JP3489258B2 - Image forming device - Google Patents

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 in particular, a plurality of reference patches having different densities are formed on a photoconductor, and the densities are measured to perform a required gradation correction. The present invention relates to an image forming apparatus provided with a tonality correcting unit.

[0002]

2. Description of the Related Art An electrostatic latent image formed by writing a light beam such as a laser modulated by a digital signal corresponding to an image in a copying machine, a printer, a facsimile or the like is visualized by a toner and transferred to a transfer medium. In the image forming apparatus that obtains the reproduced image by performing the gradation correction of the reproduced image so that the gradation becomes linear with respect to the density and the reflectance according to the reading characteristics and the developing characteristics of the original document that is the source image. Means for applying are provided.

FIG. 12 is an overall configuration diagram for explaining a color copying machine as an example of an image forming apparatus to which the present invention is applied. 10 is a scanner section, 20 is an image processing section, and 30 is an exposure optical section (ROS). Reference numeral 40 denotes an image forming unit. Further, 11a is a document table on which a document as a source image is placed, 44 is a photoconductor, 44a is a charging device, 44b is a cleaner device, 44c is a discharge lamp, 45 is a rotary developing device, 46 is a transfer device, and 46a is a transfer device. Corotron, 46
b is a peeling corotron, 46c is a static eliminating corotron, 46d
Is a transfer drum, 46e is a suction device, 47 is a toner dispensing device, 48 is an electrometer, 49 is a paper tray for storing paper as a transfer medium, 50 is a paper transport device, 51 is a fixing device, and 51a and 51b are transfer rolls. Is.

In FIG. 1, the structure of this color copying machine is roughly divided into a scanner section 10 for reading an original,
Image processing unit 2 for processing the image data of the read document
0, an exposure optical unit 30 that drives a laser according to the processed image data to irradiate the photoconductor 44 with a light beam, and an image forming unit 40 that creates a reproduced image.

First, in the scanner section 10, the document table 11
The original placed on a is irradiated with an exposure lamp, and the reflected light is photoelectrically converted and given to the image processing unit 20 as a digital signal. The image processing unit 20 subjects the digital signal corresponding to the image to image processing such as color conversion and gradation correction, and outputs the digital signal to the exposure optical unit 30.

The exposure optical unit 30 modulates the laser in accordance with the input image signal, and the scanning optical system forms the image forming unit 40.
An electrostatic latent image is formed on the surface of the photoconductor 44. In this type of color copying machine, black image data is first applied to a laser to form an electrostatic latent image corresponding to black. Photoconductor 44
Is charged by the charging device 4 prior to writing with the laser beam.
The surface 4a is uniformly charged with static electricity. The electrostatic latent image formed on the photoconductor 44 is transferred to the rotary developing device 4
At position 5, toner is first developed by a black (Bk) developing device Bk to be visualized as a toner image.

The toner image developed with toner is transferred onto the paper conveyed from the paper tray 49 by the paper conveying device 50 at the position of the transfer corotron 46a by the rotation of the photosensitive member 44. The above-mentioned paper is wound from the paper carrying device 50 by the suction device 46e onto the transfer drum 46d of the transfer device 46 and is carried to a position where the transfer corotron 46a and the photoconductor 44 face each other.

After the carried black toner image is transferred onto the paper, the cleaner 44b removes the residual toner from the photoconductor 44, and the charge removal lamp 44c neutralizes the surface charge to form the next yellow image. A uniform electrostatic charge is charged by the charging device 44a. Photoreceptor 4 with uniform electrostatic charge
An electrostatic latent image 4 is formed by scanning laser light modulated with yellow image data. This electrostatic latent image is developed with yellow toner in the yellow developing device Y of the rotary developing device 45 and visualized.

On the other hand, the sheet attracted to the transfer drum 46d of the transfer device 46 is rotated again to the transfer corotron 46a by the rotation of the transfer drum 46d. When the paper is rotated to the transfer corotron 46a, the yellow toner image carried on the photoconductor 44 is transferred onto the black toner image. Thereafter, in the same manner, all of the magenta and cyan toner images are transferred onto the sheet in an overlapping manner to form a full-color toner image.

The sheet on which all the toner images have been transferred is transferred to the transfer drum 46 by the separation electric field generated by the separation corotron 46b.
The toner is fixed on the toner after being peeled from d, passed to the fixing device 51, and pressed / heated by the fixing rollers 51a and 51b to be fixed, and discharged as a permanent image from the image forming apparatus. A toner dispensing device 47 is installed in the rotary developing device to supply toner of each color.

In this type of image forming apparatus, in order to compensate for a difference in characteristics between image forming apparatuses or a change in characteristics over time, a reference gradation pattern density is detected and gradation correction characteristics are changed. Various gradation correction methods are known.

