ASM Metals HandBook Vol. 17 - Nondestructive Evaluation and Quality Control
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recording medium. These requirements are generally of the order of 500 to 3000 lines per millimeter. Thus, the
photographic emulsion must have sufficient resolution and contrast to record these interference fringes clearly. Ordinary
photographic emulsions are usually unsatisfactory, because they can resolve at most only about 200 lines per millimeter;
exceptionally fine grain emulsions are required.
Optical Path Length Variations. The difference between the path lengths of the reference beam and the object beam
must be less than the temporal coherence length of the laser beam and should be as close to zero as possible to obtain
fringes of the highest contrast. The holocamera must be isolated from vibration to the extent that there is less than one-
fourth to one-eighth wavelength variation in the object beam and reference beam path lengths during an exposure. Such
variations can be caused by the relative motion of optical elements in the beam paths, by room vibration, or by
temperature changes. Air turbulence can also cause optical path changes that contribute to reduced interference fringe
contrast.
Reference Beam Versus Object Beam Intensity Variations. In the absence of optical path length variations, the
contrast of the interference fringes is directly related to the amplitude of the reflected light from the object and the
amplitude of the reference beam. Thus, when recording a hologram, the reference and object beam intensities are adjusted
with a variable beam splitter and/or appropriate attenuating filters to obtain optimum contrast. Although optimum fringe
contrast would be obtained for equal object and reference intensities at the film plate, the variation in reflected intensities
from point to point on a diffuse object, coupled with concerns for recording linearity, dictates that the reference beam
intensity be two to ten times brighter than the object beam intensity at the film plate. A more detailed discussion of the
effects of this and other recording parameters is given in the section "Effects of Test Variables," (specifically the
discussion "Exposure Parameters" ) in this article.
Continuous-Wave (CW) Lasers Versus Pulsed Lasers. Two generic classes of laser sources are available for
holography: continuous-wave lasers and pulsed lasers. Continuous-wave lasers are characterized by the continuous
(steady-state) emission of coherent light at relatively low power. Pulsed lasers are characterized by the pulsed emission of
coherent light at relatively high power over sometimes extremely short intervals of time.
Depending on the laser power, a few seconds of exposure of a holographic plate with a CW laser can yield the same
results as a single-pulse exposure with a pulsed laser with regard to achieving an exposure level suitable for
reconstructing a holographic image. Therefore, a CW laser source can be effectively used to record holograms of objects
that are stationary throughout the duration of the exposure. A pulsed-laser source must be used to record holograms of
objects undergoing rapid changes by freezing the motion (that is, by recording information over a very small interval of
time). Regardless of which class of laser source is used for recording the hologram, a CW laser is always used for
reconstruction.
Optical Holography
Revised by James W. Wagner, The Johns Hopkins University
Holographic Recording
When visible light waves are employed in holography, the hologram is recorded with an optical system often referred to
as a holocamera (Fig. 1a). A monochromatic laser beam of phase-coherent light is divided into two beams by a variable
beam splitter. One beam--the object beam--is expanded and filtered by a lens/pinhole spatial filter that reduces the effects
of dirt and dust in the beam path. The expanded divergent beam is then directed to illuminate the object uniformly. A
portion of the laser light reflected from the object is intercepted by a high-resolution photographic plate, as shown in Fig.
1(a). The second beam--the reference beam--originating from the beam splitter diverges from a second spatial filter and is
steered onto the photographic plate directly without reflecting from the object. With either the object beam or reference
beam absent, a uniformly exposed photographic plate will result. However, with both coherent beams falling on the plate
simultaneously, an interference pattern is generated as a result of the coherent interaction of the two beams, and this
pattern is recorded by the photographic emulsion. In fact, this complex interference pattern recorded on the photographic
plate contains all the information necessary to reconstruct an extremely high fidelity, three-dimensional image of the
object.
