ASM Metals HandBook Vol. 17 - Nondestructive Evaluation and Quality Control
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about of a fringe are readily obtained with ultimate sensitivities to about achievable under near ideal conditions.
(Heterodyne sensitivities approach of a fringe.)
Fig. 30 Surface displac
ements resulting from Lamb wave propagation in an aluminum sheet. These subfringe
displacements were extracted from the holographic interferogram using phase-step (quasi-
heterodyne)
holographic interferometry.
References cited in this section
29.
M.J. Eh
rlich and J.W. Wagner, Anisotropy Measurements and Determination of Ply Orientation in
Composite Materials Using Holographic Mapping of Large Amplitude Acoustic Waves, in
Proceedings of
the Third International Symposium on Nondestructive Characterization of Materials
(Saarbrucken, West
Germany), Oct 1988
30.
J.W. Wagner and D.J. Gardner, Heterodyne Holographic Contouring for Wear Measurement of Orthopedic
Implants, in Proceedings of the International Congress on Applications of Lasers and Electro-Optics (L
os
Angeles, CA), Laser Institute of America, 1984
Optical Holography
Revised by James W. Wagner, The Johns Hopkins University
Appendix: Selection of Holographic Systems
In general, the considerations that apply to the selection of a holographic system are similar to the considerations that
apply to the selection of any other major piece of capital equipment. The selection will be based primarily on the ability
of the equipment to perform the tasks that are of chief concern. Beyond that, selection will be governed by such factors as
the space and support facilities required, personnel and training, system versatility, and cost. This section outlines the
advantages and disadvantages of purchasing holography as an outside service and of operating an in-house system.
Contract (Purchased) Holography
An initial approach for a company with little or no holographic equipment or technical skill might be to purchase the
holographic work on a contract basis from a company that performs this work as a service. This would probably be the
most economical approach for a company that has only a few parts to inspect or wishes to run only a single development
program employing holographic analysis.
The advantages of purchasing holographic work are as follows:
• No capital investment and minimal in-house technical skill are required
• The holographic work can be purchased as required, even on a daily or a per-piece basis
•
Some companies will conduct holographic developmental work at low cost if favorable results will
promote the sale of equipment
•
Purchasing holographic work is a good way to test an application prior to investing in equipment and
personnel training
• This method allows the buyer to utilize holography with a very short lead time
The disadvantages of purchasing holographic work are the following:
• Test objects must be shipped to and from the service company. Often, the travel of some in-
house
personnel may also be required for program success
• Contracted holographic work costs may be high, depending on the equipme
nt and personnel required.
This charge must be compared with the cost of suitable alternatives
•
Personnel of the service company must be educated concerning the holographic techniques and results
as applied to the application in question
In-House Holographic Systems
If the amount of work or other factors make it desirable to have in-house capability, a suitable holographic system can be
installed. This can be a commercially available system, such as a tire analyzer or a sandwich-panel analyzer, designed to
perform only one task; a commercially available system, such as a portable holocamera or a holographic interference
camera, designed for a variety of tasks; or a system built in-house from separately purchased or fabricated components.
Assembled Systems
In-house units can be purchased piece-meal or as one complete system.
The advantages of in-house assembly of a system from components are as follows:
• The system can be built to fit specific or universal needs
• In-house engineering talent can be utilized to reduce capital expenditures by 40% or more
• Such a system can utilize readily available components
•
Building a system is an excellent learning experience for the novice desiring to make future use of
holographic techniques
• The system can be built and modified to match the expanding skills of the operator
The disadvantages of in-house assembly are the following:
• Such systems usually require a long lead time to become operational
• Designing and in-house assembly of a system require either developed or p
urchased technical skill that
can be justified only when it is readily available or when many possible applications exist
Purchased Systems
Purchasing a commercial system has the advantage that the vendor can assume complete responsibility for a system to
meet the specifications of the buyer. Details of the components do not need clarification in the bid specification. A
specification, for example, need simply state that the system shall be capable of finding a flaw in a given part, of a given
length, with a specified confidence level.
The purchased system can be either a complete system designed especially for one task or a system designed for a variety
of tasks.
