Measuring device and measuring method

Abstract

A measuring device for determining the laser beam spot diameter on a surface in a laser beam scanning device configured so that a laser beam emitted from a laser light source passes through a scanning optical system equipped with a polygon mirror, reaches the surface of an object, and is used for laser beam scanning on the surface, the measuring device comprising a camera placed in place of the laser light source at the position of the laser light source, and a surface with a pattern consisting of two types of parts placed in place of the object at the position of the object, and configured to determine the laser beam spot diameter at a certain position from the change in brightness of areas corresponding to the two types of parts at that position in the image of the pattern captured by the camera.

Description

[Technical Field]

[0001] The present invention relates to a device and method for measuring the diameter of a laser beam spot in a laser beam scanning device. [Background technology]

[0002] To evaluate the imaging performance of a scanning optical system, it is necessary to measure the spot diameter at each point on the scanning surface of the laser beam. Conventionally, this has been achieved by moving a camera across the scanning surface to acquire an image of the laser beam and measure the spot diameter. When measuring the spot diameter, the camera must be moved not only in the main scanning direction but also in a direction perpendicular to the scanning surface to measure the focal depth. Depending on the path of the laser beam, it may also be necessary to move the camera in the sub-scanning direction. Furthermore, a tilt mechanism must be linked to the above-mentioned movement mechanism to make the camera's sensor light-receiving surface perpendicular to the laser beam. Moving the camera in this way requires a complex and expensive mechanism, and measuring the spot diameter takes time because the camera must be moved over a wide range in the main scanning direction (see, for example, Patent Document 1).

[0003] Therefore, there is a need for a measurement device and a measurement method that can measure the size of the spot diameter at each point on the scanning surface in a short time using a simple mechanism. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2001-013004 Summary of the Invention [Problem to be solved by the invention]

[0005] A technical object of the present invention is to provide a measuring device and a measuring method that can measure the size of the spot diameter at each point on a scanning surface in a short time using a simple mechanism. [Means for solving the problem]

[0006] A first aspect of the measuring device of the present invention is a measuring device that determines the laser beam spot diameter on a surface in a laser beam scanning device configured so that a laser beam emitted from a laser light source passes through a scanning optical system equipped with a polygon mirror, reaches the surface of an object, and is used for laser beam scanning on the surface.The measuring device comprises a camera placed in place of the laser light source at the position of the laser light source, and a surface with a pattern consisting of two types of parts placed in place of the object at the position of the object, and is configured to determine the laser beam spot diameter at a certain position from the change in brightness of areas corresponding to the two types of parts at that position in the image of the pattern captured by the camera.

[0007] The measuring device of this aspect does not require a mechanism for moving the camera in the main scanning direction, which was necessary in conventional methods. Furthermore, since the surface with the pattern has a predetermined width in the sub-scanning direction, the camera tilt mechanism and sub-scanning direction movement mechanism, which were necessary in conventional methods, are also unnecessary. Furthermore, since there is no need to move the camera, the spot diameter at each point in the laser beam scanning range can be determined in a short time. Therefore, the measuring device of this aspect can determine the size of the spot diameter at each point on the scanning surface in a short time using a simple mechanism.

[0008] The measuring device of the first embodiment of the first aspect of the present invention further comprises a processor, and the processor is configured to determine the laser beam spot diameter at a certain position from the change in brightness of areas corresponding to two types of parts at the position in the image of the pattern captured by the camera.

[0009] In the measurement device of the second embodiment of the first aspect of the present invention, the camera is a line scan camera.

[0010] In this embodiment, by using a line scan camera, it is possible to provide a measurement device that can be installed easily and at low cost.

[0011] In the measurement device according to the third embodiment of the first aspect of the present invention, a single mirror that is movable in one direction and rotatable around a central axis is disposed in place of the polygon mirror.

[0012] In this embodiment, when measuring the spot diameter of a scanning optical system with polygon mirrors of various shapes, the measurement can be performed using the single mirror described above, thereby saving the cost and labor required to prepare and install polygon mirrors of various shapes.

[0013] A second aspect of the present invention is a measurement method for determining a laser beam spot diameter on a surface in a laser beam scanning device configured so that a laser beam emitted from a laser light source passes through a scanning optical system equipped with a polygon mirror, reaches the surface of an object, and is used for laser beam scanning on the surface by rotating the polygon mirror. The method involves placing a camera in place of the laser light source at the position of the laser light source, placing a surface equipped with a pattern consisting of two types of parts in place of the object at the position of the object, capturing an image of the pattern with the camera, and determining the laser beam spot diameter at a position corresponding to the two types of parts at the position determined by the rotation angle of the polygon mirror from changes in brightness in areas of the pattern image that correspond to the two types of parts at the position.

