Emission from dielectric materials at millimeter wavelengths in passive thermal environments

The brightness of radiation escaping a two-dimensional slab of material under ambient illumination is characterized in terms of its complex dielectric constant. Transmission and reflection coefficients derive from wave optics and the application of Beer's law; the emissivity follows from detailed balancing using Kirchoff's law. The solutions are compared with intensities measured with a commercial millimeter wave imaging system. The results show that millimeter wave imaging of semi-transparent materials can be described by optical physics based on dielectric material properties. In addition, analysis of millimeter wave images of materials could provide information about their dielectric properties.