Complete System Designed for One Specific Task. The advantages of purchasing a complete system designed
especially for one task are as follows:
• Minimum in-house holographic technical skill is required
• A system that has been previously built or designed can be delivered in a short time
The disadvantages are the following:
• Special systems are u
sually expensive because of the labor required for design, construction, and
testing. Costs per unit drop rapidly, however, if more than one unit is built
• It may be difficult to modify the system for other tasks, although many holographic components can b
e
removed for use in another system
• A system that must be designed and built for a special task will require a long lead time
Multifunctional Complete System Designed for Numerous Tasks. Although a special system may be the only
means for testing a part of unusual design or size, it is often advantageous to purchase a system designed for a variety of
jobs. The advantages of this type of system are as follows:
• Such systems can be less expensive than special systems because they are in volume production,
which
reduces design costs per unit
• Such systems have flexibility in change-over to inspect different kinds of parts.
This usually requires
little more than changing the stressing fixture
• The buyer is free to make his own setups
• Delivery time is short; some systems can be purchased off the shelf
• Many vendors will train in-house personnel at no additional cost
The disadvantages are the following:
• The system will be limited by the size or weight of the test object it can handle
• The system may not be a
s stable as a specialized system because of the flexibility required in the optical
components and mounts
Optical Holography
Revised by James W. Wagner, The Johns Hopkins University
References
1. R.K. Erf et al., "Nondestructive Holographic Techniques for Structures Inspection," AFML-TR-72-
204,
Air Force Materials Laboratory, Oct 1972 (AD-757 510)
2. L. Kersch, Advanced Concepts of Holographic Nondestructive Testing, Mater. Eval., Vol 29, 1971, p 125
3. J.P. Waters, Object Motion Compensation by Speckle Reference Beam Holography, Appl. Opt.,
Vol 11,
1972, p 630
4. L. Rosen, Focused-Image Holography, Appl. Phys. Lett., Vol 9, 1969, p 1421
5. H.J. Caulfield et al., Local Reference Beam Generation in Holography, Proc. IEEE, Vol 55, 1967, p 1758
6. V.J. Corcoran et al., Generation of a Hologram From a Moving Target, Appl. Opt., Vol 5, 1966, p 668
7. D.B. Neumann et al., Object Motion Compensation Using Reflection Holography, J. Opt. Soc. Am.,
Vol
62, 1972, p 1373
8. D.B. Neumann et al., Improvement of Recorded Holographic Fringes by Feedback Control, Appl. Opt.,
Vol 6, 1967, p 1097
9. H.W. Rose et al., Stabilization of Holographic Fringes by FM Feedback, Appl. Opt., Vol 7, 1968, p a87
10. J.C. Palais, Scanned Beam Holography, Appl. Opt., Vol 9, 1970, p 709
11. R.K. Erf, Ed., Holographic Nondestructive Testing, Academic Press, 1974
12. J.Ch. Vienot, J. Bulabois, and J. Pasteur, in Applications of Holography,
Proceedings of the International
Symposium on Holography, Laboratory of General Physics and Optics, 1970
13.
J.E. Sollid and J.D. Corbin, Velocity Measurements Made Holographically of Diffusely Reflecting
Objects, in Proceedings of the Society of Photo-Optical Instrumentation Engineers, Vol 29, 1972, p a125
14. J.E. Sollid, A Comparison of Out-of-Plane Deformation and In-
Plane Translation Measurements Made
With Holographic Interferometry, in Proceedings of the Society of Photo-
Optical Instrumentation
Engineers, Vol 25, 1971, p 171
15. P. Hariharan, Quasi-Heterodyne Hologram Interferometry, Opt. Eng., Vol 24 (No. 4), 1985, p 632-638
16. W. Juptner et al., Automatic Evaluation of Holographic Interferograms by Reference Beam Shifting,
Proc.
SPIE, Vol 398, p 22-29
17. R. Dandliker and R. Thalmann, Heterodyne and Quasi-Heterodyne Holographic Interferometry, Opt. Eng.,
Vol 24 (No. 5), 1985, p 824-831
18. J.D. Trolinger et al., Putting Holographic Inspection Techniques to Work, Lasers Applic., Oct 1982, p 51-
56
19. The Optical Industry and Systems Purchasing Directory, 34th ed., Laurin Publishing Company, 1988
20.
