3. Calibration methods and means

3.1 Direct interferometric method

This is the method used to calibrate the reference shims, which are then used to connect the other shims by mechanical or interferometric comparison. Exceptionally, this may involve shims used in control or characterization procedures, as in the case of high-precision three-dimensional machines (of the order of 1 μm). Lastly, for certain applications, such as calibrating sensors on high-precision form-measuring benches or profilometers, we use step heights calibrated by direct interferometry.

The direct interferometric method is also sometimes referred to by metrologists as the "fundamental method", because the length of the standard is a direct function of the wavelength of the laser source whose frequency is known (which is...