Elliptic birefringence of TGB phases observed in reflection

An improved method for the study of light propagation along the twist axis of helical stacks of birefringent plates has been used to explain optical a ctivity of TGB phases observed in reflection. The calculation takes into ac count the pitch variations versus temperature, the number of blocks per pit ch, the thickness of the sample as well as the birefringence dispersion of smectic blocks. The results are then compared with experiments. A qualitati ve explanation, using the Poincare geometrical method, is given: the observ ed phenomena are due to the ellipticity variations of iterative vibrations propagating in TGB phases.