JP2005062712A - Beam scanner - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a beam scanner for controlling a deviation of the main scanning magnification of a beam by using one synchronous detection sensor. <P>SOLUTION: The beam scanner comprises an LD unit 1 provided with one or more LDs, a polygon mirror 2 for deflecting the beam from the LD unit 1 and scanning it on a face to be scanned 4 at an equal speed in a main scanning direction, and an fθ lens 3 for image-forming the beam deflected from the polygon mirror 2 as the beam spot on the face to be scanned 4. Planar mirrors 6a and 6b as an optical path modification means for modifying a beam optical path are arranged out of the effective exposure region of the face to be scanned of a scanning start side and a scanning completion side, and the beams of the scanning start side and the scanning completion side are detected with one synchronous detection sensor 5. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a beam scanning device used in an image forming apparatus such as a digital copying machine, a laser printer, and a laser facsimile, and an image forming apparatus including the beam scanning apparatus.

  In general, a beam scanning device scans a beam emitted from a beam light source device having a laser light emitting unit (hereinafter referred to as LD) as a beam spot by a scanning optical system including a cylinder lens, a polygon mirror, an fθ lens, and the like. An image is formed on the surface, and the polygon mirror is rotated so that the surface to be scanned is scanned at a constant speed in the main scanning direction. In particular, a multi-beam scanning device having a plurality of LDs can quickly scan the surface to be scanned and form an electrostatic latent image without increasing the number of rotations of the polygon mirror, compared to scanning using a single beam. Therefore, it is widely used in image forming apparatuses such as copying machines, laser printers, and laser facsimiles.

  In such a beam scanning apparatus, plastic is used instead of the conventional glass as the material of the fθ lens in accordance with the recent demand for cost reduction and weight reduction. However, plastic lenses are more susceptible to environmental and temporal changes than glass lenses and are liable to adversely affect the accuracy of images formed. Therefore, it is necessary to correct the scanning start timing in the main scanning direction of each beam deflected by the polygon mirror in accordance with the change in the characteristic of the fθ lens. Patent Document 1 discloses a multi-beam scanning device provided with a synchronization detection sensor for setting the scanning start timing of each beam, as shown in FIG.

  In FIG. 4, an LD unit 1 having a plurality of LDs emits a beam from each LD based on an instruction from an LD drive unit (not shown), and the emitted beam is emitted from a collimator lens (not shown) or The light passes through a cylinder lens (not shown) and enters the polygon mirror 2. The polygon mirror 2 has an accurate polygon shape, rotates at a constant speed in a constant direction, deflects the beam from the LD unit 1, and scans the scanned surface 4 at a constant speed in the main scanning direction. In this example, it is assumed that the polygon mirror 2 is a regular hexagon and rotates clockwise at a constant speed. The beam deflected by the polygon mirror 2 enters the fθ lens 3. The fθ lens 3 focuses the beam from the LD unit 1 on the scanned surface 4 as a plurality of beam spots.

  The synchronization detection sensor 5 is a beam detecting means for detecting each beam deflected by the polygon mirror 2 and is outside the effective exposure area of the surface to be scanned on the scanning start side in order to set the scanning start timing of each beam in the main scanning direction. Is provided. By shifting the time to the synchronization detection sensor 5 and making a beam incident from a plurality of LDs, the synchronization detection signal of each beam is individually generated, and based on this, the scanning start timing of each beam is matched with high accuracy.

However, in the above multi-beam scanning device, since the synchronization detection sensor 5 is provided only on the scanning start side, information on the entire writing width cannot be obtained, and the writing width shift caused by the change in the main scanning magnification is corrected. I can't. In view of this, a method for correcting a deviation in writing width by providing a plurality of synchronization detection sensors has been proposed. In Patent Documents 2 and 3, the synchronization detection sensors are arranged on the scanning start side and the scanning end side as shown in FIG. A multi-beam scanning device arranged in two places is disclosed.
Japanese Patent Laid-Open No. 9-66630 JP 2002-96502 A JP 2002-122799 A

  5 is the same as the configuration of FIG. 4 except that two synchronization detection sensors 5 are arranged on the scanning start side and the scanning end side outside the effective exposure area of the surface to be scanned. The scanning start timing of each beam is set by measuring the timing at which each beam enters the synchronization detection sensor 5 on the scanning start side. Further, the time difference between the timing at which the synchronization detection sensor 5 on the scanning start side and the scanning end side detects the beam, that is, the scanning time of each beam, is measured and compared with the reference time to thereby reduce the writing width deviation of each beam. to correct.

