JP2009031672A - Image forming apparatus - Google Patents

Abstract

In an optical scanning device that scans symmetry by reflecting a light beam on a vibrating mirror, variations in scanning due to jitter of the vibrating mirror are eliminated.
In an optical scanning apparatus using an oscillating mirror 224 that generates non-periodic jitter for each scanning, scanning light is detected by detection units 227 and 228 at the front and end of a scanning line, and the detected timing and target From the target value stored in the value storage unit, the correction amount prediction unit 215 calculates the correction magnification of the image signal to be placed on the next scanning line. Based on the correction magnification, the light beam modulation control unit 216 delays the image signal or the modulated signal by a minute time shorter than one pixel time.
[Selection] Figure 1

Description

  The present invention relates to an optical scanning device used in an image forming apparatus using an electrophotographic system, a control method thereof, and an image forming apparatus. In particular, the present invention relates to an optical scanning apparatus, a control method thereof, and an image forming apparatus that improve image quality by performing overall magnification or partial magnification correction in the main scanning direction.

  In recent years, there is a demand for downsizing and cost reduction of image forming apparatuses using electrophotographic technology. In order to reduce the size and cost of the apparatus, a configuration using a galvanometer mirror manufactured by a semiconductor manufacturing technique instead of a conventional polygon mirror has been proposed (see, for example, Patent Document 1). In this configuration, an image is formed by scanning the light beam in the main scanning direction by causing the mirror to resonate at a specific resonance frequency based on the mechanical dimension of the galvanometer mirror. This galvanometer mirror can be miniaturized by using semiconductor manufacturing technology, and a large number of mirrors can be made at one time, so that a reduction in cost can be expected.

For image formation, it is desirable to scan with a light beam at a uniform linear velocity. Therefore, if the galvanometer mirror swings at a uniform angular velocity at least within the scanning range on the photosensitive member (hereinafter referred to as a utilization scanning region), the correction optical system for scanning at the uniform linear velocity can be simplified. In view of this, a proposal has been made that a galvanometer mirror is nested, and a plurality of vibration components are combined so that the mirror swings in the scanning area to be used has a substantially constant angular velocity, and the scanning angle can be further increased. (See, for example, Patent Document 2). According to this, the correction optical system can also be made small and simple, and is suitable as a small and low-cost optical scanning device.
JP-A-7-175005 JP 2005-208578 A

  However, if the light beam is deflected by resonantly vibrating the oscillating mirror using the above technique, the vibration of the mirror (jitter) is caused by turbulence caused by air resistance during the resonant oscillation operation. It was. This jitter is manifested as a shift for each scanning line of the pixel formation position in the main scanning direction of the light beam deflected by the vibrating mirror. As a result, for example, an image formed on the print medium is distorted in the main scanning direction, such as a straight line distortion in the sub-scanning direction, and image quality is deteriorated. This is shown in an image 201 in FIG. In the image 201, the straight line in the sub-scanning direction is drawn as a straight line in the vicinity of the main scanning start position, but the length of each scanning line increases as it moves to the left, such as the line 201b and the line 201c. Distortion due to the difference appears strongly. This difference in scanning line length is caused by fluctuations in the oscillation cycle of the reflecting mirror.

  The present invention has been made in view of the above conventional example, and an object thereof is to solve the above problems. In particular, it is an object of the present invention to provide an optical scanning apparatus, a control method thereof, and an image forming apparatus that can prevent image deterioration due to the fluctuation even when the main scanning period is changed by the light beam.

  In order to achieve the above object, the present invention comprises the following arrangement. That is, a modulation unit that modulates a light beam according to an image signal, a light scanning unit that repeatedly scans a scanning surface by deflecting the light beam modulated by the modulation unit, and a light beam deflected by the light scanning unit Detection means for detecting a scanning speed on the scanning surface, and correction means for correcting an image signal used for modulation of the light beam in accordance with a change in the scanning speed detected by the detection means.

  According to the present invention, even if there is a fluctuation in the period of main scanning due to a light beam, image degradation due to the fluctuation can be prevented.

[First Embodiment]
Hereinafter, a first embodiment to which the present invention is applied will be described with reference to the accompanying drawings. In the first embodiment, a minute delay is inserted into the pixels included in the image signal, thereby preventing image degradation due to an increase in the scanning period of the light beam.

<Configuration of image forming apparatus>
FIG. 1 is a diagram illustrating a configuration of a controller 21 and an image forming unit 22 of an image forming apparatus 20 according to an embodiment of the present invention. The image forming apparatus 20 is an electrophotographic apparatus that forms an image by scanning a photosensitive drum with a light beam using a vibrating mirror. Note that scanning with a light beam may be referred to as optical scanning. In FIG. 1, a controller 21 controls the entire image forming apparatus by a CPU (not shown) and generates image data that can be output by the image forming unit 22 from print data received from a PC 10 outside the apparatus. The image generation unit 211 analyzes print data received from the outside by the controller 21, performs image processing, and generates image data. The generated image data is binary or multi-value dot image data in this embodiment. The image data is transmitted from the image generation unit 211 to the light beam driving unit 212 in accordance with the synchronization signal input from the image forming unit 22.

  The target value storage unit 213 stores a target value used for calculating the correction amount calculated by the correction amount prediction unit 215. In the present embodiment, the main scanning scan time (timing information) is stored as a target value. The scanning time that is the target value is, for example, from the start point to the end point of the use scanning area of the light beam reflected by the vibrating mirror when the galvano mirror used is vibrated (oscillated) at a specific resonance frequency. This is the time required for scanning up to. Here, the use scanning area refers to a range used for image formation, among scanning lines by a light beam reflected by a vibrating mirror. Since the available scanning area is determined by the specifications of the image forming apparatus such as the printable paper size, the scanning time can be uniquely determined if the resonance frequency of the mirror is given. Therefore, the resonance frequency of the mirror may be stored in the target value storage unit 213 instead of the scanning time. In addition, information other than the scanning time can be stored as long as it is information that can be used for prediction in the correction amount, such as the interval between main scanning lines and the correction amount given for each scan. In the present embodiment, the correction amount is a value indicating a delay time that is inserted into or removed from the image signal or the drive signal when the light beam modulation control unit 216 generates a drive signal for driving the light beam drive unit 212. It is.

  The timing information detection unit 214 outputs timing information to the correction amount prediction unit 215 using the horizontal synchronization signal output from the image forming unit 22. The timing information detection unit 214 detects the scanning speed of the light beam on the scanning surface. The correction amount prediction unit 215 calculates a correction amount prediction value from the timing information output from the timing information detection unit 214 and the target value stored in the target value storage unit, and the light beam modulation control unit 216 modulates the light beam. Output the correction amount.

