JP2021151714A - Image forming device - Google Patents

Abstract

To enable an LD to emit light with a desired light emission amount corresponding to an image signal, even after processing for correcting an inclination of a scanning line is performed, so as to prevent density unevenness of an image.SOLUTION: An image forming device comprises: a light source; a light deflection part that deflects and scans light from the light source, in a main scanning direction; an image signal output part that outputs an image signal with a pulse width corresponding to a density indication value for each pixel constituting an image pattern in one line in the main scanning direction; and a memorizing part previously memorizing a corrected density indication value in which a density indication value corresponding to an image signal of a first pixel, a pixel after a leading pixel in the one line is corrected based on a density indication value corresponding to an image signal of a second pixel, a pixel just before the first pixel. The image signal output part outputs an image signal with a pulse width corresponding to the corrected density indication value memorized in the memorizing part, as an image signal of the first pixel.SELECTED DRAWING: Figure 11

Description

The present invention relates to an image forming apparatus that deflects and scans light from a light source onto a photosensitive drum to form an image on the photosensitive drum.

Conventionally, an electrophotographic image forming apparatus has an optical scanning apparatus. The optical scanning device deflects and scans the light from the light source with a rotating multifaceted mirror such as a polygon mirror, and focuses the light on the surface of the photosensitive drum by an imaging optical system such as an fθ lens. The light focused on the photosensitive drum forms an electrostatic latent image on the photosensitive drum as the main scan is performed by rotating the rotating multifaceted mirror and the sub-scanning is performed by rotating the photosensitive drum.

Generally, a laser diode (hereinafter referred to as LD) is used as a light source. For the light emission control of the LD, a frequency modulation method in which image data is created by digital PWM and the light emission of the LD is controlled according to the image data is generally used.

However, there is a problem that the pulse width of the light emitted by the LD is non-linear with respect to the pulse width of the image data created by the digital PWM, and the desired amount of emitted light (integrated light amount) cannot be obtained. there were.

Therefore, conventionally, the light emission control of the LD is performed according to the image data based on the correction table (hereinafter referred to as LUT) for outputting the desired amount of emitted light. Specifically, in Patent Document 1, light emission control for pulse width modulation of image data within one pixel is performed based on a LUT stored in a storage means.

On the other hand, if the transfer speed of the image data is increased, the accuracy of the rising and falling edges of the LD ON / OFF signal deteriorates, and there is a problem that the pulse width becomes narrow. Therefore, in Patent Document 2, the pulse width on the transmitting side is uniformly widened to prevent the pulse width of the laser driver (LDD) or LD from being narrowed.

By the way, when scanning light is scanned into a photosensitive drum by an optical scanning device, the scanning line does not become a straight line parallel to the rotation axis of the photosensitive drum, and the shape thereof is inclined or bent. If the degree of inclination or bending of the scanning line (hereinafter referred to as a profile or the shape of the scanning line) is different for each color, registration deviation occurs. The factors include the non-uniformity of the lens of the optical scanning device including the polygon mirror and the fθ lens, the mounting position deviation, and the mounting position deviation of the optical scanning device to the image forming apparatus main body.

Patent Document 3 describes a method of correcting image data so as to cancel the inclination and bending of these scanning lines and forming the corrected image. That is, in order to correct the deviation of the actual scanning line with respect to the straight line (that is, the ideal scanning line) on the surface of the photosensitive drum parallel to the rotation axis of the photosensitive drum, the image data is offset by shifting the image data in the opposite direction by the same amount. ..

When correcting the deviation of the actual scanning line with a bitmap, gradation correction is performed to smooth the step in order to bring the corrected image data closer to the ideal image data. This smoothing is smoothed by the width and intensity of the pulse by, for example, dithering and increasing the gradation. This gradation correction for smoothing is hereinafter referred to as interpolation processing.

Japanese Unexamined Patent Publication No. 2003-145828 JP-A-2007-168332 Japanese Unexamined Patent Publication No. 2004-170755

Although the above interpolation processing can be realized by the width and intensity of the pulse, it is actually difficult to change the amount of light at high speed, so pulse width modulation (PWM) is used. In recent years, the printing speed has increased and the scanning speed has also increased, so that the light emission time of one pixel tends to be shortened. Therefore, the pulse width of the PWM signal also becomes a short cycle, and the resolution of the integrated light amount becomes rough as the pulse width decreases. In addition, due to the increasing demand for image quality, in order to complement the step difference at the time of tilt correction more smoothly, a large number of minute pulses are used to perform the complement processing.

On the other hand, it is known that the rising delay amount of the ON signal of the LD is affected by the lighting history of the LD. In the main scanning direction in which the scanning light is scanned into the photosensitive drum by the optical scanning device, the rising delay of the pixel immediately after the preceding pixel becomes narrower as the pulse width corresponding to the density indication value of the image signal of the preceding pixel in one line becomes narrower. The effect of quantity is large. It is considered that the cause of the delay in the emission of the laser beam of the LD is the influence of the parasitic capacitance existing between the LD which is the light source and the LD drive control unit which controls the drive of the LD.

Due to the influence of the parasitic capacitance existing between the LD and the LD drive control unit, the LD is different from the amount of emitted light (integrated light amount) that should be emitted, so that the image is shaded. In particular, in order to correct the deviation of the scanning line, dithering is applied to obtain gradation, and in the portion where the image data is complemented, density unevenness occurs remarkably.

Therefore, an object of the present invention is to prevent the density unevenness of the image by causing the LD to emit light with a desired amount of emitted light according to the image signal even when the process of correcting the inclination of the scanning line is performed.

In order to achieve the above object, the present invention comprises a light source capable of emitting light in response to an image signal, an optical deflection unit that deflects and scans light from the light source in the main scanning direction with respect to the photosensitive drum, and the main scanning direction. An image signal output unit that outputs an image signal having a pulse width corresponding to a density indication value for each pixel constituting the image pattern of one line, and an image signal of the first pixel that is a pixel after the first pixel in one line. With a storage unit in which the corrected density indicator value is stored in advance, in which the density indicator value corresponding to is corrected based on the density indicator value corresponding to the image signal of the second pixel which is the pixel immediately before the first pixel. The image signal output unit outputs an image signal having a pulse width corresponding to the corrected density indication value stored in the storage unit as the image signal of the first pixel.