Fig. 1 Schematics of the basic optical systems used in continuous-
wave holography. (a) Holocamera used to
record hologram of an object on a photographic plate. (b) Optical system for reconstructing a virtual image of
the object from the hologram on the photographic plate
Fine-Grain Photographic Emulsions Required for Fringe Resolution. The details of the interference pattern
recorded on the film are extremely fine, requiring that high-resolution emulsions be used in very stable recording systems.
The fixed wavelength of the light used and the varying magnitude of the offset angle (the angle between the object beam
and reference beam at any point on the photographic plate) determine the orientation and spacing of the fringes
comprising the recorded interference pattern. The fringe spacing in turn influences the resolution requirements of the
recording medium. These requirements are generally of the order of 500 to 3000 lines per millimeter. Thus, the
photographic emulsion must have sufficient resolution and contrast to record these interference fringes clearly. Ordinary
photographic emulsions are usually unsatisfactory, because they can resolve at most only about 200 lines per millimeter;
exceptionally fine grain emulsions are required.
Optical Path Length Variations. The difference between the path lengths of the reference beam and the object beam
must be less than the temporal coherence length of the laser beam and should be as close to zero as possible to obtain
fringes of the highest contrast. The holocamera must be isolated from vibration to the extent that there is less than one-
fourth to one-eighth wavelength variation in the object beam and reference beam path lengths during an exposure. Such
variations can be caused by the relative motion of optical elements in the beam paths, by room vibration, or by
temperature changes. Air turbulence can also cause optical path changes that contribute to reduced interference fringe
contrast.
Reference Beam Versus Object Beam Intensity Variations. In the absence of optical path length variations, the
contrast of the interference fringes is directly related to the amplitude of the reflected light from the object and the
amplitude of the reference beam. Thus, when recording a hologram, the reference and object beam intensities are adjusted
with a variable beam splitter and/or appropriate attenuating filters to obtain optimum contrast. Although optimum fringe
contrast would be obtained for equal object and reference intensities at the film plate, the variation in reflected intensities
from point to point on a diffuse object, coupled with concerns for recording linearity, dictates that the reference beam
intensity be two to ten times brighter than the object beam intensity at the film plate. A more detailed discussion of the
effects of this and other recording parameters is given in the section "Effects of Test Variables," (specifically the
discussion "Exposure Parameters" ) in this article.
Continuous-Wave (CW) Lasers Versus Pulsed Lasers. Two generic classes of laser sources are available for
holography: continuous-wave lasers and pulsed lasers. Continuous-wave lasers are characterized by the continuous
(steady-state) emission of coherent light at relatively low power. Pulsed lasers are characterized by the pulsed emission of
coherent light at relatively high power over sometimes extremely short intervals of time.
Depending on the laser power, a few seconds of exposure of a holographic plate with a CW laser can yield the same
results as a single-pulse exposure with a pulsed laser with regard to achieving an exposure level suitable for
reconstructing a holographic image. Therefore, a CW laser source can be effectively used to record holograms of objects
that are stationary throughout the duration of the exposure. A pulsed-laser source must be used to record holograms of
objects undergoing rapid changes by freezing the motion (that is, by recording information over a very small interval of
time). Regardless of which class of laser source is used for recording the hologram, a CW laser is always used for
reconstruction.
Optical Holography
Revised by James W. Wagner, The Johns Hopkins University
Holographic Reconstruction
In the reconstruction process (Fig. 1b), the complex interference pattern recorded on the hologram is used as a diffraction
grating. When the grating is illuminated with the reference beam only, three angularly separated beams emerge: a zero-
order, or undeflected, beam and two first-order diffracted beams. The diffracted beams reconstruct real and virtual images
of the object to complete the holographic process. The real image is pseudoscopic, or depth inverted, in appearance.
Therefore, the virtual image (also referred to as the true, primary, or nonpseudoscopic image) is the one that is of primary
interest in most practical applications of holography. In Fig. 1(b), only the first-order diffracted beam that yields the
virtual image has been shown; the other two beams were omitted for reasons of clarity. If the original object is three
dimensional, the virtual image is a genuine three-dimensional replica of the object, possessing both parallax and depth of
focus. However, if the configuration of the optical system or the wavelength of light used during reconstruction differs
from that used during recording, then distortion, aberration, and changes in magnification can occur. (The holographic
recording and reconstruction systems can be designed to minimize these effects.)
