JP2016150581A - Image formation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image formation device in which such exposure as distortion of an image is suppressed is performed.SOLUTION: An image formation device includes a photoreceptor, scan means in which an optical spot is formed on the photoreceptor by a light emitted from a light source, and by scanning the optical spot, respective pixels of a scan line are exposed so that a latent image is formed on the photoreceptor, with a scanning speed for scanning the photoreceptor using the optical spot changing in the scan line, control means which controls, for correction, brightness and emission period of the light source according to the pixels that are exposed and holding means which holds profile information representing change of the optical spot due to environment or a change of the optical spot due to position of the pixels. The control means uses the profile information for the correction control.SELECTED DRAWING: Figure 16

Description

  The present invention relates to an image forming apparatus that forms an image by scanning light, such as a laser beam printer, a copying machine, and a FAX.

  Some image forming apparatuses form an image by exposing a photoreceptor. Further, in such an image forming apparatus, there is an image forming apparatus that reflects light with a rotary polygon mirror and collects the reflected light with a scanning lens to form a light spot on the surface of the photosensitive member. By rotating the rotary polygon mirror, the light spot moves on the surface of the photoconductor in the main scanning direction (direction perpendicular to the circumferential direction of the photoconductor), thereby forming a latent image on the photoconductor.

  As the scanning lens, a lens having an fθ characteristic is mainly used. This is because the light spot moves on the surface of the photosensitive member at a constant speed when the rotary polygon mirror is rotating at a constant angular speed. However, a scanning lens having fθ characteristics is relatively large and expensive. Therefore, for the purpose of downsizing and cost reduction of the image forming apparatus, it is considered that the scanning lens itself is not used or a scanning lens that does not have fθ characteristics is used. Japanese Patent Application Laid-Open No. H10-228867 discloses a clock frequency during scanning of one scanning line so that dots formed on the surface of the photoconductor have a constant width even when the light spot does not move at a constant speed on the surface of the photoconductor. The structure which changes is disclosed.

JP 58-1225064 A

  In an image forming apparatus, it is required to perform exposure while making the LSF (Line Spread Function) profile of one pixel (dot) in the main scanning direction uniform and suppressing image distortion. This is the same even when a scanning lens having fθ characteristics is not used.

  The present invention provides an image forming apparatus that performs exposure while suppressing image distortion.

  According to an aspect of the present invention, a light spot is formed on the photosensitive member by light emitted from the photosensitive member and a light source, and each pixel of the scanning line is exposed by scanning the light spot to thereby form a latent image on the photosensitive member. The scanning speed for scanning the photosensitive member with the light spot is adjusted in accordance with the scanning means that changes in the scanning line and the pixel to be exposed. Control means for controlling, and holding means for holding profile information indicating a change due to an environment of the light spot or a change due to a position of the pixel of the light spot, and the control means includes the correction control. Further, the profile information is used.

  According to the present invention, it is possible to perform exposure while suppressing image distortion.

1 is a schematic diagram of an image forming apparatus according to an embodiment. 1 is a cross-sectional view of an optical scanning device according to an embodiment. The figure which shows the partial magnification with respect to the image height of the optical scanning device by one Embodiment. The figure which shows the exposure control structure by one Embodiment. 6 is a timing chart of image formation according to an embodiment. The figure which shows the profile of the light spot which the optical scanning device by one Embodiment forms. The figure which shows the light emission time and brightness | luminance, and LSF profile by one Embodiment. The block diagram which shows the structure of the image modulation part by one Embodiment. 4 is a time chart of a synchronization signal, screen switching information, and an image signal according to an embodiment. The figure which shows the screen used near axial image height by one Embodiment. The figure which shows the screen used near the maximum image height by one Embodiment. The figure which shows the relationship between the electric current of a light emission part, and brightness | luminance by one Embodiment. The figure which shows the relationship between the height increase and density | concentration by one Embodiment. The block diagram of the density | concentration detection sensor by one Embodiment. The figure which shows the relationship between the image data and density by one Embodiment. The figure which shows the relationship between the fluctuation ratio of a spot diameter, and the ratio of the gradient of a gradation density characteristic. 1 is a schematic diagram of an image forming apparatus according to an embodiment. 1 is a schematic diagram of an image forming apparatus according to an embodiment.

  Hereinafter, exemplary embodiments of the present invention will be described with reference to the drawings. In addition, the following embodiment is an illustration and does not limit this invention to the content of embodiment. In the following drawings, components that are not necessary for the description of the embodiments are omitted from the drawings.

<First embodiment>
FIG. 1 is a schematic configuration diagram of the image forming apparatus according to the present embodiment. The optical scanning device 400 emits scanning light 208 (hereinafter referred to as light 208) based on the image signal from the image signal generation unit 100 and the control signal from the control unit 1. The optical scanning device 400 includes a driving unit 300 that drives a light source, and is accommodated in a housing 400a. The surface of the photoreceptor 4 is charged to a uniform potential by a charging unit (not shown). By scanning and exposing the photoconductor 4 with light 208, an electrostatic latent image is formed on the surface of the photoconductor 4. A developing unit (not shown) attaches a developer to the electrostatic latent image and visualizes it as a developer image. The developer image is fed from the paper feeding unit 8 and transferred to a recording medium such as paper conveyed to a position where the roller 5 contacts the photoreceptor 4. The developer image transferred to the recording medium is thermally fixed to the recording medium by the fixing device 6, and is discharged out of the apparatus through the paper discharge roller 7. The image forming apparatus also includes a density detection sensor 30 (hereinafter referred to as a sensor 30) that detects the density of the developer image formed on the surface of the photoreceptor 4.

