JP2013117395A - Inclination measuring device and method thereof - Google Patents

Abstract

An error of a measurement result can be suppressed regardless of an inclination angle of a measurement target.
An irradiation unit includes a light emitting device and a cylindrical polarizer, and irradiates a measurement object with circularly polarized light through the polarizer. The imaging unit 20 captures the specularly reflected light from the measurement target 1 with the camera 21 through the analyzer 22. The analysis unit 30 detects the polarization state of the specularly reflected light using the output of the imaging unit 20, and calculates the angle formed by the reflection part of the specularly reflected light in the measurement object 1 with respect to the prescribed reference plane from the polarization state. . The analysis unit 30 obtains the inclination angle on the surface of the measurement object 1 by correcting the angle calculated from the polarization state of the reflected light with the relationship determined for each angle.
[Selection] Figure 1

Description

  The present invention relates to a tilt measuring apparatus and method for measuring a tilt angle of a measurement target by irradiating the measurement target with light in a polarization state and analyzing the polarization state of specularly reflected light from the measurement target.

  Conventionally, as a technique for measuring the tilt angle of a measurement target, a technique called tilt ellipsometry is known in which a measurement target is irradiated with light in a polarization state and the polarization state of specularly reflected light from the measurement target is analyzed ( For example, see Patent Document 1). Patent Document 1 describes a technique for measuring the tilt angle of an object by irradiating an object to be measured from a light source device with circularly polarized light and detecting elliptically polarized light that is reflected light with a polarization detector. ing. In the technique described in the cited document 1, a reflection sensor is attached to an object. The angle of the reflection sensor can be adjusted using an angle adjustment mechanism, and is adjusted so that circularly polarized regular reflection light irradiated on the object from the light source device is incident on the polarization detector.

JP 2011-106920 A

  If the technique described in Patent Document 1 is adopted, it is possible to measure the inclination angle of the part where the reflection sensor is attached to the object to be measured. However, when there are a plurality of parts with different inclination angles on the surface of the measurement target, it is difficult to provide a reflection sensor at each part with different inclination angles. If a plurality of reflection sensors are provided, the inclination angle is measured for each individual reflection sensor, and thus it takes a long time to measure the inclination angle.

  When the measurement target is a material having a metallic luster, it is possible to omit the reflection sensor, but the technique described in Patent Document 1 still measures the inclination angles of a plurality of parts individually. It is difficult to reduce the time required for measurement.

  Therefore, it is conceivable to employ a configuration in which circularly polarized light is irradiated on the entire measurement target and an image of the measurement target is captured as a polarization detector. In this case, for example, it is conceivable to uniformly irradiate circularly polarized light on the entire surface of the measurement object using a dome-shaped (hemispherical) irradiation means. However, it is difficult to uniformly irradiate circularly polarized light on the entire surface to be measured by forming the polarizer in a dome shape.

  Therefore, a light emitting device whose light emitting surface is a dome shape or hemisphere surrounding the measurement target, and a relatively simple polarizer such as a cylindrical shape may be disposed between the light emission device and the measurement target. It is considered. The polarizer irradiates the measurement object with circularly polarized light by emitting light incident perpendicularly to the surface as circularly polarized light.

  In the above configuration, when the polarizer is cylindrical, light incident on the polarizer along a plane perpendicular to the mouth axis of the polarizer is circularly polarized, so that the tilt angle of the measurement target is accurately measured. . That is, when the measurement target has a specific tilt angle, it is possible to make circularly polarized regular reflection light incident on the polarization detector.

  However, when this configuration is adopted, it has been found that when the measurement target is not a specific inclination angle, an error occurs in the measurement result of the inclination angle. Such an error is due to the fact that when the measurement object is not at a specific tilt angle, the light incident as a specularly reflected light on the polarization detector is not circularly polarized when the measurement object is irradiated. Conceivable.

  An object of the present invention is to provide an inclination measuring apparatus and method capable of suppressing errors in measurement results regardless of the inclination angle of a measurement target.

  An inclination measuring apparatus according to the present invention includes an irradiating unit that irradiates a measurement target with circularly polarized light, an imaging unit that images regular reflected light from the measurement target through an analyzer, and the specular reflection using an output of the imaging unit. Analyzing means for detecting a polarization state of light, and calculating an angle formed by a reflection part of the specularly reflected light in the measurement object with respect to a prescribed reference plane from the polarization state, and the analyzing means comprises polarization of reflected light An inclination angle on the surface of the measurement target is obtained by correcting the angle calculated from the state with a relationship determined for each angle.

