Scattering and backscattering

The red tones of the sky at sunset and sunrise are caused by Mie Scattering. The hues of blue, violet, and green in the sky are caused Rayleigh Scattering.

The red tones of the sky at sunset and sunrise are caused by Mie Scattering. The hues of blue, violet, and green in the sky are caused Rayleigh Scattering. Photo by Keven Law/Flickr

Image quality is often limited by scattering caused by particles present in the imaging medium. Some of the light from an object may be scattered before reaching the imaging device. In addition, unwanted light may be scattered directly into the imaging device by the particles, increasing the brightness at various parts of the image. Both of these effects contribute to a loss of image contrast and resolution.

The degree of scattering is dependent on the distance, the wavelength and the characteristics of the particles (ie. size, shape, density and refractive index). For color images taken from a forward-looking camera (ie. a camera located at a certain height above a surface and tilted downwards from the horizontal to image that surface), the distance from the camera to the imaged surface varies considerably across the image. This results in a non-uniform loss of contrast, resolution and color fidelity across the image.

Light scattering while the light travells through different media

Light scattering while the light travels through different media

The situation is further complicated by the ensemble of different particles present in most imaging media such as the atmosphere or the ocean, as well as polarization state.

A typical example of image degradation caused by scattering is that caused by aerosols such as haze, fog, mist and cloud. This is called Mie Scattering

The loss of image quality is a nuisance in many imaging applications. For example, in underwater imaging in murky water, the detection of artifacts becomes difficult due to poor image quality. Hence, imaging must be performed at close range and this usually results in a long time required to inspect a small area. Another example is in the navigation of surface ships and aircraft in bad weather. In weather conditions such as fog, visibility is low and navigation is more difficult, dangerous and slow. In surveillance applications it is more difficult to identify perpetrators. And so on…

Backscattering

The term backscatter refers to light from a light source reflecting back from particles in the lens’s field of view causing specks of light to appear in the video. Backscatter can result from snowflakes, rain or mist, or airborne dust and is particularly a problem in underwater video, 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 light source as far from the angle of the lens as possible. If possible this can be done by placing the light source high and to one side on an extendable arm. By having the light come from the side, the reflected light is primarily in the direction of the light source 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 video, the side lighting makes the backscatter less pronounced.

Backscatter can often also be removed digitally with a LYYN real-time video enhancer.