Schematic representation of the oscillation amplitude versus the tip-sample distance. For large tip sample distances, the free oscillation amplitude is measured. As the oscillating tip interacts with the surface the oscillation decreases linearly with tip sample distance. For small oscillation amplitude, the energy pumped into the cantilever by the external driving circuit is not sufficient to compensate for the losses induced by the tip-sample interaction, and the oscillation stops. If the feedback loop does not respond instantaneously to height variations as the tip is scanned over the surface, height variations will result in variations of the oscillation amplitude. For the kind of applications proposed in this study, the tip-sample system has to stay in the linear regime of the amplitude versus distance curve.