JPH1152638A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To conduct the optimum density conversion in image formation to correspond to density of a reference patch. SOLUTION: An image forming device is provided with a patch signal generating means 8 to form plural reference patches comprising different density data from each other, an optical sensor S to detect density of each reference path formed by the patch signal generating means 38, a memory means 34a to store a target density value for each reference patch by the optical sensor S in image formation and correction data to apply correction to density conversion values for conducting density conversion, and an image density control means 34 to determine the density conversion value to obtain the target density value stored in the memory means 34a based on result of detection of the density of each reference patch detected by the optical sensor S, determine a new densits conversion value using the correction data corresponding to the density conversion value, and convert density characteristics of gradation image data in image formation based on the detrained new density conversion value.

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 for converting image density characteristics and outputting the image data when outputting the image data.

[0002]

2. Description of the Related Art In an image forming apparatus such as a copying machine, when outputting an image corresponding to a target density, a density characteristic of image data is converted using a predetermined density conversion table. In JP-A-63-208368 and JP-A-4-126462, a plurality of reference patches having different densities are created, and a density conversion table is created based on the density measurement results. A technique for converting density characteristics of image data is disclosed.

[0003] In the so-called "butting method", an output tone characteristic at that time is obtained from density measurement results obtained by measuring a plurality of density reference patches.
In this method, a target density at each density point is sequentially given as an output gradation characteristic at that time for each density point.

[0004] For example, a target relationship between the density of a reference patch and a measured value of the density is determined in advance, and at a predetermined time, an actual relationship between the density of the reference patch and the measured density is determined. The same density measurement value as that obtained by the obtained predetermined reference patch is matched with the actual relationship, and the density of the actual reference patch corresponding to this is obtained. And
Based on this, a density conversion table for converting a predetermined reference patch into an actual reference patch is created.

At the time of image output, the gradation of the fetched image data is converted using the created density conversion table, so that the image output is performed at the target density corresponding to each gradation at that time. I can do it.

[0006]

However, such an image forming apparatus that performs density conversion by the so-called "butting method" has the following problems. That is, the relationship between the density on the paper in the reference patch (the resolution after being transferred and fixed on the paper) and the density detection result of the reference patch on the photoreceptor by the optical sensor is based on the image area of the reference patch to be created. Different characteristics are represented by the ratio Cin.

Therefore, even if the image density on paper is the same,
If the Cin of the reference patch is different, the result of the density detection by the optical sensor on the photoconductor has a different value.

[0008] This is due to a difference in the height direction between Cin on the photoreceptor of the reference patch. FIG. 11 is a diagram showing the size of each of the latent image of the reference patch on the photoconductor, the reference patch, and the reference patch on the paper depending on the difference in Cin.

If the image density on the paper is the same for both Cin 40% and Cin 50%, the toner images on the paper are equal as shown in FIG. Therefore, in the toner image on the photosensitive member as shown in (B), both toner particles have the same number, but Cin is different, so that the area of the electrostatic latent image on the photosensitive member is reduced as shown in (A). Differently, since the same number of toner particles are developed on the electrostatic latent images having different areas, the toner image on the photoreceptor spreads laterally in a 50% patch having a large Cin and 40% has a narrow Cin. In the patch, toner particles are piled up in the height direction.

When measuring such a reference patch on a photoreceptor with an optical sensor, the reflected light, which is reduced by the toner on the photoreceptor, is taken in by the optical sensor, and the patch density is indirectly measured. Therefore, the detection sensitivity of the optical sensor is high when the number of toner particles on the photoconductor spreads in the horizontal direction (the ratio of covering the photoconductor), and the detection sensitivity is high when the number of toner particles changes in the height direction. Low (it is difficult to detect even if it overlaps more if the toner is concealed by one toner).

Accordingly, in FIG. 11B, Cin4
Even if the number of toner particles is equal between 0% and 50% Cin, Cin
50% of the light sensor spreads horizontally and covers more of the photoreceptor, so the optical sensor has a higher density (more toner particles)
Will be detected.

Accordingly, it is an object of the present invention to provide an image forming apparatus that performs accurate density conversion in consideration of the relationship between the density on paper and the output of a photosensor for each reference patch.

