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Due to their advantages of non-destructiveness, high accuracy, and high sensitivity, optical measurement techniques have been successfully applied to measure various important physical quantities in experimental mechanics, materials science, biomechanics, etc. In order to deal with increasingly larger amounts of data and increase accuracy, the computational burden of optical measurement techniques has become heavier. In the past decade, parallel computing devices have been applied to accelerate these techniques, among which graphics processing units (GPUs) have become mainstream due to their high parallelism, cost effectiveness, short development cycle, and transparent scalability. Additionally, compute unified device architecture (CUDA), invented by NVIDIA, provides an easy-to-use C/C++ programming interface, which has opened the possibility to program GPUs without having to learn complex shading languages and the graphics pipeline. This Spotlight not only demonstrates the power of GPUs in accelerating optical measurement algorithms but also provides a hands-on approach for the acceleration of existing sequential algorithms on CUDA-capable GPUs. Readers who understand basic C/C++ programming can then attempt to integrate CUDA with their existing optical algorithms for higher computing performance.

Phase-shifting profilometry using binary patterns with projector defocusing has been widely used for high-speed 3D measurement. Recently, a ternary Gray-code based phase unwrapping method has been proposed, which enables to accurately unwrap the phase but reduces the required binary patterns. This paper presents a comparison between the ternary and the traditional binary Gray code-based phase unwrapping methods. NRF (Natl Research Foundation, S’pore) Published version

Windowed Fourier ridges algorithm can provide a quality map to assist the quality-guided phase unwrapping. Its performance for discontinuous phase maps is investigated in this paper, where the influence of window size in the algorithm is examined. Three discontinuous phase boundaries, straight, curved, and fused, are tested for both noiseless and noisy situations. Encouraging results are observed. Small window size can be used for higher boundary detection accuracy and can be enlarged if noise is heavy and/or the discontinuities are not obvious in a small area.

Phase unwrapping remains a challenging problem in optical interferometry. In this paper we only deal with noisy phase maps. The noise is removed by a windowed Fourier transform and then the sequential line scanning method is adopted for phase unwrapping. This simple approach is verified to be effective.

In this paper, the flexural behavior was investigated at different temperatures and different cross-head speeds with and without diffusion effect. The studied material is carbon fiber reinforced derakane epoxy composite. The testing temperature was set at 30, 50, 80 and 100°C to investigate the temperature effect on flexural strength. At each temperature, the testing was conducted at four different cross head speeds (0.05, 0.2, 2 and 20mm/min). The diffusion effect was studied on flexural strength with specimens soaked in sea water at 50°C to saturate. A master curve of flexural strength was formed based on time-temperature superposition principle (TTSP) with shift factor obtained by DMA test. Based on the master curve, the flexural strength was predicted with a 35% drop after using 30 years in an ideal condition without considering pressure, stress, diffusion and so on. ASTAR (Agency for Sci., Tech. and Research, S’pore) Published version

This paper introduces iFringe, a mobile application that attempts to incorporate the resource heavy fringe analysis algorithms into the smart mobile devices platform. This first step taken towards mobility in the optical processing field aims to become a catalyst for modernization of various aspects of the field as well as to diversify developments to other applications. Predominantly, the motivation of this work stems from the vastly indifferent human interactive method of mobile devices, which enable images displayed on its touch screen to be manipulated in ways that could enhance the fringe analysis experience. Furthermore, given its hardware compatibility to the conventional fringe projection system, these mobile devices could potentially serve as a much more compact replacement. However, one imperative weakness that mobile devices pose is its limited computing ability. Therefore, to examine the feasibility of incorporating the fringe analysis algorithms into a mobile platform, we have implemented two fundamental fringe analysis techniques, namely the Fourier transform fringe analysis method and the phase-shifting technique. Formulas and processing procedures such as discrete Fourier transform (DFT) and quality-guided phase unwrapping, were included in accordance to their original algorithms to test their performance and usability on a smart mobile device. Details of the implementation and the performance results will also be presented in this paper to demonstrate the practicality of these algorithms on the smart mobile device platform. Published version

