An instrument for electron beam and light transmission imaging of mass distribution in paper and fibrous webs

Abstract

An instrument was developed for the rapid measurement of local mass density (formation) in fibrous networks and films using electron beam transmission (EBT) imaging. A transmission electron microscope (80 keV) was modified for use as a beam source for irradiating 5 cm×5 cm samples of paper or other fibrous webs. Local transmission of electrons through paper (directly proportional to the mass) was measured indirectly by video imaging of the pattern emitted by a Ca(Eu)F2 cathodoluminescing window supporting the specimen. The local optical density was also determined using a diffused electroluminescent lamp. A single CCD imaging system, with a spatial resolution of 0.1 mm, was used for both the electron and light transmission methods. EBT results were calibrated using mylar samples of known grammage. The irradiation sources and the detection system were characterized to establish the limits of operation and measurement capabilities. Electron beam flux was measured directly, and the attenuation curve for mylar correlated well with Monte Carlo estimation with an upper limit of ∼85 g/m2. For EBT imaging, procedures were established to prevent disruption of images by electrostatic discharging. Correction also was made for the back-reflected light that was a function of the reflectivity, R0, of the sample. A group of samples prepared from different pulps was imaged, and the actual grammages were compared with those determined from the instrument. The results demonstrated that, with few exceptions, good correlation existed.