Volume 35, Issue 1 (January 1990)
Photoelastic Stress Analysis of Film Ribbons
Diffused polarized-light microscopy reveals detailed patterns in the film base of used typewriter ribbons inside the de-inked letter impressions. The multicolored patterns, which are observed to match the complex paper fiber configurations found at corresponding letter sites on the typed page, are theorized to be a manifestation of photoelasticity. Wavelength-dependent phase retardation differences, as a result of strain birefringence, correspond to retained microscopic deformations produced in the ribbon base polymer by paper fibers at the page surface at the instant of typing impact stress. Diffuse illumination is observed to enhance such embedded patterns relative to interfering surface features such as manufacturing striations.