Murukeshan Vadakke Matham

Digital speckle pattern interferometry for deformation analysis of inner surfaces of cylindrical specimens

Deformation study of curved engineering and technical surfaces, such as pipes and pressure vessels, has gained much importance in the recent past. Speckle interferometric techniques and their electronic and digital analogs, which are whole field techniques, have been effectively applied for practical nondestructive testing applications over the years. However, little work has been done that discusses the speckle fringe formation with a fruitful theoretical formulation to study deformation analysis of curved surfaces. We propose an extended theory for speckle fringe formation on curved surfaces, which can be applied to the study of curved engineering and technical specimens under various loading conditions such as in-plane, out-of-plane, and out-of-plane shear configurations. Simulated contours are generated by use of finite element models with similar loading conditions, and the data are analyzed and compared with the obtained experimental results.

Red, green, and blue gray-value shift-based approach to whole-field imaging for tissue diagnostics

Identification of abnormal pathology in situ remains one of the challenges of medicine. The interpretation of tissue conditions relies mainly on optical assessment, which can be difficult due to inadequate visual differences or poor color delineation. We propose a methodology to identify regions of abnormal tissue in a targeted area based on red, green, blue (RGB) shift analysis employing a simple CCD color camera and light-emitting diode illumination in a whole-field-imaging scheme. The concept involves analysis of RGB components in an image with respect to a reference set of RGB values under different illumination wavelengths. The magnitude of the gray value shift is estimated by calculating the Euclidean distance between their normalized RGB coordinates. The shift values obtained using these concepts are thereafter used to construct pseudo-colored images with high contrast, enabling easy identification of abnormal areas in the tissue. Images processed from experiments conducted with excised Wistar rat colon sample (lightly doped with Alexafluor 488) and with simulated tumor (cancer cell pellet placed on colon) showed clear localization of tumor region. This proposed approach and methodology is expected to find potential applications for the in vivo diagnosis of disease.

Photoacoustic Based Surface Plasmon Resonance Spectroscopy: An Investigation

The objective of this paper is to carry out photoacoustic investigation of surface plasmons, so as to explore the possibility of employing photoacoustic techniques in real time sensing based on plasmon resonance spectroscopy. Such sensors exploit the high sensitivity of the surface plasmon frequency to the refractive index of layers of adsorbed molecules on the surface. Generally the resonance conditions are influenced by the material adsorbed onto the thin metal film, which will be explored in this project. An in house developed photoacoustic technique is employed as the detection system and subsequent spectroscopic investigations.

Development of high-sensitive, reproducible colloidal surface-enhanced Raman spectroscopy active substrate using silver nanocubes for potential biosensing applications

Abstract. Surface-enhanced Raman spectroscopy (SERS) has emerged as one of the thrust research areas that could find potential applications in bio and chemical sensing. We developed colloidal SERS active substrate with excellent sensitivity and high reproducibility using silver nanocube (AgNC) synthesized via the solvothermal method. Finite-difference time-domain simulation was carried out in detail to visualize dipole generation in the nanocube during localized surface plasmon resonance and to locate the respective hot spots in AgNC responsible for the huge Raman enhancement. The prediction is verified by the SERS analysis of the synthesized nanocubes using Rhodamine 6G molecule. An excellent sensitivity with a detection limit of 10−17  M and a very high enhancement factor of 1.2×108 confirms the “hot spots” in the nanocube. SERS activity is also carried out for crystal violet and for food adulterant Sudan I molecule. Finally, label-free DNA detection is performed to demonstrate the versatility of SERS as a potential biosensor.

Speckle referencing: digital speckle pattern interferometry (SR- DSPI) for imaging of non-diffusive surfaces

Optical metrology has been widely employed as a key technique for modern industrial production, owing to its fast, precise and non-invasive measurement. Digital speckle pattern interferometry (DSPI) is one of these non-destructive testing methods that possess the abilities to measure surface deformation, vibration and profile. However, one of the challenges with DSPI is the incapability to address the imaging of non-diffusive surface, owing to the failure to form speckle pattern. In this paper, we demonstrate a modified DSPI system used for non-diffusive surface measurement. Experiment has been carried out to validate this modified DSPI by using metal-alloy surface as testing sample. The speckle fringe pattern generated by applying an external load was analyzed to obtain the 3-D surface deformation parameters.