In this paper we present a new method to compensate for phase aberrations and image distortion with recording single digital hologram in digital holographic microscopy. In our method, tilt is removed from the abberrated phase map first. Then an area of interest (AOI) is generated by flood filled algorithm. By fitting AOI with discrete orthogonal Zernike polynomials, error phase map in the form of a series of Zernike polynomials is obtained. Final result can be calculated by subtracting the error phase map from the abberrated phase map. Through applying our method in microlens testing, phase aberrations and image distortion introduced by microscope objective are well suppressed. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Published version

A novel quantitative 3D imaging system of silicon microstr uctures using InfraRed Transport of Intensity Equation (IR-TIE) is proposed in this paper. By reco rding the intensity at multiple planes and using FFT or DCT based TIE solver, fast and accurate phase retrieval for both un iform and non-uniform intens ity distributions is proposed. Numerical simulation and experiments confirm the accuracy and reliability of the proposed method. The application of IR-TIE for inspection of micro-patterns in visibly opaque media using 1310 nm light source is demonstrated. For comparison, micro-patterns are also inspected by the contact scanning mode Taylor Hobson system. Quantitative agreement suggests the possibility of using IR-TIE for phase imaging of silicon wafers. Keywords: infrared imaging; phase retrieval; defocuse d intensity; transport of intensity equation 1. INTRODUCTION Silicon wafers are wildly used in ultra- large-scale integration (ULSI) and micr o-electro-mechanical system (MEMS), in quality control process it’s very important to assess the yielding microstructure etched on or inside the wafer. The conventional mechanical probe processes may cause substrate scratch, moreover it costs more time due to mechanical movements. Non-invasive phase imaging techniques has gained more attractions. There are several main types of quantitative phase imaging (QPI): off-axis, phase-shifting, co mmon-path and white-light where TIE can be considered of the fourth category [1]. This technique is based on the theoretical formalism developed by Teague [2]. TIE imaging relies on the derivatives of the wave equation in the paraxial approximation. It works under coherent illumination (usual laser) also capable with partially coherent field. Direct phase retrieval from intensity measurements using the TIE has begun to enter the actual application [3, 4]. Infrared (NIR) imaging plays a big role in non-destructive testing in medicine, aerospace and semiconductor industries. For silicon wafers normally visible light cannot pass through, whereas it’s transparent in IR range. Existing 3D IR visualization systems are mainly based on interferometric methods such as optical coherence tomography (OCT) or time consuming measurement processes as in the case of IR confocal microscopy. In this paper we seek to develop a novel type of method for quan titative 3D IR imaging. Our new system can be an alternative for subsurface meas urement applications such as non-destr uctive testing and medical imaging (eye biomedical diagnostic). This prototype is non-interferometric and inexpensive compared to existing solutions. Additionally, by using tunable lens [5] (electrically monitored) the system is capable for real time imaging at millisecond temporal resolution which broadened the spectrum of possible applications. With the vigorous development of the high-precision imaging industry, the whole setup can be mounted on a commercial inverted bright-field microscope [6], it lays the foundation for functionalization and industrialization.

This paper demonstrated a simple approach for the development of advanced Nylon 12 composites by selectively modified multiwalled carbon nanotubes (MWCNTs). Prior mixing with Nylon 12, MWCNTs were modified by a nonsolvent process namely plasma treatment in order to improve its dispersion in the nylon matrix and enhance the interfacial adhesion by increasing the compatibility. A new combination of plasma treatment have been found through extensive investigations that led the nanocomposites to be extremely strong and tough even at moderately low amount of CNTs loading. Addition of only 1 wt % of modified MWCNTs, improved the tensile strength, Young’s modulus and elongation at break of the nanocomposites by ~ 65%, 61% and 68%, respectively. The FESEM images of fracture surfaces of nanocomposites showed excellent adhesion and dispersion of CNTs within the matrix, indicating the high merit of our selective plasma treatment process. Keywords: Plasma, Interface, Young’s modulus

