JP6299946B2 - Image forming method and image forming apparatus - Google Patents

Description

  The present invention relates to an image forming method and an image forming apparatus.

Conventionally, in an image forming method in which an image is formed by an electrostatic latent image, a static beam spot defined by 1 / e ² of the maximum exposure intensity is selected to reduce the beam spot from the exposure apparatus in order to improve image quality. There is known a method for reducing the diameter.

  When the beam spot from the exposure apparatus is narrowed to achieve high image quality, for example, if the writing density is 600 dpi, the latent image carrier is formed with a static beam spot diameter smaller than the minimum pixel size of 42 μm. It is necessary to scan.

  The diameter of the static beam spot is determined by an optical system provided in the exposure apparatus. Here, in order to reduce the diameter of the static beam spot to a small size, two or more lenses are required in the scanning optical system disposed after the optical deflector, which complicates the configuration of the optical system.

  Further, in order to reduce the diameter of the static beam spot to a small size, the configuration of the optical system is complicated, so that extremely strict accuracy is required for lens processing errors and assembly errors.

  As a technique for improving the image quality in the image forming method, it is possible to realize the saving of the amount of image forming material consumed when outputting the processing target image, and the image in the case where the processing for partially reducing the density is performed. Has been disclosed (see Patent Document 1).

  By the way, what is required to form a high-quality image is a high-quality electrostatic latent image. In other words, from the viewpoint of forming an electrostatic latent image, the diameter of the static beam spot is only one factor for controlling the electrostatic latent image, and also depends on the exposure pattern, the characteristics of the latent image carrier, etc. The image quality of the electrostatic latent image can be controlled.

  Therefore, by controlling the exposure pattern, it is possible to obtain an electrostatic latent image with high image quality comparable to that formed by a small-diameter static beam spot.

  An object of the present invention is to provide an image forming method capable of forming an electrostatic latent image for forming a high quality image without reducing the diameter of the static beam spot.

In the present invention, the length in the main scanning direction of the static beam spot defined by 1 / e ² of the maximum exposure intensity is ω, the minimum pixel determined by the writing density is D, and the number of pixels per area is N (N is N Natural number), a plurality of pixels arranged in the main scanning direction satisfy 1.5 × N × D ≦ ω ≦ 2 × N × D or 2 × N × D <ω <4 × N × D. For areas satisfying 2 × N × D <ω <4 × N × D divided into areas for each number of pixels, exposure is performed for the first time with the first irradiation light amount, and a static beam spot is formed. For an area that is exposed by the first scanning method to be scanned and satisfies 1.5 × N × D ≦ ω ≦ 2 × N × D, the first irradiation light amount is stronger than the first irradiation light amount, and the first The exposure is performed by a second exposure method in which exposure is performed for a second time equal to or shorter than the time and a static beam spot is scanned.

  According to the present invention, it is possible to form an electrostatic latent image for forming a high-quality image without reducing the static beam spot diameter.

1 is a central sectional view showing an image forming apparatus according to an embodiment of the present invention. It is a schematic diagram which shows the corotron type charging device of the said image forming apparatus. FIG. 2 is a schematic diagram illustrating a scorotron charging device of the image forming apparatus. It is a schematic diagram which shows the optical scanning device of the said image forming apparatus. It is XY top view which shows the scanning optical system of the said optical scanning device. It is a ZX plan view showing a scanning optical system of the optical scanning device. It is a schematic diagram which shows the profile of a static beam spot in case a static beam spot diameter is 55 micrometers. It is a schematic diagram which shows the profile of a static beam spot in case a static beam spot diameter is 70 micrometers. It is a schematic diagram showing the difference in the size of the electrostatic latent image due to the difference in the static beam spot diameter by the image forming apparatus and the exposure method. It is a schematic diagram showing the difference in the size of the electrostatic latent image due to the difference in the static beam spot diameter by the image forming apparatus. It is a schematic diagram showing the difference in the size of the electrostatic latent image when the writing density by the image forming apparatus is 1200 dpi. It is a schematic diagram which shows the relationship between the pixel by the said image forming apparatus, and the exposure method. It is a schematic diagram which shows the relationship between the pixel and exposure position by the said image forming apparatus. It is a schematic diagram of the reference example which shows the relationship between the pixel and exposure position by the said image forming apparatus. It is a schematic diagram of another reference example which shows the relationship between the pixel and exposure position by the said image forming apparatus.

