JP2006088567A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To effectively prevent a stripe from being generated caused by deviation of a light emitting position in the main scanning direction in an image forming apparatus irradiating repeatedly an image carrier with a laser light. <P>SOLUTION: The image forming apparatus forming an image on the image carrier by repeatedly scanning along the main scanning direction by a light emitting element array in which a plurality of light emitting elements are arranged at equal intervals along a sub-scanning direction, is characterized by that a decrease in a peak light exposing intensity generated by the deviation of a light exposing position in the main scanning direction by a repeated scanning is ≤20% in comparison with a case in which there exists no deviation of the light exposing position in the main scanning direction. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus such as a copying machine or a printer that forms an electrostatic latent image on a photosensitive member by overlapping scanning with a light beam, and particularly has little image unevenness.

  Surface emitting lasers can be easily arrayed to output a large number of beams from a single chip. High resolution is achieved by multi-beam scanning, in which a plurality of light beams are emitted from such a surface emitting laser and main surface of the photosensitive member is simultaneously scanned. Can be realized (Patent Document 1).

  In the multi-beam scanning, as a method for preventing array width bunting that occurs at a frequency that is highly sensitive to human eyes, there is a method for correcting the amount of light at the joint (Patent Document 2), a double exposure method, and the like. is there.

Since the double exposure method performs exposure twice along the sub-scanning direction, it has the effect of making inconspicuous streaks caused by misalignment in the sub-scanning direction of about 5 μm, such as surface tilt of the polygon mirror and beam interval error. high.
JP 2001-215423 A JP 2003-182139 A

  However, streaks caused by multi-beam scanning are caused by misalignment in the main scanning direction due to scanner motor jitter, polygon mirror vibration, video signal timing deviation, etc., in addition to those caused by misalignment of the scanning position in the sub-scanning direction. There is something that happens.

  If such vibrations occur during image formation and the scanning position in the main scanning direction shifts between the two exposures, the irradiation position on the photoconductor shifts, and the exposure energy is partially reduced. This can cause a decrease in density, resulting in streaks.

  In order to prevent the above-described streak caused by the deviation in the main scanning direction, it is necessary to design the exposure unit that exposes the photosensitive member with the light beam so that the deviation of the scanning position in the main scanning direction does not occur. there were. However, it is very difficult to design to completely prevent these deviations, and for the vibration generated in the grounding environment of the apparatus, the image forming apparatus having the exposure unit performs appropriate correction during operation, It is necessary to prevent the deviation of the scanning position in the main scanning direction.

  SUMMARY An advantage of some aspects of the invention is that it provides an image forming apparatus that can effectively prevent the occurrence of streaks due to deviation in the main scanning direction.

  The invention according to claim 1 forms an image on the image carrier by performing overlapping scanning along the main scanning direction by a light emitting element array in which a plurality of light emitting elements are arranged at equal intervals along the sub scanning direction. The image forming apparatus is characterized in that a decrease in peak exposure intensity caused by an exposure position shift in the main scanning direction due to overlapping scanning is 20% or less compared to a case where there is no exposure position shift in the main scanning direction. The present invention relates to an image forming apparatus.

  In double exposure, when the exposure position is shifted between the first scan and the second scan, the peak value of the exposure intensity is lower than when the exposure positions are matched, and the latent image on the photoconductor is lowered. Since the peak of the image potential distribution changes, the image density is lowered.

  Here, the limit value of the human eye that can detect the density difference of the image is 0.01. According to the study by the present inventors, the decrease in the exposure amount when the density difference of the image reaches the limit value is as follows. About 20%.

  In the image forming apparatus, the decrease in the peak exposure intensity caused by the exposure position deviation in the main scanning direction is 20% or less as compared with the case where there is no exposure position deviation. The non-uniformity is such that it cannot be detected by the human eye.

  Therefore, according to the image forming apparatus, there is no image defect that can be detected by the human eye due to an exposure position shift in the main scanning direction.

  According to a second aspect of the present invention, in the image forming apparatus according to the first aspect, the main scanning at the exposure position because the peak exposure intensity is 20% or less as compared with the case where there is no deviation of the exposure position in the main scanning direction. When the image coverage, which is the area occupied by the image on the image carrier, is the allowable exposure position deviation amount S in the direction is 20%, the light beam diameter L in the main scanning direction and from when a pixel is turned on to when it is turned off From the continuous lighting length T, which is the number of pixels in the main scanning direction,

S = aL ² + bT ² + c (a, b, c are constants determined experimentally)

The image forming apparatus is defined as follows.

