JPH07250244A - Image forming device - Google Patents

Abstract

PURPOSE:To improve the reproduciveness of dots or fine parallel lines at a low density part, to improve the environmental stability of gradation/color reproduction and to prevent the reproduction of low-density fine lines from being degraded. CONSTITUTION:Concerning the image forcing device provided with a pulse width modulating means for conducting the pulse width modulation of an image density signal and an image forming means for forming an image according to the outputted pulse width modulated signal, this device is provided with over two image density signal converting means 45 and 46, which are provided with different characteristics, to be cyclically operated corresponding to multi- level image density signals arranged in the main scanning direction, one image density converting means is provided with characteristics for turning the output of the image density signal to the low density part to '0' or an image density signal within a certain range not to actualize any image and further, the device is provided with correcting means 43 and 44 for correcting respective image density signals into a value decided by prescribed arithmetic with one adjacent image density signal at least.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrophotographic image forming apparatus for scanning a light beam to form a latent image on a photosensitive medium and developing the latent image with toner.

[0002]

2. Description of the Related Art In a printer or a copying machine, a digital electrophotographic system is widely adopted as a system capable of providing high speed and high image quality. In this method, pulse width modulation exposure is often performed using an analog screen generator or the like in order to perform optical scanning of a photosensitive medium using a light beam and reproduce the gradation of an image ( See, for example, Japanese Patent Laid-Open No. 1-280965). In these, from low density part to high density part,
An image is formed with a light beam spot diameter and a constant number of lines.
As a result, the exposure profile in the low-density area has a reduced contrast and becomes analog, and since the exposure amount itself is low, the reproducibility of dots and lines deteriorates, and the stability of the gradation and color reproduction environment is stable. There was a problem that the sex became worse. The above problem can be dealt with by making the light beam spot diameter sufficiently small in order to improve the contrast of the exposure profile in the low density portion. However, an image forming optical system that collects a light beam to form a light beam spot on a photosensitive medium becomes very precise and expensive, and is not suitable for practical use.

In order to solve the above-mentioned problems, a method of stabilizing each element such as stabilization of the light beam light quantity, stabilization of toner concentration in the developing device, temperature / humidity and toner concentration in the developing device are measured to develop bias. And a transfer current value are controlled to increase the stability of gradation and color reproduction with respect to the environment, and a method called process control has been proposed (for example, Japanese Patent Laid-Open No. 4-3).
7882, Japanese Patent Application Laid-Open No. 4-376776).
However, these methods require a highly accurate sensor and control mechanism, and have the drawback of being complicated and expensive.

Further, a method has been proposed in which the diameter of the light beam spot and the light intensity of the light beam are varied to suppress the deterioration of the contrast of the exposure profile and increase the reproducibility of dots and lines (for example, Japanese Patent Laid-Open No. 4-13163). Japanese Patent Laid-Open No. Hei 4
-97374 and Japanese Patent Application Laid-Open No. 4-94261). However, these methods also require a control mechanism for varying the light beam spot diameter and the light emission intensity, which is disadvantageous in that it is complicated and expensive.

In view of the above problems, the inventors of the present invention have a pulse width modulation means for pulse width modulating an image density signal, and an image forming means for forming an image in accordance with the pulse width modulation signal output from the pulse width modulation means. In the image forming apparatus having the above, two or more converting means for converting the image density signal, which have different characteristics, and which operate alternately, are provided, and at least one converting means outputs the output of the image density signal to the low density portion to 0.
An image forming apparatus characterized by having the following characteristics has been proposed (Japanese Patent Application No. 5-248474), which improves the reproducibility of dots and lines in the low-density portion and stabilizes the gradation and color reproduction. It was possible to improve the sex. However, when an attempt is made to reproduce a low-density line image, it is said that the line width becomes thin or a line is missing due to the image signal of the line image passing through the conversion means that sets the output for the low-density image to 0. It turned out to be a problem.

