JP2018043392A - Image formation apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image formation apparatus which can properly perform light amount control even in the vicinity of an end of a scanning range, and use a wider scanning range for image formation.SOLUTION: An image formation apparatus acquires an initial main-scanning light amount for each scanning position at the time when the optical beam is emitted with no correction, sets a plurality of correction points within a scanning range, decides a correction value for each scanning position by the initial main-scanning light amount, and makes the irradiation light amount constant by correcting the emission light amount in accordance with the correction value at the time of image formation. An outermost external correction point of the correction points is arranged in an outer edge region where the initial main-scanning light amount abruptly drops on the end of the scanning range. The correction value is decided so that the magnitude becomes reverse to the value of the initial main-scanning light amount of the internal correction point for the internal correction point, decided by the approximate line on the basis of the initial main-scanning light amounts at least two spots on the inner side with respect to the outer edge region for the external correction point, and decided by the approximate line on the basis of at least adjacent correction points for the position between the correction points.SELECTED DRAWING: Figure 7

Description

  The present invention relates to an image forming apparatus that forms an image with toner. More specifically, the present invention relates to an image forming apparatus that performs deflection scanning of a light beam onto an image bearing photosensitive member by rotating a rotary polygon mirror.

  Conventionally, some image forming apparatuses perform scanning of a light beam (main scanning direction) for exposure of an image bearing photosensitive member by rotating a rotary polygon mirror. In such an image forming apparatus, even if the light beam emission intensity is constant, there is a tendency that the amount of light irradiated on the irradiated surface of the image bearing photoreceptor varies depending on the position within the scanning range. . Strictly speaking, the irradiation path of the light beam differs depending on the position within the scanning range. This unevenness in the amount of irradiated light appears on the image as the density of the density depending on the position in the main scanning direction, which causes a reduction in image quality.

  As a countermeasure against this, there is a technique of multilayer coating of optical components on the irradiation path, but in recent years, as a cheaper means, it has been proposed to adjust the light emission intensity according to the scanning position. For example, in Patent Document 1, light amount adjustment according to a shading correction curve is performed by controlling an increase / decrease period that is a unit of a period for increasing / decreasing a light amount adjustment amount. As a result, smooth shading correction according to the optical characteristics can be performed to reduce density unevenness without excessively increasing the device configuration.

JP 2010-241125 A

  However, the conventional techniques described above have the following problems. The light amount cannot be adjusted well at the end of the scanning range. When deflection scanning is performed by rotation of the rotary polygon mirror, the amount of irradiation light rapidly decreases toward the outside at the end of the scanning range. For this reason, if the light amount control is performed in the vicinity of the end portion in the same manner as the central portion of the scanning range, an appropriate response cannot be made. As a result, the scanning range that can be used for image formation is substantially limited.

  The present invention has been made to solve the above-described problems of the prior art. That is, an object of the present invention is to provide an image forming apparatus capable of appropriately controlling the amount of light even near the end of the scanning range and using a wider scanning range for image formation.

  An image forming apparatus according to an aspect of the present invention includes an image bearing photoreceptor, a light emitter that emits a light beam based on image data, and a rotating multi-surface that deflects and scans the light beam emitted from the light emitter while rotating. An image forming apparatus having a mirror and an optical member that guides a light beam deflected and scanned by a rotating polygon mirror to an irradiated surface of an image bearing photoconductor, and the target when a light beam is emitted from a light emitter without correction. A pre-correction data acquisition unit that acquires an initial main scanning light amount that is an irradiation light amount for each scanning position on the irradiation surface, a correction point setting unit that sets correction points at a plurality of locations within the scanning range, an initial main scanning light amount, Based on the position of the correction point, a correction value determination unit that determines, for each scanning position, a correction value for correcting unevenness of the irradiation light amount on the irradiated surface, and the emitted light amount at the time of image formation with the light emitter according to the correction value And a light intensity correction unit The setting unit arranges the correction points that are most outward (hereinafter referred to as “external correction points”) in the outer edge region where the initial main scanning light quantity falls sharply toward the end of the scanning range. The correction value determination unit, for the position of a correction point other than the external correction point (hereinafter referred to as “internal correction point”), sets the value of the initial main scanning light quantity at that position. The correction value is determined so that the magnitude is reversed, and the correction value is determined based on an approximate line based on at least two initial main scanning light amounts inside the outer edge region for the position of the external correction point. For the positions between them, the correction value is determined based on at least an approximate line based on the correction values of the positions of the correction points on both sides of the position.

