JP2002296531A - Image forming device - Google Patents

Abstract

(57) Abstract: Provided is an image forming apparatus capable of simplifying a structure by reducing the number of members constituting an optical scanning device and manufacturing the optical scanning device at low cost. An LSU includes a semiconductor laser, a driving unit of the semiconductor laser, a polygon mirror, and a horizontal synchronization sensor. Each deflecting / reflecting surface 27a of the polygon mirror 27 is formed as a concave surface curved in an arc shape along the sub-scanning direction, and the curvature of the concave surface is set to S. The laser beam incident on the deflecting / reflecting surface 27a is deflected and reflected while being imaged in the sub-scanning direction by the concave surface, and the horizontal synchronization sensor 30 is provided at a position where the laser beam is imaged.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a laser printer provided with a laser scan unit for detecting a position at which horizontal scanning of a laser beam on a photosensitive member is started.

[0002]

2. Description of the Related Art A laser printer as an image forming apparatus using an electrophotographic technique includes a photoreceptor, a charging unit for charging an outer peripheral surface of the photoreceptor, and an outer peripheral surface of the photoreceptor selected by a laser beam as an optical signal. Exposing means for forming an electrostatic latent image on a photoconductor by exposure to light, developing means for developing the electrostatic latent image into a toner image, and a transfer device for transferring the toner image to a transfer target. It is configured.

The operation of exposing the photoreceptor by the exposing means will be described. As shown in FIG. 6, a laser beam generated from a semiconductor laser 51 as a light source of the exposing means is collimated by a collimator lens 52 in a main scanning direction. It is focused on a linear image parallel to the sub-scanning direction. Then
The laser beam is focused on the polygon mirror 54 in the sub-scanning direction by the cylindrical lens 53. Further, the laser beam is deflected and reflected by each deflecting reflection surface 54a of the rotating polygon mirror 54 so as to scan in the main scanning direction. The laser beam scanned in the main scanning direction is imaged in the sub-scanning direction on the photoconductor by the f-θ lens 55, and is scanned in the main scanning direction from a predetermined horizontal scanning start position on the photoconductor, By the exposure with the laser beam, an electrostatic latent image is formed on the photoconductor.

The exposure means is provided with a laser scan unit (hereinafter, referred to as LSU) as an optical scanning device for detecting a horizontal scanning start position of a laser beam on a photosensitive member. The LSU includes the semiconductor laser 51, a driving unit (not shown) for driving and controlling the semiconductor laser 51, a polygon mirror 54, a horizontal synchronization sensor 57, and a reflection mirror 59 for reflecting the laser beam toward the horizontal synchronization sensor 57. And a cylindrical lens 60 for imaging a laser beam incident on the horizontal synchronization sensor 57. The horizontal synchronizing sensor 57 is installed at a position outside the image forming area on the photoconductor within the main scanning range of the laser beam. Further, the horizontal synchronization sensor 57 and the drive unit of the semiconductor laser 51 are provided on separate substrates, respectively, and are connected by a harness 58 via a connector (not shown).

The operation of the LSU having the above construction will now be described. The laser beam reflected by the deflecting / reflecting surface 54a of the polygon mirror 54 out of the image forming area and further reflected by the reflecting mirror 59 toward the horizontal synchronization sensor 57 Is incident on the horizontal synchronization sensor 57. Then
The horizontal synchronization sensor 57 outputs a horizontal synchronization signal to the driving unit. The drive unit that has detected the horizontal synchronization signal counts a certain timing from the time when the output level of the horizontal synchronization signal reaches a predetermined level. The horizontal scanning is started from a predetermined position on the photosensitive member.

[0006]

However, in the above-described conventional LSU, if the output levels of the laser beams incident on the horizontal synchronization sensor 57 are different from each other, the output level of the horizontal synchronization signal output from the horizontal synchronization sensor 57 also increases. fluctuate. For this reason, if the quality of the laser beam, such as the beam diameter or the output level, differs due to, for example, a parallel error of each surface of the polygon mirror 54 with respect to the rotation axis, the time required for the output level of the horizontal synchronization signal to reach a predetermined level for each laser beam Will vary. As a result, the time from when the horizontal synchronizing signal is output to when the horizontal scanning by the semiconductor laser 51 is started varies, and the horizontal scanning start position on the photosensitive member varies.

