Volume 17 - Nondestructive Evaluation and Quality Control Part 12 docx

Optical Holography
Revised by James W. Wagner, The Johns Hopkins University

Time-Average Interferograms
The characteristic function for time-average holographic interferograms differs from that of the double-exposure case. If
it is known that the object is undergoing strictly sinusoidal motion during the time of exposure of the holographic
interferograms, then the characteristic function is J
0
(K · d), where J
0
is the zero-order Bessel function of the first kind
and d is the vector displacement. This function behaves similarly to a cosine function with regard to its zero values;
however, it is not strictly periodic with zeroes existing at regular intervals. The first and second zeroes occur when the
argument is 2.4048 and 5.5201. After that, the zero values can be approximated by those given by the asymptotic limit for
large argument (large x); that is:
J
0
(x) cos [x - ( /4)]


(Eq 6)
For example, for the third fringe, the error is 15 parts in about 8600. None of the measurements to determine the values of
or (Fig. 8) is likely to be this accurate, so use of the zero values for the asymptotic limit is generally well justified.
Writing Eq 6 in scalar form and solving for the component of d parallel to K yields the following: For the first fringe:
|d| cos = 2.4048 /(4 sin )


(Eq 7)
For the second fringe:
|d| cos = 5.5201 /(4 sin )

(Eq 8)
For succeeding fringes, with n 3:
|d| cos = (n - ) [ /(4 sin )]


(Eq 9)
To a good approximation, the first fringe represents a displacement of about 3 /(16 sin ), with succeeding fringes
representing steps of /(4 sin ).
In addition to differences in the location of zeroes, the characteristic (Bessel) function for this case dramatically decreases
in amplitude with increasing fringe order. Because of the decreasing brightness of the fringes and the limited dynamic
range of the reconstruction film, it is difficult to record much more than seven fringes in photographically produced
reconstructions. Even when a superproportional reducer is used on the reconstruction negative to increase the visibility of
the higher-order fringes, it is difficult to work to much more than 30 fringes. In addition, it is difficult to work with a
slope on the object in excess of about 0.6% with either double-exposure or time-average holograms, because of the high
frequency of the fringes produced.
Optical Holography
Revised by James W. Wagner, The Johns Hopkins University

High-Resolution Interpretation Methods
As mentioned previously, consideration has been given only to the relationship between holographic interference fringes
and object surface motion. In fact, the appearance and apparent location of fringes in a reconstructed image depend not
only on displacement but also on object surface reflectivity and fringe brightness or contrast. Therefore, the intensity at
each point in a reconstructed holographic interferogram is a function of these three variables and not simply surface
displacement. Using high-resolution methods such as phase stepping (Ref 15, 16) and heterodyning (Ref 17), one can
compute directly all three variables at each point in the image to a degree of accuracy up to 1000 times better than can be
achieved by simple fringe counting. In this way, displacements as small as 0.25 nm (2.5 Å) can be detected in principle.
To perform either of these interpretation methods, independent control of the two interfering images must be available
during reconstruction. This is a natural consequence of real-time holographic interferometry because the reference and

object beams can be altered independently. In double-exposure methods, however, a dual-reference arrangement as
described previously must be used to permit independent control.
Phase-Stepping Methods. For phase stepping, several video images are recorded of the fringe pattern with a small
phase difference introduced between the reconstructed images prior to recording each video image. The phase shift can be
performed in several ways, but perhaps the most common method is to use a mirror mounted on an electromechanical
translation device such as a piezoelectric element. If the phrase shift imposed prior to each video recording is known, then
only three images need be recorded. Because the intensity at each point on the image is known to be a function of the
three variables described above, intensity information from the three images can be used to solve a series of three
equations in three unknowns. In addition to providing automated interpretation of fringe patterns, phase stepping affords
an increase in displacement sensitivity by as much as 100-fold ( of a fringe) relative to fringe-counting methods. In
practice, most investigators report a sensitivity boost of about 30.
Heterodyning Method. Still higher holographic sensitivity can be obtained with heterodyne holographic
interferometry. As with phase stepping, independent control of the interfering images must be provided either by real-
time analysis or dual-reference methods. Instead of introducing a phase shift between several recorded images, a fixed
frequency shift is introduced in one reconstructing beam relative to the other. Typically, acousto-optic phase shifters are
used to produce a net frequency shift of the order of 100 kHz. As a result, fringes once visible in the reconstructed
interferogram are now blurred because of their apparent translation across the image field at a 100 kHz rate. A single-
point optical detector placed in the image plane can detect this fringe motion and will produce a sinusoidal output signal
as fringes pass by the detection spot. By comparing the phase of this sinusoidal signal to that obtained from some other
point on the image, the difference in displacement or contour can be electronically measured. An entire displacement map
can be obtained by scanning the optical detector over the entire image. Because scanning is required, the speed of
heterodyne holographic interferometry is relatively slow. Sensitivities approaching of a fringe have been obtained,
however.

References cited in this section
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
Optical Holography
Revised by James W. Wagner, The Johns Hopkins University

Holographic Components
The basic components of a holocamera are the:
• Light source (laser)
• Exposure controls
• Beam splitter
• Beam expanders (spatial filters)
• Mirrors
• Photographic plate or film holder
• Lenses
• Mounts for the equipment
• Tables to support the holographic system
Components and complete holographic systems are commercially available (Ref 18, 19).
Laser Sources
The characteristics of six types of lasers commonly used for holography are listed in Table 1. Helium-neon, argon, and
ruby lasers are the most common. Helium-cadmium and krypton lasers, although not used as frequently, can fulfill special
requirements for CW applications. Frequency-doubled Nd:YAG lasers are finding increasing popularity for pulsed
holographic applications.
Table 1 Wavelengths and temporal coherence lengths of the six types of laser beams in common use for
holography

Wavelengths
Temporal coherence length

Minimum
Typical
Type of laser beam

nm

Electromagnetic

spectrum
mm

in. mm

in.
Helium-neon 633 6330 Orange-red 152

6 457

18
422 4220 Deep blue 75 3 305

12
Helium-cadmium
325 3250 Ultraviolet 75 3 305

12
514 5140 Green 25 1 914

36
(a)


488 4880 Blue 25 1 914

36
(a)


Argon
(b)

(b)

(b)
25 1 914

36
(a)


