JPH0486684A - Image forming device - Google Patents

Abstract

PURPOSE:To prevent adjacent dots from being jointed to each other when a latent image is formed by modulating image density data with a reference wave where amplitude is varied periodically in a main scanning or a sub scanning direction. CONSTITUTION:A selecter circuit 331 to 333 where the image density data is selectively outputted and a reference wave generating means 377 where the reference waves of differing amplitudes are generated are combined. Then, the image density data is modulated by the reference wave where the amplitude is varied periodically in the main scanning or the sub scanning direction. Thus, density jumping due to the adjacent dots being jointed to each other when the latent image is formed can be prevented and a stable intermediate tone reproduction can be carried out.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to an image forming apparatus that reproduces halftones by recording dots using a modulated signal modulated by a reference wave signal.

[Background technology]

In the field of image forming apparatuses using electrophotography, area gradation using pulse width modulation as the gradation of an image is known as a basic concept of a halftone reproduction method in a digital system. In pulse width modulation, the laser output is set to two values, on and off, and gradations are produced by controlling the oscillation time. The advantage of this is the laser output. By setting the surface potential of the photoreceptor to approximately QV at one time, the potential change depending on the temperature and humidity can be ignored, resulting in excellent stability. However, as the pulse width becomes shorter, the peak value decreases due to the broadening of the pulse, resulting in an analog potential expression. In order to avoid this, it is necessary to keep the output constant and make the spot diameter as small as possible. However, reducing the spot diameter requires an increase in the number of scans due to a reduction in the pitch in the sub-scanning direction, resulting in a decrease in printing speed. In order to improve this, it is known to make the spot diameter super-elliptic so that the short axis is in the main scanning direction and the long axis is in the sub-scanning direction. The recording characteristics when pulse width modulation is used are shown in FIG. 7a (dashed line). FIG. 8(a) shows a recording state in which toner adheres at an intermediate density. In addition to CJ, dot concentration type and dispersed type dither methods are known for image forming apparatuses that emphasize halftone reproduction. Further, improved methods such as a combination of pulse width modulation and a dither method are known. However, even if halftone reproduction is performed using the aforementioned pulse width modulation or dot concentration dither method, for various reasons, the tone reproduction may be skipped in highlight areas or adjacent dots in high density areas. Saturation is observed when they are connected and formed. This tendency is remarkable in high-gamma photoreceptors.

【the purpose】

SUMMARY OF THE INVENTION In view of the above-mentioned problems, an object of the present invention is to provide an image forming apparatus capable of stably reproducing halftones by preventing density jumps caused by adjacent dots being connected when forming a latent image. .

[Means to solve the problem]

To achieve the above object, the present invention provides an image forming apparatus that oscillates a semiconductor laser using a modulation signal obtained by modulating image density data with a reference wave signal to form an image on an image forming body. This is characterized by modulation using a reference wave whose amplitude is periodically changed in the scanning direction. Further, the image forming apparatus forms an image on an image forming body by oscillating a semiconductor laser using a modulation signal obtained by modulating image density data with a reference wave signal, and the amplitude is determined by a selector circuit that selectively outputs the image density data. The present invention is characterized in that it is combined with a reference wave generating means that generates different reference waves.

