Refractive Index Map Research Articles

Extended depth of focus in tomographic phase microscopy using a propagation algorithm

Tomographic phase microscopy is a laser interferometry technique in which a 3D refractive index map of a biological sample is constructed from quantitative phase images collected at a set of illumination angles. Although the resulting tomographic images provide valuable information, their resolution declines at axial distances beyond about 1 microm from the focal plane. We describe an improved 3D reconstruction algorithm in which the field at the focal plane is numerically propagated to depths throughout the sample. Diffraction is thus incorporated, extending the depth of focus to more than 10 mum. Tomograms with improved focal depth are demonstrated for single HT29 cells.

Time-resolved refractive index and absorption mapping of light-plasma filaments in water

By means of a quantitative shadowgraphic method, we performed a space-time characterization of the refractive index variation and transient absorption induced by a light-plasma filament generated by a 120 fs laser pulse in water. The formation and evolution of the plasma channel in the proximity of the nonlinear focus were observed with a 23 fs time resolution.

The structural and refractive index changes in the waveguides written by femtosecond laser in Er-doped silicate glasses

Waveguides with low propagation losses were written in erbium-doped silicate glasses by tightly focused femtosecond laser pulses. Raman spectra measurements and 2-D refractive index mapping in the micro-areas near the focus were carried out. The results supported that the refractive index change induced by fs laser pulses comes from the increase of the low rank ring structures in the silicate network.

Changes in Lens Dimensions and Refractive Index with Age and Accommodation

To measure changes in human eye lens dimensions and refractive index with age and state of accommodation. MRI methods were used to measure refractive index maps and lens geometry (diameter and thickness) of an axial slice through the lens in 44 volunteers aged 18 to 59 years, with an accommodation stimulus of 0.17 D (unaccommodated state). In a subpopulation of 26 young volunteers aged 18 to 33 years, lens dimensions were also measured in an accommodated (6.67 D stimulus) state. For a subpopulation of six of the young volunteers (22 to 33 years), refractive index maps were also acquired with an accommodation stimulus of 6.67 D. Unaccommodated lens thickness increased significantly with age (T = 3.31 +/- 0.10 mm + 0.0180 +/- 0.0036 mm x Age; p < 0.0001). Lens diameter (D = 9.33 +/- 0.0033 mm) and central refractive index (nc = 1.4198 +/- 0.0067) showed no significant age dependence. Lens thickness increased (DeltaT = 0.050 +/- 0.024 mm/D) and diameter decreased (DeltaD = -0.067 +/- 0.030 mm/D) on accommodation. A slight decrease in central refractive index with accommodation was not statistically significant. The results are consistent with the Helmholtz theory of accommodation.

Tomographic phase microscopy

We report a technique for quantitative three-dimensional (3D) mapping of refractive index in live cells and tissues using a phase-shifting laser interferometric microscope with variable illumination angle. We demonstrate tomographic imaging of cells and multicellular organisms, and time-dependent changes in cell structure. Our results will permit quantitative characterization of specimen-induced aberrations in high-resolution microscopy and have multiple applications in tissue light scattering.

TEMPERATURE FIELD RECONSTRUCTION FROM PHASE-MAP OBTAINED WITH MOIRé DEFLECTOMETRY IN DIFFUSION FLAME ON SOLIDS

Two methods of temperature field reconstruction starting from refractive index map obtained with Moiré deflectometry in downward flame spread along cylindrical PMMA samples have been compared, in order to determine their suitability in this kind of combustion problems, i.e., diffusive flames on solids. This article shows the applicability of the method based on the hypothesis of the same functional behaviour (SFB) of all the variables representing the flow, which provides a good picture of the temperature field. However, the method based on numerical solutions of the Navier–Stokes equations with simple chemical kinetics, suitable for other purposes, does not provide reliable quantitative results when it is used for the computation of the spatial variation of the Gladstone–Dale (G-D) constant and the gas constant, and for the later evaluation of the temperature from the refractive index map.