647 6470 Red 25 1 914

36
Krypton
(c)

(c)

(c)
25 1 914

36
Ruby 694 6940 Deep red 0.8 0.030

914

36
1064

10640

Infrared
(d)

(d)


Nd:YAG
532
(e)


5320
(e)


Green

(e)

(d)

(d)


(a)
Typical temporal coherence length achieved when laser incorporates an etalon (selective filtering device) to make it suitable for holography.
(b)
Six other visible lines.
(c)
Nine other visible lines.
(d)
Varies with cavity design.
(e)
Frequency-doubling crystals

Helium-neon lasers are the most popular laser source when low powers are sufficient. Excitation of the gas is
achieved through glow discharge. These lasers have excellent stability and service life with relatively low cost. Another
type of laser is usually considered only when a helium-neon laser will not perform as required. A 20-mW helium-neon
laser in a stable system can conveniently record holograms of objects 0.9 m (3 ft) in diameter. (Within the limits of
coherence length and exposure time, as discussed previously, even larger objects could be recorded.) Such a laser
consumes 125 W of 110-V electrical power and operates in excess of 5000 h without maintenance. A 5-mW laser records
objects 460 mm (18 in.) in diameter and operates for more than 10,000 h without maintenance.
Helium-cadmium lasers are closely related to helium-neon lasers, with the following differences:
• Tube life is poor by comparison (approximately 1000 to 2000 h)
• The principal visible wavelength 422 nm (4220 Å) is 30% shorter (Table 1
), which provides increased
sensitivity and allows the use of recording mediums sensitive to blue light

• They have an output in the ultraviolet (325 nm, or 3250 Å), which is half the wavelength of helium-
neon lasers and produces doubly sensitive displacement measurements
• There is more danger to the eyes at the shorter wavelengths produced by helium-cadmium lasers
Argon and krypton ion lasers can be the least expensive holographic sources on the basis of light output per dollar.
Laser outputs of 1 W with 9 m (30 feet) of coherence length are available. Low-power argon lasers, however, are more
expensive than helium-neon lasers. The use of an argon laser should be considered over a helium-neon laser in the
following situations:
• When stability or dynamic conditions necessitate short exposures requiring high light power
• When the recording of large objects requires higher power to record good holograms
• When the recording medium requires high-power blue or green light
• When the holographic
system needs the higher sensitivity provided by the shorter wavelengths of the
argon laser
Argon lasers in the 1- to 4-W output range, equipped with an etalon (a selective filter required to achieve long
coherence length), are excellent holographic light sources; however, a heium-neon or a helium-cadmium laser may be
preferred for the following reasons:
• Argon lasers consume thousands of watts of electrical power and require water cooling
• Gas excitation is by electric arc, which generates high electrica
l and thermal loads on components and
makes reliability and stability lower than with a helium-neon laser
•
The output power is well above that which causes damage to the eyes, especially at the shorter
wavelengths produced by argon lasers. Most available data indicate that helium-
neon lasers are
incapable of causing the damage that could be caused by an argon laser. Therefore, safety requirements
for argon lasers must be more stringent
Krypton lasers are essentially the same as argon lasers except that the tubes are filled with krypton gas instead of argon
gas. The output wavelengths are longer (Table 1) and the power is lower than for an argon laser. A 2-W argon laser
produces 0.8 W in its most powerful line (514 nm, or 5140 Å), while the same laser device filled with krypton produces
0.5 W at 647 nm (6470 Å) and 1.3 W total. Argon and krypton gases can be combined in the tube to give custom outputs

over a wide range of wavelengths.
Ruby lasers use rods of ruby instead of a gas-filled glass tube as a lasing medium. Excitation of the medium is by
optical pumping using xenon flash lamps adjacent to the ruby rod. Ruby requires such high energy inputs to lase that the
waste heat cannot be removed fast enough to sustain continuous output. For this reason, ruby lasers are always operated in
a pulsed mode, and the output is usually measured in joules of energy per pulse (1 J of energy released per second is 1 W
of power). Peak output powers of ruby lasers exceed 10 MW, requiring extensive safety precautions.
The development of pulsed ruby lasers for holography has progressed with the need to record holograms of moving (or
highly unstable) objects. Ruby lasers have been extensively used to record the shock waves of aerospace models in wind
tunnels, for example. Most holographic interferometry done with a ruby laser uses a double-pulse technique. The tasks
that require a ruby laser are those that cannot be done with a helium-neon or an argon laser. Ruby lasers can routinely
generate 1 J, 30-ns pulses of holographic-quality and relatively long coherence length light, which is sufficient for
illuminating objects up to 1.5 m (5 ft) in diameter and 1.8 m (6 ft) deep.
The problem with ruby lasers lies in generating the two matching pulses required to record a suitable interferogram. Most
lasers can be either pulsed once during each of two consecutive flash lamp cycles or Q-switch pulsed twice in the same
flash lamp pulse to record differential-velocity interferograms. The pulse-separation time in the one flash lamp pulse
mode extends to 1 ms. Generating two matching pulses becomes more difficult as the pulse-separation time exceeds 200
ms because of the dynamic thermal conditions in the laser cavity. The result is images with contour fringes that modulate
displacement fringes, thus obscuring the information sought.
The operation of a ruby laser, when changing pulse-separation time or energy, requires the possible adjustment of flash
lamp voltages, flash lamp timing with respect to the Q-switch timing, Q-switch voltages, and system temperatures. These
conditions change as the laser system ages. Setting up the system requires many test firings to achieve stable
performance. In short, operation of the laser requires high operator skill. In addition, the high performance of these
systems requires care in keeping the optical components clean; buildup of dirt can burn the coatings on expensive optical
components. The periodic replacement of flash lamps and other highly stressed electrical and optical components is to be
expected. A helium-neon or a krypton laser is usually needed to reconstruct a ruby-recorded hologram for data retrieval.
Differences between recording and reconstruction wavelengths lead to aberrations and changes in magnification in the
reconstructed images.
Nd:YAG Lasers. Pulsed Nd:YAG lasers are constructed similarly to ruby lasers. Instead of a ruby rod, however, a
neodymium-doped yttrium aluminum garnet rod is substituted as the lasing medium. The Nd:YAG laser is more efficient
than the ruby system, but it operates in the near infrared at a wavelength of 1.064 μm (41.89 μin.). Frequency-doubling

