FURTHER CONSIDERATIONS OF THE ASTIGMATISM ERROR ASSOCIATED WITH OFF-AXIS ALIGNMENT OF AN LDA-PROBE

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