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Measurement Techniques
Published in Marvin C. Ziskin, Peter A. Lewin, Ultrasonic Exposimetry, 2020
Marvin C. Ziskin, Peter A. Lewin
Double-exposure holography reveals the surface levitation as a typical interference fringe pattern (Figures 2a and 3a). The interference fringe order, and therefore also the local height of the relief, can be uniquely defined by shadow-optical reconstruction of the hologram, i.e., the change of sign in the relative maxima and minima can be clearly identified. The data acquisition system consists of a computerized xy translational device with reflected light scanning. By means of this system the interferogram is scanned line by line to reveal the position of the interference fringe maxima and minima and their proper order.
All About Wave Equations
Published in Bahman Zohuri, Patrick J. McDaniel, Electrical Brain Stimulation for the Treatment of Neurological Disorders, 2019
Bahman Zohuri, Patrick J. McDaniel
Instead of the interference fringes falling on a simple screen, often they’re directed into a camera to produce a permanent image called an inter-ferogram. In another arrangement, the interferogram is made by a detector (like the Charge Coupled Device (CCD) image sensor used in older digital cameras) that converts the pattern of fluctuating optical interference fringes into an electrical signal that can be very easily analyzed with a computer.
Low-coherence interferometry
Published in Pablo Artal, Handbook of Visual Optics, 2017
TD OCT for retinal imaging has a limited speed of a few 100 A-scans/s. While this is sufficient for recording 2D cross-sectional images, it is too slow for 3D image acquisition in the living eye, where involuntary ocular motions cause image artifacts and distortions. The enormous progress and success of OCT in recent years can largely be attributed to a paradigm shift in OCT technology—the introduction of Fourier domain (FD) OCT (Fercher et al. 1995; Häusler and Lindner 1998; Wojtkowski et al. 2002b): instead of shifting the reference mirror, a stationary mirror is used. The light exiting the interferometer is spectrally dispersed by a grating onto a line scan camera, and a fast Fourier transform (FFT) of the spectral interference signal provides the depth profile, that is, the entire depth profile is recorded in a single shot. Another variant of the technology uses a rapidly swept laser source, and the spectral interferogram is recorded over time (Chinn et al. 1997; Lexer et al. 1997). Both techniques are massively parallel and achieve speeds and sensitivities that are 2–3 orders of magnitude better than that of TD OCT (Choma et al. 2003; de Boer et al. 2003; Leitgeb et al. 2003a). These advantages enabled 3D retinal imaging at speeds of ~30 kA-lines/s for commercial instruments and several 100 kA-lines/s to beyond 1 MA-lines/s in experimental systems (Potsaid et al. 2008; An et al. 2011; Klein et al. 2011).
Peripheral eye length measurement techniques: a review
Published in Clinical and Experimental Optometry, 2020
Ingrid Ornella Koumbo mekountchou, Fabian Conrad, Padmaja Sankaridurg, Klaus Ehrmann
The first reported measurement of peripheral EL using partial coherence interferometry was by Fercher et al. where they obtained the fundus profile by measuring in steps of 2° from 24° nasal to 22° temporal along the horizontal meridian.1991 At that time, partial coherence interferometry was found to have several advantages compared to A‐scan ultrasonography such as greater longitudinal accuracy and transverse resolution, as well as improved patient comfort and acceptance due to the non‐contact method. Other advantages include the need for only one interferometric measurement to identify path length matching and the insensitivity to small eye movements. However, disadvantages of this technology are its complex interferogram, limited speed and sensitivity of the dual‐beam, the need for additional components, and lower sensitivity.2016 The other drawbacks are that light is strongly attenuated by opaque ocular media, and there are fixation problems that can hinder the measurements.2002 Moreover, the method requires that the experimenter be skilled in checking interference fringes.1993 In addition, measurements with this setup are time‐consuming (the patient needed to look into the ‘laser’ beam the entire time and it took about 15-minutes for one measurement for a skilled operator). Hence, this method was found less applicable for a larger number of patients, especially elderly1991 and young people.
Real time evaluation of tissue optical properties during thermal ablation of ex vivo liver tissues
Published in International Journal of Hyperthermia, 2018
Vivek K. Nagarajan, Venkateswara R. Gogineni, Sarah B. White, Bing Yu
The EFPI sensor head was fabricated by laser fusion bonding of two multimode fibers, separated by a small air gap (Lc), to a borosilicate capillary tube with an outer diameter of 400 μm (Figure 1(c)). The distance between the two bonding points, the gage length (LG), is 2.0 mm. The EFPI sensor head is connected to the sensing arm of a 2 × 2 50/50 fiber-optic coupler. The 820 nm LED was used to interrogate the EFPI sensor and the returned interferogram was collected by the NIR spectrometer (NIR Spec). Thermal expansion of the borosilicate tubing alters the optical path difference between the two interfering waves that are reflected by the end-faces of the two fiber tips, shifting the peak positions of the interferogram. The temperature is determined by tracking the temperature-dependent wavelength shift in the interferogram [23]. The temperature sensor was calibrated in a water bath and was found to have an accuracy of 1 °C between 20 and 95 °C. A laptop computer with a custom LabVIEW program and embedded MATLAB scripts was used for instrument control and data acquisition and analysis.