For example, Japanese Patent Application Laid-Open No. 63-208368 discloses an apparatus in which a gradation correction characteristic is calculated based on a correspondence result between a read signal of a predetermined gradation pattern and the gradation pattern. There is.

Further, Japanese Patent Application Laid-Open No. 4-126462 discloses that the density change coefficient is corrected according to the detected density value of one or more reference patches for obtaining the reference density value.
Further, in Japanese Patent Application Laid-Open No. 3-271767, the density of a reference toner image is detected, the grid and bias potential of a charger are selected, and gradation correction data is created from data according to the selection result. A tonal correction method has been proposed.

An electrometer 48a arranged around the photoconductor 44 measures the charging potential of the photoconductor 44 to measure the charging device 4.
The image density is controlled by controlling the amount of electrostatic charge on the surface of the photoconductor 44 by 1a to a predetermined value. The optical sensor 48b is a sensor for detecting the density value of the developed reference patch and correcting the gradation of the image.

In these gradation correction methods, reference patches are created on the photosensitive member, the transfer member, or the transfer medium at some points between the minimum density level and the maximum density level, and the density value of the reference patch is set to the optical value. Measured with a sensor, create gradation correction data by linear approximation or curve approximation so that the overall gradation characteristics will be the target gradation characteristics from the difference between the target density value and the measured density value, and The difference between the characteristics and the change in the characteristics over time is corrected.

[Problems to be Solved by the Invention]

The minimum density of the image, that is, the gradation reproducibility of the highlight portion is very important because it is sensitive to human eyes, but the prior art described above has a highlight portion corresponding to the minimum density. With respect to the gradation correction, there is a problem that a gradation correction error occurs due to the low density detection accuracy of the highlight portion.

One of the causes of this problem is that the density and the output of the optical sensor are in an exponential relationship, so that the density difference corresponding to the detection error of the optical sensor for detecting the density is large in the highlight portion. This density difference occurs regardless of where the reference patch density is detected (on the photoconductor, the transfer body, or the transfer medium).

Further, in the case where a standard patch which is generally used is formed on a photoconductor and the density thereof is detected, the following causes are added to further increase the above-mentioned problem.

FIGS. 13A and 13B are schematic diagrams for explaining the ideal distribution form of the toner forming the reference patch. FIG. 13A is a patch near the minimum density, that is, a highlight, and FIG. 13B is a patch near the intermediate density. Indicates. As shown in FIGS. 7A and 7B, in principle, both the patches near the highlight and the patches near the intermediate density should have a predetermined toner amount distributed in an orderly manner to represent the predetermined density. In an actual image forming apparatus, the so-called fog toner, which is not transferred to the sheet, is present, which causes the following problems.

FIG. 14 is a schematic diagram for explaining the distribution form of the toner forming the reference patch when the fog toner is present on the photoconductor. FIG. 14A is a patch near the minimum density, that is, near the highlight, b) shows patches near the middle density. In the figure, the fog toner is indicated by a black circle for convenience.

In the figure, when the fog toner is present in the patch (a) near the minimum density and the patch (b) near the intermediate density formed on the photoconductor, the fog toner is converted into the patches (a) and (b). Since the same effect is exerted on the patch, the influence of the fog toner on the patch near the minimum density becomes larger than that on the patch near the intermediate density.

The amount of the fog toner depends on the environment and the characteristics of the developer, and the optical sensor cannot distinguish the fog toner from the patch toner near the minimum density which is actually transferred to the transfer medium. .

Therefore, if the density value of the patch near the minimum density on the photoconductor is measured by the optical sensor and the final density on the paper is predicted by this measurement result, the fog toner which is not transferred to the paper is affected. As a result, the detection accuracy (measurement accuracy) decreases, and as a result, the gradation correction accuracy in the highlight portion decreases.

FIG. 15 is an explanatory diagram of a gradation correction method in which a reference patch is formed in the vicinity of the minimum density, and FIG. 15A is a reference patch density value on the photosensitive member detected as the ideal density gradation of the image forming apparatus. From the detected reference patch densities, (c) shows the density gradation after gradation correction using the created gradation correction data. It is an explanatory view of tonality.

In FIG. 6A, A is an ideal gradation curve of density values for the output image signal, and B is a measured gradation curve obtained from the density of the reference patch. Further, in FIG. 7B, C is tone correction data obtained from the density measurement value of the reference patch, and D is input = output data.

In FIG. 6C, E is a density gradation characteristic curve after correction, and the measured gradation curve B obtained from the density of the reference patch shown in FIG. In the density gradation characteristic curve E corrected by the gradation correction data, the highlight portion is skipped more than the ideal gradation curve A. This is because the fog toner on the photoconductor was also detected when the density of the reference patch near the minimum density was measured, and the patch density was measured as a value higher than the actual value.

