JPH0457075A - Image forming device - Google Patents

Abstract

PURPOSE:To prevent a noise by halo which occurs especially in the case of shaping a beam to the beam with light intensity distribution appropriate for a pulse width modulation system and an intensity modulation system by making specified relation hold among a contrast between maximum exposure intensity in an exposure dot and the maximum exposure intensity of the halo of the exposure dot, half exposing light quantity and the number of recorded images where the halo extends. CONSTITUTION:A beam is efficiently fetched while keeping the necessary diameter thereof by a collimator lens 32 and the intensity distribution of the beam is adjusted by a density filter 34 which is provided in front of a prism and whose transmissivity in a center part is low and whose transmissivity in a peripheral part is high, then the form of the beam is shaped on the upper surface of a photosensitive body 1. An index sensor 39 senses the beam and outputs a current, which is outputted as an index signal by an index detection circuit. The surface position of a rotating polygon mirror 36 is detected, and optical scanning by a modulated digital image density signal is performed. Supposing that the contrast between the maximum exposure intensity in the exposure dot and the maximum exposure intensity of the halo of the exposure dot is R, the half exposing light quantity for having the potential of the photosensitive body is P1/2, and the number of the recorded images where the halo extends is (n), the condition of nXRXP1<P1/2 is satisfied.

Description

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

The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus that forms an image on a photoreceptor by reducing a beam from a laser light source to a minute diameter.

[Background technology]

As a scanning optical system employed in a digital image forming apparatus, there is a device that forms an electrostatic latent image on a photoreceptor using a rask scanning method in which laser light is deflected and controlled by a scanning optical system. The scanning optical system is provided with a coherent laser light source, and the diverging light flux from the laser light source is modulated by an optical modulator with an image signal from an image signal generator, for example, a page memory, under the control of a signal control system. The beam is incident on, for example, a deflector through the first optical system and is deflected by the deflector,
By scanning a uniformly charged photoreceptor with a scanning lens focused on a minute spot, high brightness and minute spora are obtained. However, the emission distribution of the laser light source has a Gaussian distribution, which is not a beam intensity distribution suitable for the above-mentioned modulation method for reproducing halftones. In other words, the emission distribution (far fill pattern) of a typical laser light source has a full width at half maximum of θ1-10~30° within the bonded surface, and a full width at half maximum of θ2-30 to 60' in the direction perpendicular to this. It is. The beam irradiated from the above-mentioned scanning optical system has a round or elliptical brightness distribution approximating a normal distribution with the tails widening from side to side. The pulse width in the main scanning direction or the sub-scanning direction or both has an extremely narrow round or elliptical shape of 20 to 100 μm. When compressed to reduce the beam diameter to a size suitable for an optical modulation method for reproducing halftones, a halo is generated, and this halo forms a latent image equivalent to noise on the photoreceptor. There's a problem. In particular, the photoconductors used in conventional analog copiers, etc., which exhibit so-called low-gamma light attenuation characteristics, in which light attenuation is large in the early stage of exposure and slow in the middle of exposure, are subject to the above-mentioned problems. The problem is significant. Examples of low-γ type photoreceptors include single-layer types such as Se and CdS;
It is known that it has a two-layer structure consisting of a charge generation layer and a charge transport layer, which is commonly used in OPC, but many photoreceptors exhibiting the above semiconductor characteristics exhibit better performance in a low electric field than in a high electric field. Generally, the photosensitivity is low, and the photosensitivity decreases as the potential decreases due to an increase in the amount of light. For this reason, in analog copying machines, this type of photoreceptor was mostly used for gradation reproduction. If an electrostatic latent image is formed on a low-gamma photoreceptor with the beam irradiated from the above-mentioned scanning optical system, the photoreceptor will generally have high sensitivity at the initial stage of exposure, will easily pick up fluctuations in the photoreceptor, and will produce sharp dots. This means that no latent image is formed. Even if an electrostatic latent image formed by such a beam is developed preferably by reversal development to form a dot image, there is a problem in that the image often has poor sharpness.

