JP3306519B2 - Image forming device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention performs high-quality printing by reflecting the density distribution of adjacent pixels on the density distribution of a recording pixel of interest. After dividing one pixel of image data into m × n (horizontal × vertical) small pixels in consideration of the data of adjacent pixels, the center of gravity is determined for each row, and the phase of the reference wave is shifted according to the center of gravity. The present invention also relates to a color image forming apparatus that performs dot recording including n small scanning lines by using a modulation signal obtained by modulating the density data of the pixel using the reference wave signal, and reproduces characters and halftones. Further, the recording device is intended to be used as a printer device or a display device.

[0002]

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

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

[0004] On the other hand, the same problem arises even when an interpolated character or figure is created from CG or font data. That is, when the edge portion is smoothly interpolated with the intermediate density using the interpolation data, the recording pixel corresponding to the edge portion is recorded as an 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 works effectively 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. It was necessary.

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 the characters become unclear, and when the recording position is modulated, moiré fringes appear to lower the resolution.

SUMMARY OF THE INVENTION In view of the above problems, it is an object of the present invention to reduce the occurrence of moiré fringes in an image formed from a scanner, CG, font data, etc., improve resolution, and form a color image in which high-quality image recording is performed. It is to provide a device.

[0008]

An object of the present invention is to calculate a density distribution corresponding to a pixel center and a pixel using the density of a pixel adjacent to a pixel of interest, and to calculate a first and a second density based on the calculated density distribution. A laser optical system for performing a second scan;
In the image forming apparatus which records the pixel center, and the second scan records between pixels, the second scan forms a reference pixel clock and a reference waveform based on a pulse signal generated from a reference clock generation circuit. Generating a clock having a phase difference at regular intervals with respect to the reference clock;
This is achieved by an image forming apparatus that performs image formation using waveforms having different phases output from a triangular wave generation circuit based on this.

[0009]

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A color image forming apparatus according to one embodiment of the present invention.
The configuration of 400 will be described. FIG. 4 is a perspective view illustrating a schematic configuration of the image forming apparatus of the present embodiment.

A 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 photosensitive member. A dot-shaped electrostatic latent image is formed from the spot light pulse-width-modulated based on the modulation signal obtained as described above, and this is reversely developed with toner to form a dot-shaped toner image. The development process is repeated to form a color toner image on the photoreceptor, and the color toner image is transferred onto a recording sheet, separated from the photoreceptor, 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 direction of an arrow, and the photoconductor 4.
01 Scorotron charger 402 to apply uniform charge on
And a scanning optical system 430, developing units 441 to 444 loaded with yellow, magenta, cyan, and black toners, a transfer unit 462 including a scorotron charger, a separator 463, a fixing roller 464, a cleaning device 470, and a neutralizer 474. Become.

The photosensitive member 401 used in this embodiment is a photosensitive member having a high γ characteristic, and FIG. 6 shows a specific configuration example thereof.

The photosensitive member 401 comprises a conductive support 401A, an intermediate layer 401B and a photosensitive layer 401C as shown in FIG. The thickness of the photosensitive layer 401C is about 5 to 100 μm, preferably 10 to 100 μm.
5050 μm. The photosensitive member 401 uses a drum-shaped conductive support 401A made of aluminum having a diameter of 150 mm.
0.1μm thick ethylene-vinyl acetate copolymer
Of the intermediate layer 401B having a thickness of 35 μm
m photosensitive layer 401C.

As the conductive support 401A, a drum made of aluminum, steel, copper, or the like having a diameter of about 150 mm is used. In addition, a belt-shaped metal sheet laminated or vapor-deposited on paper or a plastic film, or an electroconductive support is used. It may be a metal belt such as a nickel belt made by a brazing method. Further, the intermediate layer 401B withstands high charge of ± 500 to ± 2000V as a photoreceptor, for example, in the case of positive charge, prevents injection of electrons from the conductive support 401A, and has excellent light attenuation characteristics due to avalanche phenomenon. In order to obtain the intermediate layer 401B, it is desirable to have a hole mobility.
No. 188975 describes a positively charged charge transport material.
It is preferable to attach 10% by weight or less.

As the intermediate layer 401B, for example, the following resins commonly used for photosensitive layers for electrophotography can be used.

