Fernando Mendoza-Santoyo

Phase demodulation in the space domain without a fringe carrier

It is demonstrated that interference phase can be obtained from a fringe pattern using the spatial domain method known as spatial synchronous detection (direct interferometry) but without the usual fringe carrier (e.g. tilt) introduced into the interferometer. This is particularly useful in the study of transient events, where it is not always straightforward to introduce these carrier fringes. Spatial domain analysis is considerably faster than frequency domain (Fourier) methods, and could enable phase to be calculated in real time on relatively cheap personal computers. The main disadvantage of the new technique is that the sign of the recovered phase is lost, and must be inferred from object constraints.

Detection of surface strain by three-dimensional digital holography

Three-dimensional digital holography with three object-illuminating beams has been successfully used for the detection of surface strain in metallic objects. The optical setup that uses illuminating beams to irradiate the object from three directions means that all three object surface displacement components, x, y, and z, can be independently calculated and used to find the strain gradients on the surface. The results show the conversion of the complete surface displacement field into a surface strain field. The method is capable of measuring microstrains for out-of-plane surface displacements of less than 10 microm.

Multimode vibration analysis with high-speed TV holography and a spatiotemporal 3D Fourier transform method.

The combination of a high-speed TV holography system and a 3D Fourier-transform data processing is proposed for the analysis of multimode vibrations in plates. The out-of-plane displacement of the object under generic vibrational excitation is resolved in time by the fast acquisition rate of a high-speed camera, and recorded in a sequence of interferograms with spatial carrier. A full-field temporal history of the multimode vibration is thus obtained. The optical phase of the interferograms is extracted and subtracted from the phase of a reference state to yield a sequence of optical phase-change maps. Each map represents the change undergone by the object between any given state and the reference state. The sequence of maps is a 3D array of data (two spatial dimensions plus time) that is processed with a 3D Fourier-transform algorithm. The individual vibration modes are separated in the 3D frequency space due to their different vibration frequencies and, to a lesser extent, to the different spatial frequencies of the mode shapes. The contribution of each individual mode (or indeed the superposition of several modes) to the dynamic behaviour of the object can then be separated by means of a bandpass filter (or filters). The final output is a sequence of complex-valued maps that contain the full-field temporal history of the selected mode (or modes) in terms of its mechanical amplitude and phase. The proof-of-principle of the technique is demonstrated with a rectangular, fully clamped, thin metal plate vibrating simultaneously in several of its natural resonant frequencies under white-noise excitation.

Displacement measurements over a square meter area using digital holographic interferometry

Abstract. Current industrial demand for optical nondestructive testing includes the displacement analysis of large object areas. This paper reports on the use of a digital holographic interferometer to measure displacements over an area of 1.14  m2. The object under study is a framed working table covered with a Formica layer fixed to a granite bench, and it is observed and illuminated employing a high speed and high resolution camera and a continuous wave high output power laser, respectively. A stabilization procedure needs to be established as long illumination distances are required in order to retrieve the entire surface optical phase during a series of continuous deformations. As a proof of principle, two different tests are presented: the first involves a slow continuous loading process and the second a vibration condition. The wrapped phase and displacement maps are both displayed.

Quantitative comparison of tympanic membrane displacements using two optical methods to recover the optical phase

Abstract The study and quantification of the tympanic membrane (TM) displacements add important information to advance the knowledge about the hearing process. A comparative statistical analysis between two commonly used demodulation methods employed to recover the optical phase in digital holographic interferometry, namely the fast Fourier transform and phase-shifting interferometry, is presented as applied to study thin tissues such as the TM. The resulting experimental TM surface displacement data are used to contrast both methods through the analysis of variance and F tests. Data are gathered when the TMs are excited with continuous sound stimuli at levels 86, 89 and 93 dB SPL for the frequencies of 800, 1300 and 2500 Hz under the same experimental conditions. The statistical analysis shows repeatability in z-direction displacements with a standard deviation of 0.086, 0.098 and 0.080 μm using the Fourier method, and 0.080, 0.104 and 0.055 μm with the phase-shifting method at a 95% confidence level for all frequencies. The precision and accuracy are evaluated by means of the coefficient of variation; the results with the Fourier method are 0.06143, 0.06125, 0.06154 and 0.06154, 0.06118, 0.06111 with phase-shifting. The relative error between both methods is 7.143, 6.250 and 30.769%. On comparing the measured displacements, the results indicate that there is no statistically significant difference between both methods for frequencies at 800 and 1300 Hz; however, errors and other statistics increase at 2500 Hz.

