JPH01197773A - Image forming device - Google Patents

Abstract

PURPOSE:To reproduce stable gradation at any time by comparing image data with the pattern signal of a prescribed period and modulating pulse width according to the result, forming an electrostatic latent image by projecting modulated light on a photosensitive body light and adjusting a bias voltage impressed at the time of development while matching to the characteristic of a pattern signal. CONSTITUTION:The reader element 2 of a reader part A reads an original 1, converts an original image into an analog electrical signal, amplifies the result by an amplifier 3, changes it into a digital image by an A/D converter 4 and inputs it in a RAM 10 for correcting gradation used through a latch 5. The D/A converter 13 of a printer part B converts the image data corrected in the RAM 10 into an analog image signal and inputs the obtained signal in a pulse width modulator circuit 14. Then an output signal is amplified by a laser driver 15 and sent to a laser generating device 16. After that, the laser beam from the device is projected on an optical wind drum 19 through a polygon mirror 17 and a ftheta lens 18. Then a bias voltage is impressed on the drum 19 by a corona electrifier 20. At this point, the voltage is adjusted while matching to the correction in the RAM 10.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus that forms halftone images.

[Prior Art] The dither method and the density pattern method have been well known as methods for forming halftone images.

In these methods, if a small size threshold matrix is used, sufficient gradation cannot be obtained. For this reason, if a large-sized threshold matrix is used, good gradation can be expressed, but there is a problem in that the resolution decreases.

On the other hand, apart from this, a method has been proposed in which the density of one dot is varied in the depth direction, thereby forming one density expressing element in units of one dot or a plurality of dots. In particular, in printers that use electrophotography, when exposing a photoreceptor (photoreceptor drum), by changing the light intensity and lighting time of the light source, more information in the depth direction can be obtained from a single dot signal. A method of giving is used.

Here, examples of output pixels formed by changes in light intensity and lighting time are shown in FIGS. 9(a) to 9(C).

FIG. 9(a) shows the case where the light intensity is changed in four steps for each pixel (dot). In addition, (b) of the same figure shows the case where the lighting (exposure) time of the light beam scanning in the direction of the arrow is changed, and (C) of the same figure is the same as (b), but with 3 dots in the main scanning direction. The figure shows a state in which the density is changed by four types in the sub-scanning direction, with the density expressed as one density expression element.

When expressing halftones in this way, it is best to use binarization like the dither method or density pattern method mentioned above, that is, to express the image as a collection of dots with approximately the same pixel size and similar density. Instead, the output image is represented by a collection of dots each having a different shape and density, that is, each dot has information in the depth direction.

[Problems to be Solved by the Invention] However, there is a problem in that subtle differences or changes in process conditions during image formation, from the electrostatic latent image format to development, appear conspicuously in the output image. found. This is a new problem that has hardly been a problem with the conventional dither method or density pattern method. In particular, in electrophotography, as is well known, a light beam with a finite size of at least several tens of microns is used to form a latent image on a photoreceptor with a realistic amount of blur. ~Number 10
Since development is performed using toner particles having a size of .mu., it is difficult to produce an area gradation effect when providing information in the depth direction to a size of one dot or less. That is, FIG. 9, (b)
As shown in Figure 9 (a), even if processing is performed that looks like binary area gradation at the signal level, the actual print result is as shown in Figure 9 (a).
), resulting in a density gradation image like this.

The reason for this will be explained using FIG. 10. FIG. 10 shows an example of inversion development of an optical image of a laser beam scanning in the main scanning direction, with 2 dots, 1 dot, and 1/2 dot, respectively.
This shows the case where the laser beam is irradiated by a dot. As you can see from this figure, 2 dots ON and 1 dot O
Compared to the time of N, when the 1/2 dot is ON, the width of the optical image in the main scanning direction does not become sharper, but the peak light intensity decreases, and as a result, as shown in Figure 9 (a). It can be seen that a halftone image is printed.

In this way, in an application of electrophotography, in order to express density gradation with one pixel, it is necessary to use a region that is extremely unstable compared to area gradation. Therefore, as described above, it can be seen that subtle differences in the process conditions during image formation tend to appear particularly prominently in the output image.

By the way, the factors that determine image quality in electrophotographic equipment include gradation, such as the absence of background fog, sufficient maximum density, and clear and sharp line images. There are many other important factors, and in order to satisfy these requirements, process conditions during image formation are often adjusted. Specifically 2.
For example, in order to improve the fogging phenomenon, the bias voltage applied to the developing device may be adjusted,
In practice, the output of the light source or the above-mentioned development bias is often adjusted in order to make the maximum density appropriate.

