Effect of surface roughness on determination of tissue optical properties obtained by diffusion approximation

A nylon bar with different surface roughness is used as a simulation sample of biological tissue for the determination of optical properties by using the spatially resolved steady-state diffuse reflection technique. The results obtained indicate that surface roughness has some effects on the determination of the optical properties of the nylon bar. The determined reduced scattering coefficient decreases with the decrease of the surface roughness of the nylon bar and goes to a constant for the lower surface roughness, and the determined absorption coefficient increases with the decrease of the surface roughness of the nylon bar. Consequently, the optical properties of the tissues obtained by the spatially resolved steady-state diffuse reflection technique should be modified.     

Comparison of spatially and temporally resolved diffuse-reflectance measurement systems for determination of biomedical optical properties

Time-resolved and spatially resolved measurements of the diffuse reflectance from biological tissue are two well-established techniques for extracting the reduced scattering and absorption coefficients. We have performed a comparison study of the performance of a spatially resolved and a time-resolved instrument at wavelengths 660 and 786 nm and also of an integrating-sphere setup at 550-800 nm. The first system records the diffuse reflectance from a diode laser by means of a fiber bundle probe in contact with the sample. The time-resolved system utilizes picosecond laser pulses and a single-photon-counting detection scheme. We extracted the optical properties by calibration using known standards for the spatially resolved system, by fitting to the diffusion equation for the time-resolved system, and by using an inverse Monte Carlo model for the integrating sphere. The measurements were performed on a set of solid epoxy tissue phantoms. The results showed less than 10% difference in the evaluation of the reduced scattering coefficient among the systems for the phantoms in the range 9-20 cm(-1), and absolute differences of less than 0.05 cm(-1) for the absorption coefficient in the interval 0.05-0.30 cm(-1).     

Coating effects on mid-infrared reflection spectra of fabrics

Polymer films of varying thicknesses were deposited onto cotton and polyester fabric samples by dip-coating from solution. Scanning electron microscopy (SEM) images of the coated fabric samples were used to evaluate the quality of the polymer coating. The samples were analyzed by infrared diffuse reflection spectroscopy to determine the relationship between film thickness and the effect of the coating on the spectroscopy of the two fabrics. Effects observed in four limiting cases are examined: (Case I) weak coating absorption on a fabric with weak absorption at the same frequency; (Case II) strong coating absorption in a spectral region of weak fabric absorption; (Case III) weak coating absorption in a spectral region of strong fabric absorption; and (Case IV) strong coating absorption in a spectral region of strong fabric absorption. In the first case, effects were dominated by reduced scattering as the coating is added. In the second case, the strong coating absorption that was observed at low coverages plateaus at higher coverage due to depth of penetration effects. In the third and fourth cases, reduced Fresnel diffuse reflection is measured as the coating is added, consistent with the reduction of scattering observed in the first case.     

Reflection and Absorption Techniques for Optical Characterization of Chemically Assembled Nanomaterials

This review discusses recent developments in the areas of fabrication, certain types of optical characterization, and applications of a selected class of chemically assembled nanomaterials, namely i) gold and silver nanoparticles deposited onto optically transparent glass substrates; ii) thiol‐functionalized self‐assembled monolayers (SAMs); iii) chemically stabilized gold and silver nanoparticles (monolayer protected clusters, MPCs); and iv) MPCs linked to metallic substrates and adsorbates. Six linear optical techniques for the characterization of these materials are discussed: transmission localized surface plasmon resonance spectroscopy, T‐LSPR; propagating surface plasmon resonance spectroscopy, P‐SPR; polarization‐selective Fourier transform infrared reflection absorption spectroscopy, PS‐FTIRRAS; polarization‐modulation Fourier transform infrared reflection absorption spectroscopy, PM‐FTIRRAS; surface‐enhanced infrared reflection absorption spectroscopy, SEIRRAS; and infrared ellipsometry. The review focuses particularly on providing a unified treatment of these six optical techniques by using a relatively simple stratified multilayer model.     

