ible radiation, silicon is employed as it can detect radiation with wavelengths under 1 um. However, unless silicon is doped with specific impurities to define specific bandgap energy levels, it is incapable of detecting infrared radiation. Chapter Four INFRARED REGIONS As a refresher, visible light extends from wavelengths of approximately 400 to 700 nm; wavelengths shorter than visible light are referred to as ultraviolet, X rays, and gamma rays, as the wavelength decreases. The region from 700 nm to 100 um is referred to as the infrared region, with wavelengths from 700 nm to 1 um considered to represent the near-infrared region. The remainder of the infrared region is additionally subdivided into the shortwave infrared region (SWIR) from 1 to 3 um, the midwave infrared region (MWIR) from 3 to 5 um, and the longwave infrared region (LWIR), whose wavelength exceeds 8 um. As a result of strong atmospheric absorption, the region between 5 and 8 um is commonly ignored. DOPING MATERIAL The use of lead sulfide within a semiconduc- tor provides a mechanism to detect radiation under 3 um. Thus, it pro- vides a basis for developing a SWIR detector. Other frequently used mate- rial for semiconductor detectors include silicon carbide, lead selenide, and indium antimonide. Silicon carbide results in the creation of a relatively wide bandgap, which results in the ability to detect ultraviolet radiation. The use of lead selenide and indium antimonide results in the ability to detect radiation at a wavelength under 5 um and provides the foundation for an MWIR detector. Thus, the doping of impurities provides the basis for the creation of detectors literally "tuned" to different wavelengths. Types of Photodetectors There are two general types of photodetectors: (1) those that are semicon- ductor-based and have a bandgap that can be broken through by inci- dent radiation and (2) non-semiconductor-based detectors that do not employ a bandgap bridge. Semiconductor-based photodetectors are used primarily in optical communications