pixel determined to be deteriorated in image quality by the determining means should be corrected. And a correction unit that corrects an image forming condition when the image is formed based on image data corresponding to a pixel determined to have improved image quality by the determination unit with the correction value. .

According to the third and fourth aspects of the present invention, based on the reading result obtained by reading the test pattern formed by the image forming unit, the image forming condition of the image forming unit is set as the image forming unit. A correction value for correcting so as to improve the image quality is calculated for each predetermined pixel.

Further, a reading result of the test pattern formed on the basis of the image data of the test pattern that has been corrected by the compensation values, the test pattern formed on the basis of the image data before the test pattern is corrected by the correction value The read result is compared to determine whether or not the image quality is improved for each pixel, and the image forming condition corresponding to the pixel determined to have improved the image quality is corrected with the correction value.

  As a result, the image formation condition is corrected using the calculated correction value for the pixel determined to improve the image quality, so that the image quality can be reliably improved.

Note that in the image forming apparatus according to the third and fourth aspects of the invention, the correction unit forms an image based on image data corresponding to a pixel determined to have deteriorated in image quality by the determination unit. formation conditions can be prevented positively complement.

  Thereby, unnecessary correction is not performed, and deterioration of image quality can be prevented.

In the invention according to any one of claims 1 to 4 , the predetermined pixel may be one pixel or a plurality of pixels.

  When the correction value is calculated and used for each pixel, the correction can be performed with high accuracy. When the correction value is calculated and used for each of the plurality of pixels, the same correction value can be used for each of the plurality of pixels. Therefore, the memory capacity for storing the correction value can be reduced.

  In addition, a reading unit for reading a test pattern formed by the image forming unit can be provided outside the image forming apparatus, but can also be provided inside the image forming apparatus. That is, a reading unit that reads the test pattern formed by the image forming unit based on the image data of the test pattern can be further provided.

  For example, the reading unit may be provided on a paper conveyance path inside the image forming apparatus, or may be used as a reading unit that reads a document as long as the image forming apparatus has a copying function.

  The size of the test pattern may be the maximum size that can be formed by the image forming unit.

  Thereby, it is possible to correct all the pixels in the printable area.

  The test pattern image data may be image data forming a test pattern having the same density throughout the entire area.

  As a result, it is possible to easily detect streaks and unevenness generated for each predetermined pixel from the test pattern reading result.

  As described above, according to the present invention, there is an effect that various irregularities such as a color difference between right and left and a fine stripe in the recording surface can be reliably corrected.

(First embodiment)
Hereinafter, a first embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a schematic configuration diagram of an image forming apparatus 10 according to the present embodiment.

  As shown in FIG. 1, the image forming apparatus 10 includes an image reading unit 12, an image recording unit 14, and a control unit 16.

  The image reading unit 12 reads an image recorded on the paper P sandwiched between the mounting table 18 configured to include platen glass or the like and the cover 20. Specifically, the light from the light source 22 is irradiated onto the paper P, and the reflected light L is imaged on the image sensor 26 by the reflecting mirror 24 or a lens (not shown).

  The image sensor 26 includes line CCDs for R (red), G (green), and B (blue), signal processing circuits, A / D converters, and the like. Light corresponding to a line image in a predetermined direction of the paper P (direction perpendicular to the paper surface in FIG. 1) is formed on the light receiving surface of each color line CCD, and this is converted into an electric signal. Then, by performing predetermined signal processing and A / D conversion on the electric signal, digital data of a line image is obtained. By reading this line image along the direction of arrow A in FIG. 1, image data of the image recorded on the paper P (image data of each color of R, G, B) is obtained. Image data of the image read by the image sensor 26 is output to the control unit 16.

  The image recording unit 14 includes a light beam output unit 28. The light beam output unit 28 includes a light source and a lens (not shown), and the image data of each color Y (yellow), M (magenta), C (cyan), and K (black) output from the control unit 16. A light beam modulated based on the above is output. The light beams of the respective colors are output to the optical scanning devices 30Y, 30M, 30C, and 30K, respectively.

  Since each optical scanning device has the same configuration, only the optical scanning device 30Y will be described, and description of the other optical scanning devices will be omitted.

  The optical scanning device 30Y includes a polygon mirror 32, a folding mirror 34, an fθ lens (not shown), and the like. The light beam emitted from the light beam output unit 28 is deflected by the polygon mirror 32 in the main scanning direction (direction perpendicular to the paper surface in FIG. 1). The deflected light beam passes through an unillustrated fθ lens, is folded back by the folding mirror 34, and reaches the photosensitive drum 38 of the developing unit 36Y.

  The image recording unit 14 includes developing units 36Y, 36M, 36C, and 36K for each color. Since each developing unit has the same configuration, only the developing unit 36Y will be described, and description of the other developing units will be omitted.

  The developing unit 36Y includes a cylindrical photosensitive drum 38, and a charging device 40, a developing device 42, a transfer device 44, a cleaner 46, and a static elimination device 48 are provided around the photosensitive drum 38. It has become.

