When the emitted or reflected electro-magnetic energy is observed by a sensor on board an aircraft or spacecraft, the observed energy does not coincide with the energy emitted or reflected from the same object observed from a short distance. This is due to the sun's azimuth and elevation, atmospheric conditions such as fog or aerosols, sensor's response etc. which influence the observed energy. Therefore, in order to obtain the real irradiance or reflectance, those radiometric distortions must be corrected.
Radiometric correction is classified into the following three types (see Figure 9.1.1.)
(1) Radiometric correction of effects due to sensor sensitivity
In the case of optical sensors, with the use of a lens, a fringe area in the corners will be darker as compared with the central area. This is called vignetting. Vignetting can be expressed by cos
, where is the angle of a ray with respect to the optical axis. n is dependent on the lens characteristics, though n is usually taken as 4. In the case of electro-optical sensors, measured calibration data between irradiance and the sensor output signal, can be used for radiometric correction.
(2) Radiometric correction for sun angle and topography
a. Sun spot
The solar radiation will be reflected diffusely onto the ground surface, which results in lighter areas in an image. It is called a sun spot. The sun spot together with vignetting effects can be corrected by estimating a shading curve which is determined by Fourier analysis to extract a low frequency component (see Figure 9.1.2).
b.Shading
The shading effect due to topographic relief can be corrected using the angle between the solar radiation direction and the normal vector to the ground surface.
(3) Atmospheric correction
Various atmospheric effects cause absorption and scattering of the solar radiation. Reflected or emitted radiation from an object and path radiance (atmospheric scattering) should be corrected for.(see 9.2).
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