JPH02257005A - Phase shifted lateral deviation speckle polarization interferometer - Google Patents

Abstract

PURPOSE:To quantitatively analyze the deformation quantity and strain quantity of an object to be measured with a simple measuring system by inserting a double refraction wedge which is movable in parallel and a polarizing plate between the image pickup system of the lateral deviation interferometer and the surface to be measured. CONSTITUTION:The double refraction wedge plate 14 which is movable in parallel and the polarizing plate 15 are inserted between the object 13 to be measured of the lateral deviation interferometer and an image-forming lens 16. The two laterally deviated object speckle images of the object 13 to be measured are recorded in a computer 19 before and after the deformation. The computer calculates the square of the difference of the images to obtain the stripe patterns of the object deformation quantity. The wedge plate 14 is then moved laterally and the speckle images changed in the phase between the orthogonally polarized light rays are taken into the computer 19 which calculates the square of the difference from the speckle image before the deformation to obtain the speckle interference fringes shifted in the phase. A Fourier coeffts. are calculated by using the interference fringes and the physical properties of a new stock etc., by the deformation thereof are quantitatively evaluated.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a measuring device that automatically measures the amount of deformation of a rough surface object using a phase shift laterally shifted speckle polarization interferometer.

(Prior art) Measuring the amount of deformation of rough objects is extremely important not only in a wide range of academic fields such as engineering, medicine, and dentistry, but also in manufacturing industries such as electrical and electronic equipment, aviation, and composite materials. This is a measurement item.

Particularly in the manufacturing industry, deformation analysis of structures also determines the basic design concept of the target object, so there is a strong demand for a simple measurement method in real time. The current mainstream method of deformation measurement is to process and analyze multidimensional electrical signals from strain gauges, but even when object deformation is complex, such multi-point measurement is insufficient, and it is necessary to analyze surface information. It is essential to obtain it.

Furthermore, in fields such as medicine, objects are soft and it is impossible to attach strain gauges to them. Speckle interferometry is a measurement method that satisfies these requirements, and allows real-time observation of striped patterns representing contour lines of the amount of deformation. moreover,
The present inventor has already proposed in Japanese Patent Laid-Open No. 60202984 that the amount of deformation can be measured quantitatively by using a phase shift method in which an image is captured by shifting the phase of a reference light and then processed and analyzed by a computer.

(Problems to be Solved by the Invention) However, since this method has conventionally been applied to a Michelson type interferometer, it has the disadvantage that it is easily influenced by external vibrations and thermal deformation. On the other hand, a speckle lateral displacement interferometer is a method that is resistant to external disturbances.

In this method, images of the object to be measured are laterally shifted using an imaging system to interfere with each other, and the first derivative of the amount of deformation of the object to be measured is obtained as contour lines of fringes. Since this method only shifts the image laterally, it is resistant to external disturbances. However, when trying to introduce a phase shift method to quantitatively analyze the interference fringes obtained with this lateral-shifting speckle interferometer, it is necessary to construct a new Michelson interferometer for lateral-shifting the image, and The characteristics of the interferometer will be lost. Furthermore, a method using a Savart plate that uses polarization as an image lateral shifting element has been proposed, but a method using this element does not allow the introduction of a phase shift method.

(Means for Solving the Problems) The present invention uses a laterally shifted polarization interferometer to laterally shift an image of an object to be measured, and at the same time shifts the phase between orthogonal polarized lights that form the laterally shifted image. This provides a system that automatically quantitatively analyzes the amount of deformation and strain of an object to be measured by, for example, importing a plurality of speckle images into a computer, calculating and processing them.

To explain the present invention in detail, a laser beam is irradiated onto an object to be measured, and the light that is diffusely reflected from the object is focused onto the photosensitive surface of a television camera using an imaging lens. A birefringent wedge plate and a polarizing plate are inserted in the front or rear of the imaging lens system to obtain two laterally shifted object speckle images of the object to be measured. When this speckle image is recorded on a computer before and after object deformation and the square of the difference between these images is calculated, a striped pattern is obtained that does not represent the contour line of the spatial first derivative of the amount of object deformation. Next, the birefringent wedge plate placed in the imaging system is moved laterally to change the phase between the orthogonal polarized lights that create the laterally shifted image, and the speckle image is imported into a computer. By calculating the power, it is possible to create out-of-phase speckle interference fringes. By calculating the Fourier coefficient using this out-of-phase speckle interference pattern and calculating the arctangent using the calculated cosine and sine components, it is possible to quantitatively and automatically obtain the first derivative of the amount of object deformation. I can do it. This birefringent wedge can be moved using a conventional drive system such as a stepping motor, a DC motor, a Vizoet element, or a voice coil.

Further, by setting the object illumination direction to two directions symmetrical to the direction perpendicular to the object surface and using speckle interference fringes obtained from each direction, it is also possible to separately measure the inclination of object deformation and the amount of strain.

