The ultimate test of an optical component is its performance under conditions similar to those that it will encounter in service. Prior to this stage, precision dimensional measurement is the key to achieving the specification, and a range of measuring instruments has been developed specifically for the metrology of grazing incidence optics. These instruments can be calibrated to uncertainties of about 10 picometres by X-ray interferometry.
A. Franks, A. Franks,
"The Metrology Of Grazing Incidence Optics At The National Physical Laboratory", Proc. SPIE 0640, Grazing Incidence Optics, (21 October 1986); doi: 10.1117/12.964371; https://doi.org/10.1117/12.964371