A novel silicon nanotips antireflection surface for the micro Sun sensor

We have developed a new technique to fabricate an antireflection surface using silicon nanotips for use on a micro Sun sensor for Mars rovers. We have achieved randomly distributed nanotips of radii spanning from 20 to 100 nm and aspect ratio of approximately 200 using a two-step dry etching process. The 30 degrees specular reflectance at the target wavelength of 1 microm is only about 0.09%, nearly 3 orders of magnitude lower than that of bare silicon, and the hemispherical reflectance is approximately 8%. When the density and aspect ratio of these nanotips are changed, a change in reflectance is demonstrated. When surfaces are covered with these nanotips, the critical problem of ghost images that are caused by multiple internal reflections in a micro Sun sensor was solved.     

Bidirectional reflectance distribution function of diffuse extreme ultraviolet scatterers and extreme ultraviolet baffle materials

Bidirectional reflectance distribution function (BRDF) measurements of a number of diffuse extreme ultraviolet (EUV) scatterers and EUV baffle materials have been performed with the Goddard EUV scatterometer. BRDF data are presented for white Spectralon SRS-99 at 121.6 nm; the data exhibit a non-Lambertian nature and a total hemispherical reflectance lower than 0.15. Data are also presented for an evaporated Cu black sample, a black Spectralon SRS-02 sample, and a Martin Optical Black sample at wavelengths of 58.4 and 121.6 nm and for angles of incidence of 15 degrees and 45 degrees. Overall Martin Optical Black exhibited the lowest BRDF characteristic, with a total hemispherical reflectance of the order of 0.01 and measured BRDF values as low as 2 x 10(-3) sr(-1).     

Thermal Radiation Phenomena of Surfaces of Chromium and Palladium in a High-Temperature Environment

For the purpose of thermal engineering applications, thermal radiation phenomena of surfaces of chromium and palladium in a high-temperature air-oxidation process on the basis of simultaneous measurements of spectra of normal incidence hemispherical reflectance and normal emittance in the near-ultraviolet through infrared region are investigated. As the results of the spectral measurements, clear radiation interference is observed even in the spectra of the hemispherical reflectance of a rough-finished chromium surface in the oxidation process as well as in the spectra of the normal reflectance of a specular-finished surface. The relation between the chemical reactions of oxidation and reduction of a palladium surface in the 1000 K to 1100 K range and the radiation characteristics are analyzed based on the temperature hysteresis of radiation spectra and the reaction rates of oxidation and reduction. A switching function of radiation characteristics of palladium accompanied by the oxidation and reduction reactions is suggested to be applicable for engineering applications.     

Monte Carlo modeling of an integrating sphere reflectometer

The Monte Carlo method has been applied to numerical modeling of an integrating sphere designed for hemispherical-directional reflectance factor measurements. It is shown that a conventional algorithm of backward ray tracing used for estimation of characteristics of the radiation field at a given point has slow convergence for small source-to-sphere-diameter ratios. A newly developed algorithm that substantially improves the convergence by calculation of direct source-induced irradiation for every point of diffuse reflection of rays traced is described. The method developed is applied to an integrating sphere reflectometer for the visible and infrared spectral ranges. Parametric studies of hemispherical radiance distributions for radiation incident onto the sample center were performed. The deviations of measured sample reflectance from the actual reflectance as a result of various factors were computed. The accuracy of the results, adequacy of the reflectance model, and other important aspects of the algorithm implementation are discussed.     

多角度测量系统实现室外BRDF测量

为了实现室外BRDF的准确测量,设计了多角度测量系统.测量系统包括一台自动测量架和两台ASD(Analytical Spectral Devices)公司生产的野外型光谱仪,光谱仪的光谱分辨率为3nm,一台光谱仪固定在测量架平台上测量目标各方向反射,另一台光谱仪放置在地面上同时测量漫反射板反射.应用该系统进行了室外草地的测量实验,测量整个周期(66个方向点)用时10min,测量主平面(间隔5°,共31个方向点)用时2.5min.测量结果显示目标的反射为非朗伯性,并在主平面的反射方向性最强烈.     

Breath Figures as a Dynamic Templating Method for Polymers and Nanomaterials

This review describes the use of breath figures as a templating method for the fabrication of self‐assembled polymeric‐ and nanoparticle‐based micro‐ and nanostructures. If moist air is blown over a solution of a polymer or stabilized nanoparticles in an organic solvent, such as carbon disulfide, benzene, or chloroform, evaporative cooling leads to the formation of water droplets on the liquid surface. The monodisperse droplets arrange into a hexagonal array and sink into the polymer solution. Removal of the solvent and the water leaves an imprint of the water droplets as a hollow, air‐filled, hexagonally ordered, polymeric bubble array. Progress in the field of breath‐figure formation is reviewed. The application of breath figures for the generation of functional structures in chemistry and materials science is discussed.     

