Volume 17 - Nondestructive Evaluation and Quality Control Part 12 docx

• Light source laser • Mounts for the equipment • Tables to support the holographic system Components and complete holographic systems are commercially available Ref 18, 19.. Helium-ca

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 J0 (K · d), where J0 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:

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.,

• Light source (laser)

• 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

1064 10640 Infrared

Nd:YAG

532 (e) 5320 (e) Green (e) (d) (d)

(b) Six other visible lines

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 neon lasers and produces doubly sensitive displacement measurements

helium-• 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

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 electrical 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

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 variable 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

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

front-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/cm2 versus 20 J/cm2 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 components 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 decrease 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:

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 m3 (0.6 ft3) 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 be 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 face 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 Stressing Holographic Techniques

Thermal-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

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 interferogram 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 μm (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 μin.) 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 m2 (2 or 3 ft2) 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 m2 (25 ft2) 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 Survey 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 Stressing Technique

Vacuum-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 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

vacuum-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 between 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 mechanically

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 bonded 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) isolation (holographic) table

vibration-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-exposure 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 Holography to Turbine Blade Stress Analysis,"

Final Report J990798-13 under NASC Contract N00019-69-C-0271, Feb 1970 (available as AD 702 420)

23 Proceedings of the Symposium on Engineering Applications of Holography, Society of Photo-Optical

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

Fig 23 Schematic of an optical system for multiple-source holographic contouring Source: Ref 24

Alternatively, a single holographic exposure can be made with the optical system illustrated in Fig 23, and after processing and repositioning in the plate holder, a real-time contour map of the object can be generated by a similar slight rotation of the object beam steering mirror In either case, when the system is suitably arranged for proper viewing, the altitude contours on the map of the surface have a separation distance, h, given by:

(Eq 10)

where is the wavelength of the laser light source and is the angle of mirror rotation Suitable arrangement of the system requires that the line of sight through the hologram to the object be in the same plane as the angle of rotation of the object beam and be perpendicular to the mean position of the reflected object beam, as shown in Fig 23 The multiple-source technique has the advantage of providing for an almost unlimited range of contour separations However, because of the orthogonal illumination and observation directions, shadowing effects are a handicap, and no reentrant surfaces can be contoured by this technique

technique is shown in Fig 24 This system uses collimated reference and object beams to generate hologram gratings by means of two successive exposures at different settings of the object beam steering mirror Subsequent illumination of a hologram grating by only the collimated reference beam produces a simultaneous reconstruction of both the original and the rotated object beams, which propagate and interfere beyond the hologram grating The spacing of the resulting vertical-standing-wave interference planes is given by Eq 10 and can be readily calibrated by direct measurement of the fringe field on the hologram itself or by exposing a sheet of film oriented at right angles to the propagation direction of the reconstructed object beams and measuring the fringes recorded there Finally, suitable orientation of the object anywhere in the reconstructed object beams yields a real-time contour map when viewed at right angles to the object beams

Fig 24 Schematic of a real-time optical system for holographic contouring using the multiple-source technique

See text for discussion

Example 8: Real-Time Contouring of Electroplated Copper Foil Using a Source Holographic System

Multiple-The multiple-source system shown in Fig 24 was used in a study of the tensile properties of electrodeposited metal foils

In the course of this study, the shapes of permanently bulged specimens (Fig 25a) of copper foil 0.0889 mm (0.00350 in.) thick were measured The bulges, which were 29 mm (1.14 in.) in diameter, ranged from a few millimeters to half a centimeter in height; therefore, a range of sensitivities was needed

Fig 25 Example of real-time contouring using the multiple-source holographic system illustrated in Fig 24 (a)

Photograph of bulged specimen of electrodeposited copper foil 0.0889 mm (0.00350 in.) The bulge was 29 mm (1.14 in.) in diameter (b) Composite of two contour maps The specimen was rotated between recordings to expose both sides of the bulge The fringe separation in this map is 0.229 mm (0.00902 in.)