纳米PbS/SiO2气凝胶介孔组装体的制备及光学特性

对用自行研制的ＳＣＤ－Ｉ型高压釜制备的ＳiO2气凝胶与用胶体化学方法制备的ＰbS纳米粒子的组装体系的光学性能进行了研究,用透射电镜观察了其形貌,用光吸收谱仪及荧光光谱仪测定其光吸收谱及荧光谱。发现样品的光吸收边随温火温度升高由可见逐渐移到红外波段,样品的光致发光强度随退火温度上升先增强直至５７３Ｋ,而后随退火温度上升而减弱,我们认为光吸这红移是由量子限域效应引起的,而荧光强度变化与ＰbS表面缺陷及激光子的复合几率变化有关。     

Effects of turbid media optical properties on object visibility in subsurface polarization imaging

We studied the effectiveness of using polarized illumination and detection to enhance the visibility of targets buried in highly scattering media. The effects of background optical properties including scattering coefficient, absorption coefficient, and anisotropy on image visibility were examined. Both linearly and circularly polarized light were used in the imaging. Three different types of target were investigated: scattering, absorption, and reflection. The experimental results indicate that target visibility improvement achieved by a specific polarization method depends on both the background optical properties and the target type. By analyzing all polarization images, it is possible to reveal certain information about target or the scattering background.     

Diffuse reflection of ceramics coated with dielectric thin films

We have studied the diffuse reflection properties of ceramics in the presence of dielectric thin films on the surface. A simple optical model was proposed in which interference effects in a thin film were considered for light scattered out of a ceramic in various directions. Measurements were performed on angle-resolved reflection spectra of a thin-film-coated alumina ceramic in the case of normal incidence. They showed that the presence of the thin film on the ceramic's surface modified the angular distributions of scattered radiation from that of a bare ceramic, which suggested a way to tailor the scattering properties of a diffuse reflector as needed.     

纳米PbS/SiO_2气凝胶介孔组装体的制备及光学特性

对用自行研制的ＳＣＤ－Ｉ型高压釜制备的ＳｉＯ２气凝胶与用胶体化学方法制备的ＰｂＳ纳米粒子的组装体系的光学性质进行了研究．用透射电镜观察了其形貌,用光吸收谱仪及荧光光谱仪测定其光吸收谱及荧光谱．发现样品的光吸收边随退火温度升高由可见逐渐移到红外波段．样品的光致发光强度随退火温度上升先增强直至５７３Ｋ,而后随退火温度上升而减弱．我们认为光吸收边红移是由量子限域效应引起的,而荧光强度变化与ＰｂＳ表面缺陷及激子的复合几率变化有关．     

Radiative properties of snow for clear sky solar radiation

Spectral and integrated radiative properties (reflection, transmission, and the rate of heating) of finegrained wind-packed snow typical of subpolar regions are studied through a model taking into account surface reflection and volumetric multiple scattering. The surface reflection is modeled by a bidirectional reflectance distribution function applicable to powdered dielectric material. For the volumetric multiple scattering, the radiative transfer equation designed for strongly asymmetric scattering is solved. All multiple scattering parameters (single scattering albedo, various moments of the scattering phase function, and optical depth) are related to measurable physical characteristics (density, grain size, and the absorption coefficient of pure ice).Parameterized atmospheric spectral transmission coefficients for scattering and absorption by aerosols and gases are used to obtain the direct and diffuse components of solar flux, incident on the snow-cover. Calculated values of spectral and integrated visible and near infrared reflection and flux attenuation coefficients of snow are compared with observations. The rate of radiative heating at different depths within the snowcover is calculated from the net flux divergence. It is shown that the conventional method of calculating this rate using measured bulk extinction coefficients grossly underestimates the amount of heating within the top few millimeters. This study provides a better overall understanding of the radiative properties of snow under clear sky conditions in terms of the physical characteristics of the snowcover.     

Fiber-optic probe for noninvasive real-time determination of tissue optical properties at multiple wavelengths

We present a compact, fast, and versatile fiber-optic probe system for real-time determination of tissue optical properties from spatially resolved continuous-wave diffuse reflectance measurements. The system collects one set of reflectance data from six source-detector distances at four arbitrary wavelengths with a maximum overall sampling rate of 100 Hz. Multivariate calibration techniques based on two-dimensional polynomial fitting are employed to extract and display the absorption and reduced scattering coefficients in real-time mode. The four wavelengths of the current configuration are 660, 785, 805, and 974 nm, respectively. Cross-validation tests on a 6 x 7 calibration matrix of Intralipid-dye phantoms showed that the mean prediction error at, e.g., 785 nm was 2.8% for the absorption coefficient and 1.3% for the reduced scattering coefficient. The errors are relative to the range of the optical properties of the phantoms at 785 nm, which were 0-0.3/cm for the absorption coefficient and 6-16/cm for the reduced scattering coefficient. Finally, we also present and discuss results from preliminary skin tissue measurements.     