The present study describes the burst behavior of aluminum liner based prototype filament-wound hybrid riser under internal hydrostatic pressure. The main objective of present study is to developed an internal pressure test rig set-up for filament-wound hybrid riser and investigate the failure modes of filament-wound hybrid riser under internal hydrostatic burst pressure loading. The prototype filament-wound hybrid riser used for burst test consists of an internal aluminum liner and outer composite layer. The carbon-epoxy composites as part of the filament-wound hybrid risers were manufactured with [±55o] lay-up pattern with total composite layer thickness of 1.6 mm using a CNC filament-winding machine. The burst test was monitored by video camera which helps to analyze the failure mechanism of the fractured filament-wound hybrid riser. The Fiber Bragg Grating (FBG) sensor was used to monitor and record the strain changes during burst test of prototype filament-wound hybrid riser. This study shows good improvements in burst strength of filament-wound hybrid riser compared to the monolithic metallic riser. Since, strain measurement using FBG sensors has been testified as a reliable method, we aim to further understand in detail using this technique. Published version

A fibre optic sensor was developed for in-situ pressure measurement based on the principle of differential pressure in liquids. This sensor system is very simple and consists of fibre Bragg grating (FBG) done on a fibre with core diameter of 9 μm. A calibration study was carried out with a water column and the pressure sensitivity was found to be 1.636 × 10-2MPa-1. The results show that response of FBG to the rise of water level is linear and agrees well with the theoretical results. The reliability of the sensors is confirmed by repeating the measurements for three times. The sensor is useful in applications that involve in-situ resin pressure measurement in manufacturing of laminated composite materials. Published version

Optical interferometry is a popular technique for high precision measurement. It produces fringe patterns for analysis. The output fringe patterns usually have noises, and thus a filtering process for fringe patterns is necessary. A few filtering algorithms, such as windowed Fourier filtering (WFF), have been proposed specially for fringe pattern denoising. Meanwhile, filtering techniques have been intensively studied for a long time in the general image processing area. It is curious that how the filtering techniques in general image processing area perform on fringe pattern denoising. In this paper, a state-of-the-art filtering algorithm, block-matching 3D filtering (BM3D), is selected and compared with the WFF. Published version

A%675$&7Fibre Metal Lam inate (FML), a metal sandwiche d hybrid composite material is well-known for its enhanced impact properties and better damage tolerance and it has been successfully implemented in diverse engineering applications in aviation industry. With heterogeneous constituents, the stacking sequence of FML is believe to play a cri tical role to govern its overall energy absorption capability by means of controlli ng delamination of metal composite interface and plastic deformation of metal layers. As a precursor, low velocity impact experiments were conducted on interply configured transparent plastic plates in orde r to extract the significance of stacking sequence and realize the characteristics of each layer through nake d eye which is not possible in FML due to opacity of metal layer. The stack configuration constitute hard acrylic (brittle) and soft polycarbonate (ductile) plates analogous to composite (brittle) and metal (ductile) layers on FML laminate and the impact ev ent is performed on either hard or soft facing sides separately. Hard side samples resemble more protective than soft side impact sample, with large peak resistant force and expose smaller damage growth in all experimented cases. Keywords: Transparent plastics, Fibre Metal Lamina te, Low velocity impact, Impact face 1. IntroductionIn engineering applications, structures made of heterogeneous composition fulfill majority of anticipation in terms of robustness, reliability and durability. The investigation of bi-material structure often provides a fundame ntal understanding of role played by the individual material constituents and also pave the way to enhance and optimize its influence. Wear resistant structures, coated substrates, sandwich panels, and laminated composites are some of the

Generally there are two categories of noncontact laser interferometric methods commonly used in dynamic measurement, camera-based full-field interferometry and photo-sensor-based laser Doppler interferometry. The two methods have different advantages and disadvantages thus are suitable for different applications. The camera-based interferometry enjoys the valuable merit of full-field observation and measurement. In this paper, one typical full-field interferometry, digital holography, is employed to monitor the growth process of aqueous sodium chlorate crystals. The phase proportional to the solution concentration is retrieved from the holograms captured by CCD camera in real time. There exist no phase ambiguity problem in holography compared with other optical interferometric methods. On the other hand, laser Doppler interferometry is usually a point-wise measurement but with a very high temporal sampling rate. A multipoint laser Doppler interferometer is proposed for modal parameter measurement in this paper. The multiple transient vibration signals of spatially separated points on a beam structure subjected to a shock excitation are recorded synchronously. The natural frequencies and mode shapes are extracted in the signal processing stage. This paper shows that laser interferometry is able to contribute more to the practical applications in dynamic measurement related fields. Published version