JP2001136391A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device capable of always forming an image of a desired density with respect to input gradation in accordance with the change in time, etc., of sensitivity characteristic of a density sensor. SOLUTION: In long period maintenance, an image is actually formed on a recording paper to store correlation between a toner density on a drum and output density on a recording paper. In short period maintenance, a toner image is formed on the drum receiving gradation pattern data as an input to obtain correlation between input gradation and the toner density on the drum, the output characteristic of the device is obtained from both correlations thus obtained to set gradation correction characteristic in accordance with the output characteristic. Thus, even when the sensitivity characteristic of a density sensor is deviated from an actual toner density value, the deviation of a density sensor detecting values included in both of the correlation are cancelled and the output characteristic of the device can be obtained correctly.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an image forming apparatus such as a copying machine, a facsimile, and a printer using an electrophotographic process.

[0002]

2. Description of the Related Art An image forming apparatus such as a copying machine, a facsimile, and a printer using an electrophotographic process generally includes:
A photosensitive drum having photoconductivity is used as an image carrier, and an electrostatic latent image is formed on a charged photosensitive drum by irradiating a laser beam based on input image data, and this is developed with toner. To form a toner image and transfer the toner image to recording paper or the like.

In such an image forming apparatus, a laser emission characteristic with respect to a gradation value of input image data, a photosensitive characteristic of a photosensitive drum with respect to a laser beam amount, a developing characteristic which represents a toner adhesion amount with respect to a charge amount of a photosensitive drum, and the like. ,
The image density transferred to recording paper or the like is affected by the output characteristics of the apparatus. Since such output characteristics differ from device to device and change over time, in order to obtain an image density output that accurately reproduces the gradation of the input image data, such an image forming apparatus is usually At the time of initial setting or at the time of predetermined maintenance, a gradation correction amount according to the output characteristics of the apparatus is obtained.

Conventionally, as a method of easily obtaining such a gradation correction amount without actually forming an image on a recording sheet or the like, a method based on a predetermined gradation pattern signal on a photosensitive drum is used. A method has been proposed in which a tone pattern image is developed, the output characteristic of the apparatus is detected by detecting the toner density on the photosensitive drum by a density sensor such as an optical sensor, and the output characteristic is responded to (Japanese Patent Laid-Open No. Hei 7-1995). 6697
No. 2 publication).

[0005]

However, the density sensor used in such an image forming apparatus has a problem in that, when a plurality of sheets of image are formed, contamination due to toner scattered in the apparatus and detection sensitivity characteristics are not achieved. , The detection characteristics of the sensor may change over time. For this reason, even if an attempt is made to respond to the output characteristics of the apparatus based on the toner density on the photosensitive drum detected by such a density sensor,
Since the density of the toner on the photosensitive drum detected by the density sensor does not match the density of the image actually transferred to the recording paper or the like, the gradation at which the desired density is not obtained in the final output to the recording or the like is obtained. In some cases, a defect occurred.

More recently, more than one color (eg, cyan,
An image forming apparatus that reproduces a color image by mixing toners of three colors (magenta and yellow) is increasing. In such a color image forming apparatus, the gradation balance of each color is adjusted in order to accurately reproduce the color. Since it is more important to maintain proper, it is required to reproduce the image transfer density for each color toner with higher accuracy. Therefore, in such a color image forming apparatus, even if the gradation defect due to the temporal change of the detection characteristics of the density sensor is minute, it greatly affects the value of the output image. .

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and has been developed in consideration of, for example, a temporal change in detection characteristics of a density sensor.
It is an object of the present invention to provide an image forming apparatus capable of always forming an image with a desired density for an input gradation.

