JP2013163309A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of removing effect of a dark current of a PD (photo diode) for APC (auto power control), while efficiently executing a control sequence including the APC.SOLUTION: A switch 212 is turned on during an OFF mode (non-image area), and a DC component of a dark current of a PD 106 is cut by a differential circuit 213. Alternatively, the switch 212 is turned off during a VIDEO mode (image area), and a capacitor 213a in the differential circuit 213 is not charged.

Description

  The present invention relates to a technique for performing light amount control of a light beam to be exposed in an image forming apparatus that scans a light beam on a photosensitive member such as a laser beam printer.

  In an image forming apparatus having a scanning optical system, a technique has been proposed in which image formation is performed by increasing the number of lasers that emit light and simultaneously scanning a plurality of beams in order to cope with high-speed and high-resolution image formation. .

  In this method of increasing the number of lasers, the speed of image formation can be increased without increasing the rotation speed of the polygon mirror and the clock frequency for image formation, compared with the method of increasing the writing speed by increasing the speed of the polygon mirror. Higher resolution can be achieved. Therefore, if this method is adopted, the laser does not cause problems such as shortening the service life by increasing the rotation speed of the polygon mirror, raising the temperature of the scanner, and noise, and increasing the clock frequency for image formation. It is possible to increase the image formation speed and resolution without taking measures to increase the speed of the drive circuit and transmission path. In particular, since a surface emitting laser (Vertical Cavity Surface Emitting Laser: VCSEL) can arrange a plurality of light emitting points on a single chip surface, the number of beams can be increased relatively easily.

  When image formation is performed using a laser, light emitted from the laser is detected by a PD (photodiode), and light amount adjustment (Auto Power Control: APC) of the laser is performed based on the detected light. . By the way, since the surface emitting laser emits light in a direction perpendicular to the chip surface, the APC PD cannot be built in and must be arranged outside. When the APC PD is arranged outside, as shown in FIG. 2, a part of the emitted light is reflected by the half mirror 105 and APC is performed based on the output result of the PD 106 installed outside. Conceivable. In this configuration, the APC timing can be set using the non-image area in the same manner as the configuration of the PD built in the package.

  However, when the method of detecting a part of the laser light and performing APC in this way is adopted, the influence of the dark current of the PD cannot be ignored because the amount of light detected by the PD is low. In general, since the dark current of PD changes exponentially with respect to temperature, the amount of change in dark current due to temperature change becomes an error in APC. As a configuration for removing the influence of this dark current, for example, in Patent Document 1, an output voltage Va output from the position detection element 17 made of PD and subjected to current-voltage conversion by the current-voltage converter 2a is input to the differentiation circuit 6a. Thus, a configuration has been proposed in which only the voltage Vsa corresponding to the change in the output voltage Va, that is, the current Isa flowing by light irradiation is taken out, and the voltage Vda corresponding to the dark current Ida is removed.

JP-A-7-167613

  However, when a differentiation circuit is simply introduced to remove the dark current of the APC PD, the charge charged in the capacitor included in the differentiation circuit is always sufficiently discharged when the APC is executed. I had to wait until. That is, since a predetermined waiting time is always required when APC is executed, the control sequence including APC becomes inefficient. As a result, there is a possibility that the image quality cannot be maintained because the APC time cannot be secured sufficiently, or the APC is executed over a plurality of lines, leading to an increase in the APC interval.

  The present invention has been made paying attention to this point, and provides an image forming apparatus capable of removing the influence of the dark current of an APC PD while efficiently executing a control sequence including APC. The purpose is to do.

