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Reflectivity and emissivity modeling for metals and plastics at THz frequencies

The dynamic range of the signal return from metals is a significant source of image interpretation difficulty. Techniques such as logarithmic image compression have been used to improve the recognition. Alternative techniques for improvement may be developed. This development depends in part on the ability to accurately model the surface reflective behavior including phase shifts introduced by the reflection. This work presents the results of an enhanced model development. Models of high frequency behavior in materials divide into regions such as non-relaxation region, relaxation region, optical absorption and plasma frequencies. In traditional infrared and longer wavelength imaging systems, optical absorption may play a role and it is generally assumed that the system operates in or very near the relaxation region defined as frequencies significantly greater than the reciprocal of the Boltzmann relaxation time. Though typical THz frequencies are below the relaxation time, they are not far enough below to be considered completely in the non-relaxation region. This introduces a number of issues atypical of imaging in either the RF or IR regime. Further realism is gained from the incorporation of plastic into the reflectivity and emissivity model. Empirical model validation is accomplished for selected materials.