Optical scanning device and image forming apparatus

A deflector deflects a light beam emitted from a light source including a plurality of light-emitting units. A scanning optical system focuses the light beam deflected by the deflector on a scanning target surface. A monitoring photoreceiver receives a part of a light beam deflected by the deflector and directed toward an area within a scanning area outside an image area. A detecting unit individually detects emission powers of at least two light-emitting units based on an output signal of the monitoring photoreceiver in a single sweep of scanning.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to and incorporates by reference the entire contents of Japanese priority document 2008-025842 filed in Japan on Feb. 6, 2008.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical scanning device and an image forming apparatus, and more particularly, to an optical scanning device that scans a target surface with a light beam and an image forming apparatus including the optical scanning device.

2. Description of the Related Art

In recent years, improvements in a print speed (high speed) and a writing density (high density) have been required in an image forming apparatus, such as a laser printer and a digital copier. To achieve the improvements in the print speed and the writing density, in a conventional technology, an optical scanning device is employed which includes a light source having a plurality of light-emitting elements, and a scanning target surface is scanned with a plurality of light beams emitted from the light-emitting elements at one time.

A semiconductor laser has been generally used as a light source in an image forming apparatus. Although an edge-emitting laser was mainly used as the semiconductor laser, a vertical-cavity surface-emitting laser (VCSEL) has been recently used. Although the maximum number of light-emitting elements that can be arrayed in the edge-emitting laser is about four to eight, it is possible to array more than eight light-emitting elements in the VCSEL. Therefore, the VCSEL is expected to be the light source to achieve the high speed and the high density in the image forming apparatus.

If a light output of the light source fluctuates, a density variation occurs in an output image. Therefore, a conventional optical scanning device employing the edge-emitting laser monitors a light beam emitted from the edge-emitting laser in its backward direction, and performs an automatic power control (APC) thereby preventing the fluctuation of the light output. However, the surface-emitting laser does not emit a light beam in its backward direction because of its structure. Therefore, an optical scanning device employing the surface-emitting laser needs to control a light intensity of the light source in a manner different from the APC performed in the conventional technology.

For example, Japanese Patent Application Laid-open No. H9-288244, Japanese Patent Application Laid-open No. 2002-40350, and Japanese Patent Application Laid-open No. H4-321370 disclose a method of controlling a light intensity of a light source when a surface-emitting laser is employed in an optical scanning device. Specifically, a part of a light beam emitted from the surface-emitting laser is separated and guided to a photodetector by using optical elements, such as a beam splitter and a half mirror, and a drive current of the surface-emitting laser is controlled based on an output of the photodetector.

However, if the requirement of the high speed and the high density in the image forming apparatus becomes higher, it would be difficult for image forming apparatuses disclosed in the above Patent Documents to perform the APC with a sufficient accuracy and support the high speed and the high density. Furthermore, a conventional image forming apparatus has a problem that when the number of light-emitting elements included in a light source becomes larger, a time required for the APC is increased.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided an optical scanning device that scans a scanning area on a scanning target surface with a light beam and writes image data in an image area within the scanning area. The optical scanning device includes a light source including a plurality of light-emitting units; a deflector that deflects a light beam emitted from the light source; a scanning optical system that focuses the light beam deflected by the deflector on the scanning target surface; a monitoring photoreceiver that receives a part of a light beam deflected by the deflector and directed toward an area within the scanning area outside the image area; and a detecting unit that individually detects emission powers of at least two light-emitting units based on an output signal of the monitoring photoreceiver in a single sweep of scanning.

Furthermore, according to another aspect of the present invention, there is provided an image forming apparatus including at least one image carrier; and at least one optical scanning device that scans a scanning area on a scanning target surface of the at least one image carrier with a light beam modulated by image data. The optical scanning device includes a light source including a plurality of light-emitting units, a deflector that deflects a light beam emitted from the light source, a scanning optical system that focuses the light beam deflected by the deflector on the scanning target surface, a monitoring photoreceiver that receives a part of a light beam deflected by the deflector and directed toward an area within the scanning area outside the image area, and a detecting unit that individually detects emission powers of at least two light-emitting units based on an output signal of the monitoring photoreceiver in a single sweep of scanning.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1is a schematic diagram of a laser printer1000as an image forming apparatus according to an embodiment of the present invention.

The laser printer1000includes an optical scanning device1010, a photosensitive element1030, a charging unit1031, a developing roller1032, a transfer charging unit1033, a neutralizing unit1034, a cleaning unit1035, a toner cartridge1036, a feeding roller1037, a feed tray1038, a pair of registration rollers1039, a fixing roller1041, a pair of discharging rollers1042, a catch tray1043, a communication control device1050, and a printer control device1060that controls the above components. Each of the above components is arranged at a predetermined position in a printer casing1044.

The communication control device1050controls a bidirectional communication with an upper-level device, such as a personal computer (PC), via a network.

