JPH03293687A - Image forming device - Google Patents

Abstract

PURPOSE:To stably carry out latent image formation by incorporating direct current component in a modulation signal responding to white ground of an image density signal. CONSTITUTION:By impressing outputting signal from a variable DC power source 360 to a negative inputting terminal by a differensial amplifier 350, intensity modulation signal inputted to a positive terminal for the DC part has its level shifted and outputted, and this shifted level part responds to the white ground within the image. Thus, the intensified modulation signal synchronized to a picture element clock DCK is outputted by the differential amplifier 350 based on this, and this signal is made a drive signal to drive a semi conductive body laser 31 ON and OFF. Thus, the response against the drive signal is improved, the beam intensity variation which is varied due to environmental factors such as heat is decreased and the latent image formation can be stably carried out.

Description

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

The present invention relates to an image forming apparatus using electrophotography, and particularly to an image forming apparatus that forms an electrostatic latent image on a photoreceptor using exposure light that is beam-modulated using digital image data from a computer or the like.

[Background technology]

In recent years, in fields such as electrophotography, where an electrostatic latent image is formed on a photoreceptor and this latent image is developed to produce a visible image, it is possible to easily improve image quality, convert, edit, etc., and form high-quality images. Research and development of image quality forming methods using digital methods is being actively conducted. This image forming method forms a dot-shaped image by spot-exposing a digital image signal from a computer or a copy document onto a negatively charged photoreceptor using a light emitting element such as a laser, an LED array, or a liquid crystal shutter. do. As a scanning optical yarn feeding system that optically modulates light using a digital image signal, an apparatus has been proposed that uses a semiconductor laser and directly modulates the pulse width of the laser (Jikokai No. 62-39976). A beam modulated by a digital image signal has a round or elliptical brightness distribution approximating a normal distribution with the tails expanding left and right. For example, in the case of a semiconductor laser beam, usually
It has a luminance of 1 to 6 mW and an extremely narrow round or elliptical pulse width of 20 to 100 μm in either the main scanning direction or the sub-scanning direction or both on the photoreceptor. However, even if the electrostatic latent image formed by such a beam is developed preferably by reversal development to form a dot image, the image often has poor sharpness. In devices that perform multilevel modulation of the beam by direct intensity modulation or pulse width modulation, the beam intensity changes are small relative to the initial drive current of the laser semiconductor, and are easily fluctuated by environmental factors such as heat. , the linearity with respect to the drive current is poor, and there are problems with the response to the drive signal, and it is necessary to compensate with a circuit. For this reason, it has been difficult to increase the speed of intensity modulation and pulse width modulation as multilevel modulation methods. In particular, the poor linearity of intensity modulation has hindered its practical application. These photoreceptors generally have high sensitivity at the initial stage of exposure,
This is due to the fact that fluctuations in the photoreceptor are easily picked up and a sharp dot-shaped latent image is not formed.

【the purpose】

In view of the above-mentioned problems, an object of the present invention is to provide an image forming apparatus that can stably form a latent image by improving response to drive signals and reducing changes in beam intensity that vary due to environmental factors such as heat. Our goal is to provide the following.

[Means to solve the problem]

The present invention achieves the above object by using a modulation signal obtained by modulating an image density signal with a reference wave signal for a photoconductor having a light attenuation characteristic in which the photoconductor potential does not attenuate in the early stage of exposure but rapidly attenuates in the middle of exposure. The image forming apparatus forms an image by oscillating a semiconductor laser, and is characterized in that a modulation signal corresponding to a white background of the image density signal has a DC component. Furthermore, if the exposure intensity corresponding to a white background is approximately equal to or less than the half-reduced exposure light amount, further effects can be exhibited. Further, the image density signal is intensity-modulated with a reference wave signal. Further, the present invention is characterized in that a modulation signal obtained by amplitude modulating a high frequency signal using a modulation signal obtained by modulating the image density signal with a reference wave signal is used. Furthermore, the image density signal is pulse width modulated using a reference wave signal.

