Electrostatic Detection Apparatus
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ESDA (from Electrostatic Detection Apparatus), is a piece of equipment commonly used in forensics, to reveal indented impressions on paper which may otherwise go unnoticed. It is a non-destructive technique (will not damage the evidence in question) thus allowing further test to be carried out. It is a sensitive technique, and has been known to detect the presence of fresh fingerprints.
When writing is fashioned on a sheet of paper resting upon other pages, the impressions produced are indented onto those below. It is these indentations which we detect using ESDA, thus allowing us to match the original document to its source (such as a ransom note, or a threat to extort) to the offender's notepad.
For instances where two or more handwriting styles can be found mixed into a single document, and features of one handwriting style depart from the features of another's, ESDA can help reveal the differences in pressures employed between the individuals responsible for the writing samples otherwise unified on a single exemplar.
[edit] See also
Electron Microscope - A more expensive but more precise way of comparing passages of text, Electron Microscopes help determine whether fingerprints are underneath the ink on a piece of paper, or on top of it. This is because the flow of ink on paper depends, to a large extent, on the paper's unique capacity for capillary action, and the fact that a particular kind of ink will always flow differently on top of fingerprints than upon blank, unwritten paper. Although it can be expensive, it can also determine which fingerprint is above another fingerprint, and which mark is just underneath.
