Accurate force determinations in atomic force microscopy require the precise measurement of cantilever deflections. A limiting factor in making these measurements is the calibration of the optical lever detection system, particularly when the tip cannot be pressed against a hard surface. Here we show that, for a given instrument, a fixed displacement of the photodiode detector produces a change in the detector voltage that correlates strongly with optical lever sensitivity. This provides a simple method for optical lever sensitivity calibration not requiring contact of the tip with any surface. The data also suggest that differences in the shape of the laser spot on the photodiode are a major source of variability in optical lever sensitivity.
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