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Non-destructive evaluation

Assessing the quality of materials

Non-destructive evaluation (NDE) refers to sensing, imaging and analysis techniques that are used to assess the quality of materials, components and structures. It encompasses non-destructive testing, condition monitoring and some areas of quality control and process monitoring (PM). Since it allows objects to be examined without destroying or damaging it can save money and resources.   

NPL serves industry and academia by providing measurements and data analysis, consultancy and research. We have state-of-the-art facilities and expertise in many NDE areas, and are ideally placed to help resolve complex NDE challenges and give advice on technical problems.  

What is the difference between NDE and NDT?

The terms are often used interchangeably. However, NDE is a broader term than non-destructive testing (NDT), which often implies the determination of mechanical properties only.

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Ultrasound – contact, time of flight/velocity, scanning acoustic microscopy (SAM), C scan water tank – flat, rotational and programmable surface following

  • Scope/range: Contact 1-30 MHz compression and/or shear wave probes. SAM smallest 5 µm spatial sampling, 120 µm beamwidth. C scan – 0.5-50 MHz probes, inspection area 0.65 m x 0.65 m.
  • Measured parameter(s):Time of flight, amplitude
  • Derived parameter: thickness maps, intensity maps, cross-sections. Defect detection – cracks, voids, porosity, delaminations, sample thickness, ultrasonic velocity
  • Suitability: Contact handheld probes and non-contact water bath inspections. Spot measurements or mapping, volumetric interrogation.

Find out more about non-destructive testing of advanced materials 

Resonant ultrasound spectroscopy

  • Scope/range: 5 kHz-10 MHz frequency sweep
  • Measured parameter(s): resonant frequency peak detection, characteristic spectrum from resonant peaks      
  • Derived parameter: elastic engineering properties of anisotropic materials using inverse analysis methods.
  • Suitability: Contact, whole-body resonance of small volume with well-defined geometry, suitable for any non-lossy materials (such as metals and ceramics).

Find out more about non-destructive testing of advanced materials

Impact excitation

  • Scope/range:10-200 kHz frequency range
  • Measured parameter: FFT analysis of acoustic signal detected by microphone from impacted component or model specimen
  • Derived parameter: elastic engineering properties of regular geometry isotropic materials and instrumented ‘tap’ NDT test for any component or material
  • Suitability: Contact, point measurements, manual or electromagnetic striking possible, comparative assessment for NDT purposes.

Find out more about non-destructive testing of advanced materials

Optical techniques

Laser shearography – heat/vacuum/pressure/direct loading

  • Scope/range: dependent on camera lens and desired resolution, large range of shear vectors possible (1-180° and wide range of shear magnitudes)
  • Measured parameter(s): maps changes in the out-of-plane displacement gradient of a surface due to an applied stress   
  • Derived parameter: defect detection – voids, delaminations
  • Suitability: Non-contact large area imaging, surface or near surface depending on material (up to ~ 5 mm depth commonly).

Find out more about laser shearography of advanced materials

Digital Image Correlation (DIC) 

  • Scope/range: measurement of external surface deformations of an object by capturing differences between images - a ‘before and after’ image.  Objects can be micrometres in size (using scanning electron microscopy) through mm and up to km in size using panoramic image capture techniques. Resolutions to 1/20th of a pixel.
  • Measured parameter: surface deformation (height if using 3D DIC) in plane displacements, in plane strain and shear. Can be caused by thermal or mechanical changes to an object. Additions and subtractions to a surface like corrosion or loss of material caused by weathering or damage.
  • Derived parameter: Full field displacement and strain. Full field correlation coefficient, i.e. similarity measurements.
  • Suitability: Generally non-contact standoffs depending on imaging modality and optical setup. Can use any wavelength. Resolutions typically sub-pixel to 1/20th pixel. Image panoramas can be generated to be >250 GPixels. Efficient image processing solutions for large image sets. Examples include residual stress measurements at micrometres scale in films, crack propagation in fluid cells during mechanical testing, fatigue material coupon testing, measurement of movement in fixtures during testing, large scale concrete testing in the laboratory, in-situ measurements of nuclear decommissioning structures, measurements of the effect of loading on highway bridges, measurements on the interior of rail tunnels, measurements of vibration and movement in oil and gas processing, measurement of fire damage to piers and more.