Because of shadowing, only one side or half of the specimen could be contoured at a time, but multiple recordings with the specimen revolved to expose all sides were made and are shown combined in Fig 25(b) For this contouring, the fringe separation was 0.229 mm (0.00902 in.); other gratings were made for separations up to 5 mm (0.20 in.) and down

to 0.1 mm (0.004 in.) Because no high-precision repositioning is required and the reconstructed fringe field is permanent, this represents a convenient and practical means of real-time contouring However, if a single exposure is recorded in the hologram and is accurately repositioned to provide an initially clear field with appropriately balanced beams, simple adjustments of the object beam steering mirror provide the capability of a real-time variation of the sensitivity (fringe separation) of the contour map when needed

Multiple-Wavelength Contouring. Constant-range contours measured from an origin at the center of the hologram can be generated with the multiple-wavelength technique (Ref 24) Because a change in wavelength produces changes in position as well as in magnification and the phase of the object images, the multiple-wavelength technique is more complicated than the multiple-source technique One configuration has been demonstrated in which both the illumination and viewing directions lie along a line through the center of the hologram and the object (Ref 25) This configuration yields fringes with separations, r, given by:

(Eq 11)

where λ1 and λ2 are the wavelengths of the two laser light sources Because λ1 and λ2 are not arbitrarily variable, only discrete sensitivities are available, all of which are quite high However, because of the more normal illumination conditions, the problems of shadowing associated with the multiple-source technique are completely eliminated

Multiple-Index Contouring. In this technique, the gaseous or liquid medium surrounding an object is changed between successive exposures of the hologram (for double-exposure interferometry) (Ref 25, 26) This changes the optical-path length by slightly altering the index of refraction of the medium and is most readily accomplished by the use

of an immersion cell with windows through which the object can be illuminated and viewed simultaneously In the optical

system for the multiple-index technique (Fig 26), lenses L1 and L2 act as a telescope with its viewing direction normal to the cell window The aperture between the lenses is used to limit aberrations The hologram can be constructed at any plane to the right of the beam splitter (for example, at the aperture, as shown in Fig 26) When this system is used, the

contour separation distance, Δh, is given by:

(Eq 12)

where λ is the wavelength of the laser light source and n1 and n2 are the values of the index of refraction of the cell media used for the two exposures

Fig 26 Schematic of the optical system for multiple-index holographic contouring

Example 9: Multiple-Index Holographic Contouring Used to Examine Wear of Articulating Surfaces of an Artificial Knee Implant

Figure 27 shows the results of a multiple-index holographic-contouring technique applied to the lower (tibial) component

of an artificial knee implant (Ref 27, 28) The material is a high-density polymer, and the intent of the holographic

contouring was to examine the articulating surfaces of these two sockets for wear resulting from in vitro testing Because

wear tracks are not visible from the interferometric images, high-sensitivity heterodyne holographic techniques were again applied Figure 28 shows the results of the heterodyne analysis over a region of one of the knee sockets From these scans a gouge in the material is clearly observed; the details are revealed upon further amplification of the phase-difference signal (Fig 28b)

Fig 27 Dual refractive index contour hologram of an artificial knee implant component

Fig 28 Results of heterodyne holographic analysis (a) Surface gouge in a portion of an artificial knee implant

(b) Higher-resolution scanning that reveals details of the gouge

Multiple-Index Method Versus Multiple-Wavelength Method. The multiple-index technique offers results equivalent to those of the multiple-wavelength technique (Note that if the effective wavelengths in Eq 11 are defined as

λ1 = λ/n1 and λ2 = λ/n2, Eq 11 becomes Eq 12.) The two techniques have the same types of restrictions and advantages with regard to sensitivity, but the multiple-index optical system is easier to arrange Both techniques can be performed in either the double-exposure or the real-time mode

References cited in this section

24 B.P Hildebrand and K.A Haines, Multiple-Wavelength and Multiple-Source Holography Applied to

Contour Generation, J Opt Soc Am., Vol 57 (No 2), 1967, p a155-162

25 J.S Zelenka and J.R Varner, Multiple Index Holographic Contouring, Appl Opt., Vol 8 (No 7), 1969, p

1431-1434

26 T Tsuruta, N Shiotake, J Tsujuichi, and K Matsuda, Holographic Generation of Contour Map of

Diffusely Reflecting Surface by Using Immersion Method, Jpn J Appl Phys., Vol 6, 1967, p 661-662

