PAPER PHYSICS: A method for measurement of the directional emittance of paper in the infrared wavelength range

A method for measuring the directional emittance of paper and board samples was developed. The available literature showed that the influence of temperature and observation angle on the emittance of dry and moist paper had not been investigated in detail. Methods adapted for such investigations were not available. In the developed method, the emittance of a sample is determined by comparing its infrared radiation with the radiation emitted from a reference surface with known emittance. In order to investigate the influence of the wavelength range, two cameras, operating in the mid-wavelength and long-wavelength infrared range, respectively, were used. The method allows for the adjustment of the directional emission angle in a range from 0° down to 80°, and variation of the sample temperature between 30°C and 100°C. A study was performed to evaluate the method. Here, the directional emittance of handsheets made from thermo-mechanical pulp was measured at different wavelength ranges, sample temperatures and emission angles. The obtained emittance values and trends were in agreement with previous experimental work and theoretical predictions. The emittance of the samples was also measured using Fourier-Transform Infrared spectroscopy. Given the methodological differences between the two measurement approaches, the results were in good agreement.