Therefore, in order to make the horizontal scanning start position on the photosensitive member constant, a laser beam having a constant output level must be incident on the horizontal synchronizing sensor 57. Specifically, the formed laser beam Must be incident on the horizontal synchronization sensor 57. Therefore, a cylindrical lens 60 for forming an image of the laser beam in the sub-scanning direction must be provided between the semiconductor laser 51 and the horizontal synchronization sensor 57 for the image formation.

Further, by passing the laser beam deflected and reflected by the polygon mirror 54 through the f-θ lens 55, the laser beam focused in the sub-scanning direction can be made incident on the horizontal synchronization sensor 57. Become. However, in order to form a laser beam on the horizontal synchronization sensor 57, the distance until the laser beam deflected and reflected by the polygon mirror 54 reaches the photosensitive member and the horizontal synchronization sensor 5
The distance to reach 7 must be set approximately equal.

Therefore, in order to increase the distance between the polygon mirror 54 and the horizontal synchronization sensor 57, the laser beam must be bent in the image forming apparatus.
9 had to be provided in the LSU. Alternatively, the LSU becomes large to secure the distance. Therefore, in any case, there are problems that the number of members constituting the LSU increases, the structure becomes complicated, and the manufacturing cost of the image forming apparatus increases.

The present invention has been made by paying attention to such problems existing in the prior art. An object of the present invention is to provide an image forming apparatus which can simplify the structure by reducing the number of members constituting the optical scanning device and can be manufactured at low cost.

[0011]

According to another aspect of the present invention, there is provided an image forming apparatus having a light source unit and a deflecting / reflecting surface. A deflector rotatably provided to scan the optical signal generated in the main scanning direction on the image carrier in a main scanning direction, and generates a horizontal synchronization signal when the deflected optical signal enters the deflector. An optical scanning device comprising a horizontal synchronization sensor for causing the light source unit to start scanning in a main scanning direction by an optical signal from a predetermined position on the image carrier when the horizontal synchronization signal is input, A concave surface curved at least along the sub-scanning direction so that the optical signal deflected and reflected by the deflecting unit forms an image on a horizontal synchronization sensor in at least a sub-scanning direction which is a direction orthogonal to the main scanning direction. Features formed in It is intended to.

According to a second aspect of the present invention, there is provided an image forming apparatus comprising:
According to the first aspect of the present invention, the driving unit of the light source unit and the horizontal synchronization sensor are installed on the same substrate.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment in which the present invention is embodied in an image forming apparatus (laser color printer) having a laser scanning unit (LSU) as an optical scanning apparatus will be described below with reference to the drawings. This image forming apparatus can form a full-color image using four color toners of yellow (Y), cyan (C), magenta (M), and black (K). FIG. 1 is a schematic diagram schematically showing the inside of the image forming apparatus. The main scanning direction in the specification indicates the axial direction of the photoconductor 11 in FIG. 2, and the sub-scanning direction is relative to the main scanning direction. The direction is orthogonal in two dimensions.

First, an outline of the inside of the image forming apparatus will be described. As shown in FIG. 1, the inside of the image forming apparatus includes a photosensitive member 11 as a cylindrical image carrier, a charging roller 12 as a charging unit, a developing roller 10 for four colors, an exposure unit 15 having an LSU, It is mainly constituted by a cleaning means (not shown) and the intermediate transfer device 13.

In the image forming apparatus having the above structure, first, the photosensitive layer is uniformly charged by bringing the charging roller 12 into contact with the outer peripheral surface of the photoconductor 11. The exposure unit 1 receives an image forming signal from a control unit (not shown).
When a selective exposure corresponding to the image forming signal is performed on the photoconductor 11 by the laser beam of No. 5, an electrostatic latent image is formed on the photoconductor 11. Further, the four color developing rollers 10
Thus, the toner of each color is sequentially applied to the electrostatic latent image and developed.

The intermediate transfer device 13 includes a plurality of rollers 14
An intermediate transfer belt 18 and the like are stretched around the belt. Then, the toner image formed on the photoconductor 11 is transferred to the intermediate transfer belt 18 between the photoconductor 11 and a primary transfer backup roller 17 disposed at a position facing the photoconductor 11. Next, at the pressure contact portion between the secondary transfer roller 19 and the intermediate transfer belt 18, the toner image transferred to the intermediate transfer belt 18 is transferred to the paper 23. After secondary transfer,
The toner image is fixed on the paper 23 by passing the paper 23 through a fixing device (not shown), and then the paper 23 is discharged out of the image forming apparatus.