The photosensitive element1030is a cylindrical member, and has a photosensitive layer on its surface. Specifically, the surface of the photosensitive element1030is a scanning target surface. The photosensitive element1030is rotated in a direction indicated by an arrow shown inFIG. 1.

The charging unit1031, the developing roller1032, the transfer charging unit1033, the neutralizing unit1034, and the cleaning unit1035are arranged near the surface of the photosensitive element1030in this order along the rotation direction of the photosensitive element1030.

The charging unit1031uniformly charges the surface of the photosensitive element1030.

The optical scanning device1010causes the surface of the photosensitive element1030charged by the charging unit1031to be irradiated with a light beam that is modulated based on image data received from the upper-level device. Thus, a latent image corresponding to the image data is formed on the surface of the photosensitive element1030. The latent image formed on the surface of the photosensitive element1030is then conveyed toward the developing roller1032with the rotation of the photosensitive element1030. The configuration of the optical scanning device1010will be described later in detail.

The toner cartridge1036contains toner, and the toner is supplied from the toner cartridge1036to the developing roller1032.

The developing roller1032then applies the toner to the latent image formed on the surface of the photosensitive element1030thereby developing the latent image. The latent image to which the toner is applied (hereinafter, referred to as “toner image” as appropriate for convenience of explanation) is conveyed toward the transfer charging unit1033with the rotation of the photosensitive element1030.

The feed tray1038contains one or more recording sheets (hereinafter, “recording media”)1040. The feeding roller1037is arranged near the feed tray1038. The feeding roller1037feeds one recording medium1040from the feed tray1038at a time, and conveys the fed recording medium1040to the registration rollers1039. The registration rollers1039temporarily hold the recording medium1040fed by the feeding roller1037, and convey the recording medium1040into a space between the photosensitive element1030and the transfer charging unit1033in accordance with the rotation of the photosensitive element1030.

A voltage with a reverse polarity with respect to a polarity of the toner is applied to the transfer charging unit1033, so that the toner on the surface of the photosensitive element1030is electrically attracted to the recording medium1040. Thus, the toner image formed on the surface of the photosensitive element1030is transferred to the recording medium1040. The recording medium1040with the toner image is then conveyed to the fixing roller1041.

The fixing roller1041applies heat and pressure to the recording medium1040, so that the toner is fixed to the recording medium1040. The recording medium1040is then conveyed to the catch tray1043via the discharging rollers1042, and is stacked on the catch tray1043.

The neutralizing unit1034removes residual charges from the surface of the photosensitive element1030.

The cleaning unit1035removes toner (residual toner) remaining on the surface of the photosensitive element1030. The surface of the photosensitive element1030from which the residual toner is removed is conveyed again to a position at which the surface is arranged parallel to the charging unit1031.

FIG. 2is a schematic diagram of the optical scanning device1010. The optical scanning device1010includes a scanning lens11aon a deflector side, a scanning lens11bon an image plane side, a polygon mirror13, a light source14, a coupling lens15, an aperture plate16, a cylindrical lens17, a front-side synchronization detection sensor18F, a rear-side synchronization detection sensor18E, an APC mirror19, an APC photoreceiver20, and a scanning control device22(the scanning control device22is not shown inFIG. 2). Each of the above components is arranged at a predetermined position in a housing21. In a three-dimensional orthogonal coordinate system having X, Y, and Z axes, a longitudinal direction of the photosensitive element1030is defined as an Y axial direction, and a direction along an optical axis of each of the scanning lenses11aand11bis defined as an X axial direction.

FIG. 3is a diagram for explaining a scan start area, an image area, and a scan end area on the surface of the photosensitive element1030. An area on the surface of the photosensitive element1030to be scanned by the optical scanning device1010along the Y axial direction is referred to as a scanning area. An area that is located within the scanning area and in which image data is written is referred to as the image area. For convenience of explanation, an area that is located within the scanning area and is scanned prior to the image area is referred to as the scan start area, and an area that is located within the scanning area and is scanned after the image area is referred to as the scan end area.

FIG. 4is a schematic diagram for explaining a two-dimensional array100included in the light source14. The two-dimensional array100includes nine light-emitting elements that are arranged in a two-dimensional array on a board. A direction M shown inFIG. 4corresponds to the main scanning direction, and a direction S shown inFIG. 4corresponds to the sub-scanning direction (which corresponds to a Z axial direction shown inFIG. 2). A direction T extends with a tilt angle α (0°<α<90°) from the direction M toward the direction S.

The two-dimensional array100includes three lines of the light-emitting elements, each of the lines having three light-emitting elements arranged at equal spaces in the direction T. The light-emitting elements in the three lines are arranged at equal spaces in the direction S such that when all of the light-emitting elements are projected on imaginary lines in the direction S, projection images are arranged at equal spaces. For convenience of explanation, the top one of the three lines shown inFIG. 4is referred to as a first line, the middle one of the three lines shown inFIG. 4is referred to as a second line, and the bottom one of the three lines shown inFIG. 4is referred to as a third line. A space between the light-emitting elements means a distance between the centers of the two light-emitting elements. A space dv between the light-emitting elements in the direction M is 100 μm.