【the purpose】

In view of the above problems, an object of the present invention is to provide a beam modulation method,
Particularly, it is an object of the present invention to provide an image forming apparatus that aims to prevent noise due to a halo that occurs when a beam is shaped into a beam with a light intensity distribution suitable for a pulse width modulation method and an intensity modulation method. [Means for Solving the Problems] This invention achieves the above object by exposing a beam emitted from a laser light source through a scanning optical system equipped with a compression means for compressing the beam in the main/sub-scanning direction. In an image forming apparatus that forms an electrostatic latent image on a γ photoreceptor and performs reversal development, an image in which the contrast between the maximum exposure intensity at an exposure dot and the maximum exposure intensity of a halo of the exposure dots is reduced by R1, the photoreceptor potential by half. It is characterized by satisfying the condition RXP+<P+/□, where the number is n. The light amount contrast R here indicates the ratio between the peak P1 in the exposure distribution in dot exposure and the peak light amount of a halo generated due to beam shaping.

【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 explained. FIG. 1 is a perspective view showing a schematic configuration of an image forming apparatus according to an embodiment of the present invention. The image forming apparatus 100 of this embodiment causes a semiconductor laser 31 to oscillate with a modulation signal modulated by an image density signal from a page memory (not shown), and deflects this laser light with a polygon mirror 36 rotating at a predetermined speed. , f/θ lens 37
Then, the cylindrical lenses 35a and 35b scan the uniformly charged upper surface of the photoreceptor l by narrowing it down to a minute spot. A semiconductor laser 31 is provided as a coherent light source, a collimator lens 32 is provided as a light emitting optical system, a polygon mirror 36 and an f/θ lens 37 are provided as a deflection optical system, and a cylindrical lens 35a is provided as a surface tilt correction optical system using the polygon lens 36. It is equipped with 35b. The semiconductor laser 31 is made of GaA IAs, etc., has a maximum output of 5 mW, has an optical efficiency of 25%, and has a divergence angle of 8 to 16 degrees in the direction parallel to the junction surface and 20 to 36 degrees in the direction perpendicular to the junction surface.
°. In addition, in the image forming apparatus 100 of this embodiment, since the color toner images are sequentially superimposed on the photoreceptor 1, it is preferable to expose with light of a wavelength that is less absorbed by the colored toner, and in this case, the wavelength of the beam is 800 nm. It is. The collimator lens 32 is a lens that efficiently takes out the beam with a required diameter, and has a numerical aperture N, Al; t: 0.33.
The lens is composed of a lens with a transmittance of 97% or more, and has good spherical aberration and zyaconditioning. The density filter 34 is housed in a fixing means and provided in front of the prism 33 as shown in FIG. 1, and is a density filter having low transmittance at the center and high transmittance at the periphery. Thereby, the intensity distribution of the beam can be adjusted, and the beam is shaped into a narrow elliptical beam shape at ①: on the photoreceptor l. Note that the density filter 34 may be provided anywhere as long as it is between the collimator lens 32 and the polygon mirror 36. The deflection optical system focuses the beam (luminous flux) and reduces the perapearl sum and astigmatism difference in order to flatten the scanning surface. The polygon lens 36 has 8 polygon surfaces, and 16
By rotating at a rotation speed of 535.4 rpm, a beam is scanned over one surface of the photoreceptor. Note that the mirror is not limited to a polygon mirror, and any mirror that performs a similar function may be used. The fθ lens 37 reduces the perapard sum and astigmatism difference and eliminates field curvature in order to flatten the scanning surface. The correction optical system is provided with cylindrical lenses 35a and 35b before and after the polygon mirror 36 to reduce the pitch unevenness of the scanning line due to the surface tilt error of the polygon mirror 36, and the polygon tilt angle is 120 seconds PP. The color correction rate is l/20 or more, and the cylindrical lens 35b focuses the beam onto the upper surface of the photoreceptor 1. The synchronization system makes the beam from the deflection optical system enter the index sensor 39 via a reflection mirror (not shown). The index sensor 39 outputs a current in response to the beam, and the current is converted into current/voltage (A/V) by an index detection circuit (not shown) and output as an index signal. The surface position of the polygon mirror 36, which rotates at a predetermined speed, is detected using this index signal, and optical scanning is performed using a modulated digital image density signal, which will be described later, using a rask scanning method depending on the period in the main scanning direction. Scanning frequency is 2204.72Hz, effective printing width is 297mm