【Example】

Next, embodiments of the present invention will be described based on the accompanying drawings. First, the configuration of the image forming apparatus 100 of this embodiment will be described. FIG. 1 shows an embodiment of an image forming apparatus according to the present invention.
4 is a sectional view showing a specific example of the structure of a photoreceptor, and FIG. 4 is a sectional view showing a developing device applied to the image forming apparatus of this embodiment. The image forming apparatus 100 of this embodiment uniformly charges a photoreceptor and then performs processing such as shading correction, gradation correction, and masking correction on an image density signal that is A/D converted from a computer or a scanner. The analog image density signal obtained by D/A converting the processed digital image density signal is compared with the reference wave signal, and a dot is created by spot exposure which is intensity modulated or pulse width modulated based on the modulation signal obtained. A dot-like electrostatic latent image is formed, this is reversely developed with toner to form a dot-like toner image, the exposure and development steps are repeated to form a color toner image on the photoreceptor l, and the color toner is The image is transferred, separated, and fixed to obtain a color image. The image forming apparatus 100 includes a drum-shaped photoreceptor (hereinafter simply referred to as photoreceptor) 1 that rotates in the direction of an arrow;
A scorotron charger 2 that applies a negative charge to the top, a scanning optical system 3, and developing devices 4A, 4B, and 4C loaded with yellow, magenta, cyan, and black toners. 4D, pre-transfer charger 61. It consists of a scorotron transfer device 62, a separator 63, a fixing roller 64, a cleaning device 70, and a static eliminator 74. The main configuration of this embodiment will be explained below. As shown in FIG. 3, the photoreceptor l consists of a conductive support IA, an intermediate layer IB, and a photosensitive layer IC, and the thickness of the photosensitive layer IC is as follows.
Approximately 5 to 100 μm, preferably 10 to 50 μm
A drum-shaped conductive support IA made of aluminum with a diameter of 150 mm is used, and ethylene-
Intermediate layer I with a thickness of 0.1 μm made of vinyl acetate copolymer
A photosensitive layer IC having a thickness of 35 μm is provided on the intermediate layer IB. As the conductive support IA, aluminum, steel,
A drum made of copper or the like with a diameter of 150 mm is used, but it may also be a belt-like drum made of paper or plastic film with a metal layer laminated or vapor-deposited, or a metal belt such as a nickel belt made by electroplating. . In addition, the intermediate layer IB has a range of ±500~ as a photoreceptor.
It is desirable to have hole mobility so that it can withstand a high charge of ±2000V and, for example, in the case of positive charge, prevents electrons from being injected from the conductive support 1c and obtains excellent light attenuation characteristics due to the avalanche phenomenon. For example, the applicant's patent application No. 188975, which was previously proposed by the applicant, for the middle class IH.
It is preferable to add 10% by weight or less of the positively charged charge transport material described in the specification. 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 (Poval), 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, polyhydroquinpropyl methacrylate, etc. (6) Methylcellulose, ethylcellulose, carboxymethylcellulose, hydroxyethylcellulose, hydroquinpropylmethylcellulose, etc. Ether cellulose polymers (7) Polyethyleneimine polymers such as polyethyleneimine (8) Polyalanine, polyserine, poly-L-glutamic acid, poly-(hydroxyethyl)-L-1' rutamine, boly δ-carboxy Methyl-L-cysteine, polyproline, lysine-tyrosine copolymer, glutamic acid-lysine-alanine copolymer, polyamino acids such as fibroin, casein (9) starch acetate, hydroxyl ethyl starch, starch acetate, hydroxyethyl starch,
Starch and its derivatives (10) such as amine starch and 7-mole-7-ate starch (10) A polymer soluble in a mixed solvent of water and alcohol, such as soluble nylon and methoxymethyl nylon (type 8 nylon), which are polyamides Photosensitive layer 1G is basically In particular, phthalocyanine fine particles with a diameter of 0.1 to lpm made of a photoconductive pigment, an antioxidant, a binder resin, and a solvent for the binder resin are used without using a charge transport substance in combination. A coating solution is prepared by mixing and dispersing phthalocyanine particles with a diameter of 1 μm, and this coating solution is applied to the intermediate layer, dried, and heat-treated 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 3 does not block the color toner image. The scanning optical system 3 projects a beam that is optically modulated based on an image density signal of a predetermined bit onto the uniformly charged peripheral surface of the photoreceptor 1 to form an electrostatic latent image. The scanning optical system 3 includes a semiconductor laser 3L as shown in FIG.
Mirrors 32a to 32c1 Polygon mirrors 36, f
It consists of a θ lens 38, an inclination correction lens 37a that corrects inclination caused by the polygon mirror 36, and an index sensor 39. The index sensor 39 detects the surface position of the polygon mirror 36 rotating at a predetermined speed, and is used to perform optical scanning using modulated image data, which will be described later, by synchronizing the main scanning direction. 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. 2 is a schematic diagram showing the characteristics of a high γ photoreceptor. In the figure, Vl is the charged potential (V), V is the initial potential before exposure (V), and L is the amount of laser beam irradiation required for the initial potential V0 to attenuate to 415 (l J/cm).
