TWI429877B - Photoelastic measuring method and apparatus - Google Patents

Description

Photoelasticity measuring method and device thereof

The present invention relates to a photoelasticity measuring method and an apparatus for measuring stress or strain acting on a permeable measuring object such as a liquid crystal panel or a plasma display panel, and more particularly to a technique. This is a technique for measuring the difference between the principal stress of the substrate acting on at least two bonded substrates disposed along the minute gap, and the difference between the direction and the principal stress, which is compressed or stretched with high precision.

The following method is known for the method of obtaining the stress acting on the measurement object of the glass substrate. In the first method, the object to be measured is irradiated with light on a flat surface, and the reflected light reflected from the surface and the back surface of the object to be measured is measured, and the change in the reflected light is applied to The difference in principal stress of the object and its direction are measured. In addition, the second method is to determine the difference between the principal stress acting on the object to be measured and the direction of the change in the transmitted light transmitted through the object to be measured from the light to be measured (see Non-Patent Document: Latest Stress, Strain measurement and evaluation technology (pages 49 to 66) Supervisor: Kawada Kosan Release: Co., Ltd. Integrated Technology Center.

In addition, an optical system with a confocal mode and a reflection mode is proposed (Non-patent literature: Development of a reflective laser photoelastic experimental device for the doctoral dissertation in 2003 and application of stress evaluation to the film) Department of Research, Department of Mechanical Systems Engineering, Tokyo Denki University Doctoral course Island Jingyu).

However, the conventional methods have the following problems.

In the first method, it is possible to effectively function on a substrate having a permeability to be measured. However, for a substrate in which a plurality of materials having optical characteristics, in particular, different refractive indices, are laminated, in particular, for a two-piece bonded substrate disposed with a small gap therebetween, the difference in principal stress and the direction thereof are measured. In the case of the case, it is impossible to accurately determine the difference in the principal stress acting on either or both of the two substrates and the direction thereof.

In other words, in the case where the first method is applied to measure the difference between the principal stress of the substrate acting on either or both of the two substrates, and the reflected light reflected from the bonded substrate, All of them are synthesized from the surface and the back surface of each substrate. Therefore, even if it is necessary to obtain the required reflected light from the back surface of each substrate in order to obtain the difference in the principal stress and the direction thereof, it is not easy to separate them. Further, in the case where the second method is applied, all of the transmitted light transmitted through the bonded substrate is combined. Therefore, the transmitted light transmitted through each substrate cannot be separated.

Further, in the second method shown in Non-Patent Document 2, it is necessary to use a non-polarizing beam splitter as an element that functions as a main function in measurement. Therefore, the quenching ratio of the photoelastic signal (birefringence) is remarkably deteriorated by the influence of the non-polarizing spectroscope, and there is a problem that the micro birefringence cannot be detected.

The present invention has been made in view of the above problems, and its main object is to provide a photoelasticity measuring method and apparatus thereof for a substrate laminated with a plurality of materials having different refractive indices in optical characteristics, in particular, between The bonded substrate laminated with the small gap can accurately distinguish the substrate on which the stress acts, and can obtain the principal stress with high precision and high speed from the amount of change in birefringence due to the stress acting on the substrate. The difference is the direction and the difference between the principal stresses can be specified with high precision to compress or stretch.

In order to achieve this object, the present invention employs the configuration shown below.

A photoelastic measurement method comprising the steps of: converting a light of a predetermined frequency band into linearly polarized light from an irradiation means; and applying a modulation to the linearly polarized light in a first modulation step; The linearly polarized light that has been modulated is transmitted through the optical member having optical rotation and rotated by a predetermined angle. In the focusing step, the polarized light is irradiated to the object to be measured composed of a plurality of layers having transparency, and the lens and the lens are irradiated. The measurement object moves back and forth relatively in the optical axis direction, and focuses on a predetermined contact interface among a plurality of contact interfaces in which the layers contact each other; and the second modulation step is reflected from the measurement object. The reflected light is modulated, and the separating step is performed by separating the first polarized light in the same state as before the reflected light and the second polarized light including the elastic signal component orthogonal to the first polarized light to the second polarized light. Different light paths are outputted; the conversion step is to pass only the polarized light reflected from the focal plane in the separated second polarized light through the pinhole, and use the spectroscopic means to Spectrum converter; detecting step, detecting means for detecting the lines in the spectrum; a removal step of removing a DC component contained in a spectral signal outputted from the detection means to become an AC component; and a calculating step of determining a change amount of birefringence of each frequency from the spectral signal that becomes an AC component, and The amplitude and phase angle of the distribution of the change in the birefringence with respect to the frequency are at least one of the difference between the principal stress of each layer of the object to be measured and the difference between the direction and the principal stress, which is compression or stretching.

According to the photoelastic measurement method of the present invention, the linear polarization of the predetermined frequency band is transmitted to the predetermined contact interface of the object to be measured in a state of being rotated by a predetermined angle after being subjected to modulation and transmission through the optical member having optical rotation. (Focus) illumination. This linear polarization is applied to the step of separating the reflected light reflected from the contact interface and other interfaces and toward the separation means, and is also modulated.

Then, the reflected light is separated into a first polarized light that returns to the initial optical path and a second polarized light that is orthogonal to the first polarized light that is output to the optical path different from the first polarized light by the separating means. Next, when the second polarized light including the elastic signal outputted from the separating means passes through the pinhole, only the polarized light reflected from the focal plane of the predetermined contact interface is extracted. In other words, only the polarized light that is transmitted only through the predetermined layer is extracted.

The second polarized light is spectrally converted by means of a spectroscopic means, and the spectrum is detected by a detecting means. The DC component is removed from the signal output from the detection means to become an AC component only. Performing at least one of the following calculations, that is, the spectral signal of the alternating component is used to determine the birefringence change of each frequency The amount, the amplitude and the phase angle of the distribution of the frequency by the change amount of the plurality of birefringences, and the difference between the principal stresses of the respective layers of the object to be measured and the difference between the direction and the principal stress are compressed or stretched.

In other words, before the linearly polarized light of a predetermined frequency band is irradiated to the object to be measured, the predetermined angle is rotated by optical means, and the second polarized light including the elastic signal component is spectrally converted, thereby being continuous from the initial linearly polarized light. The ground is rotated to a predetermined corner, and the second polarized light including the elastic signal is detected by one measurement. Therefore, the measurement process can be performed at high speed.

Further, since the reflected light is modulated, and the AC component from which the DC component has been removed is extracted from the signal after the spectral change, the distribution of the amount of change in birefringence with respect to the frequency can be accurately obtained. Therefore, the difference in principal stress can be accurately obtained from the amplitude of the distribution, the direction can be accurately obtained from the phase angle of the distribution, and the difference in the principal stress can be accurately compressed or stretched with high precision.

Further, the turning step is preferably set such that the difference between the angle at which the upper limit frequency component of the linearly polarized light of the predetermined frequency band is rotated by the optical means and the angle at which the lower limit frequency component is rotated by the optical means becomes 90 or more.

