Television holography (TVH) can be defined as `the family of optical measurement techniques based on the electronic recording and processing of holograms'. Image-plane TVH was introduced in the early 1970s with the name `electronic speckle-pattern interferometry' (ESPI). Since then, TVH has undergone an impressive development and become one of the most promising optical techniques for non-destructive testing and industrial inspection. The aim of this review is to propose an original scheme for the systematic treatment of TVH and to review the existing techniques according to it. In this approach we split the measurement process into four highly independent stages (illumination and observation geometry, temporal treatment, secondary-correlogram generation and fringe-pattern analysis) and establish a common notation to formulate the corresponding techniques. Such a strategy allows the free combination of the techniques proposed for each stage as building blocks to obtain every particular variant of the whole TVH measurement process, whether it has already been reported or not, and also the incorporation of new techniques while retaining compatibility with the existing variants of the previous and following stages.

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A systematic approach to TV holography

Since shearing interferometry, also called shearography, does not depict fringes caused by object tilt, it is a tool well suited for either nondestructive testing of objects under load or for quantitative evaluation of flexural strains. In traditional shearing interferometry, observation of fringes requires optical processing of double-exposed interferograms. Hence the technique is not in real time. This paper explores the possibilities and limitations for real time shearing fringe observation using the electronic speckle pattern interferometry technique. Prospects for quantitative determination of local tilt and flexural strain using the phase shifting technique are commented on, and some preliminary results are shown.

Digital speckle pattern shearing interferometry : limitations and prospects

A Fourier optics analysis for a recently developed lateral shearing interferometry-coherent gradient sensing-is presented. The governing equations for the method are explicitly derived. The method of coherent gradient sensing is particularly suitable for investigating the mechanics of fracture of transparent and opaque solids. Several examples demonstrating the applicability of the method to quasistatic and dynamic crack-growth problems is presented.

Coherent gradient sensing: a Fourier optics analysis and applications to fracture

In a mail sorting system 10, output pixel intensities of an optical scanner 18 have their contrast locally enhanced. A contrast enhancer 24 uses statistical methods (averaging, standard deviation) coupled with empirical stretch and offset data stored in a ROM 25, to enhance pixel contrast in a pipelined processing operation.

Local adaptive contrast enhancement

Recent developments in digital speckle pattern interferometry

A speckle shear interferometer is presented which can be used to yield lateral, radial, rotational, and inversion shear fringes in real time in conjunction with a digital image processing system. A modification of the optical arrangement yields reversal or folding shear as well. Experiments are conducted on an edge-clamped diaphragm with concentrated load. Unit contrast fringes have been obtained by resorting to nonlinear processing techniques such as level slicing. The results are presented for various types of shear.

Universal digital speckle shearing interferometer

Some simple image processing techniques are suggested that can be used for enhancing the contrast of real-time digital speckle pattern interferometry fringes The techniques have been developed for the commercial Intellect 100 image processing system interfaced to a PDP-1 1 /23+ microcomputer, but they can be adapted to any commercial image processing system with slight modifications, if necessary, depending on the hardware configuration of the system

Simple Image Processing Techniques For The Contrast Enhancement Of Real-Time Digital Speckle Pattern Interferometry Fringes

In this paper, we present the developmental details of an all fiber optic phase shifting Electronic Speckle Pattern Interferometer (ESPI) and its applications in Non-Destructive Evaluation (NDE) of Glass Fiber Reinforced Plastics (GFRP). Laminates made of GFRP are studied using the developed ESPI system in its shear configuration. This provides a full-field, non-contact method for NDE, and also observation of surface strains which are proportional to displacement derivatives. Since defects in objects usually induce strain concentrations, shear ESPI reveals defects by identifying anomalies in strain concentration. Experiments have been carried out for NDE of unidirectional GFRP specimens at different fiber orientations and having programmed defects. The experimental results are compared with the theoretical predictions.

Nondestructive evaluation of fiber-reinforced polymer composites by an all-fiber-optic phase-shifting electronic speckle pattern interferometer

Sharpening of fringes in digital speckle pattern interferometry.

In certain fiber optic sensors and gyroscopes using birefringent fibers, it is essential to precisely align the principal axes of the fibers used, when two fibers are spliced or connected. The proper alignment of the linearly polarized input light wave with the birefringence axis of a fiber is crucial for maintaining the state of polarization of the guided wave. In this paper, a new method for the detection of the eigen axes and precise alignment of the principal axes between two birefringent fibers (HiBi) using shear interferometry is presented.

A. R. Ganesan

Papers

Universal digital speckle shearing interferometer

Simple Image Processing Techniques For The Contrast Enhancement Of Real-Time Digital Speckle Pattern Interferometry Fringes

Nondestructive evaluation of fiber-reinforced polymer composites by an all-fiber-optic phase-shifting electronic speckle pattern interferometer

Sharpening of fringes in digital speckle pattern interferometry.

Novel techniques for the detection of eigen axes and precise angular alignment of highly birefringent (HiBi) fibers using shear interferometry