Maxwell would have loved it

James Clerk Maxwell. He demonstrated that electric and magnetic fields are propagated in space in the form of waves. In the background, an electromagnetic field in a hyperbolic antenna.

Have you ever seen a magnetic field? Or an electric field? No, they are invisible, which can only be detected by the effects that they have on objects, for example a magnet in a magnetic field. However, Onera researchers have successfully developed a system to visualize electromagnetic fields (combining an electric field and a magnetic field). In the past, to arrive at the same result, a large number of sensors had to be deployed in many places within a room in order to define the field point by point.

A first: the simultaneous visualization of the profiles of the electric field (proportional to the voltage, below) and surface magnetic field (proportional to the current, above), in phase opposition, as the theory predicts.

The experiment corresponding to the preceding image in which we placed films (one conducting, the other one magnetic) the heating of which is recorded by an infra-red camera, for a TEM [Transverse electromagnetic] wave.
 

The Emir principle (Electromagnetic and Infra-red) is simple: an electromagnetic wave interacts with the conducting objects and heats them: this is the Joule effect. This is what happens in a microwave oven for example. In this case, a very thin, lightly conducting film is used. This film heats up slightly, and we film the heating up with an infra-red camera (capable of seeing temperature differences).

The heating is proportional to the energy of the electromagnetic field: the stronger the field, the more the film heats up. "We tested several types of films, because we wanted it to be conducting, but not too much, says François Issac, research engineer at Onera Toulouse. "A non-conducting film does not get hot, because it doesn’t interact at all with the electromagnetic field. A very good conducting film does not heat up either, all of the energy is reflected, only the "bad" conductors heat up a lot." The films tested were either polymers from the Dupont Company of Nemours containing conducting carbon, or polymers covered with a metallic deposit, or conducting inks deposited on an insulating film.

The film is stretched over the area to be analyzed. It is filmed with the infra-red camera, which yields a two-dimensional cartography. To obtain a three-dimensional cartography, we have to reproduce the measurement on different planes. "We developed a signal processing technique that eliminates interference phenomena such as convection and conduction, which transfer the heat from one area of the film to another and disturb the measurement," says François Issac.

Formation of the electric field leaving an antenna in a "patch network". Several measurement planes are used to establish a three-dimensional map.

Emir is used to measure electromagnetic fields greater than 10 to 30 volts per meter. The norms governing the maximum electromagnetic fields authorized near electrical equipment are of this order of magnitude. This technique could therefore be used to check that the units comply with the norm. "We are trying to obtain greater sensitivity, to measure fields from 3 to 10 V/m," says François Issac. "To do this, we are working on signal processing, to improve the signal to noise ratio. We are looking at the magnetic field as well as the electric field, and we are trying to improve the quality of the films, so that they produce more infra-red radiation for a given field."

Visualization by EMIR of electric field "leaks" from points of the canopy of the cockpit of a Rafale aircraft.

For the moment, Emir is used in Onera research laboratories mostly to make sure that there are no leaks when the magnetic field is to remain confined, or to make sure that it doesn’t penetrate into places where it shouldn’t, for example in the hold of a aircraft or a kerosene tank.  This measurement is relatively expensive because of the price of ultra-sensitive infra-red cameras (of the order of 50,000 €), but it is easy to implement. "The Emir technique is unique in the world, and the principle is very clever," says Emmanuel Rosencher, Scientific Director of the Physics Branch at Onera. "For the first time, we can really visualize the electromagnetic field, which usually seems so mysterious. Until now, we only measured the electric field; now we can actually see the magnetic field. The image with which we can observe the electric field and the magnetic field in quadrature phase (one is strong when the other one is weak) is quite spectacular!"

Cécile Michaut, scientific reporter.

This article is dedicated to the memory of Patrick Lévesque, Onera researcher, who passed away recently. Patrick Lévesque developed the Emir method as part of pioneering work on infra-red thermography.