Jung-Taek Oh

Polarization-sensitive optical coherence tomography for photoelasticity testing of glass/epoxy composites

We measure the spatial distribution of the mechanical stress induced inside translucent glass/epoxy composites by means of polarizationsensitive optical coherence tomography. The Stokes parameters determined from two orthogonal polarization components of the backscattered light allow the internal stress to be identified in terms of its magnitude and principal direction based on a birefringence light scattering model of glass/epoxy composites. Measurement examples show the particular case of stress concentration near a through hole and the internal structural damages caused by excessive tensile loading.

Polarization-sensitive speckle spectroscopy of scattering media beyond the diffusion limit

A scattering-media-characterization method that uses partially coherent radiation and polarization discrimination of multiply scattered light is described. The method is based on an analysis of the dependence of speckle contrast on the coherence length of the probe light. Polarization discrimination of detected speckles makes it possible to select scattered-light components that propagate in the probed medium at different distances. A theoretical analysis of the polarization-dependent speckle contrast as influenced by the probe-light coherence and parameters of the probed medium is presented. Experimental results obtained with various nondiffuse scattering samples are presented.

Two-frequency phase-shifting projection moire topography

Adopting phase-shifting technique in moire topography provides many advantages in measuring complex surface profiles with varying reflectance. However, still the so- called 2(pi) -ambiguity problem remains, which limits the maximum measurable step height difference between two neighboring sample points to be less than half the equivalent wavelength of moire fringes. To cope with the problem in this investigation, a two-wavelength scheme of projection moire topography is proposed along with necessary hardware design considerations. Test results prove that the proposed scheme is capable of finding absolute fringe orders automatically, so that the 2(pi) -ambiguity problem can be effectively overcome so as to treat large step discontinuities in measured surfaces.

Scanning projection grating moire topography

One problem with moire topography for 3D surface metrology is the so-called 2(pi) -ambiguity limiting the maximum step height difference between two neighboring samples points to be less than half the equivalent wavelength of moire fringes. To cope with the ambiguity problem, a special scheme of scanning moire technique is proposed by resorting to the frequency domain fringe analysis that is in fact originated from white light scanning interferometry. This new more principle of 3D measurement allows determining the absolute height of the surface without information on absolute fringe orders so that largely stepped surfaces are measured with a great improvement in accuracy.

Speckle contrast measurements with changeable coherence length: the method of scattering media probing

We tried to probe photon path statistics by analyzing the change of speckle contrast with different coherence length of source. The speckle contrast is strong function of both photon path statistics inside scattering media and coherence function of a source. By introducing explicit formula for speckle contrast, we can relate photon path statistics with measured speckle contrast and coherence of source in reverse manner. To realize this idea, we formulated this relationship and performed several MC simulations and basic experiments for various scattering media. Although the present result is very rough, it shows some possibility of using this method to find photon propagation statistics inside turbid media such as tissues and thin polymer film.

Three-dimensional profile measurement of fine objects by phase-shifting shadow moire interferometry

An industrial application of phase-shifting shadow moire interferometry for automatic 3D inspection of fine objects is presented. A line grating is used to generate shadow type moire fringes whose relative phases are readily determined by implementing the principle of phase shifting so that the surface height of the object can be measured. A special phase-measuring algorithm, named the A-bucket algorithm, is used which can precisely computes the relative phases even though there exists a significant level of errors in phase shifting due to miscalibration and external vibration. Finally, several experimental cases are discussed to demonstrate that a measuring accuracy in the order of 0.001 mm can practically be achieved.

Polarization-sensitive low-coherence speckle spectroscopy of scattering media

The method of scattering media probing with the use of low-coherent light source with the controllable width of emission spectrum is considered. The contrast of partially coherent speckles is suggested as the diagnostical parameter. The additional polarization discrimination of detected speckles gives the possibility to select the components of scattered field which propagate in probed medium at different distances. Experimental results obtained for weakly ordered systems characterized by non-diffuse scattering regimes are presented.

Two-Wavelength Phase-Shifting Projection

[] technique is now being extensively investigated as a fast non-contact means of three-dimensional profile measurement especially for reverse engineering. One problem with technique is so called -ambiguity problem that limits the maximum step height difference between two neighboring sampling points to be less than half the equivalent wavelength of fringes. In this investigation, a new two-wavelength scheme of projection topography is proposed and tested to cope with the -ambiguity problem. Experimental results are discussed to assess the new method in measuring large objects with high step discontinuities.

Moir\acute{e}

We present a method of projection moire specially devised for the three-dimensional inspection of printed circuit boards. This method incorporates phase-shifting technique in analyzing moire fringes so as to achieve a fine resolution of 1 micron in height measurement. Further a synchronous grating translation scheme enhances the lateral measuring resolution by inherently removing the original pattern of the reference grating in resulting moire fringes. Finally we discuss the advantages of the proposed method using several measurement results performed on the various types of solder paste silk-screened on printed circuit boards.