JP4900902B2 - Image forming apparatus and density control method - Google Patents

Description

  The present invention relates to an image forming apparatus and a density control method for detecting the density of a density detection image pattern formed during an image forming operation and performing density control based on the detected density.

  Conventionally, an electrophotographic color image forming apparatus is known as an image forming apparatus. The color image forming apparatus has a mechanism for adjusting the density of an output image when a full color image is output. In particular, when a high-quality color image is output, a mechanism for performing gradation correction (γ correction) is added to stabilize the density.

  The density detection of the output image is performed by forming a toner image having a specific halftone pattern on the photosensitive drum and measuring the amount of reflected light from the halftone pattern with a density sensor. That is, the reflected light amount is measured by the density sensor, and tone correction (γ correction) that is estimated to obtain a predetermined density (reflected light amount) is obtained.

  As the density sensor, a reflected light amount sensor including a light emitting element and a light receiving element is used. As the reflected light amount sensor, a sensor that uses infrared light and can estimate the amount of toner on the photosensitive drum irrespective of the color of the toner is used. The amount of infrared light detected by the reflected light amount sensor is substantially proportional or inversely proportional to the amount of adhered toner (toner amount of the developed image). The amount of adhered toner and the density of the output image are generally not proportional, but have a one-to-one correlation, so that the density of the output image can be estimated from the measurement value of the reflected light amount sensor.

  Such density detection operation is desirably performed during a normal image forming operation without temporarily suspending the image forming operation. For this reason, control is performed so that density detection is performed by forming a halftone pattern outside the image forming area, for example, between images. However, since the area outside the image forming area is limited, it is necessary to separate these in order to form a plurality of halftone patterns. When forming a plurality of halftone patterns for one density sensor, it is necessary to form a plurality of halftone patterns along the rotation direction of the photosensitive drum. In this case, the interval between the images on the photosensitive drum becomes wide. It is not preferable.

  In contrast, Patent Document 1 discloses a concentration control method described below. A halftone pattern is divided into a plurality of times and formed in a non-image area, for example, an area sandwiched between the leading edge and the trailing edge of an image to be formed. Then, density detection is performed, and data correction is performed in density control and gradation control based on the detection result. Thereby, even when images are continuously formed, it is possible to form a plurality of halftone patterns without causing a temporary pause.

JP 2001-109219 A

  However, the conventional image forming apparatus has the following problems. FIG. 17 is a diagram showing image forming timings of a conventional halftone pattern formed by dividing a plurality of times and an image. The number (No.) in the figure represents the type of image processing (for example, the number of patterns used for dithering) used as a method for reproducing a halftone image, and the image processing corresponding to each number is shown in the figure. A halftone pattern (patch) 101 formed in (1) is shown. FIG. 3A shows a case where a halftone pattern with one type of image processing is formed. FIG. 4B shows a case where a halftone pattern having a plurality of types (here, three types) of image processing is divided and formed. When the image processing types are a plurality of types of halftone patterns, the formation frequency for each image type decreases, and the execution interval increases. For this reason, image density fluctuation due to control delay has occurred.

  SUMMARY An advantage of some aspects of the invention is that it provides an image forming apparatus and a density control method capable of suppressing an image density fluctuation due to a control delay caused by a decrease in the formation frequency for each type of image processing.

In order to achieve the above object, an image forming apparatus according to the present invention includes density detecting means for detecting the density of a density detection image pattern formed during an image forming operation, and density control based on the detected density. In an image forming apparatus provided with density control means for performing, for each type of dither for performing gradation processing, integrated value detection means for detecting an integrated value of video count values and the density detection image pattern are formed. An image pattern forming unit that forms the density detection image pattern using a dither having the largest value among the integrated values of the detected video count values for each type of the detected dither when the timing is reached. It is characterized by having.

In order to achieve the above object, according to the density control method of the present invention, the density of the density detection image pattern formed during the image forming operation is detected by the density sensor, and density control is performed based on the detected density. In the density control method, for each type of dither for performing gradation processing, the step of detecting the integrated value of the video count value and the detection of the detected value in accordance with arrival of the timing for forming the density detection image pattern Forming the image pattern for density detection using a dither having the largest value among the integrated values of the video count values for each type of the dither .