(1) polyvinyl alcohol (povar),
Vinyl polymers such as polyvinyl methyl ether and polyvinyl ethyl ether; (2) polyvinylamine, poly-
Nitrogen-containing vinyl polymers such as N-vinylimidazole, polyvinylpyridine (quaternary salt), polyvinylpyrrolidone, vinylpyrrolidone-vinyl acetate copolymer, (3) polyether polymers such as polyethylene oxide, polyethylene glycol and polypropylene glycol, (4)
Acrylic polymers such as polyacrylic acid and salts thereof, polyacrylamide and poly-β-hydroxyethyl acrylate; and (5) methacrylic acids such as polymethacrylic acid and its salts, polymethacrylamide and polyhydroxypropyl methacrylate Polymer, (6) methylcellulose, ethylcellulose, carboxymethylcellulose, hydroxyethylcellulose, ether cellulose-based polymer such as hydroxypropylmethylcellulose,
(7) polyethyleneimine 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-alanine copolymer, silk fibroin, polyamino acids such as casein, (9) starch acetate, hydroxyethyl ethyl starch, hydroxyethyl starch Starch, such as amine starch and phosphate starch, and derivatives thereof, (10) soluble polyamide polyamide,
A polymer soluble in a mixed solvent of water and alcohol, such as methoxymethyl nylon (8 type nylon).

Basically, the photosensitive layer 401C is prepared by using a phthalocyanine fine particle of 0.1 to 1 μm in diameter made of a photoconductive pigment, an antioxidant, and a binder resin without using a charge transport material in combination with a binder resin solvent. A coating solution is prepared by mixing and dispersing, the coating solution is applied to an intermediate layer, dried, and if necessary, heat-treated to form a coating solution.

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

In this embodiment, the color toner image is transferred to the photosensitive member 401.
A photoconductor and an infrared semiconductor laser having a spectral sensitivity on the infrared side are used so that the beam from the scanning optical system is not blocked by the color toner image because the beam is superimposed on the top.

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

FIG. 5 is a graph showing characteristics of the high γ photoreceptor. In the figure, V ₁ is a charging potential (V), V ₀ is an initial potential (V) before exposure, and L ₁ is an irradiation light amount (μJ / cm) of a laser beam required for the initial potential V ₀ to attenuate to 4/5. ^2), L ₂ represents a light quantity of the laser beam initial potential V ₀ which is required for attenuation to 1/5 (μJ / cm ^2).

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), is L ₂ / L ₁ = 1.2. The photosensitive member potential at the exposed portion is 10 V.

The light sensitivity at the position where the light decay curve corresponds to the middle stage of exposure where the initial potential (V ₀ ) is attenuated to １ ／ is E
When the light sensitivity at a position corresponding to the initial stage of exposure in which the initial potential (V ₀ ) is attenuated to 9/10 is E9 / 10, (E1 / 2) / (E9 / 10) ≧ 2 Preferably, a photoconductive semiconductor which gives a relationship of (E1 / 2) / (E9 / 10) ≧ 5 is selected. Here, the light sensitivity is defined by the absolute value of the amount of potential decrease with respect to the minute exposure amount.

In the light decay curve of the photosensitive member 401, as shown in FIG. 5, the absolute value of the differential coefficient of the potential characteristic, which is the photosensitivity, is small when the light amount is small, and increases sharply as the light amount increases. Specifically, as shown in FIG. 5, the light decay curve has a slightly poor sensitivity characteristic during the initial period of exposure and shows a substantially flat light decay characteristic, but from the middle to late stage of the exposure, the light decay curve changes to an extremely high level. The sensitivity becomes an ultra-high γ characteristic that decreases almost linearly. It is considered that the photoconductor 401 specifically obtains a high γ characteristic by utilizing an avalanche phenomenon under high charging of +500 to + 2000V. In other words, the carriers generated on the surface of the photoconductive pigment in the early stage of the exposure are effectively trapped in the interface layer between the pigment and the coating resin, and the light attenuation is surely suppressed. As a result, extremely rapid after the middle stage of the exposure. It is understood that an avalanche phenomenon occurs.