[0008]

In order to solve the above-mentioned problems, an image forming apparatus according to the present invention comprises: a gradation correcting unit for correcting gradation of input image data based on a predetermined gradation correction characteristic; Developing means for developing a toner image on the image carrier based on the corrected image data, a density sensor for detecting the density of the toner developed on the image carrier, and a toner image developed on the image carrier. Transfer means for transferring to recording paper or the like;
At the time of long-term maintenance, when a gradation pattern image is developed on the image carrier with predetermined gradation pattern data as an input image, the toner density value detected by the density sensor and the gradation pattern image on the image carrier A correlation storage means for storing in advance the correlation between the toner density on the image carrier and the final output density, which is obtained from the image density value of each portion of the gradation pattern image transferred when the image is transferred to recording paper or the like, At the time of short-term maintenance, a gradation pattern image is developed on the image carrier using predetermined gradation pattern data as input image data, and a toner density value is detected for each part of the gradation pattern image by the density sensor. From the tone pattern data and the detected toner density value, a correlation between the input image gradation and the toner density on the image carrier is obtained. Between the input image gradation and the toner density value on the image carrier, and the correlation between the toner density on the image carrier and the final output density stored in the correlation storage means in advance. And a gradation correction characteristic setting means for setting gradation correction characteristics in the gradation correction means (claim 1).

According to this image forming apparatus, the correlation between the toner density on the image carrier during the long-term maintenance and the final output density is stored in advance, and the correlation between the input image gradation and the toner density value on the image carrier during the short-term maintenance is stored. A correlation is obtained, and by taking these two correlations into account, the relationship between the input image data and the final output density can be obtained via the toner density value on the image carrier detected by the density sensor.
In this image forming apparatus, the correlation between the input image data thus obtained and the final output density is set as the output characteristic of the apparatus, and the gradation correction characteristic is set so as to correspond to the output characteristic.

As described above, in this image forming apparatus, the relationship between the gradation of the input image and the final output density can be obtained through the toner density detected by the density sensor. Even if the toner density value to be measured is shifted from the actual toner density value on the image carrier, both of the above correlations include this shift, the shift is canceled out, and the density sensor Irrespective of the change in the sensitivity characteristic of the above, the relationship between the input image data and the final output density can be obtained with high accuracy. Therefore,
According to the output characteristics thus obtained, it is possible to set the gradation correction characteristics with which a desired output gradation can be accurately obtained with high accuracy.

In this specification, a case in which maintenance processing involving image formation on recording paper or the like is performed is referred to as a long-term maintenance processing, in which a toner image is developed on an image carrier, but image formation on recording paper or the like is performed. Is called short-term maintenance processing.

Further, in such an image forming apparatus, the input image data is color image data having a plurality of color components, and the gradation correcting means performs gradation correction on the input image data for each color component. The correlation storage unit stores the correlation for each color component, and the tone correction characteristic setting unit is configured to set the tone correction characteristic for each color component, thereby providing a color image forming apparatus. be able to.

In the color image forming apparatus, it is more important to properly maintain the gradation balance of each color in order to accurately reproduce the color. However, according to such a configuration, the input image which may differ for each color component By appropriately correcting the gradation between the gradation and the final output density for each color component, a color image having a desired color tone can be accurately reproduced.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment in which an image forming apparatus according to the present invention is applied to a color copying machine will be described below.

FIG. 1 is a schematic configuration diagram of a color copying machine according to the present embodiment. The color copying machine 10 includes an image reading unit 20 that reads an image of a color original P placed on a contact glass, and an image processing unit that performs predetermined image processing using the image read by the image reading unit 20 as an input image. 30, an image forming unit 40 that forms an image on the recording paper by forming the image data formed by the image processing performed by the image processing unit 30, and a gradation correction characteristic used for the gradation correction process in the image processing unit 30 during a predetermined maintenance. A gradation correction characteristic setting processing unit 50 for performing a rewriting process;
0, a process control unit 90 that controls and controls the image processing unit 30, the image forming unit 40, and the gradation correction characteristic setting processing unit 50
And

The image reading unit 20 includes a contact glass 21
The color original P placed thereon is irradiated with light from the exposure lamp 22, and the reflected light is reflected by the reflecting mirrors 23a, 23b, 23c and the lens 24, and further R (red), G (green), B (blue) The image information of the color original P is separated into R, G, and B components by receiving light with a photoelectric conversion element 25 such as a CCD through a color separation filter of
Color image data R, G, B proportional to the light intensity of each component
Read. The color image data thus read is A / D converted into a digital value, and is used as input image data to the image processing unit 30.

The image processing unit 30 is configured as a computer circuit including an arithmetic unit, a storage unit, and the like (not shown). The image processing unit 30 performs various types of image processing on the input image data read by the image reading unit 20, and forms output image data to be used for image forming processing by the image forming unit 40. Each process in the image processing unit 30 will be described later in detail.