According to the image forming apparatus according to the present onset bright, it detects the toner usage accurately, by increasing the frequency of forming density detecting image pattern using more dithering of toner usage, image patterns for the concentration detection Image density fluctuation due to a control delay caused by a decrease in the formation frequency of the image can be suppressed.

  Embodiments of an image forming apparatus and a density control method according to the present invention will be described with reference to the drawings. The image forming apparatus of this embodiment is applied to a color image forming apparatus.

[First Reference Example]
FIG. 1 is a longitudinal sectional view showing an internal configuration of a color image forming apparatus according to a first reference example . The image forming apparatus 50 includes a photosensitive drum 1 that is an image carrier, and forms an image by an electrophotographic method. The photosensitive drum 1 is rotationally driven in the direction of arrow a in the figure, and its surface is uniformly charged by a primary charger (charging roller) 2 during the rotation process.

  Further, the exposure device (laser scanner) 3 emits laser light corresponding to the yellow image pattern as the first color, and an electrostatic latent image of the first color is formed on the surface of the photosensitive drum 1. Here, the laser light is generated according to the laser output signal. The magnitude of this laser output signal is determined by the LUT 25 described later. In the LUT 25 (see FIG. 3), a laser output signal that can be formed at a desired density is described based on the relationship between the laser output signal and the density level that are obtained in advance.

  The latent image formed on the photosensitive drum 1 by the laser light is developed by the developing device 4y containing yellow toner facing the photosensitive drum 1 as the photosensitive drum 1 rotates, and is visualized as a yellow toner image. The developing devices 4y, 4m, 4c, and 4k are supported by a rotary support (rotary drum) 5 and a predetermined developing device rotates and moves to a position facing the photosensitive drum 1 prior to development. The toner image obtained by the development is transferred (primary transfer) to the surface of an intermediate transfer belt (belt-shaped intermediate transfer body) 6 rotating in the direction of arrow b at substantially the same speed as the photosensitive drum 1. This primary transfer is performed by a primary transfer bias applied to the primary transfer roller 7 a inside the intermediate transfer belt 6. On the other hand, untransferred toner remaining on the photosensitive drum 1 is removed by the blade 8 (cleaning).

  The process including such charging, exposure, development, and primary transfer is performed for each color of magenta, cyan, and black after yellow. As a result, a color image obtained by superimposing toner images of four colors of yellow, magenta, cyan, and black is obtained on the intermediate transfer belt 6.

  The four color toner images superimposed on the intermediate transfer belt 6 are collectively transferred (secondary transfer) to the surface of the transfer material sent to the intermediate transfer belt 6 by the pickup roller 9 at a predetermined timing. This secondary transfer is performed by a secondary transfer bias applied to the secondary transfer roller 7 b inside the intermediate transfer belt 6. The transfer material onto which the four color toner images have been transferred is sent to the fixing device 11 by the conveyor belt 10. In the fixing device 11, the heated and pressurized toner is melted and fixed to the transfer material to form a final full color image.

  An operation unit 20 is provided on the upper portion of the color image forming apparatus 50. The operation unit 20 includes a liquid crystal operation panel (touch panel display) 20a with a touch sensor, mode setting buttons 20b such as “automatic gradation correction frequency” and “image processing selection”. The user can input various conditions such as the type of image, the number of sheets, and the automatic gradation correction frequency via the operation unit 20.

FIG. 2 is a diagram showing a configuration in the vicinity of the photosensitive drum 1 and the intermediate transfer belt 6. In this reference example , the density detection image pattern Q is formed in the non-image area sandwiched between the leading edge and the trailing edge of the image formed on the surface of the photosensitive drum 1 during continuous printing. The non-image area is not limited to the area sandwiched between the leading edge and the trailing edge of the image, and may be an area on the side of the image depending on the size of the image to be formed.