27 J.W Wagner, High Resolution Holographic Techniques for Visualization of Surface Acoustic Waves,

Mater Eval., Vol 44 (No 10), 1986, p 1238-1242

28 J.W Wagner, Examples of Holographic Versus State-of-the-Art in the Medical Device Industry, in

Holographic Nondestructive Testing (NDT) Status and Comparison With Conventional Methods: Critical Review of Technology, Vol 604, Conference Proceedings, Los Angeles, CA, Society of Photo-Optical

Instrumentation Engineers, 1986

Optical Holography

Revised by James W Wagner, The Johns Hopkins University

Characterization of Composite Materials

The high-speed pulsed holographic interferometry of transient acoustic waves is used to help determine material properties and to identify certain types of defects in graphite-reinforced epoxy laminate sheets (Ref 29)

Example 10: Holographic Interferometry of Composite Sheet Material

Large-amplitude acoustic waves were generated in the sheet materials by direct laser pulse excitation for thin sheets or by laser detonation of a very small chemical explosive in thicker ones Double-pulsed holographic exposures were made, with the first exposure at the instant of excitation and the second exposure occurring after several microseconds of delay The resulting interferograms are shown in Fig 29

Fig 29 Double-exposure pulsed hologram showing displacements associated with large-amplitude acoustic

waves traveling in a centrally excited composite sheet

In this case, the composite sheet was six plies thick, with the fibers running in only one direction in each ply The ply orientation stacking sequence was 02-904-02

From the holographic reconstruction, one observes flexural waves (asymmetric Lamb waves) emanating from the point of excitation They are clearly least attenuated in the directions of the reinforcing fibers From their velocities, one can compute the flexural stiffness and estimate the effective Young's modulus as a function of direction in the material Again, in cases where features smaller than a single fringe must be resolved or when it is not practical to use high excitation forces, phase stepping or heterodyne techniques can be applied (Ref 30)

Figure 30 shows the displacements associated with flexural wave propagation in a sheet where the peak displacement amplitude is about one-third of an interferometric fringe These displacement data were extracted using phase-stepping methods on a hologram in which not even a single complete fringe was observed With phase stepping, sensitivities to

about of a fringe are readily obtained with ultimate sensitivities to about achievable under near ideal conditions (Heterodyne sensitivities approach of a fringe.)

Fig 30 Surface displacements resulting from Lamb wave propagation in an aluminum sheet These subfringe

displacements were extracted from the holographic interferogram using phase-step (quasi-heterodyne) holographic interferometry

References cited in this section

29 M.J Ehrlich and J.W Wagner, Anisotropy Measurements and Determination of Ply Orientation in

Composite Materials Using Holographic Mapping of Large Amplitude Acoustic Waves, in Proceedings of the Third International Symposium on Nondestructive Characterization of Materials (Saarbrucken, West

Germany), Oct 1988

30 J.W Wagner and D.J Gardner, Heterodyne Holographic Contouring for Wear Measurement of Orthopedic

Implants, in Proceedings of the International Congress on Applications of Lasers and Electro-Optics (Los

Angeles, CA), Laser Institute of America, 1984

Optical Holography

Revised by James W Wagner, The Johns Hopkins University

Appendix: Selection of Holographic Systems

In general, the considerations that apply to the selection of a holographic system are similar to the considerations that apply to the selection of any other major piece of capital equipment The selection will be based primarily on the ability

of the equipment to perform the tasks that are of chief concern Beyond that, selection will be governed by such factors as the space and support facilities required, personnel and training, system versatility, and cost This section outlines the advantages and disadvantages of purchasing holography as an outside service and of operating an in-house system

Contract (Purchased) Holography

An initial approach for a company with little or no holographic equipment or technical skill might be to purchase the holographic work on a contract basis from a company that performs this work as a service This would probably be the most economical approach for a company that has only a few parts to inspect or wishes to run only a single development program employing holographic analysis

The advantages of purchasing holographic work are as follows:

• No capital investment and minimal in-house technical skill are required

• The holographic work can be purchased as required, even on a daily or a per-piece basis

• Some companies will conduct holographic developmental work at low cost if favorable results will promote the sale of equipment

• Purchasing holographic work is a good way to test an application prior to investing in equipment and personnel training

• This method allows the buyer to utilize holography with a very short lead time

The disadvantages of purchasing holographic work are the following:

• Test objects must be shipped to and from the service company Often, the travel of some in-house personnel may also be required for program success

• Contracted holographic work costs may be high, depending on the equipment and personnel required This charge must be compared with the cost of suitable alternatives