R.C. Grubinskas, "State of the Art Survey on Holography and Microwaves in Nondestructive Testing,"
MS 72-9, Army Materials and Mechanics Research Center, Sept 1972, p a40-46
21. R. Aprahamian et al., "An Analytical and Experimental Study of
Stresses in Turbine Blades Using
Holographic Interferometry," Final Report AM 71-5 under NASC Contract N00019-70-C-0590, July 1971
22. J. Waters et al.,
"Investigation of Applying Interferometric Holography to Turbine Blade Stress Analysis,"
Final Report J990798-13 under NASC Contract N00019-69-C-0271, Feb 1970 (available as AD 702 420)
23. Proceedings of the Symposium on Engineering Applications of Holography, Society of Photo-
Optical
Instrumentation Engineers, 1972
24. B.P. Hildebrand and K.A. Haines, Multiple-Wavelength and Multiple-
Source Holography Applied to
Contour Generation, J. Opt. Soc. Am., Vol 57 (No. 2), 1967, p a155-162
25. J.S. Zelenka and J.R. Varner, Multiple Index Holographic Contouring, Appl. Opt.,
Vol 8 (No. 7), 1969, p
1431-1434
26.
T. Tsuruta, N. Shiotake, J. Tsujuichi, and K. Matsuda, Holographic Generation of Contour Map of
Diffusely Reflecting Surface by Using Immersion Method, Jpn. J. Appl. Phys., Vol 6, 1967, p 661-662
27. J.W. Wagner, High Resolution Holographic Techniques fo
r Visualization of Surface Acoustic Waves,
Mater. Eval., Vol 44 (No. 10), 1986, p 1238-1242
28. J.W. Wagner, Examples of Holographic Versus State-of-the-
Art in the Medical Device Industry, in
Holographic Nondestructive Testing (NDT)--Status and Comparison
With Conventional Methods: Critical
Review of Technology, Vol 604, Conference Proceedings, Los Angeles, CA, Society of Photo-
Optical
Instrumentation Engineers, 1986
29. M.J. Ehrlich and J.W. Wagner, Anisotropy Measurements and Determination of Ply Orient
ation in
Composite Materials Using Holographic Mapping of Large Amplitude Acoustic Waves, in
Proceedings of
the Third International Symposium on Nondestructive Characterization of Materials
(Saarbrucken, West
Germany), Oct 1988
30. J.W. Wagner and D.J. Ga
rdner, Heterodyne Holographic Contouring for Wear Measurement of
Orthopedic Implants, in Proceedings of the International Congress on Applications of Lasers and Electro-
Optics (Los Angeles, CA), Laser Institute of America, 1984
Optical Holography
Revised by James W. Wagner, The Johns Hopkins University
Selected References
• R.J. Collier, C.B. Burckhardt, and L.H. Lin, Optical Holography, Academic Press, 1971
• R.K. Erf, Ed., Holographic Nondestructive Testing, Academic Press, 1974
• C.M. Vest, Holographic Interferometry, John Wiley & Sons, 1979
Speckle Metrology
F.P. Chiang, Laboratory for Experimental Mechanics Research, State University of New York at Stony Brook
Introduction
MEASUREMENT TECHNIQUES or devices traditionally employ either a regular geometric pattern or a constant as a
base transducer. The change of the regularity of the pattern or the response of the base is converted into the measured
quantity. Rulers, pressure and strain gages, moiré and interferometric methods, and so on, are all representative of these
devices or techniques. Since the 1970s, techniques have been developed that employ random patterns as transducers.
Measurement is made not by knowing the deviation from regularity but by determining the correlation between two
random signals. This has opened a new field of activity in which new measurement instruments are being developed. In
the discussion that follows, the basic principle of the random speckle technique will be described, together with its
application to the metrological measurement of metals. Speckle metrology is also known as laser speckle photography
(LSP).
Speckle Metrology
F.P. Chiang, Laboratory for Experimental Mechanics Research, State University of New York at Stony Brook
Laser Speckle Patterns
Speckle patterns occur naturally in everyday life. From the metrological point of view, they did not attract attention until
the advent of the laser. When a coherent laser beam illuminates an optically rough surface (that is, not a mirrorlike
surface), the reflected wavelets from each point of the surface mutually interface to form a very complicated pattern. The
process is analogous to the ripple pattern formed on the surface of a pond that has been disturbed by raindrops. The only
difference is that water speckles form on the surface of the pond, but laser speckles fill the entire space covered by the
reflected wavelets. A typical laser speckle pattern is shown in Fig. 1.