  2A and 2B are configuration diagrams of the optical scanning device 400 according to the present embodiment. FIG. 2A shows a cross section in the main scanning direction, and FIG. 2B shows a cross section in the sub-scanning direction. The main scanning direction is a direction in which the light 208 is scanned on the surface of the photoconductor 4, and the sub-scanning direction is a direction orthogonal to the main scanning direction on the surface of the photoconductor 4. In the present embodiment, light (light flux) 208 emitted from the light source 401 is shaped into an elliptical shape by the aperture stop 402 and enters the coupling lens 403. The light that has passed through the coupling lens 403 is converted into substantially parallel light and enters the anamorphic lens 404. The substantially parallel light includes weakly convergent light and weakly divergent light. The anamorphic lens 404 has a positive refractive power in the cross section in the main scanning direction, and converts an incident light beam into convergent light in the cross section in the main scanning direction. The anamorphic lens 404 condenses the light beam in the vicinity of the deflecting surface 405a of the deflector 405 and forms a long line image in the main scanning direction in the cross section in the sub scanning direction.

  The light that has passed through the anamorphic lens 404 is reflected by the reflecting surface 405 a of the deflector (rotating polygon mirror) 405. The light 208 reflected by the reflecting surface 405 a passes through the imaging lens 406 and forms a light spot on the surface to be scanned 407 of the photoreceptor 4. The imaging lens 406 is an imaging optical element. In the present embodiment, the imaging optical system is configured by only a single imaging optical element (imaging lens 406). By rotating the deflector 405 at a constant angular velocity in the direction of arrow A by a driving unit (not shown), the light spot moves in the main scanning direction on the surface to be scanned 407, thereby scanning the photoreceptor 4. As shown in FIG. 2, the light spot scans the surface to be scanned 407 of the photoreceptor 4 by a distance W in the main scanning direction to expose each pixel of one scanning line. An electrostatic latent image is formed on the scanned surface 407 by moving in the sub-scanning direction by the rotation of the photoconductor 4 on the surface of the photoconductor 4 and exposing a plurality of scanning lines in the sub-scanning direction.

  A beam detector (BD) sensor 409 and a BD lens 408 are synchronization optical systems that determine the timing for writing an electrostatic latent image on the scanned surface 407. The light that has passed through the BD lens 408 enters the BD sensor 409 including a photodiode and is detected. Based on the timing at which light is detected by the BD sensor 409, the write timing is controlled.

  The light source 401 is, for example, a semiconductor laser. The light source 401 of this embodiment includes one light emitting unit. However, it is possible to use a light source 401 including a plurality of light emitting units that can independently control light emission. When a plurality of light emitting units are provided, the generated plurality of light beams reach the scanned surface 407 via the coupling lens 403, the anamorphic lens 404, the deflector 405, and the imaging lens 406, respectively. On the surface to be scanned 407, light spots corresponding to the light beams are formed at positions shifted in the sub-scanning direction. Note that the various optical members of the optical scanning device 400 such as the light source 401, the coupling lens 403, the anamorphic lens 404, the imaging lens 406, and the deflector 405 described above are housed in the housing 400a shown in FIG.

  As shown in FIG. 2, the imaging lens 406 has two optical surfaces, an entrance surface 406a and an exit surface 406b. The imaging lens 406 scans the light deflected by the deflecting surface 405a on the scanned surface 407 with a predetermined scanning characteristic. In addition, the imaging lens 406 makes the light spot on the scanned surface 407 a predetermined shape. In addition, the imaging lens 406 establishes a conjugate relationship between the vicinity of the deflection surface 405a and the vicinity of the scanned surface 407 in the cross section in the sub-scanning direction. Thus, the surface tilt is compensated, that is, the scanning position deviation in the sub-scanning direction on the surface to be scanned 407 when the deflection surface 405a tilts is reduced.

  The imaging lens 406 according to the present embodiment is a plastic mold lens formed by injection molding, but a glass mold lens may be adopted as the imaging lens 406. Since the molded lens can be easily molded into an aspherical shape and is suitable for mass production, the productivity and optical performance can be improved by adopting the molded lens as the imaging lens 406.

  The imaging lens 406 according to the present embodiment does not have so-called fθ characteristics. That is, when the deflector 405 is rotated at a constant angular velocity, the light spot does not move on the scanned surface 407 at a constant velocity. In this manner, by using the imaging lens 406 having no fθ characteristics, the distance D1 in FIG. 2A can be shortened, that is, the imaging lens 406 can be disposed close to the deflector 405. It becomes. Further, the imaging lens 406 not having the fθ characteristic has a smaller length LW in the main scanning direction and a thickness LT in the optical axis direction than the imaging lens having the fθ characteristic. Therefore, the housing 400a of the optical scanning device 400 can be reduced in size by the imaging lens 406 having no fθ characteristics. Further, some lenses having fθ characteristics have a sharp change in the shape of the entrance surface and the exit surface of the lens in the cross section in the main scanning direction, and there is a possibility that good imaging performance cannot be obtained. On the other hand, the shape of the imaging lens 406 that does not have the fθ characteristic is less likely to have such a steep change, and thus good imaging performance can be obtained.

The scanning characteristic on the scanned surface 407 by the imaging lens 406 of the present embodiment is expressed by the following formula (1).
Y = K / B · tan (B · θ) (1)
Y in Expression (1) is the position (image height) of the light spot in the main scanning direction of the surface to be scanned 407, and when the light spot is on the optical axis (hereinafter simply referred to as the axis), that is, Y = 0 when there is a light spot at the center of the scanning line. Further, θ in Expression (1) is a scanning angle (scanning field angle) by the deflector 405, and θ = 0 corresponds to a case where the light spot is on the optical axis. Further, K in the equation (1) is an imaging coefficient on the axis, and B is a scanning characteristic coefficient that determines the scanning characteristic of the imaging lens 406. By the imaging lens 406, the light spot scans a range of Y = −Ymax to + Ymax. In FIG. 2, Ymax is W / 2. Hereinafter, the maximum absolute value of the image height Y, that is, Y = −Ymax and Ymax will be referred to as the maximum image height. The image height Y = 0 is referred to as an on-axis image height.

