The basic concepts of optical holography stem from the 1949 work of Gabor and the 1962 work of Leith and Upatnieks. In 1965, Homan suggested the application of holography to Mach-Zehnder interferometry and Powell and Stetson invented holographic interferometry as we know it today. This gave us the ability to optically determine deformation and displacement of diffusely reflecting opaque objects. The basic principles of quantitative interpretation of fringes produced by holographic interferornetry are now well known and can be expressed in a number of formulations. A variety of refinements and extensions have been developed during the past two decades including optical derotation, the introduction of fiber optics, the application to computer tomography, the development of heterodyne holographic interferometry and the introduction of various recording media.

Holographic Interferometry was thought to be a powerful tool for a lot of applications, since it was invented by Stetson and Powell /II. Although till today only few industrial applications - mainly in Holographic Non-Destructive Testing (HNDT) -are known, this is still valid. There are tasks, which can be solved by means of this technique better and more economically than by conventional methods. For example, it is nearly impossible to calculate the deformation behaviour of complex parts of pressure vessels till today: The very complex form would lead to a very long calculation time, using e.g. Finite-Element-Methods (FEM). Even when a calculation is performed, it is necessary to prove the calculation by experimen-tal stress analysis. For complex objects it needs up to 1000 strain gauges. This means several months of preparing time and approximately 100.000$ costs for one result. In this application holographic interferometry could do the job for less than half the amount of costs.

A pulse ruby laser is used to record double exposure holograms of large vibrating military vehicle components. Various methods are described to load the structures using small portable shakers. Several examples are presented which analyze the global fringe patterns of Army vehicles and relate them to the bending waves that are propagated from the track and suspension to the hull and turret. The temporal coherence requirements of the laser are described along with its mechanical and thermal stability for reliable operation in the laboratory.

Herein we demonstrate the versatility of optical fibers in holometry. Real time interferometric fringes causes by rigid body motion can be largely compensated by judiciously moving the object beam emanating from a fiber.

This paper describes some steps taken to implement practical holo-interferometric measurement for use in an industrial setting. These include the use of optical fiber cables and bundles to greatly simplify optical setups and allow holo-interferometric measurements to be carried out in remote or otherwise inaccessible locations; the use of special holocameras to produce holographic images in situ within a few seconds; and perhaps most importantly, a method for using an image digitizing system with a minicomputer to automatically access and analyze holo-interferometric fringe patterns.

We describe several applications of cineholography to objects scattering light by reflection: single-exposure cineholograms on 126-mm films at a repetition rate of 25 holograms per second for 3-D movies and for flight simulators applications, double-exposure cineholograms for medical and industrial applications (NDT). Limitations of cineholography are also described. The light source used for cineholograms recording is a frequency-doubled pulse YAG-laser.

Remote distance sensing using an all-fiber Mach-Zehnder interferometer has been demonstrated. The system employed a frequency modulated, coupled-cavity semiconductor laser with constant output power. Accuracy of 0.5% was obtained in absolute distance measurements, and relative displacements on the order of 0.1 mm were detected. The same interferometer was also used to characterize the tuning properties of semiconductor lasers.

A TV system has been developed for detecting and displaying a two-dimensional hologram image that permits concomitant (real-time) hologram interferometry. The optics employed are the same as for Electronic Speckle Pattern Interferometry (ESPI) wherein a uniform reference field interferes at zero offset angle with a speckled image of an object. Rather than detect spatial modulation to indicate interference, as in ESPI, this system modulates the phase of the reference beam by 120° between picture frames to vary speckle irradiance. Each incoming frame is compared with the two previous frames, and only pixels that vary in brightness are passed as white pixels in the video output. The use of binary valued pixels allows inexpensive data processing at standard video rates.

