Gudmunn A. Slettemoen

Electronic speckle pattern interferometric system based on a speckle reference beam

A new electronic speckle pattern interferometric (ESPI) technique is introduced. The technique is based on a reference beam combined with a large aperture optical system. The basic principles are described and compared with conventional ESPI setups. The new interferometer is easy to adjust, it is invulnerable to dust and scratches on the optical components, and is very compact. It is well-suited for practical engineering applications. Light sensitivity and fringe quality are comparable with the conventional ESPI features. Superior fringe pattern can be obtained by use of a new speckle reduction technique to be described.

Interferometric measurements of high temperature objects by electronic speckle pattern interferometry

Electronic speckle pattern interferometry with a cw laser has been used to study objects at very high temperatures. Interference patterns were observable to 1700°C, while high contrast interference fringes could be recorded up to 1550°C. These temperature limits were mainly set by the onset of surface reactions and melting of the objects. Examples on how the technique can be used to study deformations, oxidation shell growth, and melting zones are presented.

General Analysis of Fringe Contrast in Electronic Speckle Pattern Interferometry

A general analysis of signal processing in electronic speckle pattern interferometry is presented. The effects of limited T.V.-camera resolution is included, and it is shown that the T.V.-monitor brightness is given by essentially the same expression as for full T.V.-camera resolution. One term in this expression is the fringe signal, while the others represent noise. The optimal reference-to-object intensity ratio, which maximizes the fringe signal, is determined and the dependence on the size of the aperture is given. The effect of a double slit aperture is compared with the effect of a circular aperture.

Maximal fraction of acceptable measurements in phase-shifting speckle interferometry: a theoretical study

The interference between a uniform reference wave and a speckle object wave results in variable fringe contrast and background level. Taking these variations into account, we optimize system parameters of phase-shifting speckle interferometry. The results show that the optimal reference intensity should always be equal to one fourth of the detector’s saturation level. The optimal reference to the object-intensity ratio shows an increase from one up to, in most practical cases, six as a chosen interference dynamic range increases from its minimum value. The dependence of a maximal fraction of acceptable measurements on the dynamic range is calculated. Numerical examples indicate that we may hope for a readout accuracy in the range of 1/50th to 1/100th of a fringe period and still cover more than half of the image area with acceptable data. These data are taken without spatial averaging and have maximum resolution.

Interferometric comparison of displacements by electronic speckle pattern interferometry

Using the image waves from two similar objects as, respectively, object wave and reference wave in an electronic speckle pattern interferometer (ESPI) setup, it is possible to obtain interference patterns that indicate the amount of relative displacement between the two objects. Experimental results demonstrating direct interferometric subtraction and addition of vibration patterns are presented. Possible applications of the technique are also discussed.

Electronic speckle pattern interferometric measurements of the basilar membrane in the inner ear

A setup incorporating electronic speckle pattern interferometry, reference beam phase modulation, chopped laser light, and a simple microscope has been used to observe vibrations of the basilar membrane in human temporal bones. A postmortem effect ensures that no special treatment of the semitransparent membrane is required. Amplitudes (down to 20 A) and phases of the membrane motion are analyzed both in real time and video recordings. Observations include phase variations across the membrane and motions of Mossbauer sources (pieces of cobalt) on the membranes.

An ESPI Contouring Technique In Strain Analysis

A new electronic speckle pattern interferometric (ESPI) contouring technique, which is a variation of the holographic two beam illumination method, is described. Together with necessary mathematical derivations, experimental test results are reported. The system is an integrated part of a deformation measuring system where contour data will be used in strain analysis.

First-order statistics of displayed speckle patterns in electronic speckle pattern interferometry

The first-order probability density functions of displayed speckle patterns in electronic speckle pattern interferometry are calculated. We show that in specular-reference-beam setups the brightness in the displayed pattern is always (χ)2 distributed. For speckle-reference-beam setups the probability density depends on the effective resolution of the recorded speckle pattern. This function is calculated in the case of fully resolved patterns and shown to approach the (χ)2 density function as the size of the aperture increases. The statistics obtained with specular-and speckle-reference-beam setups are compared. General expressions are derived for the average value and the standard deviation of the monitor brightness. In specular-reference-beam setups the speckle contrast is found always to be equal to 2, and in speckle-reference-beam setups it is found to be in the region from 2 to 5. In setups in which speckle-reduction techniques have been used the speckle contrast decreases below these numbers. We also calculate this reduced speckle contrast.

Optimal signal processing in electronic speckle pattern interferometry

Abstract A theoretical investigation of the signal processing by ESPI is presented. Both electronic and optical noise are taken care of, and the optimum of ESPI with regard to the reference/object-ratio is found. In the limit of small electronic noise contributions this optimum is reached when the noise on the reference beam is equal to the noise on the object beam.

Some Industrial Applications Of ESPI

Electronic Speckle Pattern Interferometry - ESPI - may be described as dynamical hologram interferometry based on video recording, processing and presentation. The technique has been used to solve various problems in Norwegian industry and industrially related research. Work has been mainly concentrated on vibration testing and measurement, but recent measurements on crack opening and locations of high strains have also attracted interest.