JP2898124B2 - Speckle image displacement measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for measuring a displacement (movement amount) of a speckle image which appears when an object is irradiated with a laser beam, that is, a speckle image displacement measuring apparatus.

[0002]

2. Description of the Related Art When a laser beam is irradiated on a rough surface of an object, a spot-like pattern called a speckle image appears in a space on the reflection side. Since the speckle image moves (displaces) in a fixed relationship with the displacement of the object, the displacement of the object can be known in a non-contact manner by measuring the displacement of the speckle image (for example, Tokiko Sho 61-2
No. 7681).

On the other hand, as a method of measuring the displacement of an image, a method is known in which the degree of coincidence between a reference image (image before movement) and a current image (image after movement) is used as an evaluation value. That is, for two images captured by an image sensor such as a CCD image sensor, that is, a reference image and a current image,
A method is known in which evaluation values related to the degree of coincidence, such as a cross-correlation function, a sum of squares of a difference, and a sum of absolute values of a difference, are calculated while being shifted by one pixel, and a position having the highest degree of coincidence is set as a displacement (movement amount) of the image. Have been. Then, in order to further increase the resolution, regression is performed on parabolic approximation or the like using the evaluation value of the position with the highest degree of coincidence and the evaluation values of positions in the vicinity thereof, and the position of the vertex is estimated, and the pixel sampling interval The following resolution is achieved (interpolation method).

[0004]

However, when a displacement is obtained at a resolution equal to or less than the pixel interval using such an interpolation calculation method, an error is generated due to the speckle image being a speckle image, and a sufficient error is not necessarily obtained. Measurement accuracy could not be obtained.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of such a problem, and has been developed in consideration of the amount of movement of a speckle image projected on an image sensor and the amount of movement of image data before and after movement. In a speckle image displacement measuring device that measures a degree of coincidence of image data as an evaluation value at a sampling interval of a pixel of an image sensor or less, a plurality of correction tables corresponding to a speckle average diameter, and the plurality of correction tables And means for performing correction using one correction table selected from the following. Further, it is desirable to provide a means for adjusting the average speckle diameter.

[0006]

When a displacement is obtained with a resolution equal to or less than the pixel interval using an interpolation calculation method, a periodic error occurs between the actual displacement and the displacement obtained by measurement. The inventors have found that the magnitude of the error varies depending on the average diameter of the speckles. Therefore, a plurality of correction tables corresponding to the average speckle diameter are prepared, and the average speckle diameter of the speckle image measured before the measurement is obtained, and the displacement measured by the correction table corresponding to the average speckle diameter is obtained. The amount can be corrected, and a more accurate displacement can be obtained. In addition, if a means for controlling the average speckle diameter is provided, the average diameter of the speckle can be adjusted according to a desired correction table, so that strict correction can be performed, and the displacement amount after correction is more accurate. Becomes

[0007]

FIG. 1 is a block diagram showing a speckle image displacement measuring apparatus according to an embodiment of the present invention. This device includes a CCD camera 1, an image quality improvement circuit 2, and an image displacement calculation circuit 3. Here, the CCD camera 1 has a screen size of 512 × 512 pixels and a pixel size of 11 × 1.
1 μm, and an interlace method of 25 frames / sec is adopted. The image displacement calculation circuit 3 includes four image processing units 31 to 34 and a control circuit 35 that controls these and performs predetermined arithmetic processing.
Each of the image processing units 31-34 includes an image memory (video RAM) 31a-34a and a control unit 31b-34b for controlling writing and reading of data.

Here, a method in which the image displacement calculating circuit 3 estimates the displacement (movement amount) of the two images with a resolution equal to or less than the pixel will be described with reference to FIG. First, a block of an arbitrary size (here, 3 × 3 pixels) is set at an arbitrary position in the speckle image. Next, as shown in equation (1), an image R larger than the block before displacement (here, 5 × 5 pixels)
With reference to (x, y), the displaced block image D (x, y)
y) and the absolute value of the difference between each pixel is integrated.

