JPH0829664A - Method for detecting position of measured object - Google Patents

Abstract

PURPOSE:To provide a method for detecting the position of a measured object which is excellent in detecting accuracy even when diffracted light exists and hardly requires the adjustment of an optical axis. CONSTITUTION:This method is constituted so that the position of the measured object 1 is detected by forming an axially symmetric mirror image by synthesizing both light beams after dividing a reflected light beam into two, reversing at least either light beam so that either light beam and the other light beam may become the mirror image and making the light intensity of both beams identical; and comparing output from each photodiode at the time of receiving the synthesized light beam by a quadripartite photodiode 2 in the case of detecting the position of the measured object 1 by irradiating the measured object with the light beam and comparing the output from each photodiode at the time of receiving the reflected light beam from the measured object by the quadripartite photodiode 2. By such constitution, the detecting accuracy is excellent even when the diffracted light exists and the adjustment of the optical axis is hardly required.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of detecting a position of a measuring object, and more particularly to a method of detecting a position of a measuring object used in a roughness meter or a focus error circuit in an automatic focusing circuit.

[0002]

2. Description of the Related Art Various methods are known as a method for automatically detecting the focus position of a pickup lens used for an optical disk, a CD or the like. For example, the astigmatism method, critical angle method, knife edge method, and Foucault method can be mentioned, and all of them are highly sensitive methods that can be used for minute displacement sensors. Further, both use a two-division or four-division photodiode, and obtain the focus error signal by taking the differential of the light quantity output. Here, the most basic method, the astigmatism method, will be outlined (see Toyohiko Yatagai, Introduction to Applied Optical Measurement, p120, Maruzen Publishing). Astigmatism means that the focal position of the lens is 2 on the image plane.
It refers to aberrations that have different values in more than one direction, and the astigmatism method uses that property. Here, as shown in FIG. 11A, astigmatism is created by the spherical lens 51 and the cylindrical lens 52. In the y direction of FIG. 11A, the cylindrical lens 52 acts as a flat plate and has no light focusing effect. However,
In the x direction, the focusing effects of both the spherical lens 51 and the cylindrical lens 52 overlap. Therefore, the divergent light from the point B on the object side has different focusing distances on the image side in the x direction and the y direction. That is, the light collecting distance in the x direction is closer to the lens than the light collecting distance in the y direction. In FIG. 11A, a position where light is collected in the x direction is set as P point, and a position where light is collected in the y direction is set as Q point. The cross-sectional shape of the light beam changes from the vertical direction to the horizontal direction from point P to point Q. In the meantime, there is a position where the cross-sectional shape of the light beam is a perfect circle, and that position is designated as point S. When the light spot moves closer to the spherical lens 51 on the optical axis from the position of the point B to the point A, the cross-sectional shape of the light beam at the point S changes vertically. On the contrary, when it moves to the point C, the cross-sectional shape of the light beam at the point S changes to be horizontally long. A 4-division photodiode is placed so that the optical axis coincides with the position of point S. Then, the light beam becomes as shown in FIG. That is, each photodiode output of the four-division photodiode is a, b, c, d, and a, c, b
And d are the outputs of the diagonal photodiode,
A focus error signal as shown by the following equation is obtained.

[0003]

[Equation 1] FIG. 12 shows changes in the output of the focus error signal e given by the above equation 1 with respect to the distance of the light spot from the spherical lens 51. In optical discs, CDs, etc., the pickup lens is made to follow the light spot so that the focus error signal e becomes 0 to realize automatic focusing. Further, this astigmatism method can be applied as a roughness meter by utilizing the fact that the focus error signal e gives the displacement of the object. In this case, since light is used as a contact terminal, it has the feature that it can be measured in a non-contact manner, and is called the optical contact method. The roughness meter using the optical stylus method has a vertical roughness measuring range of 1 μm and a resolution of 2 n.
With performance of m (Kimiyuki Mitsui et al., Development of high-sensitivity non-contact roughness meter, PP136-141, Journal of Japan Society for Precision Engineering 53/2
/ 1987). By the way, most of the objects to be measured by such a roughness meter are rough surfaces. Therefore, especially when measuring a large cross-section curve such as a turned surface or a step having a sharp edge, strong light may be incident on a specific element of the four-division photodiode due to the influence of diffracted light, which may cause a measurement error. Therefore, in the optical probe method using the divided photodiodes, a method of removing the influence of diffracted light by taking a differential using two divided photodiodes is used. FIG. 13 shows a position detection device A01 of a measuring object used in such a roughness meter.

