ASM Metals HandBook Vol. 17 - Nondestructive Evaluation and Quality Control
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a subsequent inspection. Another example would be a coating process that needs a rapid thickness measurement to ensure
proper thickness.
Most quantitative methods are still under development. Nearly all the quantification methods must be tailored to specific
applications. Although some methods can be approached from a theoretical basis, most require an empirical development
effort involving reference standards, multiple timebased measurements, and interpretive methods or rules (Ref 5, 13).
Reference standards are as necessary for thermal methods as they are for other NDE methods. Standards ensure
consistent performance of the thermal activation methods and the temperature-sensing materials or equipment. Reference
standards are essential for quantitative techniques.
A reference standard should closely represent the thermal and surface characteristics of the components or items
inspected. For example, if the subject is a solder joint, the reference should be a solder joint with the same contact area,
heat sink, wire sizes, and other characteristics. The standard must also provide an anomaly-free condition and a second
unacceptable condition associated with a size or the presence of an anomaly. A standard for quantitative methods requires
a progression of anomaly sizes and locations. The standard needs to cover the potential accept/reject conditions that can
exist in the components to be inspected. This typically necessitates a test program to establish the anomaly sizes and
locations that are unacceptable for proper operation of the components to be inspected (Ref 16).
Multiple time-based measurements are needed to establish correlations between a response to thermal activation
and anomaly size and location (Ref 5, 16). For pulsed or modulated thermal excitation, the time delay of the temperature
response can be recorded as a relative phase change, which may be related to a parameter such as coating thickness (Ref
1) or delamination depth (Ref 3). Alternatively, variations in isotherm position with time can be monitored as a function
of anomaly size and depth to establish known time-based response characteristics. Figure 5 shows how the effusivity
(which is a function of the pulsed heat input and the time-dependent temperature change) directly over a 20 × 20 mm (0.8
× 0.8 in.) void in a carbon-epoxy laminate varies with time for void depths of 0.3, 1.12, and 2.25 mm (0.012, 0.044, and
0.088 in.).
Fig. 5 Experimental apparent-effusivity curves, after correction, of the carbon-
epoxy laminate sample in the
regions with embedded defects of 20 × 20 mm (0.8 × 0.8 in.) area at three depths. A, 0.3 mm (0.012 in.); B,
1.12 mm (0.044 in.); C, 2.25 mm (0.088 in.). Source: Ref 2
Quantitative interpretation rules are needed before the time-based data can be used in an evaluation. A simple
example is the phase change response as a function of coating thickness of yttria-stabilized zirconium on a nimonic
substrate, as shown in Fig. 6. For an acceptable coating thickness of 0.45 to 0.6 mm (0.018 to 0.023 in.), the data show
that a 0.5 Hz modulation of the thermal excitation will supply the necessary phase sensitivity and that acceptable coating
thickness will be indicated by phase angles ranging from 5 to 13°. This information then provides the accept/reject criteria
for tests conducted under these conditions.
Fig. 6 Variations in the phase of temperature changes versus the coating thickness of a yttria-
stabilized
zirconia coating on a nimonic substrate. The phase of the temperature changes are relative to the modulation
(0.5 and 5 Hz) of the thermal excitation. Source: Ref 1
For a three-variable situation in which void thickness, width and depth beneath a surface are determined, at least three
time-based measurements are needed (Ref 5). The resulting calculations relating defect geometry and location are
complicated and can be performed only with computer assistance.
References cited in this section
1.
D.P. Almond, P.M. Patel, and H. Reiter, The Testing of Plasma Sprayed Coatings by Thermal Wave
Interferometry, Mater. Eval., Vol 45, April 1987, p 471-475
2. D.L. Balageus, A.A. Déom, and D.M. Boscher, Characterization and Nondestructive Testing of Carbon-
Epoxy Composites by a Pulsed Photothermal Method, Mater. Eval., Vol 45, April 1987, p 461-465
3. J.C. Murphy and G.C. Watsel, Photothermal Methods of Optical Characterization of Materials,
Mater.
Eval., Vol 44, Sept 1986, p 1224-1230
5.