According to the method of the present invention, since the difference between the corners of the polarized light generated by the upper limit frequency and the lower limit frequency of the linearly polarized light is set to 90 or more, the obtained phase is also in the range of 90 or more. Therefore, it is easy to approximate the sin curve of the obtained phase with an alternating current waveform. In addition, since the unevenness of the distribution can be determined, for example, in the region from 0° to 90°, it is easy to specify the difference in the principal stress of the predetermined layer acting on the measurement target depending on the unevenness state. Shrink or stretch.

Further, in the method of the invention, it is preferable to have the following steps.

That is, a reflected light detecting step is provided in which only the polarized light reflected from the focal plane in the first polarized light that is separated and returned to the initial optical path passes through the pinhole and is detected; The amount of change in the intensity of the light detected by the polarized light detecting step and the intensity of the light detected by the reflected light in the reflected light detecting step, which is obtained in advance, is again adapted to the change The amount is corrected for the light intensity of the polarized light detected from the second polarized light obtained by the actual measurement.

According to the method of the present invention, only the polarized light reflected by the focal plane is extracted from the first polarized light separated by the optical means. Therefore, the amount of change in the light intensity of the two polarized lights extracted from the first polarized light and the second polarized light is known. Therefore, the correction amount for correcting the amount of change attenuated by the second polarized light B including the elastic signal component can be obtained and corrected. In addition, when the irradiation side of the measurement target is defined as the first surface of the first layer and the number is given in this order, the amount of change in the light intensity of the reflected light from the first surface of the first layer does not include birefringence. Since the amount of change is small, the light intensity of the reflected light reflected from the first surface of the first layer can be used to correct the second polarized light obtained by the actual measurement by the amount of light, thereby improving the measurement accuracy.

Moreover, the method invention preferably has the following steps.

That is, the conversion step is performed, and the first step is separated in the separation step and returned to the first optical path. In the polarized light, only the polarized light reflected from the focal plane passes through the pinhole and is spectrally converted by the spectroscopic means; and the reflected light detecting step detects the spectrum; the calculating step further detects the second polarized light detected in the detecting step Each frequency of the spectrum is divided by the spectrum detected in the reflected light detecting step, and the amount of change in birefringence is corrected for each frequency.

According to the method of the present invention, only the polarized light reflected by the focal plane is extracted from the first polarized light separated by the optical means. By spectrally converting the first polarized light, the amount of attenuation of the spectral signal when the second polarized light is spectrally converted is known. Therefore, the correction amount for correcting the attenuation amount can be obtained and corrected.

Further, in the method of the present invention, it is preferable to apply a difference between each of the principal stresses in the pressing state and the non-pressing state of the outermost layer of the object to be measured, and to specifically determine the difference based on the deviation of the two values. The difference in the principal stress of the object is preferably compressed or stretched.

According to the invention of the present invention, it is easy to specifically act on the stress system compression or stretching of a predetermined layer belonging to the object to be measured. For example, when stress acts on a plate material such as a glass substrate, warpage generally occurs. In this state, the compressive stress acts on the inner side of the concave bend, and the tensile stress acts on the outer side of the bend. Further, in the case where a plurality of layers are laminated, the same phenomenon occurs. That is, the compressive stress acts on the layer on the concave curved side, and the tensile stress acts on the outer layer.

Here, the difference between the principal stresses of a given layer obtained in the non-pressed state The direction has an uncertainty of π/2 degrees. Therefore, when the outermost layer is measured in the pressed state, if the stress difference of the layer becomes large, it is known that the layer is in a stretched state. On the other hand, if the stress difference becomes small, it is known that the stress is in a compressed state.

Further, in each of the method inventions, the optical means is preferably crystal.

The crystal is optically active. If a linearly polarized light is transmitted through the crystal and the polarization angle θ is rotated in the outward path, -θ is rotated in the loop. Therefore, the polarization angle does not change during the round trip. Further, since the rotation angle based on the rotation is dependent on the frequency of the light, if broadband light is used, the measurement of birefringence can be performed at a wide range without rotating the device. Therefore, in the present invention, the reflected light reflected by the object to be measured can be guided to the downstream side without being processed, while still containing the elastic signal. Therefore, the difference between the principal stress and the direction thereof and the difference between the principal stresses can be accurately obtained to be compressed or stretched with high precision.

Further, in order to achieve the object, the present invention adopts the configuration shown below.

A photoelasticity measuring apparatus includes: a holding means for holding a measurement target composed of a plurality of layers having light transmissivity; and an irradiation means for irradiating the measurement target with light of a predetermined frequency band; The optical means transmits the light; the first modulation means applies a modulation to the polarized light; and the second optical means has an optical rotation, and the step of transmitting the modulated polarized light to a predetermined angle is performed. a lens that transmits the polarized light and focuses on an interface of a predetermined layer of the object to be measured; The moving means moves the lens and the holding means relatively back and forth along the optical axis direction of the polarized light; and the second modulation means applies a modulation to the reflected light reflected from the focal plane of the measuring object; And separating the first polarized light that has returned from the initial optical path and the second polarized light that is orthogonal to the first polarized light to a different optical path and outputs the same; the member that forms the pinhole, which is separated In the second polarized light, only the polarized light reflected from the focal plane passes; the spectroscopic means performs spectral conversion on the second polarized light passing through the pinhole; the first detecting means detects the spectrum; and the removing means removes The DC component included in the spectral signal outputted by the first detecting means becomes an alternating current component, and the calculating means performs a change amount of birefringence of each frequency from the spectral signal which becomes an alternating current component, and changes from the birefringence The amplitude and the phase angle of the distribution of the frequency versus the frequency are specific to at least one of the difference between the principal stress of each layer of the object to be measured and the difference between the direction and the principal stress, which is the compression or the stretching.

According to this configuration, the light of a predetermined frequency band irradiated from the irradiation means to the object to be measured held by the holding means is modulated by the first modulation means after passing through the first optical means, and the second optical means is used to circumvent the light. The shaft is rotated to a predetermined angle. By moving the polarizing light while moving the holding means and the object to be measured back and forth along the optical axis direction by the moving means, the coke is moved. Point to the established interface. The amount of change caused by birefringence when the reflected light reflected by the focal plane and other interfaces is reflected by the second modulation means and transmitted through the separation means is transmitted through each layer of the measurement object. The polarized light acts as the first polarized light and returns to the starting optical path. The reflected light generated by the amount of change due to birefringence is used as the second polarized light, and is output to an optical path different from the first polarized light.

When the second polarized light outputted from the separating means passes through the pinhole, only the polarized light reflected from the predetermined interface by the alignment focus is extracted, and the spectral change is performed by the spectroscopic means. The spectrum is detected by means of detection and the spectral signal is transmitted to the calculation means. In other words, it is possible to obtain the detection data of the same amount as when the linearly polarized light is continuously rotated by a predetermined angle around the optical axis by the influence of the rotation angle of the linear polarization by the second optical means.

The calculation means removes the DC component contained in the spectral signal, extracts only the AC component, and extracts the amplitude of the birefringence from the spectral signal to the frequency distribution, and obtains the amplitude of the distribution and the phase angle thereof, and acts on the measurement target. The difference between the principal stresses of the layers of the object and the difference between the direction and the principal stress are compressed or stretched. That is, the method description can be suitably implemented.