As shown in FIG. 13, this conventional device A01
Then, the laser light from the laser diode 61 is passed through the lens 6
2, the polarized beam is projected through the polarization beam splitter 63, the quarter-wave plate 64, and the lens 65 onto the measurement object 66. The reflected light is condensed by the lens 65, converted into S-polarized light by the quarter-wave plate 64, and reflected by the polarization beam splitter 63. Then, it is divided into two by the beam splitter 67 to form two beams, and these beams are made to reach two four-divided photodiodes A and B through the cylindrical lenses 68 and 69 which are rotated by 90 degrees with respect to each other. . Due to the arrangement of the cylindrical lenses 68 and 69, the four-divided photodiodes A and B receive a beam image in which one is vertically long and the other is horizontally long. The differential signal S of the four-division photodiodes A and B at this time is expressed by the following equation.

[Equation 2] Here, A1, A2, A3 and A4 are photodiodes A
, B1, B2, B3, B4 represent the outputs of the respective light receiving elements of the photodiode B. When there is no influence of diffracted light, the absolute values of the first term and the second term in the above equation 2 are the same and the signs are different. Therefore, the actuation signal S is larger than that of a single 4-division photodiode The signal output is doubled. Further, when the diffracted light is in the state as shown in FIG. 14, for example, the diffracted light intensity component is added to the output signals of A1 and A3 and B1 and B3 of the four-division photodiodes A and B, respectively. Both the first and second terms in 2 become large. If the signal intensity of the diffracted light on the four-division photodiode A is EA, the focus error signal output of only the four-division photodiode A is given by the following equation.

(Equation 3)

From the above equation 3, the focus error signal in the presence of diffracted light is 1 / (1 + k) times the output of the focus error signal in the absence of diffracted light, and k / (k
The signal will have an offset of +1). Therefore, when there is diffracted light, the focal point does not become 0 only by the single 4-division photodiode. However, the operation signal S for the 4-division photodiode B is given by the following equation.

[Equation 4] At this time, since the light beams reaching the four-division photodiodes A and B have exactly the same output ratio of the diffracted light and the original signal light, k and j in the above equations 4 and 6 have the same values. Show. Therefore, the above equation 5 is as follows.

(Equation 5) The first and second terms in the parentheses in the above equation 7 have the same absolute value and different signs, so that the following equation is obtained.

(Equation 6) As described above, in the conventional device A01, the output is 2 / (1 + k) times the focus error signal of the single unit, and the offset is canceled. Therefore, the influence of diffracted light is reduced.

[0006]

Among the conventional methods for detecting the position of a measuring object as described above, for example, in the astigmatism method,
When the measurement target is at the in-focus position when the four-division photodiode is attached or during maintenance adjustment, the value of Equation 1 above, which is the photodiode output, is set to 0. It is necessary to adjust the two directions for adjusting the three axes. This optical axis adjustment is a troublesome work and takes time. In the conventional device A01, 2
Since four 4-division photodiodes are used and the actuation signal S is taken, this troublesome optical axis adjustment is required only twice. However, also in this conventional device A01, if disturbance light such as diffracted light is incident on a specific photodiode during adjustment, the focus error signal of the four-division photodiode has an offset as shown in Equation 3 above. Therefore, the method of aligning the optical axis by setting the output of the 4-divided photodiode to 0 makes the optical axis adjustment incomplete. In order to solve such a problem in the prior art, the present invention improves the method for detecting the position of a measuring object, can detect the position with high accuracy even in the presence of diffracted light, and requires little optical axis adjustment. It is an object of the present invention to provide a method of detecting a measurement target.

[0007]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention is directed to each measuring element when a measuring object is irradiated with light and the reflected light is received by a detector composed of a plurality of detecting elements. In the method for detecting the position of the measuring object by comparing the outputs from the above, the reflected light is divided into two, and at least one light is inverted so that one light and the other light are mirror images of each other, After making the light intensities of both of the above lights the same, create a mirror image of the axis symmetry by combining the two lights, and compare the output of each light detecting element when the combined light is received by the detector. The position detecting method is configured to detect the position of the measuring object. Furthermore, it is a position detection method of a measurement target in which at least one of the two split lights is reflected by a mirror and the other light is reflected by a rectangular prism.