  The light beam modulation control unit 216 generates a modulation signal for modulating the light beam from the image data output from the image generation unit 211 and the modulation correction amount output from the correction amount prediction unit 215 to generate a light beam. It transmits to the drive part 212.

  The light beam modulation control unit 216 inserts or deletes a delay corresponding to a part or all of one pixel in an image signal as described in, for example, Japanese Patent Application Laid-Open Nos. 2000-238342 and 2000-355122. . By doing this, the scanning line length in the main scanning direction can be enlarged or reduced partially or entirely by the time obtained by multiplying the inserted or deleted delay by the number of times. For example, in the present embodiment, a delay time (referred to as a minute pixel piece) of 1/20 of a time corresponding to one pixel (that is, one period of the pixel clock) is inserted / deleted according to the correction amount. Thereby, the scanning line length in the main scanning direction is partially enlarged / reduced, and the drawing time is adjusted.

  However, the technique for adjusting the scanning line length is not limited to this example. For example, the object can be achieved even if the frequency of the pixel clock that is the synchronizing signal of the image data is changed in whole or in part in the main scanning. For example, when the clock is generated by a programmable PLL, the oscillation frequency can be controlled by changing the reference voltage applied to the voltage-controlled oscillator constituting the PLL. In this case, however, there is a delay time after the signal for changing the transmission frequency is input until the PLL is locked and the transmission frequency is stabilized, and the delay time is indeterminate. For this reason, the adjusting means for inserting or deleting the minute pixel pieces in the image signal has higher responsiveness, and this method is preferable when the scanning line length is changed during scanning. However, in the present embodiment, since the scanning reflecting mirror is a vibrating mirror, if the effective scanning is only in one direction, a blanking period from the end position of one scan to the start position of the next scan occurs. . When the scanning line length is controlled for each scanning line, the correction value can be calculated and the pixel clock frequency can be changed during the blanking period. Therefore, it is not inconvenient to use the method of changing the pixel clock.

  Now, the light beam driving unit 212 drives the light beam unit 225 according to the modulation signal instructed by the light beam modulation control unit 216. The vertical synchronization signal generation unit 222 of the image forming unit 22 outputs a synchronization signal in the sub-scanning direction of the photosensitive drum 226, that is, a vertical synchronization signal to the image generation unit 211. The vertical synchronization signal is generated, for example, by detecting a mark provided at an inner position with a sensor if it hinders image formation on the photosensitive drum. Thus, an image signal can be output in synchronization with the image writing start position.

  The horizontal synchronization signal generation unit 221 is based on the light beam detection information from the writing start side light beam timing detection unit 227 and the writing end side light beam timing detection unit 228 installed in the vicinity of the photosensitive drum 226. Is generated. The generated horizontal synchronization signal is transmitted to the image generation unit 211, the timing information detection unit 214, and the mirror drive unit 223.

  The vibration mirror drive unit 223 applies a drive signal having a frequency corresponding to the resonance frequency of the vibration mirror, for example, to the vibration mirror 224 to drive the vibration mirror 224. The vibrating mirror 224 scans the photosensitive drum 226 in the main scanning direction by reflecting the light beam emitted from the light beam unit 225 while rotating about the axis periodically according to the drive signal. The driving method of the oscillating mirror 224 may be an electrostatic force, electromagnetic force, bimetal, piezoelectric element, or a combination thereof, but other driving methods may be used.

  The light beam unit 225 blinks the light beam using the light beam driving signal received from the light beam driving unit 212. The flashing light beam is reflected (ie, deflected) by the vibrating mirror 224 and scanned on the photosensitive drum 226 via the linear velocity conversion optical system 229 to expose the photosensitive drum 226. Here, the oscillating mirror 224 rotates at a substantially equal angular velocity in the directed scanning region. The constant linear velocity conversion optical system 229 converts a substantially constant angular velocity scanning of the vibrating mirror into a uniform linear velocity scanning. As a result, the light beam scans the photosensitive drum 226 at a constant speed.

  The photosensitive drum 226 is charged in advance prior to scanning with a light beam (that is, exposure), and an electrostatic latent image drawn depending on the presence or absence of electric charge is formed on the surface of the photosensitive drum 226 by exposure. The image forming unit 22 develops the electrostatic latent image with toner, transfers the toner image to a printing medium, performs a fixing process, and outputs the image.

<Relationship between scanning line and horizontal sync signal>
3 shows the temporal change in the scanning position of the light beam by the vibrating mirror 224 in FIG. 1, the timing information detected by the writing start side light beam timing detection unit 227 and the writing end side light beam timing detection unit 228, and the scanning line length. It is a figure explaining the relationship. In FIG. 3, the horizontal direction indicates time, and the vertical direction indicates the position on the scanning line.

In FIG. 3, t1 _n , t2 _n , and t3 _n are an n-th scanning (n is a natural number) writing start side light beam timing detection unit 227 (hereinafter also referred to as a start point side detection unit 227) and a writing end side light beam. This is timing information detected by the timing detection unit 228 (hereinafter also referred to as an end point detection unit 228). In FIG. 3, with reference to the detection time of the return light by the start point side detection unit 227, t _{1 n} is the time until the detection of the scanning line light by the start point side detection unit 227, and t 2 _n is the scan by the end point side detection unit 228. The time until the detection of the line light is indicated, and t3 _n indicates the time until the return light is detected by the end point side detection unit 228. A light beam traveling from the start point side to the end point side of the scanning line is referred to as scanning line light, and the opposite light beam is referred to as return light. The return light does not affect the charge of the photosensitive drum 226 before the photosensitive drum 226 is scanned by the return light, for example, at timing t3 _n , and is detected by the start point side and end point side detection units 227 and 228. It is dimmed to the light intensity possible. When light is detected by each detection unit, whether it is scanning line light or return light can be determined by storing which detection unit has performed the immediately preceding detection. For example, when light is detected by the start point side detection unit 227, if detection immediately before that is performed by the start point side detection unit 227, it is detection of scanning line light. When light is detected by the start point side detection unit 227, if the detection just before that is performed by the end point side detection unit 228, it is detection of return light. When light is detected by the end point side detection unit 228, if detection immediately before that is performed by the start point side detection unit 227, it is detection of scanning line light. When light is detected by the end point side detection unit 228, if light detection is performed by the end point side detection unit 228, it is detection of return light. If it is possible to determine whether scanning light or return light is present, it is possible to determine at which timing in FIG. 3 the detection of light corresponds. Therefore, if the detection unit that was detected immediately before cannot be identified, it cannot be determined whether the detected light is scanning line light or return light. Remember. Next, when light is detected by any one of the detection units, it is determined whether the light is a return light or a scanning line light.

