ISO 8203-4:2025

International
Standard
ISO 8203-4
First edition
Fibre-reinforced plastic
2025-09
composites — Non-destructive
testing —
Part 4:
Laser shearography
Composites plastiques renforcés de fibres — Contrôle non
destructif —
Partie 4: Shearographie laser
Reference number
© ISO 2025
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Basic method . 5
4.1 Principle of operation .5
4.2 Measurements . .8
4.3 Stress excitation methods .8
5 Equipment . 9
5.1 Shearing camera .9
5.1.1 General .9
5.1.2 Imaging optics .9
5.1.3 Shearing optics .9
5.2 Stress excitation .9
5.3 Camera and FRP component supports .10
5.4 Coherent illumination .10
5.5 Data capture and analysis .11
5.6 Calibration or reference defect artefacts .11
5.7 Equipment qualification .11
6 Specimen preparation . .11
6.1 Cleaning .11
6.2 Visual inspection .11
6.3 Surface coating . 12
6.4 Reference markers . 12
7 Inspection procedure .12
7.1 General . 12
7.2 Camera settings . 12
7.3 Image projection and shear calibration . 13
7.4 Illumination . 13
7.5 Exposure . 13
7.6 Shear selection . .14
7.7 System verification .14
7.8 Stress application .14
7.9 Shearogram acquisition . 15
7.10 Image analysis/processing . 15
7.11 Shearogram interpretation .17
7.12 Resolution/sensitivity .18
8 Image and defect analysis . 19
8.1 Defect sizing .19
8.2 Depth determination .21
8.3 Defect locating .21
9 Test report .22

iii
Foreword
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The procedures used to develop this document and those intended for its further maintenance are described
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of ISO document should be noted. This document was drafted in accordance with the editorial rules of the
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This document was prepared by Technical Committee ISO/TC 61, Plastics, Subcommittee SC 13, Composites
and reinforcement fibres.
A list of all parts in the ISO 8203 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
Introduction
Laser shearography can be used to detect and measure a wide range of manufacturing and in-service defects
in fibre-reinforced plastic (FRP) materials. The technique has the advantages of being fast, non-contact and
suitable for inspecting large areas, with portable, real-time, and in-situ measurement capabilities. Laser
shearography enables detection of imperfections/discontinuities which are detrimental to the performance
or ultimate strength of the material. This document specifies the principal method for laser shearography
non-destructive testing (NDT) of FRP composites via monitoring of out-of-plane surface displacement
gradients, using coaxial illumination and observation perpendicular to the part surface. Defect detection
relies on local influences on the global surface deformation from sub-surface anomalies under the selected
loading regime.
v
International Standard ISO 8203-4:2025(en)
Fibre-reinforced plastic composites — Non-destructive
testing —
Part 4:
Laser shearography
1 Scope
This document specifies the principal method for out-of-plane laser shearography non-destructive testing
(NDT) of fibre-reinforced polymer (FRP) composites.
This document is applicable to both monolithic and sandwich FRP laminate constructions, with or without
curved surfaces, fibre-reinforced thermoset and thermoplastic matrix composites incorporating uni- or
multi-directional reinforcements in either a continuous or discontinuous format; including but not limited
to woven fabrics, stitched fabrics, short fibre, or particulate filled, honeycomb or foam cores, as well as
combination or hybrid reinforcements.
This document is not applicable to variations on this basic configuration.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
IEC 60825-1, Safety of laser products — Part 1: Equipment classification and requirements
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
calibration target
deformable membrane of known and adjustable out-of-plane displacement which provides a reproducible
reference fringe pattern for assessing the shearography system operation, stability, limits, and sensitivity
3.2
coherent light source
monochromatic beam illumination having uniform phase over a length known as the coherence length
3.3
decorrelation
loss of phase data caused by test part deformation exceeding the optical measurement resolution or
excessive relative motion of the part in either translation, expansion or tilt between images or phase steps

