JP2006309140A - Image forming apparatus and method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus and an image forming method by which image quality on recording material can be improved. <P>SOLUTION: The image forming apparatus is provided with two gradation correction tables 118A and 118B corresponding to two kinds of processing screens. The respective gradation correction tables are properly corrected and updated on the basis of a value obtained by converting a result obtained by detecting a patch image formed by applying the corresponding screen by a density sensor 60 into image density on the recording material by OD value conversion tables 124A and 124B corresponding to the processing screen. Thus, an image is formed while optimally keeping gradation reproducibility on the recording material in accordance with the respective screens. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image forming apparatus and an image forming method for performing signal processing on an image signal and forming a toner image according to the processed signal.

  In an electrophotographic image forming apparatus such as a printer, a copying machine, and a facsimile machine, a small test image (patch image) having a predetermined image pattern is formed as needed, and the image density is adjusted by a density sensor. A predetermined image quality can be stably obtained by detecting and adjusting the operating condition of each part of the apparatus based on the detection result.

  For example, in the image processing apparatus described in Patent Document 1 according to the application of the present applicant, gradation correction characteristics of the apparatus are controlled based on the density detection result of the patch image. That is, the density of a test image of a predetermined pattern is detected by a patch sensor, and the gamma characteristic of the apparatus is grasped from the detection result, and a correction conversion table for tone correction is generated. The image signal is converted into laser drive pulse width data while referring to the correction conversion table newly generated in this way. As a result, it is possible to obtain an optimum printing result corresponding to the variation of the gamma characteristic of the apparatus.

JP 2000-333012 A (Page 6, FIG. 7)

  In a general image forming apparatus, in order to prevent the recording material from being consumed every time an adjustment operation is performed, an image of a test image carried on an image carrier such as a photoconductor, a transfer belt, or a transfer drum A configuration is employed in which the density is detected by a density sensor. However, the density of the test image temporarily carried on the transfer medium is not exactly the same as the image density when the test image is finally transferred and fixed on the recording material.

  Furthermore, according to the knowledge of the present inventor, an image forming apparatus configured to perform screen processing by selectively using one of a plurality of types of processing screens prepared for an image signal given from the outside. In this case, the difference between the image density on the transfer medium and the image density on the recording material varies depending on the type of processing screen used.

  The gradation correction technology in the conventional image forming apparatus includes such an error due to the difference in density on the transfer medium and the recording material, and the best image quality on the final recording material such as recording paper. This leaves room for further improvement.

  The present invention has been made in view of the above problems, and an object thereof is to provide an image forming apparatus and an image forming method capable of improving the image quality on a recording material.

  In order to achieve the above object, an image forming apparatus for forming an image according to an image signal on a recording material includes a signal process including a screen process that selectively uses one of a plurality of different process screens. A signal processing means for producing an image formation control signal by applying the image signal to the image signal, and an intermediate transfer medium, forming a toner image corresponding to the image formation control signal and carrying it on the intermediate transfer medium Image forming means for forming the image by transferring and fixing the toner image on the recording material, and detection means for detecting the amount of toner carried on the intermediate transfer medium as the toner image, The signal processing by the signal processing means includes a detection result by the detection means for a toner image as a patch image formed using the processing screen, and the processing scan. Characterized in that from said intermediate transfer medium which is determined in advance corresponding to the lean including gradation correction process based on the transfer-fixing property on the recording material.

  In order to achieve the above object, the image forming method according to the present invention performs an image forming control signal by performing signal processing including screen processing using one of a plurality of different types of processing screens on the image signal. A signal processing step of forming a toner image, an image forming step of forming a toner image according to the image formation control signal and carrying the toner image on an intermediate transfer medium, and transferring and fixing the toner image on the intermediate transfer medium onto a recording material A transfer fixing step, and in the signal processing step, a detection result of a toner amount forming a toner image as a patch image formed using a processing screen to be used and carried on the intermediate transfer medium; The signal processing including gradation correction processing based on transfer and fixing characteristics from the intermediate transfer medium to the recording material, which is obtained in advance corresponding to the processing screen, is performed on the image signal. Characterized in that to perform on.

  Here, the transfer and fixing characteristics are the amount of toner constituting the toner image carried on the intermediate transfer medium, and the image density on the recording material after the toner image is transferred and fixed on the recording material. It is a characteristic that represents the relationship.

  In the conventional gradation correction technology, the difference between the toner amount detection result on the intermediate transfer medium and the image density on the final recording material is not taken into consideration, and as a result, In some cases, the image quality is not always the best. Therefore, in the above invention, not only the detection result of the toner amount on the intermediate transfer medium but also the fixing transfer characteristic indicating the relationship between the toner amount on the intermediate transfer medium and the image density on the recording material is considered. Perform gradation correction processing. By doing this, the image density on the recording material, not on the intermediate transfer medium, is fed back to the processing contents of the gradation correction processing, so that the image quality on the recording material can be improved. Further, considering that the transfer and fixing characteristics vary depending on the type of processing screen used, the gradation correction processing is performed for each processing screen. In other words, the content of the gradation correction processing is individually determined for each processing screen, and the content is based on the detection result of the patch image formed using the processing screen and the transfer and fixing characteristics corresponding to the processing screen. Determined based on. Thereby, the image quality can be further improved.

  This gradation correction processing can be performed, for example, based on the detection result by the toner amount detection unit for each of a plurality of regions formed at different gradation levels in the toner image. In other words, the toner amount in a toner image formed at a plurality of gradation levels is detected for each gradation level, and the gamma characteristic of the apparatus is obtained from these detection results, and gradation correction processing is performed to compensate for this. Therefore, it is possible to form an image of good quality with excellent gradation characteristics without being affected by the gamma characteristic of the apparatus. A toner image formed for such a purpose includes a plurality of toner images having different gradation levels and spaced apart from each other, and gradation levels in a stepped or continuous manner in one continuous toner image. Any of the toner images may change.

  When a toner image composed of a plurality of image areas with different gradation levels is formed as the patch image, the gradation level setting value for each image area is set according to the processing screen to be used. It is desirable. In other words, when performing gradation correction processing by discretely sampling gamma characteristic curves that originally vary continuously with respect to the gradation level at several gradation levels, the gradation used when forming a toner image It is desirable to set the level setting value according to the processing screen. This is because the shape of the gamma characteristic curve differs depending on the processing screen to be used, and the sampling points necessary for accurately estimating the overall gamma characteristic do not necessarily match between the processing screens. Therefore, by setting sampling points individually for each processing screen, the gradation correction processing can be performed with high accuracy for each processing screen.

  Further, for example, the toner amount on the intermediate transfer medium detected by the toner amount detecting means is converted into an image density on the recording material based on the transfer fixing characteristic, and the gradation correction is performed based on the conversion result. Processing may be performed. That is, by performing the conversion process based on the transfer and fixing characteristics on the toner amount detected in the toner image, the image density of the toner image on the recording material can be estimated. Then, by performing gradation correction processing based on the image density thus estimated, it is possible to improve the image quality on the recording material. In addition, the gamma characteristic and transfer / fixing characteristic of the apparatus vary greatly depending on the apparatus configuration, whereas the image density on the recording material is determined by the properties of the toner and the recording material and does not depend much on the apparatus configuration. Therefore, if the processing sequence of the gradation correction processing is configured so that the toner amount detection result is converted into the image density on the recording material as described above, the same configuration can be obtained between apparatuses having different configurations. A processing sequence can be applied, and the development cost of the apparatus can be reduced.