  The density of the density detection image pattern Q is detected by measuring the reflected light amount from the density detection image pattern Q with the reflected light amount sensor 12. The reflected light amount sensor 12 includes a light emitting element and a light receiving element, and detects only regular reflected light from the photosensitive drum 1. The amount of reflected light is measured at the timing when the density detection image pattern Q formed in the non-image area on the photosensitive drum 1 passes through the measurement range. Then, based on this reflected light amount, gradation correction (γ correction) that is estimated to obtain a desired constant density (reflected light amount) is required.

The toners used in this reference example are yellow, magenta, and cyan color toners in which color materials of each color are dispersed using a styrene copolymer resin as a binder. The photosensitive drum 1 is an OPC drum having a reflectance of near infrared light (960 nm) of about 40%, but may be an amorphous silicon photosensitive drum or the like as long as the reflectance is approximately the same.

  FIG. 3 is a block diagram showing the configuration of a signal processing circuit that processes the output signal of the reflected light amount sensor 12. This signal processing circuit is mainly composed of an A / D conversion circuit 15, a density conversion circuit 16 and a CPU 17. Reflected light (near infrared light) from the photosensitive drum 1 is converted into an electrical signal by a reflected light amount sensor (photosensor) 12. This electric signal has a voltage of 0 to 5 V, and is converted into an 8-bit digital signal by the A / D conversion circuit 15. Further, the 8-bit digital signal is converted into a density signal by the density conversion circuit 16. In the density conversion circuit 16, a relationship between an 8-bit digital signal (output signal) corresponding to the amount of reflected light and a density signal (image density) is registered as a table 16a.

  Further, the operation unit 20, ROM 18, RAM 19, I / O interface 21, LUT 25, etc. are connected to the CPU 17. The CPU 17 performs an image forming process to be described later according to a control program stored in the ROM 18.

  The operation of the color image forming apparatus having the above configuration will be described. Here, density detection and gradation control during continuous printing are shown. FIG. 4 is a graph showing the relationship between the output signal of the reflected light amount sensor 12 and the image density when the density of the density detection image pattern Q formed on the photosensitive drum 1 is changed stepwise by the area gradation of each color. is there. The output of the reflected light amount sensor 12 in a state where the toner is not attached to the photosensitive drum 1 is set to 5V, that is, 255 level. As the area coverage by each toner increases and the image density increases, the output of the reflected light amount sensor 12 decreases. By preparing a table dedicated to each color having such characteristics and converting the output of the reflected light amount sensor into a density signal, the density of each color can be read with high accuracy. As described above, the table dedicated to each color is registered in the density conversion circuit 16 as the table 16a.

  The laser output when forming the density detection image pattern Q corresponds to 64 levels of density for each color. At this time, as described above, the laser output signal is determined using the LUT 25.

In this reference example , during normal image formation, a density detection image pattern (patch) is formed in a non-image area, the density is detected, and the table data of the LUT 25 is corrected as needed. The table data of the LUT 25 used for laser output when forming a patch is equivalent to normal image formation at that time. That is, the result corrected by the control until the previous time is used as the table data of the LUT 25. FIG. 5 is a graph showing the characteristics of the LUT 25.

  Although the density signal of the density detection image pattern Q is corrected to 64 levels by the LUT 25, the image characteristics of the image forming apparatus are unstable and may always change. For this reason, the density of the measurement result is not always 64 levels. The table data of the LUT 25 is corrected based on the deviation amount ΔD between the density signal and the density of the measurement result. Here, the shift amount ΔD is a shift amount between the target value obtained from the density detection image pattern Q formed using the previous LUT 25 and the density obtained from the patch formed using the current LUT 25. .

FIG. 6 is a graph showing the characteristics of a general density signal correction table (basic LUT correction table) when the density shift of the density detection image pattern Q with respect to the 64-level density signal is 1. In this reference example , this basic LUT correction table is stored in the ROM 18 in advance. At the time of control, all density signals in the basic LUT correction table stored in the ROM 18 are calculated by ΔD times, and an LUT correction table corresponding to the deviation amount ΔD is created. Then, the table data of the γLUT correction table that cancels the characteristics (pattern) of the created LUT correction table is added to the table data of the LUT 25 to correct the LUT 25. The rewriting (correction) of the LUT 25 is performed at the timing when the creation of the LUT correction table corresponding to the shift amount ΔD is completed for each color.

