This figure is a schematic drawing of the SFA3 Basic Unit ready for use for normal force measurements. The shapes of the interacting surfaces and the separation between them are measured by analyzing the optical interference fringes (known as FECO fringes) produced when white light passes through the two surfaces.
The distance between the surfaces is controlled by a four-stage mechanism of increasing sensitivity from millimeters to ångstroms. The stiffness of the force-measuring spring can be adjusted during experiments to enable forces of greatly differing magnitudes to be measured. Dynamic measurements are conducted with surfaces in motion using one of the attachments described elsewhere.