JP2016150582A - Image formation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress an unevenness in image concentration which is caused by fluctuation of exposure energy distribution on a photoreceptor that occurs after electrical fθ correction, in a configuration using a scan lens having no fθ characteristic.SOLUTION: According to image data 8 after a concentration correction process 121, a screen growth sequence of a screen process 122 is changed. To be specific, an unevenness in image concentration is suppressed by performing a screen process that decreases percentage of isolated dots in a case where the image data concentration is low, and by performing a screen process such that percentage of white isolated dots is decreased in a case where the image data concentration is high.SELECTED DRAWING: Figure 7

Description

  The present invention relates to an image forming apparatus such as an LBP, a digital copying machine, and a digital FAX, and more particularly to an image forming apparatus that forms an image using a laser beam.

  An electrophotographic image forming apparatus has an optical scanning unit for exposing a photoreceptor. The optical scanning unit emits a laser beam based on the image data, reflects the laser beam with a rotating polygon mirror, and transmits the scanning lens to irradiate and expose the photosensitive member. An electrostatic latent image is formed on the photoconductor by performing scanning that moves the spot of the laser beam formed on the surface of the photoconductor charged by rotating the rotary polygon mirror.

  The scanning lens is a lens having a so-called fθ characteristic. In the fθ characteristic, a laser beam spot on the surface of the photoconductor moves at a constant speed on the surface of the photoconductor when the rotary polygon mirror rotates at a constant angular velocity. As described above, the optical characteristic is to form an image of the laser beam on the surface of the photosensitive member. By using the scanning lens having the fθ characteristic in this way, it is possible to perform appropriate exposure using an image clock with a fixed period.

  A scanning lens having such an fθ characteristic is relatively large and expensive. Therefore, for the purpose of reducing the size and cost of the image forming apparatus, it is considered to use a scanning lens that does not use the scanning lens itself or does not have the fθ characteristic.

  In Patent Document 1, even when the laser beam spot on the surface of the photoconductor does not move on the surface of the photoconductor at a constant speed, the dots formed on the surface of the photoconductor are scanned one time so as to have a constant width. It is disclosed that electrical correction is performed to change the image clock frequency.

JP 58-125064 A

  However, in a configuration that uses a scanning lens that does not have fθ characteristics, image density unevenness due to fluctuations in exposure energy distribution on the photoconductor that occurs after electrical fθ correction as described in Patent Document 1 is suppressed. Was desired.

  The present invention has been made in view of the above-described conventional example, and provides an image forming apparatus that suppresses uneven image density due to fluctuations in exposure energy distribution on a photoreceptor without using a scanning lens having fθ characteristics. With the goal.

  In order to solve the above problems, the present invention has the following configuration. That is, a pixel width correction unit that corrects a time interval corresponding to one pixel of image data, a partial magnification correction unit that corrects an image clock, a luminance correction unit that corrects laser luminance, and the density and scanning speed of the image data And image processing means for changing the image processing in the main scanning direction, and the image processing means changes the screen growth order of the screen processing in accordance with the density of the image data and the scanning speed in the main scanning direction. .

  According to the present invention, the exposure energy distribution on the photosensitive member can be obtained without applying a scanning lens having an fθ characteristic by performing screen processing for changing the isolated dot ratio according to the image height on the image data. The image density unevenness due to the fluctuation of the image can be suppressed.

1 is a schematic configuration diagram of an image forming apparatus. (A) Main scanning sectional view of an optical scanning device. (B) Sub-scan sectional view of the optical scanning device. The characteristic graph of the partial magnification with respect to the image height of an optical scanning device. (A) The figure which shows the optical waveform of the comparative example 1, and main scanning LSF. (B) The figure which shows the optical waveform of the comparative example 2, and main scanning LSF. (C) The figure which shows the optical waveform and main scanning LSF of Example 1. FIG. The electric block diagram which shows exposure control structure. (A) Time chart of synchronization signal and image signal. (B) A time chart of a BD signal and an image signal, and a diagram showing a dot image on a scanned surface. FIG. 3 is a block diagram illustrating an image modulation unit according to the first embodiment. (A) The figure which shows an example of a screen. (B) The figure explaining a pixel and a pixel piece. The time chart regarding operation | movement of an image modulation part. (A) The figure which shows an example of the image signal input into a halftone process part. (B) The figure which shows a screen. (C) The figure which shows an example of the image signal after a halftone process. (A) The figure explaining insertion of a pixel piece. (B) The figure explaining the extract of a pixel piece The graph which shows the characteristic of the electric current of a light emission part, and a brightness | luminance. The time chart explaining partial magnification correction and brightness correction. Explanatory drawing of the stationary spot diameter and spot profile of an optical scanning device. FIG. 3 is a diagram illustrating an optical waveform and main scanning LSF according to the first embodiment. The figure which shows the other example of an optical waveform and main scanning LSF. The figure which shows LSF in an on-axis image height and the most off-axis image height. The figure which shows exposure energy distribution of the continuous 3 dots in an on-axis image height. The figure which shows exposure energy distribution in case the number of continuous dots is 1, 3 and 5. FIG. The figure which shows the exposure energy distribution at the time of extracting the center dot from 3 continuous dots. The figure which shows the exposure energy distribution in case the dot to be outlined is one, three, and five. FIG. 5 is a diagram illustrating an example of a growth order of screen processing according to the first embodiment. FIG. 6 is a block diagram illustrating an image modulation unit according to the second embodiment. FIG. 10 is a diagram illustrating an example of a method for controlling the isolated dot width in the second embodiment. The figure which shows the density irregularity of the main scanning direction in the image data density FIG. 4 is a diagram illustrating an example of screen processing according to the first embodiment. FIG. 10 is a diagram illustrating an example of dot control in the second embodiment. FIG. 10 is a block diagram illustrating an image modulation unit according to the third embodiment. FIG. 10 is a diagram illustrating a density correction table changing method according to the third embodiment.

[Embodiment 1]
First, operations and effects of partial magnification correction and luminance correction of the electrophotographic image forming apparatus according to the first embodiment will be described.

<Image forming apparatus>
FIG. 1 is a schematic configuration diagram of the image forming apparatus 9. The laser driving unit 300 in the optical scanning device 400 that is an optical scanning unit includes an image signal output from the image signal generation unit 100 (hereinafter referred to as the image signal generation unit 100) and a control output from the control unit 1. Based on the signal, scanning light 208 (hereinafter referred to as laser light 208) is emitted. The photosensitive member 4 (hereinafter referred to as the photosensitive drum 4) charged by a charging unit (not shown) is scanned with a laser beam 208 to form a latent image on the surface of the photosensitive drum 4. Then, a colorant (for example, toner) is attached to the latent image by a developing unit (not shown) to form a toner image corresponding to the latent image. The toner image is transferred to a recording medium such as paper fed from the paper feeding unit 8 and conveyed to a position where the roller 5 contacts the photosensitive drum 4. The toner 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.

<Optical scanning device>
2A and 2B are cross-sectional views of the optical scanning device 400 according to the first embodiment. FIG. 2A shows a main scanning cross section, and FIG. 2B shows a sub-scanning cross section. The main scanning section is a plane including the path of scanning light. The sub-scan section is a plane orthogonal to the main scan section, and in FIG. 2, it is also orthogonal to the main scan line.

  In the first embodiment, laser light (light beam) 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 beam 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 main scanning section, and converts an incident light beam into convergent light in the main scanning section. The anamorphic lens 404 condenses the light beam in the vicinity of the deflecting surface 405a of the deflector 405 in the sub-scan section, and forms a long line image in the main scanning direction. In this example, the deflector 405 is constituted by a reflecting mirror (rotating polygonal mirror) that rotates at a constant angular velocity.

  The light beam that has passed through the anamorphic lens 404 is reflected by a deflecting surface 405a (hereinafter referred to as a reflecting surface 405a) of a deflector (polygon mirror) 405. The light beam reflected by the reflecting surface 405 a passes through the imaging lens 406 as the laser beam 208 and enters the surface of the photosensitive drum 4. The imaging lens 406 is an imaging optical element having an incident surface (first surface) 406a and an exit surface (second surface) 406b. In the first embodiment, the imaging optical system is configured by only a single imaging optical element (imaging lens 406). The surface of the photosensitive drum 4 on which the light beam that has passed (transmitted) through the imaging lens 406 is incident is a scanned surface 407 that is scanned by the light beam. The imaging lens 406 forms an image of a light beam on the surface to be scanned 407 to form a predetermined spot-like image (hereinafter referred to as a spot). By rotating the deflector 405 at a constant angular velocity in the direction of arrow A by a drive unit (not shown), the spot moves on the scanned surface 407 in the main scanning direction, and an electrostatic latent image is formed on the scanned surface 407. To do. The main scanning direction is a direction parallel to the surface of the photosensitive drum 4 and orthogonal to the moving direction of the surface of the photosensitive drum 4. The sub-scanning direction is a direction orthogonal to the main scanning direction and the optical axis of the light beam.