[0009]

(Equation 1)

This integration process is executed a plurality of times with the position shifted by one pixel. FIG. 2 (a), (b), (c)
Indicates that the integration process has been performed three times, shifted twice in the x direction. FIG. 2D shows the integration result S (u,
The value of v) is shown for each position. If the resolution is the same as the pixel sampling interval, the minimum integration result S
From the position (u _min , v _min ) where (u, v) is obtained, the displacement (movement amount) before and after the movement of the speckle image can be obtained. Here, the minimum value S (u _min ,
v _min ) and the value S (u _min −1,
v _min ), a parabola passing through S (u _min +1, v _min ) is obtained, and the position of the vertex is estimated below the pixel interval.
Here, although the integration process is performed only in the x direction, S (u _min , v _min
If -1) and S (u _min , v _min +1) are obtained and parabolic regression is performed, the position in the y direction can be estimated with a resolution equal to or less than the pixel interval.

However, if an attempt is made to obtain a resolution equal to or less than the pixel interval using such an interpolation calculation method, the speckle image does not match the actual displacement due to a speckle pattern. And it turned out that the degree of the mismatch depends on the magnitude of the average speckle diameter. FIG. 3 (a)
(B) and (c) are measurement results showing this state. In these figures, the horizontal axis represents the displacement of the object, and the vertical axis represents the displacement of the speckle image measured using the above-described interpolation method. 3A shows a case where the average speckle diameter is 10 μm, FIG. 3B shows a case where the average diameter is 14 μm, and FIG.
The case of m is shown. It can be seen that the displacement of the speckle image should originally be linear with respect to the movement of the object (in-plane displacement), but in any case, it will meander. Also, it can be seen that the degree of meandering is greater as the average speckle diameter is smaller. Therefore, the image displacement calculation circuit 3
Has a means for correcting the displacement measurement result obtained by the above-described interpolation method according to the average speckle diameter. That is, it includes a plurality of correction tables according to the average diameter of speckles, and an arithmetic unit for correcting the displacement obtained by the interpolation method using one of the correction tables.

When a lens imaging system is used (when a camera lens is used), the average speckle diameter is determined by three factors: the F number (aperture) of the CCD camera 1, the imaging magnification, and the wavelength of laser light. It is determined. That is, assuming that the F number of the CCD camera is F, the imaging magnification is m, and the wavelength of the laser beam is λ, the average speckle diameter can be represented by (1 + m) λF. The F number can be detected using an encoder on the camera lens. Since the imaging magnification is determined by the arrangement of the optical system, the position of each component constituting the optical system can be marked and measured. Further, the wavelength of the laser light can be known from a laser oscillator.
On the other hand, when the camera lens is not used, the average speckle diameter is 0.6Lλ / L, where L is the distance between the object 4 and the CCD camera 1, D is the laser beam diameter at the object irradiation unit, and λ is the wavelength of the laser light. D. The laser beam diameter may be measured after branching the laser beam with a half mirror. As other detection methods of the average speckle diameter, take the intensity profile of a part of the speckle image and examine it, or take the autocorrelation of the speckle image,
A method of examining the correlation value profile can be considered.

The image data supplied to the image displacement calculation circuit 3 is subjected to image quality improvement by the image quality improvement circuit 2. CC
Image noise output from the D camera 1 contains shot noise at the time of CCD reading and noise on an analog line. Therefore, if the image displacement calculation circuit 3 calculates using the image data output from the CCD camera 1 as it is, noise is also integrated, and the reliability of the value of the integration result S (u, v) is low. Therefore, if the position of the vertex of the regression parabola is estimated using the result below the pixel interval, the error based on noise is very large. The image quality improvement circuit 2 is a circuit for removing shot noise at the time of CCD reading and noise on an analog line from image data, and executes an image averaging (recursive filter) process as shown in Expression (2). .

Vn = Vn-1 + (Vin-Vn-1) / N (2) where Vn: output image, Vn-1: output image one frame before, Vin: input image, N: average addition Is a number. By averaging the images, it is possible to reduce the above-described random noise from the images.

Next, the overall operation of this embodiment will be described.
A laser beam 5 from an unillustrated He-Ne laser (50 mW) is collimated and applied to the object 4. CCD
The camera 1 captures a speckle image generated by the irradiation of the laser light 5. An image signal obtained by imaging by the CCD camera 1 is converted into an improved image signal that has been subjected to image quality improvement processing by an image quality improvement circuit 2, and each image processing unit 3
1 to 34 are input in parallel. The first image processing unit 31 uses the horizontal synchronization signal HD and the vertical synchronization signal VD as masters and the image processing unit 3 as other slaves.
2-34 and output to the CCD camera 1. In each image processing unit, a control unit 31b composed of a 32-bit CPU
-34b controls generation of addresses of the image memories 31a-34a in hardware, and selects external synchronization / internal synchronization,
Execute the setting of the vertical blanking period.