In FIG. 3, the inter-sensor scanning time t _n of the _n - _th scanning is t _n = t 2 _n −t _{1 n} . In FIG. 3, it is assumed that the scanning on the photosensitive drum 226 by the vibration operation of the vibration mirror 224 is converted to a substantially constant linear velocity by the constant linear velocity conversion optical system 229. Here, the scanning speed at the pixel position i is v _ni , the number of pixels in one scanning is m, and the time for scanning the scanning area to be used (the hatched portion in the figure) is used scanning time t _{u_n} , pixel drawing time t _{p_n} , pixel drawing time the proportion of use scanning time (correction factor) is a _n and. The scan line length of the nth scan in the use scan area (this is referred to as the use scan line length) L _{u — n} and the correction magnification an are _expressed by the following equations. The pixel drawing time is a time required for output from the first pixel to the last pixel drawn in one scanning line. Ideally, from the first pixel to the last pixel coincides with the use scanning area. For this reason, the pixel clock is determined so that the pixel drawing time is equal to the use scanning time. However, as described in the prior art section, the pixel drawing time is not necessarily equal due to fluctuations in the vibration cycle of the vibration mirror.

(Formula 1-1)
t _p — _n = a _n × t _u — _n (Formula 1-2).

During one scan, the linear velocity change within the scan time t _n of the n-th scan is considered to be almost unchanged even if the influence of jitter due to the air resistance of the oscillating mirror 224 is taken into consideration. Assuming that it can be represented by the representative value v _n , Equation 1-1 can be approximated as follows.
L _u — _n = v _n × t _p — _n (Formula 1-1 ′)
Here, if the scanning speed within the scanning time t _n of the n-th scan is constant and does not change, inter-sensor scanning between the writing start side light beam timing detection unit 227 and the writing end side light beam timing detection unit 228 is performed. The following expression 1-3 is also established by using the line length L.
L = v _n × t _n (Formula 1-3)
Therefore, the drawing time t _{p_n} of the n-th scan can be obtained from the expression 1-1 ′ and the expression 1-3 as the following expression 1-3 ′.
_{t p_n = t n × L u_n} / L ... ( formula 1-3 ').

FIG. 4 is a diagram for explaining the temporal change in the target light beam scanning position when there is no jitter and the temporal change in the optical beam scanning position when jitter occurs in the nth scan. In FIG. 4, a graph indicated by a broken line 301 indicates a temporal change in the scanning position of the target light beam when there is no jitter. On the other hand, a graph indicated by a solid line 401 shows an example of a temporal change in the light beam scanning position when jitter occurs in the nth scan. In FIG. 4, times T1 and T2 are target timing values for temporal changes in the scanning position of the target light beam. Times T <b> 1 and T <b> 2 indicate target times at which the scanning line light is detected by the start point side detection unit 227 and the end point side detection unit 228 on the basis of the timing at which the return light is detected by the start point side detection unit 227. If the time for scanning the interval L between the start point detection unit 227 and the end point detection unit 228 (that is, the inter-sensor scan line length) L is the target inter-sensor scan time T, T = T2−T1. Note that the reference of T1 and T2 may be any time as long as they are the same timing. Times t1 _n and t2 _n are timing values of temporal changes in the scanning position of the light beam detected in the nth scan. The times t1 _n and t2 _n indicate the times when the scanning line light is detected by the start point side detection unit 227 and the end point side detection unit 228, respectively, with reference to the timing at which the return light is detected by the start point side detection unit 227. . In other words, it can be said that the ideal values of the times t1 _n and t2 _n are the times T1 and T2, respectively. In the n-th scan, if the time to scan the inter-sensor scan line length L is the inter-sensor scan time t _n , t _n = t 2 _n −t _{1 n} .

<Prediction of correction amount>
5, 6, and 7 are diagrams showing the scanning position within the target sensor scanning time T in FIG. 4, and show the relationship between the scanning time, the pixel drawing time, and the formed scanning line length. In the image forming apparatus 20 according to the present embodiment, the light beam is modulated so that the pixel drawing time T _p and the scanning time _Tu coincide with each other, thereby forming an image having no positional deviation in the use scanning area.

5, without jitter for each scan, scans the available scanning area at its target scanning time T _u, and, when drawing the image data in the pixel writing time T _p, the case where the scanning line length L _u Show. In this case, T _u = T _p . Therefore, one line of the image is formed in the entire use scanning area.

FIG. 6 shows a case where there is jitter for each scan, and the pixel drawing time T _{p_n at} the nth scan is not corrected. In FIG. 6, the scan speed of the nth scan is constant within the scan time of the use scan area (referred to as the use scan time) _{tu_n} , but the n scan, the n + 1 scan, the n + 2 scan, and so on. It is said that it changes due to jitter every time. That is, the scanning speed and the usage scanning time change due to the influence of jitter for each scanning, and the scanning of the usage scanning area L _u is achieved in the n-th scanning by taking t _{u —} n. Similarly, in n + 1 scan th achieving scanning of _t u_n _{+ 1} takes in use scanned area L _u, in the n + 2 scan th achieving scanning of _t u_n _{+ 2} rests in use scanned area L _u.

Expression 1-3 ′ that gives the drawing time t _{p_n} for the n-th scan is modified as follows to obtain the use scanning time L _{u_n} for the n-th scan.
L _u — _n = L × t _{p — n} / t _n (Formula 1-3 ″)
Here, (the case of a _n = 1) If not corrected for jitter, the ideal pixel writing time T _p at each scanning is as the following equation.
t _u — _n = t _p — _n = T _u = T _p (Equation 1-4)
From Equation 1-3 "and Equation 1-4,
L _u — _n = L × T _p / t _n (Formula 1-5)
It becomes.
In Expression 1-5, L and T _p are constant values determined from the resonance frequency unique to the vibrating mirror and the specifications of the image forming apparatus. In other words, Expression 1-5 indicates that the use scanning line length, that is, the length of one line of an image formed on one scanning line is reduced in inverse proportion to the measured inter-sensor scanning time t _n .

This will be described in more detail using specific numerical values. In FIG. 6, the inter-sensor scanning line length L = 252 mm, the scanning line length L _u = 210 mm, the target scanning time T = 120 μsec, the target use scanning time T _u = T _p = 100 μsec, and n = 10. In addition, the inter-sensor scan time t ₉ of the ninth scan is set to 120.525 μsec, and the inter-sensor scan time t ₁₀ of the tenth scan is set to 120.520 μsec. Further, the inter-sensor scanning time t ₁₀ of the eleventh scan is set to 120.525 μsec, and the inter-sensor scanning time t ₁₀ of the twelfth scan is set to 120.530 μsec.

