JPH02219667A - Digital type image recorder - Google Patents

Abstract

PURPOSE:To obtain a copy image having an appropriate gamma by a method wherein a latent image forming means by which an irradiation minimum electric potential is varied within a pulse width modulation control range and a means which switches a charged electric potential on an image forming material according to a mode are established. CONSTITUTION:When an original is irradiated with a light source 85, reflected light is conducted to an optical information conversion unit 11 via a reflecting mirror 87, and V mirrors 88, 89. It is converted to an image signal, and various image processings are applied thereof with an image processing part 1B. Thereafter, it is outputted to an image exposure means 5 of a printer part 1C. Then, it is deflection scanned with a deflecting system 55A, and is electrically discharged by a PCL 20. Thereafter, an electrostatic latent image corresponding to the image signal is formed by scanning on an image forming material 10 joined by uniform electric charge, and is developed with a developing machine to form a monochromatic toner image. A CPU changes a unit cycle and grid voltage of pulse width modulation, charged electric potential, and bias voltage of the image processing part 1B, according to a mode. Consequently, gamma characteristics of the image forming material can be made suitable for each of a photographic mode and a line drawing mode.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an electrophotographic digital image recording apparatus, particularly a digital monochrome reversal type digital image recording apparatus.

[Background of the invention]

As is well known, a digital image recording device, for example, an electrophotographic copying machine for monochrome images, uses an image forming body having a photoreceptor layer on its surface and a pre-charging exposure lamp (hereinafter referred to as PCL) to eliminate static electricity. The image forming apparatus includes a latent image forming means for forming an electrostatic latent image of the document on the image forming body uniformly charged by the charger. Digital devices using semiconductor lasers are often used as writing means for forming latent images in printers.

An electrostatic latent image is formed by irradiating the image forming body with a laser beam modulated by image information obtained by reading an image of the original, and this electrostatic latent image is developed by a developing means. The monochrome toner image on the image forming body, which has been developed and becomes a visible image, is transferred onto recording paper, and this is fixed by applying pressure and/or heat by a fixing means, thereby completing a copy of the original document.

Recently, in addition to monochrome binary images, there are also copy images.
In order to express halftones, the image information is multivalued, and the laser beam is modulated with the image data to form an electrostatic latent image with multiple levels of potential, making it possible to obtain copies with gradation. It looks like this.

In the digital image recording apparatus using the laser beam described above, a digital image recording method using reversal development in which toner is attached to the portion of the image forming body irradiated with the laser beam shortens the usage time of the laser light source. It is commonly used because it can increase the copying speed (especially when the original is a document).

In addition, in recent years, organic photoconductors (hereinafter referred to as oPCs) have been increasingly used as photoreceptors for image forming bodies, as they are less toxic than Se, etc., and are easier to form photoreceptor layers to reduce manufacturing costs. It became so.

However, in the case of OPC, unevenness in layer thickness is likely to occur during the manufacturing process, and therefore, uniform charging cannot be achieved with a charger using a corotron discharger. So O
In the case of PC, a scorotron type charger is used which can control the charging potential of the photoreceptor by its grid.

There are two types of oPCs, one with positive charge polarity and the other with edible type.The OPC generally used at present is the edible type OPC, and in the case of reversal development, negatively charged toner is used. Therefore, the toner image formed on the image forming body is transferred onto recording paper by a transfer device to which a high positive voltage is applied, and this recording paper is separated from the image forming body by a separator to which a high negative voltage is applied. Ru.

The laser beam used in the writing means of a general digital image recording device has a spot diameter of about one pixel (dot) of the image, so the MTF (Modulation) of the latent image formed on the image forming body is Trans
-fer Function) is limited. So,
When the spatial frequency of an image signal becomes high, sufficient gradation cannot be expressed by pulse width modulation in which the unit period is one pixel width.

Therefore, in order to express gradation, it is necessary to perform modulation based on a period of approximately two pixel width.

However, if the unit of modulation cycle is a cycle with a width of two pixels or more, there is no problem with things that require gradation such as photographs, but in the case of text (line drawings), for example, thin lines Since the recording quality deteriorates due to skipping, the modulation period is set to about one pixel width.

[Problem to be solved by the invention]

When multi-leveling is performed using the pulse width modulation method, the pulse width is set to be shorter than the light emission time of one pixel width even if the unit period is two to three pixel widths. When modulating with this short pulse width, the peak value of the optical energy distribution for the short pulse decreases, resulting in a behavior similar to optical intensity modulation. Because of these properties, the potential distribution of a latent image obtained by exposure with a pulse width modulated laser beam is as shown in FIG.

