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

A theoretical analysis of the positional accuracy of an LDA measuremen t volume using an off-axis alignment of a probe relative to an interna l flow has been carried out. General performance parameters of measure ments with a 1D-probe with an off-axis alignment are derived. In parti cular, the change in the fringe spacing, the relationship between the shift of the measurement volume and the probe movement, and the genera l 2-dimensional traversing properties of the measurement volume are co nsidered. The displacement between the two LDA measurement volumes of a 2D-probe caused by astigmatism has been determined from exact calcul ations of the laser beam transmission. The benefits of a water-filled prism to ensure the coincidence of the measurement volumes and to redu ce the effect of astigmatism on the data rate and the distortion of th e measurement volume has been shown. A calculation procedure for the u se of such a prism is given.