), L2 represents the amount of laser beam irradiation (μJ/cm2) required for the initial potential V0 to attenuate to 115. The preferred range of L+/Lx is 1.0≦Ll/L2≦1.5. In this example, V, -1000 (V), vo-9
50(V), L,/Lz=1.2. Further, the potential of the photoreceptor at the exposed portion is 1OV. 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 defined as E1/2,
When the photosensitivity at a position corresponding to the initial stage of exposure when the initial potential (vo) is attenuated to 9/lO is E9/10, (E 1 / 2 )/(E 9 /10)≧2 preferably (E A photoconductive semiconductor is selected that provides the relationship: 1/2)/(E9/10)≧5. In addition, here,
Photosensitivity is defined by the absolute value of the amount of potential drop with respect to a small amount of exposure. In the light attenuation curve of the photoreceptor 1, as shown in FIG. 2, 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. 2, the light attenuation curve exhibits poor sensitivity characteristics and almost flat light attenuation characteristics for a short period of time3 at the beginning of exposure, but from L1 to L2 in the middle of exposure, - and becomes ultra-high sensitivity, resulting in ultra-high gamma characteristics that descend almost linearly. Specifically, the photoreceptor l is +500 to +2000
It is thought that high gamma characteristics are obtained by utilizing the avalanche phenomenon under highly charged V. In other words, 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 light attenuation is reliably suppressed, resulting in an extremely rapid avalanche in the middle stage of exposure. It is understood that a phenomenon occurs. The developing units 4A, 4B, 4C, and 4D have a common configuration as shown in FIG. 4, except for the color of developer loaded therein.The configuration of the developing device 40 will be described below as a representative. The developing device 40 includes a sleeve 43 that includes a magnet roller 44 having N and S poles that rotates in a developing tank formed by a lower casing 42 and an upper casing 41, and a fixing member 46 fixed to the upper casing 41 that is also pressed against the sleeve 43. a screw-shaped first scraper 45 made of a
and second stirring members 47 and 48, and a sleeve cleaning roller 49. The first stirring member 47 is shaped to be conveyed toward the front of the page, and the second stirring member 48 is shaped to be conveyed toward the back of the page. A wall is provided between the stirring members 47 and 48 so that the developer does not stagnate. In addition, the scraper 45
Instead, a thin layer forming means made of a magnetic plate or a magnetic bar may be provided. The sleeve cleaning roller 49 rotates in the direction of the arrow, and removes the developer that has passed through the development area and consumed toner into the sleeve 4.
Scrape from 3. Therefore, the developer conveyed to the development area can be replaced, and the development conditions are stabilized. The sleeve 43 is provided with a developing bias circuit 80 that applies a voltage having a DC bias component via a protective resistor (not shown) to prevent fogging. The developer used here is a two-component developer, and the toner has a particle size of 1 to 20 μm, and is a mixture of silica fine particles treated with a charge control agent or an amine compound and other additives. be done. Like the toner, it is advantageous for the carrier constituting the developer to have a small particle size from the viewpoint of image resolution and gradation reproducibility. For example, when the carrier in the developer layer has a small particle size of 5 to 50 μm, a magnetic brush of uniform height can be formed. The development bias circuit 80 supplies an alternating current bias to cause the toner to vibrate between the sleeve 43 and the photoreceptor 1 in the development region where the toner conveyed by the sleeve can be moved to the photoreceptor 1 by electrostatic force. It is equipped with an AC power supply and a high-voltage DC power supply that supplies DC bias. This example T
DC=800V, VAC=700V. It is 3KHz. In this way, the developing bias circuit 80
generates an oscillating electric field between the sleeve 43 and the photoreceptor 1, so the developer particles vibrate between the sleeve 43 and the photoreceptor 1, preventing the developer from coming into contact with the photoreceptor 1. Since a toner image is formed by toner particles on the photoreceptor l even if the toner image is removed, the previous toner image is not destroyed. In non-contact development, since the developer does not come into contact with the latent image, it is difficult to develop fine latent images, but by creating a steep latent image using a high γ type photoreceptor, it is possible to Developability can be improved by improving latent image formation. From this, the present invention using a high γ type photoreceptor is more effective not only in contact development but especially in non-contact development. 6- Next, the structure of the developer used in this example will be described. (Developer formulation) Toner charge control agent: 0.2 to 1.0 parts by weight Colorant: 3 to 15 parts by weight The above composition is mixed, ground, crushed, and then classified to obtain a toner with a weight average particle size of 3 μm. ing. Silica was used as an external additive for the toner. Further, the amount of charge of the toner is 20 μc/g. The following may be used to provide spectral characteristics that prevent the amount of transmitted light from the writing system from decreasing due to light absorption by toner. Benzidinne Yel