  FIG. 7 is a flowchart showing an image output processing procedure accompanied with creation of an LUT correction table. This processing program is stored in the ROM 18 and is executed by the CPU 17 along with a normal image forming process. FIG. 7 shows control when an image is formed by one type of image processing.

  First, using the γLUT correction table obtained by the previous control, the table data of the LUT 25 is corrected according to Equation (1) (step S21). The correction of the table data is performed by adding the table data of the γLUT correction table created so as to cancel the characteristics of the previous LUT correction table to the table data of the LUT 25.

LUT = LUT + γLUT correction table (1)
Then, the correction result table data is set in the LUT 25 (step S22). Laser output is performed using the set LUT 25 to form an image (step S23). After the image is formed, a density detection image pattern Q is formed in a non-image area (between the images) on the photosensitive drum 1, which is an area between the trailing edge of the image and the leading edge of the next image. The density of the density detection image pattern Q is read by the density sensor 12 (step S24). A deviation amount ΔD between the read density and the target value density (64 levels) is calculated (step S25). An LUT correction table is created using the calculated deviation amount ΔD and the basic LUT correction table (see FIG. 6). Further, a γLUT correction table that cancels the characteristics of the LUT correction table is created (step S26).

  Thereafter, it is determined whether or not to continue the print job (step S27). If the print job is continued, the process returns to step S21. On the other hand, when ending the print job, this process is ended.

  As described above, FIG. 7 shows a case where image output is performed by correcting the LUT by one type of image processing. Next, a case where an image is output by correcting the LUT by a plurality of types of image processing will be described.

In the color image forming apparatus 50 of this reference example , it is possible to form an image subjected to a plurality of image processing. For example, in the image portion, low-line-number dither is used and importance is attached to halftone uniform stability. In the character part, a high line number dither is used to reproduce the edge part firmly.

This reference example shows a case where 133-line dither is used for the image portion, 190-line dither is used for the graphic portion, and 266-line dither is used for the character portion. In this reference example , dithering with the same number of lines for all colors is used, but there is no particular limitation, and dithering with different numbers of lines for each color may be used. In any case, multiple dithers are used for each color.

  Further, since the area between images (non-image area) on the photosensitive drum 1 is narrow, it is impossible to form the density detection image patterns Q for all the image processes for each color in the area between one image. Therefore, the density detection image pattern Q to be formed is divided and assigned to areas (non-image areas) between a plurality of images.

  For example, a density detection image pattern (patch) Q is formed with a 133-line dither used for an image portion of four colors between the first and second images that are continuously printed. A patch is formed between the second and third images with a 190-line dither used in the graphic portion. A patch is formed between the third and fourth images with a 266-line dither used for the character portion. Between the fourth and fifth images, a patch is formed again with 133-line dither used for the image portion. Similar operations are sequentially repeated.

In addition, the LUT is prepared for each dither (133 line, 190 line, 266 line) and controlled independently, but it is also controlled independently for each color. That is, in this reference example , a total of 12 LUTs consisting of 4 colors × 3 dithers are held, and 12 types of density detection image patterns Q are formed. Then, each shift amount (density change) ΔDn (n is a number representing a dither type different for each color) is calculated, and the LUT is corrected.

When the three types of dither density detection image patterns Q are sequentially formed, it may be inconvenient. That is, the frequency of dither control for each color is once every three times. Depending on the user, it is desirable that the frequency of dither patch formation is high for images where density and color stability are important. Therefore, in this reference example , the frequency of forming the density detection image pattern Q in each dither is variable.

