JP2008131618A - Image formation apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image formation apparatus capable of carrying out correction control with a proper output concentration. <P>SOLUTION: An image formation apparatus has a controller unit having a halftone processing part generating image reproduction data from an input gradation value, and a printing engine forming an image based on the image reproduction data. The controller unit generates the image reproduction data from patch image data having a plurality of input gradation values to make the printing engine form a patch image, and generates a correction gamma table based on a sensor value of the patch image from a concentration sensor in the printing engine and a sensor value-in print concentration conversion table. Further, the controller unit corrects the sensor value-in print concentration conversion table based on a deviation between the sensor value of the patch image of the minimum input gradation value and a sensor reference value of paper whiteness. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an image forming apparatus such as a printer, a copier, and a facsimile, and more particularly to an image forming apparatus that can prevent deterioration in image quality due to aging and variations in characteristics of a print engine.

  An image forming apparatus such as a printer, a copier, or a facsimile is provided with a controller unit that receives image data having gradation data corresponding to pixels from a host computer or a built-in scanner and generates image reproduction data from the input gradation values. , And a print engine which is supplied with image reproduction data and forms an image based on the data.

  The controller unit has a halftone processing unit that outputs image reproduction data from an 8-bit input gradation value, for example, and the halftone processing unit expresses the gradation of the input gradation value by the area ratio of the printed dots. Image reproduction data suitable for the print engine is output based on a predetermined gamma characteristic such as an AM screen (for example, multi-value dither method) or an FM screen (for example, error diffusion method) expressed by the density of printed dots. . On the other hand, in the case of electrophotography, for example, in the case of electrophotography, a latent image is formed by irradiating energy light to a charged photosensitive drum based on image reproduction data, and toner is attached to the latent image to form a toner image. Then, it is transferred to a printing medium such as paper.

  In such an image forming apparatus, an ideal linear relationship is desired in which the input gradation of the image data and the output density on the print medium (on the paper) match. At the design stage, a conversion table for converting the input tone value of the halftone processing unit into image reproduction data is designed so that the input tone and the density on the paper have a linear relationship based on the characteristics of the print engine. Is done. However, the linear relationship may not be maintained due to variations in the characteristics of the print engine at the time of shipment from the factory, or due to variations in the characteristics of the print engine due to aging or environmental changes.

  Therefore, based on the image data for forming the patch image, a patch image having a plurality of gradation values is formed on the photosensitive drum or intermediate transfer medium of the print engine, and the output density of the patch image is read by the density sensor. The relationship between the input tone value and the read output density (exactly the density on the paper) is acquired, and a correction gamma table is created so that the relationship becomes an ideal linear characteristic. It has been proposed to correct the conversion table. Alternatively, it has been proposed to correct an input tone value using a correction gamma table and supply the corrected input tone value to a halftone processing unit.

  When generating the correction gamma table, the image forming apparatus reads the output density of the patch image with a density sensor built in the engine, and measures the sensor value when the patch image is measured in advance and the patch image as a print medium (paper). The sensor value at the time of correction is converted into the on-paper density based on the sensor value / on-paper density conversion table showing the relationship with the on-paper density value printed on the paper and the density on the paper measured by the colorimeter. Then, the correction gamma table is created so that the relationship between the acquired input gradation and the on-paper density has an ideal linear characteristic.

For example, it is as described in the following Patent Documents 1, 2, and 3.
JP 2000-56525 A JP 2006-33402 A JP 2004-102239 A

  As described above, the patch image is read by the density sensor, the relationship between the on-paper density corresponding to the read sensor value and the input tone value of the patch image is acquired, and the relationship is corrected so that the relationship becomes an ideal linear characteristic. Since the gamma table is created, an appropriate correction gamma table cannot be created unless the relationship between the input gradation value and the density on the paper acquired via the sensor is appropriate.

  However, if there is a variation in the characteristics of the density sensor that detects the output density of the patch image due to a request for cost reduction, the relationship between the acquired input tone value and the density on the paper will be different from the actual print engine characteristics. Correction to ideal linear characteristics cannot be performed.

  In particular, when the output density value in the low gradation area and the output density value in the high gradation area are not accurate due to the characteristic fluctuation of the density sensor, if the characteristic connecting these two points is corrected to a linear characteristic, It is expected that the actual output density for the gradation area will deviate from the linear characteristics.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an image forming apparatus capable of solving the above-described problems and performing appropriate output density correction control.

