An infra-red absorption spectrum produced by a section of a birefringent crystal often has superimposed upon it a channeled spectrum when the beam is plane polarized, or is partially polarized. This channeling is absent when the electric vector <i>E</i><sub><i>i</i></sub> of the incident polarized beam parallels one of the principal index axes of the crystal, and usually appears with greatest visibility when <i>E</i><sub><i>i</i></sub> makes a ±45° angle with such an axis. However, even in the latter instance, it does not occur provided the index axes in the crystal surface are parallel and perpendicular, respectively, to the plane of incidence of the dispersing prisms, unless there is some uncompensated optical rotation in the spectrograph system. The maxima and minima of the channeled spectrum are caused by a combination of a wave plate interference action within the crystal section and an analyzer action involving reflection losses at the prism faces. Optical rotation in the system also usually causes the visibility cont rast between maxima and minima to vary, with one or more zero visibility positions resulting.
JOSEPH W. ELLIS and LEONARD GLATT, "Channeled Infra-Red Spectra Produced by Birefringent Crystals," J. Opt. Soc. Am. 40, 141-142 (1950)