  Embodiments of an image forming method according to the present invention and an image forming apparatus according to the present invention that executes the image forming method will be described below with reference to the drawings.

First, the configuration of the image forming apparatus according to the present invention will be described.

  FIG. 1 is a central sectional view showing an image forming apparatus according to an embodiment of the present invention. FIG. 1 shows a schematic configuration of a laser printer 1000 as an image forming apparatus according to the present invention.

  The laser printer 1000 includes an optical scanning device 1010, a photosensitive drum 1030, a charging device 1031, a developing device 1032, a transfer device 1033, a charge removal unit 1034, a cleaning unit 1035, and a toner cartridge 1036.

  The laser printer 1000 also includes a paper feed roller 1037, a paper feed tray 1038, a fixing device 1041, a paper discharge roller 1042, a paper discharge tray 1043, a communication control device 1050, and a printer control device 1060.

  Note that the above-described components of the laser printer 1000 are accommodated in predetermined positions inside the printer housing 1044.

  The communication control device 1050 controls bidirectional communication with a host device (for example, an information processing device such as a personal computer) via a network or the like.

The printer control device 1060 has a CPU (Central
Processing Unit) and ROM (Read Only Memory). The printer control device 1060 includes a RAM (Random Access Memory) and an A / D (Analog / Digital) converter. Here, the printer control device 1060 comprehensively controls each unit in response to a request from the host device, and sends image information from the host device to the optical scanning device 1010.

  Here, the printer control device 1060 has functions of a dividing unit and a determining unit in the image forming apparatus according to the present invention.

  The ROM stores a program written in a code readable by the CPU and various data used when executing this program.

  The RAM is a temporarily writable memory for work of the CPU.

  The A / D converter converts an analog signal into a digital signal.

  The photosensitive drum 1030 is a latent image carrier of a cylindrical member, and a photosensitive layer is formed on the surface thereof. That is, the surface of the photoconductor drum 1030 is a scanned surface. The photosensitive drum 1030 is rotated in the direction of the arrow in FIG. 1 by a driving mechanism (not shown).

  The charging device 1031 uniformly charges the surface of the photosensitive drum 1030.

  FIG. 2 is a schematic diagram showing a corotron charging device of the image forming apparatus. FIG. 3 is a schematic diagram showing a scorotron charging device of the image forming apparatus. Here, the charging device 1031 may be the corotron type charging device shown in FIG. 2, or may be the scorotron type charging device shown in FIG. 3 as an example, or a roller type charging device (not shown). Also good.

  The optical scanning device 1010 scans and exposes the surface of the photosensitive drum 1030 charged by the charging device 1031 with a light beam modulated based on image information from the printer control device 1060, and exposes the surface of the photosensitive drum 1030. An electrostatic latent image corresponding to the image information is formed.

  Here, the optical scanning device 1010 corresponds to an exposure unit in the image forming apparatus according to the present invention.

  The electrostatic latent image formed by the optical scanning device 1010 moves in the direction of the developing device 1032 as the photosensitive drum 1030 rotates. Details of the optical scanning device 1010 will be described later.

  The toner cartridge 1036 stores toner (developer). The toner is supplied from the toner cartridge 1036 to the developing device 1032.

  The developing device 1032 attaches the toner supplied from the toner cartridge 1036 to the latent image formed on the surface of the photosensitive drum 1030, and visualizes the electrostatic latent image. Here, an image to which toner is attached (hereinafter also referred to as “toner image”) moves in the direction of the transfer device 1033 as the photosensitive drum 1030 rotates.

  Recording paper 1040 is stored in the paper feed tray 1038. A paper feed roller 1037 is disposed in the vicinity of the paper feed tray 1038.

  The paper supply roller 1037 takes out the recording paper 1040 from the paper supply tray 1038 one by one. The recording paper 1040 is sent out from the paper feed tray 1038 toward the gap between the photosensitive drum 1030 and the transfer device 1033 in accordance with the rotation of the photosensitive drum 1030.

  A voltage having a polarity opposite to that of the toner is applied to the transfer device 1033 in order to electrically attract the toner on the surface of the photosensitive drum 1030 to the recording paper 1040. With this voltage, the toner image on the surface of the photosensitive drum 1030 is transferred to the recording paper 1040. The recording paper 1040 on which the toner image is transferred is sent to the fixing device 1041.