The control circuit 35 controls each image processing unit 31
For -34, the parameters of the number, size, and position of the blocks to be tracked are written into the memory in the control unit 31b-34b, and the measurement is started.
Are operated in parallel. Each image processing unit 31-34
Import the speckle image that has been subjected to image improvement processing,
The calculation of the expression (1) is performed for each block,
Write the end flag and displacement result to a specific address.
The control circuit 35 continuously captures the displacement results from the image processing units 31-34, removes errors based on the average speckle diameter using a correction table, and stores them as a displacement distribution and a strain distribution. FIG. 4A is a graph showing raw data of displacement obtained by using the interpolation method, and FIG. 4B is a graph showing correction using a correction table selected according to the average speckle diameter at that time. It is a graph which shows the result. In both graphs, the horizontal axis indicates the amount of movement (in-plane displacement) of the object, and the vertical axis indicates the measurement results at pixel intervals.

If a CPU for graphic display is used as the control unit 31b-34b, block transfer of a reference image is performed by writing the transfer source, transfer destination, and size in a specific register. I can do it. Further, the calculation time of the equation (1) can be reduced by assembling the calculation with an assembler.

When the average speckle diameter is an intermediate value between the average speckle diameters corresponding to a plurality of correction tables prepared in advance, the correction is performed by adopting the closest correction table or by using the nearest complement. The average value of the correction table is calculated and the result is used for correction. On the other hand, if the speckle average diameter control means is provided, the speckle average diameter can be adjusted according to a desired correction table, so that strict correction can be performed, and the displacement amount after correction can be further increased. It will be accurate. As a method of controlling the average diameter of speckle, there is a method of controlling the wavelength by using a tunable laser as a laser light source. When a camera lens is used, a method of controlling an F number using a camera lens capable of automatically controlling an F number (aperture), mounting each optical component on a table having an automatic guide function such as a linear stage, and setting each optical component There is a method of controlling the imaging magnification by adjusting the position. When a camera lens is not used, the camera can be mounted on a linear stage and controlled to adjust only the distance without changing the angular relationship of the object, or the beam diameter can be adjusted by passing the laser beam through a movable rotary aperture. There is a method to adjust. In the embodiment shown in FIG.
Can control the linear stages 6 and 7.

[0019]

As described above, according to the speckle image displacement measuring apparatus of the present invention, the measured displacement is corrected by using the correction table corresponding to the average speckle diameter, so that a more accurate displacement is obtained. be able to. In addition, if a means for controlling the average speckle diameter is provided, the average diameter of the speckle can be adjusted according to a desired correction table, so that strict correction can be performed, and the displacement amount after correction is more accurate. Becomes

[Brief description of the drawings]

FIG. 1 is a block diagram showing a speckle image displacement measuring apparatus according to an embodiment of the present invention.

FIG. 2 is a method (interpolation method) in which an image displacement calculation circuit 3 estimates displacement (movement amount) of two images with a resolution equal to or less than a pixel interval.
FIG.

FIG. 3 is a graph showing a relationship between a movement of an object and a measured value of a speckle image displacement.

FIG. 4 is a graph showing a relationship between a movement of an object, a measured value of a speckle image displacement, and a correction value thereof.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... CCD camera 2 ... Image quality improvement circuit 3 ... Image displacement calculation circuit 5 ... Laser light

Claims (2)

(57) [Claims] 1. A speckle which measures a moving amount of a speckle image projected on an image sensor at a sampling interval of a pixel of an image sensor or less as an evaluation value of a degree of coincidence between image data before moving and image data after moving. In the image displacement measuring device, a plurality of correction tables corresponding to a speckle average diameter, and a unit that corrects the measured displacement using one or two or more correction tables selected from the plurality of correction tables. A speckle image displacement measuring device comprising: 2. The speckle image displacement measuring apparatus according to claim 1, further comprising means for adjusting a speckle average diameter of the speckle image.