  In order to achieve the above object, the image forming apparatus of the present invention includes a light emitting unit that emits a light beam for forming an electrostatic latent image on a photosensitive member, and the light beam that scans the photosensitive member. A deflecting means for deflecting a beam; and a light beam emitted from the light emitting means during a non-latent image forming period other than a latent image forming period in which the light beam scans an area where the electrostatic latent image is formed on the photosensitive member. And a photoelectric conversion means for outputting a signal of a level corresponding to the amount of light of the received light beam, a generation circuit for generating a current corresponding to the level output by the photoelectric conversion means, and a generation circuit The extracted current is input to extract an alternating current component included in the current, and the light amount of the light beam is controlled based on the value of the alternating current component included in the current extracted by the extraction circuit. And having a control means for, and interrupting means for interrupting the current input to the extraction circuit in the latent image formation period.

  According to the present invention, it is possible to efficiently perform APC with high accuracy by removing the influence of the dark current of the PD for APC and effectively using the non-image area. As a result, it is possible to form a high-quality image without light quantity unevenness.

1 is a block diagram showing a schematic configuration of an image forming apparatus according to a first embodiment of the present invention. It is a figure which shows an example of a structure at the time of applying front APC using a half mirror to the scanning optical system which deflects and scans a some laser beam with a polygon mirror. It is the figure which showed typically the output change of PD by a temperature change. It is a figure which shows an example of a structure of the light quantity detection circuit containing a differentiation circuit. FIG. 5 is a diagram illustrating an example of transition of output current of a PD before / after passing through a differentiating circuit when the switch of FIG. 4 is not provided. FIG. 5 is a diagram showing an example of transition of the output current of the PD before / after passing through the differentiating circuit when the switch of FIG. 4 is provided and the injection signal to the differentiating circuit is cut off using the shutdown signal / SD in the VIDEO mode. is there. 5 is a flowchart showing a part of a procedure of image drawing processing executed by the CPU of FIG. 4. It is a figure which shows an example of a structure of the light quantity detection circuit contained in the image forming apparatus of the 2nd Embodiment of this invention. It is a figure which shows an example of transition of the output current of PD before / after passing the differentiation circuit at the time of switching the gain of an amplifier circuit using the gain switching signal / GAIN at the VIDEO mode. 6 is a flowchart illustrating a part of a procedure of image drawing processing executed by a CPU included in an image forming apparatus according to a second embodiment of the present invention. It is a figure which shows an example of a structure of the light quantity detection circuit contained in the image forming apparatus of the 3rd Embodiment of this invention. It is a figure which shows an example of transition of the output current of PD before / after passing through the differentiation circuit at the time of providing the clamp circuit of FIG.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

(First embodiment)
FIG. 1 is a block diagram showing a schematic configuration of an image forming apparatus according to a first embodiment of the present invention. In FIG. 1, an electrophotographic color image forming apparatus is exemplified as the image forming apparatus. .

  As shown in the figure, the image forming apparatus according to the present embodiment selectively feeds the transfer paper S from the paper feed units 1, 2, or 3, and the fed transfer paper S is conveyed by a plurality of transports. It is relayed by the roller pairs 8, 9, 10 and 11 and sent to the registration roller pair 12. However, the transfer sheet S fed from the sheet feeding unit 3 is relayed by the transport roller pair 8 and sent to the registration roller pair 12.

  A long intermediate transfer belt (endless belt) 13 that is an intermediate transfer member is disposed downstream of the registration roller pair 12 in the paper conveyance direction. A plurality of photosensitive drums 14 to 17 that form and carry color toner images of different colors are sequentially arranged along the rotation direction of the intermediate transfer belt 13.