Further, the present invention comprises a light source capable of emitting light according to an image signal, a light deflection unit that deflects and scans light from the light source in the main scanning direction with respect to a photosensitive drum, and an image pattern of one line in the main scanning direction. An image signal output unit that outputs an image signal having a pulse width corresponding to the density indicator for each constituent pixel, and a density indicator corresponding to the image signal of the first pixel that is a pixel after the first pixel in one line. The image signal output is provided with a storage unit in which a correction value for correcting the image signal of the second pixel, which is a pixel immediately before the first pixel, is stored in advance based on a density indicator value corresponding to the image signal of the second pixel. The unit corrects the density indication value of the first pixel with the correction value stored in the storage unit, and as an image signal of the first pixel, a pulse corresponding to the density indication value corrected by the correction value. It is characterized by outputting an image signal having a width.

According to the present invention, the light source can emit light with a desired amount of emitted light according to the image signal, and uneven density of the image can be prevented.

Cross-sectional view showing the overall configuration of the image forming apparatus (A) and (b) are diagrams schematically showing scanning lines of an image formed on a sheet. A perspective view showing the overall configuration of the optical scanning apparatus in the image forming apparatus. Block diagram showing the control circuit of the optical scanning device Timing chart of image formation by optical scanning device The figure which shows the characteristic of an optical output with respect to a drive current The figure which shows the characteristic of the amount of emitted light with respect to an image signal (A) and (b) are diagrams showing a LUT generation image. (A) to (c) are diagrams showing an image of transfer processing. (A) and (b) are diagrams showing the relationship between the image signal and the optical waveform during the transfer process. Flowchart of PWM pulse width correction The figure which shows the correction table of PWM pulse width correction (A) and (b) are diagrams showing the relationship between the image signal and the optical waveform after PWM pulse width correction during the transfer process. The figure which shows the calculation table of PWM pulse width correction (A) and (b) are block diagrams showing parasitic capacitance existing between the LD and the LD drive control unit. (A) to (c) are diagrams showing the relationship between the image signal and the optical output according to the parasitic capacitance voltage.

Hereinafter, preferred embodiments of the present invention will be described in detail exemplarily with reference to the drawings. However, the dimensions, materials, shapes, relative arrangements, etc. of the components described in the following embodiments should be appropriately changed depending on the configuration of the apparatus to which the present invention is applied and various conditions. It is not intended to limit the scope of the invention to those alone.

[Example 1]
(Configuration of the entire image forming apparatus)
The image forming apparatus 1 in this embodiment will be described. FIG. 1 shows the overall configuration of the image forming apparatus 1. The image forming apparatus 1 includes an optical scanning apparatus 2 (2a, 2b, 2c, 2d), an image control unit 5, a reader scanner unit 500, an image forming unit 503 including a photosensitive drum 25 as a photoconductor, a fixing unit 504, and a supply unit. It is composed of a feed / transport unit 505.

The reader scanner unit 500 illuminates the document placed on the platen and optically reads the image of the document, converts the image into an electric signal, and generates an image signal. The image control unit 5 has an image processing unit 403 and an image signal output unit 404 (see FIG. 4), which will be described later, and controls light emission of the optical scanning device 2 (2a, 2b, 2c, 2d) and generates an image signal. ..

The image-creating unit 503 rotates and drives the photosensitive drum 25, which is a photoconductor, and charges the surface of the photosensitive drum 25 by the charger 513. In the optical scanning device 2 (2a, 2b, 2c, 2d), a light source emits light in response to the image signal to expose the surface of the charged photosensitive drum 25, thereby causing an electrostatic latent image on the surface of the photosensitive drum 25. To form. Then, the developing apparatus 512 supplies toner as a developing agent to convert a latent image formed on the surface of the photosensitive drum 25 into a visualization image (toner image). The toner image formed on the surface of the photosensitive drum 25 is transferred to the intermediate transfer body 511 by the primary transfer unit 520. After that, the cleaning unit 522 performs cleaning to collect the toner remaining on the surface of the photosensitive drum without being transferred.

The image forming unit 503 realizes image forming by having four process units (image forming stations) of such a series of electrophotographic processes. The image forming unit 503 has four process units arranged in the order of yellow (Y), magenta (M), cyan (C), and black (K). The four process units sequentially execute magenta, cyan, and black image formation operations after a predetermined time has elapsed from the start of image formation at the yellow station. By this timing control, the full-color toner image is transferred to the intermediate transfer body 511. The toner image formed on the intermediate transfer body 511 is transferred by the secondary transfer unit 521 to the sheet 514 transported from the feeding / transporting unit 505. The fixing unit 504 fixes the toner image transferred to the sheet by the secondary transfer unit to the sheet 514 by heat and pressure.

(Photosensitive drum and optical scanning device)
The optical scanning apparatus 2 in this embodiment will be described. FIG. 3 is a perspective view showing an optical scanning device 2 as an exposure means and a photosensitive drum 25 as a photoconductor.

The optical scanning device 2 as an exposure means includes a laser diode (hereinafter, LD) 201 as a light source, a collimator lens 202, an aperture 203, a cylindrical lens 204, a polygon mirror (rotating polymorphic mirror) 205 as an optical deflection unit, and an fθ lens. It is composed of 206.

The LD201 used in the optical scanning apparatus 2 in this embodiment is a light source capable of emitting light in response to an image signal in order to form an electrostatic latent image on the photosensitive drum 25. Although not shown, the LD201 emits a laser beam corresponding to the image data to one side. The laser beam emitted by the LD201 on one side is applied to the photosensitive drum 25 to form a latent image on the photosensitive drum 25 according to the image data. The LD201 also emits laser light (rear light) at a constant ratio of laser light (front light) to the other side, which is in the opposite direction to the one side. The laser light emitted by the LD201 to the other side is incident on the photodiode (PD302 shown in FIG. 4).

