  A student laboratory experiment is presented that introduces the concept of a
lock-in measurement through the exploration of the relationship between the
power detected from a modulated light source and the distance between the
source and the detector. The lock-in measurement is done in software using time
streams for both the reference and the detector signals. The achievable
experimental sensitivity is shown to increase several orders of magnitude over
the simple case without using modulation. In addition, the student becomes
familiar with computational techniques pertinent to large data sets.
