Reflectometry

Reflectometry uses the reflection of waves at surfaces and interfaces to detect or characterize objects.

There are many different forms of reflectometry. They can be classified in several ways: by the used radiation (electromagnetic, ultrasound, particle beams), by the geometry of wave propagation (unguided versus wave guides or cables), by the involved length scales (wavelength and penetration depth versus size of the investigated object), by the method of measurement (continuous versus pulsed, polarization resolved, ...), and by the application domain.

Used radiation

Electromagnetic radiation of widely varying wavelength is used in many different forms of reflectometry:

• Radar and Lidar: Reflections of electromagnetic pulses are used to detect the presence and to measure the location and speed of objects like aircraft, missiles, ships, cars.
• 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.
• X-ray reflectometry: is a surface-sensitive analytical technique used in chemistry, physics, and materials science to characterize surfaces, thin films and multilayers.

Propagation of electric pulses in cables is used to detect and localize defects in electric wiring.[1][2]

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.[3] From the reflected wave, it is possible to determine some properties of the sample that is desired to characterize. Applications include medical ultrasonography and nondestructive testing.

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.

Skin reflectance: In anthropology, reflectometry devices are often used to gauge human skin color through the measurement of skin reflectance. These devices are typically pointed at the upper arm or forehead, with the emitted waves then interpreted at various percentages. Lower frequencies represent lower skin reflectance and thus darker pigmentation, whereas higher frequencies represent greater skin reflectance and therefore lighter pigmentation.

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:

• In time-domain reflectometry (TDR), one emits a train of fast pulses, and analyzes the magnitude, duration and shape of the reflected pulses.
• Frequency-domain reflectometry (FDR):[4][5] 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.
• Ellipsometry is the polarization-resolved measurement of light reflections from thin films.

References

1. Smail, M.K.; Hacib, T.; Pichon, L.; Loete, F. (2011), "Detection and Location of Defects in Wiring Networks Using Time-Domain Reflectometry and Neural Networks", IEEE Transactions on Magnetics, 47 (5): 1502–1505, Bibcode:2011ITM....47.1502S, doi:10.1109/TMAG.2010.2089503
2. Furse, C.; Haupt, R. (2001), "Down to the wire: The hidden hazard of aging aircraft wiring", IEEE Spectrum, 38 (2): 35–39, doi:10.1109/6.898797
3. McClements, D.J.; Fairley, P. (1990), "Ultrasonic pulse echo reflectometer", Ultrasonics, 29 (1): 58–62, doi:10.1016/0041-624X(91)90174-7
4. Soller, B.J.; Gifford, D.K.; Wolfe, M.S.; Froggatt, M.E. (2005), "High resolution optical frequency domain reflectometry for characterization of components and assemblies", Optics Express, 13 (2): 666–674, Bibcode:2005OExpr..13..666S, doi:10.1364/OPEX.13.000666
5. Furse, C.; C.C., You; Dangol, R; Nielsen, M.; Mabey, G.; Woodward \first6=R. (2003), "Frequency-Domain Reflectometery for on-Board Testing of Aging Aircraft Wiring", IEEE Trans. Electromagn. Compat., 45 (2): 306–315, doi:10.1109/TEMC.2003.811305