Detection of micromechanical deformation under rigid body displacement using twin-pulsed 3D digital holography

Twin-pulsed digital holography in its 3D set up is used to recover exclusively the micro-mechanical deformation of an object. The test object is allowed to have rigid body movements such as rotation and translation, with the result that the fringe patterns contain information of the latter and the object deformation, a feature that may significantly modify the interpretation of the results. Experimental results from a flat metal plate subject to micro stress and a displacement in the x-z plane are presented to demonstrate that using this optical method it is possible to recover exclusively the contribution of the micro stress.

Quantitative analysis of elasticity changes in UV radiated skin

Skin cancer is one of the most common forms of cancer and thus a big public health problem worldwide. However, although it is well known that UV radiation is the main cause of skin cancer, there are currently only a few quantitative studies about the effects of UV radiation on the skin. One of the main objectives of the research project we are conducting is centered on the study of the change in elasticity and/or stiffness that the skin suffers after being irradiated with UV, parameters that are being analyzed and quantified with the aim of using these elasticity properties as a powerful clinic tool. Preliminary studies on the elasticity of animal skin samples aged inside a UV radiation chamber were conducted using Digital Holography Interferometry, a non-invasive optical technique that has been gaining importance as an alternative way to study elasticity properties, mainly due to its sensitivity and precision for measuring displacements and strains. Controlled sound waves were used to stimulate the skin samples, with the results suggesting that the longer it is exposed to UV radiation the greater is its increase in stiffness, confirming the importance of not selfoverexposing to solar radiation. The research work proposed in the project represents a great opportunity to contribute to the knowledge of other skin diseases as well as skin cancer. A deeper analysis of the change in elasticity between healthy skin and skin with cancer will be discussed in a later publication.

Qualitative characterization of collagen hydrogel membranes using digital holographic microscopy and SHG microscopy

Crosslinked collagen hydrogel membranes are widely used in engineering tissue to promote the performance of biological processes involved in the healing of wounds, mainly in the presence of chronical diseases such as diabetes. One of the standard techniques used in biology to measure mechanical properties of hydrogels and tissues is based on a methodology called rheology. Rheological studies consist on the measurement of stress, strain and the ratio of stress to strain of several biological membranes to determine their viscoelastic properties. In this research work we propose as a proof of concept the use of digital holographic microscopy (DHM) and second harmonic generation (SHG) microscopy to compare qualitatively some basic image properties of collagen hydrogel membranes. Once demonstrated that this comparison is equivalent, we study under controlled excitation the biomechanical properties of collagen membranes by means of the analysis of the optical phase that results from comparing two consecutives holograms from a reference and deformed state. At this stage of characterization, an in depth study between rheological and holographic results will be performed in the near future.

Speckle noise reduction in digital speckle pattern interferometric fringes by nonlocal means and its related adaptive kernel-based methods

Digital speckle pattern interferometry (DSPI) is widely used in many scientific and industrial applications. Besides its several advantages, one of the basic problems encountered in DSPI is the undesired speckle noise existing in the fringe pattern. In this paper, we demonstrate the performance of nonlocal means (NLM) and its related adaptive kernel-based filtering methods for speckle noise reduction in DSPI fringes. The NLM filter and its related kernel-based filters such as NLM-average, NLM-local polynomial regression, and NLM-shape adaptive patches are implemented first on simulated DSPI fringes, and their performances are quantified on the basis of peak signal-to-noise ratio (PSNR), mean square error (MSE), and quality index (Q). Further, their effectiveness and abilities in reducing speckle noise are compared with other speckle denoising methods. These filtering methods are then employed on experimental DSPI fringes. The obtained results reveal that these filtering methods have the ability to improve the PSNR and Q of the DSPI fringes and provide better visual and quantitative results. It is also observed that the proposed filtering methods preserve the edge information of the DSPI fringes, which is evaluated on the basis of the edge preservation index of the resultant filtered images.

Fully automated digital holographic interferometer for 360 deg contour and displacement measurements

Abstract. A fully automated optomechatronic system based on an out-of-plane sensitivity digital holographic interferometer is proposed to measure both the 360-deg object’s contour and its surface displacements due to sound stimulation. The digital holograms to measure the surface contour are acquired using the two points of illumination method whose optimal height sensitivity for this particular case is Δz=2.98  mm. This method renders a phase map that has a tilt produced by the angle change of the object beam relative to the optical axis, a tilt that is removed by subtracting the phase difference from a physical reference plane tangentially located at the back of the object, which contrasts with former methods that locate this plane usually in the middle portion of the object. By using rotation and averaging matrices, the 360-deg object’s contour digital reconstruction was obtained, and with it, the displacements around the object can be accurately placed on its surface. The main contribution of the proposed fully automated system is to get a 360-deg object’s contour and its surface displacements in a hands-free rapid and accurate evaluation.