Abstract
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 ~20–90 μm 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 μm. 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.
Highlights
Paper is a ubiquitous product used in a wide variety of applications, from printing books and currency, to packaging and cleaning. It is generally made from a web of cellulose fibers that is processed into a thin sheet but can differ in composition and quality parameters, such as strength, density, and thickness depending on the requirements of the application
For measurements of paper thickness, sensors based on magnetic reluctance or
From the optical coherence tomography (OCT) image, the optical thickness was calculated by finding the peaks of the surface and mirror signals, indicated by red, dashed lines
Summary
Paper is a ubiquitous product used in a wide variety of applications, from printing books and currency, to packaging and cleaning. It is generally made from a web of cellulose fibers that is processed into a thin sheet but can differ in composition and quality parameters, such as strength, density, and thickness depending on the requirements of the application. To ensure that these parameters comply with the needs of an application, industrial process control is needed. For measurements of paper thickness, sensors based on magnetic reluctance or
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