When the equation (1) is differentiated by the scanning angle θ, the following equation (2) indicating the moving speed of the light spot with respect to the position in the main scanning direction of the surface to be scanned 407, that is, the scanning velocity is obtained.
dY / dθ = K / (cos ² (B · θ) (2)
From equation (2), when θ = 0, that is, the scanning speed of the light spot at the on-axis image height is K. When equation (2) is divided by K, the following equation (3) is obtained.
(DY / dθ) / K = 1 / (cos ² (B · θ) (3)
Equation (3) represents the ratio of the scanning speed of the light spot at each scanning angle to the scanning speed of the light spot at the on-axis image height. Since the image height corresponds to the scanning angle, Expression (3) indicates the ratio of the scanning speed of the light spot at the on-axis image height to the scanning speed of the light spot at each image height. The following formula (4) obtained by subtracting 1 from the formula (1) is thus the amount of deviation of the scanning speed at each image height with respect to the scanning speed of the light spot at the on-axis image height (hereinafter referred to as partial magnification). Is shown.
(DY / dθ) / K = (1 / (cos ² (B · θ)) − 1 = tan ² (B · θ) (4)
From the expressions (3) and (4), it can be seen that in the imaging lens 406 according to the present embodiment, the scanning speed of the light spot changes depending on the image height of the deflector 405. That is, the scanning speed of the optical scanning device 400 according to the present embodiment changes within the scanning line.

  FIG. 3 is a graph of partial magnification with respect to image height. As shown in FIG. 3, as the absolute value of the image height Y increases, the scanning speed increases, so that the partial magnification increases. For example, when the partial magnification is 30%, when light is irradiated for a unit time, the irradiation length in the main scanning direction on the surface to be scanned 407 becomes 1.3 times on the axis. Therefore, if the pixel width in the main scanning direction is determined at a constant time interval determined by the period of the image clock, the pixel density varies depending on the position of the light spot in the main scanning direction. Further, if the light emission luminance of the light source 401 is constant, the exposure amount varies depending on the scanning position of the light spot due to the difference in scanning speed. Specifically, the exposure amount per unit length decreases as the scanning speed increases. Therefore, in order to obtain good image quality, it is necessary to perform partial magnification correction and luminance correction for correcting the total exposure amount per unit length.

  FIG. 4 is a configuration diagram of exposure control of the image forming apparatus according to the present embodiment. The image signal generation unit 100 receives print information from a host computer (not shown) and generates a VDO signal 110 corresponding to image data (image signal). The control unit 1 controls the image forming apparatus. Note that the control unit 1 also controls the luminance (light emission intensity) of the light source 401 by controlling the drive unit 300. The driving unit 300 causes the light emitting unit 11 of the light source 401 to emit light by supplying a current to the light emitting unit 11 of the light source 401 based on the VDO signal 110.

  When the image signal generation unit 100 is ready to output an image signal for image formation, the image signal generation unit 100 instructs the control unit 1 to start printing through the serial communication 113. When the preparation for printing is completed, the control unit 1 transmits a TOP signal 112 that is a synchronization signal in the sub-scanning direction and a BD signal 111 that is a synchronization signal in the main scanning direction to the image signal generation unit 100. When receiving the synchronization signal, the image signal generation unit 100 outputs the VDO signal 110 that is an image signal to the driving unit 300 at a predetermined timing. The image signal generation unit 100 and the control unit 1 shown in FIG. Details of the constituent blocks in the drive unit 300 will be described later.

  FIG. 5 is a timing chart of various synchronization signals and image signals when an image forming operation corresponding to one page of the recording medium is performed. Note that time elapses from left to right in the figure. “HIGH” in the TOP signal 112 indicates that the leading edge of the recording medium has reached a predetermined position. When the image signal generation unit 100 receives “HIGH” of the TOP signal 112, the image signal generation unit 100 transmits the VDO signal 110 in synchronization with the BD signal 111. The light source 401 emits light based on the VDO signal 110 to form an electrostatic latent image on the photosensitive member 4. In FIG. 5, for simplicity of illustration, the VDO signal 110 is described as being continuously output across a plurality of BD signals 111. However, actually, the VDO signal 110 is output in a predetermined period of time from when the BD signal 111 is output until the next BD signal 111 is output. The BD signal 111 is a signal indicating a reference for the start timing of each scanning line.

  FIG. 6 shows an LSF profile of one pixel (dot) in the main scanning direction when partial magnification correction and luminance correction of Patent Document 1 are performed. 6A shows an on-axis image height, that is, an LSF profile at Y = 0, and FIG. 6B shows a maximum image height, that is, an LSF profile at Y = Ymax. Further, FIG. 6C shows the LSF profile of FIG. 6A and FIG. 6B superimposed. 6A to 6C, the resolution is 600 dpi, and the width of one dot in the main scanning direction is 42.3 μm. The partial magnification at the maximum image height is 35%. In the configuration of Patent Document 1, when the light is emitted for the time T3 at the brightness P3 at the axial image height, the light is emitted at the brightness 1.35 × P3 for the time 0.74 × T3 at the maximum image height. As shown in FIG. 6C, when comparing the one-dot LSF profile at the axial image height and the maximum image height, the peak of the integrated light amount is lower at the maximum image height than at the axial image height. The width of the skirt part of the profile is wide. That is, the LSF profiles do not match. More specifically, the LSF profile varies depending on the image height, that is, the position of the light spot in the main scanning direction.