The image forming section 40 is provided with an image transfer section 4 for recording paper fed from a paper feed cassette 411, 412, 413 or a manual feed tray 414 provided at the lower part of the apparatus.
2, forming a toner image based on the output image data;
The toner image is fixed by the heating roller 431 and the pressure roller 432 of the fixing unit 43, and then is discharged to the discharge tray 44. Reference numeral 45 denotes a double-sided unit for reversing the front and back surfaces of a recording sheet on which an image is formed on one side when forming images on both sides of the recording sheet and conveying the recording sheet to the image transfer unit 42 again.

The image transfer section 42 is arranged around a rotatable photoconductive drum 421 having a photoconductive property and sequentially from a charging wire (not shown) to which a high voltage is applied in the rotation direction. A charger 422 for applying a predetermined potential to the surface of the photoconductor drum by corona discharge of the photoconductor drum 421 and a laser beam corresponding to output image data from the laser oscillator 423a via the mirror 423b to selectively set the potential of the surface of the photoconductor drum 421. An exposure device 423 for attenuating the electrostatic latent image to form an electrostatic latent image, a developing device 424 for developing the electrostatic latent image formed on the surface of the photoconductor drum 421 with toner,
A transfer unit 425 for transferring the toner image formed on the photosensitive drum 421 to recording paper, a charge removing unit 426 for removing charges on the surface of the photosensitive drum 421, and a cleaner 427 for removing residual toner on the surface of the photosensitive drum 421 are provided. Have been.

In the copying machine 10, in order to form a color image, the exposing device 423 is configured to form an electrostatic latent image for each color component of output image data.
Reference numeral 24 denotes four developing units 424c, 424m, 424y, and 424k that develop toner images of C (cyan), M (magenta), Y (yellow), and K (black).
The transfer device 425 further includes a photosensitive drum 421
The toner image of each color developed on the surface is temporarily transferred to the transfer belt 42.
5a on the transfer belt 425a.
(Cyan), M (magenta), Y (yellow) and K
The toner image of each color (black) is superimposed and then secondary-transferred onto the recording paper conveyed on the transfer roller 425b.

A density sensor 48, which is an optical sensor for detecting the density of the toner image developed on the photosensitive drum 421, is disposed between the developing device 424 and the transfer device 425. The density sensor 48 is used for calculating output characteristics of the apparatus in long-term maintenance processing and short-term maintenance processing to be described later.

The tone correction correction characteristic setting processing section 50 is configured as a computer circuit including an arithmetic section, a storage section, and the like (not shown). This tone correction characteristic setting processing unit 50
Is performed at the time of initial setting, when environmental conditions such as temperature and humidity around the apparatus are changed, and when predetermined maintenance is performed such as when a predetermined number of image forming processes are performed (for long-term maintenance and At the time of short-term maintenance), a maintenance process is performed to adapt the tone correction characteristics used for the tone correction process in the image processing unit 30 to the output characteristics of the image forming unit 40 at that time. Each process in the tone correction characteristic setting processing unit 50 will be described later in detail.

The overall processing control unit 90 includes the image reading unit 2
0, the image processing unit 30, the image forming unit 40, and the gradation correction characteristic setting processing unit 50 are controlled and controlled to perform image reading, image processing, image formation, gradation correction characteristic setting processing, and the like. An operation panel 91 is connected to the overall processing control unit 90 so that an operation signal from an operator can be input as appropriate.

Next, referring to FIG.
The 0 and gradation correction characteristic setting processing unit 50 will be described.

The image processing section 30 includes a shading correction section 31, a color correction processing section 32, a color space conversion section 33, a black generation processing section 34, an output system compatibility correction section 35, an output color selection section 36,
A scaling unit 37 and a spatial filter unit 38 are provided.

The shading correction section 31 applies the exposure distribution characteristic of the exposure lamp 22 and the sensitivity distribution characteristic of the photoelectric conversion element 25 in the image reading section 20 to the R, G, B input image data read by the image reading section 20. And so on.

