JPH0595460A - Image forming method - Google Patents

Abstract

PURPOSE:To increase the density of a character area, to improve the gradation of a halftone area and to obtain satisfactory images having a wide dynamic range, in the case of forming images with the scan of laser beams undergone modulation of its intensity. CONSTITUTION:Image density data corresponding to picture elements from an image density data storage circuit 210 are read by a reader circuit 220, and an intensity modulation circuit 260 generates intensity modulation signals corresponding to the image density data. Thus, in the case of controlling the light emission of a semiconductor laser 431, an image discrimination circuit 240 discriminates whether the images are in the character area or the halftone area, and in the character area, the duty ratio of a control pulse used for intensity modulation is increased rather than in the halftone area.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming method for recording an image using a laser scanning optical system.

[0002]

2. Description of the Related Art An image forming method is well known in which a photosensitive member is scanned with one or a plurality of laser beams modulated by a recording signal and information based on the recording signal is recorded on the photosensitive member. .. The scanning means of the above method is a technical means for performing dot exposure in which a laser beam is reflected by a rotary polygon mirror that rotates at a constant speed by a motor, transmitted through an fθ lens, and scanned on a photosensitive member in a form narrowed down to a minute spot. It has been known.

Further, the image forming means is a photosensitive drum which rotates at a constant speed by a motor, and the drum shaft and the scanning direction of the laser beam are set in parallel, and the rotation of the photosensitive drum serves as sub-scanning. It is known that a latent image of an image is formed on the peripheral surface of the photosensitive drum that is uniformly charged in advance with the scanning of.

In such an image forming method, for example, the light intensity distribution of the beam spot on the photosensitive drum by the laser beam emitted from the semiconductor laser which emits light by the control pulse having a constant pulse width corresponding to the pixel is shown by the solid line in FIG. Is almost a Gaussian curve, and when a portion having a predetermined threshold value (s) or more is developed, a potential change to which toner adheres occurs. Therefore, in the latent image formed by the beam spot, the dot diameter of the developed toner image changes as the light intensity of the beam spot changes.

To reproduce a halftone image, a modulation signal that changes according to the density data corresponding to a pixel is obtained, the semiconductor laser is driven by this intensity modulation signal, and a laser beam emitted from a semiconductor laser with a changing light intensity is obtained. When a latent image is formed on the photoconductor by forming a latent image, the size of the latent image dot changes depending on the light intensity and a multi-valued minute circular or elliptical latent image can be obtained. The dot-shaped toner image obtained by developing this latent image can form a halftone image having gradation similar to the halftone dot obtained by halftone dot printing using a screen in printing.

[0006]

However, in the above-mentioned image forming method, in the area of a character image or line drawing consisting of thin lines (hereinafter this area is referred to as a character area), the area of a halftone image (hereinafter this area is an intermediate area). Unlike the control area), the number of times the laser spot passes through the same location is small, so the potential change is small. Therefore, when the image formation is performed with the same maximum writing light amount of the semiconductor laser, the dot size and the toner adhesion amount are different between the character area and the halftone area, and the gradation of the halftone area is appropriately maintained. With the maximum writing light quantity, the density of the character area can only be low. Further, if the writing maximum light amount of the semiconductor laser is increased so that the density of the character area is sufficiently high, there is a problem that the halftone area is quickly saturated and the gradation is deteriorated.

An object of the present invention is to obtain an image having a wide dynamic range by sufficiently increasing the density of a character region and improving the gradation of a halftone region in image formation by laser beam scanning with intensity modulation. That is.

[0008]

SUMMARY OF THE INVENTION The above object is to provide a character area in an image forming method in which an image is recorded by performing intensity modulation in which a light amount is changed only for a control pulse time according to density data corresponding to a pixel using a laser beam. This is achieved by an image forming method characterized in that the duty ratio of the control pulse in the character region is made larger than the duty ratio of the control pulse in the halftone region based on the discrimination of the halftone region.

