Microwaves holography revealed by photothermal films and lock-in IR thermography: Application to NDE of dielectric and radar absorbing materials

Abstract

The amplitude and phase space distributions of EM (electromagnetic) fields (X-and Ku bands) are imaged and measured using microwaves interferometry revealed by photothermal films and lock-in infrared thermography. Such EM fields imaging is a powerful tool for NDE (non Destructive Evaluation) of dielectric and radar absorbing materials. The absorption of an electromagnetic (EM) field E, H, by materials of complex permittivity, E = E" -iE", complex permeability, 11 = 11' -ill", and real part of the conductivity, d, produces heat with a dissipated volume power given by the well known formula: Nomenclature(1) where E and H are the modulus of the electric and magnetic fields. The resulting heating can be detected by IR thermography (technique called EMIR, for ElectroMagnetic InfraRed). It is controlled by the materials thermal properties and by heat transfer to the surroundings. The analysis of the space distribution and time evolution of the temperature increase allows to identify the heat sources, Pabs(x,y,z), which are controlled by the strength, space distribution and orientation of the EM field, and by the EM properties of the material, which acts as an array of photothermal sensors. If these properties of the material are known, identification of these distributed heat sources can finally lead to the knowledge of the intensity distribution of the incident EM field. The identification is optimized when the photothermal transducer is a thin film.Such a technique is a close mixing of electromagnetism and thermics. The use of both thin resistive photothermal films and lock-in IR thermography allows to image, in a quantitative way, the intensity of EM fields, with a dynamic range sufficiently extended for EM purposes. Reference [1] describes the optimisation of these films and the performances of the technique: space and time modulation transfer functions and sensitivity to the electric field. Need of a phase measurementThe main limitation of the technique was essentially due to the fact that the photon-heat conversion only allows to measure field intensity (square of the amplitude). Such a measurement impedes to reconstruct the full field, since the phase remains unknown. It is obvious that an EMIR method allowing phase measurements would be a real progress, especially for nearfield characterization of antennas. It is the reason why phase measurement was needed. Guided by optical techniques which very often use interferometric arrangements, the idea was to produce microwaves interferences for several phase values and to reveal them by photothermal films. By combining several intensity measurements it is possible to deduce both amplitude and relative phase distributions. Here the method will be very shortly presented, since we have recently presented it, with more details, in ref. [2,3].

Single-sided interferometric EMIR method for NDE of structures

“…To measure the phase, ONERA has developed since 1994 a holographic method in which two coherent plane waves are interfering, one of them being used as the reference [5,6]. It is a double-sided method, the two needed antennas or the antenna and an EM mirror being respectively on each side of the inspected structure.…”

Nondestructive Evaluation of Cooperative Structures (NDECS)

“…This is achieved by cameras working in various spectral domains: ultraviolet, visible, near or far infrared, associated with coherent light illumination (interferometric optical techniques like Electronic Speckle Pattern Interferometry [1]) or not (stimulated thermography [2], visible image correlation techniques [3]…). These techniques can image fields of displacements, deformation, temperature… of structures submitted to various types of solicitations: loads [4], fatigue [5], vibrations [6,7], radiant energy deposition [8,9], eddy-current [10], electromagnetic fields [11], etc. Sometimes the imaging technique is based on a more subtle effect, like the Faraday effect (influence of a magnetic field on the optical index) used to image the interaction of eddy-current with a defect like a crack in metallic structure [12].…”

<title>Latest developments in the EMIR technique of infrared imaging electromagnetic fields</title>

“…Due to the physical principle of the photonheat conversion, only the squared magnitude (intensity) of the electc field is directly measured, but the use of microwave holography and the combination of several intensi images allow to obtain relative phase image [5,6], with the restriction that the EM fields have just one component in the plane of the film.…”