  FIG. 8 is a diagram showing an input screen for automatic gradation correction frequency displayed on the touch panel display 20 a of the operation unit 20. When the operator presses the “automatic gradation correction frequency” mode setting button 20b provided on the operation unit 20, an input screen for automatic gradation correction frequency is displayed on the touch panel display 20a. On this input screen, the priority for each dither can be input, and the control frequency (frequency of forming the density detection image pattern Q) is determined according to the input priority. For example, if the priority is set to 10 for a 133 line dither, the priority is set to a value of 5 for a 190 line dither, and the priority is set to a value of 1 for a 266 line dither, The control frequency is as follows. That is, during the control of 16 (= 10 + 5 + 1) times, the control frequency of the 133 line dither is 10 times, the control frequency of the 190 line dither is 5, and the control frequency of the 266 line dither is 1.

  FIG. 9 is a flowchart showing a pattern formation frequency determination processing procedure. This processing program is stored in the ROM 18 and is executed by the CPU 17 prior to the start of the image forming process. First, it is determined whether or not the “automatic gradation correction frequency” mode setting button 20b has been pressed (step 31). If the mode setting button 20b has not been pressed, this processing is terminated as it is. On the other hand, when the mode setting button 20b is pressed, the automatic tone correction frequency input screen (see FIG. 8) is displayed on the touch panel display 20a (step S32). When the priority of control for each dither is input via the input screen by the operator, the priority of control for each dither that has been input is acquired (step S33). Based on the acquired priority, the control frequency for each dither is determined (step S34). The determined control frequency for each dither is stored in the RAM 19. Further, as described above, the control for each dither determined is 10 times for the 133-line dither control frequency, 5 times for the 190-line dither control frequency, and 1 time for the 266-line dither control frequency. If you want to. Then, this process is complete | finished.

  FIG. 10 is a flowchart showing an image forming process procedure involving a plurality of types of LUT correction. This processing program is stored in the ROM 18 and executed by the CPU 17. First, the control frequency (formation frequency of the density detection image pattern Q) stored for each dither stored in the RAM 19 is read (step S41). Based on the read control frequency for each dither, the order of forming the density detection image pattern Q for each dither and for each color and the total number of controls ALL for each dither are set (step S42). As the order of forming the density detection image pattern Q, for example, the order of formation corresponding to the control frequency for each dither may be determined in the order of Y, M, C, and K colors. Then, image output processing with LUT correction for each dither is performed (step S43), and this processing ends. It should be noted that the formation order commensurate with the control frequency for each dither and for each color can be arbitrarily set unless particularly biased. For example, the control order may be determined based on color, or the control order may be determined based on dither. Further, the control order may be determined regardless of the color and dither.

  FIG. 11 is a flowchart showing the image output processing procedure in step S43. First, the number n indicating the type of image processing to be LUT correction and the total number of controls ALL for each dither are acquired (step S51). Here, the initial value of the number n is the value 1.

  Then, using the γLUT correction table obtained by the previous control, the table data of the LUT 25 is corrected according to the equation (1) (step S52). The correction result table data is set in the LUT 25 (step S53). Laser output is performed using the LUT 25 to form an image (step S54). After the image is formed, a density detection image pattern Q is formed in a non-image area (between the images) on the photosensitive drum 1, which is an area between the trailing edge of the image and the leading edge of the next image. The density of the density detection image pattern Q is read by the density sensor 12 (step S55).

  A deviation amount ΔDn between the read density and the target density (64 levels) is calculated (step S56). Using this calculated deviation amount ΔDn, a γLUT correction table is created (step S57). The γLUT correction table is created as follows. That is, all density signals in the basic LUT correction table stored in the ROM 18 are calculated by ΔDn times, and an LUT correction table corresponding to the deviation amount ΔDn is created. Then, a γLUT correction table having table data that cancels the characteristics (pattern) of the created LUT correction table is created.

  Thereafter, it is determined whether or not to continue the print job (step S58). When continuing the print job, it is determined whether or not the number n is less than the total number ALL (step S59). If the total number is less than ALL, the number n is incremented by 1 (step S60), and the process returns to step S52. On the other hand, when the number n reaches the total number ALL, the number n is set to the initial value 1, and the process returns to step S52. On the other hand, when the print job is terminated in step S58, the present process is terminated.

  Thus, each priority input from the operation unit 20 is sent to the CPU 17, and the CPU 17 determines the control frequency of each dither while referring to the data stored in the ROM 18 and the RAM 19. Based on this result, the color image forming apparatus 50 performs an image forming operation.