Next, a color image forming apparatus according to the present invention will be described. In this color image forming apparatus, one pixel of interest of 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 data of the target pixel multiplied by a constant K in accordance with the distribution. The image formation is performed by displacing the phase of the reference wave of each row of the small pixel based on the image density data of the small pixel obtained by the above, thereby displacing the writing position of the dot of n rows or n / 2 rows. . Displacing the dot writing position is referred to as recording position modulation. Further, the process of converting the target pixel into image density data of small pixels divided into m × n is referred to as a resolution improvement process (RE process). High density recording can be performed by this RE processing. In this case, a high γ photoreceptor is particularly effective for forming a latent image accurately in response to a reference wave.

In the present invention, this RE processing is performed for the density data of the target pixel which is equal to or higher than the first threshold, that is, equal to or higher than a specific density. In other words, in many areas corresponding to the highlight part, the RE processing is not performed on the background part of the document, and the m × n small pixels have uniform density. In the case of a CRT, this data display is possible.

However, in the case of laser recording described later, since uniform display is difficult, a reference wave whose density center is at the center is selected. As a result, it is possible to maintain uniformity in the highlight portion and prevent generation of a noise-free image.

On the other hand, when the density gradient is large in the high density portion, if a reference wave whose density recording position is not at the center is selected, a dot is formed over the adjacent pixels.

In order to prevent the density fluctuation and the recording dot collapse between the pixels due to this, a specific second density is used even in the high density area.
If the threshold value is equal to or larger than the threshold value, the reference wave whose density center is at the center is selected.

In the case of a CRT, since uniform display is possible, m × n small pixels are processed as a uniform density. That is, no RE processing is performed.

That is, in a color image forming apparatus that performs high-density pixel recording, specific density data of a pixel of interest is determined based on density distribution data within the pixel of interest determined according to density data of a pixel adjacent to the pixel of interest. In the color image forming apparatus, the recording position modulation is performed based on the determined density distribution when the threshold value is equal to or more than 1.

Further, it is preferable that the recording position modulation is performed based on the determined density distribution when the specific density data of the pixel is equal to or less than a second threshold value.

FIG. 9A is a plan view in which the target pixel is m5 and adjacent pixels including the target pixel m5 are m1 to m9 when the target pixel m5 is divided into 4 × 4.
Assuming that the coordinates of the target pixel are (i, j), the coordinates of the adjacent pixels are (i−1, j−1) as shown in FIG.
To (i + 1, j + 1). FIG. 9B shows the target pixel m.
FIG. 14 is an enlarged view showing a case where 16 small pixels are represented by S511 to S544 when 5 is divided into 4 × 4 small parts. m1
m9 and S511 to S544 also represent the density of that part.

The RE processing according to the present invention will be described in detail. Taking the case where the target pixel m5 is divided into 4 × 4 small pixels as an example, of each small pixel, the density of m1 is reflected in S511, the density of m2 is reflected in S521 and S531, and the density of S2 is reflected in S541. Reflects the density of m3, reflects the density of m4 in S512, reflects the density of m5 in S522 and S532, and reflects the density of m6 in S542. Further, the density of m4 is reflected in S513, the density of m5 is reflected in S523 and S533, and the density of m5 is reflected in S54.
The density distribution is performed such that the density of m6 is reflected in 3, the density of m7 is reflected in S514, the density of m8 is reflected in S524 and S534, and the density of m9 is reflected in S544. Therefore, the densities of S511 to S544 are determined by the equation (1).

[0037]

(Equation 1)

In the formulas, K is a constant which is also called the intensity of the RE processing, and a numerical value in the range of 0.1 to 0.9 is used.

The preceding term of the above equation is obtained by multiplying the density of the pixel of interest m5 by K. S511 is m1, and S521 and S531 are m2.
S541 is m3, S512 and S513 are m4, S522, S
M5 for 532, S523 and S533, m6 for S542 and S543, S
514 is assigned according to the ratio of m7, S524 and S534 are assigned m8, and S544 is assigned according to the ratio of m9.

The latter term, S0, ie, the term of (1-K) × m5 / 16, is obtained by uniformly distributing the remaining density of the target pixel m5 to each of the small pixels, and adopts a blur factor. .

FIG. 1 is a block diagram showing an embodiment of an image processing circuit used in the color image forming apparatus of the present invention, FIG. 2 is a block diagram showing a reference wave phase determining circuit of this embodiment, and FIG. FIG. 3 is a block diagram illustrating a modulation circuit according to the present embodiment.

