The Christiansen effect occurs when a suspension of particles in a transparent medium is observed at a wavelength where the refractive indices of the particles and of the medium are equal, thus producing an optically homogeneous medium with optical bandpass or filter characteristics. It is shown that soil-derived atmospheric dusts and other particulates can exhibit the Christiansen effect at IR wavelengths and that their spectra can be simulated by equivalent, precipitated films of the particulates on optical substrates. Useful approximation equations are given. The implications of these observations should be considered by atmospheric physicists and IR systems designers.
Hugh R. Carlon, "Christiansen effect in IR spectra of soil-derived atmospheric dusts," Appl. Opt. 18, 3610-3614 (1979)