【Example】

The configuration of the image forming apparatus of this embodiment will be explained. FIG. 4 is a perspective view showing a schematic configuration of the image forming apparatus of this embodiment. The image forming apparatus uniformly charges a drum-shaped photoreceptor (hereinafter simply referred to as photoreceptor) as an image forming body using a scorotron charger 2, and then converts it into a dot-shaped static image using a binarized spot light using a scanning optical system 3o. An electrostatic latent image is formed, and this is reversely developed using a developing device to form a dot-shaped toner image, and the above-mentioned charging, ! A color toner image is formed on the photoreceptor 1 by repeating the light and development steps, and the color toner image is transferred, separated, and fixed to obtain a color image. The image forming apparatus includes a photoreceptor 1, a scorotron charger 2, developers 4 to 7, a scanning optical system 30, an image processing circuit 300. It consists of a reference clock generation circuit 400 and drivers 500 and 600. FIG. 6 is a sectional view showing a specific example of the structure of a high-gamma photoreceptor. The main configuration of this embodiment will be explained below. As shown in FIG. 6, the photoreceptor 1 consists of a conductive support IA, an intermediate layer IB, and a photosensitive layer IC. The thickness of the photosensitive layer IC is
Approximately 5 to 100 μm, preferably 10 to 502 m
It is. The photoreceptor I uses a drum-shaped conductive support IA made of aluminum with a diameter of 150 mm, and a 0.1 μm thick layer made of ethylene-vinyl acetate copolymer is placed on the support IA.
An intermediate layer IB with a thickness of 35μ is formed on this intermediate layer IB.
It is constructed by providing m photosensitive layer ICs. As the conductive support IA, aluminum, steel,
A drum made of copper or the like with a diameter of about 150 mm is used, but there are also belts made of paper, plastic film, laminated or vapor-deposited with a metal layer, or metal belts such as nickel belts made by the electroplating method. You can. Moreover, the intermediate layer IB is ±5 as a photoreceptor.
It can withstand high charging of 00 to ±2000V, for example, in the case of positive charging, prevents electrons from being injected from the conductive support 1c, and provides excellent light attenuation characteristics due to the avalanche phenomenon.
It is desirable that the intermediate layer I
For example, in B, the patent application 1988-188 that the present applicant proposed earlier.
It is preferable to add 10% by weight or less of the positively charged charge transport material described in No. 9't'5. As the intermediate layer IB, the following resins, which are usually used in photosensitive layers for electrophotography, can be used. (1) Vinyl polymers such as polyvinyl alcohol (Hoval), polyvinyl methyl ether, polyvinylethyl ether, etc. (2) Polyvinylamine, poly-N-vinylimidazole, polyvinylpyridine (quaternary salt), polyvinylpyrrolidone, vinylpyrrolidone-vinyl acetate Nitrogen-containing vinyl polymers such as copolymers (3) Polyether polymers such as polyethylene oxide, polyethylene glycol, and polypropylene glycol (4) Acrylic acids such as polyacrylic acid and its salts, polyacrylamide, and poly-β-hydroxyethyl acrylate (5) Methacrylic acid-based polymers such as polymethacrylic acid and its salts, polymethacrylamide, and polyhydroxypropyl methacrylate (6) Ether fibers such as methylcellulose, ethylcellulose, carboxymethylcellulose, hydroxyethylcellulose, and hydroxypropylmethylcellulose (7) Polyethyleneimine polymers such as polyethyleneimine (8) Polyalanine, polyserine, poly-L-glutamic acid, poly-(hydroxyethyl)-L-glutamine,
Poly-δ-carboxymethyl-stine, polyproline, lysine-tyrosine copolymer Glutamic acid-lysine-alanine copolymer Polyamino acids such as silk fibroin and casein (9) Starch acetate, hydroxyl ethyl starch, starch acetate, hydroxyethyl starch,
The photosensitive layer 1C is basically made of starch such as amine starch and phosphate starch and its derivatives (1O), soluble nylon which is polyamide, and soluble in a mixed solvent of water and alcohol such as methoxymethyl nylon (type 8 nylon). is a photoconductive pigment without a charge transport material. A coating solution is prepared by mixing and dispersing 7-thalocyanine fine particles with a diameter of l=lpm, an antioxidant, and a binder resin into 7-talocyanine fine particles with a diameter of 0.1 to 1 μm using a binder resin solvent. It is formed by applying a coating liquid to the intermediate layer, drying it, and subjecting it to heat treatment if necessary. In addition, when a photoconductive material and a charge transport substance are used together, a small amount of charge of 115 or less, preferably 1/1000 to 1/10 (weight ratio) of the photoconductive pigment and the photoconductive pigment A photosensitive layer is constructed by dispersing a photoconductive material consisting of a transport substance, an antioxidant, and a binder resin. In this embodiment, since the color toner image is superimposed on the photoreceptor, a photoreceptor having spectral sensitivity on the long wavelength side is required so that the beam from the scanning optical system does not block the color toner image. The light attenuation characteristics of the high γ photoreceptor of this example will be explained below. FIG. 5 is a graph showing the characteristics of a high γ photoreceptor. In the figure, Vl is the charged potential (V), vo is the initial potential before exposure (V), Ll is the amount of laser beam irradiation required for the initial potential v0 to attenuate to 475 (μJ/cm2),
