2D-displacement measurement with real time image processing is developed. The target of camera image is a lattice pattern drawn on the paper sheet. The range of measurement depends only on the size of target sheet, so that high resolution power as well as large measuring range are realized with proposal method. The histogram distribution of the image of the lattice pattern is calculated, and the position of the grid lines are detected from the peak positions of the distribution. By continuously performing this detection, it is possible to calculate the movement of the image. From the target grid lines spacing (in mm) and the grid lines spacing on the image (in pixels), the calculation of the displacement is done automatically. The features of this method are that real-time measurement can be performed with high-speed processing, measurement can be easily performed simply by setting the grid line interval (mm), and measurement can be performed with sub-pixel accuracy. In this research, the measurement accuracy and the tolerance to poor image quality of the proposed method were evaluated. As a result, it has been shown that the position of the grid line can be detected with sub-pixel accuracy even in a poor-quality image. As an application of this method, the relative displacement between the base and tested building was obtained with the measurement during 3-dimensional vibration tests.
Birefringence measuring equipment currently used has either a high-sampling-rate or large-area measurement without high-spatial resolution. A birefringence distribution measurement of 10 kilopixels or more with high-spatial resolution at a sampling rate of 10 kHz or more has yet to be achieved. To develop the elemental technology to achieve this, this study proposes preliminary equipment, namely, a circular polariscope with a polarized laser, high-speed camera, and photoelastic modulator (PEM). The light source used was a 5-mW He–Ne laser with a wavelength of 632.8 nm; the light-receiving element was a high-speed camera with a photographing speed of 200 kHz, and harmonic analysis of the light intensity was achieved with a PEM. To demonstrate the possibility of high-speed measurement with the proposed equipment, the birefringence of a variable-wave plate was measured using one pixel of the high-speed camera. As a result, the maximum error of the measured value was 5.3% in the birefringence range of 10–20 nm of the specimen, 7.9% between 20–40 nm, and 4.0% between 40–60 nm. To show that measurement is also possible when the imaging range is expanded, the birefringence distribution in the area of 60 × 60 µm of the 40-nm wave plate was measured using 8 × 8 pixels of the high-speed camera. This method returned a smooth monotonously varying birefringence distribution close to 40 nm. The result shows that expanding the 8×8 pixels to 10 kilopixels can achieve the aforementioned research goals.
This paper introduces the principle and execution of a new method for measuring of distributed minute birefringence
based on simple polarimetry with phase-shifting method. The new method requires only three stepped photoelastic data
although conventional phase-stepping methods require four or more. To evaluate the new method experimentally, two
precise crystal wave plates having nominal retardation ± tolerance of 79.1±3.5 and 10.0±4.7 nanometers were used as
specimens. The experimental averages of the distributed retardation in the specimens with standard deviations were
found to be 80.2±15.0 and 18.8±7.06 nanometers. To estimate the measurement accuracy of the angular orientations of
the distributed birefringence in the specimens, the angular positions of the rotation stage for the specimens were rotated
intermittently 45 or 30 degrees at a time during the experiment. As a result, the averages of measured offsets of the
angular orientations were found to be 30.1±8.14 for the specimen of 79.1 nanometers with standard deviations. It is
concluded that the new method has potential of measuring for distributed minute birefringence.
We developed an optical birefringence measurement equipment by using a photoelastic modulator and a polarized laser. A He-Ne infrared laser is used as a light source to measure the optical birefringence in silicon wafers. We explain the theory and process of the measurement of stress in silicon wafers. The magnitude of principal stress difference and also the direction of the principal stress are obtained simultaneously and quantitatively using our experiment. The optical birefringence of (100), (111) and (110) face silicon stressed specimens were measured. From the experimental results, the photoelastic constant depends on the crystalline orientation. By the stress-strain analysis of silicon single crystal, it was found that the relation between the principal strain difference and the retardation was independent of crystalline orientation.
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