WO2014174574A1

WO2014174574A1 - Material testing device, and test piece holder and inspection method used in same

Info

Publication number: WO2014174574A1
Application number: PCT/JP2013/061789
Authority: WO
Inventors: 泰範川口; 田窪　健二
Original assignee: 株式会社島津製作所
Priority date: 2013-04-22
Filing date: 2013-04-22
Publication date: 2014-10-30

Abstract

A material testing device capable of accurately capturing the moment at which a test piece breaks is provided. A material testing device (1) provided with a support (11) for supporting a test piece (2), a presser (10) for applying a load to the test piece (2), a light source (21) for irradiating light from the front-surface side of the test piece (2) onto the range to be observed on the test piece (2), and an imaging element (22) for detecting, from the front-surface side of the test piece (2), light from the range to be observed on the test piece (2), wherein a half mirror (23) and a retroreflector (40) that is disposed behind the test piece (2) and retroreflects light in the same direction as the incident direction are provided and the material testing device (1) is configured such that light from the light source (21) is irradiated onto the range to be observed on the test piece (2) via the half mirror (23) and after the light that has passed through the range to be observed on the test piece (2) is retroreflected by the retroreflector (40), the light passes through the range to be observed on the test piece (2) again and is detected by the imaging element (22) after passing through the half mirror (23).

Description

Material testing apparatus, and test piece holder and inspection method used therefor

The present invention relates to a material test apparatus for observing the strength of a test piece formed of a light-transmitting material such as glass or resin (plastic), and a test piece holder and an inspection method used therefor.

Conventionally, as a material strength test, a three-point or four-point bending strength test as defined in Japanese Industrial Standard JIS R 1601 is generally used. FIG. 3A is a diagram for explaining a three-point bending strength test, and FIG. 3B is a diagram for explaining a four-point bending strength test. As shown in FIG. 3A, in the three-point bending strength test, a plate-shaped test is performed with the tip of the pressure body (anvil) 101 as a load point and the tips of the two supports (anvils) 103 as fulcrums. The maximum bending stress when the piece 102 is bent by applying a load W is measured. On the other hand, as shown in FIG. 3B, in the four-point bending strength test, the tips of the two pressure bodies (anvils) 101 are used as the load points, and the tips of the two support bodies (anvils) 103 are used as the fulcrums. The maximum bending stress when the plate-shaped test piece 102 is bent by applying a load W is measured.

Here, FIG. 4 is a schematic configuration diagram showing a conventional material testing apparatus for performing a four-point bending strength test. One direction horizontal to the ground is defined as an X direction, a direction horizontal to the ground and perpendicular to the X direction is defined as a Y direction, and a direction perpendicular to the X direction and the Y direction is defined as a Z direction. The material test apparatus 101 includes a machine base 7, a casing 14 that rotatably supports two

screw shafts

5 and 13 whose vertical direction (Z direction) is a rotation axis, and a central portion on the upper surface of the machine base 7. A support base 112 provided; a crosshead 8 that can be screwed into the

screw shafts

5 and 13 and moved vertically with respect to the machine base 7 as the

screw shafts

5 and 13 rotate; A load cell 109 fixed at the center of the lower surface and an arithmetic control unit 3 that performs control and arithmetic processing are provided.

Inside the machine base 7, a motor 4 controlled by a control signal from the arithmetic control unit 3, a belt 61 that transmits the rotational force from the motor 4 to the

screw shafts

5 and 13, and an interlocking pulley 62 are connected. A gear mechanism 6 is arranged. Thereby, when the motor 4 rotates, the two

screw shafts

5 and 13 rotate, and the crosshead 8 is moved in the vertical direction with respect to the machine base 7.

Two plate-like supports (anvils) 111 that are parallel to each other are formed on the upper surface of the support 112 so as to be aligned in the X direction. Incidentally, X-direction distance between the two support 111 has a _{R 1} (e.g., 10 cm). On the other hand, two plate-like pressurizing bodies (anvils) 110 are formed on the lower surface of the load cell 109 so as to be aligned in the X direction. Incidentally, X-direction distance between the two

pressing body

110, 110 has a short _{R 2} (e.g. 5 cm) than _{R 1} described above. The plate-shaped test piece 2 is placed with the upper ends of the two supports 111 as a fulcrum and the lower ends of the two pressurizers 110 are supported by the support 111 and the pressurizer 110. As a load point, a load W is applied to a predetermined part of the test piece 2.

