JP2013217982A - Image forming apparatus, image forming method, and computer program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To properly adjust the amount of light emission of a light-emitting device in forming an image having density in accordance with the amount of light emission of the light-emitting device on an image carrier.SOLUTION: On a photoconductor drum 21, or in an area of attention common to a plurality of transfer materials p to which developing materials are attached by the photoconductor drum 21, a plurality of images having different densities from each other are formed. Density of each of the plurality of images formed is measured, and a threshold of a drive current of a light-emitting device that emits light for exposing the area of attention (specific image area) when densities of the images in the area of attention start to change is identified on the basis of the amount of change in density of each of the measured images. Subsequently, the drive current is corrected with the threshold taken as a reference.

Description

  The present invention relates to an image forming apparatus such as a printer, a scanner, a copying machine, or a multifunction machine in which these functions are integrated. More specifically, the present invention relates to a technique for appropriately controlling each light emitting element even when the light source at the time of image formation has a plurality of light emitting elements and light emission of adjacent light emitting elements becomes a problem.

  Patented as an image forming apparatus that forms an image without density unevenness by detecting density unevenness of an image caused by variations in light emitted from a plurality of light emitting elements and adjusting the intensity of light emitted by each element. An image forming apparatus disclosed in Document 1 is known. In this image forming apparatus, a test image is formed based on image data representing an image having a uniform density in the main scanning direction. Based on the density data representing the density distribution in the main scanning direction of the test image, the light quantity variation in the main scanning direction of the light emitted from the light source is corrected.

  More specifically, the density data is represented by a matrix in which pixels in the main scanning direction in which the test image is read are rows and pixels in the sub scanning direction are columns. Then, by obtaining the deviation between the target density of the density data in the main scanning direction and the measured density, the position of the light emitting element having the variation is specified. Thereafter, the light amount of the specified light emitting element is individually corrected.

JP 2002-307745 A

  Since the image forming apparatus disclosed in Patent Document 1 uses a test image, it is possible to detect variations in the amount of light emitted from a plurality of light emitting elements using existing equipment without providing a light amount sensor or the like. it can. Further, since the variation is adjusted based on the finally printed image density, the density unevenness including the deviation in the main scanning direction due to the characteristics of the toner image forming medium and the transfer material is corrected.

However, the image forming apparatus disclosed in Patent Document 1 does not consider the light emission characteristics of the individual light emitting elements, particularly the threshold value of the drive current when the density starts to change. In particular, the influence of light emission of adjacent light emitting elements when a plurality of adjacent light emitting elements emit light simultaneously is not considered. Therefore, although it is possible to correct density unevenness, there is no guarantee that the density (before and after correction) is the target density. In particular, when an image having a multi-gradation density is formed, the influence is great.
The present invention solves such a conventional problem, detects a threshold value of power for driving each light emitting element, and thereby enables appropriate light emission control, an image forming apparatus, an image forming method, and a computer program The issue is to provide

  The image forming apparatus of the present invention that solves the above problems is an image forming apparatus that forms an image having a density corresponding to the light emission amount of a light emitting element on an image carrier. The image forming apparatus includes image forming means for forming a plurality of images having different densities in a common region of interest of the image carrier. In addition, it has a measuring means for measuring the density of each formed image. Further, based on the measured amount of change in the density of each image, the threshold current of the drive current of the light emitting element that emits light that exposes the region of interest when the image density in the region of interest begins to change And a control means for specifying the value of.

  In the image forming method of the present invention, when control is performed to form an image having a density corresponding to the light emission amount of the light emitting element on the image carrier, a plurality of different densities are respectively preliminarily applied to a common region of interest of the image carrier. And measuring the density of each formed image and, based on the measured amount of change in the density of each image, the area of interest when the density of the image in the area of interest begins to change. This is a method of specifying the threshold current value of the drive current of the light emitting element that emits the light to be exposed.

  A computer program according to the present invention is a computer program for causing a computer apparatus to operate as an image forming apparatus that forms an image having a density corresponding to the amount of light emitted from a light emitting element on an image carrier. Based on the image forming means for forming a plurality of images having different densities on the common attention area of the carrier, the measuring means for measuring the density of each formed image, and the amount of change in the density of each measured image, A computer program that functions as a control unit that specifies a threshold current value of a drive current of a light emitting element that emits light for exposing the region of interest when an image density in the region of interest starts to change.

