Backscatter
Encyclopedia
In physics
, backscatter (or backscattering) is the reflection of wave
s, 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
.
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 other cases, the scattering intensity is enhanced in backward direction. This can have different reasons:
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 rate
s 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.
. 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.
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
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.
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
, backscatter (or backscattering) is the reflection of wave
Wave
In physics, a wave is a disturbance that travels through space and time, accompanied by the transfer of energy.Waves travel and the wave motion transfers energy from one point to another, often with no permanent displacement of the particles of the medium—that is, with little or no associated mass...
s, particles, or signals back to the direction they came from. It is a diffuse reflection
Diffuse reflection
Diffuse reflection is the reflection of light from a surface such that an incident ray is reflected at many angles rather than at just one angle as in the case of specular reflection...
due to scattering
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...
, as opposed to specular reflection
Specular reflection
Specular reflection is the mirror-like reflection of light from a surface, in which light from a single incoming direction is reflected into a single outgoing direction...
like a mirror. Backscattering has important applications in astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
, photography
Photography
Photography is the art, science and practice of creating durable images by recording light or other electromagnetic radiation, either electronically by means of an image sensor or chemically by means of a light-sensitive material such as photographic film...
and medical ultrasonography
Medical ultrasonography
Diagnostic sonography is an ultrasound-based diagnostic imaging technique used for visualizing subcutaneous body structures including tendons, muscles, joints, vessels and internal organs for possible pathology or lesions...
.
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:- Diffuse reflection from large particles and Mie scattering, causing alpenglowAlpenglowAlpenglow is an optical phenomenon. When the Sun is just below the horizon, a horizontal red glowing band can sometimes be observed on the opposite horizon...
and gegenscheinGegenscheinThe gegenschein is a faint brightening of the night sky in the region of the antisolar point.- Explanation :Like the zodiacal light, the gegenschein is sunlight reflected by interplanetary dust...
, and showing up in weather radarWeather radarWeather radar, also called weather surveillance radar and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, estimate its type . Modern weather radars are mostly pulse-Doppler radars, capable of detecting the motion of rain droplets in addition to the...
; - inelastic collisions between electromagnetic waves and the transmitting medium (Brillouin scatteringBrillouin scatteringBrillouin scattering, named after Léon Brillouin, occurs when light in a medium interacts with time dependent optical density variations and changes its energy and path. The density variations may be due to acoustic modes, such as phonons, magnetic modes, such as magnons, or temperature gradients...
and Raman scatteringRaman scatteringRaman scattering or the Raman effect is the inelastic scattering of a photon. It was discovered by Sir Chandrasekhara Venkata Raman and Kariamanickam Srinivasa Krishnan in liquids, and by Grigory Landsberg and Leonid Mandelstam in crystals....
, important in fiber optics, see below; - elastic collisions between accelerated ions and a sample (Rutherford backscattering)
- Bragg diffractionBragg's lawIn physics, Bragg's law gives the angles for coherent and incoherent scattering from a crystal lattice. When X-rays are incident on an atom, they make the electronic cloud move as does any electromagnetic wave...
from crystals, used in inelastic scattering experiments (neutron backscatteringNeutron backscatteringNeutron backscattering is one of several inelastic neutron scattering techniques. Backscattering from monochromator and analyzer crystals is used to achieve an energy resolution in the order of μeV...
, X-ray backscattering spectroscopy); - Compton scatteringCompton scatteringIn physics, Compton scattering is a type of scattering that X-rays and gamma rays undergo in matter. The inelastic scattering of photons in matter results in a decrease in energy of an X-ray or gamma ray photon, called the Compton effect...
, used in Backscatter X-rayBackscatter X-rayBackscatter X-ray is an advanced X-ray imaging technology. Traditional X-ray machines detect hard and soft materials by the variation in transmission through the target. In contrast, backscatter X-ray detects the radiation that reflects from the target...
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 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 anglePhase angle (astronomy)Phase angle in astronomical observations is the angle between the light incident onto an observed object and the light reflected from the object...
of less than 90°.
In other cases, the scattering intensity is enhanced in backward direction. This can have different reasons:
- In alpenglowAlpenglowAlpenglow is an optical phenomenon. When the Sun is just below the horizon, a horizontal red glowing band can sometimes be observed on the opposite horizon...
, red light prevails because the blue part of the spectrum is depleted by Rayleigh scatteringRayleigh scatteringRayleigh scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. It can occur when light travels through...
. - In gegenscheinGegenscheinThe gegenschein is a faint brightening of the night sky in the region of the antisolar point.- Explanation :Like the zodiacal light, the gegenschein is sunlight reflected by interplanetary dust...
, constructive interference might play a role (this needs verification). - Coherent backscatteringCoherent backscatteringIn physics, coherent backscattering is observed when coherent radiation propagates through a medium which has a large number of scattering centers of size comparable to the wavelength of the radiation....
is observed in random media; for visible light most typically in suspensionSuspension (chemistry)In chemistry, a suspension is a heterogeneous fluid containing solid particles that are sufficiently large for sedimentation. Usually they must be larger than 1 micrometer. The internal phase is dispersed throughout the external phase through mechanical agitation, with the use of certain...
s like milk. Due to weak localizationWeak localizationWeak localization is a physical effect which occurs in disordered electronic systems at very low temperatures. The effect manifests itself as a positive correction to the resistivity of a metal or semiconductor....