  A beam detect (hereinafter referred to as 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 beam that has passed through the BD lens 408 enters the BD sensor 409 including a photodiode and is detected. The control unit 1 controls the writing timing based on the timing when the light beam is detected by the BD sensor 409. The laser beam 208 is, for example, pulse width modulated in accordance with the signal level of the image signal, and the writing timing is a timing at which the image signal is input to the modulation unit that modulates the laser beam 208 and is modulated.

  The light source 401 is a semiconductor laser chip. The light source 401 of the first embodiment is configured to include one light emitting unit 11 (see FIG. 5). However, the light source 401 may include a plurality of light emitting units that can independently control light emission. Even when a plurality of light emitting units are provided, a plurality of light beams generated therefrom 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, spots corresponding to the respective light beams are formed at positions shifted in the sub-scanning direction.

  Various optical members 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 a housing (optical box) 400a (see FIG. 1) of the optical scanning device 400. Is done.

<Imaging lens>
As shown in FIG. 2, the imaging lens 406 has two optical surfaces (lens surfaces), an incident surface (first surface) 406a and an exit surface (second surface) 406b. The imaging lens 406 has a configuration in which the light beam deflected by the deflection surface 405a scans the scanned surface 407 with desired scanning characteristics in the main scanning section. The imaging lens 406 has a configuration in which the spot of the laser beam 208 on the scanned surface 407 has a desired shape. Further, the imaging lens 406 has a conjugate relationship between the vicinity of the deflection surface 405a and the vicinity of the surface to be scanned 407 in the sub-scan section. Thus, the surface tilt is compensated (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 first 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 does not have a so-called fθ characteristic. That is, there is no scanning characteristic that moves the spot of the light beam passing through the imaging lens 406 at a constant speed on the scanned surface 407 when the deflector 405 is rotating at a constant angular velocity. As described above, by using the imaging lens 406 having no fθ characteristic, the imaging lens 406 can be disposed close to the deflector 405 (at a position where the distance D1 is small). Further, the imaging lens 406 not having the fθ characteristic can be made smaller in the main scanning direction (width LW) and the optical axis direction (thickness LT) than the imaging lens having the fθ characteristic. For this reason, the housing 400a (see FIG. 1) of the optical scanning device 400 is downsized. In addition, in the case of a lens having fθ characteristics, the shape of the entrance surface and exit surface of the lens when viewed in the main scanning section may have abrupt changes. Image performance may not be obtained. On the other hand, since the imaging lens 406 does not have the fθ characteristic, there is little abrupt change in the shape of the incident surface and the exit surface of the lens when viewed in the main scanning section, so that favorable imaging performance is achieved. Can be obtained.

The scanning characteristic of the imaging lens 406 according to the first embodiment is expressed by the following formula (1).
Y = (K / B) tan (Bθ) (1)
In Expression (1), the scanning angle (scanning field angle) by the deflector 405 is θ, the light beam condensing position (image height) on the scanned surface 407 is Y [mm], and the axial image height. The image formation coefficient at is K [mm], and the coefficient (scanning characteristic coefficient) for determining the scanning characteristic of the imaging lens 406 is B. In the first embodiment, the on-axis image height refers to the image height on the optical axis (Y = 0 = Ymin), and corresponds to the scanning angle θ = 0. The off-axis image height indicates the image height (Y ≠ 0) outside the central optical axis (when the scanning angle θ = 0), and corresponds to the scanning angle θ ≠ 0. Further, the most off-axis image height refers to the image height (Y = + Ymax, −Ymax) when the scanning angle θ is maximum (maximum scanning field angle). The scanning width W, which is the width in the main scanning direction of a predetermined region (scanning region) where a latent image can be formed on the scanned surface 407, is expressed as W = | + Ymax | + | −Ymax |. The center of the scanning area is the on-axis image height and the end is the most off-axis image height.

  Here, the imaging coefficient K is a coefficient corresponding to f at the scanning characteristic (fθ characteristic) Y = fθ when parallel light enters the imaging lens 406. In other words, the imaging coefficient K is a coefficient for making the condensing position Y and the scanning angle θ proportional to each other in the same manner as the fθ characteristic when a light beam other than parallel light enters the imaging lens 406.

  Supplementing the scanning characteristic coefficient B, in the limit of B → 0, Equation (1) becomes Y = Kθ, which corresponds to the scanning characteristic Y = fθ of the imaging lens used in the conventional optical scanning device. Further, the equation (1) when B = 1 is Y = K tan θ, which corresponds to the projection characteristic Y = f tan θ of a lens used in an imaging device (camera) or the like. That is, by setting the scanning characteristic coefficient B in the range of 0 ≦ B ≦ 1 in the expression (1), a scanning characteristic between the projection characteristic Y = ftan θ and the fθ characteristic Y = fθ can be obtained.

Here, when the equation (1) is differentiated by the scanning angle θ, the scanning speed of the light beam on the scanned surface 407 with respect to the scanning angle θ is obtained as shown in the following equation (2).
dY / dθ = K / cos ² (Bθ) (2)
Further, when the equation (2) is divided by the velocity dY / dθ = K at the on-axis image height, the following equation (3) is obtained.
((DY / dθ) / K) −1 = 1 / cos ² (Bθ) −1 = tan ² (Bθ) (3)
Expression (3) expresses a deviation amount (partial magnification) of the scanning speed of each off-axis image height with respect to the scanning speed of the on-axis image height. In the optical scanning device 400 according to the first embodiment, the scanning speed of the light beam is different between the on-axis image height and the off-axis image height except when B = 0.

  FIG. 3 shows the relationship between the image height and the partial magnification when the scanning position on the scanned surface 407 according to the first embodiment is fitted with the characteristic of Y = Kθ. In the first embodiment, by applying the scanning characteristic shown in Expression (1) to the imaging lens 406, as shown in FIG. 3, the scanning speed gradually increases from the on-axis image height toward the off-axis image height. The partial magnification is increased because of the higher speed. The partial magnification of 30% means that the irradiation length in the main scanning direction on the irradiated surface 407 is 1.3 times the irradiation length in the vicinity of the axial image height when light is irradiated for a unit time. Therefore, if the pixel width in the main scanning direction is determined at a fixed time interval determined by the cycle of the image clock, the pixel density differs between the on-axis image height and the off-axis image height.

  Further, as the image height Y moves away from the on-axis image height and approaches the most off-axis image height (as the absolute value of the image height Y increases), the scanning speed gradually increases. Thus, it takes more time to scan the unit length when the image height is near the most off-axis image height than the time taken to scan the unit length when the image height on the scanned surface 407 is near the axial image height. The time is shorter. This is because, when the light emission luminance of the light source 401 is constant, the unit length when the image height is near the off-axis image height is larger than the total exposure amount around the unit length when the image height is near the on-axis image height. This means that the total exposure amount around is smaller.

  Thus, in the case of having the optical configuration as described above, there is a possibility that variations in the partial magnification in the main scanning direction and the total exposure amount per unit length are not appropriate for maintaining good image quality. Therefore, in the first embodiment, in order to obtain a good image quality, the above-described partial magnification correction and luminance correction for correcting the total exposure amount per unit length are performed.

  In particular, if the scanning width W is constant, the angle of view increases as the optical path length from the deflector 405 to the photosensitive drum 4 becomes shorter. Therefore, the scanning speed between the on-axis image height and the most off-axis image height described above. The difference becomes larger. According to the inventor's earnest study, the optical configuration is such that the change rate of the scanning speed is 20% or more so that the scanning speed at the most off-axis image height is 120% or more of that at the on-axis image height. In the case of such an optical configuration, it is difficult to maintain good image quality due to the influence of variations in partial magnification in the main scanning direction and the total exposure amount around the unit length.

  The change rate C (%) of the scanning speed is a value represented by C = ((Vmax−Vmin) / Vmin) * 100, where Vmin is the slowest scanning speed and Vmax is the fastest scanning speed. In the optical configuration of the first embodiment, the slowest scanning speed is obtained at the on-axis image height (center portion of the scanning region), and the fastest scanning speed is obtained at the most off-axis image height (end portion of the scanning region).