The material test apparatus 101 is used to detect the moment when the test piece 2 is cracked and broken, and the detection method includes acoustic emission (acoustic sound) generated at the moment of breakage. Output) via a microphone (for example, see Patent Document 1) and a method for detecting a sudden change in the load W (test force) applied to the test piece 2 (for example, see Patent Document 2). Is used.
In addition to observing not only the moment of destruction but also the state of occurrence and progress of the destruction by continuing to shoot with a high-speed video camera (imaging device) while irradiating light on the observation area of the specimen 2. Has also been done.

JP 2001-194346 A JP 2012-251901 A

However, in the static test, the only time at which the fracture of the test piece 2 is photographed (recording is completed) is to detect the fracture of the test piece 2.
In addition, paper is arranged on the back side of the test piece 2, light is irradiated from the front side of the test piece 2 to the observation target range of the test piece 2, and the light transmitted through the test piece 2 is irregularly reflected by the paper, In the case of side-illumination (epi-illumination) in which light transmitted through the test piece 2 is detected again from the surface side of the test piece 2, the light detected by the high-speed video camera is too diffuse due to reflection by paper. The yield of light is low, and the amount of light necessary for photographing may not be secured. Specifically, even if a bright continuous light source such as metahalide illumination is used, the actual irradiation is about 1 million lux. For example, in high-speed shooting such as 10 million frames / second, the time per frame is The amount of light is insufficient because it is as short as 100 nanoseconds.
On the other hand, a pulsed light source such as strobe lighting that can obtain a sufficient amount of light can irradiate more than 10 million lux in about 1 to 10 milliseconds. It is necessary to match (timing signal for causing the strobe to emit light), and in such an intensity test, it is impossible to predict the time when the test piece 2 is broken (breaking detection timing). Since the irradiation time of the light source cannot be matched, it cannot be said that it is an effective means.
Furthermore, light is irradiated from the back side of the test piece 2 to the observation target range (the region between the two supports 111 and 111) of the test piece 2 shown in FIG. In the case of transmitted illumination in which the detected light is detected from the front surface side of the test piece 2, it is necessary to arrange an illumination optical system on the back surface side of the test piece 2, but the light from the light source is blocked by the support 111. As a result, the entire observation target range of the test piece 2 may not be irradiated. That is, the conventional material testing apparatus 101 cannot secure a sufficient space for disposing the illumination optical system on the back side of the test piece 2.

In view of the above situation, as a result of studying an illumination method with a continuous light source that can improve the yield of light detected by a high-speed video camera, the present inventors have found that the back side of the test piece is recursive. Light is transmitted through the test piece again by placing a reflective material, irradiating the observation target area of the test piece with light from the surface side of the test piece, retroreflecting the light transmitted through the test piece with the retroreflective material It has been found that it is effective to perform coaxial epi-illumination that detects from the surface side of the test piece.

That is, the material testing apparatus according to the present invention includes a support that supports a test piece, a pressurizing body that applies a load to the test piece, and a region to be observed on the test piece that is irradiated with light from the surface side of the test piece. A material testing apparatus comprising: a light source unit configured to detect light from an observation target range of the test piece from a surface side of the test piece; and is disposed between the light source unit and the image sensor. A half mirror that guides light from the light source unit in a predetermined direction and guides light from a direction opposite to the predetermined direction to the imaging device, and is disposed on the back side of the test piece, and emits light in the same direction as the incident direction. A retroreflective member for retroreflecting the light, and the light from the light source unit is irradiated to the observation target range of the test piece via the half mirror, and the light transmitted through the observation target range of the test piece is recursively After being retroreflected by a reflective member Again transmitted through the object of observation range of the test piece, via the half mirror so that detected on the imaging device.

