Non-contact Imaging Technique Research Articles

NUMERICAL ASPECTS OF THE SECONDARY MAGNETIC FIELD MAPPING IN MAGNETIC INDUCTION TOMOGRAPHY

Magnetic Induction Tomography (MIT) belongs to the noncontact electromagnetic imaging techniques. This paper focuses on determination of a secondary magnetic field map calculated with the help of the Biot-Savart law around the low-conductivity object. The inclusions of various shapes and different electrical conductivities values and two measurement planes are considered. In each case the objects’ single maximal cell volume with assumed uniform eddy current density has been determined. In order to keep the relative error below 1% the object should be divided in most cases into elements with maximal cell volume equal to 0.244 mm3 for yz − plane, and 0.03 mm3 for xy − plane.

Comparison Study between RMS and Edge Detection Image Processing Algorithms for a Pulsed Laser UWPI (Ultrasonic Wave Propagation Imaging)-Based NDT Technique.

In this study, a non-contact laser ultrasonic propagation imaging technique was applied to detect the damage of plate-like structures. Lamb waves were generated by an Nd:YAG pulse laser system, while a galvanometer-based laser scanner was used to scan the preliminarily designated area. The signals of the structural responses were measured using a piezoelectric sensor attached on the front or back side of the plates. The obtained responses were analyzed by calculating the root mean square (RMS) values to achieve the visualization of structural defects such as crack, corrosion, and so on. If the propagating waves encounter the damage, the waves are scattered at the damage and the energy of the scattered waves can be expressed by the RMS values. In this study, notch and corrosion were artificially formed on aluminum plates and were considered as structural defects. The notches were created with different depths and angles on the aluminum plates, and the corrosion damage was formed with different depths and areas. To visualize the damage more clearly, edge detection methodologies were applied to the RMS images and the feasibility of the methods was investigated. The results showed that most of the edge detection methods were good at detecting the shape and/or the size of the damage while they had poor performance of detecting the depth of the damage.

Noncontact and Wide-Field Characterization of the Absorption and Scattering Properties of Apple Fruit Using Spatial-Frequency Domain Imaging.

Spatial-frequency domain imaging (SFDI), as a noncontact, low-cost and wide-field optical imaging technique, offers great potential for agro-product safety and quality assessment through optical absorption (μa) and scattering (μ) property measurements. In this study, a laboratory-based SFDI system was constructed and developed for optical property measurement of fruits and vegetables. The system utilized a digital light projector to generate structured, periodic light patterns and illuminate test samples. The diffuse reflected light was captured by a charge coupled device (CCD) camera with the resolution of 1280 × 960 pixels. Three wavelengths (460, 527, and 630 nm) were selected for image acquisition using bandpass filters in the system. The μa and μ were calculated in a region of interest (ROI, 200 × 300 pixels) via nonlinear least-square fitting. Performance of the system was demonstrated through optical property measurement of ‘Redstar’ apples. Results showed that the system was able to acquire spatial-frequency domain images for demodulation and calculation of the μa and μ. The calculated μa of apple tissue experiencing internal browning (IB) were much higher than healthy apple tissue, indicating that the SFDI technique had potential for IB tissue characterization.

Distribution of Anterior Chamber Angle Width and Correlation With Age, Refraction, and Anterior Chamber Depth-The Gutenberg Health Study.

Scheimpflug imaging allows quantitative analysis of the width of the anterior chamber angle. We report the population-based distribution of the anterior chamber angle width using this noncontact imaging technique and investigate associated factors. A population-based cross-sectional study was carried out in Germany. A comprehensive ophthalmologic examination including refraction, biometry, and Scheimpflug imaging was performed. Automated measurement of the anterior chamber angle was performed in each anterior chamber quadrant. Exclusion criteria were previous ocular surgery or inadequate image quality. Association analyses were carried out to determine independently associated systemic and ocular factors for anterior chamber angle width using a generalized estimating equation model. A total of 3014 subjects (48% female) with a mean age of 58.6 ± 10.4 years were included in this study. The mean anterior chamber angle width was 32.6° ± 5.5°. Statistical analysis revealed an independent association between a smaller anterior chamber angle and female sex, higher age, and more hyperopic refraction. When including biometric parameters, shallow anterior chamber depth, shorter axial length, higher central corneal thickness, and lower corneal power were independently associated with a narrower mean anterior chamber angle width. These parameters are considered risk factors for angle-closure glaucoma.