The color correction processing unit 32 adapts the input image data configured as a signal corresponding to the light intensity of R (red), G (green) and B (blue) for each pixel to the user's preference. The color correction processing for the adjusted color tone is performed.

The color space conversion unit 33 includes R (red), G
(Green) and B (blue) image data in RGB format according to the light intensity are converted into C (cyan), M (magenta), and Y
A color space conversion process (density conversion process) is performed on a CMY format signal (density recording signal) corresponding to the density of (yellow).

The black generation processing section 34 generates black image data K from color image data in CMY format.

The output-system-adaptive color correction unit 35 is set according to the output characteristics in order to respond to the characteristics (output characteristics) of the image forming unit 40, such as the deviation of the toner color from the theoretical color and the development characteristics. The color image data C,
A correction process for converting M, Y, and K into C ′, M ′, Y ′, and K ′ is performed. The gradation correction processing in the output system compatible color correction unit 35 will be described later.

The output color selection unit 36 determines that the image forming unit 40
Since the developing process is performed for each color, a color to be developed next is selected.

The scaling unit 37 performs an image enlargement / reduction process according to a set magnification or the like.

The spatial filter section 38 performs edge enhancement processing in the case of a character area and performs smoothing processing in the case of a photograph area and a halftone dot area, and outputs output image data to the image forming section 40.

On the other hand, the gradation correction characteristic setting section 50 includes a gradation pattern data storage section 51, a gradation correction characteristic setting section 52, a correlation calculation section 53 between the drum and the recording paper, a correlation storage section 54, and an input- A correlation calculating section 54 between the drums is provided.

The gradation pattern data storage section 51 stores predetermined gradation pattern data in which n different gradation areas (patches) are formed. The gradation pattern data is obtained by extracting a plurality of representative gradations from the output gradation area of the device, and is represented, for example, as data in the following table.

[0036]

[Table 1]

FIG. 5 shows an example of a state in which a gradation pattern image based on the gradation pattern data is developed on the photosensitive drum 421, and FIG. 6 shows an example in which the same gradation pattern image is formed on recording paper P or the like. An example of each state is shown. As shown in these figures, the gradation pattern images appear as patches A1, A2, A3,.

As will be described later, the gradation correction characteristic setting section 52 sets the gradation correction characteristic used in the output system adaptation correction section 35 of the image processing section 30 during long-term maintenance and short-term maintenance. Conformity correction unit 35
Is to be stored.

Drum-recording paper correlation calculator 53
In the long-term maintenance, the density value (toner density on the drum) obtained by detecting the density of each pattern portion formed on the photosensitive drum 421 based on the gradation pattern data by the density sensor 48 and the photosensitive drum 421 The upper gradation pattern image is formed on a recording sheet, and the image density (final output density) on the recording sheet obtained by reading the image with the image reading unit 20 is used to determine the toner density on the drum and the final output density during long-term maintenance. Correlation (drum-
(Correlation between recording papers).

The correlation storage unit 54 is used to set new tone correction characteristics during long-term maintenance and short-term maintenance. The correlation is stored.

The input-drum correlation calculating section 54 calculates
At the time of long-term maintenance and short-term maintenance, the tone value (input tone value) of each portion of the tone pattern data and the density of each pattern portion formed on the photosensitive drum 421 based on the tone pattern data are determined by the density. The correlation between the input image gradation and the toner density on the drum (correlation between the input and the drum) is calculated from the density value (toner density on the drum) detected by the sensor 48.

Next, the setting processing of the gradation correction characteristic performed by the gradation correction characteristic setting section 50 will be described.

The process of setting the gradation correction characteristic is a relatively infrequent maintenance process such as a factory shipment or a maintenance process involving overhaul (hereinafter, such a maintenance process is referred to as a “long-term maintenance process”). And a maintenance process (hereinafter, such a maintenance process is referred to as a “short-term maintenance process”) automatically performed by the apparatus for each predetermined number of image forming processes. .

First, the long-term maintenance processing will be described with reference to FIG.