L2 represents the amount of laser beam irradiation (μJ/cm'') required for the initial potential V0 to attenuate to 115. The preferable range of Lx/L+ is 1, 0≦L 2 / L 1≦1.5. In this example, v, -1000 (V), v o-
950 (V), L2/L+=1.2. Further, the potential of the photoreceptor in the exposed area is 10V. The photosensitivity at the position corresponding to the middle period of exposure where the optical attenuation curve attenuates the initial potential (Vo) to 1/2 is set as E17,
The photosensitivity at the position corresponding to the initial stage of exposure when the initial potential (vo) is attenuated to 9/10 is E,7. A photoconductive semiconductor is selected which, when bound, provides the relationship (E 1/2)/(E I/l.)≧2, preferably (E17□)/(E!/l.)≧5. Note that here, the photosensitivity is defined as the absolute value of the amount of potential drop with respect to the minute exposure amount. In the optical attenuation curve of the photoreceptor I, as shown in FIG. 5, the absolute value of the differential coefficient of the potential characteristic, which is photosensitivity, is small when the amount of light is small, and attenuates steeply as the amount of light increases. Specifically, as shown in FIG. 5, in the early stage of exposure, the light attenuation curve exhibits a light attenuation characteristic that is almost flat due to poor sensitivity characteristics for some period L8, but in the middle stage of exposure. From the beginning to the end, the sensitivity decreases and becomes ultra-high sensitivity, resulting in an ultra-high γ characteristic that decreases almost linearly. Specifically, the photoreceptor 1 has a voltage of +500 to +2000 V.
It is thought that high gamma characteristics are obtained by utilizing the avalanche phenomenon under highly charged conditions. In other words, the carriers generated on the surface of the photoconductive pigment in the early stage of exposure are effectively packed into the interface layer between the pigment and the coating resin, and the light attenuation is reliably suppressed. It is understood that an avalanche phenomenon occurs. The reference clock generation circuit 400 is a pulse signal generation circuit that generates a pulse signal with the same repetition period as the pixel clock, and is used in the image processing circuit 300, drivers 400 and 50.
0, thereby synchronizing the rotational speed of the photoreceptor l and the scanning of the scanning optical system 30. For convenience, this clock is referred to as the reference clock DCK. The photoreceptor 1 has a motor M1 mounted on its drive shaft. The motor Ml is rotationally controlled by a driver 500 to rotate the photoreceptor l at a predetermined speed. Further, the polygon mirror 34 has a motor M at its rotation center.
34 drive shafts are mounted. The motor M34 is configured to rotate at a predetermined speed by a driver 600. The drivers 500 and 600 include a reference clock generation circuit 400.
By inputting a reference clock DCK from , the rotational phases of the photoreceptor 1 and the polygon mirror 34 are synchronized. The scanning optical system 30 includes an index detection circuit (not shown), and detects the surface position of the polygon mirror 34 rotating at a predetermined speed based on the index signal from the index sensor 38. The device performs optical scanning using a modulated digital image density signal, which will be described later, by causing the semiconductor laser 31 to oscillate with a pulse-width modulation signal modulated with image density data from the page memory 310 (see Figure 1). , this laser beam is deflected by a polygon mirror 34 rotating at a predetermined speed, and f
The θ lens 35 and the cylindrical lenses 33 and 36 scan the uniformly charged upper surface of the photoreceptor l into a fine spot. A polygon mirror 34 is provided as a deflection optical system.
and an fθ lens 35 are provided, and a cylindrical lens 33.3 is provided as a surface tilt correction optical system using a polygon lens 34.
6 is provided. The semiconductor laser 31 is made of GaA IAs or the like, and since the color toner images are sequentially superimposed on the photoreceptor 1, exposure with light of a wavelength that is less absorbed by the colored toner is preferable, and the wavelength of the beam in this case is 800 nm. FIG. 1 is a block diagram showing an image processing circuit employed in an image forming apparatus according to an embodiment of the present invention. The image processing circuit 300 is a circuit that drives the semiconductor laser 31 with a modulation signal obtained by pulse width modulating image density data, and includes a page memory 31O1 readout circuit 320 and a binarization circuit 330. The page memory 310 is a RAM (RAM) that stores data in page units.
It has a capacity to store multivalued image density data equivalent to at least one page (one screen). In addition, if the device is used for color printing, multiple colors such as yellow, magenta,
This means that a page memory is provided that stores image density signals corresponding to cyan and black color components. Note that the image density data has been subjected to shading correction, density conversion,
This image has been subjected to γ correction, masking processing, etc. The readout circuit 320 uses the index signal as a trigger to send image density data to the binarization circuit 330 in synchronization with the reference clock DCH. The binarization circuit 330 is a circuit that generates a drive signal by changing the modulation rate for each scanning line and pixel, and selects image density data from the selection circuits 331 to 333 and the D/A conversion circuit 33.
