Search

An inner surface of straight pipe has been investigated using digital holographic technique. Both contour images for surface profile and color reconstructed images are used to the detection of artificial defects made on the inner wall of copper pipe. For the inspection, a cone-shaped mirror glued to one end of a metal rod is inserted into the pipe and illuminated from the other open end of the pipe. The new algorithm in which a deviation of the cone-shaped mirror from the pipe axis can be directly evaluated from the experimental height profile reduces the amount of calculation for correcting the distorted experimental profile. The pipe inspection has been done using the developed images of both the color intensity image and height profile for the inner wall of pipe.

Digital holographic profilometry using multiwavelength from laser diodes is applied to an inspection of inner surface of straight copper pipe and the detection of artificial defects such as a hole, rust and a scratch in its wall. To obtain the inner surface profile, a cone-shaped mirror attached to a rod having two acrylic spacers is inserted into the pipe and illuminated by the collimated laser beam from the other open end of the pipe. The measurement has been performed by shifting the mirror stepwise along the pipe. Distribution of an optical path length in the alignment is calculated and used to compensate for a distortion in the profile due to a positional misalignment of the mirror. The new algorithm to obtain the positional error is adopted for the compensation process and the shape and positions of defects in the inner wall can be investigated.

A digital holographic method for evaluation of an inner surface profile of tubes is proposed. For surface contouring, two wavelength method using an injection-current induced wavelength change of a laser diode is used. To obtain the inner surface profile, a cone-shaped mirror is inserted into the tube. Distribution of an optical path length in the experimental alignment is calculated and utilized to compensate for a distortion occurring in the experimental profile of the surface due to a deviation of a mirror position. The shape of two seals pasted on the inner surface can be evaluated by the method with a digital image processing. It is shown that the proposed method can visualize and detect three dimensional shape of defects on the inner surface of tubes.

Digital holographic technique has been applied to analyze the spatial distribution of polarization state of light transmitted through anisotropic objects by successive hologram recording using polarization switching of the reference wave by an optical fiber Faraday rotator. For a test object, a quarter wave plate (QWP) is used and the distribution of polarization ellipse over its field is analyzed for various azimuth of the QWP. By inserting a polarizer into a part of the object wave as a reference area, a phase drift of the reference waves caused by polarization switching and environmental disturbances can be estimated and compensated for by using the phase of the known polarization area. The principal axis and ellipticity are calculated and compared with their theoretical values. A good agreement between the experimental and theoretical values is observed. For an application, a photoelastic effect induced in a compressed PMMA specimen is also investigated.

This paper describes noncontacting material testing that detects displacement of laser speckle patterns by a digital speckle correlation technique. It tracks displacement of two separated positions on a specimen by separate movable heads having a laser diode and a CCD camera. Each head is driven under the feedback control that cancels the speckle displacement detected by a real-time correlator. The position of each head is monitored by an electronic encoder to derive elongation of the specimen. Strain is evaluated from the ratio of the elongation to the original interval of the measured positions. Stress-strain curve obtained from this method agreed well with a conventional method using line markings mechanically attached on the specimen. The measurement could be carried out under the same conditions and speed as the conventional material test at 500 mm/min.

Digital holographic technique has been applied to analyze the distribution of polarization state of transmitted light at the object surface by use of orthogonally polarized reference beam produced by a flint glass fiber Faraday rotator. The object composed of a glass plate and a layered sellophane tape is employed and the polarization distribution over the object surface is analyzed. It is shown that the proposed system can be applied for the whole field analysis of polarization state.

A new extensometer has been developed which needs no attachment of line markers or mechanical attachment on a specimen. An expanded beam from a laser diode is incident on the marker position of a specimen which is imaged by a lens on a C-MOS image sensor. The resultant laser-speckle patterns are analyzed by two-dimensional digital correlation at the rate of 20 frames per second. It provides speckle displacement by means of a phase-only-correlation device which uses only phase of Fourier transform of the image. In-plane displacement of the marker position is tracked by moving a head containing the laser and the image sensor under the feedback control that compensates for the speckle displacement detected. Two positions on the specimen are tracked by a pair of the heads. From rubber specimens which had a marker distance of 20 mm and were elongated at the velocity of 500 mm/sec we observed good agreement in load-strain curves with the results from the conventional methods using mechanical trackers.

A new type of Faraday effect optical current transformer has been developed which uses a square Flint (FR) glass block with dielectric-coated total reflection surfaces as the sensing element. Numerical calculation has shown that coating two dielectric layers of 45.59 nm-thick Ta 2 O 5 and 448.35 nm-thick SiO 2 films on an FR glass surface gives rise to no-retardation total internal reflection for the 45 degree(s) incident angle light beam at (lambda) equals840 nm and has large tolerances for film thickness, wavelength and incident angle. A fiber-linked current transformer has been constructed and exhibited high performances including high isolation from surrounding currents and mechanical stability.

A closed loop phase shifting Fizeau-type interferometer is constructed by using the direct frequency modulation (FM) of a laser diode. The interferometer is servo controlled fully in the phase domain where optical phases are detected by a two-frequency optical heterodyne method. Detailed study on stabilization of the interferometer under feedback control is conducted both experimentally and theoretically. The interferometer showed a good stabilization against the applied vibration up to 200 Hz. The profile measurement of a mirror surface was conducted by the phase shifting analysis algorithm and a good measurement reproducibility of (lambda) /60 in the root mean square value was obtained for 10 times measurements in a period of 20 min.