  In an electrophotographic image forming apparatus, an image density in a certain section is defined as an area occupied by an image, that is, an image coverage. When the deviation in the main scanning direction is the same, the change in image density is small when the image coverage is large, and large when the image coverage is small. Therefore, in a highlight image with an image coverage of about 20%, the change in image density with respect to the shift amount in the main scanning direction is large.

  The image forming apparatus obtains the relationship between the allowable exposure position deviation amount S, the light beam diameter L, and the continuous lighting length T for the image coverage having the largest image change with respect to the exposure position deviation. In such image coverage, by obtaining the light beam diameter L and the continuous lighting length T that give the allowable exposure position deviation amount S, the allowable exposure position in which image defects such as stripes do not occur in all image coverage. The deviation amount S can be determined.

  According to a third aspect of the present invention, in the image forming apparatus according to the second aspect, an image is formed on an image carrier using toners of four colors of yellow, magenta, cyan, and black, and the continuous lighting length is set. The present invention relates to an image forming apparatus in which a screen having the smallest allowable exposure position deviation amount S obtained from the relationship T and the light beam diameter L among the four types of screens is a yellow screen.

  In an electrophotographic image forming apparatus, an image is formed as a collection of dots, in other words, a mesh pattern, or in other words, a screen shape. The shape of the dots forming the screen is likely to be affected by the exposure position deviation in the main scanning direction. In other words, the influence of the exposure position deviation in the main scanning direction is difficult to appear. Some have a strong position shift.

  Here, when an image is formed with four colors of yellow, magenta, cyan, and black, the screen is rotated so that image defects such as streaking (banding) and moire do not occur, and screens having different forms for each color are formed. Must be selected. Therefore, it is not possible to select an optimal screen for all colors, that is, a screen that is strong in exposure position deviation.

  Therefore, in order to make the density change due to the exposure position shift inconspicuous, it is preferable to select a screen that is weak in the position shift for yellow, which is known to have the smallest weight given to the density change vision.

  The image forming apparatus has an effect of making the density change due to the exposure position shift inconspicuous by applying a screen weak to the position shift to yellow.

  According to a fourth aspect of the present invention, in the image forming apparatus according to any one of the first to third aspects, the ratio of the light emission amount in the two exposures is changed in the overlapping scanning in the main scanning direction. The present invention relates to an image forming apparatus capable of performing the above.

  The image forming apparatus is suitable to be installed and used in a place with a lot of vibration because the deformation of the exposure energy distribution is reduced and the influence of the exposure position deviation in the main scanning direction can be minimized. is there.

  As described above, according to the present invention, there is provided an image forming apparatus that can effectively prevent the occurrence of streaks due to misalignment in the main scanning direction in an exposure apparatus that uses multiple exposure.

  1. Embodiment 1

  A color image forming apparatus using an electrophotographic process as an example of the image forming apparatus of the present invention will be described below.

  As shown in FIG. 1, the color image forming apparatus 1000 according to the first exemplary embodiment exposes the surface of the photoreceptor 1, the photoreceptor 1 rotating in the direction of the arrow, the charger 2 that charges the surface of the photoreceptor 1. An exposure device 3, a developing device 4 for developing with toner, a primary transfer device 5 for primary transfer of a toner image, an intermediate transfer belt 6 to which a toner image is transferred by the primary transfer device 5, and an intermediate A secondary transfer device 7 that transfers the toner image on the transfer belt 6 to the paper, a paper tray 8 that stores the paper, a paper transport roll 9 that transports the paper in a predetermined direction, and a cleaning device 11 that removes the remaining toner. And an image processing unit 40 for generating data for driving a surface emitting laser array 12 to be described later based on the image data.

  The surface of the photoreceptor 1 is charged by a charger 2. The exposed surface of the charged photoreceptor 1 is selectively exposed by the exposure device 3 to generate an electrostatic latent image. The developing device 4 visualizes the electrostatic latent image using toner and forms a toner image. The toner image formed on the photoreceptor 1 is transferred onto the intermediate transfer belt 6 by the primary transfer device 5.

  The toner image on the intermediate transfer belt 6 is transferred by the secondary transfer device 7 onto the paper transported from the paper tray 8 by the paper transport roll 9 or the like. The fixing device 10 melts and fixes the toner image transferred to the paper. The toner remaining on the photoreceptor 1 after the primary transfer is collected from the surface of the photoreceptor 1 by the cleaning device 11.

  The color image forming apparatus forms a full-color image by repeating charging, exposure, development, and primary transfer four times for each of Y (yellow), M (magenta), C (cyan), and K (black). . At this time, the developing device 4 switches the color of the toner to be developed by rotating 90 degrees for each cycle.