crystals with efficiencies of approximately 50% are used to produce light at a more useful green wavelength of 532 nm
(5320 Å). All of the pulsed modes of operation available with the ruby system are also available with Nd:YAG system.
The reconstruction of pulsed holograms can be performed with an argon ion laser at 514 nm (5140 Å). Owing to
somewhat better thermal properties, continuous-wave Nd:YAG lasers are available with power capabilities well over 50
W (multimode), but their application in holography is still quite limited.
Exposure Controls
Most holographic systems control light by means of a mechanical or electrical shutter attached to the laser or separately
mounted next to the laser. More sophisticated systems have photodetectors in the optical system and associated
electronics that integrate the light intensity and close the shutter when the photographic plate or film has been properly
exposed. Holographic systems that require strobing capabilities use acousto-optic modulators that can modulate the laser
beam at rates up to at least 10 MHz and with 85% efficiency. It should be noted that a strobed system with a 5% duty
cycle will have an effective brightness of 5% of normal (a 20-mW laser is effectively a 1-mW laser).
Beam Splitters
A piece of flat glass is usually a sufficient beam splitter for a production holographic system designed for recording only.
If the system is to be used for recording and reconstruction or for real-time analysis, there are two approaches:
•
The less expensive system uses a beam splitter that splits 20 to 30% of the light into the reference beam;
a variable attenuator or a filter wheel is used to adjust the reference beam to the proper intensity
• The more expensive approach is to use a var
iable beam splitter, which consists of a wheel that varies the
split from 95-to-5% to 5-to-95% as the wheel is rotated
Beam Expanders and Spatial Filters
Beam Expanders. Expansion of the narrow laser beam is required to illuminate the test object as well as the
holographic film. A short focal length converging lens is often used for this purpose, ultimately causing the beam to
diverge for distances greater than the focal length of the lens. For high-power pulsed-laser sources, a diverging lens must
be used because the field strengths may become so intense at the focus of a converging lens that dielectric breakdown of
the air may occur.
Spatial Filters. An unfiltered expanded laser beam usually displays diffraction rings and dark spots arising from
extraneous particles on the beam-handling optical components. These rings and spots detract from the visual quality of
the image and may even obscure the displacement-fringe pattern. For most CW holographic systems, laser powers are

sufficiently low that spatial filters can be used to clean up the laser beam. Spatial filters basically consist of a lens with a
short focal length and an appropriate pinhole filter. By placing a pinhole of the proper size at the focal point of the lens,
only the laser light unscattered by dust and imperfections on the surfaces of the optical components can pass through the
pinhole. The result is a uniform, diverging light field.
Pinhole Size and Alignment Specifications. A good spatial filter uses a high-quality microscope objective lens; a
round, uniform pinhole in a foil of stainless steel or nickel; and a mount that allows the quick and stable positioning of the
lens and pinhole. A complete analysis of the best pinhole size includes the factors of beam diameter, wavelength, and
objective power. If the pinhole is too small, light transmission will suffer, and alignment will be very sensitive. As the
pinhole size is increased, alignment is easier to achieve and maintain. As the pinhole size becomes too large, it begins to
allow off-center, scattered light to pass through, with the result that the diverged beam will contain diffraction rings and
other nonuniformities associated with dust and dirt. The pinhole will then begin to transmit information to construct the
diffraction field of the particle. This does not prevent the recording of holograms; it only generates unwanted variations in
light intensity. As a general rule, the magnification power of the objective multiplied by the pinhole diameter (in microns)
should equal 200 to 300.
The position of the pinhole should be adjusted at a laser power level below 50 mW. A misaligned pinhole at a high power
level can be burned by the intense point of light, rendering the pinhole useless. High-magnification spatial filters require
the most care. With proper alignment, standard pinholes will function without degradation when the laser output power in
watts multiplied by the objective magnification does not exceed 20.
Mirrors
Most holographic mirrors are front-surface coated. Second-surface-coated mirrors are generally unsatisfactory, because of
losses at the front surface and the fact that the small reflection that occurs at the front surface generates unwanted fringes
in the light field, which interfere with interpretation. Mirrors for holography do not need to be ultraflat. Inexpensive front-
surface-coated mirrors are readily available in sizes up to 610 mm (24 in.). Metal-coated mirrors are usually the least
expensive, but they can cause a 15% loss of reflected light. Dielectric-coated mirrors have greater than 99.5% reflectivity,
but are much more expensive than metal-coated mirrors and are sensitive to the reflection angle. All mirrors, like all other
optical components, require some care with regard to cleanliness. The cleaning of some mirrors is so critical that in many
applications it is best to use inexpensive metal-coated mirrors, which can be periodically replaced. The manufacturer
should be consulted in each instance as to the proper procedure for cleaning each particular type of mirror. Because
mirrors reflect light rather than transmit it, they are a particularly sensitive component in a holographic system. They must
be rigidly mounted and should be no larger than necessary.

Photographic Plate and Film Holders
Photographic plate and film holders perform the following two functions:
• They hold the plate (or film) stable during holographic recording
• They permit precise repositioning of the plate (or film) for real-time analysis
The first function is not difficult to achieve, but the second function is. If real-time analysis is not required, glass plates or
films will work in almost any holder or transport mechanism. Real-time work requires special considerations. The
problems inherent in real-time work can be handled by the use of replaceable plate holders, in-place liquid plate
processors, and nonliquid plate processing.
Replaceable Plate Holders. With replaceable plate holders, the photographic plate is placed in the holder, exposed,
and removed for processing. After processing, the plate is put back in the holder; the plate must be as close as possible to
its original position in the holder to permit real-time analysis. Some plate holders have micrometer adjustments to dial out
residual fringes. As a production method, the use of replaceable plate holders is very slow.
In-place plate processing is accomplished by using a liquid-gate plate holder (termed a real-time plate holder),
which has a built-in liquid tank with appropriate viewing windows. The plate is immersed in the liquid in the tank
(usually water) and allowed to soak for 15 to 30 s. Upon exposure, the tank is drained of the immersing liquid, and the
plate is developed in place by pumping in the proper sequence of developing chemicals. After the plate is developed, the
developing chemicals are replaced with the original immersing liquid, and the hologram is viewed through the gate of
liquid. This procedure not only permits processing of the plate without disturbing its position but also eliminates the
problem of emulsion swelling and shrinking, which causes residual fringes in many real-time setups. Plate development
can take less than 30 s; total processing time is 1 min or less. Commercial systems are available that cycle the appropriate
liquids through the cell as well as provide film advance for holographic films in a continuous-roll format.
Another holographic camera system permitting in-place development uses a thermoplastic recording medium that is
developed by the application of heat. Such systems are available from at least two commercial suppliers. One system
permits erasure and reexposure of the thermoplastic film plate with cycle times of just under 1 min. The plates can be
reexposed at least 300 times. These systems and the high-speed liquid-gate processing systems mentioned above
eliminate many of the inconveniences associated with holographic film handling and processing.
Nonliquid Plate Processing. Other in-place processing systems have been devised. Nonliquid plate processing using
gases for self-development holds much promise for holographic recording. Photopolymers are promising as production
recording media because they can generate a hologram quickly and inexpensively. For one photopolymer film, the
photopolymer is exposed at a much higher energy level than is a silver emulsion (2 to 5 mJ/cm