Further, according to the invention of the present invention, the second optical means is such that the difference between the angle at which the upper limit frequency component of the linearly polarized light of the predetermined frequency band is rotated by the optical means and the angle at which the lower limit frequency component is rotated by the optical means becomes 90 or more. good.

According to this configuration, since the difference in the angle of rotation due to the upper limit frequency and the lower limit frequency of the linearly polarized light is set to 90° or more, the obtained phase is obtained. Also located in the range of 90° or more. That is, the method description can be suitably implemented.

Further, according to the invention of the present invention, there is provided a member for forming a pinhole, wherein only the polarized light reflected from the focal plane passes through the first polarized light separated in the separating step and returned from the initial optical path; And the second detecting means detects the first polarized light passing through the pinhole; and the calculating step further determines the polarized light detected in the polarized light detecting step for the light intensity of the reflected light from the irradiation means obtained in advance The amount of change in the two light intensities of the polarized light detected by the reflected light detecting step is preferably corrected based on the amount of change in the light intensity of the polarized light detected from the second polarized light obtained by the actual measurement.

According to this configuration, only the polarized light reflected by the focal plane is extracted from the first polarized light separated by the optical means. Therefore, the amount of change in the light intensity of the two polarized lights extracted from the first polarized light and the second polarized light is known. Therefore, the correction amount for correcting the amount of change attenuated by the second polarized light B including the elastic signal component can be obtained and corrected. In addition, when the irradiation side of the measurement target is defined as the first surface of the first layer and the number is given in this order, the amount of change in the light intensity of the reflected light from the first surface of the first layer does not include birefringence. Since the amount of change is small, the light intensity of the reflected light reflected from the first surface of the first layer can be used to correct the second polarized light obtained by the actual measurement by the amount of light, thereby improving the measurement accuracy.

Similarly, there is a member that forms a pinhole that is separated in the separation step and is in the starting light In the first polarized light returning, only the polarized light reflected from the focal plane passes; the conversion means performs spectral conversion on the first polarized light passing through the pinhole; and the third detecting means detects the spectrum; The calculation step is further divided by the frequency of the spectrum of the second polarized light detected in the detecting step by the spectrum detected in the reflected light detecting step, and the amount of change in the frequency corrected birefringence is preferably.

According to this configuration, only the polarized light reflected by the focal plane is extracted from the first polarized light separated by the optical means. By spectrally converting the first polarized light, the amount of attenuation of the spectral signal when the second polarized light is spectrally converted is known. Therefore, the correction amount for correcting the attenuation amount can be obtained and corrected.

This structure can also specifically compress or stretch the difference in the principal stress of the object to be measured by including the following members.

In other words, the structure further includes a moving mechanism for moving the pressing member to the outermost layer of the measuring object and pressing the pressing position at the front end of the pressing member and the waiting position separated by the non-contact state; The difference between the principal stresses in the pressed state and the non-pressed state is applied to the outermost layer of the object to be measured, and the difference in the principal stress acting on the object to be measured is compressed or pulled according to the deviation of the two values. Stretching is better.

Moreover, the structure preferably has: The first polarization separating means separates the first polarized light separated by the separating means into polarized light toward the pinhole and polarized light in the other direction, and the fourth detecting means sets the plurality of photosensitive elements in a two-dimensional array. The light-receiving element is configured to detect polarization separated by the first polarization separation means in another direction; the driving means is to tilt the holding means; and the driving control means is detected by the detecting means The position of the polarized light is used to determine the amount of tilt of the object to be measured held by the holding means, and the driving means is operated in accordance with the amount of tilt to maintain the parallelism of the holding means.

According to this configuration, since the parallelism of the object to be measured held by the holding means is maintained, the first polarized light reflected by the object to be measured overlaps with the optical path of the initial linearly polarized light. In other words, the first and second polarized lights reflected from the focal plane are reliably passed through the respective pinholes without the optical path deviation, and the polarization of the measurement target can be detected with high precision.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

Fig. 1 is a schematic structural view showing an apparatus of an embodiment using the photoelasticity measuring method of the present invention.

In the apparatus of the present embodiment, the measurement object W is provided such that the two transparent glass substrates W1 and W2 such as a liquid crystal panel or a plasma display are overlapped with a small gap therebetween; and the light source 1 is a plane The object to be measured W held on the placing table 11 where the opening H is formed in the center is irradiated with light. In addition, the first polarization detecting unit 2, the Faraday rotator 3, the 1/2 wavelength plate 4, the mirror 5, the BDP (Beam Displacing Prism) 6, and the photoelasticity are provided on the optical path from the light source 1 to the measurement object W. The modulator 7, the crystal 8, and the movable table 10 equipped with the objective lens 9 and movable back and forth, and the second polarization detecting portion 13. Hereinafter, each structure will be described in detail.

The light source 1 utilizes a SLD (Super Luminescent Diode) in the near-infrared region or a random polarizer having a predetermined frequency band such as a semiconductor laser or a white LED (Light Emitted Diode). Further, the light source 1 corresponds to the irradiation means of the present invention. Further, the frequency of the above-mentioned "light" has the same meaning as the speed of light/wavelength, and in the calculation of the present embodiment, the frequency and the wavelength are used in the same meaning.

The first polarization beam splitter 12 splits the light from the light source 1 into a straight forward direction and a vertical direction. In this case, the irradiation light is adjusted to become a horizontal polarization component in advance. On the other hand, a very small amount of vertical polarization component is contained outside the measurement system. Further, in the present embodiment, the 1⁄2 wavelength plate 4 and the Faraday rotator 3 are used to convert the reflected light reflected from the measurement target W and return to the same optical path to the vertical polarization component, and to be directed toward the first polarization detection. The photodiode 25 of the portion 2. Therefore, no horizontal component is contained.

In other words, the first polarization beam splitter 12 branches the polarization component formed by the vertical component reflected by the measurement target W in the vertical direction and faces the photodiode 25 of the first polarization detecting unit 2 . In other words, the orthogonal light linearly polarized light is reflected from the respective surfaces of the glass substrates W1 and W2 of the measurement target W and the back surfaces thereof and returned to the same optical path as the initial linearly polarized light. (hereinafter referred to as "first polarized light A") is returned to the first polarizing beam splitter 12.

The Faraday rotator 3 rotates the polarizing surface by 45° while transmitting the horizontal polarized light from the light source side. In addition, the polarizing surface which is reflected by the measurement target W and returns to the vertical polarization component of the same optical path is rotated in the direction opposite to the outward path by 45° with respect to the 1/2 wavelength plate described later, and the polarized light in the vertical direction is obtained. Linear polarization of the composition.

The 1/2 wavelength plate 4 is in an outward path, and has a predetermined inclination angle with respect to a linearly polarized light that is rotated by 45° with respect to the polarization plane of the Faraday rotator 3. In the case of the present embodiment, it is inclined by about 45 and becomes vertical polarized light. Further, the 1⁄2 wavelength plate 4 corresponds to the first optical means of the present invention. Further, in the circuit, the polarization of the vertical component is rotated by 45°.

