JPH0655772A - Image forming device - Google Patents

Abstract

PURPOSE:To obtain a color image forming device which can conduct high-quality image recording with an improved resolving power. CONSTITUTION:Image density data from an image density data storage circuit 210 is read by a reading circuit 220. In a pixel data conversion circuit 240, the data is subjected to a RE processing, wherein a target pixel is divided into small pixels and the density of the target pixel is distributed per small pixel in accordance with the distribution of adjacent pixel density data including the target pixel, and density data of a pixel center and a part adjacent to a pixel (a pixel-to-pixel part) are found. As a reference wave, a triangular wave B is generated by delaying a reference clock DCK0 by 1/2 period from a B clock obtained by a doubling inversion circuit 281 at twice the frequency of an original pixel clock. Density data per color is modulated on a reference wave signal by modulation circuits 260A, 260B. Pulse width modulation signals positioned on a pixel center and a pixel-to-pixel part are generated. In first and second scanning by two laser beams based on the moldulation signals, an image is recorded in main scanning and sub-scanning directions by an image exposure of a pixel center and a pixel-to-pixel part.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is intended to perform high image quality recording by reflecting the density distribution of adjacent pixels on the density distribution of recording pixels of interest. The image data for one pixel is divided into m × n (horizontal × vertical) small pixels in consideration of the data of the adjacent pixels, and then two lines are generated by the modulation signal obtained by modulating the density data of the pixel by the special reference wave signal. The present invention relates to a color image forming apparatus for performing dot recording consisting of small scanning lines to reproduce characters and halftones. The recording device is intended for use as a printer device or a display device.

[0002]

2. Description of the Related Art In the field of electrophotographic image forming apparatuses, a document image is read by a scanner as an image signal, and the image signal is subjected to gradation correction, A / D conversion, and shading correction to refer to image density data. A digital image obtained by halftone reproduction by modulating with a wave signal is obtained.

An image signal obtained by reading a document image with a scanner has an edge portion of the image read as a halftone density due to an aperture of a solid-state image pickup device incorporated in the scanner. When a latent image is formed on the photoconductor using the image density data obtained from this image signal, the recording pixels corresponding to the edge portion of the latent image are averagely recorded in the recording pixels when the density is intermediate. Therefore, the sharpness of the image is reduced and the image is recorded. Conventionally, it has been known that the image signal is subjected to MTF correction by sharpening with a differential filter, a Laplacian filter or the like. However, this means that only the edges of the image are emphasized, and the uniformity of the halftone image is relatively reduced.

On the other hand, even if an interpolated character or figure is created from CG or font data, there is a similar problem. That is, when the edge portion is smoothly interpolated by the intermediate density with the interpolation data, the recording pixel corresponding to the edge portion is recorded as the average density in the pixel, so that the resolution of the recorded image is reduced.

[0005]

For the above reasons, the intermediate density processing which effectively works at the image edge portion and the recording position modulation for displacing the recording position corresponding to the density distribution of the pixel adjacent to the target pixel are performed. Was needed.

However, in the color image forming apparatus, if the intermediate density processing is performed for each color, the color tone may change.
There is a problem that characters become unclear, and when recording position modulation is performed, moire fringes appear and the resolution deteriorates.

In view of the above problems, an object of the present invention is to reduce the occurrence of moire fringes in an image made from a scanner, CG, font data, etc., improve the resolution, and form a color image for high-quality image recording. To provide a device.

[0008]

The above object is to provide an image forming apparatus using a laser optical system with respect to the main scanning direction.
The pixel center and the pixel are scanned and recorded, the recording density is changed according to the density distribution of adjacent pixels, and the recording is performed in the sub-scanning direction as well as between the pixel center and the pixel. Is achieved by an image forming apparatus.

[0009]

EXAMPLE A color image forming apparatus which is an example of the present invention
The configuration of 400 will be described. FIG. 5 is a perspective view showing a schematic configuration of the image forming apparatus of this embodiment.

The color image forming apparatus 400 differentially amplifies an analog image density signal and a reference wave signal obtained by D / A converting digital image density data from a computer or a scanner after uniformly charging a photoconductor. The dot-shaped electrostatic latent image is formed by the spot light whose pulse width is modulated based on the obtained modulation signal, and the dot-shaped electrostatic latent image is reversely developed with toner to form the dot-shaped toner image. The development process is repeated to form a color toner image on the photoconductor, the color toner image is transferred onto a recording paper, separated from the photoconductor, and fixed to obtain a color image.

The color image forming apparatus 400 includes a drum-shaped photoconductor (hereinafter, simply referred to as a photoconductor) 401 having a photoconductor layer provided on a drum-shaped support that rotates in the arrow direction, and the photoconductor 4.
Scorotron charger 402 that gives a uniform charge on 01
From the scanning optical system 430, the developing devices 441 to 444 loaded with yellow, magenta, cyan, and black toner, the transfer device 462 including a scorotron charger, the separator 463, the fixing roller 464, the cleaning device 470, and the static eliminator 474. Become.

