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.

What is claimed is: 
     
         1 . A method of measuring the thickness of a web, which has a first side and a second side, that comprises the steps of:
 a. supporting the web so that the web has a free standing portion where the web has an exposed outer surface on the first side and an inner surface on the second side;   b. directing a laser beam at a selected angle of incidence onto a single spot on the exposed outer surface on the first side, wherein the laser beam comprises substantially monochromatic radiation having a wavelength in the 3-50 micron range;   c. scanning the laser beam through a selected wavelength range as the laser beam is directed onto the spot on the exposed outer surface;   d. measuring the intensity of an interference pattern that finis from superposition of radiation that emerges upon exposure to the web; and   e. extracting the thickness of the web by utilizing the relationship among the laser beam incident angle, wavelength, index of refraction of the web, and web thickness.   
     
     
         2 . The method of  claim 1  wherein step d comprises measuring the intensity of an interference pattern that forms from the superposition of radiation that is reflected from the exposed outer surface on the first side and from the inner surface on the second side. 
     
     
         3 . The method of  claim 1  wherein step d comprises measuring the intensity of an interference pattern that forms from the superposition of radiation that is transmitted directly through the web and radiation that is transmitted through the second side after internal reflections from inner surfaces of the second and first sides of the web. 
     
     
         4 . The method of  claim 1  wherein step (e) comprises fitting the interference pattern intensity distribution with wavelength to a mathematical relationship by using web thickness as a fitting parameter. 
     
     
         5 . The method of  claim 1  wherein step (e) comprises measuring the occurrence of interference minima. 
     
     
         6 . The method of  claim 1  wherein the index of refraction is independently measured. 
     
     
         7 . The method of  claim 1  wherein the web has a thickness in the range of 10 microns to 200 microns. 
     
     
         8 . The method of  claim 1  where the web comprises paper, plastic made of polyethylene, polypropylene, polyethylene terephthalate, polytetrafluoroethylene or polyvinyl chloride. 
     
     
         9 . The method of  claim 1  comprising using a tunable quantum cascade laser as the substantially monochromatic light source. 
     
     
         10 . The method of  claim 1  wherein the monochromatic radiation has a wavelength in the 8-25 micron range. 
     
     
         11 . The method of  claim 1  wherein step b employs focusing elements that direct the laser beam onto an area of the web as the laser beam is scanned through the selected wavelength range. 
     
     
         12 . The method of  claim 1  wherein step b employs a beam steering mirror system. 
     
     
         13 . The method of  claim 1  wherein step c employs an external cavity quantum cascade laser with a wavelength scan controller selecting the wavelengths emitted by the laser. 
     
     
         14 . The method of  claim 1  wherein step d employs optics that directs intensity variations of the laser-illuminated area onto an infrared-sensitive detector. 
     
     
         15 . A non-contacting caliper sensor for measuring the thickness of a web of scattering material having a first side and a second side, comprising:
 a. substantially monochromatic laser that provides a beam of incident radiation that has a wavelength in the 3-50 micron range;   b. means for directing the incident radiation toward a single spot on an exposed outer surface on the first side of the sheet wherein the incident radiation reaches a fixed position on the exposed surface at an angle of incidence of from 0 to 60 degrees with respect to the web surface normal;   c. means for detecting the interference pattern which forms by superposition of a first radiation reflected from the exposed outer surface and a second radiation reflected from an inner surface of the second side; and   d. means for analyzing an interference pattern to calculate the thickness of the web.   
     
     
         16 . The non-contacting caliper sensor of  claim 15  wherein the laser is a dynamically tunable quantum cascade laser that is sequentially tuned to emit at selectable wavelengths substantially transmitted by the web. 
     
     
         17 . The non-contacting caliper sensor of  claim 15  wherein the laser source comprises more than one tunable quantum cascade laser, each emitting in a different narrow wavelength range that is multiplexed together through a system of mirrors and dichroic beam splitters to create a laser source that can be scanned over a broad wavelength range. 
     
     
         18 . The non-contacting caliper sensor of  claim 15  wherein the means for directing the incident radiation onto the web comprises a steering mirror system that sets the angle of incidence. 
     
     
         19 . The non-contacting caliper sensor of  claim 15  wherein the means For detecting the interference pattern from the web comprises a steering mirror system and focusing optics to focus the interference pattern onto an infrared radiation sensitive detector. 
     
     
         20 . The non-contacting caliper sensor of  claim 15  where the web comprises paper plastic made of polyethylene, polypropylene, polyethylene terephthalate, polytetrafluoroethylene or polyvinyl chloride.