Reflectometry

Reflectometry is a noninvasive technique that allows the analysis of properties of a medium. This technique is based on the reflection of waves at the interface of interest. Waves propagate into a medium (according to the laws of propagation into the medium studied) and when they encounter a discontinuity (impedance break), part of their energy is reflected back to injection point. The analysis of the reflected signal can infer information about the system or the medium under consideration.

Application fields

The fields of application are very diverse and involve many sectors:

• Radar: Reflectometry has been used extensively in radar systems for measuring distance between aircraft and the target or to detect the presence of another aircraft.
• Diagnostic cables: Reflectometry is a commonly used method for determination of properties of cables. In particular, it allows the detection, localization and characterization of electrical defects,
• Characterization of rheological properties: The rheological properties are determined by deformation under the effect of force applied to the material. Rheology allows to determine parameters such as viscosity, viscoelasticity or determine the thixotropic nature, shear thinning or shear thickening.
• Characterization of Semiconductor and Dielectric Thin Films: Analysis of reflectance data utilizing the Forouhi Bloomer dispersion equations can determine the thickness, refractive index, and extinction coefficient of thin films utilized in the semiconductor industry.
• Monitoring of cells adhesion or cell cultures,
• Nondestructive testing (NDT): The nondestructive inspection technique are widely used in industry to reliably detect the presence of various anomalies in materials as corrosion evaluation or inspection of composite materials,
• Medical imaging: The most known techniques are probably medical ultrasonography or magnetic resonance imaging (MRI).

Different reflectometry techniques

Many techniques are based on the principle of reflectometry and are distinguished by the type of waves used and the analysis of the reflected signal. Among all these techniques, we can classify the main but not limited to:

• Time-domain reflectometry (TDR): this term is used to describe a technique of observing the time-dependent response of an electromagnetic field applied to a sample of interest. Generally, a train of fast pulses is appropriately generated and applied to a transmission line. The waveform in the line is observed at some point (by a sampling oscilloscope or other data acquisition system). By analyzing the magnitude, duration and shape of the reflected waveform, the nature of the impedance variation in the transmission system can be determined.
• Frequency-domain reflectometry (FDR): this technique is based on the transmission of a set of stepped-frequency sine waves from the sample. As for the TDR, these waves propagate until the sample and are reflected back to the source. Several types of FDR exist and are commonly used in radar applications or characterization of cables/wires. The signal analysis is focused rather on the changes in frequency between the incident signal and the reflected signal.
• Neutron reflectometry: is a neutron diffraction technique for measuring the structure of thin films, similar to the often complementary techniques of X-ray reflectivity and ellipsometry. The technique provides valuable information over a wide variety of scientific and technological applications including chemical aggregation, polymer and surfactant adsorption, structure of thin film magnetic systems, biological membranes...
• X-ray reflectometry: is a surface-sensitive analytical technique used in chemistry, physics, and materials science to characterize surfaces, thin films and multilayers.
• Ultrasonic reflectometry: A transducer generates ultrasonic waves which propagates until it reaches the interface between the propagation medium and the sample. The wave is partially reflected at the interface and partially transmitted into the sample. The waves reflected at the interface travel back to the transducer, then the impedance of a sample is determined by measuring the amplitude of the wave reflected from the propagation medium/sample interface. From the reflected wave, it is possible to determine some properties of the sample that is desired to characterize.