  In addition, based on the detection result of the detection unit for the toner image as the patch image formed by the image forming unit, a control process for determining processing characteristics of the gradation correction process by the signal processing unit is executed. Control means may be further provided, and the signal processing means may execute the gradation correction processing with the processing characteristics determined by the control processing. As described above, the characteristics of the gradation correction processing are determined in advance based on the actual measurement result of the patch image, so that the signal processing means simply performs the signal processing with the determined processing characteristics. The content becomes simple.

  For example, there is further provided conversion means for converting the toner amount on the intermediate transfer medium detected by the detection means into an image density on the recording material based on the transfer fixing characteristics, and the control means includes the control process. In the above, the processing characteristic may be determined based on a conversion result by the conversion means. In this way, the gradation reproducibility on the recording material is improved by determining the processing characteristics of the gradation correction processing using the value obtained by converting the detection result on the intermediate transfer medium into the image density on the recording material. Thus, an image with excellent image quality can be formed. In this case, it is more preferable to provide a plurality of conversion means corresponding to each of the plurality of processing screens in order to cope with different transfer and fixing characteristics depending on the type of processing screen used.

  As a more specific processing mode of the control processing, for example, the following can be performed. That is, the image forming unit sequentially forms a plurality of patch images to which the plurality of processing screens are respectively applied, and the control unit displays the detection result of the detection unit for each of the plurality of patch images as the patch. Conversion is performed by the conversion means corresponding to the processing screen applied to image formation, and the processing characteristics corresponding to each of the plurality of processing screens are determined based on the result.

  In such a control process, the following series of processes is performed on one process screen. First, the processing screen is applied to form a patch image on the intermediate transfer medium, and the toner amount detection result is converted into the image density on the recording material by a conversion unit corresponding to the processing screen. Based on the conversion result, the gradation correction processing characteristic corresponding to the processing screen is determined. By doing so, the signal processing means can perform gradation correction processing with processing characteristics suitable for each processing screen to be used.

  Here, for example, each of the patch images is formed with an image pattern in which the gradation value gradually increases or decreases along a predetermined direction on the surface of the intermediate transfer medium, and the detection unit mutually detects the patch image in the predetermined direction. The toner amount may be detected at a plurality of positions at different positions, and the control unit may determine the processing characteristics based on the detection results at each of the plurality of positions. In this way, the gradation reproduction characteristics in the apparatus, that is, the correspondence between the gradation value and the image density can be obtained, and the gradation correction characteristic can be determined from the result.

  The conversion means can be realized by, for example, providing a conversion table from the amount of toner on the intermediate transfer medium to the image density on the recording material in addition to the calculation based on a predetermined mathematical expression. . In this case, the toner amount detected on the intermediate transfer belt can be easily converted into the image density on the recording material by referring to the conversion table. Further, in order to cope with the temporal change of the transfer and fixing characteristics, the conversion table may be updated or corrected as appropriate according to the operation status of the apparatus.

(First embodiment)
FIG. 1 is a diagram showing a first embodiment of an image forming apparatus according to the present invention. FIG. 2 is a block diagram showing an electrical configuration of the image forming apparatus of FIG. This apparatus forms a full color image by superposing four color toners (developers) of yellow (Y), cyan (C), magenta (M), and black (K), or only black (K) toner. The image forming apparatus forms a monochrome image using In this image forming apparatus, when an image signal is given to the main controller 11 from an external device such as a host computer, the CPU 101 provided in the engine controller 10 controls each part of the engine unit EG in response to a command from the main controller 11. Then, a predetermined image forming operation is executed, and an image corresponding to the image signal is formed on the sheet S.

  In the engine unit EG, the photosensitive member 22 is provided to be rotatable in the arrow direction D1 in FIG. A charging unit 23, a rotary developing unit 4 and a cleaning unit 25 are arranged around the photosensitive member 22 along the rotation direction D1. The charging unit 23 is applied with a predetermined charging bias, and uniformly charges the outer peripheral surface of the photoconductor 22 to a predetermined surface potential. The cleaning unit 25 removes the toner remaining on the surface of the photosensitive member 22 after the primary transfer, and collects it in a waste toner tank provided inside. The photosensitive member 22, the charging unit 23, and the cleaning unit 25 integrally constitute the photosensitive member cartridge 2, and the photosensitive member cartridge 2 is detachably attached to the apparatus main body as a whole.

  Then, the light beam L is irradiated from the exposure unit 6 toward the outer peripheral surface of the photosensitive member 22 charged by the charging unit 23. The exposure unit 6 exposes the light beam L onto the photoconductor 22 in accordance with an image signal given from an external device to form an electrostatic latent image corresponding to the image signal.

  The electrostatic latent image thus formed is developed with toner by the developing unit 4. That is, in this embodiment, the developing unit 4 is configured as a support frame 40 that is rotatably provided about a rotation axis center orthogonal to the paper surface of FIG. A yellow developing device 4Y, a cyan developing device 4C, a magenta developing device 4M, and a black developing device 4K are provided. The developing unit 4 is controlled by the engine controller 10. Based on the control command from the engine controller 10, the developing unit 4 is driven to rotate, and the developing units 4Y, 4C, 4M, and 4K are selectively brought into contact with the photoreceptor 22 or have a predetermined gap. When the developing roller 44 is positioned at a predetermined developing position facing the space, the developing roller 44 provided in the developing unit and carrying the toner of the selected color is disposed to face the photosensitive member 22, and the developing roller 44 is disposed at the facing position. Then, toner is applied to the surface of the photosensitive member 22. As a result, the electrostatic latent image on the photosensitive member 22 is visualized with the selected toner color.

  The toner image developed by the developing unit 4 as described above is primarily transferred onto the intermediate transfer belt 71 of the transfer unit 7 in the primary transfer region TR1. The transfer unit 7 includes an intermediate transfer belt 71 stretched between a plurality of rollers 72 to 75, and a drive unit (not shown) that rotates the intermediate transfer belt 71 in a predetermined rotation direction D2 by rotationally driving the roller 73. It has. When a color image is transferred to the sheet S, each color toner image formed on the photosensitive member 22 is superimposed on the intermediate transfer belt 71 to form a color image and taken out from the cassette 8 one by one. Then, the color image is secondarily transferred onto the sheet S conveyed along the conveyance path F to the secondary transfer region TR2.

  At this time, in order to correctly transfer the image on the intermediate transfer belt 71 to a predetermined position on the sheet S, the timing of feeding the sheet S to the secondary transfer region TR2 is managed. Specifically, a gate roller 81 is provided on the transport path F on the front side of the secondary transfer region TR2, and the gate roller 81 rotates in accordance with the timing of the circumferential movement of the intermediate transfer belt 71. S is sent to the secondary transfer region TR2 at a predetermined timing.