• Personnel of the service company must be educated concerning the holographic techniques and results

as applied to the application in question

In-House Holographic Systems

If the amount of work or other factors make it desirable to have in-house capability, a suitable holographic system can be installed This can be a commercially available system, such as a tire analyzer or a sandwich-panel analyzer, designed to perform only one task; a commercially available system, such as a portable holocamera or a holographic interference camera, designed for a variety of tasks; or a system built in-house from separately purchased or fabricated components

Assembled Systems

In-house units can be purchased piece-meal or as one complete system

The advantages of in-house assembly of a system from components are as follows:

• The system can be built to fit specific or universal needs

• In-house engineering talent can be utilized to reduce capital expenditures by 40% or more

• Such a system can utilize readily available components

• Building a system is an excellent learning experience for the novice desiring to make future use of holographic techniques

• The system can be built and modified to match the expanding skills of the operator

The disadvantages of in-house assembly are the following:

• Such systems usually require a long lead time to become operational

• Designing and in-house assembly of a system require either developed or purchased technical skill that can be justified only when it is readily available or when many possible applications exist

Purchased Systems

Purchasing a commercial system has the advantage that the vendor can assume complete responsibility for a system to meet the specifications of the buyer Details of the components do not need clarification in the bid specification A specification, for example, need simply state that the system shall be capable of finding a flaw in a given part, of a given length, with a specified confidence level

The purchased system can be either a complete system designed especially for one task or a system designed for a variety

of tasks

Complete System Designed for One Specific Task. The advantages of purchasing a complete system designed especially for one task are as follows:

• Minimum in-house holographic technical skill is required

• A system that has been previously built or designed can be delivered in a short time

The disadvantages are the following:

• Special systems are usually expensive because of the labor required for design, construction, and testing Costs per unit drop rapidly, however, if more than one unit is built

• It may be difficult to modify the system for other tasks, although many holographic components can be removed for use in another system

• A system that must be designed and built for a special task will require a long lead time

Multifunctional Complete System Designed for Numerous Tasks. Although a special system may be the only means for testing a part of unusual design or size, it is often advantageous to purchase a system designed for a variety of jobs The advantages of this type of system are as follows:

• Such systems can be less expensive than special systems because they are in volume production, which reduces design costs per unit

• Such systems have flexibility in change-over to inspect different kinds of parts This usually requires little more than changing the stressing fixture

• The buyer is free to make his own setups

• Delivery time is short; some systems can be purchased off the shelf

• Many vendors will train in-house personnel at no additional cost

The disadvantages are the following:

• The system will be limited by the size or weight of the test object it can handle

• The system may not be as stable as a specialized system because of the flexibility required in the optical components and mounts

Optical Holography

Revised by James W Wagner, The Johns Hopkins University

References

1 R.K Erf et al., "Nondestructive Holographic Techniques for Structures Inspection," AFML-TR-72-204,

Air Force Materials Laboratory, Oct 1972 (AD-757 510)

2 L Kersch, Advanced Concepts of Holographic Nondestructive Testing, Mater Eval., Vol 29, 1971, p 125

3 J.P Waters, Object Motion Compensation by Speckle Reference Beam Holography, Appl Opt., Vol 11,

1972, p 630

4 L Rosen, Focused-Image Holography, Appl Phys Lett., Vol 9, 1969, p 1421

5 H.J Caulfield et al., Local Reference Beam Generation in Holography, Proc IEEE, Vol 55, 1967, p 1758

6 V.J Corcoran et al., Generation of a Hologram From a Moving Target, Appl Opt., Vol 5, 1966, p 668

7 D.B Neumann et al., Object Motion Compensation Using Reflection Holography, J Opt Soc Am., Vol

62, 1972, p 1373

8 D.B Neumann et al., Improvement of Recorded Holographic Fringes by Feedback Control, Appl Opt.,

Vol 6, 1967, p 1097

9 H.W Rose et al., Stabilization of Holographic Fringes by FM Feedback, Appl Opt., Vol 7, 1968, p a87

10 J.C Palais, Scanned Beam Holography, Appl Opt., Vol 9, 1970, p 709

11 R.K Erf, Ed., Holographic Nondestructive Testing, Academic Press, 1974

12 J.Ch Vienot, J Bulabois, and J Pasteur, in Applications of Holography, Proceedings of the International

Symposium on Holography, Laboratory of General Physics and Optics, 1970

13 J.E Sollid and J.D Corbin, Velocity Measurements Made Holographically of Diffusely Reflecting

Objects, in Proceedings of the Society of Photo-Optical Instrumentation Engineers, Vol 29, 1972, p a125

