Spectroscopic ellipsometry measurements were performed in the region 0.5 - 5.0 eV on single crystals of cuprates with incommensurate-type structure. Three components of the dielectric function were determined by making use of the measurements made on two developed planes of orthorhombic crystals. The main optical transitions contributing to the optical response of the crystals of various chemical compositions and structural parameters were revealed. The relationship between optical anisotropy and crystal structure was analysed.