ISSN: 0022-1198
CODEN: JFSCA
Page Count: 4

Photoelastic Stress Analysis of Film Ribbons
Daniels, JR
Forensic document examiner, TX

Abstract

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.

Keywords:
questioned documents, typewriters, microscopy, typewriter ribbons, impressions

Paper ID: JFS12816J
DOI: 10.1520/JFS12816J
ASTM International is a member of CrossRef.