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 as from a mirror. Backscattering has important applications in astronomy, photography, and medical ultrasonography. The opposite effect is forward scatter, e.g. when a translucent material like a cloud diffuses sunlight, giving soft light.

Backscatter on Resciesa Val Gardena
Backscatter in photography, showing a Brocken spectre within the rings of a glory

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:

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:

Backscattering properties of a target are wavelength dependent and can also be polarization dependent. Sensor systems using multiple wavelengths or polarizations can thus be used to infer additional information about target properties.

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, provided the wavelength is larger than the particle diameter (Rayleigh scattering). 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, but non-Rayleigh (Mie scattering) effects can confuse interpretation. 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. Dual polarization weather radars measure backscatter at horizontal and vertical polarizations to infer shape information from the ratio of the vertical and horizontal signals.

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.

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][2]

See also


  1. ^ "Flash reflections from floating dust particles". Fuji Film. Archived from the original on July 27, 2005. Retrieved 19 June 2017.
  2. ^ Cynthia Baron. Adobe Photoshop Forensics: Sleuths, Truths, and Fauxtography. Cengage Learning; 2008. ISBN 1-59863-643-X. p. 310–.
Acoustic seabed classification

Acoustic seabed classification is the partitioning of a seabed acoustic image into discrete physical entities or classes. This is a particularly active area of development in the field of seabed mapping, marine geophysics, underwater acoustics and benthic habitat mapping. Seabed classification is one route to characterizing the seabed and its habitats. Seabed characterization makes the link between the classified regions and the seabed physical, geological, chemical or biological properties. Acoustic seabed classification is possible using a wide range of acoustic imaging systems including multibeam echosounders, sidescan sonar, single-beam echosounders, interferometric systems and sub-bottom profilers. Seabed classification based on acoustic properties can be divided into two main categories; surficial seabed classification and sub-surface seabed classification. Sub-surface imaging technologies use lower frequency sound to provide higher penetration, whereas surficial imaging technologies provide higher resolution imagery by utilizing higher frequencies (especially in shallow water).

Ambient backscatter

Ambient backscatter uses existing radio frequency signals, such as radio, television and mobile telephony, to transmit data without a battery or power grid connection. Each such device uses an antenna to pick up an existing signal and convert it into tens to hundreds of microwatts of electricity. It uses that power to modify and reflect the signal with encoded data. Antennas on other devices, in turn, detect that signal and can respond accordingly.

Initial implementations can communicate over several feet of distance, even with transmission towers up to 10.5 kilometres (6.5 mi) away. Transmission rates were 1k bits per second between devices situated 0.45 metres (1 ft 6 in) apart inside and 0.75 metres (2 ft 6 in) apart outside, sufficient to handle text messages or other small data sets. Circuit sizes can be as small as 1 sq. mm.This approach would let mobile and other devices communicate without being turned on. It would also allow unpowered sensors to communicate, allowing them to function in places where external power cannot be conveniently supplied.

Backscatter (email)

Backscatter (also known as outscatter, misdirected bounces, blowback or collateral spam) is incorrectly automated bounce messages sent by mail servers, typically as a side effect of incoming spam.

Recipients of such messages see them as a form of unsolicited bulk email or spam, because they were not solicited by the recipients, are substantially similar to each other, and are delivered in bulk quantities. Systems that generate email backscatter may be listed on various email blacklists and may be in violation of internet service providers' Terms of Service.

Backscatter occurs because worms and spam messages often forge their sender addresses. Instead of simply rejecting a spam message, a misconfigured mail server sends a bounce message to such a forged address. This normally happens when a mail server is configured to relay a message to an after-queue processing step, for example, an antivirus scan or spam check, which then fails, and at the time the antivirus scan or spam check is done, the client already has disconnected. In those cases, it is normally not possible to reject the SMTP transaction, since a client would time out while waiting for the antivirus scan or spam check to finish. The best thing to do in this case, is to silently drop the message, rather than risk creating backscatter.

Measures to reduce the problem include avoiding the need for a bounce message by doing most rejections at the initial SMTP connection stage; and for other cases, sending bounce messages only to addresses which can be reliably judged not to have been forged, and in those cases the sender cannot be verified, thus ignoring the message (i.e., dropping it).

Backscatter (photography)

In photography, backscatter (also called near-camera reflection) is an optical phenomenon resulting in typically circular artifacts on an image, due to the camera's flash being reflected from unfocused motes of dust, water droplets, or other particles in the air or water. It is especially common with modern compact and ultra-compact digital cameras.

Caused by the backscatter of light by unfocused particles, these artifacts are also sometimes called orbs, referring to a common paranormal claim. Some appear with trails, suggesting motion.