This long-term maintenance process is performed every time a predetermined number of image forming processes are performed, when a predetermined period has elapsed since the last long-term maintenance process, or when environmental conditions such as temperature and humidity have changed. Then, the processing general control unit 90 and the like notify the operator that long-term maintenance should be performed, and this is performed when the operator responds to this. In this apparatus, a counter that counts the number of images formed since the end of the previous long-term maintenance process,
It is equipped with a timer for measuring the time elapsed since the last long-term maintenance process, a temperature sensor and humidity sensor for detecting the installation environment conditions of the device, and a sound or an indicator light on a display panel. Is input to the process integration control unit 90.

In this long-term maintenance process, the characteristics (particularly, output characteristics) of the apparatus during long-term maintenance are detected, and a gradation correction amount (tone correction characteristics) corresponding to the output characteristics is detected.
Is set, and the correlation between the on-drum toner density and the final output density, which is used for later short-term maintenance processing, is calculated and stored. This long-term maintenance process is based on the C, M, Y, K
Since the color output is performed, the output is performed for each color in order.

In this long-term maintenance process, a gradation pattern image is developed on the photosensitive drum 421 based on the gradation pattern data stored in the gradation pattern data storage unit 51 (step S10). Specifically, the gradation pattern data stored in the gradation pattern data storage unit 51 is transmitted to the upstream side of the output system matching correction unit 35 of the image processing unit 30, and further, the output color selection unit 36 and the scaling process. The development is performed on the photosensitive drum 421 in the image forming unit 40 via the unit 37 and the spatial filter unit 38. In the image forming process that is performed first in the long-term maintenance process, the output system adaptation correction unit 35 does not perform the tone correction process according to the tone correction characteristics.

The toner image of the gradation pattern image developed on the photosensitive drum 421 is supplied to the density sensor 4 for each patch.
The density value is detected by the step 8 (step S20). The detected toner density value on the photosensitive drum (the value detected by the sensor) is sent to the correlation calculation section 53 between the drum and the recording paper and the correlation calculation section 55 between the input and the drum (see FIG. 2). ).

Next, the gradation pattern image on the photosensitive drum 421 is formed on a recording sheet in the image forming section 40 (step S30). Then, the recording paper on which the gradation pattern image is formed is read by the image reading unit 20 (step S40). Specifically, the recording paper on which the gradation pattern image has been formed is placed on the contact glass 21 of the image reading unit 20 by an operator who performs a long-term maintenance process, and the image reading unit 20 operates on the floor above the recording paper. The key pattern image is read. The image data of the gradation pattern image read in this way is sent to the image processing unit 30, and passes through the shading correction unit 31, the color correction processing unit 32, the color space conversion unit 33, and the black generation processing unit 34, and the output system compatibility correction It is sent to the unit 35 and further sent to the drum-recording paper correlation calculating unit 53 (FIG. 2).
reference).

When the toner density value on the photosensitive drum 421 and the image density value on the recording paper are sent to the correlation calculating section 53 in this way, the correlation calculating section 53
The correlation between the toner density value on the recording paper 21 and the image density value on the recording paper is calculated (step S50), and the calculated correlation is stored in the correlation storage unit 54 (step S6).
0). This correlation may be of any data configuration as long as it can describe the correspondence between the two density values. For example, a tabular data configuration in which both density values of each patch are directly described, A form of a function that expresses both density values of each patch on the vertical axis and the horizontal axis can be given.

On the other hand, the input-drum correlation calculating section 5
5, the toner density value on the photosensitive drum 421 (detected value by the sensor) sent from the density sensor 48 and the tone value of each patch of the tone pattern data sent from the tone pattern data storage unit 51, The correlation between the input gradation and the toner density value on the drum based on the input gradation is calculated (step S70). As for this correlation, similarly to the above-described correlation between the toner density value on the photosensitive drum 421 and the image density value on the recording paper, a data configuration capable of describing the correspondence between the two density values (tone values). If so, various configurations can be adopted.

In this way, the correlation between the toner density value on the photosensitive drum 421 and the image density value on the recording paper and the correlation between the input gradation and the toner concentration value on the drum based on the input gradation are obtained. If so, these correlations are sent to the tone correction characteristic setting section 52. Then, in the gradation correction characteristic setting section 52, an output characteristic at the time of long-term maintenance is obtained according to both correlations, and an appropriate gradation correction characteristic suitable for the output characteristic is set (step S80).

Specifically, the correlation between the two and the output characteristics of the apparatus will be described with reference to the conceptual diagram shown in FIG.