PSPPG: Polarization Sensitive Photo-plethysmography

Photo-plethysmography (PPG) is a non-contact non-invasive optical imaging technique which uses intensity and colour based information of transmitted or reflected light for bio-signal analysis. The major application areas are blood pulsation and blood oxygen saturation measurement. One of the major concerns of conventional PPG for blood pulsation measurement is its accuracy. Reflection based PPG contains a useful backscattered light component from the superficial and deep layer tissues of the skin and non-informative direct reflection from the skin surface. The direct reflection component reduces accuracy of blood pulsation measurement. This paper presents a technique, termed as Polarization Sensitive PPG (PSPPG), which removes the direct reflection component using polarizing filters. Experimental results show 30% improvement in the accuracy of blood pulsation measurement over conventional PPG. Further, statistical analysis of the measured results shows that the PSPPG is more accurate and reliable in comparison to conventional PPG for better bio-signal analysis.

A simple and non-contact optical imaging probe for evaluation of corneal diseases.

Non-contact imaging techniques are preferred in ophthalmology. Corneal disease is one of the leading causes of blindness worldwide, and a possible way of detection is by analyzing the shape and optical quality of the cornea. Here, a simple and cost-effective, non-contact optical probe system is proposed and illustrated. The probe possesses high spatial resolutions and is non-dependent on coupling medium, which are significant for a clinician and patient friendly investigation. These parameters are crucial, when considering an imaging system for the objective diagnosis and management of corneal diseases. The imaging of the cornea is performed on ex vivo porcine samples and subsequently on small laboratory animals, in vivo. The clinical significance of the proposed study is validated by performing imaging of the New Zealand white rabbit's cornea infected with Pseudomonas.

3D nondestructive testing system with an affordable multiple reference optical-delay-based optical coherence tomography.

Optical coherence tomography (OCT) is emerging as a powerful noncontact imaging technique, allowing high-quality cross-sectional imaging of scattering specimens nondestructively. However, the complexity and cost of current embodiments of an OCT system limit its use in various nondestructive testing (NDT) applications at resource-limited settings. In this paper, we demonstrate the feasibility of a novel low-cost OCT system for a range of nondestructive testing (NDT) applications. The proposed imaging system is based on an enhanced time-domain OCT system with a low cost and small form factor reference arm optical delay, called multiple reference OCT (MR-OCT), which uses a miniature voice coil actuator and a partial mirror for extending the axial scan range. The proposed approach is potentially a low-cost, compact, and unique optical imaging modality for a range of NDT applications in a low-resource setting. Using this method, we demonstrated the capability of MR-OCT to perform cross-sectional and volumetric imaging at 1200 A-scans per second.

Non-contact imaging of venous compliance in humans using an RGB camera

We propose a technique for non-contact imaging of venous compliance that uses the red, green, and blue (RGB) camera. Any change in blood concentration is estimated from an RGB image of the skin, and a regression formula is calculated from that change. Venous compliance is obtained from a differential form of the regression formula. In vivo experiments with human subjects confirmed that the proposed method does differentiate the venous compliances among individuals. In addition, the image of venous compliance is obtained by performing the above procedures for each pixel. Thus, we can measure venous compliance without physical contact with sensors and, from the resulting images, observe the spatial distribution of venous compliance, which correlates with the distribution of veins.

Toward an Automated Setup for Magnetic Induction Tomography

Magnetic induction tomography is a noncontact electromagnetic imaging technique that has potential applications in security, industry, and medicine. This paper describes an automated setup developed to image metallic objects. The instrument employs a pair of coils in the Helmholtz configuration for the driving field and a planar array of 400 sensor coils. The sample object is imaged via phase variation measurements between the driver and sensor coils' potential difference, due to inductive coupling between the coils and sample object. The imaging system is automated via LabVIEW. A multiplexer with 400 channels automatically connects each sensor coil to a dual-phase lock-in amplifier, so that measurements of voltage phase-difference between the sensor and driver coils could be taken. These measurements were used to generate an image of the sample object. The planar geometry of the sensor coil array makes the system scalable to a full 3-D imaging system, by simply adding two more driver and sensor assemblies orthogonally to the existing one. However, this requires the ability to image objects at a finite distance from the array plane. An experiment was conducted to explore the imaging capability for various heights of sample-lift-off above the array. This proved that the system is scalable to a full 3-D imaging system.

Visualization of hidden delamination and debonding in composites through noncontact laser ultrasonic scanning

Abstract This study proposes a complete noncontact laser ultrasonic wavefield imaging technique to automatically detect and visualize hidden delamination and debonding in composite structures. First, ultrasonic wavefield is obtained from a target structure by scanning a Nd:YAG pulse laser beam for ultrasonic wave generation and measuring the corresponding ultrasonic responses using a laser Doppler vibrometer. Then, hidden damages are identified and visualized through adoption of a standing wave filter, which can isolate damage-induced standing waves from the obtained wavefield. The proposed technique has following advantages over the existing techniques: (1) it does not require any sensor installation; (2) it is noninvasive, rapidly deployable and applicable to harsh environments; and (3) it can visualize damage with high spatial resolution without any baseline data, which enables automated and intuitive damage diagnosis. The feasibility of the proposed technique is demonstrated by visualizing a debonding in a carbon fiber reinforced plastic aircraft wing and a delamination in a glass fiber reinforced plastic wind turbine blade. Furthermore, the effects of temperature and static loading variations on the proposed technique are also examined.