In a system (apparatus) that expresses halftones by giving density information in the depth direction using pulse width modulation to the actual dot size of one dot or less, various aspects during image formation as described above are required. It has been found that changing the process conditions has a significant effect on the gradation (for example, the tone in pale areas is blown out or black areas are crushed).

The present invention has been made in view of these problems, and it is an object of the present invention to provide an image forming apparatus that can form a good output image even when the conditions at the time of image formation are changed.

[Means for Solving the Problem] In order to solve this problem, the present invention includes the following configuration.

That is, a pulse width modulation means compares image data with a pattern signal of a predetermined period and forms a pulse width modulation signal based on the image data; a latent image forming means for forming an electrostatic latent image; a developing means to which a bias is applied for developing the electrostatic latent image; an amount of charge on the photoreceptor; an amount of light irradiated onto the photoreceptor;
The image forming apparatus includes a changing means for changing at least one of the bias values applied to the developing means, and an adjusting means for adjusting characteristics of the pattern signal based on the changed value.

[Function] In the image forming apparatus of the present invention having such a configuration, when the latent image forming means forms a latent image based on the pulse modulation signal formed by the pulse width modulating means and the developing means develops the latent image, the changing means Accordingly, at least one of the amount of charge on the photoreceptor, the amount of light irradiated to the photoreceptor, and the bias value applied to the developing means is changed, and the adjustment means adjusts the characteristics of the pattern signal based on the changed values. It is something that can be adjusted by means.

[Embodiments] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

Outline of the configuration and explanation of the operation of the image forming apparatus (Figures 1 and 2) Figure 1 is a block diagram of the image forming apparatus of Example 1, which is applied to an electrophotographic digital copying machine. It shows. The entire apparatus is roughly divided into a reader section A that reads an original image and a printer section B that forms an output image. In the reader section A, various constituent units are controlled under the control of the CPU 6, and this control processing is executed according to a program stored in the 80M7. In this embodiment, as an example, a digital multilevel image signal is converted to an analog level, and this is compared with a triangular wave to perform pulse width modulation of the image signal and obtain information in the depth direction per dot. We will use the method of giving.

A normal copying operation according to the configuration shown in the figure is as follows.

First, the original 1 is read by the reading element (COD) 2 of the reader section A, and the original image is converted into an analog electrical signal. This electrical signal is amplified by amplifier (AMP) 3, and A/
The D converter 4 converts it into an 8-bit (=256 gradation) digital image signal, and it passes through the latch 5 to the R signal for use in gradation correction.
Input to AMl0. Furthermore, the image data corrected in the RAM 10 is converted into an analog image signal via the D/A converter 13 of the printer section B, and the signal is sent to the pulse width modulation circuit 1.
Enter 4.

Here, the operation of the pulse width modulation circuit 14 will be explained using FIG. 2.

In the figure, the "image signal" is an analog image signal output from the D/A converter 13, and the "pattern signal" is a triangular wave signal generated within the pulse width modulation circuit 14. The "image signal" and the "pattern signal" are synchronized as shown in the figure, and their levels are compared by a comparator (not shown) in the circuit 14, and the resulting pulse signal (pulse width modulation signal) becomes the PWM signal output from the pulse width modulation circuit 14.

Note that this is an example of pulse width modulation operation, and is suitable for relatively high-speed image signal processing. On the other hand, if the image signal is relatively slow, the D/A converter 13 may not be used, for example, by generating a digital pattern signal that is sufficiently faster than the digital image signal and digitally comparing these signals. It is also possible to generate pulse width modulated signals directly from digital image signals.

Now, the PWM output from the pulse width modulation circuit 14 output
The signal is amplified by the laser driver 15 and then input to the laser generation circuit 16, and this amplified signal is used to control ON10 F F of the laser beam. The laser beam emitted from the laser generation circuit 16 passes through the polygon mirror 17.
The light is irradiated onto a photoreceptor (photosensitive drum) 19 through an optical system consisting of an f-〇 lens 18 and the like. The photoreceptor 19 is uniformly charged by the corona charger 2o and then exposed to the aforementioned laser to form an electrostatic latent image on its surface. This electrostatic latent image is visualized by a developing device 21, and then transferred onto a transfer material 27 by a transfer charger 24, and the transfer material 27 is separated from the photoreceptor 19 by a separation charger 25, and then fixed. It is fixed by the container 26. On the other hand, the photoconductor 1 is not transferred.
The toner remaining on the photoreceptor 9 is collected by the cleaner 22, and the electrical history of the photoreceptor 19 is also erased by the pre-exposure lamp 23, and the next printing cycle starts again.