  By adopting such a configuration, a plurality of beams can be controlled by controlling a change in main scanning magnification of a plurality of beams and a shift in scanning start timing due to a characteristic change of an fθ lens due to a difference in wavelength due to a variation in LD or a change in environmental temperature. Thus, the scan width and the scan start position can be made to coincide with each other, and image deterioration can be prevented. However, in Patent Documents 2 and 3, since the synchronization detection sensors are arranged in two places, it is necessary to secure the arrangement space of the synchronization detection sensors, and two expensive sensors are required, so that the cost is also low. Disadvantageous.

  In view of the above problems, the present invention provides a beam scanning apparatus capable of controlling a change in main scanning magnification of a beam and a shift in scanning start timing using a single synchronization detection sensor, and an image forming apparatus including the beam scanning apparatus. With the goal.

  To achieve the above object, the present invention provides a scanning optical system that deflects and scans a surface to be scanned with a beam emitted from a light source unit, and a beam detection that detects a beam scanned in the main scanning direction by the scanning optical system. A beam that detects the beam on each of the scanning start side and the scanning end side outside the effective exposure area of the surface to be scanned using the beam detection unit, and uses the detection signal for synchronization in the main scanning direction. In the scanning apparatus, the optical path changing means for changing the beam optical path is arranged so that the beam on the scanning start side and the scanning end side is detected by one beam detecting means.

  According to the present invention, in the beam scanning apparatus configured as described above, the optical path changing means is provided on a scanning start side and a scanning end side outside the effective exposure region of the surface to be scanned.

  According to the present invention, in the beam scanning apparatus having the above-described configuration, the optical path changing unit has a beam optical path distance from the light source unit to the beam detecting unit that is substantially the same as a beam optical path distance from the light source unit to the scanned surface. It is characterized by being provided at a position where

  According to the present invention, in the beam scanning apparatus configured as described above, a scanning time measurement is performed in which the scanning time of each beam is measured using the beam detection timing on the scanning start side and the beam detection timing on the scanning end side detected by the beam detection means. And a correction means for correcting the scanning magnification by comparing the scanning time with a reference time.

  According to the present invention, in the beam scanning apparatus having the above-described configuration, the time from the beam detection timing on the scanning start side to the beam detection timing on the scanning end side, and from the beam detection timing on the scanning end side to the beam detection timing on the scanning start side. By providing a difference in time, the scanning time measuring means can select only the time from the beam detection timing on the scanning start side to the beam detection timing on the scanning end side.

  According to the present invention, in the beam scanning apparatus configured as described above, the optical path changing means is a mirror.

  According to the first configuration of the present invention, the change in the main scanning magnification of the beam is changed by using only one synchronization detection sensor as in the case where the synchronization detection sensors are provided at two positions on the scanning start side and the scanning end side. It can be controlled with high accuracy.

  According to the second configuration of the present invention, in the beam scanning apparatus of the first configuration, the optical path changing means is provided on the scanning start side and the scanning end side, so that the position of the synchronization detection sensor is scanned. It can be installed to be the same distance as the distance to the surface, and the detection accuracy of the synchronous detection sensor is improved.

  Further, according to the third configuration of the present invention, in the beam scanning device of the first or second configuration, the optical path changing unit is configured such that the beam optical path distance from the light source unit to the beam detection unit is from the light source unit to be scanned. By providing at a position that is substantially the same as the beam optical path distance to the surface, the detection accuracy of the synchronous detection sensor is improved.

  According to the fourth configuration of the present invention, in the beam scanning apparatus having the first to third configurations, each beam is detected using the beam detection timing at the start of scanning and the beam detection timing at the end of scanning. The scanning time can be measured, and the scanning time can be compared with the reference time to correct the scanning magnification.

  Further, according to the fifth configuration of the present invention, in the beam scanning apparatus of the fourth configuration, only the time from the beam detection timing on the scanning start side to the beam detection timing on the scanning end side can be measured. The scanning time of each beam can be easily measured by the scanning time measuring means, and the scanning magnification can be corrected more easily by comparing the scanning time with the reference time.