2
versus 20 J/cm
2
or less
for silver emulsions). After exposure, the hologram is ready to use. However, to prevent further photoreaction during
viewing, the hologram is fixed by a flash of ultraviolet light.
Lenses
Lenses are required in some holographic systems. If the function of the lens is to diverge or converge a light beam, almost
any quality of lens will suffice. However, if precise, repeatable control is desired, the lenses may need to be diffraction
limited. Analysis of a proposed holographic system is sometimes best done by trial and error or by use of the best possible
components, rather than by attempting a complicated mathematical computation.
Lenses can be antireflection coated if needed or desired. For example, lenses used in pulsed ruby systems for diverging a
raw beam should be fused-silica negative lenses with a high-power antireflection coating. Some low-power ruby laser
systems, however, have operated satisfactorily with uncoated lenses. Guidelines for using lenses with ruby lasers are
available from the laser manufacturer.
Mounts
Mounts for the holographic components should be carefully chosen. Mounts that require adjustments should be kept to as
few as possible. All fixed mounts should be bolted or welded in place. Some attention should be given to mount material;
aluminum, for example, is generally a good material, but because of its high coefficient of thermal expansion, an
alternative material might be more suitable in a given application. Mounts that are rugged and rigidly built should be
selected. Holographic components should be positioned as close to the supporting structure as is practical.
Holographic Tables
Holographic components must be mounted with sufficient rigidity and isolation from ambient vibration to maintain their
dimensional relationships within a few millionths of an inch during recording and real-time analysis. As discussed
previously, the use of pulsed lasers to generate double-exposure holograms requires very little vibration isolation so long
as the separation between exposures is short. When vibration isolation is required, the designer must exercise care in the
design of the structure used to support the holographic system in order to isolate the structure from outside excitation.
This design involves the three following considerations:
• Building the structure with sufficient rigidity to reduce the deflection of componen
ts to within

holographic limits
•
Building the structure with sufficient damping capacity to absorb excitation energy and to prevent
excessive resonant-vibration amplitudes
• Building the structure with sufficient mass to increase inertia and therefore decre
ase response from
outside driving forces
Small, low-cost holographic systems are usually supported by one of a variety of vibration-isolation tables, which float on
three or four rubber air bladders or small inner tubes. The holographic components are screwed, clamped, magnetically
held, or simply set in place on the table. As the size of the holographic system (and therefore the size of the table)
increases, more care is needed to maintain stability. There are three basic types of large holographic tables:
• Honeycomb tables
• Slabs
• Weldments
Honeycomb tables, made of honeycomb-core sandwiched panels, are extensively used for holography. They can
range in thickness from 50 mm to 0.9 m (2 in. to 3 ft). The outer skin is usually a ferromagnetic stainless steel, but for
increased temperature stability, an outer skin of Invar can be used. Honeycomb tabletops weigh less than 10% as much as
a steel table of equivalent rigidity. They can be fabricated from vibration-damping materials to make them acoustically
dead. The tables are usually floated on three or four air mounts. Air mounts (generally forming a leg for the table) are
large air cylinders with rolling-diaphragm pistons that contact the table. A servovalve inputs or exhausts air at the cylinder
to maintain constant height of the leg and keeps the table level as components are moved about. Air mounts provide
excellent isolation by virtue of their low resonant frequencies, typically 1 to 2 Hz. The holographic components can be set
on the honeycomb table, attached with magnetic clamps, or screwed down utilizing an array of drilled-and-tapped holes in
the upper-skin centers.
Most solid tabletops used in holography are flat within 0.025 mm (0.001 in.) or less, while honeycomb tables (1.2 × 2.4
m, or 4 × 8 ft) are flat within 0.10 to 0.25 mm (0.004 to 0.010 in.). This difference does not hamper the performance of
most holographic systems.
Slabs are the least costly type of support for a large holographic system. They are usually made from steel or granite and
floated on a vibration-isolation system. Many low-cost supports have been laboratory constructed by floating a surplus
granite or steel surface plate on an array of tire tubes. It is usually difficult to dampen vibrations that reach the surface of a

slab. For this reason, the performance of a slab degrades when the test objects are large and/or the ambient noise level is
high (particularly from air-conditioning systems that emit low-frequency noise). This problem can be minimized by
selecting a material (such as gray iron) that has naturally high damping capacity rather than a material that has a ring.
Most components that are attached to three-point mounts need not be rigidly attached to the slab, but other components
(and particularly the object being vibrated or otherwise stressed) need to be rigidly mounted to ensure stability. To
facilitate the mounting of components, the slab top may require tapped holes, T-slots, or a coating of tacky wax, or it may
need to be ferromagnetic.
Weldments are heavily braced frames or plates generally designed as part of a portable or otherwise special system
used to analyze very large or unusual test objects. Weldments are generally used where slabs or honeycomb tables are not
suitable, although a slab or honeycomb-core sandwich panel may be part of the structure for mounting the components.

