JP2005138307A - Quantity of light correction method for light emitting element and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a quantity of light correction method for light emitting elements which can suppress generation of image formation defects such as exposure irregularities regardless of irregularities of characteristics of optical lenses and irregularities of beam shapes of individual light emitting elements, and to provide an image forming apparatus to which the method is applied. <P>SOLUTION: First, an MTF value for every LED 11 is obtained for each LED 11 of an optical write device 227 mounted on a digital copying machine 1 (S1). Then, a small section MTF value A for every LED 11 is obtained (S2). A large section MTF value B for every LED 11 is obtained (S3). Subsequently, a differential MTF value (A-B) showing a difference between the small section MTF value A and the large section MTF value B is obtained for every LED 11 (S5). An appropriate quantity of light correction value D is obtained based on the differential MTF value (A-B). An emission quantity of light for every LED 11 is corrected with the use of the obtained quantity of light correction value D (S6). <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a light amount correction method for a light emitting element for uniformizing an exposure amount for each light emitting element in an exposure apparatus, and an image forming apparatus to which this method is applied.

  In the image forming apparatus used in the office field among the electrophotographic image forming apparatuses, optical writing in which light emitting elements such as LEDs and ELs are arranged on an array in consideration of miniaturization, noise reduction, and power saving. The apparatus is often used as an exposure apparatus. By using an optical writing device in which light emitting elements are arranged on an array as an exposure device, the optical path length of the optical system can be shortened to reduce the size of the device, and silence can be achieved by eliminating the rotating mechanism. be able to.

  However, in the optical writing device in which the light emitting elements are arranged on the array, unevenness in the exposure amount may occur in the axial direction of the photosensitive member due to the difference in characteristics of each light emitting element.

  Therefore, in the prior art, in order to make the exposure amount uniform in the axial direction of the photoconductor, the light emitting operation of each light emitting element is controlled so that the product of the light amount emitted from each light emitting element and the light emission time is uniform. In some cases, methods were used.

Further, in the prior art, there has been a technique in which an amplitude transfer function (MTF) that is lens resolution is measured and used for light amount correction in an optical writing device (for example, see Patent Document 1).
JP 2001-138568 A

  However, even if the amount of light emitted from each light emitting element is made uniform, exposure unevenness may occur in the output image. Similarly, even if the overall variation of MTF is made uniform, uneven exposure may occur. Causes of this unevenness of exposure include variations in characteristics of an optical lens such as a SELFOC lens, variations in the amount of light in units of chips on which light emitting elements are mounted, and variations in the shape of beams emitted from the respective light emitting elements. It has been difficult to eliminate these variations even when correction is made to equalize the amount of light emitted from each light emitting element.

  An object of the present invention is to provide a light-emitting element light amount correction method capable of suppressing the occurrence of image formation defects such as uneven exposure regardless of variations in characteristics of optical lenses and beam shapes of individual light-emitting elements. An image forming apparatus to be applied is provided.

  The present invention has the following configuration.

(1) Light emitted from a plurality of light emitting elements arranged in the axial direction of the photosensitive drum is converged on a peripheral surface of the photosensitive drum via a plurality of optical lenses arranged in the axial direction, A light amount correction method for a light emitting element used in an exposure apparatus for exposing a photosensitive drum,
Measuring beam characteristics of the plurality of light emitting elements, and obtaining a corresponding MTF value for each light emitting element;
For each of the plurality of light emitting elements, averaging MTF values in a limited section including itself to obtain an average MTF value;
For each of the plurality of light emitting elements, obtaining a difference MTF value indicating a difference between the MTF value for each light emitting element and the average MTF value;
Correcting the light emission amount for each light emitting element based on the difference MTF value so that the light emission amount for each light emitting element considering beam characteristics is uniform;
It is characterized by including.

  In this configuration, the difference between the MTF value corresponding to each light emitting element and the average MTF value obtained by averaging the MTF values in the limited section including itself determines the light amount correction amount. Used. The limited section constitutes a part of the arrangement of all light emitting elements.

  If the difference MTF value is used, the degree of deviation of the MTF value corresponding to each light emitting element is grasped. For this reason, the simple light amount correction value applied to each light emitting element is further corrected by the light amount correction amount obtained based on the differential MTF value. And the light quantity correction | amendment which considered the beam characteristic for each light emitting element is applied by making the obtained value into the light quantity correction value applied for every light emitting element.

  Here, the MTF value corresponding to each light emitting element means the MTF value of the path from each light emitting element to the peripheral surface of the photosensitive drum. As a typical example of the limited section including itself, there is a section of a predetermined range extending in the arrangement direction around the light emitting element whose beam characteristics are measured. Furthermore, the beam characteristic is a property of a beam obtained by comprehensively combining characteristics of an optical lens such as an imaging lens, a light amount in a chip unit on which a light emitting element is mounted, a beam shape emitted from each light emitting element, and the like. .

  The MTF (Modulation Transfer Function) is a value calculated by the maximum value MAX and the minimum value MIN of the emission output of the emission beam at an arbitrary spatial frequency, and is represented by MTF = (MAX−MIN) / (MAX + MIN). Is done.