The photosensitive member 401 used in this embodiment is a photosensitive member having a high γ characteristic, and a concrete configuration example thereof is shown in FIG.

As shown in FIG. 7, the photoconductor 401 comprises a conductive support 401A, an intermediate layer 401B and a photoconductive layer 401C. The thickness of the photosensitive layer 401C is about 5 to 100 μm, preferably 10
~ 50 μm. As the photoconductor 401, a drum-shaped conductive support 401A made of aluminum having a diameter of 150 mm is used.
0.1μm thick with ethylene-vinyl acetate copolymer on top
Intermediate layer 401B is formed, and a film thickness of 35μ is formed on this intermediate layer 401B.
The photosensitive layer 401C of m is provided.

As the conductive support 401A, a drum of aluminum, steel, copper or the like having a diameter of about 150 mm is used. In addition to this, paper, a belt-like one in which a metal layer is laminated or vapor-deposited on a plastic film, or an electric carrier is used. It may be a metal belt such as a nickel belt made by the Chu method. Further, the intermediate layer 401B withstands a high charge of ± 500 to ± 2000 V as a photoconductor, for example, in the case of positive charge, it blocks injection of electrons from the conductive support 401A and has excellent light attenuation characteristics due to avalanche phenomenon. It is desirable to have hole mobility so that the intermediate layer 401B can be obtained, for example, in Japanese Patent Application No. 61-
The positively charged type charge transport material described in 188975
It is preferable to attach 10% by weight or less.

As the intermediate layer 401B, for example, the following resins which are usually used in a photosensitive layer for electrophotography can be used.

(1) Polyvinyl alcohol (Poval),
Vinyl polymers such as polyvinyl methyl ether and polyvinyl ethyl ether, (2) polyvinyl amine, poly-
N-containing vinyl polymers such as N-vinylimidazole, polyvinylpyridine (quaternary salt), polyvinylpyrrolidone, vinylpyrrolidone-vinyl acetate copolymer, (3) polyether-based polymers such as polyethylene oxide, polyethylene glycol, polypropylene glycol, (4)
Acrylic acid-based polymers such as polyacrylic acid and its salts, polyacrylamide and poly-β-hydroxyethyl acrylate, (5) Polymethacrylic acid and its salts, methacrylic acid-based such as polymethacrylamide and polyhydroxypropylmethacrylate Polymer, (6) Ether cellulose polymer such as methyl cellulose, ethyl cellulose, carboxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl methyl cellulose,
(7) Polyethyleneimine-based polymers such as polyethyleneimine, (8) Polyalanine, Polyserine, Poly-L-glutamic acid, Poly- (hydroxyethyl) -L-glutamine,
Poly-δ-carboxymethyl-L-cysteine, polyproline, lysine-tyrosine copolymer, glutamic acid-lysine
-Polyamino acids such as alanine copolymer, silk fibroin and casein, (9) starch and its derivatives such as starch acetate, hydroxyethyl ethyl starch, hydroxyethyl starch, amine starch, phosphate starch, and (10) polyamide soluble nylon. ,
A polymer soluble in a mixed solvent of water and alcohol such as methoxymethyl nylon (8 type nylon).

The photosensitive layer 401C basically comprises a phthalocyanine fine particle having a diameter of 0.1 to 1 .mu.m, which is made of a photoconductive pigment, an antioxidant and a binder resin, without using a charge transport material together, and a solvent for the binder resin. It is formed by mixing and dispersing to prepare a coating liquid, coating the coating liquid on the intermediate layer, drying and optionally heat treatment.

When the photoconductive material and the charge transport substance are used in combination, the photoconductive pigment and 1 of the photoconductive pigment are used.
/ 5 or less, preferably 1/1000 to 1/10 (weight ratio) of a small amount of a charge transport substance, and dispersed in a photoconductive material, an antioxidant and a binder resin to form a photosensitive layer.
By using such a high-γ photoconductor, a sharp latent image can be formed despite the spread of the beam diameter, and recording with high resolution can be effectively performed.

In this embodiment, the color toner image is transferred to the photosensitive member 401.
Since they are superposed on each other, a photosensitive member having infrared spectral sensitivity and an infrared semiconductor laser are used so that the beam from the scanning optical system is not blocked by the color toner image.

Next, the light attenuation characteristics of the high γ photoconductor used in this embodiment will be described.

FIG. 6 is a graph showing the characteristics of the high γ photoconductor. In the figure, V ₁ is a charging potential (V), V ₀ is an initial potential (V) before exposure, and L ₁ is a laser beam irradiation light amount (μJ / cm) required for the initial potential V ₀ to be attenuated to 4/5. ² ) and L ₂ represent the irradiation light amount (μJ / cm ² ) of the laser beam required for the initial potential V ₀ to be attenuated to ⅕.

