Quasistationary field of thermal emission and near-field radiometry

Abstract

We provide a theory of radiometry measurements of the quasistationary (near) field of thermal emission from a heated conducting medium. It explains why the Rytov effect, which essentially is a drastic growth of the thermal field energy near the medium surface, cannot be detected experimentally. However, we discovered a measurable near-field effect: the effective depth of formation of the received emission proves to be less than the skin-layer depth, depending on the size of the receiving antenna and its height above the surface. For such measurements highly effective antennas of a small aperture size are necessary. We developed and investigated a variety of microwave antennas whose parameters were fairly suitable for near-field radiometry. The measurements conducted with these antennas yielded experimental evidence of the fact that the quasistationary thermal field really exists. Near-field radiometry opens further opportunities for investigating media. In particular, we demonstrate here a technique for retrieval of the subsurface temperature profile in water with the help of near-field measurements.