Further considerations of the astigmatism error associated with off-axis alignment of an LDA-probe

Abstract

 Off-axis alignment of an LDA-probe in internal flow measurement is always associated with the appearance of astigmatism and this may have a large effect on the measurement accuracy. In addition to the earlier investigations reported by Zhang and Eisele (1995a, 1996) it is shown that the improper off-axis alignment of the probe leads to separation of laser beams in the formation of the measurement volume. Quantitative calculations made in this paper give a sound basis for estimating this separation and the results imply that the error arising from beam separation is very serious both for LDA- and for PDA-measurements. In addition, the analysis in Sect. 3 reveals that the reduction of the data rate from astigmatism due to off-axis alignment of the probe depends on the focal length of the receiving optics. This relationship has been validated by experiment.