Graduate Thesis Or Dissertation

Experimental and analytical characterization of bidirectional reflectance data for engineering materials

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  • Bidirectional reflectance is a fundamental property used for the analysis of radiative heat transfer. It is a form of reflectivity in which the spectral and directional dependencies of both the beam incident on a surface and the beam reflected from the surface are specified. A new system has been designed expressly for measuring bidirectional reflectance of engineering materials. This apparatus features a unique goniometer design in which the source is fixed, the sample is rotated to change the incident beam directions, and the sensor (which moves with the sample) is rotated about the sample to change the measured reflected beam direction. The system was used to measure the bidirectional reflectance of a surface consisting of flat-white paint on an aluminum substrate. The data for this quasi-diffuse surface were taken over a non-uniform grid. Algorithms are presented for interpolating and reporting values on a uniform grid which is appropriate for use in numerical radiative transfer codes. Bidirectional reflectance data can be used in the form of a look-up table although storage requirements will be large and resolution will be limited by grid resolution. Alternatively, the data can be modeled as continuous functions of the incident beam angle. A simple model of reflectance as the sum of a diffuse and a specular component is described. A second model with a diffuse component and a quasi-specular component is also presented which includes spread in both the polar and azimuthal directions. Both models are continuous and are based on curve fits to the data.
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