However, the configuration of LD201 is not limited to the above-mentioned configuration. For example, a part of the laser light (front light) emitted by the LD201 on one side may be incident on the photodiode. In this case, the LD201 is configured to emit laser light on one side and not emit laser light on the other side.

In the optical scanning device 2, the laser light a emitted by the LD 201 is made into parallel light by the collimator lens 202 and the aperture 203. Then, the laser beam a becomes an elliptical image long in the rotation axis direction of the photosensitive drum 25 by the cylindrical lens 204, and is incident on the polygon mirror 205 with a predetermined beam diameter.

The polygon mirror 205 is an optical deflection unit (rotary multifaceted mirror) that deflects and scans the laser beam a from the LD201 in the main scanning direction (the direction of the arrow shown by the dotted line in FIG. 3) with respect to the surface of the photosensitive drum 25. The polygon mirror 205 rotates at an equal angular velocity in the direction of the arrow shown in FIG. 3, and along with this rotation, the incident laser light a (light beam) is continuously changed in angle and reflected. The reflected (deflected) laser light a is focused by the fθ (f-theta) lens 206 and is irradiated to the photosensitive drum 25.

On the other hand, the fθ lens 206 corrects the distortion aberration so as to guarantee the temporal linearity of scanning in the direction of the rotation axis at the same time as focusing. Therefore, the laser beam a is coupled and scanned on the surface of the photosensitive drum 25 at a constant velocity in the direction of the rotation axis of the photosensitive drum 25 (the direction of the arrow shown by the dotted line in FIG. 3). The BD (Beam Detector) sensor 209 is a sensor that detects a part of the reflected light (laser light a) from the polygon mirror 205 via the BD mirror 208. The detection signal (BD signal) of the laser beam a by the BD sensor 209 is used as a synchronization signal for synchronizing the rotation of the polygon mirror 205 and the writing of the image data.

(Control circuit of optical scanning device)
The control circuit of the optical scanning device in this embodiment will be described. FIG. 4 is a block diagram showing a control circuit of the optical scanning device in this embodiment. This control circuit performs laser light intensity control (auto power control, hereinafter referred to as APC) in a non-image region with reference to a BD signal, and performs light emission control according to the image signal in the image region.

The operation of the control circuit will be described with reference to FIG. First, the concentration is detected in advance by the concentration sensor 408. The density sensor 408 inputs the detected waveform signal to the light amount setting unit 407. The light amount setting unit 407 stores the light amount value in the CPU 401 as a target value of the light amount that gives an appropriate toner density.

Next, the APC operation will be described with reference to FIG. The APC operation is performed in a non-image region outside the image region of the photosensitive drum 25 in the direction of the rotation axis of the photosensitive drum 25 (see FIGS. 3 and 5). The CPU 401 issues an instruction to execute the APC operation to the APC control unit 406. The APC control unit 406 inputs the drive current of the LD 201 to the LD drive control unit 405. _{The LD drive control unit 405 determines the drive current I LD} of the LD 201 according to the target light amount, and the LD 201 emits a laser beam. The amount of laser light from the LD is detected by the PD 302 and input to the APC control unit 406. The LD drive control unit 405 adjusts the drive current of the LD201 so that the detected light amount becomes a predetermined value notified from the light amount setting unit 407 to the CPU 401. The details will be described below.

The PD 302 generates a monitor current according to the amount of light of the LD 201, and the monitor current is input to the APC control unit 406. Inside the APC control unit 406, the monitor current is converted into a voltage value (monitor voltage) by current-voltage conversion. The monitor voltage is compared with a preset APC reference voltage. Here, the APC voltage to be compared with the monitor voltage is a predetermined value notified from the light amount setting unit 407 to the CPU 401, as described above. The APC control unit 406 controls the APC control voltage V _apc according to the comparison result, and adjusts the monitor voltage so that it becomes the APC reference voltage. The LD drive control unit 405 _{generates the drive current I LD} of the LD 201. At this time, the LD 201 and PD 302 are connected to a common power supply 409.

The APC operation described above, so that the light emission amount of the laser beam from the LD201 becomes a predetermined value, the drive current _{I LD} of LD201 is automatically adjusted. The timing of the APC operation is controlled by the APC control unit 406. The CPU 401 sets the timing of the APC operation for the APC control unit 406. The APC control unit 406 performs an internal clock counting operation based on the input timing of the BD signal, and controls the timing of the APC operation based on the count value set by the CPU 401. The timing of the APC operation is set so that it is performed in the non-image area.

By the above operation, the APC operation is executed in the non-image region, and the amount of emitted light of the laser light is controlled to the amount of light corresponding to the APC reference voltage.

The light amount value determined by the APC operation is held by the LD drive control unit 405. This is done in the OFF region shown in FIG.

Next, the operation in the image area (the image area shown in FIGS. 3 and 5) will be described. In the image region, the light emission control of the LD201 according to the image signal will be described. Emission control of LD201 in accordance with the image signal, the drive current _{I LD,} which is determined by the APC operation, when the image signal is on to emit LD201 to drive the LD201, image signals emit LD201 When off It is a control that blinks the LD201 by making it non-luminous. The details will be described below.

First, the PC 400 or the reader scanner unit 500 inputs image data to the image processing unit 403. The image data is data represented by an RGB color space. The image processing unit 403 converts the image data from the RGB color space to the YMCK color space. Further, the image processing unit 403 performs screen processing on the image data converted into the YMCK color space. Screen processing is processing that expresses gradation by adjusting the number and density of dots and lines. By performing this screen processing, even if the gradation expression by the light emission control of the LD201 is binary, it is possible to express a full color of 256 gradations for each color. However, in this embodiment, it is assumed that the gradation expression by the light emission control of the LD201 is performed with 16 gradations.