  First, the laser light (light beam) emitted from the laser unit 22 is applied onto each of the photosensitive drums 14 to 17 in the order of the uppermost photosensitive drum 14 to the photosensitive drum 15, the photosensitive drum 16, and the photosensitive drum 17. ) To form an electrostatic latent image on the photosensitive drums 14-17. At this time, in each of the photosensitive drums 14 to 17, the charging potential is adjusted by the potential control so that the latent image potential becomes a desired potential. The formed electrostatic latent images are visualized and developed into toner images of respective colors by developing units 23 to 26 respectively prepared on the photosensitive drums 14 to 17. Further, the developed toner image is transferred between the photosensitive drum 14 and the transfer charger 90 and the transfer portion between the photosensitive drum 15 and the transfer charger 91 while the intermediate transfer belt 13 rotates. By sequentially passing through a transfer portion between the photosensitive drum 16 and the transfer charger 92 and a transfer portion between the photosensitive drum 17 and the transfer charger 93, the image is transferred onto the intermediate transfer belt 13 in an overlapping manner. The

  On the other hand, the transfer sheet S is subjected to secondary transfer on the intermediate transfer belt 13 by a registration roller pair 12 that starts to rotate at a timing for aligning the position of the toner image on the intermediate transfer belt 13 and the leading edge of the transfer sheet S. The toner image is transferred onto the surface of the transfer paper S by being sent to the secondary transfer portion T2, which is a contact portion between the roller 40 and the secondary transfer counter roller 13b. The transfer sheet S that has passed through the secondary transfer portion T2 is sent to the fixing device 35 by the intermediate transfer belt 13. The transfer sheet S is heated by the fixing roller 35A and pressurized by the pressure roller 35B in the process of passing through the nip formed by the fixing roller 35A and the pressure roller 35B in the fixing device 35. The transferred toner image is fixed on the surface of the transfer paper S. The fixing-processed transfer sheet S that has passed through the fixing device 35 is sent to the discharge roller pair 37 by the conveying roller pair 36 and further discharged onto the discharge tray 38 outside the apparatus.

  Next, a configuration for realizing APC in the image forming apparatus according to the present embodiment will be described. In this embodiment, as APC, a laser beam emitted from a semiconductor laser using a half mirror is separated into a laser beam guided to a photosensitive drum and a laser beam incident on a light receiving sensor, and the light quantity of the laser light incident on the light receiving sensor. Based on the above, an APC system is employed that controls the amount of laser light emitted from the semiconductor laser.

  FIG. 2 shows an example of a configuration in which a plurality of laser beams are deflected by a polygon mirror as an APC using this half mirror and applied to a scanning optical system in which the deflected laser beams scan a photosensitive drum. Is shown.

  As shown in the figure, the output light of the semiconductor laser 101 passes through the collimator lens 102, the aperture stop 103 and the cylindrical lens 104, and is reflected on the reflecting surface of the polygon mirror 107, and then the toric lens 108a and the diffractive optical element. 108b passes through and is imaged on the photosensitive drum 111 (corresponding to one of the photosensitive drums 14 to 17). The collimator lens 102 converts the diverging laser light emitted from the semiconductor laser 101 into a substantially parallel light beam. The aperture stop 103 limits the passing light flux. The cylindrical lens 104 has a predetermined refractive power only in the sub-scanning direction, and focuses the light beam that has passed through the aperture stop 103 on the reflection surface of the polygon mirror 107 within the sub-scanning section. The polygon mirror 107 (deflecting means) is rotated at a constant speed by a driving device (not shown) such as a motor, and deflects the laser light imaged on the reflecting surface. The optical element 108 includes a refracting part and a diffractive part, and has an fθ characteristic. The refracting portion is composed of a toric lens 108a having different powers in the main scanning direction and the sub-scanning direction, and the lens surface in the main scanning direction of the toric lens 108a has an aspherical shape. The diffractive portion is composed of a long diffractive optical element 108b having different powers in the main scanning direction and the sub-scanning direction. The laser beam that has passed through the optical element 108b scans the photosensitive drum 111.

  The PD (photodiode) sensor 110 is a beam detection sensor that detects laser light that scans outside the image area. The laser beam deflected by the polygon mirror 107 is reflected on the reflection mirror 109 and scans on the light receiving surface of the PD sensor 110. The PD sensor 110 generates an image writing timing signal on the photosensitive drum 111 by detecting the incident laser light.