  In this inclination measuring apparatus, the irradiating means is disposed between a light emitting apparatus that irradiates light to the measuring object from all directions surrounding an optical axis of the imaging means, and the light emitting apparatus and the measuring object. It is preferable that the imaging unit includes a polarizer having a circular cross section perpendicular to the optical axis and emitting light incident perpendicularly to the surface as circularly polarized light.

  In this tilt measuring apparatus, the polarizer is more preferably cylindrical.

  In this inclination measuring apparatus, the relationship is a regression obtained from a plurality of combinations of the angle calculated from the polarization state of the reflected light and the known inclination angle of the object with respect to an object whose inclination angle is known. It is preferably represented by the formula.

  The tilt measurement method according to the present invention irradiates the measurement target with circularly polarized light from the irradiation unit, images regular reflection light from the measurement target through the analyzer with the imaging unit, and reflects the reflected light reflected by the measurement target. In calculating the angle formed by the analyzing means with respect to the prescribed reference plane by the part that reflected the reflected light in the measurement object from the polarization state, the angle calculated from the polarization state of the reflected light is a relationship determined for each angle. The inclination angle on the surface of the measurement object is obtained by correcting with the above.

  According to the configuration of the present invention, since the angle calculated based on the specularly reflected light received by the imaging unit is corrected by the relationship determined for each angle, the inclination angle of the measurement object is obtained. Regardless of the angle, there is an advantage that the occurrence of errors can be suppressed.

It is a schematic block diagram which shows embodiment. It is a figure explaining a problem same as the above. It is a figure which shows the relationship between a calculated angle and an actual angle in the same as the above. It is a figure which shows the relationship between the angle after correction | amendment, and a measurement error in the same as the above.

  As shown in FIG. 1, the tilt measurement apparatus according to the present embodiment includes an irradiation unit 10 that irradiates the measurement target 1 with circularly polarized light, and an imaging unit 20 that captures specularly reflected light from the measurement target 1. It is preferable that the measurement object 1 has a high reflectance like a metal. The output of the imaging means 20 is given to the analysis means 30. The analysis unit 30 detects the polarization state of the specularly reflected light from the measurement target 1 using the output of the imaging unit 20, and uses the polarization state detected for the reflective part of the specularly reflected light, and the reflection site is a prescribed reference plane. An angle formed with respect to is calculated. Further, as will be described later, the analysis unit 30 has a function of obtaining an inclination angle for each reflection site in the measurement target 1 by correcting the calculated angle. In the following description, it is assumed that the reference plane is a plane parallel to the upper surface of the table on which the measurement object 1 is placed and is set to be orthogonal to the optical axis of the imaging unit 20. It is also possible to set so as to be inclined with respect to the optical axis of the means 20.

  The irradiation unit 10 is configured so that circularly polarized light can be irradiated onto a portion of the measurement target 1 that exists in the field of view of the imaging unit 20. As shown in FIG. 1, the irradiation unit 10 of the present embodiment includes a light emitting device 11 that irradiates light to the measurement target 1 from all directions surrounding the optical axis of the imaging unit 20, the light emission device 11, the measurement target 1, and the like. And a polarizer 12 disposed between the two.

  The light emitting device 11 has a hemispherical or dome-shaped light emitting surface surrounding the measurement target 1, and irradiates light almost uniformly on the surface of the measurement target 1 in the field of view of the imaging means 20. The light source used for the irradiating unit 10 is preferably lit using a DC stabilized power supply so that the light output does not fluctuate while the imaging unit 20 is imaging. In addition, the irradiation unit 10 may be configured to either continuously irradiate the measurement object 1 with light or to output light only for a specified short time to irradiate the measurement object 1 with flash light. When the irradiating means 10 generates a flash, the light emission amount is controlled so that the light amount for each light emission does not change.

  The polarizer 12 has a configuration in which a polarizing plate that converts light from the light emitting device 11 into linearly polarized light and a quarter-wave plate are overlapped, and the light emitted from the light emitting device 11 is circularly polarized. Convert. In this type of polarizer 12, a circularly polarizing film having flexibility formed in a sheet shape is processed into a desired shape, or a large number of non-flexible optical elements are formed into a desired shape. It is formed by arranging. The polarizer 12 in the illustrated example is formed in a cylindrical shape, and is arranged so that the direction of the mouth axis is parallel to the optical axis of the imaging means 20. The polarizer 11 may basically be rotationally symmetric with the optical axis of the imaging unit 20 as the axis of symmetry, but preferably has a circular cross section perpendicular to the optical axis of the imaging unit 20. That is, the polarizer 12 preferably has a truncated cone shape other than the cylindrical shape. In addition, it is also possible to use the polarizer 12 whose cross section is not circular, such as a polygonal cross section. However, if the cross section of the polarizer 12 is a polygonal shape, a region where circularly polarized light cannot be obtained is formed at the corner portion, so that the application is limited.