The light intensity in the reconstructed image depends on the diffraction efficiency of the hologram. This efficiency is a
function of several recording parameters, the most significant of which is the type of recording medium (film) used.
Under ideal conditions, gelatin or polymeric films, which modify only the phase of the reconstructing light without
absorbing it, can provide nearly 100% holographic diffraction efficiency. Commercially available holocameras using
thermoplastic phase holograms, which can be developed by a thermal process without removing the film from its holder,
provide diffraction efficiencies up to about 20%. High-resolution black-and-white film plates, which are used to form
absorption holograms, have theoretical efficiencies up to 6.25%. For a typical hologram of this type, however, the
intensity is usually less than 3% of the incident reconstructing light intensity.
Once reconstructed, the light in the image beam can be used just as one would use the light from an illuminated object
viewed through a window. Pictures of the reconstructed image can be recorded photographically or electronically.
Optical Holography
Revised by James W. Wagner, The Johns Hopkins University
Interferometric Techniques of Inspection
When optical holography is applied to the inspection of parts, the generation of a three-dimensional image of the object is
of little value. Furthermore, for opaque materials, optical holography is strictly limited to surface observations. Therefore,
if optical holography is to be implemented for nondestructive evaluation and inspection, supplementary means must be
used to stress or otherwise excite test objects so as to produce surface manifestations of the feature of interest. It is for the
measurement of such manifestations that the techniques of optical holography have been further developed to form the
subfield of holography termed optical holographic interferometry.
As in conventional interferometry, holographic interferometric measurements can be made with great accuracy (to within
a fraction of the wavelength of the light being used). Although conventional interferometry is usually restricted to the
examination of objects possessing highly polished surfaces and simple shapes, holographic interferometry can be used to
examine objects of arbitrary shape and surface condition. Because holographic interferometry produces a three-
dimensional fringe-field image, which can be examined from many different perspectives (limited only by the size of the
hologram), a single holographic interferogram is equivalent to a series of conventional two-dimensional interferograms.
As with conventional interferometry, holographic interferometry is the comparison of a test wave front with a known
master wave front. Deviations of the test wave front from the master result in the appearance of interferometric fringes. In
contrast to conventional interferometry, in which the master wave front is generated by appropriate imaging optics and
mirrors, holographic interferometry systems use a holographically recorded image of the test object itself as a master
against which the test object is compared after exposure to, for example, an applied stress. Depending on the inspection
technique used, the interferometric comparison of the test object with its reference image can be made continuously (real
time) or at selected instants in time (multiple exposure). This latter technique takes advantage of the fact that more than
one hologram can be made on the same recording medium. Periodic vibratory motion can be examined with time-average
holographic interferometry, while high-speed displacements or deformations can be studied with pulse methods.
The following sections will provide brief and qualitative descriptions of the basic holographic interferometry methods. A
discussion of the quantitative information that can be obtained with these methods is presented later.
Real-Time Interferometry
In the real-time technique of holographic interferometry, a CW laser is used to make a hologram of the object in its
natural or undisturbed state or in some other useful reference state, and the holographic reconstruction is compared to the
object itself as it is deformed in real time. With the hologram being used as an observation window, it is possible to
observe the resulting interference pattern produced by the interaction of the recorded object beam (also referred to as the
stored, or frozen, beam) with the modified, real-time object beam (sometimes referred to as the live beam). With this
method, it is possible to observe interference fringes resulting from differential, stroboscopic, and time-averaged motion
of the object surface. Stroboscopic visualization can be obtained by chopping the laser source at a frequency and phase
linked to the object excitation source. For time-varying fringe fields, time averaging is performed by the observer's eye,
permitting the identification of nodes and antinodes of displacement when the object is excited at some vibratory
resonance.
