Here a laser light beam is expanded in one direction only with the help of a cylindrical lens. This so called light sheet is then used to illuminate a flow seeded with tracer particles (smoke or tiny oil droplets in air, bubbles in water). In such a light sheet the light intensity detected in the plane of an observing camera will depend on the flow geometry and the concentration of the particles in the light sheet (if only one particle size is used for visualization). This technique provides a fast mean for real time flow visualization. At the Institute for Thermal Turbomachinery and Machine Dynamics a high-speed video camera (Kodak Ektapro 1000) might also be used for flow visualization. Depending on the laser power even larger areas can be illuminated and the flow visualized.