The preferred range of L ₂ / L ₁ is _{1.0 <L 2 / L 1 ≦} 1.5.

In this embodiment, V ₁ = 1000 (V), V ₀ = 950
(V), L ₂ / L ₁ = 1.2. The photoconductor potential of the exposed portion is 10V.

The photosensitivity at the position corresponding to the mid-exposure period when the light attenuation curve attenuates the initial potential (V ₀ ) to 1/2 is E.
When the photosensitivity at the position corresponding to the initial exposure stage when the initial potential (V ₀ ) is attenuated to 9/10 is E9 / 10, (E1 / 2) / (E9 / 10) ≧ 2 Preferably, a photoconductive semiconductor that gives a relationship of (E1 / 2) / (E9 / 10) ≧ 5 is selected. Here, the photosensitivity is defined by the absolute value of the potential decrease amount with respect to the minute exposure amount.

As shown in FIG. 6, in the light attenuation curve of the photoconductor 401, the absolute value of the differential coefficient of the potential characteristic, which is the photosensitivity, is small when the amount of light is small, and increases sharply as the amount of light increases. Specifically, as shown in FIG. 6, the light attenuation curve shows an almost flat light attenuation characteristic in the early stage of exposure due to the poor sensitivity characteristic for a short period of time. The sensitivity becomes an ultra-high γ characteristic that decreases almost linearly. Specifically, the photoconductor 401 is considered to obtain a high γ characteristic by utilizing the avalanche phenomenon under the high charge of +500 to + 2000V. That is, the carriers generated on the surface of the photoconductive pigment in the early stage of exposure are effectively trapped in the interface layer between the pigment and the coating resin, and the light attenuation is surely suppressed. It is understood that an avalanche phenomenon occurs.

Next, the color image forming apparatus of the present invention will be described. In this color image forming apparatus, one pixel of interest in the image density data is formed by m × n (horizontal × vertical) small pixels. Replacing the distribution of density data of adjacent pixels including the target pixel with the distribution of m × n small pixels in the one pixel, and distributing the data of the target pixel multiplied by a constant K according to the distribution. Based on the image density data of the small pixels obtained by the above, it is converted into the image density data between the center of the pixel and the pixel, and the writing of the dots in two rows is performed, and the cycle is twice the pixel clock. By performing pulse width modulation using a triangular wave delayed by / 2 cycle as a reference wave, the image formation is performed by displacing the writing position of the image by the dots spread between pixels and the dots spread from the pixel center. Displacement of the image writing position is referred to as recording position modulation. The process of converting the pixel of interest into image density data of small pixels obtained by dividing the pixel of interest into m × n is referred to as resolution improving process (RE process). High density recording can be performed by this RE processing. In this case, the high γ photoconductor is particularly effective for forming a latent image in response to the reference wave accurately.

In the present invention, this RE processing is performed when the density data of the pixel of interest is equal to or higher than the first threshold value, that is, equal to or higher than a specific density. That is, in many areas corresponding to the highlight portion, the RE processing is not performed on the background portion of the document, and the m × n small pixels have uniform density. In the case of a CRT, this data can be displayed.

However, in the case of laser recording, which will be described later, it is difficult to perform uniform display, so a reference wave whose center of concentration is at the center is selected. As a result, it is possible to prevent the generation of a noisy image while maintaining the uniformity in the highlight portion.

On the other hand, in the case of a high density portion, if the density gradient is large, if a reference wave in which the density recording position is not in the center is selected, dots are formed across adjacent pixels.

In order to prevent the density fluctuation and the collapse of the recording dots between pixels due to this, a specific second value is set even in the high density area.
If it is equal to or more than the threshold value of, the reference wave having the center of concentration at the center is selected.

In the case of a CRT, since uniform display is possible, m × n small pixels are processed as uniform density. That is, RE processing is not performed.

That is, in the color image forming apparatus for performing high-density pixel recording, the specific density data of the target pixel is determined by the density distribution data in the target pixel determined corresponding to the density data of the pixel adjacent to the target pixel. The color image forming apparatus is characterized in that an image is formed according to the determined density distribution when the threshold value is equal to or more than 1.

Further, it is preferable that the color image forming apparatus is characterized in that an image is formed according to the determined density distribution when the specific density data of the pixel is equal to or less than the second threshold value.

FIG. 10A is a plan view showing the target pixel as m5 and adjacent pixels including the target pixel m5 as m1 to m9 when the target pixel m5 is divided into 4 × 4.
If the coordinates of the pixel of interest are (i, j), the coordinates of the adjacent pixels are (i-1, j-1) as shown in FIG.
~ (I + 1, j + 1). FIG. 10B shows the target pixel m
16 is an enlarged view showing a case where each of the 16 small pixels when 5 is divided into 4 × 4 small portions is represented by S511 to S544. FIG. m1-
m9 and S511 to S544 also represent the density of that portion.