The use scanning line length L _{u —} 10 for the 10th scan is expressed by Equation 1-5.
L _u — ₁₀ = 252 × 100 / 120.520 = 209.0939263 (mm)
The use scanning line length L _{u —} 11 of the eleventh scan is _obtained from the expression 1-5.
L _u — ₁₁ = 252 × 100 / 120.525 = 209.08552520 (mm)
The used scanning line length L _{u —} 12 for the twelfth scan can be _calculated from Equation 1-5.
L _u — ₁₂ = 252 × 100 / 120.530 = 209.0765784 (mm)
It becomes.

In Therefore correction amount prediction unit 215, a sensor between the scanning time t _n-1 in the n-1 scan, by the target to the sensor between the scan time T, to obtain a correction factor a _n for the pixel writing time. Further, the correction amount for the scanning line to be corrected is calculated from the correction magnification. In the present embodiment, the correction amount includes a time corresponding to a minute pixel piece to be inserted (that is, a delay time) and an insertion position. The light beam modulation control unit 216 modulates the light beam using the image data and the correction amount given by the correction amount prediction unit 215, and realizes correction of the pixel drawing time t _{p_n} for the nth scan. That is, the light beam modulation control unit 216 corrects the pixel drawing time by performing modulation so that a part of the pixel data placed on one scanning line is enlarged or reduced in the main scanning direction. Since the minute pixel piece has a predetermined size, the number of pixels to be enlarged or reduced is determined according to the correction magnification. Therefore, the position of the pixel to be enlarged or reduced is selected so that the pixels to be enlarged or reduced are uniformly distributed in the use scanning area.

FIG. 7 shows a case where there is jitter for each scan, and the pixel drawing time t _{p_n for} the scan line n is corrected. FIG. 7 is a diagram illustrating a case where the light beam is modulated using the correction amount prediction of the present embodiment and the pixel drawing time is corrected. In FIG. 7, the occurrence of jitter for each scan is the same as in FIG. In the present embodiment, the correction factor a _n of n scan th is defined as follows by n-1 scan th sensor between the scanning time t _n-1 and the target to the sensor between the scan time T.
a _n = t _n-1 / T (Formula 1-6).

Pixel writing time t _{p_n} is the target utilization scanning time T _u, is corrected by multiplying a correction factor a _n. That is,
t _p — _n = an × T _u (Equation 1-7)
Thus, the pixel drawing time t _{p_n} for the nth scan is given. From Expressions 1-6 and 1-7, the pixel drawing time t _{p_n} is
t _p — _n = t _n−1 × T _u / T (Formula 1-7 ′)
It becomes. When Expression 1-2 and Expression 1-7 ′ are applied to Expression 1-3 ″ and the use scanning line length L _{u —} n of the n-th scan is obtained, the following Expression 1-8 is obtained.

(Formula 1-8).
the scanning time t _n-1 between the sensors in the n-1 scan, if the n first scanning between the sensor scanning time period t _n is corrected match, n scan th use scan line length L _{U_n} the target The use scanning line length L _u = L × T _u / T. In other words, even if the scanning speed is changed for each scanning line due to jitter, if the change in scanning speed between adjacent scanning lines is small, the change in the scanning line length due to the jitter of the vibrating mirror is corrected and the image quality is deteriorated. Can be prevented.

<Correction Operation in Image Forming Apparatus>
Now, the above description is organized according to the configuration of FIG. 1 as follows. When the start point detection unit 227 and the end point detection unit 228 detect the light beam, the detection signal is output. When the detection signal is output by the starting point side detection unit 227, that is, when the light beam is detected, the timing information detection unit 214 starts measuring time. At the same time, it is determined whether the light beam detected by the start point detection unit 227 is scanning line light or return light. The determination method is as described above. If the immediately preceding detection is performed by the start point side detection unit 227, it is determined that it is scanning line light. For this purpose, the timing information detection unit 214 stores the detection unit from which the latest detection signal is input. If the result of determination is that the detected light beam is a return beam, the time measurement is canceled. In the case of scanning line light, if a light beam is detected by the end point side detection unit 228, the time during measurement is read, the inter-sensor scanning time t _u-1 is obtained, and the value is notified to the correction amount prediction unit 215. .

Correction amount prediction unit 215, first obtains the correction factor a _n of a next scan line n in accordance with Equation 1-6. Correction amount prediction unit 215 further includes a size (delay time) k of micro pixel piece is predetermined, the number of pixels m contained in the available scanning area, and a correction factor a _n, insertion (or deletion) should do The number x of small pixel pieces is calculated. If the size of one pixel is 1,
a _n × m = (mx) + (1 + k) × x (Formula 1-9)
It becomes. Therefore, x = (a _n −1) · m / k. The number x of minute pixel pieces is obtained according to this equation, the interval m / x is obtained, and the value is stored. When the correction magnification is less than 1, x is negative. In this case, the light beam modulation control unit 216 deletes instead of inserting a small pixel piece. The calculation of the correction amount is completed by the time scanning of the next scanning line is started.

  The light beam modulation control unit 216 modulates the light beam according to the image signal when scanning the next scanning line. At that time, correction is performed according to the correction amount stored in the correction amount prediction unit 215. That is, referring to the insertion / deletion interval m / x of the minute pixel piece, if the interval m / x is positive, the time k corresponding to the minute pixel piece is inserted at that interval. That is, the period per pixel is extended or delayed. On the other hand, if negative, the time k is deleted. That is, the cycle per pixel is shortened. In this case, in order to delete the time k, it is necessary for the light beam modulation control unit 216 to pre-read the image signal for one line. Therefore, as an example of the configuration of the light beam modulation control unit 216, the light beam modulation control unit 216 has a buffer that stores an image signal for one line. When the image signal stored in the buffer is output for correction of the light beam, the image signal is output at a timing delayed by time k from the original video clock period, or advanced by time K at every interval m / x. To output. In this way, the signal speed at which the image signal is output is changed in accordance with the correction magnification, that is, the correction coefficient.

  The image signal output in this way is modulated by PWM or the like as necessary in the light beam modulation control unit 216, and a light beam drive signal is generated by the output signal and transmitted to the light beam drive unit 212.

  Note that the correction may be performed on the drive signal applied to the modulated light beam driving unit without correcting the image signal. In this case, in order to delete the minute pixel piece, the drive signal is buffered for one line as a digital signal, for example, and transmitted to the light beam drive unit while performing correction.