3.4
defect
any structural imperfection or localised material inhomogeneity in an FRP composite material which affects
the expected ultimate mechanical performance
EXAMPLE Sandwich core crush, sandwich core separation, sandwich skin disbond, inclusion, delamination,
porosity, impact damage, cracking, voids, moisture, or fibre waviness/wrinkling.
3.5
displacement derivative
displacement gradient
spatial rate of change of displacement given by the relative deformation change between pairs of target
surface points separated by a distance defined by the shear vector, directly related to strain
3.6
filtering
mathematical process for reducing/smoothing image noise and removing uniform background fringes, used
to enhance defect detection in laser shearography
3.7
fringe contrast
measure of interference quality; it is reduced by low illumination intensity, high signal to noise ratio,
decorrelation (3.3) and/or restriction of the CCD bit depth available to fringes due to excess brightness of
ambient lighting
3.8
fringe pattern
light and dark speckle intensity bands resulting from constructive and destructive interference in the
shearogram
Note 1 to entry: Each intensity band corresponds to paths of equal change in displacement gradient or phase shift.
3.9
indication
notable feature in the uniform background phase map which suggests the presence of a defect
Note 1 to entry: These can be localised concentrations, severe gradients, discontinuities, or other anomalies in the
fringe/phase pattern which cannot otherwise be attributed to the expected underlying structure
3.10
interferogram
interference pattern which results from imaging the target surface speckle pattern (3.25) with a shearing
element to create an overlapping double image with an offset defined by the shear vector
Note 1 to entry: The interferogram defines the surface interference map of the part for a given load/deformation
condition.
3.11
interferometer
device for interfering coherent light
EXAMPLE The Michelson interferometer which consists of two mirrors and a beam splitter combining light at the
image plane.
Note 1 to entry: For purposes of laser shearography, one mirror tilts, acting as the shearing element.
3.12
laser shearography
method of measuring changes in the out-of-plane surface displacement gradient of a component under a low
applied stress compared to a reference condition using self-referencing, common path interferometry
Note 1 to entry: The mathematical comparison between different stress states, and therefore different surface
deformation states, enables defect identification.

3.13
FRP laminate
laminate made of only one material system in a single process, as opposed to hybrid materials containing
mixed fibre types or secondary bonded materials which contain adhesive bondlines
3.14
natural defect artefact
NDA
FRP component containing naturally formed defects either in-service or by deliberate impact/overstressing
of a test part
Note 1 to entry: Enables the inspection setup to be optimised for a given material/lay-up/geometry/surface finish
combination with highly representative defects of unspecified dimensions.
Note 2 to entry: NDAs can be investigated by other NDT methods prior to shearography to enable comparisons of
results.
3.15
optically rough surface
roughness of the order of the illuminating light wavelength
3.16
out-of-plane displacement
deformation of the test part surface along an axis which is normal to the plane of the surface
3.17
phase shift
change in the phase position of a wave, in radians
3.18
projection
spatial calibration
calibration of the shearogram image dimensions from pixels to engineering units for accurate dimensioning
of indications in the shearogram using software callipers
Note 1 to entry: Achieved by either manual definition of the measured versus pixel length of an object in the field of
view or automatically using in-built laser pointers of fixed separation.
3.19
reference defect artefact
RDA
FRP component containing artificially created defects designed to be representative of those occurring
naturally in composites
Note 1 to entry: The location and size of defects are well-controlled such that it can be used to evaluate the sensitivity
and limits of detection for a given material/defect/surface finish combination, optimise the inspection setup or to
investigate “fingerprint” indications for use in identifying the nature of unknown defects during inspection; once
characterised, an RDA can then be used as a standard to test repeatability of the inspection system and thereby
effective and consistent functioning of the inspection equipment.
3.20
sandwich FRP laminate constructions
comprised of relatively thin FRP laminate skins adhesively bonded either side of a thick but lightweight
material core (usually foam or honeycomb) to form a very rigid structure
3.21
shear calibration target
object with a high contrast geometrical pattern used to verify image offsets generated by the shearing element
Note 1 to entry: Such targets incorporate visual markers with known geometrical offsets of both angle and size.