In-Situ Optical Coherence Tomography (OCT) for the Time-Resolved Investigation of Crystallization Processes in Polymers

By application of optical coherence tomography (OCT), an interferometric noncontact imaging technique, the crystallization of a supercooled poly(propylene) melt in a slit die is monitored. Both the quiescent and the sheared melt are investigated, with a focus on experiments where solidification and flow occur simultaneously. OCT is found to be an excellent tool for that purpose since the resultant structures are strongly scattering, which is a prerequisite for application of that method. The resulting images enable for the first time to directly monitor structure development throughout the whole experiment, including final cooling to room temperature. By rendering the setup polarization-sensitive, information on the birefringence of the pertinent structures is obtained.

Imaging Defects in a Plate with Full Non-Contact Scanning Laser Source Technique

Previously, we used contact receiving transducers in the scanning laser source (SLS) technique for imaging defects on a plate in order to achieve a high signal-to-noise (SN) ratio. Herein, we developed a fast non-contact defect imaging technique that employs the SLS technique for in-line product inspection. Leaky Lamb waves from a plate were detected with a sufficiently large SN ratio by using low-frequency air-coupled transducers. Spurious images caused by reflected waves in the plate were removed by synthesizing images from multiple receivers. Defect images were compared for different repetition frequencies of laser emission. Images were found to be distorted at high repetition frequencies (2/3kHz) owing to reverberations in the plate. [doi:10.2320/matertrans.I-M2014817]

Noncontact laser ultrasonic crack detection for plates with additional structural complexities

This article presents a new noncontact laser ultrasonic wavefield imaging technique for detecting subsurface cracks in metallic plates with additional structural complexities. The proposed technique offers noncontact, automated, and baseline-free crack diagnosis for complex metal structures with potential to field structural health monitoring applications. First, a complete noncontact laser ultrasonic wavefield imaging system is introduced, and its working principle is presented. Then, a self-referencing frequency–wavenumber (f-k) filter is developed for instantaneous crack detection. The self-referencing f-k filter isolates crack-induced features from the ultrasonic wavefield images obtained only from the current state of the target structure using the noncontact laser ultrasonic wavefield imaging system. Finite element analyses are employed to investigate the characteristics of laser-generated ultrasonic waves and validate the proposed self-referencing f-k filter. Finally, the proposed technique is experimentally validated using asymmetrically tapered and vertically stiffened aluminum plates. The numerical and experimental results confirm that subsurface cracks are well identified and localized. The uniqueness of this study lies in that crack damage in plates even with additional structural features can be autonomously detected without using baseline data from the pristine condition of a target structure and with no sensor placement.

Contactless and non-destructive differentiation of microstructures of sugar foams by hyperspectral scatter imaging

Abstract A hyperspectral scatter imaging system was developed for the contactless acquisition of spatially resolved diffuse reflectance profiles for the optical characterization of turbid food products in the wavelength range from 500 to 960 nm. To investigate the potential of this concept sugar foams with different microstructures, but with similar chemical compositions, have been prepared by applying different mixing times to the same mixtures of sugar and albumin. Hyperspectral scatter images have been acquired from these samples and the absorption and reduced scattering coefficients have been derived from spatially resolved reflectance profiles based on the diffusion approximation of the radiative transfer equation describing the light propagation in turbid media. The estimated reduced scattering coefficients μ s ′ spectra clearly reflected the effect of the different mixing times on the foam microstructure. On the other hand, similar absorption coefficient spectra were observed, confirming the identical chemical composition of the sugar–albumin matrix. These results indicate that the hyperspectral scatter imaging technique has potential as a non-contact and rapid method for online quality control and process monitoring of foamed food products. Industrial relevance The presented non-contact and non-destructive hyperspectral scatter imaging technique has potential for many applications in food industry domain. The technique is suitable for on-line scanning of food samples owing to the line-scanning operation in which all the wavebands are recorded simultaneously by the CCD camera. Such non-contact online screening ability makes the hyperspectral technique become highly desirable for industrial application in sorting and grading of food products by means of optical properties related to food quality attributes. A hyperspectral scatter imaging system can be easily integrated into industrial conveyor belt system for on-line food quality assessment and monitoring of industrial processes.