<Specific example>
This will be described in more detail using specific numerical values. In FIG. 7, as in FIG. 6, the inter-sensor scanning line length L = 252 mm, the scanning line length L _u = 210 mm, the target scanning time T = 120 μsec, the target use scanning time T _u = T _p = 100 μsec, and n = 10. To do. In addition, the inter-sensor scan time t ₉ of the ninth scan is set to 120.525 μsec, and the inter-sensor scan time t ₁₀ of the tenth scan is set to 120.520 μsec. Further, an inter-sensor scanning time t ₁₁ of the 11th scan is set to 120.525 μsec, and an inter-sensor scanning time t ₁₂ of the 12th scan is set to 120.530 μsec.

The correction magnification, pixel drawing time, and scanning line length for the nth scan, n + 1th scan, and n + 2th scan are as follows. The correction magnification a _{10 for the} tenth scan is a ₁₀ = t ₉ /T=1200.525/120=1.004375 from Equation 1-6. The correction magnification a _{11 for the} eleventh scan is a ₁₁ = t ₁₀ /T=1200.520/120=1.004333 from Equation 1-6. The correction magnification a ₁₂ of the twelfth scan is a ₁₂ = t ₁₁ /T=1200.525/120=1.004375 from Equation 1-6. These values are obtained by the correction amount prediction unit 215.

When this correction magnification is applied, the pixel drawing time t _{p —} 10 for the tenth scan is corrected to t _p — ₁₀ = a ₁₀ × T _u = 1.004375 × 100 = 100.4375 (μsec) from Equation 1-7. The pixel drawing time t _{p —} 11 for the eleventh scan is _corrected to t _p — ₁₁ = a ₁₁ × T _u = 1.004333 × 100 = 100.4333 (μsec) from Equation 1-7. The pixel drawing time t _{p —} 12 for the twelfth scan is _corrected from Equation 1-7 so that t _p — ₁₂ = a ₁₂ × T _u = 1.004375 × 100 = 100.4375 (μsec).

As a result of this correction, the scanning line length L _{u —} 10 for the tenth scan is _expressed as L _u — ₁₀ = (252 × 100 × 120.525) / (120 × 120.520) = 210.0087122 (mm) from Equation 1-8. Become. The scanning line length L _{u —} 11 of the eleventh scan is L _u — ₁₁ = (252 × 100 × 120.520) / (120 × 120.525) = 209.999912881 (mm) from Expression 1-8. The scanning line length L _{u —} 12 for the twelfth scanning is _expressed as L _u — ₁₂ = (252 × 100 × 120.525) / (120 × 120.530) = 209.99998585 (mm) from Equation 1-8. In this way, the use scanning line length of each scanning line is corrected so as to be substantially equal.

  In order to realize this correction, in the light beam modulation control unit 216, the correction magnification is 1.004375 in the 10th scan, the correction magnification is 1.004333 in the 11th scan, and the correction magnification is 1.004375 in the 12th scan. The output light beam is controlled as follows.

Here, for example, the number of pixels in the main scanning direction is set to m = 5000 and k = 1/20. In the above embodiment, 10 correction factor a _n of scanning and 12th scanning eyes is a _n = 1.004375. Therefore, a _n = 1.004375, when the m = 5000, k = 1/ 20 are substituted into equation 1-9, and x = 437.5. That is, in the 10th scan and the 12th scan, the correction magnification can be set to 1.004375 by performing the control of inserting the minute pixel piece 438 times in one scan. Preferably, 438 small pixel pieces are uniformly distributed in the scanning line. That is, since 5000 / 438≈11.4, a minute pixel piece is inserted every 11 pixels. For evenness, every 11 pixels and every 12 pixels may be alternately repeated. Similarly, in the eleventh scan, x≈433, and the correction magnification can be set to 1.004333 by inserting 1/20 minute pixel pieces 433 times in one scan.

FIG. 8 is a graph showing how t1 and t2 vary from scan to scan due to jitter due to air resistance. When the scanning time for scanning the n scan th and t1 _n, t2 _n, t1 _n, t2 _n although both located above and below the variance for each scan, the variation amount compared to the maximum value / minimum value of the variation micro Can be considered. Further, it can be seen that the fluctuation does not fluctuate rapidly from the maximum value to the minimum value, but fluctuates slowly within a certain fluctuation range. That is, the fluctuation range for each scan can also be regarded as being minute compared to the maximum value / minimum value of the fluctuation. Here, if the difference in scanning time between the (n−1) th scanning and the nth scanning is Δt _n ,
t _n-1 = t _n −Δt _n (Formula 1-10)
It is. From Formula 1-8,

... (Formula 1-8 ')
It becomes.

Expression 1-8 ′ indicates that Δt _n / t _n is an error when the correction according to the present embodiment is performed. Here, since the relationship Δt _n << t _n is established, it can be said that the error is very small.

  As described above, the correction amount in the next scan is predicted from the immediately preceding scanning timing information, the light beam is modulated according to the correction amount, and the main scanning overall magnification is corrected with a uniform distribution in the main scanning direction. By doing this, it is possible to correct the low frequency component of the scanning time jitter that changes every scan, and to obtain a good image in which the pixel formation position shift at the image edge is not noticeable.

<Modification>
In this embodiment, the image signal is corrected by inserting or deleting a minute pixel piece. However, as described above, it can also be performed by changing the frequency of the video clock which is the synchronizing signal of the image signal. For example, when the video clock is generated using a phase lock loop (PLL) circuit, the frequency to be transmitted can be controlled by a reference potential applied to a voltage controlled oscillator (VCO) used in the PLL circuit. Accordingly, if the inter-sensor scanning time t _n of the n-th scan is obtained, the ratio T / t _n with respect to the target inter-sensor scanning time T is obtained, and the frequency of the pixel clock is multiplied by T / t _n. Change the reference voltage. For example, when the light beam modulation control unit 216 generates a video clock, the light beam modulation control unit 216 performs this change. This change is made during the blanking period of the scan, and the change of the video clock frequency is completed at the start of the nth scan.

[Second Embodiment]
(Example of correcting partial magnification by inserting pixel pieces)
Embodiments to which the present invention is applied will be described below with reference to the accompanying drawings. In the first embodiment, the main scanning overall magnification is corrected with the scanning speed in one scanning constant, whereas in this embodiment, the main scanning is performed by assuming that the scanning speed in one scanning changes at a uniform acceleration. (In this example, it is divided into two), and the partial magnification is corrected for each division.

  The configuration of the entire apparatus is the same as that of the first embodiment, and the configurations of the controller 21 and the image forming unit 22 of the image forming apparatus 20 in the embodiment of the present invention are as shown in FIG.