Time-resolved diffuse optical spectroscopy: a differential absorption approach

A method is presented for the estimate of spectral changes in the absorption properties of turbid media from time-resolved diffuse optical spectroscopy. The method relies on the hypothesis of constant scattering over the wavelength range of interest, but no limitations come from the sample size and shape as the method is derived directly from the Beer-Lambert law. The effects of a moderate spectral dependence of the scattering properties and of the non-ideal instrument response function were investigated theoretically, and the results were confirmed experimentally, showing that the method can be profitably applied in cases of practical interest.     

Design and testing of a white-light, steady-state diffuse reflectance spectrometer for determination of optical properties of highly scattering systems

We present a steady-state radially resolved diffuse reflectance spectrometer capable of measuring the absorption and transport scattering spectra of tissue-simulating phantoms over an adjustable 170-nm wavelength interval in the visible and near infrared. Measurements in a variety of phantoms are demonstrated over the relevant range of tissue optical properties, and the accuracy of the instrument is found to be approximately 10% in both scattering and absorption. Monte Carlo simulations designed to test the accuracy of the instrument are presented that support the experimental findings.     

Dynamic light scattering in subdiffusive regimes

For describing the fluctuating signal scattered from a multiple-scattering system, diffusive-wave spectroscopy makes use of a diffusion model that provides the path-length distribution of scattered waves for a specific geometry. Using the recently introduced optical path-length spectroscopy, we show that the diffusion model fails to describe wave propagation in the low-order multiple-scattering regime. We propose a new methodology with which to obtain information about the dynamic properties of nondiffusive scattering systems. We use optical path-length spectroscopy to obtain experimentally the path-length distribution of optical waves scattered from dynamic colloids, which are multiply scattering but not in the diffusion limit. The experimental results show that, with this new technique, the accuracy of dynamic measurements is significantly improved in subdiffusive scattering regimes.     

Optical characterization method for black pigments applied to solar-selective absorbing paints

We propose a novel, to our knowledge, method for characterizing the optical properties of pigment particles or powders. Measurements of the diffuse and the total transmittance as well as the diffuse and the total reflectance are used to obtain effective scattering and absorption coefficients per unit length for the particles that are dispersed in a continuous matrix. For dilute dispersions in the single-scattering regime scattering and absorption cross sections of the particles were obtained. The method was applied to two pigments, namely, FeMnCuO(x) and black carbon. The data were obtained by use of pellets consisting of low concentrations of FeMnCuO(x) or black-carbon pigments dispersed in a KBr matrix. The pigment volume concentrations used to evaluate the scattering and the absorption coefficients ranged from 0.053% to 0.530% for FeMnCuO(x) and 0.076% to 0.310% for the black carbon. These ranges were found to exhibit the linear dependence of the coefficients as a function of volume fraction, as given by single-scattering theory.     

Spatially resolved absolute diffuse reflectance measurements for noninvasive determination of the optical scattering and absorption coefficients of biological tissue

The absorption and transport scattering coefficients of biological tissues determine the radial dependence of the diffuse reflectance that is due to a point source. A system is described for making remote measurements of spatially resolved absolute diffuse reflectance and hence noninvasive, noncontact estimates of the tissue optical properties. The system incorporated a laser source and a CCD camera. Deflection of the incident beam into the camera allowed characterization of the source for absolute reflectance measurements. It is shown that an often used solution of the diffusion equation cannot be applied for these measurements. Instead, a neural network, trained on the results of Monte Carlo simulations, was used to estimate the absorption and scattering coefficients from the reflectance data. Tests on tissue-simulating phantoms with transport scattering coefficients between 0.5 and 2.0 mm(-1) and absorption coefficients between 0.002 and 0.1 mm(-1) showed the rms errors of this technique to be 2.6% for the transport scattering coefficient and 14% for the absorption coefficients. The optical properties of bovine muscle, adipose, and liver tissue, as well as chicken muscle (breast), were also measured ex vivo at 633 and 751 nm. For muscle tissue it was found that the Monte Carlo simulation did not agree with experimental measurements of reflectance at distances less than 2 mm from the incident beam.     

Estimating optical absorption, scattering, and Grueneisen distributions with multiple-illumination photoacoustic tomography

While photoacoustic methods offer significant promise for high-resolution optical contrast imaging, quantification has thus far proved challenging. In this paper, a noniterative reconstruction technique for producing quantitative photoacoustic images of both absorption and scattering perturbations is introduced for the case when the optical properties of the turbid background are known and multiple optical illumination locations are used. Through theoretical developments and computational examples, it is demonstrated that multiple-illumination photoacoustic tomography (MI-PAT) can alleviate ill-posedness due to absorption-scattering nonuniqueness and produce quantitative high-resolution reconstructions of optical absorption, scattering, and Gruneisen parameter distributions. While numerical challenges still exist, we show that the linearized MI-PAT framework that we propose has orders of magnitude improved condition number compared with CW diffuse optical tomography.     