  In the image forming apparatus according to the above aspect, the initial main scanning light quantity is acquired at times other than the time of image formation. That is, the data of the irradiation light amount for each scanning position on the irradiated surface when a light beam is emitted from the light emitter with a constant light emission amount is acquired. This data represents unevenness in the amount of light that occurs when the light beam is scanned without correction. Then, a correction value for correcting unevenness in the amount of irradiated light on the irradiated surface is determined for each scanning position. This correction value is determined based on the initial main scanning light quantity and the position of the correction point. The scanning of the light beam at the time of image formation is performed with the emitted light amount corrected according to this correction value. Thereby, unevenness of the irradiation light quantity on the irradiated surface is suppressed.

  Here, in this aspect, when setting the correction point, the outermost external correction point is arranged in the outer edge region. There, the initial main scanning light quantity steeply drops toward the end of the scanning range. The correction value is determined by three methods in relation to the correction point. First, for the position of the internal correction point, the correction value is determined so as to cancel the magnitude of the value of the initial main scanning light quantity at that position. That is, a correction value having a large value is determined for a correction point having a smaller initial main scanning light quantity value than other correction points. Conversely, a correction value having a smaller value is determined for a correction point having a larger initial main scanning light quantity value than the other correction points. Second, for the external correction point, a correction value is determined based on approximate lines based on at least two initial main scanning light amounts closer to the inner side than the outer edge region. Thirdly, for the position between the correction points, a correction value is determined based on at least an approximate line based on the initial main scanning light quantity at the positions of the correction points adjacent to the position. Thereby, deflection scanning can be performed with a substantially constant irradiation light amount over a wide scanning range just to the inner side of the outer edge region.

  In the image forming apparatus according to the aspect described above, the correction point setting unit is configured to arrange all the internal correction points closer to the inner side than the outer edge region, and the correction value determination unit is configured so that the position of the external correction point is the same. It is preferable that the “two places” when determining the correction value are two places, that is, the position of the outermost one of the internal correction points and the reference position outside thereof. . In this way, an approximate expression is obtained based on the correction values of “two places” that are inside the outer edge region but as far as possible, and the correction value of the external correction point is determined by this approximate expression. For this reason, correction based on the initial main scanning light amount at a position just inside the outer edge region is performed. Therefore, a substantially constant amount of irradiation light can be obtained over a wide scanning range.

  In the image forming apparatus of the above aspect, the correction value determination unit may be configured such that the reference position is a position where the distance from the end of the scanning range is a predetermined fixed value. . As a result, appropriate correction can be realized with relatively simple control.

  The reference position may be a position where the initial main scanning light amount is equal to or greater than a predetermined minimum light amount. Alternatively, the reference position may be a position where the initial main scanning light amount reaches the first maximum value from the end of the scanning range. As a result, the position inside the outer edge region where the initial main scanning light quantity is high is surely set as the reference position, so that the correction accuracy is high.

  In the image forming apparatus of the above aspect, the correction value determination unit may determine a correction value for the position of the external correction point based on an approximate line based on three or more initial main scanning light amounts inside the outer edge region. It may be configured. By using a higher-order approximation expression, correction with higher accuracy can be performed.

  Alternatively, in the image forming apparatus of the above aspect, the correction point setting unit preliminarily designates a correction point candidate location that can be a correction point within the scanning range, and the correction point is determined from the correction point candidate location by the initial main scanning light amount. It is more preferable that the configuration is such that the densely arranged areas where the rate of change with respect to the scanning position is large and the sparsely arranged areas where the rate of change is small. As a result, an area where the change in the initial main scanning light amount is drastic is corrected with high accuracy, and the calculation amount and the storage amount as a whole are not excessive.

  Alternatively, in the image forming apparatus of the above aspect, the rotary polygon mirror performs deflection scanning of the light beam at a plurality of scanning speeds, and includes a reference clock generation unit that generates a reference clock for each of the scanning speeds at the plurality of levels. The maximum value of the ratio of the reference clock frequency to each scanning speed is within 1.01 times the minimum value, and the correction point setting unit is preferably configured to set the correction point according to the reference clock. This makes it possible to achieve both reduction in storage capacity and appropriate correction within the practical range of the original oscillation frequency. Of course, it is better if the ratio between the scanning speed and the frequency of the reference clock is constant for any scanning speed.

  According to this configuration, there is provided an image forming apparatus capable of appropriately controlling the amount of light even near the end of the scanning range and using a wider scanning range for image formation.

