The forces of a photonic force microscope based
on optical tweezers are determined using the Boltzmann distribution
of thermally driven position fluctuations. As a fundamental
method, only a minimum of information about the
system is required, i.e. the solution’s temperature. This calibration
method provides detailed information about the trapping
potential with nanometer resolution. In contrast to other
calibration methods, no numerical hydrodynamic corrections
are necessary. The lateral force constant of the optical trap
was measured along the optical axis and found to decrease
linearly by  8%=mm for a latex bead 600 nm in diameter.
Oscillations in the lateral force constant were found along the
optical axis close to the surface.