  According to the sixth configuration of the present invention, in the beam scanning device having any one of the first to fifth configurations, the beam path can be changed easily and at low cost by using the optical path changing means as a mirror. be able to.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a configuration diagram of a beam scanning apparatus according to the first embodiment and a block diagram of an image forming apparatus including the beam scanning apparatus. Portions common to FIG. 4 of the conventional example are denoted by the same reference numerals and description thereof is omitted. In the present embodiment, a plane mirror is disposed as an optical path changing means for changing the beam optical path outside the effective exposure area of the scanning start side and the scanning end side, and both the beams at the start and end of the scan are arranged. Detection is performed by one synchronization detection sensor. In the following embodiments, a multi-beam scanning device including a plurality of light sources will be described. However, the configuration is exactly the same even when there is one light source.

  A method for correcting a change in main scanning magnification according to the present embodiment will be described. The following correction is performed at a timing at which beam irradiation based on an image signal is not performed on the photoconductor (scanned surface). In FIG. 1, a beam emitted from a specific LD of the LD unit 1 is incident on a polygon mirror 2. Here, the polygon mirror 2 is a regular hexagon, and rotates at a constant speed in the clockwise direction. However, the polygon mirror 2 is not limited to a regular hexagon, and other shapes can be used as long as they are regular polygons. Further, the rotation direction and rotation speed of the polygon mirror 2 can be appropriately set according to the specifications of the beam scanning device.

  The beam deflected by the polygon mirror 2 enters the fθ lens 3 and then forms an image as a beam spot on the surface to be scanned 4 and is scanned in the main scanning direction (arrow direction in the figure). At this time, the beam is detected on the scanning start side and the scanning end side outside the effective exposure area of the surface to be scanned 4 using the synchronization detection sensor 5, but the beam on the scanning start side is reflected by the plane mirror 6a and synchronized. The beam incident on the detection sensor 5 and reflected on the scanning end side is reflected by the plane mirror 6 b and enters the synchronization detection sensor 5. The synchronization detection sensor 5 outputs a signal to the time counting unit 7 according to the timing at which the beam is detected. By measuring the detection timing of the synchronization detection sensor 5 on the scanning start side, that is, the side where the image signal starts to be written, and the scanning end side, ie, the side where the image signal is finished, at this time, the time counting unit 7 measures the scanning of each beam. Time can be measured. In the present embodiment, the synchronization detection sensor 5 is disposed outside the effective exposure area of the scanned surface on the scanning start side. However, the synchronization detection sensor 5 affects the scanning end side or scanning of the scanned surface. It is also possible to place it in other places that are not.

  For example, if the synchronization detection sensor 5 is disposed at the intersection of a beam incident on the scanning start mirror 6a from the fθ lens 3 in FIG. 1 and a beam incident on the synchronization detection sensor 5 from the scanning end mirror 6b, scanning is performed. It is also possible to employ a configuration in which the start side mirror 6a is not provided. However, since the beam diameter is set to be the narrowest on the surface to be scanned 4, in order to improve the detection accuracy of the synchronization detection sensor 5, the beam distance from the LD unit 1 to the synchronization detection sensor 5 is as low as possible. It is desirable to install the synchronization detection sensor 5 at a position where the beam distance from the unit 1 to the scanned surface 4 is the same. On the other hand, since it is often difficult to arrange the synchronization detection sensor 5 in the vicinity of the scanned surface 4, the synchronization detection sensor 5 is provided from the LD unit 1 by providing a mirror 6 a on the scanning start side and turning the beam back. It is preferable to adopt the configuration of the present embodiment in which the beam distance up to can be matched with the beam distance to the scanned surface 4. In this embodiment, the beam optical path on the scanning start side and the scanning end side is changed using the two mirrors 6a and 6b. However, if the two beams are detected by the single synchronization detection sensor 5, the mirror is used. The number of arrangements and the arrangement positions are not limited to this.

  As the synchronization detection sensor 5, various optical sensors such as a photodiode, a phototransistor, and a photo IC can be used. Although only the case where a plane mirror is used as the optical path changing unit is shown here, an optical path changing unit such as a prism can be used instead of the plane mirror.