[0014] According to the measurement method of this aspect, the camera main scanning direction movement mechanism required in conventional methods is not required. Furthermore, since the patterned surface has a predetermined width in the sub-scanning direction, the camera tilt mechanism and sub-scanning direction movement mechanism required in conventional methods are also not required. Furthermore, since there is no need to move the camera, the spot diameter at each point in the laser beam scanning range can be determined in a short time. Therefore, according to the measurement method of this aspect, the spot diameter at each point on the scanning surface can be determined in a short time using a simple mechanism.

[0015] In the measurement method of the first embodiment of the second aspect of the present invention, the pattern is a plurality of pinholes formed on the surface, and the camera captures an image of the plurality of pinholes. The laser beam spot diameter at a certain position is determined from the change in brightness of the area of ​​the image corresponding to the pinhole at that position and its surroundings, which is determined by the rotation angle of the polygon mirror.

[0016] In the measurement method of the second embodiment of the second aspect of the present invention, the pattern is a checkerboard pattern, and an edge spread function of brightness in areas of the image corresponding to two types of parts at a certain position determined by the rotation angle of the polygon mirror is calculated, and a line spread function is calculated using the edge spread function, and the laser beam spot diameter at a position on the surface of the object corresponding to the position is calculated using the line spread function.

[0017] In the measurement method of the third embodiment of the second aspect of the present invention, the surface is positioned so that the pattern covers the entire range of the laser scan.

[0018] According to this embodiment, the laser beam spot diameter can be obtained at any position within the entire range of laser scanning.

[0019] In the measuring method according to the fourth embodiment of the second aspect of the present invention, the image collection by the camera is carried out in synchronization with the rotation angle of the polygon mirror.

[0020] According to this embodiment, the laser beam spot diameter at a desired position in the laser scanning range can be measured in a short time. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 2 is a diagram illustrating an example of a scanning optical system. [Figure 2] 1 is a diagram showing an example of a laser beam spot diameter measuring device according to the present invention; [Figure 3] 1 is a perspective view showing an example of a laser beam spot diameter measuring device according to the present invention; [Figure 4]FIG. 10 is a diagram showing an example of a surface having a pattern. [Figure 5] FIG. 10 is a diagram showing another example of a surface having a pattern. [Figure 6] 3 is a flowchart illustrating a method for measuring the spot diameter of a laser beam according to the present invention. [Figure 7] 1 in the case where the two types of portions are formed in the checkered pattern shown in FIG. 4. [Figure 8A] FIG. 11 is a plot of the change in brightness in the z-axis direction around a boundary extending in the y-axis direction between two portions of a pattern image. [Figure 8B] FIG. 10 is a diagram showing the edge spread function ESF(z) expressed by equation (2). [Figure 9] FIG. 10 is a diagram showing a normal distribution corresponding to the edge spread function ESF(z) expressed by equation (2). [Figure 10] FIG. 2 is a diagram showing a square cross section perpendicular to the rotation axis of the polygon mirror. [Figure 11] FIG. 10 is a diagram showing the sides of a square whose ends are the point at coordinates (0, r) and the point at coordinates (r, 0). [Figure 12] 12 is a diagram showing a state in which the side shown in FIG. 11 has been rotated clockwise around the origin O by an angle θ. [Figure 13] FIG. 2 is a diagram showing a cross section of a regular pentagon perpendicular to the rotation axis of the polygon mirror. [Figure 14] FIG. 2 is a diagram showing a cross section of a regular pentagon perpendicular to the rotation axis of the polygon mirror. [Figure 15] FIG. 1 is a diagram showing a device that replaces polygon mirrors of various shapes with a single mirror. DETAILED DESCRIPTION OF THE INVENTION

[0022] FIG. 1 shows an example of a scanning optical system. The scanning optical system 100 includes a laser light source 101, an incident optical system 103, a polygon mirror 105, and an imaging optical system (fθ lens) 107. The incident optical system 103 includes a collimator 1031 and a cylinder lens 1033. The laser beam emitted from the laser light source 101 is collimated by the collimator 1031 and focused by the cylinder lens 1033 onto the reflecting surface of the polygon mirror 105 in a plane that includes the optical axis of the incident optical system 103 and is parallel to the direction of the rotation axis of the polygon mirror 105. The reflected light is deflected by the polygon mirror 105 and focused by the fθ lens 107 onto a surface 109 of an object (photosensitive drum). As the polygon mirror 105 rotates, the laser beam scans the surface 109. In this example, the polygon mirror 105 has a square cross section perpendicular to the rotation axis.

