Optical properties of normal and carcinomatous bronchial tissue

To understand better the optical characteristics and autofluorescence properties of normal and carcinomatous bronchial tissue, we measured the absorption coefficient, scattering coefficient, and anisotropy factor from 400 to 700 nm. We made the measurements by using an integrating sphere with a collimated white-light beam to measure total reflectance and transmittance of samples. The unscattered transmittance of the samples was measured through polarized on-axis light detection. The inverse adding-doubling solution was utilized to solve the equation of radiative transfer and to determine the absorption coefficient and reduced scattering coefficient. The scattering coefficient and anisotropy factor were derived from the unscattered transmittance of the sample and the reduced scattering coefficient. The measured parameters allow us to simulate photon propagation in normal bronchial and tumoral tissue by using Monte Carlo modeling.     

Optical sectioning using a fiber probe with an angled illumination-collection geometry: evaluation in engineered tissue phantoms

We present a fiber optic probe that combines polarized illumination and detection with an angled distal probe geometry to detect the size-dependent scattering at a specific depth within epithelium. Analysis of the scattering signal by use of Mie theory allows the extraction of scatterer size and size distribution-key parameters for precancer detection. The probe was evaluated in two tissue phantoms: polystyrene beads atop collagen gel and multiple layers of cancer cells atop collagen. We also present in vivo measurements in the oral cavity of normal volunteers. The sizes of scatterers extracted from the scattering spectra corresponded to independently measured values.     

Optical component interface scatter characterization by selective polarization extinction

A procedure for the selective extinction of the scattered light based on "null ellipsometry" [R. M. A. Azzam and N. M. Bashara, Ellipsometry and Polarized Light (North-Holland, 1977)] is presented. The technique allows scattering measurement from individual layers of a multilayer component by extinguishing the scattered light from the other layer interfaces. The technique is easily applicable to multilayer components with nearly identical surface profiles at every interface and little significant bulk scattering. Analysis is provided to determine the conditions required to extinguish the light from the excluded interfaces isolating the scattered light from the desired interface. An analysis of sensitivity of the extinction conditions to experimental parameters and to layer optical thickness is also provided. Experimental data collected using the technique are compared with measurements made using a white-light optical surface profilometry.     

Simulation of the depolarization effect in porous silicon

We describe a radiative transfer (RT) equation for the simulation of optical scattering effects in a nanostructured semiconductor for spectroscopic ellipsometry (SE). As an example, we chose porous silicon (PS), whose pores are considered to act as light scatterers. We examined the effects of pore radius, slab thickness, and incident angle. The volume scattering effect in the internal morphology of the PS generates incoherent light, leading to depolarization. By simulating the four Stokes parameters through the RT equation, we could theoretically assess the degree of polarization that is essential for SE measurements of some nanostructured semiconductors.     

Optical characterization of sputtered semitransparent zirconium nitride films

Thin semi-transparent ZrN films have been prepared using reactive dc magnetron sputtering. The films had thickness from 11 to 43 nm and were grown on heated and room temperature glass substrates. The optical constants, N=n+ik, of the thin films have been determined with an RT inversion method in the wavelength interval 0.40 to 2.0 μm. The thickness of the films was determined from the photometric measurements. The optical properties of the thin films on glass were compared to opaque and thin ZrN films grown on single crystalline Si. The Drude parameters were calculated from the measured optical constants in the relaxation region of the thin films. The relaxation time, τ, of the thin films was found to increase with film thickness, substrate temperature and substrate crystallinity. The relaxation time is the mean free time for the electrons between collisions and a long relaxation time corresponds to a film with high optical quality. The observed decrease of τ with decreasing film thickness can be explained by the higher statistical probability of the electrons in a thin film to collide with the two surfaces of the film. Another explanation to the decrease of τ with film thickness is scattering from grain boundaries and lattice impurities. The higher optical quality of films grown on heated substrates is probably due to an increased grain size. The measured optical constants were compared with calculated optical constants, using the Drude model, and the optical behaviour of thin ZrN films was found to be well described by the screened free-electron model.     

Ocean optics estimation for absorption,backscattering, and phase function parameters

We propose and test an inverse ocean optics procedure with numerically simulated data for the determination of inherent optical properties using in-water radiance measurements. If data are available at only one depth within a deep homogeneous water layer, then the single-scattering albedo and the single parameter that characterizes the Henyey-Greenstein phase function can be estimated. If data are available at two depths, then these two parameters can be determined along with the optical thickness so that the absorption and scattering coefficients, and also the backscattering coefficient, can be estimated. With a knowledge of these parameters, the albedo and Lambertian fraction of reflected radiance of the bottom can be determined if measurements are made close to the bottom. A simplified method for determining the optical properties of the water also is developed for only three irradiance-type measurements if the radiance is approximately in the asymptotic regime.     