  In order to achieve the above object, according to an aspect of the present invention, an image forming apparatus includes a controller unit having a halftone processing unit that generates image reproduction data from an input gradation value based on a predetermined gamma characteristic; A print engine that forms an image having an output density corresponding to the input gradation value based on the image reproduction data. Further, the controller unit generates image reproduction data from patch image data having a plurality of input gradation values, causes the print engine to form the patch image, and detects a sensor value of the patch image by a density sensor in the print engine. The correction gamma table is generated based on the sensor value / on-paper density conversion table indicating the relationship between the sensor value of the patch image and the density of the on-paper image when the patch image is printed on the print medium. Based on this correction gamma table, the halftone processing unit generates image reproduction data from the input gradation value and performs density correction. When the sensor value of the patch image with the minimum input gradation value is different from the sensor reference value of paper white, the controller unit stores the sensor value / on-paper density conversion table with the sensor of the patch image with the minimum input gradation value. The correction gamma table is generated based on the sensor value of the patch image and the corrected sensor value / on-paper density conversion table.

  The inventors have found that the sensor value of the density sensor varies in a low density region including white where a patch image is not formed by toner. Therefore, according to the aspect of the present invention, if the sensor value of the patch image of the low density region where the variation of the density sensor is large, particularly the minimum input gradation value of the paper white is different from the sensor reference value of the paper white, it is based on the deviation. Then, the sensor value / on-paper density conversion table is corrected. As a result, the variation component of the sensor value can be removed from the density on the paper corresponding to the minimum input tone value, and a correction gamma table is generated based on the input tone / output density characteristics excluding the variation component of the density sensor. can do. Thereby, the image output apparatus can be appropriately corrected to an ideal linear characteristic.

  According to a preferable aspect of the first aspect, the controller unit adds the deviation to the sensor value / on-paper density conversion table over the range from the density of the paper white to the maximum density. Do as follows.

  According to a preferred aspect of the first aspect, the controller unit adds the deviation to the sensor value / on-paper density conversion table for the paper white density, and adds the deviation to the maximum density on the paper. The addition is not performed, but the deviation is added at a rate corresponding to the density on the paper between the density of the paper white and the maximum density on the paper.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the technical scope of the present invention is not limited to these embodiments, but extends to the matters described in the claims and equivalents thereof.

  FIG. 1 is a configuration diagram of an image forming apparatus according to the present embodiment. FIG. 1 shows the configuration of a printer as an example of an image forming apparatus. In addition to printers, copiers and facsimiles with built-in scanner functions are similar image forming apparatuses.

  The printer 20 is connected to the host computer 10 via a network or the like, and receives print data from the printer driver 12 installed in the host computer 10. The printer 20 includes a controller unit 200 that generates image reproduction data based on print data, and a print engine 250 that forms an image based on image reproduction data.

  The controller unit 200 includes a CPU 202 that controls the entire controller unit, a memory 206 that stores image data, an interface IF that receives print data, and a color conversion unit that converts image data based on the print data from RGB to CMYK. 208, a halftone processing unit 210 that converts input gradation values of color-converted CMYK image data into image reproduction data based on a predetermined gamma characteristic, and a pulse width modulator 212 that performs pulse width modulation on the image reproduction data. And have. Further, in the nonvolatile memory 214, a screen gamma table 216, a correction gamma table 218, a sensor value / paper density conversion table 220, and paper white sensor reference value data 222 as initial values are recorded. The halftone processing unit 210 refers to a screen gamma table 216 having characteristics corresponding to an AM screen or FM screen, and converts an input gradation value into image reproduction data.

  The paper white sensor reference value is a sensor value when a patch image having a minimum input gradation value corresponding to paper white is measured by a density sensor (described later) built in the engine at the factory shipment stage. The on-paper density value is a density on paper obtained by printing a patch image having the same minimum input gradation value on a printing medium (paper) and measuring with a colorimeter. In the case of a paper white image, since there is no toner adhesion by the print engine, it is not affected by aging of the print engine or individual differences. Therefore, the sensor value or the on-paper density value for such a paper white image is stored in the non-volatile memory 214 as a reference value obtained by measurement in advance, not by the sensor value, and the sensor value is used in automatic image density control. Instead of using it.

