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Results for “Refractometry”

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At least 1,243 records · Page 69Linked to original sources
PubMed1992

Modeling the power of the aging human eye.

A hypothesis is presented that may explain why the aging eye does not become myopic with age. The power of the eye lens is predicted with a modeling approach to determine how the form of the refractive-index gradient within the lens can change to maintain a constant power in spite of age-related curvature increase. Methods used include published age-dependent data on the optical parameters of the eye, a mathematical model of the lens based on elliptical isoindicial contours, and a refractive-index profile that can be expressed as a power series in the distance from the lens center. The kinds of change in profile required to prevent the eye from becoming myopic as its lens grows are shown.

Aging↗
PubMed2003

Projected index computed tomography.

Projected index computed tomography (PICT) is a new imaging technique that provides a computed reconstruction of the index of refraction of a sample. PICT makes use of data from standard optical coherence tomography images taken from several view angles to determine a mapping of the refractive indices of the sample. A rectilinear propagation model is assumed, so the data are understood to be related to the line integral of the refractive index in the beam paths. These data thus provide a set of angular projections of the sample. The spatial distribution of the index of the object may then be reconstructed by use of standard filtered backprojection techniques. The resultant PICT images are free of the spatial distortion that is inherent in standard optical cross-sectional images and correspond well to the manufactured dimensions of specific samples.

Image Processing, Computer-Assisted↗
PubMed2004

Ultracompact biochemical sensor built with two-dimensional photonic crystal microcavity.

We report an experimental demonstration of an ultracompact biochemical sensor based on a two-dimensional photonic crystal microcavity. The microcavity, fabricated on a silicon-on-insulator substrate, is designed to have a resonant wavelength (lambda) near 1.5 microm. The transmission spectrum of the sensor is measured with different ambient refractive indices ranging from n = 1.0 to n = 1.5. From observation of the shift in resonant wavelength, a change in ambient refractive index of delta(n) = 0.002 is readily apparent. The correspondence between absolute refractive index and resonant wavelength agrees with numerical calculation to within 4% accuracy. The evaporation of water in a 5% glycerol mixture is also used to demonstrate the capability for in situ time-resolved sensing.

Biosensing Techniques↗
PubMed2004

Schematic eye with a gradient-index lens and aspheric surfaces.

A new schematic eye with aspheric surfaces and a radially varying refractive-index distribution lens is proposed. Image quality and spherical aberration are determined by use of ray tracing, and the results are presented as spot diagrams and compared with five existing model eyes. The proposed model provides the best image quality and lowest spherical aberration.

Computer Simulation↗
PubMed2004

Linear optical coherence tomography system with a downconverted fringe pattern.

Linear optical coherence tomography (LOCT) systems are a simple and robust alternative to time-domain optical coherence tomography systems, but a detector with approximately 10(4) pixels is needed for an imaging depth of 2 mm. We present a new system for LOCT with a special mask attached to the image sensor. The mask essentially performs a downconversion of the spatial frequencies by multiplication with a second spatial frequency. This reduces the fringe frequency of the optical coherence tomography signal so that the signal can be sampled with fewer pixels.

Equipment Design↗
PubMed2004

Phantom validation and in vivo application of an inversion procedure for retrieving the optical properties of diffusive layered media from time-resolved reflectance measurements.

An experimental validation of an inversion procedure for retrieving the optical properties of layered media from multidistance time-resolved reflectance measurements is presented. The results cover a wide range of optical properties, showing excellent effectiveness and reliability of the procedure in reconstructing the optical properties of a two-layered medium. The optical properties of the first layer and the absorption of the second layer could be retrieved with excellent precision, whereas the reduced scattering coefficient of the second layer was reconstructed with a large error. The first layer thickness could be retrieved with an error less than 50%. An example of retrieval of the optical properties of muscle from in vivo measurements during an arterial occlusion is shown.

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↗
PubMed2004

Superresolution of three-dimensional optical imaging by use of evanescent waves.

We simulate a three-dimensional optical diffraction tomography experiment in which superresolution is achieved by illuminating the object with evanescent waves generated by a prism. We show that accounting for multiple scattering between the object and the prism interface is mandatory to obtain superresolved images. Because the Born approximation leads to poor results, we propose a nonlinear inversion method for retrieving the map of permittivity of the object from the scattered far field. We analyze the sensitivity to noise of our algorithm and point out the importance of using incident propagative waves together with evanescent waves to improve the robustness of the reconstruction without losing the superresolution.

Algorithms↗
PubMed2005

X-ray phase-attenuation duality and phase retrieval.

Phase retrieval is the key to quantitative x-ray phase-contrast imaging. To retrieve the phase image of an x-ray wave field, in general one needs multiple phase-contrast images. We have made a new observation of phase-attenuation duality for soft tissues, and we show how only a single phase-contrast image is needed for successful phase retrieval based on this duality. The phase-retrieval formula based on a single phase-contrast image of inhomogeneous soft tissue is derived and presented. We show the striking enhancement of the tissue contrast in simulated phase images that this new approach produces.