FIG. 2 is a timing chart showing the relationship between the output signal from the synchronization detection sensor and the measurement time. As can be seen from the figure, since the beam is detected by one synchronization detection sensor 5 in the present invention, the time T ₁ from the start of writing (scanning start side) to the end of writing (scanning end side), and from the end of writing. Two types of time, T ₂ until writing start, are measured. Therefore, by providing a clear time difference between T ₁ and T ₂ in advance, only the scanning time T ₁ from writing start (scanning start side) to writing end (scanning end side) necessary for correcting the scanning magnification can be easily performed. Can be selected.

The scanning time T ₁ can be measured continuously for each LD. Usually, in order to reduce the variation in measurement, the scanning time T ₁ is measured a plurality of times for one LD, and the scanning time is obtained from the average value. Similarly, the scanning time is obtained for the LDs. In this case, it is more preferable to provide a time for completely shutting off the output from the synchronization detection sensor during the measurement for each LD because it is possible to easily determine which LD the measured time is. Here, the scan time is determined by providing a clear time difference between the two types of times T ₁ and T ₂ in advance, but other determination methods, for example, the measured order is odd or even numbered. You may make it discriminate | determine two types of time according to whether there exists.

  The scanning time measured by the time counting unit 7 is transferred to the comparison control unit 8. The comparison control unit 8 includes data on the reference scanning time, compares the transferred scanning time with the reference time, and calculates the ratio of deviation from the reference time. The video clock generated from the reference clock generator 9 is corrected in the write clock generator 10 in accordance with the ratio of the deviation. For example, when the measured scanning time is longer than the reference time, the beam position is closer to the swing angle of the standard polygon mirror 2, so that the image signal is output later and the image is extended. There is a need. Therefore, the video clock is delayed, that is, the frequency of the video clock is lowered in accordance with the ratio of the scanning time deviation. On the contrary, when the measured scanning time is shorter than the reference time, the video clock is set earlier, that is, the frequency of the video clock is increased in accordance with the ratio of the scanning time deviation.

  In this way, the write clock is generated by correcting the frequency of the reference video clock for each LD, and transmitted to the LD driver 11. The LD driving unit 11 converts the frequency of the image signal transmitted from the image signal generating unit (not shown) to the LD driving unit 11 to the writing clock frequency, thereby matching the main scanning magnifications of the LDs and degrading the image. Can be prevented.

  Although the setting of the scanning start position of each LD is not described in detail here, the starting position can be set according to the beam detection timing on the scanning start side detected by the synchronization detection sensor 5. When the response speed of the synchronization detection sensor 5 is sufficiently larger than the interval of each beam in the main scanning direction and the scanning equivalent speed, an accurate synchronization detection signal can be obtained for each beam, and the scanning start timing is independent for each LD. Can be set to When the synchronization detection sensor 5 having a relatively low response speed is used, a synchronization detection signal is obtained from the beam of a specific LD, and the other LDs are shifted by a predetermined time from the scanning start timing of the specific LD. It is desirable to provide timing setting means. Examples of the timing setting means include a correspondence table of delay times accompanying temperature changes, and based on this, the scanning start positions of the LDs can be matched.

FIG. 3 is a flowchart showing a main scanning magnification correction procedure in the image forming apparatus according to the present embodiment. The procedure for correcting the main scanning magnification of each LD according to the steps of FIG. 3 will be described in more detail with reference to FIG. First, the LDs of the LD unit 1 are turned on one by one at a timing at which image formation is not performed on the surface to be scanned 4 such as a photoconductor, and the detection timing of writing start (scanning start side) and the end of writing (scanning end side). By detecting the detection timing with the synchronization detection sensor 5, the time counting unit 7 measures the scanning time (T ₁ in FIG. 2) from the start of writing to the end of writing (step 1). The measured scanning time T ₁ is transmitted to the comparison control unit 8.

Next, the comparison control unit 8 compares the scanning time T ₁ with the reference time T ₀ (step 2). Here, first, it is determined whether T ₁ and T ₀ are substantially equal (step 3). If T ₁ and T ₀ are substantially equal, the process is terminated, and the reference clock from the reference clock generator 9 becomes the write clock at the same frequency.

If T ₁ and T ₀ are different, it is determined whether T ₁ is smaller than T ₀ (step 4). When T ₁ is smaller than T ₀ , the image is enlarged in the main scanning direction. Therefore, the frequency of the reference clock is equal to the ratio of the shift amount of T _{1 to} the reference time T ₀ in the write clock generator 10. Is generated (step 5).