T.V. Baughn and D.B. Johnson, A Method for Quantitative Characterization of Flaws in Sheets by Use of
Thermal Response Data, Mater. Eval., Vol 44, June 1986, p 850-858
13.
H. Kaplan and R. Friedman, Two New Portable Infrared Instruments for Plant Inspection, Mater. Eval.,
Vol
39, Feb 1980, p 175-179
16.
J.H. Williams, Jr., and R.J. Nagem, A Liquid Crystal Kit for Structural Integrity
Assessment of Fiberglass
Watercraft, Mater. Eval., Vol 41, Feb 1983, p 202-210
Thermal Inspection
Grover Hardy, Wright Research and Development Center, Wright-Patterson Air Force Base; James Bolen, Northrop Aircraft Division
Applications
The applications discussed below provide examples of how thermal methods have been used. For ease of discussion, the
applications have been divided into the following categories:
• Hot and cold equipment
• Process control
• Liquid intrusion
• Disbonds, delaminations, and voids
• Electronic devices
• Research
Hot and Cold Equipment. Many types of equipment that conduct or generate heat during operation are likely
candidates for thermographic methods. Heating ducts, steam lines (Ref 13), radiators (Ref 17), heat exchangers (Ref 15),
exhaust systems/chimney stacks (Ref 13), and refrigeration systems are heat transfer devices for which thermal inspection
techniques can be used during periodic inspections for leaks, clogged passages, and missing or defective insulation.
Furnaces, ovens, salt baths, autoclaves, reaction stacks, and hot manufacturing equipment (such as presses and rolling
mills) may also require periodic inspection for unnecessary heat losses. Thermographic techniques are also used to inspect
cryogenic tanks.
Other equipment, such as bearings, slides, brakes, transmitting antennas, and electrical equipment, generates heat during
operation. Localized hot areas are usually a symptom of a mechanical or electrical malfunction, and early detection
provides the opportunity to replace the defective components during regularly scheduled maintenance or before more
serious damage occurs.
Process Controls. Thermal inspection methods are appropriate for certain processes in which the products are above
room temperature as they exit a process. Examples are heat-set and heat-shaped plastics, hot-worked metal components,
hot coating processes, and weld components. Components can be monitored as they exit the process. Abnormal
temperatures would indicate an out-of-control process, and corrections can be made to prevent the production of large
numbers of defective parts. An example of an actively heated process control method is the technique for monitoring
phosphate coating thickness using infrared absorption characteristics (Ref 18).
Liquid Intrusion. Water or fuel intrusion in honeycomb structures is a significant problem for aircraft maintenance.
Inspections can be performed immediately after flight to detect such liquids as the structure warms to ambient
temperature. Water intrusion in roofs can also be detected in the evening as ambient temperature drops. This method has
been reported to be successful in detecting leaks and retained moisture in insulation (Ref 19). The level of fluids in a
sealed tank can also be determined thermographically. This is accomplished by either heating or cooling the tank and then
noting the location of a sharp differential in resulting tank temperature.
Disbonds, Delaminations, and Voids. Modern fabrication techniques rely heavily on the use of bonding for
structures and protective coatings. Thermal techniques are good candidates for the detection of disbonds, delaminations,
and voids in thin laminates, honeycomb to thin face sheets, and protective coatings (Ref 15). As the specimen thickness
increases, thermal inspection becomes less effective because the possible depth of the defect may be greater.
Thermal inspection may also be difficult with bonded metal structures, such as adhesive-bonded aluminum. The radiative
heating of aluminum is relatively inefficient because of the low surface absorptivity, and the thermal-emission signal is
low because of the low emissivity. For such reasons, as well as to avoid reflective noise, the aluminum parts must
normally be black-painted prior to thermal inspection. Moreover, the high thermal diffusivity of aluminum requires a high
thermal-power injection to produce a visible thermal pattern before thermal uniformization is reached within the structure.
Nevertheless, the possibility of evaluating adhesive-bonded aluminum structures at the rapid pace afforded by thermal
inspection is attractive, particularly for online applications (Ref 20). The following two examples describe the thermal
inspection of adhesive-bonded aluminum after black-painting.