FIG. 8 is a diagram showing an example of the light energy distribution according to various pulse widths as a latent image potential distribution when the laser spot diameter on the image forming body is made approximately equal to the pixel width. The vertical axis in the figure represents the surface potential (V) of the image forming body, and the horizontal axis represents the length in units of pixel width.

Due to the above properties, an example of the relationship between the copy density of an image obtained by exposure with a pulse width modulated laser beam and the pulse width of the laser beam is as shown in FIG. The vertical axis of the figure shows the copy density, and the horizontal axis shows the duty ratio (%) of the pulse width. As shown in the figure, as the duty ratio decreases, the gamma gradually increases, resulting in hard contrast and poor gradation, making it unsuitable for originals that require gradation, and gamma is low for originals with line drawings. It wasn't appropriate.

Therefore, in conventional devices with a constant charging potential, in order to handle both originals with rich gradation and originals with line drawings, it is necessary to correct the image signal using a complex correction circuit. There wasn't.

SUMMARY OF THE INVENTION An object of the present invention is to solve these problems and provide a digital image recording apparatus that can obtain a copy image having an appropriate gamma with a simple configuration.

[Means to solve the problem]

The above object is an electrophotographic recording apparatus that forms a latent image on a charged image forming body by a writing means using pulse width modulation, and a latent image forming means in which the lowest irradiation potential changes within a pulse width modulation control range. This is achieved by a digital image recording apparatus characterized by having: and means for switching the charging potential on the image forming body depending on the mode.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a diagram showing a general dark configuration of a digital image recording apparatus which is an embodiment of the present invention. This digital image recording apparatus is a digital copying machine consisting of a scanner section IA, an image processing section IB, and a printer section IC.

In FIG. 1, in the scanner part IA, when a copy button provided on a digital copying machine is turned on, a document on a document table 81 is optically scanned by an optical system.

This optical system is composed of a carriage 84 provided with a light source 85 and a reflecting mirror 86, and mirrors 88 and 89. A halogen lamp is used as the light source 85. It is also possible to use a commercially available green fluorescent lamp instead of the halogen lamp, and in this case, it is lit with a high frequency power source of about 40 KHz to prevent flickering. Further, in order to maintain a constant temperature of the tube wall or promote warming up, it is necessary to keep the tube wall warm with a heater using a POSISTOR.

A standard white plate 97 is provided on the upper surface side of the left lI#A section of the document table 81. This is because the image signal obtained by optically scanning the standard white plate 97 is used as a standard white signal, and thereby the image signal obtained thereafter is normalized.

The carriage 84 and the V-mirrors 88 and 89 are caused to travel on slide rails (not shown) at predetermined speeds and in predetermined directions, respectively, by a stepping motor 90. ■The speed of the mirrors 88 and 89 is 172 times the speed of the carriage 84, so that the lens 12 is lower than the reading image surface.
The distance is kept constant.

A light source 85 illuminates the document, and the reflected light is reflected by a reflecting mirror 87. (2) Guided to the optical information conversion unit 11 via mirrors 88 and 89. The optical information conversion unit) 11 includes a lens 12 and a CCD 13 on which an optical image is formed, and the optical image is converted into an image (electrical) signal.

The image signal is subjected to various image processing in the image processing section IB, and then outputted to the image exposure means 25, which is a writing means of the printer section 1C, which will be described later.

As the deflector 55A of the image exposure means 5, a galvano mirror is used.
In addition to a rotating mirror or the like, an optical deflector made of a crystal or the like may be used. The laser beam modulated by the image signal is deflected and scanned by this deflector 55A. When deflection scanning is started, the beam scanning is detected by a laser beam index sensor (not shown), and laser beam modulation using an image signal is started.

As the image signal, the image information (copy data) of the document described above and the print data supplied from the outside can be selectively used.

The pulse width modulated laser beam is made to scan over an image forming member (photosensitive drum) 10 which has been charged with a negative charge by a charger 21 after being neutralized by a PCL 20 .

Here, due to the main scanning by the laser beam and the sub-scanning by the rotation of the image forming body 10, an electrostatic latent image corresponding to the image signal is formed on the image forming body 10.

This electrostatic latent image is formed by a developing roller 24 containing black toner.
Developed by a developing device 23 to which a predetermined developing bias voltage is applied from a high-voltage power supply, a monochrome toner image is formed.