As a countermeasure, it is possible to fix the gradation of the lowest density and control only the gradation of the intermediate density portion.

FIG. 16 is an explanatory diagram of a gradation correction method in which a reference patch is not formed in the vicinity of the minimum density, and (a) is a reference patch density value on the photoconductor detected as the ideal density gradation of the image forming apparatus. From the detected reference patch densities, (c) shows the density gradation after gradation correction using the created gradation correction data. It is an explanatory view of tonality.

In FIG. 4A, A is an ideal gradation curve of the density value for the output image signal, and B is a measured gradation curve obtained from the density of the reference patch. Here, since the reference patch is not created in the vicinity of the minimum density, there is no measured value in the highlighted portion.

Further, in FIG. 3B, C is tone correction data obtained from the density measurement value of the reference patch, and D is input =
This is the output data. Similar to (a), since the reference patch is not created in the vicinity of the minimum density, the gradation correction data of the highlight portion is at the position of input = output = 0.

In FIG. 7C, E is the density gradation characteristic curve after correction, and the highlight portion is too much compared with the ideal gradation curve A. This is because the reference patch is not created in the vicinity of the lowest density and the gradation reproduction correction of the lowest density portion is not performed.

An object of the present invention is to solve the above-mentioned problems of the prior art, and to form an image with a correction means capable of accurately correcting the gradation of the density of the entire image from the highlight portion near the minimum density to the high density. To provide a device.

[Means for Solving the Problems]

The configuration of the present invention for achieving the above object will be described below in association with the reference numerals of the embodiments. That is, the first aspect of the invention according to claim 1 is for each gradation of an image.
The patch creating means 25 for creating the reference patches of a plurality of densities among the densities determined accordingly on the image carrier, and the density measuring means 48 for measuring the density of the created reference patch.
a, and 48b, the highlight tone correction condition input means 61 for inputting the highlight tone correction condition from an external corresponding to the vicinity of the minimum value of the output density of the image forming apparatus, the image forming instrumentation
Based on the difference between the density value measured by the density measuring means and a predetermined target density value except near the minimum value of the density output by the device.
To create the gradation correction data and output it from the image forming apparatus.
In the vicinity of the minimum value of the density, the highlight gradation correction condition input from the highlight gradation correction condition input means and the density
At least a gradation correction data generating means 29 for generating gradation correction data from the difference between the density value measured by the density measuring means and a predetermined target density value, and the gradation correction data generating means 29 generates the gradation correction data. It is characterized in that a gradation correction unit 23 for correcting the gradation of an image signal to be image-formed is provided by using the gradation correction data thus obtained.

According to a second aspect of the present invention, the gradation correction data generating means 29 stores a highlight gradation correction condition input from the highlight gradation correction condition input means 61. A light gradation correction condition storage unit 29d is provided, and the highlight gradation correction conditions stored in the highlight gradation correction condition storage unit and the density measurement values of a plurality of reference patches other than the vicinity of the minimum density by the density measurement unit are measured. It is characterized in that the gradation correction data is generated by using the difference between the target density value and the predetermined target density value.

Further, the third invention according to claim 3 is
The gradation correction data generation means 29 determines the highlight gradation correction condition according to the difference between the measured values of a plurality of patch densities other than near the minimum density measured by the density measurement means and a predetermined target density value. A highlight gradation correction condition changing unit 29e for changing the highlight gradation correction condition stored in the storage unit is provided.

The image carrier for forming the reference patch is
The transfer device (intermediate transfer member) is not limited to the photoconductor,
Alternatively, it includes a transfer medium (paper or the like), and also includes one that measures the density of the prepared reference patch with an optical sensor or the like.

[Action]

In the configuration of the first aspect of the invention, the patch creating means 25 uses the dark color determined for each gradation of the image.
A plurality of densities of reference patches are prepared on the image carrier. The density measuring means 48a and 48b measure the density of the created reference patch and give the output to the arithmetic unit 41. The highlight gradation correction condition input means 61 comprises a control panel or the like, and inputs a highlight gradation correction condition corresponding to the vicinity of the minimum value of the density measured by the density operation measuring means from the outside. Gradation correction data generation means 29
Is for creating gradation correction data based on the difference between the density value measured by the density measuring means and a predetermined target density value except near the minimum density value output from the image forming apparatus, and form
In the vicinity of the minimum value of the density output by the composition apparatus, the density was measured by the highlight gradation correction condition input means and the density measuring means.
Gradation correction data is created from the highlight gradation correction condition input from the difference between the density value and a predetermined target density value . Then, the gradation correction unit 23 corrects the gradation of the image signal to be image-formed by using the gradation correction data generated by the gradation correction data generation unit.

