Terahertz Time-of-Flight Tomography Beyond the Axial Resolution Limit: Autoregressive Spectral Estimation Based on the Modified Covariance Method

Zhai, Min, Alexandre Locquet, Cyrielle Roquelet, and D. S. Citrin. "Terahertz Time-of-Flight Tomography Beyond the Axial Resolution Limit: Autoregressive Spectral Estimation Based on the Modified Covariance Method." Journal of Infrared, Millimeter, and Terahertz Waves (2020): 1-14.

Abstract

We present a time-of-flight tomography method for exceeding the naïve axial (i.e., depth) resolution limit of terahertz (THz) deconvolution by autoregressive spectral extrapolation (AR) based on the modified covariance method (AR/MCM). In contrast to Wiener filtering combined with wavelet denoising, AR/MCM does not discard any frequency components in the low signal-to-noise (SNR) regions of the measured data, and unlike the AR approach based on the Burg method (AR/BM), no peak splitting (single peaks in the impulse response function appearing as double peaks) as well as frequency bias (spectral peaks shifted with respect to their correct positions) is observed after deconvolution. After verifying the advantages of AR/MCM over Wiener filtering in conjunction with wavelet denoising as well as over AR/BM, using synthetic data, AR/MCM is employed to reconstruct a single layer of mill scale on a steel coupon from experimental THz time-of-flight tomography data. The reconstruction shows good agreement with the film thickness obtained from destructive cross-sectional measurements. In addition, unlike AR/BM, optimizing the parameters to obtain stable reconstruction is straightforward relying of Akaike’s information criterion suggesting that AR/MCM can be an easier to implement for THz nondestructive characterization of stratigraphy under noisy conditions, particularly when estimates of the stratigraphy may not a priori be available.

Full paper can be found at https://link.springer.com/article/10.1007/s10762-020-00722-1

… that minimizes the AIC. Experiment. The measurement in this work is carried out using a pulsed, broadband THz time domain system from TeraView Ltd. (TPSSpectra 3000), shown schematically in Fig. 1. The GaAs photoconductive …

Detection of Melamine in Foods Using Terahertz Time-Domain Spectroscopy

Seung Hyun Baek, Heung Bin Lim, and Hyang Sook Chun

Abstract

The aim of this study was to investigate the feasibility of detecting melamine in foodstuffs using terahertz imaging. The terahertz (THz) spectra and images of melamine mixtures were obtained in the frequency range of 0.1–3 THz at room temperature using THz time-domain spectroscopy. Characteristic absorption peaks of melamine were found at 2, 2.26, and 2.6 THz, and these peaks showed the same frequencies in the different food matrices. At 2 THz, the THz images of melamine were dose-dependently distinguishable from those of food components with or without the packaging materials used. The calibration curve of melamine showed a regression coefficient (R2) of >0.913 and a detection limit of <13%. These results suggest that terahertz imaging has the potential to be used for the qualitative detection of melamine in food as a nondestructive analytical tool.

This study was performed using TeraView's TPS spectra 3000 system. (TeraView, Cambridge, UK)

Full Article: http://pubs.acs.org/doi/abs/10.1021/jf501170z

Detailed non-destructive evaluation of UHMWPE composites in the terahertz range

Optical and Quantum ElectronicsDOI 10.1007/s11082-013-9836-4

N. Palka, D. Miedzinska

Abstract

We report on the terahertz analysis of an internal structure of an ultra-high molecular weight polyethylene (UHMWPE) composite material, which is based on the HB10-tape from Dyneema®. This type of composite is very hard and resistant and therefore it is often used to manufacture personal armors such as bulletproof vests and helmets. The multilayer structure of the UHMWPE composite was investigated by means of a raster scanning time domain spectroscopy technique in a reflection configuration. The mechanism of the formation of many shifted in time THz pulses (reflected from the internal layers of the sample) originates from the periodic modulation of the refractive index along the propagation of the radiation. This modulation is connected with alternate layers of fibers, each having different direction (perpendicular to each other). As a result we obtained the detailed three dimensional profile of the 3.3-mm thick sample with all 74 layers clearly visible. Thicknesses of all layers, having around 45 μm each, were determined. Moreover, it is also possible to identify internal defects i.e. delaminations in the internal structure of this composite material.

This study was performed using TeraView's TPS Spectra 3000 system, with the XY Reflectance imaging module. (TeraView, Cambridge, UK)


Source: http://link.springer.com/article/10.1007%2Fs11082-013-9836-4/fulltext.html

Application of terahertz pulsed imaging to analyse film coating characteristics of sustained-release coated pellets

• M. Haaser^a, b, c, 
• Y. Karrout^d, 
• C. Velghe^d, 
• Y. Cuppok^d, 
• K.C. Gordon^e, 
• M. Pepper^c, f, 
• J. Siepmann^d, 
• T. Rades^a, g, , , 
• P.F. Taday^c, 
• C.J. Strachan^a, h

• ^a School of Pharmacy, University of Otago, Dunedin, New Zealand
• ^b Cavendish Laboratories, University of Cambridge, Cambridge, United Kingdom
• ^c TeraView Ltd., Cambridge, United Kingdom
• ^d College of Pharmacy, Université Lille Nord de France, Lille, France
• ^e Department of Chemistry and MacDiarmid Institute, University of Otago, Dunedin, New Zealand
• ^f Department of Electronic Engineering, University College London, London, United Kingdom
• ^g Department of Pharmacy, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark
• ^h Division of Pharmaceutical Technology, Faculty of Pharmacy, University of Helsinki, Helsinki, Finland
• • http://dx.doi.org/10.1016/j.ijpharm.2013.05.039, How to Cite or Link Using DOI

