JP2022096270A - Image forming apparatus and calibration method for the same - Google Patents

Abstract

To perform calibration such that engine characteristics to be the premise of calibration for an image processing unit becomes engine characteristics in which tone jump is not likely to occur.SOLUTION: An image forming apparatus comprises: an image processing unit that represents an image to be printed as a set of pieces of pixel data and outputs gradation values according to respective pieces of pixel data; an image forming unit that converts the output gradation values into scan signals and forms an image with a density according to the scan signals; and a control unit that controls the image processing unit and the image forming unit. The control unit controls the image processing unit and the image forming unit to print a two-dimensional gradation pattern for calibration for the image forming unit, the two-dimensional gradation pattern in which a predetermined parameter affecting the density of the image formed by the image forming unit in a main scanning direction related to the scan signals is made constant and the gradation value created by the image processing unit is changed, and the gradation value is made constant in a sub-scanning direction and the parameter is changed.SELECTED DRAWING: Figure 3

Description

The present invention relates to an image forming apparatus including an image processing unit and an image forming unit that forms an image based on the output thereof, and a method for calibrating the image forming unit.

The maximum density and gradation characteristics (also called gamma characteristics) of the printing device, and in the case of color, the characteristics between individual devices related to color tone, etc. are made uniform, and the maximum density and gamma characteristics with respect to the environment and the passage of time. The calibration function for stabilizing is known. Since the color tone is determined by the combination of the primary color densities of yellow, magenta, and cyan, it is important to make the gamma characteristics of each primary color uniform.
A predetermined test pattern is stored in the memory of the printing device, the stored test pattern is printed at the time of calibration, and the maximum density and gamma characteristics (color) of the printing device are referred to by referring to the printed test pattern. In the case, it is a function to correct the color tone) so that it approaches the target.
As for the method of printing a test pattern and performing calibration, for example, the following is known. Print the patch on the reference chart prepared in advance and adjust the density. Gradation output of each CMYK is performed by applying a gamma curve whose characteristics are changed little by little based on the density adjustment result. The output gradation chart is visually compared with the reference chart of the gamma curve, and the gradation that seems to be the most suitable is selected and registered (see, for example, Patent Document 1).

Japanese Unexamined Patent Publication No. 2002-237960

In recent image forming devices, an image processing unit that digitally processes an image to represent an image as a set of pixels and performs image processing using pixel data corresponding to each pixel, and an image forming based on the output of the image processing unit. That is, it is usually provided with an image forming unit for printing. The image forming unit is also called an engine. The engine has a mechanism and circuit for printing and a processor that controls the mechanism and circuit.
When calibrating the image forming device, the maximum density of the image processing unit, the gamma characteristic, and in the case of color, the color tone, etc., under the assumption that the characteristics (engine characteristics) of the image forming unit are in a constant state. It is usual to adjust such characteristics.
However, if the engine characteristics are extremely biased due to the surrounding environment such as temperature and humidity or deterioration over time, if adjustment is made only with the gradation value, the density difference between adjacent specific gradation values will be large and it will be visually recognized as a tone jump. On the contrary, it may occur that the densities of adjacent specific gradation values do not differ.
It is rare that the engine characteristics are extremely biased to the extent that a tone jump or the like occurs, and since the generated tone jump is minor, it has not been regarded as a problem in the past.
However, with the widespread use of image forming apparatus, it has come to be used in a wide range of environments, while the image quality has been improved, and it is desired to deal with this problem.
The present invention has been made in consideration of the above circumstances, and the engine characteristics that are the premise of calibration for the image processing unit can be calibrated so that the engine characteristics are less likely to cause tone jumps. It provides an image forming apparatus.

The present invention corresponds to an image processing unit that represents an image to be printed as a set of pixel data and outputs a gradation value corresponding to each pixel data, and converts each output gradation value into a scanning signal and responds to the scanning signal. It includes an image forming unit that forms an image of density, an image processing unit, and a control unit that controls the image processing unit, and the control unit is a two-dimensional gradation pattern for calibration of the image forming unit. Then, the gradation value generated by the image processing unit is changed by keeping a predetermined parameter that affects the density of the image formed by the image forming unit constant in the main scanning direction related to the scanning signal, and the gradation value generated by the image processing unit is changed. Provided is an image forming apparatus for printing a two-dimensional gradation pattern in which the gradation value is constant and the parameter is changed.
Further, from different viewpoints, the present invention has an image processing unit that outputs a gradation value according to pixel data, and an image forming unit that converts the gradation value into a scanning signal and prints a density corresponding to the scanning signal. The control unit that controls the image forming apparatus including In, a step of printing a two-dimensional gradation pattern in which the gradation value is kept constant and the parameters are changed, and a step of reading the two-dimensional gradation pattern printed by using an image reading unit, are read. A step in which a parameter in the sub-scanning direction in which the density of the two-dimensional gradation pattern at each gradation level in the main scanning direction most closely resembles a predetermined reference gradation pattern is used as a parameter value of the image forming unit in subsequent printing. Provided is a method for calibrating an image forming apparatus including.

