JPH10145598A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce an error in image density control due to paper sheet kind difference in a high-light part and the dispersion of a density measuring instrument while evading excessive density correction in a high density part by correcting a density according to a correcting amount which is decreased as the density of a reference patch is increased. SOLUTION: When a density conversion table is generated, a correction color patch is printed and outputted from an image forming part. The correction color patch print is set in a platen. A reading part measures the color patch densities of respective 24 colors so as to obtain a present gradation property. The density measurement result is transmitted to an image density control means and it is judged whether or not a problem exists in the measurement result. Unless the problem exists, the image density control means compares present gradation property with prescribed target gradation property, generates the density conversion table based on the comparison result and stores it in a memory. The density of the image is converted based on the density conversion table at the time of outputting the image.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus, and more particularly, to forming a plurality of reference patches having different densities on a sheet of paper and creating a density conversion table based on the densities of the formed plurality of reference patches. The present invention also relates to an image forming apparatus that performs image density control for converting density characteristics of image data based on a created density conversion table.

[0002]

2. Description of the Related Art Conventionally, a plurality of reference patches having different densities are formed on a sheet, a density conversion table is created based on the densities of the formed plurality of reference patches, and a density conversion table is created based on the created density conversion table. Many techniques relating to image density control for converting density characteristics of image data have been proposed (see Japanese Patent Application Laid-Open No. Sho 64-41375).

In an image formed by an image forming apparatus, the reproducibility of a low-density region (highlight portion) is particularly sensitive to human eyes. Therefore, it is important to stabilize the highlight portion even in the above-described image density control. It is.

However, to stabilize the highlight part,
There are two major problems: The first problem is that even if toner is developed / transferred / fixed on the same amount of paper due to the type of paper, the result of measuring the reference patch density on the paper (especially the reference Measurement result) is different, resulting in an error in the density conversion table. For this reason, in the above-described image density control, image density control is generally performed on the assumption that a specific color dedicated paper is used. However, the customer does not always use this specific color dedicated paper. When different papers are used, an error occurs in the image density control result especially in the highlight portion.

FIG. 13 is a diagram for explaining this problem.
FIG. 13A shows the measurement result of the yellow component of the reference patch density at 24 points on a color-dedicated paper (hereinafter, referred to as J paper) on which the same amount of toner has been developed / transferred / fixed, as a curve J1.
Curve R1 shows the measurement results of the yellow component of the reference patch density at 24 points on the recycled paper (hereinafter referred to as R paper). As is apparent from FIG. 13 (A), the density measurement value of the highlighted portion is higher in the R paper having a yellow base than in the J paper.

FIG. 13B shows a density conversion table created based on the reference patch density measurement results shown in FIG. 13A. In FIG. 13B, a density conversion table for J paper is shown as a curve J2, a density conversion table for R paper is shown as a curve R2, and a curve without conversion is shown as a curve E. Of these, the density conversion table for R paper (curve R2) is a density conversion table in the direction of greatly reducing the density of the highlighted portion based on the result of FIG. However, the density of the highlight portion is not actually high, but a density conversion table in the direction of greatly reducing the density of the highlight portion is created due to the color of the background. Therefore, this density conversion table (curve R2)
14, the curve A1 indicating the density gradation after the image density control in FIG. 14 has a lower density in the highlight portion than the curve A2 indicating the target when the correction is performed on the J paper. I will.

As a technique for detecting the background of a sheet,
Japanese Patent Application Laid-Open No. Sho 62-296669 discloses a technique for performing color correction (color masking processing) by detecting a plurality of colors and a paper background by using IIT. , A technique for performing color correction (color masking processing) has been proposed, but both of these techniques are intended for color masking processing, and a technique for creating a density conversion table and controlling image density as in the present application. Are essentially different.

[0008] The second problem is that the density measurement result varies due to an error of a measuring device for measuring the reference patch density. For example, there is almost no problem if a commercially available density measuring device is used.However, when a simple optical sensor or an image reading means for reading an original image is used as the density measuring device, the measurement results especially in the highlight portion may be repeatedly repeated. There is.

[0009] For example, FIG.
An example of a measurement result (curve B02) of a reference patch density of a point and a target value (curve B01) are shown. In FIG. 15A,
The highlight of the measurement result of the curve B02 is lower (thinner) than the target value of the curve B01. However, the portion where the horizontal axis is “0” (= Cin 0% portion, where Cin means the formation density condition of the reference patch and CinN% means the formation density condition N% of the reference patch) Therefore, when color paper is used, the measurement result should always be equal to the target value, and the difference between the above measurement result (curve B02) and the target value (curve B01) is the variation of the measuring instrument. Is the cause. Therefore, when a density conversion table is created using the measurement result (curve B02) in FIG. 15A, as shown by a curve L01 in FIG. Become. When the image density control is performed by using such a density conversion table, as shown in FIG. 15C, the density after the image density control (curve D03) is higher than the target density (curve D02) in the rise of the highlight portion. Will be faster.

