This is the first of two special issues of Optical Engineering on optical techniques in experimental mechanics, which I have been kindly asked by Dr. Caulfield to organize. The first issue deals with coherent optical methods, and the second with incoherent optical methods. The division is somewhat arbitrary because many light sources are partially coherent and some methods require the use of both coherent and incoherent light sources. In essence, I have called a method a coherent optical technique if a laser is used as the primary information gathering device and if, in the process, some sort of the coherent property of the light is utilized. Otherwise, it is classified as an incoherent optical technique.

The basic principles of the subjective laser speckle method are reviewed with an analysis of its sensitivity and limitation. Emphasis is on the method's application to various solid mechanics problems. Examples cited include stress concentration due to discontinuities, determination of thermal stresses, flexure of plates, vibration of composites, fracture of aluminum, interior strain determination, impact stresses, nondestructive testing of debond in lavprpri nnmnncitpc atom

This paper presents a new optical method referred to as shearography. Shearography is an interferometric method which employs an image-shearing camera. It permits full-field measurement of derivatives of surface displacements. The object under study is illuminated by laser light, and it is imaged by an image-shearing camera. The image-shearing camera produces two laterally sheared images in the image plane. When a photographic film in the image plane is doubly exposed with the object being deformed between the exposures, the processed photograph yields a fringe pattern which depicts the surface displacement gradients. Theory of the method as well as its applications to strain meausrements and nondestructive testing are presented.

The development of scattered light speckle (SLS) metrology from its inception in 1975 to the present is discussed. Beginning as a technique with potential for determining the state of strain at a point in the interior of a transparent body, refined SLS techniques are now used to measure the entire field of displacement on an interior plane in a transparent solid or the instantaneous field of velocity within a volume of moving liquid.

Electronic speckle pattern interferometry (ESPI) enables static and dynamic surface displacements to be measured in real time to an accuracy of the order of a wavelength of light. The principles of the technique are outlined and the different configurations of interferometers described. Some applications of the technique are discussed.

In the past two decades, speckle photography and its electronic counterpart have been extensively used for displacement measurement purposes, the applications ranging from civil engineering over biomechanical field to astronomy. In most applications, a more or less sophisticated imaging process is involved. The simplest way of normal photographing, however, is placing the emulsion in contact with the object; this concept can also be used for speckle correlation purposes. If coherent illumination is used, one can speak about objective speckles; it will be shown that incoherent illumination can also be used, during both recording and data interpretation stages.

Considerable interest in speckle metrology and a consistent search to refine these techniques to suit the intricate engineering environment has led to an active investigation into the basic aspects of speckle movements in three-dimensional space induced by small displacements and deformations of the object surface. The model exploited in this paper is based on the notion of the local invariance of the speckle structure in such a case; it is thus intimately related to the concept of homology and draws closely on the theories aiming to reconcile speckle metrology and holographic interferometry techniques. The laws of speckle motions issuing from this model are applied to major known techniques in speckle photography (focused or defocused), speckle interferometry, and speckle shearing interferometry in an attempt to make as objective an assessment as possible of these methods. The study unveils the influence of parasite movements, field angles, and the geometrical parameters at the recording on the measured component, thereby paving the way for the optimization of a method to a given deformation problem and forging a better understanding of the phenomenon underlying these techniques.

Digital imaging techniques are utilized as a measure of surface displacement components in laser speckle metrology. An image scanner which is interfaced to a computer records and stores in memory the laser speckle patterns of an object in a reference and deformed configuration. Subsets of the deformed images are numerically correlated with the references as a measure of surface displacements. Discrete values are determined around a closed contour for plane problems which then become input into a boundary integral equation method in order to calculate surface traction in the contour. Stresses are then calculated within this boundary. The solution procedure is illustrated by a numerical example of a case of uniform tension.

An automatic computer analysis system for interferometric patterns is developed. A digital interferogram is stored on disk memory, and fringe skeletons of the interferogram are extracted, based on the two-dimensional gray level fringe peak detection method. Extracted fringe skeletons are thinned, and then their fringe orders are determined, during which process the fringe pattern is visually inspected. The least-square interpolation method is applied to determine the fractional fringe orders of pixels between fringe skeletons. All the procedures of analyzing fringe data, except for the fringe order determination, are automatically performed. A man-machine interactive method with a light pen is used to inform the determined fringe orders to the computer. A representative example of the fringe pattern analysis is presented.