[0013]

SUMMARY OF THE INVENTION The present invention is an image forming apparatus made to solve such a problem. That is, the image forming apparatus of the present invention includes: a reference patch creating unit that creates a plurality of reference patches each including different density data; a density detecting unit that detects the density of each reference patch created by the reference patch creating unit; Storage means for storing a target density value for each reference patch in the density detection means in the formation and correction data for correcting the density conversion value for performing density conversion, and for each reference patch detected by the density detection means A density conversion value for obtaining the target density value stored in the storage means is calculated based on the density detection result, and a new density conversion value is calculated using correction data corresponding to the density conversion value. Density control means for converting density characteristics of gradation image data in image formation based on a new density conversion value.

In the present invention, the correction data corresponding to the density conversion value for obtaining the target density value stored in the storage means is stored in the storage means using the density detection result for each reference patch detected by the density detection means. Then, a new density conversion value is calculated using the correction data to convert the density characteristics of the gradation image data in the image formation. And the density characteristic conversion in consideration of the relationship.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the image forming apparatus of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram illustrating the present embodiment, and FIG. 2 is a configuration diagram illustrating the present embodiment. In the present embodiment, an image forming apparatus mainly including a color copying machine will be described as an example.

As shown in FIG. 2, a color copying machine 1 according to this embodiment is roughly divided into a reading section 2 for reading an image of a document placed on a document table, and an image for processing the read image data. It comprises a processing section 3, a ROS (Raster Output Scanner) optical section 4 for driving a laser in accordance with the processed image data to irradiate a light beam to the photoconductor 50, and an image forming section 5 for forming an image.

Around the photoreceptor 50 of the image forming section 5, a charging device 51, an electrometer 52 for measuring the potential of the photoreceptor surface, a rotary developing device 53, a transfer device 54, and a cleaner device 55 along the rotation direction. And a discharge lamp 56 are arranged in order. The rotary developing device 53 includes a toner dispensing device 53a that supplies toner images of each color of Bk (black), Y (yellow), M (magenta), and C (cyan) to corresponding developing devices of the rotary developing device. Is provided.

The transfer device 54 has a configuration in which a transfer corotron 54b, a charge removing corotron 54c, a peeling corotron 54d, and the like are arranged along a transfer drum 54a. (Not shown), the toner image formed on the photoconductor 50 is transferred.

Further, a fixing device 7 is disposed below the transfer device 54 in the sheet conveying direction so as to fix the toner image transferred to the sheet.

Next, the function of each part of the color copying machine will be described with reference to FIG. First, in the reading unit, the original is irradiated with the exposure lamp L, and the reflected light is read by the CCD 21 and amplified by the amplifier 22 to an appropriate level. Thereafter, the A / D converter 23 outputs 8 bits or 10 bits.
Convert to digital image data of bits.

After the shading correction unit 24 and the gap correction unit 25 perform predetermined correction on the image data, the density converter 26 converts the reflectance data into density data.
The converted image data is sent to the image processing unit.

The image processing section performs basic image processing as a color copying machine, that is, color conversion, black reproduction (UCR),
MTF processing, etc., and Y (yellow), M (magenta), C
Image data of four colors (cyan) and B (black) is generated.
That is, color conversion is performed by the color conversion unit / UCR and MTF processing unit 31, and then gradation conversion of each color is performed by the gradation conversion unit 32 in accordance with the gradation characteristics of the reading unit and the image forming unit.

The density conversion table 33 is used for control by an image density control means 34 described later.
This is table data for performing density control of image data. Next, the image data is converted into analog data by a D / A converter 35, compared with a signal of a predetermined period sent from a triangular wave generator by a comparator 36, and subjected to pulse width modulation to perform binarization.

The patch signal generating means 38 generates a plurality of reference patch image signals having different densities, which are patches for controlling image density. Further, in the selector 39, D /
One of the image data converted into analog data by the A converter 35 and the patch image signal generated by the patch signal generating means 38 is selected and sent to the comparator 36.

The selector 39 selects analog image data at the time of normal copying, and receives an instruction to create a patch from the arithmetic unit 57 of the image forming unit, and outputs the patch signal generating means 3.
When the patch image signal is output from the control unit 8, the patch image signal is selected.

The ROS optical unit is an arithmetic unit 57 of the image forming unit.
A laser light amount varying device 41 that controls the laser light amount and a laser drive circuit 42. The laser drive circuit 42 turns on / off the laser 43 based on the binary data sent from the comparator 36 of the image processing unit.
Control.

This laser light is deflected by the polygon mirror 44 and guided to the photoreceptor 50 of the image forming section via the fθ lens 45 and the reflection mirror 46.

