JPH11327223A - Device and method for forming image - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make the maximum density value which can be reproduced constant regardless of the number of lines of pulse width modulation and to obtain optimum image quality by switching a target value every plural number of lines in a pulse width modulation means by a density control means. SOLUTION: In a 200-line mode, a developing bias DV1 corresponding to a target value Dtg1 is obtained based on a density curve 21 obtained by using a Dmax sensor through an identical process to a conventional process. A target value Dtg2 in a 600-line mode is set higher than the value Dtg1. Thus, the developing bias DV2 of the 600-line mode corresponding to the target value Dtg2 is obtained based on a density curve 22. When an image forming that the 200-line and the 600-line modes coexist is executed, an APC (light quantity stabilizing) reference value is made constant regardless of the number of lines and the developing biases DV1 and DV2 are properly switched according to the number of lines to carry out the image forming. Thus, the stable maximum density is secured regardless of the number of lines and the optimum image forming can be realized.

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 an image forming method, and more particularly to an image forming apparatus and an image forming method for forming an image by an electrophotographic method.

[0002]

2. Description of the Related Art In a conventional image forming apparatus for forming an image by an electrophotographic method, a method of performing pulse width modulation (PWM) on image data and controlling laser driving based on the obtained modulated pulse is known. Are known.

FIG. 9 shows a relationship between input image data to a PWM circuit for performing PWM and a light emission amount of a laser in a conventional image forming apparatus, that is, a light emission amount characteristic with respect to the input image data. According to the figure, when image formation is performed by, for example, 200-line PWM (hereinafter, 200-line mode), as shown at 91, good linearity can be obtained.

However, image formation by 600-line PWM (hereinafter, referred to as 60-line PWM) corresponding to the recent demand for higher resolution.
In the case of performing (0-line mode), reproducibility of a minute pulse becomes difficult in the vicinity of the maximum density, so that a density jump occurs as shown by 92 and linearity is lost.

Therefore, in the conventional image forming apparatus, in order to avoid the problem of the occurrence of the density jump, when expressing a halftone as a pseudo halftone such as dither in the 600-line mode, FIG. As shown at 93, the emission light amount characteristic is controlled so that the entire range of the input image data is expressed using only the laser emission amount range having good linearity. As a result, in high-resolution image formation in the 600-line mode or the like, although there is a slight decrease in the dynamic range, the occurrence of a density jump is avoided.
The linearity of the laser light intensity characteristic was maintained.

[0006]

However, in the above-described conventional image forming apparatus, when one image is output by selecting the number of lines according to its pattern, that is, the number of lines according to the characteristics of each area, for example, the above-described image forming apparatus is used. As can be easily understood from 91 and 93 shown in FIG. 9, there is a problem that the maximum reproducible density value in the same image changes depending on the number of lines, such as 200 lines and 600 lines.

In order to make the maximum density values in both lines equal, the light emission amount characteristic of the 200-line mode shown by 91 in FIG. 9 is changed by changing the PWM adjustment value to the 600-line mode shown by 93 in FIG. , The dynamic range is impaired even in the 200-line mode. This is an unacceptable problem in the 200-line mode in which the gradation is more important than the resolution.

Further, as described above, if the light amount corresponding to the maximum image data is controlled to be about 80% or less of the maximum light amount in the characteristics of the 200-line mode, a uniform line width cannot be secured. There was also the problem of getting lost.

In order to control the maximum amount of laser light emission in accordance with the number of lines, an APC (Automatic Power Control)
l) If the APC light quantity specified by the control is switched in accordance with the number of lines, there is a problem that the cost increases due to an increase in hardware of the APC circuit.

In the case where prints of 200 lines and 600 lines are mixed on a plurality of sheets, every time the number of lines is switched, a toner image of a sample pattern for density control is formed on, for example, an intermediate transfer member. Then, the maximum density control is performed by detecting the density. Therefore, there is a problem that the throughput is reduced, the waiting time of the user until all the printing is completed becomes long, and the toner consumption is increased due to frequent pattern generation.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problem, and provides an image forming apparatus and an image forming method for obtaining an optimum image quality by setting the maximum reproducible density value to be constant regardless of the number of lines of PWM. The purpose is to:

[0012]

As one means for achieving the above object, the image forming apparatus of the present invention has the following arrangement.

