{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,4,1]],"date-time":"2026-04-01T04:31:03Z","timestamp":1775017863536,"version":"3.50.1"},"reference-count":23,"publisher":"MDPI AG","issue":"4","license":[{"start":{"date-parts":[[2022,2,17]],"date-time":"2022-02-17T00:00:00Z","timestamp":1645056000000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"name":"DTU PoC Fund","award":["N\/A"],"award-info":[{"award-number":["N\/A"]}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>In industrial paper production, online monitoring of a range of quality parameters is essential for ensuring that the performance and appearance of the final product is suitable for a given application. In this article, two optical sensing techniques are investigated for non-destructive, non-contact characterization of paper thickness, surface roughness, and production defects. The first technique is optical coherence tomography based on a mid-infrared supercontinuum laser, which can cover thicknesses from ~20\u201390 \u03bcm and provide information about the surface finish. Detection of subsurface voids, cuts, and oil contamination was also demonstrated. The second technique is terahertz time domain spectroscopy, which is used to measure paper thicknesses of up to 443 \u03bcm. A proof-of-concept thickness measurement in freely suspended paper was also demonstrated. These demonstrations highlight the added functionality and potential of tomographic optical sensing methods towards industrial non-contact quality monitoring.<\/jats:p>","DOI":"10.3390\/s22041549","type":"journal-article","created":{"date-parts":[[2022,2,17]],"date-time":"2022-02-17T20:26:41Z","timestamp":1645129601000},"page":"1549","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":18,"title":["Non-Contact Paper Thickness and Quality Monitoring Based on Mid-Infrared Optical Coherence Tomography and THz Time Domain Spectroscopy"],"prefix":"10.3390","volume":"22","author":[{"ORCID":"https:\/\/orcid.org\/0000-0001-8545-5765","authenticated-orcid":false,"given":"Rasmus Eilk\u00e6r","family":"Hansen","sequence":"first","affiliation":[{"name":"DTU Fotonik, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-5272-7031","authenticated-orcid":false,"given":"Thorsten","family":"B\u00e6k","sequence":"additional","affiliation":[{"name":"DTU Fotonik, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark"}]},{"given":"Simon Lehnskov","family":"Lange","sequence":"additional","affiliation":[{"name":"DTU Fotonik, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark"}]},{"given":"Niels M\u00f8ller","family":"Israelsen","sequence":"additional","affiliation":[{"name":"DTU Fotonik, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark"},{"name":"NORBLIS ApS, Virumgade 35 D, 2830 Virum, Denmark"}]},{"given":"Markku","family":"M\u00e4ntyl\u00e4","sequence":"additional","affiliation":[{"name":"Valmet Oyj, Keilasatama 5, FI-02150 Espoo, Finland"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-8041-9156","authenticated-orcid":false,"given":"Ole","family":"Bang","sequence":"additional","affiliation":[{"name":"DTU Fotonik, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark"},{"name":"NORBLIS ApS, Virumgade 35 D, 2830 Virum, Denmark"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-2883-7908","authenticated-orcid":false,"given":"Christian Rosenberg","family":"Petersen","sequence":"additional","affiliation":[{"name":"DTU Fotonik, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark"},{"name":"NORBLIS ApS, Virumgade 35 D, 2830 Virum, Denmark"}]}],"member":"1968","published-online":{"date-parts":[[2022,2,17]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","unstructured":"Waller, M.H. (2001, January 17\u201321). On-line papermaking sensors: An historical perspective. Proceedings of the 12th Fundamental Research Symposium, Oxford, UK.","DOI":"10.15376\/frc.2001.2.785"},{"key":"ref_2","doi-asserted-by":"crossref","unstructured":"Busch, S.F., Probst, T., Duschek, L., Wilk, R., Voitsch, M., Fender, F., Lubbecke, S., Gartner, G., Wallace, V.P., and Koch, M. (2013, January 1\u20136). Inline monitoring of paper thickness in an industrial setting. Proceedings of the 2013 38th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), Mainz, Germany.","DOI":"10.1109\/IRMMW-THz.2013.6665803"},{"key":"ref_3","first-page":"229","article-title":"Papermaking Process Online Measurement and Control of Paper Ash Content","volume":"174","author":"Xiao","year":"2014","journal-title":"Sens. Transducers"},{"key":"ref_4","unstructured":"Osten, W., Gorecki, C., and Novak, E.L. (2005). An Online Laser Caliper Measurement for the Paper Industry, Proceedings of the Optical Measurement Systems for Industrial Inspection IV, Munich, Germany, 13\u201317 June 2005, SPIE."},{"key":"ref_5","doi-asserted-by":"crossref","unstructured":"Petersen, C.R., Rajagopalan, N., Markos, C., Israelsen, N.M., Rodrigo, P.J., Woyessa, G., Tidemand-Lichtenberg, P., Pedersen, C., Weinell, C.E., and Kiil, S. (2021). Non-Destructive Subsurface Inspection of Marine and Protective Coatings Using Near- and Mid-Infrared Optical Coherence Tomography. Coatings, 11.","DOI":"10.3390\/coatings11080877"},{"key":"ref_6","doi-asserted-by":"crossref","first-page":"11","DOI":"10.1038\/s41377-019-0122-5","article-title":"Real-time high-resolution mid-infrared optical coherence tomography","volume":"8","author":"Israelsen","year":"2019","journal-title":"Light Sci. Appl."