low) G (C, 1° 21090), Benzine Yellow GR (C, 1, 21100), Permanent Yellow O
-(Permanent Yellow) DHG (Hoechst product), Brilliant Carmine (Bri
lliant Carmine) 6 B (C, 1, 15
850), Rhodamine 6G Lake (C,
1,45160) Rhodamine B Lake (C, 1,451
70), Phthalocyanine Blue Non-Crystal (
Phthalocyanine Blue non C
rystal) (C, 1,74160), Phthalocyanine Green (C, 1,74260), Carbon Black, Fat Yellow 5G, Fat Yellow 3G1 Fat Red G1 Fat Red H
RR1 fat red 5B, fat black HB,
ZaponFast - Black B1 Zapon Fast - Black B1 Zapon Fast Blue HFL. Zapon First Red BB, Zapon First Red GE, Zapon First Yellow G
1 Quinacridone Red (C, 1,465000) carrier (resin coated carrier) Core 2 Ferrite coating resin: Styrene acrylic (4: 6) Magnetization 7 Qemu/g Weight average particle size 30 μm (spherical) Specific gravity
5.2g/c+n' specific resistance 1013Ω・
A developer was prepared by mixing at least 1 cm of the above composition. The configuration and operation of the first drive circuit of the scanning optical system will be described below. FIG. 5 is a block diagram showing the first drive circuit of the scanning optical system. As shown in FIG. 5, the drive circuit 300 of the scanning optical system 3 forms an intensity modulation signal based on an image density signal (image density data) from a computer or scanner, and drives the semiconductor laser 31 with the modulation signal. . Note that the laser drive circuit 300 may be provided with means for feeding back a signal corresponding to the amount of beam light from the semiconductor laser 31, and may be driven so that the amount of light is constant. As shown in FIG. 5, the drive circuit 300 includes a reference wave signal generation circuit 310, a buffer circuit 320, and a differential amplifier circuit 340.
．． 350, DC variable power supply 360 and D/A converter 330
It consists of The reference wave signal generation circuit 310 generates a triangular wave by an integrator including a variable resistor 311 and a capacitor 312. Furthermore, the triangular wave has a capacitor 313 and a protective resistor 31.
5 to the base terminal of the transistor 321. The reference wave signal generation circuit 310 has two variable resistors. That is, the variable resistor 311 is for adjusting the amplitude of the triangular wave. The variable resistor 314 is for adjusting the bias or offset of the triangular wave. The triangular wave passes through the buffer circuit 320 and is then sent to the differential amplifier 34.
Input to the positive input terminal of 0. The differential amplifier 340 is connected to the buffer circuit 320 as described above.
A reference wave passed through the reference wave and an analog density signal obtained by D/A converting a digital image density signal consisting of predetermined bits, for example 8 bits, by a D/A converter 330 are differentially amplified. The obtained intensity modulation signal is then output to the input terminal of the differential amplifier 350. The differential amplifier 350 has a negative input terminal connected to the variable DC power supply 3.
By applying the output signal from 60, the intensity modulation signal input to the positive terminal is level-shifted by the DC component and output. This level shift corresponds to the white background in the image. As a result, the differential amplifier 350 outputs an intensity modulated signal synchronized with the pixel clock DCK. This signal becomes a drive signal for driving the semiconductor laser 31 on and off. FIG. 6 is a time chart showing waveforms of various parts of the first drive circuit. In the figure, (a) is a reference pulse SCK, and the pulse SCK is synchronized with the pixel clock DCK. The signal shown by the broken line in (b) is an analog density signal that has been D/A converted after color correction and gradation correction, and the signal shown by the solid line is a reference wave signal that is the output signal from the buffer 320. (C) is a modulation signal whose intensity is modulated by the differential amplifier 340. The density signal corresponding to the recording pixel and the reference signal are synchronized, and an intensity modulation signal corresponding to the image density is generated. (d) is a signal obtained by level-shifting the intensity modulation signal. As described above, this level shift corresponds to the white background in the image. FIG. 7 is an explanatory diagram for explaining the relationship between the level-shifted intensity modulation signal from the drive circuit of the first embodiment and the amount of light emitted from the semiconductor laser. In the figure, the graph shows input/output characteristics showing the relationship between the input current to the semiconductor laser 31 and the amount of light emitted. A
is the region of spontaneous emission, a is the threshold current,
If a current larger than this threshold current α is inputted, stimulated emission will occur. In other words, it is a region where stimulated emission occurs. (a) shows the current input to the semiconductor laser 31. As described above, a current whose level is shifted by an amount corresponding to the white background in the image is input to the semiconductor laser. This improves the startup performance of the semiconductor laser 31. (b) shows the semiconductor laser 31 according to the input current.
