JPH03208681A - Image forming apparatus - Google Patents

Abstract

PURPOSE:To output an image including a middle tone stably at all times with high quality by detecting the image density of an electrostatic latent image, detecting the density of a specific pattern based on the detecting result of the image density, and correcting the density corresponding to the kind of the image based on the detected density of the specific pattern. CONSTITUTION:In response to an instruction from a CPU 15, a specific pattern generat ing circuit 22 generates a signal to form a mat black pattern to a laser driver 7, so that an image of a mat black pattern is formed on a photoreceptor 10. The density of this image is detected by a densitometer 23 and taken into the CPU 15 through an A/D converter 26. The CPU 15 compares the image density with a preset reference density data. When the detected density is lower than the reference density, the CPU 15 instructs a bias power source 14 to lower the bias output. On the other hand, if the detected density of the image is higher than the reference density, the CPU 15 instructs the bias power source 14 to raise the bias output. Thereafter, in correcting the density of an image of a middle tone, the CPU 15 forms a density correction data from the detected density data in the same manner as above, and outputs the data to a RAM 4. As a result, a digital-digital conversion is performed to an image data 1 in accordance with the density correction data, and the corrected data is input to a terminal P of a comparator 5.

Description

[Detailed description of the invention] ? INDUSTRIAL APPLICATION FIELD The present invention relates to an image forming apparatus for recording image data on a recording medium such as paper, and more particularly to an image forming apparatus for outputting high quality images including halftones.

[Prior Art] When an electrophotographic image forming apparatus is used for a long time, the sensitivity characteristics of the photoreceptor drum deteriorate, resulting in the overall output image becoming pale, and details and so-called solid black areas being faithfully reproduced. It will no longer be done. Furthermore, even if the image forming apparatus is not used for a long period of time, the image quality deteriorates due to fluctuations in the operating environment, charging, photosensitive drum, developing device, image exposure amount, etc.

Conventionally, in order to deal with this type of problem, automatic surface potential control has been used to control the surface potential of the photosensitive drum to be constant, and to form a solid black image on the photosensitive drum so that the density of the solid black image is constant. A developing bias control method for controlling the developing bias of a developing device was proposed in 1999, and some machines have implemented this proposal.

[Problems to be Solved by the Invention] However, in recent years, there has been an increasing demand for image forming apparatuses that can output halftone images.
The control method described above was insufficient to obtain stable, high-quality images.

There is a drawback that if the surface potential or the solid black image part is controlled, the halftone image will fluctuate, and if the halftone image part is controlled, the solid black image will fluctuate. There is.

SUMMARY OF THE INVENTION In view of the above-mentioned drawbacks of the prior art, an object of the present invention is to provide an image forming apparatus that can consistently output images including halftones with high quality in a stable manner.

[Means for Solving the Problems] In order to solve the above-mentioned problems and achieve the objects, the image forming apparatus of the present invention provides an image forming apparatus that forms a visible image that includes at least solid black and halftone image types. The forming device includes: a forming means for forming an electrostatic latent image of a specific pattern on an image carrier; a developing means for developing the electrostatic latent image corresponding to the specific pattern formed by the forming means; a detecting means for detecting the image density of the developed electrostatic latent image; a determining means for determining the density of a specific pattern according to the detection result of the detecting means; and a determining means for determining the density of the specific pattern based on the density determined by the determining means. a correction means for performing density correction according to the type of the
and forming means for forming a visible image based on the density correction of the correction means.

[Operation] According to this configuration, the forming means forms an electrostatic latent image of a specific pattern on the image carrier, the developing means develops the electrostatic latent image corresponding to the specific pattern formed by the forming means, The detecting means detects the image density of the electrostatic latent image developed by the developing means, the determining means determines the density of the specific pattern according to the detection result of the detecting means, and the correcting means determines the density of the specific pattern according to the detection result of the determining means. Based on the density correction according to the type of image, the forming means forms a visible image based on the density correction by the correction means.

[Example] Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

<First example> First, the first embodiment will be explained.

FIG. 1 is a block diagram showing the configuration of a laser printer according to a first embodiment of the present invention, and FIG. 2 is a diagram illustrating a dither matrix used in the first embodiment. In Figure 1, 1
2 is a page memory that stores one page of image data; 3 is a line buffer that holds and outputs the image data in page memory 2 line by line; 4 is a halftone image density A RAM that performs density gradation correction, 5 a comparator that compares image data with a predetermined threshold value and binarizes it, 6 a dither pattern generation circuit that generates a dither pattern, and 22 a signal for forming a solid black pattern. The generated specific pattern generation circuits are shown respectively.