  The four color toner images are superimposed on the intermediate transfer belt 6. Accordingly, the paper transport roll 9 does not transport the paper to the secondary transfer device 7 until the four-color image formation is completed. When the secondary transfer device 7 is in contact with the intermediate transfer belt 6, the secondary transfer device 7 is retracted so as not to touch the intermediate transfer belt 6 until the sheet is conveyed.

  As shown in FIG. 2, the exposure apparatus 3 includes a surface emitting laser array 12 that emits a plurality of beams and a polygon mirror 19 that scans the plurality of beams in the main scanning direction.

  The surface emitting laser array 12 generates a plurality of beams. In FIG. 2, only two beams are shown for simplicity. The surface-emitting laser array 12 that can be easily arrayed can generate several tens of beams, and the arrangement of the beams is not limited to one column, and can be arranged two-dimensionally. . In the present embodiment, the surface emitting laser array 12 is two-dimensionally arranged and the number of laser elements is 2 m [pieces]. Here, in the surface emitting laser array 12, the light emitting elements are two-dimensionally arranged as shown in FIG.

  As shown in FIG. 2, the beam emitted from the surface emitting laser array 12 is converted into substantially parallel light by the collimating lens 13. The half mirror 14 separates a part of the beam and guides it to the light quantity detection sensor 16 via the lens 15. Unlike the edge-emitting laser, the surface-emitting laser array 12 cannot emit a beam from the back side of the resonator. Therefore, in order to obtain a monitor signal for light quantity control, a part of the beam emitted from the surface emitting laser array 12 is separated and then guided to the light quantity detection sensor 16 as described above.

  The beam that has passed through the half mirror 14 is shaped by the aperture 17. The aperture 17 is preferably arranged in the vicinity of the focal position of the collimating lens 13 in order to uniformly shape a plurality of beams.

  The beam shaped by the aperture 17 is imaged in the form of a long line in the main scanning direction near the reflecting surface of the polygon mirror 19 by the cylinder lens 18 having power only in the sub-scanning direction, and the direction of the polygon mirror 19 by the folding mirror 20 Reflected in.

  The polygon mirror 19 is rotated by a scanner motor (not shown) to deflect and reflect the beam in the main scanning direction. The beam deflected and reflected by the polygon mirror 19 is imaged on the photosensitive member 1 in the main scanning direction by the Fθ lenses 21 and 22 having power only in the main scanning direction, and moves on the photosensitive member 1 at a substantially constant speed. To be imaged. The beams that have passed through the Fθ lenses 21 and 22 are imaged on the photoreceptor 1 by the cylinder mirrors 24 and 25 having power only in the sub-scanning direction.

  Further, since the exposure apparatus 3 needs to synchronize the scanning start on each reflecting surface of the polygon mirror 19, the pickup mirror 26 that reflects the beam before the scanning start and the beam reflected by the pickup mirror 26 are detected. And a synchronizing light amount detection sensor 27.

  The 2m channel LD driver 30 described later drives the surface emitting laser array 12 based on the input image data, and controls the amount of light of each laser to be a predetermined amount by a laser driving amount control unit (not shown).

  As shown in FIG. 4, the image processing unit 40 includes an image controller 100 that outputs m-line image data, a line buffer / timing controller 110 that outputs 2m-line bitmap data, and data timing for 2m channels. A 2m channel timing adjustment circuit 120 that performs adjustment and an M / C controller 130 that generates a page synchronization signal PS are provided.

  The M / C controller 130 generates a page synchronization signal indicating the start of image formation. When the line buffer / timing controller 110 detects the page synchronization signal, the page synchronization signal PS corresponding to the page synchronization signal, the line synchronization signal LS corresponding to the synchronization signal (SOS) supplied from the exposure apparatus 3, and the image Supply to the controller 100. The image controller 100 outputs image data for m lines in response to the page synchronization signal PS and the line synchronization signal LS.

  The line buffer / timing controller 110 has a line buffer for m lines, and outputs data for every 2 m lines while shifting the data by m lines.

  For example, in the Nth scan, the first m lines of the line buffer are updated with the data at the (N−1) th scan, and the second m lines of data are updated with data supplied from the image controller 100. Update. In the (N + 1) th scan, the first m lines of the line buffer are updated with the Nth scan data, and the latter m line data are updated with the (N + 1) th scan data.

  As shown in FIG. 5, the line buffer / timing controller 110 includes an m-line bitmap interface 111, an m-line X-pixel FIFO memory 112, a first m-bit data latch 113, and a second m-bit data latch 114. And.