Example 1: Thermal Inspection of Adhesive-Bonded Aluminum Sheets
(Ref 15). Figure 7 shows the detection by a transmission configuration of a 1.5 × 1.5 cm (0.6 × 0.6 in.) lack-of-adhesive
defect on a lap joint with nearly 1 mm (0.04 in.) thick skin. The hotter portions on the left and right of the thermal image
correspond to the single-sheet material, which was heated at a faster rate than the adhesive joint. The heating rate must be
sufficiently fast to avoid significant thermal propagation from the hot single-sheet to the adhesive point. Properly shaped
shading masks on the lamp side can be used to avoid single-sheet over-heating when the sample geometry is repetitive. A
line heating and sample displacement configuration of the type described in Ref 21 can also be used for the fast-response
testing of longitudinal joints.
Fig. 7 Thermographic inspection of adhesive-
bonded aluminum sheets. (a) Experimental configuration. (b)
Thermal image of a 15 × 15 mm (0.6 × 0.6 in.) disbonding in an image area of 40 × 50 mm (1.6 × 2.0 in.).
Courtesy of P. Cielo, National Research Council of Canada
Example 2: Cold-Thermal-Wave Technique for the Thermal Inspection of an
Adhesive-Bonded Aluminum Structure
(Ref 15). Rapid back heating procedures are difficult to apply when analyzing complex structures of the type shown in
Fig. 8. In this case, a cold-thermal-wave approach was applied by which the whole structure was previously warmed up
uniformly by nearly 10 °C (20 °F) above ambient, and an ambient air jet was used to rapidly cool the inspected face. The
thermal image shows a relatively warmer central area corresponding to the adhesive-bonded strip (oriented vertically in
the thermograph). The cooler area in the center of the strip corresponds to a lack-of-adhesive unbond.
Fig. 8 Cold-thermal-wave technique for inspecting a preheated, adhesive-
bonded aluminum structure. (a)
Experimental configuration. (b) Thermal image of a 10 × 10 mm (0.4 × 0.4 in.) disbonding.
Courtesy of P.
Cielo, National Research Council of Canada
Electronic devices typically fail by local overheating, corrosion, or poor solder joints. Infrared microscopes have been
used to inspect microelectronics for overheating, and laser-generated heat has been used to inspect circuit-board solder
joints. More conventional thermal inspection methods have been used to inspect solder joints during cool-down after
fabrication or during circuit operation. Solar cells have also been screened for defects with thermal techniques.
Research. Several investigators have used the heat generated by permanent deformation to track the onset of failure in
tensile overloading and crack propagation in fatigue testing (Ref 7, 8, 9). Samples of components can be loaded to failure
under thermographic monitoring to pinpoint the origin and propagation of deformation (Ref 7).
References cited in this section
7.
B.I. Sandor, D.T. Lohr, and K.C. Schmid, Nondestructive Testing Using Differential Infrared
Thermography, Mater. Eval., Vol 45, April 1987, p 372-395
8. Y. Huang, J. Xu, and C.H. Shih, Applications of Infrared Techniques to Research on Tensile Tests,
Mater.
Eval., Vol 38, Dec 1980, p 76-78
9.
Y. Huang, S.X. Li, S.E. Lin, and C.H. Shih, Using the Method of Infrared Sensing for Monitoring Fatigue
Process of Metals, Mater. Eval., Vol 42, July 1984, p 1020-1024
13.
H. Kaplan and R. Friedman, Two New Portable Infrared Instruments for Plant Inspection, Mater. Eval.,
Vol
39, Feb 1980, p 175-179
15.
P. Cielo et al., Thermographic Nondestructive Evaluation of Industrial Materials and Structures,
Mater.
Eval., Vol 45, April 1987, p 452-460
17.
E.P. Papadakis, H.L. Chesney, and R.G. Hurley, Quality Assurance of Aluminum Radiators by Infrared
Thermography, Mater. Eval., Vol 42, March 1984, p 333-336
18.
T.M. Mansour, Nondestruct
ive Thickness Measurement of Phosphate Coatings by Infrared Absorption,
Mater. Eval., Vol 41, March 1983, p 302-308
19.
E. Feit, Infrared Thermography Saves Shelburne Middle School Time and Money, Mater. Eval.,
Vol 45,
April 1987, p 400-401
20.