  FIG. 12 is a diagram showing the image formation timing of the image and the density detection image pattern Q. In the figure, the image pattern Q with 133-line dither is designated as No. 1 and the image pattern Q with 190-line dither is No. 2 and image pattern Q with 268-line dither is No. Shown as 3. At this time, the detection image pattern Q is formed evenly, for example, in all 16 times so that the control is not biased (for example, only No. 1 is not repeated first). As a result, the frequency of dither that the user considers particularly important can be set appropriately, and control for each dither that matches the user's needs becomes possible.

  FIG. 13 is a graph showing the density transition of an image using 133-line dither applied to the image in the image portion. In the figure, the solid line 80a indicates the density when the dither control frequency is changed, and the dotted line 80b indicates the density when the dither control frequency is not changed. It can be seen that the density change is less when the dither control frequency is changed than when the dither control frequency is not changed. As described above, the density stability of the image that the user considers important is greatly improved. On the other hand, there is a concern that the density variation of the 266-line dither image is large. However, since it is an image of a character portion or the like and the user determines that the influence of the fluctuation in density is hardly affected by the visibility, there is no particular problem.

As described above, according to the color image forming apparatus in the first reference example, by changing the control frequency according to the type of image processing (dithering), a highly stable image that matches the user's needs is always stable. Can get to.

[Second Reference Example ]
In the first reference example , the user directly selects the control frequency for each dither. In the second reference example , the user selects an image attribute. Since the configuration of the color image forming apparatus is the same as that of the first reference example , the description thereof is omitted by using the same reference numerals. Here, parts different from the first reference example will be described.

FIG. 14 is a diagram showing an input screen for automatic gradation correction frequency displayed on the touch panel display 20a of the operation unit 20 in the second reference example . When the operator presses the “automatic gradation correction frequency” mode setting button 20 b provided on the operation unit 20, an automatic gradation correction frequency input screen is displayed on the touch panel display 20 a of the operation unit 20. On this input screen, the priority for each image attribute can be input, and the control frequency is determined according to the input priority. For example, if the priority is set to a value of 10 for image portion dither, the priority is set to a value of 5 for graphic portion dither, and the priority is set to a value of 1 for character portion dither, The control frequency is as follows. That is, during the control of all 16 times, the control frequency of the image portion is 10 times, the control frequency of the graphic portion is 5 times, and the control frequency of the character portion is 1 time. At this time, the detection image pattern Q is uniformly formed during all 16 times so that dither control is not biased with each image attribute.

According to the color image forming apparatus of the second reference example, it is possible to appropriately set the control frequency of image attributes that the user considers particularly important. That is, it is possible to increase the control frequency for the image portion and the graphic portion in which density variation and color variation are particularly important. As described above, by varying the control frequency in accordance with the attribute of the image, it is possible to always stably obtain a highly stable image that matches the user's needs.

[Third Reference Example ]
In the third reference example , the control frequency of each dither appropriately adjusted for each image attribute is set in the color image forming apparatus in advance. The user then selects the dither used for each image attribute. Since the configuration of the color image forming apparatus is the same as that of the first reference example , the description thereof is omitted by using the same reference numerals. Here, parts different from the first reference example will be described.

FIG. 15 is a diagram showing an image processing selection input screen displayed on the touch panel display 20a of the operation unit 20 in the third reference example . This input screen is displayed when the operator presses the “image processing selection” mode setting button 20b. Here, a case is shown in which the image image dither is set to 133-line dither, the graphic image dither is set to 190-line dither, and the text image dither is set to 266-line dither. Further, as the control frequency input in advance, the priority is set to a value of 10 in the case of dithering in the image portion, the priority is set to a value of 5 in the case of dithering in the graphic portion, and priority is set in the case of dithering in the character portion. Set the degree to the value 1.

According to the color image forming apparatus of the third reference example , the control frequency of the image attribute that the user thinks that is usually important is appropriately set, and the image portion and the graphic portion in which density fluctuation and color fluctuation are particularly important are controlled. It becomes possible to increase the frequency. In this way, the control frequency for each dither can be varied by the user selecting the dither used for each image attribute.

