Backscatter


Backscatter

In physics, backscatter (or backscattering) is the reflection of waves, particles, or signals back to the direction they came from. 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.

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Backscatter of waves in physical space

Backscattering occurs in quite different physical situations. The incoming waves or particles can be deflected from their original direction by quite different mechanisms:

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 optical fibers, light can only propagate forward or backward. Forward Brillouin or Raman scattering would violate momentum conservation, so inelastic scattering in optical fibers cannot be anything else but backscattering;
  • 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:

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]

Backscatter on the internet

A form of backscatter can happen when someone who is using an IP address scrambler executes a DDoS (Or Dos) attack. This happens because the pings will return to the IP address that is being created by the scrambler, as opposed to the real host server.

See also

References


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Look at other dictionaries:

  • Backscatter — bezeichnet die Rückstreuung von Wellen, Partikeln oder Signalen. Der Begriff wird u. a. angewendet in: den Natur /Ingenieurwissenschaften, siehe Rückstreuung im Internet, siehe Backscatter (E Mail) der Sicherheitstechnik (Backscatter X ray… …   Deutsch Wikipedia

  • backscatter — i. A form of ionospheric propagation of electromagnetic waves via the E and F layers. These layers are highly ionized and have properties of reflecting electromagnetic waves back to the earth. Normally, almost all electromagnetic signals are… …   Aviation dictionary

  • backscatter — also backscattering noun Date: 1940 the scattering of radiation or particles in a direction opposite to that of the incident radiation due to reflection from particles of the medium traversed; also the radiation or particles so reversed in… …   New Collegiate Dictionary

  • backscatter — Refers to a portion of the laser energy that is scattered back in the direction of the seeker by an obscurant. See also laser …   Military dictionary

  • backscatter — /bak skat euhr/, n. Physics. the deflection of nuclear particles or of radiation in a scattering process through an angle greater than 90°. Also, backscattering. [1955 60; BACK2 + SCATTER] * * * …   Universalium

  • backscatter — 1. noun a) The deflection of particles and/or radiation by nuclear and/or electromagnetic forces through angles greater than 90 degrees to the original direction of travel. b) The particles and/or radiation deflected in this manner. 2. verb …   Wiktionary

  • backscatter — Secondary radiation deflected more than 90° from the primary beam. See scattered radiation. * * * back·scat·ter (bakґskat ər) in radiology, radiation deflected by scattering processes at angles greater than 90 degrees to the original… …   Medical dictionary

  • backscatter — n. deflection of nuclear particles or deviation of radiation in a scattering procedure through angles greater than ninety degrees; radiation or particles deflected in this manner v. scatter radiation by atoms of the medium through which it passes …   English contemporary dictionary

  • backscatter — noun 1》 Physics deflection of radiation or particles through an angle of 180°. 2》 Photography light from a flashgun or other light source that is deflected directly into a lens. verb Physics deflect (radiation or particles) through an angle of… …   English new terms dictionary

  • backscatter — back•scat•ter [[t]ˈbækˌskæt ər[/t]] n. 1) phs the deflection of radiation by scattering in a direction opposite to the direction of incidence 2) phs radiation scattered in this manner • Etymology: 1955–60 …   From formal English to slang


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