All of these techniques give us a way to accurately measure a signal that is proportional to the force exerted
on a polystyrene bead. In order to determine the force directly, the instrument must be calibrated. In the
viscous drag force calibration, we apply a known force and measure the resulting signal as a function of the
applied force. If the position of the bead as a function of force is also measured, the results can then be fit
to Eq. 4 to determine the trap stiffness k. In the Brownian motion calibration, we take advantage of our
knowledge of the frequency spectrum of position fluctuations in an optical trap to obtain k directly