It shows the amount of light emitted from the In the figure, the dashed-dotted line indicates the half-decreased exposure light amount of the high γ photoreceptor used in this example. In other words, since it is a high γ photoreceptor, the exposure amount of the semiconductor laser 31 is reduced by half, that is, the potential V. A latent image will not be formed if the exposure light amount is less than the amount of exposure light required to reduce the amount of light to 1/2 VO. For this reason, even if the level-shifted DC component is higher than the threshold current σ, it is made to correspond to a white background. In other words,
In this embodiment, the semiconductor laser 31 is oscillated even in the portion corresponding to the white background. (C) shows the exposure dot distribution recorded on the photoreceptor l. The haze + dot I; + right corner + W quarter position is shown by a broken line, and the portion beyond this is formed as a latent image due to the high-gamma photoreceptor characteristics. In other words, a latent image consisting of dots of different sizes according to the density signal is obtained, and a dot-like image density distribution formed by developing the latent image is shown. Furthermore, a sharp and small dot-shaped latent image can be formed. In reality, the exposure dot distribution is wider than the blur of the scanning optical system 3, so the diameter of the recording dots tends to be large in high density areas and small in the low density areas. The image forming process of the image forming apparatus 100 of this embodiment will be described below. Image exposure is performed by the beam light from the scanning optical system 3, and electrostatic latent images corresponding to each color are formed on the drum-shaped photoreceptor 1. Among the electrostatic latent images corresponding to the respective colors, the electrostatic latent image corresponding to yellow is formed by irradiation with laser light that is optically modulated by yellow data (digital density data). The electrostatic latent image corresponding to yellow is developed by the first developing device 4A, and a first dot-shaped toner image (yellow toner image) with extremely high sharpness is formed on the photoreceptor l. This first toner image is not transferred onto the recording paper P, but is charged again onto the photoreceptor 1 by the Sufrotron charger 2. Next, the laser light is optically modulated by magenta data (digital density data), and the modulated laser light is irradiated onto the photoreceptor l to form an electrostatic latent image. This electrostatic latent image is developed by the second developing device 4B to form a second toner image (magenta toner image). In the same manner as described above, the toner images are sequentially developed by the third developing device 4C to form a third toner image (cyan toner image), and a three-color toner image that is sequentially stacked on the photoreceptor 1 is formed. Finally, a fourth toner image (black toner image) is formed, and four-color toner images are sequentially stacked on the photoreceptor l. 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 without fringes can be formed, and as a result, a toner image with high sharpness can be obtained. can. These four-color toner images are transferred onto the recording paper P fed from the paper feeder by the action of the transfer device 62 after the photoreceptor 1 is charged by the charger 61 (this may be omitted). The recording paper P carrying the transferred toner image is separated from the photoreceptor 1 by a separation electrode 63, conveyed by a guide and a conveyor belt, carried into a fixing device 64, heated and fixed, and discharged onto a paper discharge tray. On the other hand, the toner remaining on the surface of the photoreceptor 1 after the transfer is removed by the blade, fur brush, or magnetic brush of the cleaning device 70, which was released during the toner image formation, and is not used during the toner image formation. The static electricity is removed by a lamp or a corona static eliminator 74, so that there is no problem in the formation of the next multicolor image. Note that the lamp and the static eliminator 74 may be located before cleaning. Other shapes can also be used as the reference wave. When a reference wave having one cycle of two triangular waves of different sizes so as to coincide with twice the synchronization of recording pixels was selected as the reference wave, a clear image with high gradation was obtained. Furthermore, when the same triangular wave was used (one cycle was twice the recording pixel cycle), a clear image with high resolution was obtained. In general, high-gamma photoreceptors tend to reproduce images with high contrast, and having multiple peaks as described above within one cycle of the reference wave is effective for improving gradation. When the same triangular wave was used as the reference wave (one period was the same as the recording pixel period), a clear image with high resolution was obtained. Further, the period of the reference wave can be set larger than the recording pixel synchronization. For example, similarly preferable results can be obtained even if the period is four times that of the recording pixels. In this embodiment, intensity modulation using the semiconductor laser 31 has been described, but the invention is not limited to this, and intensity modulation of concentration using other light emitting elements such as an LED array can be similarly performed. A similar effect can be obtained by optically modulating the signal. FIG. 8 is a block diagram showing a drive circuit of the second embodiment of the scanning optical system 3. The drive circuit 300 intensity-modulates an analog image density signal with a reference wave signal, modulates a carrier wave signal with the intensity-modulated signal, and applies a level-shifted modulation signal to the semiconductor laser 31. Signal generation circuit 310