In the image forming apparatus according to the present invention, the control unit changes the gradation value generated by the image processing unit by keeping a predetermined parameter that affects the density of the image formed by the image forming unit constant in the main scanning direction, and performs sub-scanning. Since a two-dimensional gradation pattern in which the gradation value is kept constant in the direction and the parameters are changed is printed, the engine characteristics that are the premise of calibration for the image processing unit are such that the engine characteristics are less likely to cause tone jumps. Can be calibrated.
The calibration method of the image forming apparatus according to the present invention also has the same effect.

It is explanatory drawing which shows the mechanical structure of the color multifunction device which is an image forming apparatus by this embodiment. It is a block diagram which shows the electric structure of the color multifunction device shown in FIG. It is explanatory drawing which shows an example of the 2D gradation pattern which the color multifunction device shown in FIG. 1 prints for calibration. It is a graph which shows the example of the print density of a single color with respect to the gradation value as the engine characteristic corresponding to the engine characteristic (1)-(3) of the two-dimensional gradation pattern shown in FIG. It is explanatory drawing which enlarges and shows a part of the engine characteristic shown in FIG. It is a graph showing an example of engine characteristics different from FIG. 4, and is an explanatory diagram showing a residual sum of squares with a target value of density at each gradation level and a residual sum of squares with a target value of slope of density. In this embodiment, it is a flowchart which shows an example of the calibration process which a control part executes. It is a graph which shows an example of the engine characteristic different from FIG.

Hereinafter, the present invention will be described in more detail with reference to the drawings. It should be noted that the following description is exemplary in all respects and should not be construed as limiting the invention.
(Embodiment 1)
<< Configuration example of image forming apparatus >>
FIG. 1 is an explanatory diagram showing a mechanical configuration of a color multifunction device which is an image forming apparatus according to this embodiment. FIG. 2 is a block diagram showing an electrical configuration of the color multifunction device shown in FIG.
As shown in FIG. 1, the multifunction device 100 has an image reading unit 111 for reading a document and an image forming unit 115 for forming an image. Further, the multifunction device 100 has a feed tray 18 at the lower end thereof, and a discharge tray 39 above the image forming unit 115 and below the image reading unit 111.
Further, the multifunction device 100 includes a circuit board of the control unit 101 on the right side of the back surface portion, and a document transfer unit 103 that conveys the document to the reading unit above.
Further, as shown in FIG. 2, the multifunction device 100 includes an operation unit 105 that receives a user's operation.

Here, the internal configuration of the multifunction device 100 shown in FIG. 1 will be described.
The multifunction device 100 prints a color image in which four colors of yellow (Y), magenta (M), cyan (C), and black (BK) are superimposed on a print sheet. Alternatively, a monochrome image using a single color (for example, black) is printed on a print sheet. For this reason, four developing units 12, a photoconductor drum 13, a charging unit 14, a drum cleaning unit 15, and the like, which are related to electrophotographic image formation, are provided. These units constitute process units Py, Pm, Pc, and Pk for each color.

Below the process units Py, Pm, Pc, and Pk, an optical scanning unit 11 as an exposure unit that exposes and scans the photoconductor drum 13 corresponding to each color with a laser beam is arranged. Further, the multifunction device 100 includes a circuit board of the image processing unit 116 that sends gradation values to the optical scanning unit 11 below the discharge tray 39.
The image processing unit 116 processes the pixel data of the original image read by the image reading unit 111 and the print data provided from the outside under the control of the control unit 101, and the gradation value corresponding to the pixel data of the image to be printed. To generate. Then, the generated gradation value is sent to the optical scanning unit 11.

The optical scanning unit 11 converts the transmitted gradation value into a scanning signal. The optical scanning unit 11 includes a laser drive circuit 11d that drives a laser element based on a scanning signal. The laser drive circuit 11d generates a laser beam having an emission intensity based on the scanning signal, and exposes and scans the photoconductor drum 13 with the generated laser beam. The optical scanning unit 11 scans the laser beam in a direction parallel to the central axis of the photoconductor drum 13 (main scanning direction). Scanning in the sub-scanning direction is realized by rotating the photoconductor drum 13 and moving the peripheral surface. The scanning signal corresponds to the pattern of the electrostatic latent image formed on the surface of the photoconductor drum 13. The image processing unit holds a data table (γ-look-up table 116t) that determines the gamma characteristics related to the exposure of each of the Y, M, C, and BK colors.
The control unit 101 controls scanning of the photoconductor drum 13 by the laser beam from the image processing unit 116 and the optical scanning unit 11 and driving of the laser element by the laser drive circuit 11d.

Further, the primary transfer roller 16 is arranged so as to be in contact with the photoconductor drum 13 of each process unit Py, Pm, Pc, Pk via the intermediate transfer belt 21. The intermediate transfer belt 21 is driven in synchronization with the photoconductor drum 13 of each color and orbits in the arrow direction C (sub-scanning direction) shown in FIG. Then, four toner images formed for each color by each process unit Py, Pm, Pc, and Pk are superimposed and conveyed, and sent to the secondary transfer unit 23. The secondary transfer unit 23 has a secondary transfer roller 23a.
A color correction sensor 41 is arranged between the process unit Pk and the secondary transfer unit 23 so as to face the intermediate transfer belt 21. The color correction sensor 41 is a reflection type photo sensor that detects the level of reflection of the toner patch transferred to the intermediate transfer belt 21.