The exposure section 15 having the LSU will be described in detail. As shown in FIGS. 2 and 3, the exposure section 15 includes a semiconductor laser 25 as a light source section and a drive section (not shown) for the semiconductor laser 25. ), A collimator lens 26, a polygon mirror 27 as a deflecting unit, an f-θ lens 28,
It comprises a reflection mirror 29 and a horizontal synchronization sensor 30. The LSU includes a semiconductor laser 25, a driving unit of the semiconductor laser 25, a polygon mirror 27, and a horizontal synchronization sensor 30.

The exposure unit 15 is accommodated in a casing 31. A semiconductor laser 25 is provided on one side of the casing 31. The collimator lens 26 is disposed on an extension of a laser beam as an optical signal generated from the semiconductor laser 25. The laser beam generated from the semiconductor laser 25 is focused in the main scanning direction and the sub-scanning direction shown in FIG. 4 as shown in FIG. It becomes parallel to the scanning direction and the sub-scanning direction.

As shown in FIGS. 2 and 3, a polygon mirror 27 provided on an extension of the laser beam that has passed through the collimator lens 26 has a substantially hexagonal prism shape, and the laser beam is directed onto the photoreceptor 11 in the main scanning direction. It is rotatably provided by a motor 32 to deflect horizontally. Each surface of the polygon mirror 27 is formed with a deflecting / reflecting surface 27a. As shown in FIG. 4, each deflection reflection surface 27a
Are each formed in a concave surface curved in an arc shape along the sub-scanning direction, and the curvature of the concave surface is set to S.

When the laser beam focused by the collimator lens 26 is incident on the deflecting / reflecting surface 27a, it is imaged in the sub-scanning direction by the concave surface. At this time,
The distance from the reflection point of the laser beam on the concave surface to the imaging position of the laser beam is set to L. The curvature S
Is smaller, the distance L is shorter, and if the curvature S is larger, the distance L is longer.

As shown in FIGS. 2 and 3, the f-θ lens 28 is disposed on an extension of the laser beam deflected and reflected along the main scanning direction by the deflective reflection surface 27a of the polygon mirror 27. . The f-θ lens 28 includes a first scanning lens 28a and a second scanning lens 28b, and the first and second scanning lenses 28a and 28b scan the laser beam deflected and reflected on the photoconductor 11 in the sub-scanning direction. It has a function of forming an image in the direction.

The f-θ lens 28 has a function of correcting the laser beam deflected and reflected along the main scanning direction so as to scan the laser beam on the photoconductor 11 at an equal angle in the main scanning direction. And a function of correcting the beam diameter in the sub-scanning direction to a constant diameter to correct the curvature of the image surface on the photoconductor 11. As shown in FIG. 2, the f-θ lens 28
The reflection mirror 29 is on the extension of the laser beam passing through
The laser beam that has passed through the f-θ lens 28 is reflected by the reflection mirror 29 and is exposed on the photoconductor 11 in the main scanning direction.

As shown in FIGS. 2 and 3, the horizontal synchronization sensor 30 is installed at a position adjacent to the semiconductor laser 25, and a driving unit of the semiconductor laser 25 and the horizontal synchronization sensor 30 are provided.
Are installed on the same substrate 33. As shown in FIG. 4, the distance from the reflection point of the laser beam on the deflection / reflection surface 27a to the horizontal synchronization sensor 30 is set to L, and an image is formed on the horizontal synchronization sensor 30 in the sub-scanning direction.
A laser beam having a constant beam diameter and a constant output level is always incident.

When the laser beam imaged in the sub-scanning direction is incident on the horizontal synchronization sensor 30, the horizontal synchronization sensor 30 outputs a horizontal synchronization signal to the drive unit. The drive unit that has detected the horizontal synchronization signal counts a certain timing from the time when the output level of the horizontal synchronization signal reaches a predetermined level. At the point in time when the certain timing is counted, the driving unit generates a laser beam from the semiconductor laser 25 to the image forming area on the photoconductor 11, and performs horizontal scanning from a predetermined position on the photoconductor 11 in the main scanning direction. Let it start.

