All matter at temperatures different from absolute zero is radiating electromagnetic energy. Most natural solid and fluid media met in the terrestrial environment obey approximately to the spectral behavior of thermal radiation within limited bandwidths as it is described by the Planck radiation law for the so-called black body. At a temperature of 300 K the spectral maximum of the radiative intensity is at a wavelength of about 10 microns i.e. well in the infrared range. In the microwave range (wavelength between one millimeter and several decimeters), the intensity is several orders of magnitude less than this maximum value. In reality no material behaves like a black body, and regarding only a narrow spectral range the various media appear in different gradations of gray. Therefore in general the radiative intensity received from a given real material is composed of a part emitted by thermal radiation and a part of reflected radiation originating from the surroundings. If the ratio of these two parts varies strongly with wavelength, the material appears “colored” i.e. with a characteristic spectral signature. The ratio of emitted and reflected intensities of an object in the natural environment depends on a variety of factors as: The complex permittivity of the medium, its homogeneity, or the mixing ratio with other media (air, water), the surface roughness, the temperature, the look angle, the polarization and the wavelength for which the radiometer is sensitive.