Backscatter

For other uses, seeBackscatter (disambiguation).

Backscatter in photography, showing aBrocken spectrewithin the rings of aglory

Inphysics,backscatter(orbackscattering) is thereflectionofwaves,particles,orsignalsback to the direction from which they came. It is usually adiffuse reflectiondue toscattering,as opposed tospecular reflectionas from amirror,although specular backscattering can occur at normal incidence with a surface. Backscattering has important applications inastronomy,photography,andmedical ultrasonography.The opposite effect isforward scatter,e.g. when atranslucentmaterial like a cloud diffusessunlight,givingsoft light.

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 reflectionfrom large particles andMie scattering,causingalpenglowandgegenschein,and showing up inweather radar;
• Inelastic collisionsbetween electromagnetic waves and the transmitting medium (Brillouin scatteringandRaman scattering), important in fiber optics, see below;
• Elastic collisionsbetween accelerated ions and a sample (Rutherford backscattering)
• Bragg diffractionfrom crystals, used in inelastic scattering experiments (neutron backscattering,X-ray backscattering spectroscopy);
• Compton scattering,used inBackscatter X-rayimaging.
• Stimulated backscatter,observed innon-linear optics,and described by a class of solutions to thethree-wave equation.

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;
• inastronomy,backscattered light is that which is reflected with aphase angleof less than 90°.

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

• Inalpenglow,red light prevails because the blue part of the spectrum is depleted byRayleigh scattering.
• Ingegenschein,constructive interference might play a role.
• Coherent backscatteringis observed in random media; for visible light most typically insuspensionslike milk. Due toweak localization,enhanced multiple scattering is observed in back direction.
  • TheBack Scattering Alignment(BSA) coordinate system is often used inradarapplications
  • TheForward Scattering Alignment(FSA) coordinate system is primarily used in optical applications

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 behindradarsystems. Inweather 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 fromhailand largegraupel(solidice) due to their sizes, but non-Rayleigh (Mie scattering) effects can confuse interpretation. Another strong return is from meltingsnowor wetsleet,as they combine size and water reflectivity. They often show up as much higherratesofprecipitationthan actually occurring in what is called abrightband.Rainis a moderate backscatter, being stronger with large drops (such as from athunderstorm) and much weaker with smalldroplets(such asmistordrizzle).Snowhas 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 infiber opticsapplications to detect optical faults. Light propagating through afiber-optic cablegradually attenuates due toRayleigh scattering.Faults are thus detected by monitoring the variation of part of the Rayleigh backscattered light. Since the backscattered lightattenuatesexponentiallyas it travels along theoptical fiber cable,the attenuation characteristic is represented in alogarithmic scalegraph.If theslopeof 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

Main article:Backscatter (photography)

Light from asmartphoneflash reflectingsandparticles.

The term backscatter in photography refers to light from aflashorstrobereflecting 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 asorb 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 lightreflectionto 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.

See also

• Backscatter (email)
• Backscatter X-ray(in security scanning applications, e.g. at airports)
• Forward scattering
• Scattering
• Electron backscatter diffraction