  Further, the sheet S on which the color image is formed in this manner is fixed with the toner image by the fixing unit 9 and is conveyed to the discharge tray portion 89 provided on the upper surface portion of the apparatus main body via the pre-discharge roller 82 and the discharge roller 83. The Further, when images are formed on both sides of the sheet S, when the rear end portion of the sheet S on which the image is formed on one side as described above is conveyed to the reversal position PR behind the pre-discharge roller 82. The rotation direction of the discharge roller 83 is reversed, whereby the sheet S is conveyed in the direction of the arrow D3 along the reverse conveyance path FR. Then, the sheet is again placed on the transport path F before the gate roller 81. At this time, the surface of the sheet S to which the image is transferred by contacting the intermediate transfer belt 71 in the secondary transfer region TR2 is first transferred. It is the opposite surface. In this way, images can be formed on both sides of the sheet S.

  In addition, the apparatus 1 includes a display unit 12 controlled by the CPU 111 of the main controller 11 as shown in FIG. The display unit 12 is constituted by, for example, a liquid crystal display, and in accordance with a control command from the CPU 111, the operation guidance to the user, the progress of the image forming operation, the occurrence of an abnormality in the apparatus, the replacement time of any unit, etc. A predetermined message for notification is displayed.

  In FIG. 2, reference numeral 113 denotes an image memory provided in the main controller 11 for storing an image given from an external device such as a host computer via the interface 112. Reference numeral 106 is a ROM for storing a calculation program executed by the CPU 101, control data for controlling the engine unit EG, and the like. Reference numeral 107 is a RAM for temporarily storing calculation results in the CPU 101 and other data. is there.

  A cleaner 76 is disposed in the vicinity of the roller 75. The cleaner 76 can be moved toward and away from the roller 75 by an electromagnetic clutch (not shown). Then, the blade of the cleaner 76 abuts on the surface of the intermediate transfer belt 71 that is stretched over the roller 75 while moving to the roller 75 side, and the toner that remains on the outer peripheral surface of the intermediate transfer belt 71 after the secondary transfer. Remove.

  Further, a density sensor 60 is disposed in the vicinity of the roller 75. The density sensor 60 is provided to face the surface of the intermediate transfer belt 71 and measures the image density of the toner image formed on the outer peripheral surface of the intermediate transfer belt 71 as necessary. Based on the measurement results, this apparatus uses the operating conditions of each part of the apparatus that affect the image quality, such as the developing bias applied to each developing device, the intensity of the exposure beam L, and the tone correction characteristics of the apparatus. Adjustments are being made.

  The density sensor 60 is configured to output a signal corresponding to the density of a predetermined area on the intermediate transfer belt 71 using, for example, a reflective photosensor. The CPU 101 can detect the image density of each part of the toner image on the intermediate transfer belt 71 by periodically sampling the output signal from the density sensor 60 while rotating the intermediate transfer belt 71. The “image density” of the toner image referred to here is the density of the toner image temporarily carried on the intermediate transfer belt 71, that is, the amount of toner constituting the toner image. Therefore, it is not the same as the image density (optical density) defined on the sheet S that has been transferred and fixed. This point will be described in detail later.

  FIG. 3 is a diagram showing a gradation processing block of the image forming apparatus. The main controller 11 includes functional blocks such as a color conversion unit 114, a gradation correction unit 115, a halftoning unit 116, a pulse modulation unit 117, gradation correction tables 118A and 118B, and a correction table calculation unit 119.

  In addition to the CPU 101, the ROM 106, and the RAM 107 shown in FIG. A gradation characteristic detecting unit 123 that detects a gradation characteristic indicating a gamma characteristic is provided.

  In the main controller 11 to which the image signal is given from the host computer 100, the color conversion unit 114 converts the RGB gradation data indicating the gradation level of the RGB component of each pixel in the image corresponding to the image signal into the corresponding CMYK. Conversion into CMYK gradation data indicating the gradation level of the component. In this color conversion unit 114, the input RGB gradation data is, for example, 8 bits per pixel per color component (that is, representing 256 gradations), and the output CMYK gradation data is similarly 8 bits per pixel per color component ( That is, it represents 256 gradations). The CMYK gradation data output from the color conversion unit 114 is input to the gradation correction unit 115.

  The gradation correction unit 115 performs gradation correction on the CMYK gradation data of each pixel input from the color conversion unit 114. That is, the gradation correction unit 115 refers to one of the two types of gradation correction tables 118A and 118B registered in advance in the nonvolatile memory, and from the color conversion unit 114 according to the gradation correction table. The input CMYK gradation data of each pixel is converted into corrected CMYK gradation data indicating the corrected gradation level. The purpose of the gradation correction is to compensate for the change in the gamma characteristic of the engine unit EG configured as described above, and to keep the overall gamma characteristic of the image forming apparatus always ideal.

  The corrected CMYK gradation data corrected in this way is input to the halftoning unit 116. The halftoning unit 116 performs halftoning processing by a screen method on the given gradation data, generates half-bit CMYK gradation data of 8 bits per pixel, and inputs it to the pulse modulation unit 117. . The content of the halftoning process varies depending on the type of image to be formed. That is, based on a determination criterion such as whether the image is a monochrome image, a color image, a line drawing, or a graphic image, the processing screen is prepared from a plurality of types of processing screens prepared in advance (two types of screens 116A and 116B in this example). An optimum image is selected by the switching means 200, and halftoning processing is performed using the selected screen.

  FIG. 4 is a diagram illustrating an example of a halftoning processing screen. A first screen (screen A; reference numeral 116A in FIG. 3) shown in FIG. 4A is a screen suitable for an image that requires high resolution. That is, this screen A is a screen having a halftone dot structure having an inclination angle of 60 degrees with respect to the scanning direction (main scanning direction) of the light beam L onto the photosensitive member 22, and has a halftone dot compared to the screen B described later. The pitch P1 is small. Thus, since the screen A has a relatively fine pitch P1, it is a screen suitable for an image that requires high resolution, for example, a character-based image.

  On the other hand, the second screen shown in FIG. 4B (screen B; reference numeral 116B in FIG. 3) has the same inclination angle of 60 degrees, but its pitch P2 is larger than that of the screen A. For this reason, it is superior to the screen A in terms of halftone expression. That is, the screen B is a screen suitable for photographs, natural images, and graphic images.

  Then, the halftoning unit 116 determines what type of image the image to be formed is based on the input image signal, selects one of the two types according to the image, and selects half of the image. Perform toning. As a result, in the formed image, toner dots are arranged with periodicity corresponding to the applied screen pitch, and the interval between the toner dot portions also corresponds to this pitch. In addition, about a screen, it is not limited to said 2 types, You may further provide the other screen from which an inclination angle and a pitch differ.

  In this embodiment, two gradation correction tables 118A and 118B corresponding to the two types of screens A and B are provided. This is to cope with the fact that the gradation level of the output image with respect to the input gradation level has non-linearity, and the curve differs slightly depending on the type of screen used. Thus, by preparing a gradation correction table corresponding to each of different screens, excellent gradation reproducibility can be obtained regardless of which screen is selected and used. In this embodiment, when the screen to be used for the halftoning process is determined, one table corresponding to the screen to be used is selected by the switching unit 201 out of the two gradation correction tables 118A and 118B, and based on that table. Thus, the above gradation correction is performed.