The control unit 101 is a circuit board mainly composed of a processor and a memory. The control unit 101 controls the operation of each part of the multifunction device 100 including the image forming unit 115 by the processor executing the control program stored in the memory. The operations of the document transport unit 103 and the image reading unit 111 are also included in the control target. Further, the memory includes a non-volatile memory for storing the target value related to the correction. Further, the control unit 101 receives a signal from the temperature / humidity sensor 43 according to the ambient temperature / humidity. The temperature / humidity sensor 43 is arranged on the same circuit board as the control unit 101, for example.

Further, as shown in FIG. 2, the image forming unit 115 includes a charging power supply 14v that supplies a charging voltage to the charging unit 14. The control unit 101 controls the charging voltage via the charging power supply 14v.
Further, the image forming unit 115 includes a development bias power supply 12v that supplies a development bias voltage to the development unit 12. The control unit 101 controls the development bias voltage via the development bias voltage power supply 12v.

Further, the image forming unit 115 includes a primary transfer power supply 16v that supplies a primary transfer voltage to the primary transfer roller 16. The control unit 101 controls the primary transfer development bias voltage via the primary transfer power supply 16v.
Further, the image forming unit 115 includes a secondary transfer power supply 23v that supplies a secondary transfer power supply to the secondary transfer roller 23a. The control unit 101 controls the secondary transfer development bias voltage via the secondary transfer power supply 23v.

The control unit 101 forms a toner patch at a position facing the color correction sensor 41 in the depth direction orthogonal to the traveling direction of the intermediate transfer belt 21. In synchronization with the timing at which the toner patch formed and transferred to the intermediate transfer belt 21 passes through the portion facing the color correction sensor 41, the control unit 101 reads the color correction sensor 41 and determines the reflection level of the toner patch to pass through. To detect. Based on the density of the toner patch read by the color correction sensor 41, the control unit 101 controls the stabilization of the engine characteristics so that the maximum density of each color of Y, M, C and BK and the gamma characteristic become the target characteristics. I do.
The transfer unit 20 includes a primary transfer roller 16, a primary transfer power supply 16a, an intermediate transfer belt 21, a drive mechanism for the intermediate transfer belt 21, and a secondary transfer unit 23 corresponding to each color.

In each process unit Py, Pm, Pc, Pk, a toner image is formed as follows. The drum cleaning unit 15 removes and recovers residual toner on the surface of the rotating photoconductor drum 13. The drum cleaning unit 15 according to this embodiment uses a cleaning blade to remove residual toner on the surface of the photoconductor drum 13. After that, the charging unit 14 uniformly charges the surface of the photoconductor drum 13 to a predetermined potential. The charging unit 14 according to this embodiment uniformly charges the surface of the photoconductor drum 13 by using a charging roller to which a voltage is applied. Then, an electrostatic latent image is formed by exposing the surface of the photoconductor drum 13 charged by the optical scanning unit 11.

After that, the developing unit 12 develops the electrostatic latent image. The developing unit 12 according to this embodiment develops by having a developing roller 12a arranged to face the photoconductor drum 13 carry a developer composed of toner and a carrier and bring it into contact with the surface of the photoconductor drum 13. Magnetic brush development). The toner used corresponds to each color of yellow (Y), magenta (M), cyan (C), and black (BK). As a result, toner images of each color are formed on the surface of each photoconductor drum 13. Each color toner image formed is transferred to the intermediate transfer belt 21 by the transfer mechanism. The transfer mechanism according to this embodiment transfers the toner image on the photoconductor drum 13 to the intermediate transfer belt 21 by applying the primary transfer voltage to the primary transfer roller 16.

The belt cleaning unit 22 removes and recovers the residual toner of the intermediate transfer belt 21. In this embodiment, the belt cleaning unit 22 uses a cleaning blade to remove residual toner on the surface of the intermediate transfer belt 21. The toner images of each color formed on the surface of each photoconductor drum 13 are sequentially transferred to the intermediate transfer belt 21 by the primary transfer roller 16 and superposed, and the toner images of each color are superimposed on the intermediate transfer belt 21. The superimposed color toner images are transferred to the secondary transfer unit 23 and collectively transferred to the printing sheet.

The print sheet is fed from the feed tray 18 to the secondary transfer unit 23 by the pickup roller 33. Alternatively, the paper is fed from the lower paper feed desk (not shown in FIG. 2) to the secondary transfer unit 23 via the sheet transport path R1. Alternatively, it is fed from the manual feed tray 19 and fed to the secondary transfer unit 23. Note that FIG. 1 shows a state in which the manual feed tray 19 is folded and accommodated. When using the manual feed tray 19, the stored manual feed tray is tilted substantially horizontally and a printing sheet is placed on the manual feed tray 19.

In front of the secondary transfer unit 23, a resist roller 34 that temporarily stops the print sheet and aligns the tips of the print sheets is arranged. After temporarily stopping the print sheet, the resist roller 34 transfers the print sheet to the secondary transfer unit 23 at a timing synchronized with the toner image transferred on the intermediate transfer belt 21.