1 is a cross-sectional view illustrating a schematic configuration of an image forming apparatus according to an embodiment. FIG. 3 is a plan view illustrating a schematic configuration of a print head unit. It is sectional drawing which shows schematic structure of a print head part. 2 is a block diagram illustrating a configuration of a control system of the image forming apparatus according to the embodiment. FIG. It is a graph which shows an example of the nonuniformity of initial stage main scanning light quantity. It is a graph explaining the setting of the correction value with respect to the initial main scanning light quantity with unevenness. It is a graph which expands and shows the left shoulder vicinity of the graph of FIG. It is a graph which expands and shows the right shoulder vicinity of the graph of FIG. It is a graph (near left shoulder) which shows the irradiation light quantity on the to-be-irradiated surface by correction | amendment in embodiment. It is a graph (right shoulder vicinity) which shows the irradiation light quantity on the to-be-irradiated surface by correction | amendment in embodiment. It is a flowchart which shows the determination procedure of the correction value in embodiment. It is a graph (fixed position system) which expands and shows the left shoulder vicinity of the graph of FIG. FIG. 7 is an enlarged graph showing the vicinity of the right shoulder of the graph of FIG. FIG. 7 is a graph (minimum value method) showing the vicinity of the left shoulder of the graph of FIG. 6 in an enlarged manner. FIG. 7 is an enlarged graph showing the vicinity of the right shoulder of the graph of FIG. FIG. 7 is a graph (peak position method) showing an enlargement of the vicinity of the left shoulder of the graph of FIG. 6. FIG. 7 is an enlarged graph showing the vicinity of the right shoulder of the graph of FIG. FIG. 7 is a graph (second-order approximation method method) showing an enlarged view of the vicinity of the left shoulder of the graph of FIG. 6. FIG. 7 is an enlarged graph showing the vicinity of the right shoulder of the graph of FIG.

  DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments embodying the present invention will be described below in detail with reference to the accompanying drawings. In this embodiment, the present invention is applied to the image forming apparatus 1 shown in FIG. The image forming apparatus 1 in FIG. 1 includes a main body 2 and a reading unit 3. The main body unit 2 includes a paper feeding unit 4 and an image forming unit 5. An image is formed on the paper supplied from the paper supply unit 4 by the image forming unit 5. The image forming unit 5 includes a print head unit 6, four photosensitive drums 7, an intermediate transfer belt 8, and a fixing device 9. The reading unit 3 is provided with an automatic document feeder 10 and a scanner unit 11. In addition, the image forming apparatus 1 is provided with an operation display panel 12.

  The characteristic point of the image forming apparatus 1 of this embodiment is an exposure system by the print head unit 6. The print head unit 6 will be described with reference to FIGS. As shown in FIG. 2, the print head unit 6 is provided with light emitters 13 for four colors (Y, M, C, and K). The light emitter 13 is a light source that emits a light beam for exposing the photosensitive drum 7 based on the image data. An electrostatic latent image based on image data is formed on the photosensitive drum 7 by irradiating the surface of the photosensitive drum 7, that is, the irradiated surface with the light beam from the light emitter 13. Based on the electrostatic latent image, a toner image is carried on the surface of the photosensitive drum 7 through a known development process. The toner image is transferred onto a sheet through a known transfer process and fixed by a fixing device 9.

  The print head unit 6 is also provided with collimator lenses 14 and reflecting mirrors 15 for each of the four colors. The collimator lens 14 condenses the light emitted from the light emitter 13 in a beam shape. The print head unit 6 is further provided with a reflecting mirror 16, a polygon mirror (rotating polygonal mirror) 17, and an Fθ lens 18 that are common to the four colors. The four colors of light are collected by the reflecting mirror 15 onto the reflecting mirror 16 and reflected toward the polygon mirror 17. In the drawing, the four reflecting mirrors 15 are actually arranged with steps so as not to obstruct each other's optical paths, although they are drawn in a simple manner. Alternatively, a half mirror may be used as the reflecting mirror 15. The polygon mirror 17 changes the reflection direction of the light beam by rotating, that is, deflects and scans the light beam. The Fθ lens 18 is an optical member that guides the light beam reflected by the polygon mirror 17 to the photosensitive drum 7.

  Further, the print head unit 6 is provided with reflecting mirrors 19 and 20 and an optical sensor 21. The reflecting mirror 19 reflects the light beam deflected and scanned by the polygon mirror 17 toward the photosensitive drum 7. The reflecting mirror 20 is arranged at a position where the light beam deflected and scanned by the polygon mirror 17 hits the reflecting mirror 19 just before hitting it. When the light beam is reflected by the reflecting mirror 20, the reflected light is guided to the optical sensor 21. When the optical sensor 21 receives the reflected light, a scanning synchronization signal is generated. That is, the timing at which the optical sensor 21 receives the reflected light is used as the drawing reference timing.

