Steady-state directional diffuse reflectance and fluorescence of human skin

We present numerical simulations predicting the directional diffuse reflectance and autofluorescence from human skin. Skin is modeled as a seven-layered medium, with each layer having its own optical properties and fluorophore concentrations. Both collimated and diffuse monochromatic excitation at 442 nm are considered. In addition, the effect of an index-matching cream used to eliminate total internal reflection within the skin is assessed. We compute the intensity distributions of the excitation and fluorescence light in the skin by solving the radiative transfer equation using the modified method of characteristics. It was found that the use of an index-matching cream reduces the directional fluorescence signal while increasing the directional diffuse reflectance from the skin for collimated excitation. On the other hand, both the fluorescence and diffuse reflectance increase for diffuse excitation with an index-matching cream. Moreover, the directional fluorescence intensity obtained by use of collimated excitation is larger than that obtained by use of diffuse excitation light. This computational tool could be valuable in designing optical devices for biomedical applications.     

Optical throughput of the Sagnac interferometer with a modified large optical path difference

The basic principle of Sagnac interferometer with modified large optical path difference is expounded on in this paper. According to the Fresnel formula, electromagnetic field energy, and energy flux, the transmittance and reflectance of each interface of a Sagnac interferometer are calculated, respectively, and then the exact expressions of the optical throughput changing with the incident angle, the angle of the incident plane, and paper plane (the bottom plane of Sagnac interferometer) and Sagnac interferometer acute angles are given. Furthermore, we analyze the effects of various parameters on the optical throughput by computer simulation, and some important conclusions are obtained. This work is of great scientific significance to the static, real-time simultaneous detection of upper atmospheric wind field.     

Angular distribution of diffuse reflectance in biological tissue

We measured angular-resolved diffuse reflectance in tissue samples of different anisotropic characteristics. Experimental measurements were compared with theoretical results based on the diffusion approximation. The results indicated that the angular distribution in isotropic tissue was the same as in isotropic phantoms. Under normal incidence, the measured angular profiles of diffuse reflectance approached the Lambertian distribution when the evaluation location was far away from the incident point. The skewed angular profiles observed under oblique incidence could be explained using the diffuse model. The anisotropic tissue structures in muscle showed clear effects on the measurements especially at locations close to the light incidence. However, when measuring across the muscle fiber orientations, the results were in good agreement with those obtained in isotropic samples.     

Characterization of spatial and temporal variations in the optical properties of tissuelike media with diffuse reflectance imaging

We describe a method to characterize spatial or temporal changes in the optical properties of turbid media using diffuse reflectance images acquired under broad-beam illumination conditions. We performed experiments on liquid phantoms whose absorption (mu(a)) and reduced scattering (mu(s)') coefficients were representative of those of biological tissues in the near infrared. We found that the relative diffuse reflectance R depends on mu(a) and mu(s)' only through the ratio mu(a)/mu(s)' and that dependence can be well described with an analytical expression previously reported in the literature [S. L. Jacques, Kluwer Academic Dordrecht (1996)]. We have found that this expression for R deviates from experimental values by no more than 8% for various illumination and detection angles within the range 0 degrees-30 degrees. Therefore, this analytical expression for R holds with good approximation even if the investigated medium presents curved or irregular surfaces. Using this expression, it is possible to translate spatial or temporal changes in the relative diffuse reflectance from a turbid medium into quantitative estimates of the corresponding changes of (mu(a)/mu(s)')(1/2). In the case of media with optical properties similar to those of tissue in the near infrared, we found that the changes mu(a)/mu(s)' should occur over a volume approximately 2 mm deep and 4 mm x 4 mm wide to apply this expression.     

Spatially correlated fluorescence/AFM of individual nanosized particles and biomolecules

Individual fluorescent polystyrene nanospheres (<10-100-nm diameter) and individual fluorescently labeled DNA molecules were dispersed on mica and analyzed using time-resolved fluorescence spectroscopy and atomic force microscopy (AFM). Spatial correlation of the fluorescence and AFM measurements was accomplished by (1) positioning a single fluorescent particle into the near diffraction-limited confocal excitation region of the optical microscope, (2) recording the time-resolved fluorescence emission, and (3) measuring the intensity of the excitation laser light scattered from the apex of an AFM probe tip and the AFM topography as a function of the lateral position of the tip relative to the sample substrate. The latter measurements resulted in concurrent high-resolution (approximately 10-20 nm laterally) images of the laser excitation profile of the confocal microscope and the topography of the sample. Superposition of these optical and topographical images enabled unambiguous identification of the sample topography residing within the excitation region of the optical microscope, facilitating the identification and structural characterization of the nanoparticle(s) or biomolecule(s) responsible for the fluorescence signal observed in step 2. These measurements also provided the lateral position of the particles relative to the laser excitation profile and the surrounding topography with nanometer-scale precision and the relationship between the spectroscopic and structural properties of the particles. Extension of these methods to the study of other types of nanostructured materials is discussed.     