[0006]

The present invention eliminates the problems and drawbacks of the prior art described above, improves the reproducibility of dots and lines in low density areas, and stabilizes the gradation and color reproduction in the environment. In addition to improving the property, it is intended to prevent deterioration of reproduction of low-density thin lines.

[0007]

In order to achieve the above object, an image forming apparatus of the present invention (claim 1) is a pulse width modulating means for pulse width modulating an image density signal, and its pulse width modulating means. And an image forming device for forming an image in accordance with the pulse width modulation signal output by the image forming device, comprising two or more image density signal converting devices having different characteristics for converting a multi-valued image density signal. The at least one image density converting means has a characteristic of converting an image density signal corresponding to a low density portion of the input image density signal into an image density signal of 0 or a range which is not visualized, and One or more image density signal converting means outputs each image density signal to the image density signal in the image forming apparatus that periodically operates with respect to the multivalued image density signals arranged in the main scanning direction. Before entering the conversion means and is characterized in that a least one of the image density signal and correcting means for correcting the determined value by the calculation of the adjacent thereto (43, 44 in FIG. 1).

According to the present invention (claim 2), in the image forming apparatus of the invention, the image forming means includes a light beam scanning means for relatively scanning a light beam with respect to a photosensitive medium, and the light beam scanning means. And an image forming optical system for forming a light beam spot of a predetermined size on the photosensitive medium by condensing the light beam, the distance between adjacent pixels in the main scanning direction when forming the low density portion. If d _p (mm) and the spot diameter (1 / e ² ) of the light beam in the main scanning direction on the photosensitive medium is d _B (mm), then d _B ≤ (1/3) d _p It is characterized by.

[0009]

The light beam scanning means scans the light beam relative to the photosensitive medium. The imaging optical system forms a light beam spot of a predetermined size on the photosensitive medium. Then, the pulse width modulation means determines the on / off time of the light beam according to the image density signal, and thereby a latent image corresponding to the image density signal is formed on the photosensitive medium. This latent image is later visualized with powder toner or liquid toner to form an image. 2A, 2B, and 2C are exposure energies on the photosensitive medium when the photosensitive medium is exposed using the light beam scanning means, the imaging optical system, and the pulse width modulation means. It shows a profile. When the ratio of the distance d _p (mm) between adjacent pixels and the light beam spot diameter d _B is D, the results are obtained when the values of D are 1/1, 1/2, and 1/3, respectively. . The results are obtained when the light beam spot diameter d _B (mm) is constant. Further, in electrophotography, a bias potential is applied during development in order to prevent toner from adhering to the base. FIG. 2 also shows a boundary line corresponding to the bias potential as the reversal development for developing the exposed portion. As is obvious in FIG. 2A, the pulse width (%)
As is decreased, the contrast of the exposure energy profile decreases and becomes analog. The amount exceeding the boundary line corresponding to the bias potential is reduced, and dots and lines are not reproduced. As can be seen from FIG. 2, the value of D is 1 /
The decrease in contrast is suppressed as it is reduced to 1, 1/2, and 1/3. From this, the light beam spot diameter d _B
If the number of lines is small and the value of D is small when the constant is set, the reproduction of dots and lines in the low density area becomes good, and the stability of gradation and color reproduction with respect to the environment increases. I understand. Further, as is conventionally known, it is important to reproduce the low density portion in designing the image quality, and it is necessary to reproduce at least a pulse width of 10%. As can be seen from FIG. 2, when the value of D is 1/1, dots and lines are not reproduced at a pulse width of 10%.
When the value of D becomes 1/3, dots and lines start to be stably reproduced at a pulse width of 10%. From this, set the value of D to 1/3 or less, i.e., by a _{d B ≦ (1/3) d p} , stable image reproduction in the low density portion is performed.

