The portions of the electromagnetic spectrum that are most useful for remote sensing can be defined as follows: The ultraviolet extends from approximately 0.1 to 0.4 microns, the visible from 0.4 to 0.7 microns, the near infrared from 0.7 to 1.0 microns, the shortwave infrared from 1 to 3 microns, the midwave infrared from 3 to 5 microns, the long-wave infrared from 5 to 15 microns, and the far infrared from 15 to 100 microns. These ranges are typically defined in terms of wavelength, but other ranges can be defined in terms of frequency as well. Thus, the submillimeter range encompasses wavelengths from 100 to 1000 microns or frequencies from 3 terahertz to 300 gigahertz. The millimeter range extends from 300 to 30 gigahertz or 1 millimeter to 1 centimeter, and the microwave region from 30 to 1 gigahertz or 1 to 30 centimeters.
Within these spectral regimes, there are "window bands" of low atmospheric absorption (in which imagers typically operate) and "absorption bands" of relatively high atmospheric absorption (in which sounders operate). There are relatively few applications for remote sensing in the ultraviolet because of its strong absorption by ozone below 0.3 microns (ozone monitoring is an obvious exception). The midwave infrared is unique in that, during daytime, it is a confusing mix of reflected solar and emitted thermal radiation. The submillimeter or terahertz regime (between the electro-optical and microwave regimes) is only beginning to be explored for remote-sensing purposes.