National Physical Laboratory

X-ray Computed Tomography

Driven by the need for quality control of complex three-dimensional engineering components, and enabled by progress in medical imaging and consumer graphics processing, X-ray computed tomography (XCT) has been increasingly used for industrial inspection in the last decade. A rise in the demand to have, more intricate, parts manufactured using Additive Manufacturing (AM), has limited the use of conventional dimensional measuring systems for verification of these parts. XCT is invaluable in the evaluation, in both dimensional and material integrity, of these complex AM parts.

The most important merit of the technique is that it allows the measurement of the geometry of external (surface) and internal features without the need to disassemble a component. Whilst many XCT instruments are used for non-destructive testing, they are increasingly being used in industry as dimensional metrology tools, akin to the conventional co-ordinate measuring machine.

XCT events


Title: Dimensional X-ray Computed Tomography Conference
Date: 13 July 2017
Venue: WMG, University of Warwick, Coventry, UK
More info:


Title: Dimensional X-ray Computed Tomography Conference 2016
Date :10 May 2016
Venue: NPL, Teddington, UK
More info:

XCT: 225kV system

XCT system at NPL

The NPL XCT system was installed in 2013. The system employs a cone-beam X-ray source with the maximum energy of 225 kV. Details of the system are as follows:

  • Cone-beam X-ray computed tomography
  • Maximum 225 kV
  • Temperature control of ±1°C
  • 40 cm × 40 cm scintillation detector
  • 200 µm × 200 µm pixel size on detector
  • Magnification up to ~120
  • Spatial frequency: up to 3 µm

Typical penetration depth of materials are given below (an energy of 200 kV is chosen in the estimation):

Material Atomic number Approximate depth
of penetration (cm)
Carbon 6 14.3
Aluminum 13 9.0
Titanium 22 5.0
Iron 26 2.6
Nickel 28 2.1
Copper 29 2.1

Development of XCT reference samples at NPL

NPL has designed a series of reference samples with the aim to:

  • Investigate systematic errors associated with the XCT technology
  • Verify the performance of XCT systems
  • Investigate the factors that influence the quality of AM
  • Evaluate XCT as a method for robust dimensional analysis of AM parts

Reference sample made by selective laser melting:

Camera image:

Reference sample made by selective laser melting - camera image
   CT image:

Reference sample made by selective laser melting - CT image


Calibrated two sphere reference sample:

Calibrated two sphere reference sample
   Ball plate:

XCT: Ball plate


Hole plate:

Hole plate


NPL can provide customers with both qualitative and quantitive analysis of single and multi-material samples. Using our other in-house capabilities, we are in the unique position of being able to correlate XCT images and analysis to other physical dimensions, such as: dimensional through one of our many traceable CMM systems, 3D external surface generation through optical methods (fringe projection or laser line projection), material properties, analysis of inclusions through microwave scanning, acoustical and thermal analysis, is to name only a few from the wide range of available systems.

Examination of the assembled internal structure of manufactured parts, in a range of discrete points in manufacturing. This allows the customer to pinpoint steps in the manufacturing process that may be responsible for particular issues or problems.


NPL has collaborated at both international and national levels with: NMIs, academia and industry on a variety of projects. The list of collaborators includes the following, but not exhaustive: PTB, NIST, NMIJ, BAM, Bath University, Huddersfield University, Padua University, Southampton University, University College London (UCL), Nikon, Simpleware, VolumeGraphics.

Research projects

  • Investigation of influence parameters and systematic errors
  • Development reference objects for XCT
  • Investigation of geometrical errors of the XCT systems and development of geometrical error model to calibrate XCT system
  • Metrology for Additive Manufacturing (AM)
  • Evaluation of 3D weaving in advanced manufacturing
  • Property of powder in advanced and additive manufacturing

Research team


NPL is an active member of a selection of external panels, groups and legal entities, including: BSI, ISO, XCT User Group.

Recent publications

  • Use of Morphological Method to Investigate the Influence of Surface Texture on Dimensional Measurement of Additively Manufactured Parts
    S. Lou, S. B. Brown, W. Sun, W. Zeng, H. S. Abdul-Rahman, X. Jiang, P. J. Scott
    Joint Special Interest Group meeting between euspen and ASPE Dimensional Accuracy and Surface Finish in Additive Manufacturing, KU Leuven, BE, October 2017
  • Extraction of Process Signature Features from Additive Manufactured Metal Surfaces
    S. Lou, W. Sun, W. Zeng, H. S. Abdul-Rahman, X. Jiang, P. J. Scott
  • Application and systematic errors of X-ray computed tomography associated with dimensional metrology
    Wenjuan Sun, Stephen Brown, Peter Woolliams, Michael McCarthy, Jan White, Moataz Attallah
    Conference: 55th Annual Conference of the British Institute for Non-Destructive Testing, Nottingham 2016
  • An Enhanced Approach to Calculate the presampled Modulation Transfer Function using Edge Samples with Surface Imperfections for X-ray Imaging Systems
    Wenjuan Sun, Nadia Flay, Anastasios Konstantinidis, Stephen Brown, Richard Leach
    Measurement Science and Technology
  • How beam hardening influences the dimensional geometry in X-ray computed tomography
    H. Corcoran, S. Brown, S. Robson, R. Speller, M. McCarthy
    7th Conference on Industrial Computed Tomography, Leuven, Belgium (iCT 2017)
  • Characterisation of a scintillation flat panel detector employed by an X-ray computed tomography system: measurements of MTF, NPS and DQE
    Sun W, Konstantinidis A, Billas I, McCarthy M, Brown S, Corcoran H
    Proceedings of 16th euspen International Conference, Nottingham, UK, 2016
  • Characterisation of X-ray source for X-ray computed tomography for dimensional metrology
    Corcoran H, Sun W, Brown S, Robson S, Speller R, McCarthy M
    International Conference on Industrial Computed Tomography, Wels, Austria
  • Investigation of the focal spot drift in industrial cone-beam X-ray computed tomography
    Flay N, Sun W, Brown S B, Leach R K, Blumensath T
    The e-Journal of Nondestructive Testing, 20, 9 (2015)
  • A preliminary study of characteristics of X-ray computed tomography
    Sun W, Flay N, Brown S, McCarthy M, Corcoran H, Ferrucci M
    Proceedings of Digital Industrial Radiology and Computed Tomography, Ghent, Belgium (2015)
  • Towards geometrical calibration of X-ray computed tomography systems - a review
    Ferrucci M, Leach RK, Giusca C, Dewulf W, Carmignato S.
    Measurement Science and Technology, 26, 092003 (2015)
  • A new method of performance verification for x-ray computed tomography measurements
    Leonard F, Brown S, Withers P, Mummery P, McCarthy M
    Measurement Science and Technology, 25 (2014)
  • Application of the optical transfer function in x-ray computed tomography - a review
    Flay N, Leach R K
    NPL Report ENG 41 (2012)
  • An overview of industrial x-ray computed tomography
    Sun W, Brown S B, Leach R K
    NPL Report ENG 32 (2012)
Last Updated: 29 Nov 2018
Created: 3 Apr 2013


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