  When the image data is supplied from the image controller 100, the m-line bitmap interface 111 supplies m-line bitmap data to the m-line X pixel FIFO memory 112 and the second m-bit data latch 114. At the same time, the m-line X pixel FIFO memory 112 reads out the data written in the previous main scan and supplies it to the first m-bit data latch 113.

  The first m-bit data latch 113 and the second m-bit data latch 114 latch and output each data in synchronization with the P clock. The data output from the first m-bit data latch 113 and the second m-bit data latch 114 are combined and supplied to the 2m channel timing adjustment circuit 120 as 2m line data X pixel data.

  As shown in FIG. 6, the m-line X pixel FIFO memory 112 is reset when a page synchronization signal PS for starting image formation is supplied, and starts data transfer when the page synchronization signal PS becomes active.

  The image controller 100 outputs the image data of line numbers 1 to m at the timing of the line synchronization signal LS0 and supplies it to the m-line bitmap interface 111. The m-line bitmap interface 111 writes the bitmap data of line numbers 1 to m to the m-line X pixel FIFO memory 112 and supplies it to the second m-bit data latch 114. At the same time, the m-line X pixel FIFO memory 112 reads out data for m lines and supplies it to the first m-bit data latch 113.

  The outputs of the first m-bit data latch 113 and the second m-bit data latch 114 are combined and output as bitmap data for 2m lines.

  Here, at the timing of the line synchronization signal LS 0, no data is stored in the m-line X-pixel FIFO memory 112. Therefore, the second m-bit data latch 114 outputs only data corresponding to the line numbers 1 to m.

  At the timing of the next line synchronization signal LS1, the image controller 100 supplies the image data of line numbers (m + 1) to 2m to the m-line bitmap interface 111. The m-line bitmap interface 111 writes the bitmap data of line numbers (m + 1) to 2m to the m-line X pixel FIFO memory 112 and supplies it to the second m-bit data latch 114. At the same time, the m-line X pixel FIFO memory 112 reads the bitmap data of line numbers 1 to m and supplies it to the first m-bit data latch 113.

  As a result, at the timing of the line synchronization signal LS1, bit map data of line numbers 1 to 2m is output from the line buffer / timing controller 110.

  With the configuration as described above, the line buffer / timing controller 110 performs control so that the second half of the data for 2 m lines always corresponds to a new scanning line. That is, the line buffer / timing controller 110 shifts data for 2 m lines (however, m lines for the first scan) by shifting by m lines for each line synchronization signal LS. More specifically, by outputting data of line numbers 1 to 2m, data of line numbers (m + 1) to 3m, data of line numbers (2m + 1) to 4m, etc. for each main scanning period, the same data is used. Overwriting is realized.

  The 2m channel timing adjustment circuit 120 shown in FIG. 4 outputs the data in the main scanning direction of each scanning line in consideration of the fact that the light emitting points of the surface emitting laser array 12 are not arranged in a line in the sub scanning direction. The timing is adjusted, and the timing-adjusted bitmap data is supplied to the 2m channel LD driver 30 of the exposure apparatus 3.

  The color image forming apparatus configured as described above uses a surface emitting laser array 12 having 2 m of laser elements arranged two-dimensionally, and the scanning line interval q on the photoreceptor 1 is 2 m at the Nth scan. The main scanning of the two beams is performed to form a scanning line, and the amount of movement in the sub-scanning direction is P = m · q, and the main scanning of the next 2m beams is performed in the next (N + 1) th scanning. At this time, the same data is supplied to the same scanning line in the area exposed in the Nth scanning and the (N + 1) th scanning.

  In the Nth, (N + 1) th, and (N + 2) th scans, as shown in FIG. 7, 8 lines are formed simultaneously. At this time, every time one scan is completed, four lines (movement amount P = 4q) are sent in the sub-scan direction, and the next scan is performed. Therefore, the areas exposed overlappingly between scans occur in a 4-line cycle.

  In the (N + 1) th scan, exposure is performed with a shift of half the scan width (for 4 lines) with respect to the Nth scan. Similarly, in the (N + 2) th and subsequent scans, exposure is performed with a shift of half the scan width of the previous scan. Therefore, as a whole, each scanning line is scanned twice (double exposure).

  Further, as shown in FIG. 8, the exposure areas adjacently overlapped between the scans are the Nth and (N + 2) th, (N + 1) th and (N + 3) th, (N + 2) th and (N + 4) th,. Occurs during scanning. These adjacent overlapping exposure areas are further exposed at the (N + 1) th, (N + 2) th, (N + 3) th,... Scanning. Therefore, although scanning is performed twice per line (double exposure) as a whole, scanning is performed three times in an area where adjacent lines overlap.