Next, the image processing unit 403 inputs the screen-processed image data to the image signal output unit 404. The image signal output unit 404 converts the image data of each color of YMCK into an image signal having a predetermined frequency that blinks (lights or does not emit) the LD201. Next, the image signal output unit 404 outputs an image signal to the LD drive control unit 405 at a predetermined timing based on the BD signal. At this time, the image signal output unit 404 performs PWM control on the image signal according to the image data, and the pulse width (Duty) is determined for each gradation. The LD drive control unit 405 controls the light emission of the LD 201 according to the image signal. Drive current and light intensity of the LD201 is a relationship as shown in FIG. 6, when the image signal is in the ON state drives the LD201 in drive current I _LD, which is determined by the APC operation, the image signal is in the OFF state In this case, the LD201 is driven by the bias current I _BIAS . Bias current _{I BIAS,} it is possible to suppress desirably LD201 is adjusted to less than the threshold current _{I TH} of light emission start, the light emission delay time closer to the threshold current _{I TH} LD201. In the image region, as described above, the BD signal is used as a reference timing, and _{the light emission control of the LD} 201 is performed based on the drive current ILD determined by the APC operation and the blinking pattern of the image signal.

FIG. 5 briefly shows the timing of the above operation. In FIG. 5, the BD signal and the APC signal are turned on by L and turned off by H. The image signal is turned on by H and turned off by L. Therefore, the light emitting state is an off state in the section where the APC signal is L and the image signal is L, and the APC signal is H and the image signal is L. Then, in the image region, the blinking (light emission or non-light emission) of the LD201 is controlled by setting the APC signal to H and the image signal to be H or L.

(Laser emission: LUT control)
In order to explain the light emission control of the LD201, FIG. 6 shows the characteristics (IL characteristics) of the light output [mW] with respect to the drive current [mA] of the LD201. As shown in FIG. 6, LD 201 does not start the emission of the laser beam to the threshold current _I TH [mA]. For example, in the configuration LD210 emits a laser beam on one side and the other side, to the threshold current I _TH is LD201 does not emit laser beam a on one side. Therefore, the LD201 does not emit the laser light on the other side, and the laser light is not detected by the PD 302 (see FIG. 4). On the other hand, when the flow larger driving current the threshold current _I TH [mA], LD201 starts oscillating and starts emission of the laser beam. LD210 is a structure that emits a laser beam on one side and the other side, the larger drive current than the threshold current I _TH, LD 201 emits a laser beam a on one side. Therefore, the LD201 also emits a laser beam on the other side, and the laser beam is detected by the PD 302 (see FIG. 4). Thus, in a given LD 201, when receiving a drive current _I LD [mA], the predetermined light output [mW] is output. A light output [mW] when it was driven by a predetermined drive current _I LD [mA], the emitted amount of light emitted by a predetermined image signal Duty (PWMDUTY) (integrated light quantity) satisfies the request value on the photosensitive drum surface The range of the drive current _ILD [mA] is determined as described above.

Next, as the light emitting characteristics of the LD 201 and the LD drive control unit 405, the PWM characteristics of the LD having the IL characteristics shown in FIG. 6 will be described. PWM characteristics shown in FIG. 7, when driven by the drive current I _LD showing the LD201 in FIG 6, the one pixel which is a predetermined period, the light emission amount when the continuous lighting in the respective Duty of (integrated light quantity) It shows the relationship. According to FIG. 7, it is shown that the LD 201 has an insensitive body that does not emit laser light when the image signal Duty is low. Therefore, it is shown that a pulse width of an image signal Duty 1 or more is required for emitting laser light. Further, it has been shown that when the image signal Duty is high, the amount of emitted light of the laser light is saturated. Therefore, it is shown that the amount of emitted light of the laser light is saturated at a pulse width of the image signal Duty 2 or more, and the amount of emitted light (integrated light amount) is equivalent to 100% of the image signal Duty. Further, in the section from the image signal Duty1 to the image signal Duty2 (the section from the start of light emission to the saturation of LD201), it is shown that the characteristic of the amount of emitted light with respect to the image signal Duty is not a perfect linear response. ing.

As shown in FIG. 7, the characteristic of the amount of emitted light with respect to the image signal Duty is not a perfect linear response. Therefore, when it is assumed that the gradation expression by the light emission control of the LD201 is performed in 16 gradations, even if the image signal Duty is divided into 16 equal parts and assigned to each gradation, the linear gradation expression cannot be performed. I understand. Therefore, in order to control the light emission of the LD201 with 16 gradations, it is necessary to correct the image signal based on the LUT (Look Up Table). 8 (a) and 8 (b) show a method of determining the LUT. From the characteristics of the amount of emitted light with respect to the image signal Duty acquired in advance (FIG. 8 (a)), by associating the image signal Duty with 16 gradations of the emitted light linearly with the input data (FIG. 8 (b)), 16 The light emission control of LD201 is performed by the gradation. For example, the input data corresponding to the amount of emitted light of the LD201 when the image signal Duty is 100% is 15, the input data is 14 if the gradation is one step lower than the amount of emitted light of the LD201 at this time, and so on. Each tone corresponds to it. Further, the input data defined here corresponds to the concentration indicated value described later. In this embodiment, the correction table (FIG. 8B) of the image signal duty that outputs the amount of emitted light of the 16 gradations is referred to as ILUT.

Next, the method of setting the ILUT acquired in FIG. 8 will be described. The ILUT is stored in advance in the memory 402, which is the storage unit shown in FIG. The ILUT stored in the memory 402 is passed to the CPU 401, and the CPU 401 sets the ILUT in the image signal output unit 404. The image signal output unit 404 sets a PWM Duty (image signal Duty) corresponding to the amount of emitted light of each pixel constituting the image data by referring to the ILUT with respect to the image data after screen processing passed from the image processing unit 403. decide.