4A-334D, comparators 335A-335D. Synthesis circuit 336, reference wave generation circuit 377, amplifier circuit 33
It consists of 88 to 338D. The binarization circuit 330 performs D/A conversion of the image data once in synchronization with the reference clock DCK using D/A conversion circuits 334A to 334D, and compares the density signal with the triangular wave to generate a pulse width modulated signal. This is the circuit to obtain. The selection circuit 331 alternately sends out image density data in units of one scanning line from the output terminals based on the index signal. The select circuit 332333 selects output terminals D1 to D2 based on the reference clock DCH.
Image density data corresponding to pixels is sent out alternately. On the other hand, the reference wave generation circuit 377 is a circuit that generates and outputs a triangular wave from the reference clock DC). The triangular waves are amplified by different amplification factors in amplifier circuits 338A to 338D, and the triangular waves with different amplitudes are sent to D/A conversion circuits 334A to 338D.
334D. D/A conversion circuits 334A to 334
D is an image density signal obtained by D/A converting image density data using a reference clock DCK, and is output to comparators 335A to 335A.
335D. In this example, the triangular waves with different amplitudes are arranged in order of magnitude: 334A>3340>334B>
334D was set. As described above, the comparators 3358 to 335D send out modulation signals with different duty ratios for image density signals of the same level by applying reference wave signals of different amplitudes. This prevents blurring in highlight areas and high density areas in halftone reproduction. The combining circuit 336 includes the aforementioned modulation circuits 334A to 334D.
This is a circuit that synthesizes modulated signals from. Next, the operation of the image processing circuit 300 will be explained. FIGS. 2(a)-'(f) and FIGS. 3(a)-(f) are time charts showing signals of each part of the image processing circuit of this embodiment. 2(a) and 3(a) respectively show the index signal, and FIG. 2(b) shows the reference clock D.
It shows CK. The readout circuit 320 reads data from the page memory 310 based on the reference clock DCH using the index signal as a trigger.
The image density data for the scanning line is read out and sent to the selection circuit 331. The selection circuits 331 to 333 are
When the first index signal is input, it enters the set state. That is, the select circuit 331 sends out image density data from the output terminal until the next index signal is input. Further, the select circuit 331 sends out high level data from the output terminal D2. This high level data is unrecorded density data corresponding to a white background. As a result, the select circuit 332 selects the reference clock DC.
Based on K, image density data and high level data corresponding to one pixel are alternately sent out from output terminals , D2 and input to D/A conversion circuits 335A and 335B. On the other hand, the select circuit 333 sends out high level data from output terminals D+ and D2. Note that the image density data indicates a light density on the high level side, and a deep density on the low level side. The modulation operations in the comparators 335A to 335D will be explained below. FIG. 2(C) shows the modulation operation in the comparator 335A, and FIG. 2(d) shows the modulation operation in the comparator 335Bl.
The second rgJ(e) shows the modulation operation in the comparator 335c, and FIG. 2(f) shows the modulation operation in the comparator 335D. In the figure, solid lines indicate reference waves, and broken lines indicate image density data (everything in the figure indicates the same intermediate density level) and high level data. In other words, the page memory 31
The image density signal from 0 is input to the comparators 335A and 335B, and the intermediate level image density signal is input every cycle of the reference clock DCK while inputting the first index signal.
Enter the information alternately. Further, triangular waves of different amplitudes are input to the comparators 335A and 335B as described above. This results in
Comparators: 335A and 335B are circuits that synthesize drive signals with different duty ratios for each cycle of the reference clock DCK, as shown in FIGS. 3(b) and 3(c), for the image source i signal of the same level. 336. On the other hand, a high-level image density signal (density data that is not recorded and corresponds to a white background at the time of output) is input to the comparators 335C and 335D. This allows comparator 3
35C and 335D input drive signals in the OFF state to a synthesis port M 336 as shown in FIGS. 3(d) and 3(e). In this way, the synthesis circuit 336 scans 242 scans with different duty ratios for image density data of the same level.
A drive signal corresponding to the number of seconds is input to the semiconductor laser 31. The scanning optical system 3o forms a latent image of varying toad diameter on the photoreceptor l by one scanning line using a drive signal based on an image density signal of the same level. When the next index signal is input to the image processing circuit 300, the image density signal from the page memory 310 is inputted alternately to the comparators 335C and 335D with intermediate level image density signals every cycle of the reference clock DCK. . Also, a comparator 335C. As mentioned above, triangular waves of different amplitudes are input to 335D. As a result, the comparators 335G and 335D operate as shown in FIG.