  The end face of the polarizer 12 is open, and an opening window 13 is formed in a portion of the light emitting device 11 that intersects the axis of the polarizer 12 in the mouth axis direction so that the measuring object 1 can be directly viewed from the imaging means 20. ing. The imaging means 20 receives the specularly reflected light from the measurement object 1 surrounded by the polarizer 12 through the opening window 13.

  The imaging unit 20 includes a camera 21 including an imaging element (not shown) in which pixels are arranged on lattice points of a two-dimensional lattice, such as a CCD image sensor or a CMOS image sensor, and between the measurement target 1 and the camera 21. And an analyzer 22 arranged at the same position. The camera 21 includes a light receiving optical system, and causes reflected light from the measurement target 1 to enter the imaging element through the light receiving optical system. The camera 21 outputs a grayscale image with pixel values as grayscale values. If necessary, a filter may be provided in front of the camera 21 to reduce environmental light other than light emitted from the light emitting device 11. An interference filter is preferably used for this type of filter.

  The analyzer 22 is disposed so as to be rotatable around a rotation center parallel to the optical axis of the camera 21, and is driven by a rotating device 23 having a driving source such as a motor. The analyzer 22 has a reference position for rotation, and an angle sensor 24 is attached to the analyzer 22 in order to measure the rotation angle of the analyzer 22 with respect to the reference position. As the angle sensor 24, a rotary encoder, a potentiometer, or the like that detects the rotation angle of the analyzer 22 is used. Further, the angle sensor 24 may be arranged to directly detect the rotation from the analyzer 22 or may be arranged to indirectly detect the rotation of the analyzer 22 from the rotating portion of the rotating device 23. Good.

  The imaging unit 20 outputs the grayscale image captured by the camera 21 in association with the rotation angle of the analyzer 22 measured by the angle sensor 24. That is, the output of the imaging means 20 is an image captured by the camera 21 when the rotation angle of the analyzer 22 detected by the angle sensor 24 is a predetermined angle, and as a result, a combination of the rotation angle and the image. Become. In other words, the pixel value for each pixel of the camera 21 reflects the polarization state for each reflection part of the regular reflection light in the measurement target 1.

  The output of the imaging means 20 is given to the analysis means 30. The analysis unit 30 includes a processor (for example, a microcomputer) that executes a program for realizing the functions described below as a hardware element. A storage unit 31 is provided for storing a set of an image acquired from the imaging unit 20 and a rotation angle of the analyzer 22 at the time of imaging. For example, the rotation angle of the analyzer 22 is changed in a range of 0 to 170 degrees at intervals of 10 degrees. In this case, the storage unit 31 stores a total of 18 images for each rotation angle.

  By the way, it is known that the received light intensity received by the imaging means 20 in the above-described configuration periodically changes according to the rotation angle of the analyzer 22. If attention is paid to one pixel of the image picked up by the image pickup means 20, the gray value at 18 points changes in a sine wave shape. Using the phase, amplitude (one half of the difference between the maximum and minimum values), and the center value (average value of the maximum and minimum values) obtained by changing the gray value for each pixel, an ellipse is obtained. It becomes possible to calculate the ellipticity angle of polarization (the arctangent of ellipticity) and the direction of the axis (azimuth angle).

  Further, the phase at which the received light intensity received by the imaging unit 20 changes varies according to the inclination angle formed by the reflection part that generates specularly reflected light in the measurement object 1 with respect to the prescribed reference plane. This is because the circularly polarized light applied to the measurement target 1 is specularly reflected according to the angle incident on the surface of the measurement target 1 to become elliptically polarized light having an axis with a direction corresponding to the tilt angle. The ellipticity angle of the elliptically polarized light is determined by the inclination angle of the measurement object 1, and the azimuth angle of the elliptically polarized light is determined by the azimuth angle of the measurement object 1. The azimuth angle of the measuring object 1 means an angle around the optical axis of the imaging means 20.

  As described above, since the ellipticity angle and the azimuth angle of the elliptically polarized light can be converted into the inclination angle and the azimuth angle of the measurement object 1, when the relationship between the rotation angle of the analyzer 22 and the pixel value for each pixel is obtained, The tilt angle and azimuth angle of the part corresponding to the pixel in the measurement object 1 can be calculated. That is, when the surface of the measuring object 1 is divided into minute surfaces according to the resolution of the imaging means 20 (camera 21), a normal vector for each minute surface is obtained based on the inclination angle and azimuth angle for each minute surface. . Further, when the normal vector for each minute surface is obtained, the three-dimensional shape of the measurement object 1 can be obtained by connecting the minute surfaces.