  Although omitted in FIG. 2, as shown in FIG. 3, the print head unit 6 is provided with reflecting mirrors 22 to 27 in addition to the above. Like the reflecting mirror 19, the reflecting mirrors 22 to 27 reflect the light beam deflected and scanned by the polygon mirror 17 toward the photosensitive drum 7. However, while only one reflecting mirror 19 (for K color) is used, reflecting mirrors 22 and 23 (for Y color), reflecting mirrors 24 and 25 (for M color), reflecting mirrors 26 and 27 ( (For C color) is a pair of two each. Although not appearing in FIG. 3, the reflecting mirrors 22, 24, and the reflecting mirror 26 are each provided with a reflecting mirror similar to the reflecting mirror 20, and both direct the light beam toward the optical sensor 21. To reflect. As shown in FIG. 3, the polygon mirror 17 is provided with a drive motor 28.

  FIG. 4 shows a block diagram of a control system of the image forming apparatus 1. The control system of the image forming apparatus 1 is configured around the overall control unit 29. The overall control unit 29 controls the overall operation of the image forming apparatus 1. The above-described print head unit 6 and operation display panel 12 are also connected to the overall control unit 29. In addition to the above, the overall control unit 29 is also connected with a reading control unit 30 and an image processing unit 31. The reading control unit 30 has a function of controlling the reading unit 3 and acquiring image data of a document. The image processing unit 31 is a part that performs data processing for image forming processing such as color space conversion and density correction on the image data acquired by the reading control unit 30. Image data for image formation is supplied from the image processing unit 31 to the print head unit 6.

  The print head unit 6 is provided with a drawing control unit 32 and a storage unit 33 in addition to the light emitter 13 and the drive motor 28 which are hardware elements. Note that the collimator lens 14, the reflecting mirrors 15, 16, 19, 20, 22 to 27, the polygon mirror 17, the Fθ lens 18, and the optical sensor 21 are omitted in FIG. The drawing control unit 32 performs light emission control of the light emitter 13 and rotation control of the drive motor 28 for image formation. The storage unit 33 stores various data necessary for that purpose.

  The light emission control of the light emitter 13 by the drawing control unit 32 includes blinking control based on image data and brightness control. Among these, the feature point as the image forming apparatus 1 of the present embodiment is the latter. The purpose of the brightness control of the light emitter 13 in this embodiment is to make the irradiation light amount of the light beam on the irradiated surface of the photosensitive drum 7 uniform within a scanning range as wide as possible. That is, when deflection scanning by the polygon mirror 17 is performed with the emission light amount from the light emitter 13 being constant, the irradiation light amount (initial main scanning light amount) on the irradiated surface is not constant, for example as shown in FIG. As described above, the main scanning position is uneven. This unevenness in the amount of irradiated light is caused by the fact that the reflective surface of the polygon mirror 17 and the material of the Fθ lens 18 are not completely uniform, but there is a limit to improvement by improving the quality of these optical components. Therefore, in this embodiment, the unevenness of the irradiation light quantity is suppressed by controlling the brightness of the light emitter 13.

  That is, as shown in FIG. 6, the correction value is set so as to cancel the strength of the initial main scanning light quantity. Then, the brightness of the light emitter 13 is controlled so that the amount of light emitted from the light emitter 13 becomes a value proportional to the correction value. As a result, the irradiation light quantity on the irradiated surface is made substantially constant. Of course, the correction value is set to a small value for a scanning position with a large initial main scanning light quantity, and is set to a large value for a scanning position with a small initial main scanning light quantity. This is to cancel the magnitude of the initial main scanning light quantity. Here, the correction value is set so as to be proportional to the inverse of the initial main scanning light quantity at that position. Instead of using the reciprocal number, the correction value may be determined so that the sum of the initial main scanning light quantity and the correction value is constant.

  In the image forming apparatus 1 of the present embodiment, correction points are further set within the scanning range. In the example of FIG. 6, the main scanning position where the shaded rectangle of “correction point arrangement” is arranged is the position set as the correction point. How to arrange the correction points within the scanning range will be described later. For the position of the correction point, a correction value is set in proportion to the inverse of the initial main scanning light amount as described above. For the position between the correction points, basically, a correction value is set by calculation from an approximate expression. The approximate expression is given as a polynomial defined by correction values at the positions of a plurality of correction points (for example, correction points on both sides).

  Here, when the initial main scanning light amount in FIG. 6 is viewed in detail, in the outer edge region Z near the end of the scanning range, the light amount sharply decreases toward the outside of the scanning range. This is an unavoidable phenomenon that occurs when the reflecting surface is switched by the rotation of the polygon mirror 17. In this embodiment, since a scan range as wide as possible is used for image formation, correction points are also arranged in this outer edge region. Of course, the correction points (hereinafter referred to as “external correction points”) arranged in the outer edge area Z are the most out of the scanning range in the entire correction points. However, this external correction point is not set to a correction value proportional to the reciprocal of the initial main scanning light amount as described above, but is set by a special method described later.

