Thanks for the suggestions and help. In the end I did this by writing a simple macro, derived from a tool setting macro, that would take up to 225 probe readings and write them to a tool table, which I then exported as a text file and opened in Excel for analysis. Slightly clunky but does the job. Measuring the performance of a Sharp photointerrupter, 87% of 100 measurements were at exactly the same point, the rest being within +/- step at 1600 steps/mm, so better than a micron. I will repeat the exercise with: different supply voltages to the opto; on positive edge triggering rather than negative (i.e. with the obstruction moving out of the gap); with and without ambient light; and with my "conductive" probe.