JPH0217043B2 - - Google Patents

Abstract

PURPOSE:To improve measuring accuracy, in a spectrum interference method, by imparting difference in light paths to one light path, and computing the phase, i.e., the displacement data, of each measuring point based on the change in brightness at each measuring point at this time. CONSTITUTION:The speckle pattern of the surface of a data medium before deformation is stored in a memory pattern. Then, the speckle pattern of the surface of the data medium after the data medium is deformed is stored in the memory every time voltages are applied to an electro-optical crystal 5 step by step, with the length of the path of laser light L3 being changed. Based on the stored data, the difference between the brightness of a picture element before the deformation and the brightness after the deformation is obtained. The difference is changed in correspondence with the change in the length of the light path. By using the phase terms of all the picture element surfaces of the data medium 7 and continuity of the change in the neighboring picture elements, the state of the displacement within the surface can be determined on the entire surface to be measured.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a device for non-contact measurement of in-plane displacement of an object to be measured, and particularly to a device for measuring in-plane displacement of an information medium such as magnetic tape or paper. The present invention relates to a measuring device suitable for measuring with precision.

[Conventional technology]

As a means for non-contactly measuring the in-plane displacement of an object to be measured, there is a method using, for example, spectral interferometry.

In this measurement method, laser light is irradiated from two directions symmetrical to the normal to the surface of the object to be measured, and the light scattered from the surface of the object to be measured is imaged on an image sensor using a lens. Then, the difference in brightness of each pixel of the speckle pattern before and after the deformation of the object to be measured is determined. When this is visually displayed on a monitor, the equidisplacement line of the displacement component in the direction parallel to the line of intersection between the plane formed by the central axes of the laser beams irradiated from the two symmetrical directions and the surface of the object to be measured is shown. Appears.

[Problem to be solved by the invention]

However, in the conventional measurement method using speckle interferometry, when calculating the difference in brightness of each pixel, it gives uniformly displaced fringes on average, but the speckle itself shows the effect of noise. The quality of the obtained image was poor, and the displacement distribution state could not be detected between the stripes, making it impossible to perform highly accurate measurements.

SUMMARY OF THE INVENTION An object of the present invention is to provide an in-plane displacement measuring device that measures in-plane displacement of an object to be measured in a non-contact manner with high accuracy.

[Means to solve the problem]

In order to solve the above problems, the present invention provides a laser oscillator that emits laser light, a beam expander that expands the beam diameter of the laser light emitted from the laser oscillator, and a laser light that has passed through the beam expander. a beam splitter that branches into two sets; an optical system configured to irradiate the two sets of branched laser beams onto the surface of the object to be measured from two directions having equal angles with respect to the normal to the surface of the object to be measured; a lens and a photographing element that capture speckle patterns generated when the irradiated laser beam is scattered by unevenness on the surface of the object to be measured; a storage device that stores image information converted into electrical signals by the photographing element; At an in-plane displacement measurement position equipped with a computer that calculates information corresponding to in-plane displacement based on this image information and a monitor that visually displays the information calculated by this computer, An electro-optic crystal is placed in the optical path of one of the two sets of laser beams and changes the equivalent optical path length of the optical path by applying a voltage to the electro-optic crystal. In addition to being provided with an electro-optic crystal drive circuit that applies voltage to the crystal, the computer stores image information before the object to be measured is deformed, and issues an optical path length change command to the electro-optic crystal drive circuit after the object to be measured is deformed. The optical path when the difference in brightness of each pixel of the stored image information before and after the deformation of the measured object becomes zero by repeating storage of image information multiple times with the electro-optic crystal changing the equivalent optical path length. Based on this change, the laser light wavelength, and the angle between the direction of the laser beam irradiated to the workpiece and the normal direction of the workpiece, the displacement of the workpiece can be calculated from the measurement surface. The feature is that the calculation is performed for all pixels corresponding to , and information corresponding to the in-plane displacement is obtained.

