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scatter (radiation) by the atoms of the medium through which it passes

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2014/11/05 02:43:26」(JST)

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In physics, backscatter (or backscattering) is the reflection of waves, particles, or signals back to the direction from which they came. It is a diffuse reflection due to scattering, as opposed to specular reflection like a mirror. Backscattering has important applications in astronomy, photography and medical ultrasonography.

Backscatter of waves in physical space

Backscattering can occur in quite different physical situations, where the incoming waves or particles are deflected from their original direction by different mechanisms:

Diffuse reflection from large particles and Mie scattering, causing alpenglow and gegenschein, and showing up in weather radar;
inelastic collisions between electromagnetic waves and the transmitting medium (Brillouin scattering and Raman scattering), important in fiber optics, see below;
elastic collisions between accelerated ions and a sample (Rutherford backscattering)
Bragg diffraction from crystals, used in inelastic scattering experiments (neutron backscattering, X-ray backscattering spectroscopy);
Compton scattering, used in Backscatter X-ray imaging.

Sometimes, the scattering is more or less isotropic, i. e. the incoming particles are scattered randomly in various directions, with no particular preference for backward scattering. In these cases, the term "backscattering" just designates the detector location chosen for some practical reasons:

in X-ray imaging, backscattering means just the opposite of transmission imaging;
in inelastic neutron or X-ray spectroscopy, backscattering geometry is chosen because it optimizes the energy resolution;
in astronomy, backscattered light is that which is reflected with a phase angle of less than 90°.

In other cases, the scattering intensity is enhanced in backward direction. This can have different reasons:

In alpenglow, red light prevails because the blue part of the spectrum is depleted by Rayleigh scattering.
In gegenschein, constructive interference might play a role (this needs verification).
Coherent backscattering is observed in random media; for visible light most typically in suspensions like milk. Due to weak localization, enhanced multiple scattering is observed in back direction.
- The Back Scattering Alignment (BSA) coordinate system is often used in radar applications
- The Forward Scattering Alignment (FSA) coordinate system is primarily used in optical applications

Radar, especially weather radar

Backscattering is the principle behind radar systems.

In weather radar, backscattering is proportional to the 6th power of the diameter of the target multiplied by its inherent reflective properties. Water is almost 4 times more reflective than ice but droplets are much smaller than snow flakes or hail stones. So the backscattering is dependent on a mix of these two factors. The strongest backscatter comes from hail and large graupel (solid ice) due to their sizes. Another strong return is from melting snow or wet sleet, as they combine size and water reflectivity. They often show up as much higher rates of precipitation than actually occurring in what is called a brightband. Rain is a moderate backscatter, being stronger with large drops (such as from a thunderstorm) and much weaker with small droplets (such as mist or drizzle). Snow has rather weak backscatter.

Backscatter in waveguides

The backscattering method is also employed in fiber optics applications to detect optical faults. Light propagating through a fiber optic cable gradually attenuates due to Rayleigh scattering. Faults are thus detected by monitoring the variation of part of the Rayleigh backscattered light. Since the backscattered light attenuates exponentially as it travels along the optical fiber cable, the attenuation characteristic is represented in a logarithmic scale graph. If the slope of the graph is steep, then power loss is high. If the slope is gentle, then optical fiber has a satisfactory loss characteristic.

The loss measurement by the backscattering method allows measurement of a fiber optic cable at one end without cutting the optical fiber hence it can be conveniently used for the construction and maintenance of optical fibers.

Backscatter in photography

The term backscatter in photography refers to light from a flash or strobe reflecting back from particles in the lens's field of view causing specks of light to appear in the photo. This gives rise to what are sometimes referred to as orb artifacts. Photographic backscatter can result from snowflakes, rain or mist, or airborne dust. Due to the size limitations of the modern compact and ultra-compact cameras, especially digital cameras, the distance between the lens and the built-in flash has decreased, thereby decreasing the angle of light reflection to the lens and increasing the likelihood of light reflection off normally sub-visible particles. Hence, the orb artifact is commonplace with small digital or film camera photographs^[1]

References

^ "The Truth Behind 'Orbs'".

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English Journal

Ultrasonic backscattering in polycrystals with elongated single phase and duplex microstructures.

Lobkis OI, Yang L, Li J, Rokhlin SI.SourceThe Ohio State University, Department of Materials Science and Engineering, Edison Joining Technology Center, 1248 Arthur E. Adams Dr., Columbus, OH 43221, United States.
Ultrasonics.Ultrasonics.2012 Aug;52(6):694-705. Epub 2011 Dec 20.
An analytical solution for a three dimensional integral representation of the backscattering (BS) coefficient in polycrystals with elongated (generally ellipsoidal) grains is obtained; it is a natural generalization of the known explicit result for the BS coefficient in polycrystals with spherical g
PMID 22209089

Ultrasonic evaluation of acute impact injury of articular cartilage in vitro.

Virén T, Timonen M, Tyrväinen H, Tiitu V, Jurvelin JS, Töyräs J.SourceDepartment of Applied Physics, University of Eastern Finland, Kuopio, Finland; Department of Orthopaedics, Traumatology and Hand surgery, Kuopio University Hospital, Kuopio, Finland.
Osteoarthritis and cartilage / OARS, Osteoarthritis Research Society.Osteoarthritis Cartilage.2012 Jul;20(7):719-26. Epub 2012 Mar 30.
OBJECTIVE: The aim of the study was to investigate whether high frequency ultrasound technique, originally designed for arthroscopic use can be utilized to detect traumatic cartilage injuries.METHODS: A total of four intact osteochondral plugs were prepared from eight patellas for parallel compariso
PMID 22465982

Japanese Journal

Nonideality of drain electrode and ballistic performance of MOSFET

Natori Kenji
Jpn. J. Appl. Phys. 54(4), 044102, 2015-03-12
… The realistic drain, however, backscatters some of the carriers into the channel once again, and the backscattering is intensified if elastic scattering is dominant within the drain. … The backscattering strongly affects the ballistic performance of MOSFETs. … The channel length dependence of the ballistic performance of MOSFETs is explored in consideration of the backscattering. …
NAID 150000110486

Novel InGaPBi single crystal grown by molecular beam epitaxy

Yue Li,Wang Peng,Wang Kai,Wu Xiaoyan,Pan Wenwu,Li Yaoyao,Song Yuxin,Gu Yi,Gong Qian,Wang Shumin,Ning Jiqian,Xu Shijie
Appl. Phys. Express 8(4), 041201, 2015-03-06
… Rutherford backscattering spectrometry confirms that the majority of Bi atoms are located at substitutional lattice sites. …
NAID 150000110393

Low temperature activation of Au/Ti getter film for application to wafer-level vacuum packaging

Wu Ming,Moulin Johan,Lani Sébastien,Hallais Géraldine,Renard Charles,Bosseboeuf Alain
Jpn. J. Appl. Phys. 54(3), 030220, 2015-01-28
… The outdiffusion kinetics of titanium was modelled and characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Rutherford backscattering spectrometry (RBS) at various temperatures. …
NAID 150000110175