  The photosensitive drum 38 rotates in the direction of arrow B in FIG. 1 and is uniformly charged by the charger 40. The light beam emitted from the optical scanning device 30Y is irradiated between the charger 40 and the developing device 42, and scans the photosensitive drum 38 in the main scanning direction (axial direction of the photosensitive drum 38). Sub-scanning is performed by rotating the photosensitive drum 38 in the arrow B direction. As a result, an electrostatic latent image corresponding to the image is formed on the photosensitive drum 38. The electrostatic latent image formed on the photosensitive drum 38 is developed with toner by the developing device 42. As a result, a toner image corresponding to the image is formed on the photosensitive drum 38.

  The transfer device 44 is disposed to face the photosensitive drum 38 with the intermediate transfer belt 50 interposed therebetween. The intermediate transfer belt 50 is conveyed in the direction of arrow C in FIG. The toner image formed on the photosensitive drum 38 is transferred to the intermediate transfer belt 50 by the transfer device 44.

  The other developing units also perform the same processing, and a color image is formed on the intermediate transfer belt 50 by sequentially transferring the toner images of the respective colors onto the intermediate transfer belt 50.

  On the other hand, the paper P discharged from the paper tray 54 is transported in the direction of arrow D in FIG. Then, when the paper P is conveyed to the transfer roller 58 and the paper P is nipped with the intermediate transfer belt 50, a predetermined transfer voltage is applied, and the color image formed on the intermediate transfer belt 50 is transferred to the paper P. .

  The sheet P on which the color image has been transferred is subjected to a fixing process by the fixing device 59 and is discharged to a discharge tray (not shown).

  The control unit 16 includes an image signal conversion unit 60, a correction calculation unit 62, a test image data generation circuit 64, and a selector 66.

  The test image data generation circuit 64 generates test image data of a test pattern as shown in FIG. 2A for creating a correction table described later.

  The image signal converter 60 converts (corrects) the input image data using the correction table and outputs the converted image data to the light beam output unit 28. Here, the correction table is a table showing a correspondence relationship between the input gradation value and the correction gradation value at each pixel position in the main scanning direction. By correcting the image data using this correction table, streak and The occurrence of unevenness can be reduced.

  The correction calculator 62 creates a correction table used by the image signal converter 60.

  The selector 66 selects image data output from another device, image data of a document read by the image reading unit 12 (image data of CMYK colors), or a test image data generation circuit according to the input operation mode. One of the test image data output from 64 is selected and output to the image signal converter 60.

  More specifically, when the operation mode is the normal operation mode, the selector 66 selects image data output from another device or image data of a document read by the image reading unit 12 and selects an image signal conversion unit. 60. As a result, an image based on the image data input to the control unit 16 is printed on the paper P.

  When the operation mode is the test mode, the test image data output from the test image data generation circuit 64 is selected and output to the image signal converter 60. As a result, a test image is printed on the paper P. The operation mode can be specified by, for example, the operator performing a predetermined operation from an operation unit (not shown).

  Here, an outline of the correction table creation processing performed by the image forming apparatus of the present embodiment will be described with reference to FIG.

  As shown in FIG. 2A, the test image data generating circuit 64 outputs test pattern image data having a uniform density on the entire surface. At this time, since the correction table is not set, the image signal converter 60 outputs the image data to the light beam output device 28 through without performing the correction by the correction table. Thereby, the first test pattern is printed (first test pattern output). The first test pattern is read by the image reading unit 12 to detect gradation characteristics (density) for each pixel. At this time, when an output image having streaks or unevenness is obtained as shown in the upper part of FIG. 2B, the correction calculation unit 62 is uniform over the entire surface as shown in FIG. A correction table having input / output characteristics so as to obtain a density output image is created. For example, as shown in the lower part of FIG. 2B, a correction gradation value that has a density lower than the density indicated by the input image data is set for a portion that is higher than the original density. On the contrary, correction is performed so that a correction gradation value is output so that a density lower than the density indicated by the input image data is output for a portion whose density is lower than the original density. Create a table.

  Next, the test image data generation circuit 64 outputs the image data of the same test pattern as described above again. The image signal converter 60 corrects the image data using the correction table created above and outputs the corrected image data to the light beam output device 28. As a result, the second test pattern is printed (second test pattern output). The second test pattern is read by the image reading unit 12 to detect gradation characteristics (density) for each pixel.

  Here, the correction calculation unit 62 compares the first and second reading results at each pixel position, and determines whether or not the target value has been approached (image quality has been improved).

  As shown in the upper part of FIG. 2C, the portion where the image quality has been improved is that the correction gradation value of the correction table obtained above is made effective (so that correction is applied), and the other portion, The correction table created above is corrected so that the correction amount is 0 (that is, the input gradation value and the correction gradation value are set to the same value) for the part where the image quality deteriorates or the part where the image quality does not change. (Refer to the lower part of FIG. 2C).