That is, a modulating means for performing pulse width modulation on an image signal for each of a plurality of lines, and an image forming means for forming an image by emitting a laser beam according to the pulse width modulated image signal. Detecting means for detecting the density of the sample image formed by the image forming means, and detecting the density of the image formed by the image forming means to be a predetermined target value based on the density of the sample image. Density control means for performing density control, wherein the density control means switches the target value for each of a plurality of lines in the pulse width modulation means.

Further, the image forming apparatus further comprises holding means for holding the density control result of the density control means for each of the plurality of lines, wherein the image forming means forms an image based on the density control result held by the holding means. It is characterized by forming.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment according to the present invention will be described below in detail with reference to the drawings.

First Embodiment FIG. 1 shows a schematic configuration of a printer as an image forming apparatus according to the present embodiment. In FIG. 1, reference numeral 201 denotes a host computer (hereinafter, a host) which is an information source of image information to be printed, and 202 denotes a printer main body. Printer body 20
2 is an interface connected to the host 201 (hereinafter referred to as an interface).
(I / F) 203, a frame buffer 204 that holds image information transferred from the host 201 as image data to be printed, and a mask or UCR process performed on the output of the frame buffer 204 to convert the output into a signal suitable for a printer engine. Production functions (hereinafter R
F) A controller unit 206 including a unit 205, and an engine unit 207 that receives output data from the controller unit 206 and prints it on recording paper.

The operation of the printer having the above configuration will be described below. The multi-valued image data input from the host 201 is taken in and held by the frame buffer 204 via the I / F 203, and the frame buffer 2 is synchronized with the recording speed of the engine unit 207 during printing.
The image data is read from the image data 04. The read image data is converted by the RF unit 205 according to the characteristics of the engine unit 207. For example, if the image data is input as RGB signals, the image data is converted into a signal that can be handled by the engine unit 207, that is, if the engine unit 207 performs full color printing of YMCK, image data of four colors of YMCK. At this time, masking and UCR processing optimized for the process characteristics of the engine unit 207, that is, the toner characteristics and the developing bias-density characteristics are performed. Then, the engine unit 207 prints out the image data that has undergone the above-described conversion process.

FIG. 2 shows the engine unit 207 particularly related to the halftone processing in the above-described printer apparatus of the present embodiment.
The detailed configuration of is shown.

The Y, M, C, and K image data generated by the RF unit 205 in the controller unit 206 through the above-described process is sent to the engine unit 207, where the pulse width modulator (hereinafter, PWM circuit) 102, When the laser 105 emits light via the laser driver 103, the photosensitive drum 10
6, latent images of respective colors are sequentially formed. The laser driver 103 is provided with a well-known light amount stabilizing circuit (hereinafter, APC circuit) 104 for stabilizing the light amount of the laser 105 in addition to a circuit for driving the laser 105 by a PWM signal. In the present embodiment, the processes after the formation of the latent image during the image formation are not particularly different from those of the conventional example, and thus the detailed description is omitted here.

Hereinafter, the PWM processing in this embodiment will be described.

According to the PWM circuit 102 of the present embodiment,
As shown in FIG. 3, the output pulse width changes in proportion to the level of the input data. In the engine unit 207, the dot size is controlled as shown in (6) of FIG. 3 by controlling the light emission amount of the laser 105 based on the pulse width, and halftone is reproduced.

In order to stably reproduce such a halftone, it is essential to stabilize the maximum density to be reproduced. Therefore, in the printer of the present embodiment,
Prior to the image forming operation, Dmax control for stabilizing the maximum density is performed. Hereinafter, the Dmax control in the present embodiment will be described with reference to FIGS. 4 and 5.

In FIG. 4, reference numeral 21 denotes a Dmax sensor;
Is a medium on which a patch for density measurement such as a photosensitive drum or an intermediate transfer member is formed, and 25 (25a, 25b) is a medium 2
4 is a patch pattern formed on the fourth pattern. The Dmax sensor 21 irradiates the patch pattern 25 with the internal LED 22, the reflected light is read by the photo sensor 23, and the density is calculated by a processing unit (not shown).