Find out more about DIC of advanced materials

Stereo line scan measurement

  • Scope/range: variant of DIC using a pair of accurately aligned line scan cameras to measure height maps. Ranges from fields of view of 10’s mm to several m with resolutions of micrometres to 10’s micrometres. Ideally for production line or rotating objects or surveying 
  • Measured parameter(s): height perpendicular to surface and in direction of line of view.
  • Derived parameter: height maps built up of lines by moving perpendicular to line scan. Using accurate repositioning allows differential line scan heightmaps to be compared to determine differences in height or shape.    
  • Suitability: Works well for cylindrical storage units such as in nuclear decommissioning, objects on production lines, tunnel walls using train mounted cameras.

Surface topography measurement

  • Scope/range: Measurement of external surface topography on objects with nominally primitive forms. Object size scale can range from millimetre to metre scale.
  • Measured parameter(s): 3D Surface topography. Height maps on standard 2D grids. Advanced scanning strategies are possible.
  • Derived parameter: Surface texture, form and waviness. Surface roughness. Profile and areal parameters.
  • Suitability: Contact and non-contact systems. Point, line and area scanning methods. Off-line, in-line and on-machine capability. Traceability to the SI metre. Many data post-processing options for feature/defect detection and dimensional measurement.

2D optical imaging

  • Scope/range: Contrast optimised 2D optical imaging of external surfaces of objects. High dynamic range capability. Object can range from millimetre to metre scale.
  • Measured parameter(s): 2D images at various magnification levels, with wide range of resolution and pixel sampling. Pixel bit depths up to 16 bit in greyscale. Wide range of optics for illumination and collection.
  • Derived parameter: Lateral dimensions. Spatial distribution of features.
  • Suitability: Non-contact systems. Point, line and area scanning methods. Off-line, in-line and on-machine capability. Traceability to the SI metre for lateral scales. Many data post-processing options for feature/defect detection and dimensional measurement.

Thickness mapping

  • Scope/range: Optical measurement of thickness of objects with nominally planar geometry. Object size can range from micrometre to millimetre scale.
  • Measured parameter:3D surface topography of opposite faces of object in single coordinate system. Coating & multilayer thickness measurement possible for some material systems.
  • Derived parameter: Thickness. Coating thickness.
  • Suitability: Non-contact system. Point scanning methods. Off-line, in-line and on-machine capability. Traceability to the SI metre. Many data post-processing options for feature/defect detection and dimensional measurement.

Thermal techniques

Thermography – Passive, active, flash, long pulse, lock in.

  • Scope/range: Flash (millisecond duration), long pulse (~10 s ) and lock in (sine/square wave) excitations
  • Measured parameter: Heat flow/cooling of sample
  • Derived parameter: Defect detection – voids, delaminations, layer thickness
  • Suitability: Non-contact large area imaging, surface or near surface typically but penetration depends on material and surface quality.

Find out more about thermography for advanced materials 

Non-contact thermometry - Radiation thermometry, quantitative thermal imaging, phosphor thermometry, fibre optic thermometry and acoustic thermometry

  • Scope/range: -196 °C to 3000 °C
  • Measured parameter: Apparent radiance temperature
  • Derived parameter: Surface temperature
  • Suitability: Non-contact, requires an observation path to the target surface.

Find out more about temperature measurement 

Contact thermometry - Thermocouples and resistance thermometers

  • Scope/range: -196 °C to 3000 °C
  • Measured parameter(s): Surface temperature
  • Derived parameter: Surface temperature
  • Suitability: Contact, surface adhesion of a thermometer is required

Find out more about temperature measurement  

X-ray techniques

2D X-ray

  • Scope/range:110 kV source with digital scanning bed, 83 µm resolution
  • Measured parameter: Absorption/penetration of radiation.
  • Derived parameter: Defect detection – voids, cracking, porosity
  • Suitability: Non-contact, but ionising radiation so confined to cabinet with associated sample size limitations (~200 x 200 mm). Volumetric interrogation.