[0023] The z-axis is defined as the direction of the rotation axis of polygon mirror 105, the y-axis is defined as the scanning direction of the laser beam, and the x-axis is defined as being perpendicular to the y-axis and z-axis. The y-axis and z-axis directions are also called the main scanning direction and sub-scanning direction, respectively. Figure 1 is a diagram showing a cross section perpendicular to the z-axis and including the optical axis of incident optical system 103.

[0024] To evaluate the imaging performance of the scanning optical system described above, it is necessary to determine the laser beam spot diameter at each point on the surface 109. Conventionally, this has been achieved by moving a camera 201 over the surface 109 to acquire images of the laser beam and measure the spot diameter. To determine the spot diameter, the camera 201 must be moved not only in the y-axis direction (the main scanning direction) but also in the x-axis direction (to measure the focal depth). Depending on the path of the laser beam, it may also be necessary to move the camera 201 in the z-axis direction (the sub-scanning direction). Furthermore, a tilt mechanism must be linked to the above-mentioned movement mechanism to make the sensor light-receiving surface of the camera 201 perpendicular to the laser beam. Moving the camera 201 in this way requires a complex and expensive mechanism. Furthermore, because the camera 201 must be moved over a wide range in the y-axis direction, measuring the spot diameter takes time.

[0025] FIG. 2 is a diagram showing an example of a laser beam spot diameter measurement device 200 according to the present invention. FIG. 2 is a diagram showing a cross section perpendicular to the z-axis and including the optical axis of a cylindrical lens 1033. In the spot diameter measurement device 200, a camera 201 is arranged in place of the laser light source 101, the collimator 1031 is removed, and a patterned surface 203 is arranged in place of the photosensitive drum. The patterned surface 203 will be described later. A light source 205 is arranged on the side of the surface 203 opposite to the side irradiated with the laser beam, irradiating the surface 203 from behind the surface 203. A processor 207 for processing images is connected to the camera 201.

[0026] 3 is a perspective view showing another example of a laser beam spot diameter measuring device 200 according to the present invention. The cross section of the polygon mirror 105 in this example, taken perpendicular to the rotation axis, has a regular hexagonal shape.

[0027] FIG. 4 is a diagram showing an example of a patterned surface 203. The pattern in FIG. 4 is a checkered pattern. As an example, the checkered surface 203 may be a chrome mask commercially available from Equa Co., Ltd. The dimensional accuracy of the above chrome mask is smaller than the diffraction limit, which will be explained later. The Equa Co., Ltd. website is as follows: https: / / equa-web.jp / pages / 27 / Alternatively, surface 203 may be formed by a display in which the pixel spacing is smaller than the diffraction limit, as will be explained later.

[0028] Figure 5 shows another example of a patterned surface 203. The pattern in Figure 5 is a number of pinholes arranged at regular intervals. The diameter of the pinholes is set to be smaller than the diffraction limit. The diffraction limit (diameter) can be expressed by the following equation, where λ is the wavelength of light and NA is the numerical aperture of the imaging optical system. d=1.22·(λ / NA) In the case of the imaging optical system of a scanning optical system, the value of d is about 50 micrometers. As an example, a pinhole with a diameter of 1 micrometer is commercially available from ThorLabs. The ThorLabs website is as follows: https: / / www.thorlabs.co.jp / thorproduct.cfm?partnumber=P1HK By providing a plurality of pinholes at regular intervals in, for example, a stainless steel plate using a method similar to that for the pinholes described above, a surface 203 having the pattern shown in FIG. 5 can be obtained.

[0029] The above pattern is composed of two types of parts, and the two types of parts are configured to have different brightness values ​​when used. In the case of the checkered pattern in Figure 4, the two types of parts are the parts represented by white and the parts represented by black. In the case of the pattern formed by multiple pinholes in Figure 5, the two types of parts are the parts represented by pinholes and the rest.