In the configuration of the second aspect of the invention, the highlight gradation correction condition storage means 29 d provided in the gradation correction data generating means 29 has the high level input from the highlight gradation correction condition input means 61. The light gradation correction condition is stored. The highlight gradation correction condition stored in the highlight gradation correction condition storage means and the difference between the density measurement value of a plurality of reference patches other than near the minimum density measured by the density measurement means and a predetermined target density value. And are used to generate gradation correction data.

Further, in the configuration of the third invention,
The highlight gradation correction condition changing means 29e provided in the gradation correction data generating means 29 is arranged to measure a plurality of patch densities other than near the minimum density measured by the density measuring means and a predetermined target density value. The highlight tone correction condition stored in the highlight tone correction condition storage means is changed according to the difference.

【Example】

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a schematic diagram for explaining the overall configuration including a gradation correction unit of an image forming apparatus according to the present invention. 10 is a scanner unit, 20 is an image processing unit, 30 is an exposure optical unit (ROS unit), 40 Is an image forming unit, and
2 corresponds to the part with the same reference numeral.

In the figure, 11 is a document, 12 is an exposure lamp, 13 is a CCD, 14 is an amplifier, 15 is an A / D converter, 16 is a shading correction circuit, 17 is a gap correction circuit, 18 is a density conversion circuit, 21 is a color conversion circuit, 22
Is a first gradation correction means, 23 is a second gradation correction means, 24 is a D / A conversion circuit, 25 is a patch preparation means (reference patch preparation means), 26 is a selector, 27 is a comparator, 28 is a triangular wave generation Reference numeral 29 is a gradation correction data generating means, 31 is a laser drive circuit, 32 is a laser, 32 is a polygon mirror, 34 is an fθ lens, 35 is a reflection mirror, 36 is a laser light quantity varying device, 41 is an arithmetic unit, and 42 is Charge amount varying device, 43 developing bias varying device, 44 photoconductor, 4
4a is a charging device, 44b is a cleaner device, 44c is a discharge lamp, 45 is a rotary developing device, 46 is a transfer device, 47 is a toner dispensing device, 48a is an electrometer,
48b is an optical sensor. Reference numeral 61 is a highlight tone correction condition input means installed on a control panel or the like.

In this image forming apparatus, the scanner unit 10
Reads the manuscript 11, the image processing unit 20 processes the read image data, and the exposure optical unit (ROS unit) 30 drives the laser with the processed image data and the electrostatic latent image on the photoconductor by the laser light. To form. The image forming unit 40 develops the electrostatic latent image with toner, transfers the toner onto a transfer member, fixes the toner, and discharges it as a permanent image outside the apparatus.

First, in the scanner section 10, the exposure lamp 1
The document is illuminated by 2 and the reflected light is photoelectrically converted by CCD 13 and amplified by amplifier 14 to an appropriate level. The amplified read signal is converted into 8-bit digital image data by the A / D converter 15, and shading correction is performed by the shading correction circuit, and gap correction is performed by the gap correction circuit 17. Thereafter, the density conversion circuit 18 converts the reflectance data into density data and sends the density data to the image processing unit.

In the image processing section 20, basic image processing as a color copying machine, that is, color signal conversion by the color conversion circuit 21, black reproduction (UCR), MTF processing, etc. are executed, and yellow, magenta, cyan, and Converted to image data of four black colors. Further, the first gradation correction unit 22 corrects each color gradation according to the gradation of the scanner unit 10 and the image forming unit 40. Since the contents of these various corrections are known, detailed description thereof will be omitted. The second gradation correction unit 23 performs gradation based on the gradation correction data from the gradation correction data generation unit 20 based on the condition input from the highlight gradation correction condition input unit 61 and the input from the arithmetic unit 41. Make a correction.

The tone-corrected image data is converted into analog data by the D / A converter 24 and input to the comparator 27 through the selector 26. A triangular wave generator 28 applies a triangular wave having a predetermined period as shown in FIG. 2 to the comparator 27. The image data is compared with the triangular wave and pulse width modulated to be converted into binary image data. It That is, FIG. 2 is an explanatory diagram of binarization of image data in the comparator of FIG. 1, in which (a) is analog image data,
(B) is a triangular wave with a predetermined period.

In the figure, the input analog image data (a) is compared with the triangular wave (b), and the part where the analog image data (a) is larger than the triangular wave (b) is "0" value, and the smaller part is "1". The laser is turned off (O
FF), binary image data that is turned on when the value is “1” is sent from the comparator 27 to the laser drive circuit 31.

In FIG. 1, a patch creating means 25 for creating a plurality of reference patches having different densities is provided. Data of a plurality of reference patch patterns having different densities (for example, those shown in FIG. 13) are stored in the patch creating means 25, and the patch pattern is created from the patch creating means 25 in the reference patch creating mode. It is given from the comparator 27 to the laser drive circuit 31 through the selector 26.