The image processing circuit 1000 of this embodiment is a circuit constituting a driving circuit of a scanning optical system, and includes an image data processing circuit 100, a modulation signal generation 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 includes an input circuit 110 composed of a computer and a font data generation circuit 12.
0, font data storage circuit 130, interpolation data generation circuit 14
0, a 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 generation circuit 120. The font data generation circuit 120 selects an address signal from the four types of input signals and sends it to the font data storage circuit 130. The font data storage circuit 130 sends 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 jaggies and jumps in the image density data generated at the edge portion of the font data using the intermediate density, and sends the result to the image density data storage circuit 210 composed of a frame memory. 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 font is subjected to bitmap development in each frame memory with each color having the same shape and different density ratio.

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 gamma correction circuit 233, a reference wave phase determination circuit 240, and a selection circuit 250A. , 250B, modulation circuit 26
0A, 260B, reference clock generation circuit 280, triangular wave generation circuit
290, a delay circuit group 291 and the like.

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

The reading circuit 220, the image density data of image density data storage circuit for consecutive one scan line units continuously in synchronization with the reference clock DCK ₀ with an index signal is a scanning head of the signal as a trigger (page memory) 210 From the reference wave phase determination circuit 240 and the image determination circuit 2
Send to 31.

The latch circuit 230 is a circuit that latches the image density data for the time during which the process of the reference wave phase determination circuit 240 described later is executed.

The reference clock generation circuit 280 is a pulse generation circuit that generates a pulse signal having the same repetition cycle as the pixel clock and outputs the pulse signal to the readout circuit 220, the triangular wave generation circuit 290, the delay circuit group 291 and the modulation circuits 260A and 260B. I do. For convenience the clock referred to as a reference clock DCK _0.

Reference numeral 290 denotes a triangular wave generating circuit, which is a reference clock DCK ₀
Performing waveform shaping of the triangular wave phi ₀ reference is the reference wave with the same period and pixel clock based on. Further, in the delay circuit group 291, the reference clock DCK _{0 is} fixed at regular intervals (in this example, 1/5
A plurality of clocks DCK _{1 to} DCK ₄ having a phase difference are generated at each cycle, and based on this, the triangular waves φ _{1 to} φ ₄ (here, triangular waves φ ₁ , 2 which are advanced by 1/5 cycle) are reference waves having different phases. A triangular wave φ ₂ advanced by / 5 cycle, a triangular wave φ ₃ delayed by 1/5 cycle, and a triangular wave φ ₄ delayed by 2/5 cycle are output.

The select circuits 250A and 250B have input units for the triangular waves φ _{1 to} φ ₄ out of phase with the reference triangular wave φ ₀ ,
One of the triangular waves is selected by a selection signal from a reference wave phase determination circuit 240, which will be described later, and modulated by the modulation circuits 260A and 260B.
To the input terminal T.

The modulation circuits 260A and 260B have the same circuit configuration as shown in FIG. 3, and include a D / A conversion circuit 261 and a comparator 262, and the reference triangular wave φ ₀ or a triangular wave whose phase is shifted by 1/5 cycle. And the latch circuit
The image density data input via 230 is used as the reference clock DCK
_{This is a} circuit that performs D / A conversion by a D / A conversion circuit 261 in synchronization with ₀ and compares the triangular waves input from the selection circuits 250A and 250B as reference waves to obtain a pulse width modulation signal.

The reference wave phase determination circuit 240 comprises a one-line delay circuit 242, a one-clock delay circuit 243, and an arithmetic processing circuit 241 as shown in FIG.
A delay of two scanning times is applied to the image density data of the first one scanning line of three scanning lines of the image density data sent for each one scanning line, and the image density data of the middle one scanning line is used. Delays one line scanning time, and does not delay the image density data of the last one scanning line. Further, each image density data is delayed by two reference clocks or one reference clock by one clock delay circuit 243, and all image density data of pixels including the target pixel and adjacent to the target pixel are simultaneously calculated. Processing circuit
Send to 241.