  In the present invention, based on the amount of change in the density of a plurality of images formed in a common attention area of the image carrier, the threshold value of power for driving the light emitting element when the density of the image in the attention area starts to change Is identified. This threshold value is a threshold value unique to each light emitting element that does not consider light emission of other light emitting elements in the region of interest. Therefore, it is possible to provide an image forming apparatus that allows appropriate light emission control of the light emitting element in the region of interest by increasing / decreasing the driving current of the light emitting element with reference to this threshold value.

1 is a diagram illustrating an internal structure of an image forming apparatus to which the present invention is applied. FIG. 3 is an operation procedure diagram of the image forming apparatus of the present embodiment. The block diagram of an optical scanning device. The figure which showed the relationship (IP characteristic) of the emitted light quantity with respect to the drive current of a laser light source. The figure explaining APC control with respect to the laser light source provided with the several light emission point. (A) shows an example of laser A control, and (b) shows an example of laser B control. The figure which showed the original IP characteristic and the IP characteristic detected in the case of Fig.5 (a). FIG. 4 is an explanatory diagram of sensors and the like provided on the photosensitive drum in order to realize the control of the present embodiment. FIG. 4 is a timing chart from when a reference position is detected until an image is formed. FIG. 3 is a functional block diagram from when a reference position is detected until an image is formed. Explanatory drawing of the test image A. FIG. The figure which shows the test image B in an Example. The figure which shows the test image C. FIG. FIG. 3 is a diagram illustrating a density adjustment flow of the image forming apparatus according to the present exemplary embodiment. The figure which showed a mode that the result of density | concentration detection was output as density data. The figure showing the result of having measured the density | concentration of the test image A. FIG. The figure showing the result of having measured the density | concentration of the test image B. FIG. The figure showing the result of having performed the density | concentration measurement of the test image C. FIG. The figure which showed the relationship between the drive current supplied to a laser element, and measurement density | concentration.

[Image forming apparatus]
First, an image forming apparatus to which the present invention is applied will be described. FIG. 1 is an internal structure diagram of an image forming apparatus according to this embodiment. FIG. 2 is an operation procedure diagram of the image forming apparatus, and S represents processing or state transition.

  The image forming apparatus 1 sequentially conveys the originals stacked on the original feeder 11 onto the surface of the original table glass 12 one by one (S501). When the document is conveyed, the lamp of the scanner 13 is turned on, the scanner unit 14 moves to a predetermined position, and the light from the light source is irradiated toward the document (S502). This light is reflected by the document and becomes reflected light r1. The reflected light r <b> 1 passes through the lens 18 via the mirrors 15, 16, and 17 and reaches the image sensor unit 19. The image sensor unit 19 converts the reflected light r1 into an image signal r2, and then temporarily stores the image signal r2 in an image memory or the like (S503). Then, the image signal r2 is read at a predetermined timing and input to the optical scanning device 20 (S504). In some cases, the image signal r2 may be directly input to the optical scanning device 20 without being stored in the image memory or the like.

  The image forming apparatus 1 includes a photosensitive drum 21 as an example of a photosensitive member. The photosensitive drum 21 is rotationally driven by a motor (not shown). The photosensitive drum 21 is charged by the charger 22 (S505). Based on the image signal r2, a laser element provided inside the optical scanning device 20 emits light and outputs a laser beam. This laser beam is guided to the photosensitive drum 21 charged by the charger 22. At that time, the laser beam is deflected by an optical system member to be described later so that the laser beam scans the photosensitive drum 21 in a predetermined direction (S506). In this manner, a latent image based on the image signal r2 is formed on the photosensitive drum 21 by the laser light irradiating the photosensitive drum 21 (S507). This latent image is developed by the developing device 23 (S508). The transfer material p, for example, paper is conveyed in the direction of the arrow from the transfer material stacking sections 24 and 25 to the conveyance mechanism 30 in synchronization with the latent image. Then, the developed toner image is transferred to the surface of the transfer material p in the transfer unit 26 (S509). The transferred toner image is fixed to the transfer material p by the fixing unit 27 (S510), and discharged from the paper discharge unit 28 to the outside of the apparatus (S511).