References cited in this section
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
Optical Holography
Revised by James W. Wagner, The Johns Hopkins University

Types of Holographic Systems
There are basically two types of holographic systems: stationary and portable. Both will be discussed in this section.
Stationary Holographic Systems
A holographic system is considered stationary when it is of such size, weight, or design that it can be utilized only by
bringing the test object and required stressing fixtures to the system for analysis. Stationary systems are usually dependent
on building services, requiring compressed air for the vibration-isolation system, electric power for the laser and other
electronic components, and running water for processing the holograms and cooling the laser (for example, as required for
an argon laser). Most stationary systems operate in a room with light and air control to achieve high stability and the low

light levels required for recording, processing, and viewing holograms. As the size of the table increases, the stability
requirements become more difficult to satisfy. As a result, the cost of a stationary system generally increases
approximately exponentially with object size.
Stationary systems are used in the following cases:
• Production line inspection of small objects
• Where required flexibility in the type and size of test objects is needed for developmental work
• Inspection of a large or awkward structure that cannot be holographed by a portable system
Portable Holographic Systems
A portable holographic system can be moved to the test object and operated with minimal setup time. A portable system
built for the Apollo lunar exploration program was designed to record holograms of lunar soil. It was battery powered,
weighed 7.89 kg (17.4 lb), and occupied less than 0.017 m
3
(0.6 ft
3
) of area. A portable holographic system used for
developmental work, particularly in wind tunnels, has been transported throughout the United States by semitrailer truck.
The components of the system are mounted in cabinets or in frames on wheels, and upon arrival at the test site, the system
is unloaded and set up in several hours. A portable system used to inspect sandwich-structure helicopter-rotor blades is
shown schematically in Fig. 5; a portable system used to inspect sandwich panels is shown in Fig. 10.

Fig. 10 Portable holographic analyzer for the inspection of sandwich panels with use of vacuum stressing.
See
description in text.
Reflection Holographic Systems. In a simplified type of portable system, a tripod-mounted laser and spatial filter
project light directly through a holographic plate fastened to the test object. A reflection hologram is formed by
interference between the light traveling through the plate and the light reflected back to the plate from the object. In this
system, the reference beam and the object beam strike the emulsion from opposite sides of the plate, resulting in the
reconstruction of a virtual image produced by reflection. This configuration differs from the holographic systems
described earlier, in which the two recording beams strike the plate from the same side and, during reconstruction, the
virtual image is produced by transmission.

An important consideration in designing reflection holographic systems is that reflection holograms are more sensitive
than transmission holograms to photographic emulsion shrinkage, which may take place during the development and
drying processes. This shrinkage causes the image formed during reconstruction to be produced at a slightly shorter
wavelength (a hologram recorded with red laser light will reconstruct best in yellow or green light). Unless the
reconstructing light matches this shorter wavelength, the image will be quite faint. Therefore, white light, which contains
all the required wavelengths for efficient image production, is often used as the reconstructing reference beam instead of
laser light.
Because, as described above, the film plate also serves as a beam splitter and can be mounted to the test object itself,
reflection holographic systems can be quite insensitive to object vibration. The major critical stability requirement is the
relationship between the object and the holographic plate fastened to it. If the emulsion shrinkage is fairly uniform and the
holographic plate is in close proximity to the test object, the image formed will be bright and clear. The plate must be
dried carefully, however, to avoid variations in emulsion thickness, which would cause variation in the color of the image.
Because the diffraction of light changes with color, variations in color will cause smearing of the image and loss of
resolution. The greater the distance from the object to the plate, the greater the smearing.
Portable systems can be used in the following cases:
• Field inspection
•
When the size or configuration of a test object is such that it is more practical to attach the holographic
system to the object than vice versa
• When the test object is in an environment or in a structure required as part of the experiment
Portable holographic systems are usually designed to inspect a specific part or a range of small parts. The questions that
establish criteria for designing a portable system are the following:
• How is the system to be powered?
• How is stability between the
system and the test object to be maintained both during and between
exposures?
• How can critical adjustments be made or eliminated?
• How is the photographic plate to be handled and processed?
• How can the holographic components and the stressing fixture b
e designed into a workable system

within the definition of portable?

Optical Holography
Revised by James W. Wagner, The Johns Hopkins University

Typical Holographic Testing Applications
Among the applications for which holographic testing is utilized are the following:
• Inspection of sandwich structures for debonds
• Inspection of laminates for unbonded regions
• Inspection of metal parts for cracks
• Inspection of hydraulic fittings
• Measuring of small crack displacements
• Vibration analysis of turbine and propeller blades
• Holographic contouring
• Characterization of composite materials
Optical Holography
Revised by James W. Wagner, The Johns Hopkins University

Inspection of Sandwich Structures for Debonds
A sandwich structure usually consists of face sheets separated by a lightweight core. The face sheets are designed to carry
in-plane loads; the core is designed to stiffen the face sheets and prevent them from buckling and to carry normal loads in
compression or shear. The core can be a solid material, such as balsa wood, or a cellular material, such as foam plastic or
honeycomb construction. The face sheets and core are usually held together by an adhesive or braze material.
The extensive use of sandwich structures in widely varying applications has created some unusual inspection problems.
The main area of interest is the quality of the attachment of the face sheets to the core; large areas of structures must be
inspected inexpensively for unbonded or unbrazed areas (debonds) and for poorly bonded or brazed regions. The
inspection for poorly bonded or brazed regions has not yet been satisfactorily accomplished on a production basis, but is
currently the subject of various research programs. A secondary area of interest is the edge-closure assembly, which
surrounds the sandwich structure. The configuration of the closure varies depending on whether the sandwich is brazed or
bonded and on the types of materials involved. Sandwich structures cause inspection problems because of the number of

parts being joined together and the abrupt changes in thickness or solidity of the assembly. An additional complication
arises in designing a holographic inspection system for sandwich structures in that structures range in area from several
square inches to several square feet and their contours vary from simple flat panels to complex curved shapes, such as
helicopter rotor blades. Both sides and all edges of the structures must be inspected.
When inspecting sandwich structures, it is necessary to determine which stressing technique or combination of techniques
will best detect the types of flaws likely to be present. If several techniques are chosen, it must be decided whether to
apply them simultaneously or sequentially. The two stressing techniques that have been found to work well for the routine
inspection of sandwich structures are thermal stressing (Fig. 11) and vacuum stressing (Fig. 4), although stress for
inspection can be provided by acoustical loading (Fig. 3), fatigue loading (Fig. 5), or impact loading.

