AFTE Store - A Simple Method for Examining Deep Toolmarks Using the Scanning Electron Microscope

The scanning electron microscope (SEM) is presented as a useful alternative to the optical microscope for comparison of toolmarks when the marks are deep or the surface too convoluted, or when because of illumination difficulties the marks are not observable by traditional optical microscopy. To obtain a large depth of focus (to ~3mm at 100X) and to enhance topographical detail, the SEM is best operated using the low-angle backscattered electron signal rather than the usual secondary electron signal for imaging. The standard Everhart-Thornley secondary electron detector is used as a backscatter detector so that the method requires no additional equipment. Although imaging with high-angle backscattered electron signals are commonly used in electron microscopy, low-angle backscattered electron signals are not. The low-angle signal duplicates the highly oblique lighting used in optical microscopy. To demonstrate our method, a comparison of firing pin impressions in 9mm cartridge cases imaged with both the SEM and the conventional optical microscope is presented. A brief tutorial is presented which explains how the SEM can simulate both diffuse and oblique lighting as used in optical microscopy.