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Primary length bar interferometer

Measure length standards at the highest level of accuracy

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. Long gauge blocks are often used 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.
All gauge blocks and length bars are calibrated according to various specification standards such as ISO 3650, and are re-calibrated periodically to maintain their accuracy.

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 SI 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.

The system is controlled by a PC using Windows-based software written by NPL in C++, which makes the measurement process fully automated. Traceably 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 Primary Length Bar Interferometer, designed by NPL and sold commercially under license by Hexagon Metrology, offers the highest accuracy interferometric calibration of long gauge blocks and length bars up to 1 m.


  • Length of items accommodated: 100 mm to 1000 mm
  • Measurement uncertainty (k = 2): from 50 nm (100 nm at 1000 mm in suitable environments)
  • Maximum loading: 3 gauge blocks/length bars/Hoke gauges

Standards measured:

  • gauge blocks (ISO 3650 and 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:

  • Calibration of internal optics using calibrated reference flat
  • Automatic background temperature monitoring (and logging to file)
  • Result printout of 3D topography
  • Result printout of entire set or selected units
  • 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
Phone: +44 (0) 1952 681372

Visit the Hexagon Metrology website to learn more

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