1. Field of the Invention
The invention relates to a process for measuring the flow vectors in gas currents containing optically detectable particles wherein a focusing means focuses at least two spatially separated beams in at least two focusing point in a measuring volume.
2. Description of the Related Art
Processes for measuring flow vectors in gas currents have been known to employ light of a light source focused by a focusing means in the flow channel at two focusing points positioned in a close succession (U.S. Pat. No. 3,941,477). Particles contained in the gas current are illuminated in traversing the focusing points. Due to the stray radiation reflected by the particles, a start pulse is produced when the first focusing point is traversed, while a stop pulse is produced during the traversing of the second focusing point. From the time interval between the start pulse and the stop pulse, it is possible to determine the component of the particle speed vector in direction of the straight line traversing the focusing points. By moving the focusing device, said direction may be varied thus permitting the detection of flow vectors having different directions. However, by said method, it is only possible to measure the components of the flow vectors which extend in a normal plane relative to the optical axis of the focusing means. The component extending in the direction of the optical axis may not be determined. Said process is designated to 2d-process to refer to the two-dimensional vector measurement.
A further development of said process is designated as 3d-process, by which the vector component extending in direction of the optical axis may be detected as well (British Pat. No. 2,109,548) and in which four laser beams are used two of which each form a beam pair. The beams of each pair being directed to two focusing points situated in the same normal plane relative to the optical axis. Due to the differences of the direction of incidence of the beams directed to a focusing point, the flow directions measured by means of the beam pairs are determined differently. From said difference of direction, one may draw a conclusion concerning the flow component in direction of the optical axis of the system. The expenditure and the laser capacity required by said known process are quite considerable.