Next, 7 indicates a laser driver, and a laser diode 8
Controls the on/off operation. The laser diode 8 is located after the laser driver 7, and when turned on, irradiates the rotating polygon mirror 9 with laser light. polygon mirror 9
is provided at a position where it can scan the photoreceptor 10 by converting the irradiated laser light into scanning light while rotating in the direction shown by the arrow. The photoreceptor 10 is provided so as to be rotationally driven in the direction shown by the arrow. A charger 11 for uniformly charging the photoconductor 10 is disposed in front of the position where the scanning light from the polygon mirror 9 is received, and a charger 11 is disposed facing the photoconductor 10 in front of the position where the scanning light from the polygon mirror 9 is received. A developing device 12 for developing the latent image and a developing sleeve 13 for applying a developing bias are arranged. 14 is the developing sleeve 1
3 shows the developing bias power supply that applies the developing bias,
15 is a CP that operates with a program (following the flowchart in FIG. 4 described later) that controls the value of the developing bias.
It shows U. Furthermore, the photoconductor 1 is removed from the developing sleeve 13.
A concentration detector 23 is disposed at a position advanced in the rotation direction of 0. 26 indicates an A/D converter, and the concentration detector 23
A transfer charger 16 that transfers a developed pattern to a transfer material (recording paper) 17 supplied from a supply cassette (not shown) is disposed downstream of the density detector 23 that detects the density. Heat fixing rollers 18 and 19 are provided in the feeding direction of the recording paper 17 from the transfer charger 16 for thermally fixing the developed pattern transferred onto the recording paper 17. Also, the photoreceptor 10
A cleaner 20 and a pre-exposure 2I are arranged in this order after the transfer charger 16.

Next, as an operation, image data 1 is read by a league (
(not shown), a magnetic disk device (not shown), or an external device such as a controller, and is stored in the page memory 2. In the first embodiment, this image data 1 will be explained as 4-bit data. The image data arranged as a 4-bit multilevel signal in the page memory 2 is sequentially read out to the line buffer 3 when the printer starts printing, and after being synchronized with the video signal, it is stored in the RAM 4, which is a look-up table. undergo digital-to-digital conversion.

Image data 1 is converted in RAM 4 and then sent to terminal P of comparator 5 in synchronization with a reference clock (not shown). On the other hand, data of a dither matrix as shown in FIG. 2, for example, is transferred to the terminal Q of the converter 5 from the dither pattern generating circuit 6 in synchronization with the reference clock. Here, if the data at the terminal P is P and the data at the terminal Q is Q, the output data R from the comparator 5 is P≧Q
When P<Q, it is sent to the laser driver 7 as ゜゜O゜゛. The laser driver 7 turns the laser diode 8 ON and OFF in response to the output signal R of "1" and "o".

The laser light emitted from the laser diode 8 is converted into scanning light by a rotating polygon mirror 9, and scans the photoreceptor IO. A portion of this scanning light is received by a beam detector (not shown), and a signal used as a video signal or a synchronization signal for the dither pattern generator 6 is generated from the beam detector.

After the photoreceptor 10 is uniformly charged by a charger l1, an electrostatic latent image is formed on the surface of the photoreceptor by receiving the aforementioned scanning light, and then the latent image is developed by a developer 12. This development uses a one-component toner reversal development method, and toner is attached to the area irradiated with laser light. A developing bias output from a developing bias power supply 14 is applied to the developing sleeve 13 .

current l 1 The value of the image bias is controlled by the CPU 15.

Note that the CPU 15 exchanges signals in each circuit, processes detected density data, and controls the operation of each part of the printer. The developed pattern on the photoreceptor 10 is transferred onto the recording paper 17 by the transfer charger 16, and then transferred to the recording paper 17 by the heat fixing roller 1.
The image is fixed on the recording paper 17 in steps 8 and 19. The developer remaining without being transferred to the surface of the photoreceptor 10 is collected by a cleaner 20.

Furthermore, the charge on the photoreceptor 10 is erased by pre-exposure 21, and the same image forming process as described above is repeated again.

Next, image density correction in this printer will be explained in detail.

FIG. 3 is a side sectional view explaining the configuration of the density detector 23 of the first embodiment, FIG. 4 is a flowchart 1 2 explaining the operation of image density correction processing of the first embodiment, and FIG. A diagram illustrating the relationship between step wedge density data and detected density during density correction according to the first embodiment. FIG. 6 is a diagram illustrating the relationship between image density data and output data during density correction according to the first embodiment. It is. In the first embodiment, as shown in FIG. 4, the solid black pattern density is set to 1.3 to
It is shown as converging to 1.4.