Backscatter X-ray

Backscatter X-ray is an advanced X-ray imaging technology. Traditional X-ray machines detect hard and soft materials by the variation in x-ray intensity transmitted through the target. In contrast, backscatter X-ray detects the radiation that reflects from the target. It has potential applications where less-destructive examination is required, and can operate even if only one side of the target is available for examination.

The technology is one of two types of whole-body imaging technologies that have been used to perform full-body scans of airline passengers to detect hidden weapons, tools, liquids, narcotics, currency, and other contraband. A competing technology is millimeter wave scanner. One can refer to an airport security machine of this type as a "body scanner", "whole body imager (WBI)", "security scanner" or "naked scanner".

Belt of Venus

The Belt of Venus, Venus's Girdle, or antitwilight arch is an atmospheric phenomenon visible shortly before sunrise or after sunset, during civil twilight, when a pinkish glow extending roughly 10–20° above the horizon surrounds the observer.

In a way, the Belt of Venus is actually alpenglow visible above the horizon during twilight, near the antisolar point. Like alpenglow, the backscatter of reddened sunlight also creates the Belt of Venus. Unlike alpenglow, the sunlight refracted by the fine particulates that cause the rosy arch of the Belt hovers high in the atmosphere and persists long after sunset or before sunrise.

As twilight progresses, the glow is separated from the horizon by the dark band of Earth's shadow, or "dark segment." The arch's light pink color is due to the backscatter of reddened light from the rising or setting Sun. A very similar effect can be seen during a total lunar eclipse. The zodiacal light, which is caused by the diffuse reflection of sunlight from the interplanetary dust in the Solar System, is also a similar phenomenon.

The name of the phenomenon alludes to the cestus, a girdle or breast-band, of the Ancient Greek goddess Aphrodite, customarily equated with the Roman goddess Venus. Since the greatest elongation (separation) between Venus and the Sun is only about 46 degrees, the planet Venus, even when visible, is never located opposite the sun and hence is never located in the Belt of Venus.

Clutter (radar)

Clutter is a term used for unwanted echoes in electronic systems, particularly in reference to radars. Such echoes are typically returned from ground, sea, rain, animals/insects, chaff and atmospheric turbulences, and can cause serious performance issues with radar systems.

Denial-of-service attack

In computing, a denial-of-service attack (DoS attack) is a cyber-attack in which the perpetrator seeks to make a machine or network resource unavailable to its intended users by temporarily or indefinitely disrupting services of a host connected to the Internet. Denial of service is typically accomplished by flooding the targeted machine or resource with superfluous requests in an attempt to overload systems and prevent some or all legitimate requests from being fulfilled.In a distributed denial-of-service attack (DDoS attack), the incoming traffic flooding the victim originates from many different sources. This effectively makes it impossible to stop the attack simply by blocking a single source.

A DoS or DDoS attack is analogous to a group of people crowding the entry door of a shop, making it hard for legitimate customers to enter, disrupting trade.

Criminal perpetrators of DoS attacks often target sites or services hosted on high-profile web servers such as banks or credit card payment gateways. Revenge, blackmail and activism can motivate these attacks.

Electron backscatter diffraction

Electron backscatter diffraction (EBSD) is a microstructural-crystallographic characterisation technique to study any crystalline or polycrystalline material. The technique involves understanding the structure, crystal orientation and phase of materials in the Scanning Electron Microscope (SEM). Typically it is used to explore microstructures, revealing texture, defects, grain morphology and deformation. It can be combined with complementary techniques within the SEM for phase discrimination. Traditionally these types of studies have been carried out using X-ray diffraction (XRD), neutron diffraction and/or electron diffraction in a TEM.

Experimentally EBSD is conducted using a SEM equipped with an EBSD detector containing at least a phosphor screen, compact lens and low light CCD camera. Commercially available EBSD systems typically come with one of two different CCD cameras: for fast measurements the CCD chip has a native resolution of 640×480 pixels; for slower, and more sensitive measurements, the CCD chip resolution can go up to 1600×1200 pixels. The biggest advantage of the high-resolution detectors is their higher sensitivity and therefore the information within each diffraction pattern can be analysed in more detail. For texture and orientation measurements, the diffraction patterns are binned in order to reduce their size and reduce computational times. Modern CCD-based EBSD systems can index patterns at up to 1800 patterns / second. This enables very rapid and rich microstructural maps to be generated. Recently, CMOS detectors have also been used in the design of EBSD systems. The new CMOS-based systems permit pattern indexing faster than CCD-based predecessors. Modern CMOS-based EBSD detectors are capable of indexing patterns up to 3000 patterns / second.