For convenience of explanation, to specify each of the light-emitting elements, the three light-emitting elements, from the upper left to the lower right inFIG. 4, in the first line are referred to as light-emitting elements v1to v3, the three light-emitting elements in the second line as light-emitting elements v4to v6, and the three light-emitting elements in the third line as light-emitting elements v7to v9.

Each of the light-emitting elements v1to v9is a VCSEL in a wavelength band of 780 nanometers. That is, the two-dimensional array100is a surface-emitting laser array.

FIG. 5is a schematic diagram of a circuit board kb on which the two-dimensional array100is mounted. The scanning control device22is also mounted on the circuit board kb.

As shown inFIG. 2, the coupling lens15converts a light beam emitted from the light source14into a substantially parallel light beam.

The aperture plate16includes an aperture, and defines a beam diameter of the light beam passed through the coupling lens15in at least a direction corresponding to the sub-scanning direction (the Z axial direction). The light source14and the coupling lens15are fixedly mounted with the same supporting member made of aluminum.

The cylindrical lens17focuses the light beam passed through the aperture of the aperture plate16near a reflecting surface of the polygon mirror13in the direction corresponding to the sub-scanning direction (the Z axial direction).

An optical system arranged on an optical path between the light source14and the polygon mirror13is also called a pre-deflector optical system. The pre-deflector optical system includes the coupling lens15, the aperture plate16, and the cylindrical lens17.

The polygon mirror13includes four mirrors each having an inscribed circle diameter of 7 millimeters, and each of the mirrors functions as the reflecting surface. The polygon mirror13deflects the light beam passed through the cylindrical lens17while being rotated at a constant speed around an axis parallel to the direction corresponding to the sub-scanning direction (the Z axial direction).

The scanning lens11ais arranged on an optical path of the light beam deflected by the polygon mirror13.

The scanning lens11bis arranged on an optical path of the light beam passed through the scanning lens11a. The surface of the photosensitive element1030is irradiated with the light beam passed through the scanning lens11bwhereby a light spot is formed on the surface of the photosensitive element1030. The light spot is moved in the longitudinal direction of the photosensitive element1030in accordance with the rotation of the photosensitive element1030. That is, the surface of the photosensitive element1030is scanned. A direction in which the light spot is moved is the main scanning direction.

An optical system arranged on an optical path between the polygon mirror13and the photosensitive element1030is also called a scanning optical system. The scanning optical system includes the scanning lenses11aand11b. A reflecting mirror that reflects the optical path can be arranged on at least either one of the optical path between the scanning lens11aand the scanning lens11band the optical path between the scanning lens11band the photosensitive element1030.

A part of the light beam that is deflected by the polygon mirror13, passed through the scanning optical system, and directed toward the scan start area enters the APC photoreceiver20via the APC mirror19as a light beam used for monitoring (hereinafter, “monitoring light beam”). The APC photoreceiver20is arranged near the light source14.

FIGS. 6 and 7are schematic diagrams for explaining the APC photoreceiver20. As shown inFIG. 6, the APC photoreceiver20includes nine light-receiving elements pd1to pd9. A direction M′ shown inFIG. 6corresponds to the main scanning direction, and a direction S′ shown inFIG. 6corresponds to the sub-scanning direction (the Z axial direction). The light-receiving elements pd1to pd9are integrally formed. A magnification of the optical system in the main scanning direction is 10×, and a distance dp between the two light-receiving elements in the direction M′ is 1000 μm.

As shown inFIG. 7, the light-receiving elements pd1to pd9correspond to the light-emitting elements v1to v9, respectively, and the light-receiving elements pd1to pd9are arranged such that the light-receiving elements pd1to pd9receive light beams sp1to sp9emitted from the light-emitting elements v1to v9, respectively. Each of the light-receiving elements pd1to pd9is of a size to receive only the light beam emitted from the corresponding one of the light-emitting elements v1to v9.

Each of the light-receiving elements pd1to pd9outputs a signal (photoelectric conversion signal) depending on an amount of received light. An output signal from each of the light-receiving elements pd1to pd9is fed to the scanning control device22.

A part of the light beam that is deflected by the polygon mirror13, passed through the scanning optical system, and directed toward the scan start area enters the front-side synchronization detection sensor18F via a mirror.

A part of the light beam that is deflected by the polygon mirror13, passed through the scanning optical system, and directed toward the scan end area enters the rear-side synchronization detection sensor18E via a mirror.

Each of the front-side synchronization detection sensor18F and the rear-side synchronization detection sensor18E outputs a signal (photoelectric conversion signal) depending on an amount of received light. An output signal from each of the front-side synchronization detection sensor18F and the rear-side synchronization detection sensor18E is fed to the scanning control device22.

