Christiansen filter

The Christiansen filter (named after Danish physicist Christian Christiansen) is a narrow bandpass or monochromatic optical filter based upon a coarse transparent powder immersed in a transparent liquid. The liquid and powder have differing values of dispersion, while having identical refractive indices at one particular wavelength. As a result, light of that wavelength passes through the filter unaffected, while light of all other wavelengths is scattered by refraction. A typical filter is composed of glass particles immersed in a mixture of benzene and carbon disulfide. The wavelength of operation can be varied by changing the percentage of the two components of the liquid mixture, or by varying the temperature.