[Implementation of the form]
This embodiment is characterized in that each change the control frequency appropriately according to the image of the dither, always obtain a good density change by performing control at an optimal frequency. Since the configuration of the color image forming apparatus is the same as that of the first reference example , the description thereof is omitted by using the same reference numerals.

As a method of obtaining the image amount, in this embodiment, a video count value that integrates the image input signal for each pixel is employed. The integrated value of the video count value is detected at the timing of forming the density detection image pattern Q, and the dither image pattern Q that is the maximum value is formed.

  For example, the integrated value of the video count value is detected at the timing of forming the yellow density detection image pattern Q. When dither 1 has a value of 152, dither 2 has a value of 26, and dither 3 has a value of 58, an image pattern Q of yellow dither 1 is formed. Then, the integrated value is reset to 0 after the formation of the image pattern Q.

Incidentally, the video count value of the present embodiment is divided by 224. Next time, when dither 1 is 26, dither 2 is 48, and dither 3 is 84 as the integrated value of the detected video count value at the same timing of yellow, image pattern Q of yellow dither 3 Form. The same sequence is performed for colors other than yellow.

According to the color image forming apparatus of the present embodiment , it is possible to increase the control frequency of dither images that are used more frequently in the entire image. Therefore, an image with high stability can be obtained from a comprehensive viewpoint. In addition, in a more frequently used image, the amount of toner replenishment is large, the amount of toner replenishment shake is large, and the amount of developer change in the developing device is large. For this reason, the developer in the developing device becomes non-uniform and the stability is difficult. In contrast, by changing the control frequency as in the present embodiment, this problem can be solved.

The present invention is not limited to the configuration of the above embodiment, any one so long as it is a structure that functions functions indicated in the claims, or the configuration of the present embodiment has can be achieved Even if it exists, it is applicable.

For example, in each of the above embodiments , every time the density of the density detection image pattern Q formed by specific image processing (dither) is detected, only the LUT corresponding to the specific dither is rewritten. In addition to LUTs corresponding to specific dithers, LUTs corresponding to other dithers may be rewritten by a predetermined coefficient (feedback rate). For example, if the amount of deviation ΔD = 15 as a result of patching with 133-line dither, the LUT correction table is multiplied by the value 15 and rewritten. Further, the LUT correction table of the 190-line dither which is the other dither is rewritten by multiplying the value 15 × 0.8 (coefficient). Furthermore, the LUT correction table for dithering 266 lines is multiplied by a value of 15 × 0.6 (coefficient) and rewritten. Thereby, the responsiveness of density control can be improved. FIG. 16 is a table showing coefficients when rewriting the LUT correction table for each dither. In this table, the size of the numbers (Nos. 1 to 7) means that the number of dither lines is large. The closer the number of dither lines, the larger the dither coefficient to be rewritten (0.80 etc.).

In the above-described embodiment , the case where the present invention is applied to a color image forming apparatus has been described. However, it is needless to say that the present invention may be applied to a monochrome image forming apparatus to perform automatic gradation control. In addition to the original printing apparatus, the image forming apparatus may be a facsimile machine having a printing function, a multifunction machine (MFP) having a printing function, a copy function, a scanner function, or the like.

The object of the present invention is also achieved by the following. That is, a storage medium storing software program codes for realizing the functions of the form of implementation described above, a system or device. The computer (or CPU, MPU, etc.) of the system or apparatus reads out and executes the program code stored in the storage medium.

In this case, the program code itself read from the storage medium realizes the functions of the form of implementation described above. The program code and a storage medium storing the program code constitute the present invention.

  Further, for example, a floppy (registered trademark) disk, a hard disk, a magneto-optical disk, a CD-ROM, a CD-R, or a CD-RW can be used as a storage medium for supplying the program code. Further, an optical disc such as a DVD-ROM, a DVD-RAM, a DVD-RW, a DVD + RW, a magnetic tape, a nonvolatile memory card, a ROM, or the like can be used. Alternatively, the program code may be downloaded via a network.