FIG. 8is a schematic diagram for explaining equivalent positions of the APC photoreceiver20, the front-side synchronization detection sensor18F, and the rear-side synchronization detection sensor18E near the photosensitive element1030. A reference numeral20′ shown inFIG. 8denotes an equivalent position of the APC photoreceiver20. A reference numeral18F′ shown inFIG. 8denotes an equivalent position of the front-side synchronization detection sensor18F. A reference numeral18E′ shown inFIG. 8denotes an equivalent position of the rear-side synchronization detection sensor18E. Specifically, the APC photoreceiver20is located closer to a scan start position than the front-side synchronization detection sensor18F.

FIG. 9is a block diagram for explaining the configuration of the scanning control device22. The scanning control device22includes a central processing unit (CPU)210, a flash memory211, a random access memory (RAM)212, a gain control amplifier (GCA)213, an interface (I/F)214, a pixel-clock generating circuit215, an image processing circuit216, a frame memory217, line buffers2181to2189, a write control circuit219, and a light-source drive circuit221. Arrows shown inFIG. 9indicate flows of representative signals and data, and do not indicate the entire connection relationships between the blocks.

The I/F214is a communication interface that controls a bidirectional communication with the printer control device1060. Image data is fed from the upper-level device via the I/F214.

The pixel-clock generating circuit215generates a pixel clock signal.

Image data (hereinafter, “raster data”) that is rasterized by the CPU210is temporarily stored in the frame memory217.

The image processing circuit216reads the raster data from the frame memory217, performs a predetermined halftoning process, generates dot data for each of the light-emitting elements v1to v9, and outputs the dot data to the line buffers2181to2189corresponding to the light-emitting elements v1to v9.

The write control circuit219determines a timing at which a scanning operation is started based on an output signal from the front-side synchronization detection sensor18F. The write control circuit219then reads the dot data for each of the light-emitting elements v1to v9from the line buffers2181to2189in accordance with the timing at which the scanning operation is started, superimposes the dot data on a pixel clock signal generated by the pixel-clock generating circuit215, and generates data that is separately modulated for each of the light-emitting elements v1to v9.

The light-source drive circuit221drives each of the light-emitting elements v1to v9depending on the modulated data from the write control circuit219.

The GCA213receives output signals of the light-receiving elements pd1to pd9, and performs a level adjustment on each of the output signals, so that an output deviation between the light-receiving elements pd1to pd9is reduced.

The flash memory211stores therein various computer programs that are written in codes readable by the CPU210and various data including emission properties of the light-emitting elements v1to v9.

The RAM212is used as a working memory.

The CPU210operates in accordance with a computer program stored in the flash memory211, and controls the optical scanning device1010.

For example, the CPU210causes the light-emitting elements v1to v9to simultaneously emit light beams at each predetermined timing with an output signal from the rear-side synchronization detection sensor18E as a synchronization signal, and individually detects emission powers of the light-emitting elements v1to v9based on output signals from the light-receiving elements pd1to pd9via the GCA213. Relations between the output signals from the light-receiving elements pd1to pd9via the GCA213and the emission powers of the light-emitting elements v1to v9are acquired in advance, and stored in the flash memory211.

The CPU210then controls a drive current of each of the light-emitting elements v1to v9via the light-source drive circuit221based on a detection result of the emission power of each of the light-emitting elements v1to v9, so that the emission power of each of the light-emitting elements v1to v9becomes a desired level. Thus, the CPU210performs the APC.

Furthermore, the CPU210adjusts a pixel clock cycle based on output signals of the front-side synchronization detection sensor18F and the rear-side synchronization detection sensor18E, thereby obtaining a desired scanning length.

As described above, in the optical scanning device1010, the light source14includes the nine light-emitting elements v1to v9. The polygon mirror13deflects a light beam emitted from the light source14. The scanning optical system focuses the light beam deflected by the polygon mirror13on the surface of the photosensitive element1030. The APC photoreceiver20receives a part of the light beam that is deflected by the polygon mirror13and is projected onto the scan start area. The scanning control device22individually detects the emission powers of the light-emitting elements v1to v9based on an output signal from the APC photoreceiver20, and controls the drive signal of each of the light-emitting elements v1to v9based on a detection result.

In the optical scanning device1010, the APC photoreceiver20includes a monitoring photoreceiver. Moreover, the scanning control device22includes a detector and a control device.

At least a part of processes performed by the CPU210in accordance with computer programs can be constructed by hardware, or all of the processes can be constructed by hardware.

As described above, in the optical scanning device1010, because a part of the light beam deflected by the polygon mirror13and directed toward the scan start area is used as the monitoring light beam, it is possible to obtain a larger amount of monitoring light beam than that in a conventional technology without decreasing a light use efficiency upon writing image data. Furthermore, because the APC photoreceiver20includes a plurality of light-receiving elements corresponding to a plurality of light-emitting elements, it is possible to detect fluctuation of a light intensity of each of the light-emitting elements in one scanning operation. The drive signals of the light-emitting elements can be individually and promptly controlled at the same timing whereby the fluctuation of the light intensities is reduced. That is, the APC can be performed with a higher speed and a higher accuracy than that in the conventional technology. Therefore, it is possible to scan the surface of the photosensitive element1030with a plurality of light beams in a stable manner.