On the other hand, the recording paper P fed out from the paper feeding device 41 via the feed roller 42 and the timing roller 43 is conveyed to the surface of the image forming body IO in a state in which the timing is synchronized with the rotation of the image forming body 10. . The black toner image is transferred onto the recording paper P by a transfer device 30 to which a high voltage is applied from a high voltage power source, and separated by a separator 31.

The separated recording paper P is conveyed to the fixing device 32 and subjected to a fixing process using heat and/or pressure to obtain a monochrome image.

The image forming body 10 after the transfer is cleaned by the cleaning device 26 and prepared for the next image forming process.

The scorotron-type charger 21 used here consists of a scorotron-type discharger having a cross section as shown in FIG. a grid stretched between and the image forming body IO;
213 is an elongated frame-shaped shield member made of a conductive material such as a sheet metal. This charger 21 can freely control the charging of the surface of the image forming body 10 by applying a bias voltage to the grid 212 (hereinafter referred to as grid voltage) while applying a high voltage to the discharge wire 211. - Various types of charging can be applied.

For example, if a negative high voltage of several KV to 10 KV is applied to the discharge wire 211 and the grid voltage is -600V, the charged potential on the surface of the image forming member 10 will be -600 V, and if the grid voltage is Ov, the surface of the image forming member 10 will be becomes Ov, and static electricity can be removed.

The cleaning device 26 includes a blade 27 made of urethane rubber or the like that is pressed against the image forming body 1O to clean residual toner, and a predetermined DC voltage is applied to the cleaning device 26 to facilitate recovery of the toner cleaned by the blade 27, thereby forming an image. A metal roller 28, an auxiliary cleaning roller 29, etc. are provided which are disposed in a non-contact manner on the surface of the body 1O. The auxiliary cleaning roller 29 is arranged so as to rotate and press in the opposite direction to the rotation of the image forming body 1O in order to remove unnecessary toner that is left behind when the blade 27 is released from pressure after cleaning.

FIG. 2 is a plan view showing a main part of a deflection scanning system using a semiconductor laser of the image exposure means 5 of FIG. 1.

A laser beam modulated by the image signal from the image processing section IB is emitted from the semiconductor laser 51. This laser beam enters a polygon 55, which is, for example, an octahedral rotating polygon mirror, via mirrors 52 and 53. The laser beam deflected by the polygon 55 is irradiated onto the surface of the image forming body 10 through the fθ lens 54, so that it travels along the surface of the image forming body 10 at a constant speed in a predetermined direction a. 56.57 is a cylindrical lens for tilt angle correction,
58 is a laser beam index sensor that detects scanning of the laser beam.

By scanning the laser beam as described above, an electrostatic latent image is formed on the negatively charged image forming body IO.

This electrostatic latent image is made into a visible toner image by reversal development as described below.

FIG. 4 shows potential changes on the surface of the image forming body during reversal development when an edible type OPC is used as the photoreceptor of the image forming body IO. FIG. 5 is a diagram showing a model of two-component developer particles used in this developing device 23. In the figure, v
H is the potential of the unexposed portion of the image area on the surface of the image forming member 10 that is uniformly charged, VB is the development bias voltage applied to the developing roller 24 of the developing device 23, and VL is the potential of the well where the charge is removed by laser light. The potential of the exposed portion PH decreases in absolute value as shown in FIG.

Referring to FIG. 4, the image area of the image forming body 10 is uniformly charged by the charger 21 and has a predetermined negative surface potential VH (1).

Exposure portion PH given image exposure by laser beam
The potential increases (the absolute value of the potential decreases) (2). In this case, the pulse width modulated laser beam exhibits the properties shown in FIG. 8, so the brightness modulation appears to be superimposed, and the potential of the exposed portion PH (this is called the minimum irradiation potential) is equal to the pulse width. Varies depending on duty ratio.

The electrostatic latent image thus formed is developed by the developing roller 24 to which a negative bias voltage VB is applied.

The developing bias voltage VB is set so that the absolute value is VH>VB>VL, and an example in which negative WOPC is used as the photoreceptor and the image signal is binary is VH#-60.
It is set to be 0V, VB# -480V, and VLt -100V.

The developer is stirred in the developing device 23, and as shown in FIG. 5, the carrier C is positively charged and the toner T is negatively charged.
The toner T adheres to the surface and is transported to the development area by the development roller 24 to which the development bias voltage VB is applied.

Therefore, toner T has a relatively high potential (low absolute value)
The toner image is formed by flying toward the exposed portion PH and developing. The area where this toner image is formed has an absolute potential value of DP due to the adhesion of the negatively charged toner T.
Although the potential increases by U, it usually does not reach the same potential as the unexposed area (3).