As a polarizing element that has both the functions of horizontally shifting an image and shifting the phase of light, the birefringent wedge plate described above is the simplest element, but a combination of a Wollaston prism, a Soleil-Vivanet compensator, and a Savard plate is also available. Alternatively, combinations of polarizing elements can be considered, such as a combination of Wollaston prisms and silk. In particular, the combination of Wollaston prisms has the characteristic that the amount of lateral shift of the image can be varied.

(Function) The present invention will be further explained using the drawings. FIG. 4 shows a basic optical system representing the measurement principle of the present invention. A rough surface object 41 illuminated with a laser is imaged using a birefringent wedge plate 42 and a lens 43 to obtain a speckle image. At this time, the main axis of the birefringent wedge plate is indicated by an arrow (-) in the figure. Further, a is a wedge angle. Therefore, since the refractive index of the ordinary ray and the extraordinary ray are different in the birefringent wedge plate,
The respective orthogonal polarized lights diffusely reflected at two points P and Q on the object plane are combined at one point p on the image plane 44. Therefore, in this imaging system, two images shifted by δX in the X-axis direction are observed. Next, in order to obtain a light interference term, a polarizing plate 45 is inserted between the birefringent wedge plate and the lens, and its polarization angle is set to 45° with respect to the principal axis of the wedge plate. By adjusting this polarizing plate, the intensities of the two laterally shifted images can be made equal. In this way, the two points on the object interfere and form a speckle image, so this image is captured using a television camera, and the A/D
After conversion, record it on the computer.

This image before object deformation is S. Let S be the speckle image after binding and object deformation. When the birefringent wedge plate is moved in the X-axis direction after the object is deformed, the thickness of the wedge changes, and the phase difference between the orthogonal polarized lights formed by the two images changes accordingly. Therefore, for example, the phase difference between polarized lights is 90°, 1
Move the wedge so that the angles are 80° and 270°, record speckle images S2, S3, and S4 on a computer, and use the computer to calculate the square of the difference between these images (Si 5o) J+ =
By calculating 1 to 4), speckle interference fringes 1 to 14 can be created. The interference fringes I represent the isolines of the spatial first derivative of object deformation. Also, since the phases of the obtained interference fringes are shifted by 90°, the cosine component C of the interference fringes is defined as II
At I3, the sine component S can be found at T=I2. Therefore, by calculating the arctangent using these sine and cosine components, it is possible to directly determine the phase of light that is proportional to the spatial first derivative of object deformation. In the method described above, four interference fringes were used, but the Fourier coefficients are calculated using N (≧3) interference fringes 1, (i = 1 to N), and the sine and cosine components of the interference fringes are calculated. By calculating the arctangent, the spatial first derivative of the amount of object deformation can be found.

(Examples) Example 1 Such a device can be applied to an interferometer as shown in FIG. 1, which mainly measures the slope of deformation of an object that causes out-of-plane deformation. The light from the laser 11 is spread by the lens 12 and then irradiated onto the object to be measured 13, and the object surface is passed through the birefringent wedge plate 14, the polarizing plate 15, and the imaging lens 16, and an object image is formed on the photosensitive surface of the television camera 17. image. Here, a quartz crystal is used as the birefringent wedge plate, and its movement is
This is performed by a wedge plate drive device 20, the main axis is in the XX' direction, and the wedge angle is 12 degrees. An image from a television camera is recorded by a digital frame memory 18 and then transferred to a computer 19 for recording. Next, after the object is deformed by w (x, y) in two axial directions, a speckle image is recorded on a computer. At this time, when the number of times the birefringent wedge plate is moved in the X-axis direction is i, the difference between orthogonal polarized light The phase of is 360° ・i/
Adjust the moving distance of the wedge plate so that it becomes N. Here, N is the total number of speckle images recorded after the object is deformed, and the speckle image at this time is S.

Using images before and after object deformation, a computer calculates the square of the difference (Si
-3°)2, it is possible to obtain interference fringes (2) whose phase is shifted by 360°·l/N. Next, using these interference fringes, the cosine component C−Σ■,・cos(2πi/N)
and the sine component S-ΣIt ・cos (2πi/N)
is calculated, and Σ represents calculating the sum of 1 to N for i. Using these S and C, the arctangent jan
-' (S/C), it is possible to obtain the phase difference of light that is proportional to the spatial first derivative w/x·δX of the amount of deformation. Here, δX is the amount of lateral shift between the two images on the object plane. Therefore, the obtained data is proportional to the slope of object deformation, and the amount of out-of-plane deformation can also be determined by integrating the obtained data.

In Figure 1 above, one birefringent wedge plate was used, but
By combining several polarizing elements, it is possible to change the amount of lateral shift.

First, FIG. 2A shows a lateral shift element using a Wollaston prism 21. As shown in the figure, this element consists of overlapping wedge plates with the main axes of the crystals perpendicular to each other, and by doing so, the bending angle of the polarized light that creates the two images is symmetrical with respect to the optical axis. . A phase shift between polarized lights can be achieved by moving one of the two prisms parallel to the plane of the paper (·→). In this Wollaston prism, the deflection angle of the two polarized lights is constant, so the amount of lateral shift between the two images cannot be varied. Therefore, as shown in Fig. 2B, if two Wollaston prisms 21.21' are used, by changing the distance d between the two prisms,
You can change the amount of horizontal shift of the image. In this case, the phase difference between the interfering polarized lights can be changed by moving any one of the four birefringent wedge plates parallel to the plane of the paper (.-).