Moreover, in the optical scanning device1010, the monitoring light beam is reflected by the APC mirror19, and received by the APC photoreceiver20arranged near the light source14. With this configuration, a distance between the APC photoreceiver20and the scanning control device22can be short, so that signal delay can be reduced and the APC can be performed further promptly.

Furthermore, because the light-receiving elements pd1to pd9are integrally formed, it is possible to simplify an assembling operation and an adjustment operation of the APC photoreceiver20.

Moreover, the optical scanning device1010includes the GCA213as a signal control device to reduce the output deviation between the light-receiving elements pd1to pd9. Therefore, algorism for detecting the fluctuation of the light intensity can be simplified, and high-speed processing can be achieved.

Furthermore, the optical scanning device1010includes two synchronization detection sensors, i.e., the front-side synchronization detection sensor18F and the rear-side synchronization detection sensor18E. Therefore, the scanning length can be adjusted with a high accuracy. That is, an accuracy of a pixel position can be improved.

Furthermore, because the laser printer1000includes the optical scanning device1010that can scan the surface of the photosensitive element1030with a plurality of light beams in a stable manner, it is possible to form a high-density image at a high speed.

FIG. 10is a schematic diagram for explaining a case where the APC photoreceiver20, the front-side synchronization detection sensor18F, and the rear-side synchronization detection sensor18E are arranged near the photosensitive element1030. If the signal delay is not really acknowledged as a problem, the APC photoreceiver20can be arranged on the scan start area. In such a case, the APC mirror19does not need to be arranged. The front-side synchronization detection sensor18F can be arranged on the scan start area. Moreover, the rear-side synchronization detection sensor18E can be arranged on the scan end area.

FIG. 11is a schematic diagram for explaining a case where two APC photoreceivers20are arranged. It is possible that the two APC photoreceivers20are arranged on the scan start area and the scan end area. In such a case, the detection accuracy of the emission power of each of the light-emitting elements v1to v9can be further improved by using output signals of the APC photoreceivers20.

Although it is explained above that the two-dimensional array100includes the nine light-emitting elements v1to v9, the present invention is not limited to this configuration.

Although it is explained above that the GCA213is arranged in the scanning control device22, the present invention is not limited to this configuration. The GCA213can be arranged in the APC photoreceiver20.

FIG. 12is a schematic diagram of a light-receiving element pd20according to a modification 1 of the APC photoreceiver20.FIG. 13is a block diagram for explaining the configuration of a scanning control device22A when the light-receiving element pd20is employed. Although it is explained above that the APC photoreceiver20includes a plurality of light-receiving elements corresponding to a plurality of light-emitting elements, the present invention is not limited to this configuration. As shown inFIG. 12, the light-receiving element pd20can be used instead of the light-receiving elements pd1to pd9. The light-receiving element pd20has a rectangle shape whose longitudinal direction extends along the direction M′ corresponding to the main scanning direction. In this case, as shown inFIG. 13, the scanning control device22A is used instead of the scanning control device22. The scanning control device22A includes the CPU210, the flash memory211, the RAM212, the I/F214, the pixel-clock generating circuit215, the image processing circuit216, the frame memory217, the line buffers2181to2189, the write control circuit219, the light-source drive circuit221, a turn-on clock generating circuit223, an amplifier (AMP)224, and a latch circuit225.

FIG. 14is a schematic diagram for explaining the size of the light-receiving element pd20. The light-receiving element pd20has a size in the direction S′ corresponding to the sub-scanning direction such that it can simultaneously receive the light beams sp1to sp9emitted from the light-emitting elements v1to v9.

FIGS. 15 to 18are timing charts for explaining an example 1 of the APC when the light-receiving element pd20is employed.FIG. 15is a timing chart for explaining turning on/off of each of the light-emitting elements v1to v9.FIG. 16is a timing chart for explaining outputs of the APC photoreceiver20.FIG. 17is a timing chart for explaining outputs of the AMP224.FIG. 18is a timing chart for explaining outputs of the latch circuit225. As shown inFIG. 15, the CPU210of the scanning control device22A causes the light-emitting elements v1to v9to be sequentially turned on one by one at a timing of a leading edge of a turn-on clock from the turn-on clock generating circuit223. Specifically, the CPU210increases the number of light-emitting elements that are turned on by one. To reduce influence of thermal interference that can occur between the light-emitting elements v1to v9, for example, the light-emitting elements v1to v9are turned on in the order of v1, v3, v7, v9, v5, v2, v8, v4, and v6.

As shown inFIG. 16, an output level of the APC photoreceiver20is increased in a stepwise manner. An output signal of the APC photoreceiver20is amplified by the AMP224. The light-receiving element pd20has a size in its longitudinal direction such that signals shown inFIG. 16can be output in one scanning operation.

As shown inFIG. 17, the latch circuit225latches an output signal of the AMP224at a timing of a trailing edge of the turn-on clock.

