Specific design requirements for a reliable slope and curvature measurement standard

In the domain of surface metrology, direct altitude measurements are increasingly being challenged by slope and curvature measuring methods because of the numerous advantages of the latter measurands. Reference standards are needed to assess the quality of these slope and curvature measuring systems and thus to allow their spread into industry. Up to now, no specific slope or curvature measurement standard has been defined; rather, existing standards are designed in terms of altitude profile specifications. This paper details our experience on a reference manufactured piece intended for deflectometric slope measurement validation. An important discrepancy between the piece specifications and the measurements led us to cross-check our deflectometric measurements with differential interferometry. The results obtained using the two measurement methods matched very well. A plausible explanation of the discrepancy between the piece specifications and the measurement results is that small altitude variations may have considerable effects on slopes and curvatures. This real example raises the question of the specific design features for slope and curvature measurement standards and highlights the importance of the chosen altitude profile.