Abstract <p>Description of the dynamics and structure of heterogeneous liquid and gas flows based on the system of fundamental equations of fluid mechanics stimulated the development of both new observation techniques and classical schlieren methods. The improvement of numerical methods for data processing, as well as the sources and receivers of light in a wide spectral range, expands the possibilities of obtaining and analyzing visual images of flows. Both classical schlieren instruments and original developments acquire new qualities. The selection of the shape of the source and the visualizing diaphragm permits recording the fields of various components of the refractive index and density gradient in stratified flows (examples are given). Of particular interest is the development of instruments with high temporal and spatial resolution, which makes it possible to simultaneously record large-scale components (jets, wakes, vortices, waves), and ligaments, i.e., mathematical images of fine components of stratified flows. Examples of schlieren visualization of 2D stratified wakes and their calculations based on a reduced system of fundamental equations are given.</p>