The mirror 5 is disposed in an inclined posture in which the mirror surface faces the measurement object W. In other words, in the linearly polarized light reflected by the measurement target W, the component that changes the polarization state due to the influence of the stress in the process of transmitting the measurement target W passes through the BDP 6 described later, and the first orientation and the initial state are made. The other polarized light B of the other direction output of the polarized light A is reflected and directed toward the second polarization detecting unit 24.

The BDP 6 transmits all of the linearly polarized light that has arrived and faces the downstream side. In addition, when the polarized light reflected from the object to be measured W is transmitted, the linearly polarized light (the first polarized light A which is a vertical component) having the same polarizing surface as that at the time of the incident is returned to the same optical path, and the object to be measured is transmitted. In the process, only the component that changes the polarization state due to the influence of the stress acting on the object W is extracted (water) The second polarized light B) of the flat component is output in a direction different from the first polarized light A. In other words, the second polarized light B is output toward the mirror 5 provided on the upstream side. Further, BDP6 corresponds to the separation means of the present invention.

The photo-elasticity modulator 7 has a function of changing the detection signal value of the second polarization B detected by the second polarization detecting unit 24 linearly with respect to the photoelastic amount of the measurement target W. In other words, the photoelastic transducer 7 is provided in a state in which the birefringence in which the residual birefringence of the glass substrate W1, W2 or the objective lens 9 constituting the measurement target W is smaller is applied, and only the alternating current component is extracted. Specifically, when the phase retardation caused by the birefringence of the objective lens 9 or the optical system is Φ 0 , the direction thereof is θ 0 , and the phase delay caused by the birefringence of the photoelastic modulator 7 is δ 0 , The AC component becomes as shown in the following formula. Wherein, Φ 0 / δ 0 | < 0.1, (Φ 0) ² is a negligible amount. Perform the following calculations. The details will be described later.

From [δ 0sin π ft + (1/2) Φ 0sin(2(θ 0-θ r))] ² → [δ 0+(1/2)Φ 0sin(2(θ 0.θ r))] ² . [δ 0+(1/2)Φ 0sin(2(θ 0.θ r))] ² =δ 0 Φ 0sin(2(θ 0.θ r))

Here, the modulation with the modulation frequency set to 100 Hz is applied and measured. The parameter θ r of the above formula changes depending on the wavelength of the crystal 8 and the wavelength. That is, the rotation angle θ r according to the optical rotation is A + B / λ ² with respect to the wavelength λ, and is proportional to the reciprocal of the square of the wavelength, and θ r has a one-to-one relationship with the wavelength. When it is split and detected by the first line sensor 20 which will be described later, each pixel position corresponds to the wavelength. The above-described distribution of frequencies means a distribution in which the θ r is a variable. Further, the photoelastic transducer 7 corresponds to the first and second modulation means of the present invention.

In the case where the polarized light is reciprocally transmitted, the crystal 8 has an optical rotation that rotates a predetermined angle by the frequency of the transmitted path around the optical axis. Further, the rotation angle differs depending on the upper limit and the lower limit of the frequency band of the light irradiated from the light source 1. Therefore, in the present embodiment, in order to easily approximate the phase obtained by the sliding member 19 described later, the length is set such that the difference in the angle between the two frequencies becomes an optical path length of 90 or more. For example, when the linear polarizing surface is changed within a range of 180°, it becomes as follows. In the case where the wavelength region is 770 to 820 mm, the rotation angle is 12.3°/mm at the lower limit of 770 mm, and the rotation angle is 10.8°/mm at the upper limit of 820 mm. Therefore, the length (optical path length) of the crystal 8 becomes 120 mm.

The objective lens 9 is attached to the movable table 10. In other words, the focus position of the polarized light toward the measurement target W can be displaced in conjunction with the movement of the movable table 10 before and after the movement.

The placing table 11 is formed with an opening to hold the rectangular end portion of the measuring object W and pass the light through the central portion. Further, the placing table 11 corresponds to the holding means of the present invention.

The second polarization detecting unit 13 is composed of a lens 14 , a plate 16 on which the pinhole 15 is formed, a lens 17 , a diffraction grating 18 , an objective lens 19 , and a first line sensor 20 .

The pinhole 15 of the plate-like object 16 moves the objective lens 9 before the measurement target object W forward and backward along the optical axis, and the focus is on the surface or the back surface of each layer of the glass substrates W1 and W2 constituting the measurement object W. Each focal point Among the reflected light reflected back, only the second polarized light separated by the BDP 6 passes. Further, in the focal point of the present embodiment, there is a contact interface between the surface of the glass substrate W1 and the air layer in contact with the surface, an air layer located in the gap between the glass substrates W1, W2, and a back surface or glass substrate of the contact glass substrate W1. The contact interface of the surface of W2, and the contact interface of the back surface of the glass substrate W2 and the surface of the placement stage 11.

The diffraction grating 18 is of a reflective type and splits the second polarized light from the mirror 5 and irradiates the first line sensor 20. Further, the diffraction grating 18 corresponds to the spectroscopic means of the present invention.

The first line sensor 20 is constructed to arrange a plurality of detecting elements in a one-dimensional array shape. Examples of the first line sensor 20 include 512, 1024, and 2048 pixels. Further, the first line sensor 20 corresponds to the first detecting means of the present invention.

Next, the first polarization detecting unit 2 is composed of a first polarization beam splitter 12, a lens 21, a plate member 23 in which the pinholes 22 are formed, a second non-polarizing beam splitter 24, and an optical diode 25.

The lens 21 condenses the linearly polarized light coming from the first polarization beam splitter 12 and focuses the contact on the pinhole 22 of the plate member 23.

The photodiode 25 receives the first polarized light A that has passed through the pinhole 22, and converts it into a detection signal of the light intensity, and transmits it to the control unit 27, which will be described later. Further, the photodiode 25 corresponds to the second detecting means of the present invention.

The second non-polarizing beam splitter 24 is non-polar and splits the light into a straight forward direction and an orthogonal direction. That is, the light that has passed through the pinhole 22 is branched. Make Each of the lights faces the photodiode 25 and the parallelism detecting portion 26.

The parallelism detecting unit 26 detects the occurrence state of the bending or warpage of the measuring object W held by the placing table 11. As shown in Fig. 2, the parallelism detecting unit 26 arranges the four photodiodes 26a to 26d adjacently in a two-dimensional array shape, and the optical axes of the linearly polarized lights coming from the second non-polarizing beam splitter 24 are adjacent to each other. The center point C is formed to be equally received by the four photodiodes 26a to 26d. The linearly polarized light received by the four photodiodes 26a to 26d is converted into a light intensity detection signal and transmitted to the control unit 27. That is, the parallelism detecting unit 26 detects the deviation of the optical path of the reflected light. Further, the parallelism detecting unit 26 corresponds to the fourth detecting means of the present invention.

The control unit 27 includes a lock-in amplifier 28, a calculation processing unit 29, a drive control unit 30, an operation unit 31, and the like. Hereinafter, each structure will be specifically described.

The lock-in amplifier 28 filters out the DC component from the spectral signal of the second polarized light B transmitted from the first line sensor 20, and extracts only the AC component. This operation will be described in detail below. Further, the lock-in amplifier 28 corresponds to the removal means of the present invention.

