Ole Johan Lokberg

Vibration phase mapping using electronic speckle pattern interferometry

The contours of constant phases across sinusoidally vibrating objects have been mapped using reference wave phase modulation in time-average electronic speckle pattern interferometry.

Detection and measurement of small vibrations using electronic speckle pattern interferometry

We describe a new method for real-time detection and measurement of small vibrations, based on phase modulation in time-average electronic speckle pattern interferometry. The modulation frequency is shifted relative to the vibration frequency, which makes the intensity of the reconstructed image vary at the difference frequency. The amplitude detection limits are about 20 A by visual observation and 0.1 A by photoelectric measurement using a lockin technique. No auxiliary system for fringe stabilization is required. At higher amplitude levels we can study the deformation of the object in slow motion. Measurements on different objects, including human ear preparation, are presented.

Vibration measurement on the human ear drum in vivo

This paper describes the application of electronic speckle pattern interferometry to vibration measurements on the human ear drum in vivo. The technique makes use of a continuous wave laser, reference wave phase modulation, and videorecording of interferograms for subsequent analysis. Preliminary results, like frequency response and amplitude and phase measurements, are presented to illustrate the ability of this technique.

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.

Sound in flight: measurement of sound fields by use of TV holography

The integrated amplitude and phase distribution of the sound field propagating from sinusoidally vibrating objects in air have been recorded by time-averaged TV holography. The behavior of the sound field is observed in real time by use of dynamic phase modulation. The magnitude and direction of the field are found by acoustic phase stepping and image processing. The field is three dimensional, and recordings of several cross sections are necessary for a complete description, but in many cases valuable information can be obtained by two-dimensional projections. The technique has been used to study sound propagation and details in the sound emission from extended sources.

Use of chopped laser light in electronic speckle pattern interferometry

Shortened exposure of each TV frame is used in conjunction with time-average ESPI to study the sinusoidal vibrations of extremely unstable objects. The effect of short exposures upon the fringe functions is calculated. It is shown how sinusoidal phase modulation techniques can be used at all realistic exposure values. We also discuss how double-exposure ESPI with a cw laser can be used to study more random movements, e.g. of biological objects. The lag characteristic of the TV target is used to extend the time separation. Fringe patterns illustrating applications of these techniques are presented.

Tomographic reconstruction of sound fields using TV holography

Combining TV holography recording with acoustic phase stepping and image processing, we measure the integrated density distribution in sound fields that propagate in air. We record a given number of two-dimensional cross sections that are tomographically backprojected to give the amplitude and phase distributions of the emitted sound field. The validity of the procedure is demonstrated.

Holographic vibration measurement using a tv speckle interferometer with silicon target vidicon

Abstract A basic realization of a TV speckle interferometer applying a silicon target vidicon is described. The increased detectivity compared with other vidicons reduces the laser power requirements. This enables stroboscopic operation in a fairly straightforward manner with a He-Ne laser source. Time average- and stroboscopic fringe patterns for a vibrating test object are presented.

Long-Distance Electronic Speckle Pattern Interferometry

Electronic speckle pattern interferometry has been used to measure the vibrations and deformations of objects located far from the optical head. Vibration recordings were made for a total path-length difference of up to 200 m with the object outdoors in bright sunshine. This limit was determined by practical considerations. For deformation recordings, turbulence and mechanical instabilities create a problem, and a more realistic limit is a path-length difference of about 50 m under stable, indoor conditions.

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

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 J(o)(2) 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 J(o)(2) function and improves the light economy.