  The part where the image quality has deteriorated is, for example, a part where the density is erroneously detected at the time of reading the first test pattern or a temporary streak having no reproducibility is generated and it is not necessary to correct it originally. is there. Therefore, the correction amount is set to 0 in this portion.

  Further, a portion where no change in image quality is observed is a portion in which streaks and unevenness generated in the output results of the first and second test patterns are not improved by correcting the image data. The correction amount is set to zero. Further, since the image quality of such a portion is not improved even if the image data is not corrected with the correction table, the value of the correction table created at the time of the first test pattern reading is used as it is for such a portion. May be.

  By correcting the input image data using the correction table created in this way, it is possible to selectively correct only a portion having a correction effect and output an image.

  The correction table creation process in the test mode will be described in detail below. Here, a case where a correction table is created using the test image T1 shown in FIG.

  The test image T1 is an image used when creating a correction table for correcting color unevenness in the main scanning direction M. From the top, a black test pattern 68K, a cyan test pattern 68C, and a yellow test pattern 68Y are used. And a magenta test pattern 68M. Each test pattern has the same density in the main scanning direction M, and in the sub-scanning direction S, the density gradually increases step by step for each of a plurality of predetermined gradations from the lower limit to the upper limit of the gradation range. A plurality of density patterns are included. Each density pattern corresponds to the uniform density test pattern shown in FIG. In the present embodiment, a case will be described in which the number of pixels in the main scanning direction M is 7000, the gradation range of each color is 0 to 255, that is, the image data has a configuration of 8 bits for each color.

  The test image T1 shown in FIG. 3A is configured to include, for example, a belt-like density pattern having the main scanning direction M as the longitudinal direction for each of the densities N1 to N10 obtained by dividing the gradation range into 10 stages. Yes. Note that the test image is not limited to such a configuration, and any test image may be used as long as each density in the gradation range can be confirmed at each pixel position.

  The test image data of the test image T1 output from the test image data generation circuit 64 by the selector 66 is output to the image signal converter 60. When outputting the first test image in the test mode, the image signal converter 60 outputs the test image data through (without correction) to the light beam output unit 28 and outputs the second test image. In this case, test image data corrected using a correction table created based on the first test image read result is output. As a result, the test image T1 is printed on the paper P by the image recording unit 14.

  The operator sets the paper P on which the test image T1 is printed on the image reading unit 12, and instructs the image reading by performing a predetermined operation from an operation unit (not shown). Thus, the test image T1 is read by the image reading unit 12, and the image data, that is, the image data of each color of R, G, and B is output to the correction calculation unit 62. The correction calculation unit 62 creates correction table data based on the image data, and sets the created correction table data in the correction table of the image signal conversion unit 60.

  FIG. 4 is a flowchart illustrating a control routine executed by the control unit 16 when the operation mode is the test mode.

  First, in step 100, the first test image T1 is output. Specifically, the test image data of the test image T1 output from the test image data generation circuit 64 is selected by the selector 66, and is output to the image signal conversion unit 60. The image signal conversion unit 60 performs the test through. The image data is output to the light beam output unit 28. Accordingly, the test image T1 before correction is printed by the image recording unit 14.

  When the test image T1 is printed, the operator causes the image reading unit 12 to read the printed test image T1 as described above. Accordingly, in step 102, the test image data of the test image T1 read by the image reading unit 12 is input and converted into YMCK image data. This conversion can be performed using a predetermined conversion table or the like.

  Hereinafter, the pixel value of each pixel constituting the test image data (YMCK image data) of the test image read by the image reading unit 12 is output gradation value, and the test image data generation circuit 64 outputs the output gradation value. A pixel value of each pixel constituting the test image data to be performed is referred to as an input gradation value.

  In step 104, target gradation values for the input gradation values N1 to N10 are obtained. Specifically, first, an average value of output gradation values for each input gradation value in an area where the black test pattern 68K is recorded is obtained. That is, for each of the input gradation values N1 to N10, an average value of output gradation values for 7000 pixels is obtained and set as a target gradation value for the input gradation value. It should be noted that the target tone values for the input tone values other than the input tone values N1 to N10 may be obtained by interpolation from the target tone values for the input tone values before and after being obtained. For example, the target gradation value for the input gradation value between the input gradation values N1 and N2 may be interpolated based on the target gradation values obtained for the input gradation values N1 and N2.

  FIG. 6A shows actual gradation characteristics and target gradation characteristics for each pixel position X in the main scanning direction M. FIG. In FIG. 9A, the solid line is the actual gradation characteristic, and the dotted line is the target gradation characteristic. As shown in FIG. 6, when there is a difference between the actual gradation characteristic and the target gradation characteristic, it is necessary to correct the pixel position.

  Therefore, in step 106, for each pixel position X in the main scanning direction M, a correction gradation value for each input gradation value is calculated by a predetermined arithmetic expression, and the correction table, that is, the input gradation value at each pixel position is calculated. Data of a correction table representing a correspondence relationship with the correction gradation value is created. The correction table data obtained here is referred to as first correction table data.