This is the effective exposure width of 306 mm or more. The above is the general configuration of the scanning optical system 30 of this embodiment, and the details of the main part configuration of this embodiment are described below in FIGS. 2(a) to 2(a).
This will be explained based on FIG. 2(c). FIGS. 2(a) and 2(b) are plan views showing a density filter, which is a main component of this embodiment, and FIG. 2(c) is a graph showing the transmittance of the density filter according to this embodiment. can be,
FIG. 5(a) is an exposure intensity distribution diagram of a focused spot,
Figures 5(b) and (c) are similar to Figures 2(a) and (b).
FIG. 3 is a schematic diagram showing a recording trace formed on a photoreceptor by a condensed light spot in response to the above. The transmittance of the density filter 34 decreases near the center, as shown in FIG. 2(c). Scanning optics 3 mentioned above
0, when the density filter shown in FIG. 2(a) is used, the diameter of the beam imaged on the photoreceptor 1 by the deflection optical system is 35±7 μm in the main scanning direction, and is not used in either the sub-scanning direction. It was compressed by about 20% compared to the original case, and as a result, the writing density in the main and sub-scanning directions was set to 800 dpi. This makes it suitable for employing intensity modulation as a multilevel recording method. However, in the scanning optical system 30 of this embodiment, a concentric halo is generated in the direction in which the beam is compressed. Next, a density filter suitable for pulse width modulation as a modulation method will be explained. The density filter has a stripe shape as shown in FIG. 2(b), and is provided between the thin linear lens 35a and the collimator lens 32, and its transmittance is shown in FIG. 2(c).
), the transmittance increases near the center. Due to the above-described configuration of the scanning optical system 30, the beam diameter on which the photoreceptor l is imaged by the deflection optical system is 20±7 μm in the main scanning direction and 40±7 μm in the sub-scanning direction, and only the main scanning direction is compressed. The writing density in the main and sub-scanning directions was set to 800 dpi. This makes it suitable for adopting pulse width modulation as a multilevel recording method. In addition, by using these density filters, the main/
Since the beam shape in the sub-scanning direction can be adjusted, variations in the scanning optical system including the semiconductor laser can be corrected.
Also, the shape can be adjusted against thermal fluctuations. The main configuration of this embodiment will be explained below. FIG. 4 is a sectional view showing a specific example of the structure of the high-gamma photoreceptor. Photoreceptor 1 includes a conductive support IA, an intermediate layer IB, and a photosensitive layer I.
The thickness of the photosensitive layer 1c is approximately 5 to 100 μm, preferably 10 to 50 μm, and the diameter is 150 μm.
An intermediate layer IB made of ethylene-vinyl acetate copolymer with a thickness of 0.1 μm is formed on the support IA using a drum-shaped conductive support IA made of aluminum, and a film is formed on the intermediate layer IB. It is constructed by providing a photosensitive layer 1c with a thickness of 35 μm. 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 by laminating or vapor depositing a metal layer on paper or plastic film, 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 high charging of ±2000 V, for example, in the case of positive charging, prevents injection of electrons from the conductive support IC and obtains excellent light attenuation characteristics due to the avalanche phenomenon. Therefore, for the intermediate layer IB, for example, the present applicant has proposed patent application No. 61-1'889.
The positively charged charge transport substance described in the specification of No. 75 was
It is preferable to add 0% by weight or less. 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, and polyhinylethyl ether (2) Polyvinylamine, poly-N-vinylimidazole, polyvinylpyridine (quaternary salt), polyvinylpyrrolidone, and vinylpyrrolidone - Nitrogen-containing vinyl polymers such as vinyl acetate pyrrolidone (3) Polyether polymers such as polyethylene oxide, polyethylene glycol, polypropylene glycol, etc. (4) Polyacrylic acid and its salts, polyacrylamide, poly-β-hydroxyethyl acrylate, etc. Acrylic acid polymers (5) Methacrylic acid polymers such as polymethacrylic acid and its salts, polymethacrylamide, polyhydroxyglobil methacrylate, etc. (6) Methylcellulose, ethylcellulose, carboquine methylcellulose, hydroxyethylcellulose, hydroquine Ether cellulose polymers such as propyl methylcellulose (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-
Polyamino acids such as lysine-alanine copolymer, silk fibroin, and casein (9) Starch acetate, hydroxyl ethyl starch, starch acetate, hydroxyethyl starch,
