As was mentioned in Chapter 2, the development of IR detectors were connected with thermal detectors at the beginning. The initial spectacular applications of thermal detectors in astronomy are noted in Figure 19.1 [1]. In 1856, Charles Piazzi Smyth [2,3], from the peak of Guajara on Tenerife, detected IR radiation from the Moon using a thermocouple. In the early 1900s, infrared radiation was successfully detected from the planets Jupiter and Saturn and from some bright stars such as Vega and Arcturus. In 1915, William Coblentz [2] at the U.S. National Bureau of Standards developed thermopile detectors, which he uses to measure the infrared radiation from 110 stars. However, the low sensitivity of early infrared instruments prevented the detection of other near-IR sources. Work in infrared astronomy remained at a low level until breakthroughs in the development of new, sensitive infrared detectors were achieved in the late 1950s.