In addition to the automatic switching of the duty ratio of the control pulse, it can be arbitrarily changed by an external input as a preferred embodiment.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The configuration of an image forming apparatus of an embodiment to which the present invention is applied will be described. FIG. 2 is a perspective view showing a schematic configuration of the image forming apparatus of this embodiment.

The image forming apparatus 400 has an intensity based on a modulation signal obtained from an analog image density signal obtained by D / A converting digital image density data from a computer or a scanner after uniformly charging a photoconductor. A dot-shaped electrostatic latent image is formed by the modulated beam spot, and this is inversely developed with toner to form a dot-shaped toner image, and the charging, exposure and development steps are repeated, and a color toner image is formed on the photoconductor. Is formed, and this color toner image is transferred onto a recording paper, separated and fixed to obtain a color image.

The image forming apparatus 400 includes a drum-shaped photoconductor (hereinafter, simply referred to as a photoconductor) 401 that rotates in the arrow direction, and a scorotron charger 402 that applies a uniform charge to the photoconductor 401. , Scanning optical system 430, developing devices 441 to 444 loaded with yellow, magenta, cyan, and black toners, transfer device 462 including a scorotron charger, separator 463, fixing roller 46.
4. A cleaning device 470 and a static eliminator 474.

The scanning optical system 430 collimates the laser light emitted from the semiconductor laser 431 into parallel laser light into a laser beam. This laser beam is reflected and deflected by a rotary polygon mirror 434 that rotates at a constant speed, and is formed into a minute spot on the peripheral surface of a photosensitive body 401, which is a drum-shaped photosensitive body uniformly charged, by an fθ lens 435 and cylindrical lenses 433 and 436. Image exposure is performed by scanning with a beam spot focused on. Here, the fθ lens 435 is a correction lens for performing optical scanning at a constant speed, and the cylindrical lenses 433 and 436 are lenses that correct the variation of the spot position due to the surface tilt of the rotary polygon mirror 434. Reference numeral 437 is a scanning mirror that reflects the laser beam, 438 is an index mirror, and 439 is an index sensor. The surface position of the rotary polygon mirror 434 that rotates at a predetermined speed is detected by the index signal from the index sensor 439, and the cycle in the main scanning direction is detected. As a result, the beam spot scans the photoconductor 401 parallel to the drum axis.

FIG. 1 is a block diagram showing an embodiment of an image processing circuit used in an image forming apparatus to which the present invention is applied.

The image processing circuit of this embodiment is a circuit which constitutes the drive circuit of the scanning optical system 430 of FIG. 2, and comprises an image data processing circuit 100, a modulation signal generating circuit, and a raster scanning circuit 300.

The image data processing circuit 100 is a circuit for interpolating and outputting the edge portion of the font data, and comprises an input circuit 110 and a font data generating circuit 12 formed of a computer.
0, font data storage circuit 130, interpolation data generation circuit 14
Consists of 0, the character code signal from the input circuit 110,
The size code signal, the position code signal and the color code signal are sent to the font data generating circuit 120. The fond data generation circuit 120 selects an address signal from four types of input signals and sends it to the font data storage circuit 130. The font data storage circuit 130 sends the font data corresponding to one character corresponding to the address signal to the font data generation circuit 120. The font data generation circuit 120 sends the font data to the interpolation data generation circuit 140. The interpolation data generation circuit 140 interpolates jaggedness or jumps of the image density data generated at the edge portion of the font data using the intermediate density and sends the interpolated data to the image density data storage circuit 210 composed of a frame memory. The generated color is converted into density data of corresponding colors yellow (Y), magenta (M), cyan (C), and black (BK) according to the color code. In this way, the font is bitmap-developed in each frame memory in a state where each color has the same shape but different density ratios.

The modulation signal generation circuit includes an image density data storage circuit 210, a read circuit 220, a latch circuit 230, an image discrimination circuit 240, a selection circuit 250, an intensity modulation circuit 260, a reference clock generation circuit 280, a pulse A generation circuit 281, It is composed of a pulse B generation circuit 282.

