Absolute Reflectance Measurements Research Articles

変形シャープ・リトル法による高精度絶対反射率測定

変形シャープ・リトル法による高精度絶対反射率測定

Neural network applications to the rainfall rate extraction in the presence of attenuation

The paper addresses the problem of the reconstruction of the rainfall field using weather radar observables. It is well known that at the C band and especially at the X band the reconstruction of the rainfall rate profile along the range using absolute (ZH) and differential (ZDR) reflectivity measurements is significantly affected by the attenuation coefficients (i.e. αH and αD. This problem has been long and extensively studied and iterative attenuation correction techniques based on a cumulative procedure were developed, in which the attenuation at nth cell is estimated using the attenuation corrected reflectivity values at previous cell. Usually the attenuation coefficients αH, αD are estimated using non linear parametrizations with (ZH, ZDR), or, if phase measurements are available, using linear parametrizations with the specific differential phase shift KDp. In this work novel approaches based on neural networks (N.N.) have been used.First, to estimate αH, αD from (αH, ZDR, and ZDP; the N.N. estimators have shown better performance (often, slightly better) in comparison to the best ones known.Second, N.N. have been implemented to extract the range rainfall rate profile. The input to the network is a vector containing the attenuated measurements of (αH, ZDR, and ZDP in a number of range cells while the output is the estimated profile of the rainfall rate. In this way a global compensation of the attenuation is implemented.

Macroporous silicon photonic crystals at 1.55 µm

The limits of macroporous silicon etching are pushed a step further to obtain a complete photonic bandgap centred at 1.55 µm wavelength. A submicrometre period triangular photonic crystal of air holes is etched over a large area of silicon and is characterised by absolute reflection measurements for two propagation directions and both polarisations. The large area covered and the quality of the photonic crystal obtained demonstrate the feasibility of realising high density integrated devices at telecommunication wavelengths.

SCATTERING AND DIFFRACTION OF LIGHT BY PARTICLES IN PLANETARY REGOLITHS

The nature of diffraction and scattering by particles in planetary regoliths is discussed. It is pointed out that the Percus–Yevick approximation to the particle radial distribution function is not valid for a regolith, so that structure factor models based on it are incorrect. A simple analytic expression, valid for general shape and size distributions, for the decrease in scattering cross section caused by the proximity of other particles is derived. It is shown that the classical diffraction pattern of an isolated particle does not exist in a regolith, and that the diffracted light is indistinguishable from the incident irradiance. Hence, to an excellent approximation diffracted light may be treated as unscattered. It is shown that back scattering particles are common in nature and that this conclusion is consistent with the theoretical models published in the paper by Mishchenko and Macke ( JQSRT, 1997, 57, 767). It is demonstrated that when exact solutions of the radiative transfer equation are used to analyze observational data sets with a small range of phase angles they retrieve particle asymmetry parameters that are erroneously negative, but that if the range is sufficiently large, approximate reflectance models retrieve correctly positive asymmetries. Hence, assertions that approximate models retrieve erroneously negative asymmetries are false. However, most laboratory and planetary observations are poorly suited to constrain the particle scattering properties in the forward direction, so that many planetary regoliths may have particle phase functions with forward scattering lobes, in addition to back scattering ones. It is pointed out that the radiative transfer equation has inherent deficiencies that make it intrinsically incapable of calculating exact descriptions of scattering by a close-packed regolith of large particles and also that the accuracies of typical absolute reflectance measurements are not high. For both reasons it is pointless to be overly concerned with the relative accuracies of exact and approximate regolith scattering models based on the equation of radiative transfer.

Retrieving land surface reflectances using the ATSR‐2: A theoretical study

The measurement of absolute land reflectance from Earth‐orbiting satellites requires the separation of the signal due to the reflection of solar radiation by the surface from that due to scattering by the atmosphere. Thus for accurate but generally applicable quasi‐operational retrieval techniques it is necessary to derive atmospheric parameters, such as the aerosol type and loading, simultaneously with the surface reflectances. The assumption of large‐scale horizontal homogeneity across an image, or the presence of very low reflectance pixels within an image, is avoided, since these are not necessarily applicable in the general case. In this paper, results of simulated retrievals using the visible and near‐infrared channels of the ATSR‐2 satellite radiometer are presented on the basis of techniques which exploit the dual look and other features of the instrument. The pixel‐based retrieval scheme comprises a fast representation of atmospheric scattering and a simple surface reflectance approximation. The effect of the atmospheric approximation on the accuracy of retrieval of the surface reflectance and atmospheric optical depth is tested by using a full multiple‐scattering radiative transfer model with simpler surface model. The effect on retrieval accuracy of the surface model is tested using a full surface vegetation canopy reflectance model with inclusion of the hot‐spot effect, combined with a successive orders of scattering atmospheric radiative transfer model, taken to second order. The results indicate that the scheme could provide accurate retrieval of surface reflectances even where no extraneous information on the surface bidirectional reflectance or on the atmospheric optical thickness is available. The scheme is also applicable to the AATSR instrument to be launched on the ENVISAT (Environmental Satellite) and, more widely, to other satellite radiometers with multiple views of the Earth's surface.