, buffer circuit 320, differential amplifier 340, D/A converter 330, variable resistor 371, variable gain amplifier 370,
High band power amplifier 380, carrier signal generation circuit 380
It consists of The reference wave generation circuit 310 generates a triangular wave by an integrator including a variable resistor 311 and a capacitor 312. Further, the triangular wave is input to the base terminal of the transistor 321 via the capacitor 313 and the protective resistor 315. Reference wave generation circuit 310 has two variable resistors. That is, the variable resistor 311 is for adjusting the amplitude of the triangular wave. The variable resistor 314 is for adjusting the bias or offset of the triangular wave. The triangular wave (shown by a solid line in FIG. 9(b), which will be described later) passes through the buffer circuit 320 and is output to the positive input terminal of the differential amplifier circuit 340. On the other hand, the digital image density signal consisting of 8 bits is D/A
The analog image density signal D/A converted by the converter 330 is outputted to the negative input terminal of the differential amplifier 340 mentioned above. As a result, the differential amplifier 340 intensity-modulates the analog image density signal with the reference wave signal, and inputs the modulated signal to the variable gain amplifier 370 via the variable resistance element 371. This signal is a carrier wave having a frequency of 600 MHz or higher, which is amplitude-modulated by a variable gain amplifier 370 , and a high-frequency voltage amplified by a high-band power amplifier 380 is applied to the electrode of the semiconductor laser 31 . Note that the high-band amplifier 380 also has a function of level-shifting the modulated signal using a DC component. FIGS. 9(a) to 9(d) are time charts showing waveforms of various parts in the drive circuit of this embodiment. In the figure, (a) is a reference pulse SCK, and the pulse SCK is synchronized with the pixel clock DCK. The signal shown by the broken line in (b) is an analog density signal that has been D/A converted after color correction and gradation correction, and the signal shown by the solid line is a reference wave signal that is the output signal from the buffer 320. (c) is an intensity modulated signal intensity-modulated by the differential amplifier 340. The density signal corresponding to the recording pixel and the reference signal are synchronized, and an intensity modulation signal corresponding to the image density is generated. (d) shows a modulated signal obtained by amplitude modulating the carrier signal with the intensity modulated signal, and here shows that the level is further shifted by a high band power amplifier. FIG. 10 is an explanatory diagram for explaining the relationship between the amount of light emitted from the semiconductor laser and the level-shifted intensity modulation signal from the drive circuit of the second embodiment. A is a region where spontaneous emission occurs, α is a threshold current, and if a current larger than this threshold current σ is input, stimulated emission occurs. In other words, it is a region where stimulated emission occurs. (a)
indicates the current input to the semiconductor laser 31. As described above, a current whose level is shifted by an amount corresponding to the white background in the image is inputted to the semiconductor at once. This improves the startup performance of the semiconductor laser 31. (b) shows the amount of light emitted from the semiconductor laser 31 according to the conducting current. In the figure, the dashed-dotted line indicates the half-decreased exposure light amount of the high γ photoreceptor used in this example. In other words, since it is a high-gamma photoreceptor, no latent image is formed if the amount of exposure from the semiconductor laser 31 is the amount of light necessary for halving the exposure potential 1/2Vo. For this reason, even if the level-shifted DC component is higher than the threshold current a, it will correspond to a white background. In other words, in this embodiment, the semiconductor laser 31 is caused to oscillate even in a portion corresponding to a white background. (c) shows the exposure dot distribution recorded on the photoreceptor 1. FIG. The position of the half-decreased exposure light amount in this exposure dot distribution is indicated by a broken line, and a portion beyond this point is formed as a latent image due to the high γ photoreceptor characteristics. In other words, a latent image consisting of dots of different sizes according to the density signal is obtained, and a dot-like image density distribution formed by developing the latent image is shown. Furthermore, a sharp and small dot-shaped latent image can be formed. In reality, the exposed dot diameter is widened due to the blurring of the scanning optical system, so the recorded dot diameter tends to be large in high-density areas and small in low-density areas. Next, a third example will be shown. As shown in FIG. 11, the drive circuit 300 includes a reference wave signal generation circuit 310, a buffer circuit 320, and a differential amplifier circuit 34.
0.350, DC variable power supply 360 and D/A converter 33
Consists of 0. The reference wave signal generation circuit 310 generates a triangular wave by an integrator including a variable resistor 311 and a capacitor 312. Furthermore, the triangular wave has a capacitor 313 and a protective resistor 31.
