EMIR: a photothermal tool for electromagnetic phenomena characterization

The heat-photon conversion phenomenon can be used to obtain a thermal image of an electromagnetic field. The electromagnetic field is partially absorbed by a sensitive paint or by a coating deposited on structures or on thin films. A map of the temperature increase of this absorbing medium is an image of the electric or magnetic intensity field distribution, depending on the electric and magnetic properties of the medium. A brief history of the various techniques used to obtain thermal images of electromagnetic fields is first presented. Emphasis is then put on infrared thermography which has been preferentially used in the past 20 years. An analysis of the thermal problems involved is presented. It appears that the solution to these problems is the key for the enhancement of the technique and for really quantitative work. Original solutions have been developed at ONERA, based on the combined use of optimised thin films with controlled electric conductivity, very sensitive infrared cameras, lock-in infrared thermography, and microwave interferometry. In these conditions, quantitative images of both amplitude and phase are obtained. Such an electromagnetic field imaging technique is a powerful tool which has no equivalent and which can be used for several types of applications such as: i) antenna radiation pattern characterization; ii) mode propagation characterization in waveguides; iii) study of absorption phenomena in complex materials; iv) nondestructive evaluation of dielectric structures (electromagnetic windows) or radar absorbing materials; v) knowledge of surface currents distribution on metallic structures.