  The print engine 250 includes an engine controller 252, light emitting means 254 including a laser diode or a line-shaped light emitting element, a photosensitive drum 256, a developing unit, a transfer unit, and a fixing unit (not shown). The engine controller 252 irradiates a beam from the light emitting means 254 based on the image reproduction data generated by the controller unit 200 to form a latent image on the photosensitive drum 256, and the developing unit converts the latent image into a toner image. The toner image is transferred to a printing medium, for example, a sheet by a transfer unit, and the toner image is fixed on the sheet by a fixing unit.

  The print engine 250 further detects the density of the toner image formed on the photosensitive drum or an intermediate transfer medium (not shown) by a built-in density sensor 260, and the sensor interface S-IF in the controller unit 200 detects the density of the toner image. The value is output.

[Automatic image density control]
The controller unit 200 of the printer has an automatic image density control program 204, and by executing this program, control is automatically performed so that the output image density by the print engine with respect to the input gradation value maintains optimum characteristics. Do. The outline of this automatic image density control is as follows.

  First, a patch image having a predetermined gradation step from a low density to a high density is formed by the print engine 250 at a predetermined timing such as when the printer is turned on or when the photosensitive drum or the developing unit is replaced. Specifically, the controller unit 200 generates image reproduction data from the patch image data, and the print engine 250 forms a patch image 258 corresponding to the image reproduction data on the photosensitive drum 256. The patch image 258 is the above-described toner image, and may be formed on an intermediate transfer medium (not shown). Then, the density sensor 260 measures the density of the patch image 258 and the sensor value is supplied to the controller unit 200.

  The automatic image density control program 200 obtains an output density value on the paper based on the sensor value, and obtains an input tone / output density characteristic indicating the relationship between the input tone value of the patch image data and the output density value. Then, a correction gamma table 218 for correcting the input gradation value is generated and recorded in the nonvolatile memory 214 so that the input gradation / output density characteristic has an ideal linear relationship. Then, the halftone processing unit 210 refers to the corrected screen gamma table 216 obtained by correcting the screen gamma table 216 based on the corrected gamma table 218, and converts the input gradation value data into image reproduction data. Alternatively, the halftone processing unit 210 corrects the input tone value data with reference to the correction gamma table 220, and converts the corrected input tone value data into image reproduction data with reference to the screen gamma table 216.

  FIG. 2 is a flowchart showing a control procedure by the automatic image density control program. 3 to 5 are diagrams showing various graphs generated in the automatic image density control. The automatic image density control will be described in detail with reference to these, and then the problem and automatic image density control according to the present embodiment will be described.

  In FIG. 2, as described above, the automatic image density control program generates a patch image (S10). The patch image has an input gradation value obtained by dividing 256 printable gradations at equal intervals of 255 / (Pmax-1) according to the number of patch gradations Pmax, using a screen gamma table used during normal printing. It consists of a patch pattern.

  Next, a sensor value is acquired for the patch image in the photosensitive drum 256 in the print engine (S12). FIG. 3A is an example of a graph showing the sensor value with respect to the input tone value of the patch pattern. In FIG. 3A, the horizontal axis represents input gradation values from 0 to 255, and the vertical axis represents sensor values from 0 to 1023.

  Here, in this specification, the sensor value means a standardized sensor value. The standardized sensor value means that the detected sensor value (voltage value) is 10 bits (0 to 1023) between the standardized voltage of the sensor (for example, 5V) and the voltage when there is no received light (sensor offset voltage). This is a standardized value. Therefore, the sensor normalization voltage is normalized to 1023, the sensor offset voltage is normalized to 0, and the toner is not adhered to the photoconductor (paper white, corresponding to the minimum density) is set to a high value on the 1023 side. The solid state with 100% area ratio (corresponding to the maximum density) is normalized to a low value on the 0 side. Therefore, the normalized sensor value can be used as an absolute value substantially the same as the output voltage of the sensor, that is, the sensor value. For example, if the normalized sensor value is 200, the on-paper density is 1.42, if the normalized sensor value is 120, the on-paper density is 1.63, and 240 corresponds to 1.38. Hereinafter, the normalized sensor value is simply referred to as a sensor value.

With respect to the input gradation value on the horizontal axis in FIG. 3A, the gradation value Pi (0 ≦ i <Pmax) of the patch pattern is obtained by the following equation. In this expression, INT is a truncation process, and according to this expression, the input gradation values of the patch pattern having the number of gradations Pmax are arranged at equal intervals in the 0 to 255 gradation range.
P _i = INT ((i * 255.) / (Pmax-1) + 0.5)
As shown in FIG. 3A, the sensor value is about 720 at the input gradation value 0 corresponding to the minimum density, and the sensor value is about 160 at the input gradation value 255 corresponding to the maximum density.