Hard X-ray phase imaging and tomography using a grating interferometer

An interferometric technique for hard X-rays is presented. It is based on two transmission gratings and a phase-stepping technique, and it provides separate radiographs of the phase and absorption profiles of bulk samples. Tomographic reconstruction yields quantitative three-dimensional maps of the X-ray refractive index and of the attenuation coefficient, with a spatial resolution down to a few microns. The method is mechanically robust, it requires little monochromaticity, and can be scaled up to large fields of view. These are important prerequisites for use with laboratory X-ray sources. Numerous applications ranging from wave front sensing to medical radiography are presently under investigation.

Application of two-dimensional spectral surface plasmon resonance to imaging of pressure distribution in elastohydrodynamic lubricant films

What we believe to be a novel two-dimensional spectral surface plasmon resonance imaging technique determining pressure distribution in elastohydrodynamic lubricant films is presented. This technique makes use of the spectral characteristics associated with the surface plasmon resonance (SPR) effect, and it provides more spectral information in refractive index mapping than conventional contrast SPR imaging. Two-dimensional imaging is demonstrated and applied to a highly pressurized liquid lubricant trapped inside an elastohydrodynamic lubrication (EHL) dimple. The hydrostatic pressure inside the EHL dimple causes a localized change of the refractive index of the lubrication oil. This also results in a shift in the spectral SPR absorption dip. By monitoring the color changes within the SPR image and calibrating with lubricants of known refractive index profiles, we can obtain a direct measurement of the refractive index distribution within the EHL dimple. PB 2400 lubricant dimples were studied in our experiments. The proposed SPR imaging approach is irrespective of the absolute lubricant film thickness h, therefore overcoming the major limitations of a conventional optical interference technique. With further development of the two-dimensional refractive index mapping technique, widespread applications in various fields are possible, including high-throughput sensors and the detection of bioaffinity interactions.

Spatial refractive index measurement of porcine artery using differential phase optical coherence microscopy

We describe a methodology to record spatial variation of refractive index of porcine renal artery using differential phase optical coherence microscopy (DP-OCM). The DP-OCM provides quantitative measurement of thin specimen phase retardation and refractive index by measuring optical path-length changes on the order of a few nanometers and with a lateral resolution of 3 microm. The DP-OCM instrumentation is an all-fiber, dual-channel Michelson interferometer constructed using a polarization maintaining (PM) fiber. Two-dimensional en face dual-channel phase images are taken over a 150 x 200 microm region on a microscopic slide, and the images are reconstructed by plotting a two-dimensional refractive index map as the OCM beam is moved across the sample. Because the DP-OCM can record transient changes in the optical path-length, the system may be used to record quantitative optical path-length alterations of tissue in response to various stimuli. A fiber-based DP-OCM may have the potential to substantially improve in vivo imaging of individual cells for a variety of clinical diagnostics, and monitoring applications.

A Refractive Index Mapping Operator for Assimilation of Occultation Data

Abstract This paper describes the details of a fast, linear, forward-inverse refractive index mapping operator that can be used for assimilation of occultation data of various kinds into NWP models. Basically, the mapping consists of the integration of the refractive index along finite straight lines, mimicking the observational geometry as well as the subsequent retrieval of a refractive index profile, assuming spherical symmetry. Line integrals are discretized such that the refractivity is evaluated along the horizontal at fixed levels that can be chosen to coincide with the pressure levels of an NWP model. Integration of the hydrostatic equation at a large number of locations is thereby avoided. The mapping operator is tested using an idealized model of a weather front with large horizontal gradients. Mapped refractivity profiles are compared with retrieved refractivity profiles obtained via accurate 3D ray tracing simulations of GPS radio occultation events with ray path tangent points near the weather front. The simulations indicate that the mapping is a good representation of occultation measurements, including the influence large horizontal gradients have on retrieved refractivity profiles. To further the results, a simple ad hoc modification is introduced to approximately account for the ray path bending near the tangent points. The forward-inverse mapping allows for the near cancellation of otherwise crude approximations—for example, straight-line propagation—and the general concept could perhaps be adapted for the development of fast and accurate observation operators for the assimilation of other types of remote sensing data.

