OFF-AXIS ALIGNMENT OF AN LDA-PROBE AND THE EFFECT OF ASTIGMATISM ON MEASUREMENTS

A theoretical analysis of the positional accuracy of an LDA measurement volume using an off-axis alignment of a probe relative to an internal flow has been carried out. General performance parameters of measurements with a 1D-probe with an off-axis alignment are derived. In particular, the change in the fringe spacing, the relationship between the shift of the measurement volume and the probe movement, and the general 2-dimensional traversing properties of the measurement volume are considered. The displacement between the two LDA measurement volumes of a 2D-probe caused by astigmatism has been determined from exact calculations of the laser beam transmission. The benefits of a water-filled prism to ensure the coincidence of the measurement volumes and to reduce the effect of astigmatism on the data rate and the distortion of the measurement volume has been shown. A calculation procedure for the use of such a prism is given.