For an EBSD measurement a flat/polished crystalline specimen is placed in the SEM chamber at a highly tilted angle (~70° from horizontal) towards the diffraction camera, to increase the contrast in the resultant electron backscatter diffraction pattern. The phosphor screen is located within the specimen chamber of the SEM at an angle off approximately 90° to the pole piece and is coupled to a compact lens which focuses the image from the phosphor screen onto the CCD camera. In this configuration, some of the electrons which enter the sample backscatter and may escape. As these electrons leave the sample, they may exit at the Bragg condition related to the spacing of the periodic atomic lattice planes of the crystalline structure and diffract. These diffracted electrons can escape the material and some will collide and excite the phosphor causing it to fluoresce.

Inside the SEM, the electron beam is focussed onto the surface of a crystalline sample. The electrons enter the sample and some may backscatter. Escaping electrons may exit near to the Bragg angle and diffract to form Kikuchi bands which correspond to each of the lattice diffracting crystal planes. If the system geometry is well described, it is possible to relate the bands present in the diffraction pattern to the underlying crystal phase and orientation of the material within the electron interaction volume. Each band can be indexed individually by the Miller indices of the diffracting plane which formed it. In most materials, only three bands/planes which intercept are required to describe a unique solution to the crystal orientation (based upon their interplanar angles) and most commercial systems use look up tables with international crystal data bases to perform indexing. This crystal orientation relates the orientation of each sampled point to a reference crystal orientation.

While this 'geometric' description related to the kinematic solution (using the Bragg condition) is very powerful and useful for orientation and texture analysis, it only describes the geometry of the crystalline lattice and ignores many physical processes involved within the diffracting material. To adequately describe finer features within the electron beam scattering pattern (EBSP), one must use a many beam dynamical model (e.g. the variation in band intensities in an experimental pattern does not fit the kinematic solution related to the structure factor).

Full body scanner

A full-body scanner is a device that detects objects on a person's body for security screening purposes, without physically removing clothes or making physical contact. Depending on the technology used, the operator may see an alternate-wavelength image of the person's naked body, or merely a cartoon-like representation of the person with an indicator showing where any suspicious items were detected. For privacy and security reasons, the display is generally not visible to other passengers, and in some cases is located in a separate room where the operator cannot see the face of the person being screened. Unlike metal detectors, full-body scanners can detect non-metal objects, which became an increasing concern after various airliner bombing attempts in the 2000s.

Starting in 2007, full-body scanners started supplementing metal detectors at airports and train stations in many countries.

Three distinct technologies have been used, though the use of Backscatter X-ray has now been discontinued in many countries:

Millimeter wave scanners use non-ionizing electromagnetic radiation similar to that used by wireless data transmitters, in the Extremely High Frequency (EHF) radio band (which is a lower frequency than visible light). The health risks posed by these machines are still being studied, and the evidence is mixed, though millimeter wave scanners do not generate ionizing radiation.

Backscatter X-ray machines use low dose penetrating radiation for detecting suspicious metallic and non-metallic objects hidden under clothing or in shoes and in the cavities of the human body. There has been considerable debate with regard to how safe this technology is.

Through-body X-Ray security scanners that emit a high level of radiation have been supplied by the US to at least two African countries.Passengers and advocates have objected to images of their naked bodies being displayed to screening agents or recorded by the government. Critics have called the imaging virtual strip searches without probable cause, and have suggested they are illegal and violate basic human rights. However, current technology is less intrusive and because of privacy issues most people are allowed to refuse this scan and opt for a traditional pat-down.


Gegenschein (German pronunciation: [ˈɡeːɡənʃaɪn] German for "countershine") is a faintly bright spot in the night sky, around the antisolar point. The backscatter of sunlight by interplanetary dust causes this optical phenomenon.


Heiligenschein (German for "halo" or "aureola", pronounced [ˈhaɪlɪɡənˌʃaɪn]) is an optical phenomenon in which a bright spot appears around the shadow of the viewer's head. In photogrammetry and remote sensing, it is more commonly known as the hotspot.

This diffuse reflection is due to the opposition surge, the reduction in the proportion of shadows viewed at angles close to the backscatter direction. It may also be created when the surface on which the shadow falls has retroreflective optical properties. Both dry regolith and dewy grass are known to exhibit these characteristics. Nearly spherical dew droplets act as lenses to focus the light onto the surface behind them. When this light scatters or reflects off that surface, the same lens re-focuses that light into the direction from which it came. This configuration is sometimes called a cat's eye retroreflector. Any retroreflective surface is brightest around the antisolar point.

The glory is a similar halo effect caused by a different mechanism.