  Here, since each scanning line is formed by two exposures, the light amount of the beam per one time can be halved. The amount of charge generated in the overlapping exposure areas of the (N + 1) th and (N + 2) th scans is also relatively reduced to about half. As a result, the density change on the image could be reduced. Further, since the amount of movement in the sub-scanning direction between each scan is also halved, the generation period of the overlapped exposure region generated between the scans is also halved. Accordingly, the image streak along the main transport direction is not visible, and a high-quality image is obtained.

  However, in the color image forming apparatus 1000 as well, due to the jitter of the scanner motor that rotates the polygon mirror 19, the vibration of the polygon mirror 19 itself, the timing shift of the image signal input to the image processing unit 40, etc. A pixel position shift in the main scanning direction, that is, an exposure position shift may occur on the surface.

  Originally, the scan start position in the main scanning direction should be the same in any of the (N−1) th, Nth and (N + 1) th scans. For example, in the Nth and N + 1th scans, the main scan direction When the exposure position shift occurs, the density of the image drawn by the Nth and N + 1th scans is lower than the density of the image drawn by the (N-1) th and Nth scans where no exposure position shift has occurred. Since this continues during scanning as shown in FIG. 9, streaks along the main scanning direction occur.

  When an exposure position shift in the main scanning direction occurs between the first scan and the second scan, or the Nth scan and the (N + 1) th scan, as shown in FIG. 10, the exposure energy peaks as compared to when there is no exposure position shift. A certain exposure peak is lowered.

  Then, when the relationship between the exposure amount peak and the image density when the image is formed under the condition of image coverage of 20%, for example, the relationship as shown in FIG. 11 is obtained.

  Here, the limit value of the human eye for detecting the density difference is 0.01, and the exposure density peak is 100%, that is, the image density corresponding to the exposure quantity when there is no exposure position deviation in the main scanning direction is 0. 2. Therefore, if the image density is 0.19 (= 0.2−0.01) or higher, the human eye cannot detect the image density. The exposure peak corresponding to an image density of 0.19 is 80% of the exposure when there is no exposure position shift in the main scanning direction.

  Therefore, in the color image forming apparatus 1000, if the reduction of the exposure amount peak is within 20% compared to the exposure amount when there is no exposure position deviation in the main scanning direction, it is detected by human eyes. It can be seen that the image unevenness does not occur as much.

  The full-color toner image is formed by forming a mesh pattern, that is, a screen with the Y, M, C, and K toners in the exposure device 3 and the developing device 4 and superimposing them. The relationship between the deviation of the exposure position in the main scanning direction and the change in density differs depending on the screen form.

  Examples of screens used to form toner images are shown in FIGS. 12A, 12B, and 12C. In FIG. 12, the screen shown in (A) is 200C, the screen shown in (B) is 200R, and the screen shown in (C) is 300C. Here, C indicates a cluster type screen and R indicates a rotation type screen. That is, 200C is a cluster type screen having a pitch of 200 line pairs per inch. All the screens shown in FIG. 12 have an image coverage of 20%. As shown in FIG. 12, a screen 200C. In the screen 200R and the screen 300C, the angles of the halftone dots with respect to the main scanning direction are about 70 degrees, about 27 degrees, and about 0 degrees, respectively, and the average continuous lighting distances are 126 μm, 280 μm, and 84 μm. Here, as shown in FIG. 13, the continuous lighting length is a length obtained from the number of pixels from when the light beam is turned on to when it is turned off, but the continuous lighting length is as in the screens 200C, 200R, and 300C. When the length varies depending on the location, an average continuous lighting length that is an average value of these values is obtained.

  For the screen 200C, the relationship between the image coverage size and the image density reduction when the exposure position deviation in the main scanning direction between the first exposure and the second exposure is fixed to 30 μm is obtained, and the result is shown in FIG. Show. As is apparent from FIG. 14, the decrease in image density is maximized when the image coverage is 20%.

  Next, for each of the screens 200C, 200R, and 300C, the relationship between the exposure position shift amount s in the main scanning direction and the image density difference De when the image coverage is fixed to 20% is obtained and shown in FIG. As is clear from FIG. 15, the screen 200R has a smaller image density change De for the same exposure position deviation amount s than the screens 200C and 300C, and is strong against exposure position deviation in the main scanning direction.