  The CMYK gradation data after halftoning input to the pulse modulation unit 117 is a multi-value signal indicating the size and arrangement of toner dot portions of CMYK colors to be attached to each pixel. The unit 117 uses the halftone CMYK gradation data to create a video signal for pulse width modulating the exposure laser pulses of the CMYK color images of the engine unit EG, and the engine controller via a video interface (not shown) 10 is output. Upon receiving this video signal, the laser driver 121 controls ON / OFF of the semiconductor laser of the exposure unit 6 to form an electrostatic latent image of each color component on the photosensitive member 22. In this way, image formation corresponding to the image signal is performed.

  Further, in this type of image forming apparatus, the gamma characteristic of the apparatus changes for each apparatus and also in the same apparatus depending on the use situation. Therefore, in order to eliminate the influence of the variation in gamma characteristics on the image quality, a gradation control process for updating the contents of the gradation correction table described above based on the actual measurement result of the image density is executed at a predetermined timing. . In this embodiment, two types of gradation correction tables 118A and 118B are provided corresponding to each of the two types of screens, and the gradation control processing described below is performed for each gradation correction table. It is done individually. That is, the output from the correction table 119 calculation unit is given to one of the gradation correction tables 118A and 118B corresponding to the screen to be used by the switching unit 202 that is linked to the switching unit 201.

  FIG. 5 is a flowchart showing the gradation control process. The gamma characteristic of the engine unit EG is not always constant but varies with time. For this reason, the tone correction unit 115 needs to change the content of the tone correction described above in accordance with changes in device characteristics. In this embodiment, such a request is met by appropriately updating the gradation correction tables 118A and 118B in accordance with fluctuations in apparatus characteristics. The gradation correction tables 118A and 118B are updated by executing the gradation control process shown in FIG. In this gradation control process, first, a patch image having a predetermined image pattern is formed (step S101). At this time, the image signal corresponding to the patch image is subjected to a halftoning process using a screen corresponding to the gradation correction table to be updated. That is, in the gradation control process for updating the gradation correction table 118A, a halftoning process using the screen A corresponding to the table is performed on the image signal. Similarly, in the gradation control process for updating the gradation correction table 118B, the screen B corresponding to the table is applied. In this way, by actually applying the corresponding screen to form a patch image, it is possible to execute gradation correction processing that is more realistic and to obtain excellent gradation reproducibility.

  FIG. 6 is a diagram illustrating an example of a patch image. The patch image in the gradation control process is desirably an image having a plurality of gradation levels. For example, as shown in FIG. 6 (a), a patch image Ip1 composed of a plurality of patch image pieces having different gradation levels, or as shown in FIG. 6 (b), a gradation level in a continuous image. It is possible to use a patch image Ip2 that gradually changes. In the example of FIG. 6A, the patch pieces may be formed so as to contact each other. When the patch image shown in FIG. 6A is used, the following should be further considered.

  FIG. 7 is a diagram showing the relationship between the input gradation level and the toner amount constituting the image. As described above, there is a non-linear relationship between the input gradation level and the toner amount constituting the image that is actually visualized. Under such a relationship, the tone level of each patch piece is carefully selected in order to accurately estimate the overall curve from the toner amount detection results of patch images formed typically at several tone levels. There is a need. That is, in the region where the slope of the curve is small in the relationship between the gradation level and the toner amount, the step of the gradation level may be made relatively coarse, whereas in the region where the slope is large, it is desirable to make the step small. More specifically, since the shape of the approximate curve can be predicted to some extent, the toner amount of each patch piece indicated by the predicted curve is distributed at approximately equal intervals on the toner amount axis of FIG. It is desirable to determine the gradation level of each patch piece. Furthermore, since the shape of the curve also varies depending on the type of screen, it is desirable that the setting value of the gradation level when forming each patch piece is determined individually according to the type of screen.

  For example, when a patch image Ip1 composed of four patch pieces is formed using a screen having the characteristic shown by the curve P in FIG. 7, the intervals between the toner amounts T1, T2, T3, and Tmax of the patch pieces are approximately equal. It is desirable that the patch pieces are formed at the gradation levels L1, L3, L5, and Lmax so as to be equal. Similarly, when a patch image is formed using a screen having the characteristics shown by the curve Q in FIG. 7, it is desirable that the respective patch pieces are formed at the gradation levels L2, L4, L6, and Lmax.

  Returning to FIG. 5, the description of the gradation control process will be continued. The signal output from the density sensor 60 is sampled for each gradation level of the patch image thus formed (step S102). Since noise may be mixed in these sampling data, appropriate data correction processing is performed (step S103). This data correction is a process for suppressing the influence of noise on the result, but since it is not essential in the present invention, a detailed description thereof will be omitted.

  What is output from the density sensor 60 is a signal whose level changes in accordance with the amount of light emitted from the intermediate transfer belt 71 opposed to the sensor 60. That is, the amount of light emitted from the surface of the intermediate transfer belt 71 with no toner attached is the largest, and the amount of light decreases as the amount of toner covering the surface increases. Therefore, the output level of the density sensor 60 is the highest when no toner adheres to the intermediate transfer belt 71, and generally decreases as the amount of adhered toner increases (in the case of black toner). However, in the case of color toner, the output level may increase on the contrary when the toner amount is large. On the other hand, the patch image composed of the toner attached to the intermediate transfer belt 71 in this manner has a higher density as the toner amount increases. As described above, it is difficult to say that the level of the output signal from the density sensor 60 directly represents the density of the patch image.

Therefore, in this embodiment, an “evaluation value” of the patch image described below is introduced as a numerical value that more directly indicates the density of the patch image (step S104). This evaluation value is a value that is zero when the density of the patch image is zero, that is, when there is no toner, and increases as the amount of toner as the patch image increases. Specifically, for example, the evaluation value G can be calculated by the following formula:
G = 1− (Ps−Pmin) / (Pmax−Pmin)
Here, the symbol Ps is sampling data (after data correction) for the patch image. Symbol Pmax is sampling data for the surface of the intermediate transfer belt 71 to which no toner is attached, and is the largest value as the sensor output. Pmin is sampling data in a state where the intermediate transfer belt 71 is completely covered with toner, and is a minimum value as a sensor output.

  Under such a definition, the evaluation value G is zero because the sampling data Ps is the maximum value Pmax when no toner is attached to the intermediate transfer belt 71. As the amount of toner increases, the sampling data Ps decreases, so the evaluation value G increases. When the toner completely covers the intermediate transfer belt 71, the sampling data Ps becomes the minimum value Pmin, and the evaluation value G is 1. It becomes. That is, by using the evaluation value G defined in this way, the patch image density is expressed as a normalized value from 0 corresponding to the minimum density to 1 corresponding to the maximum density.

