The Christiansen Filter Effect in the Infrared

A brief discussion is given of the history, construction, and applications of Christiansen light filters for use in the visible and ultraviolet regions of the spectrum. These filters, which consist of a powder of some crystalline or glassy substance suspended in a transparent medium, depend for their action upon the equality of refractive indices of powder and medium for some one wave-length of light. The theory of the effect in the infrared is briefly discussed, and methods are given for preparing and mounting the powders. The principal factors affecting the shape and size of the transmission peak, namely, the angle at which the two dispersion curves cross, the size of particle used, the thickness of the powder layer, and the solid angle included by the light striking and leaving the filter are discussed. Experimental results include transmission curves and wave-lengths of the transmission maxima for the following crystalline powders dispersed in air: quartz, MgO, calcite, marble, CaS${\mathrm{O}}_4$, eleven alkali halides, and three monovalent thallium halides. Curves are also given for quartz, MgO, and NaCl dispersed in organic liquids. The transmission peaks measured lie between $3\mu$ and $90\mu$. In conclusion are discussed possible applications of the Christiansen effect in the infrared.