3.22
shearing camera
combination of optical imaging system (lens, aperture, focus, zoom), shearing element/device (prism, wedge
or Michelson interferometer with tilting mirror) and image sensor (CMOS or CCD for older shearography
systems)
Note 1 to entry: The system can also contain other optical components e.g. filters, polarisers, phase stepping mirrors, etc.
3.23
shear vector
vector defining the magnitude and direction of separation of overlapping point pairs on the object surface which
are imaged onto a single point in the shearogram; this double image offset is produced by the shearing element
3.24
shearogram
result of a laser shearography measurement, namely the subtracted image giving the differential phase shift
between interferograms from the loaded and reference states, and therefore represents the change in the
out-of-plane displacement gradient of the surface relative to the shear vector
Note 1 to entry: Defines the change in the surface interference map of the part between different load/deformation
conditions and can be presented as either speckle fringe patterns, phase maps or unwrapped phase maps.
3.25
speckle pattern
statistically random granular intensity pattern consisting of small uncorrelated regions (speckles) produced
by the interference of diffuse scattered coherent light reflected from an optically rough surface
Note 1 to entry: The intensity of a speckle at a given point results from the coherent superposition of contributions
from many independent regions on the surface with different path lengths.
3.26
stress excitation
means of applying controlled, low-level loads to incrementally change the deformation of the part surface for
laser shearography inspection; usually achieved by applying heat, pressure, vacuum, or direct mechanical
static loads
3.27
phase stepping
temporal phase shifting
method of determining phase values for a given stress state; multiple interferogram intensity measurements
are carried out for each stress condition by the introduction of defined regular phase shift intervals
between each measurement, usually by changing the light path length by some fraction of the illuminating
wavelength, often π/2 known as 4 stepping (or 2π/3 called 3 stepping)
Note 1 to entry: Phase is then determined from the four-quadrant inverse tangent (arctan) function; as the multiple
measurements are captured consecutively, it takes a finite time to perform the live phase determination for each
condition.
3.28
wrapped phase image
shearogram where phase shift of the inspected surface is represented by a colour or greyscale conversion
modulated by 2π and displayed in the range -π to +π; contains discontinuous jumps due to the arctangent
function used for phase determination
3.29
unwrapped phase image
shearogram where the phase shift of the inspected surface is represented by a colour or greyscale conversion
demodulated and displayed in the range -nπ to +nπ; results in a continuous phase signal profile

4 Basic method
4.1 Principle of operation
The basic principles of the laser shearography inspection method are described. An optically rough FRP
inspection surface is illuminated by diffuse monochromatic laser light. The light is scattered from the test
surface and reflected into the shearing camera to create speckle interference at the image plane. This is
shown schematically in Figure 1; the lens may be located between the object and the shearing mirror or,
alternatively, between the shearing mirror and imaging plane.
Key
1 test object
2 lasers
3 illumination
4 imaging optics
5 shearing optics
6 lens
7 beam splitter
8 piezo driven mirrors
9 image sensor
10 PC software control and data acquisition / analysis
Figure 1 — Michelson interferometer based shearography system
Shearing optics in the camera are adjusted to produce a second, offset speckle image of the target, also
focussed onto the image plane. As a result, every point on the target surface is paired with another point
which is laterally displaced by the shear vector, shown in Figure 2. For every point in the field of view, light
from the overlapping pairs of points in the sheared images interfere to generate an interferogram which
characterises the object surface state. The intensity of the superposed light waves depends on their relative
phase difference, as depicted in Figure 3.

Key
1 shear vector definition
2 unsheared images
3 sheared images for shear vector offsets +45° at 25 mm and -60° at 10 mm
Figure 2 — Convention for shear vector definition and examples of overlapped images for two
different shear vector offsets
Key
1 interfering overlapping points P and P’
P original image
P’ sheared image
Figure 3 — Interference of light waves from overlapping sheared points within an interferogram
A small proof load is then applied and the new interferogram exhibits the changed optical path length, and
corresponding phase shift, caused by the change in the relative surface displacement at the overlapped
points. The phase shift is therefore proportional to the out-of-plane surface displacement derivative (slope
or gradient) with respect to the shear vector, shown in Figure 4.

Key
1 phase shift between original (reference) and deformed states at points P and P’
2 phase shift (Δφ)
Figure 4 — Measured phase shift (Δφ) between the original and deformed interferograms for the
same point in the field of view
Each interferogram can then be compared to that from a previously stored reference condition, either an
unloaded or different prior load/deformation state, to create a real-time relative phase map shearogram, as
seen in Figure 5.
Key
1 initial speckle interferogram (phase state φ )
2 deformed speckle interferogram (phase state φ )
3 resulting wrapped phase change map (Δφ = φ - φ )
2 1
Figure 5 — Initial and deformed speckle interferograms and resulting wrapped phase change map
between ±π for a shear vector angle of 45°
The resulting shearogram, which exhibits discontinuous steps every 2π (wrapped), can then be converted
into the continuous profile of the deformation gradient. This unwrapped phase map displays the overall
direction of surface displacement, either toward or away from the shearing camera, shown in Figure 6.