  Next, the sensor value is converted into an on-paper density value (S14). That is, the on-paper density value (output density value) of each patch pattern is obtained using the sensor value / on-paper density conversion table 220 obtained in advance at the factory shipment stage. FIG. 3B is an example of a sensor value / on-paper density conversion table. The horizontal axis is a value obtained by converting the sensor value of 0 to 1023 into the sensor value of 0 to 255, and the vertical axis is the on-paper density value. This conversion table is generated based on the obtained on-paper density and the sensor value of the patch pattern obtained by previously forming a patch image on the paper of the printing medium and measuring the density on the paper with a colorimeter.

  FIG. 4A is a graph in which the sensor value is converted into the on-paper density value (output density value) from FIGS. 3A and 3B, and the input tone value of the patch pattern and the density value on the paper are associated with each other. It is. This is the input gradation / output density characteristic indicating the relationship between the input gradation value and the output density value based on the sensor value, and ideally a linear relationship is desirable. Note that the sensor value includes characteristic variations of the density sensor, and includes a sensor value reversal phenomenon such that the output density decreases as the input gradation increases. Therefore, predetermined reverse correction and smoothing processing are performed in order to remove such characteristic variation of the density sensor.

  In FIG. 4A, the density value (white circle) obtained by actually measuring the patch image with the colorimeter is superimposed on the density value (black circle) converted from the sensor value. In this example, the on-paper density value converted from the sensor value is subjected to signal processing, and as a result, it almost matches the actually measured density value. In FIG. 4A, an ideal linear characteristic IDEA is indicated by a one-dot chain line. Compared with this linear characteristic IDEA, it can be understood that the actual output density is low, that is, a light image is formed.

  Next, the automatic image density control program generates a correction gamma table such that the input tone / output density characteristics of FIG. 4 become linear characteristics IDEA (S18). This correction gamma table is a table for correcting the obtained on-paper density value shown in FIG. 4A to a desired density value. Here, the output density is linear according to the input gradation value. Create a table for correction. FIG. 4B is an example of a correction gamma table. In subsequent normal printing, the input gradation is corrected based on the correction gamma table (S20).

  The meaning of this correction gamma table is that an output density 56.73 can be obtained if the input gradation value 64 before correction is used as the input gradation value 56.73 after correction. In other words, before correction, the on-paper density value obtained for a certain input tone value is lower than the ideal value according to the input tone / output density characteristics of FIG. 4A, and the output image is printed lightly. As a result, it is possible to output a darker image by converting the input gradation value 56.73 to 64 using the correction gamma table and inputting the corrected input gradation value 64 to the halftone processing unit. It becomes like this.

  FIG. 5 is a diagram showing a halftone processing unit using a correction gamma table. This halftone processing unit 210 converts the corrected input tone value INa = 56.73 into the pre-correction input tone value INb = 64 by the correction gamma table 218, and converts it by the screen gamma table 216 to convert the image reproduction data. Generate. The output density D printed by this image reproduction data is 56.73, which is the same gradation value as the corrected input gradation value, and has a linear characteristic.

  The halftone processing unit 210 may correct the screen gamma table 216 based on the corrected gamma table 218 and convert the corrected input tone value INa into image reproduction data using the corrected screen gamma table.

  FIG. 6 is a graph summarizing the automatic image density control described above. In the first quadrant, a sensor value 600 with respect to an input gradation value based on a patch image is shown, and a graph obtained by inverting the sensor value in the graph of FIG. 3A (= maximum normalized sensor value−sensor value). Yes, substantially the same. The reason why the sensor value graph 600 with respect to the input gradation value is the inversion value of the sensor value is indicated by the inversion value, and the sensor inversion value is simply increased with respect to the input gradation value, so it is intuitively understood. This is because it becomes easy. The input gradation value on the horizontal axis corresponds to the input gradation value INb before correction in FIG. In the second quadrant, a graph of sensor value and on-paper density value (sensor value / on-paper density conversion table) 602 is shown, and the sensor value is an inverted value in the graph of FIG. is there. Then, an ideal input tone value and on-paper density value (output density value) graph 604 is shown in the third quadrant, and the maximum on-paper density value Dmax (Dmax = 1.6) converted from the sensor value and the on-paper level. There is a linear relationship with the minimum density value D0 (D0 = 0.1).