(2) Light emitted from a plurality of light emitting elements arranged in the axial direction of the photosensitive drum is transmitted to the peripheral surface of the photosensitive drum via a plurality of optical lenses arranged at a predetermined pitch in the axial direction. A light amount correction method for a light emitting element used in an exposure apparatus for converging and exposing the photosensitive drum,
Measuring beam characteristics of the plurality of light emitting elements, and obtaining a corresponding MTF value for each light emitting element;
For each of the plurality of light emitting elements, averaging the MTF values in a small section smaller than the pitch to obtain a small section MTF value;
For each of the plurality of light emitting elements, averaging MTF values in a limited section larger than at least the pitch to obtain a large section MTF value;
Obtaining a difference MTF value indicating a difference between the small section MTF value and the large section MTF value for each of the plurality of light emitting elements;
Correcting the light emission amount for each light emitting element based on the difference MTF value so that the light emission amount for each light emitting element considering beam characteristics is uniform;
It is characterized by including.

  In this configuration, the light amount correction value for each light emitting element is obtained using the difference MTF value indicating the difference between the small section MTF value and the large section MTF value, and the light emission amount of each light emitting element is corrected by this light amount correction value. Is done. The limited section constitutes a part of the arrangement of all light emitting elements.

  That is, by taking the difference between the MTF value that is less susceptible to noise caused by the strand pitch of the optical lens and the MTF value that is reliably affected by noise caused by the strand pitch of the optical lens, the optical It is easy to grasp how the noise caused by the lens strand pitch affects the MTF value of each light emitting element. For this reason, based on the differential MTF value, a light amount correction amount for equalizing the light emission amount in consideration of noise caused by the strand pitch of the optical lens is calculated.

(3) The small section is larger than an arrangement length of two or more light emitting elements and smaller than the pitch.

  In this configuration, the small section is set in a range larger than the arrangement length of the two or more light emitting elements and smaller than the pitch of the optical lens. Here, the reason why the lower limit of the range of the small section is the arrangement length of two or more light emitting elements is to reduce the influence of noise during MTF measurement.

(4) The light emitting elements are formed as one chip every predetermined number,
The limited section may be at least longer than an arrangement length of the predetermined number of light emitting elements.

  In this configuration, variations in the amount of light for each chip on which a predetermined number of light emitting elements are mounted appear in the large section MTF value and the differential MTF value calculated based on the large section MTF value.

(5) The correction amount D applied to the light amount correction of each light emitting element is
When A is a small section MTF value, B is a large section MTF value, and C is a simple light amount correction amount that does not consider beam characteristics,
D = C [1+ (A−B)]
It is characterized by being.

  In this configuration, the correction amount D applied to the light amount correction of each light emitting element is calculated by a simple calculation formula D = C [1+ (A−B)]. Here, the simple light amount correction amount C means, for example, the light amount correction amount for each light emitting element obtained by the light amount measurement performed without using an optical lens or the like.

(6) The correction amount D applied to the light amount correction of each light emitting element is
When A is a small section MTF value, B is a large section MTF value, C is a simple light quantity correction amount that does not take beam characteristics into account, and k is an arbitrary coefficient that changes according to the use state of the exposure apparatus.
D = C [1 + k (AB)]
It is characterized by being.

  In this configuration, the correction amount D applied to the light amount correction of each light emitting element is calculated by a simple calculation formula of D = C [1 + k (AB)]. Here, k is an arbitrary coefficient that varies depending on the use state of the exposure apparatus. Since the correction amount D increases or decreases in proportion to the value of k, the correction amount D corresponding to a plurality of types of process conditions can be obtained by changing the value of k depending on the process conditions in the image forming apparatus.

(7) The correction amount D applied to the light amount correction of each light emitting element is
A is a small section MTF value, B is a large section MTF value, C is a simple light quantity correction amount that does not take beam characteristics into account, k is an arbitrary coefficient that changes according to the use state of the exposure apparatus, and n is a characteristic of the photosensitive drum. With an arbitrary coefficient that changes accordingly,
D = C [1 + k (A−B) ⁿ ]
It is characterized by being.

In this configuration, the correction amount D applied to the light amount correction of each light emitting element is calculated by a simple calculation formula D = C [1 + k (A−B) ⁿ ]. Here, n is an arbitrary coefficient that varies depending on the characteristics of the photosensitive drum. A typical example of the characteristics of the photosensitive drum is the sensitivity of the photosensitive drum. For example, when the differential MTF value (A−B) is positive, the light amount correction is emphasized by setting n to a value exceeding 1, and the light amount correction is moderated by setting n to a value less than 1.

(8) Light emitted from a plurality of light emitting elements arranged in the axial direction of the photosensitive drum is transmitted to the peripheral surface of the photosensitive drum via a plurality of optical lenses arranged at a predetermined pitch in the axial direction. A light amount correction method for a light emitting element used in an exposure apparatus for converging and exposing the photosensitive drum,
The correction amount D applied to the light amount correction of each light emitting element is:
A ′ is the average value of the optical lens diameter in a minute section, B ′ is the average value of the optical lens diameter in a large section, C is a simple light quantity correction amount that does not take beam characteristics into account, and k is according to the use state of the exposure apparatus. An arbitrary coefficient that changes, where n is an arbitrary coefficient that changes according to the characteristics of the photosensitive drum,
D = C [1 + k (A′−B ′) ⁿ ]
It is characterized by being.