  The corrected gradation value calculated by the predetermined arithmetic expression is a value such that when the image of the corrected gradation value is printed on a sheet, the density becomes the density of the target gradation value. For example, when the input gradation value is DI, the output gradation value is DO, the target gradation value is DM, and the correction gradation value is DR, the predetermined arithmetic expression can be expressed by the following expression as an example.

DR = DI + (DM-DO) (1)
FIG. 6B shows an example of the gradation characteristic (relationship between the input gradation value and the correction gradation value) at each pixel position X in the correction table. For example, for a pixel whose output gradation value tends to be larger than the target gradation value, such as the gradation characteristic at pixel position X = 2 shown in FIG. 6A, as shown in FIG. Then, the correction table data is created so that the correction gradation value becomes smaller by that amount. For example, when the input tone value DI is 50, the output tone value DO is 53, and the target tone value DM is 52, the corrected tone value DR calculated by the above equation (1) is 49.

  By converting the image data using the correction table created by calculating the correction gradation value in this way, the output gradation value can be substantially matched with the target gradation value, color variation is reduced, and image quality is reduced. Can be improved. The predetermined arithmetic expression is not limited to the above expression (1), and can be set as appropriate according to the characteristics of the apparatus.

  By performing the above process for each color of C, Y, and M in the same manner, the first correction table data for each color can be created.

  In step 108, the created first correction table data for each color is set in the correction table of the image signal converter 60.

  As shown in FIG. 7, the image signal converter 60 includes a position information output unit 70 and correction tables 72C, 72M, 72Y, and 72K for the respective colors. When the image data is output to the light beam output device 28 via the correction table, the CMYK color image data constituting the image data, that is, the input gradation values are sequentially input to the position information output unit 70, and the input is performed. Based on the order and paper size information, the pixel position X in the main scanning direction M of the input gradation value is determined and output to each correction table. Each correction table outputs a corrected gradation value corresponding to the input pixel position X and the input gradation value to the light beam output device 28.

8A shows the configuration of the cyan correction table 72C, and FIG. 8B shows a numerical example of the correction table 72. The numerical value in the frame indicates the correction gradation value. In FIG. 5B, the pixel position and the input gradation value are thinned out. As shown in FIG. 5A, the cyan correction table 72C inputs the pixel position X in the main scanning direction M and the input gradation value Cin of the pixel position X, and corrects the input gradation value Cin. The corrected gradation value Cout is output. The other color correction tables have the same configuration. Subsequently, in step 110, a second test image T1 is output using the test image data corrected using the set correction table.

  Specifically, the test image data of the test image T1 output from the test image data generation circuit 64 by the selector 66 is selected and output to the image signal conversion unit 60. The image signal conversion unit 60 uses the correction table. The test image data corrected in the above is output to the light beam output unit 28. As a result, the corrected test image T1 is printed by the image recording unit 14.

  When the test image T1 is printed, the operator causes the image reading unit 12 to read the printed test image T1. Accordingly, in step 112, the test image data of the corrected test image T1 read by the image reading unit 12 is input and converted into YMCK image data.

  In step 114, second correction table data is created. FIG. 5 is a flowchart of a processing routine for creating the second correction table data.

  In step 200, 1 is set to the variable i. A variable i is a variable for designating each pixel position in the main scanning direction.

  In step 202, a difference α between the output gradation value before correction of the pixel position (i) and the target gradation value and a difference β between the corrected output gradation value and the target gradation value are calculated.

  In step 204, α and β are compared, and if α is larger than β, it can be determined that the image quality has been improved since the color variation is smaller after the correction than before the correction, and the pixel position (i) is corrected. The gradation value is set to the same value as the correction gradation value of the first correction table data. On the other hand, if α is less than or equal to β in step 204, it can be determined that the image quality has deteriorated or the image quality has not changed since the color variation has increased or has not changed after the correction. The corrected gradation value at the pixel position (i) is set to the same value as the input gradation value. That is, the correction amount is set to zero.

  In step 210, it is determined whether or not the variable i matches the maximum number of pixels. In the present embodiment, since the number of pixels in the main scanning direction M is 7000, it is determined whether or not the variable i matches 7000. If it is determined that they do not match, the variable i is incremented in step 212, the process returns to step 202, and the above steps are repeated. If it is determined in step 210 that the variable i matches the maximum number of pixels, this processing routine ends.

  By performing the above-described process for each of the C, Y, and M colors in the same manner, the second correction table data for each color can be created.

  In step 116, the created second correction table data for each color is set in the correction table of the image signal converter 60.

  In the normal operation mode, the image data corrected by the correction table of the image signal conversion unit 60 is output to the light beam output unit 28. As a result, the image recording unit 14 prints an image based on the image data corrected by the correction table on the paper P. The image printed on the paper P in this manner is an image in which the occurrence of color unevenness and fine stripes in the main scanning direction M is reduced.