Optical properties of potassium acid phthalate

Potassium acid phthalate (KAP) crystals are promising as substrates for the growth of highly oriented films of conjugated polymers with exceedingly high and fast nonlinear optical response. We report the KAP optical properties (real and imaginary parts of the refractive index ñ = n + ik) in the near infrared and visible range deduced by ellipsometric measurements and direct inversion of transmittance and near-normal incidence absolute reflectance measurements. In the infrared region n was also deduced by the interference fringes.

Influence of the Number of Bilayers on the Optical Performances of X-UV Multilayer Interferential Mirrors

The influence of the number of bilayers on the optical performances of actual X-UV multilayer interferential mirrors (MIMs) has been studied in order to emphasize the experimental restrictions in the designing of “thick” mirrors used for the development of etched multilayer gratings. Several sets of samples (W/C, Mo/Si) with increasing number of bilayers have been manufactured in the very same conditions by means of a sputtering technique. X-ray diffraction characterization at Cu-Kαradiation (λ = 1.54018 Å) exhibits technical constraints in the achievement of multilayer structures with large number of bilayers. We obtain a gradual loss of reflectivity for deposition times greater than 1h 30 min to 2h without significant drift of the MIM's geometrical parameters (period and division parameter). In the same time, absolute reflectivity measurements at Cu-Lαradiation (λ = 1.333 nm) emphasize satisfying optical and spectroscopic performances of W/C thick samples (150 bilayers).

Influence of the Number of Bilayers on the Optical Performances of X-UV Multilayer Interferential Mirrors

The influence of the number of bilayers on the optical performances of actual X-UV multilayer interferential mirrors (MIMs) has been studied in order to emphasize the experimental restrictions in the designing of "thick" mirrors used for the development of etched multilayer gratings. Several sets of samples (W/C, Mo/Si) with increasing number of bilayers have been manufactured in the very same conditions by means of a sputtering technique. X-ray diffraction characterization at Cu-Kα radiation (λ = 1.54018 Å) exhibits technical constraints in the achievement of multilayer structures with large number of bilayers. We obtain a gradual loss of reflectivity for deposition times greater than 1h 30 min to 2h without significant drift of the MIM's geometrical parameters (period and division parameter). In the same time, absolute reflectivity measurements at Cu-Lα radiation (λ = 1.333 nm) emphasize satisfying optical and spectroscopic performances of W/C thick samples ( 150 bilayers).

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.

Optical properties of Mg, from UV to IR, using ellipsometry and reflectometry

Magnesium films have been traditionally used for UV filters. However, scarce information has been reported on its optical properties [1–3] and the problems it presents in the production of the films. Being a Drude material, magnesium is a suitable material for studying the photon-surface plasmon interaction by attenuated total reflectance, but the dielectric function is poorly known for the IR-visible region. In this paper we present our values for the optical properties of a Mg thin film obtained from ellipsometric and absolute reflectance measurements. The dielectric function is estimated using Drude modeling, extending the previously reported data into the infrared.

Ab-plane optical properties of Tl2Ba2CuO6+ delta.

We report on absolute ab-plane reflectivity measurements of the single-layer high-${\mathit{T}}_{\mathit{c}}$ superconductor ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{CuO}}_{6+\mathrm{\ensuremath{\delta}}}$ in both the normal and superconducting states. The reflectivity has been measured from 40 to 40000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and the optical conductivity has been calculated from Kramers-Kronig analysis. The unusual form of the low-frequency reflectivity leads to an intense low-frequency feature in the optical conductivity. In the superconducting state, the spectral weight is strongly suppressed at frequencies below \ensuremath{\sim}800 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ due to the formation of a superfluid fraction. The value of the London penetration depth has been estimated to be 2400\ifmmode\pm\else\textpm\fi{}200 \AA{}, larger than that from muon-spin-resonance (\ensuremath{\mu}SR) experiments.

Laboratory characterization of Wölter x-ray optics

We have conducted characterization measurements of a Wölter grazing incidence x-ray microscope. The measurements were carried out on 5% sectors of a Wölter x-ray optic in a laboratory utilizing a new, very versatile, high brightness x-ray source. Absolute reflectance measurements as a function of x-ray energy were made with Si(Li) detectors to acquire continuum spectra prior to and after reflecting off the Wölter optic. Spatial resolution characteristics of the Wölter were mapped out using backilluminated pinholes or grids imaged onto film or an x-ray charge-coupled device camera. The implications of the asymmetric point spread function are considered.