As shown in FIG. 3(e), drive signals with different duty ratios are input to the synthesis circuit 336 every cycle of the reference clock DCK. On the other hand, comparators 335A and 335B
A high-level image density signal is input to the input signal. As a result, the comparators 335A and 335B operate as shown in FIG.
b), as shown in FIG. 3(c), the off-state drive signal is output to the combining circuit 336. In this way, from the synthesis circuit 336, l scans 942 with different duty ratios are sent to image density data of the same level.
A drive signal corresponding to the number of seconds is input to the semiconductor laser 31. The scanning optical system 30 forms a latent image of only one scanning line on the photoreceptor l in proximity to the aforementioned scanning line and with a varying toad diameter, using a drive signal based on an image density signal of the same level. At this time, since the diameters of adjacent dot-shaped latent images on the photoreceptor l are changed, the latent images are not connected at the same time and no density jump occurs. In addition, there are no skips in the highlighted area and it is recorded reliably. The recording characteristics (broken line) are shown in FIG. 7(b). The dynamic range is significantly expanded compared to the comparative example in which pulse radiation modulation is used (solid line a in the figure). FIG. 8(b) shows a recording state in which toner adheres at an intermediate density. The image forming process of the image forming apparatus 100 will be described below. First, the photoreceptor 1 is uniformly charged by the scorotron charger 2. An electrostatic latent image corresponding to yellow is formed on the photoreceptor l.
Yellow data (8bi
It is formed by irradiation with a laser beam optically modulated according to the digital density data of t. The electrostatic latent image corresponding to yellow is reversely developed by the first developing device 4, and a first dot-shaped toner image (yellow toner image) with extremely high sharpness is formed on the photoreceptor 11. This first toner image is not transferred onto the recording paper P, but is charged again onto the photoreceptor l by the scorotron charger 2. Next, the laser light is optically modulated using magenta data (3-bit digital density data), and the modulated laser light is irradiated onto the photoreceptor 1 to form an electrostatic latent image. This electrostatic latent image is reversely developed by the second developing device 5 to form a second toner image (magenta toner image). In the same manner as described above, the third developing device 6 sequentially performs reversal development to form a third toner image (cyan toner image).
Three-color toner images are formed on the photoreceptor 1 in a sequential manner. Finally, a fourth toner image (black toner image) is formed, and four-color toner images are sequentially stacked on the photoreceptor 1. These four-color toner images are transferred onto recording paper P fed from a paper feeder by a transfer device (not shown) after the photoreceptor L is charged by a charger (this may be omitted). Ru. The recording paper P carrying the transferred toner image is separated from the photoconductor 1 by a separation electrode (not shown), is conveyed by a guide and a conveyor belt, is carried into a fixing device, is heated and fixed, and is discharged onto a paper discharge tray. . In this embodiment, four types of reference waves are set so that large and small dots are easily connected in the sub-scanning direction, and the amplitude of the reference waves is set to 334A>334B>3'34C>334D.
It is also possible to make it easier for large and small dots to connect in the main scanning direction. Also, the amplitude of the reference wave is 334A>334D>
334B>334C so that large and small dots can be easily connected in the diagonal direction. In addition, two types of reference waves are used so that the dots in the main scanning direction can be easily connected (33
4A = 334B > 334C = 334D), or set it so that the dots in the sub-scanning direction are easily connected (334A-
334C>334B=334D). In this case, the coefficient of the reference wave decreases, resulting in a slight decrease in gradation reproduction. In particular, for high-gamma photoreceptors, it is difficult to record at low illuminance areas (light intensity Ll) corresponding to highlights, and there are overlapping areas in adjacent dot exposures that correspond to high-density areas at a constant light intensity. If it is exceeded, there is a problem in that a density jump occurs due to the formation of a latent image due to a steep potential drop. This embodiment works extremely effectively on this high-gamma photoreceptor. According to the image forming apparatus 100 of the present embodiment, the photoreceptor has excellent high gamma characteristics, and this excellent high gamma characteristic is achieved by repeating the charging, exposure and development process many times from above the toner image. Even when the latent images are formed by overlapping them, a latent image is stably formed. In other words, even if a beam is irradiated from above the toner image based on a digital signal, a dot-shaped electrostatic latent image with high sharpness and no fringes is formed, and as a result, a toner image with high sharpness and gradation is formed. can be obtained. Furthermore, the image forming apparatus 100 of the present embodiment combines different reference waves and image density data to ensure reliable recording of highlight portions and prevent density jumps.