  As described above, for each correction tone value of the first correction table data created based on the first test image reading result for each pixel position, the first test image reading result and 2 It is determined whether or not the image quality is improved by comparing with the result of reading the test image for the second time, and only the correction gradation value that improves the image quality is validated to create the second correction table data. As a result of the setting, pixels that have been determined to have improved image quality are corrected by the correction gradation value, and pixels that have deteriorated image quality and no change in image quality are not corrected by the correction gradation value. Therefore, the image quality can be improved reliably, and the situation where the image quality deteriorates due to the correction can be prevented.

FIGS. 9A and 9B show a case where only cyan is printed at a density within a predetermined range without correcting the image data using the correction table, and a case where the image data is corrected using the correction table as in the present embodiment. The relationship between the position in the main scanning direction M and the lightness L ^* in the case of printing only cyan with a density in the range is shown. Where FIG. (A) is the lightness L ^* of 88 to 91, FIG. (B) is the lightness L ^* indicates the case of 64-67 respectively. As can be seen from the figure, the variation in brightness L ^* is large without correction, whereas the variation in brightness L ^* is small with correction.

  In this way, by correcting the input tone value for each pixel, color unevenness such as left and right color differences and fine stripes can be reduced, and image quality can be improved.

  In the present embodiment, the test image T1 has a density pattern in which the density gradually increases in multiple gradations from the lower limit to the upper limit of the gradation range in the sub-scanning direction S. A density pattern in which the density gradually increases or decreases may be used. Thereby, color unevenness can be reduced more effectively.

  Further, in the present embodiment, the case where correction is performed for each pixel in the main scanning direction M has been described. However, correction may be performed for each of a plurality of pixels in the main scanning direction M. For example, when correcting every 10 pixels, an average value of output gradation values is obtained for each range of pixel positions X = 1 to 10, 11 to 20... 6991 to 7000, and based on the average value of each range. A target gradation value is obtained. Then, a correction table is created by obtaining corrected gradation values for each range based on the obtained target gradation values. Therefore, since the same correction gradation value can be used within each range, the capacity of the correction table can be reduced.

  FIG. 10 shows the results of experiments conducted by the present inventors on the relationship between the correction resolution (correction unit) and the appearance of color unevenness visually. As shown in FIG. 10, if the correction resolution is at least 50 lines / 1 inch (corrected every 12 pixels) or more, the color unevenness is not a concern, and the correction resolution is 100 lines / 1 inch (every 6 pixels). (Correction) Above, it was found that the color unevenness was not visible. Therefore, the correction resolution is set to 50 lines / inch or more, preferably 100 lines / inch or more.

  Further, when the correction gradation value is interpolated when the correction table thinned out with respect to the pixel position X and the input gradation value is created like the correction table shown in FIG. 11, the pixel position X and the input gradation value are interpolated. It is also possible to interpolate from the corrected gradation values around the area. For example, the pixel position to be interpolated is Xa, the input gradation value to be interpolated is Da, the pixel position where the corrected gradation value is calculated and the pixel positions before and after the pixel position Xa are X1, X2, and the corrected gradation value. Are input gradation values calculated before and after the input gradation value Da, D1 and D2, the correction gradation value of the input gradation value D1 at the pixel position X1 is DH1, and at the pixel position X2. When the correction gradation value of the input gradation value D1 is DH2, the correction gradation value of the input gradation value D2 at the pixel position X1 is DH3, and the correction gradation value of the input gradation value D2 at the pixel position X2 is DH4, The corrected gradation value DHa of the input gradation value Da at the pixel position Xa can be obtained by the following equation.

DHa = A1 + (A2-A1) * (Da-D1) / (D2-D1) (2)
However, A1 = DH1 + (DH2-DH1) * (Xa-X1) / (X2-X1)
A2 = DH3 + (DH4-DH3) * (Xa-X1) / (X2-X1)
For example, in the correction table of FIG. 11, the corrected gradation value DHa of the input gradation value Da = 100 at the pixel position Xa = 4000 is the four corrected gradation values (93, 92, 109) shown in the rectangular frame of FIG. 108) from the above equation (2), in this case DHa = 98.

  In this way, by providing the interpolation means for interpolating the corrected gradation values for the pixel positions and input gradation values that do not exist in the correction table, for example, in the image signal converter 60, the capacity of the correction table can be reduced.

  Further, in the present embodiment, a case has been described in which a correction table for reducing color unevenness in the main scanning direction M is created using the test image T1 as shown in FIG. As in the case of the main scanning direction M, a correction table for reducing color unevenness in the sub-scanning direction S is created using the test image T2 as shown in FIG. It may be.