Fig. 1 Typical laser speckle pattern
Speckle Metrology
F.P. Chiang, Laboratory for Experimental Mechanics Research, State University of New York at Stony Brook
Recording and Delineation of Speckle Displacement
The speckle methods that will be described in the following sections originated from a physical phenomenon first
observed by Burch and Tokarsky (Ref 1). They found that when two random patterns are superimposed with an in-plane
displacement between them and illuminated with a coherent beam of light, the far field diffraction spectrum is that of a
single speckle pattern modulated by a series of Young's fringes whose orientation and spacing are identical to those
formed by two point sources located at the ends of the displacement vector experienced by the speckles.
When applying the laser speckle technique, a coherent laser beam is used to illuminate a metal surface (or any optically
rough surface). Depending on the information sought, one selects a certain section of the resulting volumetric speckle
pattern to be photographed by a camera. When the surface is deformed, the speckle pattern moves accordingly. The
spatial movement of the speckle is quite complicated and is governed by a set of three equations (Ref 2).
Conceptually, laser speckles can be viewed as tiny particles attached to the surface by rigid but massless wires. They shift
as the surface shifts and tilt in much the same way as light rays are tilted by a mirror. The displaced speckles are
photographed again on the same film through double exposure, or a separate recording is made and then superimposed on
the first one. The two superimposed speckle patterns are called a specklegram. The speckle displacement can be
delineated by sending a narrow laser beam at the points of interest and receiving their corresponding far field diffraction
spectrum on a screen. The experimental arrangement is shown in Fig. 2(a). The observer sees a circular diffraction halo
(assuming the aperture of the camera lens is circular) modulated by a series of uniformly spaced straight fringes, that is,
the Young's fringes. The intensity distribution of the diffraction spectrum can be expressed as follows (Ref 3):
(Eq 1)
where u is the position vector at the receiving plane, k = 2π/λ with λ being the wavelength of light, d is the displacement
vector at the point of probing, L is the distance between the specklegram and the receiving screen, and I
h
is the so-called
halo function, which is the diffraction spectrum of a single speckle pattern.
Fig. 2
Pointwise Fourier processing that results in Young's fringes representing the displacement vector at the
point of probing. (a) Schematic of the optical setup. (b) Resulting Young's fringe in a diffraction halo
Equation 1 can be converted into (Ref 3):
(Eq 2)
where |d| is the magnitude of the displacement vector and S is the fringe spacing.
These fringes represent the displacement vector of a small cluster of speckles illuminated by the laser beam (Fig. 2b). The
displacement of the speckles can be converted into the displacement of the specimen, and depending on the experimental
arrangement, different information can be obtained.
Measurement of In-Plane Deformation
For this application, the recording camera is directly focused on the flat surface of the specimen. (The method can also be
applied to measuring interior in-plane displacement if the material is transparent to the radiation used, as discussed in Ref
4.) Under either a plane-strain or (generalized) plane-stress condition, the speckle movement is largely confined to the
plane of the surface. Therefore, the observer again photographs the speckle pattern formed on the surface after the
specimen has been subjected to deformation. The resulting specklegram can be probed point by point to yield the
displacement information, as described in the section "Recording and Delineation of Speckle Displacement" in this
article.
Alternatively, the observer can obtain full-field displacement information by using the following optical spatial filtering
scheme. The specklegram is placed inside a convergent laser beam, as shown in Fig. 3(a). The diffraction spectrum at the
focal plane is also governed by Eq 1. At the focal plane, a mask with a small hole situated at u is used to block all light
except that passing through the small aperture. A second lens is used to receive the filtered light and forms an image of
the specimen surface, which is now modulated by a series of fringes governed by (Ref 3):
(Eq 3)
where is the angle between the displacement vector d and the aperture vector u. Therefore, the fringes are contours of
displacement component resolved along the direction u. Displacement contours along any direction can be obtained by
simply varying the direction of the aperture, and the sensitivity of the contour can be changed continuously by sliding the
aperture along the direction of the position vector of the aperture. An example of this application is also shown in Fig.
3(b), in which the fringes are contours of opening (that is, vertical) displacement of a cracked aluminum specimen.
Fig. 3 Optical spatial filtering used to generate full-
field displacement contour (isothetic) fringes. (a)
Components of
optical spatial filter setup. (b) Fringe pattern obtained for an aluminum specimen containing a
crack. Each fringe represents 0.005 mm (0.0002 in.) in vertical displacement.