Find out more about non-destructive testing of advanced materials

X-ray computed tomography

  • Scope/range: Up to 225 kV. Volumetric imaging of samples of up to 180 mm × 180 mm × 180 mm, up to 3 μm resolution
  • Measured parameter: X-ray attenuation
  • Derived parameter: Non-contact; non-destructive, volumetric visualisation
  • Suitability: Non-contact, non-invasive. Provides volumetric data. Can be used for form, thickness, distance, surface texture, structure evaluation, and defect evaluation.

Find out more about X-ray computed tomography

2D radiography

  • Scope/range: 2D imaging of samples up to 400 mm × 400 mm, up to 3 μm resolution
  • Measured parameter: X-ray attenuation
  • Derived parameter: 2D non-contact
  • Suitability: Non-contact, projection images. Used for defect inspection.

Electrical techniques

Eddy current

  • Scope/range: 60 kHz conductivity testing, 0.5-1 MHz and 4-6 MHz probes
  • Measured parameter(s): Changes to eddy currents induced in material.
  • Derived parameter: Crack detection in metallic/conductive samples, non-conductive layer thickness.
  • Suitability: Contact point measurements, orientation and probe handling are very important. Requires a conductive material.

Find out more about eddy current testing of advanced materials

Magnetic induction/eddy current

  • Scope/range: Multiple probes for magnetic and eddy current based thickness measurements (as well as duplex coating measurements)
  • Measured parameter: Stand-off distance to underlying magnetic or conductive substrate
  • Derived parameter: Coating layer thickness measurement on metallic or ferromagnetic substrates
  • Suitability: Contact point measurements, requires initial calibration curve determination. Suitable for many coating/substrate material combinations.

Find out more about magnetic induction testing of advanced materials

AC (eddy current) conductivity

  • Scope/range: 2 MS/m to 60 MS/m (3.45 %IACS­ to 103 %IACS), 10 kHz to 100 kHz, UKAS accredited at 60 kHz, in accordance with BS EN 2004-7
  • Measured parameter: AC (Eddy current) conductivity
  • Derived parameter: Supply and calibration of reference materials. Calibration of AC conductivity instruments
  • Suitability: Contact used to determine the state of heat treatment and effects of work hardening of materials/components in service. Used for detection of surface defects such as cracks (rail) and insight into other material effects including stress.

DC conductivity - using various 4-point probe techniques

  • Scope/range: 1 MS/m to 60 MS/m (3.45 %IACS­ to 103 %IACS), UKAS accredited in 2023
  • Measured parameter: DC (bulk) resistivity
  • Derived parameter: Resistivity of bars, block, wires etc.
  • Suitability: Contact, used to support material characterisation for above in accordance with ASTM and MIL standards.Using various 4-point probe techniques

Magnetic techniques

Magnetic field - meters, gaussmeters, magnetometers, indicators, field @ distance, on-site surveys

  • Scope/range: 1 nT to 3 T (0.8 mA/m to 2.4 MA/m), UKAS accredited
  • Measured parameter: DC magnetic flux density (T) and DC magnetic field strength (A/m)
  • Derived parameter: Calibration of instruments. Characterisation of magnetic materials.
  • Suitability: Variety of applications where magnetic field of environment, components or systems is required. Systems can be used to characterise non-magnetic magnetic materials (EMC/magnetic cleanliness).

Relative magnetic permeability

  • Scope/range: µr = 1.0002 to 2.5 UKAS accredited
  • Measured parameter: DC relative magnetic permeability µr
  • Derived parameter: Supply and calibration of reference materials. Calibration of instruments and indicators.
  • Suitability: Used to characterise material properties during fabrication and in use. Used to assess pipe condition prior to welding, or the effect of stress on materials/components in use.

Magnetic induction spectroscopy

  • Scope/range: Swept low frequency ~0.5-5 kHz depending on material, on multi-layered structures with wall thickness up to ~1 cm
  • Measured parameter: Impedance from eddy currents
  • Derived parameter: Wall thickness, corrosion
  • Suitability: Contact method although some lift-off is acceptable. Inspection of wall thickness of multi-layered structures e.g. corrosion under insulation challenges.