  The “image density” of the patch image here accurately indicates the amount of toner constituting the image, and as described above, as a result of the transfer and fixing of the patch image to the sheet S. It is not the same as the density of the image on the obtained sheet S. The reason is as follows. In the patch image on the intermediate transfer belt 71, the toner particles constituting the image are merely attached on the intermediate transfer belt 71 by electrostatic force. On the other hand, in the image fixed on the sheet S, the toner is fused to the sheet S. Due to the difference in the state of toner constituting the image, the density detection result in the toner image temporarily held on the intermediate transfer belt 71 does not necessarily match the final image density on the sheet S. do not do.

  The ultimate purpose of gradation correction is to optimize the quality of the image formed on the sheet S. For this purpose, the processing content of the gradation correction processing is preferably determined by feeding back the density detection result of the patch image on the sheet S. However, forming a patch image on the sheet S for this purpose consumes an extra sheet S and additionally provides a means for detecting the patch image density after being fixed on the sheet S. It is not realistic because it is necessary to provide it. In an image forming apparatus equipped with a scanner for reading an original such as a copying machine or a multifunction machine, the scanner can be used for such a purpose, but the sheet S on which a patch image is formed can be read by the scanner. It is necessary to transport to the correct position.

  Therefore, in this image forming apparatus, using the “transfer fixing characteristic” obtained in advance, the image density on the sheet S (optical density; optical density; based on the patch image density detection result (evaluation value) on the intermediate transfer belt 71. (OD value), the image density on the sheet S is substantially fed back to the gradation correction process. That is, as shown in FIG. 5, the evaluation value G of the patch image on the intermediate transfer belt 71 obtained previously is converted into an OD value on the sheet S (step S105), and the OD value is converted to the correction table calculation unit 119. provide feedback. Then, the gradation correction table 119 performs a predetermined calculation so that the gradation level represented by the input image signal matches the actual gradation level of the image on the sheet S. The contents of 118A and 118B are updated (step S106). As described above, by performing the gradation control process and updating the gradation correction tables 118A and 118B as needed, it is possible to stably form an image with good image quality regardless of fluctuations in apparatus characteristics. .

  The “transfer fixing characteristic” is a value detected on the intermediate transfer belt 71 as a value representing the amount of toner constituting a toner image (in this embodiment, an “evaluation value” corresponds to this, but for example, sampling This is a characteristic that quantitatively represents the relationship between the data itself and the image density (optical density) when the toner image is fixed on the sheet S.

  FIG. 8 is a diagram for explaining the principle of gradation correction processing in this image forming apparatus. As described above, in this image forming apparatus, the main controller 11 performs predetermined signal processing (including gradation correction processing) on an image signal given from an external device. The signal subjected to the signal processing is given to the exposure unit 6 as an exposure control signal via the engine controller 10, and the exposure unit 6 forms an electrostatic latent image on the photoconductor 22 in accordance with the signal. The electrostatic latent image is visualized as a toner image by the developing unit 4, and the toner image is transferred and fixed onto the sheet S via the intermediate transfer belt 71.

  The density sensor 60 detects the density (exactly the amount of toner) of the patch image carried on the intermediate transfer belt 71. In the conventional tone correction technology of the image forming apparatus, the toner amount detection result on the intermediate transfer belt detected by the density sensor is fed back to the main controller. In other words, the conventional gradation correction technique is configured to correct the gamma characteristic of the engine unit up to the intermediate transfer belt. In such a method, since the transfer and fixing characteristics from the intermediate transfer belt to the sheet are not considered, the difference between the toner amount on the intermediate transfer belt and the image density on the sheet cannot be corrected. The image quality above is not always the best.

  In contrast, in the present embodiment, the toner amount on the intermediate transfer belt 71 detected by the density sensor 60 is converted into an OD value on the sheet S and then fed back to the main controller 11. This is substantially equivalent to feeding back the density detection result on the sheet S. By doing so, the gradation correction processing in the main controller 11 is processing contents that comprehensively correct both the gamma characteristic of the engine unit EG and the transfer fixing characteristic from the intermediate transfer belt 71 to the sheet S. It will have. As a result, in this embodiment, the best image quality on the sheet S can be obtained.

  Specifically, as shown in FIG. 3, OD value conversion tables 124A and 124B in which transfer and fixing characteristics are tabulated are provided in the engine controller 10, and the gradation characteristic detecting unit 123 is based on the output of the density sensor 60. The corresponding OD value is obtained by referring to this table 124A or 124B from the obtained toner amount (evaluation value) of the patch image. Since the transfer and fixing characteristics are slightly different depending on the screen to be used, in this embodiment, two OD value conversion tables 124A and 124B are provided corresponding to each of the two types of screens A and B, and are switched. The table is appropriately switched by the switching means 203 linked with the means 200 to 202. For example, the difference in transfer and fixing characteristics can be considered as follows.

  FIG. 9 is a diagram showing the relationship between screen pitch and image density. More specifically, FIG. 9A schematically shows an image pattern and its toner amount when an image having a gradation level of 50% is formed using a screen with a coarse pitch. FIG. 9B shows a case where the image is formed with a fine pitch screen. When an image having the same gradation level is formed, if a screen having a coarse pitch is used, relatively wide dots that are discretely arranged at intervals corresponding to the screen pitch, as shown in FIG. It is formed.

  On the other hand, when a screen with a fine pitch is used, as shown in FIG. 9B, the pitch between dots is narrow, and the width of each dot is narrow. In such an image formed with a fine pitch, toner scattering tends to occur when the image is transferred from the intermediate transfer belt 71 to the final sheet S, which is a recording material. That is, the toner distribution is local on the intermediate transfer belt 71 as shown by the solid line in FIG. 9B, but in the state after being transferred and fixed on the sheet S, the toner distribution in FIG. As indicated by the broken line, the dot width becomes wider. On the other hand, in an image (FIG. 9A) formed using a screen with a coarse pitch, the dot width is small and the number of lines is originally small, so the change of the toner scattering on the dot width is small. Due to such a difference in phenomenon, it is considered that the transfer and fixing characteristics differ depending on the type of screen used.

  Actually, according to the experiment of the present inventor, in an image formed using a fine pitch screen, the dot width on the sheet S is wider than the dot width on the photosensitive member 22 or the intermediate transfer belt 71, A phenomenon has been observed in which the image density on the sheet S is higher than an image of the same gradation level formed using a screen with a coarse pitch. Therefore, providing an OD value conversion table for each type of screen is extremely useful for obtaining excellent gradation reproducibility regardless of the screen to be used.

  FIG. 10 is a diagram showing an example of the OD value conversion table. In the actual OD value conversion tables 124A and 124B, the characteristic curve shown in FIG. 10 is stored in the ROM 106 as a numerical table. As shown in FIG. 10, there is not much difference in transfer and fixing characteristics between the two screens in the high density region, but the same toner detected on the intermediate transfer belt 71 is in the screen A having a fine pitch in the low density region. The OD value conversion table is created so that the image density on the sheet with respect to the amount increases.

  Further, it is desirable to prepare a plurality of sets for the OD value conversion tables 124A and 124B in advance, and select one of them according to the operation history of the apparatus. The reason will be described below.