  In addition, according to the inventor's earnest study, it is known that the change rate of the scanning speed is 35% or more in the case of an optical configuration with an angle of view of 52 ° or more. The conditions for the angle of view to be 52 ° or more are as follows. For example, in the case of an optical configuration that forms a latent image having a short side width of an A4 sheet in the main scanning direction, the optical path from the deflection surface 405a to the scanned surface 407 when the scanning width W = 214 mm and the scanning field angle is 0 °. Length D2 (see FIG. 2) = 125 mm or less. In the case of an optical configuration that forms a latent image having a short side width of the A3 sheet in the main scanning direction, the optical path length D2 from the deflection surface 405a to the scanned surface 407 when the scanning width W = 300 mm and the scanning field angle is 0 °. (See FIG. 2) = 247 mm or less. In the image forming apparatus 9 having such an optical configuration, by using the configuration of the first embodiment described below, good image quality can be obtained even when the imaging lens 406 having no fθ characteristic is used. Is possible.

<Exposure control configuration>
FIG. 5 is an electric block diagram showing an exposure control configuration in the image forming apparatus 9. An image modulation unit 101 included in the image signal generation unit 100 receives print information from a host computer (not shown) under the control of the CPU 102, and generates a VDO signal 110 corresponding to the image data (image signal). The image signal generator 100 also has a function of correcting the pixel width. The control unit 1 controls the image forming apparatus 9 and controls the light amount of the light source 401 as a luminance correction unit. The laser driving unit 300 causes the light source 401 to emit light by supplying current to 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 as a sub-scanning synchronization signal and a BD signal 111 as a main scanning synchronization signal to the image signal generation unit 100. When receiving the synchronization signal, the image modulation unit 101 of the image signal generation unit 100 outputs the VDO signal 110 that is an image signal to the laser driving unit 300 at a predetermined timing.

  Main constituent blocks of the image signal generation unit 100, the control unit 1, and the laser driving unit 300 will be described later.

  FIG. 6A 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. 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 receiving “HIGH” of the TOP signal 112, the image signal generation unit 100 transmits the VDO signal 110 in synchronization with the BD signal 111. A light source 401 emits light based on the VDO signal 110 to form a latent image on the photosensitive drum 4.

  In FIG. 6A, 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.

<Partial magnification correction method>
A partial magnification correction method by the image signal generation unit 100 will be described. Prior to the description, the factors of the partial magnification and the correction principle will be described with reference to FIG. FIG. 6B shows a dot image formed by the timing of the BD signal 111 and the VDO signal 110 and the latent image on the surface to be scanned 407. Time elapses from left to right in the figure.

  When the image signal generator 100 receives the rising edge of the BD signal 111, it transmits a VDO signal 110 after a predetermined timing so that a latent image can be formed at a desired distance from the left end of the photosensitive drum 4. The light source 401 emits light based on the VDO signal 110, and a latent image corresponding to the VDO signal 110 is formed on the scanned surface 407.

  Here, a case where a dot-shaped latent image is formed by causing the light source 401 to emit light for the same period at the on-axis image height and the most off-axis image height based on the VDO signal 110 will be described. The size of this dot corresponds to one dot of 600 dpi (width of 42.3 μm in the main scanning direction). As described above, the optical scanning device 400 has an optical configuration in which the scanning speed of the end portion (most off-axis image height) is higher than the central portion (axial image height) on the scanned surface 407. As shown in the latent image A, the latent image dot1 having the most off-axis image height is enlarged in the main scanning direction as compared with the latent image dot2 having the on-axis image height. Therefore, in the first embodiment, as the partial magnification correction, the cycle and time width of the VDO signal 110 are corrected according to the position in the main scanning direction. That is, the partial magnification correction shortens the light emission time interval of the most off-axis image height as compared with the light emission time interval of the on-axis image height. The image height latent image dot4 is made the same size. By such correction, dot-shaped latent images corresponding to the respective pixels can be formed at substantially equal intervals in the main scanning direction.

  Next, specific processing of partial magnification correction for shortening the irradiation time of the light source 401 by the amount of increase in partial magnification as it moves from the on-axis image height to the off-axis image height will be described with reference to FIGS. FIG. 7 is a block diagram illustrating an example of the configuration of the image modulation unit 101. The density correction processing unit 121 stores a density correction table for printing an image signal received from a host computer (not shown) with an appropriate density. The halftone processing unit 122 performs screen (dither) processing on the input multi-level parallel 8-bit image signal and performs conversion processing for expressing the density in the image forming apparatus 9.

  FIG. 8A shows an example of a screen, in which density expression is performed by a 200-line matrix 153 of 3 pixels for main scanning and 3 pixels for sub scanning. The white part in the figure is the part that does not cause the light source 401 to emit light (off), and the black part is the part that causes the light source 401 to emit light (on). The matrix 153 is provided for each gradation, and the gradation increases in the order indicated by the arrows (the density increases). In the first embodiment, one pixel 157 is a unit for dividing image data in order to form one dot of 600 dpi on the scanned surface 407. As shown in FIG. 8B, in the state before the pixel width is corrected, one pixel is composed of 16 pixel pieces each having 1/16 the width of one pixel, and the light source 401 is turned on for each pixel piece.・ Can be switched off. That is, a gradation of 17 steps can be expressed by one pixel. The PS conversion unit 123 is a parallel-serial conversion unit, and converts the parallel 16-bit signal 129 input from the halftone processing unit 122 into a serial signal 130. The FIFO 124 receives the serial signal 130, accumulates it in a line buffer (not shown), and outputs it as a VDO signal 110 to the subsequent laser driver 300 as a serial signal after a predetermined time. The write / read control of the FIFO 124 is performed by controlling the write enable signal WE131 and the read enable signal RE132 based on the partial magnification characteristic information received by the pixel piece insertion / extraction control unit 128 from the CPU 102 via the CPU bus 103. . The PLL unit 127 supplies a clock (VCLKx16) 126 obtained by multiplying the frequency of the clock (VCLK) 125 corresponding to one pixel by 16 times to the PS conversion unit 123 and the FIFO 124.

  Next, operations after the halftone process in the block diagram of FIG. 7 will be described using a time chart regarding the operation of the image modulation unit 101 in FIG. 9. As described above, the PS conversion unit 123 takes in the multi-value 16-bit signal 129 from the halftone processing unit 122 in synchronization with the clock 125, and sends the serial signal 130 to the FIFO 124 in synchronization with the clock 126.

  The FIFO 124 takes in the signal 130 only when the WE signal 131 is valid “HIGH”. When shortening the image in the main scanning direction for correcting the partial magnification, the pixel piece insertion / extraction control unit 128 partially disables the WE signal so that the serial signal 130 is not taken into the FIFO 124. To control. That is, the pixel piece is thinned out. FIG. 9 shows an example in which one pixel piece is thinned out from the 1st pixel and one pixel piece is constituted by 15 pixel pieces in a structure in which one pixel is normally constituted by 16 pixel pieces.

  Further, the FIFO 124 reads out the accumulated data in synchronization with the clock 126 (VCLKx16) only when the RE signal 132 is valid “HIGH”, and outputs the VDO signal 110. When making the image longer in the main scanning direction to correct the partial magnification, the pixel piece insertion / extraction control unit 128 partially disables the RE signal 132 to be “LOW”, so that the FIFO 124 does not update the read data, The data one clock before the clock 126 is continuously output. That is, the pixel piece having the same data as the data of the adjacent pixel piece on the upstream side in the main scanning direction processed immediately before is inserted. FIG. 9 shows an example in which one pixel is normally composed of 16 pixel pieces, and two pixel pieces are inserted into the second pixel, and 18 pixel pieces are formed. The FIFO 124 used in the first embodiment has been described as a circuit configured to continue the previous output, instead of the output being in the Hi-Z state when the RE signal is invalid “LOW”.

  FIGS. 10 and 11 are diagrams illustrating the image from the parallel 16-bit signal 129, which is an input image of the halftone processing unit 122, to the VDO signal 110, which is the output of the FIFO 124, using image images.

  FIG. 10A is an example of a multi-level parallel 8-bit image signal input to the halftone processing unit 122. Each pixel has 8-bit density information. The pixel 150 has density information of F0h, the pixel 151 has 80 h, the pixel 152 has 60 h, and the white background portion has density information of 00 h. FIG. 10B shows a screen, which is a screen that grows from the center at 200 lines as described in FIG. FIG. 10C shows an image of the image signal of the parallel 16-bit signal 129 after the halftone process, and each pixel 157 is composed of 16 pixel pieces as described above.

