Three different image-processing methods based on the time-averaged technique were compared by the electronic speckle pattern interferometry (ESPI) technique for vibration measurement The three methods are the video-signal-addition method, the video-signal-subtraction method, and the amplitude-fluctuation method Also, errors introduced by using the zero-order Bessel function directly into the analysis of the fringe pattern were investigated The video-signal-addition method has been the most generally used ESPI technique for vibration measurement However, without additional image and/or signal-processing procedures, the fringe pattern obtained directly by the video-signal-addition method is rather difficult to observe The reason for poor visibility of the experimentally obtained fringe pattern with this method is explained To increase the fringe pattern's visibility without additional image and/or signal processes, we tried two video-signal-subtraction methods One of the two methods is the video-signal-subtraction method that has normally been used in the static applications The other method, called the amplitude-fluctuation method, and its associated theory are reported here

Vibration measurement by the time-averaged electronic speckle pattern interferometry methods

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Experimental analysis and computational modelling of damage and fracture

Vibration observation is a major application of digital speckle-pattern interferometry (DSPI), which is a variation on electronic speckle-pattern interferometry (ESPI). DSPI processes speckle patterns in a computer rather than with a frame grabber and analog electronics as in ESPI. A new method of observing vibration fringes is presented and compared with existing techniques as well as some variations on them. Fringe contrast and signal-to-noise ratio are used as a means of comparison since these quantities are dependent on the techniques used. This new technique involves continuously subtracting a reference frame containing only self-interference terms and no cross interference term from the time-averaged data frames of the vibrating object. This reference frame is created by vibrating a reference mirror at a high amplitude while the object is at rest. Comparisons of calculated fringe contrast with four other observation methods show that this method yields extremely good fringe contrast. Experimental results are shown for this new technique as well as for the most commonly used vibration-observation technique. These results show that the new technique is far superior to all the other methods for moderately unstable objects, which may slowly drift or deform in time.

/pdf/vibration-observation-techniques-for-digital-speckle-pattern-56te440i0i.pdf

Vibration-observation techniques for digital speckle-pattern interferometry

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

TV holograms for spatial phase stepping are formed with a small angular offset between the object and the reference beams to give a spatial frequency bias to the pattern recorded by the TV camera. It is common to set the bias so that there is a 90° or 120°phase shift between adjacent pixels and to use the irradiance of three or more adjacent pixels to evaluate the phase of the interference. We report the Fourier-transform evaluation of such recordings to obtain their phase data. We also demonstrate the direct calculation of the phase difference between successive recordings without intermediate calculation of the random phase of each hologram. This technique is proposed as an approach to pulsed TV holography.

Fourier-transform evaluation of phase data in spatially phase-biased TV holograms

The optical configuration comprises an imaging lens with the reference point source on the optical axis in the centre of the exit pupil of the lens. The spatial frequency band is determined by the aperture and is independent of object size. In spite of the similarity to inline image plane holograms, image and intermodulation wavefront terms are separated by means of slit apertures in the recording step and matched spatial filters in the reconstruction step. Reconstruction is performed with thermal light, the interferograms can be displayed or printed directly on photographic paper.

https://iopscience.iop.org/article/10.1088/0022-3735/8/7/011/pdf

K Biedermann

Papers

A recording and display system for hologram interferometry with low resolution imaging devices