The arithmetic processing circuit 241 calculates the density data of the small pixels m5 and m8 based on the RE processing. The density data of the obtained small pixel of m8 will be referred to as S811 to S844 in the same order as the small pixel of m5. And
The density sum of the area including the two rows of small pixels S512 to S543 in FIG. 9B is defined as the density data for the first small scan line as the first scan, and the adjacent small pixels S514 to S544 of m5 and m8. And m
The density sum of the area including the small pixels in the two rows of S811 to S841 of 8 is calculated as the density data for the second small scanning line that is the second scanning. The first small scan line of the two small scan lines scans the center of the original pixel (m5), and the second small scan line scans the boundary between m5 and m8.
The scanning corresponding to the scanning (however, the position is shifted by 1 / 4L) is performed.

In the second scan, the boundary between the pixels m2 and m5 may be scanned using the sum of the densities of the small pixels S214 to S244 and S511 to S541 adjacent to the pixels m2 and m5.

The arithmetic processing circuit 241 further performs an arithmetic operation for obtaining the position of the center of gravity from the density data distribution of each small scanning line, and outputs the sum of the density data to the MTF correction circuit 232. Select signals 250A, 250 from the output terminals OA, OB.
Output to B.

FIG. 10 is a diagram showing an example of the positional relationship between the triangular waves having different phases and the target pixel. In the figure, h1 ~
h5 indicates a divided area when the target pixel m5 is divided into five equal parts in the main scanning direction.

[0057] The signal for selecting the reference triangular wave phi ₀ without phase displacement when the gravity center position is h3 region is a central m5, triangular wave advances 1/5 cycle phase when the center of gravity is h2 region phi ₁ Is selected, the signal for selecting the triangular wave φ ₂ whose phase is advanced by 2/5 cycle when the center of gravity is in the h1 area, and the triangle wave φ ₃ whose phase is delayed by 1/5 cycle when the center of gravity is in the h4 area.
Is selected when the center of gravity is in the h5 region.
A signal for selecting the triangular wave φ ₄ delayed by / 5 cycle is output from the output terminal OA to the selection circuit 250A. Similarly, from the output terminal OB, a triangular wave selection signal for the second small scanning line determined by the density centroid of the second small scanning line is supplied to the select circuit 250B.
Output to

Further, the arithmetic processing circuit 241 may correspond to each laser driver 301 in accordance with the density sum for each small scanning line, if necessary.
A, a signal for controlling the light emission output of 301B can be transmitted. Thereby, the maximum light emission amount of the laser light emitting units 431A and 431B of the semiconductor laser 431 can be controlled, and the balance of the recording density can be changed according to the type of the image to improve the image quality. FIG. 15 is a graph showing an example of the relationship between the driving current of the semiconductor laser and the laser emission output.

On the other hand, the image discriminating circuit 231 discriminates whether the image data of the target pixel is the first or second threshold value, and determines that the area is outside the first and second threshold values. In this case, the reference wave phase determination circuit 240
There triangular wave selected is not output, and sends a selection signal to output only the reference triangular wave phi ₀ select circuit 250A, the 250B, M
The TF correction circuit 232 is not operated. As a result, the read circuit 2
The image density data outside the threshold value read from 20 is not corrected by the MTF correction circuit 232, corrected by the γ correction circuit 233, and then modulated by the modulation circuits 260A and 260B via the latch circuit 230.
Sent to

As a result, a highly uniform and noise-free image can be formed in the highlight and high density area.

The image discriminating circuit 231 further discriminates whether the image is a character area or a halftone area under the above conditions. This discrimination is performed by 5 × including the pixel of interest.
This is performed by changing the density in five pixels. If the change in density is large, the pixel of interest is determined to be a character area, and if it is small, it is determined to be a halftone area. If it is determined that it is a character area of a character or a line drawing, a selection signal for outputting the triangular wave selected by the reference wave phase determination circuit 240 to the modulation circuits 260A and 260B for all color components is output to the selection circuits 250A and 250B. , M
The TF correction circuit 232 and the γ correction circuit 233 are disabled and the image density data is not processed and the modulation circuit 2 is passed through the latch circuit 230 without processing.
60A, 260B. As a result, clear characters and edge portions having no change in color tone are reproduced. When it is determined that the image is in the halftone area, a selection signal similar to that in the character area is output only for the achromatic component, that is, black data,
For other components, the selection signal that outputs only the reference triangular wave φ ₀ is output to the selection circuits 250A and 250B without outputting the triangular wave selected by the reference wave phase determination circuit 240, and the MTF correction circuit
232, the gamma correction circuit 233 is operated. As a result, the image density data other than black read by the reading circuit 220 is corrected by the MTF correction circuit 232 and the γ correction circuit 233, and then sent out to the modulation circuits 260A and 260B via the latch circuit 230.