Reduction of skylight reflection effects in the above-water measurement of diffuse marine reflectance

Reflected skylight in above-water measurements of diffuse marine reflectance can be reduced substantially by viewing the surface through an analyzer transmitting the vertically polarized component of incident radiance. For maximum reduction of effects, radiometric measurements should be made at a viewing zenith angle of ~45 degrees (near the Brewster angle) and a relative azimuth angle between solar and viewing directions greater than 90 degrees (backscattering), preferably 135 degrees . In this case the residual reflected skylight in the polarized signal exhibits minimum sensitivity to the sea state and can be corrected to within a few 10(-4) in reflectance units. For most oceanic waters the resulting relative error on the diffuse marine reflectance in the blue and green is less than 1%. Since the water body polarizes incident skylight, the measured polarized reflectance differs from the total reflectance. The difference, however, is small for the considered geometry. Measurements made at the Scripps Institution of Oceanography pier in La Jolla, Calif., with a specifically designed scanning polarization radiometer, confirm the theoretical findings and demonstrate the usefulness of polarization radiometry for measuring diffuse marine reflectance.     

Optical approach to angular displacement measurement based on attenuated total reflection

An optical approach for angular displacement measurement (ADM) based on the attenuated total reflection technique is presented. As a laser beam is incident upon a planar optical waveguide, an m line is obtained by scanning the incident angle. Theoretical analysis shows that the m line sharply shifts with a tiny variation of the thickness of the waveguided layer. And the specific schemes for ADM, which are based on the angular interrogation and the intensity measurement, are analyzed. The calculated result of sensitivity demonstrates that the intensity measurement is more efficient than the angular interrogation. Furthermore, small incident angles indicate higher sensitivity to the angular displacement than relatively large incident angles for the intensity measurement.     

Geometrical-optics approximation of forward scattering by gradient-index spheres

By means of geometrical optics we present an approximation method for acceleration of the computation of the scattering intensity distribution within a forward angular range (0-60 degrees ) for gradient-index spheres illuminated by a plane wave. The incident angle of reflected light is determined by the scattering angle, thus improving the approximation accuracy. The scattering angle and the optical path length are numerically integrated by a general-purpose integrator. With some special index models, the scattering angle and the optical path length can be expressed by a unique function and the calculation is faster. This method is proved effective for transparent particles with size parameters greater than 50. It fails to give good approximation results at scattering angles whose refractive rays are in the backward direction. For different index models, the geometrical-optics approximation is effective only for forward angles, typically those less than 60 degrees or when the refractive-index difference of a particle is less than a certain value.     

Laboratory-based bidirectional reflectance distribution functions of radiometric tarps

Laboratory-based bidirectional reflectance distribution functions (BRDFs) of radiometric tarp samples used in the vicarious calibration of Earth remote sensing satellite instruments are presented in this paper. The results illustrate the BRDF dependence on the orientation of the tarps' weft and warp threads. The study was performed using the GSFC scatterometer at incident zenith angles of 0 degrees, 10 degrees, and 30 degrees; scatter zenith angles from 0 degrees to 60 degrees; and scatter azimuth angles of 0 degrees, 45 degrees, 90 degrees, 135 degrees, and 180 degrees. The wavelengths were 485 nm, 550 nm, 633 nm, and 800 nm. The tarp's weft and warp dependence on BRDF is well defined at all measurement geometries and wavelengths. The BRDF difference can be as high as 8% at 0 degrees incident angle and 12% at 30 degrees incident angle. The fitted BRDF data show a very small discrepancy from the measured ones. New data on the forward and backscatter properties of radiometric tarps are reported. The backward scatter is well pronounced for the white samples. The black sample has well-pronounced forward scatter. The provided BRDF characterization of radiometric tarps is an excellent reference for anyone interested in using tarps for radiometric calibrations. The results are NIST traceable.     

Influence of the propagation-plane inclination angle on the fluorescent intensity: Study of the emission of small-thickness samples and determination of the maximum emission angle

A theoretical study of the fluorescent intensity emitted by a small-thickness multicomponent specimen is carried out and a strong dependence on the inclination angle of the propagation plane is found. The existence of one angular value _M at which the total fluorescent emission achieves a maximum is demonstrated, and it is found that such an angle depends essentially on the sample thickness and composition. Calculating _M by means of the theoretical model, it is possible to select the optimal excitation detection configuration to reach the maximum fluorescent intensity compatible with the characteristics of thickness and composition of the observed sample. Inversely, the thickness of a sample of known composition can be estimated by means of the experimental determination of the angle _M. However, it was found that this thickness measuring method has a very poor sensitivity. The theoretical results have been confirmed by Monte Carlo simulation.     