  In the APC method employed in this embodiment, a part of the laser light reflected by the half mirror 105 is guided to the PD 106 that functions as a light amount detection sensor. Since the PD 106 outputs a signal having a level corresponding to the amount of received laser light, the signal output from the PD 106 is monitored, and feedback control is performed to change the amount of laser light emitted from the semiconductor laser to the target light amount. To control. On the other hand, in the APC system that receives laser light emitted in a direction opposite to the laser light guided to the photosensitive drum that is not employed in the present embodiment, the PD installed in the package of the semiconductor laser 101 is used for light quantity detection. Use as a monitor. The present invention may be applied to this APC system.

  In addition, APC is usually responsible not only for light amount adjustment but also for setting a bias current value. If the mode for adjusting the amount of light is, for example, mode APC_H, the bias current value is determined to converge to a value lower than the amount of light adjusted by mode APC_H (for example, a value that is 1/4 of mode APC_H). For example, mode APC_L is required. When the current value and light amount converged in mode APC_H are I_H and P_H, respectively, and the current value and light amount converged in mode APC_L are I_L and P_L, respectively, the bias current Ib is determined by the following equation (1).

Ib = {− P_H × (I_H−I_L) + I_H × (P_H−P_L)} / (P_H−P_L) (1)
Next, the influence of the dark current of the PD 106 on the APC accuracy will be described.

  In general, the dark current of the PD changes exponentially with respect to the temperature. However, since the amount of light incident on the PD 106 is weak, the influence of the dark current of the PD 106 cannot be ignored.

  FIG. 3 is a diagram schematically showing changes in the output of the PD due to temperature changes. As shown in the figure, even when the amount of light incident on the PD is constant, the PD output current at the high temperature is larger than the PD output current at the low temperature. ΔI in the figure is the amount of change in dark current due to temperature. When negative feedback control is performed in APC, the dark current increases as the temperature increases, thereby increasing the PD output current (monitor current). Therefore, even if APC is performed, the laser light amount is lower than the target light amount. Will be controlled. Therefore, in this embodiment, a differentiation circuit is introduced to remove the DC component of the dark current contained in the PD output current.

  4A and 4B are diagrams showing the configuration of the light quantity detection circuit 210 including the differentiating circuit 213. FIG. 4A is a block diagram, and FIG. 4B is a circuit diagram. In FIG. 9A, in addition to the light amount detection circuit 210, a semiconductor laser drive circuit 200 that controls the output light amount of the semiconductor laser 101 according to the signal level output from the light amount detection circuit 210, and the light amount detection circuit 210. A CPU (Central Processing Unit) 220, a semiconductor laser 101, and a half mirror 105 that control the operation are also described.

  The light quantity detection circuit (light quantity detection means) 210 includes a PD (photoelectric conversion means) 106, an amplification circuit 211 (generation circuit) that amplifies the output current from the PD 106, and an output current from the amplification circuit 211 in a subsequent differentiation circuit 213. A switch (shut-off means) 212 for controlling whether or not to flow through and a differentiation circuit (extraction circuit) 213 are configured. When the output current is supplied from the amplifier circuit 211, the differentiating circuit 213 removes the direct current component included in the output current and extracts the alternating current component.

  When the shutdown signal / SD becomes “1 (Hi)”, the switch 212 is turned off and does not flow the output current from the amplifier circuit 211 to the differentiation circuit 213. On the other hand, when the shutdown signal / SD becomes “0 (Lo)”, the switch 212 is turned on and allows the output current from the amplifier circuit 211 to flow to the differentiation circuit 213. Hereinafter, the reason why the switch 212 that operates in this manner is provided will be described.

  FIG. 5 is a diagram showing an example of the transition of the output current of the PD 106 before / after passing through the differentiating circuit 213 when the switch 212 is not provided. In the illustrated example, a case where a plurality of beams emit light with a certain amount of light in the image region (that is, in the VIDEO mode of the control sequence (latent image formation period)) is shown.