Another example of this type of application is illustrated in Fig. 4, which shows the thermal strain patterns of a heated
aluminum plate. The plate is heated with a torch at the lower left-hand corner until it is red hot. It is then left to cool by
natural convection. A thermocouple is embedded nearby to monitor the temperature. The patterns are the result of
differential thermal strain from the temperatures indicated in Fig. 4. Figure 4(a) shows the displacement of the contour
fringes along the x-direction; Fig. 4(b), those along the y-direction. These fringe patterns are governed by:
(Eq 4)
(Eq 5)
where u and v are the displacement components along the x- and y-directions, respectively, and u
x
and u
y
, are the distances
of the filtering aperture along the x- and y-directions, respectively. As the plate cools, the temperature tends toward
equilibrium, as evidenced by the uniformity of the fringe patterns at lower temperatures. Thermal strains measured by the
speckle method compared quite well with the existing data, as shown in Fig. 5.
Fig. 4 Thermal strain determination using laser speckle method. (a) u field. (b) v
field. Fringes show the
residual strain field between two temperature levels of an aluminum plate torch heated to red hot and allowed
to cool by natural convection. Fields u and v are the contour fringe displacements in the x- and y-
directions,
respectively.
Fig. 5 Comparison between theore
tical and experimental thermal strain relaxation at a point as a function of
time
Measurement of Out-of-Plane Deformation
The laser speckle method can also be used to measure the out-of-plane deformation of a specimen with a slightly different
experimental arrangement. Instead of photographing the speckles on or very close to the surface, one can photograph the
speckles at a certain finite distance (a few centimeters, for example) from the surface. When the surface experiences out-
of-plane deformation, the spatial speckles are tilted, resulting in speckle displacement at the defocused plane. By
recording these speckles before and after deformation, the resulting specklegram is processed in the same way as
described in the section "Measurement of In-Plane Deformation" in this article. The resulting fringes are governed by
(Ref 3):
(Eq 6)
(Eq 7)
where w/ x and w/ y are the slopes of the surface along the x- and y-directions, respectively, and A is the defocused
distance. An example of this application is shown in Fig. 6, in which the fringes are the slope fringes w/ x of a
clamped thin circular plate transversely loaded at the center. A comparison between the experimental result and the
theoretical result is also shown in Fig. 6.
Fig. 6 Slope fringes of a clamped circular plate under concentrated load
References cited in this section
1.
J.M. Burch and J.M.J. Tokarski, Production of Multiple Beam Fringes From Photographic Scatterers,
Optica
Acta, Vol 15 (No. 2), 1968
2.
D.W. Li and F.P. Chiang, Laws of Laser Speckle Movement, Opt. Eng., Vol 25 (No. 5), May 1986, p 667-
670
3.
F.P. Chiang, A Family of 2-D and 3-D Experimental Stress Analysis Techniques Using Laser Speckles,
Solid
Mech. Arch., Vol 3 (No. 1), 1978, p 1-32
4.
F.P. Chiang, A New Three-
Dimensional Strain Analysis Technique by Scattered Light Speckle
Interferometry, in The Engineering Uses of Coherent Optics,
E.R. Robertson, Ed., Cambridge University
Press, 1976, p 249-262
Speckle Metrology
F.P. Chiang, Laboratory for Experimental Mechanics Research, State University of New York at Stony Brook
White Light and Electron Microscopy Speckle Methods
Laser speckle is a natural result of coherent radiation, but speckle pattern can also be artificially created. For example, the
aerosol particles of black paint sprayed onto a white surface (or vice versa) create a pattern quite similar to laser speckles.
White Light Method. Another efficient way of creating artificial speckle is to cover the surface with a layer of
retroreflective paint. When illuminated by incoherent white light (or any visible light), the individual glass beads
embedded in the paint converge the light rays and send them back in approximately the same direction. When imaged and
recorded on film, the beads appear as sharply defined, random speckles. These patterns can be used for metrological
measurements as well. Any speckle pattern can be used if it yields a distinct pattern and can be recorded for future
reference. A few examples are given in Ref 6. These are classified as white light speckles because they are observable
under the illumination of incoherent white light. The identical procedures used for laser speckles can be adapted to record
and process white light speckles. The only exception is that because white light speckles are located on the surface of the
specimen, they cannot be tilted in the same way as laser speckles. However, through the use of defocused photography,
depth information can nevertheless be obtained (Ref 6).