【Effect of the invention】

The present invention provides an image forming apparatus that forms an image on an image forming body by oscillating a semiconductor laser using a modulation signal obtained by modulating image density data with a reference wave signal. An image forming device that uses modulation using a reference wave whose amplitude is changed to prevent adjacent dots from being connected when forming a latent image, and can reproduce stable halftones without being affected by density jumps. were able to provide the following.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an image processing circuit employed in the image forming apparatus of this embodiment, and FIGS. 2(a) to (f) and 3(a) to (f) are images of this embodiment. A time chart showing the signals of each part of the processing circuit, FIG. 4 is a perspective view showing the schematic configuration of the image forming apparatus of this embodiment, FIG. 5 is a graph showing the characteristics of the high γ photoreceptor, and FIG. 6 is the high γ photoreceptor. FIG. 7, which is a cross-sectional view showing a specific example of the structure of the body, is a graph showing gradation characteristics when using each reference wave. Figure 8(a) and Figure 8(b)
1 is a diagram showing a recording state when each reference wave is used. ■... Photoreceptor 31 as an image forming body... Semiconductor laser 100... Image forming devices 331-333... Selector circuits 335A-335D... Comparator 377... Reference wave generation circuits 338A-338D ...Amplifier circuit DCK...Reference clock Figure 5

Claims (1)

[Claims] In an image forming apparatus that forms an image on an image forming body by oscillating a semiconductor laser using a modulation signal obtained by modulating image density data with a reference wave signal, the amplitude of the image density data is periodically changed in the main scanning or sub-scanning direction. An image forming apparatus characterized in that the image forming apparatus performs modulation using a reference wave.