Based on these considerations, the present inventors have made the invention of Japanese Patent Application No. 5-248474 as described above,
The image density signal is converted so that the number of lines is substantially reduced in the low density portion. That is, the two or more image density signal converting means have different characteristics as shown in FIGS. 5 (a) and 5 (b), for example. One conversion means is shown in FIG.
Since it has a characteristic that the output of the image density signal to the low density portion is 0 as in (b), there is no output when the low density signal comes. These conversion means operate alternately in a time-division manner, and as shown in FIG. 8A, the pulse width modulated output has a low number of lines for the low-density portion and a medium-high density portion. The number of lines has been increased. However, as shown in FIG. 8B, when trying to reproduce the low density line image,
As the image signal of the line image passes through the conversion means in which the output for the low-density image is set to 0, the line width becomes thin,
The problem of missing lines arises.

Therefore, in the present invention, each image density signal is corrected to a value determined by calculation with at least one image density signal adjacent thereto before being input to the image density signal converting means. Solve a problem. The correction means for that purpose, for example, takes in each image density signal arrayed in the main scanning direction together with at least one adjacent image, and performs arithmetic operations such as arithmetic mean of the taken image density signals as the arithmetic operation. Since each image density signal corrected as a result of the calculation has a value influenced by the adjacent image density, even an image density signal of a low density thin line is input across the plurality of image density signal converting means. It becomes possible. Therefore, according to the present invention, it is possible to prevent the conversion result from becoming 0 by being converted only by the image density signal converting means that makes the output for the low density image 0.

In the above description, for simplification of the description, the characteristic of the image density signal converting means is the characteristic in which the output to the low density portion of one converting means is zero. Generally, in an electrophotographic apparatus involving scanning of a light beam, a semiconductor laser does not respond to a minute input signal, and a developing bias potential is given to suppress toner adhesion to a base. Even if the output of the conversion unit is not 0, there is a value in a range that is not visualized as an output image. That is, the characteristic of the image density signal converting means is not an absolute condition that it has a characteristic that the output to the low density portion of one converting means is 0,
For example, as shown in FIGS. 9 (a) and 9 (b), the output to the low-density portion of one of the converting means may be converted into a value in a range not visualized. In the present invention, the intention is to reduce the number of lines by the characteristic that the output to the low-density portion of one of the conversion means is set to a value in the range that is not visualized.

As described above, the present invention improves the reproducibility of dots and lines in the low density area,
In addition to improving the stability of color reproduction in the environment,
It is possible to prevent deterioration of reproduction of low-density thin lines.

[0014]

【Example】

(First Embodiment) FIG. 3 shows a first embodiment of the image forming apparatus of the present invention.
It is a figure which shows the structure of the Example of this. A charger 2, a rotary developing device 3, a transfer drum 4, a cleaner 5 and the like are arranged around the photoconductor 1 rotating in the direction of the arrow. Photoreceptor 1
In the dark area, the charger 2 uniformly charges the toner. The light beam scanning device 20 scans the light beam on the photoconductor 1. In addition, the light beam is generated by the light beam pulse width modulation device 3 according to the concentration signal supplied from the current reading unit 10 or the like.
Turned on and off by 0. As a result, the photoconductor 1 is exposed and an electrostatic latent image is formed. The spot diameter (1 / e ² ) of the light beam in the main scanning direction on the photoconductor 1 is 42 μm.
set to m.

The rotary developing device 3 is composed of four developing devices having yellow, cyan, magenta, and black toners, respectively. Each developing device adopts a reversal developing method using two-component magnetic brush development. Average toner particle size is 7μ
m was used. The rotary developing device 3 is appropriately rotated to develop the electrostatic latent image with toner of a desired color. At this time, a bias voltage is applied to the developing roll to suppress toner adhesion to the white background portion. The transfer drum 4 is rotated by mounting a sheet on the outer circumference. The developed toner image on the photoconductor is transferred to the transfer device 4
It is transferred to the sheet 4P by b. Yellow, cyan,
An electrostatic latent image is formed, developed, and transferred for each of magenta and black. The toner on the paper obtained by this operation is fixed by the fixing device 9 to form a multicolor image.