Here, the “predetermined direction” is an arbitrary direction predetermined by a designer or the like, for example, a horizontal direction. The half mirror according to the present invention reflects light from the light source unit in a predetermined direction, transmits light from a direction opposite to the predetermined direction and guides it to the imaging device, or transmits light from the light source unit to the predetermined direction. In addition to being transmitted in the direction, it is possible to reflect light from the direction opposite to the predetermined direction and guide it to the image pickup device, so-called coaxial epi-illumination.
In addition, the “observation target range” is an arbitrary region determined in advance by a designer or the like, for example, a region surrounded by a support where a crack (crack) progresses due to a strong stress load, or pressurization It becomes an area surrounded by the body.
The “surface side of the test piece” may be the upper surface of the test piece or the lower surface of the test piece. Therefore, when the “surface side of the test piece” is the upper surface of the test piece, the “back surface side of the test piece” is the lower surface of the test piece, while the “surface side of the test piece” is the lower surface of the test piece. The “back side of the test piece” is the upper surface of the test piece.
In addition, the “support” may be any material that can support the test piece, and examples thereof include two plate-like bodies and a cylindrical body. The “pressurizing body” may be anything that can apply a load to the test piece. For example, one or two plate-shaped bodies, a cylindrical body, a rod-shaped piston (columnar shape) Body) and the like.

According to the material testing apparatus of the present invention, since the retroreflective member is arranged on the back side of the test piece instead of the irregular reflection by paper, the light can reach the image pickup device with high yield by utilizing the retroreflectivity of the reflection. As a result, even if a high-speed video camera is used as the image sensor, the amount of light is not insufficient, and the visibility of the image can be improved.

(Means and effects for solving other problems)
In the above invention, the retroreflective member is a flexible plate-like body having a plurality of microprisms formed on the surface, or a flexible plate-like body having a plurality of glass beads arranged on the surface. It may be arranged between the test piece and the support or between the test piece and the pressurizing body.
According to the material testing apparatus of the present invention, since the retroreflective member is thin and flexible, it can be sandwiched between the test piece and the support body without being affected by the deformation of the test piece. Furthermore, the configuration of the apparatus can be realized at a low cost.

In the invention described above, the pressure body is two plate-shaped pressure bodies parallel to each other, and the light from the half mirror is transmitted between the two plate-shaped pressure bodies. You may make it provide the specular reflector which irradiates the observation object range of a test piece, and guides the light from the observation object range of the test piece to the half mirror.
In the above invention, the support is two plate-like supports parallel to each other, and the distance between the two plate-like supports is the two plate-like pressures. You may make it shorter than the distance between bodies.

In the above invention, the light source unit may be a continuous light source, and the imaging device may be a high-speed video camera.
Here, the “high-speed video camera” is, for example, a camera capable of high-speed shooting with an exposure time of the order of 100 nanoseconds.

Furthermore, the test piece holder of the present invention includes a light source unit that irradiates light from the surface side of the test piece to the observation target range of the test piece, and light from the observation target range of the test piece. An image sensor that detects from the light source, and a half that is disposed between the light source unit and the image sensor and guides light from the light source unit in a predetermined direction and guides light from a direction opposite to the predetermined direction to the image sensor. A test piece holder used in a material testing apparatus including a mirror, and is disposed on a back side of the test piece, a support body that supports the test piece, a pressurizing body that applies a load to the test piece, and And a retroreflective member that retroreflects light in the same direction as the incident direction.
And the inspection method of this invention irradiates light from the surface side of the said test piece to the support body which supports a test piece, the pressurization body which loads a load to the said test piece, and the observation object range of the said test piece A light source unit, an image sensor that detects light from the observation target range of the test piece from the surface side of the test piece, and the light source unit and the image sensor are arranged, and the light from the light source unit is predetermined. A retroreflective method for retroreflecting light in the same direction as the incident direction, the inspection method being used in a material testing apparatus that includes a half mirror that guides light in a direction opposite to the predetermined direction to the imaging element. An arrangement step of disposing a member on the back side of the test piece, and light from the light source unit is irradiated to the observation target range of the test piece via the half mirror, and passes through the observation target range of the test piece. The reflected light is the retroreflective part In after being retroreflective, is transmitted through the observation target range of the test piece again via the half mirror is to include a detection step is detected on the imaging device.

The schematic block diagram which shows the material testing apparatus which concerns on embodiment of this invention. Sectional drawing of the principal part of the material testing apparatus seen from the direction different from FIG. The figure for demonstrating a bending strength test. The schematic block diagram which shows the conventional material test apparatus.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that the present invention is not limited to the embodiments described below, and it is needless to say that various aspects are included without departing from the gist of the present invention.