14 J.E Sollid, A Comparison of Out-of-Plane Deformation and In-Plane Translation Measurements Made

With Holographic Interferometry, in Proceedings of the Society of Photo-Optical Instrumentation Engineers, Vol 25, 1971, p 171

15 P Hariharan, Quasi-Heterodyne Hologram Interferometry, Opt Eng., Vol 24 (No 4), 1985, p 632-638

16 W Juptner et al., Automatic Evaluation of Holographic Interferograms by Reference Beam Shifting, Proc SPIE, Vol 398, p 22-29

17 R Dandliker and R Thalmann, Heterodyne and Quasi-Heterodyne Holographic Interferometry, Opt Eng.,

Vol 24 (No 5), 1985, p 824-831

18 J.D Trolinger et al., Putting Holographic Inspection Techniques to Work, Lasers Applic., Oct 1982, p

51-56

19 The Optical Industry and Systems Purchasing Directory, 34th ed., Laurin Publishing Company, 1988

20 R.C Grubinskas, "State of the Art Survey on Holography and Microwaves in Nondestructive Testing,"

MS 72-9, Army Materials and Mechanics Research Center, Sept 1972, p a40-46

21 R Aprahamian et al., "An Analytical and Experimental Study of Stresses in Turbine Blades Using

Holographic Interferometry," Final Report AM 71-5 under NASC Contract N00019-70-C-0590, July 1971

22 J Waters et al., "Investigation of Applying Interferometric Holography to Turbine Blade Stress Analysis,"

Final Report J990798-13 under NASC Contract N00019-69-C-0271, Feb 1970 (available as AD 702 420)

23 Proceedings of the Symposium on Engineering Applications of Holography, Society of Photo-Optical

Instrumentation Engineers, 1972

24 B.P Hildebrand and K.A Haines, Multiple-Wavelength and Multiple-Source Holography Applied to

Contour Generation, J Opt Soc Am., Vol 57 (No 2), 1967, p a155-162

25 J.S Zelenka and J.R Varner, Multiple Index Holographic Contouring, Appl Opt., Vol 8 (No 7), 1969, p

1431-1434

26 T Tsuruta, N Shiotake, J Tsujuichi, and K Matsuda, Holographic Generation of Contour Map of

Diffusely Reflecting Surface by Using Immersion Method, Jpn J Appl Phys., Vol 6, 1967, p 661-662

27 J.W Wagner, High Resolution Holographic Techniques for Visualization of Surface Acoustic Waves,

Mater Eval., Vol 44 (No 10), 1986, p 1238-1242

28 J.W Wagner, Examples of Holographic Versus State-of-the-Art in the Medical Device Industry, in

Holographic Nondestructive Testing (NDT) Status and Comparison With Conventional Methods: Critical Review of Technology, Vol 604, Conference Proceedings, Los Angeles, CA, Society of Photo-Optical

Instrumentation Engineers, 1986

29 M.J Ehrlich and J.W Wagner, Anisotropy Measurements and Determination of Ply Orientation in

Composite Materials Using Holographic Mapping of Large Amplitude Acoustic Waves, in Proceedings of the Third International Symposium on Nondestructive Characterization of Materials (Saarbrucken, West

Germany), Oct 1988

30 J.W Wagner and D.J Gardner, Heterodyne Holographic Contouring for Wear Measurement of

Orthopedic Implants, in Proceedings of the International Congress on Applications of Lasers and Optics (Los Angeles, CA), Laser Institute of America, 1984

Electro-Optical Holography

Revised by James W Wagner, The Johns Hopkins University

Selected References

• R.J Collier, C.B Burckhardt, and L.H Lin, Optical Holography, Academic Press, 1971

• R.K Erf, Ed., Holographic Nondestructive Testing, Academic Press, 1974

• C.M Vest, Holographic Interferometry, John Wiley & Sons, 1979

Speckle Metrology

F.P Chiang, Laboratory for Experimental Mechanics Research, State University of New York at Stony Brook