  Here, the screen 200R that is resistant to the exposure position shift in the main scanning direction has a continuous lighting length of 280 μm, which is longer than 126 μm of the screen C and 84 μm of the screen 300C. Accordingly, when the relationship between the continuous lighting length of the screen and the amount of decrease in the exposure peak intensity when the exposure position deviation in the main scanning direction is fixed to 20 μm, as shown in FIG. 17 and FIG. It can be seen that the amount of decrease in the exposure peak intensity is smaller when the lighting length is as large as 50 μm than when the lighting length is as small as 10 μm. Further, when the relationship between the exposure position deviation amount in the main scanning direction and the reduction amount of the exposure peak intensity is obtained for each of the cases where the continuous lighting length of the screen is 10 μm, 30 μm, and 50 μm, as shown in FIG. It can be seen that when the continuous lighting length is 50 μm, the decrease in the peak exposure amount is the smallest.

  The image density also changes depending on the beam diameter in the main scanning direction. For example, the change in the image density difference De when the diameter of the light beam in the main scanning direction when the exposure position shift amount s in the main scanning direction is fixed at 17 μm or 30 μm is changed between 50 μm and 200 μm is obtained. As shown in FIG. 19, the image density difference De is smaller when the diameter of the light beam in the main scanning direction is larger in both cases where the exposure position deviation amount s is 17 μm and 30 μm, in other words, the exposure position in the main scanning direction. Strong against misalignment. This is also clear from FIGS. 20, 21, and 22 showing the relationship between the exposure position shift amount s (μm) and the exposure intensity peak (%) for a beam diameter of 50 μm and a beam diameter of 100 μm.

  However, if the beam diameter becomes excessive, the density of the image in the low density region increases and the gradation reproducibility deteriorates. Therefore, when the beam diameter in the main scanning direction is increased, the beam diameter in the sub-scanning direction is reduced. It is preferable to perform gradation correction (TRC).

  From the above matters, in the color image forming apparatus 1000, the exposure position deviation amount at which the decrease in the peak value of the exposure intensity is 20% from the continuous lighting length T and the beam diameter L in the main scanning direction, that is, the allowable exposure position. The deviation amount S is expressed by the following formula:

S = aL ² + bT ² + c

Can be defined by The constants a, b, and c can be obtained experimentally.

  A procedure for performing tolerance design of the exposure apparatus 3 based on the exposure position shift amount S in the main scanning direction given by the above formula will be described.

  For example, in the exposure apparatus 3, if the cylinder mirrors 24 and 25 are bent, the light beam is distorted in the main scanning direction as shown in FIG. As shown in FIG. 24, there is a proportional relationship between the deflection amount of the cylinder mirrors 24 and 25 and the exposure position deviation amount s in the main scanning direction.

  In the exposure apparatus 3, there are various factors that affect the exposure position deviation amount s in addition to the deflection of the cylinder mirrors 24 and 25, so the sum of the exposure position deviation amounts s caused by each factor is the allowable exposure position deviation amount. The allowable exposure position deviation amount S for each factor is a large ratio for a factor having a large influence on the exposure position deviation amount s, and a small ratio for a factor having a small influence on the exposure position deviation amount s so as to be smaller than S. Can be sorted.

  Further, since the color image forming apparatus 1000 uses toner of four colors Y, M, C, and K as shown in FIG. 1, in order to avoid image defects such as banding, screen rotation, that is, different colors are used. It is necessary to select a different screen for the toner. Therefore, it is not possible to use a screen that is optimal for all the four colors, in other words, a screen that is resistant to misalignment of the exposure position in the main scanning direction.

  On the other hand, among the four color images, it is known that the image weighted Y is the smallest for visual perception.

  Therefore, in the color image forming apparatus 1000, for the Y image, the image processing unit 40 is controlled so as to select a screen that is weak in exposure position deviation in the main scanning direction.

  Further, in the case of performing double exposure, the density change due to the exposure position shift in the main scanning direction can be made inconspicuous also by adjusting the ratio of the exposure intensity in the two scans.

  When performing double exposure, for example, the ratio of the exposure amount between the first scan and the second scan is normally 100%: 100%, but the exposure amount for the first scan is maintained while keeping the total amount of light constant. Is increased to 180% and the exposure power for the second scan is decreased to 20%, as shown in FIGS. 25 and 26, even if a 20 μm deviation occurs in the exposure position in the main scanning direction, The collapse is reduced, and as a result, the influence of the exposure position shift s can be reduced.

  In the case of performing adjacent exposure in which the surface-emitting laser array is scanned in parallel, if a shift in the sub-scanning direction occurs between the arrays, the exposure amount is reduced at the array boundary as shown in FIG. There was a problem.