As shown in FIG. 9, the gradation of the input image data is subjected to gradation correction in accordance with a predetermined gradation correction characteristic, and is then developed on the drum 421 as a toner image. This toner image is expressed as a toner density, and the final output image density on the recording paper is obtained by transferring this toner image onto the recording paper.

In order to obtain a desired final output gradation with respect to the input image gradation by the above gradation correction, each step of charging, exposure, development and the like until the input image data is developed on the drum 421 is performed. The tone correction characteristics are set in accordance with the output characteristics of the entire device, which includes both the device status at the time of (1) and the device status in the transfer process until the toner image on the drum 421 is transferred onto the recording paper. Is required.

Here, the sensor output value is the density sensor 48
It is considered that there is a deviation from the actual density value of the toner image developed on the drum 421 due to the individual difference of the toner image or the influence of the secular change of the detection sensitivity characteristic.

In the image forming apparatus according to this embodiment,
Since the correlation between the input and the drum and the correlation between the drum and the recording paper use the sensor detection values including substantially the same deviation from the toner density on the drum 421, the sensor detection values included in both correlations are used. It is possible to obtain an output characteristic that matches the state of the device before and after the development on the drum 421 from the correlation, regardless of the deviation, that is, the individual difference of the density sensor 48 or the secular change of the detection sensitivity characteristic. it can. Then, an appropriate gradation correction characteristic can be set according to the output characteristic.

In order to absorb the output characteristics, the gradation correction characteristic may be set so that the gradation correction is performed so that the output gradation formed on the image is linear with respect to the input gradation. However, it is desirable that the gradation is corrected so that the gradation detected by the human eye looks linear or a natural gradation distribution. This gradation correction characteristic (gamma data)
If the horizontal axis indicates the input gray scale and the vertical axis indicates the gray scale after the gray scale correction, it is illustrated as a curve in FIG. 7. Specifically, in this embodiment, the output gray scale with respect to the input gray scale is shown. May be set as a table in which is described. The set gradation correction characteristic is stored in the output system matching correction unit 35 of the image processing unit 30.

After the long-term maintenance processing is performed as described above, the short-term maintenance processing described later or the next long-term maintenance processing is performed in the gradation correction processing in the output system adaptation correction section 35 of the image processing section 30 thereafter. Up to this point, the gradation correction processing is performed according to the gradation correction characteristics.

Next, the short-term maintenance processing will be described with reference to FIG.

The short-term maintenance process is performed every time a predetermined number of image forming processes are performed, when a predetermined period has elapsed since the previous maintenance process, or when environmental conditions such as temperature and humidity have changed. Then, the processing general control unit 90 or the like automatically determines a maintenance time (a short-term maintenance time) at which the maintenance processing is performed, and is performed according to a start signal from the processing general control unit 90. For this reason, in this apparatus, a counter that counts the number of images formed after the end of the short-term maintenance processing, a timer that measures the time elapsed from the end of the short-term maintenance processing, a temperature sensor and a humidity sensor that detect the installation environment conditions of the apparatus, These outputs are configured to be input to the processing general control unit 90.

In this short-term maintenance process, the toner image to be developed on the photosensitive drum 421 is not transferred to the recording paper as described later, so that the operator can automatically operate the apparatus without any trouble. It can be carried out.

In this short-term maintenance processing, a gradation correction amount (gradation correction characteristic) is set according to the characteristics (particularly, output characteristics) of the apparatus during the short-term maintenance. Similar to the long-term maintenance process, the short-term maintenance process is performed sequentially for each color because the apparatus outputs four colors of C, M, Y, and K.

In this short-term maintenance process, first, a gradation pattern image is developed on the photosensitive drum 421 based on image data obtained by correcting gradation pattern data with gradation correction characteristics set at the time of long-term maintenance. (Step S110). Specifically, the tone pattern data stored in the tone pattern data storage unit 51 is sent to the output system adaptation correction unit 35, and the tone pattern data is converted into a tone with the tone correction characteristics set during long-term maintenance. The image data is corrected and sent to the image forming unit 40 via the output color selecting unit 36, the scaling unit 37, and the spatial filter unit 38. Then, a gradation pattern image is developed as a toner image on the photosensitive drum 421 based on the image data.