An automatic method of measuring surface strain by electro-optical techniques is described. It is a real-time and point-by-point method in which a specimen surface is illuminated by a thin laser beam and the displacements of the speckle pattern resulting from the surface deformation are detected by linear image sensors followed by a microcomputer. The displacements are calculated as the peak positions of the cross-correlation functions of image sensor signals before and after the deformation. By adopting a differential system consisting of two image sensors positioned symmetrically about the surface normal we can eliminate the effect of rigid body motion. The sensitivity depends on the surface to sensor distance, while the gauge length is given by the diameter of the laser spot. These relations derived from physical optics are verified by experiments using a metal plate specimen subject to tensile strain that is monitored by an electrical resistance strain gauge. The minimum strain value measured was a few ten microstrains with a gauge length of 1 mm. We could also measure thermal strain by this method.

Most of the laser speckle methods utilize subjective speckles in that speckles are recorded through a lens whose aperture determines the admitted spatial frequencies. Objective speckles are those intrinsically formed by the scattering wavelets. In this paper we propose the use of far-field objective speckles generated from a ground glass to measure surface displacement and strain. The object's surface is coated with a photosensitive material and placed inside the speckle field. Double exposure is made on the photosenstive coating before and after the application of load. Young's fringes are generated by probing the surface coating with a narrow laser beam. Surface strain can then be calculated from these fringes. The method can be applied to doubly-curved surfaces and opaque materials.

In conventional holographic interferometry, the observed fringe patterns are determined by the object displacement and deformation, and by the illumination and observation configurations. The obtained information may not be in the most convenient form for further data processing. To overcome this problem, and to create new possibilities, holographic fringe patterns can be changed by modifying the optical setup. As a result of these modifications, well-known procedures of the moire method can be applied to holographic interferometry. Components of displacement and components of the strain tensor can be isolated and measured separately. Surface contours and slopes can also be determined.

Recent progress in high-sensitivity moire interferometry is reviewed. Interference patterns reveal full-field contour maps of in-plane displacements. Sensitivity corresponds to moire with 1200 lines/mm (30,480 //in.) for most examples, but approaches the theoretical limit of X/2 displacement per fringe in one demonstration. Techniques for producing cross-line phase gratings on specimens are described, as well as use of real and virtual reference gratings. Carrier patterns and optical filtering are used to cancel initial or no-load patterns. Diverse applications are illustrated.

A pulsed ruby laser is used with the techniques of holographic interferometry to record stress waves and cracks propagating in two and three-dimensional specimens. The problems discussed include the development of surface waves from a buried explosive source, the effects of surface discontinuities on surface waves, geometric effects from multiple explosions, the observation of propagating cracks from an explosive source, and the determination of dynamic stress intensity factors. The examples demonstrate the utility of the holographic method in problems with dynamic loads.

During the past decade the optical measurement technique of image derotated hologram interferometry has evolved to a point where it has become an extremely useful laboratory tool for studying the resonant structural response of rotating bodies such as turbomachinery bladed disks. The results obtained using derotated hologram interferometry to study a simple bladed disk are presented. Both stationary and traveling disk modes of vibration were recorded. Also presented is an extension of the technique to quantitatively measure traveling mode shapes using a triple-pulsed laser. Combining moire analysis with image derotation is also discussed, and some preliminary experimental results are given.

Although holographic interferometry is in theory capable of measuring the three-dimensional deformation of a strained body, in practice its most frequent use lies in the evaluation of the warping and twisting of a surface, for which only unidirectional viewing through a small sized hologram is required. The alternative technique of defocused speckle photography gives values of local surface tilting, from which the change in shape of its profile along any direction in the surface can be obtained by integration. An experiment per-formed to compare the two techniques showed that with careful adjustment of the focus condition, speckle photography is capable of measuring the out-of-plane displacement with an absolute accuracy approaching that of holographic interferometry.