Embodiment The method described in Embodiment 1 is used when the object to be measured mainly undergoes out-of-plane deformation. Even if the object deforms mainly in the plane,
Since the measurement sensitivity of the optical system is large with respect to out-of-plane displacement, it is not possible to obtain only the distortion component with the measurement system described above. Therefore, here we provide a method of separately measuring the inclination in the direction perpendicular to the measurement surface of the amount of deformation and the strain component, which is the first derivative of the in-plane deformation, using two-beam illumination. FIG. 3 shows an overview of the measurement system of this embodiment. Semi-transparent mirror 31 for light from the laser
The lens 12. is divided into two parts from the normal direction of the object 13'.
12' to irradiate symmetrically. These illumination lights are designated as IL and IL2. Images are imported as follows. Before object deformation, the object is illuminated alternately with IL and IL2, and each speckle image S, is obtained. and records Sho. Next, after the object is deformed, the object is alternately illuminated with the illumination lights IL and IL2, and at this time, the birefringent wedge plate is moved N times to record N speckle images with different phases in each direction. Let the speckle images of the two illumination lights be 311~SIN and S21~S2N, respectively, and calculate the square of the difference between those images to obtain the interference fringe I++- with a phase shift of 1-.
(S++ S+o)2 and I Zi-(s2i 5z
o)" (i = 1 to N) can be obtained by calculation. Using these speckle interference fringes, the sine components St, Sz and cosine components C1, C2 due to each illumination light can be calculated as shown in Example 1.
As in the case of , the optical phase differences φ5 and φ2 are obtained by calculating the Fourier coefficients and calculating the respective arc tangents.

The phase of this light is φ+=2π(sinθ・uZ
x+(1+CO8θ)・w/x)/λ・δX, and φ
2= 2yc (-sinθ・u/ x+ (1+co
sθ)-w/x)/λ·δX. Here, θ is the angle with respect to the normal direction of the symmetrical illumination light. Therefore, finding the sum of phases φ, +φ2 is 2π/λ・(1+cosθ)・
w/x·δX, and only the slope of the out-of-plane displacement is determined. Furthermore, finding the phase difference φ, -φ2, 2π/λ・si
nθ・u/x・δX, and the distortion component can be measured separately.

The phase difference and sum are the Fourier coefficients C4 and C obtained earlier.
2, S6, S2 to create a new sum sine s, -3
, C2+C, S2 and the cosine component as C9-ClC25IS2
In the case of difference, sine S=S + C2CIS
2 and the cosine component as C--C, C2S, S, and find the arctangent jan-'(S+/C,) and tan-', respectively.
It can be directly obtained by calculating (S-/C-).

(Effects of the Invention) As described above, according to the present invention, a phase shift laterally shifted speckle polarization interferometer can be configured, and the spatial first derivative of the amount of deformation of a rough surface object can be automatically quantitatively analyzed. In particular, since the lateral shift of the object image and the phase shift of the light can be performed with a single polarizing element, the measurement system is extremely simple. Furthermore, by using a combination of polarizing elements, the amount of lateral shift of the image can be made variable. Since this interferometer is a common optical path interferometer, it has the characteristic that it is resistant to external disturbances and can perform stable measurements. Furthermore, by using illumination from a direction symmetrical to the object plane, the slope of displacement and the amount of strain can be measured separately, making it possible to directly measure the two-dimensional distribution of strain, which was previously impossible. can. If the amount of object deformation can be measured stably in this way, it will become possible to quantitatively evaluate the physical properties of composite materials and new materials, providing important data not only for the materials industry but also for system design. Become.

[Brief explanation of drawings]

FIG. 1 is a system configuration diagram showing a first embodiment of the present invention suitable for measuring the inclination of out-of-plane deformation of an object, FIG. 2A, and FIG. 2B.
The figure shows the configuration of a transverse shift element using a Wollaston prism instead of the birefringent wedge plate used in Figure 1. Figure 3 shows a book suitable for measuring strain components of in-plane deformation of an object using two-beam illumination. FIG. 4, a system configuration diagram showing a second embodiment of the invention, shows the components of an optical system showing the measurement principle of the invention. (Explanation of symbols) 11... Laser light source, 12.12', 43... Lens, 13.13'... Measured object, 14.42...
・Birefringent wedge, 15.45...Polarizing plate, 16...
・Imaging lens, 17...TV camera, 19...Computer, 1B...Digital frame memory, 20...
- Wedge plate drive device, 21.21'...Wollaston prism, 31...semi-transparent mirror, 44...imaging surface.

Claims (1)

[Claims] A laser device that irradiates a laser beam onto a surface of an object to be measured, an imaging device that focuses on the surface of the object to be measured, a birefringent wedge that can be moved in parallel and polarized light inserted between the surface to be measured and the imaging device. A phase-shifted side-shifting speckle polarization interferometer consisting of a plate.