In all instruments the raw data measured by the sensors (i.e., transducers or detectors) has to be converted into physical magnitudes. The process of inference of such conversion is known as calibration. The calibration can be performed in either two ways: 1) by comparing a known magnitude of one physical process with the measurement of the instrument or 2) by establishing a conversion relation with another calibrated instrument.
The calibration of the Minitweezers involves three independent magnitudes: time, distance and force. The rest of magnitudes (e.g. pulling rate, motor velocity, etc.) measured by the instrument are obtained from combinations of those three ones. One interesting aspect of the optical tweezers is that the calibration of the force can be performed in different ways, allowing to cross-check the calibration procedures. During the following explanation, we use square brackets to represent measured and converted magnitudes, and bare symbols for calibration factors and offsets.