  When forming a plurality of images, the above procedure is repeated. Operation instructions, confirmation and overall control of the image forming apparatus 1 are performed by an operation unit 40 having a display. The operation unit 40 is a kind of computer device, and performs the above-described control by executing a predetermined control program. For example, the control system designates the size of the transfer material p and the number of images to be formed, and performs control for characteristic density adjustment described later. In the monitoring system, the size of the transfer material p designated as described above and the number of formed images are displayed on the display.

[Optical scanning device]
A configuration example of the optical scanning device 20 is shown in FIG. The optical scanning device 20 includes a control unit 209. The control unit 209 is a kind of computer device, and controls the operation of the constituent members of the optical scanning device 20 in cooperation with the operation unit 40 by reading and executing a predetermined program.

  The laser light source 201 turns on or turns off the laser light at a predetermined timing corresponding to the image signal r2 sent from the control unit 209. The photo detector 208 (hereinafter referred to as “PD”) detects the laser beam and transmits a detection signal c 1 corresponding to the amount of the laser beam to the control unit 209. The control unit 209 detects the light amount of the laser light based on the detection signal c1, and performs subsequent light amount control based on the detected light amount.

The laser light from the laser light source 201 is deflected and scanned through the optical system member and reaches the surface of the photosensitive drum 21. That is, the laser light is shaped into parallel light by the collimator lens 202, then condensed by the cylindrical lens 203, and reaches the photosensitive drum 21 through the polygon mirror (rotating polygon mirror) 204, the scanning lens A 205, and the scanning lens B 206. .
The polygon mirror 204 is rotationally driven by a driving mechanism such as a motor, and deflects the laser light (in the direction indicated by the arrow) so that the laser light scans the photosensitive drum 21 in a predetermined direction. The deflected laser light (scanning light) is corrected by the scanning lens A 205 and the scanning lens B 206 so as to maintain a constant speed. The light detection sensor 207 generates a horizontal synchronization signal in response to detecting the scanning light, and outputs the horizontal synchronization signal to the control unit 209. The control unit 209 controls the emission timing of the laser light according to the generation timing of the horizontal synchronization signal.

[APC control]
In the image forming apparatus of this embodiment, a driving current exceeding a threshold current described later is supplied to the light emitting element. Thus, toner does not adhere to the area on the photosensitive drum exposed by the emitted laser light. In the image area ga emitted in a state where a bias current equal to or less than the threshold current is supplied to the light emitting element, the developer (toner) adheres to the transfer material p in the area where the laser light is extinguished. In the region where the laser light is lit, the developer does not adhere to the transfer material p. Forming a latent image with laser light and allowing the developer to adhere is called “exposure”, and the above exposure method is called background exposure (hereinafter referred to as “BAE”). .
The controller 209 constantly supplies a driving current to the laser light source 201 that does not cause laser oscillation in order to improve light emission characteristics (for example, rise characteristics) when the laser light source 201 is turned on or off during BAE. Such a drive current is called a bias current. For this reason, even if the control unit 209 controls the laser light source 201 to be turned off, the laser light source 201 emits a slight amount of light. Such a state is referred to as a “bias light emission state”. Since the charged potential on the photosensitive drum is not changed by the laser light emitted from the laser light source 201 in the bias emission state, the toner adheres to the charged potential region on the photosensitive drum. As a result, the toner adheres to the area on the transfer material p corresponding to the area not exposed by the laser beam.

  The light emission amount (light emission intensity) of the laser light source 201 affects the density of the formed image. The image density also varies depending on variations in sensitivity characteristics or charging characteristics of the photosensitive drum 21. Therefore, this type of image forming apparatus generally performs APC (Auto Power Control) control for detecting the light emission amount when the laser light source 201 is turned on and keeping the light emission amount constant during the non-image formation time. It is. The non-image forming time is a time during which no image is formed during one scan. The APC control is performed by the control unit 209 based on the detection signal c1 from the PD 208. That is, when it is desired to reduce the image density, the current flowing through the laser light source 201 is increased with reference to the threshold current Ith. In BAE, if the current flowing through the laser light source 201 increases, the amount of toner attached to the transfer material decreases and the image density decreases. Thereby, density correction is performed. Here, the “threshold current Ith” is the current value of the drive current when the laser element of the laser light source 201 starts to oscillate.

  However, even if APC control is performed, for example, when the laser light source 201 includes a plurality of light emitting points and they are close to each other, there is a problem that they are affected by other light emitting points. That is, the threshold current Ith is determined to be a value different from the original value due to the influence of the bias current of another light emitting point, and the density correction may not be performed correctly.