X-ray phase imaging with a grating interferometer

Using a high-efficiency grating interferometer for hard X rays (10-30 keV) and a phase-stepping technique, separate radiographs of the phase and absorption profiles of bulk samples can be obtained from a single set of measurements. Tomographic reconstruction yields quantitative three-dimensional maps of the X-ray refractive index, with a spatial resolution down to a few microns. The method is mechanically robust, requires little spatial coherence and monochromaticity, and can be scaled up to large fields of view, with a detector of correspondingly moderate spatial resolution. These are important prerequisites for use with laboratory X-ray sources.

Refractive index distribution and optical properties of the isolated human lens measured using magnetic resonance imaging (MRI)

Using a non-invasive MRI technique for measuring the refractive index distribution through the crystalline lens, refractive index maps were obtained through 20 intact isolated human lenses (7–82 years). Focal length measurements, obtained by simulated light ray propagation through each index map were found to be in agreement with direct measurements performed on a scanning laser apparatus. With increasing age, the refractive index profiles became flatter in the central region, accompanied by steepening of the profile in the periphery. This appears to be an important mechanism underlying the observed changes in power and longitudinal aberration of the human lens.

Measuring optical properties of an eye lens using magnetic resonance imaging.

We compare the focal lengths of porcine lenses measured optically and by using a novel MRI technique. The geometric properties of the lenses were also measured and compared. The MRI technique exploits the dependence of both the lens refractive index and relaxation rates on the local protein concentration. By measuring the refractive index and corresponding values of R2 (=1/T2) for samples of lens homogenates with different protein concentrations, the dependence of refractive index on R2 was determined empirically. R2 maps, constructed from monoexponential fits to multiecho images of a slice through the lens containing the optical axis, were converted to refractive index maps using this relationship. A simulated ray trace through the refractive index map provided an estimate of lens focal length that was compared to a direct optical measurement of focal length using a laser ray-tracing method. It was found that the mean focal lengths estimated from the two techniques agreed within experimental uncertainty. The refractive index profile along the optical axis was found to be well described by a simple function of the form n=n0 + n1 x ra where r is the (normalized) lens radius.

Correlation between specific heat and change of refractive index formed by laser spot heating of tellurite glass surfaces

Correlation between the specific heat and the refractive index change formed by laser spot heating on tellurite glass surfaces has been investigated. The ternary tellurite glasses of TeO 2–Na 2O–Al 2O 3, TeO 2–Na 2O–GeO 2 and TeO 2–Na 2O–TiO 2 doped with 2 mol% of CoO were irradiated by a green light beam spot (532 nm) from a second harmonic generator of a Q switch pulse YAG laser. The refractive index map of the glass surface was measured with a He–Ne laser beam using a scanning ellipsometric technique, showing the formation of the refractive index dot patterns lower than the matrix by 0.05–0.22. The specific heat of the glasses was measured by CO 2 laser flashing method. The glasses containing Al 2O 3 possessed the lowest specific heat among the three glass systems, giving the largest refractive index change on the glass surfaces. A good linear correlation between the reciprocal of the specific heat and the refractive index change was found in cases where the refractive index change was less than 0.1, suggesting that the refractive index change derived from the change of the fictive temperature of the glasses.