Recent advances in video camera and processing technology, coupled with the development of relatively inexpensive but powerful mini- and micro-computers are providing new capabilities for the experimentalist. This paper will present an overview of current areas of application and an insight into the selection of video/computer systems. The application of optical techniques for most experimental mechanics efforts involves the generation of fringe patterns that can be related to the response of an object to some loading condition. The data reduction process may be characterized as a search for fringe position information. These techniques include methods such as holographic interferometry, speckle metrology, moire, and photoelasticity. Although considerable effort has been expended in developing specialized techniques to convert these patterns to useful engineering data, there are particular advantages to the video approach. Other optical techniques are used which do not produce fringe patterns. Among these is a relatively new area of video application; that of determining the time-history of the response of a structure to dynamic excitation. In particular, these systems have been used to perform modal surveys of large, flexible space structures which make the use of conventional test instrumentation difficult, if not impossible. Video recordings of discrete targets distributed on a vibrating structure can be processed to obtain displacement, velocity, and acceleration data.

A description is given of the history and principal developers of the technique of T.V. Holographic Interferometry. The principles of the method are next outlined and recent work involving the use of pulsed holographic lasers combined with a C.O.D. video holographic interferometer is described as applied to the study of automobile disk brakes rotor vibrations. Finally, an account is given of future trends and developments in this area of optical technology.

The application of electronic phase measurement techniques (heterodyne and quasi-heterodyne fringe interpolation) to dimensional profiling of 3-D objects is discussed. The contour fringes are generated by interference of two illumination sources or by two-wavelength holography. The use of a micro-computer for automated data acquisition and processing allows to determine the object shape for a large number of points and for arbitrarily shaped contour fringe surfaces. Experimental results are reported with both, heterodyne and quasi-heterodyne phase measurement, offering a typical accuracy for object depth measurement of 0.2% of the lateral extension of the object.

A system designed to provide a low-cost entry to the area of computer-assisted holo-graphic fringe interpretation, making maximum use of the computational and display capabil-ities of a standard personal computer with a color graphics adapter is described. The system includes a 7MHz video digitizer and four software modules. The software modules provide for video frame capture and storage, rapid analysis of general characteristics, computer-assisted data entry and least squares fitting to a polynomial model, as well as graphic model display and data extraction.

This presentation deals with theoretical and experimental analysis of holographic fringe patterns. More specifically, double-exposure holograms of a cylinder undergoing internal pressure changes were recorded for a known illumination and observation geometry. The experimental results obtained from holograms were compared with theoretically predicted fringe patterns. The prediction of the fringe patterns was achieved using a theoretical model, based on the fringe vector theory of holographic analysis. This model was developed specially for analysis of the fringe patterns as a function of object surface shape, its loading conditions, as well as illumination and observation geometry. The model was tested, using computer simulation, under a number of different loading conditions and recording/reconstruction geometries. In these computer tests, only one quarter of the cylinder was considered because of the symmetry of the obiect. The computer simulated results show the effect that changes in hologram illumination and observation directions, for a given loading condition, have on the appearance of the fringe patterns. The correlation between the experimentally observed fringe patterns and those predicted theoretically is good.

A complete experimental determination of the stress and strain fields in an arbitrary deformed structure is generally unavailable. However, for two dimensional elasticity problems, such determinations are possible since in those cases one needs only to solve for three stresses (two normal and one shear). In fact, such determinations have been conducted quite often. By using isochromatic and isoclinic photoelastic data, the shear difference and numerical iteration techniques (1) and the least squares techniques (2) have been successfully used for complete stress field determinations of two dimensional elasticity problems. Though the shear difference technique can be particularly sensitive to cumulative errors resulting from numerical integration, the least squares technique is not affected by this and appears to yield better accuracy. The methods just cited use both experimental data and one or more mechanics conditions(e.g., the equations of equilibrium) to determine the stress field. However, the stress field can also be obtained from experimental data alone for planar elasticity problems, if there is enough of it to solve for the three stresses. For example, the Moire* technique or the combination of isochromatic, isoclinic, and isopachic data (for transparent models) can be used for such determinations. Further, with the marriage of advanced image processing equipment to computers, such analyses using this type of data can be conveniently conducted. It is even possible that such analyses could be more accurate than those using the combined experimental/numerical techniques cited above. The purposes of this report are two fold: i) to describe a single apparatus for obtaining isochromatic, isopachic, and isoclinic results for complete stress field determinations of two dimensional transparent models, and ii) to compare experimental and theoretical stress field values for an antisymmetrically loaded beam obtained using that apparatus.

