The road towards accurate optical width measurements at the industrial level

Dimensional metrology based on optical vision systems is commonly used at the industrial level, for example in optical coordinate measurement machines (CMMs). Measurement uncertainties of well below 1 μm are more and more required. Thus for calibration samples a measurement uncertainty of the order of 100 nm is required. To enable accurate and traceable measurements both unidirectional (e. g. distance) measurements and bidirectional measurements (e. g. width or diameter) have to be performed for system calibrations and the reverification of the performance [1]. However, up to now the national metrology institutes (NMIs) are not able to offer internationally recognized bidirectional calibrations, which are covered by the Mutual Recognition agreement (MRA [2]) and documented in the key comparison database [3]. Currently only one NMI offers optical widths calibrations on suitable user-supplied samples, including line scales, as well.