This will be described in detail. First, how to determine the threshold current Ith will be described with reference to FIG. FIG. 4 shows the relationship (I-P characteristics) of the light emission form (for example, the light emission amount) of the laser light source 201 with respect to the drive current of the laser light source 201. The horizontal axis is the current value, and the vertical axis is the light emission amount.
In the APC control, a target light amount is determined, and a change value from the threshold current Ith is detected so that the light emission amount becomes the target light amount. Therefore, first, the threshold current Ith is determined as follows. That is, the current value I1 when the APC control is performed with respect to the laser light source 201 with the first target light amount (P1) is stored. Next, the current value I2 when the APC control is performed with the second target light quantity (P2) is stored. Then, a straight line connecting the point (I1, P1) and the point (I2, P2) in FIG. 4 is extended to obtain a current value when the light emission amount P becomes zero. The current value at this time is detected as a threshold current Ith.

  APC control when the laser light source 201 includes a plurality of light emission points is performed for each light emission point. This is because usually only one PD 208 is provided for a plurality of light emitting points. Therefore, the APC control for the laser light source 201 provided with a plurality of light emitting points and in the vicinity thereof has the content shown in FIG.

  FIG. 5A shows that the laser B is in the bias emission state when the APC control of the laser A is performed. Similarly, FIG. 5B shows that when the APC control of the laser B is performed, the laser A is in a bias emission state. As described above, when the APC control of the laser A is performed, not only the light of the laser A but also the light of the laser B in the bias emission state, that is, the bias light is incident on the PD 208. Similarly, when the APC control of the laser B is performed, not only the laser B light but also the bias light of the laser A in the bias emission state is incident on the PD 208. As a result, the control unit 209 detects a current value different from the original threshold current Ith shown in FIG. 4 as a threshold current.

FIG. 6 shows a relationship (IP characteristic) of the light emission amount with respect to the drive current in the state of FIG. As in FIG. 4, the horizontal axis represents the current value, and the vertical axis represents the light emission amount. The thin line is the same as the characteristic shown in FIG. This is the original IP characteristic A. The characteristic indicated by the thick line is the IP characteristic A ′ detected in the state of FIG.
When the APC control is performed on the laser A with the first target light amount (P1) in the same procedure as in FIG. 4, the actual light emission amount is the first target light amount due to the influence of the bias light of the laser B. P1 ′ is smaller than the light amount (P1) (P1 = P1 ′ + bias light of the laser B). Therefore, the current value I1 ′ when the laser A has a light emission amount P1 ′ is smaller than the current value I1 when the light emission amount P1.

  Similarly, when the APC control is performed on the laser A with the second target light amount (P2), the actual light emission amount of the laser A due to the bias light of the laser B is the second target light amount (P2). The light emission amount P2 ′ is smaller (P2 = P2 ′ + laser B bias light). Therefore, the current value I2 'when the laser A emits light with the light emission amount P2' is smaller than the current value I2 when the light emission amount P2. Then, when the threshold current Ith is obtained from the point (I1 ′, P1) and the point (I2 ′, P2) by the above-described method, the threshold current Ith ′ smaller than the original threshold current Ith is obtained. It will be detected. The same applies to the case of FIG.

  Therefore, if the APC control is performed based on the detected threshold current Ith ′, the image density cannot be corrected accurately. Therefore, in the present embodiment, in order to accurately detect the light emission characteristics of the laser light source 201, density adjustment is performed using a plurality of image patterns having different target densities.

  As a premise, a reference position is set on the photosensitive drum 21. In order to detect the reference position, a home position (hereinafter referred to as “HP”) sensor 211 is installed at or near the photosensitive drum 21. Further, a protrusion 212 is provided on the photosensitive drum 21 so as to pass through the photo interrupter every time the photosensitive drum 21 rotates once. The HP sensor 211 is, for example, a photointerrupter, that is, a sensor having opposed light emitting / receiving units. However, substitution with a sensor having the same function is not prevented.

