The Christiansen filter - A centennial retrospective review Part 1. Fundamental principles and applications as monochromator and polarizer

Abstract

The filter devised by Christiansen in 1884 consists of two phases, one of which is dispersed in the other. Christiansen originally used glass grains immersed in an organic liquid, but other combinations were developed later. Such systems are only optically homogeneous for radiation in a narrow band of wavelengths which depends on temperature and pressure. Radiation of other wavelengths are more or less strongly refracted or scattered on passing through the filter because of the differences in refractive index of the two phases. Because chromatic effects arise when visible light traverses such filters, other systems were then investigated in which these effects could be further observed. After reviewing these works, the applications of the Christiansen filter as monochromator are considered, as well as the methods of separating the directly transmitted radiation from the scattered ones, and the spectral properties of such filters. Whilst such filters can easily be replaced by others more convenient to use in the ultraviolet and visible ranges they still have uses in the infrared. After reviewing other applications of the Christiansen filter such as their use as polarizers or thermometers, the numerous theoretical works on the behaviour of the filter are described. The basic model, the assumptions made and the mathematical approximations involved are considered together with experimental tests of the validity of theory. It is shown that, because of the serious difficulties involved in the theoretical treatment of a realistic model of the filter, no equations now exist which are valid over the whole range from visible to ultraviolet and their development in the future is unlikely.