FIG. 1 is a schematic configuration diagram showing a material testing apparatus according to an embodiment of the present invention. FIG. 2 is a cross-sectional view of the main part of the material testing apparatus viewed from a direction different from FIG. The test piece 2 used in this embodiment is a glass substrate, and a case where a four-point bending strength test is performed on the glass substrate to observe the breakage of the glass substrate in a bright field will be described. In addition, the same code | symbol is attached | subjected about the thing similar to the conventional material test apparatus 101 mentioned above.

The material testing apparatus 1 is attached to a machine base 7, a casing 14 that rotatably supports two

screw shafts

5, 13 whose vertical direction (Z direction) is a rotation axis, and a central portion on the upper surface of the machine base 7. Removable support base 12 (part of the test piece holder) and

screw shafts

5 and 13 can be screwed to move up and down with respect to the machine base 7 as the

screw shafts

5 and 13 rotate. , A load cell 9 (part of the test piece holder) that can be attached to and detached from the center of the lower surface of the cross head 8, a coaxial epi-illumination unit 20 for photographing the test piece 2, and a specular reflector 30 And an arithmetic control unit 3 that performs control and arithmetic processing, and a retroreflecting material 40 (a part of the test piece holder) disposed on the back side of the test piece 2.

Two plate-like supports (anvils) 11 that are parallel to each other are formed on the upper surface of the support base 12 so as to be aligned in the X direction. Incidentally, X-direction distance between the two

support

11, 11 has _{R 2} (for example, 5 cm). On the other hand, two plate-like pressure bodies (anvils) 10 that are parallel to each other are formed on the lower surface of the load cell 9 so as to be aligned in the X direction, and the X direction between the two

pressure bodies

10 and 10 is formed. The distance is R ₁ (for example, 10 cm) longer than R ₂ described above. The plate-shaped test piece 2 is placed with the upper end portions of the two support bodies 11 as fulcrums and the lower end portions of the two pressurization bodies 10 by the support body 11 and the pressure body 10. A load W is applied to a predetermined part of the test piece 2 as a load point.

The coaxial epi-illumination unit 20 includes a light source unit 21, an image sensor 22, and a flat half mirror 23. The light source unit 21 may be a light source that can obtain a necessary amount of light, and is a continuous light source such as metahalide illumination, for example. In addition, a halogen lamp, a xenon lamp, a laser, an LED, a fluorescent lamp, a light bulb, or the like may be used. Further, the wavelength of the emitted light is not specified, and may be white light or light having a specific wavelength. Furthermore, it is not limited to visible light, but may be infrared rays or ultraviolet rays.

The image sensor 22 is, for example, a high-speed video CCD camera capable of high-speed shooting with an exposure time on the order of 100 nanoseconds. In addition to this, a solid-state imaging device such as a MOS or CMOS, a photodiode array, or the like may be used.
In order to cause the image sensor 22 to detect the light beam with the same optical axis as the light beam emitted from the light source unit 21, a half mirror is provided in front of the image sensor 22 (Y direction) and below the light source unit 21 (−Z direction). 23 is arranged. As a result, 50% of the light beam traveling in the −Z direction is changed to the Y direction by the half mirror 23, and 50% of the light beam traveling in the −Y direction is transmitted through the half mirror 23 and guided to the image sensor 22. It has come to be.

The specular reflector 30 has a flat plate shape (for example, 5 cm × 7.1 cm × 7.1 cm), and is disposed between the two

pressure bodies

10 and 10. The light beam traveling in the Y direction is arranged to change the traveling direction to the -Z direction by the specular reflector 30, and the light beam traveling in the Z direction is arranged to change the traveling direction to the -Y direction by the specular reflector 30. Yes. Therefore, the light beam from the half mirror 23 is irradiated to the observation target range (the region between the pressurizing bodies 10 and 10) of the test piece 2, and the light beam from the observation target range of the test piece 2 is guided to the half mirror 23. ing.