In this configuration, the correction amount D applied to the light amount correction of each light emitting element is calculated by a simple calculation formula D = C [1 + k (A′−B ′) ⁿ ]. That is, the light amount correction amount applied to each light emitting element is calculated on the basis of the difference between the light amount correction amount that easily causes the variation in the diameter of the optical lens and the light amount correction amount that does not easily cause the variation in the diameter of the optical lens. The

(9) Light emitted from a plurality of light emitting elements arranged in the axial direction of the photosensitive drum is transmitted to the peripheral surface of the photosensitive drum via a plurality of optical lenses arranged at a predetermined pitch in the axial direction. In an image forming apparatus having an exposure device for convergence,
An exposure apparatus to which the light quantity correction method for a light emitting element according to any one of (1) to (8) is applied is provided.

  In this configuration, the image forming apparatus includes an exposure apparatus in which an appropriate light amount correction is performed.

  According to the present invention, the following effects can be obtained.

(1) It is possible to obtain a light amount correction value reflecting the light transmission characteristics of the path from each light emitting element including the optical lens to the photosensitive drum. In other words, even when the convergence state of the light beam emitted by the optical lens changes due to this transfer characteristic, it is possible to correct the amount of light in consideration of the beam characteristic including the change in the convergence state, thereby suppressing the occurrence of uneven exposure. .

(2) Since the light amount correction amount considering the variation in the characteristics of the optical lens can be applied to each light emitting element, the occurrence of exposure unevenness can be suppressed regardless of the variation in the characteristics of the optical lens. It becomes possible.

(3) Since the small section MTF value and the differential MTF value calculated based on the small section MTF value are less affected by noise of the MTF measuring instrument, a more accurate light amount correction value can be applied to each light emitting element. become.

(4) With the light amount correction value obtained based on the differential MTF value, it is possible to equalize the light emission amount for each light emitting element in consideration of the variation in the characteristics of the optical lens and the variation in the light amount for each chip.

(5) Using the differential MTF value, a light amount correction amount to be applied for each light emitting element can be obtained quickly and easily.

(6) Appropriate light amount correction can be applied to each light emitting element regardless of the process conditions of the image forming process.

(7) Optimal light amount correction can be applied to each light emitting element in accordance with the sensitivity of the photosensitive drum.

(8) Appropriate light amount correction can be applied to each light emitting element regardless of variations in the diameter of the optical lens.

(9) Since the occurrence of uneven exposure in the exposure apparatus can be suppressed, an output image with high reproducibility can be obtained.

  Hereinafter, a digital copying machine equipped with the optical writing device of the present invention will be described as an embodiment of the present invention with reference to the drawings. In the following description, the term “paper” is interpreted to include all sheet materials, recording paper, transfer paper, and the like.

  FIG. 1 shows a schematic configuration of a digital copying machine 1 provided with an optical writing apparatus according to the present invention. As shown in FIG. 1, the digital copying machine 1 includes a document reading unit 110, an image forming unit 210, a multistage sheet feeding desk 300, and a post-processing device 260.

  The document reading unit 110 includes a document table 111 made of transparent glass, an automatic document feeder 112 disposed above the document reading unit 110, and an optical system unit that reads an image of a document on the document table 111.

  The automatic document feeder 112 is a device that automatically feeds a plurality of documents set on a document setting tray onto the document table 111 one by one. The optical system unit is disposed below the document table 111 and scans and reads an image of the document placed on the document table 111. The optical system unit includes a first scanning unit 113, a second scanning unit 114, an optical lens 115, and a CCD line sensor 116 that is a photoelectric conversion element.

  The first scanning unit 113 includes an exposure lamp unit that exposes the surface of the document, a first mirror that reflects a reflected light image from the document in a predetermined direction, and the like. The second scanning unit 114 includes a second mirror and a third mirror that guide reflected light from the document reflected from the first mirror to the CCD line sensor 116. The optical lens 115 forms an image of the reflected light from the original on the CCD line sensor 116.

  In addition, the document reading unit 110 reads an image of a document automatically conveyed by the automatic document conveying device 112 at a predetermined document reading position by an operation related to the automatic document conveying device 112. The document image read by the document reading unit 110 is sent as image data to an image data input unit (not shown). Subsequently, this image data is subjected to predetermined image processing and then temporarily stored in the memory of the image processing unit.

  The image data stored in the memory is read in response to an output instruction and transferred to an optical writing device 227 (described later) configured by an LED writing head or the like that is a solid scanning method of the image forming unit 210.

  A manual feed tray 254, a paper cassette 251, and a duplex unit 255 are provided below the image forming unit 210. Further, a multi-stage paper feed desk 300 having paper cassettes 252 and 253 is provided below.

  A paper transport path is formed from each of the paper cassettes 251 to 253 and the manual feed tray 254 to the post-processing device 260 via an image forming position described later. Further, the paper fed from the paper cassettes 251 to 253 or the manual feed tray 254 or the duplex unit 255 is supplied to the image forming unit 210 by a transport unit 250 having a transport roller.

  The duplex unit 255 communicates with a switchback path 221 that reverses the sheet, and is used when image formation is performed on both sides of the sheet. The duplex unit 255 can be replaced with a normal paper cassette, and the duplex unit 255 can be replaced with a normal paper cassette.