FIG. 9 is a diagram illustrating an example of a temporal change in the scanning position of the light beam in the present embodiment. In FIG. 9, a broken line 301 indicates a temporal change in the scanning position of the target light beam when there is no jitter, and a solid line 901 indicates the light beam scanning position when jitter occurs in the use scanning area at the nth scan. The time change of is shown. In FIG. 9, T ₁ , T ₂ , and T ₃ are timing target values in the case where a temporal change in the scanning position of the target light beam is realized. In the absence of jitter, T ₁ = T ₃ −T ₂ . Further, t _1n , t _2n , and t _3n are timing values detected in the n-th scan, and a time for scanning the inter-sensor scanning line length L at this time is an inter-sensor scanning time t _n . t _n = t _2n −t _1n . T _{1 to} T ₃ and t _{1 to} t ₃ may be based on the same timing as each other. In the example of FIG. 9, the timing at which the return light is detected by the start point side detection unit 227 is used as the base point.

  In FIG. 9, a characteristic is that the graph 901 in the case where there is jitter is not point-symmetric with respect to the scanning line center position 902. When there is no jitter, it is point-symmetric about the scanning line center position 902. That is, when there is jitter, the time required for the light beam to reach the scanning line center position from the start point detection unit is different from the time required for the light beam to reach the end point detection unit from the scanning line center position. For this reason, when the light beam is modulated with image data supplied in synchronization with the pixel clock at a constant rate, the portion of the image that should originally be at the center position of the scanning line is shifted from the center position of the scanning line. In FIG. 9, in order to prevent the displacement of the scanning line center position, the writing start side drawing time correction magnification and the writing end side drawing time correction magnification are respectively determined by correction amount prediction with the scanning line center position as a boundary. Then, the light beam may be modulated, and the pixel drawing time may be corrected at each correction magnification. In order to prevent misalignment more accurately, it is only necessary to shorten a section to which one correction magnification is applied, perform a correction amount prediction for each section, and correct the drawing time. In the present embodiment, in order to simplify the implementation, a correction method in two right and left sections will be described, particularly by dividing the scan at the scan center position. Of course, the method of the present embodiment can be similarly applied even when divided at an arbitrary scanning position.

Here, Formula 1-1 is redefined in accordance with a case where the scanning speed changes at a constant acceleration (for example, a line 901 in FIG. 9). The scanning speed at the pixel position i is v _ni , the number of pixels in one scanning is m, the time for scanning one pixel at the pixel position i is the scanning time t _{u_ni} , and the time for drawing one pixel at the pixel position i is the pixel drawing time t _{p_ni.} And Further, the ratio (correction magnification) of the pixel drawing time and the use scanning time at the pixel position i is assumed to be _ani . The scanning line length L _{u —} n and the correction magnification an _i of the n-th scanning in the use scanning area are expressed by the following equation 2-1.

... Formula 2-1.
t _{p_ni} = a _ni × t _{u_ni} Equation 2-2
In other words, the pixel drawing time _{tp_ni} is a time obtained by multiplying the scanning time by the correction magnification, and the distance (that is, the scanning length per pixel) when the time is scanned at the scanning speed at the pixel position is calculated per scanning. As a result, the n-th scanning line length L _{u —} n is obtained.

In FIG. 9, the linear velocity change within the scanning time t _n of the n-th scanning changes continuously due to the influence of jitter due to the air resistance of the vibrating mirror 224. Assuming that the scanning speed (starting point side scanning speed) in the writing start side light beam timing detection unit 227 is v _nB and that the scanning speed v _nB changes with the constant acceleration α, the scanning speed v _n (t) at time t is Is given as follows.
v _n (t) = v _nB + αt Equation 2-3.

The scanning speed in the write end side light beam timing detection unit 228 (end point scanning speed) and v _nE, v the ratio between _nB and v _nE, the starting point side and the end point respectively sensor range with a transit time of When approximated by the reciprocal ratio, Equation 2-4 is established.

... Formula 2-4.
That is,

  ... Formula 2-4 '.

Here, when the end point side scanning speed v _nE is _obtained by the inter-sensor scanning time t = t2 _n −t1 _n in the expression 2-3, the following expression 2-5 is obtained.

... Formula 2-5.
From Formula 2-4 and Formula 2-5,

... Formula 2-6.
Further, from the formula of the distance of the uniform acceleration motion, the scanning line position function L _n (t) at the time t is as follows.

  ... Formula 2-7.

Here, the starting point side scanning speed v _nB is obtained. In this case as well, when approximated by the ratio of the reciprocal number to the target scanning speed v _T as in Expression 2-4, the following expression is established.

... Formula 2-8.
Therefore,

... Formula 2-8 '.
Since the target scanning speed v _T is a speed at which the inter-sensor scanning line length L is scanned with the target inter-sensor scanning time T,
v _T = L / T.
Therefore,

... Formula 2-8 ".
Substituting into Equation 2-4 ′,

  ... Formula 2-4 ".

... Formula 2-7 '.
From Formula 2-7 ′, the time required for each pixel position in Formula 2-1 can be obtained, and magnification correction at each pixel position can be controlled.

Here, in this embodiment, for further simplification, the quadratic equation of Expression 2-7 is approximated by linear approximation that is bent at the center of the scanning line. Therefore, FIG. 10 shows a method of _obtaining the pixel drawing time t _{u_n_B} from the writing start position of the use scanning area to the scanning line center position and the pixel drawing time t _{u_n_E} from the scanning line center position to the writing end position of the use scanning area. It explains using. In FIG. 10, if the n-th scan is scanned at the speed v _nB up to the center of the scan line, the time t _{u — n — B} required to scan half of the inter-sensor scan line length, that is, the distance L / 2 is From 2-8 ”

  ... Formula 2-9.

On the other hand, when scanning from the scanning center to the writing end position with v _nE , similarly, the time t _{u_n_E} required to scan the scanning line length L / 2 is _obtained from the equation 2-4 ″:

  ... Formula 2-10.

However, t1 _n , t2 _n , and t3 _n are timing information that cannot be obtained unless the nth scan is scanned. For this reason, the timing information t1 _n−1 , t2 _n−1 , t3 _n−1 obtained in the _n− 1th scan is replaced with t1 _n , t2 _n , t3 _n respectively, and the nth scan The pixel drawing time is determined.

At this time, the correction magnification an _{n_B} on the writing start side and the correction magnification an _{n_E} on the writing end side are _ratios between the pixel drawing times t _{u_n_B} and t _{u_n_E} and the time T / 2 that is half the target sensor scanning time T. It is. Therefore, the following equation holds.

  ... Formula 2-9 '.

  ... Formula 2-10 '.