The RE processing in the present invention will be described in detail. Taking the case where the pixel of interest m5 is divided into 4 × 4 small pixels as an example, the density of m1 is reflected in S511 of each small pixel, the density of m2 is reflected in S521 and S531, and the density of m2 is reflected in S541. Reflects the concentration of m3, reflects the concentration of m4 in S512, reflects the concentration of m5 in S522 and S532, and reflects the concentration of m6 in S542. In addition, the concentration of m4 is reflected in S513, the concentration of m5 is reflected in S523 and S533, and S54
The concentration distribution is performed such that the concentration of m6 is reflected in 3, the concentration of m7 is reflected in S514, the concentration of m8 is reflected in S524 and S534, and the concentration of m9 is reflected in S544. Therefore, the densities of S511 to S544 are determined by the equation (1).

[0036]

[Equation 1]

In the equation, K is a constant which should be called the strength of RE treatment, and a numerical value in the range of 0.1 to 0.9 is used.

The previous term in the above equation is obtained by multiplying the density of the pixel of interest m5 by K by m1 in S511 and m2 in S521 and S531.
, S541 for m3, S512, S513 for m4, S522, S
532, S523, S533 for m5, S542, S543 for m6, S
514 is m7, S524 and S534 are m8, and S544 is m9.

The term of S0 = (1−K) × m5 / 16 in the latter section is obtained by evenly distributing the remaining densities of the target pixel m5 to the respective small pixels, which means that blurring factors are incorporated. .

FIG. 1 is a block diagram showing an embodiment of an image processing circuit used in a color image forming apparatus of the present invention, FIG. 2 is a block diagram showing an image data processing circuit of this embodiment, and FIG. FIG. 4 is a block diagram showing the modulation circuit of the embodiment, and FIG. 4 is a block diagram of the double inversion circuit of this embodiment and its time chart.

The image processing circuit 1000 of this embodiment is a circuit which constitutes a drive circuit of a scanning optical system, and comprises an image data processing circuit 100, a modulation signal generating circuit 200 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 is composed of 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 font 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. 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. As for the generated color, the corresponding color is converted into density data of yellow (Y), magenta (M), cyan (C), and black (BK) according to the color code. In this way, the fonts are bitmap-developed in each frame memory in the state where each color has the same shape but different density ratios.

The modulation signal generation circuit 200 includes an image density data storage circuit 210, a read circuit 220, a latch circuit 230, an image discrimination circuit 231, an MTF correction circuit 232, a γ correction circuit 233, an image data conversion circuit 240, a selection circuit 250A, 250B, select circuit 251, modulation circuits 260A and 260B, reference clock generation circuit 28
It is composed of a 0, 2 double inverting circuit 281, a reference triangular wave generating circuit 290, a triangular wave B generating circuit 291, and the like.

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 in page units, and at least one page (for one screen). It has a capacity for storing corresponding multi-valued image density data. Further, if it is an apparatus adopted for a color printer, it will have a page memory for storing image density signals corresponding to a plurality of color components, for example, yellow, magenta, cyan and black color components.

The reading circuit 220 uses the index signal, which is the signal at the beginning of scanning, as a trigger to generate continuous image density data in units of one scanning line in synchronization with the reference clock DCK ₀ , and an image density data storage circuit (page memory) 210. Read from the image data conversion circuit 240 and the image discrimination circuit 2
Send to 31.

The latch circuit 230 is a circuit for latching the image density data only while the image data conversion circuit 240, which will be described later, is executing the processing.

The reference clock generation circuit 280 generates a pulse signal having the same repetition period as the pixel clock, and the readout circuit 220, the reference triangular wave generation circuit 290, and the double inverting circuit 28.
1, output to the select circuit 251. For convenience, this clock is referred to as a reference clock DCK ₀ .

FIG. 4 (a) is a block diagram showing the configuration of the double inverting circuit 281 and FIG. 4 (b) is a time chart showing the waveforms at each part. The 2-stage double inverting circuit 281 is shown in FIG.
A delay line that delays 90 °, as shown in (a).
2811, Ex-OR element 2812, and inverting amplifier 2813. When the reference clock DCK ₀ (a) is input to the input terminal 281a,
As shown in FIG. 4B, the reference clock (b) and the reference clock (b) delayed by 90 degrees (1/4 cycle) by the delay line 2811.
Is exclusive-ORed by the Ex-OR circuit 2812 to obtain a clock (c) having a frequency doubled. This clock is inverted by the inverting amplifier 2813, and the clock signal shown in FIG. 4B (d) (this is called B clock) is output terminal 28.
It is output from 1b.