Design of angle-tolerant multivariate optical elements for chemical imaging

Multivariate optical elements (MOEs) are multilayer optical interference coatings with arbitrary spectral profiles that are used in multivariate pattern recognition to perform the task of projecting magnitudes of special basis functions (regression vectors) out of optical spectra. Because MOEs depend on optical interference effects, their performance is sensitive to the angle of incidence of incident light. This angle dependence complicates their use in imaging applications. We report a method for the design of angle-insensitive MOEs based on modification of a previously described nonlinear optimization algorithm. This algorithm operates when the effects of deviant angles of incidence are simulated prior to optimization, which treats the angular deviation as an interferent in the measurement. To demonstrate the algorithm, a 13-layer imaging MOE (IMOE, with alternating layers of high-index Nb2O5 and low-index SiO2) for the determination of Bismarck Brown dye in mixtures of Bismarck Brown and Crystal Violet, was designed and its performance simulated. For angles of incidence that range from 42 degrees to 48 degrees, the IMOE has an average standard error of prediction (SEP) of 0.55 microM for Bismarck Brown. This compares with a SEP of 2.8 microM for a MOE designed by a fixed-angle algorithm.     

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.     

Relations between the critical angles and the optical tensor of a biaxial material

The behavior of the critical angles between a high-index isotropic medium and a biaxial crystal with arbitrary orientation of the optical tensor has been theoretically analyzed and numerically modeled. The results indicate that, as the biaxial crystal is rotated around an axis perpendicular to the interface, two critical angles appear, corresponding to the excitation of two eigen modes, which periodically vary with a period of pi. An optical procedure for fully characterizing the optical tensor of a biaxial crystal is suggested on the basis of the twist-angle dependence of these critical angle. This procedure simply requires the measurement of the p- to s-conversion reflectivity against the sample rotation angle, with just one polished surface of a biaxial crystal.     

Spectrophotometer for noninvasive measurement of intrinsic fluorescence and reflectance of the ocular fundus

A spectrophotometer for noninvasively measuring the intrinsic fluorescence and the reflectance of the ocular fundus is described. The instrument uses multichannel spectral analysis for recording fluorescence emission spectra (500-800 nm) with seven excitation wavelengths between 430 and 550 nm and for the determination of fundus reflectance spectra (400-800 nm). Measurements are performed from a discrete fundus area, with a spatial resolution of a 1-2° visual angle. Calibration procedures are detailed. Representative fluorescence and reflectance spectra obtained from five normal subjects indicate that the fluorescence originates from within the fundus layers. Although the absolute fundus fluorescence measurement is affected by lens absorption and ocular refraction, it is minimally influenced by the strong fluorescence of the crystalline lens.     

Effects of tissue surface curvature and incident light angle on diffuse correlation spectroscopy

Diffuse correlation spectroscopy is an emerging technique for the continuous, non-invasive monitoring of blood flow in biological tissues. However, the influence of oblique incidence and surface curvature has not been fully discussed. In this paper, we study the effects of the incident angle and surface curvature on diffuse correlation spectroscopy measurement and the blood flow index (BFI). For semi-infinite plane with lower absorption, oblique incidence angle has slightly stronger influence. Larger incident angles lead to high values and a reduced decay rate of the normalized electric field temporal autocorrelation. When the radius of curvature is more than 10?cm, the BFI estimation error reduced to less than 5% and very close to semi-infinite plane case. Besides, for the surface with small radius of curvature, larger incident angles sometimes may cause smaller estimated errors of the BFI. This work may help improve BFI estimation accuracy from diffuse correlation spectroscopy.     

Realization of High Performance AR‐coatings with Dust‐ and Water‐Repellent Properties

Hydrophobic coatings enable the manufacture of easy‐to‐clean surfaces having dust‐ and water‐repellent properties. In this work, a hydrophobic coating is deposited as a top layer on an antireflective (AR) multilayer system to produce low reflectance optical surfaces at a normal incident angle in the visible spectrum with dust‐ and water‐repellent properties for applications in precision optics. It is shown that the hydrophobic coating can be considered, from an optical point of view, as two adjacent thin layers having specific thicknesses and densities. In fact, the hydrophobic layer is one monolayer comprising molecular chains with anchoring groups responsible for the chemical bond with the substrate material and functional groups responsible for the water‐ and oil‐repellent properties. Their optical constants are determined and included in the final coating design. High performance AR coatings having an average reflectance of 0.14% at 7° incident angle in the 400‐680nm spectral range together with a pleasing purplered reflex color are produced. Coated lenses exhibit an excellent abrasion resistance, environmental stability, resistance to cleaning agents, homogeneity and water repellence with contact angles against water higher than 110°.     