<Correction Operation in Image Forming Apparatus>
Now, the above description is organized according to the configuration of FIG. 1 as follows. When the light beam is detected by the start point side detection unit 227, the timing information detection unit 214 starts measuring time. At the same time, it is determined whether the light beam detected by the start point detection unit 227 is scanning line light or return light. The determination method is as described in the first embodiment. If the immediately preceding detection is performed by the end point side detection unit 228, it is determined that it is return light. If it is return light, then if a light beam is detected by the starting point side detection unit 227, the timer is read and stored as time t _1n . Next, if a light beam is detected by the end point detection unit 228, the timer is read and stored as time t _2n . Next, when the light beam is detected by the end point side detection unit 228, the timer is read and stored as time t _3n . Then, the stored value is notified to the correction amount prediction unit 215.

First, the correction amount prediction unit 215 _obtains correction magnifications _{an_B} and _{an_E} for the next scanning line n according to Equations 2-9 ′ and 2-10 ′. The value stored in the target value storage unit is used as the time T1. Further, the correction amount predicting unit 216 further calculates a use scanning _threshold from the predetermined size (delay time) k of the small pixel piece, half m / 2 of the number of pixels included in the use scanning area, and the correction magnification an_B. The number _xb of minute pixel pieces to be inserted (or deleted) in the first half is calculated. If the size of one pixel is 1,
a _{n — B} × m / 2 = (m / 2−x _b ) + (1 + k) × x _b (Formula 2-11)
It becomes. Therefore, x _b = (a _n — _B −1) · m / 2k. Obtains the number x _b of minute pixel piece in accordance with this equation, and stores the value in search of the interval m / 2x _b. When the correction magnification is less than 1, _xb is negative. In this case, the light beam modulation control unit 216 deletes the minute pixel piece instead of inserting it. The calculation of the correction amount is completed by the time scanning of the next scanning line is started.

For the second half of the scanning line, x _E is obtained in the same manner, and the interval m / 2x _E is obtained. The obtained value is transmitted to the light beam modulation correction unit 216.

The light beam modulation correction unit 216 modulates the light beam according to the image signal when scanning the next scanning line. At that time, correction is performed according to the correction amount stored in the correction amount prediction unit 215. That is, in the first half of the scan, with reference to the insertion or deletion interval m / 2x _b of minute pixel piece, in the second half Referring to m / 2x _E. If the interval is positive, a delay k corresponding to a minute pixel piece is inserted. If the interval is negative, the time k is deleted. The boundary between the first half and the second half of the scanning line can be notified to the light beam modulation correction unit 216 by the timing information detection unit 214 and the correction amount prediction unit 215. For example, the scanning speed v _nB of the first half of the scanning line of _interest is obtained from the immediately preceding scanning line by Expression 2-8 ′, and the scanning time L / 2v _nB of the first half is further obtained. Then, the time when the time L / 2v _{nB has} elapsed from the time when the scanning line light is detected by the start point side detection unit 227 is the time when the intermediate position of the use scanning area is scanned, so that the light beam modulation control unit 216 is notified accordingly. Notice. At this timing, the light beam modulation control unit 216 switches between the correction on the first half side and the correction on the second half side.

<Specific example>
The second embodiment will be described in more detail using specific numerical values. In FIG. 10, the inter-sensor scanning line length L = 252 mm, the utilization scanning line length L _u = 210 mm, the target inter-sensor scanning time T = 120 μsec, the target utilization scanning time T1 = T3−T2 = 80 μsec, and n = 10. Further, the scanning time of the ninth scan is t1 ₉ = 80.525 μsec, t2 ₉ = 201.050 μsec, and t3 ₉ = 281.010 μsec. The scanning time for the tenth scan is t1 ₁₀ = 80.520 μsec, t2 ₁₀ = 201.040 μsec, and t3 ₁₀ = 281.0000 μsec. The scanning time for the eleventh scan is t1 ₁₁ = 80.525 μsec, t2 ₁₁ = 201.050 μsec, and t3 ₁₁ = 281.025 μsec. The scanning time of the 12th scan is t1 ₁₂ = 80.530 μsec, t2 ₁₂ = 201.060 μsec, and t3 ₁₂ = 281.025 μsec.

10 correction factor a _{10_E} correction factor a _{10_B} and write end side of the scanning th writing start side, respectively _{a 10_B = t1 9 /T1=80.525/80=1.0065625,a 10_E =} (t3 9 -t2 ₉ ) / T1 = (280.510-201.050) /80=0.9995000. 11 correction factor a _{11_E} correction factor a _{11_B} and write end side of the scanning th writing start side, respectively _{a 11_B = t1 10 /T1=80.520/80=1.0065000,a 11_E =} (t3 10 -t2 ₁₀ ) / T1 = (280.500-201.040) /80=0.9995000. 12 correction factor a _{12_E} correction factor a _{12_B} and write end side of the scanning th writing start side, respectively _{a 12_B = t1 11 /T1=80.525/80=1.0065625,a 12_E =} (t3 11 -t2 ₁₁ ) / T1 = (280.525-201.050) /80=0.996875. The calculation of the number of corrected pixels is the same as in Expression 1-9.

For example, when the number of main scanning pixels up to the center position at the 10th scan is m = 2500, when inserting (deleting) k = 1/20 minute pixels, a _{10_B} = 1.0065625, m = 2500, k Substituting = 1/20 into Equation 1-9 gives x _B = 328.125. a _{10_E} = 0.9995000, when the m = 2500, k = 1/ 20 are substituted into equation 1-9, and x _E = -25.000. That is, at the 10th scan, a desired correction magnification control can be performed by performing control of inserting a minute pixel piece 328 times on the writing start side and deleting the minute pixel piece 25 times on the writing end side.

Similarly, a x _B = 325 Substituting a _{11_B} = 1.0065000, the m = 2500, k = 1/ 20 in Equation 1-9. a _{11_E} = 0.9995000, when the m = 2500, k = 1/ 20 are substituted into equation 1-9, and x _E = -25. If a ₁₂ — _B = 1.0065625, m = 2500, and k = 1/20 are substituted into Equation 1-9, x _B = 328.125 is obtained. a _{12_E} = 0.9996785, when the m = 2500, k = 1/ 20 are substituted into equation 1-9, and x _E = -15.625.

  That is, in the eleventh scan, control is performed such that the minute pixel piece is inserted 325 times on the writing start side and the minute pixel piece is deleted 25 times on the writing end side. In the twelfth scan, control is performed so that the minute pixel piece is inserted 328 times on the writing start side and the minute pixel piece is deleted 16 times on the writing end side. With this control, it is possible to correct the image signal with different correction magnifications in the first half and the second half of the scanning line.

