Investigations of transparent objects like stationary plasmas on the basis of hologra- phic interferometry are difficult because of the relatively small fringe shift caused by these phase objects. For tomographic reconstruction procedures the phase shifts derived from the interferograms have to be particularly accurate , otherwise the reconstruction of the refrac- tive index distribution of the inhomogeneous plasmas is not reliable. The highly resolved phase profiles can be obtained by the use of spatial heterodyning, providing an accuracy of 2n/20 in the determination of the phase. A test of the optical arrangement and the computer-tomographic reconstruction algorithms is performed. The method is applied to diagnostics of different plasmas, such as high frequency plasmas and dc-glow-discharges. A special feature of these investigations is the use of resonance interferometry, in order to determine spatial distributions of particle densities of certain atoms in distinct states.