The CPU210detects a light intensity of each of the light-emitting elements v1to v9based on an output signal of the latch circuit225. As shown inFIG. 18, the light intensity of the light-emitting element v1is detected based on Vp1, and the light intensity of the light-emitting element v3is detected based on (Vp2-Vp1). Furthermore, the light intensity of the light-emitting element v7is detected based on (Vp3-Vp2), and the light intensity of the light-emitting element v9is detected based on (Vp4-Vp3). The light intensity of the light-emitting element v5is detected based on (Vp5-Vp4), and the light intensity of the light-emitting element v2is detected based on (Vp6-Vp5). The light intensity of the light-emitting element v8is detected based on (Vp7-Vp6), and the light intensity of the light-emitting element v4is detected based on (Vp8-Vp7). Then, the light intensity of the light-emitting element v6is detected based on (Vp9-Vp8).

The CPU210individually detects the emission powers of the light-emitting elements v1to v9based on detection results of the light intensities of the light-emitting elements v1to v9.

The CPU210controls a drive current of each of the light-emitting elements v1to v9via the light-source drive circuit221, so that the emission power of each of the light-emitting elements v1to v9becomes a desired level.

In the above case, it is possible to individually detect the emission powers of all of the light-emitting elements v1to v9in one scanning operation, and performs the APC with a higher speed and a higher accuracy than that in the conventional technology.

When the APC photoreceiver20includes the light-receiving element pd20, if a large amount of offset signal component or noise component caused by the light-receiving element pd20is contained in an output signal of the APC photoreceiver20, it is possible that at least one of the light-emitting elements v1to v9is selected as an offset light-emitting element, and at least two target light-emitting elements for detection among the light-emitting elements v1to v9are turned on one by one together with the offset light-emitting element in one scanning operation whereby the emission powers of the two target light-emitting elements can be individually detected based on output signals of the APC photoreceiver20. In this manner, it is possible to reduce influences of an offset signal and a noise caused by the light-receiving element pd20on the output signal of the APC photoreceiver20, and to improve a signal-to-noise ratio of the output signal of the APC photoreceiver20.

FIGS. 19 to 22are timing charts for explaining an example 2 of the APC when the light-receiving element pd20is employed and the light-emitting element v1is selected as the offset light-emitting element.FIG. 19is a timing chart for explaining turning on/off of each of the light-emitting elements v1to v9.FIG. 20is a timing chart for explaining outputs of the APC photoreceiver20.FIG. 21is a timing chart for explaining outputs of the AMP224.FIG. 22is a timing chart for explaining outputs of the latch circuit225.

The CPU210of the scanning control device22A causes the light-emitting elements v1, v2, and v3to be turned on at a timing of a leading edge of a turn-on clock p1from the turn-on clock generating circuit223.

The CPU210then causes only the light-emitting element v1to be turned off at a timing of a leading edge of a turn-on clock p2. The CPU210then detects the light intensity of the light-emitting element v1based on a difference between an output of the latch circuit225when the light-emitting elements v1, v2, and v3are turned on and an output of the latch circuit225when only the light-emitting element v1is turned off.

The CPU210causes the light-emitting element v1to be turned on at a timing of a leading edge of a turn-on clock p3, and causes the light-emitting element v3to be turned off at the same timing. That is, the CPU210causes the light-emitting elements v1and v2to be turned on. The CPU210then determines the light intensity of the light-emitting element v2by subtracting the detected light intensity of the light-emitting element v1from the light intensity obtained from the output of the latch circuit225when the light-emitting elements v1and v2are turned on.

The CPU210then causes the light-emitting elements v3to v9to be sequentially turned on one by one at timings of leading edges of turn-on clocks p4to p10while causing the light-emitting element v1to be turned on. The CPU210then determines the light intensity of each of the light-emitting elements v3to v9by subtracting the light intensity of the light-emitting element v1from the light intensity obtained from the output of the latch circuit225when each of the light-emitting elements v3to v9is turned on.

FIG. 23is a table for explaining selection patterns of the offset light-emitting element in the example 2 of the APC. The offset light-emitting element (pattern) can be changed over in every one scanning operation. Thus, it is possible to prevent the decrease in the life of the two-dimensional array100.

Furthermore, the offset light-emitting element can be changed over during one scanning operation. In such a case, the offset light-emitting element can be changed over such that the offset light-emitting element is prevented from being adjacent to the target light-emitting elements.FIG. 24is a timing chart for explaining an example 3 of the APC when the light-receiving element pd20is employed. The light-emitting element v1is selected as the offset light-emitting element when the light-emitting elements v3, v5to v9are the target light-emitting elements, and the light-emitting element v9is selected as the offset light-emitting element when the light-emitting elements v2and v4are the target light-emitting elements. Thus, even if the distance between the light-emitting elements is small, it is possible to reduce the influence of heat caused by other light-emitting elements.

