Forces in nature are measured (with no exception) in an indirect manner from the change of a second magnitude that keeps a known relation with the former. In general, the elastic property of a certain material is used as a connection between the measurement of the applied force and its deformation under the effect of the load (for example, the extension or the compression of a spring in a dynamometer). The calibration of the instrument is therefore strongly dependent on the physical properties of the probe. This has some clear drawbacks. Specifically, when the conditions of the experiment (the temperature, for instance) are modified, the calibration changes. Thus, the use of the instrument is typcally restricted to certain environmental condtions. On the other hand, the extend of the measurement is limited to the regime where the elastic properties of the instrument are maintained.
Unfortunately, this not only happens at the macroscopic scale but also at the microscopic level. In paritcular, in biology, there exist many different techniques to measure forces between cells or molecules and they all use this approach to obtain a measurement:
Atomic force microscope
In some cases (for magnetic tweezers, for example), the deformation of the material is substituted by a modification of a certain field although with the same result, the force is obtained from a change in position. For most techniques, the harmonic approximation of the force, F = -kx, is the key element that links a fundamental magnitude such as the displacement (or the deformation) with the force.
The measurement of forces with optical traps is subjected, in general, to the physical properties of a probe
The measurement of the force is thereby intimately connected to the origin of the force itself.
We give a description of the conditions required to implement the method based on the direct measurement of the light momentum changes in single beam gradient traps, combining the strength of both techniques.
We give technical aspects and further details about the implementation of the method. We analyze in depth the important features that the instrument must fulfill in order to measure forces according to the method described in the previous paper.
We show that, in fact, back-focal-plane inteferometry is in essence the single-beam version of the method based on the direct measurement of the light momentum changes.
Force detection >