5 to the base terminal of the transistor 321. The reference wave signal generation circuit 310 has two variable resistors. That is, the variable resistor 311 is for adjusting the amplitude of the triangular wave. The variable resistor 314 is for adjusting the bias or offset of the triangular wave. The triangular wave passes through the buffer circuit 320 and is sent to the comparator 3.
Input to the positive input terminal of 41. The comparator 341 is connected to the buffer circuit 32 as described above.
A digital image density signal consisting of a reference wave passed through 0 and predetermined bits, for example 8 bits, is D/A converted by a D/A converter 330.
Compare with the converted analog concentration signal. The obtained pulse width modulation signal is then output to the input terminal of the differential amplifier 350. The differential amplifier 350 has a negative input terminal connected to the variable DC power supply 3.
By applying the output signal from 60, the intensity modulation signal input to the positive terminal is level-shifted by the DC component and output. This level shift corresponds to the white background in the image. As a result, the differential amplifier 350 outputs a pulse width modulated signal having a DC component synchronized with the pixel clock DCK. This signal becomes a drive signal for driving the semiconductor laser 31 on and off. FIG. 12 is a time chart showing waveforms of various parts of the third drive circuit. In the figure, (a) is a reference pulse SCK, and the pulse SCK is synchronized with the pixel clock DCK. The signal shown by the broken line in (b) is an analog density signal that has been D/A converted after color correction and gradation correction, and the signal shown by the solid line is a reference wave signal that is the output signal from the buffer 320. (C) is a modulation signal subjected to pulse width modulation by the comparator 341. The density signal corresponding to the recording pixel and the reference signal are synchronized, and a pulse width modulation signal corresponding to the image density is generated. (d) is a signal obtained by level shifting the pulse width modulation signal. As described above, this level shift corresponds to the white background in the image. FIG. 13 is an explanatory diagram for explaining the relationship between the level-shifted pulse width modulation signal from the drive circuit of the third embodiment and the amount of light emitted from the semiconductor laser. In the figure, the graph shows input/output characteristics showing the relationship between the input current to the semiconductor laser 31 and the amount of light emitted. A
is the spontaneous emission region, α is the threshold current,
If a current larger than this threshold current a is inputted, stimulated emission will occur. In other words, it is a region where stimulated emission occurs. (a) shows the current input to the semiconductor laser 31. As described above, a level diaded current corresponding to the white background in the image is input to the semiconductor laser. This improves the startup performance of the semiconductor laser 31. (b) shows the semiconductor laser 31 according to the input current.
It shows the amount of light emitted from the In the figure, the dashed-dotted line indicates the half-decreased exposure light amount of the high γ photoreceptor used in this example. In other words, since it is a high γ photoreceptor, the amount of exposure from the semiconductor laser 31 is reduced by half, that is, the potential V. If the amount of exposure light is less than the amount of exposure light required to make the value 1/2Vo, no latent image will be formed. For this reason, even if the level-shifted DC component is higher than the threshold current σ, it is made to correspond to a white background. In other words,
In this embodiment, the semiconductor laser 31 is oscillated even in the portion corresponding to the white background. (C) shows the exposure dot distribution recorded on the photoreceptor 1. The position of the half-decreased exposure light amount in this exposure dot distribution is indicated by a broken line, and a portion beyond this point is formed as a latent image due to the high γ photoreceptor characteristics. In other words, although the exposure dot distribution is an elliptical beam, it has become wider due to the blurring of the scanning optical system.The position of the half-reduced exposure light amount in this exposure dot distribution is shown by a broken line, and the area beyond this point is is formed as a latent image due to the high γ photoreceptor characteristics. In other words, a latent image consisting of dots of different sizes according to the density signal is obtained, and a dot-like image density distribution formed by developing the latent image is shown. Furthermore, a sharp and small dot-like latent image can be formed. In reality, the exposure dot distribution is wider than the blur of the scanning optical system 3, so the diameter of the recording dots tends to be large in high density areas and small in the low density areas. In this embodiment described above, the print density can be changed depending on the print content. That is, by shifting the DC component of the reference wave, the concentration can be changed. Although the case where the DC component is larger than the threshold current a is shown, it can also be set smaller than this. In this embodiment, the noise is stable against changes in temperature and changes in the amount of optical feedback, and no spike-like noise occurs.