  In the fixing device 1041, heat and pressure are applied to the recording paper 1040, whereby the toner is fixed on the recording paper 1040. Here, the recording paper 1040 on which the toner is fixed is sent to a paper discharge tray 1043 via a paper discharge roller 1042 and is sequentially stacked on the paper discharge tray 1043.

  The neutralization unit 1034 neutralizes the surface of the photosensitive drum 1030.

  The cleaning unit 1035 removes the toner remaining on the surface of the photosensitive drum 1030 (residual toner). The surface of the photosensitive drum 1030 from which the residual toner has been removed returns to a position facing the charging device 1031.

Configuration of Optical Scanning Device Next, the configuration of the optical scanning device 1010 will be described.

  FIG. 4 is a schematic diagram showing the optical scanning device 1010. As shown in the figure, the optical scanning device 1010 includes a light source 11, a coupling lens 12, an aperture plate 13, a cylindrical lens 14, a polygon mirror 15, a scanning optical system 20, a scanning control device (not shown). ). The optical scanning device 1010 is assembled at a predetermined position of an optical housing (not shown).

  In the following description, the direction along the longitudinal direction (rotation axis direction) of the photosensitive drum 1030 is defined as the Y-axis direction of the XYZ three-dimensional orthogonal coordinate system, and the direction along the rotation axis of the polygon mirror 15 is defined as the Z-axis direction. The direction perpendicular to both the Y axis and the Z axis is taken as the X axis direction.

  In the following description, the direction corresponding to the main scanning direction of each optical member is referred to as “main scanning corresponding direction”, and the direction corresponding to the sub scanning direction is referred to as “sub scanning corresponding direction”.

  The light source 11 has, for example, 25 light emitting units (not shown) arranged two-dimensionally. Here, the light emitting units are arranged so that the intervals between the light emitting units are equal when all the light emitting units are orthogonally projected onto a virtual line extending in the sub-scanning corresponding direction.

  In the present embodiment, the “light emitting portion interval” refers to the distance between the centers of two light emitting portions.

Each light emitting unit is, for example, a VCSEL (Vertical Cavity
It is composed of a surface emitting laser such as Surface Emitting LASER. That is, the light source 11 is constituted by a surface emitting laser array. In addition, the number of light emission parts is not limited to the above-mentioned 25 pieces.

  The coupling lens 12 is disposed on the optical path of the light emitted from the light source 11 and controls the light into parallel light or substantially parallel light.

  The aperture plate 13 has an aperture and shapes the light that has passed through the coupling lens 12.

  The cylindrical lens 14 forms an image of the light that has passed through the opening of the aperture plate 13 in the vicinity of the deflection reflection surface of the polygon mirror 15 in the Z-axis direction.

  The optical system disposed on the optical path between the light source 11 and the polygon mirror 15 is also called a pre-deflector optical system.

  The polygon mirror 15 is a four-sided mirror that rotates about a rotation axis that is orthogonal to the longitudinal direction (rotation axis direction) of the photosensitive drum 1030. Here, each mirror surface of the polygon mirror 15 is a deflection reflection surface. The polygon mirror 15 rotates at a constant speed and deflects light from the cylindrical lens 14 at a constant angular velocity.

  FIG. 5 is an XY plan view showing a scanning optical system of the optical scanning device. FIG. 6 is a ZX plan view showing a scanning optical system of the optical scanning device. The scanning optical system 20 is disposed on the optical path of the light deflected by the polygon mirror 15. For example, as shown in FIGS. 5 and 6, the optical scanning device 1010 includes a first scanning lens 21, a second scanning lens 22, a folding mirror 24, a synchronization detection mirror 25, and a synchronization detection sensor 26. have.

  The first scanning lens 21 is disposed on the optical path of the light deflected by the polygon mirror 15.

  The second scanning lens 22 is disposed on the optical path of the light that passes through the first scanning lens 21.

  The folding mirror 24 folds the optical path of the light passing through the second scanning lens 22 in a direction toward the photosensitive drum 1030.

  That is, the light deflected by the polygon mirror 15 is applied to the photosensitive drum 1030 via the first scanning lens 21, the second scanning lens 22, and the folding mirror 24, thereby forming a light spot on the surface of the photosensitive drum 1030. .