Optical properties of an optically rough coating from inversion of diffuse reflectance measurements

A method is developed for determining the optical properties of an optically rough coating on an opaque substrate from reflectance measurements. A modified Kubelka-Munk two- flux model is used to calculate the reflectance of the coating as a function of the refractive index, absorption coefficient, scattering coefficient, and thickness. The calculated reflectance is then fitted to measurements using a spectral projected gradient algorithm, allowing the optical properties to be obtained. The technique is applied to titanium dioxide coatings on a titanium substrate. Realistic values of refractive index and absorption coefficients are generally obtained. Quantities that are useful for solar water-splitting applications are calculated, including the depth profile of absorption and the proportion of the incident photon flux absorbed in the coating under solar illumination.     

Simple calibration plane-invariant method for complex permittivity determination of dispersive and non-dispersive low-loss materials

Complex scattering parameter measurements generally require meticulous measurements of materials since (a) the measured phase generally differs from the actual value, (b) a shift in calibration (reference) plane results in large errors and (c) the phase uncertainty of the reflection scattering parameters of low-loss materials greatly increases when the material thickness is of integer multiples of one-half guided wavelength. To overcome all these drawbacks, the authors propose an amplitude-only method for complex permittivity determination of dispersive and non-dispersive medium- and low-loss materials. The method utilises amplitude-only measurements of two identical samples where the length of the second sample is twice as long as that of the first. There are three main advantages of this method. First, it eliminates multi-valued terms that produce multiple solutions in the process of extracting the complex permittivity and, thus, allows one to measure unique complex permittivity. Secondly, it removes the dependency of the calibration plane on measurements and phase uncertainty in the measurements of the reflection scattering parameters. Thirdly, it uses an objective function, which depends on only one variable for its goal. This is very important since fast, dynamic and accurate computations could be achieved by using this objective function. The method is verified by amplitude-only and complex scattering parameter measurements of two low-loss samples fitted into a waveguide section in the X-band (8.2?12.4 GHz).     

Light scattering by wood fibers

We consider the scattering of light by single wood fibers both theoretically and experimentally. We describe the size and the shape distributions and the internal structure and chemical composition of the wood fibers. We have modeled the random shape of the hollow, cylindrical wood fiber by using multivariate lognormal statistics. We have computed wood-fiber absorption and scattering cross sections, asymmetry parameters, and scattering phase matrices in the ray-optics approximation. Finally, we have provided experimental results from angular scattering measurements for wood fibers and present what we believe is the first comparison between these measurements and ray-optics computations for Gaussian random wood-fiber models. In spite of the complicated internal structure of the wood fiber, our model together with the ray-optics treatment explains the scattering measurements surprisingly well.     

Optical and structural characterization of silver islands films on glass substrates

Metal island films (MIFs) of Ag on glass substrates were fabricated by the e-beam evaporation technique. The dependence of the surface plasmon (SP) absorption properties on the deposition mass thickness and substrate temperature was quantified. The structural and optical characterization of the MIFs, obtained using spectrometry, grazing-incidence small-angle X-ray scattering (GISAXS) and atomic force microscopy (AFM) evidences the evolution of SP characteristics depending on the fabrication parameters: the red shift of the absorption peaks with the increase of deposition thickness accompanied by peak widening and the blue-shift of peaks with the increase of deposition temperature followed by the peak narrowing. These findings were explained by the differences in the concentration, shape and size of the obtained silver islands.     

Simple Mathematical Models for Temporal,Spatial, Angular,and Attenuation Characteristics of Light Propagating Through the Atmosphere for Space Optical Communication:

