Primary Length Bar Interferometer
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The Primary Length Bar Interferometer, designed by NPL and sold commercially by Hexagon Metrology, offers the highest accuracy interferometric calibration of long gauge blocks and length bars up to 1 m.
Overview
Length bars and long gauge blocks are among the most commonly used standards for maintaining traceability in dimensional metrology. Long gauge blocks are becoming increasingly popular as standards for calibrating or verifying the performance of Co-ordinate Measuring Machines, because they can cover the length range up to 1 m with calibration uncertainties as small as 100 nm. However, before use, all gauge blocks and length bars must themselves be calibrated according to various specification standards such as ISO 3650. They must also be re-calibrated periodically to maintain their accuracy.
The NPL Primary Length Bar Interferometer was designed to measure length standards at the highest level of accuracy. The instrument can measure length, flatness, variation in length and thermal expansion coefficient of gauge blocks and length bars from 100 mm up to 1000 mm in length, with uncertainties of less than 100 nm.
Instrument features
The instrument operates as an absolute length measuring interferometer using multiple-wavelength phase-stepping interferometry with three frequency-stabilised He-Ne lasers providing the measurement wavelengths. By calibrating the frequencies of these lasers by direct comparison with a primary standard such as an iodine-stabilised reference laser, direct traceability to the realisation of the metre is maintained.
The interferometer is of a folded Twyman-Green design, housed in a semi-sealed, temperature-controlled chamber which is used to minimise temperature gradients and refractive index variation. Up to three bars/gauges can be measured at once. Each bar/gauge to be measured is supported horizontally on two supports featuring integral platinum resistance thermometers which are used to measure the temperature of the bar/gauge. A small platen is wrung directly to the end of the bar/gauge and used as a reference surface in the interferometer. The length of the bar/gauge is measured as the distance from the surface of the platen to the centre of the exposed face of the bar/gauge, in accordance with the relevant standards (for example ISO 3650, BS 5317, BS 1790, ASME B89.1.9-2002).
The system is controlled by a PC using Windows-based software written by NPL in C++, which makes the measurement process fully automated. Calibrated sensors are used to measure the air temperature, pressure, humidity and carbon dioxide content to correct the laser wavelength for the refractive index of air. Temperature measurements of the bars/gauges are used to calculate the length at the standard reference temperature of 20 °C.
The instrument was first developed at NPL and is now sold commercially by Hexagon Metrology Ltd.
Specifications
Length of items accommodated: 100 mm to 1000 mm
Measurement uncertainty (k = 2): from 50 nm (100 nm at 1000 mm) (in suitable environment)
Maximum loading: 3 gauge blocks/length bars/Hoke gauges
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. Other types of NPL-designed interferometer have been sold to twenty five countries worldwide and a Primary Length Bar Interferometer was recently installed in National Metrology Institute in the Asia Pacific Region.
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
Contact the Instrumentation team for more information