Key
1 fringe/intensity difference
2 wrapped phase shearogram
3 unwrapped phase shearogram
Figure 6 — Shearogram representations from actual data
4.2 Measurements
The method is performed with single-sided access where illumination and interrogation are applied on the
same inspected surface. Stress excitation, suitable for generating deformation on the inspected side, shall be
introduced via the optimum route for the component and application. For this method of operation, different
measurements are considered:
— Magnitude map of the phase shift of reflected light from the inspected surface resulting from stress-
induced deformation changes. This measurement is approximately related to the out-of-plane
displacement gradient of the inspected surface. This type of measurement may be used for defect
detection and characterisation.
Measurements are ideally performed by comparison with a reference standard measured before or
after the test component to verify system operation.
— Effective defect size.
NOTE The size observed can be influenced by material properties, defect type and defect depth,
4.3 Stress excitation methods
Suitable excitation methods are required to induce the required changes in surface displacement of the
composite part. Several proven approaches are available.
— Vacuum – for large, flat or gently curving structures where close contact allows a transparent vacuum
hood to be sealed onto the surface to create a local partial vacuum to generate surface flexure.
— Pressure – for pipes or closed vessels where internal pressure changes generate global surface bending.
Alternatively, a vacuum chamber can also be used.
— Thermal – for any shape of part where three-dimensional thermal expansion gradients occur in the
region of heating producing bending.
— Mechanical – for any part where small mechanical loads can be readily applied in either tension,
compression, flexure, shear or torsion.

5 Equipment
5.1 Shearing camera
5.1.1 General
The shearing camera shall consist of:
— the optics required to recreate an image of the target in the image plane,
— a shearing element to generate a second identical image onto the image plane,
— any additional components required to create a common path interferometer where the reference and
secondary images interfere after following approximately equivalent distances.
These can include mirrors, filters, beam-splitters, polarisers, etc., as necessary.
In accordance with the manufacturer’s instructional guide, the optical surfaces should be regularly checked
for dirt or damage and cleaned with a soft lens cloth as necessary to remove dust and debris which would
otherwise degrade image quality and inspection results. Damaged optics shall be noted and replaced/
repaired. All lenses, filters and other exposed optical and connection surfaces shall be kept covered or
stored when not in use.
5.1.2 Imaging optics
The camera shall be equipped with suitable mounting connections for interchanging imaging lenses. Screw
or bayonet fittings are most common, but the connection shall enable appropriate adaptors to be used for
any front-end lens type.
Either fixed or variable zoom lenses should be used provided these have fully adjustable aperture and
focussing controls to enable optimum imaging of the inspected part with the selected field of view.
The camera shall be capable of capturing the images focussed onto the image plane with a user-defined
acquisition time (exposure time).
5.1.3 Shearing optics
The camera shall be equipped with shearing optics capable of generating a second, offset and overlapping
image of the target object onto the image plane where it can interfere with the original undeviated image.
NOTE Examples include an optical wedge, biprism or Michelson interferometer with tilting mirror.
The shear optics shall be adjustable in angle/magnitude shear vector settings. The shear vector controls
shall be computer-controlled and be verified independently using the double image generated.
5.2 Stress excitation
The stressing system shall be capable of introducing appropriate proof loads to the inspected part, sufficient
to create small surface deformations, without causing damage.
All systems shall supply variable output levels to allow operator control of the stress applied to the target
component. For example, vacuum loading stress is controlled by the pressure steps available and thermal
loading by the heating time period.
For differential pressure, vacuum or direct loading methods, the system shall be able to hold a steady state
at each pre-selected level for the duration of a measurement step.
NOTE 1 Inline pressure gauges or load cells provide independent corroboration of the excitation levels achieved.