FIGS. 25 to 28are timing charts for explaining an example 4 of the APC when the light-receiving element pd20is employed and the light-emitting elements v1and v2are selected as the offset light-emitting elements.FIG. 25is a timing chart for explaining turning on/off of each of the light-emitting elements v1to v9.FIG. 26is a timing chart for explaining outputs of the APC photoreceiver20.FIG. 27is a timing chart for explaining outputs of the AMP224.FIG. 28is a timing chart for explaining outputs of the latch circuit225.

The CPU210of the scanning control device22A causes the light-emitting elements v1and v2to be turned on at a timing of the leading edge of the turn-on clock p1from the turn-on clock generating circuit223.

The CPU210then causes the light-emitting element v3to be turned on at a timing of the leading edge of the turn-on clock p2. The CPU210detects the light intensity of the light-emitting element v3based on a difference between an output of the latch circuit225when the light-emitting elements v1and v2are turned on and an output of the latch circuit225when the light-emitting elements v1to v3are turned on.

The CPU210then causes the light-emitting elements v4to v9to be sequentially turned on one by one at timings of the leading edges of the turn-on clocks p3to p8while causing the light-emitting elements v1and v2to be turned on. The CPU210then determines the light intensity of each of the light-emitting elements v4to v9based on a difference between an output of the latch circuit225when each of the light-emitting elements v4to v9is turned on and an output of the latch circuit225when only the light-emitting elements v1and v2are turned on.

The CPU210causes only the light-emitting elements v1and v3to be turned on at the timing of the leading edge of the turn-on clock p9, and detects the light intensity of the light-emitting elements v2based on a difference between an output of the latch circuit225when the light-emitting elements v1and v3are turned on and an output of the latch circuit225when the light-emitting elements v1to v3are turned on.

The CPU210then causes only the light-emitting elements v2and v3to be turned on at the timing of the leading edge of the turn-on clock p10, and detects the light intensity of the light-emitting element v1based on a difference between an output of the latch circuit225when the light-emitting elements v2and v3are turned on and an output of the latch circuit225when the light-emitting elements v1to v3are turned on.

FIG. 29is a table for explaining selection patterns of the offset light-emitting elements in the example 4 of the APC. The offset light-emitting elements (patterns) can be changed over in every one scanning operation. Thus, it is possible to prevent the decrease in the life of the two-dimensional array100.

FIGS. 30 to 33are schematic diagrams for explaining an APC photoreceiver200according to a modification 2 of the APC photoreceiver20. The APC photoreceiver200includes three light-receiving elements pdA, dpB, and dpC instead of the light-receiving elements pd1to pd9. As shown inFIGS. 31 to 33, the light-receiving element pdA can receive three light beams sp1, sp4, and sp7. The light-receiving element pdB can receive three light beams sp2, sp5, and sp8. The light-receiving element pdC can receive three light beams sp3, sp6, and sp9.

In this case, as shown inFIG. 31, the light-emitting elements v1, v2, and v3are turned on in the first scanning operation. Then, the emission power of the light-emitting element v1is detected based on an output signal of the light-receiving element pdA, the emission power of the light-emitting element v2is detected based on an output signal of the light-receiving element pdB, and the emission power of the light-emitting element v3is detected based on an output signal of the light-receiving element pdC.

As shown inFIG. 32, the light-emitting elements v4, v5, and v6are turned on in the next scanning operation. Then, the emission power of the light-emitting element v4is detected based on an output signal of the light-receiving element pdA, the emission power of the light-emitting element v5is detected based on an output signal of the light-receiving element pdB, and the emission power of the light-emitting element v6is detected based on an output signal of the light-receiving element pdC.

As shown inFIG. 33, the light-emitting elements v7, v8and v9are turned on in the next scanning operation. Then, the emission power of the light-emitting element v7is detected based on an output signal of the light-receiving element pdA, the emission power of the light-emitting element v8is detected based on an output signal of the light-receiving element pdB, and the emission power of the light-emitting element v9is detected based on an output signal of the light-receiving element pdC.

Because the emission powers of the three light-emitting elements can be individually detected in one scanning operation, the emission powers of all of the light-emitting elements can be individually detected in the three scanning operations. In this case, the APC can be performed with a higher speed and a higher accuracy than that in the conventional technology.

Although it is explained in the above embodiment that the image forming apparatus is the laser printer1000, the present invention is not limited to this configuration. It is possible to form a high-density image at a high speed if an image forming apparatus includes the optical scanning device1010.

For example, it is possible to employ an image forming apparatus in which a medium (for example, a sheet) that develops an image with a laser beam is directly irradiated with a laser beam.

Moreover, it is possible to employ an image forming apparatus that uses a silver halide film as an image carrier. In such a case, a latent image is formed on the silver halide film by an optical scanning operation, and the latent image can be developed by an operation similar to a developing operation in a conventional silver halide photographic process. The developed image can be then transferred onto a photographic paper by an operation similar to a printing operation in the conventional silver halide photographic process. Such an image forming apparatus can be implemented as an optical press or an optical plotter that forms a computed tomography (CT) scan image or the like.