FIG. 5 shows an example of reading by the Dmax sensor 21. In FIG. 5, the horizontal axis represents the developing bias, and the vertical axis represents D.
Indicates the density value read by the max sensor. In the conventional printer device, for example, eight patches as shown in FIG. 4C with different developing biases are formed on the medium 24 with respect to patch patterns of the same density. Then, by interpolating the density measurement values at eight points read by the Dmax sensor 21, the density curve 11 shown in FIG. 5 is obtained. Then, based on the density curve 11, a developing bias DV1 corresponding to a predetermined Dmax target value Dtg1 is obtained. The density curve 11 shown in FIG.
Is an example of reading a patch pattern formed by 200 lines.

The thus obtained developing bias DV1
By performing image formation by using, it is possible to secure a stable maximum density while minimizing the influence of environmental changes and the like.

In the present embodiment, the maximum density control is performed by the above-described method. However, as described in the conventional example, the maximum density value differs depending on the number of lines, so that the maximum density value differs for each number of lines. Must be equal. Therefore, in the present embodiment, the maximum density value for each line number is made substantially equal by the following configuration.

FIG. 6 shows an engine unit 2 according to this embodiment.
At 07, a detailed block configuration including a configuration realizing the maximum density control for each line number is shown.

In FIG. 1, image data is stored in a PWM circuit 1.
When the signal is input to 02 and modulated, a pulse having a pulse width corresponding to the image data is output. PWM
The circuit 102 has two modes, a 200-line mode and a 600-line mode, and is switched by the switch 10 according to a mode signal output from a control unit (not shown). The laser driver 103 drives the laser 105 in response to the pulse, and the laser 105 emits light to form a latent image on a photosensitive drum (not shown).

On the other hand, a part of the laser light is received by a photodiode 5 for monitoring the amount of light, and the received light current is converted into a voltage by an I / V converter 6, supplied to an integration circuit 7, and Is supplied to the well-known APC circuit to stabilize the amount of emitted light.

The output of the integration circuit 7 and a predetermined APC reference voltage are supplied to the comparator 8, and the output of the comparator 8 is applied to a pulse width adjustment input terminal of the PWM circuit 102. Thereby, the pulse width adjustment is performed in the PWM circuit 102.

On the other hand, the reflection voltage of the patch pattern detected by the Dmax sensor 11 is converted into a density value by a Dmax processing unit 12, and a developing bias 12a corresponding to a Dmax target value described later is calculated. The developing bias 12a is supplied to a developing device 14 via a developing high voltage circuit 13. Here, the Dmax target value input to the Dmax processing unit 12 includes the Dmax target value Dtg1 for 200 lines input from the terminal 15 in the switch 18 and the Dmax target value 600 input from the terminal 16 in the switch 18.
The line Dmax target value Dtg2 can be switched. . still,
The switch 18 is also switched according to a mode signal output from a control unit (not shown).

Next, a process for stabilizing the maximum density regardless of the number of lines in the above configuration will be described with reference to FIG.

In the 200-line mode shown in FIG. 7, the developing bias DV1 corresponding to the target value Dtg1 is obtained based on the density curve 21 obtained by using the Dmax sensor 11 by the same process as in the prior art.

On the other hand, in the 600-line mode, Dmax
The density curve 22 obtained using the sensor 11 is 200
The characteristic shows that the slope is gentler than the density curve 21 in the linear mode. As described in the conventional example, this difference in the density curve characteristics occurs because the range of the actual laser light emission amount is limited to the light amount range in which the density jump does not occur (for example, about 80% of the maximum light amount) in the 600-line mode. .

In the 200-line mode and the 600-line mode, as described above, since the image reproduction characteristics are different due to the difference in resolution, the target value of the maximum density is also different. That is, in the 200-line mode and the 600-line mode, the highest density value to be targeted for matching the highest density value actually reproduced naturally differs. Therefore, in the present embodiment, the target value Dtg2 in the 600-line mode
Is set higher than Dtg1 as shown in FIG. Thus, the developing bias D in the 600-line mode corresponding to the target value Dtg2 based on the density curve 22
V2 is obtained.

More specifically, based on the control of the switch 18 shown in FIG.
, It is possible to apply an optimum developing bias DV2 to the developing device 14 in the 600-line mode.

