A holographic interferometer is used to measure diffusion coefficients of binary molten salt mixtures. The composition of such transparent mixtures is proportional to the refractive index. The composition changes cause local variations of the refractive index during diffusion. These variations are visualized by interference fringes, which are photographed by a CCD camera. Exact detection of the intensity extreme values of the fringes is essential to calculate diffusion coefficients. As irregular illumination and noise disturb the detection, adaptive filter algorithms are applied to reduce noise sufficiently while maintaining all relevant information.