National Physical Laboratory

Gauge Block Interferometer

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NPL Gauge Block Interferometer

The 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.

 

Software control of the interferometer
Software control of the interferometer

 

Traceability to the SI is assured using two frequency-stabilised lasers which can be calibrated by reference to iodine stabilised lasers, used to realise the metre. Temperature measurement of the gauge blocks and the ambient air is performed continually using calibrated platinum resistance thermometers and a resistance bridge, under software control. Correction for ambient refractive index is achieved using additional calibrated sensors for air pressure and humidity (as well as the air temperature).

 

Schematic layout of the instrument
Schematic layout of the instrument

 

Specifications

Length of items accommodated: 0.5 mm to 300 mm

Measurement uncertainty (k = 2): from 20 nm (40 nm at 100 mm) (in suitable environment)

Maximum loading: 12 gauge blocks

Standards measured:

gauge blocks (ISO 3650 & similar)

length bars (BS 5317, BS 1790)

Hoke gauges (ASME B89.1.9-2002)

Parameters measured:

central length (deviation from nominal)

flatness

variation in length

fo, fu parameters of ISO 3650

Other features:

1. Calibration of internal optics using calibrated reference flat

2. Automatic background temperature monitoring (and logging to file)

3. Result printout of 3D topography

4. Result printout of entire set or selected units

5. Automatic storing of results in measurement file (and conversion to XLS)

Worldwide use

The partnership between NPL and Hexagon (formerly Brown & Sharpe Ltd) who sell the interferometers has been in place for twenty five years. During this time, the instrument has established itself as the de facto commercial interferometer for gauge block interferometry. Sales of instruments have been made around the world. Typical customers include foreign National Metrology Institutes, accredited laboratories and high technology laboratories and organisations.

Commercial enquiries for supply of interferometers:

Gary Brice
Hexagon Metrology
Metrology House
Halesfield 13
Telford
Shropshire
TF7 4PL

Email: gary.brice@hexagonmetrology.com
Phone: +44 (0)1952 681372

Visit the Hexagon Metrology website to learn more

Publications

Lewis, A J, Measurement of length, surface form and thermal expansion coefficient of length bars up to 1.5 m using multiple-wavelength phase-stepping interferometry, Meas. Sci. Technol., 1994, 5, 694 - 703

Lewis A J, Pugh D J, Interferometer light source and alignment aid using single-mode optical fibres, Meas. Sci. Technol., 1992, 3, 929-930

Lewis A J, Absolute length measurement using multiple-wavelength phase-stepping interferometry, PhD Thesis, 1993, Imperial College, University of London

Lewis A J, Key Comparison CCL-K2 - Measurement of long gauge blocks - Final Report, Metrologia, 2003, 40 Tech. Suppl. 04004

Lewis A J, Final Report on EUROMET Key Comparison EUROMET.L-K2: Calibration of long gauge blocks, Metrologia, 2006, 43 Tech. Suppl. 04003

Lewis A J, Hughes E B, Aldred P J E, Long-term study of gauge block interferometer performance and gauge block stability, Metrologia, 47, 473


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