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Abstract
A large number of A2BX4 compounds possess a structurally incommensurate phase. Studies of linear optical properties belong to the most frequently used methods for their investigations. In this contribution we give a review of these results. The paper concentrates on the following topics: fluctuation phenomena near the normal-incommensurate (IC) phase transition; changes in absorption spectra; specific features of light propagation in the IC phase; behaviour of birefringence and its connection with the order parameters in the IC phase as well as in the lock-in phase C; use of birefringence for elucidating the transformation process IC-C and for determining the phase diagrams; memory effects. Areas where additional data or theoretical analyses would be worthwhile are indicated.