Spectroscopic anisotropy micro-ellipsometry (SAME) is employed for a rapid and precise measurement of lateral dimensions of periodic structures showing form birefringence. For this, the ellipsometric parameters Delta and Psi were measured as a function of the Euler angle alpha which describes sample rotation about the surface normal. Analysis of the resulting Delta(alpha) and Psi(alpha)-curves, the so-called SAME curves, allows determination of the components of the permittivity tensor of the form birefringent pattern in the UV-VIS range. From this, the volume fractions of the pattern constituents can be deduced. Once the volume fractions are known the critical dimensions (CD) can be calculated using the pitch of the periodic structures as an additional input. As an example, results for determination of CD of sub-micron lithographic patterns used in dynamic random access memory fabrication are given. The lateral pattern dimensions averaged over the ellipsometer spot size of approximately 120 mum could be determined with an accuracy in the nm range either for lines and spaces-by quantitative simulation of the SAME curve or for more complex pattern, e.g. deep trench patterns, by applying empirical calibration curves of Delta(alpha) or lambda(max) (Psi).