Hemispherical and diffuse reflectance and transmittance of a slightly inhomogeneous film bounded by rough, unparallel interfaces

The optical reflectance and transmittance of an ideal thin film are calculated in a well-known way. As far as a non-ideal thin film is concerned - i.e., a slightly inhomogeneous thin film bounded by rough, unparallel interfaces - three categories of spectral coefficients can be defined, i.e.: specular reflectance and direct transmittance (light intensity flux along the optical axis), hemispherical reflectance and transmittance (light intensity flux integrated over the solid half angle π), and diffuse reflectance and transmittance (light intensity flux scattered around the optical axis) coefficients. In this paper a model recently introduced for the specular and direct coefficients is generalized to calculate also the hemispherical and diffuse coefficients of a non-ideal film.     

Approximate phase conjugation with a retroreflecting array in microscopy

Some experiments on image recovery in microscopy are described. The aperture stop of a microscope objective is partially obscured, and a retroreflecting array is used as an approximate phase conjugator to return the beam through the unobstructed part of the aperture. An application is made to the measurement of the absolute reflectance of small samples by use of a variant of Strong's method.     

Fractal Web Design of a Hemispherical Photodetector Array with Organic-Dye-Sensitized Graphene Hybrid Composites

The vision system of arthropods consists of a dense array of individual photodetecting elements across a curvilinear surface. This compound-eye architecture could be a useful model for optoelectronic sensing devices that require a large field of view and high sensitivity to motion. Strategies that aim to mimic the compound-eye architecture involve integrating photodetector pixels with a curved microlens, but their fabrication on a curvilinear surface is challenged by the use of standard microfabrication processes that are traditionally designed for planar, rigid substrates (e.g., Si wafers). Here, a fractal web design of a hemispherical photodetector array that contains an organic-dye-sensitized graphene hybrid composite is reported to serve as an effective photoactive component with enhanced light-absorbing capabilities. The device is first fabricated on a planar Si wafer at the microscale and then transferred to transparent hemispherical domes with different curvatures in a deterministic manner. The unique structural property of the fractal web design provides protection of the device from damage by effectively tolerating various external loads. Comprehensive experimental and computational studies reveal the essential design features and optoelectronic properties of the device, followed by the evaluation of its utility in the measurement of both the direction and intensity of incident light.     

A digital microfluidic method for in situ formation of porous polymer monoliths with application to solid-phase extraction

We introduce the marriage of two technologies: digital microfluidics (DMF), a technique in which droplets are manipulated by application of electrostatic forces on an array of electrodes coated by an insulator, and porous polymer monoliths (PPMs), a class of materials that is popular for use for solid-phase extraction and chromatography. In this work, circular PPM discs were formed in situ by dispensing and manipulating droplets of monomer solutions to designated spots on a DMF device followed by UV-initiated polymerization. We used PPM discs formed in this manner to develop a digital microfluidic solid-phase extraction (DMF-SPE) method, in which PPM discs are activated and equilibrated, samples are loaded, PPM discs are washed, and the samples are eluted, all using microliter droplets of samples and reagents. The new method has extraction efficiency (93%) comparable to that of pipet-based ZipTips and is compatible with preparative sample extraction and recovery for on-chip desalting, removal of surfactants, and preconcentration. We anticipate that DMF-SPE may be useful for a wide range of applications requiring preparative sample cleanup and concentration.     

Nonintrusive measurements of temperature and size of single falling raindrops

A nonintrusive laser technique, based on the detection of a rainbow, is presented that permits one to determine simultaneously the temperature and size of droplets. Therefore the Airy theory for a rainbow and a calibration rainbow pattern at isothermal conditions are applied. Rainbow patterns coming from droplets in the millimeter range have been recorded on a linear CCD array. It has been found that the sphericity of the droplets plays an important role for this rainbow-based technique.     

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 and microstructural properties of anodically oxidized aluminum

Scanning electron microscopy, Auger electron spectroscopy, emissometer and reflectometer measurements have been made on anodic oxide coatings on aluminum prepared from solutions of phosphoric, chromic and sulfuric acid. The anodic oxide films are very porous, with the porosity greatest in films which were produced in phosphoric acid. A structural model for the oxide coatings has been generated from the data, and microstructural parameters for the model were defined. The IR emittance as well as the spectral hemispherical reflectance of the oxides have been measured as a function of film parameters, with specific emphasis on optimization of optical properties for use of the coatings as the matrix material for solar absorber coatings.     

Reflectance properties of pressed Algoflon F6: a replacement reflectance-standard material for Halon

The standard ultraviolet through short-wave infrared (200-2500-nm) diffuse-reflectance material, Halon PTFE, type G-80, is no longer available. Therefore an equivalent diffuse-reflectance standard material must be found. Algoflon F6 is shown here to be an appropriate replacement through the presentation of measurements of various spectral-reflectance properties of Halon and Algoflon F6. The measurements include spectral hemispherical reflectance, spectral bidirectional reflectance factor (BRF), sample BRF repeatability, and sample lifetime.     