  Thus, the reason why the LSF profile differs depending on the image height is that the profile of the stationary spot indicated by the broken line in FIGS. 6A and 6B differs depending on the image height. The stationary spot profile is a light spot profile at a certain moment. That is, the LSF profile of one pixel is obtained by integrating the light spot profiles in one pixel.

  In the configuration described in Patent Document 1, the LSF profile varies depending on the image height because the shape (profile) of a stationary spot generated at each moment on the scanned surface 407 by the imaging lens 406 varies depending on the image height. Therefore, in this embodiment, in addition to the partial magnification correction and the luminance correction, the light emission time of the light source 401 is corrected (light emission time correction). Thereby, the reproducibility of a fine image is improved.

  FIG. 7A shows a one-dot optical waveform and LSF profile according to Patent Document 1, and FIG. 7B shows a one-dot optical waveform and LSF profile according to this embodiment. Here, the optical waveform indicates the light emission time and luminance of one dot, and shows the three on-axis image height, intermediate image height, and maximum image height. The intermediate image height is an image height between the on-axis image height and the maximum image height. 7A and 7B, the scanning time for one pixel (42.3 μm) at the on-axis image height is T3, and the luminance at this time is P3. In FIGS. 7A and 7B, the partial magnification at the maximum image height is 35%. Therefore, the scanning time for one pixel at the maximum image height is 0.74T3. In Patent Document 1, since the partial magnification is 35%, the light emission time at the maximum image height is 0.74T3 which is equal to the scanning time of one pixel. In this embodiment, the light emission time is not corrected based on the partial magnification as in Patent Document 1, but light is emitted for a time shorter than the scanning time of one pixel except for Y = 0. When Y is not 0, the luminance is not corrected based on the partial magnification, but is corrected based on the light emission time. In other words, light is emitted with a luminance higher than the luminance of Patent Document 1, which is obtained by multiplying the luminance at Y = 0 by the partial magnification. For example, in FIG. 7B, at the maximum image height, light is emitted only by 0.22T3 which is shorter than the scanning time of 0.74T3 for one pixel. Therefore, the luminance at the maximum image height is set to 1 / 0.22 times the axial image height, that is, 4.50P3. With this configuration, as shown in FIG. 7B, the difference in the shape of the LSF profile of one pixel due to the difference in the position in the main scanning direction is reduced. As described above, in the present embodiment, the light emission time correction is performed while performing the partial magnification correction, and in addition, the luminance correction is performed in consideration of the light emission time correction. Details will be described below.

  FIG. 8 is a configuration diagram of the image modulation unit 150 of the image signal generation unit 100. The halftone processing unit 186 performs light emission time correction. The halftone processing unit 186 holds a screen corresponding to each image height, and performs halftone processing by selecting a screen to be used based on the screen switching information 184 output from the SCR switching unit 185. The SCR switching unit 185 generates screen switching information 184 based on the BD signal 111 and the image clock signal 125 that are synchronization signals. FIG. 9 shows the relationship between the BD signal 111 and the screen switching information 184. In the present embodiment, the scanning line is divided into n regions according to the absolute value of the image height, and the halftone processing unit 186 holds the screen corresponding to each region. Each region is referred to as region 1 to region n, a screen corresponding to the region including the on-axis image height is referred to as SCRn, and a screen corresponding to the region including the maximum image height is referred to as SCR1. For areas other than the area including the maximum image height and the area including the on-axis image height, the screens of SCR2 to SCRn-1 are used in the order closer to the area including the maximum image height. The SCR switching unit generates the screen switching information 184 by determining the development scanning area based on the image clock signal 125 with the timing of the BD signal 111 as a reference.

  FIG. 10 is an example of SCRn used in the range including the on-axis image height, and FIG. 11 is an example of SCR1 used in the range including the maximum height increase. As representatively shown in FIGS. 10 and 11, SCRk (k = 1 to n) is a 200-line matrix, and gradation representation is performed by 16 pixel pieces obtained by dividing each pixel into 16 parts. In accordance with the density information represented by the multi-value parallel 8-bit data of the VDO signal 110, the area of the screen composed of 9 pixels is changed. The matrix 153 is provided for each gradation, and the gradation increases in the order indicated by the arrows in FIGS. 10 and 11 (the density increases). As shown in FIG. 11, the SRC 1 is set so that the pixel pieces of the 16 sections of each pixel are not completely lit even in the highest gradation (maximum density) matrix.

As an example, a case where the light emission time at the maximum image height is set to 0.22T3 as shown in FIG. 7B will be described. By executing the partial magnification correction, the scanning time corresponding to one dot (pixel) is 0.74T3. For this reason, in order to limit the maximum light emission time to 0.22T, the light emission may be set in the section corresponding to 0.22 / 0.74 out of the 16 sections of one pixel. That means
16 × (0.22 / 0.74) = 4.75 [section]
It becomes. Therefore, a screen that illuminates pixel pieces of up to about five sections may be used.

  Next, luminance correction will be described. The light emission time of one pixel is shortened as the absolute value of the image height Y is increased by the light emission time correction already described. Accordingly, if the luminance is constant, the total exposure amount (integrated light amount) for one pixel decreases as the absolute value of the image height Y increases. In the present embodiment, luminance correction is performed to compensate for the decrease in the total exposure amount. That is, the luminance of the light source 401 is corrected so that the total exposure amount (integrated light amount) for one pixel is constant at each image height.