Multiple-Exposure Interferometry
The double-exposure, or differential, technique of holographic interferometry involves the use of either a continuous-
wave or a pulsed laser to record two holograms--one of the object in its natural or undisturbed state (or in some other
useful reference state) and the other of the object in some changed or disturbed state. Both holographic recordings are
made on the same piece of film so that in the reconstruction process, the two recorded object beams interfere with each
other to produce a fringe field in which the contour and spacing describe the changes that occurred between the two
exposures. Because both the master and the test images are holographically recorded, high-speed disturbances such as
stress wave propagation or ballistic impact can be examined using the double-exposure technique and double-pulsed laser
sources. Laser pulse durations of several nanoseconds can be used to freeze surface motion that cannot be readily
observed in real time.
One important variation of the double-exposure recording process incorporates two angularly separated reference beams.
One of the beams is used for each exposure. When the resulting hologram is reconstructed by either beam alone, the
image in the primary diffraction order contains no interference fringes, because only a single corresponding object beam
is reconstructed. With both reference beams illuminating the hologram, the reconstructed image appears as one that might
be obtained from a conventional double-exposure hologram with a single reference beam. The advantage of the dual-
reference double-exposure technique derives from the ability to independently access the two reconstructed images to be
interfered. By changing the frequency or phase of one reconstructing beam relative to the other, the observed fringes can
be made to move in such a manner as to facilitate the quantitative interpretation of the interferometric information with
measurement accuracies approaching 1/1000 of a fringe.
The stroboscopic technique of holographic interferometry involves the multiple recording of certain predetermined
positions of an object in periodic motion, usually at the vibrational antinodes, the positive and negative maximum
amplitudes of vibration. A pulsed laser or a continuously chopped CW laser synchronized with the source of vibrational
excitation is ordinarily used to make these recordings. In the reconstruction process, fringes of high contrast are produced
that enable the measurement of large amplitudes of vibration. By appropriately timing the laser pulses during holographic
recording, the phase relationships of the antinodes can also be obtained.
Time-Average Interferometry
The continuous-exposure technique (time-average technique) of holographic interferometry permits mapping of the
displacements of the surface of an object in periodic motion. A CW laser is ordinarily used to make this recording.
Because the holographic exposure time is usually much greater than the period of object motion, the hologram is a record
of the time-averaged irradiance distribution at the hologram plane. In the fringe field resulting upon reconstruction, bright
fringes correspond to nodal regions (regions of zero or very small amplitudes of vibration). Fringe order increases as one
approaches antinodal regions (regions of large amplitudes of vibration). By using appropriate fringe analysis techniques,
the amplitudes of vibration for each point on the surface of the object can be determined. Because the fringe contrast
decreases as the amplitude of motion increases, the displacement amplitudes analyzed in this way should not be too great.
Holographic Contouring
In addition to being a powerful tool for studying the response of an object of complex shape to a variety of applied
stresses, holographic interferometry also provides a convenient means of evaluating the shape itself. With the appropriate
techniques, it is possible to produce interferometric fringes that are a function of surface contour rather than surface
deformation or displacement. Principally, two double-exposure recording methods are employed for this purpose: dual
wavelength and dual refractive index methods.
The dual wavelength method for holographic contouring requires that the two holographic exposures be made at
slightly different laser wavelengths. A CW gas laser such as an argon ion laser is usually used for recording purposes
because the laser cavity can be easily tuned to any of several operating wavelengths. Upon reconstruction of the hologram
with one or the other of the two recording wavelengths, a fringe pattern, directly related to the surface relief of the object,
is observed superimposed on the image.
The dual refractive index method requires that the object to be contoured be viewed through a flat window in a test
cell or chamber containing the object. Between the holographic double exposures, the refractive index of the medium
surrounding the object in the chamber is changed. The contour fringe rate is determined by the change in refractive index.