Algorithms↗
PubMed2005

Bulk optical parameters of porcine skin dermis at eight wavelengths from 325 to 1557 nm.

We introduce a method with which to obtain accurately the bulk optical parameters of fresh biological tissues in vitro by combining measurements of the sample surface profiles, reflectance, and transmittances with Monte Carlo-based inverse calculations. The bulk optical parameters of fresh porcine dermis tissue were determined at eight wavelengths from 325 to 1557 nm and were found to be much different from those determined without consideration of surface roughness.

Algorithms↗
PubMed2005

Nanolayer characterization through wavelength multiplexing of a microsphere resonator.

We optically characterize nanolayer (<150 nm) formation in situ on a silica microsphere in an aqueous environment by simultaneously following the shifts of whispering-gallery modes at two wavelengths. This approach was inspired by layer perturbation theory, which indicates that these two measurements can be used to determine independently both the thickness and the optical dielectric constant. The theory is verified for extreme cases and used to characterize a biophysically relevant hydrogel nanolayer with an extremely small excess refractive index of 0.0012.

Biosensing Techniques↗
PubMed2005

In situ determination of refractive index and size of Bacillus spores by light transmission.

Light-extinction measurements in the wavelength range of 400 to 1000 nm are performed in situ on Bacillus subtilis endospores during heat-shock-induced activation. Simultaneous information on particle size and refractive indices during activation is calculated from the transmission spectra by use of the Gaussian ray approximation of anomalous diffraction theory. During activation the refractive index of the core decreases from 1.51 to 1.39, and the size increases from 0.38 to 0.6 microm.

Bacillus subtilis↗
PubMed2005

Dynamic instability of self-induced bidirectional waveguides in photorefractive media.

We report on the experimental observation of a dynamic instability in the interaction of counterpropagating self-trapped beams in a photorefractive strontium barium niobate crystal. While the interaction of copropagating spatial optical solitons exhibits only transient dynamics, resulting in a final steady state, the counterpropagating geometry supports a dynamic instability mediated by intrinsic feedback. Experimental observations are compared with and found to be in qualitative agreement with numerical simulations.

Artifacts↗
PubMed2005

Theory of light propagation incorporating scattering and absorption in turbid media.

A general theoretical approach to the description of light propagating through turbid media is proposed. The theory is a modification of the two-flux model of Kubelka-Munk (KM), extending its applicability to media systems containing an absorptive component. The modified KM model takes into account the influence of internal scattering on the total path length and accommodates a wide range of absorption influences. Experimental results obtained for dyed-paper systems illuminated by diffuse light are demonstrated to be qualitatively and quantitatively reproduced by the theory.

Absorption↗
PubMed2005

Compact Fourier-transform volume holographic spectrometer for diffuse source spectroscopy.

We present a new idea for diffuse source spectroscopy using a Fourier-transform volume holographic spectrometer formed by a Fourier-transform lens, a volume hologram, and a CCD. We show that this spectrometer can operate well under spatially incoherent light illumination. Furthermore, this spectrometer is less bulky, less sensitive to input alignment, and potentially more appropriate for implementation of highly sensitive spectrometers than conventional spectrometers.

Diffusion↗
PubMed2005

Development of a multiwavelength Raman fiber laser based on phase-shifted fiber Bragg gratings for long-distance remote-sensing applications.

We propose a simple and flexible multiwavelength Raman fiber laser based on phase-shifted fiber Bragg gratings (FBGs) and a tunable chirped FBG. Using a simple multiwavelength Raman laser configuration with a single phase-shifted FBG and a tunable chirped FBG, we readily achieved simultaneous two-channel sensing probes with a high extinction ratio of more than 50 dB over a 50-km distance. The multiwavelength output was stable and the peak fluctuation was less than 0.5 dB. The temperature and strain sensitivities are estimated to be 10.1 pm/degrees C and 7.5 pm/microstrain, respectively.

Environmental Monitoring↗
PubMed2005

Optical biosensor with dispersion compensation.

Dispersion limits performance in many optical systems. In surface plasmon resonance (SPR) biosensors, the sensing area is an optical element in which the dispersion depends on the effective refractive index of the biochemical compounds to be measured. We report a method of compensating for wavelength dispersion in SPR biosensors employing two integrated diffractive optical coupling elements in a polymer substrate. The dispersion compensation is achieved over the whole dynamic measurement range and provides a biosensor more robust to wavelength fluctuations than prism-coupler SPR systems. The concept can readily be employed in other types of sensor measuring refractive-index changes.

Algorithms↗
PubMed2005

Synthesis of MFI zeolite films on optical fibers for detection of chemical vapors.

We report the development of a novel zeolite-incorporated optical fiber sensor and demonstrate its capability for in situ detection of chemical vapors. The sensor comprises a polycrystalline silicalite thin film grown upon the cleaved end face of a standard single-mode optical fiber. The sensor device operates by measuring the optical reflectivity of the zeolite crystals, which changes reversibly in response to the amount of chemical vapor adsorbed in its crystalline microporous structure. The sensor has been successfully demonstrated for measuring the concentration of isopropanol vapor in mixtures with nitrogen gas.

2-Propanol↗