As described above, according to the present embodiment,
When forming an image in which the 200-line / 600-line mode is mixed, the APC reference value is fixed irrespective of the number of lines, and the developing biases DV1 and DV2 are appropriately switched according to the number of lines to form the image. 200 where emphasis should be placed on tone reproduction
Even in the line mode, a stable maximum density is ensured irrespective of the number of lines without lowering the maximum light amount, and optimal image formation is possible.

Since the switching of the Dmax target value and the Dmax processing unit 12 in FIG. 6 are usually realized by software, no increase in cost due to an increase in hardware occurs.

<Second Embodiment> Hereinafter, a second embodiment according to the present invention will be described.
An embodiment will be described.

FIG. 8 is a block diagram showing the configuration of the engine unit in the image forming apparatus according to the second embodiment. In the figure, the same components as those in FIG. 6 shown in the above-described first embodiment are denoted by the same reference numerals, and description thereof will be omitted.

In the second embodiment, 200 lines and 6 lines are used.
A storage unit 17 for storing a developing bias for line 00 is provided. Normally, Dmax control needs to be performed once when a large environmental change occurs or every time several hundred pages of image are formed. In the second embodiment, when image formation with 200 lines and image formation with 600 lines are mixed, image formation is performed according to the number of lines with reference to the development bias information held in the storage unit 17. . Therefore, in the Dmax processing unit 12, the control corresponding to each line number is performed at least once at the timing of performing the Dmax control, and the obtained developing bias may be stored in the storage unit 17.

As described above, according to the second embodiment, by storing and maintaining the developing bias in accordance with the number of lines, even when forming an image in which both the 200-line mode and the 600-line mode are formed, Since only one Dmax control is required for each line number, not only the throughput of image formation is improved, but also the increase in toner consumption due to the generation of a sample pattern for density control can be suppressed.

The Dmax processing unit 1 in the second embodiment
2 is also realized by software, so that there is no cost increase due to an increase in hardware.

[0044]

[Other Embodiments] Even if the present invention is applied to a system including a plurality of devices (for example, a host computer, an interface device, a reader, a printer, etc.), an apparatus (for example, a copying machine) Machine, facsimile machine, etc.).

Another object of the present invention is to provide a storage medium storing a program code of software for realizing the functions of the above-described embodiments to a system or apparatus, and to provide a computer (or CPU) of the system or apparatus.
And MPU) read and execute the program code stored in the storage medium.

In this case, the program code itself read from the storage medium realizes the functions of the above-described embodiment, and the storage medium storing the program code constitutes the present invention.

As a storage medium for supplying the program code, for example, a floppy disk, hard disk, optical disk, magneto-optical disk, CD-ROM, CD
-R, a magnetic tape, a nonvolatile memory card, a ROM, or the like can be used.

When the computer executes the readout program code, not only the functions of the above-described embodiment are realized, but also an OS (Operating System) running on the computer based on the instruction of the program code. ) May perform some or all of the actual processing, and the processing may realize the functions of the above-described embodiments.

Further, after the program code read from the storage medium is written into a memory provided on a function expansion board inserted into the computer or a function expansion unit connected to the computer, based on the instructions of the program code, It goes without saying that the CPU included in the function expansion board or the function expansion unit performs part or all of the actual processing, and the processing realizes the functions of the above-described embodiments.

[0050]

As described above, according to the present invention, A
By making the PC reference value constant irrespective of the number of lines and switching the target value of the density control by the number of lines, the maximum reproducible density value is kept constant irrespective of the number of lines, and the optimum image quality can be obtained without increasing the cost Can be.

The maximum density control information obtained for each number of lines is stored. If prints with different numbers of lines are mixed, an image is formed using the stored information, whereby each line is printed. The density control corresponding to the number needs to be performed at least once. As a result, not only the throughput of the apparatus is improved, but also the consumption of toner due to the generation of a sample pattern for density control can be suppressed.

[0052]

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a schematic configuration of a printer according to an embodiment of the invention.

FIG. 2 is a block diagram showing a configuration of an engine unit including a configuration for realizing halftone processing in the embodiment.

FIG. 3 is a diagram illustrating a PWM modulation process according to the embodiment.

FIG. 4 is a diagram showing a configuration for performing Dmax control in the present embodiment.

FIG. 5 is a diagram showing an example of a density curve obtained by Dmax control in the embodiment.