  FIG. 11 is a diagram showing an example of transfer and fixing characteristics. As shown in FIG. 10, the transfer / fixing characteristic can be expressed as a transfer / fixing characteristic curve on a coordinate plane with the toner amount (evaluation value) on the intermediate transfer belt and the OD value on the sheet as axes. This transfer and fixing characteristic curve varies depending on the operation history of the apparatus. For example, the particle size distribution and charging characteristics of the toner in the developing device are different between a state where there is a sufficient amount of toner in the developing device (solid line) and a state where the remaining amount of toner is low (broken line), As a result, the transfer and fixing characteristic curves are also different. Also, the transfer and fixing characteristic curve varies depending on the degree of deterioration of the photosensitive member 22 and the developing roller 44.

  As described above, the transfer fixing characteristic curve varies depending on the operation history of the apparatus, causing the following problems. That is, even if the toner amount detected in the toner image on the intermediate transfer belt 71 is the same, the image density as a result of fixing the toner image on the sheet S differs depending on the operation history of the apparatus. May end up. In the example of FIG. 11, when the evaluation value of the patch image on the intermediate transfer belt 71 is G1, the image density when this patch image is fixed on the sheet S is OD1 depending on the remaining amount of toner in the developing device. Will change from OD2 to OD2. Therefore, if the content of the conversion process from the evaluation value to the OD value is always the same, it is not possible to cope with the fluctuation of the transfer fixing characteristic, and as a result, the image quality on the sheet S is not stable. Arise.

  Therefore, assuming that the transfer and fixing characteristics fluctuate according to the operation history of the apparatus, prepare several candidates for the OD value conversion table in advance, and when converting the evaluation value to the OD value, If one of the most suitable transfer fixing characteristics estimated from the operation history of the apparatus at that time is selected and referred to, such a problem can be solved. Alternatively, only one type of table may be set in advance, and the contents may be corrected and updated as needed according to the operation history of the apparatus.

  Further, as an index for judging the operation history of the apparatus, the usage time of the photosensitive member 22 and the usage time of the developing device 4Y can be used in addition to the toner remaining amount in the developing device. In addition, even if the remaining amount of toner is the same, the characteristics change due to toner deterioration, so the rotation time of the developing roller 44 may be used as a parameter representing the degree of toner deterioration. These parameters may be used in appropriate combination.

  Furthermore, the degree of deterioration of the apparatus may be determined based on the density detection result of the test image formed under specific image forming conditions. For this purpose, the density detection result for at least a part of the patch image described above may be used.

  As described above, in this embodiment, the toner amount detection result of the patch image on the intermediate transfer belt 71 obtained from the output signal of the density sensor 60 is used as the image density (on the sheet S) using a prepared conversion table. OD value), and gradation correction processing content is determined based on the result. Therefore, gradation correction is performed to correct the gamma characteristics of the entire apparatus including the characteristics of only the engine unit EG and the characteristics of transfer and fixing to the sheet S. As a result, the image forming apparatus Then, the image quality in the sheet S can be improved.

  Further, gradation correction tables 118A and 118B are provided for each screen to be used, and the contents thereof are based on the detection results of patch images formed using the corresponding screens and the corresponding OD value conversion tables 124A and 124B. By updating as needed, gradation correction can be performed in a preferable mode according to the state of the apparatus and according to the screen used, and an image can always be formed with good gradation reproducibility.

  Further, by changing the OD value conversion tables 124A and 124B according to the operation history of the apparatus, it is possible to cope with the change in the transfer and fixing characteristics according to the operation history of the apparatus. Therefore, it is possible to stably obtain good image quality.

  Further, as described in the present embodiment, by converting the output of the density sensor 60 into an on-sheet OD value by the engine controller 10 and transmitting it to the main controller 11, the following effects can be obtained. That is, the image density (OD value) defined on the sheet S is uniquely determined from the reflectance of light and the like, and is a value that does not depend on the configuration of the apparatus that forms the image density. As described above, the parameters independent of the hardware configuration of the apparatus are given, so that the signal processing content in the engine controller 11 can be determined regardless of the hardware configuration of the apparatus. This not only simplifies the processing content in the main controller 11, but also makes it possible to apply signal processing with the same content to a device having an engine part having a different configuration, resulting in the development and development of the device. It can also contribute to reduction of maintenance costs. On the other hand, the toner amount detection result on the intermediate transfer belt 71 changes depending on the characteristics of the toner used, the density sensor, and the like. Therefore, the contents of the signal processing based on this need to be changed according to the configuration of the apparatus.

  As described above, in this embodiment, the main controller 11 and the engine unit EG function as “signal processing means” and “image forming means” of the present invention, respectively. Further, the intermediate transfer belt 71 and the density sensor 60 function as the “intermediate transfer medium” and “detecting means” of the present invention, respectively. In addition, the gradation characteristic detection unit 123 and the OD value conversion tables 124A and 124B function as a “conversion unit” of the present invention. In the present embodiment, the sheet S corresponds to the “recording material” of the present invention. Furthermore, in the present embodiment, the exposure control signal given from the main controller 11 to the engine unit EG via the engine controller 10 corresponds to the “image formation control signal” of the present invention.

(Second Embodiment)
Next, a second embodiment of the image forming apparatus according to the present invention will be described. The configuration of the apparatus of this embodiment is the same as that of the first embodiment described above. The basic operation is also the same as that in the first embodiment. However, in the first embodiment, gradation control processing (FIG. 5) is performed individually for each processing screen, whereas in the second embodiment, the overall operating conditions of the apparatus are adjusted and each processing screen is selected. The gradation control process is performed as a series of processes.

  FIG. 12 is a flowchart showing a control process in the second embodiment. This process is executed at a predetermined timing such as immediately after the apparatus is turned on, after returning from sleep, or immediately after any unit is replaced. In this process, first, the development bias is optimized (step S201). More specifically, a solid image as a patch image is formed with each developing bias while changing and setting the developing bias in multiple stages. Then, based on the density detection results, an optimum value of the developing bias at which the solid image density becomes a predetermined target density is calculated.

  Next, the exposure power is optimized (step S202). Specifically, the development bias is fixed to the optimum value described above, and a line image composed of, for example, 1 on 10 off isolated one dot line is formed as a patch image at each exposure power while changing and setting the exposure power in multiple stages. To do. Then, based on those density detection results, an optimum value of exposure power at which the line image density becomes a predetermined target density is calculated.

  The above-described solid image and line image have simple and known image patterns, and are not subjected to halftoning by screen processing. Therefore, on the recording material from the sampling result by the density sensor 60 on the intermediate transfer belt 71. The image density can be easily estimated. Therefore, in the above process, the sampling result by the density sensor 60 does not have to be converted into the image density on the recording material. In other words, if the output target value of the density sensor corresponding to the target image density is determined and the optimum values of the developing bias and the exposure power are obtained so that the output of the density sensor 60 becomes the target value, the substantial value is substantially obtained. In other words, an operation condition is required in which the image density matches the target density.

  When the operating conditions of the apparatus are determined in this way, the following processing is performed under the optimum developing bias and optimum exposure power that are obtained. Subsequently, gradation control processing for determining gradation correction characteristics for each screen is performed (steps S203 to S209). First, one of the two screens (screen A) is applied to form the gradation patch image Ipa (step S203), and then the other screen (screen B) is applied to form the gradation patch image Ipb. (Step S204). Both gradation patch images are formed as follows.

