Thickness Determination of Web Product by Mid-infrared Wavelength Scanning Interferometry

Abstract

Non-contacting caliper measurements of free-standing sheets detect mid-IR interferometric fringes created by the reflection of light from the top and bottom surfaces of the sheet. The technique includes directing a laser beam at a selected angle of incidence onto a single spot on the exposed outer surface and scanning the laser beam through a selected wavelength range as the laser beam is directed onto the exposed outer surface and measuring the intensity of an interference pattern that forms from the superposition of radiation that is reflected from the exposed outer surface and from the inner surface. Alternatively, the intensity of an interference pattern formed from the superposition of radiation that is directly transmitted through the web and radiation that is transmitted through the web after internal reflections from the internal surfaces of the web. Thickness can be extracted from the fringe separation in the interference pattern.

Description

FIELD OF THE INVENTION[0001]The present invention relates generally to interferometry techniques for non-contacting thickness or caliper measurements of a moving sheet such as porous polymer and paper and more particularly to methods of detecting mid-IR interferometric fringes created by the reflection of light from the top and bottom surfaces of the sheet and thereafter extracting the thickness from the fringe separation.BACKGROUND OF THE INVENTION[0002]Caliper is one of the most important quality specifications of paper and plastic products. Traditional commercial on-line caliper measurement requires the measuring heads to physically touch the web. Contacting the web causes a number of issues with the two most significant ones being the marking of the sheet and the accumulating of dirt on the measuring heads, which leads to measurement drift and inaccuracy. More advanced techniques make use of laser triangulation or confocal microscopy techniques but they still require a measuring...

AI Technical Summary

Benefits of technology

[0016]In a preferred embodiment, radiation in the mid-infrared wavelength (3-50 microns), which is preferably in the 8-25 micron range, is directed into the paper web and interferometric fringes created by the reflection of the light at the top and bottom surfaces of the web are recorded. In comparison with radiation of shorter wavelengths, mid-IR wavelengths are less affected by scattering in the paper which makes the inventive technique suitable to applications unsuitable to prior art techniques. Web thickness in the range of 20 microns to 2-3 mm can be measured if the caliper sensor wavelength is extended to the far-IR (typically having a wavelength of 50 microns to 1 mm) or terahertz range (typically having a wavelength of 100 microns to 1 mm). The web does not come into contact with the measurement head in which the caliper sensor is positioned. The measurement can be performed in a reflection geometry requiring only one measurement head.

Problems solved by technology

Contacting the web causes a number of issues with the two most significant ones being the marking of the sheet and the accumulating of dirt on the measuring heads, which leads to measurement drift and inaccuracy.

Moreover, prior art optical techniques are not suitable to all paper products because they are very sensitive to the scattering properties of the sheet.

In addition, achieving better than 1 micron accuracy is a challenge as these techniques rely on the difference between two independent distance measurements.

This is difficult to achieve in the paper scanner environment where the measurement heads are exposed to frequent temperature changes and the positions of the paper and heads are subject to constant fluctuations.

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