FIG. 34is a schematic diagram of a color printer2000according to the embodiment. It is also possible to employ the color printer2000including a plurality of photosensitive elements.

Each of the photosensitive elements C1, M1, K1, and Y1is rotated in a direction indicated by an arrow shown inFIG. 34. The charging devices C2, M2, K2, Y2, the developing devices C4, M4, K4, Y4, the transferring devices C6, M6, K6, Y6, and the cleaning units C5, M5, K5, Y5are arranged around the photosensitive elements C1, M1, K1, Y1in the order along the rotation direction of the photosensitive elements C1, M1, K1, Y1. Each of the charging devices C2, M2, K2, and Y2uniformly charges the surface of the corresponding photosensitive element. The surface of the corresponding photosensitive element charged by each of the charging devices C2, M2, K2, and Y2is irradiated with a light beam emitted from the optical scanning device2010whereby an electrostatic latent image is formed on the surface of the photosensitive element. The electrostatic latent image is then developed by the corresponding developing device, so that a toner image is formed on the surface of the photosensitive element. The toner images of the four colors are transferred onto a recording medium in a superimposed manner by the transferring devices C6, M6, K6, and Y6, and then the toner images are fixed to the recording medium by the fixing unit2030.

The optical scanning device2010includes, for each of the four colors, a light source (not shown) similar to the light source14, a APC photoreceiver (not shown) similar to the APC photoreceiver20, a front-side synchronization detection sensor (not shown) similar to the front-side synchronization detection sensor18F, a rear-side synchronization detection sensor (not shown) similar to the rear-side synchronization detection sensor18E, and a scanning optical system (not shown) similar to the scanning optical system described above. In the following description, the same reference numerals are used to explain the same components as those in the optical scanning device1010.

FIG. 35is a schematic diagram for explaining arrangement positions of the APC photoreceiver20, the front-side synchronization detection sensor18F, and the rear-side synchronization detection sensor18E in the optical scanning device2010. The APC photoreceiver20and the front-side synchronization detection sensor18F are arranged in the scan start area of each of the photosensitive elements C1, M1, K1, and Y1. Furthermore, the rear-side synchronization detection sensor18E is arranged on the scan end area of each of the photosensitive elements C1, M1, K1, and Y1.

The photosensitive elements K1is irradiated with a light beam emitted from the light source14corresponding to the block color via the scanning optical system corresponding to the block color. The photosensitive elements C1is irradiated with a light beam emitted from the light source14corresponding to the cyan color via the scanning optical system corresponding to the cyan color. The photosensitive elements M1is irradiated with a light beam emitted from the light source14corresponding to the magenta color via the scanning optical system corresponding to the magenta color. The photosensitive elements Y1is irradiated with a light beam emitted from the light source14corresponding to the yellow color via the scanning optical system corresponding to the yellow color.

The APC photoreceiver20and the front-side synchronization detection sensor18F each corresponding to the black color receive a part of the light beam that is emitted from the light source14corresponding to the black color and is directed toward the scan start area of the photosensitive element K1. The APC photoreceiver20and the front-side synchronization detection sensor18F each corresponding to the cyan color receive a part of the light beam that is emitted from the light source14corresponding to the cyan color and is directed toward the scan start area of the photosensitive element C1. The APC photoreceiver20and the front-side synchronization detection sensor18F each corresponding to the magenta color receive a part of the light beam that is emitted from the light source14corresponding to the magenta color and is directed toward the scan start area of the photosensitive element M1. The APC photoreceiver20and the front-side synchronization detection sensor18F each corresponding to the yellow color receive a part of the light beam that is emitted from the light source14corresponding to the yellow color and is directed toward the scan start area of the photosensitive element Y1.

The rear-side synchronization detection sensor18E corresponding to the black color receives a part of the light beam that is emitted from the light source14corresponding to the black color and is directed toward the scan end area of the photosensitive element K1. The rear-side synchronization detection sensor18E corresponding to the cyan color receives a part of the light beam that is emitted from the light source14corresponding to the cyan color and is directed toward the scan end area of the photosensitive element C1. The rear-side synchronization detection sensor18E corresponding to the magenta color receives a part of the light beam that is emitted from the light source14corresponding to the magenta color and is directed toward the scan end area of the photosensitive element M1. The rear-side synchronization detection sensor18E corresponding to the yellow color receives a part of the light beam that is emitted from the light source14corresponding to the yellow color and is directed toward the scan end area of the photosensitive element Y1.

The optical scanning device2010performs the APC on the light source14for each of the four colors in the same manner as described in the above embodiment.

In this manner, the APC can be performed with a higher speed and a higher accuracy than that in the conventional technology. Thus, the color printer2000can form a high-density color image at a high speed.

The optical scanning device1010can be used in the color printer2000for each of the colors instead of the optical scanning device2010.

According to one aspect of the present invention, the APC can be performed with a higher speed and a higher accuracy than that in the conventional technology.

Furthermore, according to another aspect of the present invention, a high-density image can be formed at a high speed.