Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “INTERFEROMETRY”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 955 records · Page 53Linked to original sources
PubMed1998

Effect of healed tympanic membrane perforations on umbo velocity in the rat.

Heterodyne laser interferometry was used to measure tympanic membrane (TM) velocity at the umbo during acoustic stimulation in 2 groups of rats. The control group had a normal TM, while in the other group the TM was perforated and then allowed to heal for at least 28 days. Umbo velocity functions for constant sound pressure level stimuli were obtained for test tones between 0.3 and 40.0 kHz in each animal. The results revealed that velocity was the same in the control and healed TMs below 3.0 kHz. Above 5.0 kHz, the velocity response in the healed ear was between 3 and 12 dB smaller than in the control ears. Histologic evaluation of the healed perforation revealed a thick fibrous reaction between the epidermal and lamina propria layers. The results indicated that the added mass of the scar tissue changed the middle- and high-frequency TM responses.

Acoustic Stimulation↗
PubMedDate not supplied

Determination of arbitrary tooth displacements.

The noninvasive, modern technique based on the method of double-exposure hologram interferometry was used to measure arbitrary displacements of teeth in the three dimensional space. The experimental studies were carried out on an idealized model of the maxillary central incisor. The results show that the experimental data, based on component loading, are inadequate to accurately predict tooth displacement from an arbitrary force acting in the three dimensional space.

Alveolar Process↗
PubMed1996

Determination of lubricating film thickness for permeable hydrogel and non-permeable polyurethane layers bonded to a rigid substrate with particular reference to cushion form hip joint replacements.

The lubricating film thickness in a model of compliant layered bearings, using both permeable hydrogels and non-permeable polyurethane elastomers for total hip joint replacements, has been measured using optical interferometry, under both entraining and squeeze-film motion. The film thickness in the lubricated contact was measured for both water and a 40 per cent glycerol solution in water as a function of entraining velocity and squeeze-film time. The measured lubricating film thickness for the permeable hydrogel was compared to that of the non-permeable polyurethane elastomer and little difference was found when the lubricating film thickness was sufficiently large (greater than 150 nm). Comparison of the experimental results and the theoretical predictions based upon elastohydrodynamic lubrication analysis showed good agreement in the entraining experiments where the film thickness was greater than 150 nm. In the squeeze-film experiments the experimental measurements were greater than the theoretical predictions for all squeeze times due to the formation of a central pocket of fluid which was not predicted by the simple theory used. This also occurred for the hydrogels for films greater than 150 nm. For longer squeeze times the film thickness for the hydrogel fell below the theoretical prediction. This was considered to be due to the permeability of the hydrogel reducing the film thickness when the film thickness was less than 150 nm. The permeability of the hydrogel was not modelled in the theoretical lubrication analysis used in this study.

Biomechanical Phenomena↗
PubMed2002

High-speed, high-sensitivity, gated surface profiling with closed-loop optical coherence topography.

We describe and experimentally demonstrate a novel (to our knowledge) surface profiling technique, for which we propose the term closed-loop optical coherence topography. This technique is a scanning beam, servo-locked variation of low-coherence interferometry. It allows for the sub-wavelength-resolution tracking of a weakly scattering macroscopic-scale surface, with the surface profile being directly output by the controlling electronics. The absence of significant real-time computational overhead makes the technique well suited to high-speed tracking. The use of a micrometer-scale coherence gate efficiently suppresses signals arising from structures not associated with the surface. These features make the technique particularly well suited to real-time surface profiling of in vivo, macroscopic biological surfaces.

Animals↗
PubMed2003

Dispersion in a grating-based optical delay line for optical coherence tomography.

Optical coherence tomography (OCT) is a high-resolution imaging technology based on low-coherence interferometry. When OCT imaging is performed in biological tissue, dispersion almost inevitably occurs. We quantify the group-velocity dispersion that a grating-based optical delay line may induce and its contribution to the axial point-spread function of OCT. Among the practical reasons for modeling the dispersion in grating-based optical delay line is that, at maximum compensation, it can provide insight into the dispersive properties of tissues.

Computer Simulation↗
PubMed2005

Dispersion control with a Fourier-domain optical delay line in a fiber-optic imaging interferometer.

Recently, Fourier-domain (FD) optical delay lines (ODLs) were introduced for high-speed scanning and dispersion compensation in imaging interferometry. We investigate the effect of first- and second-order dispersion on the photocurrent signal associated with an optical coherence imaging system implemented with a single-mode fiber, a superluminescent diode centered at 950 nm +/- 35 nm, a FD ODL, a mirror, and a layered LiTAO3 that has suitable dispersion characteristics to model a skin specimen. We present a practical and useful method to minimize the effect of dispersion through the interferometer and the specimen combined, as well as to quantify the results using two general metrics for resolution. Theoretical and associated experimental results show that, under the optimum solution, the maximum broadening of the point-spread function through a 1-mm-deep specimen is limited to 57% of its original rms width value (i.e., 8.1 microm optimal, 12.7 microm at maximum broadening) compared with approximately 110% when compensation is performed without the specimen taken into account.