, enhanced multiple scattering is observed in back direction.- The Back Scattering AlignmentBack scattering alignmentThe Back Scattering Alignment is a coordinate system used in coherent electromagnetic scattering. The coordinate system is defined from the viewpoint of the wave source, before and after scattering. The BSA is most commonly used in radar, specifically when working with a Sinclair Matrix because...
(BSA) coordinate system is often used in radar applications - The Forward Scattering AlignmentForward scattering alignmentThe Forward Scattering Alignment is a coordinate system used in coherent electromagnetic scattering.The coordinate system is defined from the viewpoint of the electromagnetic wave, before and after scattering...
(FSA) coordinate system is primarily used in optical applications
- The Back Scattering Alignment
Radar, especially weather radar
Backscattering is the principle behind radar systems.In weather radar
Weather radar
Weather radar, also called weather surveillance radar and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, estimate its type . Modern weather radars are mostly pulse-Doppler radars, capable of detecting the motion of rain droplets in addition to the...
, 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
Hail
Hail is a form of solid precipitation. It consists of balls or irregular lumps of ice, each of which is referred to as a hail stone. Hail stones on Earth consist mostly of water ice and measure between and in diameter, with the larger stones coming from severe thunderstorms...
and large graupel (solid
Solid
Solid is one of the three classical states of matter . It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a...
ice
Ice
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...
) due to their sizes. Another strong return is from melting snow
Snow
Snow is a form of precipitation within the Earth's atmosphere in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by...
or wet sleet
Rain and snow mixed
Rain and snow mixed is precipitation composed of rain and partially melted snow. This precipitation can occur where the temperature in the lower part of the atmosphere is slightly above the freezing point...
, as they combine size and water reflectivity. They often show up as much higher rate
Rate
In mathematics, a rate is a ratio between two measurements, often with different units. If the unit or quantity in respect of which something is changing is not specified, usually the rate is per unit time. However, a rate of change can be specified per unit time, or per unit of length or mass or...
s of precipitation
Precipitation (meteorology)
In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...
than actually occurring in what is called a brightband. Rain
Rain
Rain is liquid precipitation, as opposed to non-liquid kinds of precipitation such as snow, hail and sleet. Rain requires the presence of a thick layer of the atmosphere to have temperatures above the melting point of water near and above the Earth's surface...
is a moderate backscatter, being stronger with large drops (such as from a thunderstorm
Thunderstorm
A thunderstorm, also known as an electrical storm, a lightning storm, thundershower or simply a storm is a form of weather characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere known as thunder. The meteorologically assigned cloud type associated with the...
) and much weaker with small droplets (such as mist
Mist
Mist is a phenomenon of small droplets suspended in air. It can occur as part of natural weather or volcanic activity, and is common in cold air above warmer water, in exhaled air in the cold, and in a steam room of a sauna. It can also be created artificially with aerosol canisters if the...
or drizzle
Drizzle
Drizzle is a light rain precipitation consisting of liquid water drops smaller than those of rain, and generally smaller than 0.5 mm in diameter. Drizzle is normally produced by low stratiform clouds and stratocumulus clouds. Precipitation rates due to drizzle are on the order of a millimetre...
). Snow
Snow
Snow is a form of precipitation within the Earth's atmosphere in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by...
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 scatteringRayleigh scattering
Rayleigh scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. It can occur when light travels through...
. Faults are thus detected by monitoring the variation of part of the Rayleigh backscattered light. Since the backscattered light attenuates exponentially
Exponential function
In mathematics, the exponential function is the function ex, where e is the number such that the function ex is its own derivative. The exponential function is used to model a relationship in which a constant change in the independent variable gives the same proportional change In mathematics,...
as it travels along the optical fiber cable, the attenuation characteristic is represented in a logarithmic scale
Logarithmic scale
A logarithmic scale is a scale of measurement using the logarithm of a physical quantity instead of the quantity itself.A simple example is a chart whose vertical axis increments are labeled 1, 10, 100, 1000, instead of 1, 2, 3, 4...
graph
Graph of a function
In mathematics, the graph of a function f is the collection of all ordered pairs . In particular, if x is a real number, graph means the graphical representation of this collection, in the form of a curve on a Cartesian plane, together with Cartesian axes, etc. Graphing on a Cartesian plane is...
. If the slope
Slope
In mathematics, the slope or gradient of a line describes its steepness, incline, or grade. A higher slope value indicates a steeper incline....
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 flashFlash (photography)
A flash is a device used in photography producing a flash of artificial light at a color temperature of about 5500 K to help illuminate a scene. A major purpose of a flash is to illuminate a dark scene. Other uses are capturing quickly moving objects or changing the quality of light...
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
Reflection (physics)
Reflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...
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
Backscatter on the internet
A form of backscatter can happen when someone who is using an IP addressIP address
An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...
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
- ScatteringScatteringScattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...
- Backscatter X-rayBackscatter X-rayBackscatter X-ray is an advanced X-ray imaging technology. Traditional X-ray machines detect hard and soft materials by the variation in transmission through the target. In contrast, backscatter X-ray detects the radiation that reflects from the target...
(in security scanning applications, e.g. at airports)