Interlaboratory comparison measurements of aspheres

Abstract

The need for high-quality aspheres is rapidly growing, necessitating increased accuracy in their measurement. A reliable uncertainty assessment of asphere form measurement techniques is difficult due to their complexity. In order to explore the accuracy of current asphere form measurement techniques, an interlaboratory comparison was carried out in which four aspheres were measured by eight laboratories using tactile measurements, optical point measurements, and optical areal measurements. Altogether, 12 different devices were employed. The measurement results were analysed after subtracting the design topography and subsequently a best-fit sphere from the measurements. The surface reduced in this way was compared to a reference topography that was obtained by taking the pointwise median across the ensemble of reduced topographies on a Cartesian grid. The deviations of the reduced topographies from the reference topography were analysed in terms of several characteristics including peak-to-valley and root-mean-square deviations. Root-mean-square deviations of the reduced topographies from the reference topographies were found to be on the order of some tens of nanometres up to 89 nm, with most of the deviations being smaller than 20 nm. Our results give an indication of the accuracy that can currently be expected in form measurements of aspheres.

Round robin comparison study on the form measurement of optical freeform surfaces

“…Thereafter, we performed bilateral comparisons of the processed measurements obtained in the first step. The procedure for obtaining processed measurements is similar to that described in [6].…”

“…Concerning manufacturing machines for optical surfaces which can remove material of single-nanometre thickness, *Correspondence: ines.fortmeier@ptb.de 1 Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116 Braunschweig, Germany Full list of author information is available at the end of the article measurement accuracy needs to be improved in order to improve manufacturing accuracy. Currently, no general reference measurement system with the required accuracy exists [6].…”

“…In this situation, comparisons are a key factor in gaining new insights. Comparisons [6][7][8][9] have shown that, while some progress has been achieved, further improvements are also necessary. Most of the specimens used in these comparison studies were conventional spherical or aspherical surfaces made of glass.…”

“…Most of the specimens used in these comparison studies were conventional spherical or aspherical surfaces made of glass. The study with the largest variety of instruments and measurement data was published anonymously [6].…”

Research regarding tactile scanning versus optical scanning

“…The results show that some measurement systems may have been influenced by systematic errors and that "the data does not suggest that one measurement principle is superior to another" [6]. Another study [7] that found good agreement between measuring machines, compared measurements of microgrears with a micro-CMM, computer tomography, optical CMM and a gear measuring instrument.…”