NCJ Number
114397
Journal
Journal of the Forensic Science Society Volume: 28 Issue: 4 Dated: (1988) Pages: 237-242
Date Published
1988
Length
6 pages
Annotation
A new viewpoint is advanced concerning the principle of electrostatic imaging for the detection of indented impressions on documents.
Abstract
In electrostatic imaging, the document is placed between an earthed, sintered metal bed and transparent polyester film. The film is charged with a corona charging unit, while the electrostatic image of the indented impression is formed on the film. Previous theories on the principle underlying this technique have proposed that indented impressions could be developed because of variations in the thickness of the paper. However, this explanation is inconsistent with several experimental observations. It is proposed that the image developed is due to variation in the texture of the paper bearing the impression rather than to variations in the thickness and density of the paper. Due to pressing and indentation, the surface of the paper becomes smoother at the location of the impression and contacts the polymer film more closely. As a result, the capacitance at this location is greater than elsewhere and the electrostatic potential is lower, thus, producing a latent electrostatic image of the indented impression. 3 figures and 2 references. (Author abstract modified).