The thermometric spectrum that was described by William Herschel in his 1800 experiment is today termed the infrared radiation. The principle developments in such detectors began during the 1940s driven by the needs of the military. However, it was not until 1958 that the military exclusion for infrared imaging was lifted, and medicine along with many other industrial applications was free to develop. Infrared sensor technology is an aspect of thermal imaging that has dramatically improved over the last 50 years. The practical outcome is that speed and resolution is dependant, so the higher the scanning speed, the lower the resolution of the final image. There are now a wide range of electronic sensors, and thermal cameras built around uncooled bolometers can be effective for medical use. There are a limited number of materials that can be used to transmit infrared radiation. Few are, however, stable enough for practical use.