[0030] Generally, surface 203 is disposed in place of an object such as a photosensitive drum, and is, for example, the surface of a plate-like object. Surface 203 is disposed perpendicular to the x-axis. A light source 205 is disposed on the side of the object having surface 203 opposite to the side irradiated with the laser beam, and illuminates surface 203 from behind the object.

[0031] FIG. 6 is a flow chart illustrating a method for measuring the spot diameter of a laser beam according to the present invention.

[0032] In step S1010 of FIG. 6, a camera 201 is placed in place of the laser light source 101 at the position of the laser light source 101, and a surface 203 having a pattern consisting of two types of parts is placed in place of an object such as a photosensitive drum at the position of an object such as a photosensitive drum.

[0033] In step S1020 of FIG. 6, the camera 201 captures an image of the pattern on the surface 203 having a pattern consisting of two types of parts.

[0034] In step S1030 of Figure 6, the laser beam spot diameter at a certain position is determined from the change in brightness of areas of the pattern image corresponding to two types of portions at that position, which are determined by the rotation angle of polygon mirror 105. The rotation angle of polygon mirror 105 is the angle around the rotation axis of polygon mirror 105, and this rotation angle determines the position of the laser beam spot in the y-axis direction on surface 109 in scanning optical system 100. In step S1020, if image acquisition by camera 201 is performed in synchronization with the rotation angle of polygon mirror 105, the laser beam spot diameter at a desired position in the laser scanning range of surface 203 can be measured in a short time. A rotary encoder, for example, may be used to synchronize image acquisition by camera 201 with the rotation angle of polygon mirror 105.

[0035] FIG. 7 is a flowchart for explaining step S1030 in FIG. 1 when the two types of portions are formed in the checkered pattern shown in FIG.

[0036] In step S2010 of FIG. 7, the edge spread function of brightness in the area corresponding to two types of portions at a certain position in the pattern image is calculated.

[0037] FIG. 8A is a plot of the change in brightness in the z-axis direction around the boundary extending in the y-axis direction between two types of portions in a pattern image. The horizontal axis of FIG. 8A represents the position in the z-axis direction (sub-scanning direction). The unit of the horizontal axis is millimeters. The vertical axis of FIG. 8A represents the relative brightness value. The reason that multiple points exist at approximately the same coordinate along the horizontal axis in FIG. 8, which plots the change in brightness, is because the change in brightness along the horizontal axis (z-axis direction) was plotted at multiple positions on the boundary (side of the rectangle) extending in the y-axis direction in the image corresponding to one rectangle of the checkerboard pattern. The spot diameter in the sub-scanning direction is calculated using the data shown in FIG. 8A. To calculate the spot diameter in the main scanning direction (y-axis direction), a plot of the change in brightness in the y-axis direction around the boundary extending in the z-axis direction between two types of portions in the pattern image with different brightness values ​​is used instead of FIG. 8A.

[0038] In the method shown in FIG. 7, since the change in brightness in the y-axis direction or the z-axis direction is observed, the camera 201 may be a line scan camera in the y-axis direction or the z-axis direction.

[0039] The edge spread function ESF(z) is calculated by fitting the relationship between the horizontal and vertical axis values ​​of the multiple points plotted in Figure 8A with a function of z. Here, the relationship between the vertical and horizontal axis values ​​of the multiple points is fitted with the cumulative distribution function CDF of a normal distribution. Specifically, the edge spread function ESF(z) is expressed by the following equation, and a, b, c, and d are calculated by fitting.

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[0040] Fig. 8B is a diagram showing the edge spread function ESF(z) expressed by equation (2). The horizontal axis of Fig. 8B represents the position in the z-axis direction (sub-scanning direction). The unit of the horizontal axis is millimeters. The vertical axis of Fig. 8B represents the relative value of brightness.

[0041] 7, the line spread function is calculated using the edge spread function. Since the derivative of the cumulative distribution function CDF is a normal distribution function, the line spread function LSF(z) can be expressed by the following equation when the maximum value is normalized to 1:

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[0042] FIG. 9 is a diagram showing a normal distribution corresponding to the edge spread function ESF(z) expressed by equation (2). The horizontal axis in FIG. 9 indicates the position in the z-axis direction (sub-scanning direction). The unit of the horizontal axis is millimeters. The vertical axis in FIG. 9 indicates the relative value of the normal distribution value corresponding to the brightness. As mentioned above, the maximum value of the normal distribution is 1.