  In this embodiment, the target gradation value for each input gradation value is the average value of the output gradation values at all the pixel positions. However, the present invention is not limited to this, and the most frequently used output gradation value is the target. It may be a gradation value. FIG. 12 shows an example of the output gradation value DO at each pixel position when the input gradation value is DI = 50 when the test image T1 shown in FIG. In this example, the density on both ends tends to decrease. This is because the transfer roller 58 contacts the paper P unevenly. Therefore, when the target gradation value is an average value of all pixel positions (52 in the example of FIG. 12), the target gradation value is lower than the most frequently used output gradation value (54 in the example of FIG. 12). It may not be possible to correct properly. Therefore, when the transfer roller has the above-described characteristics, even if the density of both ends of the paper P is lowered by setting the most frequently used output gradation value as the target gradation value, Color unevenness can be made inconspicuous.

  In the present embodiment, the correction is performed so that the color unevenness is reduced in the main scanning direction or the sub-scanning direction. However, the correction may be made so that the color unevenness is reduced in both directions. In this case, for example, as in the image forming apparatus 10A shown in FIG. 13, an image signal conversion unit 60A including a correction table for the main scanning direction M and an image signal conversion unit 60B including a correction table for the sub-scanning direction S are connected in series. It may be configured to be connected to. That is, the correction gradation value of each color output from the image signal conversion unit 60A is input as the input gradation value of the image signal conversion unit 60B. Thereby, color unevenness can be reduced in any direction.

  In the present embodiment, when a test image is printed on the paper P, the operator sets the image on the image reading unit 12 to read the test image, and a correction table is created based on the image data of the read image. However, as in the image forming apparatus 10B shown in FIG. 14, a scanner 74 for reading a test image is provided at a position where the paper P is discharged, and is automatically read when the test image is discharged. May be. Thereby, an operator's effort can be saved.

  Further, as in the image forming apparatus 10 </ b> C shown in FIG. 15, a scanner 76 for reading a test image formed on the intermediate transfer belt 50 is provided in front of the transfer roller 58, and an image of the test image read by the scanner 76. A correction table may be created based on the data. Thereby, it is not necessary to print a test image on the paper P, and the paper can be saved.

  In the case where the image recording unit 14 is a device that can perform screen processing by switching between a plurality of screens, the color unevenness may vary depending on the screen. In this case, a correction table may be created according to the screen, and the image data may be corrected using each correction table. FIG. 16 shows an image forming apparatus 10D that corrects image data corresponding to each screen in an apparatus that can record an image by selecting one of two screens having different screen line numbers, for example. It was.

  As shown in FIG. 16, the image forming apparatus 10 </ b> D includes image signal conversion units 60 </ b> C and 60 </ b> D corresponding to each screen, and includes a selector 66 (switching unit) and a light beam output unit 28 of the image recording unit 14. A screen instruction signal for designating a screen is input. The screen instruction signal is, for example, a signal corresponding to the print mode (high image quality mode, high speed mode) selected by the user.

  The selector 66 selects an image signal conversion unit corresponding to the input screen instruction signal, and outputs image data to the selected image signal conversion unit. The image data of each color corrected by the selected image signal conversion unit is output to the light beam output unit 28. The light beam output unit 28 performs screen processing using a screen corresponding to the screen instruction signal.

  In such a configuration, a test image is printed for each screen, a correction table is created by the correction calculation unit 62 in the same manner as described above, and the created correction table is set in the image signal conversion units 60C and 60D. Note that an apparatus configuration in which the above technologies are appropriately combined may be employed.

(Second Embodiment)
In the first embodiment, a 1-input 1-output type correction table is created for each color and the image data is corrected for each color. In such a case, color unevenness can be reduced in the case of an image of only one color. However, color unevenness may not be reduced in the case of an image in which a plurality of colors are superimposed.

  Therefore, in the present embodiment, a case will be described in which image data is corrected by creating a 3-input 3-output correction table so that color unevenness can be reduced even when each color is superimposed. In addition, the same code | symbol is attached | subjected to the same part as 1st Embodiment, and detailed description is abbreviate | omitted.

FIG. 17A shows, as an example, each pixel position and lightness L ^* when a cyan-only image of a predetermined density is printed without correction and when it is printed with correction using a one-input one-output correction table. In FIG. 5B, the green image obtained by superimposing two colors of cyan and yellow having a predetermined density is printed without correction, and is corrected with the correction table of 1 input 1 output type and printed. The relationship between each pixel position and the lightness L ^* in the case is shown.

  As shown in FIG. 6A, in the case of only cyan, if the image data is corrected using the 1-input 1-output type correction table as described above, the brightness will be almost the same at any pixel position, and color unevenness will occur. Can be reduced.

  However, as shown in FIG. 5B, when cyan and yellow are overlapped, even if correction is performed using a 1-input 1-output correction table, the brightness varies at each pixel position, and color unevenness can be reduced. It may disappear.

  In such a case, in addition to the image data of the test image T1 as shown in FIG. 3 (A), as shown in FIG. 18, the process black, which is a color obtained by combining all of yellow, magenta, and cyan in order from the top. Test pattern 68PB, red test pattern 68R which is a color combining yellow and magenta, green test pattern 68G which is a color combining yellow and cyan, and blue test pattern 68B which is a color combining cyan and magenta The test image data generation circuit 64 stores the image data of the test image T3 (third test image) configured in the above.