This review article summarizes applications of the interferometric strain/displacement gage (ISDG). The laser-based ISDG measures in-plane surface deformation between two small reflecting surfaces by monitoring the interference fringe pattern motion. The short gage length (100 micrometers) and noncontacting nature are especially attractive in certain laboratory situations. Following a discussion of the measurement procedures, applications in dynamic loading, fatigue and fracture, and high temperature environments are reviewed.

A review of basic techniques for recording various features of two-dimensional stress fields with photoelastic holography is presented. The emphasis is placed on the relationship between polarization of the object and reference beams in simple transmission holographic systems and on the type of fringe pattern produced, as well as its interpretation in terms of the stress field. After describing established techniques for single-exposure, double-exposure, time-average, and real-time holography of photoelastic models, the paper concludes with early results of a new method for direct optical separation of the stress components.

Radar cross sections per unit area of ground snow were calculated for linearly polarized incident radiation at 8.6, 17, 35, and 95 GHz. The snow was modeled as a plane-parallel medium consisting of ice grains immersed in a homogeneous substance having varying amounts of liquid water-like absorption. Individual ice grains were treated as independent Rayleigh scattering centers. Calculations of the radar cross sections were made using known solutions of the radiative transfer equation. These were then compared with experimental data. Curves are given which can be used to find radar cross sections once the grain size and the water content of the snow are known.

The Magnetic Field Satellite (MAGSAT) was launched into low earth orbit on October 30, 1979, and reentered on June 11, 1980. It included a vector magnetometer which was successfully used to accurate-ly map the magnitude and direction of the magnetic field over most of the surface of the earth. To achieve the accuracy sought required arc second precision determination of the relative orientations of the three sensor axes in a coordinate system defined by optical references attached to the magnetometer. The calibration procedure consisted basically of accurate and repeatable positioning of the vector magnetometer within a unique magnetic test facility which nulls the earth's magnetic field, then generates magnetic fields of selected orientations and strengths. Analysis of the magnetometer sensor outputs together with data on the accurate positioning and optical reference alignment gave the axes orientations. We used precision theodolites and methods related to surveying techniques to do the accurate positioning and optical component alignment measurements.

A simple on-line technique for monitoring the power absorbed by a cooled target irradiated by a high power laser beam is investigated. A description of the device and some experimental results are reported and discussed.

Fluorescent lighting is part of our environment. The authors have determined the intensity of such light on the corneas of working radiologists. No assessment has yet been made of the effects of such lighting over prolonged and continued periods of exposure which are frequently encountered in many work activities. Additional data is needed.

A computational model of the deterministic and stochastic processes involved in remote sensing is being developed as a tool for studying multispectral sensor systems and concepts. The goal is to improve the efficiency of sensor systems for routine worldwide monitoring of earth resources and the environment. Preliminary computational results are presented for simple models of the natural variability of atmospheric radiative transfer and surface reflectance. These results illustrate the dependence of classification accuracy on the selection of sensor spectral channels and data processing algorithms.

The dependence of the signal-to-noise ratio (SNR) on three important focal plane design functions frame time, time delay and integration (TDI), and pulse shaping is examined. A general focal plane model is developed that permits the SNR to be parameterized separately in terms of the three functions. The focal plane model treats both staring mosaic sensors and scanning line sensors as special cases of the model parameters and accounts for the pres-ence of multispectral detectors arranged in either a side-by-side or a sandwiched configuration. Both noise in the detectors (including photon noise) and charge coupled device (CCD) noise that arises in transferring the signal off the focal plane are considered, but it is assumed that the detectors are not limited by 1 /f noise. General parametric expressions for the SNR in terms of frame time, the number of samples per blur spot (pulse shaping), and the number of TDI steps are derived, and specific results for both a mosaic focal plane and a scanning line focal plane are presented. It is found that, regardless of the focal plane design, the SNR increases at least as the square root of the frame time and decreases at least as the square root of the number of samples per blur spot. The behavior of the SNR in terms of the number of TDI steps is more complicated and depends on the specific focal plane configuration. It can be stated generally, however, that the presence of CCD noise ultimately causes the SNR to peak and then decrease with increasing TDI. The peak SNR occurs at large TDI steps for scanning line sensors and at small TDI steps for mosaic sensors.