The selector 26 is used for the D / A converter 2
The analog image data from 4 and the patch pattern data from the patch creating means 25 are switched. That is, the selector 27 selects the analog image data from the D / A converter 24 during normal image formation (during copying), and when the arithmetic unit 41 of the image forming unit 40 gives an instruction to create a reference patch. The reference patch data from the patch creating means 25 is sent to the comparator 27 and binarized.

The exposure optical unit (ROS unit) 30 includes a laser light amount varying device 36 and a laser driving circuit 31 which are controlled by the arithmetic unit 41 of the image forming unit 40 to vary the output light amount of the laser 32. The laser drive circuit 31 turns on / off the laser based on the binarized data sent from the comparator 27.

The laser light from the laser 32 is deflected by the polygon mirror 33 and guided to the photoconductor 44 of the image forming section 40 via the fθ lens 34 and the reflection mirror 35. Around the photoconductor 44 that constitutes the image forming section 40,
Charging device 44a, rotary developing device 45, transfer device 4
6, a cleaner device 46b, a charge eliminating lamp 44c, an electrometer 48a for measuring the potential on the photoconductor 44, and an optical sensor 48b as a densitometer measuring means for measuring the density of a reference patch formed on the photoconductor 44 are arranged. ing.

Further, the rotary developing device 45 is provided with a toner dispense device 47 for supplying the respective toners to the developing devices Bk, Y, M and C of the respective colors. further,
The image forming unit 40 controls the entire image formation, and an arithmetic unit 41 that controls toner dispensing and image forming conditions according to a reference patch creation instruction and the output of the optical sensor 48b.
A charge amount varying device 42 that changes the charge amount of the charging device 44a under the control of the computing device 41 and a developing bias varying device 43 that changes the developing bias are provided.

As in FIG. 12, the image forming section 40 is provided with a fixing device and a sheet conveying device, but they are omitted in FIG. In the above structure, image formation is performed according to a well-known xerographic process. That is, the surface of the rotating photosensitive member 44 is uniformly charged with, for example, a negative charge by the charging device 44a. A latent image of the first color (here, black: Bk) is formed on the surface of the charged photoconductor 44 by scanning the laser beam.

This latent image is the first image of the rotary developing device 45.
The portion written by the laser light is developed by the black toner negatively charged by the developing device Bk of the color to be visualized. The developed toner image is transferred to the paper tray (4 in FIG. 12).
9) is conveyed by the sheet conveying device (same as 50), is wound around the transfer drum 46d of the transfer device 46, and is transferred to the sheet sent to the portion of the transfer corotron (same as 46a).

After the transfer, the toner remaining on the photoconductor 44 without being transferred is removed by the cleaner device 46b, and the charge is removed by the charge removing lamp 44c, and the charging device 4 is again charged.
The image of the second color (yellow) is formed by superimposing the toner image of the first color on the sheet, which is uniformly negatively charged by 4a and previously transferred. Thereafter, similarly, the third color (magenta) and the fourth color (cyan) toner images are sequentially transferred and superposed on the paper.

After the transfer of all color toner images is completed through the above transfer process, the paper is peeled off (see FIG. 1).
No. 46b), the toner is separated from the transfer drum and is conveyed to the fixing device (the same 51) and fixed. A static elimination corotron (same as 46c) is also arranged around the transfer drum to eliminate excess charges on the paper or on the transfer drum after transferring each color or peeling the paper. In this way, full-color image formation is performed.

Next, an embodiment of gradation correction by the gradation correction device according to the present invention will be described. In the image forming apparatus shown in FIG. 1, when the mode for executing the gradation correction is entered, the image forming section 40 gives a patch forming instruction to the patch forming means 25. The patch creating means 25 generates patch data of plural densities, and the selector 26 selects the patch data and sends it to the comparator 27. 2 in comparator 27
The digitized data is given by the laser drive circuit 31 of the exposure optical unit 30, and latent images of a plurality of reference patches having different densities are formed on the photoconductor 44, and toner development is performed.

The plurality of reference patches prepared in the embodiment of the present invention correspond to image areas of 15%, 20%, 30% and 50% in the gradation of 256 gradations of 0 to 255 of each color, 38/25.
There are four patches at 6, 5/256, 76/256, 128/256 levels. The density value of each of the developed four reference patches of each color is measured by the optical sensor 48b. The measurement result and the target value of each patch density are calculated by the arithmetic unit 4
1 to the gradation correction data generating means 29.

FIG. 3 is an explanatory diagram of the highlight gradation correction condition input means provided on the control panel, and is designed to instruct the user to input the highlight gradation reproducibility condition. This highlight gradation correction condition input means is for Y, M, and
The correction conditions for C and K (Bk) can be set. The input value is sent to and stored in the gradation correction data generating means 29.

