TYPES OF IMAGERY

TYPES OF IMAGERY All things (with a temperature above absolute zero) emit radiation in the form of electromagnetic waves. The wavelengths emitted by each object depend primarily on the object’s temperature. Higher temperatures cause electrons to vibrate faster and therefore produce shorter wavelengths. The sun emits radiation at several different wavelengths, and the range of these wavelengths is known as the electromagnetic spectrum (fig. 1-10, view A). Electromagnetic wavelengths in the visual and infrared region are usually measured in micrometers. A micrometer is equal to one-millionth of a meter and is represented by the symbol µm. Micrometers are also referred to as microns. The sun emits a maximum amount of radiation at wavelengths near 0.5 µm. The earth, which is obviously much cooler, emits most of its radiation at longer wavelengths of between 4 and 25 µm. For this reason, the earths radiation is referred to as long-wave radiation and the sun’s energy is referred to as shortwave radiation. The atmosphere is a strong absorber of radiation at certain wavelengths and is relatively transparent to others. Generally, the atmosphere is transparent to wavelengths associated with incoming solar radiation; but because of the presence of water vapor, carbon dioxide, and other elements, the atmosphere is largely opaque to the outgoing terrestrial wavelengths. When data signals from a satellite sensor scan are compiled at the satellite receiver on earth, the pixels and scan lines form an image. The image composed of measurements of energy in the visual range forms a visual image. The data from sensors that measure energy in the infrared band compose an infrared image. ELECTROMAGNETIC SPECTRUM Figure 1-10.—(A) The electromagnetic spectrum, and (B) the distribution of solar energy incident to the earth. 1-10