[0010] In order to correct the above-described error in the highlighted portion, the relative value between the measurement result of the background portion and the measurement result of another patch is obtained by using a relative density method which is one of the general density measurement methods. To create a density conversion table based on the relative values.

[0011]

However, the influence of the paper background is large in the highlight portion, but hardly in the high-density portion, and the measurement result of the reference patch generally shows a slope between the highlight portion and the high-density portion. The difference in the measurement result of the highlight part is larger than the error of the measurement result of the high-density part. It is difficult to appropriately control the image density of both portions.

For example, FIG. 16A shows a measurement result (curve B12) and a target value (curve B11) of the reference patch density at 24 points on a sheet when the relative density method is used. Here, the measurement result and the target value each take a relative concentration, and C
Since the value at in0% was adjusted to 0, and the relative concentration with Cin0% was taken over the entire concentration range, the relative concentration difference was large on the high concentration side where there was almost no difference in absolute concentration. In the density conversion table obtained from the target value obtained by taking the relative density and the measurement result, as shown by the curve L02 in FIG. 16B, the density is largely reduced on the high density side.

When image density control is performed using such a density conversion table, as shown in FIG. 16C, the density after image density control (curve D13) becomes the target density (curve D1).
Compared with 2), the rising of the highlighted portion is almost the same, but the target density (curve D1)
The density is much lower than in 2).

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and it is possible to avoid excessive density correction in a high-density portion and to reduce the difference in the type of paper in a highlight portion and a density measuring device. An object of the present invention is to provide an image forming apparatus capable of reducing an error in image density control caused by variation.

[0015]

According to another aspect of the present invention, there is provided an image forming apparatus, comprising: a reference patch forming unit configured to form a plurality of reference patches having different densities on an image carrier; Density measuring means for measuring the density of each reference patch formed by the reference patch forming means and the background density of the image carrier; and measuring the background density measured by the density measuring means with a predetermined background density. Background correction amount calculating means for calculating a reference background correction amount from the target value;
Either the density measurement value of each reference patch or the predetermined density target value of each reference patch is set to a value that does not exceed the reference background correction amount calculated by the background correction amount calculation means and the density of the reference patch is high. Density correction means for correcting with a correction amount that becomes lower as the density increases, based on the density measurement value and the density target value corrected by the density correction means,
Or, based on the corrected density target value and the density measurement value, density conversion means for converting the density characteristics of the image data,
It is characterized by having.

Further, in the image forming apparatus according to the second aspect,
2. The image forming apparatus according to claim 1, wherein the density correction unit performs correction with the correction amount set based on a ratio of density conditions corresponding to each reference patch and the reference background correction amount. .

Further, in the image forming apparatus according to the third aspect,
2. The image forming apparatus according to claim 1, wherein the density correction unit performs correction with the correction amount set based on a ratio according to a density measurement value of each reference patch and the reference background correction amount. 3. I do.

Further, in the image forming apparatus according to the fourth aspect,
4. The image forming apparatus according to claim 1, wherein the plurality of reference patches include a reference patch having a density of 0, and the density correction unit measures the density of the reference patch having a density of 0. 5. The value or the density target value is corrected by a correction amount equal to the reference background correction amount.

Further, in the image forming apparatus according to the fifth aspect,
5. The image forming apparatus according to claim 1, wherein the density measuring unit includes an image reading unit that reads an image of a document.

In the image forming apparatus according to the first aspect of the present invention, a plurality of reference patches having different densities are formed on the image carrier by the reference patch forming means, and the density of each of the formed reference patches and the base density of the image carrier are formed. Is measured by a concentration measuring means. Note that the image carrier is a general concept including paper, and in the present invention, a plurality of reference patches are formed on paper as an image carrier, and the density of each of the formed reference patches and the background density are measured.

Then, a reference background correction amount is calculated by the background correction amount calculating means from the measured background density measurement value and a predetermined background density target value. Further, a correction amount that does not exceed the calculated reference background correction amount and decreases as the density of the reference patch increases increases the density measured value of each reference patch or the density target value of each predetermined reference patch. One of them is corrected by the density correcting means.