The calculation processing unit 29 mainly performs two types of processing. In the first processing, in order to calculate the correction amount of the deviation of the optical path, the polarized light reflected from the focal plane is affected by the warpage or the like of the measurement target W held by the placing table 11, and the first and second polarization detecting units can be used. The photodiode 25 of the 2, 13 and the first line sensor 20 are received. In the second process, the amount of change in birefringence caused by the stress acting on the predetermined glass substrate W1 or W2 of the measurement target W, the difference between the principal stress acting on the predetermined glass substrate, and the direction thereof, and the Main should The difference in force is compression or stretching. These specific processes will be described in detail in the following description of the operations. Further, the calculation processing unit 29 corresponds to the calculation means of the present invention.

The drive control unit 30 transmits a drive signal according to the condition set by the operation unit 31 to the actuator 32. That is, the placing table 11 is inclined on the plane. Further, the actuators 32 each correspond to the driving means of the present invention.

The control unit 27 including these various configurations, in addition to the above-described processing, collectively controls the operations of the respective drive mechanisms and the like in accordance with the initial setting conditions.

Next, according to the flowchart shown in FIG. 3, the direction of action of the difference between the principal stress and the principal stress of each of the glass substrates W1 and W2 acting on the object W to be measured by the apparatus of this embodiment will be described, and the main direction will be described. The difference in stress is the action and processing of one wheel of compression or stretching. In addition, the case where stress acts on both of the glass substrates W1 and W2 which comprise the object to be measured W is demonstrated as an example.

The operator operates the operation unit 31 to set and input measurement conditions such as the total thickness of the measurement target W, the frequency band to be irradiated from the light source 1, and the modulation frequency (step S1). Further, in the case of the present embodiment, the wavelength band is 770 to 820 nm, and the resonance frequency belonging to the modulation frequency is set to 100 Hz.

When the condition setting is completed, the measurement object W is placed and held on the placement table 11, and the operation of each drive mechanism is controlled to be in a state where measurement can be started. At this time, the movable table 10 is operated, and the focus position of the objective lens 9 is aligned with the outermost surface of the measurement object W. When these measurement conditions are ready, the operator enters The line is tested for illumination (step S2).

At this time, linearly polarized light is irradiated from the light source 1 to the measurement target W, and the reflected first polarized light A is received by the parallelism detecting unit 26, and the respective photodiodes 26a to 26d are transmitted to the calculation processing unit 29 of the control unit 27. The detection signal of the light intensity.

The calculation processing unit 29 determines whether or not the optical path from the light source 1 to the measurement target W is different from the light intensity and the average value of each of the photodiodes 26a to 26d (step S3). When it is confirmed that there is a deviation in the optical path, the calculation processing unit 29 obtains a correction amount for correcting the deviation due to the influence of the warpage or the like of the measurement target W, and performs signal conversion (step S4). This correction signal is transmitted to the drive control unit 30. The drive control unit 30 operates the actuator 32 based on the signal, and tilts the placement table 11 so that the reflected light is uniformly applied to the respective photodiodes 26a to 26d of the parallelism detecting unit 26 (step S5).

When the deviation correction processing of the optical path deviation is completed, the test irradiation is performed. If the optical path deviation is eliminated at this time, the measurement is started (step S6). If the optical path deviation is not eliminated, the deviation correction processing from step S2 is repeated.

When the measurement is started, light is irradiated from the light source 1 to the measurement target W. This light is branched by the first polarization beam splitter 12, linearly advanced, rotated by 45 degrees by the Faraday rotator 3, and further imparted a tilt angle of 45° by the 1/2 wavelength plate 4 to become linearly polarized light composed of vertical components.

This linear polarized light passes through the BDP 6 and passes through the photoelastic transducer 7. At this time, a modulation of 100 Hz is applied. The linear polarized light after modulation is transmitted through the crystal 8. At this time, a predetermined rotation angle determined by the difference between the rotation angles due to the upper limit and the lower limit of the set frequency band is given to the linearly polarized light. That is, in the case of the present embodiment, the difference in the corner angle becomes 180°. The linearly polarized light passing through the crystal 8 reaches the measurement object W.

While the light is being irradiated, the movable table 10 advances only the thickness of the measurement object W, and the first polarized light A and the second polarized light B reflected by the front and back surfaces of the glass substrates W1 and W2 are directed toward the photodiode 25 and the first 1-line sensor 20.

At this time, when stress acts on the glass substrates W1 and W2, the reflected light reflected by each of the focal planes includes the second polarized light B. That is, as shown in Fig. 4, the reflected light R1 reflected by the contact interface (focus surface P1) between the surface of the glass substrate W1 and the air layer, and the air layer and the glass substrate which are minute gaps between the glass substrate W1 and the glass substrate W2 The reflected light R2 reflected by the contact interface (focus surface P2) on the back surface of W1, the reflected light R3 reflected from the contact interface (focus surface P3) of the surface of the air layer and the glass substrate W2, and the glass substrate W2 and the placing table The reflected light R4 reflected by the contact interface (focus surface P4) of 11 includes an elastic signal of the detection signal of the second polarized light B.

The reflected light reflected by each of the focal planes P1 to P4 returns to the same optical path as the initial light, and is modulated by the photoelastic transducer 7 and transmitted through the BDP 6. At this time, the second polarized light B generated by the influence of the stress and the first polarized light A which is not affected are separated. The separated first polarized light A is received by the optical diode 25 on the upstream side. Light that is outputted differently from the second polarized light B and the first polarized light A The path is guided by the mirror 5 to the pinhole 15, and after passing, it reaches the diffraction grating 18.

The second polarized light B that has reached the diffraction grating 18 is spectrally converted and reflected, and goes to the first line sensor 20. The spectral signal of the second polarized light B detected by the first line sensor 20 is input to the lock-in amplifier 28 of the control unit 27.

Further, since there are four focal planes P1 to P4 in this embodiment, although the light intensity detected by the photodiode 25 should generate four sharp peaks, as shown in Fig. 5, three sharp peaks appear. This phenomenon is caused by the interference of the light reflected from the glass surface before and after the air layer to increase the light intensity, and the reflected light R2 and R3 reflected by the focal planes P2 and P3 are combined.

Next, the lock-in amplifier 28 and the calculation processing unit 29 obtain a spectral signal from which the DC component is removed from the detected spectral signal, and obtain a change amount of the birefringence of each focal plane based on the spectral signal (step S7). Further, the surface of the glass substrate W1, the boundary between the glass substrates W1 and W2, and the back surface of the glass substrate W2 are each P1, (P2+P3), and P4, and the detection signals there are set to I0x, I0y, I1x, and I1y, respectively. , I2x, I2y. Here, the component detected by the photodiode 25 is indicated by the symbol x, and the component detected by the first line sensor 20 is indicated by the symbol y.

First, the second polarized light B from the position P1 is obtained as a signal in the form of a spectrum. The signal from the position P1 can be expressed by the following equation, and the equation (2) is a curve with respect to the frequency (=light speed/wavelength) shown in Fig. 6 when the modulation signal is δ 0 .

