Around 1920, Case investigated the thallium sulde photoconductor-one of the rst photoconductors to give a response in the near IR region to approximately 1.1 µm . The next group of materials to be studied was the lead salts (PbS, PbSe, and PbTe), which extended the wavelength response to 7 µm. The PbS photoconductors from natural galena found in Sardinia were originally fabricated by Kutzscher at the University of Berlin in the 1930s . However, for any practical applications it was necessary to develop a technique for producing synthetic crystals. PbS thin-lm photoconductors were rst produced in Germany, next in the United States at Northwestern University in 1944, and then in England at the Admiralty Research Laboratory in 1945 . During World War II, the Germans produced systems that used PbS detectors to detect hot aircraft engines. Immediately after the war, communications, re control, and search systems began to stimulate a strong development effort that has extended to the present day. The Sidewinder heat-seeking infrared-guided missiles received a great deal of public attention. After 60 years, low-cost, versatile PbS and PbSe polycrystalline thin lms remain the photoconductive detectors of choice for many applications in the 1-3 µm and 3-5 µm spectral range. Current development with lead salts is in the focal plane arrays (FPAs) conguration.