The development and recent status of ESPI or TV holography are described with examples of applications within industrial and biomedical research. Future developments within instrumental design and application areas are discussed.

The use of holographic interferometry (HI) has been available for whole field testing of components for nearly twenty years. As an experimental technique it was limited to a laboratory environment but with the incorporation of a pulsed laser the possibility existed to attempt this type of analysis on the component operating in its usual environment. This should have provided the scientific and engineering community with the necessary tool to investigate many different difficult problems. The reality is holographic interferometry consumed billions of dollars globally, produced few real results and interest dwindled. Why? A number of reasons have been proposed which include experimental difficulties, data reduction and cost. Combined with the vast research effort in holography, interest in the allied field of speckle interferometry developed a video based technique which could rival holographic interferometry. This technique known as electronic speckle pattern interferometry (ESPI)1 provided equipment with similar capabilities to holographic interferometry but used the television camera directly as the imaging transducer rather than the photographic system. However this technique, which overcame a number of the experimental problems of holographic interferometry, still did not find immediate favour as an analysis tool. The end user was still not hammering on the door of the research laboratories. Laser costs have reduced and supporting digital electronics have substantially reduced to the extent where fully ruggedised commercial ESPI equipment costs approximately $30-50K. This makes it comparable with other experimental analysis packages, the cost argument therefore no longer strictly holds. The availability of image processing computer's has enabled a great deal of research into automated fringe data reduction and analyis, this was reviewed at a recent symposium by the Fringe Analysis Special Interest Group 4. The final major problem now outstanding is the availability of a hand�held portable instrument capable of providing the computer with suitable data. The work this paper reports is an effort in this direction, to produce such a system. The application areas envisaged are: (i) displacement measurement of strain fields (ii) vibration measurement of resonant objects (iii) vibration measurement of non-resonant conditions (iv) refractive index changes in fluids, eg convective heat and fluid flow.

Conditions in the inner ear for interferometric measurements are quite different than those encountered in other mechanical systems. (i) The inner ear is mechanically not stable, due to blood pulsations and breathing artifacts; (ii) access to the inner ear is limited by anatomical constraints making it difficult to visualize the structures of interest; (iii) vibration amplitudes to be measured in the inner ear are very low; (v) the structures in the inner ear are nearly transparent therefore the reflectivity is low, attempts to change this reflectivity artificially usually alters the response characteristics; (vi) cells are subject to light damage if the incident light intensity is too high. This limits the laser power that can be utilized in the interferometer. A heterodyne interferometer specially designed to measure vibrations in the living inner ear is described. Theoretical and experimental characteristics of this instrument are discussed in detail, particularly the noise characteristics which determine the limit of sensitivity. It has been shown that the measurement accuracy of this interferometer is not affected by the low frequency animal movements. This system does not require attachment of a reference mirror on the animal allowing precise determination of the measuring point. It has a high linearity and dynamic range. Its vibration sensitivity is high (3x10-16 m for 1 Hz bandwidth) even under the condition of low light reflectivity (0.02%), with 0.5 mrg incident laser power. Therefore vibrations of nearly transparent membranes and structures can be measured directly.

Vacuum speckle interferometry can be used for long term monitoring of local deformation and strain in the vicinity of hydride blisters as they evolve on the surface of hot samples of zirconium pressure tubes used in nuclear reactors. A video camera and frame grabber were used to provide computer access to the speckle pattern for fringe pattern extraction, image processing, and data analysis. Evidence was seen for strain around the blister during thermal expansion, and for change of strain with time when heated. Decorrelation could be seen at the blister edge as it dissolved at 340 C.

