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Developments in Dimensional Metrology
in X-ray Computed Tomography at NPL
Wenjuan Sun and Stephen Brown
10th May 2016
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Possible factors influencing XCT measurements
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Components Influencing variables Possible effects
X-ray source voltage
current
focal spot
Inherent filtration
(materials thickness)
Stability of focal spot
spectrum shape
stability of intensity
affect the resolution
affect the X-ray spectrum
affect projection image
Detector exposure, Image lag
pixel size, resolution
linearity and efficiency
Others(dynamic range etc.)
affect image quality
Mechanical parts alignment of the x-ray source, the detector
and the sample stage.
scale (for example, source to detector
distance, source to object distance)
deviations cause partially blurred
reconstructions
cause error in pixel value
Environmental temperature, humility, etc.
Software reconstruction algorithm
surface determination algorithm
correction algorithms: beam hardening,
scattering corrections.
affect dimensional measurements
Operators measurement and analyses approach can significantly affect the results
Most frequent questions: • What are the best settings? (voltage, current, average of images, etc.)
• Should we use physical filters or not? What are the differences of
having different thickness of filters?
• What are the influences of sample orientation and position?
• What is the influence of shading correction? Linear or nonlinear
correction?
• How should we evaluate the quality of measurements?
• What is the smallest feature that can be measured?
• …
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Influence factors: X-ray source (focal spot)
• Stability during scan
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target
Part of X-ray
tube that
consists of X-Y
coils and focus
coil
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Influence factors: X-ray source (focal spot)
• Stability during scan
Material:
Tungsten carbide
Sphere diameters:
Sphere 1: 0.7954 mm
Sphere 2: 0.7951 mm
Centre to centre distance:
0.9795 mm (k = 2)
Reference sample calibrated by
traceable contact CMM at NPL.
Two-sphere reference sample
Projection images
Boundary
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Cold start
(without system warm up)
Hot start
(with system warm up)
Sphere centre drift
along x-axis
Sphere centre drift
along y-axis
Sphere separation
(reflect variation of
X-ray source along
beam axis)
Influence factors: X-ray source (focal spot)
Reference:
Sun W, et al. 2015 A preliminary study of characteristics of X-ray computed tomography, Proc. of Digital
Industrial Radiology and Computed Tomography (Ghent, Belgium)
Flay N, et al. 2015 Investigation of the focal spot drift in cone-beam industrial X-ray computed tomography,
Proc. of Digital Industrial Radiology and Computed Tomography (Ghent, Belgium)
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Detective quantum efficiency (DQE): describes the ability of an X-ray detector to transfer
the signal-to-noise ratio (SNR) from the radiation field to the resulting digital image
DQE results of the scintillation detector at 70 kV (top-left), 90 kV (top-right), 120 kV
(bottom-left) and 150 kV (bottom-right). lp/mm is line pair per millimetre
Influence factors: detector
Reference: Sun W, et al. 2016 Characterisation of a scintillation flat panel detector employed by an X-ray
computed tomography system: measurements of MTF, NPS and DQE, Proc. of EUSPEN (Nottingham)
Spatial resolution
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0
5
10
15
0 200 400 600
Sp
ati
al
reso
luti
on
: µ
m
Current:µA
Influence of magnification Influence of current
0
5
10
15
20
0 100 200 300
Sp
ati
al
reso
luti
on
: µ
m
Voltage: kV
Influence of voltage
1
10
100
1000
1 10 100Sp
ati
al
reso
luti
on
: µ
m
Magnification
Slanted edge sample
50 100 150 2000
5
x 104
pixel
gra
y v
alu
e
-30 -20 -10 0 10 20 30-0.2
0
0.2
0.4
0.6
0.8
1
1.2
x
y
data
fitted curve
Edge response function
Line spread function
0 5 10 15 20 25 300
0.2
0.4
0.6
0.8
1
m
Modulation transfer function
(MTF)
Reference: Sun W, et al. 2015 A preliminary study of characteristics of X-ray computed
tomography, Proc. of Digital Industrial Radiology and Computed Tomography (Ghent, Belgium)
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Geometrical error modeling for
coordinate measurements by XCT
Modelling volumetric errors due to detector angular misalignments
Modelling radiographic binning errors due
to detector angular misalignments
Ref: Ferrucci M, et al. 2016 Evaluating the effects of detector angular misalignments on simulated
computed tomography data Precis. Eng.
