  The full characterization of a stringed musical instrument requires measuring
the motion of the strings in at least two dimensions. Traditionally this has
been done using electromagnetic means or by optical transmission. However in
many instruments the strings are not made of steel, nor are the strings easily
accessible on both sides. In this work it is shown that string positions can be
measured by inexpensive optical reflection sensors which neither require
metallic strings nor obtrusive access. We have demonstrated a versatile and
non-invasive technique for measuring the position of a vibrating string in a 1
mm^2 area with an accuracy of tens of microns for thin strings, and better than
0.1 mm for thick ones.