A charging device 51, an electrometer 52, a rotary developing device 53, a transfer device 54, a cleaner device 55, and a static elimination lamp 56 are arranged around the photoreceptor 50 in the image forming section in this order. The electrometer 52 measures the potential on the photoconductor 50 in order to control the photoconductor potential. The optical sensor S detects the density of the patch transferred onto the belt B of the intermediate transfer device 56, and the arithmetic device 57 controls image forming conditions according to the output of the electrometer 52 and the optical sensor S. The charge amount varying device 58 changes the charge amount of the charging device 51 under the control of the arithmetic device 57. Further, the developing bias variable device 53b changes the developing bias.

In the color copying machine 1 of the present embodiment having such a configuration, the density of the reference patch formed on the photoreceptor 50 is detected by the optical sensor S, and the density according to the image area ratio Cin of the reference patch is detected. It is characterized in that the conversion table 33 is created and the optimum density conversion is performed during image formation.

Here, the structure of the optical sensor will be described with reference to the sectional view of FIG. The optical sensor S is a reflection type sensor, and has a structure in which a light emitting diode (LED) as a light emitting unit and a photodiode as a light receiving unit are incorporated in a sensor main body having a housing structure.

In the optical sensor S arranged opposite to the surface of the photoreceptor 50, light is emitted from the LED to the reference patch T formed on the surface of the photoreceptor 50, the reflected light is received by the photodiode, and the reflected light is received. And outputs an electric signal corresponding to the light amount as a detection value. That is, as the amount of reflected light from the reference patch T increases, a larger detection value can be obtained.

The reference patch is composed of a collection of minute halftone images or linear images, and the density conditions are different depending on the size of halftone dots or the line thickness.

FIG. 4 is a diagram for explaining binarization of image data by pulse width modulation in the comparator 36 (see FIG. 1). That is, the input analog image data is compared with a triangular wave, and a portion where the analog image data is larger than the triangular wave is set to "0" to turn off the laser, and a portion where the analog image data is small is set to "1" and the laser is turned on. The data is sent from the comparator 36 to the ROS optical unit.

In the color copying machine 1 having such a configuration, an image is formed according to a well-known xerographic process. That is, first, the rotating photoconductor 50 is uniformly negatively charged by the charging device 51, and a first color latent image is formed on the surface of the photoconductor 50 by the laser beam.

In the portion of the latent image written with the laser beam, the first color (Bk:
The negatively charged black toner is developed by the developing device of (black). Next, the developed image is transferred from the sheet tray 8 by the sheet transfer device 6 to a sheet (not shown) wound around the transfer drum 54a and transferred by the transfer corotron 54b.

At this time, the image remaining on the photoreceptor 50 without being transferred is removed by the cleaner device 55 and the photoreceptor 50 is removed.
Is discharged by the discharge lamp 56. Next, the photoconductor 50 is uniformly negatively charged again by the charging device 51, and the image formation of the second color (Y: yellow) is performed in the same manner as the first color.

The third color (M: magenta) and the fourth color
Developed images of four colors up to the color (C: cyan) are transferred to transfer drum 54
a is sequentially transferred to the sheet on a. After the transfer, the sheet is peeled off from the transfer drum 54a by the peeling corotron 53d and is fixed by the fixing device 7 to complete the color copy. Also,
A static elimination corotron 54c is arranged around the transfer drum 54a, and removes extra charges on the sheet and the transfer drum 54a after the transfer of each color or after the sheet is separated.

As the control of the toner dispensing device 53a in the color copying machine 1, the density of the toner dispensing control reference patch formed on the photoconductor 50 is measured by the optical sensor S, and the toner dispensing amount is adjusted. Do.

For example, immediately after turning on the power of the color copying machine 1,
The arithmetic unit 57 sends the patch signal generating means 3 for every 0 copies.
A signal for generating a patch is sent to 8, and a selector 39 selects a signal for generating a reference patch for toner dispensing control (for example, an image area ratio of 50%) from the patch signal generating means and sends it to the comparator 36.

Then, in the same procedure as the image forming process by the color copying machine 1 described above, the above-mentioned reference patches are formed on the non-image portion on the photoreceptor 50 for each color. Photoconductor 50
The optical sensor S that measures the density of the reference patch formed above irradiates light from the LED as shown in FIG. 3 and receives reflected light from the reference patch by a photodiode to measure the density.

