The laser light sheet technique has been used for many years to visualize flow fields. The laser beam may be expanded up to a plane light sheet by using a cylinder lens or a moving mirror, a scanner. The disadvantage of the cylinder lens is the resulting nonuniform light intensity distribution (Gaussian distribution along the spreading angle). In single phase flows, it is necessary to add tracer particles that follow the fluid motion. In the case, where single particles are observed on one or more images, the direction of particle motion and its velocity can be estimated by determining the change of particle location during a fixed time period. Another application of the laser light sheet technique is in the field of aerodynamics. With fog generators or smoke wires, a large number of very small particles follow the movement of the fluid elements. The motion of the fluid elements with different particle concentrations show the streamlines. In these applications of the laser light sheet technique, the flow velocities or streamlines can only be determined in the directions of the light plane. A common disadvantage of the optical measuring technique is the need of optical access to the flow field. This is required for both the transmitting and the receiving side. For those problems in fluid dynamics where no walls exist, e.g. free jets or sprays, the optical access is always possible.