  As shown in the figure, when the VIDEO mode is completed and the control sequence is switched to the OFF mode, only the dark current (ΔI in the figure) is output from the PD 106, but the DC component of the dark current is generated by the differentiation circuit 213. Removed. However, the capacitor 213a (see FIG. 4B) constituting the differentiating circuit 213 is charged with an amount of charge corresponding to the amount of light emitted from the semiconductor laser 101 immediately before the end of the VIDEO mode. During the latent image formation period), a discharge current flows from the capacitor 213a for a time t. Since the APC cannot be performed while the discharge current is flowing, the APC is performed after waiting for t seconds from the end of the VIDEO mode. That is, a waiting time of t seconds occurs. Since the time t changes according to the amount of charge charged in the capacitor 213a, the time t changes according to the light emission amount immediately before the VIDEO mode ends, that is, the output current amount from the PD 106. Therefore, when the output current from the PD 106 is small, the discharge time is short, but when the output current from the PD 106 is large, a long discharge time is required. For this reason, it is necessary to set the longest discharge time as the time t assuming that all the beams are fully lit. However, if the output current from the PD 106 is not passed through the differentiating circuit 213 in the VIDEO mode, the capacitor 213a of the differentiating circuit 213 is not charged in the VIDEO mode, so there is no need to take a longer discharge time than necessary. In addition, the number of light-emitting elements that perform APC can be increased while the laser beam is scanning the non-image area. Therefore, the image forming apparatus of the present embodiment is provided with a switch 212 as shown in FIG. 4 so that the output current from the PD 106 does not flow to the differentiating circuit 213 in the VIDEO mode.

  FIG. 6 shows an example of transition of the output current of the PD 106 before / after passing through the differentiating circuit 213 when the switch 212 is provided and the injection current to the differentiating circuit 213 is cut off using the shutdown signal / SD in the VIDEO mode. FIG.

  As shown in the figure, by setting the shutdown signal / SD to “0 (Lo)” in the VIDEO mode, the injection current to the differentiation circuit 213 is cut off even when the beam is emitted. Thereby, the waiting time t can be set short, and the number of light emitting elements that perform APC can be increased during a period in which the laser light scans the non-image area.

  FIG. 7 is a flowchart showing a part of image drawing processing executed by the CPU 220 and relates to on / off control of the switch 212.

  As shown in the figure, when the image drawing process is started, the CPU 220 first sets the shutdown signal / SD to “Hi” (/ SD = 1) and turns on the switch 212 (step S1). Next, the CPU 220 determines whether or not the video mode is selected (step S2). If the video mode is selected, the CPU 220 sets the shutdown signal / SD to “Lo” (/ SD = 0) and turns off the switch 212 (step S3). ). On the other hand, if not in the VIDEO mode, the CPU 220 sets the shutdown signal / SD to “Hi” (/ SD = 1) and turns on the switch (step S4).

  In this embodiment, the switch 212 is provided between the amplifier circuit 211 and the differentiation circuit 213. However, the position of the switch 212 is not limited as long as it is before the differentiation circuit 213. For example, the switch 212 may be provided immediately after the PD 106.

  Further, an amplifier shutdown function in the amplifier circuit 211 may be used instead of providing the switch 212.

(Second Embodiment)
The image forming apparatus according to the present embodiment is different from the image forming apparatus according to the first embodiment only in the configuration of the light amount detection circuit. Therefore, the hardware of the image forming apparatus according to the present embodiment adopts the hardware of the image forming apparatus according to the first embodiment, that is, the hardware shown in FIGS.

  FIG. 8 is a diagram showing a configuration of the light amount detection circuit 310 included in the image forming apparatus of the present embodiment, where FIG. 8A is a block diagram and FIG. 8B is a circuit diagram. In FIG. 8, the same elements as those in FIG. 4 are denoted by the same reference numerals, and the description thereof is omitted.