FIG. 4 is a block diagram for explaining the configuration around the gradation correction data generating means in the first embodiment of the image forming apparatus according to the present invention, in which 29a is a difference circuit, 29b is table data, and 29c is correction data. Creation circuit, 29d
Denotes a memory, and the same reference numerals as those in FIG.

In the figure, gradation correction data generating means 2
Reference numeral 9 denotes a difference calculation circuit 29a for calculating the difference between the density measurement result input from the calculation device 41 and the target density, table data 29b showing the relationship between the reading value of the optical sensor and the gradation offset amount, and highlight gradation correction condition input means. The memory 29d stores the correction conditions set from 61, and the correction data creation circuit 29c.

The gradation correction data generated by the gradation correction data generating means 29 is 256 gradation correction data, that is, the input 256 gradations are converted into an output of 256 gradations according to the relationship of FIG.

That is, FIG. 5 is an explanatory diagram of an example of 256 gradation correction data for converting an input gradation into an output gradation, where F is correction data and G is input (X) = output (Y). Is a straight line. Here, the value set by the highlight gradation correction condition input means is the gradation correction data of 256 gradations created by the gradation correction data generating means, that is, the input 256.
It corresponds to the output 256 gradation Y intercept of the relationship of converting the gradation (X axis) to the output 256 gradation (Y axis), and all initial values are “0”. Of course, the value to be set and input is not limited to this, and may be the rising point of the input 256 gradations (X axis) or the virtual patch measurement density of the highlight patch. Next, 2 in the gradation correction data generation means
A method of creating gradation correction data of 56 gradations will be described.

FIG. 6 is a flow chart for explaining an algorithm for creating gradation correction data in the first embodiment of gradation correction according to the present invention. In the figure, Cin is the image area, Radc-delta is the difference between the reading value of the optical sensor and the target value, PT-TRC is the correction amount at each image area point, and P-LUT is the look-up table (level). Key adjustment value table).

[0064] First, in S-1, an image area (C _in) 15% to each of colors, 20%, 30%, the difference R of the target density value of 50% of the patch density measurement results and the image area (C _in)
Calculate adc-delta (CIN).

FIG. 7 is an explanatory diagram of table data showing the relationship between the read value Radc-delta of the optical sensor and the gradation offset amount stored in the gradation correction data generating means.
Next, in S-2 of FIG. 6, the table No. of each image area (C _in ) from Radc-delta (CIN) is referred to with reference to the table data shown in FIG. Ask for.

At this time, Cin100% changes in the same manner as the concentration fluctuation of Cin50%. Is calculated from Radc-delta (50%). further,
In S-3, the table No. is selected from the table. According to each C
A gradation offset amount of in (15%, 20%, 30%, 50%, 100%) is obtained for each of the photo mode and the character mode, and the photo mode / input from the highlight gradation correction condition input means in S-4 The value of the highlight gradation correction condition for each character mode is the gradation offset amount of Cin0%,
Correction amount PT-TRC at each Cin point in S-5
Calculate "CIN".

The correction amount PT-TRC "CIN" is calculated by
For each Cin, the gradation offset amount of each Cin is added to the Cin value to obtain the value. For example, Cin 15% (38
(/ 255 level) gradation offset amount is +3,
The PT-TRC “15” = 38 + 3 = 41 level.

Then, at S-6, the PT-
The data between the TRCs is obtained by linear approximation calculation connecting each point, and the whole gradation correction data P-LUT (0 to 255) of 256 gradations is calculated.
0 or less than 255 beyond the range of 255, 25 above 255
Set to 5. This is the gradation correction data, that is, 0 to 255.
256 gradations for the input 256 gradations of P-LUT
The data of (0 to 255) is obtained as shown in FIG.

That is, FIG. 8 is an explanatory diagram of the gradation data of 256 gradations calculated when the highlight gradation correction condition is the initial value 0, G is gradation correction data, and H is input signal =
The straight line of the output signal is shown. This gradation data is the second one in FIG.
It is transferred to the gradation correction means 23. As described above, the image data is corrected by this gradation correction data to form an image.

However, when an adapted image is created using the gradation correction data obtained here, the density gradation after each color gradation correction is corrected correctly by the correction by the patch density measurement at the intermediate density (Cin 15%) or more. However, since the highlight gradation correction condition is the initial value “0” here,
The highlight reproducibility is the same as in the case where the gradation reproduction correction itself of the highlight portion described in the section of the problem of the conventional technique is not performed (see FIG. 16). It may be incorrect.

Therefore, in the present invention, the created highlight reproducibility is confirmed, and the highlight gradation correction condition input means 6 is used.
1 sets the highlight gradation correction condition for each color.
That is, when the density is high, it is on the-side, and when it is light, it is +.
Turn it to the side.