Refractive index patterning of tellurite glass surfaces by ultra short pulse laser spot heating

Dot patterns of refractive indices were formed by the laser pulse irradiation on the tellurite glasses. The ternary tellurite glasses of TeO2-Na2O-Al2O3, TeO2-Na2O-GeO2 and TeO2-Na2O-TiO2 doped with 2 mol% of CoO were irradiated by a femtosecond pulse laser beam (800 nm) or by a green light beam (532 nm) from a second harmonic generator of a Q switch pulse YAG laser. The refractive index map of the glass was composed with an He-Ne laser beam by an scanning ellipsometric technique at a resolution of 100 μm × 50 μm, indicating that the spots possessing refractive index lower by about 0.05–0.38 than the surroundings were formed at the region irradiated by the laser beam. The irradiation of the femtosecond laser beam generated the dot patterns roughly equivalent to the beam size. The change of refractive index could be tunable by adjusting laser power, suggesting that the process could be applied to optical recording.

Particle classification from light scattering with the scanning flow cytometer.

The differential light-scattering pattern, an indicatrix, provides the most complete characterization of the optical properties of a particle. Particle classification can be performed on the basis of particle parameters retrieved from the indicatrices. This classification extends the ability of flow cytometry in particle recognition. The scanning flow cytometer (SFC) permits an acquisition of traces of light scattering signals, i.e., native SFC traces, from single particles. The acquired native SFC traces are transformed into indicatrices. The performance of the SFC in measurements of indicatrices has been demonstrated for the following particles: lymphocytes, erythrocytes, polystyrene particles, and milk-fat particles. The structure and profile of the indicatrix for each particle type have been found to be unique. Classification of polystyrene particles has been performed on the basis of the map formed by particle refractive index and size. The polystyrene particles were classified using this map into different size categories ranging from 1.4-7 microm, with a size deviation of 0.07 microm. The method based on analysis of native SFC traces shows better performance in particle classification than the method based on the particle refractive index and size map. The classification performance of the SFC will be useful, for example, for particle sorting and particle identification, and with additional fluorescent measurements may have applications in multiparameter particle-based immunoassay.

High-sensitivity mapping of refractive index across a surface using a dual-beam differential reflectance technique

With any waveguide device accurate knowledge of the optical characteristics is necessary for device design and specification; hence it is important from a practical and theoretical point of view to establish an efficient and precise method for measuring the refractive index profile of optical waveguides. For the majority of waveguide applications accurate knowledge of the absolute refractive index is rarely required but in contrast the difference between two adjacent regions is essential. The technique presented here addresses this second requirement. Measurements can be performed at any wavelength on the basis of intensity measurements of reflected signals from two adjacent points on the surface of a material and, since it is a non-contact measurement, it is also possible to record reflectance changes in situ. Samples can be heated, illuminated or processed in other ways, with a constant record of the refractive index changes which occur or are being made, mapped across an entire surface to an accuracy in of . The spatial mapping of is limited only by the focused spot diameters and the need for a separation between them, the surface quality and the noise associated with the combined detector and PSD electrical noise. We used and ZnSe at various temperatures to calibrate and measure small refractive index changes.

Laser Reviews. Surface Plasmon Resonance and Its Application to Laser Microscopy.

Surface plasmons are electromagnetic waves localized and propoagating along metal-dielectric interfaces. They can be excited by a p-polarized light in an attenuated total reflection geometry. The surface plasmon resonance can be used in laser microscopy that provides two dimensional maps of refractive index and/or thickness of samples with very high sensitivity. Spatial light modulators using the surface plasmon resonance are also described.

Refractive index mapping of concentration profiles

We report an optical microscopy technique for mapping variations in refractive index across an electrode surface that result from non-uniform electrochemical generation of a soluble species

THE USE OF POTENTIAL REFRACTIVE INDEX IN SYNOPTIC-SCALE RADIO METEOROLOGY

Abstract The concept of potential refractive index is examined in the light of earlier work and of the present emphasis on synoptic-scale radio meteorology. A new potential refractive index is defined and illustrated. It is concluded that the new parameter has operational utility in long-range microwave radio and radar transmission as well as in refractive index mapping.