User Manual - pancroma

51. Haze Reduction – Dark Area Method________________________________________________________________As solar light traverses the atmosphere it is scattered. One type of scattering iscalled Rayleigh scattering. In this type of scattering, electromagnetic radiation(EMR) interacts with matter whose size is much smaller than the wavelength ofthe EMR. Haze can also be caused by atmospheric aerosols. An aerosol is asuspension of fine particles or liquid droplets in a gas. Examples are smoke,oceanic haze, air pollution, clouds, soot and photochemical smog.Short wavelengths of visible light (mainly violet and blue) are most affected byRayleigh scattering, which is inversely proportional to the fourth power of thewavelength, Longer wavelengths (red, near infrared (NIR), infrared (IR) etc. arerelatively unaffected and pass through the atmosphere without interacting at amolecular level. A second type of scattering called Mie scattering affects longerwavelengths of light. In this type of scattering, electromagnetic radiation interactswith much larger aerosol particles and can be scattered in the same way thatshorter wavelengths are scattered by gas molecules. The wavelengths affecteddepend on the size, shape and texture of the particles encountered by theradiation.The top of atmospheric (TOA) radiance sensed by orbiting satellites is the resultof a complicated series of interactions between the EMR, the atmosphere andthe ground both downward toward the earth from the sun and from the earth tothe satellite sensor. This is shown in the Figure 1 where L1 is the atmosphericradiation, L2 is the reflected radiation and L3 is adjacency radiation. L4 and L5represent backscattered radiation that represents a net loss of available sensedradiation. Only radiation component 2 contains information from the currentlyviewed pixel. Normally, the task of atmospheric correction is the removal ofcomponents 1 and 3 and the retrieval of component 2.141