  FIG. 11 shows an example of extending an image by inserting a pixel piece, focusing on the area 158 of eight pixels in the main scanning direction of FIG. 10C with respect to the serial signal 130, and shortening the image by thinning out the image piece. An example is shown. FIG. 11A shows an example in which the partial magnification is increased by 8%. A main image is scanned by changing the pixel width so that the partial magnification is increased by 8% by inserting a total of eight pixel pieces at equal or substantially equal intervals into a group of 100 consecutive pixel pieces. Can stretch in the direction. FIG. 11B shows an example in which the partial magnification is reduced by 7%. A total of seven pixel pieces are extracted from a group of 100 consecutive pixel pieces at equal or substantially equal intervals, thereby changing the pixel width to reduce the partial magnification by 7% and main scanning the latent image. Can be shortened in the direction. As described above, in the partial magnification correction, by changing the pixel width whose length in the main scanning direction is less than one pixel, dot-shaped latent images corresponding to each pixel of the image data are substantially equally spaced in the main scanning direction. To be able to form. Note that the substantially equal intervals in the main scanning direction include those in which the pixels are not completely arranged at equal intervals. That is, as a result of performing partial magnification correction, there may be some variation in the pixel interval, and it is only necessary that the pixel interval is an equal interval on average within a predetermined image height range. As described above, when pixel pieces are inserted or extracted at equal or substantially equal intervals, when the number of pixel pieces constituting a pixel is compared between two adjacent pixels, the difference in the number of pixel pieces constituting the pixel is 0 or 1. For this reason, variations in the image density in the main scanning direction when compared with the original image data can be suppressed, and a good image quality can be obtained. Further, the position where the pixel piece is inserted or extracted may be the same position for each scanning line (line) in the main scanning direction, or the position may be shifted.

  As described above, the scanning speed increases as the absolute value of the image height Y increases. Therefore, in the partial magnification correction, the above-described pixel piece is inserted or thinned out according to the image height so that the image becomes shorter (the length of one pixel becomes shorter) as the absolute value of the image height Y increases. In this manner, latent images corresponding to the respective pixels can be formed at substantially equal intervals in the main scanning direction, and the partial magnification can be appropriately corrected.

  The partial magnification information is information that tabulates the correspondence between the image height and the partial magnification shown in FIG. 3, for example. Therefore, the pixel piece insertion / extraction control unit 128 determines the image height during processing by accumulating the number of image clocks based on, for example, the BD signal, and determines the pixel piece to be inserted, which is determined by the partial magnification according to the determined image height. The WE signal is controlled according to the number, and the RE signal is controlled according to the number of pixel pieces to be thinned out. Thereby, control of the pixel width according to the image height is realized. A detailed configuration and procedure will be described below with reference to FIG. 13 together with luminance correction.

<Brightness correction>
Next, brightness correction will be described with reference to FIGS. 5, 12, and 13. The reason for performing the luminance correction is that the partial exposure correction corrects the length of one pixel to be shorter as the absolute value of the image height Y becomes larger. ) Decreases as the absolute value of the image height Y increases. In the luminance correction, the luminance of the light source 401 is corrected so that the total exposure amount (integrated light amount) for one pixel becomes constant at each image height.

  The control unit 1 in FIG. 5 includes an IC 3 including a CPU core 2, an 8-bit DA converter 21 and a regulator 22, and constitutes a luminance correction unit together with the laser driving unit 300. The laser driving unit 300 includes a memory 304, a VI conversion circuit 306 that converts a voltage into a current, and a laser driver IC 9, and supplies a driving current to the light emitting unit 11 that is a laser diode of the light source 401. In the memory 304, partial magnification characteristic information is stored, and correction current information (correction current information) to be supplied to the light emitting unit 11 is stored. The partial magnification characteristic information is partial magnification information corresponding to a plurality of image heights in the main scanning direction. The partial magnification information is information that tabulates the correspondence between the image height and the partial magnification shown in FIG. For example, the image height may be indicated for each section on the scanning line, and may be information indicating a partial magnification for each section. This interval may be determined based on, for example, image quality evaluation, or one portion where the integer part has the same value when the number of pixel pieces to be inserted or thinned according to the partial magnification is obtained. It is good also as the section. Instead of the partial magnification information, characteristic information of the scanning speed on the surface to be scanned may be used. The correction current information is information for correcting the laser current value IL as described below. In this example, the correction current information is held as information independent of the partial magnification characteristic information. For this reason, for example, a section to which one correction value is applied can be determined independently. Since the correction current information correlates with the partial magnification characteristic information, the correction of the laser current value and the correction of the laser pulse width per pixel can be performed based on either one of the information.

  Next, the operation of the laser driving unit 300 will be described with reference to FIG. Based on the correction current information for the light emitting unit 11 stored in the memory 304, the IC 3 adjusts and outputs the voltage 23 output from the regulator 22. 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 and decreases in the main scan in synchronization with the BD signal 111. Then, the luminance correction analog voltage 312 is converted into a current value Id 313 by the subsequent VI conversion circuit 306 and output to the laser driver IC 9. In the first embodiment, the IC 3 mounted on the control unit 1 outputs the brightness correction analog voltage 312. However, the DA converter is mounted on the laser drive circuit 300, and the brightness correction analog voltage is near the laser driver IC 9. 312 may be generated.

  The laser driver IC 9 controls ON / OFF of light emission of the light source 401 by switching the switch 14 between the current IL flowing through the light emitting unit 11 or the dummy resistor 10 according to the VDO signal 110. The laser current value IL (third current) supplied to the light emitting unit 11 is obtained by subtracting the current Id (second current) output from the VI conversion circuit 306 from the current Ia (first current) set by the constant current circuit 15. Current. The current Ia flowing through the constant current circuit 15 is feedback controlled by a circuit inside the laser 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 desired luminance Papc1. To adjust automatically. 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 while the light emitting unit 11 emits light in order to detect the BD signal outside the print area (see FIG. 13) of the laser emission amount 316 for each main scan. A method for setting the VI conversion output 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 laser 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, the current obtained by subtracting the VI conversion output current value Id output from the VI conversion circuit 306 from the current Ia necessary for emitting light with a desired luminance is supplied to the light emitting unit 11 as the laser drive current IL. It is the composition to do. With this configuration, the laser drive current IL does not flow more than Ia. Note that the VI conversion circuit 306 constitutes a part of the luminance correction means.

  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. 11 is an example of a current-luminance graph under a standard temperature environment, and a graph 52 is an example of a current-luminance graph under a high temperature environment. In general, it is known that when the environmental temperature changes, the laser diode changes the current Ia necessary for outputting a predetermined luminance, but the efficiency (slope in the figure) hardly changes. That is, in order to emit light with the predetermined luminance Papc1, a current value indicated by point A is required as current Ia in a standard temperature environment, whereas a current value indicated by point C is required as current Ia in a high temperature environment. It becomes. As described above, the laser driver IC 9 automatically adjusts the current Ia supplied to the light emitting unit 11 so that the predetermined luminance Papc1 is obtained 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, by subtracting the predetermined currents ΔI (N) and ΔI (H) from the current Ia for emitting light at the predetermined luminance Papc1, 0.74 times Papc1. The brightness can be reduced. Since the efficiency hardly changes even when the environmental temperature changes, ΔI (N) and ΔI (H) are substantially the same current. In the first embodiment, the luminance of the light emitting unit 11 is gradually increased from the central portion (axial image height) to the end portion (most off-axis image height) (the absolute value of the image height Y increases). In the center portion, light is emitted at the luminance indicated by the points B and D in FIG. 12, and at the end portion, light is emitted at the luminance indicated by the points A and C.

  The luminance correction is performed by subtracting the current Id corresponding to the currents ΔI (N) and ΔI (H) from the current Ia automatically adjusted to emit light with 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 or 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. The extent to which the current IL is increased will be described in detail below. In this way, the partial magnification can be appropriately corrected.

<Explanation of partial magnification correction and brightness correction>
FIG. 13 is a timing chart for explaining the partial magnification correction and the luminance correction described above. In the memory 304 of FIG. 5, partial magnification characteristic information 317 of the optical scanning device 400 is stored. The partial magnification characteristic information 317 may be measured and stored in individual apparatuses after the optical scanning apparatus 400 is assembled. If there is little variation between the individual apparatuses, representative characteristics are stored without being measured individually. You may do it. In the example of FIG. 13, the partial magnification characteristic information 317 includes a partial magnification corresponding to each section obtained by dividing the main scanning line into 27 sections. The CPU core 2 reads the partial magnification characteristic information 317 from the memory 304 via the serial communication 307 and sends it to the CPU 102 in the image signal generation unit 100 via the serial communication 113. Based on this information, the CPU 102 generates partial magnification correction information 314 and sends it to the pixel piece insertion / extraction control unit 128 in the image modulation unit 101 of FIG. In FIG. 13, since the change rate C of the scanning speed indicated by the partial magnification characteristic information 317 is 35%, a partial magnification of 35% is generated at the most off-axis image height when the on-axis image height is used as a reference. Is described as an example. In this example, the partial magnification correction information 314 has a 17% point with zero magnification correction, an off-axis image height of -18% (-18/100), and an on-axis image height of + 17% (+17/100). It is said. Therefore, as shown in the figure, in the main scanning direction, the image length is shortened by thinning out the pixel pieces near the edge where the absolute value of the image height is large, and the pixel piece is inserted near the center where the absolute value of the image height is small. It is an area that extends its length. As described with reference to FIG. 11, in order to perform correction of −18% at the most off-axis image height, the pixel piece 18 section is thinned out with respect to the pixel piece 100 section, and the on-axis image height is corrected by + 17%. Inserts 17 pixel pieces into 100 pixel pieces. Thus, when viewed on the basis of the vicinity of the on-axis image height (center), it is substantially the same as that of the pixel piece 35 section extracted with respect to the pixel piece 100 section near the most off-axis image height (end). The partial magnification for 35% can be corrected. That is, the most off-axis image height is 0.74 times the on-axis image height during the period in which the spot of the laser beam 208 moves on the scanning surface 407 by the width of one pixel (42.3 μm (600 dpi)).

