User:Schenad/背散射

与背向散射相关的摄影作品：光环与布羅肯奇景。

在物理学中，背向散射（或背散射、反向散射）是指波、粒子或信号沿其入射的相反方向发生的反射现象。背向散射是一种散射造成的漫反射，与镜面反射相对。背向散射在天文學、摄影和医学超声检查中有着重要的应用。与背向散射相反的现象叫做正向散射，例如像云一般的透明物体对太阳光的散射造成的柔光效應。

物理学中的背向散射

背向散射见诸于物理学中的各个领域，常常是因为一束波或粒子因不同的物理机制被折射：

• 大颗粒的漫反射和米氏散射导致朝霞和对日照的出现，可见于气象雷达；
• 电磁波和传导介质之间的非弹性碰撞：布里渊散射和拉曼效应，对光纤中的光学很重要；
• 加速后的离子和样品的彈性碰撞：卢瑟福背散射；
• 晶体的布拉格定律，应用于非弹性散射实验（中子背向散射和X射线背向散射谱学）；
• 康普頓散射，应用于X射线背向散射成像。

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:

• 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.
  • The Back Scattering Alignment (BSA) coordinate system is often used in radar applications
  • The Forward 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 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.

另见

• 散射
• 正向散射
• 背向散射X光机