Animals↗
PubMed2005

Phase extraction from a single fringe pattern based on guidance of an extreme map.

A method for automatic phase extraction from a single fringe pattern based on the guidance of an extreme map is introduced. The method uses an adaptive weighted filter to reduce noise and enhance contrast and to locate the fringe extremes. Wrapped phase values are calculated by use of an arccosine function obtained from the extreme map. With this method, wrapped phase values can be efficiently demodulated from a single fringe pattern without the need for assigning fringe order or interpolating fractional fringe order. The validity of the method is demonstrated by use of closed-fringe patterns generated by digital speckle interferometry.

Algorithms↗
PubMed1993

Aberration-free measurements of the visibility of isoluminant gratings.

We developed a new apparatus and psychophysical technique to extend isoluminant contrast-sensitivity measurements to high spatial frequencies. The apparatus consists of two identical laser interferometers that are designed to produce phase-locked two-color interference fringes on the retina without the influence of diffraction and most aberrations in the eye. However, even with interferometry, transverse chromatic aberration of the eye can produce a wavelength-dependent phase shift in the interference fringes, which can be exaggerated by head movements. To reduce the effect of head movements, isoluminant red and green interference fringes of equal spatial frequency and orientation were drifted slowly in opposite directions to guarantee a purely isochromatic (in phase) and a purely isoluminant (out of phase) stimulus during each cycle of stimulus presentation. With this technique we found that observers could resolve red and green stripes at spatial frequencies higher than 20 cycles per degree (c/deg) (20-27 c/deg), substantially higher than has previously been reported. This places a lower bound on the sampling density of neurons that mediate color vision. At all spatial frequencies, even those above the isoluminant resolution limit, a relative phase of the red and the green components could be found that obliterated the appearance of luminance modulation at the fringe frequency. Above the resolution limit, red-green-isoluminant interference fringes are seen as spatial noise, which may be chromatic aliasing caused by spatial sampling at some stage in the chromatic pathway.

Color Perception Tests↗
PubMed1996

Coherent imaging of the cone mosaic in the living human eye.

A new system for the recording of high-resolution images of the cone mosaic in the living room fovea has been developed. The experimental method is inspired by stellar speckle interferometry, used in astronomy to resolve binary stars. Series of short-exposure images of small areas of the fovea are registered under coherent illumination. These images show speckle patterns that have some correlation with the topography of the cone mosaic and retain high-resolution information. Such correlation is better revealed in the power spectrum (square modulus of the Fourier transform). The signal-to-noise ratio is increased, without loss of high frequencies, by averaging the power spectra of a number of such speckle patterns. The average power spectra show, in most of the cases, an elliptical ring (or hexagon), whose mean radius corresponds to the characteristic spatial frequency of the cone mosaic (or the inverse of the mean row-to-row cone spacing) at a given retinal location. Good results are obtained in the five normal observers tested, at various retinal eccentricities, up to 1 visual degree, including the center of the fovea for two eyes. We find a decrease in the spatial frequency of the mosaic with the eccentricity and an important intersubject variability, in agreement with anatomical studies.

Adult↗
PubMed2000

Interferometric data analysis based on Markov nonlinear filtering methodology.

For data processing in conventional phase shifting interferometry, Fourier transform, and least-squares-fitting techniques, a whole interferometric data series is required. We propose a new interferometric data processing methodology based on a recurrent nonlinear procedure. The signal value is predicted from the previous step to the next step, and the prediction error is used for nonlinear correction of an a priori estimate of the parameters phase, visibility, or frequency of interference fringes. Such a recurrent procedure is correct on the condition that the noise component be a Markov stochastic process realization. The accuracy and stability of the recurrent Markov nonlinear filtering algorithm were verified by computer simulations. It was discovered that the main advantages of the proposed methodology are dynamic data processing, phase error minimization, and high noise immunity against the influence of non-Gaussian noise correlated with the signal and the automatic solution of the phase unwrapping problem.

Algorithms↗
PubMed2000

Network approaches to two-dimensional phase unwrapping: intractability and two new algorithms.

Two-dimensional (2-D) phase unwrapping, that is, deducing unambiguous phase values from a 2-D array of values known only modulo 2pi, is a key step in interpreting data acquired with synthetic aperture radar interferometry. Noting the recent network formulation of the phase unwrapping problem, we apply here some well-established ideas of network theory to formalize the problem, analyze its complexity, and derive algorithms for its solution. It has been suggested that the objective of phase unwrapping should be to minimize the total number of places where unwrapped and wrapped phase gradients differ. Here we use network constructions to show that this so-called minimum L0-norm problem is NP-hard, or one that complexity theory suggests is impossible for efficient algorithms to solve exactly. Therefore we must instead find approximate solutions; we present two new algorithms for doing so. The first uses the network ideas of shortest paths and spanning trees to improve on the Goldstein et al. residue-cut algorithm [Radio Sci. 23, 713 (1988)]. Our improved algorithm is very fast, provides complete coverage, and allows user-defined weights. With our second algorithm, we extend the ideas of linear network flow problems to the nonlinear L0 case. This algorithm yields excellent approximations to the minimum L0 norm. Using interferometric data, we demonstrate that our algorithms are highly competitive with other existing algorithms in speed and accuracy, outperforming them in the cases presented here.