On the other hand, when T ₁ is larger than T ₀ , the image is reduced in the main scanning direction, so that the write clock generation unit 10 uses the reference for the ratio of the shift amount of T _{1 to} the reference time T ₀ . A write clock with a reduced clock frequency is generated (step 6).

In step 3, whether T ₁ and T ₀ are substantially equal is determined to be normal if it is within an allowable magnification error range, and the clock frequency is set when the time difference is more than that. This is because correction is performed only when practically necessary to speed up the process.

  An example of correction in the image forming apparatus including the beam scanning apparatus of FIG. 1 will be specifically described with reference to Table 1. Here, the fθ lens 3 has a focal length of 200 mm when the beam wavelength is 780 nm, the resolution of the scanned surface 4 is 600 dpi, the process speed is 200 mm / sec, the number of polygon mirrors 2 is 6, and scanning is started. The image height for detecting the side beam and the end beam is ± 160 mm. The LD unit 1 is assumed to include two LDs, LD1 and LD2.

基準の走査速度が０．９８９４７８ｍｍ／μｓｅｃであるとき、走査開始−終了間距離（走査幅）は１６０ｍｍ×２＝３２０ｍｍより、走査開始−終了間の基準時間Ｔ₀は３２０（ｍｍ）／０．９８９４７８（ｍｍ／μｓｅｃ）＝３２３．４０３μｓｅｃである。いま、ＬＤ１の波長が７８５ｎｍ、ｆθレンズ３の焦点距離が２００．００２ｍｍであり、計測された走査時間Ｔ₁が３２３．４００μｓｅｃとなった場合、Ｔ₁は基準時間Ｔ₀に対し９９．９９９０％である。走査時間Ｔ₁が基準時間Ｔ₀より短いということは、走査速度が速いということであり、書込クロックを早くして画像を縮める必要がある。

When the reference scanning speed is 0.989478 mm / μsec, the scanning start-end distance (scanning width) is 160 mm × 2 = 320 mm, and the scanning start-end reference time T ₀ is 320 (mm) / 0. 9898478 (mm / μsec) = 323.403 μsec. Now, when the wavelength of LD1 is 785 nm, the focal length of the fθ lens 3 is 200.002 mm, and the measured scanning time T ₁ is 323.400 μsec, T ₁ is 99.9990% with respect to the reference time T _0. It is. That the scanning time T ₁ is shorter than the reference time T ₀ means that the scanning speed is fast, and it is necessary to shorten the image by increasing the writing clock.

Therefore, the reference video clock may be corrected by the ratio of T _{1 to} the reference time T ₀ . That is, 23.37350 MHz / 99.9990 (%) = 23.37373 MHz. Further, by performing the same correction for LD2, the scanning widths of LD1 and LD2 can be set to the reference values.

  The present invention includes a scanning optical system that deflects and scans a surface to be scanned with a beam emitted from a light source unit, and a beam detection unit that detects a beam scanned in the main scanning direction by the scanning optical system, In a beam scanning device that uses a beam detection means to detect a beam on each of a scanning start side and a scanning end side outside the effective exposure area of the surface to be scanned and uses the detection signal for synchronization in the main scanning direction, The optical path changing means to be changed is arranged so that the beam on the scanning start side and the scanning end side is detected by one beam detecting means. As a result, a beam scanning device that uses only one synchronization detection sensor to control the change in the main scanning magnification of the beam with high accuracy in the same manner as when the synchronization detection sensors are provided at two locations on the scanning start side and the scanning end side. Can be provided. In addition, since the degree of freedom of the location where the synchronization detection sensor is arranged also increases, it contributes to the reduction in weight, size and cost of the beam scanning device.

  In order to improve the detection accuracy of the synchronization detection sensor, it is desirable to install the synchronization detection sensor at a position where the beam optical path distance to the synchronization detection sensor is as equal to the beam optical path distance to the scanned surface as much as possible. By providing the changing means on the scanning start side and the scanning end side, the synchronization detection sensor can be installed at a position where the beam optical path distance is substantially the same as the surface to be scanned, and the detection accuracy of the synchronization detection sensor is improved.

