Reflectometry is a general term for the use of the reflection of

wave In physics, mathematics, engineering, and related fields, a wave is a propagating dynamic disturbance (change from List of types of equilibrium, equilibrium) of one or more quantities. ''Periodic waves'' oscillate repeatedly about an equilibrium ...

s or pulses at surfaces and interfaces to detect or characterize objects, sometimes to detect anomalies as in fault detection and

medical diagnosis Medical diagnosis (abbreviated Dx, Dx, or Ds) is the process of determining which disease or condition explains a person's symptoms and signs. It is most often referred to as a diagnosis with the medical context being implicit. The information ...

. 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 Penetration depth is a measure of how deep light or any electromagnetic radiation can penetrate into a material. It is defined as the depth at which the intensity of the radiation inside the material falls to 1/ ''e'' (about 37%) of its original ...

in relation to size of the investigated object), by the method of measurement (continuous versus pulsed, polarization resolved, ...), and by the application domain.

Radiation sources

Electromagnetic radiation In physics, electromagnetic radiation (EMR) is a self-propagating wave of the electromagnetic field that carries momentum and radiant energy through space. It encompasses a broad spectrum, classified by frequency or its inverse, wavelength ...

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

Radar Radar is a system that uses radio waves to determine the distance ('' ranging''), direction ( azimuth and elevation angles), and radial velocity of objects relative to the site. It is a radiodetermination method used to detect and track ...

: Reflections of radiofrequency pulses are used to detect the presence and to measure the location and speed of objects such as aircraft, missiles, ships, vehicles. **

Lidar Lidar (, also LIDAR, an acronym of "light detection and ranging" or "laser imaging, detection, and ranging") is a method for determining ranging, ranges by targeting an object or a surface with a laser and measuring the time for the reflected li ...

: Reflections of light pulses are used typically to penetrate ground cover by vegetation in aerial archaeological surveys. ** Characterization of semiconductor and dielectric thin films: Analysis of reflectance data utilizing the Forouhi Bloomer dispersion equations can determine the thickness,

refractive index In optics, the refractive index (or refraction index) of an optical medium is the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...

, and extinction coefficient of thin films utilized in the

semiconductor A semiconductor is a material with electrical conductivity between that of a conductor and an insulator. Its conductivity can be modified by adding impurities (" doping") to its crystal structure. When two regions with different doping level ...

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 and reflection at discontinuities in cables is used in time domain reflectometry (TDR) to detect and localize defects in electric wiring. ** Skin reflectance: In anthropology, reflectometry devices are often used to gauge

human skin color Human skin color ranges from the Dark skin, darkest brown to the Light skin, lightest hues. Differences in Human skin, skin color among individuals is caused by variation in pigmentation, which is largely the result of genetics (inherited from o ...

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. * Acoustic reflectometry: the reflection of sound waves is used. One application is the use of a tympanometer (a specialised acoustic reflectometer) to diagnose medical conditions of the ear. ** Ultrasonic reflectometry: A

transducer A transducer is a device that Energy transformation, converts energy from one form to another. Usually a transducer converts a signal in one form of energy to a signal in another. Transducers are often employed at the boundaries of automation, M ...

generates acoustic waves at ultrasonic frequency which propagate until they 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

acoustic impedance Acoustic impedance and specific acoustic impedance are measures of the opposition that a system presents to the acoustic flow resulting from an acoustic pressure applied to the system. The International System of Units, SI unit of acoustic impeda ...

of the sample is determined by measuring the

amplitude The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of am ...

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. Applications include

medical ultrasonography Medical ultrasound includes Medical diagnosis, diagnostic techniques (mainly medical imaging, imaging) using ultrasound, as well as therapeutic ultrasound, therapeutic applications of ultrasound. In diagnosis, it is used to create an image of ...

and

nondestructive testing Nondestructive testing (NDT) is any of a wide group of analysis techniques used in science and technology industry to evaluate the properties of a material, component or system without causing damage. The terms nondestructive examination (NDE), n ...

. * Neutron reflectometry: is a

neutron diffraction Neutron diffraction or elastic neutron scattering is the application of neutron scattering to the determination of the atomic and/or magnetic structure of a material. A sample to be examined is placed in a beam of Neutron temperature, thermal or ...

technique for measuring the structure of

thin film A thin film is a layer of materials ranging from fractions of a nanometer ( monolayer) to several micrometers in thickness. The controlled synthesis of materials as thin films (a process referred to as deposition) is a fundamental step in many ...

s, similar to the often complementary techniques of X-ray reflectivity and

ellipsometry Ellipsometry is an optical technique for investigating the dielectric properties (complex refractive index or dielectric function) of thin films. Ellipsometry measures the change of polarization upon reflection or transmission and compares it ...

. The technique provides valuable information over a wide variety of scientific and technological applications including chemical aggregation,

polymer A polymer () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ...

and

surfactant Surfactants are chemical compounds that decrease the surface tension or interfacial tension between two liquids, a liquid and a gas, or a liquid and a solid. The word ''surfactant'' is a Blend word, blend of "surface-active agent", coined in ...

adsorption Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which a ...

, structure of thin film

magnetic Magnetism is the class of physical attributes that occur through a magnetic field, which allows objects to attract or repel each other. Because both electric currents and magnetic moments of elementary particles give rise to a magnetic field, m ...

systems, biological membranes.

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), fast pulses are emitted, and the magnitude, duration and shape of the reflected pulses is analyzed. * Frequency-domain reflectometry (FDR): this technique is based on the transmission of a set of stepped-frequency sine waves from the sample. As with TDR, these waves propagate to the sample and are reflected at the interface back to the source. There are several types of FDR; they are commonly used in radar applications and for characterization of cables/wires. The changes in frequency between the incident signal and the reflected signal are analyzed. *

Ellipsometry Ellipsometry is an optical technique for investigating the dielectric properties (complex refractive index or dielectric function) of thin films. Ellipsometry measures the change of polarization upon reflection or transmission and compares it ...

is the polarization-resolved measurement of light reflections from thin films.

References

{{Reflist reflectometry