Polymer microlens with independent control of radius and focal length for an imaging fiber

Fabrication of microlens array using polymer reflow is beginning to be a mainstream process, whether the polymer is directly used or whether the spherical profile is transferred by plasma etching to a glass substrate as, for example, in some handphone cameras. The focus so far has been on uniformity and obtaining lenses with equal radius and equal focal length. Actually it is easy to show using a phenomenological model that the focal length is depending on the lens radius, and not much on the contact angle, an effect that can be traced to the line tension force. For a biomedical application we need to terminate a 600um diameter imaging fiber with a group of lenses of different diameters - but with similar focal length. We have devised a microfabrication process on a silicon wafer to produce the lens with variable diameter and identical focal length, while etching the silicon wafer has helped us producing a sheath to insert the optical fiber and mount the lenses on the optical fiber.

Fiber pixelated image database

Abstract. Imaging of physically inaccessible parts of the body such as the colon at micron-level resolution is highly important in diagnostic medical imaging. Though flexible endoscopes based on the imaging fiber bundle are used for such diagnostic procedures, their inherent honeycomb-like structure creates fiber pixelation effects. This impedes the observer from perceiving the information from an image captured and hinders the direct use of image processing and machine intelligence techniques on the recorded signal. Significant efforts have been made by researchers in the recent past in the development and implementation of pixelation removal techniques. However, researchers have often used their own set of images without making source data available which subdued their usage and adaptability universally. A database of pixelated images is the current requirement to meet the growing diagnostic needs in the healthcare arena. An innovative fiber pixelated image database is presented, which consists of pixelated images that are synthetically generated and experimentally acquired. Sample space encompasses test patterns of different scales, sizes, and shapes. It is envisaged that this proposed database will alleviate the current limitations associated with relevant research and development and would be of great help for researchers working on comb structure removal algorithms.

Hydrogen sensors based on Pt-loaded WO3 sensing layers

In this letter we report the enhanced sensing of platinum (Pt)-loaded tungsten oxide (WO3) sensor compared to pure WO3 sensor towards hydrogen (H2) gas at low operating temperature of 200 °C. The hydrogen sensing of pure and Pt-loaded WO3 sensors is reported at operating temperatures 200, 300 and 400 °C. The presence of Pt promotes the spillover mechanism and the dissociated H2 atoms can react with adsorbed/lattice oxygen atoms to release electrons which in turn increases the conductivity of the WO3 film. The H2 sensing mechanism of Pt-loaded WO3 sensor is investigated using micro-Raman spectroscopy. The Raman spectrum of Pt-loaded sensing layer shows a shift of symmetric stretching band from 796 to 804 cm−1 with the introduction of H2 gas.

Hybrid-modality ocular imaging using a clinical ultrasound system and nanosecond pulsed laser

Abstract. Hybrid optical modality imaging is a special type of multimodality imaging significantly used in the recent past in order to harness the strengths of different imaging methods as well as to furnish complementary information beyond that provided by any individual method. We present a hybrid-modality imaging system based on a commercial clinical ultrasound imaging (USI) system using a linear array ultrasound transducer (UST) and a tunable nanosecond pulsed laser as the source. The integrated system uses photoacoustic imaging (PAI) and USI for ocular imaging to provide the complementary absorption and structural information of the eye. In this system, B-mode images from PAI and USI are acquired at 10 Hz and about 40 Hz, respectively. A linear array UST makes the system much faster compared to other ocular imaging systems using a single-element UST to form B-mode images. The results show that the proposed instrumentation is able to incorporate PAI and USI in a single setup. The feasibility and efficiency of this developed probe system was illustrated by using enucleated pig eyes as test samples. It was demonstrated that PAI could successfully capture photoacoustic signals from the iris, anterior lens surface, and posterior pole, while USI could accomplish the mapping of the eye to reveal the structures like the cornea, anterior chamber, lens, iris, and posterior pole. This system and the proposed methodology are expected to enable ocular disease diagnostic applications and can be used as a preclinical imaging system.

A flexible image fiber probe based speckle imaging for extraction of surface features with possible application in intra-cavity inspection

Non-destructive inspection and non-invasive interrogation of surface features has always been a subject of discussion owing to the rapid advances in engineering and medical fields. Measurement of surface features which are miniature in size, inaccessible and of complex shape, has always posed challenges to conventional types of imaging and metrological systems. This paper, presents a methodology and a miniature image fiber probe configuration based on speckle technology for imaging such surface features, with possible application in intra cavity inspection. In the present work, a metal pipe is used as a test sample representing an engineering cavity. The acquired images of the intra cavity were subjected to image processing for contouring and size estimation. An analysis on the variation in the average speckle intensity, when the speckle image passes through an image fiber, is also carried out in this work. The obtained results indicate that the proposed probe configuration and related methodology can be used for inspection of cavity features and profiles of diffusive surfaces.