Algorithms↗
PubMed2004

Detection of ultrasound-modulated photons in diffuse media using the photorefractive effect.

Ultrasound-modulated optical tomography is a dual-wave sensing technique in which diffusive light in a turbid medium interacts with an imposed acoustic field. A phase-modulated photon field emanates from the interaction region and carries with it information about the optomechanical properties of the medium. We present a technique for detection of ultrasound-induced optical phase modulation using an adaptive, photorefractive-crystal-based interferometry system. Experimental results are presented demonstrating detection of ultrasound-modulated signals in highly scattering media by use of pulsed ultrasound insonation.

Image Enhancement↗
PubMed2005

Fusion of conventional ultrasound imaging and acousto-optic sensing by use of a standard pulsed-ultrasound scanner.

Acousto-optic sensing (AOS) is a dual-wave sensing technique based on the ultrasound modulation of diffuse light in a turbid medium. We experimentally demonstrate the feasibility of combining AOS and conventional ultrasound imaging by use of a commercially available pulsed-ultrasound scanner coupled with a photorefractive crystal-based optical interferometry system. Optically absorbing targets embedded in highly diffusive phantoms (mus'= 10 cm(-1)) are imaged through a thickness of 27 mm with millimeter resolution. The acousto-optic images are intrinsically coregistered with the ultrasound images.

Acoustics↗
PubMed2005

Miniature fiber-optic multicavity Fabry-Perot interferometric biosensor.

A fiber-optic sensor is designed based on multicavity Fabry-Perot interferometry for the study of optical thickness in self-assembled thin-film layers. This miniature sensor is applicable not only to the measurement of self-assembled polyelectrolyte layers but also to the immobilization of proteins such as immunoglobulin G (IgG). The binding of IgG and the corresponding antigen is observed, and the nonspecific binding characteristics are investigated. The optical thickness changes are used to evaluate the immobilization of the IgG and the immunological activities of the immobilized layers.

Biosensing Techniques↗
PubMed2005

Adaptive monogenic filtering and normalization of ESPI fringe patterns.

A technique is presented for filtering and normalizing noisy fringe patterns, which may include closed fringes, so that single-frame demodulation schemes may be successfully applied. It is based on the construction of an adaptive filter as a linear combination of the responses of a set of isotropic bandpass filters. The space-varying coefficients are proportional to the envelope of the response of each filter, which in turn is computed by using the corresponding monogenic image [Felsberg and Sommer, IEEE Trans. Signal Process. 49, 3136 (2001)]. Some examples of demodulation of real Electronic Speckle Pattern Interferometry (ESPI) images patterns are presented.

Algorithms↗
PubMed2006

Fourier transform ultraviolet-visible spectrometer based on a beam-folding technique.

A beam-folding technique in optical interferometry, where the number of beam folds used can be very large, is reported. This technique can be used as a low-cost position-tracking method in a Fourier transform spectrometer (FTS) to cover the broad spectral range from UV to IR. The main advantage gained is the simple position-tracking algorithm used in sampling the interferogram. We have developed a UV-visible FTS, whose wavelength coverage is limited only by the optical elements (350 nm(-1) microm with off-the-shelf components). Preliminary results show that it can achieve a resolution of approximately 4 cm(-1) even with a ball-bearing translation stage.

Algorithms↗
PubMed2006

High-spectral-resolution coherent anti-Stokes Raman scattering with interferometrically detected broadband chirped pulses.

To achieve high-spectral-resolution multiplex coherent anti-Stokes Raman scattering (CARS), one typically uses a narrowband pump pulse and a broadband Stokes pulse. This is to ensure a correspondence between anti-Stokes and vibrational frequencies. We obtain high-resolution CARS spectra of isopropanol, using a broadband chirped pump pulse and a broadband Stokes pulse, by detecting the anti-Stokes pulse with spectral interferometry. With the temporally resolved anti-Stokes signal, we can remove the chirp of the anti-Stokes pulse and restore high spectral resolution while also rejecting nonresonant scattering.

Equipment Design↗
PubMed2006

Frequency-comb-based interferometer for profilometry and tomography.

A novel interferometry technique using a frequency-comb light source is proposed for surface profilometry and tomography of discontinuous objects. Surface profile measurement is performed by sweeping the comb interval frequency without mechanical scanning. Step heights of 0.5 and 1.0 mm are successfully measured by use of the scheme with 9 mum accuracy.

Algorithms↗