Description of gradation correction (FIG. 3)> Next, the contents of the RAM10 used as the gradation correction means will be explained.

RAM10 stores a gradation correction table for faithfully reproducing the original image data input from C0D2 in printer section B.
-FFH (256 gradations) image signal is gradation corrected and output in 8 bits.

FIG. 3(d) shows the conversion contents of this gradation correction table.

This table is based on (a) the input characteristics of the CCD, (C) the output characteristics of the print, and (b) the desired gradation curve when copying the original. It was obtained by synthesizing it in a similar way. That is, when the gradation correction table shown in FIG. 3(d) is used, the original image is converted as shown in FIG. 3(b) to form an output image.

<Explanation of adjustment processing (Figs. 4 to 8)> By the way, in this embodiment, since image formation is performed using an electrophotographic method using a reversal development method, the portions exposed by the laser 16 are black and are not exposed. The portion will be copied and output as white. The density of the black background area and the fogging of the white text area are determined by the relative relationship between the bright area (exposed area) and the dark area (unexposed area) on the photoreceptor 19 and the developing and image bias value applied to the developing device 21. changes can be made by changing the relationship between the two. The reason for this will be explained using FIG. 4.

In this embodiment, as the developing bias, AC voltage and DC voltage are used.
A superimposed component is used, and this DC level value (
(voltage value) is VOC. Further, the potential is set to be L, which is the potential of a completely dark area (=laser always ON). In Figure 4, V o ”
500 V, V L = 50 V, VDC
is maintained at approximately 400V. Here, in order to reduce fogging, it is sufficient to increase the value of CV D ("Vo - Voc). In other words, the amount of charge can be limited to increase VO, or vDc can be decreased. Also, to increase the concentration, CVL (VDC
-VL) should be increased. That is, the concentration can be increased by reducing the VL by limiting the amount of light or the amount of charge, or by increasing the VOC.

By the way, when such an adjustment is made, the density of the halftone (vH
(equivalent to Ding) was found to be more affected. This is because C□7 (=Voc-, Vote) changes, and the reason why this appears greatly in the image is as follows. That is, in the apparatus used in this example, the ON10 FF time of the laser is finely changed by pulse width modulation, but due to the laser spot diameter, the responsiveness of the laser driver, and the characteristics of the electrophotographic method, The density cannot be sufficiently expressed by the area gradation effect as shown in FIG. 5(a), but rather is expressed by the density gradation effect as shown in FIG. 5(b). Therefore, the steep V-D characteristic, which is the development characteristic of electrophotography, as shown in FIG. 6, appears as it is, and a change in halftone density appears conspicuously.

In fact, as an example, when we re-measured the output characteristics of the printer by changing the developing bias VDC, we found that Fig. 7 (C)
As shown in Figure 2, it was found that the dotted line A changes in the fog removal direction and as shown by dotted line B in the direction of increasing density. Therefore,
In this embodiment, by adjusting the relationship between the image signal and the comparison (pattern signal) wave described in FIG. 2, this deviation in printer output characteristics is corrected and gradation change is prevented.

For example, in the initial state, the relationship as shown in Figure 8(a),
Assume that there is an image signal and a triangular wave that is a comparison wave. On the other hand, if the relationship is as shown in FIG. 3(b), the image becomes stronger overall, and conversely, if the relationship is established as in FIG. 2(c), the image becomes darker overall. Furthermore, if you do something like (d), the white side will be blurred and the black image will be blurred, and if you do something like (e), the white side will be covered and the black image will become pale. When changing the VOC of the developing bias, the relationship between the image signal and the triangular wave is changed accordingly.
Changes in halftones can be corrected by making changes as shown in FIG.

Specifically, using the configuration shown in Figure 1, the volume 12
When the vDc output of the developing bias power supply 28 is changed, the offset amount of the triangular wave (and the amplitude of the triangular wave, if necessary) of the pulse width modulation circuit 14 is changed by an amount corresponding to the change. For example,
In order to reduce "fogging", the developer 21 is set at a volume of 12.
When the bias vDc of is set to lower, the entire output image (halftone image) changes to become lighter.
In order to correct this, if the offset of the triangular wave in the pulse width modulation circuit 14 is lowered as shown in FIG. 8(C), the halftone pixel itself can be prevented from changing. Also, when trying to increase the concentration, the bias VO
When C is set in the upward width direction, by increasing the offset of the triangular wave as shown in FIG. 8(b), it is possible to always construct a good output image.

