Jane Spicer and her colleagues have also introduced microwave heating methods into the TRIR technique, and this has provided some unique capabilities compared to heating with conventional optical sources. The use of a microwave heating source has distinct advantages for optically opaque but microwave transparent materials containing localized absorbing regions, such as entrapped water in composites. For particular specimen geometries and material properties, the presence of the defect region can be imaged at higher contrast and better spatial resolution than obtainable with the surface heating technique. Since the heat has only to diffuse to the surface, the characteristic thermal transit times for the measurement are shorter. Further, the spatial resolution in these measurements is determined by the IR wavelength and not by the microwave wavelength as occurs in conventional microwave imaging techniques.

Another application of microwave TRIR is the detection of conducting fibers of carbon or metal in dielectric materials. Such small, conducting, one-dimensional structures are very efficient microwave absorbers and scatterers. TRIR detection with microwave excitation can detect and identify these fibers and determine their depth in the material and the degree of bonding between fiber and matrix. This work led to the development of the concept of using such small fibers as an embedded sensor. Such a sensor can be remotely excited using a microwave source and then remotely interrogated using an infrared detection method without any need for contact with the structure.

Experiments designed to study the interaction of microwaves with linear conductors, including carbon fibers whose diameters ranged from 10 µm < D < 500 gm, have been carried out in different polymer matrix composites. The electromagnetic interaction depends on fiber length, thickness, and microwave polarization whereas the thermal response depends on the depth of the fiber in the material, its bonding to the matrix, and on the thermal properties of the matrix. The microwave absorption is also very sensitive to the polarization of the electric field with respect to the fiber direction, thus providing another method for selectively interrogating only specific embedded sensors. The thermal response of the heated fiber can be used to provide a probe of local thermal properties, and a potential application of this is a method for monitoring the curing of composite materials.