FIG. 8 shows the timing from when the HP sensor 211 outputs a detection signal indicating the reference position to when the image is actually formed by the image forming apparatus 1. Also, the flow of each signal is shown in FIG. Referring to these drawings, the HP sensor 211 generates a pulse-like detection signal every time the photosensitive drum 21 makes one rotation, and transmits it to the control unit 209 (or the operation unit 40). The control unit 209 forms an image in a specific image area of the photosensitive drum 21 based on the detection signal. At the same time, a laser control signal and a conveyance command signal are output so that the formed image is transferred to a specific position of the transfer material p such as paper. The laser control signal is output to the laser light source 201. The conveyance command signal is output to the conveyance mechanism 30 in synchronization with the laser control signal. In the example of FIG. 8, the laser control signal is output after time T1 from the input of the detection signal. Further, a transport command signal is output after time T2.
With this configuration, the image formed in the image area of the photosensitive drum 21 and the image area (print image area) of the transfer material p onto which the image is transferred (printed) can be associated one-to-one. .

  In the present embodiment, three types of image patterns are used as test images, respectively. These are designated as test image A, test image B, and test image C. An example of the test image A is shown in FIG. 10, an example of the test image B is shown in FIG. 11, and an example of the test image C is shown in FIG. Each test image is transferred (printed) onto an output sheet as an example of the transfer material p. The output paper on which the test image A is printed is TA, the output paper on which the test image B is printed is TB, and the output paper on which the test image C is printed is TC.

  The density of the output paper TA on which the test image A is printed is the darkest, and then the test image B printed on the output paper TB and the test image C printed on the output paper TC become lighter in this order. 10 to 12, small areas partitioned by dotted lines correspond to a plurality of image areas of the photosensitive drum 21, and n × n (n is a natural number) image areas (printed images) to be printed. Area). The position of each print image area is recognized as an area [XnYn] where X is the coordinate in the main scanning direction and Y is the coordinate in the sub-scanning direction. The same applies to the plurality of image areas on the photosensitive drum 21. These image area and print image area are common to the test image A, the test image B, and the test image C.

FIG. 13 shows the density adjustment procedure in this embodiment.
The density adjustment is started by an instruction of “start density adjustment” from the operation unit 40 (S601). First, the operation unit 40 outputs an image pattern of the test image A (S602). For example, the output paper TA is discharged. As described above, the threshold value Ith ′ of the drive current supplied to the laser light source 201 (light emitting element) at this time is detected by the PD 208 to be lower than the original threshold value Ith.

  When the image pattern of the test image A is output, it is determined whether the output paper TA is set on the document table (S603). If the output sheet TA is not set on the document table (S603: no), the process waits until it is set. At this time, the operation unit 40 may perform a display prompting the user to set the output paper TA on the document table. When the output sheet TA is set on the document table (S603: yes), the output sheet TA is read by the document reader (S604). After the reading, the density is measured for each print image area of the test image A (S605).

  For density measurement, an image sensor such as a CCD (Charge Coupled Device) is used. The image sensor unit 19 described above may be used. For example, a voltage value corresponding to the density is output from the image sensor (image sensor unit 19). This voltage value (voltage output) is output to the control unit 209. This is shown in FIG. FIG. 14 shows an example in which this voltage value is output as concentration data to the operation unit 40. However, when the threshold current Ith is detected by the control unit 209, output to the operation unit 40 is not necessary.

  The control unit 209 calculates density data by comparing a voltage value (voltage output) output from the image sensor with a voltage value representing a density (target density) set as a target for each print image area of the test image A. To do. For example, it is assumed that the print image region of interest of the test image A, that is, the voltage value of the target density in the region of interest is 2.00 [V]. At this time, if the voltage value actually output from the image sensor is 2.02 [V], the density data is 1.01. The operation unit 40 records this density data in a recording table A prepared in advance. This procedure is repeated for all print image areas of the test image A (image pattern) on the output paper TA.

  The recording table A is prepared with the horizontal axis as the coordinate X in the main scanning direction and the vertical axis as the coordinate Y in the sub-scanning direction according to the image area for convenience. For example, if the density data of the region of interest [X1Y1] is “1.009”, “1.009” is stored in the corresponding region of the recording table A. If the density data of the area [X2Y1] that is the attention area is “1.011”, “1.011” is stored in the corresponding area of the recording table A. Similarly, density data of a total of n × n areas are recorded in the recording table A until reaching the area [XnYn]. An example of the contents of the recording table A created in this way is shown in FIG.

