National Physical Laboratory

Areal measurement

The modern trend in advanced manufacturing is to use three-dimensional, or areal, surface modification to alter the functionality of products. NPL is leading the way for areal surface topography measurement with good practice guides, instrumentation and artefacts to transfer traceability of measurements to industry.

An areal transfer artefact
An areal transfer artefact


There is soon to be a step-change in the way that we measure and characterise surface texture or topography. In many manufacturing operations only a line scan using a stylus instrument (a profile) is required to hunt for changes in the manufacturing process or for parts that are not in tolerance. However, the modern trend in advanced manufacturing is to attempt to control the functionality of a part by the use of deterministic, often three-dimensional patterning. By patterning or functionalising a surface, its optical, thermal, mechanical, electrical, rheological and biological function, for example, can be controlled. To control the three-dimensional nature of a surface requires a three-dimensional measurement.

To measure three-dimensional or areal topography, a stylus can be scanned in a raster fashion to produce an area grid of measured points. However, such methods can be very time consuming and can sometimes damage the surface. There is also a range of non-contact optical instruments, which are usually based on a microscope, available commercially that can measure areal surface topography. Optical instruments can take an areal measurement very quickly, over a range dictated by the numerical aperture of the instrument. To increase the scanning range a two axis translation stage can be used, and sets of areal measurement data 'stitched' together; although with a corresponding increase in measurement time and uncertainty.

Whilst the stylus instrument operating in a profile measuring mode has the support of an international standards infrastructure, the measurement of areal surface topography and the use of optical instruments is not yet covered by standards. An important aspect of the draft standards is the new group of areal surface texture parameters that are being developed. In some case these 'S-parameters' are three-dimensional equivalents of the standardised profile parameters (the most popular being Ra). However, the areal parameters are much more powerful than the two-dimensional parameters in that they can be used to characterise functional aspects of a surface topography.

The NPL Areal Instrument
The NPL Areal Instrument


NPL has developed a scanning stylus instrument, the NPL Areal Instrument, that can measure areal surface topography in an 8 mm x 8 mm x 0.1 mm range with nanometres of accuracy. The instrument uses laser interferometers to measure the position of the stylus in three axes and obtains traceability to the SI metre via the laser sources. The instrument is currently being tested before an areal surface topography measurement service is launched at NPL. NPL has also developed areal transfer artefacts that can be measured using the NPL Areal Instrument and then used in industry and academia. Also, Taylor Hobson have recently delivered the latest in their range of scanning white light interferometers to NPL, so that the issues of areal traceability for optical instruments can be addressed. The instrument has a lateral scanning range of 150 mm x 150 mm and a vertical resolution of less than one nanometre.

NPL has also developed a range of transfer artefacts that can be calibrated using the Areal Instrument.

Products & Services

NPL offers a traceable measurement service for profile surface texture




The International Organization for Standardization's ISO 213 working group 16 is in the process of drafting a suite of standards for areal measurement and the first specification standards are expected to be published in the next two years.


Customer Service tel: +44 20 8943 8631

Last Updated: 6 Oct 2016
Created: 23 Oct 2007


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