  The image forming unit 210 includes an image forming unit, a fixing unit 217, and a paper discharge roller 219 in order from the upstream side along the paper conveyance path. The image forming unit includes a photosensitive drum 222 as an image carrier, an optical writing device 227 as an exposure device, a charger 223 for charging the photosensitive drum 222 to a predetermined potential, and a static electricity formed on the photosensitive drum 222. A developer 224 that supplies toner to the electrostatic latent image to make it visible, a charger-type transfer device 225 that transfers the toner image formed on the surface of the photosensitive drum 222 to the paper, and removes the paper from the image carrier 222. A static eliminator 229 that facilitates the cleaning and a cleaning device 226 that collects excess toner are provided.

  Around the photosensitive drum 222, the charging unit 223, the optical writing device 227, the developing unit 224, the transfer unit 225, the static eliminator 229, and the cleaning unit 226 are charged, exposed, developed, and transferred. And a cleaning process is performed. At an image forming position located between the photosensitive drum 222 and the transfer device 225, an unfixed developer image based on the image data is transferred to the surface of the paper. Thereafter, the image is guided to a fixing unit 217 disposed on the downstream side of the image forming position in the paper conveyance path, and an unfixed developer image on the paper is heated and pressurized by the fixing unit 217, and the developer is applied to the paper. The image is fixed.

  On the downstream side of the fixing unit 217, the paper conveyance path branches in two directions, and one of the paper conveyance paths leads to a switchback path 221 that reverses the front and back of the paper in order to form an image again on the back side of the paper. The post-processing device 260 performs post-processing such as stapling on the paper on which the image is formed, and discharges the paper onto the lifting tray 261. In the present embodiment, a monochromatic image forming apparatus is described. However, the light quantity correction method of the present invention described later can be applied to a multicolor image forming apparatus.

  FIG. 2 shows the configuration of the optical writing device 227. The optical writing device 227 includes a plurality of LEDs 11, an LED array substrate 12, a lens array 13, and a storage unit described later. The optical writing device 227 is disposed at a predetermined position in the digital copying machine 1 so as to face the peripheral surface of the photosensitive drum 222. The optical writing device 227 functions as an exposure device that forms an electrostatic latent image on the peripheral surface of the photosensitive drum 222.

  The LEDs 11 are arranged in the axial direction (main scanning direction) of the photosensitive drum 222. In the present embodiment, the arrangement direction means the axial direction of the photosensitive drum 222. Each LED 11 blinks based on the input image data, and exposes the peripheral surface of the photosensitive drum 222 line by line. The number of LEDs 11 corresponds to the resolution of an image formed by the digital copying machine 1. For example, when the resolution is A3 width of 600 dpi, about 7000 LEDs 11 are used.

  The LED array substrate 12 includes a drive circuit (not shown). This drive circuit controls the lighting operation of each LED 11.

  The lens array 13 is disposed between the LED 11 and the photosensitive drum 222. At this time, the lens array 13 is arranged so that the lens array 13, the LED 11, and the photosensitive drum 222 are parallel to each other. The lens array 13 converges the blinking information of each LED 11 on the peripheral surface of the photosensitive drum 222. In the present embodiment, the lens array 13 is composed of a plurality of Selfoc lenses having a predetermined diameter arranged at a predetermined pitch.

  The storage unit is a rewritable nonvolatile memory attached to the optical writing device 227. This memory | storage part memorize | stores the measurement result of the light quantity from LED11 in the several measurement position mentioned later. In this embodiment, the optical writing device 227 is provided with a storage unit, but this storage unit may be provided separately from the optical writing device 227. In the case where the storage unit is provided separately from the optical writing device 227, the storage unit is always attached to the optical writing device 227 when the optical writing device 227 is attached to the digital copying machine 1. .

  In the above-described configuration, the optical writing device 227 repeats the blinking operation of each LED 11 for each line based on the image data. At this time, the light output from the LED 11 passes through the lens array 13 and converges on the peripheral surface of the photosensitive drum 222. As a result, exposure based on image data is performed line by line on the peripheral surface of the photosensitive drum 222 to form an electrostatic latent image.

  Here, when an electrostatic latent image is formed on the photosensitive drum 222, unevenness occurs in the amount of light reaching the photosensitive drum 222 due to a difference in light emission amount of each LED 11 and a difference in pitch between the LED 11 and the lens array 13. Sometimes. Due to this unevenness in the amount of light, the electrostatic latent image is also uneven, and there is a disadvantage that the image quality of the image formed on the paper is lowered.

  Therefore, in this embodiment, the light amount reaching the photosensitive drum 222 is made uniform by using the light amount correction method of the light emitting element unique to the present invention. Hereinafter, the light amount correction method of the light emitting element of the present invention will be described in detail.

  FIG. 3 shows a light quantity measurement process using the light quantity detector 20. As shown in the figure, the optical writing device 227 is attached to the light amount detector 20 in the light amount measurement process. The light quantity detector 20 includes a CCD camera 21, an automatic stage 22, and a motor 23. The light quantity detector 20 is connected to the personal computer 50 and automatically performs necessary measurement processing in accordance with a signal from the personal computer 50. In the figure, L1 indicates the arrangement length of all the LEDs 11. L2 indicates the pitch of the light image emitted from each LED 11, and L3 indicates the diameter of the light image. In this embodiment, L1 is about 300 mm (A3 width), L2 is about 42.3 μm, and L3 is 60 μm.