  Next, the operation unit 40 outputs an image pattern of the test image B (S606). For example, the output paper TB is discharged. The image pattern of the test image B is obtained by adding an addition current Ib to the threshold current Ith ′ supplied to the laser light source 201 when the control unit 209 forms the test image A for each of the image areas of the photosensitive drum 21. Form by adding. Since the drive current (= threshold current Ith ′ + Ib) supplied to the laser light source 201 is larger than the current when the image pattern of the test image A is formed, the image density is lowered under the BAE described above. To do. At this time, the addition current Ib is desirably set to a value such that the density measurement result of the test image B performed later can be distinguished from the density measurement result of the test image A. The test image B is formed in an image area specified on the basis of the detection signal of the HP sensor 211 set on the photosensitive drum 21.

  When the image pattern of the test image B is output, it is determined whether the output paper TB is set on the document table (S607). If the output paper TB is not set on the document table (S607: no), it waits until it is set. At this time, the operation unit 40 may prompt the user to set the output paper TB on the document table. When the output sheet TB is set on the document table (S607: yes), the output sheet TB is read by the document reader (S608). After reading, the density is measured for each image pattern, more specifically, for each print image area of the test image B (S609). The density data obtained by this density measurement is recorded in a recording table B prepared in advance.

  As with the recording table A, the recording table B is prepared with the horizontal axis as coordinates X in the main scanning direction and the vertical axis as coordinates Y in the sub-scanning direction for convenience. For example, if the density data of the area [X1Y1] that is the attention area is “0.205”, “0.205” is recorded in the corresponding area of the recording table B. Further, if the density data of the region [X2Y1] that is the attention region is “0.206”, “0.206” is recorded in the corresponding region of the recording table B. Similarly, density data of a total of n × n areas are recorded in the recording table B until reaching the area [XnYn]. An example of the contents of the recording table B recorded in this way is shown in FIG.

Next, the operation unit 40 outputs an image pattern of the test image C (S610). For example, the output paper TC is discharged. The image pattern of the test image C is obtained by adding an additional current to the current threshold current Ith ′ that the control unit 209 has supplied to the laser light source 201 when the test image A is formed for each of the plurality of regions on the surface of the photosensitive drum 21. It is formed by adding Ic (Ic> Ib).
Since the current (= threshold current Ith ′ + Ic) flowing through the laser light source 201 is larger than the current flowing through the laser light source 201 when the test image B is formed, the image is generated under the above-described BAE. The concentration is further reduced. At this time, the addition current Ic is desirably set to a value such that the density measurement result of the test image C to be performed later can be distinguished from the density data of the test image A and the test image B. The test image C is formed in an image area specified with reference to the output of the HP sensor 211 provided on the photosensitive drum 21.

  When the image pattern of the test image C is output, it is determined whether or not the output paper TC is set on the document table (S611). If the output paper TC is not set on the document table (S611: No), it waits until it is set. At this time, the operation unit 40 may prompt the user to set the output paper TC on the document table. If the output sheet TC is set on the document table (S611: yes), the output sheet TC is read by the document reader (S612). After reading, the density is measured for each image area of the test image C as in the case of the test images A and B (S613).

Similar to the recording table A and the recording table B, the recording table C is prepared with the horizontal axis as the coordinate X in the main scanning direction and the vertical axis as the coordinate Y in the sub-scanning direction for convenience. For example, if the density data of the region of interest [X1Y1] is “0.143”, “0.143” is recorded in the corresponding region of the recording table C. Similarly, if the density data of the area [X2Y1] that is the attention area is “0.144”, “0.144” is recorded in the corresponding area of the recording table C. Similarly, density data of a total of n × n areas is recorded in the recording table C until reaching the area [XnYn]. An example of the contents of the recording table C recorded in this way is shown in FIG.
When the recording of the density data of each area of the test image C is completed, the operation unit 40 calculates the density correction amount of each image area (print image area). (S614)

  This density correction method will be described with reference to FIG. 18, taking the above-described region [X1Y1] as an example. In FIG. 18, the current value of the drive current supplied to the laser light source 201 is on the horizontal axis, and the measured density is on the vertical axis. First, the density data A (1.009) of the image of the attention area (hereinafter referred to as “target image”) A of the image pattern of the test image A is plotted. Also, the density data B (0.205) of the image of interest B (image at the same position as the image of interest A) of the image pattern of the test image B is plotted. Further, the density data C (0.144) of the attention image C (the image at the same position as the attention image A and the attention image B) in the image pattern of the test image C is plotted. Next, a straight line L connecting the two points of density data B and density data C is calculated, and further, an intersection point X at which the value of the vertical axis of the straight line L is equal to the density data A (1.009) is calculated. The intersection point X is a point where the image density starts to change when the current value of the drive current is gradually increased from FIG. That is, it can be seen that the value on the horizontal axis of the intersection X is the original threshold current Ith.

