The far infrared is that uncomfortable region of the electromagnetic spectrum falling between 50 and 1000 microns (200 to 10 cm−1) where conventional infrared methods cease to be efficient while microwave techniques cannot yet be applied in a straightforward way. Spectroscopic work in the far infrared has been hampered by the unavailability of intense broad-band sources of radiation. Although promising developments in far-infrared laser emission may change this picture, the most useful source of far-infrared radiation is still the mercury arc lamp, which was originally used by Rubens in his pioneering work at the turn of the century. Infrared detectors are all seriously affected by thermal noise because they inherently respond to a wide range of frequencies.