The reference clock generation circuit 280 is a reference clock pulse generation circuit and generates a clock pulse having the frequency of the pixel clock. Reference clock generation circuit 280
The clock output from is referred to as a reference clock DCK ₀ for convenience, and is output to the read circuit 220, the pulse A generation circuit 281, the pulse B generation circuit 282, and the intensity modulation circuit 260.

In the pulse A generation circuit 281, the reference clock D
Based on CK ₀ , a control pulse A having a duty ratio that maximizes the gradation in the halftone region is generated to select circuit 25.
To the reference clock DC.
The control circuit B generates a control pulse B having a larger duty ratio than the control pulse A, that is, a longer pulse width, based on K ₀ ,
Send to 250.

The image density data storage circuit 210 is a normal page memory (hereinafter simply referred to as the page memory 210), a RAM (random access memory) for storing page units, and at least one page (one screen). It has a capacity for storing image density data corresponding to corresponding multi-valued pixels. Further, if the device is adopted in a color printer, it is equipped with a page memory for storing image density signals corresponding to color components of a plurality of colors, for example, yellow, magenta, cyan and black.

The readout circuit 220 reads out image density data corresponding to pixels in a unit of one scanning line continuous from the image density data storage circuit (page memory) 210 in synchronization with the reference clock DCK ₀ by using the index signal as a trigger, It is sent to the discrimination circuit 240 and the intensity modulation circuit 260.

The image discriminating circuit 240 is a circuit for discriminating whether the image is a character region or a halftone region. When it is discriminated that the image is a halftone region, the gradation characteristic of the halftone region is the best. When the control pulse A having a duty ratio is selected and it is determined that it is a character area, a control signal for selecting the control pulse B having a large duty ratio is sent to the select circuit 250.

The image discriminating circuit 240 successively differentiates the image density data corresponding to, for example, consecutive pixels, and if the differential value is α or more or -α or less when the specific value is α, the image is a character area. If the differential value is α or more, it means that the edge is on the left side of the scanning line, and if it is −α or less, it means that the edge is on the right side in the scanning line direction. be able to.

The latch circuit 230 is a circuit for latching the image density data only during the time when the processing of the image discrimination circuit 240 is being executed.

The intensity modulation circuit 260 is the selection circuit 250.
Is a circuit which has a pulse width corresponding to the duty ratio of the control pulse from the pixel, and obtains an intensity modulation signal corresponding to the image density data corresponding to the pixel. Send to 300. That is, in the case of the halftone region, a light emission control signal which has an appropriate light emission time and emits a laser beam having a light intensity corresponding to the image density data is sent to the semiconductor laser 431. A light emission control signal for emitting a laser beam having a light intensity corresponding to the image density data for a long time is sent to the semiconductor laser 431.

The dot developed with the latent image formed by the beam spot of the laser beam whose emission time has been lengthened by the control pulse having a large duty ratio becomes long in the main scanning direction as shown by the dotted line in FIG. 3, and the dot area becomes large. As a result, the image density can be increased.

The raster scanning circuit 300 includes a laser driver 30.
1. An index detection circuit and a polygon mirror driver (not shown) are provided.

The laser driver 301 is the intensity modulation circuit 260.
The semiconductor laser 431 is oscillated by a modulated signal from the semiconductor laser 431, and a signal corresponding to the laser beam light quantity from the semiconductor laser 431 when there is no signal is fed back to drive the writing maximum light quantity to be constant.

The index detection circuit detects the surface position of the rotary polygon mirror 434 which rotates at a predetermined speed based on the index signal from the index sensor 439, and the intensity-modulated image density in the raster scanning system according to the cycle in the main scanning direction. Optical scanning is performed by signals.

The polygon mirror driver rotates the DC motor at a predetermined speed and rotates the rotary polygon mirror 434 at a constant angular speed.

Next, the image forming process of the image forming apparatus 400 will be described.

