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PubMed2006

High-reflectance composite metal coatings for planar-integrated free-space optics.

For planar-integrated free-space optical (PIFSO) systems high-reflectance thin-film coatings are crucial. Evaporated metal films are preferred for their relative technological simplicity. We propose a three-layer Al-Ag-Al coating composition that combines the high reflectance of Ag with the chemical passivity of Al and its good adherence to glass. Two special measures are taken to prevent delamination: one is an anchoring of the edges of the coating in narrow ditches that are etched into the substrate and the other is the use of an adhesive Al underlayer; to reduce absorption this underlayer is implemented only in sparsely distributed discrete areas. The optical properties of such composite coatings are investigated theoretically. The fabrication complexity is only slightly increased compared to PIFSO systems with one-layer Al reflectors. In experimental tests we verified a reflectance of approximately 98% and an adherence comparable to that of simple Al coatings.

Journal Article↗

PubMed2006

Determination of scattering volume fraction and particle size distribution in the superficial layer of a turbid medium by using diffuse reflectance spectroscopy.

We have investigated the possibility of determining changes in the volume fraction of microstructure scatterers in the superficial tissue layers by using diffuse reflectance spectroscopy. To that extent we have built a two-layer optical phantom by using microparticles with various sizes in order to simulate the scattering properties of tissue microstructures. Reflectance spectral measurements were performed on a number of optical phantoms having different volume fractions of various microparticle sizes. An analytical model was developed using light-transport theory and fractal modeling approaches and was then fitted to the measured reflectance to calculate the volume fractions of the microparticles in phantoms. The results showed that we could measure changes in both the total volume fraction of the microparticles and in the overall size distribution of the microparticles with good accuracy (>80%). These results suggest the potential of using this method for measuring the volume fraction changes of tissue microstructure scatterers and applications in the detection of cancerous related morphological and structural changes.

Algorithms↗

PubMed2006

Use of hyperspectral remote sensing reflectance for detection and assessment of the harmful alga, Karenia brevis.

We applied two numerical methods to in situ hyperspectral measurements of remote sensing reflectance Rrs to assess the feasibility of remote detection and monitoring of the toxic dinoflagellate, Karenia brevis, which has been shown to exhibit unique absorption properties. First, an existing quasi-analytical algorithm was used to invert remote sensing reflectance spectra, Rrs(lambda), to derive phytoplankton absorption spectra, a(phi)Rrs(lambda). Second, the fourth derivatives of the a(phi)Rrs(lambda) spectra were compared to the fourth derivative of a reference K. brevis absorption spectrum by means of a similarity index (SI) analysis. Comparison of reflectance-derived a(phi) with filter pad measured a(phi) found them to agree well (R2=0.891; average percentage difference, 22.8%). A strong correlation (R2=0.743) between surface cell concentration and the SI was observed, showing the potential utility of SI magnitude as an indicator of bloom strength. A sensitivity analysis conducted to investigate the effects of varying levels of cell concentrations and colored dissolved organic matter (CDOM) on the efficacy of the quasi-analytical algorithm and SI found that a(phi)Rrs(lambda) could not be derived for very low cell concentrations and that, although it is possible to derive a(phi)Rrs(lambda) in the presence of high CDOM concentrations, CDOM levels influence the a(phi)Rrs(lambda) amplitude and shape. Results suggest that detection and mapping of K. brevis blooms based on hyperspectral measurements of Rrs are feasible.

Algorithms↗

PubMed2006

Laboratory measurements of spectral reflection from ice clouds of various habits.

Spectral light from 550 to 650 nm reflected from the surface of an ice cloud produced in a temperature-controlled column is measured at seven different angles between 16.7 degrees and 29.9 degrees . Cloud optical depth (tau) is determined from the extinction of a 670 nm laser and is corrected for forward scattering using a Monte Carlo ray-tracing algorithm. Reflection measurements are compared to expectations from a plane-parallel radiative transfer model with input parameters based on the measured tau and a phase function for the observed ice crystal types. The plane-parallel radiative transfer model can be used to interpret the measured reflection for tau less than about 0.4 for this particular experiment, ideal for providing a validation data set to assist with the development of satellite bidirectional remote sensing.

Journal Article↗

PubMed2006

Unpolarized emissivity with shadow and multiple reflections from random rough surfaces with the geometric optics approximation: application to Gaussian sea surfaces in the infrared band.

The emissivity from a stationary random rough surface is derived by taking into account the multiple reflections and the shadowing effect. The model is applied to the ocean surface. The geometric optics approximation is assumed to be valid, which means that the rough surface is modeled as a collection of facets reflecting locally the light in the specular direction. In particular, the emissivity with zero, single, and double reflections are analytically calculated, and each contribution is studied numerically by considering a 1D sea surface observed in the near infrared band. The model is also compared with results computed from a Monte Carlo ray-tracing method.

Journal Article↗

PubMed2006

Hyperspectral diffuse reflectance imaging for rapid, noncontact measurement of the optical properties of turbid materials.