A preliminary study on noncontact imaging inspection for internal defects of plate-type nuclear fuel by using an active laser interferometer

A non-contact optical imaging technique was preliminarily studied to detect internal defects of a plate-type nuclear fuel specimen by using an active laser interferometer. A sinusoidal and periodic thermal wave induced periodic micro deformation including internal defect information of the specimen. The laser interferometer acquired a micro deformation image for the specimen surface, the shape of which varied according to the energy of the thermal wave. An adopted flexible holder of the specimen increased the capability of defect detection by concentrating the strains at the defects. A real-time inspection image including internal defect information was produced after the signal processing to remove noises. As a nondestructive inspection system, the developed visual inspection technique using an active laser interferometer can be a valuable tool to detect the internal defects of plate-type nuclear fuel.

Optical coherence tomography for the evaluation of retinal and optic nerve morphology in animal subjects: practical considerations

Optical coherence tomography (OCT) is a noninvasive, noncontact imaging technique capable of producing high-resolution images of the retina and optic nerve. These images provide information that is useful for following the progression and/or resolution of posterior segment disease. Rapid advances in OCT technology allow the acquisition of increasingly detailed images, approaching the original goal of providing in vivo histopathology. Increases in scan acquisition speeds and axial resolution enhance the clinical diagnostic value of this modality. Adapting instrumentation designed for use in human patients for use in animals can be challenging. Each species has a unique set of adjustments that need to be made but it is possible to obtain reproducible, high-quality OCT images in a variety of animals, including rodents, dogs, cats, pigs, and monkeys. Deriving quantitative measurements from OCT instruments is hindered by software algorithm errors in detecting the edges of the distinct retinal layers. These segmentation errors occur in scans of human eyes as well in other species and arise with similar frequency with each of the different OCT instruments. Manual segmentation methods to derive optic nerve head and other structural indices have been developed for several species.

Photo-Thermal Spectroscopic Imaging of MEMS Structures with Sub-Micron Spatial Resolution

ABSTRACTWe are developing a new non-contact and non-destructive imaging technique which requires no sample preparation and provides similar content information as FTIR or Raman spectroscopy while being immune to fluorescence and offers a potentially faster scan rate and/or higher spatial resolution. It utilizes photo-thermal heating of the sample with a quantum cascade laser (or other suitable infrared laser) and measuring the resulting increase in thermal emissions by either an infrared (IR) detector or a laser probe consisting of a visible laser reflected from the sample. The latter case allows for further increases in the spatial resolution from ∼10 μm to ∼1 μm or better, with suitable experimental conditions. Since the thermal emission signal is proportional to the absorption coefficient, by tuning the wavelength of the IR laser we can directly measure the IR spectrum of the sample. By raster scanning over the surface of the sample we can obtain maps of the chemical composition of the sample surface. We demonstrate this technique by imaging the surface of a micro-fabricated flow-through chemical vapor preconcentrator consisting of a silicon frame and a suspended-perforated polyimide membrane with a pair of platinum heater traces, coated with a custom sorbent polymer for selective sorption of analyte. We measure the spatial resolution of our photo-thermal imaging system as well as discuss the conditions under which the spatial resolution can be further increased from the far-field diffraction limited resolution given by the combination of the imaging optic and IR excitation laser wavelength.

617 薄板内損傷の完全非接触高速画像化技術

Authors have studied defect imaging technique for a thin plate using a scanning laser source technique, in which Lamb waves are generated with laser emission and received by ultrasonic transducers fixed on the plate. This study described full non-contact imaging technique with air-coupled sensors in the low frequency range of 40 kHz. Moreover, fast defect imaging system using analog peak detectors were developed. Although real-time imaging of defects in a thin plate was realized, unwanted images were obtained in high speed scanning due to reverberations in the plate.

Noncontact photoacoustic imaging achieved by using a low-coherence interferometer as the acoustic detector

We report on a noncontact photoacoustic imaging (PAI) technique in which a low-coherence interferometer [(LCI), optical coherence tomography (OCT) hardware] is utilized as the acoustic detector. A synchronization approach is used to lock the LCI system at its highly sensitive region for photoacoustic detection. The technique is experimentally verified by the imaging of a scattering phantom embedded with hairs and the blood vessels within a mouse ear in vitro. The system's axial and lateral resolutions are evaluated at 60 and 30 μm, respectively. The experimental results indicate that PAI in a noncontact detection mode is possible with high resolution and high bandwidth. The proposed approach lends itself to a natural integration of PAI with OCT, rather than a combination of two separate and independent systems.

Crack imaging by pulsed laser spot thermography

A surface crack close to a spot heated by a laser beam impedes lateral heat flow and produces alterations to the shape of the thermal image of the spot that can be monitored by thermography. A full 3D simulation has been developed to simulate heat flow from a laser heated spot in the proximity of a crack. The modelling provided an understanding of the ways that different parameters affect the thermal images of laser heated spots. It also assisted in the development of an efficient image processing strategy for extracting the scanned cracks. Experimental results show that scanning pulsed laser spot thermography has considerable potential as a remote, non-contact crack imaging technique.