  Then, the pre-correction input gradation value INb and the post-correction input gradation value INa are set so that the graph 606 in the fourth quadrant of FIG. 6 has an ideal linear relationship input gradation value and on-paper density value. It is a correction gamma table showing the relationship. As described with reference to FIG. 5, the post-correction input tone value INa is converted to the pre-correction input tone value INb by the correction gamma table, and converted to image reproduction data by the screen gamma table. The relationship between the input gradation value INa = 56.73 before correction and the input gradation value INb = 64 after correction is also shown in the fourth quadrant.

[Problems of concentration sensor]
In the input gradation / output density characteristics shown in FIG. 4A described above, the on-paper density value (black circle) obtained from the sensor value and the actually measured output density value (white circle) substantially coincide. However, the sensor value of the density sensor may include variations. In particular, an error is likely to be included in the sensor value in a low density region where toner does not adhere to the photoreceptor. This is because the surface of the photoconductor has irregularities, and the sensor value varies depending on which position on the surface of the photoconductor is measured. Furthermore, it is conceivable that the low-cost density sensor itself includes variations in sensor values.

  In the broken line 600S in FIG. 6, the sensor value increases in the low input gradation value region including the minimum input gradation. When such a sensor value is acquired, the paper white density D0e (= 0.16) converted based on the sensor value / on-paper density conversion table 602 is the ideal paper white density D0 (= 0.1). Become bigger. In order to obtain a linear characteristic 610 between the paper white density D0e corresponding to the minimum input gradation based on the sensor value and the maximum on-paper density Dmax corresponding to the maximum input gradation, a correction gamma table 612 like a broken line is used. Is generated.

  That is, the linear characteristic 610 between the on-paper densities D0e and Dmax obtained by converting the sensor value 600S based on the sensor value / on-paper density conversion table 602 is different from the linear characteristic 604 in the case of the sensor value 600 in a low density region. A discrepancy occurs, and in the corrected gamma table 612, the corrected input gradation INa is corrected to a higher corrected input gradation INb in the low density region. Therefore, this correction means that the density value on paper after correction becomes too dark.

  FIG. 7 is a diagram showing the relationship between the on-paper density value and the input tone value INa of the patch image obtained as a result of the halftone processing unit generating image reproduction data based on the correction gamma table 612 of FIG. . A solid line in the figure indicates an ideal linear characteristic IDEA based on the correction gamma table 606 in FIG. 6, and a broken line in the figure indicates an input gradation / on-paper density characteristic 614 based on the correction gamma table 612 in FIG. When based on the correction gamma table 612 (characteristic 614), as described above, the on-paper density value is higher particularly in the low density area.

  In the patch image corresponding to the minimum gradation value, no toner adheres to the photosensitive drum or the intermediate transfer medium. Therefore, the sensor value corresponding to the minimum gradation value does not include a change due to the aging of the print engine or individual differences. Nevertheless, the sensor value corresponding to the minimum gradation value fluctuates because of variations in the characteristics of the density sensor itself and the irregularities on the photosensitive drum described above. When the correction gamma table 612 is generated based on the change in sensor value corresponding to such minimum gradation value (increase of 600S in FIG. 6), a low density region is obtained by a print engine in which no secular change or individual difference occurs. In this case, the on-paper density value becomes unnecessarily high. That is, the paper white sensor value Sw1 including the error adversely affects the correction gamma table 612, and inappropriate image density correction is performed.

[Automatic image density control in this embodiment]
FIG. 8 is a control flowchart according to the automatic image density control program in the present embodiment. The flowchart of FIG. 8 is compared with the paper white sensor reference value in which the sensor value (paper white sensor value) for the patch image having the minimum gradation value is stored in advance in the nonvolatile memory 214 (FIG. 1), as compared with FIG. If they are different, steps S13 and S13A for correcting the sensor value / on-paper density conversion table are added, and the other steps are the same as those in FIG. Therefore, the newly added steps 13 and 13A will be described in detail.

  First, in the present embodiment, as a premise, when the sensor value / paper density conversion table 602 is created at the time of design or factory shipment, the paper white sensor value Swth is stored in the nonvolatile memory 214 as the paper white sensor reference value. Keep it.