According to the invention, the density correction means decreases as the density of the reference patch increases based on the ratio of the density condition corresponding to each reference patch and the reference background correction amount. The correction may be performed using a correction amount set to be as follows. For example, a value obtained by multiplying the reference background correction amount by the ratio of the density condition for each reference patch can be used as the correction amount.

Further, based on the ratio according to the measured density value of each reference patch and the reference background correction amount, the density is set to decrease as the density of the reference patch increases. The correction may be performed according to the correction amount. However, the correction according to the ratio according to the actual density measured value of each reference patch as in the invention according to claim 3 is more effective than the correction using the ratio of the above-mentioned predetermined density condition for each reference patch. There is an advantage that more accurate correction can be performed in accordance with the density measurement result (density gradation) at that time.

Next, when the density measurement value of the reference patch is corrected by the density correction means as described above, the density conversion means performs image processing based on the corrected density measurement value and the density target value as follows. Convert the density characteristics of the data. For example,
A density conversion table is created based on the measured density value of each reference patch and the target density value corrected by the correction amount that decreases as the density of the reference patch increases. Here, in the highlight portion where the difference due to the difference in the type of paper (the effect of the background) and the variation of the density measuring device is strong, the correction rate based on the above-described reference background correction amount increases, and as the density increases. The correction rate based on the reference background correction amount decreases.

For this reason, the density conversion table created by the density gradation after the above-mentioned correction is performed, for example, as shown in FIG.
As shown by the curve G2 in (B), in the highlight portion, the density conversion table (curve G
In the high-density portion, which is close to 1) and is no longer affected by the difference in the type of paper (the effect of the background) or the variation of the density measuring device, the density conversion table (curve G3) using the original paper (recycled paper) is close. .

Further, the density characteristic of the image data is converted based on the created density conversion table. For example,
The density gradation when the image density control is performed by applying the density conversion table (curve G2 in FIG. 17B) is shown by a curve A3 in FIG. As is apparent from FIG. 14, in the highlight portion, the density gradation is close to the target density gradation (curve A2).

On the other hand, when the density correction means corrects the density target value of the reference patch, the density conversion means determines the density of the image data based on the corrected density target value and the measured density value in the same manner as described above. Convert characteristics. For example, a density conversion table is created based on a density target value and a density measurement value of each reference patch corrected by a correction amount that decreases as the density of the reference patch increases. Here, in the highlight portion where the difference due to the difference in the type of paper (the effect of the background) and the variation of the density measuring device is strong, the correction rate based on the above-described reference background correction amount increases, and as the density increases. Since the correction rate based on the reference background correction amount decreases, the same effect as when the density measurement value of the reference patch is corrected can be obtained.

As described above, according to the first aspect of the present invention, it is possible to avoid excessive density correction in a high density portion while causing a difference in the type of paper and a variation in a density measuring device in a highlight portion. An error in image density control can be reduced.

In the above description, a plurality of reference patches may include a reference patch having a density of 0, as in the fourth aspect of the present invention. In this case, since the density of the reference patch having the density of 0 corresponds to the density of the background, the density correction means corrects the measured density value or the target density value of the reference patch having the density of 0 with a correction amount equal to the reference background correction amount. It is desirable to do.

Further, the density measuring means may be constituted by an image reading means for reading an image of a document, as in the fifth aspect of the present invention. With the image reading means, it is not necessary to provide a dedicated density measuring device as the density measuring means, so that the number of components and the cost of the image forming apparatus can be reduced.

[0031]

BEST MODE FOR CARRYING OUT THE INVENTION

[First Embodiment] Hereinafter, a first embodiment of the present invention will be described with reference to the drawings.
An embodiment will be described.

[Overall Configuration of Color Copier] FIG. 1 is an overall configuration diagram of a color copier to which the present invention is applied, and FIG. 2 is a block diagram of a color copier to which the present invention is applied.

As shown in FIGS. 1 and 2, the color copier 1
Numeral 0 denotes a reading unit 20 for reading a document, an image processing unit 30 for processing read image data, a ROS optical unit 40 for driving a laser according to the processed image data to irradiate a light beam to a photoconductor, and forming an image. And an image forming unit 60.

As shown in FIG. 2, in the reading section 20, a document G placed at a predetermined position on the table 12 (see FIG. 1) is irradiated by an exposure lamp 22 and reflected light thereof is reflected by a CCD image sensor (hereinafter referred to as a CCD image sensor). ). The image signal read by the CCD 24 is amplified to an appropriate level by an amplifier 26, and the amplified image signal is A / D
The data is converted by the converter 28 into 8-bit digital image data. The shading correction and the gap correction are sequentially performed on the digital image data. The digital image data subjected to these corrections is converted into density data by the density converter 29 and sent to the image processing unit 30.

