Reflectometry uses the

reflection Reflection or reflexion may refer to: Science and technology * Reflection (physics), a common wave phenomenon ** Specular reflection, reflection from a smooth surface *** Mirror image, a reflection in a mirror or in water ** Signal reflection, in ...

wave In physics, mathematics, and related fields, a wave is a propagating dynamic disturbance (change from equilibrium) of one or more quantities. Waves can be periodic, in which case those quantities oscillate repeatedly about an equilibrium (re ...

s 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 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 valu ...

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 In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visible) li ...

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

Radar Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, w ...

and

Lidar Lidar (, also LIDAR, or LiDAR; sometimes LADAR) is a method for determining ranges (variable distance) by targeting an object or a surface with a laser and measuring the time for the reflected light to return to the receiver. It can also be ...

: 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 In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, or ...

, and extinction coefficient of thin films utilized in the

semiconductor A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...

industry. *

X-ray reflectometry X-ray reflectivity (sometimes known as X-ray specular reflectivity, X-ray reflectometry, or XRR) is a surface-sensitive analytical technique used in chemistry, physics, and materials science to characterize surfaces, thin films and multilayers.J. ...

: 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

cable Cable may refer to: Mechanical * Nautical cable, an assembly of three or more ropes woven against the weave of the ropes, rendering it virtually waterproof * Wire rope, a type of rope that consists of several strands of metal wire laid into a hel ...

s is used to detect and localize defects in electric wiring.

Ultrasonic Ultrasound is sound waves with frequencies higher than the upper audible limit of human hearing. Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hear it. This limit varies fr ...

reflectometry: A

transducer A transducer is a device that 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, measurement, and cont ...

generates

ultrasonic Ultrasound is sound waves with frequencies higher than the upper audible limit of human hearing. Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hear it. This limit varies fr ...

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 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 amplit ...

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 and nondestructive testing.

Neutron reflectometry 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 var ...

: 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 thermal or cold neutrons to o ...

technique for measuring the structure of

thin film A thin film is a layer of material 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 t ...

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

polymer A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...

and surfactant

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 ...

, structure of thin film magnetic systems, biological membranes.

Skin reflectance In anthropology, reflectometry devices are often used to gauge human skin color Human skin color ranges from the darkest brown to the lightest hues. Differences in skin color among individuals is caused by variation in pigmentation, which i ...

: In anthropology, reflectometry devices are often used to gauge

human skin color Human skin color ranges from the darkest brown to the lightest hues. Differences in skin color among individuals is caused by variation in pigmentation, which is the result of genetics (inherited from one's biological parents and or indivi ...

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 Time domain refers to the analysis of mathematical functions, physical signals or time series of economic or environmental data, with respect to time. In the time domain, the signal or function's value is known for all real numbers, for the ca ...

(TDR), one emits a train of fast pulses, and analyzes the magnitude, duration and shape of the reflected pulses. * 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. *

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 t ...

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

References

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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, ...