First, the photoconductor 401 is neutralized by the static eliminator 474 and then uniformly charged by the scorotron charger 402. An electrostatic latent image corresponding to yellow is formed on the photoconductor 401 by the yellow data (8bi) from the image density data storage circuit 210.
(digital density data of t) and a laser beam whose intensity is modulated by a control pulse selected by the image discrimination circuit 240. The electrostatic latent image corresponding to the yellow color is developed by the first developing device 441, and the photoreceptor 4
Dot-shaped first toner image (yellow toner image) on 01
Is formed. The first toner image is not transferred to the recording sheet, passes under the retracted cleaning device 470, and is charged again on the photoconductor 401 by the scorotron charger 402.

Then, the intensity of the laser beam is modulated by the magenta data (8-bit digital density data) and the control pulse selected by the image discrimination circuit 240, and the modulated laser light is irradiated onto the photoconductor 401 to be quiet. A latent image is formed. This electrostatic latent image is developed by the second developing device 442 to form a second toner image (magenta toner image). The third developing device 443 sequentially develops in the same manner as described above to form a third toner image (cyan toner image), and a three-color toner image sequentially laminated on the photoconductor 401 is formed. Finally, a fourth toner image (black toner image) is formed, and a four-color toner image sequentially formed on the photoconductor 1 is formed.

These four-color toner images are transferred by the transfer device 462 onto the recording paper supplied from the paper feeding device.

The recording paper carrying the transferred toner image is separated by a separator.
The sheet is separated from the photoconductor 401 by 463, is conveyed by a guide and a conveyor belt (not shown), is carried into a fixing roller 464, is heated and fixed, and is discharged to a sheet discharge tray. On the other hand, the photoconductor 401 that has finished the transfer is cleaned by the cleaning device 4
It is cleaned by 70 and prepared for the next image formation.

In the above embodiment, the image discrimination circuit 240 discriminates whether the image is a character region or a halftone region and automatically switches the duty ratio of the control pulse. However, the duty ratio can be changed by an external command. Needless to say, it may be performed independently or simultaneously with the automatic switching.

Although the above-mentioned image processing circuit has been described as a laser printer, it is not limited to this, and the color scanner 15 is used instead of the image data processing circuit 100.
1, A / D conversion circuit 152, density conversion circuit 153, masking U
If the image data processing circuit 150 including the CR circuit 154 and the like is replaced with a circuit for inputting image density data corresponding to pixels from the scanner and performing image processing, it can be applied to other image forming apparatuses such as copying machines. You can

[0038]

As described above, in the image forming method of generating the intensity modulation signal from the image density data corresponding to the pixel and recording the image by the modulation signal,
In the halftone region of the image, the control pulse used in the intensity modulation is selected to have a duty ratio that maximizes the gradation, and in the character region of the image, a control pulse having a larger duty ratio than the halftone region is selected. Therefore, it is possible to provide an excellent image forming method in which the density of the character area of the image to be formed is increased and the gradation of the halftone area is improved, and the dynamic range is wide.

[Brief description of drawings]

FIG. 1 is a block diagram showing an example of an image processing circuit of an image forming apparatus to which the present invention is applied.

FIG. 2 is a perspective view showing a schematic configuration of an image forming apparatus of the present invention.

FIG. 3 is a diagram showing a relationship between a light intensity distribution of a beam spot and a dot diameter.

[Explanation of symbols]

 100 Image data processing circuit 210 Image density data storage circuit (page memory) 230 Latch circuit 220 Readout circuit 240 Image discrimination circuit 250 Select circuit 260 Intensity modulation circuit 280 Reference clock generation circuit 281 Pulse A generation circuit 282 Pulse B generation circuit 300 Raster scan Circuit 400 Image forming device 430 Scanning optical system

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location G03G 15/04 116 9122-2H

Claims (2)

[Claims] 1. An image forming method for performing image recording by performing intensity modulation in which a light amount is changed for a control pulse time according to image density data corresponding to a pixel by using a laser beam, and based on discrimination between a character area and a halftone area. An image forming method characterized in that the duty ratio of the control pulse in the character region is made larger than the duty ratio of the control pulse in the halftone region. 2. The image forming method according to claim 1, wherein the duty ratio of the control pulse can be arbitrarily changed by an external input in addition to automatic switching.