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Radiometric calibration refers to a set of techniques that are applied during remote sensing data processing and includes corrections related to the sensitivity of the remote sensor, topography and sun angle, and atmospheric scattering and absorption.
There are usually three main parts of the CCD output to be considered; dark current, vignetting and Quantum efficiency. After a set of precise laboratory experiments and essential parameters determination, the radiance of the observed object can be retrieved from the DN value of the camera's CCD array.
Spectral data acquired by satellite sensors are influenced by a number of factors, such as atmospheric absorption and scattering, sensor-target-illumination geometry, sensor calibration, and image data processing procedures, which tend to change through times (Teillet, 1986). Targets in multi-date scenes are extremely variable and have been nearly impossible to compare in an automated mode (Kim and Elman, 1990). In order to detect genuine landscape changes as revealed by changes in surface reflectance from multi-date satellite images, it is necessary to carry out radiometric correction. Two approaches to radiometric correction are possible: absolute and relative (Lo and Yang, 1998). The absolute approach requires the use of ground measurements at the time of data acquisition for atmospheric correction and sensor calibration. This is not only costly but also impractical when archival satellite image data are used for change analysis (Hall et al., 1991). The relative approach to radiometric correction, known as relative radiometric normalization (RRN), ispreferred because no in situ atmospheric data at the time of satellite overpasses are required. This method involves normalizing or rectifying the intensities or digital numbers (DN) of multi-date images bandby-band to a reference image selected by the analyst. The normalized images would appear as if they were acquired with the same sensor under similar atmospheric and illumination conditions to those of the reference image.
^Yang, Xiajun, and C. P. Lo. "Relative radiometric normalization performance for change detection from multi-date satellite images." Photogrammetric Engineering and Remote Sensing 66.8 (2000): 967-980.