  In addition, the detection accuracy of the synchronization detection sensor is improved by providing the optical path changing means at a position where the beam optical path distance from the light source unit to the synchronization detection sensor is substantially the same as the beam optical path distance from the light source unit to the scanned surface. It can also be made.

  Also, a scanning time measuring means for measuring the scanning time of each beam using the beam detection timing on the scanning start side and the beam detection timing on the scanning end side detected by the beam detection means, and comparing the scanning time with the reference time By providing correction means for correcting the scanning magnification, the scanning time of each beam is measured using the beam detection timing on the scanning start side and the beam detection timing on the scanning end side, and the scanning time is compared with the reference time for scanning. It becomes possible to correct the magnification.

  In addition, since the beam is detected by one synchronization detection sensor, there are 2 from the start of writing (scan start side) to the end of writing (scan end side) and from the end of writing (scan end side) to the start of writing (scan start side). The time from the beam detection timing on the scanning start side to the beam detection timing on the scanning end side and the time from the beam detection timing on the scanning end side to the beam detection timing on the scanning start side will be measured. By providing a difference between the beam detection timing and the beam detection timing on the scanning start side to the beam detection timing on the scanning end side, it is easier to measure the scanning time of each beam by the scanning time measuring means. Thus, the scanning magnification can be corrected more easily.

  Further, by using a mirror as the optical path changing means, the optical path changing means has a simple configuration, and a compact and low-cost beam scanning apparatus can be provided.

  In addition, since the light source unit includes a plurality of light sources, it is possible to provide a multi-beam scanning device capable of controlling a change in main scanning magnification of a plurality of beams by using one synchronization detection sensor.

  In addition, by providing an image forming apparatus provided with the beam scanning device having the above-described configuration, it is possible to prevent image deterioration due to variations in the wavelength of the LD and environmental temperature, and to achieve good image formation, and to reduce the weight of the image forming apparatus. Contributes to downsizing, downsizing and cost reduction.

These are the block diagram of the beam scanning apparatus of 1st Embodiment of this invention, and an image forming apparatus provided with the same. These are timing charts which show the relationship between the output signal from the synchronous detection sensor used for this embodiment, and measurement time. These are the flowcharts which show the correction procedure of the main scanning magnification in the image forming apparatus of this embodiment. These are the schematic diagrams which show the structure of the conventional beam scanning apparatus. These are the schematic diagrams which show the structure of the other conventional beam scanning apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 LD unit 2 Polygon mirror 3 f (theta) lens 4 Scanned surface 5 Synchronization detection sensor 6a Plane mirror 6b Plane mirror 7 Time count part 8 Comparison control part 9 Reference clock generation part 10 Write clock generation part 11 LD drive part

Claims (6)

A scanning optical system that deflects and scans a surface to be scanned with a beam emitted from a light source unit; and a beam detection unit that detects a beam scanned in the main scanning direction by the scanning optical system. In the beam scanning device that detects the beam on each of the scanning start side and the scanning end side outside the effective exposure area of the scanned surface, and uses the detection signal for synchronization in the main scanning direction,
An optical path changing unit for changing a beam optical path is arranged so that the beam on the scanning start side and the scanning end side is detected by one beam detecting unit.   2. The beam scanning apparatus according to claim 1, wherein the optical path changing means is provided on a scanning start side and a scanning end side outside the effective exposure region of the surface to be scanned.   The optical path changing unit is provided at a position where a beam optical path distance from the light source unit to the beam detecting unit is substantially the same as a beam optical path distance from the light source unit to the scanned surface. The beam scanning apparatus according to claim 1 or 2.   Scanning time measuring means for measuring the scanning time of each beam using the beam detection timing on the scanning start side and the beam detection timing on the scanning end side detected by the beam detection means, and comparing the scanning time with a reference time. 4. The beam scanning apparatus according to claim 1, further comprising a correction unit that corrects the scanning magnification.   The scanning time measurement is performed by providing a difference between the time from the beam detection timing on the scanning start side to the beam detection timing on the scanning end side and the time from the beam detection timing on the scanning end side to the beam detection timing on the scanning start side. 5. The beam scanning apparatus according to claim 4, wherein only the time from the beam detection timing on the scanning start side to the beam detection timing on the scanning end side can be selected in the means.   6. The beam scanning apparatus according to claim 1, wherein the optical path changing unit is a mirror.

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