When the user inputs the value of the highlight gradation correction condition from the control panel, the gradation correction data creation algorithm shown in FIG. 6 is executed, and the gradation offset amount of Cin0% is the initial value "0". Instead, the value newly input from the highlight gradation correction condition input means 61 is used.

FIG. 9 shows "-" as the highlight gradation correction condition.
5 is an explanatory diagram of gradation data when “5” is set,
Is the gradation correction data based on the set PT-TRC,
Indicates a straight line of input signal = output signal. As shown,
By setting and inputting the highlight gradation correction condition by the highlight gradation correction condition input unit 61, the gradation correction data of the highlight portion which is important for the image forming apparatus and cannot be accurately detected by the measurement of the density value of the reference patch. Can be created correctly.

By forming an image using this gradation correction data, a desired density gradation can be obtained.
After that, a reference patch having a Cin of 15% or more is regularly created, and tone correction data that compensates for a temporal change of Cin of 15% or more is created by measuring the density value and executing the algorithm of FIG. According to the present embodiment, it is possible to perform highly accurate gradation correction and obtain a high quality reproduced image. Next, a second embodiment of the image forming apparatus according to the present invention will be described.

In the first embodiment, the highlight gradation property is updated only when the highlight gradation correction condition inputting means sets and inputs the highlight gradation correction condition, and thereafter, the standard of Cin of 15% or more is periodically set. It is not updated when measuring patch density. It is possible to cope with the gradation variation of the highlight portion with time between image forming apparatuses or for a long time, but it is not possible to cope with the variation of short intervals.

FIG. 10 shows a second image forming apparatus according to the present invention.
FIG. 9 is a block diagram for explaining the configuration around the gradation correction data generating means in the embodiment, in which 29e is an adder circuit and constitutes highlight gradation correction condition changing means. The same reference numerals as those in FIG. 4 correspond to the same parts.

FIG. 11 is an explanation of the table data showing the relationship between the difference (Radc-delta) between the reading value of the optical sensor and the target value and the gradation offset amount stored in the gradation correction data generating means of FIG. It is a figure. In this table,
A 0% gradation offset amount is also obtained according to 15% Radc-delta.

The 0% gradation offset amount is calculated in accordance with the 15% Radc-delta. Although variations in Cin0% and Cin15% do not always have the same tendency among the image forming apparatuses, the shortness in the same apparatus is short. The time variation of the interval is Cin0
This is based on the fact that Cin15% adjacent to% has almost the same tendency.

Then, the adder circuit 29c provided in the gradation correction data generating means 29 of FIG.
When creating gradation correction data for 6 gradations, Cin 0% in S-4
The gradation offset amount is set and input from the highlight gradation correction condition input means 61 and stored in the memory 29e, and from the table data 29b in FIG. 0 according to
%, And the sum with the data of the gradation offset amount is calculated, and this is set as the Cin0% gradation offset amount.

As a result, Cin1 which is regularly measured
Based on the density measurement result of the 5% reference patch, gradation correction data capable of coping with fluctuations at short intervals is created. In this way, according to the present embodiment, it is possible to perform more accurate gradation correction and obtain a reproduced image of high quality.

In each of the above embodiments, the reference patch on the photoconductor is read by the high sensor, but the present invention is not limited to this, and a transfer device (intermediate transfer device) or a sheet or the like may be used. The same effect can be obtained by adopting a method of measuring the reference patch on the transfer medium by a density measuring means such as an optical sensor. Further, the present invention can be applied not only to a full-color image forming apparatus but also to a single-color image forming apparatus.

【The invention's effect】

As described above, according to the present invention, the gradation correction data generating means is provided, and the actual measurement is performed from the value set and input by the highlight gradation correction condition input means and the reference patch created on the image carrier. By creating the gradation correction data using both of the density values obtained, it is possible to obtain the gradation correction data corresponding to the gradation fluctuation between the image forming apparatuses and in the highlight portion over time. It is possible to provide an image forming apparatus capable of accurately correcting highlight reproducibility, which is important for an image forming apparatus, by executing image formation using gradation data.

Further, by using the density close to the highlight portion, for example, the density value of the reference patch of 15% portion of all the gradations, a gradation correction device capable of coping with a short-term temporal gradation fluctuation of the highlight portion is provided. It is possible to provide the provided image forming apparatus.

[Brief description of drawings]

FIG. 1 is a schematic diagram illustrating an overall configuration including a gradation correction unit of an image forming apparatus according to the present invention.

FIG. 2 is an explanatory diagram of binarization of image data in the comparator of FIG.

FIG. 3 is an explanatory diagram of highlight gradation correction condition input means provided on a control panel.

FIG. 4 is a block diagram illustrating a configuration around a gradation correction data generating unit in the first embodiment of the image forming apparatus according to the present invention.