  FIG. 9 is a diagram showing the first correction of the sensor value / on-paper density conversion table. Assume that the sensor value of the patch image is detected as 600S in the figure in the automatic image density control process. According to the acquired sensor value 600S, the paper white sensor value Sw1 is increased by the difference dS from the paper white sensor reference value Swhh. If the paper white sensor value Sw1 of the sensor value 600S detected from the patch image is different from the paper white sensor reference value Swth in step S13 of FIG. 8 (YES in S13), the automatic image density control program 204 The value / paper density conversion table 602 is corrected according to the difference dS (S13A), and the paper white density D0 obtained by converting the paper white sensor value Sw1 is equal to the ideal paper white density value (D0 = 1.0). To be. If the paper white sensor value Sw1 of the sensor value 600S detected from the patch image is the same as the paper white sensor reference value Swth in step S13 of FIG. 8 (NO in S13), the automatic image density control program 204 The sensor value is converted into the on-paper density value without correcting the sensor value / on-paper density conversion table 602 at the time of design or factory shipment. Also in this case, the converted paper white density D0 becomes equal to the ideal paper white density value D0.

  As shown in FIG. 9, in this example, the deviation dS between the paper white sensor value Sw1 and the paper white sensor reference value Swth at the time of correction is changed from the paper white density to the sensor value of the sensor value / on-paper density conversion table 602. The corrected sensor value / on-paper density conversion table 602C is generated by adding over the range up to the maximum density. According to the corrected sensor value / on-paper density conversion table 602C, the paper white sensor value Sw1 acquired from the patch image is converted to the same value as the ideal paper white density value D0. Therefore, when the correction gamma table is generated so that the ideal white characteristic 614 from the paper white density value D0 to the maximum density value Dmax is generated, a broken line 616 in the fourth quadrant of FIG. 9 is obtained.

  Based on the deviation dS between the paper white sensor value Sw1 and the paper white sensor reference value Swth at the time of correction as described above, the sensor value / on-paper density conversion table 602 is corrected as indicated by the broken line 602C, thereby obtaining the paper white sensor value. The on-paper density value converted from Sw1 is equivalent to the ideal paper white density value D0. Therefore, the correction gamma table 616 generated by the ideal linear characteristic 614 based on the on-paper density value D0 is not converted to an unnecessarily high input gradation value in the low density area. Therefore, it is possible to prevent the on-paper density from becoming too high in the low density region as shown in FIG.

  The correction example of FIG. 9 is effective when the sensor value deviation dS occurs entirely from the minimum input gradation to the maximum input gradation. In this case, the sensor value deviation includes not only the print engine but the above-described unevenness of the photosensitive drum and the sensor value deviation in the minimum input gradation, and the deviation due to the characteristic fluctuation of the print engine in the high input gradation area. Inferred. Therefore, by shifting the sensor value / on-paper density conversion table entirely based on the deviation dS as indicated by the broken line 602C, the low density area including paper white is not corrected so as to increase the density unnecessarily. In the density region, the density can be corrected to be higher.

  FIG. 10 is a diagram showing the second correction of the sensor value / on-paper density conversion table. In this example, if the paper white sensor value Sw1 is different from the paper white sensor reference value Swth among the sensor values 600S acquired at the time of correction, the deviation dS is added to the sensor value / paper density conversion table 602 as the paper white density. However, addition is not performed at the maximum density on paper, and deviation dS is added between the density of white paper and the maximum density on paper at a ratio (proportional distribution) according to the density on paper to correct the corrected sensor value / density on paper. A table 602C is generated.

  That is, with respect to the difference dS of the paper white sensor value Sw1 with respect to the paper white sensor reference value Swth, 75% of the difference dS is added at 25% of the maximum density, and 50% of the difference dS is added at 50% of the maximum density. , 25% of the difference dS is added at 75% of the maximum density and is not added at 100% of the maximum density.

  If the sensor value 600S is converted to the on-paper density value by the correction sensor value / on-paper density conversion table 202C, the paper white density becomes the same as the ideal paper white density D0, and the maximum on-paper density Dmax is also set at the time of factory shipment or design. The ideal characteristic 614 connecting them is equivalent to the ideal characteristic 604 at the time of factory shipment or design. As a result, according to the correction gamma table 616, the input gradation value INb before correction corrected from the input gradation value INa after correction is not converted to an unnecessarily high density in the low density region.