  FIG. 13 is a diagram showing an image pattern of a gradation patch image in the second embodiment. FIG. 14 is a diagram showing a gradation patch image on the intermediate transfer belt. In the gradation control process, the CPU 111 provided in the main controller 11 generates image data corresponding to the image pattern Ipp shown in FIG. As shown in FIG. 13, the image pattern Ipp includes a pattern corresponding to the solid image portion Ps having the maximum gradation value (255), and a gradation image portion in which the gradation value is decreased by, for example, 4 at a predetermined pitch. It consists of a pattern corresponding to Pg. The length Ls of the solid image portion Ps is set to about the circumference of the developing roller 44 provided in each developing device 4Y, for example.

  The image data thus generated is subjected to screen processing by each of the two types of processing screens, and the processed data is sent to the engine unit EG. The engine unit EG forms a gradation patch image corresponding to the received data. At this time, the original image pattern Ipp is the same, but as a result of the screen processing having different gradation expression characteristics, the density change of the formed patch image is exemplified by the curves 13a and 13b in FIG. However, it will differ depending on the processing screen applied. That is, the density change curve (density profile) of each gradation patch image represents the gradation expression characteristic of each processing screen.

  As a result of forming the patch image in this way, as shown in FIG. 14, the gradation patch images Ipa and Ipb formed by applying the screen A and the screen B, respectively, on the intermediate transfer belt 71 are transferred intermediately. The belts 71 are arranged along the moving direction D2. As the intermediate transfer belt 71 moves, each patch image moves in the direction D2 with the solid image portion Ps at the head.

  The density sensor 60 samples each of the gradation patch images Ipa and Ipb thus formed on the intermediate transfer belt 71 (step S205). It is the solid image portion Ps that is sampled first among the patch images. The sampling result here can be used as follows, for example. The first utilization method is to verify the result of the above-described development bias and exposure power optimization (steps S201 and S202). As a result of the above processing, the solid image density should be controlled to the target density, but the final image density is not actually confirmed. Concentration may not be achieved. Therefore, by examining the sampling result in the solid image portion Ps, it is possible to verify whether the development bias and the exposure power are set appropriately. If an abnormal result is obtained, appropriate error processing such as stopping the operation of the apparatus or notifying the user can be performed.

  The second usage is to use as a reference for confirming the sampling position by the density sensor 60. The purpose of the gradation correction process is to properly maintain the relationship between the gradation value represented by the image data and the actual image density. Therefore, in the gradation control process, it is necessary to correctly grasp the correspondence between the sampling result at each point of the gradation patch image and the gradation value given to that point. Therefore, if the top portion of the gradation patch image is set as the solid image having the highest density, the output of the density sensor 60 will change abruptly when the gradation patch image reaches the position facing the density sensor 60. Therefore, by performing sampling on the basis of the time point, the sampling result and the sampling position can be easily and reliably associated with each other.

  A third utilization method is to improve the detection accuracy of the concentration profile. The density of the halftone image is evaluated on the basis of the solid image density. Specifically, by normalizing the sampling result in the gradation image portion Pg with the sampling result in the solid image portion Ps, the gradation value and the image density are Correspondence can be obtained more accurately.

  As with the first embodiment, the sampling result of each gradation patch image is subjected to a predetermined data correction process and converted into an evaluation value (steps S206 and S207). Then, the evaluation value obtained for each gradation patch image is converted into an image density (OD value) on the recording material (step S208). At this time, the conversion table 124A corresponding to the applied screen A is used for the gradation patch image Ipa. For the gradation patch image Ipb, the conversion table 124B corresponding to the applied screen B is used. Then, based on the respective conversion results, the contents of the gradation correction tables 118A and 118B corresponding to the respective screens are updated (step S209).

  As described above, in the second aspect of the image forming apparatus according to the present invention, adjustment of the general operating conditions of the apparatus and determination of gradation correction characteristics corresponding to each processing screen are executed as a series of processes. . By executing this series of processing at an appropriate timing, the image forming apparatus performs an appropriate gradation correction process according to the type of the processing screen in a normal image forming operation, so that the image quality is good. Can stably form an image. At this time, the sampling result by the density sensor 60 for the gradation patch image is processed after being converted into an OD value by a conversion table suitable for the processing screen used, so that the quality of the image on the recording material is improved. Can be the best. In this embodiment, the main controller 11 and the engine controller 10 function together as “control means” of the present invention.

  In the process of FIG. 12, the process of step S202 cannot be started until the optimum value of the developing bias is obtained in step S201. This is because the process in step S202 needs to be performed under the optimum development bias obtained in step S201. In order to obtain the optimum developing bias, it is necessary to wait until the formed solid patch image is conveyed to the position facing the density sensor 60 and a sampling result is obtained, which increases the processing time. The same applies when shifting from the calculation of the optimum exposure power to the gradation control process.

  However, when performing the above processing for a plurality of toner colors, for example, if the processing is performed in the following order, the processing can be completed in a short time. That is, when all the series of patch images are formed for one toner color, the developing device is switched immediately to form a patch image with the next toner color. In this way, it is possible to form a patch image of another toner color by effectively utilizing the time until the patch image is transported to the position facing the density sensor 60, sampled, and subjected to calculation processing. Therefore, the processing time can be significantly shortened compared with the case where the processing of FIG. 12 is executed in order for each color.

  In gradation control processing, processing for each screen is performed individually, and there is no need to wait for the progress of processing on other screens, so gradation patch images Ipa and Ipb corresponding to each screen for one toner color. May be formed continuously. That is, it is not necessary to switch the developing device during the formation of the gradation patch images Ipa and Ipb.

  The present invention is not limited to the above-described embodiment, and various modifications other than those described above can be made without departing from the spirit of the present invention. For example, in the above-described embodiment, the evaluation value (toner amount) for the patch image obtained based on the output from the density sensor 60 is converted into the OD value on the sheet S using the OD value conversion table. The conversion to the OD value may be performed by a calculation that does not use a table, for example, a mathematical expression representing the correspondence between the toner amount and the OD value. Alternatively, the step of converting to an OD value may be omitted, and the main controller 11 may determine the content of gradation correction directly from the output (or evaluation value) of the density sensor 60 and the transfer fixing characteristic.

  Further, for example, in the above-described embodiment, the density sensor 60 is configured to detect the toner amount of the toner image carried on the intermediate transfer belt 71. However, other than this, the density sensor 60 may be configured to detect the toner amount on the photosensitive member 22. In this case, the photosensitive member 22 corresponds to the “intermediate transfer medium” of the present invention, and accordingly, the “transfer fixing characteristic” of the toner on the photosensitive member 22 passes through the intermediate transfer belt 71 and the recording material. It is defined as a characteristic until it is transferred and fixed to the sheet S.