Now, the operation of exposing the photoconductor 11 by the exposing unit 15 having the above LSU will be described. First, when an image forming signal from a host computer or the like (not shown) is input to the control unit, the photoconductor 11, the developing roller 10, and the intermediate transfer belt 18 are driven to rotate, and the exposure unit 15 is driven. The outer peripheral surface of the photoconductor 11 is charged by the roller 12. Next, the drive unit performs control to generate a laser beam by the semiconductor laser 25 in accordance with the image forming signal, and the generated laser beam is focused by the collimator lens 26 in the main scanning and sub-scanning directions, and further rotates. The light is deflected and reflected by the deflection reflection surface 27a of the polygon mirror 27.

At this time, the laser beam imaged in the sub-scanning direction is incident on the horizontal synchronization sensor 30 separated by the distance L by setting the curvature S of the concave surface. Horizontal synchronization sensor 30
When a laser beam is incident on the horizontal synchronization sensor 3
From 0, a horizontal synchronization signal corresponding to the output level of the laser beam is output to the drive unit. Then, the driving unit performs control to generate a laser beam from the semiconductor laser 25 at a certain timing. At this time, since the horizontal synchronization sensor 30 and the drive unit of the semiconductor laser 25 are provided on the same substrate 33, the output level of the output horizontal synchronization signal is input to the drive unit without fluctuating due to noise. .

The laser beam generated in response to the horizontal synchronizing signal is converged by the collimator lens 26 and is deflected and reflected by the polygon mirror 27 toward the f-θ lens 28.
The deflected and reflected laser beam passes through the f-θ lens 28, is reflected by the reflection mirror 29, and is scanned in the main scanning direction from a predetermined horizontal scanning start position on the photosensitive member 11.

As shown in FIG. 5, the laser beam forms an image once in the sub-scanning direction at a position separated by a distance L,
The light enters the scanning lens 28a and the second scanning lens 28b. Therefore, the beam diameter becomes smaller as compared with the case where the laser beam focused by the collimator lens 26 is directly incident on the first and second scanning lenses 28a and 28b, and the processing accuracy of the lens surfaces of the scanning lenses 28a and 28b is reduced. Less susceptible. Further, the laser beam is emitted from the f-θ lens 28.
As a result, an image is formed on the photoconductor 11 in the sub-scanning direction, so that a variation in beam diameter in the sub-scanning direction on the photoconductor 11 is prevented, and field curvature is prevented. In addition, even if the deflecting / reflecting surface 27a is tilted due to the tilting of the polygon mirror 27, the position of the laser beam focused by the concave surface
Since the position where the image is formed on the photoconductor 11 in the sub-scanning direction has a conjugate relationship, deviation of the laser beam in the sub-scanning direction along the main scanning direction is prevented (plane tilt correction).

Then, selective exposure corresponding to the first color (for example, yellow) image forming information is performed by the laser beam, and an electrostatic latent image for yellow is formed. Thereafter, the electrostatic latent image is developed on the photoconductor 11 by a yellow developing roller 10 with yellow toner.
A yellow toner image is formed on the photoconductor 11. After that, development with the other three colors of toner is performed to form a toner image in which the four colors of toner are superimposed.

The effects exerted by the above embodiment will be described below. A concave surface is formed on the deflecting / reflecting surface 27a of the polygon mirror 27, and the laser beam deflected and reflected on the polygon mirror 27 by the concave surface forms an image in the sub-scanning direction. Therefore, the horizontal synchronization sensor 30 is located at the position where the laser beam forms an image.
Is disposed, the laser beam imaged on the horizontal synchronization sensor 30 can be made incident. Therefore, unlike the related art in which the laser beam is formed into an image by the cylindrical lens and is incident on the horizontal synchronization sensor 30, the number of parts of the LSU and the exposure unit 15 can be reduced by omitting the cylindrical lens, and the structure can be simplified. At the same time, an image forming apparatus including the LSU can be manufactured at low cost.