  The light spot on the surface of the photosensitive drum 1030 moves along the longitudinal direction of the photosensitive drum 1030 as the polygon mirror 15 rotates. Here, the moving direction (Y-axis direction) of the light spot on the surface of the photosensitive drum 1030 is the “main scanning direction”, and the rotational direction (Z-axis direction) of the photosensitive drum 1030 is the “sub-scanning direction”.

  The synchronization detection mirror 25 reflects the light before the start of writing reflected by the folding mirror 24 in a direction toward the synchronization detection sensor 26 (here, + Y axis direction). The synchronization detection sensor 26 outputs a signal (photoelectric conversion signal) corresponding to the amount of received light to the scanning control device. Here, the output signal of the synchronization detection sensor 26 is also referred to as a “synchronization detection signal”.

● Relationship between Static Beam Spot Diameter and Exposure Method Next, in the laser printer 1000, the length of the static beam spot in the main scanning direction defined by 1 / e ² of the maximum exposure intensity (hereinafter referred to as “static beam spot”). The difference in the size of the electrostatic latent image due to the relationship between the diameter and the exposure method will be described. Here, in the following description, the writing density is 600 dpi. At this time, the dimension of the minimum pixel determined by the writing density is D = 42.3 μm.

  FIG. 7 is a schematic diagram showing a profile of a static beam spot when the static beam spot diameter ω is 55 μm. FIG. 8 is a schematic diagram showing a profile of a static beam spot when the static beam spot diameter ω is 70 μm. 7 and 8, the vertical axis represents the light intensity density, and the horizontal axis represents the distance from the center of the static beam spot.

  In each beam spot diameter, the light amount of the beam spot is the same, so the area of the profile is the same.

  Here, when the speed at which the beam spot scans the latent image carrier is represented by v, the time T required for the static beam spot to scan the size D of one pixel is obtained by Expression (1).

T = D / v (1)

  Further, if the scanning optical system arranged between the optical deflector and the latent image carrier satisfies the constant velocity, the position of the beam spot on the latent image carrier is H, and the proportionality coefficient is k. The angle θ formed between the light beam deflected by the optical deflector and the optical axis of the scanning optical system can be obtained by Expression (2).

  H = kθ (2)

  Therefore, the relationship between the speed v at which the beam spot scans the latent image carrier and the angular speed r of the optical deflector satisfies the formula (3).

  v = kr (3)

  If the scanning optical system has a function of condensing parallel light beams, the proportional constant k is k = f, where f is the combined focal length in the main scanning direction of the scanning optical system.

  Here, in the image forming method according to the present invention, the exposure method in which the exposure is performed by scanning the static beam spot having the light amount (first irradiation light amount) shown in FIGS. 7 and 8 only for the time T (first time). This is called an exposure method. The first exposure method is an exposure method that is normally performed.

  In the image forming method according to the present invention, the exposure method scans and exposes a static beam spot having a light intensity (second irradiation light quantity) stronger than the first irradiation light quantity for a time t (second time) shorter than T. Is referred to as a second exposure method.

  Then, in the image forming method according to the present invention, the image forming apparatus according to the present invention executes a combination of the first exposure method and the second exposure method as described later, thereby electrostatically forming on the latent image carrier. A latent image is formed.

Next, the relationship between the pixel size exposed by the static beam spot in the image forming method according to the present invention will be described.

  FIG. 9 is a schematic diagram showing the difference in the size of the electrostatic latent image due to the difference between the static beam spot diameter ω by the image forming apparatus and the exposure method.

  In FIG. 9, (1-1) indicates the size of the electrostatic latent image when exposed by the first exposure method using a static beam spot having a static beam spot diameter ω of 55 μm.

  Further, (1-2) indicates the size of the electrostatic latent image when the static beam spot having a static beam spot diameter ω of 70 μm is exposed by the second exposure method. Here, in the size of the electrostatic latent image shown in (1-2), the second exposure method was executed with the time t being T / 4 and the second irradiation light amount being four times the first irradiation light amount.

  Further, (1-3) shows the size of the electrostatic latent image when the static beam spot having a static beam spot diameter ω of 70 μm is exposed by the first exposure method. The size of the electrostatic latent image shown in (1-3) is shown as a reference for comparison with (1-2) in which the exposure method is different.

  According to (1-2), by executing the second exposure method that irradiates a larger amount of irradiation light in a shorter time than the first exposure method, a narrow static beam spot as in (1-1) An electrostatic latent image having the same size as when the first exposure method is executed with the diameter ω can be obtained.