  Now, consider a case where the angle of inclination of the reflection part of the measurement object 1 with respect to the reference plane is approximately 45 degrees. In this case, the light that is emitted from the light emitting device 11 and is specularly reflected on the surface of the measurement object 1 and then enters the camera 21 enters the polarizer 12 at approximately 0 degrees as shown in FIG. Only the light that plays. Here, it is assumed that the surface of the measurement object 1 has a metallic luster and no diffuse reflection occurs. In the drawing, a reference plane is indicated by a symbol PL. When the traveling direction of light incident on the polarizer 12 is approximately 0 degrees, this light is incident on the surface of the polarizer 12 at a substantially right angle. Become polarized.

  On the other hand, when the reflection part in the measurement object 1 has an inclination angle of about 30 degrees with respect to the reference plane PL, only light incident on the polarizer 12 at about 30 degrees is shown in FIG. After being specularly reflected on the surface of the measurement object 1, the light enters the camera 21. When the traveling direction of light incident on the polarizer 12 is approximately 30 degrees, the light is incident on the surface of the polarizer 12 with an inclination. Therefore, it is considered that the light irradiated to the measurement object 1 does not become completely circularly polarized light but becomes elliptically polarized light having an ellipticity corresponding to the incident angle to the polarizer 12. Note that it is assumed that the azimuth angle of elliptically polarized light does not depend on the incident angle to the polarizer 12.

  Since the above-described event occurs, the tilt angle of the measurement target 1 obtained from the ellipticity angle of elliptically polarized light causes an error corresponding to the incident angle of light to the polarizer 12. In other words, the angle of the measurement object 1 calculated from the ellipticity angle of elliptically polarized light has an error corresponding to the calculated angle. And this error is considered to originate in the light irradiated to the measurement object 1 being elliptically polarized light instead of circularly polarized light.

  In consideration of the above circumstances, the analysis unit 30 corrects the angle by an angle calculation unit 32 that calculates an angle for each reflection part in the measurement target 1 from the output of the imaging unit 20 stored in the storage unit 31. And an angle correction unit 33 for obtaining an inclination angle for each reflection part in the measurement object 1. That is, since the angle obtained by the angle calculation unit 32 from the image captured by the camera 21 and the rotation angle of the analyzer 22 includes an error, the angle correction unit 33 performs correction so as to remove this error. Then, an actual inclination angle related to the measurement object 1 is calculated.

  The relationship between the angle calculated by the angle calculation unit 32 and the inclination angle of the measurement object 1 will be described below. FIG. 3 shows the relationship between the angle calculated by the angle calculation unit 32 and the tilt angle of the measurement target 1 for the measurement target 1 whose tilt angle with respect to the reference plane is known. In FIG. 3, the characteristic (1) indicates the angle calculated by the angle calculation unit 32, and the characteristic (2) indicates the true inclination angle of the measurement target 1. The irradiation means 10 and the imaging means 20 are arranged as shown in FIG. Moreover, the angle of the measuring object 1 was adjusted using the gonio stage. As can be seen from FIG. 3, the error of the angle calculated by the angle calculation unit 32 increases as the tilt angle of the measurement object 1 increases, but the relationship is almost linear.

  According to the above-described result, when the relationship determined according to the angle calculated by the angle calculation unit 32 is applied, the true inclination angle of the measurement target 1 is obtained. That is, the angle correction unit 33 has a function of correcting the angle calculated by the angle calculation unit 32 with a relationship determined for each angle.

  For example, if the angle calculated by the angle calculation unit 32 is θ with respect to the actual inclination angle Θ of the measurement object 1, the relationship between the angle θ and the inclination angle Θ is linear as θ = aΘ−b. It can be expressed by a regression equation. Here, a and b are constants. Therefore, the inclination angle Θ obtained by the angle correction unit 33 with respect to the angle θ calculated by the angle calculation unit 32 can be obtained as Θ = (θ + b) / a. For example, if a = 1.4 and b = 18, when the calculation result of the angle θ is 32 degrees, the inclination angle Θ is 35.7 degrees.

  The regression equation is basically a linear relationship, but a regression equation having another relationship can also be used. Further, instead of using the regression equation, a data table in which the angle and the inclination angle are associated with each other in the angle correction unit 33 can be used. In this case, since the angle value is a discrete value in the data table, for an angle that does not exist in the data table, an interpolation operation is performed to obtain an inclination angle.