  The CCD camera 21 measures the emitted beam from each LED 11. The automatic stage 22 is equipped with a CCD camera 21. The motor 23 moves the automatic stage 22 in a direction approaching or separating from the LED 11 (arrow Z direction in the figure) and an arrangement direction of the LEDs 11 (Y direction in the figure).

  In the light quantity measurement process, the LEDs 11 and the CCD camera 21 are arranged to face each other. The CCD camera 21 is moved in the Z direction by an automatic stage 22 that is driven by a motor 23. At this time, the CCD camera 21 stops at a predetermined distance from each LED 11. This predetermined distance corresponds to the distance between the optical writing device 227 and the peripheral surface of the photosensitive drum 222. In the present embodiment, the position of the CCD camera 21 at this time (position indicated by an arrow B in the figure) is set as a reference position.

  Subsequently, a predetermined current is applied to one of the LEDs 11 focused on the CCD camera 21 for a predetermined time. As a result, the LED 11 is lit for a predetermined lighting time, and the emitted light beam from the LED 11 is measured by the CCD camera 21. When the measurement of the LED 11 is completed, the automatic stage 22 moves in the Y direction. Then, the emission beam of the next LED 11 focused on the CCD camera 21 is measured.

  In addition, after the light emission beams of all the LEDs 11 are measured at the reference position B, the light emission beams may be measured by changing the distance between the CCD camera 21 and the LEDs 11 as necessary. In this case, the CCD camera 21 moves in the Z direction and measures the emitted light beams of all the LEDs 11 at the measurement position A or the measurement position C.

  As described above, the measurement of the emitted beam of each LED 11 at the reference position B is normally performed while moving the automatic stage 22 in the Y direction. The measurement of the light emission beam of each LED 11 at the reference position B is performed by sequentially lighting the LEDs 11 one by one so as not to cause interference with the light emission beam of the adjacent LED 11.

  In the present embodiment, the LEDs 11 are sequentially turned on one by one. However, when the CCD camera 21 has a sufficiently wide field of view, it is possible to perform measurement by turning on a plurality of LEDs 11 in the same figure. . In this case, when measuring the emitted beam of a certain LED 11, in order to prevent the beams from other LEDs 11 from interfering with each other, the LEDs 11 arranged in a line are lit at intervals of every three to six. You just have to devise it. Thereby, the work time of the light emission beam measurement by the light quantity detector 20 can be shortened.

  When the beam characteristic of the LED 11 is measured by the light quantity detector 20, the input light quantity input to the CCD camera 21 of each LED 11 is detected based on the beam shape. This input light amount corresponds to an exposure amount for exposing the peripheral surface of the photosensitive drum 222 when the optical writing device 227 is mounted on the digital copying machine 1. The input light amount detected by the CCD camera 21 is stored in a storage unit provided in the optical writing device 227.

  FIG. 4 shows the beam shape of the emitted light beam measured by the CCD camera 21. The beam shape measured by the CCD camera 21 is represented by a three-dimensional beam shape as shown in FIG. Here, an example is shown in which the measurement is performed by turning on every six LEDs 11 arranged in a row at intervals of one. By calculating the volume of the three-dimensional beam shape shown in the figure, the input light amount input from each LED 11 to the CCD camera 21 can be obtained. The amount of light that is exposed to the peripheral surface of the photosensitive drum 222 is obtained from this input light amount. By measuring the exposure amount by measuring the three-dimensional beam shape in this way, the amount of light exposed on the peripheral surface of the photosensitive drum 222 is obtained with high accuracy, and therefore the sensitivity characteristic of the photosensitive drum 222 is high. It is effective for.

  On the other hand, when the sensitivity of the photosensitive drum 222 is low, the two-dimensional beam shape may be simply measured without measuring the three-dimensional beam shape.

  FIG. 5 shows the measurement result of the two-dimensional beam shape. As shown in the figure, by calculating the area of the two-dimensional beam shape, the amount of light exposed on the peripheral surface of the photosensitive drum 222 can be obtained. In this case, since the three-dimensional light beam input to the CCD camera 21 is replaced with a two-dimensional light beam, the exposure amount measurement accuracy slightly decreases. However, by replacing the three-dimensional emission beam with the two-dimensional emission beam, the processing speed can be increased, such as shortening the measurement time.

  In the present embodiment, a simple light amount correction value C to be described later is obtained based on the difference between the light emission amount for each LED 11 obtained in the light amount measurement process and a predetermined reference light emission amount.

  The light emitted from each LED 11 reaches the peripheral surface (image surface) of the photosensitive drum 222 through a predetermined path including the lens array 13. Therefore, in the present embodiment, the MTF value corresponding to each LED 11 is measured in order to quantitatively represent the beam characteristics from each LED 11 to the peripheral surface of the photosensitive drum 222. Here, the MTF value corresponding to each LED 11 means the MTF value in the light transmission path formed at the position corresponding to each LED 11. With this MTF value, it is possible to express what characteristics the lens array 13 that transmits the light emitted from each LED 11 to the peripheral surface of the photosensitive drum 222 has.

