The influence of inhomogeneous fields on the optical properties of cry
stals is discussed on the basis of the phenomenological approach: we d
iscuss here the results of birefringences in LiNbO3 crystals under the
inhomogeneous mechanical strain (torsion) for various experimental co
nfigurations and the observation of optical activity under the torsion
al deformation in centrosymmetrical NaBi(MoO4)(2) crystals. The possib
ility of application of the torsion-induced optical effects is also me
ntioned.