  As shown in FIG. 4, the control unit 1 includes an IC 3 including a CPU core 2, an 8-bit DA converter (DAC) 21, and a regulator (REG) 22, and constitutes a luminance correction unit together with the drive unit 300. Yes. The drive unit 300 includes a memory 304, a VI conversion circuit 306 that converts voltage into current, and a driver IC 9, and supplies drive current to the light emitting unit 11 of the light source 401. The memory 304 stores partial magnification characteristic information and light emission time characteristic information, and information on the correction current supplied to the light emitting unit 11. The partial magnification characteristic information is information indicating the partial magnification with respect to the image height. The partial magnification information does not have to indicate the partial magnification itself. For example, information that can determine a partial magnification with respect to the image height, such as information indicating the scanning speed with respect to the image height, can be used. The light emission time characteristic information is light emission time information with respect to the image height.

  The IC 3 of the control unit 1 adjusts and outputs the voltage 23 output from the regulator 22 based on the correction current information for the light emitting unit 11 acquired from the memory 304 through the serial communication 307. The voltage 23 is a reference voltage for the DA converter 21. Next, the IC 3 sets the input data 20 of the DA converter 21 and outputs a luminance correction analog voltage 312 that increases or decreases in accordance with the image height in one scanning line in synchronization with the BD signal 111. The luminance correction analog voltage 312 is converted into a current value 313 by the VI conversion circuit 306 and output to the driver IC 9. In this embodiment, the IC 3 mounted on the control unit 1 outputs the brightness correction analog voltage 312. However, a DA converter is mounted on the drive unit 300, and the brightness correction analog voltage 312 is generated in the vicinity of the driver IC 9. May be.

  The driver IC 9 performs ON / OFF control of the light emission of the light source 401 by switching with the switch 14 whether the current IL is supplied to the light emitting unit 11 or the dummy resistor 10 in accordance with the VDO signal 110. The drive current value IL supplied to the light emitting unit 11 is a current obtained by subtracting the current Id output from the VI conversion circuit 306 from the current Ia set by the constant current circuit 15. The current Ia passed through the constant current circuit 15 is automatically feedback-controlled by a circuit inside the driver IC 9 so that the luminance detected by the photodetector 12 provided in the light source 401 for monitoring the light amount of the light emitting unit 11 becomes a predetermined value Papc1. It is adjusted. This automatic adjustment is a so-called APC (Automatic Power Control). The automatic adjustment of the luminance of the light emitting unit 11 is performed at a timing when the light emitting unit 11 emits light in order to detect the BD signal 111. A method for setting the current value Id output from the VI conversion circuit 306 will be described later. The variable resistor 13 is adjusted in value so as to be input to the driver IC 9 as a desired voltage when the light emitting unit 11 emits light with a predetermined luminance at the time of factory assembly.

  As described above, a current obtained by subtracting the current value Id output from the VI conversion circuit 306 from the current Ia required to emit light with a predetermined luminance is supplied to the light emitting unit 11 as the drive current IL. With this configuration, the drive current IL does not flow more than the current Ia. Note that the VI conversion circuit 306 constitutes a part of the luminance correction unit.

  FIG. 12 is a graph showing the current and luminance characteristics of the light emitting unit 11. The current Ia necessary for emitting light from the light emitting unit 11 with a predetermined luminance varies depending on the ambient temperature. A graph 51 in FIG. 10 is a graph under a standard temperature environment, and a graph 52 is an example of a graph under a high temperature environment. In general, the light emitting unit 11 such as a laser diode is known to change the current Ia required for outputting a predetermined luminance when the environmental temperature changes, but the efficiency (slope in the figure) hardly changes. ing. That is, in order to emit light with the predetermined brightness Papc1, the current value indicated by point A is required as the current Ia in the standard temperature environment, whereas the current value indicated by point C is required in the high temperature environment. As described above, the driver IC 9 automatically adjusts the current Ia supplied to the light emitting unit 11 so that the luminance becomes the predetermined luminance Papc1 by monitoring the luminance with the photodetector 12 even if the environmental temperature changes. Since the efficiency does not substantially change even when the environmental temperature changes, the luminance is 0.74 times that of Papc1 by subtracting the predetermined currents ΔI (N) and ΔI (H) from the current Ia for emitting light at the predetermined luminance Papc1. Can be lowered. Since efficiency does not change substantially even when the environmental temperature changes, ΔI (N) and ΔI (H) are substantially the same value. In the present embodiment, the luminance of the light emitting unit 11 is gradually increased from the on-axis image height toward the maximum image height. Therefore, the on-axis image height emits light at the luminance indicated by the points B and D in FIG. Light is emitted at the luminance indicated by the high A point and the C point.

  Luminance correction is performed by subtracting current Id corresponding to image height ΔI (N) and ΔI (H) from current Ia automatically adjusted to emit light at a desired luminance. As described above, the scanning speed increases as the absolute value of the image height Y increases. As the absolute value of the image height Y increases, the total exposure amount (integrated light amount) for one pixel decreases. Therefore, in the luminance correction, the correction is performed so that the luminance increases as the absolute value of the image height Y increases. Specifically, the current value Id is set so as to decrease as the absolute value of the image height Y increases, so that the current IL increases as the absolute value of the image height Y increases. In this way, the partial magnification can be appropriately corrected.

  As described above, in the present embodiment, the scanning speed of the light spot that exposes the pixels of the photoconductor 4 varies within the scanning line. More specifically, the scanning speed of the light spot increases as the absolute value of the image height increases. For this reason, as described with reference to the exposure control configuration of FIG. 4, the luminance and light emission time of the light source 401 are controlled in accordance with the pixel to be exposed. Specifically, the image modulation unit 150 holds a screen for controlling the light emission time. In addition, the control unit 1 controls the luminance of the light source 401 using information regarding the value of the correction current held in the memory 304. This screen is information indicating the light emission time for the pixel, and the value of the correction current is information indicating the luminance for the pixel, and these are collectively referred to as scanning information. The image forming apparatus uses the scanning information to control the luminance and the light emission time of the light source for the pixel to be exposed.

