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In this work, we present an alternative optical method to determine the probe-sample separation distance in a scanning near-field optical microscope. The experimental method is based in a Lloyd's mirror interferometer and offers a measurement precision deviation of ~100 nm using digital image processing and numerical analysis. The technique can also be strategically combined with the characterization of piezoelectric actuators and stability evaluation of the optical system. It also opens the possibility for the development of an automatic approximation control system valid for probe-sample distances from 5 to 500 μm. [ABSTRACT FROM AUTHOR]