【Effect of the invention】

The present invention oscillates a semiconductor laser using a modulation signal obtained by modulating an image density signal with a reference wave signal for a photoconductor having an optical attenuation characteristic in which the photoconductor potential does not attenuate in the early stages of exposure but attenuates sharply in the middle of exposure. In an image forming apparatus that forms an image by using the image density signal, the modulation signal corresponding to the white background of the image density signal has a DC component, thereby improving the response to the drive signal and reducing the fluctuation due to environmental factors such as heat. It is possible to provide an image forming apparatus that can reduce changes in beam intensity and stably form latent images. In addition, the charging potential is 172 as the exposure intensity corresponding to a white background.
By setting the amount of light required to achieve this to less than or equal to the half-reduced exposure light amount, it was possible to provide an image forming apparatus capable of forming a sharp and small latent image. In addition, by making the image density signal into a modulated signal having a DC component using a reference wave, the noise becomes stable against temperature changes and changes in the amount of optical feedback, and no spike-like noise occurs, so latent image formation is possible. It was possible to provide an image forming apparatus that can stably perform the following steps. Furthermore, by using a modulation signal in which a high frequency signal is amplitude-modulated with a modulation signal in which an image density signal is modulated with the reference wave signal, the noise becomes stable against temperature changes and changes in the amount of optical feedback, as described above, and, Since no spike-like noise is generated, an image forming apparatus capable of stably forming a latent image can be provided. By pulse width modulating the image density signal with the reference wave signal, it was possible to provide an image forming apparatus that can form a sharp and small latent image.

[Brief explanation of the drawing]

3 is a sectional view showing a specific configuration example of a high γ photoreceptor; FIG. 4 is a sectional view showing a developing device applied to the image forming apparatus of this embodiment; Fig. 5 is a block diagram showing the first drive circuit of the scanning optical system, Fig. 6 is a time chart showing waveforms of each part of the first drive circuit, and Fig. 7 is a level shift from the drive circuit of the first embodiment. FIG. 8 is a block diagram showing the drive circuit of the second embodiment of the scanning optical system 3, and FIG. )～(d
) is a time chart showing the waveforms of various parts in the drive circuit of this embodiment, and Figure 1O is a time chart for explaining the relationship between the amount of light emitted from the semiconductor laser due to the level-shifted intensity modulation signal from the drive circuit of the second embodiment. An explanatory diagram, FIG. 11 is a block diagram showing the third drive circuit of the scanning optical system, and FIG. 12 is a time chart showing waveforms of each part of the third drive circuit.
FIG. 3 is an explanatory diagram for explaining the relationship between the amount of light emitted from the semiconductor laser and the level-shifted pulse width modulation signal from the drive circuit of the third embodiment.

Claims (5)

[Claims] (1) For a photoreceptor with a light attenuation characteristic in which the photoreceptor potential does not attenuate in the early stages of exposure but rapidly attenuates in the middle of exposure,
An image forming apparatus that forms an image by oscillating a semiconductor laser using a modulation signal obtained by modulating an image density signal with a reference wave signal, characterized in that the modulation signal corresponding to a white background of the image density signal has a DC component. image forming apparatus. (2) The image forming apparatus according to claim 1, wherein the exposure intensity corresponding to a white background is approximately equal to or less than the half-decreased exposure light amount. (3) The image forming apparatus according to claim 1 or 2, wherein the image density signal is intensity-modulated with a reference wave signal. (4) The image forming apparatus according to claim 3, characterized in that a modulation signal obtained by modulating the amplitude of a high frequency signal using a modulation signal obtained by modulating the image density signal with a reference wave signal is used. (5) The image forming apparatus according to claim 1 or 2, wherein the image density signal is pulse width modulated using a reference wave signal.