A nip region is formed between the secondary transfer roller 23a of the secondary transfer unit 23 and the intermediate transfer belt 21, and a secondary transfer voltage is applied to the secondary transfer roller 23a. When the print sheet passes through the nip region, the color toner image transferred to the surface of the intermediate transfer belt 21 is transferred to the print sheet by the secondary transfer voltage. The printed sheet to which the toner image is transferred is conveyed to the fixing unit 17 and heated and pressurized while being sandwiched between the heating roller 24 and the pressurizing roller 25. As a result, the color toner image is fixed on the print sheet.

The printed sheet that has passed through the fixing unit 17 is discharged to the discharge tray 39 via the discharge roller 36. Alternatively, it is switched back once before being discharged to the discharge tray 39, and returns to the resist roller 34 via the double-sided transport path 37. Then, the toner image is transferred to the back surface side of the print sheet, and is discharged to the discharge tray 39 via the fixing unit 17 and the discharge roller 36.

≪Printing of 2D gradation pattern≫
FIG. 3 is an explanatory diagram showing an example of a two-dimensional gradation pattern printed by the color multifunction device shown in FIG. 1 for calibration. The horizontal axis of the two-dimensional gradation pattern shown in FIG. 3 shows the change in engine characteristics, and in FIG. 3, the three engine characteristics (1), (2), and (3) are shown as representative values. ..
As described above, there are various elements (parameters) that change the engine characteristics, but (1), (2), and (3) in FIG. 3 show, for example, the intensity of the laser beam with respect to the same gradation value. That is, the characteristics of the laser drive circuit 11d are different. As a result, the exposure intensity of the photoconductor for the same gradation value is different.
As can be understood from the internal configuration of the multifunction device 100 described above, it is difficult to change the engine characteristics in the main scanning direction, but it is possible to change the engine characteristics in the sub-scanning direction.

FIG. 4 is a graph showing an example of the print density with respect to the gradation value, which is the engine characteristics corresponding to (1) to (3) of the two-dimensional gradation pattern shown in FIG. It can be said that it is the state before the gamma characteristic is corrected as a result of calibration, that is, the gamma characteristic of the original engine. (1) corresponds to a state in which the gamma characteristic of the engine is convex downward. (3) corresponds to a state in which the gamma characteristic of the engine is convex upward. Compared with (1) and (3), (2) has a gamma characteristic closer to a straight line and closer to a target gamma characteristic.
Conventional calibration, that is, correction of the maximum density and gamma characteristics by the image processing unit (correction of the maximum density and gamma characteristics of each primary color in the case of color) adjusts the correspondence between the target density value and the gradation value. It is a thing. The gradation value corresponding to the value of each pixel data is determined so that the print density with respect to the pixel data has a substantially straight line (proportional) relationship.

Print density is affected by engine characteristics. However, the engine usually adjusts various parameters related to image formation by feedback control and feedforward control in order to stabilize the engine characteristics.
For example, it is a stabilization control that creates a toner patch on the intermediate transfer belt 21 and causes the color correction sensor 41 to read it, and feeds it back to the charging voltage, the development bias voltage, or the gamma characteristic of the image processing unit 116. Alternatively, it is a stabilization control by feedforward that corrects the charging voltage, the development bias voltage, the primary transfer current, and the secondary transfer current based on the ambient temperature and humidity.

If the usage environment can be supported by the stabilization control of the engine characteristics, it is sufficient to adjust only the correspondence relationship of the gradation values in the image processing unit. Further, even if the image forming apparatus is used in a harsh environment, it may be accepted even if a slight tone jump occurs if the demand for image quality is not strict.
However, there is a limit to the range in which the engine characteristics can be stabilized, and if it is used in a harsh environment beyond that range, it cannot be corrected only by adjusting the gradation value in the image processing unit.

FIG. 5 is an enlarged explanatory view showing an enlarged portion of the engine characteristics shown in FIG. 4 shown by C1 and C2. The left side of FIG. 5 shows C1 and the right side shows C2. C1 is a part of a curve showing the engine characteristic (1). C2 is part of a curve showing the target engine characteristics.
In FIG. 5, a plurality of chain lines extending in the vertical direction correspond to gradation values. The adjacent chain line is one unit of the gradation value (density scale indicating the density of the image data output by the γ-lookup table 116t inside the image processing unit 116) in the output image data, that is, the limit of the resolution of the gradation value. Equivalent to. Further, the plurality of laterally extending chain lines correspond to the maximum density (density A) and the minimum density (density B) to be printed in the portion C1. Those densities are also the maximum and minimum densities to be printed in the C2 portion. Here, the difference between the maximum concentration (concentration A) and the minimum concentration (concentration B) corresponds to one unit of the target concentration value. Here, one unit of the target density value is one unit of the gradation value in the input image data (the density scale indicating the density of the image data input to the γ-lookup table 116t inside the image processing unit 116), that is, the gradation. Corresponds to the limit of value resolution.