The ratio of the scanning period of the width of one pixel in the most off-axis image height to the on-axis image height is adjusted as follows when the change rate C of the scanning speed is used.
100 [%] / (100 [%] + C [%])
= 100 [%] / (100 [%] + 35 [%])
= 0.74
By inserting and removing such a pixel piece having a width of less than one pixel, the pixel width can be corrected and latent images corresponding to the respective pixels can be formed at substantially equal intervals in the main scanning direction.

  It should be noted that the axial image height is the reference, the pixel width is not inserted or extracted in the vicinity of the axial image height, the reference pixel width is set, and the extraction rate of the pixel piece is increased as the image height approaches the most off-axis image height. May be. Conversely, the most off-axis image height is used as a reference, the pixel piece is not inserted or extracted in the vicinity of the most off-axis image height, the reference pixel width is set, and the pixel piece is inserted as the image height approaches the on-axis image height. The proportion may be increased. However, as described above, the image quality is better when the pixel piece is inserted / extracted so that the pixel of the intermediate image height between the on-axis image height and the most off-axis image height has the reference pixel width (the width of 16 pixel pieces). Get better. In other words, the smaller the absolute value of the difference between the reference pixel width and the pixel width of the pixel from which the pixel piece is inserted or removed, the more faithful to the original image data with respect to the image density in the main scanning direction, the better the image quality. can get.

  In FIG. 13, the partial magnification characteristic information 317 is shown in the format of x / 100, but any format may be used as long as it is a value indicating the partial magnification. For example, only the number of molecules is shown. Also good. The pixel piece insertion / extraction control unit 128 determines the WE signal at the time of writing the VDO to the FIFO 124 and the RE signal at the time of reading the VDO from the FIFO 124 according to the value of the partial magnification characteristic information 317 corresponding to the main scanning section being drawn. To control. However, since the FIFO 124 is a memory in units of main scanning lines, the relative positions, that is, image heights, of input data and output data in the main scanning lines match. Since the partial magnification characteristic information 317 is common to all lines, reading and writing are not particularly distinguished, and the WE signal and the RE signal may be controlled according to the value of the partial magnification characteristic information 317 corresponding to the image height. For example, the position (image height) of the pixel being processed on the main scanning line can be known by integrating the image clock VCLK from the BD signal as described above. When the image height at a certain point in time is within a certain section determined by the partial magnification characteristic information 317, the pixel piece insertion / extraction control unit 128 refers to the partial magnification information corresponding to the section, and the value is negative. If so, the WE signal is controlled to thin out the pixel piece, and if it has a positive sign, the RE signal is controlled to insert the pixel piece. For example, when the partial magnification corresponding to a certain section is −18/100, an operation of thinning out 2.88 pixel pieces per 16 pixel pieces (that is, one pixel) is performed. In this case, for example, 2.88 may be approximated by 3, and 3 pixel pieces may be thinned out per 16 pixel pieces. Alternatively, for example, if the section includes four or more pixels, partial magnification such as thinning out three pixel pieces per one pixel (16 pixel pieces) for three pixels and thinning out two pixel pieces for the following one pixel. You may adjust so that the number of the pixel pieces thinned out per 100 pixel pieces may approximate to 18 in the section where −18/100 is given. Of course, this is the same regardless of the value of the partial magnification. The thinning pattern, that is, the pattern for turning off the WE signal is determined in advance according to the value of the partial magnification characteristic information 317, and the pixel piece insertion / extraction control unit 128 performs control for turning off the WE signal according to the pattern. Just do it. The same applies to the case where a pixel piece is inserted. In the case where a pixel piece is inserted, “−18/100” is set to “+18/100” and “thinning” is set to “insert” in the above description. If the “WE signal” is read as the “RE signal”, the signal that is turned off even when the pixel piece is inserted becomes the RE, and the control method is the same as the WE signal described above.

  FIG. 14A and FIG. 14B respectively show the integrated light amount at the on-axis image height and the integrated light amount at the most off-axis image height after dot width correction according to the image height for pixels having the same density. FIG. 14 (c) shows a diagram in which they are superimposed. Luminance correction is performed so that the integrated light quantity is uniform for pixels having the same density regardless of the image height. By making the integrated light quantity uniform, the potential of the latent image portion becomes uniform and the density of development becomes uniform. In the luminance correction, partial magnification characteristic information 317 (see FIG. 13) and correction current information in the memory 304 are read out before the printing operation. Based on these values, the CPU core 2 in the IC 3 generates the luminance correction value 315 and stores the luminance correction value 315 for one scan in a register (not shown) in the IC 3. Further, the CPU core 2 determines the output voltage 23 of the regulator 22 and inputs it to the DA converter 21 as a reference voltage. Then, in synchronization with the BD signal 111, the luminance correction value 315 stored in a register (not shown) is read and input to the DA converter 21, and the luminance correction analog voltage 312 is output from the output port of the DA converter 21 to the subsequent stage. The signal is sent to the VI conversion circuit 306 and converted into a current value Id by the VI conversion circuit 306. The current value Id is input to the laser driver IC 9 and is subtracted from the current Ia. As shown in FIG. 13, since the luminance correction value 315 varies according to the change in the irradiation position (image height) of the laser beam on the surface to be scanned, the current value Id is also changed according to the irradiation position of the laser beam. The Thereby, the current IL is controlled.

The luminance correction value 315 generated by the CPU core 2 based on the partial magnification characteristic information 317 and the correction current information is set so that the VI conversion output current value Id decreases as the absolute value of the image height Y increases. . Therefore, as shown in FIG. 13, the current IL increases as the absolute value of the image height Y increases. In other words, the VI conversion output current value Id changes during one scan, and the current IL decreases toward the center of the image (as the absolute value of the image height Y decreases). As a result, as shown in FIG. 13, the laser light amount output from the light emitting unit 11 emits light with the brightness of the most off-axis image height of Papc1, and the brightness of the on-axis image height of 0.74 times that of Papc1. It is corrected. In other words, the on-axis image height attenuates the luminance at an attenuation rate of 26% compared to the most off-axis image height. That is, the brightness of the most off-axis image height is 1.35 times the brightness of the on-axis image height. This value coincides with 135% (in FIG. 3 and the like, only an increment with respect to 100% is shown) which is a partial magnification at the most off-axis image height in this example. The attenuation rate R% of the brightness at the most off-axis image height can be expressed as follows using the change rate C of the scanning speed.
R = (C / (100 + C)) * 100
= 35 [%] / (100 [%] + 35 [%]) * 100
= 26 [%]
Since the partial magnification characteristic information 317 is a value correlated with the change rate of the scanning speed, the luminance attenuation rate according to the image height on the basis of the most off-axis image height is, for example, ((100 + Mmax) − (100 + M)) / (100 + Mmax) [%]. Here, Mmax is a partial magnification in a section on the main scanning line corresponding to the most off-axis image height, and M is a partial magnification in the target section. In the example of FIG. 13, Mmax is 35 [%]. Therefore, in the section corresponding to the most off-axis image height, the value of the correction current information is 0. In addition, since the partial magnification of the section corresponding to the on-axis image height is 0, the attenuation rate of the section is 0.259 (≈26 [%]) as described above. For example, if the partial magnification is M = 29 [%], (135-129) /135≈0.044=4.4%. Also, the input of the DA converter 21 and the rate of decrease in luminance are in a proportional relationship. For example, when the input of the DA converter 21 in the CPU core 2 is set to FFh (maximum value) and the light amount is reduced by 26%, 80 h It will be down 13% at (1/2 of the maximum value). That is, the DA converter 21 outputs the brightness correction analog voltage 312 so that one step (that is, 01h) of the input value becomes R / (DA converter resolution)%. In the above numerical example, one step corresponds to 26 / FFh≈0.1%. For example, 4.4 [%] corresponds to 2 Ch when converted to the input value of the DA converter 21. Thus, the luminance correction value 315 is a value obtained by converting the luminance attenuation rate according to the section on the main scanning line (that is, according to the image height) into the input value to the DA converter 21. In this manner, the luminance correction value 315 for each corresponding section can be determined based on the partial magnification or correction current information for each section included in the partial magnification correction information 314. In this example, it can be determined from the partial magnification characteristic information 317, but if the correction current information is provided independently, the luminance correction value can be determined independently from the partial magnification information.