The retroreflective member 40 includes a flexible plate-like body having a plurality of microprisms formed on the surface, a flexible plate-like body having a plurality of glass beads arranged on the surface, and the like. For example, glass beads / fabric (manufactured by Sumitomo 3M Co., Ltd., trade name “8910”), prism / film (manufactured by Sumitomo 3M Co., Ltd., trade name “PV9110N”), micro prism / film (manufactured by Sumitomo 3M Co., Ltd.) , Trade name “6260”) and the like. The light incident on the retroreflecting material 40 is retroreflected and reflected in the direction opposite to the incident direction. Therefore, the traveling direction of the incident light beam is different from the traveling direction of the reflected light beam by 180 °.
Then, the retroreflecting material 40 is disposed on the

supports

11 and 11 on the back side of the test piece 2. At this time, if the distance between the back surface of the test piece 2 and the retroreflecting material 40 increases, the image becomes a double image when the focal points of the light source unit 21 and the lens of the image sensor 22 do not match. There is a possibility that "double reflection" will occur. Therefore, the distance between the back surface of the test piece 2 and the retroreflecting material 40 should be small, and the test piece 2 is preferably placed on the retroreflecting material 40.

Next, an inspection method for performing a four-point bending strength test on the test piece 2 using the material test apparatus 1 according to the embodiment of the present invention will be described. First, the measurer places the retroreflecting material 40 on the upper ends of the two

supports

11 and 11 and then places the test piece 2 on the retroreflecting material 40 (placement step). Next, the measurer turns on the power of the light source unit 21 and the image sensor 22. Thereby, the light beam emitted in the −Z direction from the light source unit 21 changes the traveling direction to the Y direction by the half mirror 23. Then, the light beam traveling in the Y direction is irradiated to the observation target range of the test piece 2 in the −Z direction by changing the traveling direction to the −Z direction by the specular reflector 30. Thereafter, the light beam transmitted through the observation target range of the test piece 2 in the −Z direction is changed in the traveling direction to the reverse direction (Z direction) by the retroreflecting material 40, and this time the observation target range of the test piece 2 is changed to the Z direction. Irradiated. The light beam transmitted through the observation target range of the test piece 2 in the Z direction is changed in the traveling direction to the −Y direction by the specular reflector 30. Then, the light beam traveling in the −Y direction passes through the half mirror 23 and is guided to the image sensor 22 (detection step).

In such a state, the measurer inputs a measurement start signal to the arithmetic control unit 3 to start measurement. When a measurement start signal is input, the motor 4 rotates at a constant speed and the crosshead 8 descends at a predetermined speed by the control signal from the arithmetic control unit 3, and the support body 11 and the pressure body 10 move up and down. The interval approaches at a constant speed, and as a result, a bending test force is applied to the test piece 2 to push and bend the central portion (predetermined part) of the test piece 2 downward (−Z direction). At this time, the test force detection signal is taken into the calculation control unit 3 at regular intervals, and a predetermined calculation process is performed to calculate the test force against displacement.
When the test force exceeds the bending strength of the test piece 2, the test piece 2 is cracked and broken, but the image sensor 22 continues to be irradiated with a light beam from the observation target range of the test piece 2. Therefore, the video from the occurrence of the crack to the destruction is taken at high speed.

As described above, according to the material testing apparatus 1 of the present invention, since the retroreflective member 40 is thin and flexible, it is not affected by the deformation of the test piece 2, and between the test piece 2 and the support 11. Can be pinched. Thereby, since the retroreflective member 40 is arranged on the back side of the test piece 2 instead of irregular reflection by paper, the light can reach the image pickup device 23 with high yield due to the recursiveness of reflection, and as a result. Even if a high-speed video CCD camera having an exposure time on the order of 100 nanoseconds is used as the image pickup device 23, the light quantity is not insufficient and the visibility of the image can be improved.

<Other embodiments>
(1) In the material testing apparatus 1 described above, a configuration in which a four-point bending strength test is performed in which two plate-like pressure bodies 10 are formed on the lower surface of the load cell 9 is shown. It is good also as a structure which performs the 3 point | piece bending strength test in which the pressurization body of the plate-shaped object is formed. In such a configuration, the retroreflective material is placed on the test piece. Then, the optical axis of the coaxial epi-illumination is changed from the X direction to the Z direction with a reflecting mirror from below, and the test piece is photographed from below.

