Modern astronomy includes optical, ultraviolet, infrared and, in recent years, radio, γ-ray, and X-ray astronomy. Such a classification is justified to a certain degree. In fact, the difference in wavelength ranges causes a distinction in the methods and techniques of receiving radiation. Also, the solution of specific problems requires observation in different wavelength regions. In this respect it is possible to describe a new astronomical branch, the submillimeter one.
The submillimeter range is intermediate between the infrared and microwave regions, as shown in Table 1. The boundaries of this region are not very definite. Some authors include in the submillimeter range the wavelengths longer than 50 microns, others, those longer than 100 microns. The long wavelength edge of the range is also diffuse. Formally it is a wavelength of 1 mm, but in some cases the 2-mm or 4-mm wavelengths are also included in the submillimeter range. The measurements of submillimeter receiver performances are sometimes carried out at wavelengths up to 8 mm. The uncertainty of the boundaries is very understandable: their shifting depends on the methods of generation, transmission, and detection of radiation. In this review paper, following Martin’s (1962, 1963) terminology, wavelengths between 50 microns and 2 mm will be attributed to the submillimeter range.