The image processing unit 30 performs basic image processing as a color copying machine, that is, color signal conversion, black reproduction (UC
R), MTF processing and the like are performed, and the image data is converted into image data of four colors of yellow, magenta, cyan, and black. The converted image data of each color is subjected to gradation conversion in accordance with the gradation of the reading unit 20 and the image forming unit 60. Further, the image processing unit 30 receives image data (for example, image data (CG image data) created by computer graphics, image data stored in a CD-ROM, etc.) from an external image processing device or the like. External data input means 33 and an image density control means 31 for creating a density conversion table for converting the density characteristics of the image data. The image density control means 31 performs the gradation conversion. Image data or the external image data is subjected to density conversion based on a density conversion table. The external data input means 33 is, for example, a floppy disk reader,
It can be constituted by a CD-ROM reading device, a communication processing device for receiving data from an external image processing device or the like via a network, or the like.

The image data subjected to the density conversion is D / D
The data is converted into analog image data by the A converter 32 and sent to the comparator 39 via the selector 34. In the comparator 39, pulse width modulation is performed by comparing the sent analog image data with a signal of a predetermined period output from the triangular wave generator 38, and the analog image data is converted into binary image data. Is converted. In the pulse width modulation here, for example, as shown in FIG. 3, the input analog image data A is compared with the triangular wave B from the triangular wave generator 38, and the portion where the analog image data A is larger than the triangular wave B is " 0 "
(Laser off), and binary image data in which the portion of the analog image data A smaller than the triangular wave B is "1" (laser on) is generated, and sent from the comparator 39 to the ROS optical unit 40.

The image processing section 30 is provided with patch signal generating means 36 for generating image signals (hereinafter referred to as patch image signals) of a plurality of image density control patches (reference patches) having different densities. I have. The selector 34 selects the analog image data from the D / A converter 32 at the time of normal copying, and, when receiving an instruction to create a reference patch from the arithmetic unit 84 of the image forming unit 60 described later, generates a patch signal generating means. The patch image signal from 36 is selected, sent to the comparator 39, and binarized by the pulse width modulation as described above.

The ROS optical unit 40 is controlled by a laser drive circuit 42 for controlling on / off of the laser 46 based on the binary image data sent from the comparator 39, and a control unit 84 of the image forming unit 60 described later. And a laser light amount varying device 44 for variably controlling the laser light amount. The laser light is deflected by the polygon mirror 48 and f
Image forming unit 60 via θ lens 50 and reflection mirror 52
To the photoreceptor 62.

As shown in FIGS. 1 and 2, the image forming section 6
0, a photoconductor 62 is provided.
Around the charging device 68, an electrometer 70 for measuring the potential on the photoconductor for controlling the potential of the photoconductor, a rotary developing device 72, and measuring the patch density on the photoconductor for controlling the toner dispensing. An optical sensor 74, a transfer device 80, a cleaner device 64, and a discharge lamp 66 are provided. Further, the image forming unit 60 includes a toner dispensing device 76 that supplies toner to the developing devices of the respective colors of the rotary developing device 72, a fixing device 88, and a sheet conveying device 9.
2 is also provided.

Further, the image forming section 60 includes an arithmetic unit 84 for controlling the entire image formation and controlling image forming conditions in accordance with the outputs of the electrometer 70 and the optical sensor 74, and an arithmetic unit 84.
And a developing bias variable device 78 that changes the developing bias under the control of the arithmetic device 84. The arithmetic unit 84 includes the patch signal generator 3
6 and the selector 34 are instructed to create a reference patch.

[Outline of Image Forming Process] Next, an outline of the image forming process executed by the image forming section 60 will be described. In the image forming section 60, the following image forming processing is executed according to a well-known xerographic process. That is, the photoconductor 62 rotating clockwise in FIG.
8, the first black latent image of the first color is formed on the photoconductor 62 by the laser light from the ROS optical unit 40. This latent image is developed with black toner by the black developing device of the rotary developing device 72. The developed black toner image is transferred by a transfer corotron 80 </ b> A to a sheet (not shown) that is transported from a paper tray 90 by a paper transport device 92 and wound around a transfer drum 80. The toner image remaining on the photoconductor 62 without being transferred is removed by the cleaner device 64, and the photoconductor 6 is removed.
2 is discharged by the discharge lamp 66.