Fig. 11 Honeycomb-
core panel illustrating the detection of debond by thermal stressing. (a) Section through
the region of debond. (b) Same section as in (a) showing bulge in fa
ce sheet over the region of debond, caused
by gentle heating of the face sheet
Techniques for inspecting sandwich structures are well established and documented (Ref 20). Most inspection of
sandwich structures is being done, and probably will be done, using CW techniques, with pulsed-laser techniques being
used only for special applications.
Example 1: Detection of a Debond in a Honeycomb-Core Panel With Thermal-
Stressing Holographic Techniques.
As an example of holographic inspection using thermal stressing, assume that a honeycomb-core sandwich panel, such as
that illustrated in Fig. 11(a), contains a debond. If the face sheet over the debond is gently heated, the region over the
debond will become hotter faster because the heat in that region is not conducted away to the core. The result of this
differential-temperature field is a slight bulge in the heated face sheet (Fig. 11b). Using either real-time or double-
exposure time-lapse holographic techniques, an image will be formed in which the region of the bulge is contoured by a
set of fringes representing lines of constant displacement between the two images.
A typical set of fringes caused by a debond in a honeycomb-core sandwich panel is illustrated by the interferogram in Fig.
12, which demonstrates the sensitivity of inspection by thermal stressing. This interferogram was obtained using the
double-exposure technique and a pulsed ruby laser, which has a wavelength of 694 nm (6940 ) (Table 1). Using Eq 3, it
can be seen that each fringe, except the first, represents an out-of-plane displacement of 0.694/(2 sin 90°), or about 0.33

m (13 in.). The region above the debond in the panel, which had a face sheet 500 m (0.02 in.) thick and was heated
only about 2.8 °C (5 °F), has a maximum displacement of about 3 m (120 in.), or only about 0.6% of the face sheet
thickness.
Example 2: Holographic Detection
of a Debond in a Sandwich
Structure Using the Vacuum-
Stressing Method.
Inspection by vacuum stressing has also been found to
be effective for detecting debonds in sandwich
structures. Figure 10 shows the essential components
of a portable holographic analyzer for the inspection
of sandwich panels. This analyzer consists of two
separate sets of equipment: one set for vacuum
stressing and holographic recording (Fig. 10a) and
one set for holographic reconstruction (Fig. 10b). The
holographic recording system is mounted on the top
of a hollow supporting structure that rests on the test
panel. The recording system contains a 3-mW CW
helium-neon laser and, through the use of suitable
optical components, encompasses a 460 mm (18 in.)
diam circular field of view of the portion of the panel
surface beneath the supporting structure.
During an inspection, the vacuum chamber, which is
made of fiberglass, is lowered over the recording
system until contact is made with the surface of the
panel. A first exposure is made of the panel in its
unstressed state. A second exposure is made when the
internal pressure has been reduced by approximately 7
kPa (1 psi). The pressure of the ambient air in the core voids pushes the face sheets out at unbonded regions. The double-
exposure hologram is recorded as a circular field 8 mm (0.3 in.) in diameter on a 16 mm ( in.) film strip. The film is

then advanced, the vacuum is released, the system is moved to the next location on the panel, and the sequence is
repeated. The total time required for constructing a double-exposure hologram is usually about 1 min or less. By using a
film strip 2030 mm (80 in.) long, it is possible to record a total of more than 200 double-exposure holograms.
A typical commercial holographic analyzer for the inspection of sandwich structures is illustrated in Fig. 13. It
consists of a 3.7 × 2.4 m (12 × 8 ft) table supported on air bearings. On this table is a part-holding mounting plate, which
is supported by two 1220 mm (48 in.) diam trunnion plates. The mounting plate can be rotated and translated to view
either flat panels or curved shapes. The holographic system usually uses a 50-mW helium-neon laser, but can also use
higher-powered argon lasers. The part to be inspected is held in place on the mounting plate by a series of vacuum cups or
clamps. Thermal, vibrational, pressure or vacuum stressing can be applied. A part measuring up to 1.5 × 1.8 m (5 × 6 ft)
can be inspected with this analyzer. Other analyzers are available that are designed to handle either smaller parts or larger
parts (up to 1.8 × 6.1 m, or 6 × 20 ft).

Fig. 12 Double-exposure time-lapse inter
ferogram of a
thermally stressed honeycomb-
core sandwich panel
showing a fringe pattern (arrow) contouring a region of the
front face sheet over a debond. The panel had metal face
sheets 500 μ
m (0.02 in.) thick and was heated about 2.8
°C (5 °F) between exposures, which were made with a
pulsed ruby laser. The fringes indicate a maximum
displacement over the debond of 3 μm (120 μ
in.). The
background fringes were caused by general movement of
the face sheet due to heating.

Fig. 13 Commercial holographic analyzer used for the inspection of sandwich structures.
See text for
description.

Approximately 0.19 or 0.28 m
2
(2 or 3 ft
2
) of a part surface can be viewed in a single hologram with the analyzer
illustrated in Fig. 13. The exposure and processing of the hologram are controlled automatically by the analyzer. With the
50-mW helium-neon laser, approximately 2.3 m
2
(25 ft
2
) of surface can be inspected per hour. Either double-exposure or
real-time interferograms can be made with this analyzer. Using the real-time technique, the inspected areas can be
recorded by taking still photographs through the hologram or preferably by recording the transient patterns on videotape.
Still photographs give an indication of obvious flaws, but far smaller flaws can be detected by an experienced inspector
viewing a sweeping fringe pattern. A library of videotapes showing various flaws can be used as a training and qualifying
aid. As in other inspection techniques, standards and qualifying procedures must be established. Application and further
development of automated fringe interpretation methods could also be used to advantage for this type of inspection.

Reference cited in this section
20.

R.C. Grubinskas, "State of the Art Surv
ey on Holography and Microwaves in Nondestructive Testing," MS
72-9, Army Materials and Mechanics Research Center, Sept 1972, p a40-46
Optical Holography
Revised by James W. Wagner, The Johns Hopkins University

Inspection of Laminates for Unbonded Regions
Standard optical holographic systems are readily used to identify the presence of flaws in laminated composites and
structures. Generally, double-exposure techniques are the most convenient, although both real-time techniques and time-

average techniques (for oscillating systems) can also be used. Because the most common flaw in laminates is a lack of
bonding, the critical problem is to select the most suitable stressing technique that will reveal the presence of the
unbonded region by inducing some differential movement between the bonded and unbonded regions.
Example 3: Holographic Tire Analyzer for Detecting Flaws With Vacuum-
Stressing Technique.
An example of a laminated composite is the pneumatic tire, which is constructed of layers of rubber and fabric. (Some of
the fabrics used in modern tires contain steel wires.) The commercial holographic tire analyzer illustrated in Fig. 14 is
capable of inspecting tires ranging in size up to a maximum outside diameter of 1145 mm (45 in.) at a rate of more than
12 tires per hour when auxiliary semiautomatic options are employed. The inspection procedure uses the vacuum-
stressing technique described previously; a double-exposure interferogram is made of each quadrant of the inner walls of
the tire undergoing inspection. Flaw anomalies are manifested by minute changes in the inner-wall topography occurring
as a result of the pressure differential existing between the unvented tire flaws and the evacuated chamber.