The reference triangular wave generation circuit 290 uses the reference clock DCK.
Performing waveform shaping of the triangular wave phi ₀ reference having an apex at the center of the pixel in the pixel clock and the cycle on the basis of _0. Further, the triangular wave B generation circuit 291 receives the B clock shown in (d) of FIG. 4 (b) and generates a triangular wave BφB having the phase and cycle shown in FIG. 4 (e).

The select circuits 250A and 250B have input terminals for the reference triangular wave φ ₀ and the triangular wave BφB, and select one of the triangular waves by the selection signal from the image discrimination circuit 231 which will be described later and modulate it from the output terminal. It is sent to the input terminal T of the circuits 260A and 260B.

The select circuit 251 has input terminals for the reference clock DCK ₀ and B clock, and the image discrimination circuit 23.
One of the above clocks is selected by the selection signal from 1 and the input terminal CK of the modulation circuits 260A and 260B is selected from the output terminal.
Send to.

The modulation circuits 260A and 260B have the same circuit configuration as shown in FIG. 3, and have a D / A conversion circuit 261, a comparator 262, and an input section T for the reference triangular wave φ ₀ or triangular wave BφB. Then, the image density data input via the latch circuit 230 is D / A converted by the D / A conversion circuit 261 in synchronization with the reference clock DCK ₀ or the B clock, and the image density data input from the selection circuits 250A and 250B is input. It is a circuit that obtains a pulse width modulation signal by comparing a triangular wave as a reference wave.

As shown in FIG. 2, the image data conversion circuit 240 comprises a 1-line delay circuit 242, a 1-clock delay circuit 243, and an arithmetic processing circuit 241.
In the image density data for the first one scanning line for the three scanning lines of the image density data sent for each one scanning line, a delay of two line scanning time is added to the image density data for one intermediate scanning line. Does not delay the image density data for the last one scanning line. Further, each image density data is delayed by 2 reference clocks or 1 reference clock by the 1-clock delay circuit 243, and all image density data of pixels including the target pixel and adjacent to the target pixel are calculated at the same time. Processing circuit
Send to 241.

In the arithmetic processing circuit 241, the density data of the small pixels of m5 and m8 are calculated based on the RE processing. The obtained density data of the m8 small pixel will be referred to as S811 to S844 in the same order as the m5 small pixel. Similarly m
The small pixels in the part adjacent to m5 of 4, m6, m7 and m9 are S441.
-S444, S611-S614 S, 741 S, 911. And
Sum of S442, S443, S512, S513 in FIG. 11, S522, S523, S5
The sum of 32, S533 and the sum of S542, S543, S612, S613 are calculated as density data for the first scan line which is the first scan, and are output to the modulation circuit 260A via the MTF correction circuit 232 and the like.
Sum of S444, S741, S514, S811, S524, S821, S534, S8
The sum of 31 and the sum of S544, S841, S614, and S911 are calculated as the density data for the second scan line, which is the second scan, and MTF is calculated.
It is output to the modulation circuit 260B via the correction circuit 232 and the like. The above density data will be referred to as A density data.

Further, according to the signal of the image discrimination circuit 231, S512
The sum of the densities of the areas including the small pixels in the two rows of S to S543 is used as the density data for the first small scanning line, and the small pixels of the adjacent small pixels S514 to S544 of m5 and m8 and S811 to S841 of the m8 are set to The sum of the densities of the included area is calculated as the density data for the second small scanning line, and is output to the modulation circuit 260B via the MTF correction circuit 232 and the like. This density data will be called B density data.

The first small scan line of the two small scan lines based on the above two kinds of density data scans the center of the original pixel (m5), the second small scan line scans the boundary between m5 and m8, With these two small scans, the scan corresponding to the original one scan (however, the position is displaced by 1 / 4L) is performed.

In the second scan, the small pixels S144, S214 to S244, S314, and S74 of m1, m2, and m3 adjacent to the pixel m5 are used.
The boundary between the pixels m2 and m5 may be scanned using the density data obtained from 1, S511 to S541, S911 and the like.

Further, the arithmetic processing circuit 241 corresponds to the density sum for each small scanning line, if necessary, to each laser driver 301.
A signal for controlling the light emission output of A, 301B can be transmitted. This makes it possible to control the maximum amount of light emitted from the laser emitting units 431A and 431B of the semiconductor laser 431 and change the balance of the recording density according to the type of image to improve the image quality. FIG. 17 is a graph showing an example of the relationship between the drive current of the semiconductor laser and the laser emission output.