  On the other hand, as shown in FIG. 29, when double exposure is performed so that the exposure amount of the first exposure and the second exposure is 100%: 100%, the exposure amount due to the exposure position shift in the sub-scanning direction. Since the decrease in (exposure energy) is small and the frequency is doubled, the image density unevenness caused by the decrease in the exposure amount is hardly noticeable to human eyes.

  That is, double exposure is effective in reducing image unevenness due to exposure position shift in the sub-scanning direction.

  However, when the ratio adjustment shown in the [0084] column is performed in the double exposure method, if the exposure amount of either the first scan or the second scan is extremely small, the banding may occur as in the case of the adjacent exposure. Concerns arise.

  Therefore, in the exposure apparatus 3, for example, as shown in FIG. 27, the light quantity of the surface emitting laser array 12 is controlled so that the ratio of the exposure amount of the Nth scan and the exposure amount of the (N + 1) th scan becomes 150: 50, for example. For example, as shown in FIG. 30, image unevenness due to misalignment of the exposure position in the sub-scanning direction can be reduced as in the case of double exposure so that the exposure amounts of the first exposure and the second exposure are 100%: 100%. In addition, as described above, the image unevenness due to the exposure position shift in the main scanning direction can be reduced.

  In the color image forming apparatus 1000, instead of always performing such light amount control in the exposure device 3, it can be selected between the light amount control and the normal light amount control, and is installed in a place with a lot of vibration. In this case, the light amount control may be selected.

  As described above, the color image forming apparatus 1000 can not only form a high-quality image by lowering the visibility of the streak appearing in the image by increasing the spatial frequency at which the streak generated in the adjacent exposure occurs. The main scanning direction streaks appearing in the image due to double exposure can be reduced, and a high-quality image can be formed. As a result, it is possible to reduce the size of the apparatus without using an adjusting mechanism or a circuit as described in, for example, Japanese Patent Application Laid-Open Nos. 4-149523 and 4-149522.

  In the present embodiment, the case where the scanning density is 2400 dpi and the number of laser elements is 36 has been described, but the present invention is not limited to this.