  Similarly to the above, the test image T3 is printed on the paper P, and the image reading unit 12 reads the test image T3 printed on the paper P. The correction calculation unit 62 converts RGB data of each density pattern of the read test image T3 into YMC data.

  Next, based on the image data obtained by reading the process black test pattern 68PB (output tone values of each color of C, M, and Y), a target tone value is obtained by the same process as described above, and the obtained target tone value is obtained. Based on the above, a corrected gradation value is obtained. Using the corrected gradation value, the test image data is corrected and a test pattern is output again, and it is determined whether the corrected gradation value obtained above is valid or the correction amount is zero. A typical correction table. By performing this similarly for the test patterns 68R, 68G, and 68B, when the C, M, and Y colors are overlapped, the correction gradation values when the two colors C and M, C and Y, and M and Y are overlapped are obtained. Can be sought.

  Further, the same processing as described above is performed on the test image T1 to obtain a corrected gradation value for each color of YMCK.

  From these results, a three-input three-output correction table 80 as shown in FIG. 19A is created. FIG. 5B shows a numerical example of the correction table. In FIG. 5B, the pixel position and the input gradation value are thinned out. Note that the correction gradation values for the input gradation values of each color that do not exist in the correction table can be interpolated using, for example, the same interpolation method as described in the first embodiment for each color.

  For black, a 1-input 1-output black correction table created based on image data obtained by reading the test image T1 in the same manner as described above is used.

FIG. 20 shows pixel positions and lightness L ^* when an image in which cyan and yellow having a predetermined density are printed without correction, and when corrected with a three-input three-output correction table and printed. Showed the relationship.

  As shown in the figure, when image data is corrected using a three-input three-output type correction table, the lightness is almost the same at any pixel position, and as shown in FIG. Compared with the case where each color is corrected using this correction table, color unevenness can be greatly reduced.

  In this way, by correcting the image data using the three-input three-output type correction table, it is possible to appropriately correct not only one color image but also two-color superimposed images and three-color superimposed images. Color unevenness can be effectively reduced.

  In the first and second embodiments, the image forming apparatus that reduces the occurrence of streaks and unevenness by correcting the image data has been described as an example. However, the present invention is not limited to this. By correcting the image forming conditions of the image forming apparatus, for example, the charging amount according to the position of the photoconductor, the developing amount, the exposure amount (laser power) of the optical scanning device, etc., the occurrence of streaks and unevenness is reduced. You may apply to an apparatus. In the case of such an apparatus, first, a test pattern is output while leaving the image forming conditions at the initial setting values, and the image forming conditions are determined for each position of the photoconductor and the position in the main scanning direction based on the reading result of the test pattern. Then, the second test pattern is output under the image forming conditions corrected by the correction value, and only the correction value with improved image quality is set to be effective. The part where the image quality is deteriorated is returned to the initial image forming conditions. In addition, for the part where the image quality has not changed, the correction value obtained as described above may be set, or the initial value may be left as it is.

1 is a schematic configuration diagram of an image forming apparatus. FIG. 10 is a schematic explanatory diagram of correction table creation processing performed by the image forming apparatus. (A) is an image diagram showing an example of a test image used for reducing color unevenness in the main scanning direction, and (B) is an image diagram showing an example of a test image used for reducing color unevenness in the sub-scanning direction. is there. It is a flowchart of the process performed by a control part. It is a flowchart of the process which produces 2nd correction table data. It is a diagram which shows the gradation characteristic of each pixel position. It is a block diagram of an image signal converter. (A) is a figure which shows the correction table of 1 input 1 output type, (B) is a figure which shows the numerical example of the correction table of (A). (A) and (B) are a case where only cyan is printed with a density within a predetermined range without correcting the image data, a case where only cyan is printed with a density within a predetermined range by correcting the image data with a correction table, It is the figure which showed the relationship between the position of the main scanning direction M, and the lightness L ^* . It is a table | surface which shows the relationship between correction | amendment resolution and the appearance of a color nonuniformity. It is a figure which shows the numerical example of the correction table of 1 input 1 output type | mold. It is a figure which shows the relationship between each pixel position and output gradation value about a predetermined | prescribed input gradation value when the test image T1 is read. It is a schematic block diagram of the image forming apparatus which concerns on a modification. It is a schematic block diagram of the image forming apparatus which concerns on a modification. It is a schematic block diagram of the image forming apparatus which concerns on a modification. It is a schematic block diagram of the image forming apparatus which concerns on a modification. (A) is a diagram showing the relationship between each pixel position and lightness L ^* when a cyan-only image of a predetermined density is printed without correction and when it is corrected and printed with a one-input one-output correction table. , (B) shows each pixel position and brightness when printing a green image in which two colors of cyan and yellow of a predetermined density are superimposed without correction and when correcting and printing with a correction table of 1-input 1-output type. It is a diagram which shows the relationship with L ^* . It is an image figure which shows an example of the test image used when creating the 3 input 3 output type | mold correction table. (A) is a figure which shows a 3 input 3 output type | mold correction table, (B) is a figure which shows the numerical example of the correction table of (A). It is a figure which shows the relationship between each pixel position and the lightness L ^* at the time of printing the image which superimposed cyan and yellow of predetermined density | concentration without a correction | amendment, and correct | amending with the correction table of 3 input 3 output type | mold.

