Passive sensors capture natural radiation, either reflected or emitted, from the target. Reflected sunlight is the most common source of radiation measured by passive sensors. Passive remote sensors include photographic camera, digital camera, video camera, radiometers, and imaging spectrometers. Remote sensing data for Earth resources surveys are generally collected from air/space borne platforms. Since conceptualization of hyperspectral remote sensing or imaging spectrometry in mid-1980s several airborne and a few spaceborne sensors have been developed. The optics for radiation collection primarily comprises mirrors, lenses, and a telescopic setup to collect the radiation from the ground and focus it onto radiation-sorting optics. With the development of detector technology photomultipliers were used as detectors. Photoemissive detectors are based on the photoelectric effect, in which incident photons release electrons from the surface of the detector material. The simplest radiometers are non-imaging detectors that integrate the radiation that arrives from within a field of view and within a specific wave band.