The book highlights the problems and methods of using the infrared part of the electromagnetic vibration spectrum. The book sets out the history of the development of infrared technology, examines in detail various sources of infrared radiation, as well as the optical properties of a number of substances and the earth's atmosphere. A large section is devoted to theoretical studies of the optical properties of the medium and the infrared region of the spectrum, semiconductor physics and a description of the infrared detection mechanism. A number of chapters are devoted to mathematical analysis of the mechanism of selective detection and restrictions caused by photon noise.