Subsequently, the toner density (pattern density) of each part of the gradation pattern image developed on the photosensitive drum 421 is detected by the density sensor 48 (step S120). The detected toner density value on the photosensitive drum 421 is sent to the input-drum correlation calculation unit 55 of the gradation correction characteristic setting processing unit 50 (see FIG. 2).

The correlation calculation unit 55 calculates the toner density (detected value by the sensor) on the photosensitive drum 421 sent in this way and the tone value of each patch of the tone pattern sent from the tone pattern storage unit 51. Then, the correlation between the input gradation and the toner density value on the drum based on the input gradation is calculated (step S130). The correlation between the input and the drum calculated in this way is sent to the gradation correction characteristic setting section 52.

The tone correction characteristic setting section 52 stores the drum-type image data stored in the correlation storage section 54 during long-term maintenance.
From the correlation between the recording papers, a gradation correction characteristic for obtaining a desired final output is obtained (step S140). More specifically, the correlation between the input and the drum calculated during the short-term maintenance is based on the toner concentration on the drum that has been subjected to the gradation correction processing by the gradation correction characteristics set during the long-term maintenance. In the tone correction characteristic setting process at the time of short-term maintenance, the tone correction characteristic set at the time of long-term maintenance is corrected so as to respond to a state change from the time of long-term maintenance. In other words, in the tone correction characteristic correction process, the change in the output characteristic between the long-term maintenance and the short-term maintenance for each tone is set as the tone correction characteristic after the short-term maintenance process. In addition to the correction characteristics, a new gradation correction characteristic is set. FIG. 8 is an explanatory diagram schematically showing the gradation correction characteristic (gamma data) obtained by correcting the gradation correction characteristic (gamma data) set at the time of long-term maintenance according to the state at the time of short-term maintenance.

The tone correction characteristics thus corrected are stored in the output system adaptation correction unit 35 of the image processing unit 30, and thereafter, tone correction processing according to the state is performed according to the corrected tone correction characteristics. Will be

As described above, in the image forming apparatus according to this embodiment, the correlation between the toner density value on the photosensitive drum 421 and the final image density value on the recording paper is stored during long-term maintenance. At the time of maintenance, only the correlation between the toner density value of the toner image developed on the photosensitive drum 421 and the gradation value (gradation pattern data) of the input image on which it is based is obtained, and
It is possible to obtain output characteristics that match the state of the apparatus before and after the development of the toner image on the toner image 21. Then, since the gradation correction characteristic according to the output characteristic is set, an appropriate gradation correction characteristic suitable for the state of the apparatus can be set to obtain a desired image density output.

Since the correlation between the toner density on the drum and the final output is stored during long-term maintenance,
At the time of short-term maintenance, it is not necessary to output an image to recording paper in order to obtain this correlation. Therefore, the short-term maintenance processing can be easily performed, and can be automatically performed by the apparatus itself.

In addition, since these correlations use the same detection value of the density sensor 48 as the toner density value on the photosensitive drum 421, the deviation of the sensor output value from the actual toner density value is determined. By canceling each other, the relationship between the input gradation and the final output image density can be accurately obtained without being affected by the secular change of the sensor sensitivity characteristic or the individual difference of the sensor.

Further, during long-term maintenance,
The tone correction characteristics that are surely adapted to the state of the apparatus at that time are obtained by actually outputting on the recording paper, and during short-term maintenance, the photosensitive drum is set using the tone correction characteristics set during the long-term maintenance. Since the toner image is developed on the reference numeral 421 to determine the correlation between the input and the drum, and the tone correction characteristic set at the time of long-term maintenance is corrected in accordance with the correlation, the short-term maintenance process is simple. , An appropriate gradation correction characteristic can be set.

Although the present invention has been described with reference to the embodiment, the image forming apparatus according to the present invention is not limited to the above embodiment, and may be configured as follows.

(1) In the above embodiment, the image processing apparatus according to the present invention is applied to a digital color copying machine. However, if the image forming apparatus forms an image using predetermined input image data, the facsimile apparatus The present invention can be applied to any image forming apparatus such as a printer and a color printer.