As shown in FIG. 5, in C1, a gradation value (a + 1) one larger than the gradation value a corresponding to the minimum density (density B) corresponds to a print density even larger than the density A. However, the gradation value one step smaller than the gradation value (a + 1) is a, and when printing with the gradation value a, the density difference from the minimum density (density B) cannot be obtained. After all, the gradation value a is too lighter than the density A, and the gradation value a + 1 is too darker than the density A. No matter which gradation value is applied, printing at density A cannot be obtained.
On the other hand, in C2 corresponding to the target engine characteristic, there is a difference of 2 units between the gradation values of the maximum density (density A) and the minimum density (density B). The minimum density (density B) corresponds to the gradation value b, and the maximum density (density A) corresponds to the gradation value (b + 2).

Calibration by the image processing unit prints a gradation pattern with a predetermined gradation value, reads the gradation pattern, and uses the relationship between the gradation value obtained from the gradation value and the reading result to obtain an arbitrary target density value. It is a process of determining or updating the correspondence relationship to which any gradation value is associated with. If the difference in density to be printed by the difference of 1 unit of pixel data is smaller than 1 unit of gradation value, it is not possible to select an appropriate gradation value corresponding to both pixel data even if calibration is performed. .. That is, in the C1 part of the engine characteristic (1), even if calibration is performed by the image processing unit due to the engine characteristic in which the slope of the density with respect to the gradation value is steep, it is appropriate because of the resolution limit of the gradation value. It is not possible to associate various gradation values, and the result appears as a tone jump of the gradation pattern in the C1 portion.

The same thing can happen to the C4 portion of the engine characteristics shown in FIG. C4 is a part of the curve showing the engine characteristic (3), but the slope of the density with respect to the gradation value is steeper than that of C3 which is a part of the curve showing the target engine characteristic.
In order to avoid the tone jump caused by the resolution limit of the gradation value as shown in C1 and C4, it is necessary to set the engine characteristic closer to the target engine characteristic, for example, the engine characteristic (2).

The two-dimensional gradation pattern shown in FIG. 3 is for realizing it.
FIG. 4 is a graph in which the print density with respect to the gradation value is plotted for different engine characteristics (1), (2), and (3). It corresponds to the print density of the gradation pattern for each gradation value in the vertical direction in the two-dimensional gradation pattern shown in FIG. Although FIG. 4 shows only three engine characteristics (1), (2), and (3), the two-dimensional gradation pattern shown in FIG. 3 is not limited to this, and the gradation pattern corresponding to various engine characteristics is not limited to this. Includes. From among them, it is sufficient to find a gradation pattern that most closely matches the target engine characteristics. For example, it is the engine characteristic (2) shown in FIGS. 3 and 4. Then, if the parameter value of the engine characteristic corresponding to the gradation pattern is applied to the image forming unit to perform printing, it is possible to avoid the occurrence of tone jump due to the resolution limit of the gradation value.

When calibrating the engine characteristics, the two-dimensional gradation pattern shown in FIG. 3 can be visually confirmed to determine a parameter value that is close to the target gamma characteristic. However, manual confirmation takes time.
Since the multifunction device shown in FIG. 1 includes an image reading unit 111, the printed two-dimensional gradation pattern may be read by the image reading unit 111 to determine the optimum parameter value.
In order to realize this, for example, it is necessary to select the engine characteristic closest to the target engine characteristic from the plurality of engine characteristics shown in FIG.
In this embodiment, the optimum engine characteristics are obtained by calculating the residual sum of squares between the density at each gradation level and the density of the target engine characteristics for the gradation pattern due to each engine characteristic constituting the two-dimensional gradation pattern. The method of determination is described. Once the optimum engine characteristics have been determined, the parameter values corresponding to those engine characteristics are known. The control unit 101 may perform subsequent printing using the parameter value.

FIG. 6 is a graph showing an example of engine characteristics different from those in FIG. Then, as an example, for three different engine characteristics A, B, and C, the residual sum of squares of the density at each gradation level and the density of the target engine characteristic is shown. The residual sum of squares is a statistical quantity that represents the magnitude of the difference between the two curves, and the smaller the residual sum of squares, the smaller the difference.
In the example shown in FIG. 6, the calculation result of the residual sum of squares of the concentration according to the engine characteristic A is 6565. The calculation result of the residual sum of squares of the concentration according to the engine characteristic B is 3696. The calculation result of the residual sum of squares of the concentration according to the engine characteristic C is 33867. Of the three, the one with the smallest residual sum of squares of concentration is the engine characteristic B. Therefore, in the example shown in FIG. 6, the subsequent printing may be performed using the parameter value corresponding to the engine characteristic B.

≪Flowchart of calibration execution≫
FIG. 7 is a flowchart showing an example of the calibration process executed by the control unit 101 in this embodiment.
As shown in FIG. 7, the control unit 101 first checks whether or not an instruction to execute calibration has been received (step S11). The instruction to execute the calibration may be received via the operation unit 105 of the multifunction device 100.
Upon receiving the calibration execution instruction (Yes in step S11), the control unit controls the image processing unit 116 and the image forming unit 115 to print, for example, the two-dimensional gradation pattern shown in FIG.