Other factors considered
• Scattering
• Beam hardening
• Cone beam error
• Reconstruction algorithm
• Surface determination algorithm
• More…
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Scale check – 1D distance verification
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• Traceability: Calibrated Ball
bar
• Scale: Distance is
independent on threshold
• Downfall: measured at a fixed
height, therefore alignment
issues not highlighted
d
Ball separation vs height
85.55
85.555
85.56
85.565
85.57
85.575
85.58
85.585
180 190 200 210 220 230 240 250 260 270 280 290 300
Y position/mm
Sp
he
re s
ep
era
tio
n/m
m
Ball plate – 2D distance verification
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• Ball plate manufactured from
6 mm ruby spheres, 25 in total
• Allows measurements
between spheres in multiple
directions
• Verification of system at
different scales
• Alignment issues
Holeplate– investigation
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• Multi direction distances
• Bi-directional and uni-
directional measurements
of hole plate, immersed
inside NPL cam.
• Multi-machine
measurements
Bi-directional
Uni-directional
XCT vs Other measurement techniques
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Advantages Disadvantages
Tactile Profile/surface
data
Can be traceable
Contact method, can damage
surface
External surface only
Angle limitation
Time consuming for large area
Optical Surface data
Noncontact
External surface only
Angle limitation
Time consuming for large area
Influence of some systematic
errors on measurement not
fully understood
XCT Volumetric data
(both external
and internal)
Noncontact
Non-invasive
Systematic errors
Large amount of data
Influence of systematic errors
on measurement not
understood
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Dimensional summary - Verification
Standards
Tactile •CMMs ISO 10360
Optical •VDI/VDE 2634
XCT •VDI/VDE 2630, (ISO
10360 Part XCT under
development)
Performance verification
External dimensions •Surface roughness may
affects all measurement
systems
•Young’s modulus affects tactile
but not optical or XCT
•Optical properties may affect
optical techniques but do not
affect tactile or XCT
Internal dimensions • Surface has to be
accessible for tactile and
optical
• XCT has the ability to
measure both surfaces and
material interfaces, through
threshold values, however,
not traceable.
The surface roughness of the AM parts, limit the ability to
make confident dimensional measurements.
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An example of NPL designed test artefact
• Corkscrew distortion
during build
• Prismatic simple
geometric structures
• Equivalent internal
structure
• Allow traceability
• Effects of beam
hardening
• AM capability
• Parallel flat side
faces
• Include cube and
cylinder
• Internal cube and
cylinder
• Tooling balls
• External cylinders of
a variety of material
• Conventional
manufactured
Manufactured from XCT compliant
materials: AlSi10Mg and Al for
conventional
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Overview of dimensional tests carried
out using contact CMM • Measurements of large flat areas on the
AM surface.
• Evaluation of best fit plane
• Corkscrew distortion
• Measurement of cylinder at different
heights, above the plane ‘A’:
• (18.5, 21, 23.5, 26, 28.5) mm
• Best fit circle and standard
deviation of data
• External (and internal proposed work)
comparison measurements.
• AM allows easy to build but,
unfortunately, sometimes difficult
to measure.
A
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Dimensional measurements - Summary
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• Comparison of data from CMM,
XCT and optical.
• Sphere diameter and distance
• Circle diameters and standard
deviation (Std)
18.5
21.0 23.5
26.0 28.5
C
B
A
Position
of scans CMM Std
CMM -
Optical Std
CMM -
XCT Std
18.5 14.070 0.037 0.206 0.030 0.165 0.012
21.0 14.051 0.036 0.232 0.029 0.177 0.012
23.5 14.066 0.043 0.246 0.030 0.192 0.013
26.0 14.052 0.040 0.202 0.035 0.163 0.012
28.5 14.059 0.034 0.201 0.034 0.177 0.013
Item CMM CMM-
Optical
CMM-
XCT
A(Dia) 9.991 0.079 0.005
B(Dia) 9.991 0.188 0.006
C(Dia) 9.992 0.064 0.006
A-B 68.381 -0.063 0.014
A-C 50.769 -0.026 0.013
B-C 57.820 -0.025 0.008 All units
are in mm
Questions…. if time permits
Contact:
The National Measurement System is the UK’s national infrastructure of measurement
Laboratories, which deliver world-class measurement science and technology through four
National Measurement Institutes (NMIs): LGC, NPL the National Physical Laboratory, TUV NEL
The former National Engineering Laboratory, and the National Measurement Office (NMO).
The National Measurement System delivers world-class
measurement science & technology through these organisations
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