  The operation unit 40 repeats the above procedure for another area [XnYn]. Thereafter, the drive current is increased or decreased in order to obtain a current value corresponding to each area with respect to the calculated original threshold current Ith for all image areas on the photosensitive drum 21 (that is, correction). To do). In this way, the unevenness of the image density that has absorbed the variation in the sensitivity characteristic or charging characteristic of the photosensitive drum 21 is corrected.

As described above, in the image forming apparatus 1 of the present embodiment, image patterns having a plurality of different densities are output for all the image areas of the photosensitive drum 21. Then, the density is measured for each image pattern, more preferably for each image area, and the measured density is compared with the target density. Thereby, the threshold current Ith of the laser light source 201 can be accurately detected.
Further, according to the present invention, density correction including variations in the sensitivity characteristics or charging characteristics of the photosensitive drum 21 can be performed. Therefore, it is not necessary to measure in advance the sensitivity characteristic or charging characteristics variation data of the photosensitive drum 21, and a configuration for this purpose is provided. There is no need to prepare it separately. Therefore, the cost of the photosensitive drum 21 can be reduced.

  The test images A to C may each be stored in an image for one sheet of printing paper in order to simplify the density adjustment procedure. In this case, it is preferable that the size of the output paper is changed in accordance with the diameter (surface area) of the photosensitive drum 21. For example, if the photosensitive drum 21 is 30φ (peripheral length 94.2 mm), A4 size paper is used. Alternatively, if the photosensitive drum 21 is 80φ (circumferential length: 251.2 mm), the A3 size is used.

  When the image forming apparatus 1 forms a color image, a configuration in which image patterns of different colors are stored on one output sheet may be adopted. For example, when a photosensitive drum 21 having a diameter of 30φ (peripheral length 94.2 mm) is used, test images of yellow and magenta are stored on A4 size paper. Or it is good also as a structure which accommodates the several image pattern of the same color on one output paper. For example, when the photosensitive drum 21 having a diameter of 30φ (peripheral length 94.2 mm) is used, the test images A to C can be stored on A3 size paper. As a result, even when density correction is performed for a plurality of colors, the number of times the output paper is read by the document reader is reduced, so that troublesomeness felt by the user or the like can be reduced.

  In the present embodiment, an example in which the HP sensor 211 is used to associate the position of the image area of the photosensitive drum 21 with the position of the print image area of the test images A to C is shown, but this is not restrictive. For example, an encoder or the like for detecting the rotation position of the pulse train is provided on the photosensitive drum 21, and the count result of this pulse train is compared with the count value for one rotation of the photosensitive drum 21 in advance so as to correspond to the positions of both. Anyway.

  Alternatively, the positions of the photosensitive drums 21 may be associated with each other based on the time required for one rotation of the photosensitive drum 21 and the rotational speed or amount of rotation of the photosensitive drum 21. In this case, the above time is stored in the internal memory of the control unit 209, and this time is compared with the rotation speed or rotation amount of the photosensitive drum 21 measured using a separately prepared sensor.

  In the present embodiment, the processing example in the case where the density adjustment is completed with the correction amount based on the threshold value Ith of the driving current for each image area of the photosensitive drum 21 is shown, but this is not restrictive. For example, after the density adjustment is performed, the test image is output again, the output paper TA or the like is read by an original reader, and the density is measured to check whether a desired correction effect has been obtained. . More preferably, if a desired correction effect is obtained, the user or the like is notified through the operation unit 40 that the density adjustment has been normally completed. If the desired correction effect is not obtained, the density adjustment of this embodiment is repeated again. In this way, for example, it is possible to cope with a case where reading is not normally performed due to dirt on the platen glass or the like. Thereby, it is possible to reduce the discard amount of the transfer material p due to the failure of density adjustment.