If the measured density of the reference patch is lower than the target, the toner dispensing device 53a is driven to increase the toner density to bring the density of the reference patch closer to the target. On the other hand, if the measured density of the reference patch is higher than the target, the toner dispensing device 53a is stopped to bring the density of the reference patch closer to the target.

In the color copying machine 1, the photoconductor potential is controlled by an instruction from the arithmetic unit 57 of the image forming unit 5 immediately before the power is turned on and before the copy is started, and before the copy is started after a lapse of 30 minutes. . FIG. 5 is a flowchart of the photoconductor potential control. In the following description, reference numerals not shown in FIG. 5 refer to FIG.

That is, the fog prevention potential difference VC from the target dark potential VHS, the target exposure partial potential VLS, or the target dark potential VHS to the developing bias potential VB is stored in the arithmetic unit 57 in advance.

In this state, as shown in step S101, the grid voltage of the charging device 51 is changed to the charging amount variable device 58.
Dark potentials VH1 and VH2 when VG1 and VG2 are set
Is detected by the electrometer 52.

Next, as shown in step S102, a grid voltage VGS for obtaining the target dark potential VHS is calculated.
Next, as shown in step S103, the photoconductor 50 is charged with the grid voltage VGS obtained in step S102. Then, based on an instruction from the arithmetic unit 57, the laser driving circuit 42 is driven by the laser light amount varying device 41 with the two laser light amounts LD1 and LD2, and the two laser light amounts LD1 and LD2 are placed on the photoreceptor 50. An exposure patch is created, and each exposure partial potential VL1,
2 is detected by the electrometer 52.

Next, as shown in step S104, a laser light amount LDS for obtaining the target exposure partial potential VLS is calculated. In step S105, the developing bias potential VB is calculated from the target dark potential VHS by the difference of the fog prevention potential difference VC, and then the grid potential VG is calculated in step S106.
S, the laser light amount LDS, and the developing bias VB are set by the respective variable devices, and the process ends.

FIG. 6 is a flowchart for explaining the outline of density characteristic conversion using the density conversion table in the present embodiment. In the following description, reference numerals not shown in FIG. 6 refer to FIG.

First, as shown in step S201, the density of the reference patch created between copies is detected by the optical sensor S. As the reference patches, a plurality of types of patches having different image area ratios Cin are formed for each color.

Next, as shown in step S202,
From the detected measurement result of the optical sensor S and the target density value stored in the storage means 34a, a density conversion value is calculated using a so-called butting method.

Here, the target density value stored in the storage means 34a is a target density value corresponding to the image area ratio Cin, and is a target value for matching the density of the created reference patch. The density conversion value is the difference between the detected density detection value of each reference patch and the target density value of the reference patch.

Next, in step S203, the density conversion value is corrected by the density conversion value correction data stored in the storage means 34a in accordance with the previously calculated magnitude of the density conversion value.

In step S204, the density characteristic of the gradation image data is converted by referring to the density conversion table 33 based on the corrected density conversion value.

Next, a specific example of the above-described density conversion will be described. FIG. 7 is a diagram illustrating a relationship between the on-paper density and the optical sensor output for each reference patch. In this example, the image area ratio of the reference patch (the ratio of the black area to the entire reference patch) is 0% Cin (0), (14/256)
× 100% is Cin (14), (26/256) × 100
% To Cin (26), (38/256) x 100% to Cin
(38), (64/256) × 100% is converted to Cin (6
4), (102/256) × 100% is converted to Cin (10
2), (152/256) × 100% is converted to Cin (15
2), (202/256) × 100% is converted to Cin (20
2), (255/256) x 100% is Cin (255)
And

As described above, it can be seen that, even with the same optical sensor output, if the image area ratio Cin of the reference patch is different, the density on paper is also different. In the present embodiment, correction is performed in accordance with the magnitude of the density conversion value calculated for each image area ratio Cin of each reference patch based on this characteristic.

FIG. 8 is a diagram showing an example of density conversion value correction data. The density conversion value correction data is a value set according to the magnitude of the density conversion value L for each image area ratio Cin of the reference patch. In this example, the detected density detection value of the reference patch (the output value of the optical sensor S) and the target density value of the reference patch (a value corresponding to the output value of the optical sensor S) are used as the density conversion value L. Shows the difference between

For example, a reference patch having an image area ratio Cin of 50% is formed and its density is detected by the optical sensor S. The difference between the detected value and a target density value corresponding to the reference patch having an image area ratio of Cin 50%, ie, the density is obtained. When the conversion value L is “5”, the density conversion value correction data “4” corresponding to 8> L ≧ 4 in the Cin 50% column shown in FIG. 8 is selected. Then, the selected density conversion value correction data “4” is added to the original density conversion value L = 5 to obtain a new density conversion value “9”.