  Note that, as described with reference to FIG. 7B, when the light emission time of a pixel is specified, the luminance of the pixel can be determined from the light emission time and luminance of the pixel having the axial image height. Hereinafter, the light emission time and the luminance with respect to the pixel having the axial image height are referred to as a reference light emission time and the reference luminance, respectively, and the pixel having the axial image height is referred to as a reference pixel. The reference pixel is also the pixel at the center of the scanning line and the pixel with the longest scanning time. As shown in FIG. 7B, the luminance for the pixel can be obtained from the ratio of the light emission time of the pixel to the reference light emission time and the reference light emission time. Even when the luminance is defined instead of the light emission time of the pixel, the light emission time of the pixel can be similarly obtained. Therefore, the scanning information may include only one of the luminance of the light source and the light emission time for the pixel to be exposed. Further, as shown in FIG. 7B, the light emission time of the reference pixel is equal to the scanning time of the reference pixel. On the other hand, as shown in FIG. 7B, the light emission time of a pixel different from the reference pixel is shorter than the scanning time of the pixel. For example, in FIG. 7B, the scanning time of the pixel having the maximum image height is 0.74T3, but the light emission time is 0.22T3.

  As described above, by controlling the light emission time and luminance, it is possible to perform exposure with high accuracy while suppressing distortion without using a scanning lens having fθ characteristics. In the exposure control configuration shown in FIG. 4, the light emission time and the brightness are controlled by the image signal generation unit 100, the control unit 1, and the drive unit 300 in cooperation. However, the present invention is not limited to such a form. For example, the light emission time and the brightness may be controlled by only one control unit or by cooperation of an arbitrary number of functional blocks. it can.

  As described above, the light emission time and luminance correction control based on the characteristics of the optical scanning device 400 alone has been described. However, the positional relationship between the optical scanning device 400 and the photosensitive member 4 that is the surface to be scanned may deviate from an ideal relationship due to variations in the assembly position when the optical scanning device 400 is mounted on the image forming apparatus. As a result, the scanning characteristic on the surface of the photoreceptor 4 varies. In such a case, even if the above-described correction is performed based on the characteristics of the optical scanning device 400 alone, the light spot profile cannot be corrected appropriately.

  FIG. 13 is a graph showing an example of density measurement values of a halftone image formed in a state where the spot profile is non-uniform in the main scanning direction. FIG. 13A shows characteristics when a halftone image is formed with image data corresponding to a density of 20%. The density decreases as the absolute value of the image height increases. On the other hand, FIG. 13B shows characteristics when a halftone image is formed with image data corresponding to a density of 80%, and the density increases as the absolute value of the image height increases. Thus, if the profile of the light spot cannot be corrected appropriately, the density variation can increase as the absolute value of the image height increases. Therefore, it is necessary to correct the positional deviation with respect to the positional variation when the optical scanning device 400 is mounted on the image forming apparatus. In this embodiment, the sensor 30 is used to appropriately correct the light spot profile.

  FIG. 14 is an explanatory diagram of density detection according to the present embodiment. Three sensors 30F, 30C, and 30R are arranged along the main scanning direction of the photoconductor 4. The sensor 30 is a regular reflection type sensor including a light emitting element such as a light emitting diode (LED) and a light receiving element. The sensor 30 irradiates light from the light emitting element to the patch 31 that is a developer image for density detection formed on the photoreceptor 4, and receives reflected light by the light receiving element. At this time, since the light reflected on the toner portion of the patch 31 becomes scattered light, most of the reflected light received by the light receiving element is light regularly reflected on the surface of the photoreceptor 4. Therefore, the density of the patch 31 can be measured from the amount of light received by the sensor 30.

  In the present embodiment, the sensor 30C is disposed at the axial image height, and the sensors 30F and 30R are disposed near the maximum image height. This is because the scanning speed of the on-axis image height is stable, and even if the position of the optical scanning device 400 is slightly shifted, the profile of the light spot is not easily shifted. That is, since the density fluctuation hardly occurs at the on-axis image height, the density fluctuation near the maximum image height at which the density fluctuation is likely to occur can be measured using the sensors 30F and 30R with reference to the measurement value of the sensor 30C. .

  In the present embodiment, the number of sensors 30 is three, but the present invention is not limited to this. For example, if three or more sensors 30 are arranged, it is possible to more accurately detect density fluctuations throughout the main scanning direction. Also, since the scanning speed profile is basically symmetric, one sensor near the maximum image height can be used. For example, the structure which provides the sensor 30C and the sensor 30F may be sufficient. In this embodiment, the patch formed on the photosensitive member 4 is measured. However, in the case of an image forming apparatus equipped with an intermediate transfer member (not shown), the image is transferred from the photosensitive member 4 to the intermediate transfer member. A configuration for measuring a patch may be used. The patches 31F, 31C, and 31R are formed so as to correspond to the sensors 30. Each patch 31 is a patch of continuous gradation from low density to high density.

  FIG. 15 is an example showing a detection result of the patch 31 by the sensor 30. The graph 32 is the detection result of the sensor 30C, the graph 33 is the detection result of the sensor 30F, and the graph 34 is the detection result of the sensor 30R. As is clear from the relationship between the image height and the density in FIG. 13, the graph 33 and the graph 34 of the sensor 30F and the sensor 30R show steep gradation density characteristics compared to the graph 32 of the sensor 30C.