Contact force method

  • Scope/range: Ferromagnetic materials such as steels, tailored to suit specific requirements.
  • Measured parameter: Force required to disengage magnet from material surface
  • Derived parameter: Magnetic permeability, related to material stress
  • Suitability: Contact - Quantification of magnetic permeability provides information on material stress and can be used in asset integrity decision making

Large standoff magnetometry

  • Scope/range: Ferromagnetic materials such as steels
  • Measured parameter: Magnetic field
  • Derived parameter: Wall thickness/depth of corrosion, other flaws e.g.  cracks and dents, influence of stress
  • Suitability: Non-contact – measurement of magnetic field a distance from a structure. Can provide validation of commercial equipment or investigation into suitability for specific applications. NPL facilities can be used to create well-defined reference flaw geometries.

Find out more about magnetic NDT research

Microwave and terahertz

Microwave 2D scanner

  • Scope/range: 10.5 GHz (λ ~ 3 cm), 24.1 GHz (λ ~ 1.3 cm), 34.0 GHz (λ ~ 0.9 cm) probes
  • Measured parameter(s): Changes to standing waves’ superposed amplitudes due to differences in dielectric constant.
  • Derived parameter: Defect detection – voids, delaminations.
  • Suitability: Non-contact large area imaging (x/y scanning table). Non-conductive samples only, volumetric interrogation.

Find out more about microwave 2D scanning of advanced materials

Electrical sheet resistance, 2D conductivity and mobility

  • Scope range: Non-contact measurement of microwave sheet resistance/ conductivity of thin films
  • Measured parameter: Perturbation of high quality factor microwave dielectric resonator, measuring centre frequency and linewidth changes.
  • Derived parameter: Sheet resistance, 2D conductivity and mobility 
  • Suitability: Non-contact. Thin films, buried conducting layers on relatively low loss substrates. 

Admittance methods

  • Scope/range: 1 kHz to 10 MHz, Permittivity < 10. Solids and liquids.
  • Measured parameter: Impedance
  • Derived parameter: Complex permittivity
  • Suitability: Contact. Solid specimens must be machined to size.

VNA coaxial probe/sensor methods

  • Scope/range: 30 MHz to 50 GHz. Malleable solids and liquids.
  • Measured parameter: Complex reflection coefficient
  • Derived parameter: Complex permittivity
  • Suitability: Contact. Malleable materials do not require precise machining. Good for foodstuffs and polar liquids. Can, with difficulty, be embedded in pipes.

VNA transmission-line methods

  • Scope/range: 100 MHz to 750 GHz Permittivity < 10. Solids, powders and grains.
  • Measured parameter: Complex transmission and reflection coefficients
  • Derived parameter: Complex permittivity
  • Suitability: Usually contact, but non-contacting methods are possible. Solid specimens must be machined to size. Gels, powders and grains can be measured.


  • Scope/range: 1 MHz to 144 GHz solids and powders.
  • Measured parameters: Resonant frequency and Q-factor
  • Derived parameter: Complex permittivity
  • Suitability: Contact. Solid specimens must be machined to size. Some techniques can measure piped liquids.

Terahertz time-domain spectroscopy - transmission

  • Scope/range: 100 GHz to 5 THz Solids, liquids, powders.
  • Measured parameters: Transmission time and loss.
  • Derived parameter: Complex permittivity
  • Suitability: Non-contact. Solid specimens must be flat and plain-parallel. Liquids and powders must be contained in suitable cells. Can measure porosity.

Terahertz time-domain spectroscopy - reflection

  • Scope/range: 100 GHz to 5 THz solids.
  • Measured parameter: Reflectivity
  • Derived parameter: Spectral reflectivity, real permittivity
  • Suitability: Non-contact. Specimens must be flat.

Terahertz frequency-domain spectroscopy – transmission

  • Scope/range: 50 GHz to 2 THz solids, liquids, powders.
  • Measured parameter: Transmission loss
  • Derived parameter: Loss coefficient (approximate)
  • Suitability: Non-contact. Solid specimens must be flat and plain-parallel. Liquids and powders must be contained in suitable cells.

Terahertz time-domain spectroscopy imaging - transmission or reflection

  • Scope/range: 100 GHz to 5 THz Solids, liquids, powders.
  • Measured parameters:Transmission time and loss.
  • Derived parameter: Complex permittivity
  • Suitability: Non-contact. Solid specimens must be flat and plain-parallel. Liquids and powders must be contained in suitable cells. Spatial resolution 1-2 mm.

Find out more about terahertz consultancy

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