chapter  4
56 Pages

Image Calibration and Processing

A generic term, spectral response, is typically used to refer to the detected energy recorded as digital measurements in remote sensing imagery (Lillesand and Kiefer, 1994). Since different sensors collect measurements at different wavelengths and with widely varying characteristics, spectral response is used to refer to the measurements without signifying a precise physical term such as backscatter, radiance, or reflectance. In the optical/infrared portion of the spectrum there are five terms representing radiometric quantities (radiant energy, radiant density, radiant flux, radiant exitance, and irradiance). These are used to describe the radiation budget of a surface and are related to the remote sensing spectral response (Curran, 1985). When discussing image data, the term spectral response suggests that image measurements are not absolute, but are relative in the same way that photographic tone refers to the relative differences in exposure or density on aerial photographs. Digital image spectral response differs fundamentally from photographic tones, though, in that spectral response can be calibrated or converted to an absolute measurement to the extent that spectral response depends on the particular characteristics of the sensor and the conditions under which it was deployed. When all factors affecting spectral response have been considered, the resulting physical measurement — such as radiance (in W/m

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µ

m/sr), spectral reflectance (in percentage), or scattering coefficient (in decibels) is used. Consideration of the geometric part of the image analysis procedure typically follows; here the task is the correct placement of each image observation on the ground in terms of Earth or map coordinates.