In order to set the same distance between the laser beam passing through the f-θ lens 28 and forming an image on the photoconductor 11 and the distance to the horizontal synchronization sensor 30, the laser beam is reflected. Unlike the related art in which the light is reflected by the mirror and made incident on the horizontal synchronization sensor 30, the reflection mirror is omitted, the number of parts of the LSU and the exposure unit 15 can be reduced, the structure can be simplified, and the LSU is provided. An image forming apparatus can be manufactured at low cost.

The image formation of the laser beam in the sub-scanning direction makes it possible to suppress variations in the beam diameter and output level of the laser beam incident on the horizontal synchronization sensor 30. Therefore, by suppressing variation in the output level of the horizontal synchronization signal depending on the output level of the laser beam, variation in the time from when the laser beam is incident on the horizontal synchronization sensor 30 until the start of horizontal scanning by the semiconductor laser 25 is suppressed. Variations in the horizontal scanning start position on the photoconductor 11 can be prevented.

By adjusting the curvature S of the concave surface,
The distance L from the reflection point of the polygon mirror 27 to the image formation of the laser beam can be adjusted. Therefore, the position of the horizontal synchronizing sensor 30 is
And can be changed in accordance with the internal structure of the image forming apparatus, and the size of the image forming apparatus can be reduced.

Further, by adjusting the curvature S and the position of the horizontal synchronization sensor 30, the horizontal synchronization sensor 3
0 and the drive unit of the semiconductor laser 25 can be provided on the same substrate 33. Therefore, unlike the related art in which the horizontal synchronization sensor 30 and the driving unit are respectively mounted on separate substrates and both are connected by a harness via connectors provided on each substrate, the LSU and the exposure unit are omitted by omitting the connector and the harness. The image forming apparatus can be manufactured at a low cost by simplifying the structure of the image forming apparatus.

Further, unlike the related art in which the horizontal synchronization signal is input to the driving unit from the horizontal synchronization sensor 30 via the connector and the harness, the influence of noise on the horizontal synchronization signal via the connector and the harness can be suppressed. . Therefore, the variation in the time until the output level of the horizontal synchronization signal depending on the noise reaches a predetermined level is suppressed, and the horizontal scanning start position by the semiconductor laser 25 after the laser beam is incident on the horizontal synchronization sensor 30 is kept constant. be able to. Further, since no member is required for suppressing noise, the LSU and the exposure unit 15, that is, the image forming apparatus can be manufactured at low cost.

Since the laser beam formed by the concave surface enters the first and second scanning lenses 28a and 28b,
A laser beam having a smaller beam diameter than when the laser beam focused by the collimator lens 26 enters the first and second scanning lenses 28a and 28b. Therefore, as compared with the case where a laser beam having a large beam diameter is incident, undulation and the like due to the processing accuracy of the lens surface of the f-θ lens 28 can be reduced, and the thickness can be reduced. The manufacturing of the lens 28 can be simplified.

Even if the polygon mirror 27 is tilted, the concave surface prevents the laser beam from shifting in the sub-scanning direction along the main scanning direction on the photoconductor 11. Therefore, unlike the related art in which the deflecting / reflecting surface 27a is formed in a planar shape and the laser beam is displaced in the sub-scanning direction along the main scanning direction on the photoconductor 11 due to tilting of the shaft, pitch unevenness of an obtained image is prevented. can do.

The present embodiment can be embodied with the following modifications. In the embodiment, the horizontal synchronization sensor 30 and the drive unit of the semiconductor laser 25 are installed on the same substrate 33. However, both may be installed on separate substrates 33 and connected via a connector and a harness.

In the embodiment, the concave surface curved in the sub-scanning direction is formed on the deflecting / reflecting surface 27a. However, a concave surface curved in the sub-scanning direction and the main scanning direction may be formed. In such a configuration, the laser beam deflected and reflected by the polygon mirror 27 forms an image on the photoconductor 11 in the main scanning direction and the sub-scanning direction, and the beam diameter and output of the laser beam incident on the horizontal synchronization sensor 30 are output. Level variations can be suppressed.

In the embodiment, the laser beam is imaged on the photoconductor 11 in the sub-scanning direction by the f-θ lens 28, but the laser beam is imaged on the photoconductor 11 at least in the sub-scanning direction. If so, the optical design of the f-θ lens 28 may be changed, or a cylindrical lens or a toric lens may be provided.