As a result, an image free from moire fringes and color skipping can be formed in the halftone area, and the effect of giving sharpness and closeness to the image by the black image is produced.

By using the phase of the reference wave in common for each recording color, it is possible to guarantee the gradation of an image and prevent a change in tint. In determining the phase, G that visually matches
It is preferable to use achromatic data having a component or a G component.

A specific color such as R + 2G + B (where R is red density data, G
Is green density data, and B is blue density data. ) Is used. For convenience (R
The density data of (+ 2G + B) is represented by N.

The data used in the image discriminating circuit 231 uses data common to each color for the same reason.

The modulation circuits 260A and 260B modulate the signal of the image density data input through the latch circuit 230 with the triangular wave as the selected reference wave to generate a pulse width modulated modulation signal. For two consecutive small scanning lines (one line of original image density data)
Is sent to the raster scanning circuit 300 as one unit.

FIGS. 11A to 11D are time charts showing signals of each part of the modulation signal generation circuit 200 when the recording position is modulated.

FIG. 11A shows a page memory 210.
From the reference clock DCK _{0 using the} index signal as a trigger
After the image density data read based on is processed by the reference wave phase determination circuit 240, the MTF correction circuit 232,
Modulation circuits 260A, 26 via a γ correction circuit 233 and a latch circuit 230.
0B, and shows a part of the data that has been converted to an analog value by the D / A conversion circuit 261. A higher level indicates a lighter density, and a lower level indicates a higher density.

(B) shows a triangular wave which is a reference wave including a delayed one sequentially output from the select circuits 250A and 250B.

(C) shows the triangular wave (solid line) and the image density signal (dashed-dotted line) converted to the analog value, and shows the modulation operation in the modulation circuits 260A and 260B.

(D) shows a pulse width modulation signal generated by comparison by the comparator 262.

The raster scanning circuit 300 includes a δ 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 provided with a modulation circuit 260.
A, the two laser emitting units 431A, 43
The semiconductor laser array 431 having 1B is oscillated. A signal corresponding to the light amount of the beam from the semiconductor laser array 431 is fed back, and driving is performed so that the light amount becomes constant.

The index detecting circuit detects the surface position of the rotating polygon mirror 434 which rotates at a predetermined speed based on the index signal from the index sensor 439, and modulates the modulated image density signal by the raster scanning method according to the period in the main scanning direction. Optical scanning is performed. Scan frequency 2204.72Hz
The effective printing 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 shown in FIG. 7, a semiconductor laser array 431 in which two light emitting portions 431A and 431B are arranged in an array at regular intervals is used. Normally, the distance d between the light emitting units is 20 μm
As shown in FIG. 7, the axis passing through the center of each of the light emitting units 431A and 431B is parallel to the rotation axis of the rotary polygon mirror 434 and has a fixed angle ( θ). In this manner, the laser spots Sa and Sb of the laser beam on the photosensitive member 401 by the semiconductor laser array 431 can be scanned vertically and closely as shown in FIG. However, the positions of the laser spots Sa and Sb in the scanning direction are shifted from the main scanning direction. In order to correct this shift, a δ delay circuit 311 is inserted between the modulation circuit 260B and the laser driver 301B, the shift is corrected by taking an appropriate amount of delay, and the semiconductor laser array 431 is corrected.
The laser spots Sa and Sb emitted from the light source become Sa and Sb 'aligned perpendicularly to the main scanning direction, so that image exposure can be performed and an image can be recorded.

As shown in FIG. 12, the laser spots Sa and Sb have one center scanning the pixel center and the other center scanning the pixels.

In the above-mentioned modulation signal generation, in the case of pixels in the low-density portion and high-density portion, the recording position modulation is not performed as described above. The position of the small dots in the two lines is moved to a position along the line direction of the original character or line drawing, and as a result, the character or line drawing is clearly reproduced.

FIG. 13 shows the recording state of a line image perpendicular to the scanning direction, and FIG. 14 shows the recording state of a line image parallel to the scanning direction. In the case where there is a line image, (b) shows the case where there is a line image in the pixel.

