In the 1950's astronomers started to use Lead-sulphide (PbS) detectors to study infrared radiation in the 1 to 4 micron range. When infrared radiation in this range falls on a PbS cell it changes the resistance of the cell. This change in resistance can be measured and is related to the amount of infrared radiation which falls upon the cell. To increase the sensitivity of the PbS cell it was cooled to a temperature of 77 degrees Kelvin by placing it in a flask filled with liquid nitrogen.
A major breakthrough came in 1961, with the development of the germanium bolometer. This instrument was hundreds of times more sensitive than previous detectors and was capable of detecting all infrared wavelengths. Basically, a cool thin strip of germanium is placed in a container which has a small opening in it. When infrared radiation comes through the opening and hits the germanium, it warms the metal and changes its conductivity (a measure of how much electrical current flows through an object). The change in conductivity can be measured and is directly proportional to the amount of infrared radiation entering the container. The germanium bolometer works best at an extremely low temperature (much lower than liquid nitrogen). The best way to cool the bolometer to such a low temperature is to surround it with liquid helium which cooled it to 4 degrees Kelvin. This is only a few degrees above absolute zero. To do this a metal Dewar (similar to a well insulated thermos flask) was developed which was able to hold the liquid helium in which the germanium bolometer was immersed. This type of infrared detector is sensitive to the entire range of infrared wavelengths. To study a particular wavelength of infrared emission from astronomical objects, astronomers place filters in front of the detectors, which filter out all but the desired wavelengths.
Infrared detector technology continues to advance at a rapid rate. Astronomers now use InSb and HgCdTe detectors for the 1 to 5 micron range. These operate in a way similar to the PbS detectors but use materials which are much more sensitive to the infrared. The development of infrared array detectors in the 1980's caused another giant leap in the sensitivity of infrared observations. Basically a detector array is a combination of several single detectors. These arrays allow astronomers to produce images containing tens of thousands of pixels at the same time. Infrared arrays have been used on several infrared satellite missions. In 1983 the IRAS mission used an array of 62 detectors. Astronomers now commonly use 256x256 arrays (thats 65,536 detectors!). Due to these breakthroughs in infrared technology, infrared astronomy has developed more rapidly than any other field of astronomy and continues to bring us exciting new views of the universe.