Laser Speckle Patterns

Speckle patterns occur naturally in everyday life From the metrological point of view, they did not attract attention until the advent of the laser When a coherent laser beam illuminates an optically rough surface (that is, not a mirrorlike surface), the reflected wavelets from each point of the surface mutually interface to form a very complicated pattern The process is analogous to the ripple pattern formed on the surface of a pond that has been disturbed by raindrops The only difference is that water speckles form on the surface of the pond, but laser speckles fill the entire space covered by the reflected wavelets A typical laser speckle pattern is shown in Fig 1

Fig 1 Typical laser speckle pattern

Speckle Metrology

F.P Chiang, Laboratory for Experimental Mechanics Research, State University of New York at Stony Brook

Recording and Delineation of Speckle Displacement

The speckle methods that will be described in the following sections originated from a physical phenomenon first observed by Burch and Tokarsky (Ref 1) They found that when two random patterns are superimposed with an in-plane displacement between them and illuminated with a coherent beam of light, the far field diffraction spectrum is that of a single speckle pattern modulated by a series of Young's fringes whose orientation and spacing are identical to those formed by two point sources located at the ends of the displacement vector experienced by the speckles

When applying the laser speckle technique, a coherent laser beam is used to illuminate a metal surface (or any optically rough surface) Depending on the information sought, one selects a certain section of the resulting volumetric speckle pattern to be photographed by a camera When the surface is deformed, the speckle pattern moves accordingly The spatial movement of the speckle is quite complicated and is governed by a set of three equations (Ref 2)

Conceptually, laser speckles can be viewed as tiny particles attached to the surface by rigid but massless wires They shift

as the surface shifts and tilt in much the same way as light rays are tilted by a mirror The displaced speckles are photographed again on the same film through double exposure, or a separate recording is made and then superimposed on the first one The two superimposed speckle patterns are called a specklegram The speckle displacement can be delineated by sending a narrow laser beam at the points of interest and receiving their corresponding far field diffraction spectrum on a screen The experimental arrangement is shown in Fig 2(a) The observer sees a circular diffraction halo

(assuming the aperture of the camera lens is circular) modulated by a series of uniformly spaced straight fringes, that is, the Young's fringes The intensity distribution of the diffraction spectrum can be expressed as follows (Ref 3):

(Eq 1)

where u is the position vector at the receiving plane, k = 2π/λ with λ being the wavelength of light, d is the displacement

vector at the point of probing, L is the distance between the specklegram and the receiving screen, and Ih is the so-called halo function, which is the diffraction spectrum of a single speckle pattern

Fig 2 Pointwise Fourier processing that results in Young's fringes representing the displacement vector at the

point of probing (a) Schematic of the optical setup (b) Resulting Young's fringe in a diffraction halo

Equation 1 can be converted into (Ref 3):

(Eq 2)

where |d| is the magnitude of the displacement vector and S is the fringe spacing

These fringes represent the displacement vector of a small cluster of speckles illuminated by the laser beam (Fig 2b) The displacement of the speckles can be converted into the displacement of the specimen, and depending on the experimental arrangement, different information can be obtained

Measurement of In-Plane Deformation

For this application, the recording camera is directly focused on the flat surface of the specimen (The method can also be applied to measuring interior in-plane displacement if the material is transparent to the radiation used, as discussed in Ref 4.) Under either a plane-strain or (generalized) plane-stress condition, the speckle movement is largely confined to the plane of the surface Therefore, the observer again photographs the speckle pattern formed on the surface after the specimen has been subjected to deformation The resulting specklegram can be probed point by point to yield the displacement information, as described in the section "Recording and Delineation of Speckle Displacement" in this article

Alternatively, the observer can obtain full-field displacement information by using the following optical spatial filtering scheme The specklegram is placed inside a convergent laser beam, as shown in Fig 3(a) The diffraction spectrum at the

focal plane is also governed by Eq 1 At the focal plane, a mask with a small hole situated at u is used to block all light

except that passing through the small aperture A second lens is used to receive the filtered light and forms an image of the specimen surface, which is now modulated by a series of fringes governed by (Ref 3):

Fig 3 Optical spatial filtering used to generate full-field displacement contour (isothetic) fringes (a)

