The phenomenon of photorefraction was discovered in 1966 in studies of propagation of a fairly powerful laser beam through the electro-optic crys- tals LiNb0, LiTa0, and some other compounds. The laser beam illumi- 3 3 nating part of the sample was found to cause a local change in the refrac- tive index of the crystal, thereby leading to distortion of the beam's wave front. The light had deteriorated the initially high optical quality of the crystal, in other words, it had given rise to a nonuniform distribution of the refractive index in the illuminated region. The effect was first called...