  The determination of the correction value for the external correction point will be described with reference to FIGS. FIG. 7 is an enlarged view of the left shoulder portion (scanning start side end portion) in FIG. 6, but there is a region X in which the initial main scanning light amount is slightly raised. The outer edge region Z is a portion where the amount of light sharply decreases outside the region.

  In FIG. 7, three correction points P1, P2, and P3 are arranged. Among these, the correction point P1 that is the outermost of the scanning range is an external correction point that is arranged in the outer edge region Z. Correction points P2 and P3 (hereinafter, referred to as “internal correction points”) other than the correction point P1 are arranged within the outer edge region Z within the scanning range. Here, for the internal correction points P2 and P3, as described above, the correction values D2 and D3 are determined so as to be proportional to the reciprocals of the initial main scanning light amounts E2 and E3 at the positions. In the example of FIG. 7, since the initial main scanning light amounts E2 and E3 are both larger than 100%, the correction values D2 and D3 are both smaller than 100%.

  However, the correction value for the external correction point P1 is determined by a method different from that for the internal correction points P2 and P3. If the correction value Da1 is determined for the external correction point P1 by the same method as the correction values D2 and D3, the correction value Da1 becomes a value considerably larger than 100%. This is because the value is small because the initial main scanning light amount E1 is in the outer edge region Z. In this case, the correction value Da1 is extraordinarily large with respect to the correction values D2 and D3, and does not match the purpose of leveling the curve of the initial main scanning light quantity.

  Therefore, in this embodiment, Da1 is not adopted as the correction value for the external correction point P1, and the correction value D1 is determined by the following method. First, a position Pt (reference position) is set in a region X in FIG. The position Pt is a position inside the outer edge region Z and outside the outermost point P2 among the internal correction points. Then, the correction value Dt is determined so as to be proportional to the reciprocal of the initial main scanning light amount Et at the position Pt. Then, the correction value D1 of the external correction point P1 is determined by the equation (1). P1 and P2 in Equation 1 represent the positions of the correction points P1 and P2 in the scanning direction.

  That is, in the graph of FIG. 7, a straight line (primary approximation) connecting the point Dt and the point D2 is drawn, and the straight line is extrapolated to the position of the correction point P1 to obtain the correction value D1. That is. In FIG. 7, it seems that the correction value D1 almost coincides with the initial main scanning light amount E1 at the correction point P1, but this is just a coincidence. Further, the same determination may be made for the right shoulder (the end portion on the scanning end side) in FIG. When correction values are determined for all correction points in this way, correction values for the interval between the correction points are determined. What is necessary is just to obtain | require this with the primary approximation formula by the correction value of the correction point of both adjacent.

  With reference to FIGS. 9 (left shoulder side) and FIG. 10 (right shoulder side), the amount of light beam irradiated on the irradiated surface at the time of actual image formation based on the correction value thus determined will be described. The “corrected main scanning light quantity” shown in FIGS. 9 and 10 is obtained by multiplying the “initial main scanning light quantity” by the “correction value”. Therefore, according to the method of determining the correction value, the “main scanning light amount after correction” is almost 100% at the internal correction points and the positions between the internal correction points, and the uniformity is high. Further, even in the region outside the outermost internal correction point P2, the uniformity is still high except for the outer edge region Z and a range very close thereto. This is because the correction value D1 is appropriately determined as described above. Therefore, a wide scanning range R extending immediately before the outer edge region Z can be used for image formation.

  If the above-described Da1 is used as the correction value at the correction point P1, the corrected main scanning light quantity outside the internal correction point P2 is as shown by the two-dot chain line curve W in FIGS. That is, the corrected main scanning light amount becomes considerably high around the point Et shown in FIG. For this reason, the scanning range available for image formation is almost limited to the range from the outermost internal correction point to the outermost internal correction point. In contrast, in this embodiment, as described above, the entire wide scanning range R can be used for image formation.

  The procedure for determining the correction value described above will be described with reference to FIG. In the flow of FIG. 11, first, data of the initial main scanning light quantity is acquired (S1). The data of the initial main scanning light quantity acquired here is data as shown in the graph of FIG. This data acquisition is performed by forming a solid image and measuring the density distribution in the main scanning direction with the light emission amount from the light emitter 13 being constant.