When a high fringe rate is desired, liquids such as mixtures of water and alcohol can be used. For lower contour intervals,
the pressure of air within the test chamber can be regulated to control the refractive index.
Optical Holography
Revised by James W. Wagner, The Johns Hopkins University
Methods of Stressing for Interferometry
The nondestructive inspection of materials or structures by optical holography is potentially applicable to any problem in
which stressing the part will produce a change in its shape that is indicative of the property or flaw being sought. Because
holographic interferometry is most sensitive to displacements out of the plane of the specimen surface, stressing methods
must be sought that will cause changes in the out-of-plane direction, which are detected by the recording of a holographic
interferogram. The interferogram will consist of uniformly spaced dark and light bands (fringes), with a flaw being
characterized as a distinguishable anomaly, such as a circular set of closed fringes. The part can be subjected to any of
several types of stress, including acoustic, thermal, pressure (or vacuum), and mechanical. The type of stress used
depends on:
• The physical characteristics of the part
• The type of flaw under inspection
•
The accessibility of the part (whether the part can be tested by itself or must be tested as an integral part
of a more complex system)
Acoustic Stressing. By combining optical holography with acoustic-stressing techniques, it is possible to record
interferometrically the physical properties of relatively large surfaces as they are deformed by acoustic wave propagation
and therefore to detect flaws. The acoustic stressing is done at sonic and moderately ultrasonic frequencies (usually less
than 100 kHz). Excitation at frequencies in this range can be an important advantage of this approach over conventional
ultrasonic inspection at megahertz frequencies because the effects of grain scattering, sonic attenuation, surface
roughness, and shape complexity are all considerably minimized (see the article "Ultrasonic Inspection" in this Volume).
This increases detection probability, permits the inspection of much larger areas at one time, and generally eases the
transducer-coupling problem.
Application of acoustic-stressing systems for inspection is readily accomplished by electrically driving a piezoelectric
transducer that has been bonded to the surface of the test object or to a fixture in which the object can be solidly mounted.
The transducer vibrations are transmitted through the bond and cause flexural displacements of the component. In this
way, with a proper choice of the driving frequency, a resonant-plate mode can be established in the test object. Additional
transducers can be used as sensors to aid in establishing plate resonances. This technique of excitation has the advantage
of producing a relatively broad range of frequencies--from a few hundred hertz to several hundred kilohertz.
In situations where larger-amplitude vibrations are required, a transducer can be mechanically coupled to a single point on
the test object through a solid exponential horn (acoustic transformer). The horn consists of a cylinder whose radius
decreases exponentially from one end to the other, with a piezoelectric transducer mounted on the larger end. The smaller
end is pressed against the test object, and the transducer is driven at resonance to couple a large amount of acoustic
energy into the test object. A typical setup is shown in Fig. 2. Such drive systems are generally limited to operation within
narrow bands of frequency that are centered about the design frequency and its resonances. With this type of drive at 50
kHz, a peak displacement of about 10 m (400 in.) is obtained at the horn output. Horn-coupling excitation eliminates
the bonding process required in the simple piezoelectric technique described above and is the most practical technique of
acoustic stressing at high frequencies. This single-point method of excitation also permits the establishment of resonance
throughout the entire test object, thus permitting inspection of the full surface at one time.
Fig. 2 Typical setup for the acoustic stressing of a test object using a piezoelectrically driven exponential horn
Standing, traveling, and surface waves can be utilized for inspection. Each is discussed below.
Standing Acoustic Waves. With standing waves, surfaces over debonds and voids, for example, can readily be made
to vibrate, thus producing an easily distinguishable change in the mode pattern. Figure 3 shows two examples of areas of
debonds detected with standing acoustic waves. Resonances within the skin covering the debonded area may require the
application of increasingly higher frequencies as the size of the debonded region to be detected becomes smaller.
Broadband acoustic excitation over a band of frequencies is often used to excite motion in debonded regions that may
vary in size.