When the operator sets the output two-dimensional gradation pattern in the image reading unit 111 and instructs the operation unit 105 to read the two-dimensional gradation pattern, the control unit 101 issues the instruction. Recognize (Yes in step S15).
In response to the instruction, the control unit 101 causes the image reading unit 111 to read the set two-dimensional gradation pattern (step S17). Then, among the various engine characteristics, the gradation pattern closest to the target engine characteristic is found from the read two-dimensional gradation patterns (step S19). In this search process, the residual sum of squares of the density at each gradation level described above may be used.
Then, the control unit 101 determines the parameter value of the engine characteristic that gives the gradation pattern closest to the target engine characteristic, which is found in the process of step S19, as the optimum parameter value of the engine characteristic. The determined parameter value is applied to subsequent printing (step S21).
Further, using the relationship between the gradation value and the reading result of the gradation pattern closest to the target engine characteristic found in the process of step S19, the control unit 101 corresponds to the target density value and the gradation value. The relationship is determined or updated (step S23).
The above is an example of the process of executing calibration.

(Embodiment 2)
In the first embodiment, an example of determining the optimum engine characteristics by calculating the residual sum of squares between the density at each gradation level and the density of the target engine characteristics has been described.
Instead of the density at each gradation level, the slope of the density at each gradation level may be evaluated using the residual sum of squares.
That is, in this embodiment, the residual sum of squares of the slope of the density at each gradation level and the slope of the density of the target engine characteristic is calculated for a plurality of engine characteristics.

FIG. 6 shows the result of calculating the residual sum of squares for the slope of the density at each gradation level instead of the density at each gradation level.
The residual sum of squares of the slope of the concentration according to the engine characteristic A is 18.429. The residual sum of squares of the slope of the concentration according to the engine characteristic B is 0.583. The residual sum of squares of the slope of the concentration according to the engine characteristic C is 5.540. Of the three, the one with the smallest residual sum of squares of the slope of concentration is the engine characteristic B. Therefore, in the example shown in FIG. 6, the evaluation result using the density of each gradation level and the evaluation result using the slope of the density show the same result that the engine characteristic B is optimum. It can be said that a reasonable result can be obtained regardless of which one is used for the evaluation. However, the results of the evaluation may differ.

FIG. 8 is a graph showing an example of engine characteristics different from those in FIGS. 4 and 6. As an example, for three different engine characteristics D, B, and C, the residual sum of squares between the density at each gradation level and the density of the target engine characteristic is shown.
In the example shown in FIG. 8, the calculation result of the residual sum of squares of the concentration related to the engine characteristic D is 3495. The calculation results of the residual sum of squares of the concentrations related to the engine characteristics B and C are 3696 and 33867, respectively, as described in FIG. Of the three, the one with the smallest residual sum of squares of concentration is the engine characteristic D.

On the other hand, the residual sum of squares of the slope of the concentration according to the engine characteristic D is 0.645. The residual sum of squares of the concentration slopes with respect to engine characteristics B and C are 0.583 and 5.540, respectively, as described in FIG. Of the three, the one with the smallest residual sum of squares of the slope of concentration is the engine characteristic B.
As described above, in the example shown in FIG. 8, the evaluation result using the density of each gradation level and the evaluation result using the slope of the density are different.
In high-precision calibration, it is important to bring the gamma characteristic, especially in the case of color, the color tone due to the gamma characteristic of each primary color closer to the target, so from that point of view, the evaluation result using the slope of the density at each gradation level is It can be said that it gives more reasonable results. In fact, in the example shown in FIG. 8, the difference between the engine characteristics B and D with respect to the target engine characteristics is slight, but it can be said that the engine characteristic B is closer to the target engine characteristics.

(Embodiment 3)
In the first embodiment, it is premised that the engine characteristics are calibrated using one two-dimensional gradation pattern, but if the one two-dimensional gradation pattern cannot be used for precise calibration, A plurality of two-dimensional gradation patterns may be used.
For example, after printing a two-dimensional gradation pattern with roughly changed engine characteristics on the first sheet and tentatively determining the parameter values, the engine characteristics are finely changed centering on the tentatively determined parameter values on the second sheet. The two-dimensional gradation pattern may be printed to determine the final parameter value.
Alternatively, the first parameter value is determined by printing a two-dimensional gradation pattern in which the engine characteristics are changed by changing the exposure intensity related to the characteristics of the laser drive circuit 11d, for example, on the first sheet. do. When further adjustment is required, the second sheet is printed with a second parameter, for example, a two-dimensional gradation pattern in which the engine characteristics are changed by changing the combination of the charging voltage and the bias voltage, and the second is printed. It may be controlled to determine the parameter value.
Further, as a series of calibrations, first, as shown in FIG. 3, one or more two-dimensional gradation patterns having changed engine characteristics are printed to determine the parameter values. Subsequently, a two-dimensional gradation pattern in which the gamma characteristic of the image processing unit 116 is changed by the engine characteristic to which the determined parameter value is applied may be further printed to determine the final parameter value.