Monte Carlo-based inverse model for calculating tissue optical properties. Part I: Theory and validation on synthetic phantoms

A flexible and fast Monte Carlo-based model of diffuse reflectance has been developed for the extraction of the absorption and scattering properties of turbid media, such as human tissues. This method is valid for a wide range of optical properties and is easily adaptable to existing probe geometries, provided a single phantom calibration measurement is made. A condensed Monte Carlo method was used to speed up the forward simulations. This model was validated by use of two sets of liquid-tissue phantoms containing Nigrosin or hemoglobin as absorbers and polystyrene spheres as scatterers. The phantoms had a wide range of absorption (0-20 cm(-1)) and reduced scattering coefficients (7-33 cm(-1)). Mie theory and a spectrophotometer were used to determine the absorption and reduced scattering coefficients of the phantoms. The diffuse reflectance spectra of the phantoms were measured over a wavelength range of 350-850 nm. It was found that optical properties could be extracted from the experimentally measured diffuse reflectance spectra with an average error of 3% or less for phantoms containing hemoglobin and 12% or less for phantoms containing Nigrosin.     

In vivo time-resolved reflectance spectroscopy of the human forehead

We present an in vivo broadband spectroscopic characterization of the human forehead. Absorption and scattering properties are measured on five healthy volunteers at five different interfiber distances, using time-resolved diffuse spectroscopy and interpreting data with a model of the diffusion equation for a homogeneous semi-infinite medium. A wavelength-tunable mode-locked laser and time-correlated single-photon counting detection are employed, enabling fully spectroscopic measurements in the range of 700-1000 nm. The results show a large variation in the absorption and scattering properties of the head depending on the subject, whereas intrasubject variations, assessed at different interfiber distances, appear less relevant, particularly for what concerns the absorption coefficient. The high intersubject variability observed indicates that a unique set of optical properties for modeling the human head cannot be used correctly. To better interpret the results of the analysis of in vivo measurements, we performed a set of four-layer model Monte Carlo simulations based on different data sets for the optical properties of the human head, partially derived from the literature. The analysis indicated that, when simulated time-resolved curves are fitted with a homogeneous model for the photon migration, the retrieved absorption and reduced scattering coefficients are much closer to superficial layer values (i.e., scalp and skull) than to deeper layer ones (white and gray matter). In particular, for the shorter interfiber distances, the recovered values can be assumed as a good estimate of the optical properties of the first layer.     

Carbon Nanosheets: Mechanically Stacked 1‐nm‐Thick Carbon Nanosheets: Ultrathin Layered Materials with Tunable Optical,Chemical, and Electrical Properties (Small 7/2011)

Carbon nanosheets are mechanically stable, free‐standing two‐dimensional materials with a thickness of ≈1 nm and well defined physical and chemical properties. They are made by radiation‐induced cross‐linking of aromatic self‐assembled monolayers. Herein, a route is presented to the scalable fabrication of multilayer nanosheets with tunable electrical, optical, and chemical properties on insulating substrates. Stacks of up to five nanosheets with sizes of ≈1 cm² on oxidized silicon are studied. Their optical characteristics are investigated by visual inspection, optical microscopy, UV–vis reflection spectroscopy, and model calculations. Their chemical composition is studied by X‐ray photoelectron spectroscopy. The multilayer samples are then annealed in an ultrahigh vacuum at various temperatures up to 1100 K. A subsequent investigation by Raman, X‐ray photoelectron, and UV–vis reflection spectroscopy, as well as by electrical four‐point probe measurements, demonstrates that the layered nanosheets transform into nanocrystalline graphene. This structural and chemical transformation is accompanied by changes in the optical properties and electrical conductivity and opens up a new path for the fabrication of ultrathin functional conductive coatings.     

Determination of the optical coefficients of biological tissue by neural network

A system that incorporated a laser source and a CCD camera was used to measure spatially-resolved steady-state diffuse reflectance. Monte Carlo simulations and experiments in tissue phantoms were used to train a neural network that characterizes the reflectance data on a turbid medium. The neural network was used to extract the optical properties (scattering and absorption coefficients) of biological tissue. The accuracy of the neural network was investigated and validated. Tests on tissue-simulation phantoms showed the relative errors of this technique to be 3% for the reduced scattering coefficient and 9% for the absorption coefficients. The optical properties of human skin were also measured in vivo at 633 nm. For human skin tissue it was found that our results were in good agreement with their reference values.