  As shown in FIG. 9, the upper limit value of the static beam spot diameter ω that provides the same effect as that obtained by irradiating with a small diameter static beam spot by the second exposure method is about twice as large as one pixel. It is.

  That is, in the case of 600 dpi, if the static beam spot diameter ω is about 85 μm, using the second exposure method, the same size as that irradiated with a small diameter static beam spot by the first exposure method. The electrostatic latent image can be obtained.

  FIG. 10 is a schematic diagram showing the difference in the size of the electrostatic latent image due to the difference in the static beam spot diameter ω by the image forming apparatus.

  (2-1) indicates the size of the electrostatic latent image when exposed by the second exposure method using a static beam spot having a static beam spot diameter ω of 60 μm.

  (2-2) shows the size of the electrostatic latent image when exposed by the second exposure method using a static beam spot having a static beam spot diameter ω of 65 μm.

  (2-3) shows the size of the electrostatic latent image when exposed by the second exposure method using a static beam spot having a static beam spot diameter ω of 70 μm.

  (2-4) shows the size of the electrostatic latent image when exposed by the second exposure method using a static beam spot having a static beam spot diameter ω of 80 μm.

  (2-5) shows the size of the electrostatic latent image when exposed by the second exposure method using a static beam spot having a static beam spot diameter ω of 85 μm.

  (2-6) shows the size of the electrostatic latent image when exposed by the second exposure method using a static beam spot having a static beam spot diameter ω of 90 μm.

  As shown in FIG. 10, the size of the electrostatic latent image similar to (1-1) shown in FIG. 9 can be obtained by the second exposure method in (2-2) static beam spot diameter ω. And (2-5) static beam spot diameter ω.

  That is, the static beam spot diameter ω that can obtain the same electrostatic latent image size as that obtained by the first exposure method using the above-described (1-1) beam spot diameter by the second exposure method. The lower limit value is about 1.5 times as large as one pixel.

  As described above, when the writing density is 600 dpi, exposure is performed by the second exposure method with a static beam spot diameter ω larger than 63 μm, and exposure is performed by the first exposure method with a static beam spot diameter ω smaller than 63 μm. Can be obtained.

  In the image forming method according to the present invention, when the static beam spot diameter ω is determined based on the relationship between the pixel number N (N is a natural number) and the pixel size D per unit exposed by the second exposure method, The relationship of the following formula | equation (4) is satisfy | filled.

  1.5 × N × D ≦ ω ≦ 2 × N × D (4)

Relationship between the writing density and the unit of the number of pixels exposed by the second exposure method Next, the relationship between the writing density and the unit of the number of pixels exposed by the second exposure method in the image forming method according to the present invention will be described. .

  In the following description, the writing density is 1200 dpi. When the writing density is 1200 dpi, the size of one pixel is D = 21.6 μm. Here, T is the time required for the static beam spot to scan a distance of 21.6 μm.

  FIG. 11 is a schematic diagram showing the difference in the size of the electrostatic latent image when the writing density by the image forming apparatus is 1200 dpi. In the figure, (3-1) shows the size of the electrostatic latent image when exposed by the first exposure method using a static beam spot having a static beam spot diameter ω of 25 μm.

  Further, (3-2) is an electrostatic charge when a static beam spot having a static beam spot diameter ω of 70 μm is exposed by the second exposure method with a time t of T / 2 and an irradiation light amount of 2 times. Indicates the size of the latent image.

  Further, (3-3) is an electrostatic charge when exposure is performed using a static beam spot having a static beam spot diameter ω of 70 μm by the second exposure method with a time t of T / 4 and an irradiation light amount of 2 times. Indicates the size of the latent image.

  Further, (3-4) is an electrostatic charge when exposure is performed using a static beam spot having a static beam spot diameter ω of 70 μm by the second exposure method with a time t of T / 8 and an irradiation light amount of 8 times. Indicates the size of the latent image.

  As shown in FIG. 11, when the writing density is 1200 dpi, a thin static beam spot diameter as shown in (3-1) can be obtained regardless of the time t and the amount of irradiation light in the second exposure method. Thus, an electrostatic latent image having the same size as that of the first exposure method cannot be obtained.

  As described above, in the image forming method of the present invention, when the writing density is high, only one pixel is accurately reproduced in addition to making the static beam spot small (per pixel). Reproducibility) is difficult.