FIG. 5: 256 for converting an input gradation into an output gradation
It is an explanatory view of an example of gradation amendment data.

FIG. 6 is a flow chart for explaining an algorithm for creating gradation correction data in the first embodiment of gradation correction according to the present invention.

FIG. 7 is an explanatory diagram of table data showing the relationship between the read value Radc-delta of the optical sensor and the gradation offset amount stored in the gradation correction data generating means.

FIG. 8 is an explanatory diagram of gradation data of 256 gradations calculated when a highlight gradation correction condition has an initial value of 0.

FIG. 9 is an explanatory diagram of gradation data when “-5” is set as the highlight gradation correction condition.

FIG. 10 is a block diagram illustrating a configuration around a gradation correction data generating unit in a second embodiment of the image forming apparatus according to the present invention.

11 is an explanatory diagram of table data showing a relationship between a read value (Radc-delta) of an optical sensor and a gradation offset amount stored in the gradation correction data generating unit of FIG.

FIG. 12 is an overall configuration diagram illustrating a color copying machine as an example of an image forming apparatus to which the present invention is applied.

FIG. 13 is a schematic diagram illustrating an ideal distribution form of toner forming a reference patch.

FIG. 14 is a schematic diagram illustrating a distribution form of toner forming a reference patch when fog toner is present on a photoconductor.

FIG. 15 is an explanatory diagram of a gradation correction method in which a reference patch is created near the minimum density.

FIG. 16 is an explanatory diagram of a gradation correction method in which a reference patch is not created near the minimum density.

[Explanation of symbols]

10 ... Scanner unit, 20 ... Image processing unit,
30 ... Exposure optical unit (ROS unit), 40 ... Image forming unit, 11 ... Original document, 12 ... Exposure lamp, 13 ... CCD, 14 ... Amplifier , 15 ...
... A / D converter, 16 ... Shading correction circuit, 17 ... Gap correction circuit, 18 ... Density conversion circuit, 21 ... Color conversion circuit, 22 ... First gradation correcting means, 23 ... Second gradation correcting means, 24
.... D / A conversion circuit, 25 ... ... patch creating means (reference patch creating means), 26 ... selector, 2
7 ... Comparator, 28 ... Triangle wave generator, 29.
... Gradation correction data generating means, 31 ... Laser driving circuit, 32 ... Laser, 33 ... Polygon mirror, 34 ... f.theta. Lens, 35 ... Reflection mirror, 36 ... Laser light amount variable device, 41 ...
..Computing device, 42 ...
..Variable developing bias device, 44 ... Photosensitive member, 44
a ... Charging device, 44b ... Cleaning device,
44c ... Static elimination lamp, 45 ... Rotary developing device, 46 ... Transfer device, 47 ... Toner dispensing device, 48a ... Electrometer, 48b ...
-Optical sensor, 61 ...- Highlight gradation correction condition input means installed on a control panel or the like.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ identification code FI H04N 1/407 (58) Fields investigated (Int.Cl. ⁷ , DB name) G03G 15/00 303 G03G 21/00 370-512 B41J 2/52 G03G 15/04 G03G 15/043 H04N 1/04 106

Claims (3)

(57) [Claims] 1. A density determined corresponding to each gradation of an image.
Of the plurality of densities of the reference patches on the image carrier, the density measuring means for measuring the densities of the created reference patches, and the density of the output of the image forming apparatus near the minimum value. A highlight gradation correction condition input means for inputting a corresponding highlight gradation correction condition from the outside, and a density value measured by the density measuring means and a predetermined target except near the minimum density value output from the image forming apparatus. Tone correction data is created based on the difference in density value, and
In the vicinity of the minimum value of the density output by the unit, the highlight gradation correction condition input from the highlight gradation correction condition input means, the density value measured by the density measuring means, and a predetermined target.
Gradation correction data generating means for generating gradation correction data from the difference in density value, and using the gradation correction data generated by the gradation correction data generating means, the level of an image signal to be image-formed. An image forming apparatus comprising: a gradation correcting unit that corrects tones. 2. The gradation correction data generation means according to claim 1, further comprising a highlight gradation correction condition storage means for storing the highlight gradation correction condition input from the highlight gradation correction condition input means. A highlight gradation correction condition stored in the highlight gradation correction condition storage means and a difference between a density measurement value of a plurality of reference patches other than near the minimum density measured by the density measurement means and a predetermined target density value; An image forming apparatus provided with a gradation correction unit, wherein gradation correction data is generated by using. 3. The gradation correction data generating means according to claim 2, according to a difference between a plurality of patch density measurement values other than near the minimum density measured by the density measuring means and a predetermined target density value. And an image forming apparatus having gradation correction means, characterized in that it comprises highlight gradation correction condition changing means for changing the highlight gradation correction conditions stored in the highlight gradation correction condition storage means. .