The thermopile sensor consists of a series of 116 thermoelements, forming a sensitive region size of 545m (diameter). The sensor is hermetically sealed into a TO-5 metal housing, with an optical filter. This standard filter allows measurements to be made in the spectral range above 5 m wavelength. The thermosensor exhibits an almost white noise, comparable to an ohmic resistance. It has a constant signal versus frequency up to its frequency limit, and is directly proportional to incident radiation.