  The correction example in FIG. 10 is effective when the sensor value deviation dS occurs only in the region from the minimum input gradation to the low gradation. That is, although the paper white sensor value Sw1 includes the above-described unevenness of the photosensitive drum and the deviation of the sensor value itself, it is presumed that there is no change in the characteristics of the print engine in a high input gradation region. Therefore, by shifting the sensor value / on-paper density conversion table on the low density area side based on the deviation dS as shown by the broken line 602C, the density is not corrected to be unnecessarily high in the low density area including paper white. , No correction is made to unnecessarily increase the density even in the high density region.

  In both the correction examples of FIGS. 9 and 10, it is possible to avoid generation of on-paper density higher than necessary in a low density area by the correction gamma table 616 based on the deviation dS of the paper white sensor value. In the high density region, the influence of the correction on the image quality is small compared to the low density region. Therefore, any of the correction methods shown in FIGS. 9 and 10 can effectively avoid the deterioration of the image quality due to the correction.

  As shown in FIG. 8, the automatic image density control program 204 shown in FIG. 1 differs from the paper white sensor reference value Swth obtained at the time of design or factory shipment among the sensor values acquired from the patch image. If so, the sensor value / on-paper density conversion table 220 in the nonvolatile memory 214 is corrected. Based on the relationship between the corrected sensor value / on-paper density conversion table 220 and the input tone value and on-paper density (output density), the correction gamma table 218 is adjusted so that it has ideal linear characteristics. Generate.

  As described above, in the present embodiment, in the automatic image density control using the patch image, when the paper white sensor value is deviated from the paper white sensor reference value obtained in advance, the sensor value / paper density is accordingly adjusted. Correct the conversion table. As a result, it is possible to eliminate an adverse effect caused by variations in the paper white sensor value caused by variations in the sensor.

1 is a configuration diagram of an image forming apparatus in the present embodiment. It is a flowchart figure which shows the control procedure by an automatic image density control program. It is a figure which shows the various graphs produced | generated in automatic image density control. It is a figure which shows the various graphs produced | generated in automatic image density control. It is a figure which shows the halftone process part using a correction | amendment gamma table. FIG. 6 is a graph summarizing automatic image density control. FIG. 7 is a diagram illustrating a relationship between an on-paper density value and an input tone value INa of a patch image obtained as a result of correction based on the correction gamma table 612 in FIG. 6. It is a control flowchart figure by the automatic image density control program in this Embodiment. It is a figure which shows the 1st correction | amendment of a sensor value and on-paper density | concentration conversion table. It is a figure which shows the 2nd correction | amendment of a sensor value and on-paper density | concentration conversion table.

Explanation of symbols

20: Printer 200: Controller unit 204: Automatic image density control program 210: Halftone processing unit 216: Screen gamma table 218: Correction gamma table 220, 602: Sensor value / on-paper density conversion table 222, Swth: Paper white sensor reference value

Claims (3)

In an image forming apparatus for forming an image based on image data having an input gradation value,
A controller unit having a halftone processing unit for generating image reproduction data from the input gradation value based on a predetermined gamma characteristic;
A print engine that forms an image having an output density corresponding to the input gradation value based on the image reproduction data;
The controller unit generates image reproduction data from patch image data having a plurality of input gradation values, causes the print engine to form the patch image, and a sensor value of the patch image by a density sensor in the print engine; A correction gamma table is generated based on a sensor value / on-paper density conversion table indicating a relationship between the sensor value of the patch image and the density of the on-paper image when the patch image is printed on a print medium,
The halftone processing unit generates image reproduction data from the input gradation value based on the correction gamma table,
Further, when the sensor value of the patch image having the minimum input tone value is different from the sensor reference value of paper white, the controller unit reads the sensor value / on-paper density conversion table of the patch image having the minimum input tone value. The correction gamma table is generated based on the sensor value of the patch image and the corrected sensor value / on-paper density conversion table. An image forming apparatus. In claim 1,
The image forming apparatus according to claim 1, wherein the controller unit performs the correction by adding the deviation to the sensor value / on-paper density conversion table over a range from the density of the paper white to the maximum density. In claim 1,
The controller unit adds the deviation to the sensor value / on-paper density conversion table for the paper white density and does not perform the addition for the maximum density on the paper, and the density of the paper white and the maximum on-paper. An image forming apparatus, wherein the deviation is added at a rate corresponding to the density on the paper.

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