Imaging radar

Imaging radar is an application of radar which is used to create two-dimensional images, typically of landscapes. Imaging radar provides its light to illuminate an area on the ground and take a picture at radio wavelengths. It uses an antenna and digital computer storage to record its images. In a radar image, one can see only the energy that was reflected back towards the radar antenna. The radar moves along a flight path and the area illuminated by the radar, or footprint, is moved along the surface in a swath, building the image as it does so.Digital radar images are composed of many dots. Each pixel in the radar image represents the radar backscatter for that area on the ground: brighter areas represent high backscatter, darker areas represents low backscatter.The traditional application of radar is to display the position and motion of typically highly reflective objects (such as aircraft or ships) by sending out a radiowave signal, and then detecting the direction and delay of the reflected signal. Imaging radar on the other hand attempts to form an image of one object (e.g. a landscape) by furthermore registering the intensity of the reflected signal to determine the amount of scattering (cf. Light scattering). The registered electromagnetic scattering is then mapped onto a two-dimensional plane, with points with a higher reflectivity getting assigned usually a brighter color, thus creating an image.

Several techniques have evolved to do this. Generally they take advantage of the Doppler effect caused by the rotation or other motion of the object and by the changing view of the object brought about by the relative motion between the object and the back-scatter that is perceived by the radar of the object (typically, a plane) flying over the earth. Through recent improvements of the techniques, radar imaging is getting more accurate. Imaging radar has been used to map the Earth, other planets, asteroids, other celestial objects and to categorize targets for military systems.

NYPD X-ray vans

The New York City Police Department is reported to have a number of X-ray vans that contain X-ray equipment for inspecting vehicles. They are described as being able to see into other vehicles using Z backscatter technology.The NYPD has refused to release details of the uses and operation of these vans. The New York Civil Liberties Union have filed an amici curiae brief in support of a legal action by the journalist Michael Grabell, who is attempting to obtain more information about these vehicles.

Opposition surge

The opposition surge (sometimes known as the opposition effect, opposition spike or Seeliger effect) is the brightening of a rough surface, or an object with many particles, when illuminated from directly behind the observer. The term is most widely used in astronomy, where generally it refers to the sudden noticeable increase in the brightness of a celestial body such as a planet, moon, or comet as its phase angle of observation approaches zero. It is so named because the reflected light from the Moon and Mars appear significantly brighter than predicted by simple Lambertian reflectance when at astronomical opposition. Two physical mechanisms have been proposed for this observational phenomenon: shadow hiding and coherent backscatter.

Pencil (optics)

In optics, a pencil or pencil of rays is a geometric construct used to describe a beam or portion of a beam of electromagnetic radiation or charged particles, typically in the form of a narrow cone or cylinder.

Antennas which strongly bundle in azimuth and elevation are often described as "pencil-beam" antennas. For example, a phased array antenna can send out a beam that is extremely thin. Such antennas are used for tracking radar. See Beamforming for further details.

In optics, the focusing action of a lens is often described in terms of pencils of rays. In addition to conical and cylindrical pencils, optics deals with astigmatic pencils as well.In electron optics, scanning electron microscopes use narrow pencil beams to achieve a deep depth of field.Ionizing radiation used in radiation therapy, whether photons or charged particles, such as proton therapy and electron therapy machines, is sometimes delivered through the use of pencil beam scanning.In Backscatter X-ray imaging a pencil beam of x-ray radiation is used the scan over an object to create an intensity image of the Compton-scattered radiation.


The NASA QuikSCAT (Quick Scatterometer) was an Earth observation satellite carrying the SeaWinds scatterometer. Its primary mission was to measure the surface wind speed and direction over the ice-free global oceans. Observations from QuikSCAT had a wide array of applications, and contributed to climatological studies, weather forecasting, meteorology, oceanographic research, marine safety, commercial fishing, tracking large icebergs, and studies of land and sea ice, among others. This SeaWinds scatterometer is referred to as the QuikSCAT scatterometer to distinguish it from the nearly identical SeaWinds scatterometer flown on the ADEOS-2 satellite.


A scatterometer or diffusionmeter is a scientific instrument to measure the return of a beam of light or radar waves scattered by diffusion in a medium such as air. Diffusionmeters using visible light are found in airports or along roads to measure horizontal visibility. Radar scatterometers use radio or microwaves to determine the normalized radar cross section (σ0, "sigma zero" or "sigma naught") of a surface. They are often mounted on weather satellites to find wind speed and direction, and are used in industries to analyze the roughness of surfaces.

Whole body imaging

Whole body imaging (WBI) refers to the internal display of the entire body in a single procedure.

It may refer to one of two types of Full body scanner technologies used for security screening such as in airports:

Millimeter wave scanner

Backscatter X-ray

Infra-red thermal difference detectionIn medical imaging, it may also refer to full-body CT scan or magnetic resonance imaging.

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