  As can be seen by comparing FIG. 8B and FIG. 4B, the amplifier circuit 311 (generation circuit) adds a resistor 311b having a resistance value smaller than that of the resistor 211a in addition to the resistor 211a of the amplifier circuit 211. And both resistors 211a or 311b can be switched by a gain switching signal / GAIN. More specifically, when the gain switching signal / GAIN becomes “0 (Lo)”, the amplification circuit 311 switches to the resistor 311b to set the gain of the amplification circuit 311 to a lower magnification, and the gain switching signal / GAIN is When “1 (Hi)” is set, the gain is switched back to the original high magnification by switching to the resistor 211a. Then, by setting the gain switching signal / GAIN to “0” in the VIDEO mode, the gain of the amplifier circuit 311 is set to a low magnification. As a result, even if the PD 106 receives a high light amount in the VIDEO mode, the amplification is performed. The output after passing through the circuit 311 is kept small. This means that the waiting time t until the electric charge of the capacitor 213a in the differentiating circuit 213 is discharged can be set in a short time, as shown in FIG. Therefore, the OFF mode time after the VIDEO mode can be set to a short time, and efficient APC can be executed.

  FIG. 10 is a flowchart illustrating a part of the image drawing process executed by the CPU 220, and relates to switching control of the resistors 211a and 311b.

  As shown in the figure, when the image drawing process is started, the CPU 220 first sets the gain switching signal / GAIN to “Hi” (/ GAIN = 1), selects the resistor 211a, and increases the gain of the amplifier circuit 311. The magnification is set (step S11). Next, the CPU 220 determines whether or not the video mode is selected (step S12). If the video mode is selected, the gain switching signal / GAIN is set to “Lo” (/ GAIN = 0) to select the resistor 311b, and the amplifier circuit. The gain of 311 is set to a low magnification (step S13). On the other hand, if not in the VIDEO mode, the CPU 220 sets the gain switching signal / GAIN to “Hi” (/ GAIN = 1), selects the resistor 211a, and sets the gain of the amplifier circuit 311 to a high magnification (step S14).

  In this embodiment, two types of resistors 211a and 311b are provided as resistors for changing the gain of the amplifier circuit 311 and the degree of freedom to change the gain is “2”. Different resistance values may be provided, and these may be appropriately switched according to the temperature of the PD 106, so that the degree of freedom in changing the gain may be “3” or more.

(Third embodiment)
The image forming apparatus according to the present embodiment is different from the image forming apparatus according to the first embodiment only in the configuration of the light amount detection circuit. Therefore, the hardware of the image forming apparatus according to the present embodiment adopts the hardware of the image forming apparatus according to the first embodiment, that is, the hardware shown in FIGS.

  FIG. 11 is a diagram illustrating a configuration of a light amount detection circuit 410 included in the image forming apparatus according to the present embodiment. FIG. 11A is a block diagram, and FIG. 11B is a circuit diagram. In FIG. 11, the same elements as those in FIG. 4 are denoted by the same reference numerals, and the description thereof is omitted.

  As can be seen from a comparison between FIG. 11A and FIG. 4A, the light amount detection circuit 410 deletes the switch 212 included in the light amount detection circuit 210 and adds a clamp circuit 414 to the subsequent stage of the differentiation circuit 213. Provided. The clamp circuit 414 functions so that the capacitor 213a in the differentiation circuit 213 does not flow a discharge current.

  FIG. 12 is a diagram showing an example of transition of the output current of the PD 106 before / after passing through the differentiating circuit 213 when the clamp circuit 414 is provided. In the illustrated example, a case where a plurality of beams emit light with a certain amount of light in the image region (that is, in the VIDEO mode) is shown.

