This study reports a unique optical system for experimental, laboratory-level testing of light scattering methods for noninvasive characterization of optical fibers. This new modular system comprises of various optical, mechanical, electrical and software components enabling the control, detection, and analysis of the measurement results. Practical measurements are investigated to explore an inverse relationship between scattering data from the vicinity of a rainbow and fiber diameter/refractive index estimates.
The paper describes a hybrid method for estimating flow-velocity vector fields from Particle Image Velocimetry (PIV) images that combines a cross-correlation technique with a multiresolution estimation based on optical flow with the aim of obtaining the highest possible spatial resolution. The method offers the possibility to determine one vector per seeding particle. The manuscript examines accuracy of the estimates compared to other known methods using various standard test images. Experimental results are also presented.
This paper examines two models for image representation used for optical flow estimation in Particle Image Velocimetry (PIV). The common approach for flow estimation bases on a cross-correlation between PIV images. An alternative solution bases on an optical flow, which has the advantage of calculating vector fields with much better spatial resolution. The optical flow-based estimation requires calculations of temporal and spatial derivatives of the image intensity, which is usually achieved by using finite differences. Due to rapid intensity changes in the PIV images caused by particles having small diameters, an exact estimation of spatial derivatives using finite differences may lead to numerical errors that render data interpretation limited or even impractical. The present study aims at solving this problem by introducing two algorithms for PIV image processing, which differs in terms of a digital image representation. Both algorithms rely on a PIV image model, wherein the particle image complies with an Airy disc, which is well approximated by using a Gaussian function. Numerical analysis of sample PIV images (uniform and turbulent fields) show that both methods allow for high precision flow-velocity fields estimates in conjunction with the Lucas-Kanade algorithm.
The aim of this work is to present a technique for non-intrusive velocity vector measurements of micron-sized tracer particles following a fluid flow. The technique is based on Particle Image Velocimetry (PIV). In contrast to conventional PIV, which analyzes light scattering for incident high-energy laser pulses, the technique uses a light sheet produced by a prototype LED-based illuminator. A sequence of exposures from the flow taken by a high-speed camera is analyzed by means of a multi-scale optical flow-based algorithm developed by the authors. The LED illuminator offers the possibility to deliver high-power light pulses at microsecond levels and high-repetition rates. An integrated optics produce a lightsheet with adjustable thickness and width, enabling the user to measure velocity components either in a plane or in a volume. Compared to pulse lasers used in PIV systems, the illuminator has the advantages of low cost, safe operation, and much simpler construction. For the purposes of experimental verification, velocity vector measurements in a crosssection of a rotary water flow seeded with micron sized tracer particles have been performed. The velocity vectors have been computed using a multi-scale estimation algorithm based on optical flow and four-level pyramidal decomposition of PIV images. In order to validate our optical flow-based approach, the experimental results have been finally analyzed by means of a commercial PIV software that uses image cross-correlation for velocity field estimation.
This paper reports an application of a rainbow technique to characterize both the core and cladding diameter of a single, silica-based, step-index optical fiber. Both quantities are inversely calculated using a correlation formula from the farfield scattering pattern where multiple primary rainbows occur. A set of observations of scattering allows one to retrieve the parameters of interest. Numerical studies assume variable core (10–50 μm) as well as cladding diameter (120– 130 μm). A part of analysis shows how the temporal coherence of the incident beam of light affects the solutions.
The aim of this paper is to present a method for estimating flow-velocity vector fields using the Lucas-Kanade algorithm.
The optical flow measurements are based on the Particle Image Velocimetry (PIV) technique, which is commonly used
in fluid mechanics laboratories in both research institutes and industry. Common approaches for an optical
characterization of velocity fields base on computation of partial derivatives of the image intensity using finite
differences. Nevertheless, the accuracy of velocity field computations is low due to the fact that an exact estimation of
spatial derivatives is very difficult in presence of rapid intensity changes in the PIV images, caused by particles having
small diameters. The method discussed in this paper solves this problem by interpolating the PIV images using Gaussian
radial basis functions. This provides a significant improvement in the accuracy of the velocity estimation but, more
importantly, allows for the evaluation of the derivatives in intermediate points between pixels. Numerical analysis proves
that the method is able to estimate even a separate vector for each particle with a 5× 5 px2 window, whereas a classical
correlation-based method needs at least 4 particle images. With the use of a specialized multi-step hybrid approach to
data analysis the method improves the estimation of the particle displacement far above 1 px.
This study reports an application of a fiber-optic LED-based illumination system to solve an inverse problem in
optical measurements of characteristics of a single-mode fiber. The illumination system has the advantages of low
temporal coherence, high intensity, collimation, and thermal stability of the emission spectrum. The inverse analysis is
investigated to predict the values of the diameter and refractive index of a single-mode fiber and applies to the far field
scattering pattern in the vicinity of a polychromatic rainbow. As the inversion possibility depends considerably on the
properties of the incident radiation, a detailed discussion is provided on both the specification of the illumination system
as well as preliminary characteristics of the produced radiation. The illumination system uses a direct coupling between a
thermally-stabilized LED junction and a plastic optical fiber, which transmits light to an optical collimator. A numerical
study of fiber-to-LED coupling efficiency helps to understand the influence of lateral and longitudinal misalignments on
the output power.