  However, in the image forming method of the present invention, when the writing density is high, if it is attempted to improve the reproducibility per pixel, the toner particle size can be reduced in accordance with the size of one pixel. It is necessary to reduce the thickness of the latent image carrier.

  Further, in the image forming method of the present invention, when the writing density is high, if it is attempted to improve the reproducibility per pixel, as the toner particle size is reduced, the toner scattering is reduced. It is necessary to develop a developing machine that suppresses this. Here, by increasing the writing density, it is possible to carry out electrical correction of scan line inclination, bending, or resist position in fine units.

  However, when the writing density is high, it is difficult to improve the reproducibility per pixel due to the above factors. Therefore, in the image forming method of the present invention, when the writing density is high, if the reproducibility per area can be improved with two or more pixels as one area, the formed image is It can be said that it is a good image.

  That is, in the image forming method of the present invention, when the writing density is high, N × D indicating the size of the area of two or more pixels is converted into an electrostatic latent image by using the second exposure method. The smallest unit that can be obtained.

That is, when the writing density is 1200 dpi, the static beam spot diameter ω is 70 μm, the number of pixels N per area exposed by the second exposure method is 2, and the pixel size D is 21.6 μm.
1.5 × 2 × D ≦ ω ≦ 2 × 2 × D
Thus, Expression (4) is satisfied.

  Therefore, when the writing density is 1200 dpi, two pixels are exposed as one area by the second exposure method, and the exposure is performed by the first exposure method using a small static beam spot diameter ω. An electrostatic latent image of the same size can be obtained.

  In addition, T at this time transforms Equation (1) into Equation (5).

T = 2 × D / v (5)

  Next, the relationship between the writing density in the image forming method according to the present invention and the unit of the number of pixels exposed by the second exposure method will be described using another example having a different writing density.

  In another comparative example, the writing density is 2400 dpi. When the writing density is 2400 dpi, the size of one pixel is D = 11 μm.

In another comparative example, when the writing density is 2400 dpi, the static beam spot diameter ω is 70 μm, the number of pixels N per area exposed by the second exposure method is 4, and the pixel size D is 11 μm. Then
1.5 × 4 × D ≦ ω ≦ 2 × 4 × D
Thus, Expression (4) is satisfied.

  Therefore, when the writing density is 2400 dpi, exposure is performed by the first exposure method using a small static beam spot diameter ω by exposing four pixels as one area by the second exposure method, and An electrostatic latent image of the same size can be obtained.

  In addition, T at this time transforms Equation (1) into Equation (6).

T = 4 × D / v (6)

  As described above, in the image forming method according to the present invention, the number of pixels in the area exposed by the second exposure method is changed corresponding to the writing density. Therefore, according to the image forming method of the present invention, the electrostatic latent image having the same size as that obtained by the first exposure method using a small static beam spot diameter ω regardless of the writing density. Is obtained.

● Relationship Between Pixel and Exposure Method Next, the flow of executing the combination of the first exposure method and the second exposure method in the image forming method according to the present invention will be described based on the relationship between the pixel and the exposure method. . Here, the process of determining whether the first exposure method or the second exposure method is performed for each of the plurality of divided areas is performed by the dividing unit and the determining unit of the image forming apparatus according to the present invention. Executed.

  FIG. 12 is a schematic diagram showing the relationship between the pixels and the exposure method by the image forming apparatus. In the figure, an example is shown in which five terminal pixels P are arranged among the seven pixels exposed by the image forming method according to the present invention. In the following description, for example, the writing density is 1200 dpi and the static beam spot diameter ω is 70 μm.

  In the image forming method according to the present invention, when an electrostatic latent image of 7 pixels is obtained, a plurality of pixels arranged in the main scanning direction such as 2 pixels + 2 pixels + 3 pixels are replaced with one pixel or a plurality of adjacent pixels. It is divided into three areas composed of pixels.

  In the image forming method according to the present invention, the first exposure method or the second exposure method is determined for each of the divided and generated areas, and the determined exposure method is used for exposure. .

Here, for the area divided into two pixels, from equation (4):
1.5 × 2 × D ≦ ω ≦ 2 × 2 × D
Meet.

  Accordingly, it is determined that the area divided into two pixels is exposed by the second exposure method.