Traveling Acoustic Waves. With traveling waves,
acoustic energy will be scattered or absorbed at locations
where the test object has a flaw. This scattered energy can be
observed as an anomaly in the surface displacement in a
holographic reconstruction of the test object. It is important to
note, however, that the dynamic characteristic of traveling
waves requires pulsed-laser holographic procedures, while
standing waves are amenable to both CW laser and pulsed-
laser holography.
Surface waves (Rayleigh waves) offer another acoustic
approach, but require some sophistication in their generation
to achieve sufficient amplitude for easy holographic
observation. They are, however, easily attenuated by changes
in structure and therefore offer potential for the detection of
such flaws as surface microcracks in metal components.
Electromagnetic drive systems can also be used to excite the
test object and are generally quite easy to effect. For
inspecting nonferromagnetic materials, some preliminary
preparation, such as coating with magnetic paint, is required.
Thermal Stressing. The use of thermal stressing for
successful holographic nondestructive inspection relies on
changes in the surface deformation caused by differences in
thermal expansion that are a direct result of a flaw or feature
of interest. For example, when there is a lack of intimate
contact at a debond between two components made of
materials with different thermal conductivities, the lower rate
of heat removal from the debonded region can affect the
surface expansion in such a way as to create an anomaly in the
interferometric fringe pattern of the hologram. In general,
thermal stressing causes time-varying changes in surface
displacement and therefore is best used in conjunction with
real-time CW laser or pulsed-laser holographic techniques.
Many methods are available for applying a thermal stress to a
component under test. The method used and its applicability
depend on the particular application.
Radiant Heat Sources. Radiation from a heat lamp or a
photoflood lamp directed onto the front or back surface of the
component is one of the simplest methods.
Electrical-resistance heating tape applied to a specific area of the test object is a method for preferential thermal
stressing, as is a heat gun.
Resistance heating of the object itself can be performed by passing electrical current through it.
Hot-air heating or liquid-nitrogen cooling can also be utilized as a thermal stressing method. For hollow or
channeled components, the heated or cooled gases can be passed through the test object where accessible.
In addition to steady-state and transient heating methods, periodic heating of the object and synchronized strobing of the
real-time holographic imaging system have been effective in locating surface-breaking cracks and similar defects.
Pressure or Vacuum Stressing. One of the most effective methods of stressing hollow components for holographic
inspection is pressurization or evacuation. This method is most easily implemented in situations where access is available
to the internal cavity. However, it is not limited to this type of test object. For any sealed-off structure that is
manufactured at atmospheric pressure, external evacuation can be utilized to create a pressure differential between the
Fig. 3
Optical holographic interferograms showing
areas of debonds (arrows) that were detected with
standing acoustic waves. (a) Debonds between a
compressor f
an blade and a metal strip protecting
the leading edge. (b) Debonds between the metal
face sheet and the honeycomb-
cellular core of a
sandwich structure
cavity and the environment. An example of this latter approach is presented in Fig. 4, which shows an area of debond
between the core and an aluminum faceplate of a honeycomb-core sandwich panel.
Fig. 4 Optical holographic interferogram of a pressure-stressed honeycomb-
core sandwich panel. The sandwich
panel was stressed by the atmospheric pressure of t
he air sealed in the core cavities when the air surrounding
the panel was evacuated. An area of debond between the core and the near faceplate (of aluminum) is
indicated by the fringe pattern in the vicinity of the ink marks on the faceplate. The backgroun
d fringes were
caused by general movement of the faceplate during evacuation.
In general, very mild pressure stressing (0 to 70 kPa, or 0 to 10 psi) is sufficient to produce a surface change capable of
detection by the holographic interferometric technique. However, in some cases (for example, the inspection of diffusion
bonds and electron beam and resistance welds), large pressure differentials of the order of 700 kPa (100 psi) are required
to create enough surface deformation for holographic interferometric detection. In these cases, the possibility of
inspection by pressure stressing of hollow components when internal access is not available is precluded because external
evacuation would not provide a sufficient pressure differential.
