Reflectance of three paper grades was investigated using a pressure gauge, an imaging system, and a spectrophotometer. It was observed that under high pressure, dark-colored areas appear in paper when using the imaging system. The area of the dark pattern increases as the pressure is raised. Reflectance measurements as a function of wavelength confirmed that the dark patterns appear due to the contact of the paper and the probe window of the pressure gauge. Based on results by other researchers the amount and the nature of optical contact observed depends on structural properties of paper such as surface roughness, formation, and compressibility. Although there are differences in the spectral properties of different paper grades they all share the common feature that the reflectance is decreasing as a function of the applied pressure. An empiric result for the fine and the super-calendered (SC) paper samples measured is that they both have an exponential dependence of total reflectance as a function of applied pressure when the illumination wavelength is kept constant.
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