P. Cielo, R. Lewak, and D.L. Balageas, Thermal Sensing for Industrial Quality Control, in
Thermosense
VIII, Vol 581, Society of Photo Interpretive Engineers, 1985, p 47-54
21.
H. Tretout and J.Y. Marin, Transient Thermal Technique for IR NDT of Composite Materials, in
IR
Technology and Applications, Vol 590, Society of Photo Interpretive Engineers, 1985, p 277-292
Thermal Inspection
Grover Hardy, Wright Research and Development Center, Wright-Patterson Air Force Base; James Bolen, Northrop Aircraft Division
References
1.
D.P. Almond, P.M. Patel, and H. Reiter, The Testing of Plasma Sprayed Coatings by Thermal Wave
Interferometry, Mater. Eval., Vol 45, April 1987, p 471-475
2. D.L. Balageus, A.A. Déom, and D.M. Boscher, Characterization and Nondestructive Testing of Carbon-
Epoxy Composites by a Pulsed Photothermal Method, Mater. Eval., Vol 45, April 1987, p 461-465
3. J.C. Murphy and G.C. Watsel, Photothermal Methods of Optical Characterization of Materials,
Mater.
Eval., Vol 44, Sept 1986, p 1224-1230
4. P. Pote
t, P. Jeanin, and C. Bathias, The Use of Digital Image Processing in Vibrothermographic Detection
of Impact Damage in Composite Materials, Mater. Eval., Vol 45, April 1987, p 466-470
5. T.V. Baughn and D.B. Johnson, A Method for Quantitative Characterizat
ion of Flaws in Sheets by Use of
Thermal Response Data, Mater. Eval., Vol 44, June 1986, p 850-858
6.
E.G. Henneke II and T.S. Jones, Detection Damage in Composite Materials by Vibrothermography, in
Nondestructive Evaluation and Flaw Criticality of Composite Materials,
STP 696, R.B. Pipes, Ed.,
American Society for Testing and Materials, 1979
7.
B.I. Sandor, D.T. Lohr, and K.C. Schmid, Nondestructive Testing Using Differential Infrared
Thermography, Mater. Eval., Vol 45, April 1987, p 372-395
8. Y. Huang, J. Xu, and C.H. Shih, Applications of Infrared Techniques to Research on Tensile Tests,
Mater.
Eval., Vol 38, Dec 1980, p 76-78
9.
Y. Huang, S.X. Li, S.E. Lin, and C.H. Shih, Using the Method of Infrared Sensing for Monitoring Fatigue
Process of Metals, Mater. Eval., Vol 42, July 1984, p 1020-1024
10. D.R. Green and J.A. Hassberger, Infrared Electro-Thermal Examination of Stainless Steel, Mater. Eval.,
Vol 35, March 1977, p 39-43
11. J. Newitt, Application of Specific Thermal Imaging, Mater. Eval., Vol 45, May 1987, p 500-504
12. Infrared Thermal Testing, Product Showcase, Mater. Eval., Vol 45, April 1987, p 403-414
13. H. Kaplan and R. Friedman, Two New Portable Infrared Instruments for Plant Inspection, Mater. Eval.,
Vol 39, Feb 1980, p 175-179
14.
J.H. Williams, Jr., B.R. Felenchak, and R.J. Nagem, Quantitative Geometric Characterization of Two
Dimensional Flaws Via Liquid Crystal Thermography, Mater. Eval., Vol 41, Feb 1983, p 190-201, 218
15. P. Cielo et al., Thermographic Nondestructive Evaluation of Industrial Materials and Structures,
Mater.
Eval., Vol 45, April 1987, p 452-460
16.
J.H. Williams, Jr., and R.J. Nagem, A Liquid Crystal Kit for Structural Integrity Assessment of Fiberglass
Watercraft, Mater. Eval., Vol 41, Feb 1983, p 202-210
17.