FIG. 4 is a detailed view of the light beam scanning device 20, which is composed of a semiconductor laser 21, a collimator lens 22, a polygon mirror 23, an fθ lens 24, and the like, and further SO for detecting the light scanning start timing.
A scanning start signal generation sensor 26 that generates an S signal is provided.

As shown in FIG. 1, the pulse width modulation device 30 for turning on / off the light beam includes a triangular wave oscillator 41, a comparison circuit 42, an image density signal acquisition device 43, a calculation device 44, and a first look-up table. (LUT) 45,
It is composed of a second look-up table (LUT) 46, a signal selection device 47, and a D / A converter 48. 400
The digital image density signals arranged in the main scanning direction at a resolution of dpi are captured and held by the signal capturing device 43 in units of two data, and sent to the arithmetic device 44. The arithmetic unit 44 generates an image density signal representative of the two captured data. In the present embodiment, the two pieces of captured data are averaged and generated. The generated image density signal representative of the captured two data generates two converted image density signals by the first look-up table (LUT) 45 and the second look-up table (LUT) 46. . First look-up table (LUT) 4
5, and a second look-up table (LUT) 46
5A and 5B, the output of the second lookup table (LUT) 46 for low density image density signals is set to zero.
The first look-up table (LUT) 45 is shown in FIG.
As shown in (a), the characteristic is such that an output with an enlarged image density is obtained in the area where the input digital image density is less than 20%, and the image density is slightly in the area where the image density signal is 20% or more and less than 50%. It has a characteristic of obtaining an output of an enlarged value, and has a characteristic of obtaining a digital output value equal to the input digital value without the enlargement of the amplitude in the region of 50% or more. Second look-up table (LUT) 46
Has a characteristic that the output value becomes zero in the area where the input digital data value is less than 20% as shown in FIG. 5B, and the image density is large in the area where the image density signal is 20% or more and less than 50%. It is a property to get an expanded value output,
In the region of 50% or more, the amplitude is not expanded and the output value equal to the input digital value is obtained. The signal selection device 47 includes a first look-up table (LUT) 4
5, and a second look-up table (LUT) 46
The two image density signals converted by
/ A converter 48.

The D / A converter 48 converts the digital image density signal transmitted from the signal selection device 47 into an analog image density signal. The comparison circuit 42 compares the magnitude of the triangular waveform pattern signal with the analog image density signal to generate a pulse width modulation signal. In this embodiment, one cycle of the triangular pattern signal is set to be the same as one data cycle of the image density signal. This waveform generation process is shown in FIG.
Shown in (a) and (b). As can be seen from FIG. 6B, the image density signal of the fine line missing in the invention of the above-mentioned Japanese Patent Application No. 5-248474 is generated without being missing in this embodiment. As described above, it was possible to prevent deterioration of the reproducibility of low density thin lines. Further, as can be seen from FIG. 6A, it is understood that the line number is reduced to 200 dpi for a uniform low-density image, as in the invention of Japanese Patent Application No. 5-248474. The laser beam diameter is set to 42 μm, and d _p = 2
Since it is 5.4 / N, it satisfies d _B ≤ (1/3) d _p . As can be seen from FIG. 2, the latent image contrast was increased, and the stability of the gradation and color reproduction in the low density portion against the environment was also secured.

(Second Embodiment) In the first embodiment, the number of a plurality of image density signals captured by the signal capturing device 43 and the image density conversion for converting one image density signal calculated by the calculating means. Although the number of means is the same, it is not necessary to set the same number in the present invention. FIG. 7 shows the configuration of the second embodiment in which the number of a plurality of image density signals to be fetched is set to 4 and the number of image density converting means is set to 2, but not the same. Differences from the first embodiment will be mainly described.