  Next, a method of correcting the light spot profile will be described. As shown in FIG. 1, the sensor 30 is connected to the image signal generation unit 100. The image signal generation unit 100 acquires the gradation density characteristics measured by the sensor 30C as a reference, and compares the gradation density characteristics measured by the sensors 30F and 30R to obtain the variation of the light spot profile. In the present embodiment, as shown in FIG. 15, the gradient of the gradation density characteristic in the interval from 30% to 70% density is used. The image signal generation unit 100 calculates the ratio of the reference values of the inclinations measured by the sensors 30F and 30R using the inclination measured by the sensor 30C as a reference value. In addition, the memory 304 of the driving unit 300 stores a table (not illustrated) that associates the calculated ratio with the light spot variation ratio. The image signal generation unit 100 corrects both or one of the light emission time and the luminance determined as described above based on the fluctuation ratio of the light spot corresponding to the calculated ratio. Note that, as a method of obtaining the light spot fluctuation ratio from the calculated ratio, a calculation formula that associates the calculated ratio with the spot fluctuation ratio may be used instead of the table. FIG. 16 shows an example of the relationship between the calculated gradient ratio of the gradation density characteristic and the variation ratio of the light spot. Note that the relationship shown in FIG. 16 varies depending on the characteristics of the optical scanning device 400 and the configuration of the image forming apparatus, and therefore a unique table or calculation formula is obtained in advance for each image forming apparatus. Further, the relationship between the variation ratio of the light spot and the light emission time and luminance correction values is also obtained in advance and stored in the memory 304. The relationship between the variation ratio of the light spot and the correction value of the light emission time and the luminance may be stored as a table or stored as a calculation formula.

  In the present embodiment, patches of multiple gradations from low density to high density are formed as patches for density detection, but the present invention is not limited to this configuration. Specifically, any pattern that can detect density fluctuations according to the image height may be used. For example, the slope may be obtained from the detected densities of two types of patches formed with image data corresponding to a density of 30% and a density of 70%. Furthermore, the variation ratio of the light spot is obtained from the gradient ratio of the gradation density characteristics, but the present invention is not limited to this configuration. That is, any parameter that correlates with fluctuations in the light spot may be used. For example, a configuration that compares the detected densities of patches of specific image data or a configuration that uses a difference instead of a ratio may be used.

  As described above, in the present embodiment, the profile information indicating the change depending on the scanning position of the light spot, that is, the position of the pixel to be exposed is held. The profile information is, for example, the above-described light spot variation ratio depending on the pixel position. Then, the image forming apparatus uses the above-described scanning information and profile information in determining the luminance of the light source and the light emission time for the pixel. For example, the luminance and / or the light emission time of the light source for the pixel determined based on the scanning information are corrected based on the profile information. Note that the control unit 1 forms a patch 31 for detecting the density on the photoconductor 4, thereby detecting a change in density at each pixel in the main scanning direction to generate profile information. Specifically, sensors 31F, 31C, and 31R are provided at a plurality of positions in the main scanning direction, and a change in density at each pixel in the main scanning direction is detected based on the density detected by each sensor. In addition, the position of the sensor can be configured to be provided at least at the center and the end of the scanning line, for example. With this configuration, the light spot profile can be corrected regardless of density fluctuations due to changes in image height. As a result, it is possible to perform accurate exposure with suppressed distortion without using a scanning lens having fθ characteristics.

<Second embodiment>
Next, the second embodiment will be described focusing on the differences from the first embodiment. In the first embodiment, the variation ratio of the light spot is obtained from the density measurement result with respect to the variation of the light spot due to the positional variation of the optical scanning device 400, and the light emission time and the luminance are corrected. In the present embodiment, after the optical scanning device 400 is assembled to the image forming apparatus, the light spot is directly measured. As a method for measuring the light spot, for example, a spot measuring function of a general measuring instrument may be used. Since this method requires measurement work, the cost increases compared to the configuration of the first embodiment, but the spot can be directly measured, so that the spot can be corrected with higher accuracy. In this embodiment, the measuring instrument 500 is used as a spot information detection unit.

  FIG. 17 shows a measurement configuration of a light spot according to the present embodiment. A measuring instrument 500 for measuring the light spot is installed with the photoconductor 4 of FIG. 1 removed, and measures the profile of the light spot of the light 208. At this time, by arranging the light receiving surface of the measuring device 500 so as to coincide with the light receiving surface of the photoconductor 4, the profile of the light spot on the surface of the photoconductor 4 can be measured.

  Next, a light spot profile correction method will be described. The light spot profile information measured by the measuring instrument 500 is written in the memory 304 of the driving unit 300. The memory 304 stores a reference value of the light spot. The image signal generation unit 100 calculates the variation ratio of the light spot with respect to the height increase from the reference value of the profile of the light spot stored in the memory 304, and corrects the light emission time and the luminance based on the calculated variation ratio of the light spot. Update the value. Note that the light emission time and luminance correction method is the same as in the first embodiment, and a description thereof will be omitted. Further, the measuring instrument 500 is removed after measuring the spot, and the photosensitive member 4 is mounted.

  Note that if the image forming apparatus has a configuration in which the photoconductor 4 cannot be attached and detached, for example, the measuring device 500 may be disposed between the optical scanning device 400 and the photoconductor 4 to measure the profile of the light spot. When this configuration is used, the light receiving surface of the measuring device 500 and the light receiving surface of the photoconductor 4 do not coincide with each other. A light spot can be obtained.

  As described above, according to the present embodiment, after the optical scanning device 400 is assembled to the image forming apparatus, the profile of the light spot is directly measured, so that even when there is a variation in the position of the optical scanning device 400, it is appropriate. The light spot profile can be corrected. As a result, it is possible to perform accurate exposure with suppressed distortion without using a scanning lens having fθ characteristics.

