Analysis of force-displacement measurements obtained with a commercial
beam-reflection scanning force microscope on rigid samples in
the contact regime reveals the need for previously-unpublished
calibration procedures. The measured cantilever deflection is
linearly proportional to the sample displacement to lowest order
as expected, but the proportionality coefficient is dependent
on the cantilever/sample immersion medium via its refractive index.
An additional term which is cubic in the sample displacement becomes
significant at large cantilever deflections, amounting to about
8% of the total signal at the largest measurable deflections.
This nonlinearity apparently derives from a cross-sectional nonuniformity
in the intensity of laser light used to monitor cantilever deflections.
The importance of these considerations to studies in fluid media
and on biological and other soft materials is illustrated employing
gelatin films in air and in water. Appropriate calibration measures