(Image screen)
In electrophotographic, screen processing is generally performed when generating a halftone image. A multi-valued screen using pulse width modulation (PWM) may be used for screen processing. At this time, by using ILUT, the gradation of halftone is improved. ILUT is determined from the emission characteristics of a single duty continuous signal. However, in view of the actual image signal pattern that has been screen-processed, the duty is not a single duty but is formed by a combination of various duty.

For example, FIGS. 9 (a), 9 (b), and 9 (c) exemplify a part of image data composed of 16-gradation pixels. In FIGS. 9 (a), 9 (b), and 9 (c), the main scanning direction is the direction in which the laser beam emitted from the light source LD201 is deflected and scanned by the polygon mirror 205 which is a rotating multifaceted mirror. Yes, it is the direction of the rotation axis of the photosensitive drum 25. The sub-scanning direction is a direction orthogonal to the main scanning direction and is a rotation direction of the photosensitive drum 25.

In the image data (image pattern) shown in FIG. 9A, there is a line-shaped screen formed by density indication values of "0", "4", "12", and "14" out of 16 gradations. .. The screen determines the density (concentration indication value) according to the line thickness and the data used.

By the way, when scanning light to the photosensitive drum 25 with the optical scanning device 2, as shown in FIG. 2A, if the scanning line L1 formed on the sheet 514 becomes a straight line parallel to the rotation axis of the photosensitive drum 25. However, the shape is tilted or bent. If the degree of inclination or bending of the scanning line L1 (hereinafter, referred to as a profile or the shape of the scanning line) is different for each color, registration deviation occurs. The factors include the non-uniformity of the lens of the optical scanning device 2 including the polygon mirror 205 and the fθ lens 206, the deviation of the mounting position, and the deviation of the assembly position of the optical scanning device to the image forming apparatus main body.

Therefore, as shown in FIG. 2B, the image data (scanning line L1 shown in FIG. 2A) formed on the sheet 514 is corrected, and the corrected image (scanning line L2 shown in FIG. 2B) is corrected. ) Is formed. That is, the deviation of the actual scanning line with respect to the straight line (that is, the ideal scanning line) on the surface of the photosensitive drum parallel to the rotation axis of the photosensitive drum 25 is offset (corrected) by shifting the image data in the opposite direction by the same amount. An image consisting of the scan line L2 is formed. For example, the scanning line L1 shown in FIG. 2A has an end on the downstream side in the main scanning direction on the downstream side in the sub-scanning direction with respect to an ideal scanning line (straight line) parallel to the rotation axis of the photosensitive drum. The straight line is tilted by one line. In this case, as shown in FIG. 2B, each of the plurality of divided scanning lines L1 in the main scanning direction is shifted in the opposite direction by the same amount as the shifted amount, that is, on the upstream side in the sub-scanning direction. Make a correction to shift by one line.

The image data (image pattern) shown in FIG. 9A is an enlarged view of a part of the scanning line L1 whose shape is inclined, which is shown in FIG. 2A. On the other hand, the image data (image pattern) shown in FIG. 9B is an enlarged view of a part centered on the transfer position of the tilt-corrected scanning line L2 shown in FIG. 2B. The dotted line in the center of the main scanning direction indicates the transfer position of the image data. Here, the transfer position of the scanning line L2 is the position of the boundary between the scanning lines divided into a plurality of scanning lines in the main scanning direction. The transfer position indicates a portion where one line data is transferred in the sub-scanning direction. Such a transfer is likely to be recognized as the line shifts at the transfer position. Especially for horizontal lines, it is easy to recognize.

Next, FIG. 9C shows a case where the center of gravity of the line is changed in the sub-scanning direction by one pixel or less by using PWM. The pixels on the left side of the transfer position (pixels on the upstream side in the main scanning direction) have PWM intensities (concentration indication values) of "4", "14", and "12" from the upper part in the sub-scanning direction. In addition, the pixel on the right side of the transfer position has PWM intensities of "4", "14", and "4" from the upper part in the sub-scanning direction, and the data is exchanged with respect to the sub-scanning direction, resulting in a line screen. The center of gravity of is moved. Since such screen switching seems to have a pseudo-increased resolution, it is possible to make it difficult to recognize the deviation at the switching position. Generally, a method is adopted in which the transfer process is performed at a resolution higher than the print resolution, complemented by an area filter, and the resolution is converted to the print resolution to generate the transfer PWM data.

The main scanning line 1 and the main scanning line 2 surrounded by a thick frame shown in FIG. 9C are further cut out in the main scanning direction at an arbitrary sub-scanning position of the screen-processed image pattern. That is, it is an image pattern in a certain line.

By the way, in the main scanning line 1 and the main scanning line 2 surrounded by the thick frame of FIG. 9C, the image signals of the main scanning line 1 are in the order of "12", "14", and "4", and the main scanning line 2 The image signals are in the order of "4", "14", and "12". In this case, the image signal of the main scanning line 1 is the image signal shown in FIG. 10 (a), and the image signal of the main scanning line 2 is the image signal shown in FIG. 10 (b). Normally, the image data is pulse-width modulated through the ILUT to blink the LD201. However, the image signal of each pixel constituting the image data is input, and the LD201 is blinked by the image signal for each pixel. In this case, the image signal of the pixel of interest, which is the first pixel, is desired because the image signal of the preceding pixel, which is the second pixel immediately before the pixel of interest in the main scanning direction, causes a further rise delay. The pulse width (lighting width) may be narrower than the concentration indicated value of. This is shown by the optical waveform shown in FIG. 10 (a). Here, the pixel of interest, which is the first pixel, is a pixel after the first pixel in one line in the main scanning direction. The preceding pixel, which is the second pixel, is the pixel immediately before the first pixel in one line in the main scanning direction. As shown in FIG. 10A, since a certain pixel of interest in one line does not have the image signal of the preceding pixel (because the image signal is “0”), it corresponds to the density indication value of the image signal of the pixel of interest. The width of the optical waveform is as narrow as "2" while the pulse width is "4". That is, the light emitted by the LD201 at the pixel of interest has an optical waveform that is narrower than the density indicated value corresponding to the image signal of the pixel of interest due to the influence of the image signal of the preceding pixel immediately before the pixel of interest. Become. The width of the optical waveform is not narrowed by the continuous pulse, but as shown in FIG. 10A, there is no preceding pixel, and the pixel immediately after the image signal of "0" continues has the optical waveform with respect to the image signal. The width of is narrowed. The numbers in parentheses in the optical waveform indicate the width of the optical waveform. For example, FIG. 10A shows that the width of the optical waveform when the pulse width corresponding to the density indication value of the image signal is “4” is the width corresponding to “2”. When the density indication value of the image signal of the pixel immediately after the pixel having the width of the optical waveform corresponding to "2" is "14", the width of the optical waveform is narrowed to correspond to "13". On the other hand, in a part of the line shown in FIG. 10B, the width of the optical waveform when the density indication value of the image signal of a certain pixel of interest is "12" is considerably narrowed to "10".

