Novel Methods of Optical Metrology

As the National Physical Laboratory has oft observed, ‘If you can’t measure it, you can’t make it’. Therefore, measurement science is a core research activity within the National Facility for Ultra-Precision Surfaces.

The 10m high Test Tower is itself of itself of novel design, especially as the part is measured whilst located on the polishing machine. This theme of ‘on-machine metrology’ forms the core of the current research in surface-metrology. The particular challenge is measurement of the form of aspheric optics (surfaces that depart from a sphere or flat), and the emerging requirements in a number of sectors for free-from optics (complex surfaces that have no axis of symmetry). Classical measurement methods encounter problems of dynamic range and surface-precision. This has led to the on-going development of sub-aperture stitching interferometry, where the interferometer module is mounted in place of the tool-unit on the CNC polishing machine. The machine scans the interferometer over the surface and collects overlapping images that are then ‘stitched’ together in the computer. There are significant challenges ranging from optical design, through environmental issues, to data capture and analysis, handling of errors etc.

Other metrology methods under development fall under the title ‘profilometry’, where a measurement probe is traversed over the surface to build up a scanned map of the surface-profile. Two instruments are currently in use. One is a ‘swing arm profilometer’, developed as a collaboration between the National Physical Laboratory and UCL, which measures surface height as the probe swings across the surface. The other is the NOMM (Nanometer Optical Measuring Machine) – a pentaprism/autocollimator-based profilometer that measures surface slope and integrates the data to provide a surface profile.

Page last modified on 22 oct 10 11:38 by A Peter Doel