  The present invention is not limited to the image forming apparatus having the intermediate transfer belt 71 described above, but also includes an apparatus having another intermediate transfer medium such as a transfer drum and a transfer roller, or a toner directly from a photosensitive member to a recording material without an intermediate transfer belt. The present invention can also be applied to an apparatus configured to transfer an image. Even in these cases, the transfer and fixing characteristics between the medium where the toner amount is detected and the final recording material are obtained in advance, and gradation correction is performed based on the toner amount detection result and the transfer and fixing characteristics. As a result, it is possible to obtain the same effect, and as a result, it is possible to obtain a good image quality on the recording material.

  Further, in the configuration shown in FIG. 3, the gradation correction unit 115 and the halftoning unit 116 are described as separate functional blocks, but these may be integrated into a functional block. In this case, the first processing table in which the processing characteristics of the screen A and the characteristics of the gradation correction table 118A corresponding to the screen are integrated, the processing characteristics of the screen B, and the gradation correction table corresponding to the screen. A second processing table in which the characteristics of 118B are integrated may be provided, and one of them may be appropriately selected according to the contents of the image to be formed and used for signal processing. The aspect of the gradation control process in this case is “to correct the processing characteristics for each screen tabulated in advance by the density detection result of the patch image”.

  In the above-described embodiment, the present invention is applied to an apparatus that forms an image using toners of four colors of yellow, magenta, cyan, and black. However, the types and number of toner colors are limited to the above. It is not a thing but arbitrary. In addition to the rotary development type apparatus as in the present invention, the so-called tandem type image forming apparatus in which developing units corresponding to the respective toner colors are arranged in a line along the sheet conveying direction. The present invention is also applicable. Furthermore, the present invention is not limited to the electrophotographic apparatus as in the above embodiment, but can be applied to all image forming apparatuses.

1 is a diagram illustrating a first embodiment of an image forming apparatus according to the present invention. FIG. FIG. 2 is a block diagram showing an electrical configuration of the image forming apparatus in FIG. 1. FIG. 3 is a diagram illustrating a gradation processing block of the image forming apparatus. The figure which shows the example of the screen for a halftoning process. The flowchart which shows a gradation control process. The figure which shows the example of a patch image. FIG. 4 is a diagram illustrating a relationship between an input gradation level and an amount of toner constituting an image. FIG. 3 is a diagram for explaining the principle of gradation correction processing in this image forming apparatus. The figure which shows the relationship between the pitch of a screen, and image density. The figure which shows the example of an OD value conversion table. FIG. 6 is a diagram illustrating an example of transfer fixing characteristics. The flowchart which shows the control processing in 2nd Embodiment. The figure which shows the image pattern of the gradation patch image in 2nd Embodiment. FIG. 4 is a diagram illustrating a gradation patch image on an intermediate transfer belt.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Engine controller (control means) 11 ... Main controller (signal processing means, control means), 22 ... Photoconductor, 60 ... Density sensor (detection means), 71 ... Intermediate transfer belt (intermediate transfer medium), 123 ... Floor Tonal characteristic detection unit (conversion unit), 124A, 124B ... OD value conversion table (conversion unit, conversion table), EG ... engine unit (image forming unit), S ... sheet (recording material)

Claims (11)

In an image forming apparatus for forming an image according to an image signal on a recording material,
Signal processing means for performing image processing on the image signal by performing signal processing including screen processing using one of a plurality of different types of processing screens, and generating an image forming control signal;
Having an intermediate transfer medium, forming a toner image corresponding to the image forming control signal and carrying it on the intermediate transfer medium, and forming the image by transferring and fixing the toner image on the recording material An image forming means;
Detecting means for detecting the amount of toner carried on the intermediate transfer medium as the toner image;
The signal processing by the signal processing means includes a detection result by the detection means for a toner image as a patch image formed using the processing screen, and the intermediate obtained in advance corresponding to the processing screen. An image forming apparatus comprising a gradation correction process based on a transfer fixing characteristic from a transfer medium to the recording material.
Here, the transfer and fixing characteristics are the amount of toner constituting the toner image carried on the intermediate transfer medium, and the image density on the recording material after the toner image is transferred and fixed on the recording material. It is a characteristic that represents the relationship.   The image according to claim 1, wherein the signal processing unit performs the gradation correction processing based on a detection result by the detection unit for each of a plurality of regions formed at different gradation levels in the toner image. Forming equipment.   The image forming unit forms a toner image composed of a plurality of image areas with different gradation levels as the patch image, and the gradation level setting value for each image area depends on a processing screen to be used. The image forming apparatus according to claim 1, wherein the image forming apparatus is set as described above. A conversion means for converting the toner amount on the intermediate transfer medium detected by the detection means into an image density on the recording material based on the transfer and fixing characteristics;
The image forming apparatus according to claim 1, wherein the signal processing unit performs the gradation correction processing based on a conversion result obtained by the conversion unit. Control means for executing control processing for determining processing characteristics of the gradation correction processing by the signal processing means based on a detection result by the detection means for the toner image as the patch image formed by the image forming means Further comprising
The image forming apparatus according to claim 1, wherein the signal processing unit executes the gradation correction process with a processing characteristic determined by the control process. A conversion means for converting the toner amount on the intermediate transfer medium detected by the detection means into an image density on the recording material based on the transfer and fixing characteristics;
The image forming apparatus according to claim 5, wherein the control unit determines the processing characteristic based on a conversion result by the conversion unit in the control process. The image forming apparatus according to claim 6, further comprising a plurality of the conversion units provided corresponding to each of the plurality of processing screens. In the control process,
The image forming unit sequentially forms a plurality of patch images to which the plurality of processing screens are respectively applied;
The control means converts the detection result by the detection means for each of the plurality of patch images by the conversion means corresponding to the processing screen applied to the formation of the patch image, and based on the result, The image forming apparatus according to claim 7, wherein the processing characteristics corresponding to each of the plurality of processing screens are determined. Each of the patch images is formed with an image pattern in which gradation values gradually increase or decrease along a predetermined direction on the surface of the intermediate transfer medium,
The detection means detects the amount of toner at a plurality of locations different from each other in the predetermined direction in the patch image,
The image forming apparatus according to claim 8, wherein the control unit determines the processing characteristics based on detection results at each of the plurality of locations.   The image forming apparatus according to claim 4, further comprising a conversion table for converting the toner amount on the intermediate transfer medium to the image density on the recording material as the conversion unit. A signal processing step of performing image processing on the image signal by performing signal processing including screen processing by selectively using one of a plurality of different types of processing screens; and
An image forming step of forming a toner image according to the image forming control signal and carrying the toner image on an intermediate transfer medium;
A transfer fixing step of transferring and fixing the toner image on the intermediate transfer medium onto a recording material,
In the signal processing step,
A detection result of a toner amount constituting a toner image as a patch image formed using the processing screen to be used and carried on the intermediate transfer medium, and the intermediate transfer obtained in advance corresponding to the processing screen An image forming method, wherein the signal processing including gradation correction processing based on transfer and fixing characteristics from a medium to the recording material is performed on the image signal.
Here, the transfer and fixing characteristics are the amount of toner constituting the toner image carried on the intermediate transfer medium, and the image density on the recording material after the toner image is transferred and fixed on the recording material. It is a characteristic that represents the relationship.

Images