As described above, as the pulse width corresponding to the density indication value of the image signal of the preceding pixel in one line in the main scanning direction becomes narrower, the influence of the rising delay amount of the pixel immediately after the preceding pixel becomes larger. It is considered that the cause of the delay in the emission of the laser beam of the LD is caused by the parasitic capacitance existing between the LD 201 and the LD drive control unit 405.

15 (a) and 15 (b) show an equivalent circuit including a parasitic capacitance existing between the LD 201 and the LD drive control unit 405. FIG. 15A assumes that the LD201 is a laser diode with a common anode. FIG. 15B assumes that the LD201 is a common cathode laser diode. Further, although FIGS. 15 (a) and 15 (b) show a configuration in which one light emitting point is controlled, the LD 201 has a plurality of light emitting points, and the LD drive control unit 405 corresponds to the number of light emitting points. It may be configured to have a plurality of drive control units. Although the common of LD201 is different between FIGS. 15 (a) and 15 (b), there is no difference in the operation explanation, so the description will be made with reference to FIG. 15 (a).

In FIG. 15A, the switching unit 902 performs an OFF / ON operation by a switching signal 901 according to the input image data. There is a parasitic capacitance 904 in the path between the LD201 and the switching unit 902, and in the LD201. Further, a current leak portion 905 exists in the path between the LD 201 and the switching portion 902, and is shown as a pseudo resistance component.

Next, the operation will be described. When the switching unit 902 is in the ON state, a current is supplied from the power supply 409 and the current drive defined by the constant current source 903 is performed. When the current is driven by the constant current source 903, a current flows through the parasitic capacitance 904, and the parasitic capacitance 904 is charged with an electric charge. When the parasitic capacitance 904 is sufficiently charged, a current flows through the LD201 and light emission control is performed. Next, when the switching unit 902 is in the OFF state, the constant current source 903 and the LD201 are cut off and the current drive is not performed. When the current drive is not performed, the electric charge charged in the parasitic capacitance 904 is discharged to the leak portion 905 as a leak current. The amount of electric charge charged in the parasitic capacitance 904 is controlled by the amount of discharge from the leak unit 905 according to the OFF time of the switching unit 902.

FIG. 16 shows the operation of the circuit described with reference to FIG. 15 focusing on the relationship between the image signal, the parasitic capacitance voltage, and the optical output. When the switching unit 902 is turned on while the switching unit 902 is OFF for a sufficiently long time and the parasitic capacitance is sufficiently discharged, the LD is charged until the parasitic capacitance 904 is charged with a charge exceeding the threshold voltage Vf emitted by the LD201. Cannot start laser emission. Therefore, as shown in FIG. 16A, the optical waveform has a large rise delay. Next, after turning off the switching unit 902 while the parasitic capacitance 904 is charged with an electric charge, the LD is turned off when the electric charge becomes equal to or less than the threshold voltage Vf, and the electric charge is further discharged from the leak unit 905. If the switching unit 902 is turned on again before the electric charge is completely discharged from the leak unit 905, the charging time up to the threshold voltage Vf changes according to the amount of electric charge remaining in the parasitic capacitance 904, so that the LD201 is driven. The start time will be earlier. Therefore, the light output shown in FIG. 16 (b) or FIG. 16 (c) can be obtained. As described above, since the drive start time of the LD201 differs depending on the amount of electric charge of the parasitic capacitance 904, it can be seen that the rise time is affected by the lighting history.

Due to this effect, the LD is different from the amount of emitted light (integrated light amount) that should be emitted, so that the image is shaded. Therefore, in this embodiment, in order to emit light having a pulse width corresponding to the density indication value of the image signal from the LD201, the density indication value (pulse width) is corrected for each pixel as follows.

(PWM pulse width correction)
As described above, in order to prevent the light (width of the optical waveform) emitted by the LD201 from becoming narrower than the pulse width corresponding to the density indicated value of the image signal, the density indicated value (pulse width) for each pixel of one line. ) Is corrected. To correct the density indication value (pulse width) for each pixel in one line, the density indication value (pulse width) of the pixel of interest is indicated by the density indication value (pulse width) of the preceding pixel immediately before the pixel of interest. It is corrected to a value. The correction of the density indication value (pulse width) for each pixel of one line is performed by the image signal output unit 404 of the block diagram shown in FIG. The flowchart of the pulse width correction is shown in FIG.