On the other hand, the image discriminating circuit 231 discriminates whether the value of the image data of the pixel of interest is between the first and second threshold values or outside the threshold value, and is within the first and second threshold values. When the value is a value, a signal for outputting A density data is output to the image data conversion circuit 240, and a triangular wave B is output to the selection circuits 250A and 250B.
Send a signal to output φB. If it is determined that the area is outside the first and second threshold values, for all color components,
An image data conversion circuit for converting the signal for transmitting the B density data
A selection signal for outputting the reference triangular wave φ ₀ to 240 is sent to the selection circuits 250A and 250B, and the MTF correction circuit 232 is not operated. As a result, the image density data outside the threshold value read by the read circuit 220 is not corrected by the MTF correction circuit 232, is corrected by the γ correction circuit 233, and is then sent to the modulation circuits 260A and 260B via the latch circuit 230. To be done.

As a result, in the highlight and high density area, the image is formed with the dot in the center of the pixel, and a highly uniform and noise-free image can be formed.

Further, the image discrimination circuit 231 further discriminates whether the image is a character area or a halftone area under the above conditions. This determination is 5 × including the pixel of interest.
This is performed by changing the density at 5 pixels. When the density change is large, the pixel of interest is discriminated as a character region, and when it is small, it is discriminated as a halftone region. When it is determined that the area is a character area of a character or a line drawing, the A density data for all color components is used.
The triangular wave BφB is modulated into the MTF correction circuit 232 by the modulation circuits 260A and 260B.
The selection signal to be output to the image data conversion circuit 240 and the selection circuits 250A and 250B, and the MTF correction circuit 232, γ
Since the correction circuit 233 is inoperative, the image density data is sent to the modulation circuits 260A and 260B via the latch circuit 230 without performing MTF correction or γ correction. As a result, clear characters and edge portions with no change in color tone are reproduced.

When it is judged that the image is in the halftone area, a selection signal similar to that in the character area is output only for the achromatic color component, that is, black data, and for the other components, the B density data and the reference triangular wave φ ₀ are selected. The signal is sent to the image data conversion circuit 240 and the selection circuits 250A and 250B to activate the MTF correction circuit 232 and the γ correction circuit 233. As a result, the image density data other than black read by the read circuit 220 is corrected by the MTF correction circuit 232 and the γ correction circuit 233, and then sent to the modulation circuits 260A and 260B via the latch circuit 230.

Furthermore, the image discrimination circuit 231 selects a signal for outputting the B clock when the A density data is used and the reference clock DCK ₀ when the B density data is used for the select circuit.
Output to 251.

By the above discrimination processing, it is possible to form an image without moire fringes or color skip in the halftone area,
The black image produces the effect of adding sharpness and tightness to the image.

By using the same kind of reference wave for each recording color, it is possible to guarantee the gradation of the image and prevent the tint from changing. Note that it is preferable to use G components that visually match or achromatic data having G components for the image discrimination.

To be used for image discrimination, the data converted into density data of a specific color, for example, R + 2G + B (where R is red density data, G is green density data, and B is blue density data) is converted. I am using. For convenience (R + 2G +
The density data of B) will be represented by N.

In the modulation circuits 260A and 260B, the triangular wave which is the selected reference wave is used to modulate the signal of the image density data input through the latch circuit 230 to generate a pulse width modulated signal. For two small continuous scanning lines in parallel (one line of original image density data)
To the raster scanning circuit 300 as one unit.

In FIGS. 13A to 13C, the A data is the triangular wave B.
6 is a time chart showing a signal when recording position modulation is performed with φB as a reference wave.

In FIG. 13, (a) shows the page memory 210.
Reference clock DCK _{0 using the} index signal as a trigger
After the image density data read out based on the data is processed and converted by the image data conversion circuit 240 to become A data, it is input to the modulation circuits 260A and 260B via the MTF correction circuit 232, the γ correction circuit 233, and the latch circuit 230. A part of the analog value converted by the D / A conversion circuit 261 is shown. The higher level shows a lighter density, and the lower level shows a darker density.

(B) shows a triangular wave BφB sequentially output from the select circuits 250A and 250B. The solid line shows the triangular wave, and the alternate long and short dash line shows the image density signal converted into the analog value, showing the modulation operation in the modulation circuits 260A and 260B.

(C) shows a pulse width modulation signal generated by being compared by the comparator 262. As described above, when the triangular wave BφB is used as the reference wave and the image data is modulated, it is possible to obtain a modulation signal that spreads between pixels along with the center of the pixel.

FIG. 14 is a diagram showing a recording dot formed by the first small scanning line based on the pulse width modulation signal in which the A data is modulated, and the thick solid line represents the pixel boundary line,
(A) shows only the pixel center in the main scanning direction, and (b) and (c)
Shows the case where the recording dots are formed only between the pixels in the main scanning direction and between the pixels in the main scanning direction, and FIG. Scanning by the second small scanning line similarly records between pixels in the sub-scanning direction.

By the image recording (writing) as described above, it becomes possible to form a recording dot between the pixel center and the pixel not only in the main scanning direction but also in the sub scanning direction.