Then, the photosensitive member 62 is uniformly negatively charged again by the charging device 68, and the second color yellow image is subsequently formed. In this manner, the toner images of a total of four colors from the third color magenta to the fourth color cyan are sequentially transferred to the paper wound around the transfer drum 80. The paper to which the four color toner images have been transferred is separated from the transfer drum 80 by the separation corotron 80B, and
The color toner image is fixed on the paper, and a color copy is formed. After the transfer of each color or the peeling of the paper, the extra charge on the paper and the transfer drum 80 is discharged by the charge removing corotron 80C.
The charge is removed.

[Overview of Photoconductor Potential Control] Next, the photoconductor 6
The control of the photoconductor potential by the charge amount varying device 82, the developing bias varying device 78, and the laser light amount varying device 44 by the electrometer 70 for measuring the potential on 2 will be briefly described.

In the present embodiment, the arithmetic unit 84 of the image forming unit 60 before the start of copying immediately after the power of the color copying machine 10 is turned on and before the start of copying every 30 minutes thereafter.
Thus, the photoconductor potential control is performed according to the flowchart of FIG. The memory of the arithmetic unit 84 stores in advance a target dark potential VHS, a target exposure partial potential VLS, and a fog prevention potential difference VC from the target dark potential VHS to the developing bias potential VB.

First, at step 152 in FIG.
8 is applied to the voltage VG by the charge amount varying device 82.
The dark potential VH1 when set to 1 and the dark potential VH2 when set to the voltage VG2 are detected by the electrometer 70. In the next step 154, the target dark potential VH is calculated using the following equation (1).
The grid voltage VGS for obtaining S is calculated.

VGS = VG1 + ((VG2−VG1) × (VHS−VH1) / (VH2−VH1)) (1) In the next step 156, the photosensitive member 62 is moved to the above step 1
It is charged with the grid voltage VGS obtained in 54. And
Laser light amount LD by laser light amount variable device 44
1. The laser drive circuit 42 is driven by two kinds of laser light amounts LD1 and LD2 so that two kinds of laser light amounts L
A reference patch is formed by D1 and LD2. Further, the exposure partial potential VL of each of the two formed reference patches
1. VL2 is measured by the electrometer 70. In the next step 158, a laser light amount LDS for obtaining the target exposure partial potential VLS is calculated using the following equation (2).

LDS = LD2 − ((LD2−LD1) × (VLS−VL2) / (VL1−VL2)) (2) In the next step 160, development is performed using the following equation (3). Calculate the bias potential VB. VC indicates a fog prevention potential difference.

VB = VHS-VC (3) In the next step 162, the grid voltage VGS obtained as described above is developed into the charging amount varying device 82, and the laser light amount LDS is developed into the laser light amount varying device 44. The bias potential VB is set in the developing bias variable device 78, and the process ends.

[Overview of Toner Dispensing Control]
Dispensing control of each color toner for the rotary developing device 72 will be described. The toner dispensing control is realized by measuring the patch density for toner dispensing control on the photoconductor 62 by the optical sensor 74 and controlling the operation of the toner dispensing device 76 by the arithmetic unit 84 based on the measured patch density. I do. The toner dispensing control patch is instructed to be created by the arithmetic unit 84.

When an instruction to create a patch is issued from the arithmetic unit 84, the selector 34 selects a patch image signal for each color for toner dispense control with an image area ratio of 50% from the patch signal generating means 36 and sends it to the comparator 39. Then, a reference patch for each color having an image area ratio of 50% is formed on a non-image portion on the photoconductor 62 in the same procedure as the above-described image forming process of the color copying machine.

The optical sensor 74 for measuring the patch density on the photosensitive member 62 irradiates the reference patch P on the photosensitive member 62 with light from the LED 74A as shown in FIG. 4, and measures the reflected light with the photodiode 74B. The patch density is measured based on the measured reflected light amount.

If the patch density measured here is lower than the target value, the arithmetic unit 84 sets the toner dispensing unit 76
Is driven to increase the toner density so that the patch density approaches the target value. Conversely, when the measured patch density is higher than the target value, the arithmetic unit 84 stops the toner dispensing device 76 and brings the patch density closer to the target value.

[Outline of Image Density Control] Next, a plurality of reference patches having different densities are formed, a density conversion table is created based on the density measurement results, and density characteristics of image data are created based on the created density conversion table. Will be described with reference to a flowchart of a density conversion table creation process in FIG. 6 and a schematic diagram of image density control in FIG.

As shown in FIGS. 6 and 7, when creating the density conversion table, the arithmetic unit 84 sends a signal for creating a correction color patch to the patch signal generating means 36, and each color selector 34 sets the patch signal generating means 36. The correction color patch image signal is selected and sent to the comparator 39, and the correction color patch is printed and output on paper in the same image forming procedure as that of the above-described color copying machine (step S1).
Here, for example, as shown in FIG. 12, a correction color patch print composed of 24 gradation patches of four colors of yellow, magenta, cyan, and black and having different densities for each color is output. In addition, Y attached to the upper stage in FIG. 12 is yellow,
M indicates magenta, C indicates cyan, and K indicates black.