First, when image formation is started (S1101), among the pixels constituting the image pattern (image data) of one line in the main scanning direction, the density indicator value corresponding to the image signal of the preceding pixel, which is the second pixel, is referred to. (S1102). As described above, the second pixel is a pixel immediately after the first pixel, which is a pixel after the first pixel in one line. Next, the density indicator value corresponding to the image signal of the pixel of interest, which is the first pixel, is referred to (S1103). Then, the density indication value of the attention pixel is corrected to the corrected density indication value (pulse width) based on the density indication value of the preceding pixel immediately before the attention pixel (S1104). At this time, the image signal output unit 404 corrects the density indicator value of the pixel of interest based on the density indicator value of the preceding pixel immediately before the pixel of interest with reference to the correction table (PWM pulse width correction LUT) shown in FIG. Correct to the later concentration indication value. The correction table shown in FIG. 12 has a density indicator value of the pixel of interest, a density indicator value of the preceding pixel immediately before the pixel of interest, and a density indicator value after correction of the pixel of interest. When the density indicator value corresponding to the image signal of the preceding pixel is input from 0 to 15, and the density indicator value corresponding to the image signal of the pixel of interest is 1, the density correction value of the pixel of interest is set to 1 to 3. Correct with the density correction value after the range is corrected. The lower the density correction value corresponding to the image signal of the preceding pixel, the larger the density correction value after correction of the pixel of interest. FIG. 12 describes the case where the density indication values corresponding to the image signals of the preceding pixels are “1”, “2”, “4”, “12”, and “14”, but the PWM pulse width correction LUT precedes. A table is prepared in which the density indication values corresponding to the image signals of the pixels are "0" to "15". When the density indication value corresponding to the image signal of the pixel of interest is "0", it is not necessary to make the LD 201 emit light. Therefore, it does not have a correction table when the density indication value corresponding to the image signal of the pixel of interest is "0". After correcting the density indication value (pulse width) of the pixel of interest in this way, an image signal having a pulse width corresponding to the density indication value after the correction is output as an image signal of the pixel of interest (S1105).

As a result, as shown in FIG. 13, the width of the optical waveform faithful to the density indication value of the image signal for each pixel can be formed.

In FIG. 13A, assuming that the density indicator value corresponding to the image signal of the preceding pixel is “0”, the density indicator value is changed to “4” after the correction with respect to the density indicator value “4” corresponding to the image signal of the pixel of interest. 6 ”is corrected. Then, as the image signal of the pixel of interest, an image signal having a pulse width with the corrected density indication value set to "6" is output. Then, the optical waveform of the pixel of interest has a width corresponding to "4" corresponding to the density indication value "4" of the image signal, and a desired optical waveform width "4" can be obtained. That is, the LD201 emits light having a pulse width corresponding to the density indicated value of the image signal. Further, the attention pixel (pixel in which the density indicator value of the image signal is "4") whose density indicator value is corrected to the corrected density indicator value "6" is set as the preceding pixel, and the density indicator value of the image signal immediately after that is the density indicator value. The pixel of "14" is set as the pixel of interest. Then, the density indication value of the attention pixel is corrected to the corrected density indication value "15", and an image signal having a pulse width corresponding to the corrected density indication value is output as the image signal of the attention pixel. Then, the LD 201 emits light having a pulse width (a width corresponding to the optical waveform of "14") corresponding to the density indicated value of the image signal. Similarly, the attention pixel (pixel whose density indication value of the image signal is "12") whose density indication value is corrected to the corrected density indication value "15" is set as the preceding pixel, and the density indication value of the image signal immediately after that is used as the preceding pixel. Let the pixel of "12" be the pixel of interest. In this case, the density indicator value corresponding to the image signal of the preceding pixel is "14", and since it is sufficiently thick, no correction is applied here, and the image signal "14" having a pulse width corresponding to the density indicator value as it is is output. .. Then, the LD 201 emits light having a pulse width corresponding to the density indicated value “12” of the image signal.

On the other hand, in FIG. 13B, the density indicator value corresponding to the image signal of the preceding pixel is set to “0”, and the density indicator value after correction is set with respect to the density indicator value “12” corresponding to the image signal of the pixel of interest. Correct it as "13". Then, as the image signal of the pixel of interest, an image signal having a pulse width with the corrected density indication value set to "13" is output. Then, the optical waveform of the pixel of interest has a width corresponding to "12" according to the density indication value of the image signal. That is, the LD201 emits light having a pulse width corresponding to the density indicated value of the image signal.

By correcting the density indicator value corresponding to the image signal of the pixel of interest in this way based on the density indicator value corresponding to the image signal of the preceding pixel immediately before the pixel of interest, the LD201 can obtain a desired value according to the image signal. It is possible to emit light with the amount of emitted light, and it is possible to prevent uneven density of the image. Further, the integrated light amounts of the image signals of the lines in the main scanning direction "4", "14", "12" and "12", "14", and "4" are the same. As a result, the integrated light amount of the line screen shown in FIG. 9C matches on the left and right of the transfer position, and the density is preserved.

[Example 2]
In Example 1, as shown in FIG. 12, a configuration in which the density indication value of the pixel of interest is corrected is illustrated with reference to a correction table having a one-to-one correspondence between the preceding pixel and the pixel of interest. On the other hand, in the second embodiment, as shown in FIG. 14, referring to a correction table (calculation table) having a correction value for correcting the density indication value of the pixel of interest based on the density indication value of the preceding pixel. This configuration corrects the density indication value of the pixel of interest. In the second embodiment, the image signal output unit 404 corrects the density indication value of the attention pixel, which is the first pixel, with a correction value based on the density indication value of the preceding pixel, which is the second pixel immediately before the attention pixel. do. This correction value is stored in advance in the memory 402, which is a storage unit. Specifically, as described above, the calculation table shown in FIG. 14 is stored in the memory 402 in advance. As shown in FIG. 14, the calculation table is for correcting the density instruction value of the preceding pixel immediately before the attention pixel and the density instruction value of the attention pixel based on the density instruction value of the preceding pixel immediately before the attention pixel. It has a correction value and. The image signal output unit 404 corrects the density indication value of the pixel of interest with the correction value based on the density indication value of the preceding pixel immediately before the pixel of interest with reference to the calculation table shown in FIG. Then, the image signal output unit 404 outputs an image signal having a pulse width corresponding to the density indication value corrected by the correction value as the image signal of the pixel of interest.