FIG. 15 shows a time chart when the B density data is modulated by the reference triangular wave φ ₀ . In this case, a modulation signal that spreads only from the center of the pixel is obtained.

The raster scanning circuit 300 includes a delta delay circuit 311, laser drivers 301A and 301B, an index detection circuit (not shown), a polygon driver and the like.

The laser drivers 301A and 301B are modulation circuits 260.
Two laser emission parts 431A, 43 by the modulation signal from A, 260B
The semiconductor laser array 431 having 1B is oscillated, and a signal corresponding to the light quantity of the beam from the semiconductor laser array 431 is fed back and driven so that the light quantity becomes constant.

The index detection circuit detects the surface position of the rotary polygon mirror 434 that rotates at a predetermined speed by the index signal from the index sensor 439, and the image density signal modulated by the raster scanning method according to the cycle in the main scanning direction. Optical scanning is performed. Scan frequency 2204.72Hz
The effective print width is 297 mm or more, and the effective exposure width is 306 mm.
That is all.

The polygon mirror driver rotates the DC motor uniformly at a predetermined speed and rotates the rotary polygon mirror 434 at 16535.4 rpm.

As the semiconductor laser array 431, as shown in FIG. 8, one in which two light emitting portions 431A and 431B are arranged in an array at regular intervals is used. The distance d between the normal light emitting parts is 20 μm
Since it is difficult to make the following, as shown in FIG. 8, the axis passing through the centers of the respective light emitting units 431A and 431B is parallel to the rotation axis of the rotary polygon mirror 434, and at a constant angle with respect to the main scanning direction ( Install at an angle of θ). In this way, the laser spots Sa and Sb of the laser beam on the photoconductor 401 by the semiconductor laser array 431 can be closely scanned up and down as shown in FIG. However, for this reason, the positions of the laser spots Sa and Sb in the scanning direction deviate from the main scanning direction. In order to correct this shift, a delta delay circuit 311 is inserted between the modulation circuit 260B and the laser driver 301B, and the shift is corrected by delaying an appropriate amount and timing to correct the shift.
The laser spots Sa and Sb emitted from the laser beams become Sa and Sb ′ aligned perpendicularly to the main scanning direction, and image exposure can be performed to record an image.

FIG. 12 is a diagram showing the center position and exposure intensity of the above two small scans. The laser spots Sa and Sb are shown in FIG.
As shown in 2, one center is made to scan the pixel center and the other center is made to scan between pixels.

Next, the image forming process of the image forming apparatus 400 shown in FIG. 5 will be described.

First, the photoconductor 401 is uniformly charged by the scorotron charger 402. The electrostatic latent image corresponding to yellow is formed on the drum-shaped photoconductor 401 by the image density data storage circuit 210.
The two laser beams modulated by the yellow data (8-bit digital density data) from the inside are the cylindrical lens 433, the rotary polygon mirror 434, and the fθ lens 435,
It is formed by irradiation through a cylindrical lens 436 and a reflection mirror 437. The electrostatic latent image corresponding to yellow is developed by the first developing device 441, and a dot-shaped first toner image (yellow toner image) having extremely high sharpness is formed on the photoconductor 401. The first toner image is not transferred onto the recording paper, passes under the retracted cleaning device 470, and is charged again by the scorotron charger 402 on the photoconductor 401.

Then, the two laser beams modulated by the magenta data (8-bit digital density data) are irradiated onto the photoconductor 401 to form an electrostatic latent image. 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 401 is formed.

According to the image forming apparatus 400 of the present embodiment, the photoconductor 401 has excellent high γ characteristics, and this excellent high γ characteristic is used for charging, exposing and developing a toner image many times. The latent image is stably formed even when the toner images are repeatedly formed over each other. That is, even if a laser beam is irradiated from above the toner image based on a digital signal, a dot-shaped electrostatic latent image with high fringes and high sharpness is formed, and as a result, a toner image with high sharpness can be obtained. You can

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

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 conveyance belt, is carried into a fixing roller 464, is heated and fixed, and is discharged to a sheet discharge tray.

In the present example, as a result of an experiment in which the value of the coefficient K of the RE treatment was variously changed, the value of K was 0.1 to 0.9.
Good images were obtained in the range of. However, when K is small, the sharpness of the character is insufficient, and when K is large, the edge portion of the character or the line drawing is overemphasized. Therefore, the preferable K value range is 0.3 to 0.7. It was found to be in the range. As a result, when the manuscript is a character or a line drawing, the edge part appears clearly, and even small characters can be reproduced in detail. Moreover, the low-density portion and the high-density portion were not adversely affected. This is because the method stops recording position modulation for these pixels and effectively sets K = 0.