Starch and its derivatives such as amine starch and phosphate starch (10) Polyamides such as soluble nylon, methoxymethyl nylon (type 8 nylon) and other polymers soluble in a mixed solvent of water and alcohol The photosensitive layer IC is basically is a 0.1-1 μm diameter phthalocyanine fine particle made of a photoconductive pigment, an antioxidant, a binder resin, and a solvent for the binder resin, without using a charge transporting substance in combination. The intermediate layer is mixed and dispersed to prepare a coating liquid, and this coating liquid 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/lo (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, the beam from the scanning optical system 30 passes through the color toner image, and in order to cause a potential drop, a photoreceptor is required that has spectral sensitivity on the long wavelength side. . FIG. 3 is a schematic diagram showing the characteristics of the high γ photoreceptor employed in this example. In the figure, Vl is a charging potential (v), ■. is the initial potential (V) before exposure, and Ll is the initial potential V. The amount of irradiation light required for attenuation to 415 (/'l/cm2), L2 is the initial potential V. represents the amount of irradiation light (μJ/cm2) required for attenuation to 115. The preferred range of L I / L 2 is 1.0≦Ll/L2≦1.5. In this example, V, = 1000 (V), Vo
= 950(v), L +/L 2 = 1.2. Further, the potential of the photoreceptor at the exposed portion is 1OV. The light sensitivity at the position corresponding to the middle stage of exposure where the light attenuation curve attenuated the initial potential (Vo) to 1/2 is expressed as El/□, and the exposure at which the initial potential (Vo) was attenuated to 9/10. The light sensitivity at the position corresponding to the initial stage is E,7. A photoconductive semiconductor is selected that provides the relationship (E, 7 □)/(E , /, .)≧2, preferably (E , /2)/(E , /□.) ≧5 when closed. Note that here, the photosensitivity is defined as the absolute value of the amount of potential drop with respect to the minute exposure amount. Next, we will examine the exposure conditions for salt content using the density filter shown in FIG. 2(a) and having the exposure intensity distribution shown in FIG. 5(b). In this case, the forced exposure contrast R with the halo is l/4
It was 0. Further, the number of pixels n spanned by the halo was 12. Therefore, if solid printing is performed during laser scanning,
The cumulative amount of light irradiated on areas other than recording pixels is P2-nX
In case of RX P + Tona, n-12, R= 1/4
0, but P2-12/40 x PI. Table 1 shows the results of image evaluation by varying the Pl exposure intensity. (Table 1) Exposure conditions for salinity were investigated using the density filter shown in FIG. 2(b) and having the exposure intensity distribution shown in FIG. 5(C). In this case, the forced exposure contrast R with the halo is 1/2
It was 0. Further, the pixel n spanned by the halo was 2. Therefore, if a solid image is printed during laser scanning,
The cumulative amount of irradiation light for areas other than recording pixels is P2-nxRxP□
In this case n = 2. Since R= 1/20, P
2"" 2/20x P l. Table 2 shows the results of image evaluation by varying the Pl exposure intensity. (Table 2) Good images are indicated by ◯, images which are somewhat poor are indicated by △, and images which are poor are indicated by ×. Here, P17□ is the photoreceptor potential V. This is the half-reduced exposure light amount required to reduce the value to l/2. Regarding P2, when P17□ was exceeded, the image became blurry and the resolution was significantly reduced, and regarding Pl, when P1/2 or less, a sufficient image was not formed. From the above results, in this example, the above exposure conditions are n x P 2 < P + / 2 from the conditions of the non-image area.
RX P and Pl satisfy the condition P+>Pl/□ from the condition that a latent image is formed. In the light attenuation curve of the photoreceptor 1, as shown in FIG. 3, 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. 3, the light attenuation curve exhibits a light attenuation characteristic that is almost flat for a short period L1 during the initial period of exposure, with poor sensitivity characteristics, but from the middle period L1 to R2 of exposure, - This results in ultra-high sensitivity and 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 high charging of 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. As described above, an electrostatic latent image is formed on the high-gamma photoreceptor 1 by exposing the beam emitted from the laser light source 31 through the scanning optical system 30 equipped with a density filter that compresses it in the main/sub-scanning direction. In an image forming apparatus 100 that performs reversal development, the contrast between the maximum exposure intensity in the exposure drums 1 to 1 and the maximum exposure intensity of the halo of the exposure dots is reduced by half the exposure light amount P17 to reduce the photoreceptor potential by half R1 Recording across the halo When the number of images is n, by satisfying the condition n Noise due to the resulting halo can be prevented.