E.P. Papadakis, H.L. Chesney, and R.G. Hurley, Quality Assurance of Aluminum Radiators by Infrared
Thermography, Mater. Eval., Vol 42, March 1984, p 333-336
18. T.M. Mansour, Nondestructive Thickness Measurement of Phosphate Coatings by Infrared Absorp
tion,
Mater. Eval., Vol 41, March 1983, p 302-308
19. E. Feit, Infrared Thermography Saves Shelburne Middle School Time and Money, Mater. Eval.,
Vol 45,
April 1987, p 400-401
20. P. Cielo, R. Lewak, and D.L. Balageas, Thermal Sensing for Industrial Quality Control, in
Thermosense
VIII, Vol 581, Society of Photo Interpretive Engineers, 1985, p 47-54
21. H. Tretout and J.Y. Marin, Transient Thermal Technique for IR NDT of Composite Materials, in
IR
Technology and Applications, Vol 590, Society of Photo Interpretive Engineers, 1985, p 277-292
Thermal Inspection
Grover Hardy, Wright Research and Development Center, Wright-Patterson Air Force Base; James Bolen, Northrop Aircraft Division
Selected References
• R.E. Engelhardt and W.A. Hewgley, Thermal and Infrared Testing, in Nondestructive Testing: A Survey,
SP-5113, National Aeronautics and Space Administration, 1973, p 118-140
• F.P. Incropera and D.P. DeWitt, Fundamentals of Heat and Mass Transfer,
2nd ed., John Wiley & Sons,
1985
• W.N. Reynolds and G.M. Wells, Video-Compatible Thermography, Br. J. Non-Destr. Test.,
Vol 26 (No. 1),
1984, p 40-44
• A.J. Rogovsky, Ultrasonic and Thermographic Methods for NDE of Composite Tubular Parts, Mater. Eval.,
Vol 43, April 1985, p 547-555
Optical Holography
Revised by James W. Wagner, The Johns Hopkins University
Introduction
HOLOGRAPHY is a process for creating a whole image--that is, a three-dimensional image--of a diffusely reflecting
object having some arbitrary shape. More precisely, it is a means of recording and subsequently reconstructing wave
fronts that have been reflected from or transmitted through an object of interest. Because the entire wave front and not just
a two-dimensional image is reconstructed, an image of the original object can be viewed with full depth of field, location,
and parallax.
The process involves two steps. In the first step, both the amplitude and phase of any type of coherent wave motion
emanating from the object are recorded by encoding this information in a suitable medium. This recording is called a
hologram. At a later time, the wave motion is reconstructed from the hologram by a coherent beam in a process that
results in the regeneration (reconstruction) of an image having the true shape of the object. The utility of holography for
the nondestructive inspection of materials, components, and structures lies in the fact that this reconstructed image can
then be used as a kind of three-dimensional template against which any deviations in the shape or dimensions of the
object can be observed and measured.
In principle, holography can be performed with any wavelike radiation encompassed in the entire electromagnetic
spectrum, any particulate radiation (such as neutrons and electrons) that possesses wave-equivalent properties, and
nonelectromagnetic wave radiation (such as sound waves). The two methods currently available for practical
nondestructive inspection are:
• Optical holography, using visible light waves, which is discussed in this article
• Acoustical holography, using ultrasonic waves, which is discussed in the article
"Acoustical
Holography" in this Volume
An optical holographic system can be designed to do only one task or various tasks. A system can be purchased as a unit
or assembled in-house from components, or holography can be purchased as an outside service. The selection of a system
requires weighing the advantages and disadvantages of each type. The factors governing the selection of holographic
systems are discussed in the Appendix to this article.
Optical Holography
Revised by James W. Wagner, The Johns Hopkins University
Uses of Optical Holographic Interferometry
Optical holographic interferometry has been successfully used both in research and testing applications as a noncontacting
tool for displacement, strain and vibration studies, depth-contour mappings, and transient/dynamic phenomena analyses.
Specific applications of optical holography in nondestructive evaluation include:
• Detection of debonds within honeycombcore sandwich structures
• Detection of unbonded regions within pneumatic tires and other laminates
• Detection of cracks in hydraulic fittings
• Qualitative evaluation of turbine blades
These uses and others are described in detail in later sections of this article. The advantages and disadvantages of using
optical holographic interferometry for nondestructive inspection are described below.