We present a method and technique of using hyperspectral diffuse reflectance for rapid determination of the optical properties of turbid media. A hyperspectral imaging system in line scanning mode was used to acquire spatial diffuse reflectance profiles from liquid phantoms made up of absorbing dyes and fat emulsion scatterers over the spectral range of 450-1000 nm instantaneously. The hyperspectral reflectance data were analyzed by using a steady-state diffusion approximation model for semi-infinite homogeneous media. A calibration procedure was developed to compensate the nonuniform instrument response of the imaging system, and a curve-fitting algorithm was used to extract absorption and reduced scattering coefficients (mua and mus', respectively) for the phantoms in the wavelength range from 530 to 900 nm. The hyperspectral imaging system gave good measures of mua and mus' for the phantoms with average fitting errors of 12% and 7%, respectively. The hyperspectral imaging technique is fast, noncontact, and easy to use, which makes it especially suitable for measurement of the optical properties of turbid liquid and solid foods.

Equipment Design↗

PubMed2006

Bidirectional reflectance study on dry, wet, and submerged particulate layers: effects of pore liquid refractive index and translucent particle concentrations.

We performed extensive bidirectional reflectance measurements on dry, wet, and submerged particulate layers with various albedos to investigate the darkening effect caused by wetting with fluids. It was found that, in addition to the reduction of the refractive index contrast when there is a pore liquid (wetted), the concentration of translucent grains in a particulate layer and the surface roughness conditions of the individual grains make important contributions to the wetting-induced darkening effect. Reflectance measurements on glass-sediment mixtures confirmed that, as the concentration of translucent particles increases, the reflectance of the dry layers increases while that of the wetted layers decreases. Measurements indicate that neither the prediction made by the theory of Twomey et al. [Appl. Opt. 25, 431 (1986)] nor that of Lekner and Dorf [Appl. Opt. 27, 1278 (1988)] is sufficient.

Journal Article↗

PubMed2007

Closed equation for the normal incidence reflectance of thin films on absorbing substrates.

The optical constants of thin films can be obtained from inversion of spectrophotometric measurements by using minimization gradient methods. The computational approach of these minimization methods requires closed compact formulas for reflectance and/or transmittance. For normal incidence closed compact formulations for the direct transmittance, both for thin films on transparent or absorbing substrates, and for the reflectance of thin films on transparent substrates, are available in the literature. We report here a closed compact formula to evaluate reflectance spectra of thin films on absorbing substrates, and it is shown that for vanishing substrate absorption this new, to the best of our knowledge, approach gives the same results obtained from the formulation corresponding to thin films supported by transparent substrates.

Journal Article↗

PubMed1999

Reflectance and curvature of the inner limiting membrane at the foveola.

Light reflected specularly by the inner limiting membrane (ILM) provides information on the topography of the retinal surface. The ILM in the central part of the foveal pit acts as a concave mirror. Light reflected specularly by this mirror forms an image of the entrance pupil in front of the retina. In 15 normal subjects (ages 16-56 years) we have measured photometric and geometrical-properties of this image to derive two characteristics of the ILM: its reflectance rho at the foveola and its radius of curvature r in the central part of the fovea. rho and r are found to decrease significantly with age (p = 0.0073 and p = 0.01, respectively). The equations of the regression lines are log10 rho = -4.234 - 0.0118 age and radius r = 1484 - 13.6 age, respectively (age in years, r in micrometers).

Adolescent↗

PubMed2002

Reconstruction of gratings from noisy reflection data.

The worst-case error amplification factor in reconstructing a grating from its complex reflection spectrum is shown to be of the order 1/T(min), where T(min) is the minimum transmissivity through the grating. For a uniform grating with coupling coefficient-length product kappaL, the error amplification is exp(2kappaL). The exponential dependence on the grating strength shows that spatial characterization of gratings from a measured reflection spectrum is impossible if the grating is sufficiently strong. For moderately strong gratings, a simple regularization technique is proposed to stabilize the solution of the inverse-scattering problem of computing the grating structure from the reflection spectrum.

Journal Article↗

PubMed2002

Bidirectional reflectance distribution function of specular surfaces with hemispherical pits.

We derive the bidirectional reflectance distribution function for a class of opaque surfaces that are rough on a macroscale and smooth on a microscale. We model this type of surface as a distribution of spherical mirrors. Since our study concerns geometrical optics, it is only the aperture of the concavities that is relevant, not the dimension. The three-dimensional problem is effectively transformed into a much simpler two-dimensional one involving the possibly infinitely many reflections in a spherical mirror. We find that these types of surface show very strong backscattering when the pits are deep but forward scattering when the pits are shallow. Such surfaces also show spectral effects as a result of multiple reflections and polarization effects that are due to the orientation of the effective surface. Both this model and the locally diffuse thoroughly pitted surface model [Int. J. Comput. Vision 31, 129 (1999)] are superior to other models in that they allow for an exact treatment for physically realizable surface geometries.

Journal Article↗

PubMed2004

Device for convenient measurement of spatially varying bidirectional reflectance.