[0008]

According to the present invention, by irradiating light on the object to be measured and comparing the output from each light detecting element when the reflected light is received by the detector consisting of a plurality of light detecting elements, When detecting the position, the reflected light is split into two,
At least one light is inverted so that the one light and the other light are mirror images of each other. Then, after the light intensities of the both lights are made the same, the two lights are combined to create a mirror image of the axis symmetry. The position of the measurement target is detected by comparing the outputs of the respective light detecting elements when the combined light is received by the detector. Therefore, even if there is diffracted light, the focus error output becomes 0, so that the detection accuracy is good, and since the light is received by one detector, the optical axis adjustment is small. Further, one of the two split lights is reflected by the mirror and the other light is reflected by the rectangular prism, so that at least one of the lights is inverted. By using the optical method in this way, the actuation output can be obtained by one detector. Therefore, the focus error output does not include the offset output due to the diffracted light. Therefore, when the cross-sectional shape of the light on the detector is circular, the focus error output becomes 0, and the optical axis can be adjusted even if there is diffracted light.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments embodying the present invention will be described below with reference to the accompanying drawings for the understanding of the present invention. The following embodiments are examples of embodying the present invention and are not intended to limit the technical scope of the present invention. Here, FIG. 1 is a schematic diagram showing a schematic configuration of a position detection device A1 for a measurement object according to a first embodiment of the present invention, FIG. 2 is an explanatory diagram showing an operation principle of the device A1, and FIG. FIG. 4 is an explanatory view of a light beam image on a split photodiode, FIG. 4 is a schematic view showing a schematic configuration of a position detecting device A2 for measurement according to a second embodiment of the present invention, and FIG. 5 is a split photodiode. 6 is a schematic diagram showing a schematic configuration of a position detecting device A3 for measurement according to a third embodiment of the present invention, and FIG. 7 is a diagram showing a light beam image on a two-division photodiode. FIG. 8 is a schematic diagram showing a schematic configuration of a position detecting device A4 for coping with measurement according to a fourth embodiment of the present invention, and FIG. 9 is an explanatory diagram of a light beam image on a two-division photodiode, FIG.
FIG. 9 is a schematic diagram showing a schematic configuration of a position detection device A5 for a measurement target according to a fifth embodiment of the present invention. As shown in FIG. 1, the measurement target position detection device A1 according to the first embodiment of the present invention is an example applied to a roughness meter, and a measurement sample 1 (corresponding to the measurement target) is irradiated with light, A conventional device in that the position of the measurement target is detected by comparing the output from each photodiode when the reflected light is received by a four-division photodiode 2 (corresponding to a detector composed of a plurality of light detecting elements). It is the same as A01. However, in the present embodiment, the reflected light is divided into two, and at least one of the lights is inverted so that the one light and the other light are mirror images of each other, and the light intensities of the both lights are the same. After that, a mirror image of the axial object is created by combining the two lights, and the outputs of the photodiodes when the combined light is received by the four-division photodiode 2 are compared to measure object 1
Is different from the conventional example in that the position of is detected.

