The trap length along the beam axis for an optical trap formed with an upright, oil-immersion microscope was
measured. The goals for this effort were twofold. It was deemed useful to understand the depth to which an optical
trap can reach for purposes of developing a tool to assist in the fabrication of miniature devices. Additionally, it was
desired to know whether the measured trap length favored one or the other of two competing theories to model an
optical trap. The approach was to trap a microsphere of known size and mass and raise it from its initial trap
position. The microsphere was then dropped by blocking the laser beam for a pre-determined amount of time.
Dropping the microsphere in a free-fall mode from various heights relative to the coverslip provides an estimate of
how the trapping length changes with depth in water in a sample chamber on a microscope slide. While it was not
possible to measure the trap length with sufficient precision to support any particular theory of optical trap
formation, it was possible to find regions where the presence of physical boundaries influenced optical traps, and
determine that the trap length, for the apparatus studied, is between 6 and 7 micrometers. These results allow more
precise control using optical micromanipulation to assemble miniature devices by providing information about the
distance over which an optical trap is effective.