The advantages of using optical holographic interferometry for nondestructive inspection include the following:
• It can be applied to any type of solid material--
ferromagnetic or nonferromagnetic; metallic,
nonmetallic, or composite; electrically or thermally conductive or nonconductive; and optically
transparent or opaque
• It can be applied to test objects of almost any size and shape, provided
a suitable mechanism exists for
stressing or otherwise exciting the object
• Pulsed-laser techniques allow the inspection of test objects in unstable or hostile environments
• It has an inherent sensitivity to the displacement or deformation of at least one-
half an optical
wavelength, or about 125 nm (1250
). This permits the use of low levels of stress during inspection.
Further, special analysis techniques can provide improved sensitivity of almost 1000-fold
• It does not rely on data acquisition by either point-by-
point determinations or scanning processes;
instead, three-dimensional images of interference fringe fields are obtained o
f the entire surface (front,
back, and internal, if desired) or a large fraction thereof
•
It allows flexibility of readout. For example, in flaw detection applications, the images can be examined
purely qualitatively for localized fringe anomalies within t
he overall fringe field. If, on the other hand,
the application involves the strain analysis of an object subjected to a specific type of stress, the image
can be analyzed to yield a highly quantitative point-by-point map of the resulting surface displacements
•
It permits comparison of the responses of an object to two different levels of stress or other excitation.
The frame of reference for this differential measurement is usually, but not always, the unstressed or
natural state of the test object. This d
ifferential type of measurement contrasts with absolute types,
which are made without a frame of reference, and with comparative measurements, in which a similar,
but different, object is used as a frame of reference
• The interferograms can be reconstructed at any later time to produce three-
dimensional replicas of the
previously recorded test results
The disadvantages of using optical holographic interferometry for nondestructive inspection include the following:
• Although no physical contact with the te
st object is required to effect the interaction of the coherent
light with either the test object or the photographic plate, it is often necessary to provide fixturing not
only for the test object but also for the stressing source. The success of a hologra
phic inspection
procedure depends largely on the adequacy of design and the practical performance of both the fixturing
and the stress-imparting mechanisms
• It is limited to test objects with wall or component thicknesses that will offer sufficiently large
displacements without requiring stressing forces that will cause rigid-
body movement or damage to the
object. For sandwich structures, the thickness of the skin is the limiting factor; the maximum skin
thickness that can be tested is a function of the stressing method
•
Holographic methods are best suited to diffusely reflecting surfaces with high reflectivity. Removable
coatings are often sprayed onto strongly absorbing materials and specularly reflecting surfaces
• Although holographic interferometry is ca
pable of accurately locating a flaw within the surface area of
the test object being inspected, the cross-
sectional size of the flaw can often be only approximately
determined, and information concerning the depth of the flaw, when obtainable at all, is qu
alitative in
nature. Furthermore, a direct correspondence does not always exist between the shape of a flaw-
indicative fringe anomaly and the actual shape of the flaw and between the size of the fringe anomaly
and that of the flaw. For a given set of test
conditions in which only the type of the applied stress is
varied, the fringe anomalies indicating the presence of a flaw will vary. In many cases, flaw detection by
means of holographic inspection is impeded by the appearance of spurious fringes associated with rigid-
body motion and/or rigid-body displacements
•
Where visual interpretation of interferograms is to be performed, holographic interferometry may be
limited in its dynamic range for some applications. However, electronic methods for fringe
interpretation have been used to provide increased sensitivity and dynamic range
•
Test results sometimes cannot be analyzed because of the localization of the interference fringes in
space rather than on the surface of the test object (in realtime holographic int
erferometry) or on the
reconstructed image of the object (in double-exposure holographic interferometry)
• With the exception of holographic-
contouring applications, holographic interferometry is currently
limited to differential tests, in which the object
is compared to itself after it has been subjected to
changes in applied stress. Comparative tests, in which a given object can be compared to a standard
object, are not feasible with holographic interferometry because of inherent random variations in test
object surfaces
•
Personnel performing holographic inspection must be properly trained. The greater the sophistication of
the equipment, the greater the required operating skill. Because lasers are a necessary part of
holographic equipment, operating person
nel must adhere strictly to existing safety codes, particularly
with respect to eye safety
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
recor
d 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