  Next, the arrangement of correction points is determined (S2). In the image forming apparatus 1 of this embodiment, correction point candidate locations that can be set as correction points are determined in advance. The correction point candidate locations are the shaded square location of “Correction point placement” and the white square location of “No correction point placement” in FIG. In the example of FIG. 6, there are 46 places in total. There are also correction point candidate locations in the outer edge region Z at both ends of the scanning range. The scanning direction position of the correction point candidate location is stored in the storage unit 33.

  The process performed in S2 is to actually select some of these correction point candidate locations as correction points. In the example of FIG. 6, 30 correction point candidate locations are selected as correction points, and the remaining 16 correction point candidate locations are not selected. This correction point selection is performed so as to satisfy the following two guidelines.

  The first guideline is that correction point candidate locations in the outer edge region Z at both ends are also selected as correction points one by one. Otherwise, the correction process related to the correction point P1 described above cannot be performed. In order to execute this, it is necessary to automatically identify the range to be the outer edge area Z in the distribution of the initial main scanning light quantity acquired in S1 (for example, FIG. 6). It is already known. For example, a range to be set as the outer edge region Z is appropriately set based on a differential value of the initial main scanning light amount by the scanning position and a difference from the target light amount.

  The second guideline is to arrange a large number of correction points in a region where the unevenness of the curve is intense and a small number of correction points in a relatively flat region with respect to the distribution of the initial main scanning light quantity acquired in S1. is there. This is because in a region where the unevenness of the curve is severe, it is necessary to calculate the correction value using the approximation formula between the correction points described above with higher accuracy. Any known algorithm can be used as appropriate for extracting a region with severe irregularities from the entire scanning range. That is, it is possible to appropriately distinguish between a region with severe unevenness and a region with no unevenness by using an index such as a differential value or a secondary differential value depending on the scanning position of the initial main scanning light quantity.

  After the correction point arrangement is determined, the correction value is determined as described above for the internal correction points among the set correction points (S3). Further, the correction value is determined for the external correction point as described above (S4). Then, based on the correction value of the determined correction point, the correction value is also determined for the position between the correction points as described above (S5). The determined correction value is stored in the storage unit 33. Thereafter, at the time of image formation, the correction value is read from the storage unit 33, and the light emission amount of the light emitter 13 is corrected by the correction value. As a result, as described with reference to FIGS. 9 and 10, a uniform main scanning light amount over a wide scanning range R can be obtained on the irradiated surface of the photosensitive drum 7.

  The correction value determination process as described above is performed at least before the first image formation after the image forming apparatus 1 is manufactured. However, not only that, it is desirable that the subsequent operation is performed at an appropriate timing other than the time of image formation (immediately after turning on the power or every predetermined number of sheets). In particular, it is desirable to redo the correction value determination process when the internal temperature of the image forming apparatus 1 fluctuates beyond a certain limit or after some maintenance is performed on the scanning optical system.

  Subsequently, a variation regarding determination of a correction value for the external correction point P1 will be described. The variations described here are mainly for determining the position Pt in FIG.

  The first variation is that the position Pt is a fixed position for each model of the image forming apparatus 1. As shown again in FIGS. 12 and 13, when the vicinity of both ends of the graph of FIG. 6 is enlarged, a shoulder point V where the initial main scanning light quantity sharply decreases toward the outside can be determined. The position Pt is preferably the same position as the point V or a position inside it, and a position as close to the outside as possible. On the other hand, the distribution of the initial main scanning light quantity in the image forming apparatus 1 varies from unit to unit due to factors such as individual differences among optical components. However, it is considered that the variation is not so large for the image forming apparatuses 1 of the same model. Therefore, 20 image forming apparatuses 1 of the same model were prepared, and a graph of the initial main scanning light quantity was prepared for each, and the positions of the points V on the left and right sides were examined. The results are shown in Table 1. In Table 1, for each of the left side and the right side, the number of units for each distance in the main scanning direction between the position of the point V and the external correction points P1 and P30 (indicated by “U” in FIGS. 12 and 13) is shown. Yes.

  Looking at Table 1, the maximum value of the distance U is 1.7 mm on the left side and 2.3 mm on the right side. Therefore, in this type of image forming apparatus 1, the position Pt is a fixed position such that the left side is a position 1.7 mm inside the external correction point P1 and the right side is a position 2.3 mm inside the external correction point P30. It is. Accordingly, it is possible to appropriately correct the image forming apparatuses 1 of the same model without determining the position Pt for each image forming apparatus 1. Therefore, the correction process can be simplified.

  The second variation is that the position Pt is determined by setting the minimum value of the initial main scanning light quantity. That is, in this variation, as shown in FIGS. 14 and 15, a minimum line L required for the position Pt is set. The minimum line L is set to 102.5% in FIG. 14 (left side) and 105% in FIG. 15 (right side). Then, the position Pt is designated from the range of the region T in which the initial main scanning light quantity is the minimum line L or more. Thereby, the position Pt can be specified so that the correction value D1 is appropriately set.