<Explanation of effect of partial magnification correction and brightness correction>
FIGS. 4A to 4B and 15 are diagrams showing an optical waveform and a main scanning LSF (Line Spread Function) profile. These optical waveforms and main scanning LSF profiles respectively show the cases where the light source 401 emits light with a predetermined luminance and period at each of the on-axis image height, the intermediate image height, and the most off-axis image height. In the optical configuration of Embodiment 1, the scanning speed at the most off-axis image height is 135% of that at the on-axis image height, and the partial magnification of the most off-axis image height with respect to the on-axis image height is 35%. The light waveform is a light emission waveform of the light source 401. The main scanning LSF profile is obtained by integrating the spot profile formed on the surface to be scanned 407 in the sub scanning direction by emitting light with the above-described optical waveform while moving the spot in the main scanning direction. This indicates the total exposure amount (integrated light amount) on the scanned surface 407 when the light source 401 is caused to emit light with the above-described optical waveform.

  FIG. 4A shows a comparative example 1 in which the above-described partial magnification correction and luminance correction are not performed in the same optical configuration as that of the first embodiment. In the first comparative example, the light source 401 emits T3 light for a period of time necessary for main scanning for one pixel (42.3 μm) at the on-axis image height with the luminance P3. For this reason, it can be seen that the main scanning LSF profile is enlarged and the peak of the integrated light quantity is lowered as the axial image height is shifted to the off-axis image height.

  FIG. 4B shows a comparative example 2 in which the partial magnification correction described above is performed and the luminance correction is not performed. Partial magnification correction is performed by one pixel (42.3μm) at the on-axis image height, and by one increment of the partial magnification from the on-axis image height to the off-axis image height, based on the period T3 required for main scanning. Correction for shortening the period corresponding to is performed. The brightness is constant at P3. As the on-axis image height shifts to the off-axis image height, enlargement of the main scanning LSF profile is suppressed. However, since the irradiation time is shortened to 0.87 times the T3 at the intermediate image height and 0.74 times the T3 at the most off-axis image height, the peak of the integrated light amount is further reduced compared to FIG. I understand.

  FIG. 15 shows the first embodiment in which the partial magnification correction and the luminance correction described above are performed. For the partial magnification correction, the same processing as in Comparative Example 2 is performed. As the luminance correction, the amount of accumulated light that has decreased is reduced by shortening the light emission time of the light source 401 facing one pixel as the image magnification shifts from the on-axis image height to the off-axis image height by partial magnification correction. That is, the brightness P3 is corrected so that the brightness of the light source 401 is increased from the on-axis image height to the off-axis image height. In FIG. 15, the luminance of the most off-axis image height is 1.35 times P3, and the peak of the integrated light amount of the main scanning LSF profile is shifted from the on-axis image height to the off-axis image height as compared with FIG. The decrease in the amount is suppressed, and the enlargement is also suppressed. Although the LSF profiles of the on-axis image height, the intermediate image height, and the most off-axis image height in FIG. 15 are not completely coincident, the total exposure amount of each pixel is substantially the same, which affects the formed image. The level can be corrected with no level.

  As described above, by performing the partial magnification correction and the luminance correction, it is possible to perform exposure while suppressing image defects without using a scanning lens having fθ characteristics.

  FIG. 16 is a diagram showing another example of partial magnification correction and luminance correction. As shown in the figure, the same effect as the correction shown in FIG. 15 can be achieved by making the integrated luminance constant irrespective of the image height, thereby making the non-uniformity of density due to the image height constant. . In FIG. 16, while increasing the difference between the dot width (time reference) at the on-axis image height and the dot width at the most off-axis image height, the luminance at the on-axis image height and the most off-axis image are canceled out. The difference from high brightness is also increased. In FIGS. 15 and 16, the integrated light amount corresponds to the area of the convex portion, but if the value is constant depending on the image height, the relationship between the dot width and the luminance is changed as shown in FIG. be able to.

<Reason for print density fluctuation due to LSF profile fluctuation>
However, as described above, even when partial magnification correction and luminance correction are performed, the LSF profile of the on-axis image height does not completely match the LSF profile of the most off-axis image height. Due to this LSF profile fluctuation, print density unevenness occurs in the main scanning direction. Specifically, when the image data density is low, the print density becomes lighter as the most off-axis image height increases, and when the image data density is high, the print density becomes darker as the most off-axis image height. This phenomenon will be described with reference to FIGS.

  FIG. 17 shows the exposure energy distribution of isolated dots when partial magnification correction and luminance correction are performed. The exposure energy distribution in the case of printing an isolated dot matches the LSF profile. Although the sum of the exposure energy of the on-axis image height and the off-axis image height (integrated value in the main scanning direction) is the same, the spot diameter (light quantity distribution) is different. For example, as shown in FIG. 17, when the width of one dot is defined as “the width where the exposure energy is 0.3”, that is, the toner is developed in a portion where the exposure energy irradiated to the photosensitive drum 4 is 0.3 or more, and dots are formed. When formed, the dot width W21a of the most off-axis image height is narrower than the dot width W21b of the on-axis image height.

  18A shows the exposure energy distribution when printing three consecutive dots at the on-axis image height, and FIG. 18B shows three consecutive dots printed at the most off-axis image height. The exposure energy distribution at the time of performing is shown. Here, three consecutive dots are expressed as dot 1, dot 2, and dot 3. The exposure energy of three consecutive dots is a value obtained by integrating the LSF profiles of each dot. Thus, the exposure energy when printing a plurality of dots continuously is a value obtained by temporally integrating the LSF profiles for consecutive dots.

  FIG. 19 shows the exposure energy distribution when the number of continuous dots is 1, 3, and 5, respectively. The solid line shows the exposure energy distribution with the on-axis image height, and the broken line shows the most off-axis image height. The exposure energy distribution is shown. Here, as in the case of FIG. 17, the dot width is defined as “a width where the exposure energy is 0.3”. When printing an isolated dot, the dot width of the most off-axis image height is smaller than the dot width of the on-axis image height. On the other hand, when printing a plurality of continuous dots, the on-axis image height and the most off-axis image height have the same dot width.

  FIG. 20 shows an exposure energy distribution when the central dot (dot 2) is removed (outlined) from three consecutive dots (dot 1, dot 2, and dot 3). FIG. 21A shows the exposure energy distribution at the on-axis image height, and FIG. 21B shows the exposure energy distribution at the most off-axis image height. The exposure energy distribution is a temporal integration value of the LSF profiles of the dot 1 and the dot 3. Here, the dot width is defined as “a width where the exposure energy is 0.3”. FIG. 20A shows the exposure energy distribution at the axial image height. The width of the dot 1 is W23a, the width of the dot 3 is W23c, and the white width of the dot 2 is W23b. In the on-axis image height, each width has a relationship of “W23a = W23b = W23c”. FIG. 20B shows the exposure energy distribution at the most off-axis image height. The width of the dot 1 is W23d, the width of the dot 3 is W23f, and the white width of the dot 2 is W23e. At the most off-axis image height, each width has a relationship of “W23d = W23f> W23e”. 20A and 20B are compared, the dot width is “W23a = W23c <W23d = W23f”, and the white width is “W23b> W23e”. That is, when an isolated dot is whitened, the white dot width becomes smaller as the height of the most off-axis image becomes higher.

  FIG. 21 shows the exposure energy distribution when the number of white dots is one, three, and five. The solid line shows the exposure energy distribution with the on-axis image height, and the broken line shows the most off-axis image height. The exposure energy distribution is shown. Here, the dot width is defined as “a width where the exposure energy is 0.3”. When isolated dots are outlined, the outline width of the most off-axis image height is smaller than the outline width of the on-axis image height. On the other hand, when a plurality of continuous dots are outlined, the outline width of the most off-axis image height matches the outline width of the on-axis image height.