[Effect]

The storage device stores image information obtained by the photographing element, and the computer reads the contents of this storage device at a time before the object to be measured is deformed. Subsequently, the computer gives an optical path length change command to the electro-optic crystal drive circuit after the object to be measured has been deformed. The electro-optic crystal driving circuit applies a voltage to the electro-optic crystal in accordance with this command. This causes the electro-optic crystal to change the optical path length. Scattered light from the surface of the object in this changed state is imaged by the image sensor and stored as image information in the storage device. The computer takes in this image information. Such operations are repeated multiple times. Thereafter, the computer calculates the difference in brightness of each pixel of the stored image information before and after the deformation of the object to be measured, and calculates the amount of change in the optical path length when this difference becomes zero. Then, the amount of displacement of the object to be measured is determined based on this amount of change, the wavelength of the laser beam, and the angle between the normal direction of the object and the direction of the laser beam irradiated onto the object. This calculation is performed for all pixels corresponding to the measurement surface to obtain information corresponding to in-plane displacement.

〔Example〕

Hereinafter, one embodiment of the in-plane displacement measuring device of the present invention will be described with reference to FIG.

In FIG. 1, a laser beam L ₁ emitted from a laser oscillator 1 has its beam diameter expanded by a beam expander 2 . This expanded laser beam _L2 is then bent by a mirror 3 and split into two directions by a beam splitter 4. An electro-optic crystal 5 is placed on one optical path of the branched laser beam. Then, the two laser beams L ₃ and L ₄ are bent by mirrors 6 and 6' and irradiated onto the surface of the information medium 7 to be measured from two directions symmetrical with respect to the normal direction of the surface of the information medium. Then, the splitter pattern generated by the laser light scattered on the surface of the information medium 7 is converted into an image signal by the television camera 8. The television camera 8 includes a lens 9, an aperture 10, an image sensor 11,
It is composed of an image sensor drive circuit 12. The average size of speckles caused by laser light scattered from the surface of the information medium 7 is
It is determined by the F value (aperture ratio) of 0 and the imaging magnification. The F value is 16, the imaging magnification is 1x, and the wavelength of the laser beam is
When the diameter is 0.633 μm, the average diameter of speckles is approximately 24.7 μm. As an image sensor that captures a two-dimensional image, for example, in a CCD sensor, there is an element with a pixel size of 18 μm x 12 μm, and by using such an image sensor, the above-mentioned change in the brightness of the speckle pattern can be realized. can be detected as an output signal of each pixel constituting the image sensor 11.

FIG. 2 shows the configuration of an electric circuit that performs image processing and drives the electro-optic crystal.

The image signal of the speckle pattern obtained from the television camera 8 is converted into a digital signal by the A/D converter 13 and is stored in a random access memory (RAM).
14 in sequence. The storage location of the image information in the memory is specified by address information given by the memory control circuit 15. After the computer 17 has finished loading the information for one image, the computer 17 sends the information to the memory control circuit 1 via the interface circuit 16.
5, the image information stored in the arbitrarily accessible storage device (RAM) 14 is taken in while changing the address, and is stored in the storage device 18.
Furthermore, the computer 17 sends an optical path length change command to the electro-optic crystal drive circuit 22 via the interface 21.
As a result, the drive circuit 22 can apply a voltage to the electro-optic crystal 5 to equivalently change the optical path length of the laser beam. Electro-optic crystal 5
For example, there is LiNbO ₃ , and when a voltage is applied to this crystal, the refractive index of the laser beam changes, and accordingly, the optical path length changes equivalently.

In this way, the computer 17 operates the electro-optic crystal drive circuit 22 via the interface 21, and changes the optical path length of one of the two sets of laser beams, while changing the speckle pattern obtained at this time. The information is stored in the storage device 18. That is,
When the speckle pattern is imaged by the television camera 8 and stored in the storage device 14, the computer 17 transfers the speckle pattern to the storage device 1 via the interface 16.
4 is stored in the storage device 18. Then, the difference in brightness is determined for each corresponding pixel between the speckle pattern before deformation and the speckle pattern imaged while repeatedly changing the optical path length after deformation and stored in the storage device 18, and this value is zero. In other words, the amount of change Δx in the optical path length at which the brightness becomes equal is determined for all pixels. Then, the amount of displacement of the object to be measured is measured based on this amount of change Δx, the laser beam wavelength λ, and the angle θ between the normal direction of the object and the direction of the laser beam irradiated to the object. Calculations are performed for all pixels corresponding to the surface. Through this calculation, information corresponding to the in-plane displacement on the surface of the information medium 7 is obtained.