FIG. 6 is a block diagram showing a detailed configuration of an engine unit including a configuration for realizing maximum density control for each number of lines in the present embodiment.

FIG. 7 is a diagram for describing Dmax control in the present embodiment.

FIG. 8 is a block diagram illustrating a configuration of an engine unit according to a second embodiment of the present invention.

FIG. 9 is a diagram showing a laser emission light amount characteristic with respect to input image data obtained in a conventional PWM process.

[Explanation of symbols]

 Reference Signs List 5 photodiode 6 I / V converter 7 integrating circuit 8 comparator 10, 18 switch 11 Dmax sensor 12 Dmax processing unit 13 developing high voltage 14 developing unit 15 200 line target value 16 600 line target value 102 PWM circuit 103 laser driver 104 APC Circuit 105 Laser

Claims (15)

[Claims] 1. A modulating means for performing pulse width modulation on an image signal for each of a plurality of lines, and an image forming means for forming an image by emitting a laser beam in accordance with the pulse width modulated image signal. Detecting means for detecting the density of the sample image formed by the image forming means; and controlling the density value of the image formed by the image forming means to a predetermined target value based on the density of the sample image. An image forming apparatus comprising: a density control unit that performs density control; wherein the density control unit switches the target value for each of a plurality of lines in the pulse width modulation unit. 2. The apparatus according to claim 1, further comprising instruction means for instructing switching of the number of lines of pulse width modulation in said modulation means, wherein said concentration control means switches said target value in accordance with an instruction from said instruction means. The image forming apparatus according to claim 1. 3. The image forming apparatus according to claim 2, wherein the density control unit controls a maximum density value of an image formed by the image forming unit. 4. The image forming apparatus according to claim 3, wherein the density control unit controls an image forming condition in the image forming unit. 5. The image forming apparatus according to claim 4, wherein the image forming condition is a developing bias. 6. The modulation means performs pulse width modulation by a first number of lines emphasizing reproducibility of resolution and pulse width modulation by a second number of lines emphasizing reproducibility of gradation. The image forming apparatus according to claim 1. 7. The image forming apparatus according to claim 6, wherein the target value for the first line number is higher than the target value for the second line number. 8. The image forming apparatus according to claim 1, further comprising: a holding unit configured to hold a density control result of the density control unit for each of the plurality of lines. The image forming apparatus according to claim 1, wherein the image forming apparatus forms an image. 9. The image forming apparatus according to claim 8, wherein said image forming means forms an image based on a density control result held by said holding means when forming an image within a predetermined number of sheets. Forming equipment. 10. The apparatus according to claim 9, wherein said density control means performs density control for each of said plurality of lines at least once each time said predetermined number of images are formed by said image forming means. Image forming device. 11. The image forming apparatus according to claim 1, further comprising light amount stabilizing means for stabilizing the light emission amount of the laser based on a predetermined reference value. 12. An image forming apparatus according to claim 11, wherein a reference value in said light amount stabilizing means is constant irrespective of said plurality of lines. 13. The light quantity stabilizing means monitors the light emission quantity of the laser, and compares the output of the light quantity monitoring means with a predetermined reference value, the light quantity monitoring means generating a voltage corresponding to the light emission quantity. The image forming apparatus according to claim 12, further comprising: a comparing unit; and a stabilizing unit that stabilizes an emission amount of the laser according to the comparison result. 14. An image forming method, comprising: performing pulse width modulation on an image signal for each of a plurality of lines; and causing a laser to emit light in accordance with the pulse width modulated image signal to form an image. A sample forming step of forming a sample image; a detecting step of detecting the density of the sample image; and a density control based on the density of the sample image such that a density value of the formed image becomes a predetermined target value. An image forming method, comprising: performing the density control step; wherein in the density control step, the target value is switched for each of the plurality of lines. 15. A program code for an image forming method for forming an image by performing pulse width modulation on an image signal for each of a plurality of lines and causing a laser to emit light according to the pulse width modulated image signal. A computer-readable memory stored therein, comprising: a code of a sample forming step of forming a sample image; a code of a detecting step of detecting a density of the sample image; and a code of an image formed based on the density of the sample image. A code for a density control step of performing density control so that a density value becomes a predetermined target value that differs for each of the plurality of lines.