Holographic interferometry and laser speckle techniques make it possible to measure deformation and vibration of diffusely reflecting surfaces with sensitivities related to laser wavelengths. The relationship between these methods are discussed with views to formation of interference fringes observed and their recent advances for quantitative and automatic measurements.

Developments in Laser Doppler Velocimetry techniques and applications are making this technique to be a primary candidate for practical flow measurements. Specific application areas such as measurements in rotating machinery, combustion, flame, and three dimensional flow fields are also discussed. Developments in technology such as the use of fiber optics, lasers, and data and signal processing techniques are making the system more compact and easy to use.

We study the motion of a magnetic recording disk drive read/write slider as it drags in contact with thin film and particulate media disks. The out-of-plane displacement of the slider is measured with a laser-Doppler vibrometer (LDV), and its in-plane component is detected with a laser-Doppler anemometer (LDA). One goal is to determine if the motion of a slider/suspension that is mounted on a strain gage transducer arm is substantially different from that in a production system during constant speed sliding. Another is to verify that the vibrations detected by the strain gage transducers are actually related to motion of the slider. In addition to obtaining answers to these questions, we observe that the motion of a slider in contact sliding is indicative of the topography of the disk surface. A track "signature" is obtained, and changes in this signature may be an early indicator of disk wear.

Diffraction moire interferometry has been applied to numerous static and slowly changing stress configurations. This technique has now been extended to the study of dynamic loading events, especially to the interaction of dynamic stress waves with such flaws as cracks or with the variations in composition found in composite materials. A pulsed ruby laser was used to provide the rapid (20 ns wide), brilliant, and coherent illumination required for these studies. The technique and several specific applications are described. Key words: Dynamic strain, dynamic moire', pulsed laser, fracture, composites, ceramics.

In laser speckle velocimetry or LSV a sheet of laser light is used to illuminate a plane of interest within a volume of moving fluid which has been properly seeded to assure efficient scattering. An accurately focussed photograph of the resulting side scattered intensity field provides a pattern of (depending on the effective seeding concentration) either particle images or coherent speckle which is directly related to the existing two-dimensional flow field in the plane of interest. If the illumination is appropriately interrupted while recording particle images, the resulting photograph or LSV specklegram will include many discrete recordings of each particle image. A positive transparency print of this multiple exposure LSV particle image specklegram may readily by analyzed optically by either a point-by-point "Young's fringe" technique or a full-field "Fourier filtering" technique to produce quantitative velocity field informa-tion with superior signal-to-noise characteristics. Computer aided image processing and data interpretation techniques are also used to provide further enhancement and conve-nience. In this paper the sources and consequences of signal decorrelation and noise are discussed in detail, and compared with those obtained using double exposure LSV (either coherent speckle or particle image).

Holographic interferometry has received many plaudits as an extremely powerful flow diagnostic method while users have languished at the cumbersome methods available and long time required to extract even the elementary data from the hologram. Recent years have seen breakthroughs that, when combined, will address the problems of producing the usable raw data quickly (the processed hologram) and transferring it into a computer in a tractable form. Keys to the former are either thermoplastic devices or on-line film processors. Keys to the latter are electronic imaging and analyzing systems that can digitize data reconstructed from holograms and then perform complex operations on the digitized data. The authors have tested, individually, all of the components required for producing fully refined, on-line density data in wind tunnels employing holographic interferometry systems, and are currently developing the fully integrated system. This paper describes the state-of-the-art system concept, components, options, and potential capabilities for realizable systems.

