Optical measurement of paper moisture content with application in paper pressing

We present a novel non-contact method for measuring the moisture content of paper. In the method, paper is illuminated obliquely by light from an IR LED, and the light reflected from the paper is imaged by a short-wave infrared (SWIR) camera. Owing to the high absorptivity of liquid water to light in the 1400–1500 nm wavelength range, the intensity of light reflected off the paper diminishes sharply with increasing moisture content. We show that for a variety of paper samples (Whatman paper, NBSK, NBHK, tissue paper) and moisture contents of up to 200%, there is a monotonic relationship between reflectivity and moisture content. This relationship is independent of the wood species used to make the paper, but does differ between wood-based papers and cotton-based papers. As this method involves an optical camera, the spatio-temporal distribution of moisture content in paper can be measured. The method was validated by separately measuring moisture content in paper gravimetrically for both uniform and non-uniform moisture distributions. The measurement technique was applied to a simulated pressing operation where the inferred moisture content distribution changes with the applied pressure.