Optical studies of YBCO c - axis in plane oriented film in normal and superconductive states

Abstract We report for the first time absolute reflection measurements on YBa CuO HTSC thin film with c - axis oriented in the film plane. Measurements have been fulfilled in a wide frequency range (20 – 5500 cm −1 ) for several film temperatures (13–300 K) with different polarizations of the incident light. The film revealed a strong anisotropy reflection. Some peculiarities are observed, though general behavior of the reflectivity curves is in a fairly good agreement with previous results, obtained on the perfect monocrystal samples. One can conclude, that a simple Drude model hardly the best approach for the film data interpretations.

Absolute measurements of diffuse reflectance in the α°/d configuration

An improved theory for data reduction of absolute reflectance measurements using the third Taylor method in the alpha degrees / d configuration is presented. A brief description is given of an absolute reflectometer operating in the 0.8-2.5-microm region. The reflectometer is operated according to the improved theory. Experimental data for some widely used samples are given, as well as data showing agreement between the current measurements and those made by the U.S. National Institute of Standards and Technology.

Absolute reflectivity measurements of multilayered interference mirrors in the soft X-ray region

High reflectivity multilayer soft X-ray mirrors were deposited using a dc triode sputtering system, equipped with facilities for accurately monitoring film thickness. Absolute reflectivity measurements of Ni-C and W-C multilayers at λ = 44.79 Å (CK α) in the first and second order Bragg peaks, and at λ = 67.8 Å (BK α) in the first order only, were carried out using a special soft X-ray spectrogoniometer which enables the manufacturing conditions to be rapidly optimized without relying on synchrotron radiation. In this spectrometer the incident beam is polarized by two parallel plane multilayer mirrors fixed at an angle close to the Brewster angle (θ ≈ 45°). Thus the measured reflectivities are not affected by the large variation of the P component versus the angle. The preliminary results obtained at λ = 113 Å (BeK α) are also presented.

Absolute reflectivity measurements at 4479 Å of sputter deposited multilayer x-ray mirrors

Multilayer x-ray mirrors have been deposited using a dc triode sputtering system, which incorporates an accurate method of thickness monitoring based on the dependence of the deposition rate on the target current. Thickness can be controlled with an accuracy of better than 0.1 A. High efficiency W-C and Ni-C multilayer mirrors have been synthesized and tested at 1.54-A (CuKoalpha) and 44.79-A (CKalpha). Absolute reflectivity measurements at lambda = 44.79-A (CKalpha) have been carried out. In this case the incident beam is previously polarized by a premonochromator equipped with a pair of parallel-plane multilayer mirrors fixed at an angle close to the Brewster (theta ? 45 degrees ). Thus the measured reflectivities are not affected by a progressive variation of the P-component.

Structure and properties of W/C and Ni/C multilayer films

High reflectivity multilayered mirrors for the soft X-ray region were deposited using a triode sputtering system equipped with an accurate method of film thickness monitoring and giving very sharp interfaces and a high stack regularity. Absolute reflectivity measurements of W/C and Ni/C multilayers were carried out at λ = 44.79 A ̊ and λ = 67.8 A ̊ using a special soft X-ray spectrogoniometer which enables the manufacturing conditions to be optimized rapidly without the need for the synchrotron radiation. Magnetic studies of Ni/C multilayer films yielded information on the structure of the interfaces.

Structure and properties of W/C and Ni/C multilayer films

High reflectivity multilayered soft X-ray mirrors were deposited using a triode sputtering system, equipped with facilities for accurately monitoring film thickness, giving very sharp interfaces and a high stack regularity. Absolute reflectivity measurements of W/C and Ni/C multilayers were carried out at λ = 44.79 Å and λ = 67.8 Å using a special soft X-ray spectrogoniometer which enables the manufacturing conditions to be rapidly optimized without relying on synchrotron radiation. Magnetic studies of Ni/C multilayer films give information on the structure of the interfaces.

Optical determination of the thickness of single planar thin lipid films using the symmetrical three-layer model

The total thickness of thin lipid films can be determined by the method of absolute or relative reflectance measurements. The film is described by the symmetrical three-layer model. By assuming equal refractive indices for the hydrocarbon layer and for the organic solution, the thickness and refractive index of the head group layer can be estimated.

Flat-sample and limited-field effects in integrating sphere measurements

The effect of large, flat diffuse samples on integrating sphere measurements is calculated exactly, based on an extension of the integral equation formalism of Jacquez and Kuppenheim [ J. Opt. Soc. Am.45, 460 ( 1955)]. The theory is further extended to include the effect of a limited detector field of view, and examples for different sphere configurations are provided. A variation of Taylor’s method for absolute reflectance measurements is presented that has the advantage of an improved signal-to-noise ratio.