Apparent directional spectral emissivity determination of semitransparent materials

An inverse estimation method and corresponding measurement system are developed to measure the apparent spectral directional emissivities of semitransparent materials. The normal spectral emissivity and transmissivity serve as input for the inverse analysis. Consequently, the refractive index and absorption coefficient of the semitransparent material could be retrieved by using the pseudo source adding method as the forward method and the stochastic particle swarm optimization algorithm as the inverse method. Finally, the arbitrary apparent spectral directional emissivity of semitransparent material is estimated by using the pseudo source adding method given the retrieval refractive index and absorption coefficient. The present system has the advantage of a simple experimental structure, high accuracy, and excellent capability to measure the emissivity in an arbitrary direction. Furthermore, the apparent spectral directional emissivity of sapphire at 773 K is measured by using this system in a spectral range of 3 μm-12 μm and a viewing range of 0°-90°. The present method paves the way for a new directional spectral emissivity measurement strategy.

Inversion Methods of Optical Constants of Semitransparent Solid Materials from Transmittance Spectrograms

The direct calculation models of spectral transmittance of single and double slabs consisted of semitransparent solid materials were developed based on ray trace method, and a new inversion method of optical constants （k is extinction coefficient and n is refractive index ） of materials was proposed based on transmittance spectrograms of double slabs. Differences between the new method and two others currently used methods were studied, and application range of methods was also investigated. Optical constants of selenide glass attained in references were selected as true values, and spectral transmittances of glass simulated based on direct calculation model were regarded as experimental values. Optical constants of selenide glass were achieved by inverse models. Influences of measurement error on inverse results were also determined. The results showed that ： （ 1 ） based on transmittance spectrograms of double slabs in which thickness of single slab is the same, the new proposed method can attain optical constants of materials; （2） the effect of optical constants n and k on three inversion methods are urgent larger, but inversed calculation precision of optical constants are higher in most application ranges ; （ 3 ） the influence of measurement errors existed in experimental datum on the inverse precision of three methods are urgent distinctness.

Thermal Emission of a Disc Body of SemitransparentMaterial

By introducing the concept of radiosity intensity to diffuse surfaces, the ray tracing method is improved to analyze the thermal emission of a disc body of gray semitransparent material. The two plane sur-faces of the disc body are both specularly reflecting, and the fiank surface is either diffusely reflecting or specularly reflecting. The apparent thermal emission from one plane sllrface is investigated with considering the infiuences of the characteristic optical thickness, the dimensionless radius, the refrac-tive index of the material and the reflecting characteristics of the flank surface. The directional and hemispherical emissions show considerable differences under different refiecting characteristics of the flank surface. Moreover, in some cases, the emission not only varies with the viewing direction but also with the apparent emitting position on the plane surface. Some interesting results are presented and discussed.

Temperature-Dependent Thermal Conductivity and Absorption Coefficient Identification of Quartz Window up to 1100 K

The temperature-dependent absorption coefficient and thermal conductivity of a quartz window are obtained through experimental tests at a wide range of temperatures.A Fourier transform infrared spectrometer with a heated cavity is used for performing the transmittance measurements.The spectral absorption coefficient of the quartz window is inverted by the transmittance information at different temperatures using a genetic algorithm.Then,a quartz window-graphite plate-quartz window multilayer structure is designed,and the transient response of the structure subjected to high-temperature heating is recorded by a self-designed setup.Cooperating with the above absorption coefficient,a non-gray radiative-conductive heat transfer model is built for the multilayer structure,and the intrinsic thermal conductivity of the quartz window is identified.Finally,the effects of the temperature-dependent absorption coefficient and spectral selective features of the medium on the heat transfer characteristics are discussed.The results show that the absorption coefficient gradually increases with temperature.The intrinsic thermal conductivity of the quartz window varies from 1.35 to 2.52 W/(m·K)as the temperature rises,while the effective thermal conductivity is higher than the intrinsic thermal conductivity due to thermal radiation,specifically 26.4%higher at 1100 K.In addition,it is found that the influence of the temperature-dependent absorption coefficient on temperature of unheated side shows a trend of first increasing and then decreasing.When the absorption coefficient varies greatly with wavelength,a non-gray radiative-conductive heat transfer model should be built for the semitransparent materials.