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.

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:
 * Rayleigh scattering of electromagnetic waves from small particles, diffuse reflection from large particles, or Mie scattering in the intermediate case, 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 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:
 * 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.

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. Photographic backscatter can result from snowflakes, rain or mist, or airborne dust. Backscatter is particularly a problem in underwater photography, where particulate matter can be very dense and include plankton which would otherwise be near transparent.

Backscatter can be reduced by offsetting the direction of the photo strobe as far from the angle of the lens as possible. This is normally done by placing the light source high and to one side by placing the strobe on an extendable strobe arm. By having the light come from the side, the reflected light is primarily in the direction of the strobe instead of the camera lens. This is similar to comparing a full moon to a half moon. The full moon is when the moon is lit from almost behind the earth, creating reflection off the whole surface facing the earth. A half moon is when the moon is being lit from one side, making the reflection half the size and the light intensity much less. In photography, the side lighting makes the backscatter less pronounced.

Backscatter can often also be removed digitally after the photo is taken with photo editing software using digital filters or cloning of areas of the photo near the backscatter spots.