  FIG. 25 shows how the print density varies depending on the image data density. The horizontal axis represents the image height, and the vertical axis represents the print density. FIG. 25A shows a case where an image with a low image data density is printed. As an example, the print density when the image data density is 20% is shown. When the image data density is low, the ratio of isolated dots is large in the entire image area. As shown in FIG. 19, since the isolated dot width is smaller at the most off-axis image height than at the on-axis image height, when the ratio of isolated dots is large, the most off-axis image corresponding to the smaller isolated dot width. High concentration becomes thinner. FIG. 25B shows a case where an image having a high image data density is printed. As an example, the print density when the image data density is 80% is shown. When the image data density is high, the ratio of isolated dot whiteout is large in the entire image area. As shown in FIG. 21, since the isolated dot blank width is smaller at the most off-axis image height than the on-axis image height, the isolated dot blank width is small when the isolated dot blank ratio is large. Therefore, the density of the most off-axis image height is increased. FIG. 25C shows the print density when the image data density is 100%. When the image data density is 100%, there are no isolated dots or isolated dots. Therefore, density unevenness in the main scanning direction due to uneven light quantity distribution of isolated dots does not occur.

  As described above, print density unevenness occurs in the main scanning direction due to the fluctuation in the main scanning direction of the LSF profile of the isolated dots remaining after the fθ correction. The above isolated dots may be referred to as isolated black pixels, and isolated dot missing or isolated dot white missing may be referred to as isolated white pixels or isolated pixel missing.

<Image processing method (screen processing)>
Next, the image processing method according to the first embodiment will be described. The image processing method according to the first embodiment is characterized in that screen processing is changed according to image data density and image height. The configuration and operation of the image processing method are basically the same as when partial magnification correction and luminance correction were described. The difference is that the growth order of the screen is changed according to the image data density and the image height.

  FIG. 22 shows an example of the screen processing according to the first embodiment. FIG. 22A shows the screen growth order A. FIG. In the screen growth order A, the screen is continuously grown in the main scanning direction so as not to form isolated dots. According to the screen growth order A, the proportion of isolated dots decreases when the image data density is low. Further, when the image data density is high, the ratio of missing isolated dots decreases. FIG. 22B shows the screen growth order B. By the screen growth order B, the ratio of isolated dots increases when the image data density is low. Further, when the image data density is high, the proportion of isolated dots missing increases. As the screen processing of FIG. 22, multi-value screen processing may be performed. FIG. 22 shows an example in which the threshold value corresponding to each pixel in the threshold value matrix used for screening is one, and each pixel is binary. On the other hand, a multi-value threshold matrix of three values, four values or more can also be used. Of course, in this case as well, in the process of growing dots in the main scanning direction as shown in FIG. 22A, the dots first grow in the main scanning direction for a certain pixel row (from the upper left of FIG. 22A, 3 Fifth example) If there is no more blank in the main scanning direction, the process moves to another column and grows in that column in the main scanning direction.

  FIG. 26 shows an example of screen processing in the first embodiment. As shown in the middle part of FIG. 26, when the image data density is low, screen processing is performed so as to reduce the ratio of isolated dots in order to increase the print density of the most off-axis image height. On the other hand, when the image data density is high, screen processing is performed such that the isolated dot whiteout ratio is reduced in order to reduce the print density of the most off-axis image height. More specifically, for example, the main scanning line is divided (for example, divided into five), and the halftone processing unit 122 increases the ratio of isolated dots and missing isolated dots in the image data at the center. The screen processing shown in FIG. 22B is performed, and the screen processing shown in FIG. 22A is performed on the image data at both ends so as to reduce the ratio of isolated dots and missing isolated dots. The normal screen processing as shown in FIG. 8 is performed on the intermediate image data with the image data. The five divisions described above may be uniform or non-uniform. However, it is desirable to be symmetric about the axial image height. Further, the screen processing pattern that can change the ratio of isolated dots and isolated dot missing is not limited to five divisions, and the ratio of isolated dots and isolated dot missing is changed more smoothly. Also good. Of course, it may be divided into three. In this case, the screen processing of FIG. 22B is applied to the central portion, and the screen processing of FIG. 22A is applied to both end portions. Furthermore, even if the screen processing shown in FIG. 22A is used regardless of the image height, the ratio of isolated dots and missing isolated dots is reduced, which is in comparison with the normal screen processing shown in FIG. For example, the fluctuation of the print density due to the image height can be further reduced. In the present embodiment, since the pixel width and the laser emission luminance are adjusted in accordance with the image height in the main scanning direction, both isolated dots and isolated dot missing may be considered in the main scanning direction. Therefore, the screen processing may be anything that lowers or raises the ratio of isolated dots and missing isolated dots in the main scanning direction, and does not need to consider isolation in the sub-scanning direction.

  As in the first embodiment, by performing screen processing so that the number of isolated dots changes according to the image data and the image height, density unevenness caused by fluctuations in the LSF profile of the isolated dots in the main scanning direction. Can be suppressed.

  In the first embodiment, the screen processing method in which the isolated dot ratio is changed in an analog manner according to the image height has been described. However, the same effect can be obtained by changing the isolated dot ratio digitally with respect to the image height. can get.

<Effects of this embodiment>
In the first embodiment, partial magnification correction is performed by inserting and removing pixel pieces. However, when the partial magnification is corrected by such a method, there is an advantage that a high-quality image can be formed without using an fθ lens. Further, there are the following effects compared to changing the clock frequency as shown in Patent Document 1 in the main scanning direction. In the configuration shown in Patent Document 1, since the clock frequency is changed in the main scanning direction, a clock generation unit capable of outputting a plurality of clocks having different frequencies is required, and the cost of the clock generation unit is increased. However, in the first embodiment, partial magnification correction can be performed as long as only one clock generation unit is provided, and the cost related to the clock generation unit can be suppressed. In addition, since the magnification can be finely changed in the partial magnification correction, a higher quality image can be provided as compared with the case where the clock frequency is switched.

  Further, by performing screen processing so that the ratio of isolated dots changes according to the image height, it is possible to suppress density unevenness caused by fluctuations in the LSF profile of isolated dots in the main scanning direction. Further, by performing screen processing so as to reduce the ratio of isolated dots regardless of the image height, it is possible to suppress density unevenness caused by fluctuations in the LSF profile of isolated dots in the main scanning direction.

  In this embodiment, the dot width, brightness, etc. are corrected according to the image height. This is based on the fact that the image height correlates with the linear velocity (scanning velocity) of the laser beam that scans the surface of the photosensitive member, and the scanning velocity is decisive as a value corresponding to the image height. Therefore, the dot width, brightness, etc. may be corrected according to the scanning speed rather than the image height. Considering the case where the image height does not correlate with the scanning speed, it can be said that the scanning speed is more general. The scanning speed can be known directly, for example, by monitoring fluctuations in the rotation of the drive motor of the deflector.

[Embodiment 2]
In the second embodiment, image processing is performed after partial magnification correction and luminance correction are performed in the same manner as in the first embodiment for a phenomenon in which density unevenness occurs in the main scanning direction due to LSF profile fluctuations. Unlike the first embodiment, the image processing of the second embodiment is characterized in that the isolated dot width and the isolated dot blank width are changed according to the image height.

  The partial magnification correction method and the luminance correction method in the second embodiment are the same as those in the first embodiment. The second embodiment will be described below with reference to FIGS. 23, 24, and 27. FIG.

<Isolated dot width control>
FIG. 23 is a block diagram illustrating an example of the image modulation unit 101 according to the second embodiment. In the block diagram of FIG. 23, the description of the same parts as those in FIG. 7 is omitted. In the second embodiment, the image modulation unit 101 includes an isolated dot control unit 261. The halftone processing unit 269 transmits the image data 262 after the halftone processing to the isolated dot control unit 261. The isolated dot control unit 261 detects isolated dots and isolated dot white spots from the image data 262 and, when printing the isolated dots, transmits an isolated dot detection signal 263 to the pixel piece insertion / extraction control unit 264 in synchronization with VCLK 125. When isolated dot missing is printed, an isolated dot missing signal 265 is transmitted to the pixel piece insertion / extraction control unit 264 in synchronization with VCLK 125.

  As shown in FIG. 24, the pixel piece insertion / extraction control unit 264, when printing the isolated dots, the pixel piece insertion / extraction control unit 264 increases the width of the isolated dots toward the most off-axis image height. Insert a piece. On the other hand, when printing an isolated dot blank, the pixel piece is extracted so that the isolated dot blank width is increased toward the most off-axis image height. Thereafter, as in the first embodiment, partial magnification control is performed based on the partial magnification information 103.

