Caliper coating measurement on continuous non-uniform web using THz sensor

摘要

An analytical model simulates the propagation of radiation through a coated continuous web where layer thickness and refractive index, as variables, determine the speed and direction of transmitted radiation. The model predicts characteristics of transmitted radiation based on characteristics of incident radiation and initially assigned values for layer thicknesses. Coating thickness(s) are ascertained in a process whereby incident radiation of known characteristics is directed onto a coated web and thereafter, actual measurements of transmitted radiation are compared to predicted characteristics. Using a fitting algorithm, the assigned thickness(es) of the layer(s) of the model are adjusted and the process repeated until the actual and predicted values are within desired limits at which time, the assigned thickness(es) represent the measured calipers. Radiation measurements are obtained using terahertz time-domain spectroscopy apparatus that creates a continuous reference whereby a sample pulses' phase and amplitude can be tracked and corrected.

主权项

1. A method of measuring the thickness of a coating on a coated continuous web that comprises the steps of:
(a) developing an analytical model of the coated continuous web having one or more coating layers and a substrate layer wherein the model expresses the transmission of electromagnetic field through the coated continuous web as a function of (i) the thickness of each coating layer, (ii) the thickness of the substrate layer, (iii) the refractive indices of each coating layer and of the substrate which values are initially assigned, and (iv) angle of incident of terahertz radiation onto the coated continuous web; (b) establishing a sample measurement for a coated continuous web sample by employing a time-domain terahertz spectroscopy apparatus that includes an emitter that generates pulses of radiation and a detector that receives pulses of radiation, which comprises the steps of:
(i) positioning a beam splitter along an optical path between the emitter and the detector to form reference radiation pulses and sample radiation pulses that are directed to the sample and the reference radiation pulses do not interact with the sample;(ii) positioning a first mirror to reflect sample radiation pulses to the detector; and(iii) positioning a second mirror to reflect reference radiation pulses to the detector and using the reflected radiation pulses to monitor fluctuations in phase and amplitude of the reference radiation pulses; (c) establishing a reference measurement for the coated continuous web sample with the time-domain spectroscopy by executing steps (b)(i), (b)(ii) and (b)(iii) without placement of any sample for measurement; and (d) comparing the sample measurement to the reference measurement and iteratively assigning values for the thickness of the coating and the thickness of the substrate layers based on the model defined in step (a) so as to achieve a best-fit relationship between the sample measurement and the reference measurement to thereby determine the thickness of the coating.