In the framework of the European COoperation in the field of Scientific and Technical research action on "X-ray and Neutron Optic" (COST P7) and following the decision announced in the last International Workshop on Metrology for X-ray and neutron optic (Grenoble, April 2004), the metrology facilities of four European synchrotrons, Bessy, Elettra, ESRF and Soleil, have decided and started a program of instrument inter-comparison. Other synchrotrons are joining us and further interested Institutions are invited to participate in this open measurement comparison. The metrology instruments involved are different kinds of direct slope measurement devices, like the well known Long Trace Profiler (in house made or modified) and the Bessy N.O.M..
The Round Robin was started with 2 flat and 2 spherical mirrors (three made of Zerodur and one of fused silica) made available by Bessy and Elettra. A short radius of curvature spherical mirror of Silicon from SOLEIL was later added. First results show a very close match between the measurements of all facilities provided that the same procedures are followed. In particular, a special attention has to be given to the way of supporting the reference objects, as it will be illustrated by some examples. Another important issue is the characterization of the systematic errors of the different instruments and how they can be reduced or eliminated.
The paper expects to open a discussion on the performances of different commercial and custom made or modified profilometers, and over standard procedures for calibration testing, including the definition of standard reference surfaces.