As mentioned above,
(I) The image forming apparatus according to the present invention is an image processing unit that represents an image to be printed as a set of pixel data and outputs a gradation value corresponding to each pixel data, and converts each output gradation value into a scanning signal. An image forming unit that forms an image having a density corresponding to the scanning signal thereof, and a control unit that controls the image processing unit and the image forming unit are provided, and the control unit is used for calibrating the image forming unit. It is a two-dimensional gradation pattern, and the gradation value generated by the image processing unit is changed by keeping a predetermined parameter that affects the density of the image formed by the image forming unit constant in the main scanning direction related to the scanning signal. It is characterized in that a two-dimensional gradation pattern in which the gradation value is kept constant and the parameter is changed in the sub-scanning direction is printed.

In the present invention, the pixel data treats an image as a set of pixels, and the data representing the characteristics of each pixel is used as pixel data.
Further, the gradation value is a value of data or a signal sent by the image processing unit to the image forming unit (engine) corresponding to the pixel data, and is suitable for printing the pixel corresponding to the pixel data to the image forming unit. The value was determined in consideration of engine characteristics and the like. As an example, if the pixel data is represented by a digital value with a depth of 10 bits, the gradation value is represented with a depth of 12 bits. The correspondence between the pixel data and the gradation value is not just a proportional relationship, but the correspondence relationship is determined so as to correct the gamma characteristic of the engine element and bring it closer to the target engine characteristic. Calibration can be said to be a process of determining or updating the correspondence.

The gradation level represents a stage of each gradation to be targeted when printing a gradation pattern (including a two-dimensional gradation pattern), and is a value corresponding to (proportional to) the target density.
When simply described as concentration, it indicates the actual concentration, not the target concentration described above.
Furthermore, the scanning signal is a signal that sequentially corresponds to each pixel when the image is sequentially scanned and read or printed. Usually, it is a signal that represents the digital signal corresponding to each pixel data in the order of scanning. The main scanning direction (also referred to as the line direction) is the direction corresponding to the arrangement of adjacent pixels on the scanning signal. The sub-scanning direction is a direction orthogonal to the sub-scanning direction and corresponds to the direction in which the lines are lined up.

A predetermined parameter that affects the density of the image formed by the image forming unit is a parameter related to image forming that affects the engine characteristics. Specific embodiments thereof include exposure intensity to the photoconductor, charge potential of the photoconductor, development bias voltage, transfer current, and the like in electrophotographic image formation. The image forming unit in the above-described embodiment is an engine of a color multifunction device, and the transfer current is mainly related to the primary transfer current.

Further, a preferred embodiment of the present invention will be described.
(Ii) The image forming unit may be an electrophotographic system, and the parameter may be at least one of the exposure intensity corresponding to the scanning signal, the charging potential of the photoconductor, the development bias voltage, and the transfer current.
In this way, at least one of the parameters of exposure intensity, development bias voltage, charging potential of the photoconductor or transfer current can be optimized by calibration. A two-dimensional gradation pattern may be printed in which three parameters of the exposure intensity, the development bias voltage, the charging potential of the photoconductor, and the transfer current are fixed and only one parameter is changed. For example, a two-dimensional gradation pattern in which only the exposure intensity is changed may be printed. Alternatively, a two-dimensional gradation pattern changed by combining a plurality of parameters may be printed.

(Iii) An image reading unit for reading the printed two-dimensional gradation pattern is further provided.
The control unit subsequently sets parameters in the sub-scanning direction in which the gradation pattern in the main scanning direction of the two-dimensional gradation pattern read by the image reading unit most closely resembles a predetermined reference gradation pattern. It may be a parameter value of the image forming unit in printing.
By doing so, calibration can be performed without visual inspection by having the image reading unit read the printed two-dimensional gradation pattern.

(Iv) The control unit sets each floor of the density at each gradation level of the gradation pattern in the main scanning direction corresponding to each parameter in the sub-scanning direction of the read two-dimensional gradation pattern and the reference gradation pattern. The parameter that minimizes the residual sum of squares with the density at the tuning level may be used as the parameter value of the image forming unit.
By doing so, it is possible to determine the parameter that realizes the engine characteristic most closely to the reference characteristic by using the residual sum of squares of the density at each gradation level as the evaluation index.

(V) The control unit determines the slope of the density at each gradation level of the gradation pattern in the main scanning direction corresponding to each parameter in the sub-scanning direction of the read two-dimensional gradation pattern and the reference gradation pattern. The parameter that minimizes the residual sum of squares with the slope of the density at each gradation level may be used as the parameter value of the image forming unit.
By doing so, it is possible to determine the parameter that realizes the engine characteristic that most closely resembles the reference characteristic by using the residual sum of squares of the density gradient at each gradation level as the evaluation index.

(Vi) The control unit determines the parameter value of the image forming unit based on the printed two-dimensional gradation pattern, and then uses the determined parameter value to cause the two-dimensional order in the image processing unit. A gradation pattern having a change width smaller than the change of the adjacent gradation values of the adjustment pattern is printed, and each gradation value is adjusted so as to match the reference gradation pattern. It may be a gamma characteristic.
By doing so, it is possible to add calibration to the image processing unit using the gradation pattern in which the gamma characteristic is changed by the image processing unit using the determined parameters.

