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Application of Terahertz Imaging to Nondestructive Evaluation

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A special issue of Sensors (ISSN 1424-8220). This special issue belongs to the section "Optical Sensors".

Deadline for manuscript submissions: closed (31 December 2022) | Viewed by 19504

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Special Issue Editor

Dr. Alexandre Locquet

E-Mail Website1 Website2
Guest Editor

Unité Mixte Internationale 2958 GeorgiaTech-CNRS, Georgia Tech Lorraine, Metz, France
Interests: terahertz imaging; nondestructive evaluation; semiconductor laser dynamics; nonlinear time series analysis; photonic information processing; neuromorphic computing

Special Issue Information

Dear Colleague,

Terahertz technology shows great promise for the nondestructive evaluation of a variety of materials. This Special Issue focuses on all innovations that can improve nondestructive evaluation based on far- and near-field THz imaging as well as on the identification of THz spectroscopic fingerprints. We look forward, in particular, to investigations of new materials and samples and to improvements in resolution, penetration depth, measurement speed, and in the compacity of THz systems. The advances can rely on developments in devices (emitters, sensors), system design, or data processing methods.

Dr. Alexandre Locquet
Guest Editor

Manuscript Submission Information

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Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2600 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

THz imaging
THz spectroscopy
Nondestructive Evaluation
Signal Processing
Image Processing

Published Papers (7 papers)

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Research

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20 pages, 9934 KiB

Open AccessArticle

Terahertz Nondestructive Testing with Ultra-Wideband FMCW Radar

by Barnabé Carré, Adrien Chopard, Jean-Paul Guillet, Frederic Fauquet, Patrick Mounaix and Pierre Gellie

Sensors 2023, 23(1), 187; https://doi.org/10.3390/s23010187 - 24 Dec 2022

Cited by 7 | Viewed by 1684

Abstract

This paper presents the development, performance, integration, and implementation of a 150 GHz FMCW radar based on a homodyne harmonic mixing scheme for noncontact, nondestructive testing. This system offers high-dynamic-range measurement capabilities up to 100 dB and measurement rates up to 7.62 kHz. Such interesting characteristics make this system attractive for imaging applications or contactless sensing. Numerous samples of different materials and geometries were imaged by taking advantage of the radar’s performance. By taking into account the nonionizing capability of the system, new applicative fields such as food industry and pharmaceutical packaging were explored. Full article

(This article belongs to the Special Issue Application of Terahertz Imaging to Nondestructive Evaluation)

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14 pages, 3087 KiB

Open AccessArticle

Layer-Resolving Terahertz Light-Field Imaging Based on Angular Intensity Filtering Method

by Nanfang Lyu, Jian Zuo, Yuanmeng Zhao and Cunlin Zhang

Sensors 2021, 21(22), 7451; https://doi.org/10.3390/s21227451 - 09 Nov 2021

Cited by 1 | Viewed by 1397

Abstract

Terahertz focal plane array imaging methods, direct camera imaging and conventional light field imaging methods are incapable of resolving and separating layers of multilayer objects. In this paper, for the purpose of fast, high-resolution and layer-resolving imaging of multilayer structures with different reflection characteristics, a novel angular intensity filtering (AIF) method based on terahertz light-field imaging is purposed. The method utilizes the extra dimensional information from the 4D light field and the reflection characteristics of the imaging object, and the method is capable to resolve and reconstruct layers individually. The feasibility of the method is validated by experiment on both “idealized” and “practical” multilayer samples, and the advantages in performance of the method are proven by quantitative comparison with conventional methods. Full article

(This article belongs to the Special Issue Application of Terahertz Imaging to Nondestructive Evaluation)

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19 pages, 2943 KiB

Open AccessArticle

Fast FMCW Terahertz Imaging for In-Process Defect Detection in Press Sleeves for the Paper Industry and Image Evaluation with a Machine Learning Approach

by Maris Bauer, Raphael Hussung, Carsten Matheis, Hermann Reichert, Peter Weichenberger, Jens Beck, Uwe Matuschczyk, Joachim Jonuscheit and Fabian Friederich

Sensors 2021, 21(19), 6569; https://doi.org/10.3390/s21196569 - 30 Sep 2021

Cited by 26 | Viewed by 2638

Abstract

We present a rotational terahertz imaging system for inline nondestructive testing (NDT) of press sleeves for the paper industry during fabrication. Press sleeves often consist of polyurethane (PU) which is deposited by rotational molding on metal barrels and its outer surface mechanically processed in several milling steps afterwards. Due to a stabilizing polyester fiber mesh inlay, small defects can form on the sleeve’s backside already during the initial molding, however, they cannot be visually inspected until the whole production processes is completed. We have developed a fast-scanning frequenc-modulated continuous wave (FMCW) terahertz imaging system, which can be integrated into the manufacturing process to yield high resolution images of the press sleeves and therefore can help to visualize hidden structural defects at an early stage of fabrication. This can save valuable time and resources during the production process. Our terahertz system can record images at 0.3 and 0.5 THz and we achieve data acquisition rates of at least 20 kHz, exploiting the fast rotational speed of the barrels during production to yield sub-millimeter image resolution. The potential of automated defect recognition by a simple machine learning approach for anomaly detection is also demonstrated and discussed. Full article

(This article belongs to the Special Issue Application of Terahertz Imaging to Nondestructive Evaluation)

