A self-scanned 1024 element photodiode array and minicomputer are used to measure the phase (wavefront) in the interference pattern of an interferometer to lambda/100. The photodiode array samples intensities over a 32 x 32 matrix in the interference pattern as the length of the reference arm is varied piezoelectrically. Using these data the minicomputer synchronously detects the phase at each of the 1024 points by a Fourier series method and displays the wavefront in contour and perspective plot on a storage oscilloscope in less than 1 min (Bruning et al. Paper WE16, OSA Annual Meeting, Oct. 1972). The array of intensities is sampled and averaged many times in a random fashion so that the effects of air turbulence, vibrations, and thermal drifts are minimized. Very significant is the fact that wavefront errors in the interferometer are easily determined and may be automatically subtracted from current or subsequent wavefrots. Various programs supporting the measurement system include software for determining the aperture boundary, sum and difference of wavefronts, removal or insertion of tilt and focus errors, and routines for spatial manipulation of wavefronts. FFT programs transform wavefront data into point spread function and modulus and phase of the optical transfer function of lenses. Display programs plot these functions in contour and perspective. The system has been designed to optimize the collection of data to give higher than usual accuracy in measuring the individual elements and final performance of assembled diffraction limited optical systems, and furthermore, the short loop time of a few minutes makes the system an attractive alternative to constraints imposed by test glasses in the optical shop.

Digital wavefront measuring interferometer for testing optical surfaces and lenses

The paper presents the application of ultrasonic guided waves for fatigue crack detection in metallic structures. The study involves a simple fatigue test performed to introduce a crack into an aluminium plate. Lamb waves generated by a low-profile, surface-bonded piezoceramic transducer are sensed using a tri-axis, multi-position scanning laser vibrometer. The results demonstrate the potential of laser vibrometry for simple, rapid and robust detection of fatigue cracks in metallic structures. The method could be used in quality inspection and in-service maintenance of metallic structures in aerospace, civil and mechanical engineering industries.

https://iopscience.iop.org/article/10.1088/0957-0233/18/3/024/pdf

Fatigue crack detection in metallic structures with Lamb waves and 3D laser vibrometry

Guided ultrasonic waves have shown great potential for structural health monitoring. Various types of transducer can be used for actuating and sensing of these waves. This includes non-contact approaches such as optical/laser techniques. Classical laser methods usually involve high energy interferometers. The paper demonstrates that a commercial laser vibrometer, designed for vibration/modal analysis, can be used for crack detection in metallic structures. The study involves a simple fatigue test in order to initiate and grow a crack. Lamb waves generated by one bonded piezoceramic transducer were sensed using a multi-point scanning laser vibrometer. The results demonstrate the potential of laser vibrometry for simple, rapid and robust detection of fatigue cracks in metallic structures.

https://iopscience.iop.org/article/10.1088/0964-1726/14/6/031/pdf

Structural health monitoring using scanning laser vibrometry: II. Lamb waves for damage detection

Many optical measurement techniques provide fringe patterns as their results. The decodification processes that employ one or several fringe patterns to automatically retrieve the phase are generally designated as phase-evaluation methods. In this work, an overview of these methods will be schematically presented. Their particular performances will be compared, stressing their main advantages and drawbacks. An important group of these methods employs the well-known phase-shifting algorithms as a tool for calculating the phase. In the general form of these algorithms, the principal value of the optical phase is computed by an inverse trigonometric function whose argument is a combination of phase-shifted intensity values, provided by the modulation of one or several fringe patterns. These algorithms will be also studied in the general context of the phase-evaluation methods.

https://iopscience.iop.org/article/10.1088/0957-0233/10/3/005/pdf

Phase-evaluation methods in whole-field optical measurement techniques

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.

/pdf/a-systematic-approach-to-tv-holography-44h8p1g35o.pdf

A systematic approach to TV holography

We present a system that measures the full-field amplitude and phase distributions of objects that vibrate with small amplitudes. The system is based on TV–holography combined with sinusoidal phase modulation, discrete vibration-phase shifts, and digital image processing. Different new algorithms, based on a linear approximation of the fringe function, are discussed. Averaging techniques, used to reduce the effects of noise sources and to increase the resolution of the system, are also introduced. For one of the algorithms, tested in combination with the averaging techniques, the amplitude threshold was approximately 1/3000 of the wavelength of the applied laser light. The amplitude resolution was of the same magnitude. The phase accuracy is amplitude dependent and was about 3° for amplitudes greater than 5 nm.

Analysis of a data-based TV–holography system used to measure small vibration amplitudes

We describe a time efficient method for calculating the optical phase distribution by a phase stepping technique. The computation is performed in a PC-based frame grabber board. For a 512 × 512 pixel field the processing time is 1.7 s, which represents a substantial increase in speed compared to conventional computing in a PC. The method is used for calculations of synthetic and real data from a contouring setup.

Fast measurements of phase using a PC-based frame grabber and phase stepping technique

We analyze the sensitivity of a video-based moire technique used to measure vibrations. By use of modulated projected fringes, full-field distributions are mapped for both the vibration amplitude and the vibration phase. We also show methods for visual interpretation of the vibration fringe pattern, and we describe phase-stepping algorithms for full-field calculations of the distributions by use of digital image-processing equipment. Implementation of these methods makes it possible to measure vibration amplitude and phase over a large range of amplitudes, from < 1 μm to several millimeters. We show that the method bridges the traditional gap between holographic interferometry and moire methods.

Video-based vibration analysis using projected fringes.

A time-averaged recording of a sinusoidally vibrating object reconstructs a fringe function, which is determined by the ratio between the exposure time and the vibration period. For short exposures, compared with the vibration period, the fringe function is highly dependent on the number of vibration cycles recorded and on the starting point of the exposure in the vibration cycle. When several fringe functions that are recorded at different parts of the vibration cycle are added, the resulting averaged fringe function is similar to the normal J02 function, even at short exposures. The frequency ranges at which numerical analysis can be used in these two cases are defined, and the result of short exposures permitting digital fringe analysis, even under extremely unstable situations, is demonstrated. Extending the standard video exposure time permits recording vibrations at low frequencies as the normal J02 function and improves the light economy.

Effect and use of exposure control in vibration analysis using TV holography.

A system for automatic data acquisition and fringe analysis based on phase shifting TV- holography is described. The system has been used to analyze vibrating objects heated to high temperatures.

Geir Ove Rosvold

Papers

Analysis of a data-based TV–holography system used to measure small vibration amplitudes

Fast measurements of phase using a PC-based frame grabber and phase stepping technique

Video-based vibration analysis using projected fringes.

Effect and use of exposure control in vibration analysis using TV holography.

Computerized vibration analysis of hot objects