The laboratory techniques used for the fabrication of sub-micron gratings in GaAs are presented. A thin (~ 1250 A) film of Shipley AZ1450B photoresist on GaAs is exposed holographically with the use of the 4579 A line of an argon ion laser to produce gratings with a period of approximately 0.35 um. Data are presented that demonstrate the effects of variation of the following parameters: developer type, developer time, laser intensity, exposure time, photoresist thickness, and ion beam etching parameters. Grating efficiency measurements as a function of parameter variations indicate an optimum set of parameters for grating fabrication.

Sandwich hologram interferometry was examined for ap-plicability to the nondestructive testing of high pressure nuclear reactor components. A nonnuclear valve was used as a test specimen. While new and unflawed, it was examined and holographically characterized under the stress of pressurization. A flaw was then machined on the inside surface of the valve casing, and the valve was reexamined. This procedure was repeated, deepening the flaw each time, until it was visible as a "chink" in the fringe pattern characteristic of normal valve expansion. From this, a sensitivity limit for interferometric detection of subsurface voids has been established. Cracks were not examined because the difficulty and expense of the simulation techniques did not fit within the scope of this contract.

Air-broadening coefficients and line strengths are reported for selected ozone lines in the 8.8 to 10 um spectral region. These parameters were obtained from a least squares fit of the Voigt line shape to experimental ozone absorption spectra obtained with a tunable diode laser. Analysis of 11 lines in the vi and 18 lines in the v3 band suggest a transition-dependent coefficient although no definitive relationship was identified. The average air-broadening coefficient for the vi and v3 ozone lines is 0.077 and 0.083 cm-1/atm, respectively. The measured line strengths are typically 10% to a factor of two larger than those given in the Air Force Geophysics Laboratory compilation.

Polar molecules (such as acetonitrile, cyanoacetylene, acetone, and nitromethane) imbedded in solid inert gas matrices give rise to different types of IR absorption bands in a spectral range where intramolecular vibrations may be excluded, i.e., below 150 cm-1. Phonon bands of Ar, Kr, and Xe are activated in the range below 72 cm-1. Below 90 cm-1 translational ("rattling") and librational modes of monomer molecules relative to the cage of inert gas atoms were observed. Between 80 cm-1 and 130 cm-1 the IR active intermolecular vibrations of antiparallel dimers appear.

This paper describes a microcomputer-based hybrid processor which has been developed around two optical processors: (a) the coded-phase optical processor and (b) the confocal optical feedback system. The hybrid processor includes: (a) a microcomputer-controlled laser scanner which can produce both holographic and zero-order spatial filters, (b) an image analysis system based on video electronics which can digitize, store, analyze, and display optical output images, and (c) a liquid crystal light valve to provide a real-time input to the optical processors. The major capabilities of these three subsystems are described.

A very high resolution and high contrast laser-addressed liquid crystal color display has been developed for use in the acquisition and exploitation of high density alphanumeric and graphic data. The display system consists of two smectic liquid crystal light valves (SLCLV) which are thermally addressed by a single laser selectively. Each light valve is capable of generating 2048 X2048 addressable pixels within a 1 in. X1 in. data format. There are 8 X106 pixels available for data presentation. The laser writing system consists of the laser, a modulator which enables the formation of discrete data elements on the SLCLV, a two-axis galvanometer deflection unit, and beam splitting optics designed to multiplex the writing laser on each SLCLV. A closed-loop galvanometer deflection system can position the focused laser beam to an accuracy of 0.01 % across the screen. The laser focus lens is an f/8.0 telecentric designed to produce a 2.54 cm X2.54 cm (1 in. X1 in.) data format on the SLCLV with spot size less than 10 ,um (0.0004 in.). Color images from each light valve are superimposed onto ' the screen by a wide angle f/8.0 projection lens with 48X magnification which relays the combined color image to a 1.2 m X 1.2 m (48 in. X48 in.) screen with less than 0.1 % of distortion. The possible contrast of the display is up to 60 to 1. The overall dimension of the rear projected color display is 1.52 m X3.2 m X0.74 m (60 in. X52 in. X29 in.).

Unlike the distinguished editor of another fine optics journal whose new home is really quite old, I have a home so new that (at this writing) it is unheated, unfurnished, uncomfortable, and unsuited for editing. By the time this is published all that will have changed.