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18 pages, 6308 KiB

Open AccessArticle

Automated Inorganic Pigment Classification in Plastic Material Using Terahertz Spectroscopy

by Andrej Sarjaš, Blaž Pongrac and Dušan Gleich

Sensors 2021, 21(14), 4709; https://doi.org/10.3390/s21144709 - 09 Jul 2021

Cited by 8 | Viewed by 2182

Abstract

This paper presents an automatic classification of plastic material’s inorganic pigment using terahertz spectroscopy and convolutional neural networks (CNN). The plastic materials were placed between the THz transmitter and receiver, and the acquired THz signals were classified using a supervised learning approach. A THz frequency band between 0.1–1.2 THz produced a one-dimensional (1D) vector that is almost impossible to classify directly using supervised learning. This paper proposes a novel pre-processing of 1D THz data that transforms 1D data into 2D data, which are processed efficiently using a convolutional neural network. The proposed pre-processing algorithm consists of four steps: peak detection, envelope extraction, and a down-sampling procedure. The last main step introduces the windowing with spectrum dilatation that reorders 1D data into 2D data that can be considered as an image. The spectrum dilation techniques ensure the classifier’s robustness by suppressing measurement bias, reducing the complexity of the THz dataset with negligible loss of accuracy, and speeding up the network classification. The experimental results showed that the proposed approach achieved high accuracy using a CNN classifier, and outperforms 1D classification of THz data using support vector machine, naive Bayes, and other popular classification algorithms. Full article

(This article belongs to the Special Issue Application of Terahertz Imaging to Nondestructive Evaluation)

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13 pages, 3778 KiB

Open AccessArticle

Video-Rate Identification of High-Capacity Low-Cost Tags in the Terahertz Domain

by Florent Bonnefoy, Maxime Bernier, Etienne Perret, Nicolas Barbot, Romain Siragusa, David Hely, Eiji Kato and Frederic Garet

Sensors 2021, 21(11), 3692; https://doi.org/10.3390/s21113692 - 26 May 2021

Cited by 3 | Viewed by 2494

Abstract

In this article, we report on video-rate identification of very low-cost tags in the terahertz (THz) domain. Contrary to barcodes, Radio Frequency Identification (RFID) tags, or even chipless RFID tags, operate in the Ultra-Wide Band (UWB). These THz labels are not based on a planar surface pattern but are instead embedded, thus hidden, in the volume of the product to identify. The tag is entirely made of dielectric materials and is based on a 1D photonic bandgap structure, made of a quasi-periodic stack of two different polyethylene-based materials presenting different refractive indices. The thickness of each layer is of the order of the THz wavelength, leading to an overall tag thickness in the millimetre range. More particularly, we show in this article that the binary information coded within these tags can be rapidly and reliably identified using a commercial terahertz Time Domain Spectroscopy (THz-TDS) system as a reader. More precisely, a bit error rate smaller than 1% is experimentally reached for a reading duration as short as a few tens of milliseconds on an 8 bits (~40 bits/cm²) THID tag. The performance limits of such a tag structure are explored in terms of both dielectric material properties (losses) and angular acceptance. Finally, realistic coding capacities of about 60 bits (~300 bits/cm²) can be envisaged with such tags. Full article

(This article belongs to the Special Issue Application of Terahertz Imaging to Nondestructive Evaluation)

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20 pages, 8132 KiB

Open AccessArticle

THz-TDS Reflection Measurement of Coating Thicknesses at Non-Perpendicular Incidence: Experiment and Simulation

by Ruben Burger, Julia Frisch, Matthias Hübner, Matthias Goldammer, Ole Peters, Enno Rönneberg and Datong Wu

Sensors 2021, 21(10), 3473; https://doi.org/10.3390/s21103473 - 16 May 2021

Cited by 7 | Viewed by 3724

Abstract

Time-domain spectroscopy (TDS) in the terahertz (THz) frequency range is gaining in importance in nondestructive testing of dielectric materials. One application is the layer thickness measurement of a coating layer. To determine the thickness from the measurement data, the refractive index of the coating layer must be known in the surveyed frequency range. For perpendicular incidence of the radiation, methods exist to extract the refractive index from the measurement data themselves without prior knowledge. This paper extends these methods for non-perpendicular incidence, where the polarization of the radiation becomes important. Furthermore, modifications considering effects of surface roughness of the coating are introduced. The new methods are verified using measurement data of a sample of Inconel steel coated with yttria-stabilized zirconia (YSZ) and with COMSOL simulations of the measurement setup. To validate the thickness measurements, scanning electron microscopy (SEM) images of the layer structure are used. The results show good agreement with an average error of 1% for the simulation data and under 4% for the experimental data compared to reference measurements. Full article

(This article belongs to the Special Issue Application of Terahertz Imaging to Nondestructive Evaluation)

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Review

Jump to: Research

26 pages, 6645 KiB

Open AccessReview

THz Sensing of Human Skin: A Review of Skin Modeling Approaches

by Jiarui Wang, Hannah Lindley-Hatcher, Xuequan Chen and Emma Pickwell-MacPherson

Sensors 2021, 21(11), 3624; https://doi.org/10.3390/s21113624 - 23 May 2021

Cited by 24 | Viewed by 4184

Abstract

The non-ionizing and non-invasive nature of THz radiation, combined with its high sensitivity to water, has made THz imaging and spectroscopy highly attractive for in vivo biomedical applications for many years. Among them, the skin is primarily investigated due to the short penetration depth of THz waves caused by the high attenuation by water in biological samples. However, a complete model of skin describing the THz–skin interaction is still needed. This is also fundamental to reveal the optical properties of the skin from the measured THz spectrum. It is crucial that the correct model is used, not just to ensure compatibility between different works, but more importantly to ensure the reliability of the data and conclusions. Therefore, in this review, we summarize the models applied to skin used in the THz regime, and we compare their adaptability, accuracy, and limitations. We show that most of the models attempt to extract the hydration profile inside the skin while there is also the anisotropic model that displays skin structural changes in the stratum corneum. Full article

(This article belongs to the Special Issue Application of Terahertz Imaging to Nondestructive Evaluation)

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