  Note that the insertion and thinning of the pixel pieces are realized by controlling the WE signal and the RE signal as described in the first embodiment. Therefore, it is difficult for the pixel piece insertion / extraction control unit 264 to perform pixel piece insertion and thinning in two stages. Therefore, in addition to the normal WE signal and RE signal patterns for each section on the main scanning line as described in the first embodiment, the WE signal pattern and isolated dot missing detection when the isolated dot detection signal 263 is input. A pattern of a WE signal when the signal 265 is input is prepared. The pattern of the WE signal when the isolated dot detection signal 263 is input is obtained by reducing the number of pixel pieces to be thinned out according to the image height, and the WE signal when the isolated dot missing detection signal 265 is input. This pattern is obtained by further increasing the number of pixel pieces to be thinned out according to the image height. The degree of increase / decrease may be determined according to, for example, the characteristics of the exposure energy shown in FIGS. If the number of pixel pieces to be inserted for isolated dots is larger than the number of pixel pieces to be thinned out, it is necessary to replace not only the WE signal but also the pattern of the RE signal. The pattern of the RE signal in this case is a pattern that is turned off at the timing corresponding to the pixel piece to be inserted, as described in the first embodiment. When the isolated dot detection signal 263 or the isolated dot missing detection signal 265 is input, the WE signal pattern and the isolated dot missing detection when the isolated dot detection signal 263 is input in synchronization with the corresponding pixel. The WE signal is output by switching to the WE signal pattern when the signal 265 is input. When it is necessary to insert a pixel piece, the pattern of the RE signal when the isolated dot detection signal 263 is input is output in synchronization with the output from the FIFO 124 of the corresponding pixel. For this reason, the WE signal needs to be further delayed by the number of pixels buffered in the FIFO 110 while the WE signal is the same as in the first embodiment. The pixel piece insertion / extraction control unit 264 also performs this timing control.

  FIG. 27 shows a state in which the density is made uniform by this embodiment. That is, the change in the print density according to the image height according to the first embodiment is shown at the top, and the correction according to the image height of the isolated dot and the isolated dot missing width according to the present embodiment is shown in the middle stage. The state where the density becomes uniform regardless of the height is shown in the lower part.

<Effect of isolated dot width control according to this embodiment>
By performing the isolated dot width control of the second embodiment, it is possible to suppress the isolated dot width and the isolated dot blank width from fluctuating in the main scanning direction. Therefore, it is possible to suppress print density unevenness that occurs when the LSF profile of isolated dots fluctuates in the main scanning direction.

[Embodiment 3]
In the third embodiment, image processing is performed after partial magnification correction and luminance correction are performed in the same manner as in the first embodiment for a phenomenon in which density unevenness occurs in the main scanning direction due to LSF profile fluctuations. The image processing according to the third embodiment is characterized in that the density correction table is changed according to the image height. The partial magnification correction method and the luminance correction method in the third embodiment are the same as those in the first embodiment. The third embodiment will be described below with reference to FIGS.

<Density correction table change method>
FIG. 28 is a block diagram illustrating an example of the image modulation unit 101 according to the third embodiment. In the block diagram of FIG. 28, the description of the same parts as those in FIG. 7 is omitted. The density correction processing unit 311 stores a density correction table for printing an image signal received from a host computer (not shown) at an appropriate density, and uses the partial magnification information 103 according to the density and the image height. The density correction table is further changed.

  FIG. 29 is a diagram illustrating a density correction table correction method performed by the density processing unit 311. As described above, when the LSF profile of the isolated dot fluctuates in the main scanning direction, as shown in FIG. 29A, when the image data density is low, the density becomes light at the most off-axis image height. When the data density is high, the density is high at the most off-axis image height. Therefore, in order to obtain an appropriate image density, as shown in FIG. 29B, when the image density is low, the density of the most off-axis image height is increased, and when the image density is high, the highest axis is obtained. The density correction of the image data is performed so as to reduce the density of the external image height. For example, if the density is higher than 50% of the maximum density, that is, the density is higher than the reference value, the density of the off-axis image height is increased, and if the density is lower than the reference value, the highest density is obtained. Decrease the off-axis image height density. The degree of density correction also changes as shown in FIG. 29B according to the density. Here, the density of the image data may be determined in units of pixels, for example, or may be determined by an average density of several pixel groups (for example, 3 × 3 pixels). In order to realize these correction methods, the density correction processing unit 311 has a plurality of density correction tables corresponding to the image height as shown in FIG. 29C, and changes the correction table used according to the image height. To do. Although the linear density diagram 29 (c) shows the corrected density characteristics for each of the three image heights, this is merely an example, and the image height steps can be further increased. For example, an image height section can be provided to the same extent as the partial magnification characteristic information 314. Thus, the density correction is performed so that the pixel closer to the end in the main scanning direction is closer to the median density (in this example, 50% of the maximum density).

<Effect of density correction table change according to this embodiment>
By changing the density correction table of the third embodiment, it is possible to suppress print density unevenness that occurs when the LSF profile of isolated dots fluctuates in the main scanning direction. Further, according to the present embodiment, the configuration of the first embodiment can be realized only by changing the density characteristics at the time of halftone processing, so that the implementation is easy and inexpensive.

[Other Examples]
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 ... photosensitive drum, 9 ... image forming apparatus, 100 ... image signal generator, 400 ... light source, 405 ... deflector

Claims (11)

An electrophotographic image forming apparatus,
In accordance with the control signal modulated based on the image signal, a photoconductor, an image signal generating means for generating an image signal corresponding to the image data, a control means for generating a control signal for controlling the luminance of the laser light Laser driving means for irradiating the photosensitive member with a laser beam having a brightness, and a deflector for deflecting the laser beam to scan the surface of the photosensitive member,
The image signal generating means generates the image signal so that the length in the main scanning direction, which is the width of the pixel on the surface of the photoconductor, is constant regardless of the scanning speed,
The image forming apparatus according to claim 1, wherein the control unit generates the control signal so that an integrated light amount is uniform for pixels having the same density regardless of the scanning speed. The image signal generation means includes image processing means for performing screen processing on the image data,
2. The image processing unit according to claim 1, wherein the dot growth order by the screen processing is changed according to the scanning speed so that isolated dots and isolated dot omissions decrease as the scanning speed increases. The image forming apparatus described.   The image processing means further changes the order of dot growth by the screen processing according to the scanning speed so that isolated dots and isolated dot omissions increase as the scanning speed decreases. The image forming apparatus described in 1.   The image signal generation unit detects isolated dots and isolated dot missing from the image data, and the laser driving unit determines the detected isolated dot and isolated dot missing width according to the scanning speed in the main scanning direction. The image forming apparatus according to claim 1, wherein the laser beam is modulated such that the laser beam increases as the scanning speed increases.   The image signal generation unit includes a density correction unit for correcting the density of the image data, and the density correction unit has the density equal to or higher than a reference value according to the density of the image data and the scanning speed. In this case, the density of the image data is corrected such that the higher the scanning speed, the higher the density, and when the density is lower than a reference value, the higher the scanning speed, the lower the density. The image forming apparatus according to claim 1.   The said image signal generation means changes the width | variety of 1 pixel of image data by inserting or thinning out the pixel piece whose width | variety is smaller than 1 pixel, It is characterized by the above-mentioned. Image forming apparatus.   A latent image can be formed in a predetermined region with respect to the main scanning direction, and the speed at which the laser beam moves in the main scanning direction increases from the center to the end of the predetermined region with respect to the main scanning direction. The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus.   2. The deflector includes a reflecting mirror that rotates at a constant angular velocity, and the scanning speed is decisive according to an image height of a surface of the photosensitive member scanned by the laser beam. The image forming apparatus according to claim 1. An electrophotographic image forming apparatus that forms an electrostatic latent image by deflecting laser light with a rotating polygon mirror and scanning the surface of the photoreceptor with the laser light without using an fθ lens,
The width of the pixels formed on the surface of the photoconductor is constant regardless of the position in the main scanning direction on the surface of the photoconductor, and the laser light is controlled so that the integrated light amount for pixels having the same width is the same. Means to
An image forming apparatus comprising: means for performing screen processing on image data to be subjected to image formation using a threshold value matrix in which pixels first grow in the main scanning direction. An electrophotographic image forming apparatus that forms an electrostatic latent image by deflecting laser light with a rotating polygon mirror and scanning the surface of the photoreceptor with the laser light without using an fθ lens,
The width of the pixels formed on the surface of the photoconductor is constant regardless of the position in the main scanning direction on the surface of the photoconductor, and the laser light is controlled so that the integrated light amount for pixels having the same width is the same. Means to
In addition to controlling the laser beam, the means further controls the laser beam so that the width of the isolated pixel and the isolated pixel omission becomes larger toward the end in the main scanning direction. . An electrophotographic image forming apparatus that forms an electrostatic latent image by deflecting laser light with a rotating polygon mirror and scanning the surface of the photoreceptor with the laser light without using an fθ lens,
The width of the pixels formed on the surface of the photoconductor is constant regardless of the position in the main scanning direction on the surface of the photoconductor, and the laser light is controlled so that the integrated light amount for pixels having the same width is the same. Means to
An image forming apparatus comprising: means for performing density correction on image data to be subjected to image formation so that a pixel closer to an end in the main scanning direction is closer to a median density.

Images