The device A1 will be further embodied and its operation and basic principle will be described below. As shown in FIG. 1, the laser beam L1 emitted from the laser oscillator 3
Is converted into an appropriate beam diameter by the beam expander 4, and the S-polarized component is reflected by the polarization beam splitter 5. This S-polarized component is converted into circularly polarized light by the quarter-wave plate 6 and is condensed on the measurement sample 1 by the objective lens 7. The condensed laser light L1 is reflected by the measurement sample 1, condensed by the objective lens 7, and transmitted through the quarter-wave plate 6. The P-polarized laser light L1 is transmitted through the polarization beam splitter 5. Laser light L2 after passing is 1/2
It is rotated to an appropriate polarization angle by the wave plate 8. Then, it is divided into two laser beams of S-polarized light and P-polarized light by the polarization beam splitter 9. One laser beam L3 (S
The polarized light is transmitted through the quarter-wave plate 10 and converted into circularly polarized light, which is then reflected by the mirror 11. After passing through the quarter-wave plate 10 again, it becomes P-polarized light, and then passes through the polarization beam splitter 9. The other laser beam L4 (P-polarized light) is transmitted through the quarter-wave plate 12 and then becomes circularly polarized light, which is reflected by the rectangular prism 13. Then, it passes through the quarter-wave plate 12 again,
It becomes S-polarized light and is reflected by the polarization beam splitter 9. At this time, the cross-sectional shape of the reflected light L5 from the rectangular prism 13 is a left-right inverted image (corresponding to a mirror image) of the cross-sectional shape of the reflected light L6 from the mirror 11. Further, the half-wave plate 8 is appropriately rotated to equalize the light intensities of the laser beams L5 and L6. As a result, the light quantity distribution in the cross section of the combined light obtained by superimposing the laser lights L5 and L6 has a symmetrical relationship with the axis perpendicular to the paper surface. The combined light is guided to the four-division photodiode 2 with astigmatism by the spherical lens 14 and the cylindrical lens 15 tilted 45 degrees with respect to the paper surface, and a focus error signal is obtained.

The laser light L6 reflected by the mirror 11
Laser beam L reflected by the right-angle prism 13
The mirror 11 is positioned so as to be the same as the optical path of 5. Therefore, the laser light L reflected by the mirror 11
The sectional shape of 6 on the 4-division photodiode 2 is exactly the same when the reflected light from the measurement sample 1 does not include the diffracted light. For example, when the measurement sample 1 is in the in-focus position, the cross-sectional shape of the laser beam L6 reflected by the mirror 11 immediately before entering the spherical lens 14 is as shown in FIG. A point P1 in the figure indicates the starting point of the diffracted light that causes an error during measurement. At this time,
The sectional shape of the laser L5 reflected by the rectangular prism 13 immediately before entering the spherical lens 14 is as shown in FIG. Here, since the axis of the cylindrical lens 15 having the ability to collect light is inclined by 45 degrees with respect to the paper surface, the laser beam L6 reflected by the mirror 11 and the laser beam L5 reflected by the right-angle prism 13 are divided into four parts. The cross-sectional shape on the diode 2 is as shown in FIGS. One division axis of the four-division photodiode 2 is placed parallel to the paper surface, and the sectional shape of the combined laser light is shown in FIG.
become that way. Here, the symmetry axis X in FIG.
It is parallel to the page above. Therefore, the photodiodes 2a, 2b, which form the four-division photodiode 2,
When there is diffracted light in the photodiode 2a of 2c and 2d, diffracted light also appears in the photodiode 2b, and vice versa. Similarly, when there is diffracted light in the photodiode 2c, diffracted light also appears in the photodiode 2d and vice versa. From the above relationship, the numerator of Equation 1 for obtaining the focus error signal has the relationship of the difference in the sum of the two diagonal components, so that the effect of the bright spot due to the diffracted light is canceled.
That is, in this method, the light amounts of the laser beams L5 and L6 incident on the four-division photodiode 2 have a symmetrical distribution with respect to the optical axis. Therefore, the focus error signal of the four-division photodiode 2 when the symmetry axis and one of the two division axes of the four-division photodiode 2 coincide with each other in the conventional device A01.
Is the same as the signal for which the operation output was obtained using.

For example, if the diffracted light is distributed on the four-divided photodiodes A and B in the conventional device A01 as shown in FIG. 3A, in this method, the diffracted light is distributed as shown in FIG. 3B. become. If the light intensity of the diffracted light 1 is Ec1 and the light intensity of the diffracted light 2 is Ec2, the focus error signal is expressed by the following equation.

(Equation 7) As described above, since only one 4-division photodiode is used in this method, the optical axis of the 4-division photodiode 2 need only be adjusted once. In addition, since the differential output can be obtained by the method in which the light beams are optically symmetrical, the offset output due to the diffracted light does not appear in the focus error output. Therefore, when the cross-sectional shape of the light beam on the four-division photodiode is circular, the focus error output is 0, so that the optical axis can be adjusted even if there is diffracted light. The roughness is measured as follows.
That is, the relationship between the movement amount of the measurement sample 1 in the optical axis direction and the focus error signal is obtained in advance, the measurement sample 1 is moved in the plane, and the measurement sample 1 is determined from the relation between the movement amount and the focus error signal. The roughness can be measured by obtaining the surface shape of. In the above-described first embodiment, the case where the astigmatism method is applied as the basic principle has been described, but the case where the other basic principles are applied will be briefly described below. The second embodiment is an application of the critical angle method, which is one of the optical stylus methods.

