In this paper, we try to develop a general methodology for defining and characterising homogeneity at the outlet of any continuous mixer based on auto-correlation of samples. We also emphasize on the necessity of developing on-line non-invasive techniques, as well as systemic model of the powder flow. This is further applied for the case of a static mixer, using a special set-up that allows following the evolution of a typical homogeneity length by image analysis. Mass flow perturbations performed with binary mixtures of aspirin and semolina are studied by examining the variations of the correlograms obtained. The absence of axial dispersion of the signal is finally assessed.