In the present method, K can be used with a constant value, but it is preferable to use K by changing it according to the image (character area or halftone area). When the value in the character area is K ₁ and the value in the halftone area is K ₂ , it is preferable that K ₁ > K ₂ . That is, when the image is a character area, the value of K is large, preferably 0.9 to 0.4, and when the image is a halftone area, the value of K is small and is 0.6 to 0.1. Note that K =
0 corresponds to no recording position modulation.

In addition to the present embodiment, the number of doubling circuits excluding the inverting amplifier 2813 from the doubling inverting circuit 281 of FIG. It is also possible to form an image based on a modulation signal obtained by modulating the image data obtained by summing two small pixels in the direction by the triangular wave BφB. FIG. 16 shows the image data (a) and the modulation signal (c) obtained by the triangular wave B (b) in that case.

Although the above-described image data flow is described as a laser printer which outputs data once stored in the page memory 210, the present invention is not limited to this, and instead of the image data conversion circuit 100, a color scanner 151, A / Instead of the image data conversion circuit 150 composed of the D conversion circuit 152, the density conversion circuit 153, the masking UCR circuit 154 and the like, if a circuit for inputting image density data from the scanner and performing image processing is used, it may be a copy machine or the like. It can be applied to an image forming apparatus.

In the above embodiment, the semiconductor laser array 431 having two light emitting portions is used to perform writing scanning of two lines at a time. However, by using the semiconductor laser having one light emitting portion, for example, main scanning is performed. The same image recording as described above can be performed by doubling the speed and doubling the scanning density in the sub-scanning direction to perform scanning with one laser beam.

[0092]

As described above, in the present invention,
The image density data is calculated by the RE process that distributes the density of the target pixel according to the distribution of the density data of the adjacent pixels including the target pixel, and is converted into the image density data between the center of the pixel and the pixel, and this is converted into 2 of the pixel clock. It has twice the frequency, 1 /
By performing pulse width modulation using a triangular wave delayed by two cycles as a reference wave, two lines of dots are written with dots that spread from between pixels and dots that spread from the center of the pixel, and the pixel center and pixel in both the main scanning direction and the sub-scanning direction are written. It has become possible to record images during this period. As a result, it is possible to provide an excellent color image forming apparatus that does not change the color tone of a color image made from a scanner, CG, font data, or the like, reduces the occurrence of moire fringes, and improves sharpness. .

Further, the effect can be further improved by using the high γ photoconductor.

[Brief description of drawings]

FIG. 1 is a block diagram of an image processing circuit of an embodiment of an image forming apparatus of the present invention.

FIG. 2 is a block diagram showing an example of an image data conversion circuit of the circuit of FIG.

FIG. 3 is a block diagram showing an example of a modulation circuit of the circuit of FIG.

FIG. 4 is a block diagram showing an example of a double inverting circuit of FIG.

FIG. 5 is a perspective view showing a schematic configuration of an image forming apparatus of the present invention.

FIG. 6 is a graph showing characteristics of the high γ photoconductor used in this example.

FIG. 7 is a cross-sectional view showing a specific configuration example of a high-γ photoconductor used in this example.

FIG. 8 is a diagram showing a semiconductor laser array of the embodiment of FIG.

9 is a diagram showing a scanning locus by the semiconductor laser array of FIG.

FIG. 10 is a diagram for explaining RE processing.

FIG. 11 is a diagram for explaining a recording unit of A density data of the present invention.

FIG. 12 is a diagram showing a center position and exposure intensity of two small scans according to the present invention.

FIG. 13 is a triangular wave B obtained by converting A density data according to the embodiment of FIG.
7 is a time chart showing each signal in the case where modulation is performed according to FIG.

14 is a diagram showing an image recording state of a pixel center in the main scanning direction by the modulation signal of FIG.

FIG. 15 is a time chart when B density data is modulated by a reference triangular wave.

FIG. 16 is a time chart when a pixel clock of 4 times is used.

FIG. 17 is a graph showing an example of the relationship between the driving current of the semiconductor laser and the laser emission output.

[Explanation of symbols]

 100 Image data processing circuit 200 Modulation signal generation circuit 210 Image density data storage circuit (page memory) 220 Read circuit 230 Latch circuit 231 Image discrimination circuit 232 MTF correction circuit 233 γ correction circuit 240 Image data conversion circuit 241 Operation processing circuit 250A, 250B , 251 Select circuit 260A, 260B Modulation circuit 280 Reference clock generation circuit 281 Double multiplication inverting circuit 290 Reference triangle wave generation circuit 291 Triangle wave B generation circuit 300 Raster scanning circuit 400 Image forming device

Claims (2)

[Claims] 1. An image forming apparatus using a laser optical system, wherein a pixel center and a pixel are scanned and recorded in a main scanning direction, and the recording density is changed according to a density distribution of adjacent pixels. A characteristic image forming apparatus. 2. The image forming apparatus according to claim 1, wherein the recording is performed between the pixel center and the pixel even in the sub-scanning direction.