FIG. 1 is a schematic diagram illustrating a configuration of a color image forming apparatus according to the first embodiment. FIG. 2 is a schematic perspective view showing a configuration of an exposure apparatus provided in the color image forming apparatus shown in FIG. FIG. 3 is a plan view showing an example of a surface emitting laser array provided in the color image forming apparatus shown in FIG. FIG. 4 is a block diagram showing a configuration of an image processing unit provided in the color image forming apparatus shown in FIG. FIG. 5 is a block diagram showing a detailed configuration of the line buffer / timing controller provided in the color image forming apparatus shown in FIG. FIG. 6 is a timing chart showing the relationship between the page synchronization signal PS, scan number, access data (Data_0), SOS signal, and 2m image data (Data_1) in the line buffer / timing controller shown in FIG. FIG. 7 is an explanatory diagram showing scanning lines when the number of laser elements in the surface emitting laser array is eight in the color image forming apparatus shown in FIG. FIG. 8 is an exposure profile showing the exposure energy with respect to the position in the sub-scanning direction for the color image forming apparatus shown in FIG. FIG. 9 is an explanatory diagram for explaining the reason why streaks occur when the exposure position shift in the main scanning direction occurs in the color image forming apparatus shown in FIG. FIG. 10 is a graph comparing the magnitude of the exposure amount peak when there is no exposure position shift in the main scanning direction and when there is no exposure position shift in the color image forming apparatus shown in FIG. FIG. 11 is a graph showing the relationship between the exposure peak and the image density. FIG. 12 is a schematic view showing screen forms that can be selected when forming toner images of each color of Y, M, C, and K in the color image forming apparatus shown in FIG. FIG. 13 is an explanatory diagram regarding the definition of the continuous lighting length. FIG. 14 is a graph showing the relationship between image coverage and image density change. FIG. 15 is a graph showing the relationship between the screen form, the exposure position shift amount s in the main scanning direction, and the image density change. FIG. 16 compares the amount of exposure peak when there is no exposure position deviation in the main scanning direction and when there is no exposure position deviation in the main scanning direction when the color image forming apparatus shown in FIG. 1 is exposed under the condition of a continuous lighting length of 10 μm. It is a graph. FIG. 17 is a graph showing a decrease in exposure peak when there is an exposure position shift in the main scanning direction when the color image forming apparatus shown in FIG. 1 is exposed under the condition of a continuous lighting length of 50 μm. FIG. 18 is a graph showing the relationship between the continuous lighting length, the exposure position shift amount in the main scanning direction, and the exposure amount peak size. FIG. 19 is a graph showing the relationship between the light beam diameter and the image density difference in the exposure apparatus when the exposure shift amount s in the main scanning direction is 17 μm and 30 μm. FIG. 20 is a graph showing the relationship between the exposure position shift amount in the main scanning direction and the size of the exposure amount peak when the beam diameter of the light beam in the main scanning direction is 50 μm and 100 μm. FIG. 21 is a graph showing a decrease in exposure peak when there is an exposure position shift in the main scanning direction when the beam diameter is 50 μm. FIG. 22 is a graph showing a reduction in exposure peak when there is an exposure position shift in the main scanning direction when the beam diameter is 100 μm. FIG. 23 is an explanatory view showing that the light beam is distorted in the main scanning direction when the cylinder mirror of the exposure apparatus is bent in the color image forming apparatus shown in FIG. FIG. 24 is a graph showing the relationship between the deflection amount of the cylinder mirror and the exposure position deviation amount s. FIG. 25 shows the dew scanning when the double exposure is performed in the color image forming apparatus shown in FIG. 1 and the deviation s in the main scanning direction is 20 μm and the exposure amount is changed between the first scanning and the second scanning. It is a graph which shows the relationship between the exposure position of a direction, and exposure energy density. FIG. 26 is a graph showing the relationship between the ratio of the exposure amount between the first scan and the second scan and the magnitude of the change in image density when the deviation s in the main scanning direction is 20 μm. In FIG. 27, when the number of laser elements in the surface emitting laser array provided in the exposure apparatus is 8, exposure is performed so that the ratio of the exposure amount for the N scan and the exposure amount for the N + 1 scan is 150: 50. It is explanatory drawing which shows the scanning line at the time. FIG. 28 is a graph showing the relationship between the exposure position in the sub-scanning direction and the exposure energy when adjacent exposure is performed in the color image forming apparatus shown in FIG. FIG. 29 shows exposure in the sub-scanning direction when double exposure is performed with the ratio of the exposure amount between the Nth scan and the N + 1th scan set to 100%: 100% in the color image forming apparatus shown in FIG. It is a graph which shows the relationship between a position and exposure energy. FIG. 30 shows the exposure in the sub-scanning direction when the double exposure is performed in the color image forming apparatus shown in FIG. 1 with the ratio of the exposure amount between the Nth scan and the N + 1th scan set to 100%: 100%. It is a graph which shows the relationship between a position and exposure energy.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Photoconductor 2 Charger 3 Exposure apparatus 4 Developing apparatus 5 Primary transfer apparatus 6 Intermediate transfer belt 7 Secondary transfer apparatus 8 Paper tray 9 Paper conveyance roll 10 Fixing apparatus 11 Cleaning apparatus 12 Surface emitting laser array 13 Collimating lens 14 Half mirror 15 Lens 17 Aperture 18 Cylinder lens 19 Polygon mirror 20 Mirror 26 Pickup mirror 27 Synchronization light quantity detection sensor 30 Driver 40 Image processing units 21 and 22 f-θ lenses 24 and 25 Cylinder mirror 100 Image controller 200C Screen 200R Screen 300C Screen 1000 Color image formation apparatus

Claims (4)

An image forming apparatus that forms an image on an image carrier by performing overlapping scanning along a main scanning direction by a light emitting element array in which a plurality of light emitting elements are arranged at equal intervals along a sub scanning direction,
An image forming apparatus, wherein a decrease in peak exposure intensity caused by an exposure position shift in the main scanning direction due to overlapping scanning is 20% or less as compared to a case where there is no exposure position shift in the main scanning direction. The allowable exposure position shift amount S for the peak exposure intensity being 20% or less as compared with the case where there is no exposure position shift in the main scanning direction is 20% of the image coverage which is the area occupied by the image on the image carrier. When the light beam diameter L in the main scanning direction and the continuous lighting length T, which is the number of pixels in the main scanning direction from when a certain pixel is turned on to when it is turned off,
S = aL ² + bT ² + c (a, b, c are constants determined experimentally)
The image forming apparatus according to claim 1, defined by   An image is formed on the image carrier using toners of four colors of yellow, magenta, cyan, and black, and when the yellow toner image is formed, the relational expression is obtained from the continuous lighting length T and the light beam diameter L. The image forming apparatus according to claim 2, wherein the exposure is performed under such a condition that the allowable exposure position shift amount S obtained by the step is smaller than the shift amounts of the other three colors.   The image forming apparatus according to any one of claims 1 to 3, wherein a ratio of a light emission amount in two exposures can be changed in overlap scanning in the main scanning direction.

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