For thermal loading, a suitable defined energy/power rating device shall be used to provide constant heat
output prior to or during each measurement step.
NOTE 2 Thermistors, thermocouples or IR imaging cameras can provide independent corroboration of the
excitation levels achieved.
Suitable systems include quartz or halogen lamps, hot air guns, pneumatic or hydraulic pressure supplies,
optically transparent window hoods with a peripheral seal for partial vacuum application, direct mechanical
actuation devices to apply tension, compression, flexure, shear or torsion as appropriate.
Operator safety shall be considered for each system according to the risks presented and appropriate
precautions taken, and training provided.
All devices shall be regularly checked for function and damage and replaced/repaired before any further
measurements are attempted.
5.3 Camera and FRP component supports
The camera shall be stably supported by a tripod, framework or gantry which enables it to be positioned
normal to the part under inspection, or vice versa.
Where the FRP part to be inspected is not easily supported, i.e. not floor standing or not integrated as part
of a larger assembled structure, a separate fixture shall be used for the purpose.
The camera/FRP support systems shall permit initial adjustments of the position of the part or the camera
relative to each other, facilitating adjustments to the captured field of view and the focal position.
A minimum of one of the support systems shall provide adjustable angular movement in orthogonal planes
for optimised inspection alignment (focal plane tilt correction).
The support systems shall be sufficiently rigid and isolated from ambient vibration such that they minimise
unwanted relative movement between the camera and the part during the inspection process, a significant
source of image deterioration and decorrelation.
5.4 Coherent illumination
The FRP target surface shall be diffusely illuminated with monochromatic coherent light.
The monochromatic illumination provided shall comprise one or more coherent light sources. These shall
have orientation adjustment capability (independently or via attachment to the camera housing), such
that the imaging and illumination are both approximately normal to the part surface. This shall include
waveguide optics to steer the light from the source to expose the FRP target.
NOTE Laser diodes are commonly used.
The illuminating beam shall be expanded to improve areal coverage of the inspected surface. This should be
achieved using either variable or fixed opening angle, divergent or collimating optical lenses.
The coherent light sources shall provide sufficient power to effectively illuminate the part surface after beam
expansion and scattering, while still generating a good image signal at the image plane. Where possible,
controllable power supplies shall be provided to allow fine tuning of the part illumination. The laser power
and wavelength should be appropriate to the intended application and environment.
Safety shall be considered for the light sources according to the risks presented to the operator and others
in the working area. Appropriate precautions (local rules) shall be taken, and training provided according to
the required legislation.
Lasers may present serious optical radiation exposure hazards to eyes and skin or fire hazards depending
on their classification. The laser safety class shall not exceed Class 3R as defined within IEC 60825-1. Lasers
shall always be switched off when unattended.

All light sources and their housings shall be regularly checked for function (power and light distribution)
and damage. Malfunctioning components shall be replaced before any further measurements are attempted.
5.5 Data capture and analysis
The system control software shall store a permanent record of shearogram images and raw interferogram
data during measurement to allow post-processing at any time.
The software shall enable naming, recall and display of shearograms from any loaded state compared to its
reference for review, analysis and/or test reporting after measurements are completed. This can include
timestamps, time intervals and/or stress excitation set level for each captured dataset.
Software may enable further processing such as image subtraction, unwrapping and rewrapping,
dimensional measurement, and suitable image analysis (e.g. filtering, contrast enhancement, smoothing,
additive and/or averaging, etc.), though this can be performed separately and offline.
5.6 Calibration or reference defect artefacts
Suitable targets shall be prepared and maintained for regular system checks. These shall be durable
constructions that are resistant to creep, ageing, etc. that would otherwise change their physical properties
over time. These provide consistent, well-defined out-of-plane displacements which are used to assess the
shearography system operation.
Calibration targets usually consist of a deformable membrane supported in a rigid frame with a ball
micrometer located at the centre to apply a well-controlled known displacement. Alternatively, a solid
block with one or several holes cut partially through, leaving a small remnant wall thickness, also act as
deformable membranes where pressure or vacuum application can deform several connected membranes
simultaneously. Other similar designs are acceptable provided the displacement generated is repeatable and
the target material does not deteriorate over time.
Reference FRP targets containing artificially created defects which are designed to be representative of
those occurring naturally in composites can also be used as suitable targets. The location and size of defects
in the RDA are well-controlled such that it may be used to evaluate the sensitivity and limits of detection
for a given material/lay-up/defect/surface finish combination, in order to investigate and optimise the
inspection parameters. However, once characterised, the RDA can then be used as a standard to test the
repeatability of the inspection system.
5.7 Equipment qualification
Records shall be kept on file of equipment checks/calibrations/verifications carried out, the operator and
the date. The evaluation of performance shall be carried out periodically a minimum of every 6 months
and immediately following any incidents which are capable of producing damage or severe jarring to any
component of the test facility.
6 Specimen preparation
6.1 Cleaning
The test part shall be wiped clean prior to inspection to remove any loose debris/liquids/contamination
which can affect the sensitive surface measurements during inspection. It is essential that the surface
condition does not change between capturing data for the reference and loaded states.
6.2 Visual inspection
The FRP part (or reference standard/calibration target) surface shall be visually examined prior to
shearography inspection. A record of the test part dimensions, as well as position and size of all notable
features and damage shall be maintained.

