The EM spectrum has been arbitrarily divided into regions or intervals to which descriptive names have been applied. At the very energetic (high frequency; short wavelength) end are gamma rays and x-rays (whose wavelengths are normally measured in angstroms [Å], which in the metric scale are in units of 10^-8 cm). Radiation in the ultraviolet extends from about 300 Å to about 4000 Å. It is convenient to measure the mid-regions of the spectrum in one of two units: micrometers (µm), which are multiples of 10^-6 m or nanometers (nm), based on 10^-9 m. The visible region occupies the range between 0.4 and 0.7 µm, or its equivalents of 4000 to 7000 Å or 400 to 700 nm. The infrared region, spanning between 0.7 and 100 µm, has four subintervals of special interest: (1) reflected IR (0.7 - 3.0 µm), and (2) its film responsive subset, the photographic IR (0.7 - 0.9 µm); (3) and (4) thermal bands at (3 - 5 µm) and (8 - 14 µm). Longer wavelength intervals are measured in units ranging from mm. through cm. through meters. The microwave region spreads across 0.1 to 100 cm.; this includes all of the interval used by man-made radar systems which generate their own active radiation that is directed towards (and reflected from) targets of interest. The lowest frequency-longest wavelength region beyond 100 cm is associated with radio bands. An enlarged figure, from which the one above was derived, indicates many of the atomic or molecular mechanisms by which these different forms of radiation are generated.

Here is a generalized diagram showing relative atmospheric transmission of radiation of different wavelengths.

Blue zones mark minimal passage of incoming and/or outgoing radiation whereas white areas denote "atmospheric windows" in which the radiation experiences much reduced interactions with various molecular species and hence can penetrate the air with little or no loss by absorption. Most remote sensors on air or space platforms were selected to operate in one or more of these windows and make their measurements using detectors "tuned" to certain specific frequencies (wavelengths) that pass through the atmosphere. However, some sensors, especially those on meteorological satellites, seek to directly measure absorption phenomena, such as those associated with CO₂ and other gaseous molecules. Note that the atmosphere acts as nearly opaque to EM radiation in part of the mid-IR and all of the far-IR. In the microwave region, by contrast, most of this radiation moves through unimpeded, so that radar signals at all commonly used bands will reach the surface (although raindrops produce backscattering that allows precipitation to be detected).

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