As the density conversion table, the converted image area ratio is obtained based on the new density conversion value “9”. As a result, the density characteristics can be converted using the density conversion table values reflecting the relationship between the on-paper density and the optical sensor output for each reference patch shown in FIG.

In the example shown in FIG. 8, the image area ratio Cin of the reference patch has a value every 10% from 10% to 100%. May be interpolated using the correction data.

As the density conversion value correction data, FIG.
In addition to storing as table data as shown in
If an exponent, a logarithm, a straight line, and a formula of a straight line divided into a plurality of sections are used and only the formula and the coefficient are stored, the storage capacity of the storage unit 34a can be saved.

FIG. 9 is a diagram showing correction characteristics in this embodiment.
FIG. 10 is a diagram showing correction characteristics in a conventional example (so-called abutting method). In each figure, the horizontal axis indicates the image area ratio Cin, the vertical axis indicates the density on paper (D_OUT), and K, M, Y,
C is K (black), M (magenta), Y (yellow), C before correction
(Cyan), K, M, Y, and C on the right side show K (black), M (magenta), Y (yellow), and C (cyan) characteristics after correction. Further, each drawing shows the characteristics in four states, and the broken line in the drawing shows the target density value.

As described above, by performing the conversion of the density characteristics in the present embodiment, it can be seen that each color and each state approach the target density value as compared with the conventional correction.

Although the above embodiment has been described using an example of a color copying machine as an image forming apparatus, the same applies to a printer, a facsimile, and a so-called multifunction machine in which these are integrated. Further, in the above-described embodiment, an example in which the reference patch is formed on the photoconductor has been described. However, the same applies to a case where the reference patch is formed on an intermediate transfer body or the like other than the photoconductor.

[0063]

As described above, the image forming apparatus of the present invention has the following effects. That is, in the present invention, when performing tone correction by creating a reference patch and detecting its density, predetermined correction is applied based on the magnitude of the density conversion value calculated from the detected value and the target density value. Therefore, accurate density characteristic conversion can be performed over the entire gradation area corresponding to the density of each reference patch, and a stable density can be maintained.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating an embodiment.

FIG. 2 is a configuration diagram illustrating the present embodiment.

FIG. 3 is a cross-sectional view illustrating a structure of an optical sensor.

FIG. 4 is a diagram illustrating binarization by a comparator.

FIG. 5 is a flowchart illustrating photoconductor potential control.

FIG. 6 is a flowchart illustrating an outline of density characteristic conversion.

FIG. 7 is a diagram illustrating a relationship between on-paper density and an optical sensor output for each reference patch.

FIG. 8 is a diagram illustrating an example of density conversion value correction data.

FIG. 9 is a diagram illustrating correction characteristics according to the present embodiment.

FIG. 10 is a diagram showing correction characteristics in a conventional example.

FIG. 11 is a diagram illustrating a size on paper according to a difference in image area ratio.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Color copier, 2 ... Reading part, 3 ... Image processing part,
4 ROS optical unit, 5 image forming unit, 6 paper transport device, 7 fixing device, 8 paper tray, 33 density conversion table, 34 image density control unit, 34a storage unit,
50: photoreceptor, 53: rotary developing device, 54: transfer device

Claims (2)

[Claims] 1. A reference patch creating means for creating a plurality of reference patches each having different density data, a density detecting means for detecting the density of each reference patch created by the reference patch creating means, and the density in image formation. Storage means for storing a target density value for each reference patch in the detection means and correction data for correcting the density conversion value for performing density conversion; and a density for each reference patch detected by the density detection means. A density conversion value for obtaining the target density value stored in the storage means is calculated based on the detection result, and a new density conversion value is calculated using the correction data corresponding to the density conversion value. An image forming apparatus comprising: a density control unit configured to convert density characteristics of gradation image data in image formation based on a calculated new density conversion value. 2. The apparatus according to claim 1, wherein the correction data comprises a value for compensating for an error in the density conversion value based on an output characteristic difference generated according to the density of the reference patch in the density detection means. Image forming apparatus.