The critical angle method is a method utilizing a rapid change in reflectance at an incident angle near the critical angle, and its principle diagram is shown in FIG. 4 (a). As shown in the figure, when the light spot is at point B, which is the in-focus position, the light incident on the critical angle prism is parallel and thus totally reflected, but when the light spot deviates to become divergent light or focused light In this case, one of the light beams deviates from the condition of total reflection and the light amount distribution becomes asymmetric. The light intensity distribution is 2
The difference between the two outputs can be used as the focus error signal by capturing with the split photodiode. According to the above critical angle method, when a bright spot due to diffracted light from the measurement sample exists in the cross section of the light beam incident on the two-divided photodiode as shown in FIG. However, the focus error signal does not become 0 and an error occurs. However, the device A of this embodiment as shown in FIG.
When 2 is used, when the measurement sample is at the focus position B, 2
The cross section of the light beam incident on the split photodiode is shown in FIG.
As shown in (b), the influence of diffracted light is canceled. As another method of canceling the influence of diffracted light, a method of using two two-divided sensors is known, but according to the apparatus A2 of this embodiment, only one sensor is required. The third embodiment is applied to the knife edge method which is one of the photo-contact methods. The knife edge method is a method of obtaining a focus error signal by blocking a part of a light beam and making the light beam asymmetrical, and the principle thereof is shown in FIG. 6 (a). In the figure, when the light spot is at the focus position B, the image point of the light spot occurs on the dividing line of the split sensor, but when the light spot shifts to the position A, the light after the objective lens is It becomes divergent. However, the light beam reaches only the lower part of the two-divided photodiode because the lower part of the second lens is blocked. When the light spot shifts to the position C, the light rays after the objective lens are converged, and the light rays reach only the upper part of the two-divided photodiode. Therefore, the output difference of the two-divided photodiode becomes 0 at the position B where the light spot is the in-focus position, and A
Positive or negative is obtained at the positions of C and C, and a focus error signal is obtained.

In the above knife edge method, when the measurement sample is at the in-focus position B, an image point occurs on the dividing line of the two-division photodiode, and the bright point due to the diffracted light from the measurement sample occurs at the image point. 7A as shown in FIG. 7A, even if the measurement sample is at the in-focus position B, the focus error signal does not become 0 and an error occurs. However, FIG.
When the apparatus A3 of the present embodiment as shown in (b) is used, the cross section of the light beam incident on the two-division photodiode becomes as shown in FIG. 7 (b) when the measurement object is at the focus position B. The effect of diffracted light is canceled. The apparatus A4 of the fourth embodiment shows a case where the Foucault method, which is one of the optical stylus methods, is applied. The principle of the Foucault method is shown in FIG.
According to the Foucault method, the light rays passing through the upper split prism reach the lower two-divided photodiode, and the light rays passing through the lower split prism reach the two-divided photodiode in the same manner as the camera split rhythm. To do. Therefore, when the image point of the light spot occurs at the front of the split prism, the apex, and the publication, the cross-sectional shape of the light beam becomes A, B, and C in the figure on the two-divided photodiode, respectively. The focus error signal can be obtained by taking the output difference of the diodes as shown in the figure.