Next, the reading unit 20 of the color copying machine 10
Is used as a density measuring device for correction color patch prints, the operator sets the correction color patch prints on the mounting table (platen) 12 (step S).
2). The density measuring device for the color patch print for correction may use a densitometer other than the reading unit 20 of the color copying machine 10.

Next, the reading section 20 measures the density of 24 color patches of each color to determine the current gradation (step S3). The result of the density measurement is sent to the image density control means 31, and it is determined whether or not the measurement result has a problem (step S4). If there is no problem here, the image density controller 31 compares the current gradation with a predetermined target gradation, and creates a density conversion table based on the comparison result and stores it in the memory (step S5). On the other hand, if there is a problem with the measurement result in step S4, a warning is displayed to the operator and the processing is stopped because there is a possibility that the correction color patch print is not properly placed (step S4).
6).

After the density conversion table has been prepared as described above, the original image data sent from the reading unit 20 is subjected to color conversion and image conversion by the image processing unit 30 at the time of image output, as shown in FIG. After the gradation conversion processing, the density of the image is converted based on the created density conversion table so that the gradation of the image matches the target gradation.
Similarly, when printing image data transmitted from the outside, based on the created density conversion table, an external image data is output so that the gradation of the image matches the target gradation. Density conversion is performed on the image data.

[Execution Result of Image Density Control] The execution result of the above-described image density control will be described with reference to FIGS. FIG. 8 (B) shows 24 different black densities.
A patch conversion result (curve K11), a target value of 24 patches (curve K12), and a density conversion table (curve K) created to match the measurement result of 24 patches to the target value
13) is shown. FIG. 8A shows a density gradation (curve K01) before density control for black, a target value of the density gradation (curve K02), and the density conversion table (FIG. 8).
The density gradation (curve K03) after density control based on the curve K13) of (B) is shown. Obviously, the curve K03 is closer to the curve K02 than the curve K01. That is, by performing the density control, the density gradation can be made closer to the target value.

Similarly, for yellow, FIG.
, Measurement results of 24 patches having different densities (curve Y11),
FIG. 9A shows a target value of 24 patches (curve Y12) and a density conversion table (curve Y13) for matching the measurement result of the 24 patches to the target value, and shows a density gradation before density control shown in FIG. (Curve Y01) is changed to a target value (curve Y0).
In order to approach 2), the density conversion table (FIG. 9)
Density control is performed based on the curve Y13) of (B), and a density gradation (curve Y03) after the density control can be obtained.

FIG. 10B also shows magenta.
, Measurement results of 24 patches having different densities (curve M11),
A density conversion table (curve M13) for matching the target value of 24 patches (curve M12) and the measurement result of 24 patches to the target value is shown, and the density gradation before density control shown in FIG. (Curve M01) is changed to a target value (curve M0).
In order to approach 2), the density conversion table (FIG. 10)
Density control is performed based on the curve M13) of (B), and a density gradation (curve M03) after the density control can be obtained.

FIG. 11B shows the measurement results of 24 patches having different densities (curve C11).
A density conversion table (curve C13) for matching the target value of four patches (curve C12) and the measurement result of 24 patches to the target value is shown, and the density gradation before density control shown in FIG. (Curve C01) is changed to the target value (curve C0).
In order to approach 2), the density conversion table (FIG. 11)
Density control is performed based on the curve C13) of (B), and a density gradation (curve C03) after the density control can be obtained.

[Operation / Effect of First Embodiment] Next, the operation / effect of the first embodiment will be described.

In the first embodiment, step S5 in FIG.
When the density conversion table is created by comparing the current gradation with the target gradation, first, a reference background correction amount is obtained from the difference between the measurement result of the background (Cin 0%) and the target value ( Reference base correction amount = target base value−base measurement result).

Next, the measurement result of each of the 24 patches is corrected using the following equation (4) according to each Cin (n) (n is an integer of 1 to 24). In addition, Cin (1)
Is "0" because it corresponds to Cin 0% in FIG.
Since in (24) corresponds to Cin100% of FIG. 12, it is “100”.

Measurement result (n) = measurement result (n) + (reference base correction amount × (100−Cin (n)) / 100) (4) Further, the current gradation based on the corrected measurement result A density conversion table is created by comparing the characteristics with the target gradation characteristics.