Depolarization of diffusely reflecting man-made objects

The polarization properties of light scattered or diffusely reflected from seven different man-made samples are studied. For each diffusely reflecting sample an in-plane Mueller matrix bidirectional reflectance distribution function is measured at a fixed bistatic angle using a Mueller matrix imaging polarimeter. The measured profile of depolarization index with changing scattering geometry for most samples is well approximated by an inverted Gaussian function. Depolarization is minimum for specular reflection and increases asymptotically in a Gaussian fashion as the angles of incidence and scatter increase. Parameters of the Gaussian profiles fitted to the depolarization data are used to compare samples. The dependence of depolarization on the incident polarization state is compared for each Stokes basis vector: horizontal, vertical, 45 degrees, 135 degrees, and right- and left-circular polarized light. Linear states exhibit similar depolarization profiles that typically differ in value by less than 0.06 (where 1.0 indicates complete depolarization). Circular polarization states are depolarized more than linear states for all samples tested, with the output degree of polarization reduced from that of linear states by as much as 0.15. The depolarization difference between linear and circular states varies significantly between samples.     

Liquid-crystal tunable filter spectral imaging for brain tumor demarcation

Past studies have demonstrated that combined fluorescence and diffuse reflectance spectroscopy can successfully discriminate between normal, tumor core, and tumor margin tissues in the brain. To achieve efficient, real-time surgical resection guidance with optical biopsy, probe-based spectroscopy must be extended to spectral imaging to spatially demarcate the tumor margins. We describe the design and characterization of a combined fluorescence and diffuse reflectance imaging system that uses liquid-crystal tunable filter technology. Experiments were conducted to quantitatively determine the linearity, field of view, spatial and spectral resolution, and wavelength sensitivity of the imaging system. Spectral images were acquired from tissue phantoms, mouse brain in vitro, and human cortex in vivo for functional testing of the system. The spectral imaging system produces measured intensities that are linear with sample emission intensity and integration time and possesses a 1 in. (2.54 cm) field of view for a 7 in. (18 cm) object distance. The spectral resolution is linear with wavelength, and the spatial resolution is pixel-limited. The sensitivity spectra for the imaging system provide a guide for the distribution of total image integration time between wavelengths. Functional tests in vitro demonstrate the capability to spectrally discriminate between brain tissues based on exogenous fluorescence contrast or endogenous tissue composition. In vivo imaging captures adequate fluorescence and diffuse reflectance intensities within a clinically viable 2 min imaging time frame and demonstrates the importance of hemostasis to acquired signal strengths and imaging speed.     

A Method for the Optical Characterization of Thin Uniaxial Samples

An optical characterization procedure for small fragments of uniaxial materials is described involving the simple use of crossed polarizers with one polished face of the material. The reflectance at a fluid-uniaxial slab boundary beyond, but near, the critical angle of incident light is examined for linear incidence polarization using an orthogonal output polarizer. It is found that, as the crossed incident and output polarizers are rotated together, there are, for a given angle of incidence, particular polarization angles for which the reflectivity is a minimum. These angles give information on the optical tensor of the crystal under study. Further the intensity of the reflected light, for incidence angles beyond critical with the input and output polarizers crossed, has as a function of the incident polarization angle an oscillatory form which, when fitted to theory, can also yield the full uniaxial tensor of the material under study. This is confirmed experimentally for a thin single crystal of calcite with one polished face.     

Measurement of the complex refractive-index spectrum for birefringent and absorptive liquids

The optical constants of birefringent and/or opaque liquids, e.g., liquid crystals and magnetic fluids, are difficult to measure at wavelengths at which a strong light source such as a laser or an arc lamp is not accessible. The refractive index n and the extinction coefficient kappa of these liquids can be simultaneously evaluated from the reflectance curves that are measured in the large incident angle range. A semicylindrical sample cell allows the spectral reflectance measurement with a weak light source even at large incident angles. By using this method, we evaluated the ordinary and the extraordinary indices of a nematic liquid crystal in the continuous wavelength range of 0.55-1.60 mum. The complex refractive indices of magnetic fluids were also evaluated, and the affect of the magnetic field was demonstrated.