The density correction is performed after the power is turned on and before the image data I is obtained. When the power is turned on to the power supply section (not shown) of this printer and preparation for printing is completed, the CPU
15, the specific pattern generation circuit 22 outputs a signal for forming a solid black pattern to the laser driver 7. This solid black pattern is detected by the developer density detector 2, which will be described later.
3 is the smallest detectable area, and preferably about 10 [mml x 10 [mm] is appropriate. Note that the size is not limited to 10 [mm] x 10 [mml] as long as it does not depart from the spirit of the present invention. The solid black pattern signal goes through the same process as the signal R described above to form a solid black pattern image on the photoreceptor 10 (step Sl). The image density of this solid black pattern image is detected by the density detector 23 (step S2
). As shown in FIG. 3, the concentration detector 23 illuminates the photoreceptor 10 with a light source 24 such as an LED, and receives the reflected light with a light receiving element 25 such as a vinyl photodiode.
The reflection density on the photoreceptor 10 is detected. The density of the solid black pattern detected by the density detector 23 is determined by the A/D converter 2.
6 and is taken into the CPU 15. The CPU 15 compares the predetermined reference density data with the imported solid black pattern density data (D), and instructs the bias power supply 14 to lower the bias output when the solid black pattern density is low (D≦1.3). (step S4), and when it is dark (D≧1
．． In step 4), a command is given to raise the level (step S6). By repeating these steps, the density of the solid black pattern is converged to the reference density, and the density correction of the solid black image is completed.

Next, to correct the density of the halftone image, first, the specific pattern generation circuit 22 generates a step wedge pattern signal of 16 gradations expressed by the dither matrix shown in FIG. A 16-gradation step wedge image (density step pattern) is created (step S7). The concentration at each step is detected by the concentration detector 23 (step S8). The step wedge density data detected by the density detector 23 shows, for example, a nonlinear output image density characteristic in which the intermediate density portion is biased toward the dark side, as shown in FIG. The CPU 15 creates density correction data shown in FIG. 6 from the detected density data (step S9) and outputs it to the RAM 4 (step S10). Image data 1 undergoes digital-to-digital conversion using the density correction data shown in FIG. 6, and is input to terminal P of comparator 5. For example, if "5°" is input to the address line of RAM4 as image density data 1, the data converted to "3°" will be sent to RA.
It is output to the data line of M4, and the comparator 5 inputs its ``3''.

The image density correction explained above is shown in Figures 7, 8, and 9.
Organize using diagrams.

FIGS. 7 to 9 are diagrams illustrating characteristics due to density correction in the first embodiment.

The variation in output image density when no density correction is performed on the input image density data (here, 16 types are listed) is as shown in Figure 7, where the maximum value of the solid black image density is a z and b max, and the rate of change in halftone image density is γ. , γゎ. If only the solid black image density is corrected for these fluctuations, as shown in FIG. 8, the solid black image density amax 1
t) Wax can be kept at a constant concentration. Furthermore, if halftone image density correction is added, different rates of change γ. , γ㎎ can be made constant, a constant output image density that is linear with respect to input image density data can be obtained, and a high-quality image can thereby be obtained.

As described above, according to the first embodiment, images including halftones can always be stably reproduced with high quality.

Now, when performing solid black image density correction in the first embodiment described above, a solid black pattern was generated by the specific pattern generation circuit 22, but a step wedge pattern of a 4×4 dither matrix was generated, and the density " 1 6
” may be used as a solid black pattern.

In addition, solid black image density correction has been explained using control that converges by repeating density detection and density correction, but the required amount of development bias change is calculated from the detected density data and a predetermined solid black image density is achieved with one control. It is also possible to do so.

<Second example> Next, a second example will be described.

FIG. 10 is a block diagram showing the configuration of a laser printer according to a second embodiment of the present invention. In the second embodiment, an image density detector 27 detects the image density on the recording paper 17 after transfer.

Further, the solid black density correction is performed not by controlling the developing bias output but by controlling the voltage applied to the charger 18 that forms the electrostatic latent image. 29 is a high-voltage power supply (HV) for charging that outputs a high voltage to the charger l8.
T). The rest is the same as the first embodiment, so the explanation will be omitted.

The second embodiment has the advantage that since the image density on the recording paper 17 is detected, it can also correct image quality deterioration during transfer. However, since recording paper is used to perform image density correction, the number of corrections must be made as small as possible. Furthermore, the second embodiment also has the advantage that controlling the electrostatic latent image makes it possible to secure the electrostatic latent image necessary for development.