The toner image developed as described above is transferred to recording paper P, and then separated and fixed to form a copy image.

When we experimented with the relationship between the duty ratio of pulse width modulation and the copy density by changing the charging potential of the image forming body 10, we found that the a curve (v H- -550V) has a high gamma and sharp contrast, but the b curve (VH・-6
50V), the result was that the gradation was good. That is, it has been found that the lower the absolute value of the charged potential is, the gamma characteristic is suitable for line drawings, and the higher the absolute value is, the gamma characteristic is suitable for those requiring gradation.

Therefore, in the present invention, when the selection button for the photo mode that requires gradation is turned on, and when the selection button for the line drawing mode is turned on.
In this case, the unit period of pulse width modulation, charging potential VH, and development bias voltage VB are changed as shown in Table 1.

Table 1 The unit period is the pixel width.
is the grid voltage applied to 11. (Image forming body lO
(The charging potential attenuates due to leakage while moving from the position of the charger 21 to the developing area.) As described above, the CPU of this device controls the unit period of pulse width modulation of the image processing section IB, the grid voltage VG, The charging potential VH and bias voltage VB are switched depending on the mode.

However, when the charging potential changes due to the above switching, if a pre-transfer exposure lamp is not installed,
If a thin recording paper P is used, poor separation may occur depending on the environmental conditions (temperature and humidity). In order to avoid this, the DC bias voltage of the separator 31 is switched in conjunction with the switching of the grid voltage VG.

For example, if the grid voltage VG is 1750v, set the DC bias voltage to -2, OKV, and the grid voltage vG
In the case of -650V, the DC bias voltage should be switched to -1.5KV.

FIG. 6 is a sectional view showing one example of the separator 31, and FIG. 7 is a sectional view showing another example of the separator 31. FIG. 6 shows an example in which a DC bias voltage is superimposed on an AC bias voltage and applied to the discharge wire 311, and FIG. 7 shows an example in which the DC bias voltage is applied to the back plate 312 and the AC bias voltage is applied to the discharge wire 311. This is an example in which the voltage is applied to . The AC bias voltage is 2.7KV, 500H2 in both cases.

〔Effect of the invention〕

According to the present invention, as explained above, since the charging potential on the surface of the image forming body is switched depending on the mode,
Since the gamma characteristics of the image forming body can be made suitable for both photo mode and line drawing mode, it is possible to obtain excellent copy images with a simple configuration that does not require a special gamma correction circuit. A type image recording device can be provided.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing a schematic configuration of an embodiment of the present invention, Fig. 2 is a plan view showing the main parts of a deflection scanning system using a semiconductor laser, and Fig. 3 is a sectional view showing a scorotron-type charger. , FIG. 4 is a diagram showing the change in potential on the surface of the image forming body during reversal development, FIG. 5 is a diagram showing a model of two-component developer particles, and FIGS. 6 and 7 are examples of separators. A cross-sectional view, Figure 8 shows the potential distribution of a latent image obtained by pulse width modulated laser beam exposure, and Figure 9 shows the copy density of the image obtained by pulse width modulated laser beam exposure and the pulse width of the laser beam. FIG. 10 is a diagram showing the relationship between the copy density and the pulse width of the laser beam when the charging potential of the image forming body is changed. IA: Scanner part IB: Image processing section I
C... Printer section 5... Image exposure means 10.
... Image forming body 21 ... Charger 23 ...
Developing device 24...Developing roller 26...
Cleaning device 27...Blade 30...Transfer device 31...Separator 32...Fixer 212...Grid 311...Discharge wire VB...Bias voltage VH...Charging potential

Claims (6)

[Claims] (1) In an electrophotographic recording apparatus that forms a latent image on a charged image forming body by a writing means using pulse width modulation, a latent image forming means (1) in which the lowest irradiation potential changes within a pulse width modulation control range; A digital image recording apparatus comprising: means for switching the charging potential on the surface of the image forming member depending on a mode. (2) The digital image recording apparatus according to claim 1, wherein the charging potential switching means switches a grid voltage of a scorotron charger. (3) The digital image recording apparatus according to claim 1, wherein the modes include a line drawing mode and a photographic mode having gradation. (4) The digital image recording apparatus according to claim 1, wherein the charging voltage is set higher in the photo mode than in the line drawing mode. (5) The digital image recording apparatus according to claim 1, wherein the writing means is a semiconductor laser optical system. (6) The digital image recording apparatus according to claim 1, wherein separation conditions are made variable in accordance with the switching of the charging potential.