On the other hand, for the area divided into 3 pixels,
1.5 × 3 × D ≦ ω ≦ 2 × 3 × D
Therefore, if the static beam spot diameter ω is 70 μm, the relationship of Expression (4) is not satisfied.

  Therefore, it is determined that the area divided into three pixels is exposed by the first exposure method. That is, in the image forming method according to the present invention, the area of the pixel exposed by the first exposure method satisfies the following expression (7).

  2 × N × D <ω <4 × N × D (7)

Next, the relationship between the pixel and the exposure position in the image forming method according to the present invention will be described.

  FIG. 13 is a schematic diagram illustrating a relationship between pixels and exposure positions by the image forming apparatus. As shown in the figure, when exposing by the second exposure method, it is desirable to expose only the central portion in the main scanning direction of the area divided into two pixels for the second time t.

  The reason for exposing only the central portion in the main scanning direction of the area divided into two pixels when the exposure is performed by the second exposure method is that the electrostatic latent image when exposed by a small diameter static beam spot by the first exposure method is used. This is because the image and the apparent exposure position are the same.

  FIG. 14 is a schematic diagram of a reference example showing a relationship between pixels and exposure positions by the image forming apparatus. This figure is a reference example in which exposure is performed only at the front end portion in the main scanning direction of an area divided into two pixels when exposure is performed by the second exposure method.

  FIG. 15 is a schematic diagram of another reference example showing the relationship between pixels and exposure positions by the image forming apparatus. This figure is a reference example in which exposure is performed only at the rear end portion in the main scanning direction of an area divided into two pixels when exposure is performed by the second exposure method.

  As shown in FIGS. 14 and 15, if the exposure is performed only at the front end portion or the rear end portion in the main scanning direction of the area divided into two pixels when the exposure is performed by the second exposure method, the exposure position is biased in the main scanning direction. The formed image may be recognized as having jaggy.

  As described above, in the image forming method according to the present invention, when the exposure is performed by the second exposure method, the image formed by exposing only the central portion in the main scanning direction of the area divided into two pixels. It is possible to prevent jaggy from occurring.

11: Light source 12: Coupling lens 13: Aperture plate 14: Cylindrical lens 15: Polygon mirror 20: Scanning optical system 21: First scanning lens 22: Second scanning lens 24: Folding mirror 1000: Laser printer (image forming apparatus)
1010: Optical scanning device (electrostatic latent image forming device)
1030: Photosensitive drum (image carrier)
1060: Printer control device

JP 2009-37283 A

Claims (4)

The length in the main scanning direction of the static beam spot defined by 1 / e ² of the maximum exposure intensity is ω,
D, the minimum pixel determined by the writing density
The number of pixels per area is N (N is a natural number)
When the number of pixels arranged in the main scanning direction is 1.5 × N × D ≦ ω ≦ 2 × N × D or 2 × N × D <ω <4 × N × D Divided into areas
An area satisfying 2 × N × D <ω <4 × N × D is exposed with a first exposure light amount for a first time and exposed by a first exposure method that scans a static beam spot. ,
An area satisfying 1.5 × N × D ≦ ω ≦ 2 × N × D is exposed for a second time equal to or less than the first time with a second light amount that is stronger than the first light amount. An image forming method comprising exposing by a second exposure method of scanning a static beam spot.   The image forming method according to claim 1, wherein when the exposure is performed by the second exposure method, the exposure is performed only at a central portion of the area in the main scanning direction. A first exposure method in which exposure is performed for a first time with a first irradiation light amount and a static beam spot is scanned, and a second irradiation light amount that is stronger than the first irradiation light amount and for a second time that is equal to or less than the first time. A second exposure method for exposing and scanning a static beam spot;
The length in the main scanning direction of the static beam spot defined by 1 / e ² of the maximum exposure intensity is ω,
D, the minimum pixel determined by the writing density
The number of pixels per area is N (N is a natural number)
When the number of pixels arranged in the main scanning direction is 1.5 × N × D ≦ ω ≦ 2 × N × D or 2 × N × D <ω <4 × N × D A dividing unit that divides the area,
An area satisfying 2 × N × D <ω <4 × N × D is exposed by the first exposure method and satisfies 1.5 × N × D ≦ ω ≦ 2 × N × D. Is determined by the second exposure method to determine exposure,
Having
An image forming apparatus.   The image forming apparatus according to claim 3, wherein the exposure unit performs exposure only in a central portion of the area in the main scanning direction when performing the second exposure method.

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