Further, by executing the program code read by the computer, not only the functions of the form of implementation described above are realized, also includes the following cases. That is, based on the instruction of the program code, an OS (operating system) running on the computer performs part or all of the actual processing. If such a function of the implementation of embodiment described above by the process are realized is also included.

Further, the program code read from the storage medium is written in a memory provided in a function expansion board inserted into the computer or a function expansion unit connected to the computer. Thereafter, based on an instruction of the program code, a CPU or the like provided with the extension function in the extension board or the extension unit performs part or all of the actual processing. If such a function of the implementation of embodiment described above by the process are realized is also included.

It is a longitudinal cross-sectional view which shows the internal structure of the color image forming apparatus in a 1st reference example . FIG. 3 is a diagram illustrating a configuration in the vicinity of a photosensitive drum 1 and an intermediate transfer belt 6. 3 is a block diagram illustrating a configuration of a signal processing circuit that processes an output signal of a reflected light amount sensor 12. FIG. 6 is a graph showing the relationship between the output signal of the reflected light amount sensor 12 and the image density when the density of the density detection image pattern Q formed on the photosensitive drum 1 is changed stepwise by the area gradation of each color. 3 is a graph showing characteristics of an LUT 25. 10 is a graph showing the characteristics of a general density signal correction table (basic LUT correction table) when the density shift of the density detection image pattern Q with respect to a 64-level density signal is 1. It is a flowchart which shows the image output processing procedure accompanied with preparation of a LUT correction table. 6 is a diagram showing an input screen for automatic gradation correction frequency displayed on the touch panel display 20a of the operation unit 20. FIG. It is a flowchart which shows a pattern formation frequency determination processing procedure. It is a flowchart which shows the image formation process sequence in the case of performing different LUT correction | amendment for every dither. It is a flowchart which shows the image output processing procedure in step S43. It is a figure which shows the image formation timing of the image and the image pattern Q for density detection. It is a graph which shows the density transition of the image using the 133 line dither applied to the image of an image part. It is a figure which shows the input screen of the automatic gradation correction frequency displayed on the touch-panel display 20a of the operation part 20 in a 2nd reference example . It is a figure which shows the input screen of the image processing selection displayed on the touch-panel display 20a of the operation part 20 in a 3rd reference example . It is a table which shows the coefficient at the time of rewriting the LUT correction table for every dither. It is a figure which shows the image formation timing of the halftone pattern and image which are divided | segmented and formed in the conventional several times.

1 Photosensitive drum 12 Reflected light quantity sensor (photo sensor)
17 CPU
20 Operation unit 25 LUT
50 color image forming apparatus

Claims (5)

In an image forming apparatus comprising: a density detecting unit that detects the density of a density detection image pattern formed during an image forming operation; and a density control unit that performs density control based on the detected density.
An integrated value detecting means for detecting the integrated value of the video count value for each type of dither for performing gradation processing;
In response to the arrival of the timing for forming the density detection image pattern, the density detection image pattern using the dither having the largest value among the integrated values of the video count values for each type of the detected dither. And an image pattern forming unit for forming the image. The integrated value of the video count value corresponding to the dither of the formed density detection image pattern is reset to 0 in response to the density detection image pattern being formed by the image pattern forming means. The image forming apparatus according to claim 1. 2. The rewrite means for rewriting a gradation correction table corresponding to the dither of the density detection image pattern each time the density detection means detects the density of the density detection image pattern. Or the image forming apparatus according to 2; The rewriting means rewrites a gradation correction table corresponding to a dither other than the dither of the density detection image pattern whose density is detected by the density detection means by applying a predetermined coefficient. The image forming apparatus according to claim 3. In a density control method for detecting a density of a density detection image pattern formed during an image forming operation by a density sensor and performing density control based on the detected density,
Detecting the integrated value of the video count value for each type of dither for performing gradation processing ;
In response to the arrival of the timing for forming the density detection image pattern, the density detection image pattern using the dither having the largest value among the integrated values of the video count values for each type of the detected dither. And a step of forming the concentration control method.

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