Two more distributions are attached:

1. sigma vs position - this is a standard deviation (SD) of the distributions.
Or the error bars in the plot of z-measured vs z-set. It is clear that at the
ends the SD is increased

2. total length vs z-position, as recovered using only the reflected light from
the interface to the opposite end of the bar. (We cut the bar ends at 45 deg
in order to reduce the reflections)
The last point at z=80 cm is the calculated length using the main light signal.

To me, the reflected light reconstruction is more precise and consistent from
point to point.

Sam