  Regarding the level setting of the minimum line L, it is necessary to satisfy two points of 100%, that is, higher than the target light amount itself and lower than the peak value of the initial main scanning light amount near the end of the scanning range. is there. In general, a portion exceeding 100% of the peak value of the initial main scanning light amount (about 104% in FIG. 14 and about 105.5% in FIG. 15) is multiplied by a predetermined coefficient less than 1, Alternatively, it may be determined by subtracting a predetermined value (about 0.5 to 1.5%) from the peak value. Further, where in the region T is determined as the position Pt may be arbitrary, but a rule such as setting the position as outward as possible may be determined.

  The third variation is that the position Pt is the peak position of the initial main scanning light quantity in the vicinity of the end of the scanning range. The setting of the position Pt by this variation will be described with reference to FIG. 16 (left side) and FIG. 17 (right side). That is, in this variation, the first position from which the inclination (position differentiation) of the initial main scanning light quantity becomes zero is defined as the position Pt from the end side of the scanning range. Thereby, the peak position of the initial main scanning light quantity becomes the position Pt. Therefore, appropriate correction can be made.

  The fourth variation is not a variation on how to determine the position Pt, but a variation on how to determine the correction value D1. In this variation, the correction value D1 is determined using a secondary approximation formula instead of using the primary approximation formula as described above. That is, as shown in FIG. 18 (left side), as an approximate expression for calculating the correction value D1, a quadratic expression connecting the three points Dt, D2, and D3 is used. The correction value D1 is determined by this quadratic approximate expression. The same applies to FIG. 19 (right side). Thereby, it is possible to determine the correction value D1 with higher accuracy and perform more appropriate light amount correction. Note that a higher order expression using more internal correction points may be used as an approximate expression. Any of the methods described above may be used for determining the position Pt.

  The fifth variation is a variation regarding the frequency of the reference clock. In this variation, there are three types of modes shown in the leftmost column of Table 2 for image formation in the image forming unit 5. The scanning speed on the photosensitive drum 7 differs depending on the image forming mode. The frequency of the reference clock is also different for each mode. In this variation, the frequency of the reference clock in each mode is determined so that the ratio between the scanning speed and the frequency of the reference clock is constant in any mode. As described above, by making the ratio of the scanning speed and the frequency of the reference clock constant regardless of the mode, the position of the correction point candidate portion described above is constant in any mode. That is, the position of the correction point is constant regardless of the mode. For this reason, it is not necessary to store the position of the correction point and the correction value of each correction point separately for each mode. For this reason, the capacity of the storage unit 33 can be small. “Us” in the main scanning speed column in Table 2 means microseconds (the same applies to Table 3).

  The sixth variation is also a variation with respect to the reference clock frequency. In this variation, there are three types of image forming modes as in the case of the fifth variation. However, unlike the fifth variation, the ratio between the scanning speed and the reference clock frequency is slightly different depending on the mode (Table 3). This means that the original oscillation frequency for realizing a different reference clock frequency for each mode is kept as low as possible.

  In this variation, three reference clock frequencies can be handled with an original oscillation frequency of about 75 MHz. This is a considerably lower frequency than in the case of the fifth variation, which requires about 750 MHz. The sixth variation is that the reference clock frequency is selected so that the decrease in the accuracy of the position of the correction point is within the allowable range within the limitation. That is, in Table 3, the maximum value of the ratio between the scanning speed and the reference clock frequency is only about 1.006 times the minimum value and is within 1.01 times. Therefore, in the sixth variation, the correction accuracy is slightly disadvantageous compared to the fifth variation, but the capacity of the storage unit 33 is still small and the original oscillation frequency is low.

  As described in detail above, according to the present embodiment, the amount of light emitted from the light emitter 13 is corrected with a correction value corresponding to the scanning position, so that the amount of uneven irradiation light on the surface to be irradiated of the photosensitive drum 7 is corrected. Is trying to reduce. Therefore, a plurality of correction points are set within the scanning range. Here, correction points (external correction points) are also set in the outer edge region Z where the light amount sharply decreases near both ends of the scanning range. For the external correction points, correction values are determined by a special method different from the internal correction points. As a result, a substantially uniform amount of irradiation light with no unevenness can be obtained on the irradiated surface in the widest possible scanning range. Thus, the image forming apparatus 1 that can use the widest possible scanning range for image formation is realized.