  In the present embodiment, for the sake of simplification, an example is shown in which each correction magnification is obtained by dividing into two sections from the writing start to the center position and from the center position to the writing end. However, more accurate correction magnification control can be performed by finely dividing the drawing position and obtaining the drawing time according to Equation 2-7 '.

[Third Embodiment]
(Prediction using least squares method)
In the first embodiment and the second embodiment, an example in which the correction amount calculation is performed using the timing information in the immediately preceding scan for the prediction of the correction amount has been described. However, the correction is performed using the timing information for the immediately preceding multiple scans. The amount may be predicted. The method will be described below. A case where the basic configuration is the same as in the case of the first embodiment will be described.

Correction factor a _n is defined from equation 1-6.
a _n = t _n-1 / T Equation 1-6.
This correction magnification is stored for a plurality of scans immediately before the scanning line of interest (that is, from the nc to the nth scan, where c is a predetermined value). At the (n + 1) th scan, an approximate curve is obtained by the least square method using these values. From this approximate curve, the correction magnification at the (n + 1) th scan is obtained, and correction is performed in the same manner as in the first embodiment using the correction magnification. Even in the same configuration as in the second embodiment, the correction factor prediction using the least square method can be applied.

  In this way, the correction magnification obtained from the measurement value of the immediately preceding scanning line is not used as it is, but a plurality of correction magnifications are obtained from the measurement values for the plurality of scanning lines, and the correction magnification to be applied is determined by the least square method. As a result, variations in scanning line length due to correction can be suppressed.

[Fourth Embodiment]
(Prediction using moving average)
In the third embodiment, the method of predicting the correction amount using the timing information for a plurality of immediately preceding scans has been described. However, another statistical method may be used for this prediction. For example, as in the third embodiment, the correction magnification is stored for a plurality of scans immediately before the scanning line of interest (that is, from nc to nth scan, where c is a predetermined value). Then, the value obtained by taking the moving average of the correction magnifications for a plurality of scans immediately before being stored is used as the correction magnification in the (n + 1) th scan. Of course, the moving average of this embodiment can also be applied to the correction method of the second embodiment in which one scanning line is divided into a plurality of portions for correction.

  Thus, the correction magnification obtained from the measurement value of the immediately preceding scanning line is not used as it is, but a plurality of correction magnifications are obtained from the measurement values for the plurality of scanning lines, and the correction magnification to be applied is determined by the moving average. As a result, variations in scanning line length due to correction can be suppressed.

1 is a diagram illustrating a configuration of an image forming apparatus. It is a figure which shows the image quality degradation by the jitter of a vibration mirror, and the effect by this invention. It is a figure which shows the relationship between the time change of a scanning position, timing information, and scanning line length. It is a figure which shows that the relationship between the time change of a scanning position, timing information, and scanning line length changes with jitter. It is a figure which shows the relationship between scanning time, pixel drawing time, and scanning line length. It is a figure which shows the relationship between scanning time, pixel drawing time, and scanning line length, Comprising: When a jitter generate | occur | produces, it is a figure explaining the case where correction | amendment is not performed. It is a figure which shows the relationship between scanning time, pixel drawing time, and scanning line length, Comprising: It is a figure explaining the case where correction | amendment is performed when jitter generate | occur | produces. It is a figure which shows the change for every scanning of the timing information in case the jitter has generate | occur | produced. It is a figure which shows that the relationship of the time change of a scanning position, timing information, and scanning line length changes with jitter, Comprising: It is a figure explaining the case where a scanning speed changes at equal acceleration within 1 scan. It is a figure which shows the relationship between a scanning time, pixel drawing time, and scanning line length, Comprising: Jitter generate | occur | produces and it is a figure explaining correction | amendment of the drawing time performed when a scanning speed changes at equal acceleration within 1 scan. is there.

Explanation of symbols

10 PC
20 image forming apparatus 21 controller 22 image forming unit 211 image generating unit 212 light beam driving unit 213 target value storage unit 214 timing information detecting unit 215 correction amount predicting unit 216 light beam modulation control unit 221 horizontal synchronizing signal generating unit 222 vertical synchronizing signal Generation unit 223 Mirror drive unit 224 Vibration mirror 225 Light beam unit 226 Photosensitive drum 227 Writing start side light beam timing detection unit 228 Writing end side light beam timing detection unit 229 Contour speed conversion optical system

Claims (9)

Modulation means for modulating the light beam according to the image signal;
An optical scanning means for repeatedly scanning the scanning surface by deflecting the light beam modulated by the modulating means;
Detecting means for detecting a scanning speed on the scanning surface of the light beam deflected by the light scanning means;
An optical scanning apparatus comprising: correction means for correcting an image signal used for modulation of the light beam in accordance with a change in scanning speed detected by the detection means.   The optical scanning unit includes a galvanometer mirror that periodically oscillates around an axis, and periodically scans a scanning surface by reflecting a light beam modulated by an image signal by the galvanometer mirror. The optical scanning device according to claim 1.   The correction means changes the signal speed of the image signal corresponding to one scan in accordance with the change of the scanning speed of the light beam so that the scanning line length scanned by the light beam modulated by the image signal is constant. The optical scanning device according to claim 1, wherein:   The correction means modulates with the image signal corresponding to one scan in accordance with a change in the scanning speed of the light beam so that the scanning line length scanned with the light beam modulated with the image signal is constant. The optical scanning device according to claim 1, wherein an output speed of a driving signal for the light beam is changed. The correction means corrects the image signal so as to extend or shorten the period of pixels included in the image signal by a time shorter than the period of the pixels,
The number of pixels to be corrected is the scanning time of the scanning line length by the light beam modulated by the image signal, which is the product of the length of time that is expanded or shortened per pixel and the number of pixels to be corrected. 5. The optical scanning device according to claim 3, wherein the number is such that a difference from a time from the beginning to the end of the image signal corresponding to one scanning line before correction.   6. The optical scanning according to claim 1, wherein the correction unit corrects the image signal for each scanning line based on a scanning speed detected for the immediately preceding scanning line. 7. apparatus.   7. The optical scanning device according to claim 1, wherein the correction unit corrects the image signal for each section obtained by dividing one scan into a plurality of sections.   The correction unit scans the correction target based on information on the scanning speed of the plurality of scanning lines detected by the detection unit before the correction target scanning line and a target value of the scanning speed stored in advance. The optical scanning apparatus according to claim 1, wherein the image signal placed on a line is corrected. An optical scanning device according to any one of claims 1 to 8,
Image generating means for inputting an image signal to the optical scanning device;
A surface is a scanning surface scanned by the optical scanning device, and a photosensitive drum that rotates each time scanning is performed;
An image forming apparatus comprising: an image forming unit that develops an image formed on the surface of the photosensitive drum and forms the image on a print medium.

Images