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

Noncontact vibration measurement using three spatial-frequency shifting coherent digital holography

A new digital coherent holographic system that works as a spatial-frequency shifter for measuring three-dimensional (3D) vibration of an object is proposed. The spatial-frequency shifter is constructed by a system of three mirrors inclined with different small angles to shift the object wave to three different frequencies in the spatial-frequency domain. By applying the Fourier transform method and appropriate filters to the hologram recorded by the camera of the system, a three-phase set of object waves corresponding to three shifted frequencies was obtained. From the relation between the phases and the relative position of the object, the position of each point on the surface of the object along the x, y, and z directions was extracted from each hologram. The same process was repeatedly applied to a series of holograms recorded by a fast camera, allowing the 3D vibration of the object to be precisely observed.

Comparison of three full-field optical measurement techniques applied to vibration analysis

Digital image correlation, deflectometry and digital holography are some of the full-field optical measurement techniques that have matured in recent years. Their use in vibroacoustic applications is gaining attention and there is a need for cataloging their performance in order to provide, to a broad community of users and potential future users, quantitative and qualitative evaluations of these three approaches. This paper presents an experimental comparison of the three optical methods in the context of vibration measurements, along with classical reference measurements provided by an accelerometer and a laser Doppler vibrometer. The study is carried out on two mechanical structures exhibiting various vibration responses when submitted to an impact.

Non-contact detection of ultrasound with light - Review of recent progress

In this article, we present an overview of recent progress in non-contact remote optical detection of ultrasound in application to nondestructive testing and evaluation of materials. The focus of the review is on the latest advances in optical detection that offer mature and robust field-applicable solutions for diagnostics and imaging of engineered structures. We provide a detailed description of these solutions, including their operation principles, their evolution from the previously known designs to commercial devices, and their contribution to solving the most important problems associated with non-contact optical detection of ultrasound. Several application examples are presented to demonstrate the capabilities of optical detection and provide ideas to a reader on how it can be used in practice. We also discuss the main challenges of modern non-contact detectors which have not yet been addressed, as well as the directions and prospects for their development.

Temporal super-resolution high-speed holographic video recording based on switching reference lights and angular multiplexing in off-axis digital holography

We develop a temporal super-resolution high-speed holographic video recording method based on the angular multiplexing in off-axis digital holography that can achieve an acquisition rate greater than the frame rate of image sensors. We realize a high-speed switching of reference lights with different incident angles using two acousto-optic modulators. We successfully double the frame rate of the hologram recording using a rotating circular protractor and demonstrate its practical application in compressed gas flow injection; we achieve a frame rate of 175,000 fps using a high-speed image sensor triggered at 87,500 Hz.

Phase derivative estimation in digital holographic interferometry using a deep learning approach

In digital holographic interferometry, reliable estimation of phase derivatives from the complex interference field signal is an important challenge since these are directly related to the displacement derivatives of a deformed object. In this paper, we propose an approach based on deep learning for direct estimation of phase derivatives in digital holographic interferometry. Using a Y-Net model, our proposed approach allows for simultaneous estimation of phase derivatives along the vertical and horizontal dimensions. The robustness of the proposed approach for phase derivative extraction under both additive white Gaussian noise and speckle noise is shown via numerical simulations. Subsequently, we demonstrate the practical utility of the method for deformation metrology using experimental data obtained from digital holographic interferometry.

Modeling of speckle decorrelation in digital Fresnel holographic interferometry

This paper presents analytical modeling of the speckle decorrelation noise in digital Fresnel holographic interferometry. The theoretical analysis is carried out by considering the complex coherence factor between two speckled images from two digitally reconstructed holograms at two different instants. The expression giving the modulus of the coherence factor is established and depends on the local surface deformation and parameters from the holographic setup. The analysis is supported by realistic simulations and experiments. Both simulations and experimental results exhibit a very good agreement with the theoretical prediction.

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.