【Effect of the invention】

In the present invention, a beam emitted from a laser light source is exposed through a scanning optical system equipped with a compression means that compresses the beam in the main/sub-scanning direction, and an electrostatic latent image is formed on a high-gamma photoreceptor for reversal development. In an image forming apparatus, the contrast between the maximum exposure intensity of an exposure dot and the maximum exposure intensity of a halo of the exposure dot is determined by the condition n x Rx P l < P l/□, where n is the number of recorded images spanned by the halo before R1. By satisfying the above requirements, it is possible to provide an image forming apparatus that can prevent noise due to a halo that occurs when a beam is shaped into a beam with a light intensity distribution suitable for a beam modulation method, particularly a pulse width modulation method and an intensity modulation method. Ta.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing a schematic configuration of an image forming apparatus according to an embodiment of the present invention, and FIG. (b) is a plan view showing the density filter which is the main part of this embodiment, FIG. 2(c) is a graph showing the transmittance of the density filter according to this embodiment, and FIG. 3 is adopted in this embodiment. 4 is a cross-sectional view showing a specific configuration example of the high γ photoreceptor, and FIG. 5(a) is an energy distribution diagram of a focused spot. (b), (c)
FIG. 2 is a schematic diagram showing a recording trace formed on a photoreceptor by a focused spot. l...High γ photoreceptor 30...Scanning optical system 31...Semiconductor laser 3
4... Density filter 100 as compression means... Image forming apparatus

Claims (1)

[Claims] Exposure is performed through a scanning optical system provided with a compression means for compressing a beam emitted from a laser light source in the main/sub-scanning direction,
In an image forming apparatus that forms an electrostatic latent image on a high-gamma photoreceptor and performs reversal development, the contrast between the maximum exposure intensity at an exposure dot and the maximum exposure intensity of a halo of the exposure dots is set to R, and the potential of the photoreceptor is halved. The half-reduced exposure light amount is P_1
_/_2, an image forming apparatus characterized in that, where n is the number of recorded images spanned by the halo, the following condition is satisfied: n×R×P_1<P_1_/_2.