Abstract

Mathematical models are developed to characterize propagation through a turbid medium at three different wavelengths in the visible and near infrared spectral range. These models are based upon relations between the temporal, angular, and spatial spread of electromagnetic unpolarized radiation, geometrical path length, particle size distribution, and the medium's propagation parameters such as Mie scattering, and absorption coefficients, Mie phase-function, and optical thickness. Calculations of the radiation characteristics were carried out using Monte Carlo simulations. Here, atmospheric particulates are used to model turbid media for optical thickness between 1 and 6, emphasizing optical communication applications, The advantage of this work is the ability to predict simply and in real time important radiation parameters relevant to any optical communication system. Results indicate very high correlation between optical thickness and propagation characteristics. For transmission, comparison is made to Bucher's model. Results are similar except for absorption effects which are not included in Bucher's model. Some important conclusions are derived such as the prediction that it is advantageous to use longer wavelength radiation through the atmosphere. In addition, there is a very dominant back scattering effect, involving up to 50% of transmitted power for optical densities as low as 6. On the other hand, power density of received scattered light is very low for conventional distances relevant to satellite optical communication, and can be neglected. On the basis of simulation results, the received radiation is of unscattered light only for any optical communication application. The dominant mechanism relating to radiation attenuation is scattering rather than absorption.     

Optical indices and transport scattering coefficient of pyrolytic boron nitride: a natural thermal barrier coating for solar shields

Absorption and scattering properties of pyrolytic boron nitride (pBN) have been characterized by infrared spectroscopy. The strong dielectric anisotropy predicted by first principles calculations is confirmed by measurements performed on a highly oriented pBN sample. Optical properties of textured samples elaborated by chemical vapor deposition were identified from normal hemispherical reflectance and transmittance spectra by applying modified two-flux and four-flux transport models. It is also shown that coating carbon–carbon composites used to build solar shields with a pBN layer having an optimal thickness could improve the protection performance.     

Optical characterization of alignment and effective refractive index in carbon nanotube films

We have characterized the thickness and effective refractive index of carbon nanotube forests by fitting reflectance measurements in the visible and near infrared ranges. The measurements were performed with polarized light. An effective medium layer consisting of a mixture of graphite and air was used to simulate the nanotube film. The proposed model accurately described the behaviour of the reflected s-polarized component (Rs), which allowed for the precise determination of the thickness and porosity of the films, in very good agreement with SEM measurements of film thickness. The p-polarized component (Rp), on the other hand, could not be described in terms of the developed model. In badly aligned samples, where there is a mixture of Rs and Rp behaviour, the model fails to fit the Rs component as well. This effect can therefore be taken as an indirect indication of lack of alignment in the samples.     

Fluorescence lifetime optical tomography in weakly scattering media in the presence of highly scattering inclusions

We consider the problem of fluorescence lifetime optical tomographic imaging in a weakly scattering medium in the presence of highly scattering inclusions. We suggest an approximation to the radiative transfer equation, which results from the assumption that the transport coefficient of the scattering media differs by an order of magnitude for weakly and highly scattering regions. The image reconstruction algorithm is based on the variational framework and employs angularly selective intensity measurements. We present numerical simulation of light scattering in a weakly scattering medium that embeds highly scattering objects. Our reconstruction algorithm is verified by recovering optical and fluorescent parameters from numerically simulated datasets.     

Optical and electronic characterization of transition layer in thin film Au-GaAs Schottky barrier

The possibility of determination of optical parameters and thickness of interfacial oxide (IO) layer by multiple-angle-of-incidence (MAI) ellipsometry has been studied for Au-IO-GaAs structures with different oxide thickness. The films parameters have been calculated by the original program of inverse ellipsometric problem solution. Parameters of oxide layer have been determined both for free GaAs surface and for that covered by Au film. Gold film was prepared by vacuum evaporation on the heated n-n+GaAs substrate. Its parameters have been determined by MAI ellipsometry and transparency measurements of Au-quartz satellite system. The investigation showed the change of optical parameters of GaAs oxide layer after the Au evaporation. In the assumption of invariable thickness of IO layer its optical parameters are : n = 1.0±0.1, k = 0.4 ÷ 0.6. The mechanism of current transport and electrical parameters of Au-IO-GaAs structures have been determined from I-V and C-V characteristics of Schottky diodes. The results of the determination of transition layer thickness to its dielectric constant ratio (d\_i)) from electrical characteristics of barrier also indicate its difference from initial parameters of oxide layer probably due to interdiffusion of Au, Ga, and As.     