In the recording position modulation, only the black component is performed in the halftone region to prevent a change in color tone, and the other color components are modulated by the triangular wave φ _{0 having} no phase displacement.

Furthermore, by sequentially shifting the phase of the reference wave in the sub-scanning direction, a dot corresponding to a halftone dot having a screen angle can be formed. For example, the screen angle is 45 ° for the yellow component and 2 for the magenta component.
6.6 °, -26.6 ° for the cyan component, and 0 ° for the black component can improve the uniformity of color reproduction and prevent the occurrence of moiré fringes.

In particular, setting the black component to 0 ° has the advantage that the recording position modulating means can be used without any change.

Next, an image forming process of the image forming apparatus 400 shown in FIG. 4 will be described.

First, the photosensitive member 401 is uniformly charged by the scorotron charger 402. An electrostatic latent image corresponding to yellow is stored on a drum-shaped photoconductor 401 in an image density data storage circuit 210.
The two laser beams modulated by the yellow data (8-bit digital density data) from the inside are converted into a cylindrical lens 433, a rotating polygon mirror 434, and an fθ lens 43.
5, formed by irradiation through a cylindrical lens 436 and a reflection mirror 437. The electrostatic latent image corresponding to the yellow color is developed by the first developing device 441, and a first toner image (yellow toner image) in a dot shape with extremely high sharpness is formed on the photoconductor 401. The cleaning device is retracted without transferring the first toner image to the recording paper.
After passing under 470, the photoconductor 401 is charged again by the scorotron charger 402.

Next, the two laser beams modulated by the magenta data (8-bit digital density data) are irradiated on the photosensitive member 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). In the same manner as described above, the toner is sequentially developed by the third developing device 443 to form a third toner image (cyan toner image), and a three-color toner image sequentially stacked on the photoconductor 401 is formed. Finally, a fourth toner image (black toner image) is formed, and a four-color toner image sequentially laminated on the photoconductor 401 is formed.

According to the image forming apparatus 400 of this embodiment, the photosensitive member 401 has excellent high γ characteristics, and this excellent high γ characteristic requires a large number of steps of charging, exposure and development from above the toner image. The latent image is formed stably even when the toner image is repeatedly formed over a long period of time. That is, even if a laser beam is irradiated from above the toner image based on the digital signal, a high-sharpness, high-dot electrostatic latent image without fringes is formed, and as a result, a high-sharpness toner image is obtained. Can be.

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

The recording paper carrying the transferred toner image is separated by a separator.
The photoconductor 401 is separated from the photoconductor 401 by 463, is conveyed by a guide and a conveyance belt, is conveyed to a fixing roller 464, is heated and fixed, and is discharged to a paper discharge tray.

In this embodiment, as a result of experiments in which the value of the coefficient K for RE processing was changed variously, the value of K was 0.1 to 0.9.
A good image was obtained within the range. However, when K is small, the sharpness of the character is insufficient, and when K is large, the result that the edge portion of the character or the line drawing is excessively emphasized is obtained. Therefore, the preferable range of the value of K is 0.3 to 0.7. Turned out to be in the range. As a result, when the original is a character or a line drawing, the edge portion clearly appears, and even small characters can be reproduced in detail. In addition, there was no adverse effect on the low density portion or the high density portion. This is because the present method stops the recording position modulation for these pixels, and effectively sets K = 0.

In the present method, it is possible to use K constant, but it is preferable to use K by changing K according to the image (character area or halftone area). Assuming that the value in the case of the character area is K ₁ and the case of 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, 0.6 to 0.1. Note that K =
0 corresponds to not performing recording position modulation.

In the present invention, the ratio of the RE processing can be arbitrarily changed by using a specific value of K.

FIG. 16 is a graph showing an example of converting the relationship between the recording position in the main scanning direction and the center of gravity, and FIG. 17 is a graph showing an example of converting the density in the sub-scanning direction.

In the arithmetic processing circuit 241, the result obtained by performing arithmetic processing from the image density data is converted by using a built-in or external ROM 245 in accordance with a conversion formula set in advance as shown in FIG. Can be changed. Similarly, the density in the sub-scanning direction can be converted as shown in FIG.

In addition to the present embodiment, five types of modulated signals obtained by modulating the density sum data with the five types of reference waves are prepared, and selected by the selection signal output from the reference wave phase determination circuit 240. The object can also be achieved by a circuit configured to transmit to the raster scanning circuit 300 after being selected by the circuit.