The digital image density signal which is sequentially read by scanning in the main scanning direction at a resolution of 400 dpi is fetched and held by the signal fetching device 53 in units of four data and sent to the computing device 54. The arithmetic unit 54 generates an image density signal representative of the fetched 4 data.
In the present embodiment, the four pieces of captured data are averaged and generated. The generated image density signal representative of the fetched four data is the first look-up table (LUT) 55,
And a second look-up table (LUT) 56 to generate two transformed image density signals. The conversion characteristics of the first look-up table (LUT) 55 and the second look-up table (LUT) 56 are shown in FIG.
It has the characteristics shown in (a) and (b), and the output for the low-density image density signal of the second look-up table (LUT) 56 is set to zero.

The signal selection device 57 converts the two image density signals converted by the first look-up table (LUT) 55 and the second look-up table (LUT) 56 into the cycle of the triangular waveform pattern signal. It is selected and taken into consideration for sweeping and transmitted to the D / A converter 58. 4 with the same cycle as the cycle of the image density signal by the triangular waveform selection signal
When the triangular wave pattern signal for 00 dpi is selected, the signal selection device 57 causes the LUT (1) → (2) →
Alternately select (1) → (2) to sweep out 4 data. Since the output of the low density image density signal of LUT (2) is 0, the low density image density signal captured at 400 dpi is converted to 200 dpi. When the 200 dpi triangular wave pattern signal having a cycle twice the cycle of the image density signal is selected, the signal selection device 5
7, the LUT (1) → (1) → (2) → (2) is selected twice, and four data are swept out. LUT
Since the output for the low-density image density signal of (2) is 0, the low-density image density signal captured at 400 dpi is converted to 100 dpi.

The D / A converter 58 includes a signal sweeping device 57.
The transmitted digital image density signal is converted into an analog image density signal. The comparator circuit 52 compares the magnitude of the triangular waveform pattern signal with the magnitude of the analog image density signal to create a pulse width modulation signal. As in the above-described embodiment, it is possible to prevent the image density signal of the low density thin line from being converted by the conversion unit that does not set the output for the low density image to 0, and the conversion result to become 0.

(Third Embodiment) In the first and second embodiments, the arithmetic unit 44 averages the captured data to generate an image density signal representative of the captured data. In the present embodiment, in a copied image, unclear thin lines such as dust and stains are not reproduced, while clear thin lines tend to be reproduced in a darker color. Fine lines are not reproduced, and clear fine lines are reproduced more densely. FIG. 10 shows the flow of calculation.

The image density signals D1 and D2 fetched by the arithmetic unit 44 are such that when D1 and D2 are not 0, that is,
If the line is not a thin line, an average calculation is performed and a representative image density signal is output. On the other hand, when D1 or D2 is 0, the average value D of the captured image density signals D1 and D2
The following calculation is performed using the ave, the threshold value Dcut for determining dust and stains, and the proportional gain K to generate a representative image density signal Dout.

Dave = (D1 + D2) / 2 Dout = 0 When Dave ≦ Dcut Dout = K × (Dave-Dcut) Dave
When>

Dcut is a density on the paper of about 0.1.
It was made possible to vary in the range of up to 0.3D. Further, the proportional gain K is made variable in the range of 1.5 to 3. By setting Dcut to 0.2D and proportional gain K to 3, 0.15D dust and stains and other unclear thin lines disappear, while 0.6D thin lines are reproduced as 1.2D clear thin lines. A favorable copy image was obtained.

In the above description, 2 data D1, D
Although the case of capturing 2 is shown, 4 data D1, D2,
When D3 and D4 are taken in, Dave = (D1 + D2 + D3 + D4) / 4.