(2) In the above embodiment, the tone correction characteristic is set in accordance with the output characteristics of the apparatus during long-term maintenance, and the tone correction characteristic is corrected during short-term maintenance. At this time, only the correlation between the drum and the recording paper may be stored, and the gradation correction characteristic may be set only during short-term maintenance.

(3) In the above embodiment, the optical density sensor 48 is used as the density sensor. However, any known sensor can be used as long as it can detect the toner density value on the surface of the photosensitive drum 421. Can be adopted.

(4) In the above embodiment, the photosensitive drum 421 is used as the image carrier, but any image carrier that develops an input image as a toner image and transfers it to recording paper or the like can be used. Can be employed.

[0078]

As described above, according to this image forming apparatus, the correlation between the toner density on the image carrier stored during long-term maintenance and the final output density, and the input image gradation and image obtained during short-term maintenance. From the correlation between the toner density values on the carrier, the relationship between the tone of the input image data indicating the output characteristics of the apparatus and the final output density is obtained, and the tone correction characteristics are set according to the output characteristics. Therefore, even if the toner density value detected by the density sensor deviates from the actual toner density value on the image carrier, the deviation of the density sensor detection value included in both correlations is offset. . Therefore, the relationship between the input image data and the final output density can be obtained with high accuracy, and the tone correction characteristic for obtaining a desired output tone can be set reliably.

[Brief description of the drawings]

FIG. 1 is an overall schematic diagram of an embodiment in which an image forming apparatus according to the present invention is applied to a color copying machine.

FIG. 2 is a configuration diagram of an image processing unit and a gradation correction characteristic setting processing unit of the image forming apparatus.

FIG. 3 is a flowchart of a long-term maintenance process.

FIG. 4 is a flowchart of a short-term maintenance process.

FIG. 5 is an example of a gradation pattern image formed on a recording sheet.

FIG. 6 is an example of a gradation pattern image developed on a photosensitive drum.

FIG. 7 is an example of a gradation correction characteristic (gamma data) set at the time of initial setting.

FIG. 8 is an explanatory diagram schematically showing a correction process of a tone correction characteristic (gamma data).

FIG. 9 shows a correlation between an input gradation and a toner density on a drum,
FIG. 4 is an explanatory diagram conceptually showing a correlation between a toner density on a drum and a final output density.

[Explanation of symbols]

 Reference Signs List 20 image reading unit 30 image processing unit 35 output system adaptation correction unit 40 image forming unit 421 photosensitive drum (image carrier) 48 density sensor 50 gradation correction characteristic setting processing unit 52 gradation correction characteristic setting unit 53 drum-recording paper Correlation calculation unit 54 Correlation storage unit 55 Correlation calculation unit between input and drum

Claims (2)

[Claims] 1. A gradation correction means for correcting gradation of input image data based on a predetermined gradation correction characteristic, and a development means for developing a toner image on an image carrier based on the gradation corrected image data. A density sensor for detecting the density of the toner developed on the image carrier; a transfer unit for transferring the toner image developed on the image carrier to recording paper; and a predetermined gradation pattern during long-term maintenance. The toner density value detected by the density sensor when the gradation pattern image is developed on the image carrier with the data as an input image, and the toner density value transferred when the gradation pattern image on the image carrier is transferred to a recording paper or the like. Correlation storage means for storing in advance the correlation between the toner density on the image carrier and the final output density, which is obtained from the image density value of each part of the gradation pattern image, Developing a gradation pattern image on the image carrier using predetermined gradation pattern data as input image data; detecting a toner density value by the density sensor for each part of the gradation pattern image; The correlation between the input image gradation and the toner density on the image carrier is determined from the detected toner density value and the correlation between the input image gradation and the toner density on the image carrier determined in this way. Tone correction characteristic setting means for setting tone correction characteristics in the tone correction means based on the correlation between the toner density on the image carrier and the final output density stored in the correlation storage means in advance; An image forming apparatus comprising: 2. The method according to claim 1, wherein the input image data is color image data having a plurality of color components, the gradation correction means performs gradation correction on the input image data for each color component, and the correlation storage means stores Memorize the correlation for each
2. An image forming apparatus according to claim 1, wherein said gradation correction characteristic setting means is configured to set said gradation correction characteristic for each color component.