Here, the MTF value is determined when V _max and V _min are the maximum value and the minimum value of the input light amount of the adjacent LEDs 11 at an arbitrary spatial frequency,
MTF value = [(V _max −V _min ) / (V _max + V _min )]
It is represented by In addition, when expressing the MTF value as a percentage,
MTF value (%) = [(V _max −V _min ) / (V _max + V _min )] × 100
It becomes. From this value, the amount of reduction in contrast on the image plane can be known.

A method of calculating the MTF value (%) will be described with reference to FIG. In the figure, a light emission beam A and a light emission beam B represented in a two-dimensional shape are shown. Here, V _max of the emission beam A is “60”, and V _min is “20”. For this reason, the MTF value (%) of the emission beam A is 50%. On the other hand, V _{max of the} emission beam B is “45”, and V _min is “15”. For this reason, the MTF value (%) of the emission beam B is 50%.

  In this embodiment, when the MTF value is measured, every two LEDs 11 are lit at intervals. When measuring the MTF value, every three LEDs 11 may be lit at one interval.

In the present embodiment, when the MTF value is measured, the adjacent LEDs 11 are caused to emit light, thereby forming a state in which interference occurs with respect to the light emitted from the adjacent LEDs 11. In this case, V _max is a value equivalent to a state where no interference occurs. However, V _min is a value that is significantly different from the state in which no interference occurs.

  Here, a method of creating light amount correction data will be described. First, all the LEDs 11 are sequentially made to emit light one by one, and the input light quantity is measured by the light quantity measuring device 20. At this time, all the light emitting elements may emit light collectively.

  FIG. 7 shows the MTF value corresponding to each LED 11. In the drawing, positions corresponding to the LEDs 11 arranged in the axial direction of the photosensitive drum 222 are indicated by the horizontal axis, and MTF values (%) corresponding to the respective LEDs 11 are indicated by the vertical axis.

  In the figure, the MTF measurement value protruding upward or downward indicates the noise n of the MTF measurement device. Here, various noises such as noise that appears in a fine cycle and noise that appears in a large cycle are included in the MTF measurement value.

  Subsequently, averaging is performed sequentially with a relatively small number of data in order to remove noise in the MTF data measured by the MTF measuring apparatus. Here, the MTF values are averaged for the LEDs 11 included in a relatively small range including the LED 11 to be measured. The value obtained by this averaging is the small section MTF value of the present invention.

  FIG. 8 shows an example of the small section MTF value of the present invention. If the MTF measuring apparatus is low in noise, averaging is not necessary. However, averaging is effective when there is a lot of noise in the MTF measuring apparatus. That is, by averaging the MTF values in a small section, the influence of noise is less likely to appear in the MTF measurement value. For this reason, in the same figure, the MTF measurement value which protrudes upward or downward like FIG. 7 does not appear.

  The range of this small section is set according to the characteristics of the LED head. In the present embodiment, the small section refers to a range corresponding to the length in the arrangement direction of the 20 LEDs 11. This is because the purpose of this embodiment is to correct the wire pitch streak of the lens pitch in the lens array 13. That is, if a range exceeding the length in the arrangement direction of the 20 LEDs 11 is set as a small section, the width of the small section exceeds the lens strand pitch. As a result, there is an inconvenience that it becomes impossible to obtain an MTF value that is not easily affected by the lens pitch streak.

  When obtaining the small section MTF value, the number of LEDs 11 in the arrangement direction is set so that the arrangement length is equal to or less than the strand pitch of the Selfoc lens. For example, in this embodiment, since one fiber of the SELFOC lens is 1 mm and each LED 11 (600 dpi) is 42.3 μm, the range of the small section may be made smaller than the arrangement length of about 22 LEDs 11.

  The range of the small section is not limited to this embodiment, and can be arbitrarily set according to the size of the strand pitch of the lens pitch in the lens array 13. As a general rule, it is desirable to make the range of the small section as small as possible within the range in which the influence of noise can be removed.

  By setting the range of the small section in this way, it is possible to obtain a light amount correction value reflecting the lens pitch unevenness that is likely to occur at the boundary of each fiber of the SELFOC lens.

  Subsequently, the MTF values are averaged over a relatively large large section to obtain the large section MTF value.

  FIG. 9 shows an example of the large section MTF value of the present invention. By taking an average over a large section, it is possible to correct exposure unevenness with a large period. Similar to the small section described above, the large section range is set in accordance with the characteristics of the LED array substrate 12. In this embodiment, the large section is for 512 pixels. This is because the purpose of this embodiment is to correct the influence of the uneven exposure amount of each chip unit (128 elements) in which the LED light emitting elements are arranged. That is, when the large section is set to 512 or less, there is a disadvantage that it is impossible to eliminate the exposure unevenness caused by the difference in the generation amount of the LED 11 for each chip unit.

  The array length of the LEDs 11 corresponding to the large section is set according to the number of LEDs 11 mounted on one chip. For example, when 256 LEDs 11 are arranged in one chip, the width of the large section may be set to be equal to or larger than the arrangement length of 256 LEDs 11.

  In this way, by setting the range of the large section, it is possible to obtain the MTF value that cancels noise and measurement errors while leaving the variation in light amount or beam shape in units of chips.

  Subsequently, a difference between the small section MTF value and the large section MTF value is calculated, and a difference MTF value corresponding to each LED 11 is obtained.

