Micrometric size measurement of biological samples using a simple and non-invasive transmission interferometric set up.

An interferometer with a minimum of optical hardware is employed to measure invasiveness the size of biological samples. Nowadays, there are several techniques in microscopy that render high quality resolved images. For instance, consider optical microscopy that has been around for over a century and has since developed in different configurations such as: bright and dark field, phase contrast, confocal, polarized, and so on. However, only a few of these use interferometry to retrieve not only the sample's amplitude but also its phase. An interesting example of the latter is digital holography which normally uses a Mach Zehnder interferometer setup. In the research work reported here a transmission digital holographic interferometer designed with a simple and minimal optical hardware, that avoids the drawback of the small field of view present in classical optical microscopic systems, is used to measure the microscopic dimensions of pollen grains. This optical configuration can be manipulated to magnify and project the image of a semitransparent sample over a neutral phase screen. The use of a collimated beam through the sample prevents geometrical distortions for high magnification values. The measurements using this novel configuration have been validated using a standard precision pattern displacement specimen with certified dimensions. As proof of principle, microscopically characterized pollen grains are placed in the transmission set up in order to estimate their dimensions from the interferometrically retrieved optical phase. Results match and thus show a relation between the sample's size and the optical phase magnitude.

3D displacement measurements of the tympanic membrane with digital holographic interferometry.

A digital holographic interferometry (DHI) system with three object-illumination beams is used for the first time to measure micro-deformations along the x, y and z axes (3D) on the tympanic membrane (TM) surface of a post-mortem cat. In order to completely and accurately measure the TM surface displacements its shape is required to map on it the x, y and z micro-deformations. The surface contour is obtained by applying small shifts to the object illumination source position. A cw laser in stroboscopic mode and a CCD camera were used and synchronized to the acoustic excitation wave that produces a resonant vibration mode on the tympanic membrane surface. This research work reports on the 3D full field of view response of the TM to sound pressure, and has as its main goal the presentation of DHI as an alternative technique to study the TM real displacement behavior when subjected to sound waves, so it can be used as a diagnostic tool to prevent and treat TM diseases.

Holographic otoscope for nanodisplacement measurements of surfaces under dynamic excitation.

We describe a novel holographic otoscope system for measuring nanodisplacements of objects subjected to dynamic excitation. Such measurements are necessary to quantify the mechanical deformation of surfaces in mechanics, acoustics, electronics, biology, and many other fields. In particular, we are interested in measuring the sound-induced motion of biological samples, such as an eardrum. Our holographic otoscope system consists of laser illumination delivery (IS), optical head (OH), and image processing computer (IP) systems. The IS delivers the object beam (OB) and the reference beam (RB) to the OH. The backscattered light coming from the object illuminated by the OB interferes with the RB at the camera sensor plane to be digitally recorded as a hologram. The hologram is processed by the IP using the Fresnel numerical reconstruction algorithm, where the focal plane can be selected freely. Our holographic otoscope system is currently deployed in a clinic, and is packaged in a custom design. It is mounted in a mechatronic positioning system to increase its maneuverability degrees to be conveniently positioned in front of the object to be measured. We present representative results highlighting the versatility of our system to measure deformations of complex elastic surfaces in the wavelength scale including a copper foil membrane and postmortem tympanic membrane. SCANNING 33: 342-352, 2011. © 2011 Wiley Periodicals, Inc.

Measurement of Young's modulus in an elastic material using 3D digital holographic interferometry.

Digital holographic interferometry using three different illumination directions is applied to quantify in full field of view the deformation of an elastic membrane under a given vibration frequency. The technique allows the separation of the object microdisplacements components in the x, y, and z directions, and the resulting data is used to find the membrane Young's modulus. The latter was compared with the result obtained using the theory of thin plates for a clamped circular membrane under a uniformly distributed load, reaching a 95% agreement. Validation of the results between these methods shows the reliability of DHI to measure the mechanical properties of an elastic material.

Digital holographic interferometer using simultaneously three lasers and a single monochrome sensor for 3D displacement measurements.

The use of digital holographic interferometry for 3D measurements using simultaneously three illumination directions was demonstrated by Saucedo et al. (Optics Express 14(4) 2006). The technique records two consecutive images where each one contains three holograms in it, e.g., one before the deformation and one after the deformation. A short coherence length laser must be used to obtain the simultaneous 3D information from the same laser source. In this manuscript we present an extension of this technique now illuminating simultaneously with three different lasers at 458, 532 and 633 nm, and using only one high resolution monochrome CMOS sensor. This new configuration gives the opportunity to use long coherence length lasers allowing the measurement of large object areas. A series of digital holographic interferograms are recorded and the information corresponding to each laser is isolated in the Fourier spectral domain where the corresponding phase difference is calculated. Experimental results render the orthogonal displacement components u, v and w during a simple load deformation.