The flow of the image data described above has been described as a laser printer that outputs data once stored in the page memory 210. However, the present invention is not limited to this. Instead of the image data processing circuit 100, the color scanner 151 and the A / A If a circuit for inputting image density data from a scanner and performing image processing is used instead of the image data processing circuit 150 including the D conversion circuit 152, the density conversion circuit 153, the masking UCR circuit 154, etc. It can be applied to an image forming apparatus.

[0096]

As described above, the target pixel is divided into small pixels, and the density of each small pixel is determined by dividing the density of the target pixel according to the distribution of the density data of the adjacent pixels including the target pixel.
The phase of the reference wave signal is selected from the image data centroid of the area where the small pixel group is divided into two in the sub-scanning direction, and the density sum signal of the divided area is modulated by the reference wave. A recording position modulation signal is generated, and a color image is recorded by two laser beams scanning between the pixel center and the pixel by using the modulation signal. Therefore, a color image formed by a scanner, CG, font data, or the like is used. It was possible to provide an excellent color image forming apparatus in which the occurrence of moire fringes was reduced and the sharpness was improved without causing a change in color tone.

The effect can be further improved by using a high γ photoreceptor.

[Brief description of the drawings]

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

FIG. 2 is a block diagram illustrating an example of a reference wave phase determination circuit of the circuit of FIG. 1;

FIG. 3 is a block diagram illustrating an example of a modulation circuit of the circuit of FIG. 1;

FIG. 4 is a perspective view illustrating a schematic configuration of the image forming apparatus of the present invention.

FIG. 5 is a graph showing characteristics of the high γ photoconductor used in the present example.

FIG. 6 is a cross-sectional view showing a specific configuration example of a high γ photoconductor used in the present embodiment.

FIG. 7 is a diagram showing a semiconductor laser array according to the embodiment of FIG. 4;

FIG. 8 is a diagram showing a scanning locus by the semiconductor laser array of FIG. 7;

FIG. 9 is a diagram for explaining RE processing.

FIG. 10 is a diagram for explaining a phase shift of a reference wave.

FIG. 11 is a time chart showing signals of respective parts of the modulation signal generation circuit of the embodiment of FIG. 1;

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

FIG. 13 is a diagram showing a recording state of a line image perpendicular to the scanning direction according to the present invention.

FIG. 14 is a diagram illustrating a recording state of a line image parallel to a scanning direction according to the present invention.

FIG. 15 is a graph showing an example of a relationship between a drive current of a semiconductor laser and a laser emission output.

FIG. 16 is a graph showing an example of a case where the relationship between the center of gravity of a small scanning line in the main scanning direction and the recording position is converted.

FIG. 17 is a graph illustrating an example of a case where the average density in the sub-scanning direction of a small scanning line is converted.

[Description of Signs] 100 Image data processing circuit 200 Modulation signal generation circuit 210 Image density data storage circuit (page memory) 220 Readout circuit 230 Latch circuit 231 Image discrimination circuit 232 MTF correction circuit 233 γ correction circuit 240 Reference wave phase determination circuit 241 Arithmetic processing circuit 250A, 250B Select circuit 260A, 260B Modulation circuit 280 Reference clock generation circuit 290 Triangular wave generation circuit 291 Delay circuit group 300 Raster scanning circuit 400 Image forming apparatus

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-3-213360 (JP, A) JP-A-2-155760 (JP, A) JP-A-63-93269 (JP, A) JP-A-62- 42693 (JP, A) JP 4-3147 (JP, B2) (58) Fields surveyed (Int. Cl. ⁷ , DB name) B41J 2/44 H04N 1/23 103

Claims (1)

(57) [Claims] 1. A laser optical system for calculating a density distribution corresponding to a pixel center and a pixel using the density of a pixel adjacent to a pixel of interest and performing first and second scanning based on the calculated density distribution. An image forming apparatus that records a pixel center in the first scan and records a pixel center in the second scan, wherein a reference pixel clock and a reference pixel signal are generated based on a pulse signal generated from a reference clock generation circuit. Shaping the waveform, generating a clock having a phase difference with respect to the reference clock at regular intervals, and forming an image using a waveform having a different phase output from the triangular wave generation circuit based on the generated clock. Image forming apparatus.