Fig. 14 Commercial holographic analyzer used for the inspection of pneumatic tires.
See text for a description
of the inspection procedure.
Example 4: Holographic Tire Analyzer for Detecting Defects Using 345 kPa (50
psi) Inflation Pressure.
A different type of holographic tire analyzer employed to obtain the interferogram shown in Fig. 15 used inflation to 345
kPa (50 psi) rather than vacuum stressing. With this analyzer, both of the sidewalls and the tread portion of the tire can be
inspected simultaneously for all unbonded regions.

Fig. 15 Double-exposure time-
lapse interferogram of a defective pneumatic tire that was stressed by inflation
to 345 kPa (50 psi) between exposures. The contoured fringe patterns (arrows) indicate regions of the sidewall
and tread where there is no bond betwe
en layers of the cured tire. General movement of the tire during
inflation caused the background fringes.
Additional Techniques for Stressing Laminates. Other laminates can be stressed by heating (as with printed
circuit boards), by ultrasonic excitation, and by the addition and removal of a mechanical load.

Example 5: Holographic Inspection of Coprene Rubber-Stainless Steel Laminate
Using Mechanical Loading Technique.
An example of the use of the mechanical-loading method is a simple rubber-steel laminate that was stressed in cantilever
bending while clamped along one edge. Holographic exposures made before and after stressing reveal a field of closely
spaced fringes (Fig. 16) due to anelastic effects that prevent full recovery. Local perturbations in this field due to an
unbonded region between the rubber sheet and the metal substrate can be readily identified as an anomaly in the fringe
pattern.

Fig. 16 Double-exposure time-lapse interferogram of a defective rubber-steel laminate that was mecha
nically
stressed between exposures showing a local perturbation in the fringe pattern (arrow) over an unbonded
region. The laminate was 125 mm (4.92 in.) long by 90 mm (3.5 in.) wide and was composed of a 0.94 mm
(0.037 in.) thick sheet of coprene rubber b
onded to a 1.93 mm (0.075 in.) thick sheet of stainless steel. The
rubber face was painted with a bright white reflecting paint to aid in holographic inspection.
Optical Holography
Revised by James W. Wagner, The Johns Hopkins University

Inspection of Metal Parts for Cracks
Optical holographic inspection has not proved to be an effective method for finding even large cracks in metal parts. The
primary reason for this is the difficulty of stressing the test object in such a manner as to create a difference in
displacement that is easily detectable optically. Speckle-pattern techniques can be useful for the detection of in-plane
displacements associated with surface cracks.
In general, only those cracks greater in length than the thickness of the part are detectable by optical holographic
inspection. Several stressing methods have been found to be useful, such as mechanical stressing by means of loading
fixtures or interference fasteners as well as thermal stressing by means of heat lamps or cold liquids or solids. Either real-
time techniques or double-exposure techniques can be used. Also, high-resolution techniques, such as phase stepping and
heterodyne holographic interferometry, have been used (see the section "Interpretation of Inspection Results" in this
article). Time-average time-lapse techniques are generally not successful for detecting the effects of cracks on vibratory
patterns.

Example 6: Holographic Inspection of Hydraulic Fittings to Detect Cracks.
An instance of the successful detection of cracks by optical holography was the inspection of small hydraulic fittings.
Radiographic studies using x-rays, eddy current testing, and other forms of nondestructive inspection did not provide
reliable detection of these small cracks or sufficient data on crack growth characteristics.
To solve the inspection problem, conventional double-exposure time-lapse holography was employed using a 15-mW
helium-neon laser and portable optical components. The plate holder was a conventional static type and held 100 × 125
mm (4 × 5 in.) glass plates. The entire optical system was mounted on a commercial 1.2 × 2.4 m (4 × 8 ft) vibration-
isolation (holographic) table.
Inspection consisted of first making a holographic exposure of a fitting held statically in a vise-type clamping fixture.
After the first exposure, a mating fitting was screwed into the test fitting to approximate normal in-use loading, and a
second exposure was then made on the same holographic plate. The resulting interferogram is shown in Fig. 17. Each
interference fringe in this reconstruction represents a displacement between exposures of approximately one-half the
wavelength of the helium-neon light (λ = 633 nm, or 6330 Å), or about 0.33 μm (13 μin.). The discontinuity in the fringe
pattern indicates that relative motion, or slippage, occurred along the front vertical edge of the fitting during loading.
Inspection at still higher loads revealed a small crack along that edge.

Fig. 17 Double-
exposure interferogram of a cracked hydraulic fitting. The fitting was stressed between
exposures by having a mating fitting screwed into it. The holocamera used a 15-mW heliumneon
laser. The
discontinuity in the fringe pattern (arrow) was caused by a small crack in the fitting.
The use of optical holographic inspection in this application permitted relatively inexperienced personnel to pinpoint
small cracks in several hydraulic fittings and to study the propagation of these cracks under varying load conditions. This
brief study program required approximately 6 h of engineering time and no special fixturing or tooling.
Example 7: Small Crack Displacement Measurements Using Heterodyne
Holographic Interferometry.
As mentioned above, the small out-of-plane displacements associated with most cracks make it difficult to apply
conventional (homodyne) holographic interferometry techniques. Still, small displacements do occur and can be
visualized holographically using high-resolution techniques. For example, Fig. 18 shows the results of a heterodyne
analysis of a region of a holographic interferogram near a surface-breaking crack. Total displacements of only 5 to 6 nm

(50 to 60 Å) were observed in this case with a background noise floor of about 0.6 nm (6 Å).