Modeling radiation characteristics of semitransparent media containing bubbles or particles

Modeling of radiation characteristics of semitransparent media containing particles or bubbles in the independent scattering limit is examined. The existing radiative properties models of a single particle in an absorbing medium using the approaches based on (1) the classical Mie theory neglecting absorption by the matrix, (2) the far field approximation, and (3) the near field approximation are reviewed. Comparison between models and experimental measurements are carried out not only for the radiation characteristics but also for hemispherical transmittance and reflectance of porous fused quartz. Large differences are found among the three models predicting the bubble radiative properties when the matrix is strongly absorbing and/or the bubbles are optically large. However, these disagreements are masked by the matrix absorption during calculation of radiation characteristics of the participating medium. It is shown that all three approaches can be used for radiative transfer calculations in an absorbing matrix containing bubbles.     

Emissivity Measurements on Metallic Surfaces with Various Degrees of Roughness: A Comparison of Laser Polarimetry and Integrating Sphere Reflectometry

Both integrating sphere reflectometry (ISR) as well as laser polarimetry have their advantages and limitations in their ability to determine the normal spectral emissivity of metallic samples. Laser polarimetry has been used for years to obtain normal spectral emissivity measurements on pulse-heated materials. The method is based on the Fresnel equations, which describe reflection and refraction at an ideally smooth interface between two isotropic media. However, polarimetry is frequently used with surfaces that clearly deviate from this ideal condition. Questions arise with respect to the applicability of the simple Fresnel equations to non-specular surfaces. On the other hand, reflectometry utilizing integrating spheres provides a measurement of the hemispherical spectral reflectance, from which the normal spectral emissivity can be derived. ISR provides data on spectral-normal-hemispherical reflectance and, hence, normal spectral emissivity for a variety of surfaces. However, the resulting errors are minimal when both the sample and the reference have a similar bidirectional reflectance distribution function (BRDF). In an effort to explore the limits of polarimetry in terms of surface roughness, room temperature measurements on the same samples with various degrees of roughness were performed using both ISR and a laser polarimeter. In this paper the two methods are briefly described and the results of the comparison are discussed.     

Optical properties (bidirectional reflectance distribution function) of shot fabric

To study the optical properties of materials, one needs a complete set of the angular distribution functions of surface scattering from the materials. Here we present a convenient method for collecting a large set of bidirectional reflectance distribution function (BRDF) samples in the hemispherical scattering space. Material samples are wrapped around a right-circular cylinder and irradiated by a parallel light source, and the scattered radiance is collected by a digital camera. We tilted the cylinder around its center to collect the BRDF samples outside the plane of incidence. This method can be used with materials that have isotropic and anisotropic scattering properties. We demonstrate this method in a detailed investigation of shot fabrics. The warps and the fillings of shot fabrics are dyed different colors so that the fabric appears to change color at different viewing angles. These color-changing characteristics are found to be related to the physical and geometrical structure of shot fabric. Our study reveals that the color-changing property of shot fabrics is due mainly to an occlusion effect.     

水性建筑反射隔热涂料的研制

利用纯丙和苯丙乳液为基料,制备了具有热反射和低导热协同作用的隔热涂料。通过对颜填料太阳光反射率和隔热性能的研究,选择以空心玻璃微珠和金红石型二氧化钛为颜填料,空心微珠的最优含量为4%～10%(wt)。研制建筑反射隔热涂料基础性能优异,可见光反射比达到90%以上,半球发射率大于85%,隔热指数大于60%。     

Measurement of Infrared Spectral Directional Hemispherical Reflectance and Emissivity at BNM-LNE

An infrared reflectometer has been designed by BNM-LNE (Bureau National de Métrologie–Laboratoire National d’Essais) to measure the spectral directional hemispherical reflectance of solid materials at ambient temperature. For opaque materials, the spectral directional emissivity can be calculated from the measured reflectance. The reflectance can be measured from 0.8 to 14 μm in five directions with an angle of 12°, 24°, 36°, 48°, and 60° with respect to the normal to the surface of the sample. The optical arrangement to collect the reflected flux is based on the Coblentz arrangement (hemispherical mirror). In fact, four mirrors cut in an hemisphere are used to collect the flux reflected by the sample. This optical arrangement was chosen to limit the angle of incidence of rays on the detector (38° instead of 90° for the Coblentz arrangement). The final expanded uncertainty (level of confidence 95%) of the reflectance is estimated to be about ±0.03 for wavelengths between 0.8 and 10 μm and ±0.04 for wavelengths over 10 μm. The values of the spectral reflectance measured on a black paint and on a white ceramic tile are compared to those measured by the two laboratories PTB (Physikalisch Technische Bundesanstalt) and NIST (National Institute of Standards and Technology). The results validate the measurements performed at BNM-LNE. Paper presented at the Fifteenth Symposium on Thermophysical Properties, June 22–27, 2003, Boulder, Colorado, U.S.A.