<Third embodiment>
Next, the third embodiment will be described with a focus on differences from the first embodiment and the second embodiment. In the first embodiment and the second embodiment, the light spot is corrected with respect to variations in the assembly position of the optical scanning device 400. However, the variation factor of the light spot is caused by factors other than the variation in the assembly position. For example, the internal temperature of the image forming apparatus rises due to the influence of continuous printing or the like, and the imaging lens 406 or the like is thermally expanded to change the imaging characteristics, whereby the light spot profile may fluctuate. In the present embodiment, such a change in the profile of the light spot due to a change in the environment of the image forming apparatus is also corrected. In the present embodiment, the temperature is used as information indicating this environment, and thus the temperature sensor 550 is provided as a temperature detection unit that measures the temperature inside the image forming apparatus.

  FIG. 18 is a configuration diagram of the image forming apparatus according to the present embodiment. The difference from the first embodiment and the second embodiment is that a temperature sensor 550 is arranged around the optical scanning device 400. Further, the influence of the positional variation of the optical scanning device 400 is corrected by the method according to the second embodiment. However, similarly to the first embodiment, the sensor 300 may be arranged and corrected.

  The temperature sensor 550 is connected to the image signal generation unit 100 and transmits the measured temperature information to the image signal generation unit 100. The memory 304 of the driving unit 300 stores a table (not shown) indicating the relationship between the temperature information measured by the temperature sensor 550 and the light spot profile on the photoconductor 4. Since there is a correlation between the thermal expansion of the imaging lens 406 and the imaging characteristics, the above table can be created by correlating the ambient temperature of the optical scanning device 400 with the profile of the light spot. The memory 304 stores a light spot reference value.

  Next, a light spot profile correction method will be described. The image signal generation unit 100 obtains a light spot profile from the temperature information measured by the temperature sensor 550 based on a table. Further, the spot fluctuation ratio is calculated from the reference value stored in the memory 304. By updating the light emission time and brightness correction values based on the calculated spot fluctuation ratio, the spot profile can be appropriately corrected. The light emission time and the luminance correction method are the same as those in the first embodiment, and a description thereof will be omitted.

  As described above, according to the present embodiment, in addition to the positional variation of the optical scanning device 400, light caused by mechanical influences including changes in the environment in which the image forming apparatus is installed and changes in operating states are included. It is possible to correct spot profile variations. As a result, it is possible to perform accurate exposure with suppressed distortion without using a scanning lens having fθ characteristics.

[Other Embodiments]
The present invention supplies a program that realizes one or more functions of the above-described embodiments to a system or apparatus via a network or a storage medium, and one or more processors in a computer of the system or apparatus read and execute the program This process can be realized. It can also be realized by a circuit (for example, ASIC) that realizes one or more functions.

  4: Photoconductor, 400: Optical scanning device, 1: Control unit, 304: Memory

Claims (14)

A photoreceptor,
Scanning means for forming a light spot on the photosensitive member by light emitted from a light source and exposing each pixel of a scanning line by scanning the light spot to form a latent image on the photosensitive member. A scanning speed at which the light spot is scanned with a scanning means that changes within the scanning line;
Control means for correcting and controlling the luminance and light emission time of the light source according to the pixel to be exposed;
Holding means for holding profile information indicating a change due to the environment of the light spot or a change due to the position of the pixel of the light spot;
With
The image forming apparatus, wherein the control unit uses the profile information for the correction control. The holding unit holds scanning information indicating a light emission time of the light source or a luminance of the light source with respect to a pixel for correcting a change in the scanning time of the pixel due to a change in the scanning speed;
The image forming apparatus according to claim 1, wherein the control unit performs the correction control based on the scanning information and the profile information. The scanning information indicates a light emission time of the light source with respect to a pixel,
The control means determines the luminance of the light source for the pixel so that the luminance of the light source for the pixel increases when the light emission time of the light source for the pixel becomes short, and based on the profile information, determines the luminance of the light source for the pixel. The image forming apparatus according to claim 2, wherein either or both of a light emission time and the determined luminance are corrected.   The image forming apparatus according to claim 3, wherein the light emission time of the light source with respect to the pixel indicated by the scanning information is shorter than the scanning time of the pixel in a pixel different from the reference pixel. The scanning information indicates a luminance of the light source with respect to a pixel,
The control means determines the light emission time of the light source for the pixel so that the light emission time of the light source for the pixel becomes shorter when the luminance of the light source for the pixel becomes higher, and the light source for the pixel based on the profile information 3. The image forming apparatus according to claim 2, wherein one or both of the luminance of light and the determined light emission time are corrected.   The image forming apparatus according to claim 5, wherein a light emission time of the light source for a pixel determined based on the scanning information is shorter than a scanning time of the pixel in a pixel different from the reference pixel.   The image forming apparatus according to claim 4, wherein the reference pixel is a pixel having the longest scanning time.   The image forming apparatus according to claim 4, wherein the reference pixel is a central pixel of the scanning line. The scanning information indicates a light emission time of the light source with respect to a pixel,
The image forming apparatus according to claim 2, wherein a light emission time of the light source for a pixel is indicated by a screen used for the pixel.   The image forming apparatus according to claim 9, wherein the screen is provided according to a gradation of a pixel. Developing means for developing a latent image formed on the photoreceptor to form a developer image;
Density detecting means for detecting the density of the developer image formed on the photoreceptor;
The profile information is generated by detecting a change in density of the developer image according to a scanning position of the photoconductor. The image forming apparatus described in 1. The density detecting means detects the density at a plurality of positions along the scanning direction of the photoconductor by the scanning means;
The image forming apparatus according to claim 11, wherein the control unit detects a change in density depending on a scanning position of the photoconductor based on a density of a developer image detected at each of the plurality of positions.   The image forming apparatus according to claim 12, wherein the density detection unit detects at least a density at a center and an end of the scanning line by the scanning unit. A temperature detecting means for detecting the temperature of the image forming apparatus;
The image forming apparatus according to claim 1, wherein the control unit generates the profile information based on a temperature detected by the temperature detection unit.

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