The results of the in-plane displacement distribution of the information medium 7 thus obtained are sent to the RAM 14 via the interface 16.
and displayed on the monitor 20.

FIG. 3 shows the principle of speckle pattern generation. When the two beams of laser beams L ₃ and L ₄ are incident on the surface of the information medium 7, the laser beams L ₃ and
It can be considered that the laser light whose brightness changes periodically is incident in a direction parallel to the intersection of the plane formed by the central axis of each of _L4 and the plane on which the laser light is incident. It can be considered that this incident laser light undergoes a phase change due to the unevenness of the surface of the information medium 7 and is scattered, and then forms an image on the image pickup device 11 through the lens 9 and the aperture 10. When the angle between the incident direction of the laser beam and the normal direction (Z-axis direction) to the surface of the information medium 10 is θ, the period in which the brightness of the laser beam fluctuates is given by λ/(2sinθ). When the wavelength λ of the laser beam is 0.63 μm and θ=45°, the fluctuation period T is 0.45 μm. Illumination light having the light intensity distribution shown in FIG. On the surface, the spread scattered lights overlap and interfere with each other, creating a speckle pattern as shown in FIG. Note that in FIG. 6, the speckle pattern on the image pickup device is shown separated from the device. In the case of this optical system, the spread of light on the image sensor due to diffraction is 24.7 μm. Further, the degree of contribution to the formation of a speckle pattern caused by interference is determined by the light intensity distribution of the illumination light of the two-beam laser, and the larger the light intensity, the stronger the influence on the formation of the speckle pattern.

FIG. 5 shows the relationship between the output signals from the pixels constituting the image sensor 11 and the displacement of the minute elements resulting from the deformation of the information medium 10.

With the displacement of the microelements, as shown in Figure 3b, parts with different roughness, that is, phase states, on the information medium 7 will have a strong influence on the brightness of the speckle, and the randomness of the surface roughness will increase. The brightness of the interference light changes depending on the nature of the interference. As shown in Figure 5 a and b, the change in the output signal of each pixel varies depending on the location, but since the scattering conditions are almost the same every 0.45 μm, the changes in the output signal of each pixel are almost the same every 0.45 μm. An output signal appears.

As shown in FIG. 4, recording was performed using the above-mentioned principle without changing the optical path length due to the electro-optic crystal 5.
The speckle pattern on the surface of the information medium before deformation is stored in the storage device 18. Next, the speckle pattern on the surface of the information medium 7 after being deformed is stored in the storage device 18 . The deformed speckle pattern is stored in the storage device 18 each time by applying a voltage to the electro-optic crystal 5 in stages and changing the optical path length of one laser beam _L3 . As shown in FIG. 3c, when the optical path length of the laser beam _L3 is changed by Δx, the light quantity distribution of the illumination light produced by the interference of the two-beam lasers changes by Δx/2sinθ=0.71Δx.
If the brightness of the pixel (i, j) on the image sensor 11 before deformation is P ₀ , the brightness of this pixel changes to P ₁ due to a minute displacement of the corresponding position of the information medium 7 .

Let the images obtained by changing the optical path length Δx according to Δx = 0.653/n×(k-1) be m ₁ to m _o , and calculate the difference between the output signals of pixels (i, j) for each image. The image m _k in which the difference in 2 is zero indicates that the light intensity distribution of the illumination light shown in Fig. 3 has shifted and the irradiation conditions on the information medium 7 surface match the state before deformation (Fig. 3 a). It shows.

The relationship between the amount of change Δx in the optical path length and the amount of displacement S of the information medium 7 in a state where the difference between the output signals of each pixel is zero is expressed by the following equation using an integer n.