(2) In the material testing apparatus 1 described above, two plate-like supports (anvils) 11 are formed on the upper surface of the support 12, and two plate-like pressure bodies are formed on the lower surface of the load cell 9. 10 shows a configuration for performing a four-point bending strength test. A cylindrical support body is formed on the upper surface of the support base, and a cylindrical pressure body is formed on the lower surface of the load cell. The configuration may be such that the “ON RING” test is performed.
(3) In the material testing apparatus 1 described above, two plate-like supports 11 are formed on the upper surface of the support base 12, and two plate-like pressure bodies 10 are formed on the lower surface of the load cell 9. The four-point bending strength test is shown. A cylindrical support is formed on the upper surface of the support base, and a rod-shaped piston (columnar pressure body) with a pointed tip is formed on the lower surface of the load cell. The punching test may be performed. In such a configuration, with the retroreflective material placed on the test piece, the test piece and the retroreflective material are fixed by being sandwiched from above and below by a test piece holder having two circular holes. Become. Then, the optical axis of the coaxial epi-illumination is changed from the X direction to the Z direction with a reflecting mirror from below, and the test piece is photographed from below.

The present invention can be used for a material testing apparatus for observing the strength of a test piece formed of a light-transmitting material such as glass or resin (plastic).

DESCRIPTION OF SYMBOLS 1 Material test apparatus 2 Test piece 10 Pressurization body 11 Support body 21 Light source part 22 Image pick-up element 23 Half mirror 40 Retroreflective member

Claims

A support for supporting the test piece;
A pressurizing body for applying a load to the test piece;
A light source unit that emits light from the surface side of the test piece to the observation target range of the test piece;
A material testing apparatus comprising: an imaging device that detects light from the observation target range of the test piece from the surface side of the test piece;
A half mirror that is disposed between the light source unit and the image sensor, guides light from the light source unit in a predetermined direction, and guides light from a direction opposite to the predetermined direction to the image sensor;
A retroreflective member disposed on the back side of the test piece and retroreflecting light in the same direction as the incident direction;
The light from the light source unit is irradiated on the observation target range of the test piece via the half mirror, and the light transmitted through the observation target range of the test piece is retroreflected by the retroreflective member, and then the test is performed. The material testing apparatus, wherein the observation target range of the piece is transmitted again, and is detected by the imaging device via the half mirror.
The retroreflective member is a flexible plate having a plurality of microprisms formed on the surface, or a flexible plate having a plurality of glass beads arranged on the surface,
The material testing apparatus according to claim 1, wherein the material testing apparatus is disposed between the test piece and the support or between the test piece and the pressure body.
The pressure body is two plate-shaped pressure bodies parallel to each other,
Between the two plate-like pressure bodies, the light from the half mirror is irradiated to the observation target range of the test piece, and the light from the observation target range of the test piece is guided to the half mirror. The material testing apparatus according to claim 1, further comprising a specular reflection material.
The support is two plate-like supports parallel to each other,
The material test apparatus according to claim 3, wherein a distance between the two plate-like supports is shorter than a distance between the two plate-like pressure bodies.
The light source unit is a continuous light source,
The material testing apparatus according to any one of claims 1 to 4, wherein the image sensor is a high-speed video camera.
A light source unit that emits light from the surface side of the test piece to the observation target range of the test piece;
An image sensor for detecting light from the observation target range of the test piece from the surface side of the test piece;
A material testing apparatus that is disposed between the light source unit and the imaging device and includes a half mirror that guides light from the light source unit in a predetermined direction and guides light from a direction opposite to the predetermined direction to the imaging device. A test piece holder used for
A support for supporting the test piece;
A pressurizing body for applying a load to the test piece;
A test strip holder comprising a retroreflective member disposed on the back side of the test strip and retroreflecting light in the same direction as the incident direction.
A support for supporting the test piece;
A pressurizing body for applying a load to the test piece;
A light source unit that emits light from the surface side of the test piece to the observation target range of the test piece;
An image sensor for detecting light from the observation target range of the test piece from the surface side of the test piece;
A material testing apparatus that is disposed between the light source unit and the imaging device and includes a half mirror that guides light from the light source unit in a predetermined direction and guides light from a direction opposite to the predetermined direction to the imaging device. An inspection method used for
An arrangement step of arranging a retroreflective member that retroreflects light in the same direction as the incident direction on the back side of the test piece;
The light from the light source unit is irradiated on the observation target range of the test piece via the half mirror, and the light transmitted through the observation target range of the test piece is retroreflected by the retroreflective member, and then the test is performed. And a detection step of detecting again by the imaging element through the half mirror through the observation target range of the piece.