Lock-in vibration retrieval based on high-speed full-field coherent imaging

The use of high-speed cameras permits to visualize, analyze or study physical phenomena at both their time and spatial scales. Mixing high-speed imaging with coherent imaging allows recording and retrieving the optical path difference and this opens the way for investigating a broad variety of scientific challenges in biology, medicine, material science, physics and mechanics. At high frame rate, simultaneously obtaining suitable performance and level of accuracy is not straightforward. In the field of mechanics, this prevents high-speed imaging to be applied to full-field vibrometry. In this paper, we demonstrate a coherent imaging approach that can yield full-field structural vibration measurements with state-of-the-art performances in case of high spatial and temporal density measurements points of holographic measurement. The method is based on high-speed on-line digital holography and recording a short time sequence. Validation of the proposed approach is carried out by comparison with a scanning laser Doppler vibrometer and by realistic simulations. Several error criteria demonstrate measurement capability of yielding amplitude and phase of structural deformations.

Comparative study of multi-look processing for phase map de-noising in digital Fresnel holographic interferometry

This paper presents a comparative study of multi-look approaches for de-noising phase maps from digital holographic interferometry. A database of 160 simulated phase fringe patterns with eight different phase fringe patterns with fringe diversity was computed. For each fringe pattern, 20 realistic noise realizations are generated in order to simulate a multi-look process with 20 inputs. A set of 22 de-noising algorithms was selected and processed for each simulation. Three approaches for multi-look processing are evaluated. Quantitative appraisal is obtained using two metrics. The results show good agreement for algorithm rankings obtained with both metrics. One singular and highly practical result of the study is that a multi-look approach with average looks before noise processing performs better than averaging computed with all de-noised looks. The results also demonstrate that the two-dimensional windowed Fourier transform filtering exhibits the best performance in all cases and that the block-matching 3D (BM3D) algorithm is second in the ranking.

Quantitative appraisal for noise reduction in digital holographic phase imaging

This paper discusses on a quantitative comparison of the performances of different advanced algorithms for phase data de-noising. In order to quantify the performances, several criteria are proposed: the gain in the signal-to-noise ratio, the Q index, the standard deviation of the phase error, and the signal to distortion ratio. The proposed methodology to investigate de-noising algorithms is based on the use of a realistic simulation of noise-corrupted phase data. A database including 25 fringe patterns divided into 5 patterns and 5 different signal-to-noise ratios was generated to evaluate the selected de-noising algorithms. A total of 34 algorithms divided into different families were evaluated. Quantitative appraisal leads to ranking within the considered criteria. A fairly good correlation between the signal-to-noise ratio gain and the quality index has been observed. There exists an anti-correlation between the phase error and the quality index which indicates that the phase errors are mainly structural distortions in the fringe pattern. Experimental results are thoroughly discussed in the paper.

Measuring vibrational motion in the presence of speckle using off-axis holography

We present a holographic laser vibrometer designed to mitigate the effects of speckle noise when measuring the vibrational motion of a rough object. We show that multiplexing the interferometric measurement across 10⁵ pixels provides a 50 dB reduction in the incoherent noise. Using a high-speed camera, this enables a displacement sensitivity of 50  fm/√Hz with a bandwidth of 12.5 kHz when measuring rough objects, representing a 20 dB improvement compared with a commercially available single-detector-based laser vibrometer. Finally, we show that the holographic vibrometer system is capable of stand-off acoustic sensing by measuring the acoustic-induced vibrations of a piece of paper with sensitivity as low as 10 dB (re 20 μPa). The ability to sensitively and noninvasively measure the vibrations of arbitrary rough surfaces could enable new applications in laser vibrometry.

Quality assessment of combined quantization-shot-noise-induced decorrelation noise in high-speed digital holographic metrology

this paper discusses on the influence of decorrelation noise induced by quantization and shot-noise when recording digital holograms at very high frame rate. A criterion based on the coherence factor of the hologram phase difference is proposed. The main parameters of interest are the ratio between the reference and the object waves and the sensor dynamics, depending on the photo-electron capacity of pixels. The study is based on a full numerical simulation of the holographic process, which provides useful rules. This leads to define the optimal conditions for recording at very-high frame rate with minimization of the decorrelation noise. Experimental results obtained with frame rate at 50kHz confirm the proposed approach.