In the Foucault method, when the measurement sample is at the focus position B, the image point of the light spot is an image point on the dividing line of the two-divided photodiode. However, when a bright spot due to the diffracted light from the image point exists as shown in FIG. 9 (a),
Even if the measurement sample is at the in-focus position B, the focus error signal does not become 0 and an error occurs. However, FIG. 8 (b)
When the apparatus A5 of the present embodiment as shown in FIG. 9 is used, the cross section of the light beam incident on the two-division photodiode when the measurement sample is at the focus position B is as shown in FIG. Will be canceled. The device A5 of the fifth embodiment shows a case where the device A5 is applied to an optical system having an autofocus mechanism using the astigmatism method to make the laser light scattered by the measurement sample enter the optical fiber (see FIG. 10). . Here, the scattered light from the measurement sample 1 is always guided by performing feedback control so that the objective lens 19b equipped with the voice coil motor is moved so that the focus error signal becomes zero. By using the apparatus A5 of the present embodiment, even if the angular distribution of scattered light is uneven, the light can be guided to the optical fiber without being affected by the uneven distribution and the adjustment can be simplified. As a result, in any case, even if the diffracted light is present, it is possible to obtain a method for detecting the position of the measurement object, which has high detection accuracy and requires less adjustment of the optical axis.

[0016]

Since the position detecting method for a measuring object according to the present invention is configured as described above, the focus error output becomes 0 even in the presence of diffracted light, so that the detection accuracy is high, and 1 Since the light is received by two detectors, there is little adjustment of the optical axis. Further, one of the two split lights is reflected by the mirror and the other light is reflected by the rectangular prism, so that at least one of the lights is inverted. By using the optical method in this way, the actuation output can be obtained by one detector. Therefore, the focus error output does not include the offset output due to the diffracted light. Therefore, when the cross-sectional shape of the light on the detector is circular, the focus error output becomes 0, and the optical axis can be adjusted even if there is diffracted light.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a schematic configuration of a position detection device A1 for a measurement target according to a first embodiment of the present invention.

FIG. 2 is an explanatory diagram showing the operating principle of the device A1.

FIG. 3 is an explanatory diagram of a light beam image on a four-division photodiode.

FIG. 4 is a schematic diagram showing a schematic configuration of a position detection device A2 for a measurement target according to a second embodiment of the present invention.

FIG. 5 is an explanatory diagram of a light beam image on a two-divided photodiode.

FIG. 6 is a schematic diagram showing a schematic configuration of a position detection device A3 for a measurement target according to a third embodiment of the present invention.

FIG. 7 is an explanatory diagram of a light beam image on a two-divided photodiode.

FIG. 8 is a schematic diagram showing a schematic configuration of a position detection device A4 for a measurement target according to a fourth embodiment of the present invention.

FIG. 9 is an explanatory diagram of a light beam image on a two-divided photodiode.

FIG. 10 is a schematic diagram showing a schematic configuration of a measurement target position detection device A5 according to a fifth embodiment of the present invention.

FIG. 11 is an explanatory diagram showing the basic principle of a conventional method for detecting the position of a measurement target.

FIG. 12 is an explanatory diagram showing changes in focus error output.

FIG. 13 is a schematic diagram showing a schematic configuration of an example of a conventional position detection device A01 for a measurement target.

FIG. 14 is an explanatory diagram of a light beam image on a four-division photodiode.

[Explanation of symbols]

A1 to A5 ... Position detection device 1 ... Measurement sample (corresponding to measurement object) 2 ... 4-division photodiode or 2-division photodiode (corresponding to detector composed of a plurality of photodetection elements)

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Katsuya Takaoka 1-5-5 Takatsukadai, Nishi-ku, Kobe-shi, Hyogo Kobe Steel Co., Ltd. Kobe Research Institute (72) Inventor Toshishi Yanai Takatsuka, Nishi-ku, Kobe-shi, Hyogo 1-5-5 Taiwan Kobe Works, Kobe Steel Co., Ltd. (72) Inventor Hiroyuki Takamatsu 1-5-5 Takatsukadai, Nishi-ku, Kobe-shi, Hyogo Kobe Steel Works, Kobe Steel Co., Ltd.

Claims (2)

[Claims] 1. A position of a measuring object is detected by irradiating the measuring object with light and comparing the output from each of the detecting elements when the reflected light is received by a detector including a plurality of analyzing elements. In the method, the reflected light is divided into two, at least one of the lights is inverted so that the one light and the other light are mirror images of each other, and the light intensities of the both lights are equalized. To produce a mirror image of the axial object by synthesizing, and to detect the position of the object to be measured by comparing the output of each of the light detecting elements when the combined light is received by the detector. Position detection method. 2. The measuring object according to claim 1, wherein at least one of the two split lights is reflected by a mirror and the other light is reflected by a right-angle prism to invert at least one of the lights. Position detection method.