[0043] In step S2030 of FIG. 7, the line spread function LSF(z) is used to calculate the diameter of the laser beam spot at the position corresponding to the position on the surface of the object. The value on the vertical axis is the peak value.

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[0044] When the two types of portions of the pattern are formed by a pinhole as shown in Figure 5, the spot diameter can be determined by creating a graph from an image of the pinhole and its surroundings, where the horizontal axis indicates the position in the y-axis direction (main scanning direction) or z-axis direction (sub-scanning direction) and the vertical axis indicates the relative value of brightness.

[0045] The patterned surface 203 is installed so that the pattern is distributed over the entire scanning range of the laser beam in the y-axis direction by the scanning optical system 100, and therefore the spot diameter of the laser beam at any position within the scanning range of the laser beam can be determined from an image of the pattern at that position according to step S1030 in FIG. 1. Therefore, the mechanism for moving the camera in the y-axis direction, which was necessary in the conventional method, is not required. Furthermore, since the patterned surface 203 has a predetermined width in the z-axis direction, the camera tilt mechanism and mechanism for moving the camera in the z-axis direction, which were necessary in the conventional method, are also not required. Furthermore, since there is no need to move the camera, the spot diameter at each point within the scanning range of the laser beam can be determined in a short time.

[0046] To measure the depth of focus, the surface 203 may be moved in the x-axis direction using a linear guide or the like.

[0047] The cross section perpendicular to the rotation axis of the polygon mirror 105 in Figure 2 is a square. There are cases where it is desired to evaluate the imaging performance of a scanning optical system using a polygon mirror 105 whose cross section perpendicular to the rotation axis is a regular pentagon or regular hexagon. In such cases, preparing polygon mirrors of various shapes and installing them in a measurement device would require additional cost and effort. Therefore, we developed a device that replaces polygon mirrors of various shapes with a single mirror in a measurement device. This device is described below.

[0048] FIG. 10 is a diagram showing a square cross section perpendicular to the rotation axis of polygon mirror 105. The shape of the cross section perpendicular to the rotation axis of polygon mirror 105 is square. In the cross section shown in FIG. 10, the position of the rotation center of polygon mirror 105 is taken as the origin O, and if one vertex of the square in the cross section of polygon mirror 105 is located on the y-axis, the coordinate of that vertex can be expressed as (0, r). Furthermore, the coordinate of one of the other vertices of the square in the cross section of polygon mirror 105 can be expressed as (r, 0). Here, r is the radius of the circumscribing circle of the square centered at the origin. In the state shown in FIG. 10, the coordinate of position P of the reflection point of the chief ray of the laser beam is expressed as (d, y).

[0049] 11 is a diagram showing the sides of a square whose ends are the point at coordinate (0, r) and the point at coordinate (r, 0). The straight lines corresponding to the above sides can be expressed by the following equations. y=-x+r Therefore, the coordinates of the reflection point P are (d, rd) is.

[0050] 12 is a diagram showing a state in which the side shown in FIG. 11 is rotated clockwise by an angle θ around the origin O. The coordinates of the point at coordinates (0, r) in FIG. 10 after rotation are

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[0051] As a result, the position of the reflection point of the laser beam's chief ray moves in the y-axis direction before and after the rotation. In other words, by moving the mirror in the y-axis direction according to the angle θ in accordance with equation (3), the change in the position of the reflection point due to the rotation of the polygon mirror can be reproduced.

[0052] Next, a case where the cross section of the polygon mirror perpendicular to the rotation axis has a regular polygon shape other than a square will be described.

[0053] Figure 13 is a diagram showing a cross section of a regular pentagon perpendicular to the rotation axis of a polygon mirror. In the state shown in Figure 13, light rays are reflected in the same way as in Figure 10. In Figure 13, a square is shown by a dashed line, with sides represented by straight lines that indicate the reflecting surfaces of the regular pentagon and with a center of rotation that coincides with the center of rotation of the regular pentagon.

[0054] Fig. 14 is a diagram showing a regular pentagonal cross section perpendicular to the rotation axis of a polygon mirror. The reflecting surface in the state shown in Fig. 14 is rotated clockwise by θ around the center of rotation O, with the reflecting surface in the state shown in Fig. 13 as the reference. Therefore, in the state shown in Fig. 14, light rays are reflected in the same way as in Fig. 12. In Fig. 14, a square is shown by a dashed line, whose sides are represented by straight lines that indicate the regular pentagonal reflecting surface and whose center of rotation coincides with the center of rotation of the regular pentagon.