Components of optical spatial filter setup (b) Fringe pattern obtained for an aluminum specimen containing a crack Each fringe represents 0.005 mm (0.0002 in.) in vertical displacement

Another example of this type of application is illustrated in Fig 4, which shows the thermal strain patterns of a heated aluminum plate The plate is heated with a torch at the lower left-hand corner until it is red hot It is then left to cool by natural convection A thermocouple is embedded nearby to monitor the temperature The patterns are the result of differential thermal strain from the temperatures indicated in Fig 4 Figure 4(a) shows the displacement of the contour

fringes along the x-direction; Fig 4(b), those along the y-direction These fringe patterns are governed by:

(Eq 4)

(Eq 5)

where u and v are the displacement components along the x- and y-directions, respectively, and u x and u y, are the distances

of the filtering aperture along the x- and y-directions, respectively As the plate cools, the temperature tends toward

equilibrium, as evidenced by the uniformity of the fringe patterns at lower temperatures Thermal strains measured by the speckle method compared quite well with the existing data, as shown in Fig 5

Fig 4 Thermal strain determination using laser speckle method (a) u field (b) v field Fringes show the

residual strain field between two temperature levels of an aluminum plate torch heated to red hot and allowed

to cool by natural convection Fields u and v are the contour fringe displacements in the x- and y-directions,

respectively

Fig 5 Comparison between theoretical and experimental thermal strain relaxation at a point as a function of

time

Measurement of Out-of-Plane Deformation

The laser speckle method can also be used to measure the out-of-plane deformation of a specimen with a slightly different experimental arrangement Instead of photographing the speckles on or very close to the surface, one can photograph the speckles at a certain finite distance (a few centimeters, for example) from the surface When the surface experiences out-of-plane deformation, the spatial speckles are tilted, resulting in speckle displacement at the defocused plane By recording these speckles before and after deformation, the resulting specklegram is processed in the same way as described in the section "Measurement of In-Plane Deformation" in this article The resulting fringes are governed by (Ref 3):

(Eq 6)

(Eq 7)

where w/ x and w/ y are the slopes of the surface along the x- and y-directions, respectively, and A is the defocused

distance An example of this application is shown in Fig 6, in which the fringes are the slope fringes w/ x of a

clamped thin circular plate transversely loaded at the center A comparison between the experimental result and the theoretical result is also shown in Fig 6

Fig 6 Slope fringes of a clamped circular plate under concentrated load

References cited in this section

1 J.M Burch and J.M.J Tokarski, Production of Multiple Beam Fringes From Photographic Scatterers, Optica Acta, Vol 15 (No 2), 1968

2 D.W Li and F.P Chiang, Laws of Laser Speckle Movement, Opt Eng., Vol 25 (No 5), May 1986, p

667-670

3 F.P Chiang, A Family of 2-D and 3-D Experimental Stress Analysis Techniques Using Laser Speckles, Solid Mech Arch., Vol 3 (No 1), 1978, p 1-32

4 F.P Chiang, A New Three-Dimensional Strain Analysis Technique by Scattered Light Speckle

Interferometry, in The Engineering Uses of Coherent Optics, E.R Robertson, Ed., Cambridge University

Press, 1976, p 249-262

Speckle Metrology

F.P Chiang, Laboratory for Experimental Mechanics Research, State University of New York at Stony Brook

White Light and Electron Microscopy Speckle Methods

Laser speckle is a natural result of coherent radiation, but speckle pattern can also be artificially created For example, the aerosol particles of black paint sprayed onto a white surface (or vice versa) create a pattern quite similar to laser speckles

White Light Method. Another efficient way of creating artificial speckle is to cover the surface with a layer of retroreflective paint When illuminated by incoherent white light (or any visible light), the individual glass beads embedded in the paint converge the light rays and send them back in approximately the same direction When imaged and recorded on film, the beads appear as sharply defined, random speckles These patterns can be used for metrological measurements as well Any speckle pattern can be used if it yields a distinct pattern and can be recorded for future reference A few examples are given in Ref 6 These are classified as white light speckles because they are observable under the illumination of incoherent white light The identical procedures used for laser speckles can be adapted to record and process white light speckles The only exception is that because white light speckles are located on the surface of the specimen, they cannot be tilted in the same way as laser speckles However, through the use of defocused photography, depth information can nevertheless be obtained (Ref 6)