I0x=K0 (1) I0y(t,θ r)=K0 K0(θ 0r)[δ 0sin2 π ft+(1/2)Φ 0sin(2(θ 0-θ r))] ² (2)

Further, K0 is a reflection intensity, K0 (θ 0r) is a spectrum of an intensity distribution of a wavelength obtained by a spectroscope, and Φ 0 is a birefringence caused by an influence of an optical element of a device such as an objective lens, and θ 0 is a direction thereof. θ r is the angle of rotation of the linearly polarized light that depends on the wavelength. The means for determining K0 (θ 0r) will be described later. δ 0 is the amplitude of the modulated signal. Further, the sizes Φ 1 , Φ 2 and δ 0 of the birefringence of the glass substrates W1 and W2 are set to | Φ 1 | < 0.1 radians, | Φ 2 | < 0.1 radians. Because (Φ 1) ² and (Φ 2) ² are considered to be small and ignored.

Here, by inputting the detection signal to the lock-in amplifier 28, the DC component is removed from the detection signal to obtain an AC component. That is, it can be represented by the following formula.

I0L=I0y(δ 0)-I0y(-δ 0)=K0 K0(θ 0r)((2 δ 0)(Φ 0sin(2(θ 0-θ r)))) (3)

Here, the timing at which δ 0 and -δ 0 are supplied to the modulation signal δ 0sin2 π ft is t1 and t2, and I0y(t1, θ r) is represented by I0y(δ 0)=I0y(t1, δ 0). In the future, the representation of I0y (δ 0) is also expressed by the same idea. Further, δ 0 is set to a known value which gives a magnitude of a phase delay of about 1 nm.

Further, the detection signal contained in the reflected light from each of the focal planes is corrected by the stress acting on the measurement target W, and the amount of attenuation from the initial light is corrected. In other words, the detection signal I0x detected by the photodiode 25 is extracted from the reflected light reflected from the focal plane P1, and corrected by the equation (3). That is, when the equation (3) is divided by I0x, (2K0(θ 0).δ 0) (Φ 0sin(2(θ 0 - θ r))) is obtained.

Next, the signal when the second polarized light B from the position (P2+P3) is spectrally changed can be expressed by the following equation. The description of the symbols of the formulas (4) and (5) will be described later.

I1x=K1 (4) I1y(t,θ r)=K1 K1(θ r)[δ 0sin2 π ft+(1/2)Φ 0sin(2(θ 0-θ r))+(1/2)Φ 1sin (2(θ 1-θ r))] ² (5)

Here, by inputting the detection signal to the lock-in amplifier 28, the DC component is removed from the detection signal, and an AC component is obtained. That is, it can be represented by the following formula, and as shown in Fig. 7, the distribution of the amount of change in birefringence is obtained.

I1yL(θ r)=I1y(δ 0)-I1y(-δ 0)=2K1 K1(θ r)δ 0[Φ 0sin(2(θ 0-θ r))+Φ 1sin(2(θ 1-θ r ))] (6)

Here, in the first measurement of the present invention, first, the amplitude δ'0 of the photoelastic modulation of the photoelastic transducer 7 is set to be large enough to negate the birefringence Φ 0 of the optical member constituting the device such as the objective lens. (| δ'0/Φ 0 |>100) and measure the wavelength-dependent reflection intensity. This gives the relationship of the following formula at the position P1. At this time, it is not necessary to adjust, and the data is scanned in the thickness direction to obtain data.

[δ'0+(1/2)Φ 0sin(2(θ 0-θ r))] ² ≒δ'0 ²

This signal is a spectrum (= δ'0 (θ r)) taken in by the spectroscope as the photoelastic signal δ'0, and is held in advance as correction data for each position. At the other positions (P2+P3) and P4, the same approximation is also established, but the omission is omitted. Including the position P1, the three spectra in the above equations are set as follows.

K0(θ r), K1(θ r), K2(θ r) (7)

Further, the constants K0, K1, and K2 correspond to signals obtained by integrating the spectrum of the equation (7) between the frequencies of use, which are obtained by the photodiode 25.

Here, in this case, the photoelastic signal having a large δ'0 is known in advance from the added stress. For example, the photoelastic signal is expressed by a retardation distance of birefringence and is set to 20 nm.

The correction signal from the first line sensor 20 in the detection signal included in the reflected light from each of the focal planes is separated from the initial light by the stress of the object to be measured W and the stress acting on the object W. The amount of attenuation. That is, the two sides are divided by the equation (7) and I1x. As a result, the following formula is obtained.

[I1yL(θ r)/I1x/K1(θ r)-I0L(θ r)/I0x/K0(θ r)]=2 δ 0[Φ 1sin(2(θ 1-θ r))] (8)

Next, a signal when the second polarized light B from the position P4 is spectrally changed can be expressed by the following equation. Further, as in the above, the distribution of the amount of change in birefringence similar to that of Fig. 7 can be obtained.

The description of the symbols of the formulas (9) and (10) will be described later.

I2x=K2 (9)

I2y(t,θ r)=K2 K2(θ r)[δ 0sin2 π ft+(1/2)Φ 0sin(2(θ 0-θ r))+(1/2)Φ 1sin(2(θ 1- θ r))+(1/2)Φ 2sin(2(θ 2-θ r))] ² (10)

Here, by inputting the detection signal to the lock-in amplifier 28, the DC component is removed from the detection signal, and an AC component is obtained. That is, it can be represented by the following formula.

I2yL(θ r)=I2y(δ 0)-I2y(-δ 0)=2K2 K2(θ r)δ 0[Φ 0sin(2(θ 0-θ r))+Φ 1sin(2(θ 1-θ r ))+Φ 2sin(2(θ 2-θ r))] (11)

Further, according to the equation (7) and the stress acting on the measurement target W, the detection signal from the first line sensor 20 in the detection signal included in the reflected light from each of the focal planes is corrected from the initial light. The amount of attenuation that is separated. That is, divide both sides by equations (7) and I2x. As a result, the following formula is obtained.

[I2yL(θ r)/I2x/K2(θ r)-I0L(θ r)/I0x/K0(θ r)]=2 δ 0[Φ 1sin(2(θ 1-θ r))+Φ 2sin(2 (θ 2-θ r))] (12)

From the equation (3), Φ 0 and θ 0 are obtained, and then Φ 1 , Φ 2, θ 1 and θ 2 are obtained from the equations (8) and (11).

Further, in each of the above mathematical expressions, I0x, I1x, and I2x are reflection components, I0y, I1y, and I2y are birefringence variations, that is, stress-dependent amounts, δ 0 is modulated amplitude, and θ 1 is a glass substrate W1. The direction of the stress, θ 2 is the direction of the stress of the glass substrate W2, and θ r is the angle of the linearly polarized light, corresponding to the wavelength of 770 to 820 nm, and K0, K1, K2, and K3 are the amounts of reflected light according to the reflection coefficient. Sin2 π ft is the modulation part of the photoelastic modulator, and the frequency f is 100 Hz.

From the equations (3), (8), and (11), the spectral signals from the glass substrates W1 and W2 including the elastic signals actually affected by the stress are obtained from the spectral signals of the respective focal planes. (8) The spectral signal (3) of the glass substrate W1 not including the elastic signal is subtracted. Similarly, the spectral signal from the back side of the glass substrate W2 is also the same, minus the spectral signal (11). Equations (3) and (8) of the signal. Thereby, only the elastic signals included in the respective glass substrates W1, W2 can be extracted.

