Spectropolarimetric Measurement of the Complex Refractive Index of Powders

The unique spectral reflectance signature of a diffusely scattering solid material is known to be related to its unique physical properties including its complex refractive index (RI) and surface roughness. This is also true of single particulate scatterers, in which case particle size and morphology also contribute to the scattering signal. Calculating these signatures based on known optophysical properties, often called the forward problem, and is possible using a number of procedures. Fresnel, Mie, and radiative transfer theories have all successfully been used to predict light scattering, albeit within certain constraints. The so-called inverse problem, wherein the scattering materials optophysical properties are derived from measurements of the scattered light, is a much less tractable problem. Again, within certain limitations, each of these theories has been applied with some success. This project endeavors to expand on these successes through the incorporation of the scattered lights polarization information to facilitate the remote identification of powders and mixed-material solids. Polarization resolved light scattering measurements with be performed using a goniometric spectropolarimeter and will serve as input to a modified version of the radiative transfer equation. The modification will incorporate the ability to exploit added information provided by polarization resolved measurement and permit sample identification. Specifically, the samples spectral RI will be derived from the measurements. The RI component spectra, i.e., n() and k(), are indicative of the scattering species and will thus serve as a way to identify unknown powder samples. The approach will be employed during this Phase I effort using samples of known optophysical properties in an effort to validate the technique. Phase II work will extend this effort through additional measurements and the design & amp; construction of a portable polarimetric sensor. The overall project objective is fabrication of an accurate sensor capable of identifying unknown powders. 10 There are many potential commercial applications of this technology. For example, the RI of pharmaceuticals is indicative of their composition, purity, effectiveness and the metabolic uptake rate of their active components. The RI is also used as an input parameter to scattering based laser particle sizers which require accurate index values to determine an accurate particle size distribution. This sizing technique is used not only for pharmaceuticals but also in the building industry to qualify cementitious materials. The refractive indices of component powders of a ceramic composite can be used to predict the stoichiometry and physical properties of the resultant composite. Lastly, the paint and cosmetic industries would benefit as the technique permits prediction of required pigment concentrations.