  FIG. 10 shows an example of the differential MTF value of the present invention. With this differential MTF value, the variation of the MTF value with the center being 0 can be converted into data. That is, the differential MTF value is data that is easily reflected during light amount correction. For this reason, by obtaining the differential MTF value, it becomes possible to correct exposure unevenness in both a small cycle and a large cycle.

Specifically, when the small section MTF value is A, the large section MTF value is B, and the differential MTF value is “A−B”, the specific light amount correction value D of the present invention in consideration of the beam characteristics of each LED 11. But,
D = C [1+ (A−B)]
It is represented by

  Here, the small section MTF value A and the large section MTF value B each take a value of 0 to 1.0. C is a simple light quantity correction value that does not consider the beam characteristics of each LED 11. The simple light amount correction value C is a light amount correction value that has been conventionally used to equalize the light emission amount of each LED 11 when the beam characteristics of each LED 11 are not considered. This simple light quantity correction value C is generally obtained by measuring the volume, area, or MTF value of the beam shape. In the present embodiment, the simple light amount correction value C obtained by the light amount measurement process shown in FIG. 3 is used.

  Further, by adding a variable value k to the above equation in consideration of the characteristics of each image forming apparatus, the equation for obtaining the light amount correction value is D = C [1 + k (A−B)]. By using this equation, the present invention can be applied to a plurality of types of image forming apparatuses. If the coefficient k is a function that takes into consideration the lifetime (life) of the image forming apparatus and the environment, it is possible to cope with changes in the operating environment regardless of the apparatus.

  The difference MTF value is reflected in the light quantity correction data by applying an arbitrary coefficient k while taking the process condition into consideration. When light amount correction is performed in increments of 3%, the correction value is changed in one step at 0 to 3%, and the correction value is changed in two steps at 4 to 6%.

  The process conditions also change depending on the use state of the digital copying machine 1 and the optical writing device 227. For example, when the use is continued for a long time, the sensitivity of the photosensitive drum 222 is lowered. Therefore, considering the deterioration of the process conditions, if the value of the coefficient k can be changed according to the process conditions, the optimum MTF correction can be performed according to the use situation.

Furthermore, the correction amount considering the sensitivity of the photoreceptor is
D = C [1 + k (A−B) ⁿ ]
Is required.

  By using this equation, the correction amount can be changed in consideration of the sensitivity of the photoreceptor. Here, if n> 1, the MTF measurement can be emphasized, and if n <1, the MTF correction can be relaxed.

In this way, the light amount correction amount D is obtained using D = C [1 + k (AB) ⁿ ], and the power supplied to each LED 11 and the power supply time are adjusted based on the light amount correction amount D. This makes it possible to equalize the light emission amount for each light emitting element in consideration of the beam characteristics. For this reason, it becomes possible to prevent image formation defects such as uneven exposure regardless of the accuracy of the lens array 13.

  In this embodiment, the range of the small section and the large section is set according to the lens pitch or the size of the chip, but the setting of the range of the small section and the large section is not limited to this embodiment. For example, when the MTF value is measured by a high-performance measuring device with little noise, the range of the small section can be set to “0”. Moreover, if the number of LEDs 11 mounted on one chip increases, the range of the large section can be expanded. In this manner, the small section and the large section can be set by selecting optimum values according to the usage situation.

  Since the sensitivity of the photosensitive drum 222 deteriorates with time, it is also effective to use the above equation for obtaining the light amount correction amount as a function of the usage period so that the difference MTF value can be changed depending on the usage period. It is.

  Further, although the coefficient depends on the process condition, the coefficient k may be changed according to the process condition or the use situation of each digital copying machine 1. By storing the optimum k value in the storage unit according to the process condition or use situation for each digital copying machine 1, it becomes possible to make the optical writing device 227 of any use model or use environment.

  FIG. 11 is a flowchart showing the light amount correction method of the present invention. For each LED 11 of the optical writing device 227 mounted on the digital copying machine 1, first, an MTF value for each LED 11 is obtained (S1). Subsequently, a small section MTF value for each LED 11 is obtained (S2). Subsequently, the large section MTF value for each LED 11 is obtained (S3). Subsequently, a difference MTF value indicating a difference between the small section MTF value and the large section MTF value is obtained for each LED 11 (S5). Subsequently, an appropriate light amount correction amount D is obtained based on the differential MTF value. Then, the light emission amount for each LED 1 is corrected using the obtained light amount correction amount (S6).

Finally, variations of the light amount correction amount D are shown. For the light amount correction amount D, A ′ is the average value of the optical lens diameter in the small section, B ′ is the average value of the optical lens diameter in the large section, C ′ is the simple light amount correction amount, and k is the use state of the exposure apparatus. An arbitrary coefficient that changes in accordance with this, and n is an arbitrary coefficient that changes in accordance with the characteristics of the photosensitive drum,
D = C [1 + k (A′−B ′) ⁿ ]
It can also be obtained using the following formula.

  In this case, the range of the small section and the range of the large section can be arbitrarily set according to the usage situation. For example, the small section is set in a range in which variations in the diameter of the optical lens such as the arrangement length of 50 LEDs 11 are likely to appear, and the large section is less likely to show variations in the diameter of the optical lens such as the arrangement length of 1000 or more LEDs 11. It should be a range. Of course, the small section and the large section can be appropriately changed according to the use situation.