},{"key":"ref_7","doi-asserted-by":"crossref","first-page":"218","DOI":"10.1007\/s10043-010-0045-0","article-title":"Optical coherence tomography as an accurate inspection and quality evaluation technique in paper industry","volume":"17","author":"Czajkowski","year":"2010","journal-title":"Opt. Rev."},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"1131","DOI":"10.1088\/0957-0233\/16\/5\/012","article-title":"Study on the use of optical coherence tomography in measurements of paper properties","volume":"16","author":"Alarousu","year":"2005","journal-title":"Meas. Sci. Technol."},{"key":"ref_9","doi-asserted-by":"crossref","first-page":"6541","DOI":"10.1364\/AO.48.006541","article-title":"Simultaneous composition and thickness measurement of paper using terahertz time-domain spectroscopy","volume":"48","author":"Mousavi","year":"2009","journal-title":"Appl. Opt."},{"key":"ref_10","doi-asserted-by":"crossref","unstructured":"Hattori, T., Kumon, H., and Tamazumi, H. (2010, January 5\u201310). Terahertz spectroscopic characterization of paper. Proceedings of the 35th International Conference on Infrared, Millimeter, and Terahertz Waves, Rome, Italy.","DOI":"10.1109\/ICIMW.2010.5612460"},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"4231","DOI":"10.1364\/OL.44.004231","article-title":"Upconversion-based mid-infrared spectrometer using intra-cavity LiNbO3 crystals with chirped poling structure","volume":"44","author":"Friis","year":"2019","journal-title":"Opt. Lett."},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"952","DOI":"10.1364\/AOP.11.000952","article-title":"Parametric upconversion imaging and its applications","volume":"11","author":"Barh","year":"2019","journal-title":"Adv. Opt. Photonics"},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"4558","DOI":"10.1364\/OL.432765","article-title":"High-resolution mid-infrared optical coherence tomography with kHz line rate","volume":"46","author":"Israelsen","year":"2021","journal-title":"Opt. Lett."},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"1519","DOI":"10.1063\/1.126082","article-title":"A cross-correlation spectroscopy in subterahertz region using an incoherent light source","volume":"76","author":"Morikawa","year":"2000","journal-title":"Appl. Phys. Lett."},{"key":"ref_15","doi-asserted-by":"crossref","first-page":"17723","DOI":"10.1364\/OE.17.017723","article-title":"Terahertz quasi time domain spectroscopy","volume":"17","author":"Scheller","year":"2009","journal-title":"Opt. Express"},{"key":"ref_16","doi-asserted-by":"crossref","first-page":"1884","DOI":"10.1364\/AO.50.001884","article-title":"Terahertz quasi-time-domain spectroscopy imaging","volume":"50","author":"Scheller","year":"2011","journal-title":"Appl. Opt."},{"key":"ref_17","doi-asserted-by":"crossref","first-page":"12851","DOI":"10.1364\/OE.25.012851","article-title":"Terahertz quasi time-domain spectroscopy based on telecom technology for 1550 nm","volume":"25","author":"Kohlhaas","year":"2017","journal-title":"Opt. Express"},{"key":"ref_18","doi-asserted-by":"crossref","first-page":"4197","DOI":"10.1364\/OL.38.004197","article-title":"Telecom technology based continuous wave terahertz photomixing system with 105 decibel signal-to-noise ratio and 35 terahertz bandwidth","volume":"38","author":"Stanze","year":"2013","journal-title":"Opt. Lett."},{"key":"ref_19","unstructured":"Lehmann, P.H. (2009). Calibration Method for Accurate Optical Measurement of Thickness Profile for the Paper Industry, Proceedings of the Optical Measurement Systems for Industrial Inspection VI, Munich, Germany, 14\u201318 June 2009, SPIE."},{"key":"ref_20","doi-asserted-by":"crossref","first-page":"2766","DOI":"10.1364\/AO.45.002766","article-title":"Terahertz birefringence and attenuation properties of wood and paper","volume":"45","author":"Reid","year":"2006","journal-title":"Appl. Opt."},{"key":"ref_21","doi-asserted-by":"crossref","first-page":"1129","DOI":"10.1364\/OL.416123","article-title":"Power sTable 1.5\u201310.5 \u00b5m cascaded mid-infrared supercontinuum laser without thulium amplifier","volume":"46","author":"Woyessa","year":"2021","journal-title":"Opt. Lett."},{"key":"ref_22","doi-asserted-by":"crossref","first-page":"296","DOI":"10.1364\/OL.43.000296","article-title":"Mid-infrared supercontinuum generation from 1.6 to >11 \u03bcm using concatenated step-index fluoride and chalcogenide fibers","volume":"43","author":"Martinez","year":"2018","journal-title":"Opt. Lett."},{"key":"ref_23","doi-asserted-by":"crossref","first-page":"1417","DOI":"10.1364\/BOE.378506","article-title":"Co-registered combined OCT and THz imaging to extract depth and refractive index of a tissue-equivalent test object","volume":"11","author":"Fitzgerald","year":"2020","journal-title":"Biomed. Opt. Express"}],"container-title":["Sensors"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/1424-8220\/22\/4\/1549\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2025,10,10]],"date-time":"2025-10-10T22:21:21Z","timestamp":1760134881000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/1424-8220\/22\/4\/1549"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2022,2,17]]},"references-count":23,"journal-issue":{"issue":"4","published-online":{"date-parts":[[2022,2]]}},"alternative-id":["s22041549"],"URL":"https:\/\/doi.org\/10.3390\/s22041549","relation":{},"ISSN":["1424-8220"],"issn-type":[{"value":"1424-8220","type":"electronic"}],"subject":[],"published":{"date-parts":[[2022,2,17]]}}}