  Note that this embodiment is merely an example, and does not limit the present invention. Therefore, the present invention can naturally be improved and modified in various ways without departing from the gist thereof. For example, the target image forming apparatus 1 is not limited to the copier type shown in FIG. 1 but may be a printer type that does not have the reading unit 3. Alternatively, it may be provided with a function for transmitting and receiving a print job via a public line. Further, the configuration of the image forming unit 5 may be of a type that does not have the intermediate transfer belt 8 (multi-cycle type or monochrome type). It should be noted that the use of optical parts (polygon mirror 17, Fθ lens 18, and reflecting mirrors) whose surfaces are coated is not excluded.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 2 Main body part 5 Image forming part 6 Print head part 7 Photosensitive drum 13 Light emitter 17 Polygon mirror (rotating polygon mirror)
18 Fθ lens 19 Reflecting mirror 22 Reflecting mirror 23 Reflecting mirror 24 Reflecting mirror 25 Reflecting mirror 26 Reflecting mirror 27 Reflecting mirror 29 Overall control unit 32 Drawing control unit (pre-correction data acquisition unit, correction point setting unit, correction value determination unit, light quantity Correction unit, reference clock generation unit)
33 Storage Dn Correction Value Pn Correction Point Z Outer Edge Area

Claims (8)

An image bearing photoconductor, a light emitter that emits a light beam based on image data, a rotating polygon mirror that reflects and deflects and scans the light beam emitted from the light emitter while rotating, and deflection scanning by the rotating polygon mirror And an optical member for guiding the light beam to the irradiated surface of the image bearing photoreceptor,
A pre-correction data acquisition unit that acquires an initial main scanning light amount that is an irradiation light amount for each scanning position on the irradiated surface when a light beam is emitted from the light emitter without correction;
A correction point setting unit for setting correction points at a plurality of locations within the scanning range;
A correction value determining unit for determining, for each scanning position, a correction value for correcting unevenness of the irradiation light amount on the irradiated surface based on the initial main scanning light amount and the position of the correction point;
A light amount correction unit that corrects the emitted light amount at the time of image formation with the light emitter according to the correction value;
The correction point setting unit has an outer edge where the initial main scanning light quantity falls steeply toward the end of the scanning range at the outermost correction point to be set (hereinafter referred to as “external correction point”). Is configured to be placed within an area,
The correction value determining unit
For the position of the correction point other than the external correction point (hereinafter referred to as “internal correction point”), a correction value is determined so that the magnitude of the initial main scanning light quantity at that position is reversed.
For the position of the external correction point, a correction value is determined based on an approximate line based on the initial main scanning light quantity at least two locations inside the outer edge region;
The position between the correction points is configured to determine a correction value based on an approximate line based on at least the correction values of the positions of the correction points adjacent to the position. Image forming apparatus. The image forming apparatus according to claim 1,
The correction point setting unit is configured to arrange all the internal correction points closer to the inside than the outer edge region,
The correction value determining unit determines the “two locations” when determining a correction value for the position of the external correction point as a position of an outermost one of the internal correction points and a reference position outside of the internal correction point. An image forming apparatus configured to have two locations. 3. The image forming apparatus according to claim 2, wherein the correction value determining unit is
An image forming apparatus, wherein the reference position is configured such that a distance from an end of a scanning range is a predetermined fixed value. 3. The image forming apparatus according to claim 2, wherein the correction value determining unit is
An image forming apparatus, wherein the reference position is configured to be a position where the initial main scanning light quantity is not less than a predetermined minimum light quantity. 3. The image forming apparatus according to claim 2, wherein the correction value determining unit is
An image forming apparatus, wherein the reference position is configured to be a position where the initial main scanning light quantity reaches an initial maximum value from an end of a scanning range. 6. The image forming apparatus according to claim 1, wherein the correction value determining unit includes:
The correction value for the position of the external correction point is determined based on approximate lines based on the initial main scanning light quantity at three or more locations inside the outer edge region. Forming equipment. The image forming apparatus according to any one of claims 1 to 6, wherein the correction point setting unit includes:
Correction point candidate locations that can be the correction points in the scanning range are designated in advance,
The correction points are selected from the candidate correction points so that the correction points are densely arranged in a region where the change rate of the initial main scanning light quantity with respect to the scanning position is large and sparsely arranged in a region where the change rate is small. An image forming apparatus characterized by that. An image forming apparatus according to any one of claims 1 to 7, comprising:
The rotating polygon mirror performs deflection scanning of a light beam at a plurality of scanning speeds,
A reference clock generator for generating a reference clock for each of the scanning speeds of the plurality of levels;
The maximum value of the ratio of the reference clock frequency to each of the scanning speeds is within 1.01 times the minimum value,
The image forming apparatus, wherein the correction point setting unit sets a correction point according to the reference clock.

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