Since the configuration of the image forming apparatus in the second embodiment is the same as that in the first embodiment, the description thereof will be omitted. Further, the flowchart of the pulse width correction is the same as that of the first embodiment. In the second embodiment, the density indication value of the pixel of interest is calculated using the correction value.

As described above, also in this embodiment, the density indicator value corresponding to the image signal of the pixel of interest is corrected based on the density indicator value corresponding to the image signal of the preceding pixel immediately before the pixel of interest. As a result, as in the first embodiment described above, the LD201 can emit light with a desired amount of emitted light according to the image signal, and uneven density of the image can be prevented. Further, also in this embodiment, similarly to the above-described first embodiment, the integrated light amount of the line screen shown in FIG. 9C matches on the left and right of the transfer position, and the density is preserved. Further, in the correction table of the first embodiment shown in FIG. 12, the number of tables is 240, but in the calculation table of the second embodiment shown in FIG. 14, the number of tables is only 16, so a storage unit for storing this in advance. The usage of the memory 402 (see FIG. 4) can be reduced.

In the above-described embodiment, four image forming stations are used in the image forming unit for forming an image, but the number of used is not limited and may be appropriately set as needed.

Further, in the above-described embodiment, the printer is exemplified as the image forming apparatus, but the present invention is not limited thereto. For example, it may be another image forming apparatus such as a copying machine or a facsimile apparatus, or another image forming apparatus such as a multifunction device combining these functions. Further, an image forming apparatus is exemplified in which an intermediate transfer body is used, toner images of each color are sequentially superimposed and transferred to the intermediate transfer body, and the toner images supported on the intermediate transfer body are collectively transferred to a sheet. , Not limited to this. An image forming apparatus may be used in which a sheet carrier is used and toner images of each color are sequentially superimposed and transferred onto the sheet supported on the sheet carrier. Similar effects can be obtained by applying the present invention to these image forming devices.

a, a'... Laser light 1 ... Image forming device 2 ... Optical scanning device 5 ... Image control unit 25 ... Photosensitive drum 201 ... LD
205 ... Polygon mirror 209 ... BD sensor 302 ... PD
401 ... CPU
403 ... Image processing unit 404 ... Image signal output unit 405 ... LD drive control unit 406 ... APC control unit 407 ... Light intensity setting unit 500 ... Reader scanner unit 503 ... Image creation unit 504 ... Fixing unit 505 ... Feeding / transporting unit

Claims (6)

A light source that can emit light according to the image signal,
An optical deflection unit that deflects and scans the light from the light source in the main scanning direction with respect to the photosensitive drum, and
An image signal output unit that outputs an image signal having a pulse width corresponding to a density indication value for each pixel constituting the one-line image pattern in the main scanning direction.
The density indicator value corresponding to the image signal of the first pixel, which is a pixel after the first pixel in one line, is the density indicator value corresponding to the image signal of the second pixel, which is the pixel immediately before the first pixel. A storage unit in which the corrected density indicator value corrected based on the above is stored in advance, and
With
The image signal output unit is an image forming apparatus characterized in that, as an image signal of the first pixel, an image signal having a pulse width corresponding to the corrected density indication value stored in the storage unit is output. .. The image signal output unit divides one line of scanning lines composed of a plurality of pixels into a plurality of scanning lines in the main scanning direction, and shifts the divided scanning lines in a sub-scanning direction orthogonal to the main scanning direction to perform the scanning. When the process of correcting the shape of the line is performed, the density indicated value of the first pixel is set to the density of the second pixel immediately before the first pixel at the boundary between adjacent scanning lines in the main scanning direction. A claim characterized in that it is corrected to the corrected density indicated value based on the indicated value, and an image signal having a pulse width corresponding to the corrected density indicated value is output as an image signal of the first pixel. Item 1. The image forming apparatus according to Item 1. The storage unit has a correction indicating a density indicating value of the first pixel, a density indicating value of the second pixel immediately before the first pixel, and a corrected density indicating value of the first pixel. The table is stored in advance
The image signal output unit refers to the correction table and sets the density indication value of the first pixel to the density indication value of the second pixel immediately before the first pixel after the correction. The image forming apparatus according to claim 1 or 2, wherein the image forming apparatus is corrected to an indicated value. A light source that can emit light according to the image signal,
An optical deflection unit that deflects and scans the light from the light source in the main scanning direction with respect to the photosensitive drum, and
An image signal output unit that outputs an image signal having a pulse width corresponding to a density indication value for each pixel constituting the one-line image pattern in the main scanning direction.
The density indicator value corresponding to the image signal of the first pixel, which is a pixel after the first pixel in one line, is the density indicator value corresponding to the image signal of the second pixel, which is the pixel immediately before the first pixel. A storage unit in which correction values for correction are stored in advance based on
With
The image signal output unit corrects the density instruction value of the first pixel with the correction value stored in the storage unit, and the density instruction value corrected by the correction value as the image signal of the first pixel. An image forming apparatus characterized by outputting an image signal having a pulse width corresponding to the above. The image signal output unit divides one line of scanning lines composed of a plurality of pixels into a plurality of scanning lines in the main scanning direction, and shifts the divided scanning lines in a sub-scanning direction orthogonal to the main scanning direction to perform the scanning. When the process of correcting the shape of the line is performed, the density indicated value of the first pixel is set to the density of the second pixel immediately before the first pixel at the boundary between adjacent scanning lines in the main scanning direction. 4. The fourth aspect of the present invention is that the correction value is corrected based on the indicated value, and an image signal having a pulse width corresponding to the density indicated value corrected by the corrected value is output as the image signal of the first pixel. The image forming apparatus according to. The storage unit uses the density indicator value of the second pixel immediately before the first pixel and the density indicator value of the first pixel as the density indicator value of the second pixel immediately before the first pixel. A correction table having a correction value for correction based on is stored in advance.
The image signal output unit refers to the correction table and corrects the density indication value of the first pixel with the correction value based on the density indication value of the second pixel immediately before the first pixel. The image forming apparatus according to claim 4 or 5.

Images