Capturing surface appearance is a challenging task because reflectance varies as a function of viewing and illumination direction. In addition, most real-world surfaces have a textured appearance, so reflectance also varies spatially. We present a texture camera that can conveniently capture spatially varying reflectance on a surface. Unlike other bidirectional imaging devices, the design eliminates the need for complex mechanical apparatus to move the light source and the camera over a hemisphere of possible directions. To facilitate fast and convenient measurement, the device uses a curved mirror so that multiple views of the same surface point are captured simultaneously. Simple planar motions of the imaging components also permit change of illumination direction and region imaging. We present the current prototype of this device, imaging results, and an analysis of the important imaging properties.

Journal Article↗

PubMed2004

Synthesis of birefringent reflective gratings.

The layer-peeling method for reconstruction of fiber and waveguide gratings is extended to the case of birefringent reflective gratings with polarization-dependent background index and polarization-dependent effective index contrast. Using a discrete grating model, we characterize the set of possible reflection and transmission Jones matrices and show that for a given wavelength, the total structure can be represented by a discrete reflector sandwiched between two retardation sections. In reflection the discrete reflector acts as a partial polarizer. A method for designing birefringent gratings is developed and tested numerically.

Journal Article↗

PubMed2004

Phase shifts that accompany total internal reflection at a dielectric-dielectric interface.

The absolute, average, and differential phase shifts that p- and s-polarized light experience in total internal reflection (TIR) at the planar interface between two transparent media are considered as functions of the angle of incidence phi. Special angles at which quarter-wave phase shifts are achieved are determined as functions of the relative refractive index N. When the average phase shift equals pi/2, the differential reflection phase shift delta is maximum, and the reflection Jones matrix assumes a simple form. For N > square root 3, the average and differential phase shifts are equal (hence deltap = 3 deltas) at a certain angle phi that is determined as a function of N. All phase shifts rise with infinite slope at the critical angle. The limiting slope of the delta-versus-phi curve at grazing incidence (partial partial differential delta/partial partial differential phi)phi=90 degrees = -(2/N)(N2 - 1)1/2 = -2 cos phic, where phic is the critical angle and (partial partial differential2 delta/partial partial differential phi 2)phi=90 degrees = 0. Therefore delta is proportional to the grazing incidence angle theta = 90 degrees - phi (for small theta) with a slope that depends on N. The largest separation between the angle of maximum delta and the critical angle is 9.88 degrees and occurs when N = 1.55377. Finally, several techniques are presented for determining the relative refractive index N by using TIR ellipsometry.

Journal Article↗

PubMed2005

Angular dependence of the specular reflectance from an isotropic polydomain medium with large domains: surprising results regarding Brewster's angle.

The angular dependence of the reflectance from an isotropic medium with large, randomly oriented, ordered domains is investigated and compared with that of an isotropic medium in which the optical properties can be characterized by a scalar dielectric function. Employing average reflectance and transmittance theory, we demonstrate that an isotropic medium consisting of optically large ordered domains exhibits only a parallel-polarized reflectance minimum, not the formal zero expected at Brewster's angle. We attribute this result to the existence of nonzero cross-polarization terms despite the random orientation and the application of an incoherent averaging scheme, which is used to mimic traditional light sources.

Journal Article↗

PubMed2005

Reflection and refraction of multipole radiation by an interface.

Reflection and refraction of electromagnetic multipole radiation by an interface is studied. The multipole can be electric or magnetic and is of arbitrary order (dipole, quadrupole). From the angular spectrum representation of the radiation emitted by the multipole, I have obtained the angular spectrum representations of the reflected and transmitted fields, which involve the Fresnel reflection and transmission coefficients. The intensity distribution in the far field is evaluated with the method of stationary phase. The result is very simple in appearance and can be expressed in terms of two auxiliary functions of a complex variable. By exchanging the Fresnel coefficients for s and p polarization, the result for an electric multipole can be obtained from the result for a magnetic multipole.

Journal Article↗

PubMed2005

Detailed analytical approach to the Gaussian surface bidirectional reflectance distribution function specular component applied to the sea surface.

A statistical sea surface specular BRDF (bidirectional reflectance distribution function) model is developed that includes mutual shadowing by waves, wave facet hiding, and projection weighting. The integral form of the model is reduced to an analytical form by making minor and justifiable approximations. The new form of the BRDF thus allows one to compute sea reflected radiance more than 100 times faster than the traditional numerical solutions. The repercussions of the approximations used in the model are discussed. Using the analytical form of the BRDF, an analytical approximation is also obtained for the reflected sun radiance that is always good to within 1% of the numerical solution for sun elevations of more than 10 degrees above the horizon. The model is validated against measured sea radiances found in the literature and is shown to be in very good agreement.

Journal Article↗

PubMed2004

High-efficiency, high-dispersion diffraction gratings based on total internal reflection.

We report a new class of high-dispersion immersed diffraction gratings for which the reflective nature of the diffraction is provided by the phenomenon of total internal reflection (TIR) regardless of grating tooth shape. Thus, the component can be fabricated from a single dielectric material and requires no metallic or dielectric film layers for high reflection diffraction efficiency. With the absence of metallic absorption, diffraction efficiencies of these TIR gratings can reach more than 99% for 15-20-nm spectral bandwidths, making them suitable for many laser-based technologies.

Journal Article↗