6.3 Surface coating
In cases where the surface is too smooth (optical roughness is required to generate speckle), too reflective/
shiny (areas of light saturation), too light/dark (exposure time controls) or does not efficiently absorb
heat (where thermal loading is introduced), a suitable coating may be used to improve shearography
measurements, provided it is agreed upon between the parties concerned.
The coating shall be stable under the selected heating/loading regime, easily removable, well-bonded onto
the part surface to accurately follow small deformations arising there, but without causing damage to the
underlying surface finish or material structural performance.
Any coating applied shall be allowed to dry completely prior to shearography inspections being performed.
A record of coating used shall be maintained.
Recommended coatings include washable graphite loaded spray, peelable rubberised spray paint, chalk
developer spray or combinations thereof.
6.4 Reference markers
A reference marker, visible in shearograms, should be placed into the image frame directly on or close to
the test surface. This acts as an independent location marker on large parts to identify inspected sections or
localise defects relative to a reference point. Removable surface tapes attached to the part at the periphery
of the image field of view is effective for this purpose or graticules on smaller image fields.
7 Inspection procedure
7.1 General
The specifics of the setup for any particular measurement are difficult to prescribe since they depend on
the application, the camera lenses available, the FRP part size, the resolution and sensitivity required etc,
making each measurement unique. Several slightly modified experiments can be performed to successfully
detect all defects accurately.
The following information provides basic guidance and consideration/precautions to take into account, but
the setup selections are ultimately subject to the skills, experience and decisions of the operator.
7.2 Camera settings
Select a suitable imaging lens for the camera. Zoom lenses accommodate variable focal lengths and
magnifications.
Position the camera and/or part facing relative to each other. The camera may be set a few degrees off
orthogonal to the target surface to minimise reflections.
Adjust the objective zoom to select the field of view (magnification) and the focus for optimising the image
clarity/edge definition.
NOTE 1 It can be useful to have a high contrast optical marker or high contrast grid covering the FRP part (or located
in the same plane) as a means of optimising the focus, assessing distortion, and determining the actual field of view.
Adjust the aperture. A smaller aperture (greater f-number) increases the depth of field, which can be useful
for optimising the focal zone especially for curved parts but reduces the illumination. This side effect can be
counteracted by greater exposure times, laser power and/or sensor gain to retain fringe contrast.
NOTE 2 The changes to the depth of field can be seen with the same high contrast grid, where distortions can often
lead to fish-eye effects leaving the image border out of focus compared to the centre until the aperture is reduced by 2
or 3 stops.
However, aperture selection also influences the spatial frequency of the speckles and therefore the spatial
averaging effects on the image sensor. Reducing the aperture lowers the spatial resolution by increasing the
speckle size according to Formula (1):
lλ
d =12, 2 (1)
s
D
where
d is the speckle diameter;
s
l is the distance from the lens to the image plane;
λ is the illuminating wavelength;
D is the aperture diameter.
7.3 Image projection and shear calibration
Once the desired optical image characteristics have been selected, calibration of the image dimensions
from pixels to engineering units is performed. This enables the accurate definition of shear offsets, defect
dimensions in the shearogram and distances measured using software callipers/gauges.
This requires correlating the pixel length to the real measured length between two defined points in the
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