Optical scattering and backscattering by organic and inorganic particulates in U.S. coastal waters

We present the results of a study of optical scattering and backscattering of particulates for three coastal sites that represent a wide range of optical properties that are found in U.S. near-shore waters. The 6000 scattering and backscattering spectra collected for this study can be well approximated by a power-law function of wavelength. The power-law exponent for particulate scattering changes dramatically from site to site (and within each site) compared with particulate backscattering where all the spectra, except possibly the very clearest waters, cluster around a single wavelength power-law exponent of -0.94. The particulate backscattering-to-scattering ratio (the backscattering ratio) displays a wide range in wavelength dependence. This result is not consistent with scattering models that describe the bulk composition of water as a uniform mix of homogeneous spherical particles with a Junge-like power-law distribution over all particle sizes. Simultaneous particulate organic matter (POM) and particulate inorganic matter (PIM) measurements are available for some of our optical measurements, and site-averaged POM and PIM mass-specific cross sections for scattering and backscattering can be derived. Cross sections for organic and inorganic material differ at each site, and the relative contribution of organic and inorganic material to scattering and backscattering depends differently at each site on the relative amount of material that is present.     

Optical extinction by closely spaced parallel cylinders inside a finite dielectric slab

The formulation for the extinction and scattering cross sections of closely spaced parallel infinite cylinders in a dielectric medium of finite thickness is presented. We consider the general case of dissimilar refractive indices for the half-spaces on both sides of the slab, and the diameter and refractive index of each cylinder can be different. The formulation accounts for the coherent scattering between the cylinders and scattering of the multiply reflected internal waves inside the slab. Discontinuity in the refractive index across the dielectric slab interfaces results in boundary reflections that modify the angular distribution of the scattered intensity in both forward and backward directions. The extinction cross section, which is derived by a formal application of the optical theorem, is shown to consist of both a forward and a backward component. The general solution is applied to obtain the formulas for the cases of cylinders in front of a reflecting plane, cylinders inside a semi-infinite dielectric medium, and cylinders in free space.     

Precision of measurement of tissue optical properties with optical coherence tomography

Accurate and noninvasive measurement of tissue optical properties can be used for biomedical diagnostics and monitoring of tissue analytes. Noninvasive measurement of tissue optical properties (total attenuation and scattering coefficients, optical thickness, etc.) can be performed with the optical coherence tomography (OCT) technique. However, speckle noise substantially deteriorates the accuracy of the measurements with this technique. We studied suppression of speckle noise for accurate measurement of backscattering signal and scattering coefficient with the OCT technique. Our results demonstrate that the precision of measurement of backscattering signals with the OCT technique can be 0.2% for homogeneously scattering media and 0.7% for skin, if spatial averaging of speckle noise is applied. This averaging allows us to achieve the precision of tissue scattering coefficient measurements of approximately +/-0.8%. This precision can be further improved by a factor of 2-3, upon optimization of OCT operating parameters.     

Measuring the optical parameters of weakly absorbing, highly turbid suspensions by a new technique: photoacoustic detection of scattered light

We present and apply a novel method, the scattering photoacoustic (SPA) technique, for measuring optical parameters in weakly absorbing, highly scattering suspensions. In this method, a solid absorber is in contact with a suspension sample to permit the photoacoustic detection of the sample's light-scattering properties. We conducted measurements conducted to determine the reduced scattering coefficients of Intralipid suspensions with a concentration range of 0.1-5%, and the results are in good agreement with those achieved by other researchers. Moreover, we also illustrate the relationship between the amplitude of the SPA signal and absorption, scattering, and detection distance. Through a study of Intralipid-ink mixes, we demonstrate that the SPA technique has the ability to determine simultaneously the absorption and reduced scattering coefficients of turbid media. This new technique has low cost and is noninvasive, and it enables on-line measurements to be made.     

Optical measurement and modelling of parison sag and swell in blow moulding

Blow moulding is a process whereby a cylindrical parison is extruded first, then pinched between the two halves of a mould and, finally, blown into the product. Parison size and shape result from complex interactions between mandrel and die geometries, processing parameters and viscoelastic properties of the polymeric material. Moreover, parison size changes with time due to sag. An innovative, contactless and online measurement technique of the parison is shown to be an effective tool to measure precisely parison diameter and thickness and to capture dimensional changes with time. This technique employs laser lighting of the parison and hinges on the refractive properties of molten polymer. Images taken with a digital camera are processed to give a precise measurement of diameter and thickness, at different time step during extrusion. Thus, parison swell and sag have been recorded for a commercial HDPE. Influence of processing parameters such as the rotational screw speed or die gap width can be brought forward. From the measurements, thickness swell is found to possess a different behaviour from diameter swell. Moreover, sag has been measured and can be modelled from a Newtonian perspective using one dimensional convected coordinates. Parison sag is shown to be governed by two parameters: an extrusion velocity and a a single coefficient of sagging susceptibility which value has been deduced from experiments.