he NPL-designed Gauge Block Interferometers, sold commercially through Hexagon Metrology, are the de facto instruments used worldwide for the highest accuracy calibration of gauge blocks and similar engineering artefact standards.
Overview
Gauge blocks are among the most commonly used standards for maintaining traceability in dimensional metrology. They are used to calibrate a wide range of instruments and other standards from micrometers to Co-ordinate Measuring Machines. Before use, all gauge blocks must themselves be calibrated according to various specification standards such as ISO 3650. Gauge blocks must also be re-calibrated periodically to maintain their accuracy.
The NPL Gauge Block Interferometer, which is manufactured and sold under licence by Hexagon Metrology, was designed by NPL to measure the length of gauge blocks, length bars and Hoke gauges up to 300 mm in length. Up to 12 gauges can be wrung to a platen which is then placed in the instrument and measured after a suitable thermal settling period. After appropriate corrections have been applied, typical measurement uncertainties of 20 nm for a 1 mm gauge and 40 nm for a 100 mm gauge can be achieved, at 95 % confidence level.
Instrument features
The instrument design is based on a Twyman-Green interferometer with a CCD camera and computer system to provide automatic analysis and result calculation using Windows-based software written by NPL in C++. Original versions of the instrument used hard-wired electronics to obtain fringe pattern intensity profiles which were then offloaded to a PC for analysis, using a program written in BASIC.
Whilst many laboratories still operate this older version of the instrument, several have taken the opportunity to have the latest phase-stepping technique (used in current instruments) installed as a retro-fit. Phase-stepping is a powerful technique for processing interference patterns and is used in the instrument to obtain 3D topographic measurements of the gauge block and platen surfaces, with nanometre resolution.