Third embodiment> Next, a third example will be described.

FIG. 11 is a block diagram showing the configuration of a laser printer according to a third embodiment of the present invention. In the third embodiment, an image density detector 30 is configured to detect the reflection density of the photoreceptor 10 immediately before the cleaner 20 after transfer. This position of the image density detector 30 has the advantage of being relatively advantageous in space for measuring the image density on the photoreceptor. The solid black image density correction controls the amount of laser light, and the control signal (indicated by rEJ in FIG. 11) is sent to the CPU 15.
and enters the laser driver 7.

The halftone image is formed using a pulse width control method that controls the lighting time of the laser beam. Therefore, the image data 1 that has been digital-to-digital converted by the RAM 4 is input to the terminal P of the analog converter 32 through the D/A converter 31.

33 indicates a triangular wave generation circuit, and its output is 32 and l9 It is input to terminal Q of the analog converter shown.

The image data is compared with a triangular wave by an analog comparator 32, and the laser diode 8 is turned on only during the time when P≧Q, thereby pulse width modulating the laser light. The density correction of the halftone image, that is, the pulse width correction, is performed while the laser beam is on, but the correction method is basically the same as in the first embodiment, and the input image data is stored in the RAM.
Correction is performed using the density correction data loaded in 4.

In the case of the third embodiment, since laser light amount control is used for solid black image density correction to correct electrophotographic recording characteristics, the correction signal can be processed by the laser driver, and the first and second There is no need to control the high voltage as in the embodiment.

Now, in the above-mentioned first to third embodiments, the dither method and the pulse width control method were used as examples of intermediate 20-tone image forming methods, but the error diffusion method and the light amount modulation method may also be used. . In addition, although the image density correction has been described as being performed when the printer is powered on, it may also be performed during rotation before printing, during rotation after printing, or between prints (between sheets).

Further, the image forming apparatus according to the present invention may be any electrophotographic apparatus other than a laser printer, such as an LED printer or an LCD printer.

[Effects of the Invention] As explained above, according to the present invention, it is possible to always obtain a high-quality image with a constant quality in which the maximum density of the image including halftones is high and the halftone gradation is good. effective.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing the configuration of a laser printer according to the first embodiment of the present invention, Fig. 2 is a diagram explaining the dither matrix used in the first embodiment, and Fig. 3 is a density diagram of the first embodiment. 4 is a flowchart explaining the operation of image density correction processing in the first embodiment; FIG. 5 is a diagram showing step wedge density data during density correction according to the first embodiment; FIG. FIG. 6 is a diagram explaining the relationship between image density data and output data during density correction according to the first embodiment, and FIGS. 7 to 9 are diagrams explaining the relationship between the detected density and the detected density. Figure 10 is a block diagram showing the configuration of a laser printer according to a second embodiment of the present invention; Figure 11 is a block diagram showing the configuration of a laser printer according to a third embodiment of the present invention. It is. In the figure, 1... image data, 2... page memory, 3
...Buffer, 4...RAM, 5.32...Comparator, 6...Dither pattern generation circuit, 7...
Laser driver, 8... Laser diode, 10...
・Photoreceptor, 11.28...Charger, 12...Developer, 13...Developing sleeve, 16...Transfer charger, 1
7...Recording paper, 22...Specific pattern generation circuit,
23,27.30...image density detector, 26...A
/D converter, 29...HVT, 31...D/A converter, 33...triangular wave generation circuit, 14...developing bias power supply, 15...cpu.

Claims (3)

[Claims] (1) In an image forming apparatus that forms a visible image including at least solid black and halftone images, a forming means forms an electrostatic latent image of a specific pattern on an image carrier, and the forming means a developing means for developing an electrostatic latent image corresponding to the specific pattern formed; a detecting means for detecting the image density of the electrostatic latent image developed by the developing means; a determining means for determining the density of a pattern; a correcting means for performing density correction according to the type of image based on the density determined by the determining means; and forming a visible image based on the density correction by the correcting means. An image forming apparatus comprising: a forming means for forming an image. (2) The correction means includes a first density correction means for correcting the density of the solid black image, and a second density correction means for correcting the density of the halftone image. The image forming apparatus according to item 1. (3) The first density correction means is a means for making the maximum density value of the input/output characteristics of image density almost constant;
3. The image forming apparatus according to claim 2, wherein the density correction means is a means for making the rate of change of the input/output characteristics of the image density substantially constant.