As a result, at Cin 0%, the reference background correction amount is entirely corrected (100%). As Cin increases, the correction ratio of the reference background correction amount decreases. At Cin 100%, the correction ratio of the reference background correction amount becomes 0. , And the reference background correction amount is not corrected.

FIG. 17 (A) shows a 2D image formed on recycled paper.
With respect to the density measurement results of the four patches, the density gradation before the above correction is performed is shown on a curve F1, and the density gradation after the above correction is performed is shown on a curve F4. Obviously, the correction rate of the reference background correction amount is high in the highlight portion where the influence of the background is strong,
It can be seen that the correction rate of the reference background correction amount decreases as the density increases.

For this reason, as shown by the curve G2 in FIG. 17B, the density conversion table created by the density gradation after the above-mentioned correction is performed on the color-only paper (J paper) in the highlight portion. Close to the conversion table (curve G1)
In the high density portion where the influence of the background is eliminated, the density conversion table (curve G3) using the original paper (recycled paper)
Become closer to

The density gradation when the image density control is performed by applying this density conversion table (curve G2 in FIG. 17B) is shown by a curve A3 in FIG. As is apparent from FIG. 14, in the highlight portion, the target density gradation (curve A
It is close to 2).

As described above, according to the first embodiment,
Density correction is performed with a correction amount that decreases as the density of the reference patch increases.This avoids excessive density correction in high-density areas, and is caused by differences in paper types and variations in density measurement devices in highlight areas. Error of the image density control can be reduced.

In the above correction, the reference background correction amount is corrected for each measurement result (n) of each of the 24 patches using the above equation (4). The correction may be performed using the following equation (5) according to each Cin (n) (n is an integer of 1 to 24), and the same effect can be obtained.

Target value (n) = target value (n) − (reference base correction amount × (100−Cin (n)) / 100) (5) Second Embodiment Next, the present invention will be described. The second embodiment will be described. In the second embodiment, in step S5 in FIG.
When the density conversion table is created by comparing the current gradation with the target gradation, the reference background correction amount is set at a ratio according to the patch Cin (n) as in the first embodiment. Instead of correction, correction is performed at a ratio according to the density measurement result of the reference patch.

That is, after calculating the reference background correction amount from the difference between the measurement result of the base (Cin 0%) and the target value (reference base correction amount = base target value−base measurement result), the measurement result of each of the 24 patches is obtained. For each measurement result (n) (n is 1-2
(Integer of 4) according to the following equation (6).

Measurement result (n) = measurement result (n) + (reference base correction amount × (measurement result (24) −measurement result (n)) /
(Measurement result (24)-Measurement result (0))
(6) Further, a density conversion table is created by comparing the current gradation with the target gradation based on the corrected measurement result.

As a result, as in the first embodiment, Cin
At 0% (= measurement result (0)), the reference background correction amount is entirely corrected (100%). As Cin increases, the correction rate of the reference background correction amount decreases, and Cin 100% (= measurement result (24)). In, the correction rate of the reference background correction amount becomes 0, and the reference background correction amount is not corrected.

According to the second embodiment, the reference background correction amount is corrected in accordance with the ratio according to the actual density measurement result. Compared to the first embodiment in which correction is performed in accordance with the ratio of Cin, more accurate correction in accordance with the density measurement result (density gradation) can be performed.

Note that also in the second embodiment, the target value of each of the 24 patches is determined by the measurement result (n) (n is 1).
(Integer of 24) according to the following equation (7), and the same effect can be obtained.

Target value (n) = target value (n) − (reference background correction amount × (measurement result (24) −measurement result (n)) / (measurement result (24) −measurement result (0))) (7) In the image forming apparatus of the above embodiment, the density of the reference patch is read by the reading unit that reads the image of the document without providing a dedicated density measuring device as a density measuring unit for measuring the density of the reference patch. Measuring. By thus using the reading unit also as the density measuring unit, it is possible to reduce the number of components and the cost of the image forming apparatus.

[0079]

According to the first aspect of the present invention, since the density correction is performed with the correction amount that decreases as the density of the reference patch increases, the highlight correction can be performed while avoiding excessive density correction in the high density portion. It is possible to reduce an error in image density control due to a difference in the type of paper in a portion or a variation in a density measuring device.

In particular, according to the third aspect of the present invention, since the correction is performed by the correction amount set based on the ratio according to the density measurement value of each reference patch and the reference background correction amount, the density measurement result (density) (Gray scale), it is possible to perform more accurate correction.

According to the fifth aspect of the present invention, it is not necessary to provide a dedicated density measuring device as the density measuring means, so that the number of components and the cost of the image forming apparatus can be reduced.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram of a color copying machine according to an embodiment.

