Frequency-Modulated Continuous-Wave (FMCW) technique has been introduced into digital holography (DH) using an injection-current-induced frequency modulation of a laser diode (LD) as a light source. Since the frequency of beat signals observed in the intensity variation of holograms captured by a high speed camera depends on the optical path length difference between the reference and the object light waves, the use of FMCW technique in DH can achieve selective reconstruction of an object located at a desired distance. As a preliminary consideration, both experimental and calculational studies were conducted for selective reconstruction of two objects at different positions. In addition, a polarization state of object wave transmitting through a PMMA sample was investigated using the technique in which the object wave interfered with two orthogonally linearly polarized reference waves having the different optical path lengths.
Digital holographic techniques to investigate drying processes of both paint films and ink dot is presented. The proposed
technique based on digital holographic interferometry can achieve both visualization of variations and analysis of
dryness of paint films in the drying process by using phase changes between two subsequent reconstructed complex
amplitudes of the reflected light from the film. To follow the drying processes, holograms are recorded at a constant time
interval. Phase-shifting digital holography has been applied to analyze the dryness of commercial paints applied on the
metal plate. For analysis of an ink dot having diameter of a few hundred micrometers, digital holographic microscopy is
applied to evaluating the time history of dryness of ink dot in the drying process. This paper describes these holographic
techniques applied to the commercially available paint and ink and presents some experimental results.
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 two laser diodes is applied to the measurement of the inner surface of a straight copper pipe and the detection of artificial defects such as holes, rust, and scratches made on the inner wall. To obtain the inner surface profile, a cone-shaped mirror (CSM) attached to a metal 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 inspection of the pipe has been performed by shifting the CSM stepwise along the pipe axis. The new algorithm in which a positional misalignment of the CSM can be directly obtained from the experimental height profile can reduce the load for calculation of correcting the distorted experimental height profile. The pipe inspection has been done using the developed images of both the intensity and height profile for the inner wall of the 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 combination method to study the drying process of paints, based on digital holography and gravimetric measurement, is proposed. The proposed method allows taking holographic measurement in a simultaneous way to compare the results obtained by the reconstructed image changes with gravimetric data. By directly comparing a phase change in the reconstructed images of a paint surface and weight change of the paint film, it is found that a stationary state of the paint surface detected by the phase change occurs in the last stage of solvent evaporation and corresponds to a dry-hard of the paint film. The proposed technique can also analyze dryness of clear coat having no scattering particle using the phase change. It is shown that the present technique can allow us to further investigate not only a film formation of clear coat but also an estimation of specific gravity of solvents by comparing directly the phase change with weight loss due to solvent evaporation in the simultaneous measurement.
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.
We propose a novel method for monitoring the drying process of a painted surface by using phase-shifting digital holography. In comparison with previous methods using speckle patterns, the proposed method can afford an intensity image for directly monitoring and local variations of drying without an imaging lens. It can also be used for surfaces of complex shapes. In addition, quantitative analysis utilizing a cross-correlation function and phase change derived from the reconstructed complex amplitude is performed and the drying time of paint for different areas and temperature is evaluated. The technique is also applied to monitoring the drying process of a complex surface of a lightbulb.
A new algorithm for fringe phase analysis is proposed for shape and deformation measurement of diffusely reflecting surfaces by digital holography. It derives the phase of the averaged conjugate product of complex amplitudes before and after wavelength change or object deformation, corresponding to the complex coherence factor, and is consistent with a theory on fringe formation in holographic interferometry. By experiments and computer simulations, it is demonstrated that the algorithm is much more immune to speckle noise in phase analysis and unwrapping than conventional methods.
Digital holography is applied to analyze the spatial distribution of the 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 preliminary object to confirm the principle of polarization analysis, a quarter wave plate (QWP) is used, and the distribution of the polarization ellipse over the object surface is analyzed for various azimuths of the QWP. By inserting a polarizer into a part of the object wave after the object as a reference area, a phase drift of the reference waves caused by polarization switching and/or environmental disturbances can be evaluated and compensated for by using the reference 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 stressed 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 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.
A simple method for surface contouring by phase-shifting digital holography is reported. Diffusely reflected light from a coherently illuminated object is recorded by a CCD with coherent in-line superposition of a reference beam subject to phase-shifting. From three phase-shifted in-line holograms the complex amplitude of the object wave at the CCD plane is derived to reconstruct phase distributions before and after wavelength shift that is provided by a mode-hopping induced by a change of injection current of a laser diode. The difference of the reconstructed phases corresponding to each of the wavelengths is proportional to surface height from the reference plane that is normal to the incident beam. This setup permitting normal incidence introduces neither the carrier component corresponding to oblique reference plane nor the shadowing effect that arise in the fringe projection and the dual incident angle methods requiring oblique illumination. In experiments plane surfaces tilted by various angles are first measured to evaluate accuracy. Then a spherical surface and a miniature valve are employed. The results are compared with one-dimensional simulations using random number model of surface roughness to exhibit good agreement.
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 Ta2O5 and 448.35 nm-thick SiO2 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.
A simple optical fiber water sensor using Tm3+:YAG fluorescent light is constructed. Highly sensitive measurement of water concentration in ethanol and methanol is achieved by use of the intensity ratio of fluorescent and pumping lights.
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