  In the present embodiment, the example in which the image pattern output from the image forming apparatus 1 is read by the document reader and the density thereof is detected has been described. For example, a CCD sensor or a density sensor may be provided inside the image forming apparatus 1 and the density may be directly detected by these sensors. By doing this, the trouble of placing the output sheets TA, TB, and TC shown in FIGS. 10 to 12 on the document reader can be saved, and the troublesomeness felt by the user or the like can be reduced.

In this embodiment, an example in which density adjustment is performed by the operation unit 40 has been described. However, the control unit 209 of the optical scanning device 20 may perform the adjustment.
Furthermore, in the present embodiment, an example in which a driving current is used by using electric power for controlling the light emission amount of the laser light source 201 is described, but the laser light source 201 may be driven by a voltage. In this case, the threshold currents Ith and Ith ′ are threshold voltages.

  The present invention can be applied to a printer, a scanner, a copier, or a multifunction machine or other image forming apparatus in which these functions are integrated, and components thereof.

DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus, 20 ... Optical scanning device, 21 ... Photosensitive drum, 22 ... Charger, 24, 25 ... Transfer material stacking part, p ... Transfer material, 26. ..Transfer section, 27... Fixing section, 28... Paper discharge section, 30. DESCRIPTION OF SYMBOLS 201 ... Laser light source, 202 ... Collimator lens, 203 ... Cylindrical lens, 204 ... Polygon mirror (rotating polygon mirror), 205, 206 ... Scanning lens. 207... Photodetection sensor, 208... Photo detector (PD), 209. 211 ... Home position (HP) sensor, 212 ... Protrusions, ga ... Image area, Ith, Ith '... Threshold current, TA, TB, TC ... Output paper.

Claims (7)

An image forming apparatus for forming an image having a density according to a light emission amount of a light emitting element on an image carrier,
Image forming means for forming a plurality of images of different densities in a common region of interest of the image carrier;
A measuring means for measuring the density of each formed image;
A threshold current value of a drive current of a light emitting element that emits light for exposing the region of interest when the image density in the region of interest starts to change based on the measured amount of change in the density of each image Control means for identifying
An image forming apparatus comprising: In addition to the light emitting element that emits light that exposes the target area, the target area receives light emitted from other light emitting elements,
The plurality of images are formed with the light emitted from the other light emitting elements by changing the light emission amount of the light emitting element that emits light for exposing the attention area, and the image A formed in the attention area, An image B having a density different from that of the image A, and an image C having a density different from that of the image A and the image B,
The control means includes density data B and density data C specified among density data A representing the density of the image A, density data B representing the density of the image B, and density data C representing the density of the image C. The power value when the extension of the change gradient coincides with the concentration data A is specified as the threshold value,
The image forming apparatus according to claim 1. A plurality of image regions including the region of interest are formed in a matrix on the image carrier,
The measuring means measures the density data A, density data B, and density data C for each image area, and tabulates a comparison result between a density represented by the density for each image area and a target density set in advance as a target. Is,
The control means corrects the drive current for each image region based on a comparison result for each image region tabulated.
The image forming apparatus according to claim 2. The image carrier is either a photoconductor on which a plurality of image areas are formed by a developer adhering to a surface thereof, or a sheet-like transfer material on which the developer is transferred through the photoconductor. And
When the image carrier is the transfer material, the plurality of images are transferred to one sheet through the photoconductor,
The image forming apparatus according to claim 3. A reference position for determining a relative position of each of the plurality of image areas is set on the photoconductor,
The image forming means specifies the position of the attention area based on the reference position.
The image forming apparatus according to claim 4. When performing control to form an image with a density corresponding to the light emission amount of the light emitting element on the image carrier,
In advance, a plurality of images with different densities are formed in a common region of interest of the image carrier,
While measuring the density of each image formed,
A threshold current value of a drive current of a light emitting element that emits light for exposing the region of interest when the image density in the region of interest starts to change based on the measured amount of change in the density of each image Characterized by specifying
Image forming method. A computer program for causing a computer apparatus to operate as an image forming apparatus that forms an image having a density according to the light emission amount of a light emitting element on an image carrier,
Said computer device,
Image forming means for forming a plurality of images of different densities in a common region of interest of the image carrier;
Measuring means for measuring the density of each formed image;
A threshold current value of a drive current of a light emitting element that emits light for exposing the region of interest when the image density in the region of interest starts to change based on the measured amount of change in the density of each image Functioning as a control means for identifying
Computer program.