FIG. 2 is a block diagram of the color copying machine of FIG.

FIG. 3 is a diagram illustrating binarization of image data by pulse width modulation.

FIG. 4 is a diagram illustrating a configuration of an optical sensor.

FIG. 5 is a flowchart showing a processing routine of photoconductor potential control.

FIG. 6 is a flowchart showing a procedure for creating a density conversion table.

FIG. 7 is a diagram for explaining an outline of image density control.

8A is a graph showing the density gradation before black density control, the target value of the density gradation, and the density gradation after density control for black, and FIG. 8B is a graph showing the density of 24 patches for black. It is a graph which shows a measured value, a target value of 24 patches, and a density conversion table.

9A is a graph showing a density tone before density control, a target value of density tone, and a density tone after density control for yellow, and FIG. 9B is a graph showing density of 24 patches for yellow. It is a graph which shows a measured value, a target value of 24 patches, and a density conversion table.

10A is a graph showing density gradation before magenta density control, a target value of the density gradation, and density gradation after magenta density control, and FIG. 10B is a graph showing the density of 24 patches for magenta. It is a graph which shows a measured value, a target value of 24 patches, and a density conversion table.

11A is a graph showing density gradation before density control, a target value of density gradation, and density gradation after density control for cyan, and FIG. 11B is a graph showing density of 24 patches for cyan. It is a graph which shows a measured value, a target value of 24 patches, and a density conversion table.

FIG. 12 is a diagram illustrating an example of a reference patch.

FIG. 13A is a graph showing the measurement results of the yellow component of the reference patch density at 24 points on each of the color-dedicated paper and recycled paper on which the toner has been developed / transferred / fixed by the same amount;
(B) is a graph showing a density conversion table created from the reference patch density measurement result of (A).

14 is a graph showing density gradations before and after image density control using the density conversion table of FIG. 13B and target density gradations.

FIG. 15A is a graph showing an example of a measurement result and a target value of the reference patch density at 24 points on the paper when the density correction is not performed, and FIG. 15B is a graph showing the reference patch density measurement of FIG. It is a graph which shows the density conversion table created by the result, and (C) is a graph which shows the density gradation before and after the image density control using the density conversion table of (B) and the target density gradation.

FIG. 16A is a graph showing an example of a measurement result and a target value of the reference patch density at 24 points on a sheet when the relative density method is used, and FIG. 16B is a graph showing the reference patch density of FIG. 7A is a graph showing a density conversion table created based on the measurement results, and FIG. 7C is a graph showing a density gradation before and after image density control using the density conversion table of FIG.

17A is a graph showing a density gradation before and after performing the correction according to the present invention, a density gradation on J paper, and a target density gradation. FIG. 17B is a graph showing the correction according to the present invention. 7 is a graph showing a density conversion table before and after the execution and a density conversion table for J paper.

[Explanation of symbols]

 Reference Signs List 10 color copying machine (image forming apparatus) 20 reading unit 30 image processing unit 31 image density control means 60 image forming unit 84 arithmetic unit

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI H04N 1/60 H04N 1/40 D 1/46 1/46 Z

Claims (5)

[Claims] 1. A reference patch forming means for forming a plurality of reference patches having different densities on an image carrier, and measuring a density of each reference patch formed by the reference patch forming means and a base density of the image carrier. Density measuring means for calculating, a base correction amount calculating means for calculating a reference base correction amount from a measured value of the base density measured by the density measuring means and a predetermined target value of the base density, Either the measured density value or the predetermined target density value of each reference patch does not exceed the reference background correction amount calculated by the background correction amount calculation means and decreases as the density of the reference patch increases. Density correction means for correcting with a correction amount; and a density target value and the density based on the density measurement value and the density target value corrected by the density correction means, or Based on the value, an image forming apparatus having a density conversion means for converting the density characteristic of the image data. 2. The apparatus according to claim 1, wherein the density correction unit performs correction with the correction amount set based on a ratio of a density condition corresponding to each reference patch and the reference background correction amount. Image forming device. 3. The density correction unit according to claim 1, wherein the density correction unit corrects the correction based on a ratio corresponding to a density measurement value of each reference patch and the reference background correction amount. Image forming apparatus. 4. A method according to claim 1, wherein the plurality of reference patches include a reference patch having a density of 0, and the density correction unit sets a density measurement value or a density target value of the density 0 reference patch equal to the reference background correction amount. The image forming apparatus according to claim 1, wherein the correction is performed by a correction amount. 5. The image forming apparatus according to claim 1, wherein the density measuring unit includes an image reading unit that reads an image of a document.