The phase is obtained from the distribution of the amount of change in birefringence based on the obtained spectral signals. That is, a sin curve composed of the alternating current components shown in Fig. 7 or Fig. 8 is obtained. From this amplitude V, the difference in principal stress is obtained. Further, the angle of the origin 0 surrounded by the broken line in Fig. 7 is calculated in the range of 0 to 90° of the sin curve, and the direction of the principal stress is obtained from the inclination at that time.

In addition, a reference model for compressing and stretching the difference between the principal stresses obtained by experimentally using the same test piece as the object W, and the seventh or eighth figure obtained by actual measurement are used. The fitting of the sin curve determines the difference in the principal stress of each of the glass substrates W1 and W2 acting on the object W to be compressed or stretched. Further, in the case of the present embodiment, FIG. The state shown is stretch, and the state shown in Fig. 8 is compression.

According to the photoelasticity measuring apparatus having the above-described structure, the main stress acting on each of the glass substrates W1 and W2 can be measured in a short time by moving the objective lens 19 while irradiating light to the measuring object W to change the focus once. The difference and its direction, as well as the difference in the principal stress, are either compressed or stretched. Further, since the moving distance between the focal points of the objective lens 19 is also short, the measurement time of each of the glass substrates W1, W2 can be realized at about 0.1 sec.

In other words, the crystal 8 having an optical component that is optically active is irradiated onto the measurement target W in a state where a predetermined rotation angle is given to the polarized light in advance, whereby the second polarized light B having an effective approximate phase (sin curve) can be detected. The amount of data.

Further, since the polarization signal is applied to the polarized light, and only the spectral signal composed of the alternating current component is removed by the lock-in amplifier 28, the phase of the reference model of the compression and stretching of the principal stress difference obtained in advance is obtained. That is, for example, positive and negative differential compression and stretching of the differential coefficient across the angle of the stress difference 0 in the range of 0 to 90°.

Further, by passing the second polarized light B through the pinhole 22, only the second polarized light B which is affected by the stress returned from each focal plane can be extracted. In other words, by using the confocal point, the second polarized light that is affected by the stress acting on any glass substrate (layer) can be measured.

Further, by using the parallelism detecting unit 26, the optical path deviation of the measurement light can be corrected, and the first polarized light A and the second polarized light B can be accurately received by the first line sensor 20.

Further, the present invention is not limited to the above-described embodiments, and can be modified and implemented as follows.

(1) In the apparatus of this embodiment, the first polarization detecting unit 2 can also have a structure in which the spectral signal of the first polarized light A can be obtained in the same manner as the second polarized light detecting unit 13. Specifically, as shown in FIG. 9, the first non-polarizing beam splitter 12, the lens 21, the plate 23 on which the pinhole 22 is formed, the lens 33, and the second non-polarized light are provided on the optical path from the light source 1 side. The detecting unit 24, the diffraction grating 34, the objective lens 35, and the second line sensor 36. Further, the parallelism detecting unit 26 is provided to detect one of the polarizations separated by the second polarization detecting unit 24.

According to this configuration, the spectral signal is obtained by causing a spectral change in the first polarized light A returning from each focal plane. Therefore, according to the frequency The unit compares the amount of change attenuated by the separation of the second polarized light B and corrects it. That is, the correction accuracy can be improved.

(2) In the apparatus of this embodiment, the direction of the principal stress can also be obtained as follows.

As shown in Fig. 10, the measurement of the direction of the stress applied from the outside of the measurement object W is performed plural times, and the direction of the stress can be obtained by detecting that the intensity becomes the minimum.

(3) In the respective embodiments, the second non-polarization detecting unit 24 and the parallelism detecting unit 26 may be omitted when the parallelism of the measurement target W is maintained.

(4) In the respective embodiments, the placing table 11 in which the opening H is formed in the center is used, but the flat material in which the opening H is not formed may be used.

The present invention can be embodied in other specific forms without departing from the spirit and scope of the invention, and the scope of the invention is not limited by the scope of the invention.

1‧‧‧Light source

2‧‧‧1st Polarization Detection Department

3‧‧‧Faraday rotator

4‧‧‧1/2 wavelength plate

5‧‧‧Mirror

6‧‧‧BDP

7‧‧‧Photoelastic modulator

8‧‧‧Crystal

9, 19‧‧‧ objective lens

10‧‧‧ movable platform

11‧‧‧Place table

12‧‧‧1st non-polarizing beam splitter

14, 17, 21‧ ‧ lens

15, 22‧‧‧ pinhole

16, 23‧‧‧ plate

18‧‧‧Diffraction grating

20‧‧‧1st line sensor

24‧‧‧2nd non-polarization detection department

25‧‧‧Light diode

26‧‧‧Parallelism Detection Department

27‧‧‧Control unit

28‧‧‧Locking amplifier

29‧‧‧Computation Processing Department

30‧‧‧Drive Control Department

31‧‧‧Operation Department

32‧‧‧Actuator

W‧‧‧Measurement object

W1, W2‧‧‧ glass substrate

A‧‧‧1st polarized light

B‧‧‧2nd polarized light

H‧‧‧ openings

In order to illustrate the invention, several forms that are presently considered suitable are illustrated, but it is understood that the invention is not limited to the illustrated structure and measures.

Fig. 1 is a schematic structural view showing an apparatus for realizing the photoelastic measurement method of the first embodiment.

Fig. 2 is a plan view showing a state of lightness of polarization of the parallelism detecting portion.

Figure 3 is a block diagram showing the measurement of the difference in principal stress and the processing and action of one of its directions.

Fig. 4 is a view showing a state of reflected light reflected by each focal plane of the measurement object.

Fig. 5 is a view showing a state of detection of the light intensity of the first polarized light.

Fig. 6 is a diagram showing a conversion of a spectral signal detected by a first line sensor.

Fig. 7 is a phase diagram showing the amount of change in birefringence obtained from the measurement results.

Fig. 8 is a view showing a state in which a compressive force acts on the object to be measured of the present embodiment.

Fig. 9 is a structural view showing a modification device.

Fig. 10 is a schematic view showing the direction of the principal stress.