  As shown in the figure, when the semiconductor laser 101 is turned on and the PD 106 is irradiated with light in the VIDEO mode, the semiconductor laser 101 is turned off and the PD 106 is no longer irradiated with light. 213a starts discharging and tries to pass a reverse current. However, the presence of the clamp circuit 414 prevents the capacitor 213a from flowing a reverse current, and the current becomes “0” A. As a result, since the time of the OFF mode after the VIDEO mode can be set freely without considering the discharge time t of the capacitor 213a, efficient APC can be executed.

101 Semiconductor laser 105 Half mirror 106 Light quantity detection sensor (PD)
210, 310, 410 Light amount detection circuit 211, 311 Amplification circuit 213 Differentiation circuit 220 CPU
414 Clamp circuit

Claims (6)

A light emitting means for emitting a light beam for forming an electrostatic latent image on the photoreceptor;
Deflecting means for deflecting the light beam such that the light beam scans the photoreceptor;
The light beam receives the light beam emitted from the light emitting means during a non-latent image forming period other than the latent image forming period for scanning the area where the electrostatic latent image is formed on the photoconductor, and the received light Photoelectric conversion means for outputting a signal of a level corresponding to the amount of light of the beam;
A generation circuit for generating a current according to the level output by the photoelectric conversion means;
An extraction circuit that receives the current generated by the generation circuit and extracts an AC component included in the current;
Control means for controlling the light amount of the light beam based on the value of the alternating current component included in the current extracted by the extraction circuit;
An image forming apparatus comprising: a blocking unit configured to block the current input to the extraction circuit during the latent image forming period. The extraction circuit is a differentiation circuit including a capacitor,
The image forming apparatus according to claim 1, wherein the blocking unit blocks a generated current from the generating circuit during the latent image forming period so that the capacitor of the differentiating circuit is not charged. A light emitting means for emitting a light beam for forming an electrostatic latent image on the photoreceptor;
Deflecting means for deflecting the light beam such that the light beam scans the photoreceptor;
The light beam receives the light beam emitted from the light emitting means during a non-latent image forming period other than the latent image forming period for scanning the area where the electrostatic latent image is formed on the photoconductor, and the received light Photoelectric conversion means for outputting a signal of a level corresponding to the amount of light of the beam;
A generation circuit for generating a current according to the level output by the photoelectric conversion means;
An extraction circuit that receives the current generated by the generation circuit and extracts an AC component included in the current;
Control means for controlling the light amount of the light beam based on the value of the alternating current component included in the current extracted by the extraction circuit;
An image forming apparatus comprising: switching means for switching the gain of the generation circuit to a lower magnification during the latent image formation period. The extraction circuit is a differentiation circuit including a capacitor,
The generation circuit includes a plurality of resistors each capable of setting a plurality of gains of different magnifications,
4. The switching unit according to claim 3, wherein the switching unit switches to a resistor that sets a low magnification gain during the latent image formation period, and switches to a resistor that sets a high magnification gain during the non-latent image formation period. Image forming apparatus. A light emitting means for emitting a light beam for forming an electrostatic latent image on the photoreceptor;
Deflecting means for deflecting the light beam such that the light beam scans the photoreceptor;
The light beam receives the light beam emitted from the light emitting means during a non-latent image forming period other than the latent image forming period for scanning the area where the electrostatic latent image is formed on the photoconductor, and the received light Photoelectric conversion means for outputting a signal of a level corresponding to the amount of light of the beam;
A generation circuit for generating a current according to the level output by the photoelectric conversion means;
An extraction circuit that includes a capacitor, receives the current generated by the generation circuit, and extracts an AC component included in the current;
Control means for controlling the light amount of the light beam based on the value of the alternating current component included in the current extracted by the extraction circuit;

An image forming apparatus comprising: a clamp circuit that operates so as not to flow a discharge current from the capacitor of the extraction circuit.   The image forming apparatus according to claim 1, wherein the light emitting unit emits a plurality of light beams for forming an electrostatic latent image on each of the plurality of photosensitive members.

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