Fringe-free holographic measurements of large-amplitude vibrations

Full-field laser heterodyne imaging vibrometry using a CMOS-DVR system

A laser heterodyne imaging vibrometry is proposed for full-field vibration measurement. The vibration responses are imaged and recorded using a CMOS camera and a digital video recorder. A digital demodulation method based on a cumulative distribution function and autocorrelation is designed to demodulate signals affected by speckle noise. The experimental investigations confirm the viability of the proposed method for vibration measurement. Meanwhile, a comparison with laser Doppler vibrometry is performed to further validate the method. The results prove the proposed vibrometry is an effective and precise option for full-field vibration measurement.

One Single Static Measurement Predicts Wave Localization in Complex Structures

A recent theoretical breakthrough has brought a new tool, called the localization landscape, for predicting the localization regions of vibration modes in complex or disordered systems. Here, we report on the first experiment which measures the localization landscape and demonstrates its predictive power. Holographic measurement of the static deformation under uniform load of a thin plate with complex geometry provides direct access to the landscape function. When put in vibration, this system shows modes precisely confined within the subregions delineated by the landscape function. Also the maxima of this function match the measured eigenfrequencies, while the minima of the valley network gives the frequencies at which modes become extended. This approach fully characterizes the low frequency spectrum of a complex structure from a single static measurement. It paves the way for controlling and engineering eigenmodes in any vibratory system, especially where a structural or microscopic description is not accessible.

Heterodyne holography with full control of both the signal and reference arms

Heterodyne holography is a variant of phase-shifting holography in which the reference and signal arms are controlled by acousto-optic modulators. In this review paper, we will briefly describe the method and its properties, and we will illustrate its advantages in experimental applications.

Multi-point vibrometer based on high-speed digital in-line holography

This paper describes a digital holographic setup based on in-line holography and a high-speed recording to get a multipoint vibrometer. The use of a high-speed sensor leads to specificities that enable the in-line configuration to be used. The case of transient vibrations is investigated through a full simulation of the holographic process. The simulation shows that the first instants are critical since distortion may occur, resulting in errors in the phase measurement. Experimental results are provided by exciting an aluminum beam with a transient signal. A comparison with the velocity measured by a pointwise vibrometer is provided. Frequency response functions are extracted and the experimental results confirm the ability of the method to provide full-field contactless measurements at the high-speed time scale evolution of the vibration.

Stochastic digital holography for visualizing inside strongly refracting transparent objects

This paper presents a digital holographic method to visualize and measure refractive index variations, convection currents, or thermal gradients, occurring inside a transparent and refracting object. The proof of principle is provided through the visualization of refractive index variation inside a lighting bulb. Comparison with transmission and reflection holography is also provided. A very good agreement is obtained, thus validating the proposed approach.

Video-rate computational heterodyne holography

We present a versatile computational image rendering software of optically acquired holograms. The reported software can process 4 Megapixel 8-bit raw frames from a sensor array acquired at a sustained rate of 80 Hz. Video-rate image rendering is achieved by streamline image processing with commercial computer graphics hardware. For time-averaged holograms acquired in off-axis optical configuration with a frequency-shifted reference beam, wide-field imaging of one tunable spectral component is permitted. This software is validated by phase-stepped hologram rendering, and noncontact monitoring of surface acoustic waves by single and dual sideband hologram rendering. It demonstrates the suitability of holography for video-rate computational laser Doppler imaging in a heterodyne optical configuration.

Evaluation of surface acoustic waves on the human skin using quasi-time-averaged digital Fresnel holograms

This paper proposes a first attempt to visualize and analyze the vibrations induced by a bone-conduction device and propagating at the surface of the skin of a human face. The method is based on a new approach in a so-called quasi-time-averaging regime, resulting in the retrieval of the vibration amplitude and phase from a sequence of digital Fresnel holograms recorded with a high image rate. The design of the algorithm depends on the ratio between the exposure time and the vibration period. The results show the propagation of vibrations at the skin surface, and quantitative analysis is achieved by the proposed approach.

Imaging velocities of a vibrating object by stroboscopic sideband holography

We propose here to combine sideband holography with stroboscopic illumination synchronized with the vibration of an object. By sweeping the optical frequency of the reference beam such a way the holographic detection is tuned on the successive sideband harmonic ranks, we are able to image the instantaneous velocities of the object. Since the stroboscopic illumination is made with an electronic device, the method is compatible with fast (up to several MHz) vibration motions. The method is demonstrated with a vibrating clarinet reed excited sinusoidally at 2 kHz, and a stroboscopic illumination with cyclic ratio 0.15. Harmonic rank up to n = ± 100 are detected, and a movie of the instantaneous velocities is reported.

Video-rate laser Doppler vibrometry by heterodyne holography

We report a demonstration video-rate heterodyne holography in off-axis configuration. Reconstruction and display of a one megapixel hologram is achieved at 24 frames per second, with a graphics processing unit. Our claims are validated with real-time screening of steady-state vibration amplitudes in a wide-field, noncontact vibrometry experiment.