  Now, it is assumed that the data table is represented by a pair of (angle, inclination angle), and data (24, 30) (31, 35) (38, 40) (45, 45) exists. Here, if the angle obtained by the angle calculation unit 32 is 32 degrees, the inclination angle is obtained as {(40−35) / (38−31)} (32−31) + 35 = 35.7 degrees.

  Here, when the measurement error was obtained for the tilt angle corrected by the angle correction unit 33, the result shown in FIG. 4 was obtained. In the range where the angle calculated by the angle calculation unit 32 is small, the variation in measurement error tends to increase, but the inclination angle error is within ± 2 degrees.

  As described above, since the angle calculated by the angle calculation unit 32 and the actual inclination angle of the measurement object 1 are linearly related, the relationship between the angle and the inclination angle is determined as a regression equation from a plurality of combinations of both. . That is, it is preferable that the angle correction unit 33 represents the relationship between the angle calculated by the angle calculation unit 32 and the actual inclination angle of the measurement target 1 by a regression equation.

  In setting such a regression equation, the case where the polarizer 12 has a cylindrical shape is compared with the case where it has another shape. When the inclination angle of the measuring object 1 is in the range of 0 to 90 degrees, when the polarizer 12 is cylindrical, the angle range is symmetric with respect to 45 degrees, whereas other shapes of the polarizer 12 are used. When used, the range of angles becomes asymmetric. As shown in FIG. 4, since the measurement error tends to increase as the angle difference with respect to the light incident perpendicularly to the polarizer 12 increases, the measurement error is suppressed when the cylindrical polarizer 12 is used. It will be. Of course, when the range of the tilt angle is limited in the measurement object 1, if the polarizer 12 having a shape matched to the range is used, the measurement error may be suppressed in the other shape of the polarizer 12. is there.

  The analysis means 30 calculates | requires the inclination angle of the site | part corresponding to each pixel about the measuring object 1 from the output of the imaging means 20 in the procedure mentioned above. Further, the obtained inclination angle is output through the output unit 34 provided in the analysis means 30. The output unit 34 outputs the inclination angle for each pixel (that is, the reflection part in the measurement object 1) as a numerical value, and sets the inclination angle for each section divided by appropriate angle intervals (5 degree increments, 10 degree increments, etc.). It is also possible to encode and output the code in association with each pixel. The output unit 34 has a function of dividing the surface of the measurement object 1 into minute surfaces, obtaining a normal vector for each minute surface, and outputting the three-dimensional shape of the surface of the measurement object 1 based on the normal vector. You may have.

DESCRIPTION OF SYMBOLS 1 Measurement object 10 Irradiation means 11 Optical radiation apparatus 12 Polarizer 20 Imaging means 22 Analyzer 30 Analysis means PL Reference plane

Claims (5)

An irradiation means for irradiating the measurement object with circularly polarized light;
Imaging means for imaging specularly reflected light from the measurement object through an analyzer;
Analyzing means for detecting a polarization state of the specularly reflected light using an output of the imaging means, and calculating an angle formed by a reflection part of the specularly reflected light in the measurement object with respect to a prescribed reference plane from the polarization state; Prepared,
The inclination measuring apparatus characterized in that the analysis means obtains an inclination angle on the surface of the measurement target by correcting the angle calculated from the polarization state of the reflected light with a relationship determined for each angle. The irradiation means includes
A light emitting device for irradiating the measurement object with light from all directions surrounding the optical axis of the imaging means;
A cross-section perpendicular to the optical axis of the imaging means arranged between the light emitting device and the measurement object is circular, and includes a polarizer that emits light perpendicularly incident on the surface as circularly polarized light. The tilt measuring apparatus according to claim 1. The inclination measuring device according to claim 2, wherein the polarizer has a cylindrical shape. The relationship is represented by a regression equation obtained from a plurality of combinations of the angle calculated by the analysis unit from the polarization state of reflected light and the known tilt angle of the object with respect to an object having a known tilt angle. The inclination measuring apparatus according to any one of claims 1 to 3, wherein While irradiating the object to be measured with circularly polarized light from the irradiating means, imaging the specularly reflected light from the object to be measured through the analyzer with the imaging means
In calculating the angle made by the analysis means with respect to a prescribed reference plane from the polarization state of the reflected light reflected by the measurement object, the part that reflected the reflected light in the measurement object,
A tilt measurement method, wherein the angle calculated from the polarization state of the reflected light is corrected by a relationship determined for each angle to determine the tilt angle on the surface of the measurement target.

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