Explanation of symbols

10, 10A, 10B, 10C, 10D Image forming device 12 Image reading unit 14 Image recording unit 16 Control unit 28 Light beam output units 30Y, 30M, 30C, 30K Optical scanning devices 36Y, 36M, 36C, 36K Development units 60, 60A , 60B, 60D, 60C, 60D Image signal conversion unit 62 Correction calculation unit 64 Test image data generation circuit 66 Selector 70 Position information output unit 72Y, 72M, 72C, 72K Correction table 74, 76 Scanner 80 Correction table

Claims (8)

Image forming means for forming an image based on the input image data;
Based on the read result obtained by reading the test pattern formed by the image forming unit based on the image data of the test pattern, the image data supplied to the image forming unit is the image data formed by the image forming unit. A calculation means for calculating a correction value for correcting so as to improve the image quality for each predetermined pixel;
The image formation based on the reading result of the test pattern formed by the image forming unit based on the image data of the test pattern corrected by the correction value and the image data of the test pattern before being corrected by the correction value A determination unit that compares the result of reading the test pattern formed by the unit to determine whether the image quality is improved for each pixel;
Without correcting the image data corresponding to the pixel determined to have deteriorated in image quality by the determination means, it is determined that there is no change in the image quality by the pixel determined by the determination means and the image determined by the determination means. Correction means for correcting image data corresponding to the selected pixel with the correction value;
An image forming apparatus including: Image forming means for forming an image based on the input image data;
Based on the read result obtained by reading the test pattern formed by the image forming unit based on the image data of the test pattern, the image data supplied to the image forming unit is the image data formed by the image forming unit. A calculation means for calculating a correction value for correcting so as to improve the image quality for each predetermined pixel;
The image formation based on the reading result of the test pattern formed by the image forming unit based on the image data of the test pattern corrected by the correction value and the image data of the test pattern before being corrected by the correction value A determination unit that compares the result of reading the test pattern formed by the unit to determine whether the image quality is improved for each pixel;
It is determined that the image quality has been improved by the determination means without correcting the pixels determined to have no change in image quality by the determination means and the image data corresponding to the pixels determined to have deteriorated by the determination means. Correction means for correcting image data corresponding to the selected pixel with the correction value;
An image forming apparatus including: Image forming means for forming an image based on the input image data;
Based on the reading result obtained by reading the test pattern formed by the image forming unit based on the image data of the test pattern, the image forming condition of the image forming unit is set to the image quality of the image formed by the image forming unit. Calculating means for calculating for each predetermined pixel a correction value for correcting so as to improve,
By the image forming unit based on the reading result of the test pattern formed by the image forming unit based on the image forming condition corrected by the correction value and the image forming condition before being corrected by the correction value. A determination means for comparing the result of reading the formed test pattern and determining whether the image quality is improved for each pixel;
Without the image forming conditions are not correct when forming an image based on image data corresponding to the pixels determined to image quality is degraded by the determining means, a pixel and it is determined that the image quality is improved by the determining means and correction means for correcting the previous Kiho positive value image forming conditions for the image formed based on image data corresponding to the pixel is determined that there is no change in quality by the determining means,
An image forming apparatus including: Image forming means for forming an image based on the input image data;
Based on the reading result obtained by reading the test pattern formed by the image forming unit based on the image data of the test pattern, the image forming condition of the image forming unit is set to the image quality of the image formed by the image forming unit. Calculating means for calculating for each predetermined pixel a correction value for correcting so as to improve,
By the image forming unit based on the reading result of the test pattern formed by the image forming unit based on the image forming condition corrected by the correction value and the image forming condition before being corrected by the correction value. A determination means for comparing the result of reading the formed test pattern and determining whether the image quality is improved for each pixel;
The image forming conditions for forming an image based on the pixel determined to have no change in image quality by the determining unit and the image data corresponding to the pixel determined to have deteriorated by the determining unit are not corrected. and correction means for correcting the previous Kiho positive value image forming conditions for the image formed based on image data corresponding to the pixels determined to image quality has been improved by the determining means,
An image forming apparatus including:   The image forming apparatus according to claim 1, wherein the predetermined pixel is one pixel or a plurality of pixels.   The image forming apparatus according to claim 1, further comprising a reading unit that reads a test pattern formed by the image forming unit based on image data of the test pattern.   The image forming apparatus according to claim 1, wherein a size of the test pattern is a maximum size that can be formed by the image forming unit.   The image forming apparatus according to claim 1, wherein the image data of the test pattern is image data for forming a test pattern in which the entire area has the same density.