Above, we have explained the case where when the developing bias is changed, the offset of the triangular wave of the pulse width modulation circuit 14 is changed accordingly, but as another example, there is a case where the emission intensity or charge amount of the image exposure light source is changed. For example, the amplitude of the triangular wave (
You can change gain) and offset at the same time,
The present invention is extremely effective for stabilizing image quality when changing the level or changing various other process elements that affect image quality.

In particular, in electrophotography, process conditions are often changed in the market due to background fog, adjustment of maximum density, and other issues related to image quality, but this embodiment prevents variations in gradation that occur at this time. As a result, not only image quality can be improved, but also operability and serviceability can be significantly improved.

In the above embodiment, it was explained that the offset of the bias V 00% triangular wave is changed by hardware according to the amount of change in the volume 12, but the CPU 6 detects this amount of change in the volume 12, and These values may be changed under control.

Furthermore, in the embodiment, an example of a reversal development method was explained, but it goes without saying that the principle of this embodiment can also be applied to a normal development method in which a dark area is developed after performing background exposure to expose the background area. stomach.

Furthermore, in addition to the laser which is the principle of this embodiment, the same implementation is possible with various electrophotographic printers, such as those using an LED array or other light source with a liquid crystal shutter added as a light source.

Furthermore, even in printers that use a recording head to form an electrostatic latent image on the surface of an insulator, which is similar to the electrophotographic method and the image forming method, there is a method that can give density information in the depth direction to each dot. If so, it can be used in exactly the same way.

[Effects of the Invention] As explained above, according to the present invention, it is possible to always perform stable gradation reproduction.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram of the configuration of this embodiment. Figure 2 is a diagram for explaining the principle of pulse width modulation. Figure 3 is the characteristics of league section A and printer section in this embodiment, and gradation correction based on them. Figure 4 is a diagram showing the relationship between the bias level of the source imager and the output image in the example, Figure 5 is a diagram showing the relationship between the laser spot and the output pixel, and Figure 6 is the diagram showing the contents of the table. Figure 7 is a diagram showing the output characteristics of a photograph. Figure 7 is a diagram to explain how the output image characteristics change when the bias level of the source imager is changed. Figures 8 (a) to (e) are A diagram for explaining the state in which the offset and amplitude of the triangular wave in the pulse width modulation circuit in the embodiment are changed. FIGS. 9(a) to (C) are diagrams showing changes in output pixels due to changes in exposure intensity and lighting time. , FIG. 10 is a diagram showing the density change of the output pixel based on the irradiation time of the negative light in the main scanning direction. In the figure, l...Original, 2...-CCD, 3...Amplifier, 4...A/D converter, 5...Latch, 6...-CP
U. 7...ROM, 8...Buffer, 9...Bidirectional buffer, 10...RAM (gradation correction table), 1
1...I10 boat, 12...Volume, 13...
・D/A converter, 14...! \ Lux width modulation circuit, 15
...Laser driver, 16-...Semiconductor laser, 17
... Polygon mirror, 18 ... f-theta lens, 19
...Photoreceptor, 20...Corona charger, 21...Developer, 22...Cleaner, 23...Pre-exposure lamp,
24... Transfer charger, 25... Separation charger, 26.
. . . Fixing roller, 27 . . . Transfer paper, 28 . . . Development bias power supply. Figure 4 Figure 5 Figure 1 Leather Figure 6 □・□-V9 + □-VW (d) Figure 8 Mechi level, white level) ・□, -v8 (C) (e) (c) 2 dots ON Figure 10 Procedure amendment (voluntary) April 1, 1986

Claims (1)

[Scope of Claims] Pulse width modulation means for comparing image data with a pattern signal of a predetermined period to form a pulse width modulation signal based on the image data, and directing light to a photoreceptor based on the pulse modulation signal. a latent image forming means that irradiates to form an electrostatic latent image; a developing means that applies a bias to develop the electrostatic latent image; the amount of charge on the photoreceptor; the amount of light irradiated to the photoreceptor; An image forming apparatus comprising: a changing means for changing at least one bias value applied to the developing means; and an adjusting means for adjusting characteristics of the pattern signal based on the changed value.