1‧‧‧Light source

2‧‧‧1st Polarization Detection Department

3‧‧‧Faraday rotator

4‧‧‧1/2 wavelength plate

5‧‧‧Mirror

6‧‧‧BDP

7‧‧‧Photoelastic modulator

8‧‧‧Crystal

9, 19‧‧‧ objective lens

10‧‧‧ movable platform

11‧‧‧Place table

12‧‧‧1st polarizing beam splitter

14, 17, 21‧ ‧ lens

15, 22‧‧‧ pinhole

16, 23‧‧‧ plate

18‧‧‧Diffraction grating

20‧‧‧1st line sensor

24‧‧‧2nd non-polarization detection department

25‧‧‧Light diode

26‧‧‧Parallelism Detection Department

27‧‧‧Control unit

28‧‧‧Locking amplifier

29‧‧‧Computation Processing Department

30‧‧‧Drive Control Department

31‧‧‧Operation Department

32‧‧‧Actuator, correction signal, detection signal (I λ 0x, I λ 1x, I λ 2x)

W‧‧‧Measurement object

W1, W2‧‧‧ glass substrate

H‧‧‧ openings

A‧‧‧1st polarized light

B‧‧‧2nd polarized light

Claims (13)

A photoelastic measurement method comprising the steps of: converting a light of a predetermined frequency band into linearly polarized light from an irradiation means; and applying a modulation to the linearly polarized light in a first modulation step; The linearly polarized light that has been modulated is transmitted through the optical member having optical transparency and rotated by a predetermined angle. In the focusing step, the polarized light is irradiated to the object to be measured, which is composed of a plurality of layers having transparency, and the lens is irradiated. The object to be measured is relatively moved forward and backward along the optical axis direction, and is focused on a predetermined contact interface among a plurality of contact interfaces in which the layers contact each other; and the second modulation step is reflected from the object to be measured The reflected light is applied to the reflected light, and the separating step is to separate the first polarized light in the same state before the reflected light from the reflected light and the second polarized light including the elastic signal component orthogonal to the first polarized light by optical means. To different optical paths and output; the conversion step is to pass only the polarized light reflected from the focal plane in the separated second polarized light through the pinhole, and use the spectroscopic means Converting the line spectrum; detecting step, detecting means for detecting the lines in the spectrum; removing step of removing a DC component from the line spectrum of the output signal of the detecting means becomes an AC component contained; and In the calculation step, the amount of change in birefringence of each frequency is obtained from the spectral signal which becomes an alternating current component, and the amplitude and phase angle of the distribution of the frequency of the change in the birefringence are specifically applied to each layer of the object to be measured. The difference between the principal stress and its direction, and the difference between the principal stresses is at least one of the calculations of compression or stretching. The photoelasticity measuring method according to claim 1, wherein the rotating step is set such that an upper limit frequency component of a linearly polarized light of a predetermined frequency band is rotated by the optical means and a lower frequency component is rotated by the optical means. The difference in angle becomes 90° or more. The photoelasticity measuring method according to claim 1, wherein the reflected light detecting step is provided, wherein only the first polarized light separated in the separating step and returned from the initial optical path is reflected from the focal plane The polarized light passes through the pinhole and is detected; the calculating step further determines the polarized light detected in the polarized light detecting step and the detected in the reflected light detecting step for the light intensity of the reflected light from the irradiation means obtained in advance. The amount of change in the two light intensities of the polarized light, and the light intensity of the polarized light detected from the second polarized light obtained by the actual measurement is corrected in accordance with the amount of change. The photoelasticity measuring method according to the first aspect of the patent application, wherein the conversion step is performed, wherein only the first polarized light separated in the separating step and returned from the initial optical path is polarized reflected from the focal plane. Performing spectral conversion through pinholes and by spectroscopic means; and The reflected light detecting step detects the spectrum; the calculating step divides the frequency of the spectrum of the second polarized light detected in the detecting step by the spectrum detected in the reflected light detecting step, and corrects the birefringence according to each frequency. The amount of change. The photoelastic measurement method according to the first aspect of the invention, wherein the difference between the principal stresses in the pressed state and the non-pressed state is applied to the outermost layer of the object to be measured, and then the deviation between the two values is obtained. The difference is the compression or stretching of the difference in the principal stress of the object to be measured. The photoelasticity measuring method of claim 1, wherein the optical means is crystal. A photoelasticity measuring apparatus includes: a holding means for holding a measurement target composed of a plurality of layers having light transmissivity; and an irradiation means for irradiating the measurement target with light of a predetermined frequency band; The optical means transmits the light; the first modulation means applies a modulation to the polarized light; and the second optical means has an optical rotation for rotating the modulated polarized light at a predetermined angle in the step of transmitting; a lens that transmits the polarized light and focuses on an interface of a predetermined layer of the object to be measured; and the moving means moves the lens and the holding means relatively back and forth along the optical axis direction of the polarized light; the second modulation means Reflecting from the focal plane of the object to be measured The reflected light is applied to the reflected light; the separating means separates the first polarized light that has returned from the initial optical path and the second polarized light that is orthogonal to the first polarized light to a different optical path, and outputs the same; a pinhole member that passes only polarized light reflected from the focal plane in the separated second polarized light; and a spectroscopic means performs spectral conversion on the second polarized light passing through the pinhole; the first detecting means The spectrum is detected, and the removing means removes the DC component contained in the spectral signal outputted by the first detecting means to become an alternating current component, and the calculating means performs birefringence of each frequency from the spectral signal which becomes an alternating current component. The amount of change, and the amplitude and phase angle of the distribution of the frequency of the change from the birefringence are specifically applied to the difference between the principal stress of each layer of the object to be measured and the difference between the direction and the principal stress. At least one of them. The photoelasticity measuring apparatus according to claim 7, wherein the second optical means is a difference between an angle at which an upper limit frequency component of a linearly polarized light of a predetermined frequency band is optically rotated and an angle at which the lower limit frequency component is optically rotated. It becomes 90° or more. The photoelasticity measuring apparatus according to claim 7, comprising: a member formed with a pinhole, wherein only the first polarized light separated in the separating step and returned from the initial optical path is from the focal plane Reflected back And the second detecting means detects the first polarized light passing through the pinhole; and the calculating step further determines that the light intensity of the reflected light from the irradiation means obtained in advance is detected in the polarized light detecting step The amount of change in the two light intensities of the polarized light detected by the reflected light detection step is corrected by the amount of change in the intensity of the polarized light detected by the second polarized light obtained by the actual measurement. The photoelasticity measuring apparatus according to claim 7, wherein the member is provided with a pinhole, and only the first polarized light separated in the separating step and returned from the initial optical path is from the focal plane The reflected polarized light passes; the conversion means performs spectral conversion on the first polarized light passing through the pinhole; and the third detecting means detects the spectrum; and the calculating step is further performed according to the second polarized light detected in the detecting step Each frequency of the spectrum is divided by the spectrum detected in the reflected light detecting step, and the amount of change in birefringence is corrected for each frequency. The photoelasticity measuring device according to the seventh aspect of the invention, further comprising a moving mechanism for bringing the pressing member into contact with the outermost layer of the object to be measured at the front end of the pressing member and pressing the non-contact state Separate waiting position movement; the calculation means to push the outermost layer of the object to be measured The difference between the principal stresses in the pressed state and the non-pressed state is determined by the difference between the two values, and the difference in the principal stress acting on the object to be measured is compressed or stretched. The photoelasticity measuring apparatus according to claim 7, wherein the first polarized light separating means separates the first polarized light separated by the separating means into polarized light toward the pinhole and polarized light in the other direction; 4 detection means, comprising a plurality of photosensitive elements arranged in a two-dimensional array shape, wherein the photosensitive elements are used for detecting polarization separated by the first polarization separation means to other directions; driving means for maintaining the And the driving control means is configured to determine the amount of tilt of the object to be measured held by the holding means by the calculating means in response to the position of the polarized light detected by the detecting means, and to operate the driving means based on the amount of tilting To maintain the parallelism of the holding means. The photoelasticity measuring device according to claim 7, wherein the second optical means is a crystal.