 Abstract

Terahertz pulsed imaging (TPI) was employed to explore its suitability for detecting differences in the film coating thickness and drug layer uniformity of multilayered, sustained-release coated, standard size pellets (approximately 1 mm in diameter). Pellets consisting of a sugar starter core and a metoprolol succinate layer were coated with a Kollicoat^® SR:Kollicoat^® IR polymer blend for different times giving three groups of pellets (batches I, II and III), each with a different coating thickness according to weight gain. Ten pellets from each batch were mapped individually to evaluate the coating thickness and drug layer thickness between batches, between pellets within each batch, and across individual pellets (uniformity). From the terahertz waveform the terahertz electric field peak strength (TEFPS) was used to define a circular area (approximately 0.13 mm²) in the TPI maps, where no signal distortion was found due to pellet curvature in the measurement set-up used. The average coating thicknesses were 46 μm, 71 μm and 114 μm, for batches I, II and III respectively, whilst no drug layer thickness difference between batches was observed. No statistically significant differences in the average coating thickness and drug layer thickness within batches (between pellets) but high thickness variability across individual pellets was observed. These results were confirmed by scanning electron microscopy (SEM). The coating thickness results correlated with the subsequent drug release behaviour. The fastest drug release was obtained from batch I with the lowest coating thickness and the slowest from batch III with the highest coating thickness. In conclusion, TPI is suitable for detailed, non-destructive evaluation of film coating and drug layer thicknesses in multilayered standard size pellets.

...Samples were measured on a TPI imaga 2000 (Teraview Ltd., Cambridge, UK).

You can download the article on ScienceDirect

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Non-destructive measurement of polymer coatings on pharmaceutical tablets using X-ray micro CT and terahertz pulsed imaging

Authors: Isabelle-Sophie Russe^1,2, Daniela Brock^1,2, Klaus Knop², Peter Kleinebudde², J. Axel Zeitler¹⁼
1 Department of Chemical Engineering and Biotechnology, University of Cambridge,  Cambridge, UK
2 Institute of Pharmaceutics and Biopharmaceutics, University of Duesseldorf, Duesseldorf, Germany


Full text: http://www.skyscan.be/company/UM2013/15.pdf

Conclusion

We conclude that TPI is a robust technique, and, that due to its very simple measurement  principle, it is an ideal measurement technique to quantify the coating thickness in process control and quality monitoring applications. The method introduced in this paper will be useful to explore whether there is an influence on  the refractive index of a given coating formulation with changes in process conditions during  coating or curing. It furthermore opens the possibility, in pharmaceutical applications and  beyond, to develop metrology standards for coatings that can be used to calibrate TPI  measurements for absolute thickness.
Beyond its use as a calibration technique for TPI the XµCT method has provided some  intriguing insight into the coating properties at extremes of the tablet geometry, at the edges  of tablets where high curvature was thought to typically prevent quantitative TPI  measurements due to strong scattering. The main limitations of the XµCT technique are the  measurement, and in particular, processing time together with the high demands in  computational power and data storage as well as the limited contrast that can be resolved  between coating and core. We therefore envisage that the main impact of this technique is likely to be in research and development and to support techniques such as TPI rather than  in routine quality control or process measurements.

...Samples were measured on a TPI imaga 2000 (Teraview Ltd., Cambridge, UK), using a point spacing grid of 200 x 200 µm and a penetration depth of 1 mm in air...

Characterisation of historic plastics using terahertz time-domain spectroscopy and pulsed imaging

ANALYTICAL AND BIOANALYTICAL CHEMISTRY

DOI: 10.1007/s00216-012-5931-9

Gianluca Pastorelli, Tanja Trafela, Phillip F. Taday, Alessia Portieri, David Lowe, Kaori Fukunaga and Matija Strlič

For more information see http://www.springerlink.com/content/218682g6809873l1/


Abstract
Terahertz (THz) time-domain spectroscopy and 3D THz pulsed imaging have been explored with regard to polymer materials, both commodity and historic polymers. A systematic spectroscopic study of a wide range of different polymer materials showed significant differences in their spectra. Polyolefins and polystyrenes generally exhibit lower absorption than other examined polymers, various cellulose derivates, poly(vinyl chloride), poly(methyl methacrylate), polyamide, hard rubber and phenol formaldehyde resin, the last of these exhibiting the most intense absorption over the entire range, 0.15–4.2 THz. It was also examined how the presence of plasticisers in poly(vinyl chloride), the presence of fillers in polypropylene, and the degree of branching in polyethylene and polystyrene affect the spectra; inorganic fillers in polypropylene affected the absorption most. With 3D THz pulsed imaging, features in polymer objects were explored, appearing either as integral parts of the material (coatings and pores in foams) or as a consequence of physical deterioration (cracks, delamination). All of these features of various complexities can be successfully imaged in 3D. Terahertz technology is thus shown to have significant potential for both chemical and structural characterisation of polymers, which will be of interest to heritage science, but also to the polymer industry and development of analytical technologies in general.


For more information on terahertz imaging visit http://www.teraview.com/products/terahertz-pulsed-spectra-3000/index.html