[0055] The square shown in Fig. 14 is rotated clockwise by θ around the center of rotation O with respect to the square shown in Fig. 13. Therefore, the reflection in the case of a regular pentagon can be replaced with the reflection in the case of a square.

[0056] In FIG. 13, if the radii of the inscribed circles of the regular pentagon and square are h, the radius of the circumscribed circle of the regular pentagon, r, can be expressed by the following equation.

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[0057] In general, the following equation for a polygon mirror having a regular polygonal cross section with N sides can be derived from equation (3) for a polygon mirror having a square cross section:

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[0058] Figure 15 is a diagram showing a device that replaces polygon mirrors of various shapes with a single mirror. The device shown in Figure 15 includes a mirror that can rotate around a central axis and a movement mechanism, and the mirror is attached to the movement mechanism so that a point on the central axis can move along a line perpendicular to the central axis. The device shown in Figure 15 is installed so that the central axis is in the z-axis direction and the line is in the y-axis direction, and by moving the mirror in the y-axis direction according to the angle θ, it is possible to reproduce the change in the position of the reflection point due to the rotation of the polygon mirror.

[0059] The spot diameter values ​​obtained by the method shown in FIG. 6 were within ±5% of the spot diameter values ​​obtained by the conventional method.

Claims

1. 1. An apparatus for evaluating the imaging performance of a scanning optical system of a laser beam scanning device in which a laser beam emitted from a laser light source passes through a scanning optical system equipped with a polygon mirror, reaches a surface of an object, and is used for laser beam scanning on the surface, comprising: a camera disposed in place of the laser light source at the position of the laser light source of the laser beam scanning device; a mechanism for causing a change in brightness at the position of the object of the laser beam scanning device, the mechanism being other than the laser light source; An apparatus configured to indirectly determine the laser beam spot diameter at a certain position, which is determined by the imaging performance, from a change in brightness at that position in an image captured by the camera.

2. 2. The apparatus according to claim 1, further comprising a processor configured to indirectly determine a laser beam spot diameter at a certain position of an image, the diameter being determined by the imaging performance, from a change in brightness at the position.

3. 2. The apparatus of claim 1, wherein the camera is a line scan camera.

4. 2. The apparatus according to claim 1, wherein the polygon mirror is replaced by a single mirror that is movable in one direction and rotatable around a central axis.

5. An apparatus as described in claim 1, wherein the mechanism for causing the change in brightness is composed of a surface having a pattern and a light source for illuminating said surface.

6. An apparatus as described in claim 1, wherein the mechanism for causing the change in brightness is composed of a surface having a pinhole and a light source for illuminating said surface.

7. The device described in claim 1, wherein the mechanism that causes the change in brightness is constituted by a display.

8. A method for evaluating the imaging performance of a scanning optical system of a laser beam scanning device in which a laser beam emitted from a laser light source passes through a scanning optical system equipped with a polygon mirror, reaches a surface of an object, and is used for scanning the surface with the laser beam by rotating the polygon mirror, the method comprising: a camera is disposed at the position of the laser light source of the laser beam scanning device in place of the laser light source; causing a change in brightness at the position of the object on the laser beam scanning device by a mechanism other than the laser light source; capturing an image of the change in brightness with the camera; A method for indirectly determining the laser beam spot diameter at a certain position on the image, which is determined by the imaging performance, from the change in brightness at that position.

9. A method for producing a change in brightness at the position of an object by using a surface with a pinhole and a light source that illuminates the surface, capturing an image of the pinhole with the camera; 9. The method according to claim 8, wherein the laser beam spot diameter at a certain position is determined from a change in brightness of a region of the image corresponding to a pinhole at that position and its periphery, the change being determined by the rotation angle of the polygon mirror.

10. A patterned surface and a light source illuminating the surface cause a change in brightness at the position of the object, The camera captures an image of the change in brightness; determining an edge spread function of brightness in a region of the image corresponding to a change in brightness at a certain position determined by the rotation angle of the polygon mirror; determining a line spread function using the edge spread function; 9. The method of claim 8, further comprising using the line spread function to determine a laser beam spot diameter on the surface of the object at a position corresponding to the position, the diameter being determined by the imaging performance.

11. 9. The method according to claim 8, wherein the image acquisition by the camera is performed in synchronization with the rotation angle of the polygon mirror.

Images