S=n·λ/2sinθ−Δx/2sinθ The second term on the right side of the above equation indicates the phase term of pixel (i, j). The measurement of this phase term is calculated for all pixel planes corresponding to the measurement surface of the information medium 7. Here, at a known displacement point on the measurement surface, for example, a fixed point, the difference in light intensity is zero when Δx = 0, the integer n becomes n = 0, and it is assumed that the in-plane displacement of adjacent points changes continuously. , find the integer term n for each point, and furthermore, if there are multiple images for which the difference between the output signals is zero, it is possible to select the image and determine the phase term Δx, and the difference between this value and the output signal is The state of in-plane displacement over the entire measurement surface of the information medium 7 can be determined from the value of the phase term Δx which becomes zero.

As described above, this embodiment has the effect that the in-plane displacement of the entire measurement surface of the information medium 7 can be measured with high precision based on the phase information at each point.

Furthermore, in the conventional measurement method using speckle interferometry, the intensity of the scattered light incident on the image sensor changes depending on the inclination of the surface to be measured, and therefore the peak value of the light amount at each pixel also differs, which appears as noise. . However, in this embodiment, instead of the difference in light amount, the change in optical path length that makes the difference in output signals zero,
That is, since a phase difference is used, noise is essentially reduced and highly accurate measurement becomes possible.

〔Effect of the invention〕

According to the present invention, the in-plane displacement of the information medium surface is
Since it is possible to measure the phase information rather than the brightness of the fringes by speckle interferometry, displacement can be measured with high precision, and the displacement state between the fringes can be continuously measured, which was previously impossible. Also,
Since the phase of each pixel is detected, it is not affected by the brightness of each pixel, and has the advantage of being able to remove speckle noise, which was a drawback of the conventional method.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an embodiment of the measuring device of the present invention, FIG. 2 is a block diagram of an electric circuit for image processing and driving the electro-optic crystal in FIG. 1, and FIGS. Figure 5 explains the principle of speckle pattern generation in the apparatus shown in Figure 1.
Figures a and b are diagrams showing changes in the output signal of a pixel due to displacement, Figure 6 is a diagram showing a speckle pattern on the image sensor, and Figure 7 is a diagram showing changes in the output signal due to a change in optical path length. . 1... Laser oscillator, 5... Electro-optic crystal, 8
...TV camera, 11...imaging device.

Claims (1)

[Claims] 1. A laser oscillator that emits laser light, a beam expander that expands the beam diameter of the laser light emitted from the laser oscillator, and the laser light that has passed through the beam expander is split into two groups. a beam splitter to irradiate the surface of the object to be measured, an optical system configured to irradiate the surface of the object to be measured with two sets of branched laser beams from two directions having equal angles to the normal to the surface of the object to be measured; A lens and an image sensor that capture a speckle pattern generated when laser light is scattered by unevenness on the surface of the object to be measured; a storage device that stores image information converted into an electrical signal by the image sensor; and a storage device that stores the image information. In an in-plane displacement measuring device comprising a computer that calculates information corresponding to in-plane displacement based on the information, and a monitor that visually displays the information calculated by the computer, the two sets are split by the beam splitter. an electro-optic crystal that is arranged in one optical path of the laser beam and changes the equivalent optical path length of the one optical path by applying a voltage; and an electro-optic crystal drive circuit that applies an electric voltage to the optical crystal, and the computer stores the image information before deforming the object to be measured, and stores the image information in the electro-optic crystal drive circuit after deforming the object to be measured. The storage of the image information in a state in which the electro-optic crystal changes the equivalent optical path length by giving the optical path length change command is repeated several times, and the brightness of each pixel of the stored image information before and after deformation of the object to be measured is determined. Find the amount of change in the optical path length when the difference between the two becomes zero, and based on the amount of change, the laser beam wavelength, and the angle between the normal direction of the object to be measured and the direction of the laser beam irradiated to the object to be measured. An in-plane displacement measuring device characterized in that the amount of displacement of the object to be measured is calculated for all pixels corresponding to the measurement surface to obtain information corresponding to in-plane displacement.