A fast-Fourier-transform method of topography and interferometry is proposed. By computer processing of a noncontour type of fringe pattern, automatic discrimination is achieved between elevation and depression of the object or wave-front form, which has not been possible by the fringe-contour-generation techniques. The method has advantages over moire topography and conventional fringe-contour interferometry in both accuracy and sensitivity. Unlike fringe-scanning techniques, the method is easy to apply because it uses no moving components.

Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry

We first provide an overview of 3-D shape measurement us- ing various optical methods. Then we focus on structured light tech- niques where various optical configurations, image acquisition tech- niques, data postprocessing and analysis methods and advantages and limitations are presented. Several industrial application examples are presented. Important areas requiring further R&D are discussed. Finally, a comprehensive bibliography on 3-D shape measurement is included, although it is not intended to be exhaustive. © 2000 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(00)00101-X)

Overview of three-dimensional shape measurement using optical methods

Guided Lamb waves for identification of damage in composite structures: A review

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

Fringe projection techniques: Whither we are?

Shape measurement of small objects using LCD fringe projection with phase shifting

This paper describes the use of optical fringe projection method for 3D surface profile and deformation measurement of micro-components. In this method, sinusoidal linear fringes are projected on a micro-component surface by a grating phase shifting projector and a long working distance microscope (LWDM). The image of the fringe pattern is captured by a high-resolution CCD camera and another LWDM and processed by phase-shifting technique. A simple procedure is described which enables calibration of the optical set-up for subsequent quantitative measurement of micro-components of unknown shapes. This method is relatively simple and accurate, and is capable of conducting fully automated measurements. In this paper, two micro-components, a micro-mirror ( 0.1 mm ×0.1 mm ) and a micro-electrode pad are used to demonstrate deformation measurement and microscopic surface contouring.

Microscopic surface contouring by fringe projection method

In situ surface roughness measurement using a laser scattering method

Results on flaw detection of glass fibre reinforced plastic beams using time- average shearography are presented here. Detection and sizing of flaws such as debonds or delaminations are successfully carried out using this technique. For easy detection of flaws, the component has to be excited at the resonance frequencies of the flaws. As the flaw gets smaller, a higher frequency is required. For flaws of the same exterior size, a deeper one will also require a higher frequency.

Flaw detection in composites using time-average shearography

A whole-field method of double-exposure speckle photography is employed to determine metal surface roughness by correlation between two speckle patterns. A movable rectangular aperture that is mounted before an image lens is shifted between the exposures, which results in a decrease in the contrast of the reconstructed Young's fringes with increasing roughness. The technique permits evaluation of the roughness of particular points on a surface as well as the average roughness of an entire surface. Four sets of random surfaces that were prepared by different machine-finishing processes and with roughnesses ranging from 0.6 to 13 µm have been tested. Different methods have been carried out to process the test data, and a practical method for the evaluation of surface roughness is proposed.

H.M. Shang

Papers

Shape measurement of small objects using LCD fringe projection with phase shifting

Microscopic surface contouring by fringe projection method

In situ surface roughness measurement using a laser scattering method

Flaw detection in composites using time-average shearography

Whole-field determination of surface roughness by speckle correlation.