Interferornetric methods for determining the electron concentration Ne in thermal plasmas are reviewed. Such methods depend on measuring the contribution to the refractive index of the free electrons for a probing em wave of suitable frequency. This contribution depends on the ratio of the wave frequency f to the plasma frequency fp ∞ Ne 1/2 and in practice (f/fp) must be significantly greater than unity. For thermaill plasmis, of interest for various devices and processes, probe frequencies in the range 10¹¹ to 10¹³ Hz are required, corresponding to wavelengths in the millimeter to mid infrared regions, which are accessible with CO2 laser-pumped gas lasers. The technique may be implemented using a two-path laser interferometer or, in the case of a strongly magnetized plasma, by using Faraday rotation of the plane of polarization. Experiments are detailed in which both these methods were used to measure Ne in seeded combustion plasmas in a mag-netohydrodynamic generator.

A technique has been developed for the non-contact measurement of the lateral motion of a diffusely reflecting surface. This technique, which is based on the use of a commercially available stabilized 2-frequency laser, is capable of sub-micron resolution, and operates with a stand-off distance of up to 50 cm. The principle of the measurement is coherent heterodyne detection of Doppler-shifted light scattered by the diffuse surface. By making two such measurements at spatially separated points on the surface, extension of the surface can be monitored. The resultant extensometer provides a non-contact replacement for conventional strain-gage extensometers, and is particularly applicable to measurements in harsh environments (high temperature, corrosive atmospheres, etc.). Frequency response in excess of 25 KHz has been confirmed, making the sensor particularly useful for closed-loop operation in fatigue testing.

Heterodyne and quasi-heterodyne holographic techniques are being used to great advantage in steady-state full field contouring and deformation studies, for example. The high dynamic range and sensitivity afforded by these techniques is also of benefit in high speed holographic applications where there is the potential for providing new holographic tools to study a variety of high speed, transient phenomena. Full field visualization of surface acoustic waves, surface perturbations from acoustic emissions, and deformation during dynamic fracture studies are but a few of the potential applications of high speed heterodyne hologram interferometry. Triple-exposure and high speed switching techniques have been developed to facilitate recording of pulsed holograms for subsequent heterodyne analysis. Additional modifications to more conventional CW recording and readout geometries have been explored as well. Some degradation in sensitivity results from increased spatial and temporal noise associated with high speed recording.

The use of fiberoptic imaging structures for holographic investigations of displacement and vibration is demonstrated. Image-plane holograms are formed at the proximal output end of an imaging multifiber. Investigations using a cw laser for double-exposed holograms of a cantilever beam, as well as time-average vibration studies, were conducted. Pairs of double-pulsed holograms of dynamic events were recorded, one through the fiber bundle, the other in a conventional manner as a standard of reference. The fringes of both holograms were practically identical. The ability to perform measurements in holographic interferometry through optical fibers should lead to further developments in medical endoscopy as well as industrial applications.

With conventional interferometric transducers, the light from a gas laser is reflected from a test piece and mixed with reference light to produce interference fringes as the test piece moves. A detector converts the light variations into electrical signals, which can then be analyzed to determine surface displacement. To determine target direction and to improve signal-to-noise characteristics, optical heterodyning is employed. This technique mixes light from the target with reference light that has been offset in frequency by, for example, an acousto-optic modulator. Thus, a stationary target gives rise to a signal at the offset frequency and a non-stationary target causes the received signal to deviate up or down from the offset frequency depending on target direction. Significant improvements in the transducer described above have been realized with the use of a semiconductor laser as the light source and by enclosing the beam paths in optical fibers. These two advances make the transducer more flexible, and also improve reliability and performance. However, the design of a solid-state, fiber-optic transducer presents a unique set of problems which must be solved before the technique can be used in an industrial environment. These problems include: 1) noise pickup in the fiber leading to the test piece; 2) feedback (or back reflections) of light into the laser diode; 3) modal noise introduced when the fibers are flexed; and 4) speckle effects which can cause signal dropout. Solutions to the above problems have been devised and implemented in a prototype interferometric transducer developed at United Technologies Research Center.