(Vii) One aspect of the present invention is an image processing unit that outputs a gradation value corresponding to pixel data, and an image forming unit that converts the gradation value into a scanning signal and prints a density corresponding to the scanning signal. A control unit that controls an image forming apparatus including The step of printing the two-dimensional gradation pattern in which the gradation value is constant in the direction and the parameter is changed, and the step of reading the two-dimensional gradation pattern printed by using the image reading unit are read. The parameter in the sub-scanning direction in which the density of the two-dimensional gradation pattern at each gradation level in the main scanning direction most closely matches the predetermined reference gradation pattern is used as the parameter value of the image forming unit in the subsequent printing. Includes a method of calibrating an image forming apparatus comprising a step.

Aspects of the present invention also include a combination of any of the plurality of embodiments described above.
In addition to the embodiments described above, there may be various variations of the present invention. These variations should not be construed as not belonging to the scope of the present invention. The invention should include claims and equivalent meaning and all modifications within said scope.

11: Optical scanning unit, 11d: Laser drive circuit, 12: Development unit, 12v: Development bias power supply, 13: Photoconductor drum, 14: Charging unit, 14v: Charging power supply, 15: Drum cleaning unit, 16: 1 primary transfer Roller, 16v: 1st transfer power supply, 17: Fixing unit, 18: Feed tray, 19: Manual feed tray, 20: Transfer unit, 21: Intermediate transfer belt, 22: Belt cleaning unit, 23: Secondary transfer unit, 23a : Secondary transfer roller, 23v: Secondary transfer power supply, 24: Heating roller, 25: Pressurized roller, 33: Pickup roller, 34: Resist roller, 36: Discharge roller, 37: Double-sided transport path, 39: Discharge tray, 41: Color correction sensor, 43: Temperature / humidity sensor 100: Complex machine, 101: Control unit, 103: Document transfer unit, 105: Operation unit, 111: Image reading unit, 115: Image forming unit, 116: Image processing unit, 116t: γ-lookup table Py, Pm, Pc, Pk: process unit, R1: sheet transfer path

Claims (7)

An image processing unit that represents an image to be printed as a set of pixel data and outputs gradation values according to each pixel data.
An image forming unit that converts each output gradation value into a scanning signal and forms an image with a density corresponding to the scanning signal.
A control unit for controlling the image processing unit and the image forming unit is provided.
The control unit is a two-dimensional gradation pattern for calibrating the image forming unit, and constantly determines a predetermined parameter that affects the density of the image formed by the image forming unit in the main scanning direction related to the scanning signal. An image forming apparatus that changes the gradation value generated by the image processing unit, keeps the gradation value constant in the sub-scanning direction, and prints a two-dimensional gradation pattern in which the parameters are changed. The image according to claim 1, wherein the image forming unit is an electrophotographic system, and the parameter is at least one of an exposure intensity, a development bias voltage, a charging potential of a photoconductor, or a transfer current corresponding to the scanning signal. Forming device. Further equipped with an image reading unit that reads the printed two-dimensional gradation pattern,
The control unit subsequently sets parameters in the sub-scanning direction in which the gradation pattern in the main scanning direction of the two-dimensional gradation pattern read by the image reading unit most closely resembles a predetermined reference gradation pattern. The image forming apparatus according to claim 1 or 2, which is a parameter value of the image forming unit in printing. The control unit is at each gradation level of the density at each gradation level of the gradation pattern in the main scanning direction corresponding to each parameter of the sub-scanning direction of the read two-dimensional gradation pattern and the reference gradation pattern. The image forming apparatus according to claim 3, wherein the parameter that minimizes the residual sum of squares with the density is used as the parameter value of the image forming unit. The control unit has a density gradient at each gradation level of the gradation pattern in the main scanning direction corresponding to each parameter of the sub-scanning direction of the read two-dimensional gradation pattern and each gradation level of the reference gradation pattern. The image forming apparatus according to claim 3, wherein the parameter that minimizes the residual sum of squares with the gradient of the density in the image forming unit is used as the parameter value of the image forming unit. The control unit determines the parameter value of the image forming unit based on the printed two-dimensional gradation pattern, and then uses the determined parameter value to send the two-dimensional gradation pattern to the image processing unit. A gradation pattern with a change width smaller than the change of adjacent gradation values is printed, and each gradation value is adjusted so as to match the reference gradation pattern, which is the gamma characteristic of the image processing unit in the subsequent printing. The image forming apparatus according to any one of claims 1 to 4. Control to control an image forming apparatus including an image processing unit that outputs a gradation value according to pixel data and an image forming unit that converts the gradation value into a scanning signal and prints a density corresponding to the scanning signal. The department
The gradation value is changed by making the parameter that affects the density of the image formed by the image forming unit constant in the main scanning direction related to the scanning signal, and the gradation value is made constant and the parameter is set in the sub-scanning direction. Steps to print the changed 2D gradation pattern,
The step of reading the two-dimensional gradation pattern printed using the image reading unit, and
The parameter value of the image forming unit in the subsequent printing is the parameter of the sub-scanning direction in which the density of the read two-dimensional gradation pattern at each gradation level in the main scanning direction is most similar to the predetermined reference gradation pattern. A method of calibrating an image forming apparatus including a step.

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