Fig. 18 Results of heterodyne holographic interferometry showing minute displacements adjacent to a surface-
breaking crack in a nickel-base superalloy.
A dual-reference, double-exposure recording technique was used where a bending stress was applied to the cracked
specimen between the holographic double exposures. The resulting hologram was reconstructed with a 100-kHz
frequency difference imposed between the two reconstructing beams so that the intensity of the image varied sinusoidally
at a 100-kHz rate.
Although each point on the image varies in intensity at 100 kHz, the relative phase of these oscillations varies from point
to point on the image, depending on the amount of displacement recorded between holographic exposures. Therefore,
when a small detector is scanned over the image, the phase of its output signal can be compared with some reference
phase from another (fixed) point on the object, as shown in Fig. 19. Because a phase difference of 360° corresponds to a
single interferometric fringe, the resulting map of phase difference is directly related to surface displacement. Electronic
phase measurement accuracy to 0.36° corresponds to of one fringe. Uncertainties resulting from the effects of
speckle and the environment do not permit meaningful measurements below this level.

Fig. 19 Dual-detector readout for heterodyne holographic interferometry
Optical Holography
Revised by James W. Wagner, The Johns Hopkins University

Vibration Analysis of Turbine and Propeller Blades
Vibration analysis using routine optical holographic techniques can significantly contribute to the inspection and
evaluation of turbine and propeller blades in both the design and manufacturing stages (Ref 21, 22, 23). A recommended
approach to turbine blade evaluation is the simultaneous holographic recording of both sides of the blade as it is excited
into vibration with shaker tables (at frequencies generally limited to less than 50 kHz), air-horn vibrators, electromagnetic
drive systems (where blade materials permit), or piezoelectric transducers (bonded or clamped to the blade). By the use of
simultaneous recording, information over and above the straightforward recording of vibrational mode patterns can be
obtained. Differences in the mode patterns for the two sides of the blade will suggest an absence of structural integrity at
those points, while differences in vibrational amplitude (for example, in hollow blades) can offer a relative measure of
blade wall thickness (that is, a check on cooling passage alignment).

Holographic techniques are applicable to all types of blades, both solid and hollow and of almost any size and shape, as
well as entire turbine wheels or propeller assemblies. However, some experimental problems may be encountered in the
dual-sided simultaneous recording of large parts, thus necessitating a two-step procedure. (Caution must then be exercised
to ensure that the driving frequency and amplitude are identical for both of the holograms.)
For blades of reasonable size (up to perhaps 100 to 125 mm, or 4 to 5 in., chord by 460 to 610 mm, or 18 to 24 in.,
length), the simultaneous recording of both sides of the blade can be accomplished by a standard holographic system
through the use of mirrors (taking care to maintain the path lengths of the reference beam and the object beam within the
coherence length of the laser). An alternative method of simultaneous recording is to use a dual holographic system. A
reasonably compact (1.2 × 1.2 m, or 4 × 4 ft) setup of a dual system can be assembled by first splitting the incoming laser
beam into two beams (one for each hologram), each of which is subsequently split again into a reference beam and an
object beam by a symmetrical arrangement of the various optical components, as shown in Fig. 20.

Fig. 20
Schematic of a compact dual holographic system for the simultaneous recording of both sides of a
turbine blade
An example of the type of data that can be obtained by simultaneous recording is illustrated in Fig. 21, which shows
interferograms of both sides of a hollow blade whose pressure wall is about 250 to 375 μm (10 to 15 mil) thinner than its
suction wall. Although the two interferograms in Fig. 21 were not recorded simultaneously, identical ultrasonic
frequencies and amplitudes were used to excite the blade during recording. This produced fringe patterns in solid regions
(the leading and trailing edges) that are essentially the same on the front and back surfaces. However, over the hollow
region of the blade, the vibrational amplitude on the pressure side is measurably larger (each fringe represents
approximately 0.30 μm, or 12 μin., of out-of-plane displacement) than that on the suction side. In addition, the vibrational
amplitude increases from the root to the tip of the blade, indicating a decreasing wall thickness for both the pressure and
the suction sides. Pressure stressing is an alternative to vibrational excitation for the holographic inspection of hollow
turbine blades for detecting weakened structural characteristics.

Fig. 21 Continuous-
exposure interferograms of both sides of a turbine blade that were recorded while the blade
was being vibrated. The interferograms were recorded at identical driving frequencies and amplitudes, such as

might be obtained with the compact dual holographic system shown in Fig. 20. The holocameras used helium-
neon lasers.
To establish the most favorable driving force and frequency for a particular blade, a somewhat extensive series of tests is
recommended (perhaps including sectioning of the blades) to correlate the results and to establish standards of
acceptance. Although such preliminary testing may be costly, for large production runs this inspection procedure could be
valuable for accurate wall thickness gaging in thin-wall structures, in which standard ultrasonic pulse-echo techniques are
the most difficult to effect.
The ability to observe one entire surface of a large test object at one time, rather than in a series of limited views, is one of
the most important benefits of using holography for nondestructive inspection. One entire surface of an 810 mm (32 in.)
diam jet engine fan assembly can be recorded in a single hologram (Fig. 22), considerably facilitating the performance of
a vibrational-mode analysis that alternatively would require transducers placed over the entire assembly.

Fig. 22 Continuous-ex
posure interferogram of one side of an 813 mm (32 in.) diam jet engine fan assembly
that was excited at a frequency of 670 Hz showing a 4-nodal-diam mode of vibration.
The holocamera used a
helium-neon laser.

References cited in this section
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 Holo
graphy 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
Optical Holography
Revised by James W. Wagner, The Johns Hopkins University

Holographic Contouring
When evaluating the shape of an opaque object, interference fringes are generated on the object that represent depth or
elevation contours. These fringes are usually generated most readily by optically interacting two images of the object that
have been slightly displaced from one another, although one method simply requires projecting onto the object a set of
fringe surfaces whose normals are roughly parallel to the line of sight of the viewer. The three principal techniques for
holographic contouring are discussed below.
Multiple-Source Contouring. In the multiple-source technique (which is not really holographic contouring, but rather
holographic interferometry), an optical system is used that incorporates a rotatable mirror for steering the object beam
(Fig. 23). A fixed contour map of the object can be generated with this system by making two holographic exposures on
the photographic plate that differ only by a slight rotation of the object beam steering mirror, which shifts the virtual
image of the object-illumination source.