In each of the embodiments described above, the characteristic of the image density signal converting means is that the output to the low density portion of one converting means is 0, but the characteristic of the image density signal converting portion is that of one converting portion. The output to the low-density portion of may be set to a value in the range where the image is not visualized. The characteristics of FIGS. 9A and 9B show examples of the characteristics of the image density signal converting means. In the apparatus shown in FIG. 3, since the semiconductor laser does not respond to a minute input signal and the developing bias potential is applied to suppress the toner adhesion to the background, the pulse width is 5% (8%). Visualization is performed for the laser lighting of 13) or more by bit digital data. In the characteristic of FIG. 9B, the output for the low density portion is set to be less than this value, and the output for the low density portion is not visualized. On the other hand, in the characteristic of FIG. 9A, there is a region where the output is set to 13 or more even in the low density portion, and the output is visualized in that region. In this way, the input image is thinned out, and it is possible to form an image in which the number of lines is lower than that of the simple halftone image of low density as in the above-described embodiments.

[0029]

According to the present invention, a plurality (N) of image density signals are taken in and one representative image density signal is obtained by calculation to be a corrected image density signal, which is converted into at least one image density conversion signal. The means has a characteristic of converting an image density signal corresponding to the low density portion of the input image density signal into an image density signal of 0 or a range not visualized, at least one of which is for the low density portion of the image density signal. Since a plurality of (N) image density signals are converted by a plurality of image density signal converting means with the output set to 0 and pulse width modulation is performed to form an image, complicated and expensive process control and variable emission intensity are performed. The stability of the gradation and color reproduction in the low density area against the environment is improved without the need for a device or a precise and expensive beam imaging optical system, and the reproducibility of low density fine lines is poor. It is possible to prevent.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of a pulse width modulation device in an image forming apparatus according to a first embodiment.

FIG. 2 is an explanatory view of the operation of the present invention.

FIG. 3 is a diagram showing a schematic configuration of an embodiment of an image forming apparatus of the present invention.

FIG. 4 is a diagram showing an example of a configuration of a light beam scanning device.

5A and 5B are diagrams showing an example of data conversion characteristics of the LUT of the present invention.

FIG. 6 is a diagram showing a waveform generation process of the pulse width modulator of the present embodiment.

FIG. 7 is a diagram showing a configuration of a pulse width modulation device in the image forming apparatus according to the second embodiment.

FIG. 8 is a diagram showing a waveform generation process of a pulse width modulator according to the invention of Japanese Patent Application No. 5-248475.

9A and 9B are diagrams showing another example of the data conversion characteristic of the LUT of the present invention.

FIG. 10 is a diagram showing a flow of calculation in the third embodiment.

[Explanation of symbols]

41 ... Triangle wave oscillator, 42 ... Comparator, 43 ... Signal acquisition device, 44 ... Arithmetic device, 45 ... First LUT, 46 ... Second LUT, 47 ... Signal selection device, 48 ... D / A converter.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location G06F 15/68 310 J

Claims (2)

[Claims] 1. An image forming apparatus comprising pulse width modulation means for pulse width modulating an image density signal, and image forming means for forming an image according to the pulse width modulation signal output from the pulse width modulation means, comprising: Two or more image density signal converting means having different characteristics for converting the binarized image density signal are provided, and at least one image density converting means is an image density signal corresponding to a low density portion of the input image density signal. Is converted into an image density signal in the range of 0 or not visualized, and the two or more image density signal conversion means are provided for multi-valued image density signals arranged in the main scanning direction. In the image forming apparatus that operates periodically, each image density signal is calculated with at least one image density signal adjacent thereto before being input to the image density signal converting means. An image forming apparatus characterized in that a correction means for correcting the value determined I. 2. The image forming apparatus according to claim 1, wherein the image forming unit is a light beam scanning unit that relatively scans the light beam with respect to a photosensitive medium, and the light beam is condensed to perform the photosensitive process. An image forming optical system for forming a light beam spot of a predetermined size on a medium, and a distance between adjacent pixels in the main scanning direction when forming a low density portion is d _p (mm), when the main scanning direction of the spot diameter on the photosensitive medium of the light beam (1 / e ²⁾ was _{d B (mm), d B} ≦ (1/3) image forming apparatus characterized by a d _p .