1 is a diagram showing a schematic configuration of a digital copying machine to which the present invention is applied. It is a figure which shows the structure of the optical writing apparatus with which this invention is applied. It is a figure which shows the light quantity measurement process using a light quantity measuring device. It is a figure which shows the light emitted from LED by the three-dimensional beam shape. It is a figure which shows the light emitted from LED by the two-dimensional beam shape. It is a figure which shows the light emitted from LED by the two-dimensional beam shape. It is a figure which shows the MTF measuring device corresponding to every LED. It is a figure which shows the small area MTF value of this invention. It is a figure which shows the large area MTF value of this invention. It is a figure which shows the difference MTF value of this invention. It is a flowchart which shows the light quantity correction method.

Explanation of symbols

1-Digital copier 11-LED
12-LED array substrate 13-lens array 210-image forming unit 222-photosensitive drum 227-optical writing device

Claims (9)

Light emitted from a plurality of light emitting elements arranged in the axial direction of the photosensitive drum is converged on a peripheral surface of the photosensitive drum via a plurality of optical lenses arranged in the axial direction, and the photosensitive drum A light amount correction method for a light emitting element used in an exposure apparatus that exposes
Measuring beam characteristics of the plurality of light emitting elements, and obtaining a corresponding MTF value for each light emitting element;
For each of the plurality of light emitting elements, averaging MTF values in a limited section including itself to obtain an average MTF value;
For each of the plurality of light emitting elements, obtaining a difference MTF value indicating a difference between the MTF value for each light emitting element and the average MTF value;
Correcting the light emission amount for each light emitting element based on the difference MTF value so that the light emission amount for each light emitting element considering beam characteristics is uniform;
A method for correcting the amount of light of a light emitting element, comprising: Light emitted from a plurality of light emitting elements arranged in the axial direction of the photosensitive drum is converged on a peripheral surface of the photosensitive drum via a plurality of optical lenses arranged at a predetermined pitch in the axial direction, A light amount correction method for a light emitting element used in an exposure apparatus that exposes the photosensitive drum,
Measuring beam characteristics of the plurality of light emitting elements, and obtaining a corresponding MTF value for each light emitting element;
For each of the plurality of light emitting elements, averaging the MTF values in a small section smaller than the pitch to obtain a small section MTF value;
For each of the plurality of light emitting elements, averaging MTF values in a limited section larger than at least the pitch to obtain a large section MTF value;
Obtaining a difference MTF value indicating a difference between the small section MTF value and the large section MTF value for each of the plurality of light emitting elements;
Correcting the light emission amount for each light emitting element based on the difference MTF value so that the light emission amount for each light emitting element considering beam characteristics is uniform;
A method for correcting the amount of light of a light emitting element, comprising:   The light amount correction method for a light emitting element according to claim 2, wherein the small section is larger than an arrangement length of two or more light emitting elements and smaller than the pitch. The light emitting elements are formed as one chip every predetermined number,
4. The light quantity correction method for a light emitting element according to claim 2, wherein the limited section is at least longer than an arrangement length of the predetermined number of light emitting elements. The correction amount D applied to the light amount correction of each light emitting element is:
When A is a small section MTF value, B is a large section MTF value, and C is a simple light amount correction amount that does not consider beam characteristics,
D = C [1+ (A−B)]
The light quantity correction method for a light emitting element according to claim 2, wherein The correction amount D applied to the light amount correction of each light emitting element is:
When A is a small section MTF value, B is a large section MTF value, C is a simple light quantity correction amount that does not take beam characteristics into account, and k is an arbitrary coefficient that changes according to the use state of the exposure apparatus.
D = C [1 + k (AB)]
The light quantity correction method for a light emitting element according to claim 2, wherein The correction amount D applied to the light amount correction of each light emitting element is:
A is a small section MTF value, B is a large section MTF value, C is a simple light quantity correction amount that does not take beam characteristics into account, k is an arbitrary coefficient that changes according to the use state of the exposure apparatus, and n is a characteristic of the photosensitive drum. With an arbitrary coefficient that changes accordingly,
D = C [1 + k (A−B) ⁿ ]
The light quantity correction method for a light emitting element according to claim 2, wherein Light emitted from a plurality of light emitting elements arranged in the axial direction of the photosensitive drum is converged on a peripheral surface of the photosensitive drum via a plurality of optical lenses arranged at a predetermined pitch in the axial direction, A light amount correction method for a light emitting element used in an exposure apparatus that exposes the photosensitive drum,
The correction amount D applied to the light amount correction of each light emitting element is:
A ′ is an average value of the optical lens diameter in a minute section, B ′ is an average value of the optical lens diameter in a large section, C ′ is a simple light quantity correction amount not including beam characteristics, and k is in accordance with the use state of the exposure apparatus. And n is an arbitrary coefficient that changes according to the characteristics of the photosensitive drum,
D = C [1 + k (A′−B ′) ⁿ ]
A method for correcting the amount of light of a light-emitting element. Exposure for converging light emitted from a plurality of light emitting elements arranged in the axial direction of the photosensitive drum on a peripheral surface of the photosensitive drum via a plurality of optical lenses arranged at a predetermined pitch in the axial direction. In an image forming apparatus including the apparatus,
An image forming apparatus comprising an exposure apparatus to which the light amount correction method for a light emitting element according to claim 1 is applied.

Images