Although the semiconductor laser 25 is used as the light source in the embodiment, a He-Ne laser, a He-Cd laser, or an Ar ion laser may be used as the gas laser.

In the embodiment, the polygon mirror 27 is formed to have the deflecting / reflecting surfaces 27a on the six surfaces. However, the number of the deflecting / reflecting surfaces 27a of the polygon mirror 27 may be changed. In the embodiment, the present invention is embodied as a color printer as an image forming apparatus, but may be embodied as a monochrome printing printer, a facsimile, or the like as an image forming apparatus provided with the image carrier of the present invention.

Further, the technical ideas that can be grasped from the above embodiment will be described below. In the image forming apparatus, the light source is provided in an exposure unit that selectively exposes the peripheral surface of the charged image carrier to the peripheral surface, and has a light source unit and a deflecting / reflecting surface. A deflecting section rotatably provided for scanningly deflecting the optical signal generated from the section in the main scanning direction on the image carrier; A horizontal synchronizing sensor that generates a horizontal synchronizing signal, and a driving unit that causes the light source unit to start scanning by an optical signal from a predetermined position on the image carrier when the horizontal synchronizing signal is input,
A concave surface curved at least along the sub-scanning direction so that the optical signal deflected and reflected by the deflecting unit forms an image on a horizontal synchronization sensor in at least a sub-scanning direction which is a direction orthogonal to the main scanning direction. An optical scanning device, wherein: With this configuration, the number of members can be reduced to simplify the structure, and an image forming apparatus including the optical scanning device can be manufactured at low cost.

The distance from the deflecting / reflecting surface to the position where the optical signal deflected and reflected by the deflecting / reflecting surface forms an image at least in the sub-scanning direction and the distance from the deflecting / reflective surface to the horizontal synchronization sensor are set to be the same. The image forming apparatus according to claim 1, wherein: With this configuration, the position of the horizontal synchronization sensor can be changed, and the layout in the optical scanning device and the image forming apparatus can be changed.

[0045]

Since the present invention is configured as described above, it has the following effects. According to the image forming apparatus of the first aspect, it is possible to simplify the structure by reducing the number of members constituting the optical scanning device and to manufacture the optical scanning device at low cost.

According to the image forming apparatus of the second aspect,
In addition to the effects of the invention according to claim 1, unlike the related art in which the horizontal synchronization sensor and the drive unit of the light source unit are respectively installed on separate substrates, and both are connected by a harness via connectors provided on each substrate. By omitting connectors and harnesses, the structure of the optical scanning device can be simplified, and the image forming apparatus can be manufactured at low cost.

[Brief description of the drawings]

FIG. 1 is a schematic diagram illustrating an image forming apparatus according to an embodiment.

FIG. 2 is a partial perspective view showing an exposure unit of the embodiment.

FIG. 3 is a partial plan view showing the LSU of the embodiment.

FIG. 4 is a partial side view showing a state in which a laser beam is incident on a horizontal synchronization sensor.

FIG. 5 is a partial side view showing a state where a laser beam is incident on an f-θ lens.

FIG. 6 is a partial plan view showing a conventional LSU.

[Explanation of symbols]

 11 Photoconductor as Image Carrier 25 Semiconductor Laser as Light Source 27 Polygon Mirror as Deflector 27a Deflection / Reflection Surface 30 Horizontal Synchronous Sensor 33 Substrate

Claims (2)

[Claims] 1. A light source unit and a deflecting / reflecting surface, which are rotatably provided to deflect an optical signal generated from the light source unit by the deflecting / reflecting surface in a main scanning direction on an image carrier. A deflecting unit, a horizontal synchronization sensor that generates a horizontal synchronization signal when an optical signal deflected and reflected by the deflecting unit enters,
An optical scanning device comprising: a driving unit for causing a light source unit to start scanning in a main scanning direction by an optical signal from a predetermined position on the image carrier when the horizontal synchronization signal is input; The deflecting reflecting surface is formed as a concave surface curved at least along the sub-scanning direction so that the formed optical signal forms an image on the horizontal synchronization sensor at least in the sub-scanning direction which is a direction orthogonal to the main scanning direction. Image forming apparatus. 2. The image forming apparatus according to claim 1, wherein the driving unit of the light source unit and a horizontal synchronization sensor are installed on a same substrate.