Mechanical Stressing. Another method for deforming a test object during holographic interferometric inspection
consists of subjecting it to simple mechanical force. There are numerous ways of applying mechanical force, but the
technique chosen will depend on the shape of the test object and the type of flaw being sought. One method that has found
relatively wide acceptance for the vibration analysis of components is the use of mechanical shakers.
Optical Holography
Revised by James W. Wagner, The Johns Hopkins University
Inspection Procedures
As previously mentioned in the section "Holographic Recording" in this article, optical holography utilizes both
continuous-wave lasers and pulsed lasers.
Continuous-Wave Techniques
Generally, in developing a procedure for the holographic inspection of a specific component or class of components, the
common CW technique of laser holography is initially used to:
• Study engineering aspects of the problem
• Evaluate various stressing methods
• Develop an effective test procedure
Subsequently, specialized adaptations of the CW technique can be designed to eliminate or minimize the potentially
deleterious effects of environmental vibration and the associated need for massive vibration-isolated tables. Indeed, for
the inspection or testing of manufactured parts from an assembly line, it may be necessary to provide sufficient vibration
isolation from the factory environment. In such situations, with adequate vibration isolation, the straightforward, widely
used and well-developed conventional holographic procedures can be implemented.
However, in many cases, inspection cannot be done with a CW technique. This is especially true when:
• The most effective stressing procedures involve noncyclic dynamic loading
• The inspection must be performed without using a stable (holographic) table
• The inspection area cannot be darkened to facilitate recording of the hologram
• Specialized test facilities must be used
In such situations, holography using a pulsed-laser beam offers one practical approach to nondestructive inspection.
Pulsed-Laser Techniques
Continuous-wave lasers are used in optical holography at widely varying power levels and wavelengths. However, when
high-speed transient stressing is used or when the test environment is unstable or hostile, a pulsed-laser technique has
been most successful for making holograms. The most common pulsed lasers used for holographic applications are flash
lamp pumped solid-state lasers such as ruby and frequency-doubled neodymium-doped yttrium aluminum garnet
(Nd:YAG).
The process of making holographic interferograms with a pulsed laser can be more complicated than it is with CW lasers.
Because laser pulse duration determines the exposure times, which may be of the order of several nanoseconds, precise
synchronization of the laser with the excitation source is usually required. An additional complication arises if a stable
environment is not used for the pulsed system. In this case, if a single exposure of a specimen is made, followed a few
seconds later by a second exposure of the same object without applying any excitation, the resulting hologram will be
found to be covered with a number of uninterpretable fringes caused by atmospheric changes and rigid-body specimen
displacements that occurred between exposures. If this procedure is used to record an interferogram in the same manner
as with the CW technique, the fringes resulting from the presence of a flaw may not even be visible. One solution to this
problem is to pulse the second exposure a sufficiently short time after the first so that no significant rigid-body motion
can occur between exposures. Elaborate switching systems have been created to perform this function. A typical pulse
sequence might be a first exposure of 20 ns, a delay of 25 s, and then a second exposure of 20 ns.
With pulsed-laser techniques, the nature of the excitation required for the detection of flaws can be quite different from
that used with CW techniques. One method is to impact the test object and thus trigger the pulse sequence. Fringes tend to
form around or in the vicinity of flaws because of a difference in the mechanical properties and therefore in the response
of the surface. These effects are entirely too brief to record by CW techniques. The same type of effects will be recorded
by pulsed-laser techniques if the component is thermally excited before the pulse sequence in such a way that the
specimen is still deforming thermally during exposure.
Because the requirement of environmental stability is greatly reduced when pulsed-laser techniques are used, the laser and
the necessary ancillary optical components can be installed in an existing testing facility to perform holographic
interferometry. The laser head and its power supply can be placed in any convenient, nearby location. A beam splitter and
several mirrors are used to generate two approximately equal optical beam paths to illuminate the test object and to
provide a reference beam, and the photographic plate is clamped in position to view the test area of interest. A portable
pulsed-laser setup is shown in Fig. 5.