Caliper sensor and method using mid-infrared interference method

摘要

Contactless caliper measurements of free standing sheets such as porous polymers and paper detect mid-infrared interference fringes created by the reflection of light from the top and bottom of the sheet. The method includes directing the laser beam at a selected incidence angle to a single spot on the exposed outer surface, wherein the laser beam comprises radiation having a wavelength in the range of 3 to 50 microns; Scanning the laser beam over a selected angular range while the laser beam is directed to the exposed outer surface; and scanning the laser beam reflected from the exposed outer surface and the inner surface And measuring the intensity of the interference pattern formed from superimposition. It is possible to derive the thickness from the spacing of the fringes in the interference pattern. Rotating and focusing elements ensure that the spot position on the sheet remains the same while varying the angle of incidence.(FIG.