Electrostatic Detection Apparatus (ESDA) illustrated above, is used to produce a visible image of latent indentations which may have been made in the paper surface, from writings on previous overlying page/s.
Writing on top page makes indentations on under-lying pages
The procedure using ESDA to produce a visible image of the indentations involves:
- Humidify the questioned document to increase sensitivity to electric charge. (Using gloves to avoid fingerprints or contamination at this sensitive stage).
- Place document upon the porous top plate of the apparatus.
- Place transparent polymer imaging film over the questioned document on top plate. Apply vacuum suction to flatten the document and imaging film.
- A high voltage charge is applied over the document. The charge registers more strongly upon the imaging sheet at the sites where paper surface fibres have been compressed by writing from overlying page/s.
- Toner is then applied to the surface of the imaging film, which retains an electrostatic charge. The toner is attracted strongly to the site of any indentations, (where the electrostatic charge has registered and remained the strongest). Dark images are recorded, showing any indentations.
- The image of any indentations may be then preserved for evidential purposes, by applying a transparent adhesive sheet on top of the imaging sheet. The examiner then removes the questioned document in undamaged condition.
Any ordinary visible writing on the questioned document, will not produce a dark image during the imaging process, and therefore will not interfere with attempted decipherment of the dark images showing the indentations on the questioned document.
The ESDA process is very sensitive, even for slight indentations. Therefore the image of the indentations upon a questioned document may reveal multiple overlying indentations from writings upon multiple overlying pages. Expert interpretation is obviously always advisable.
An ESDA image may reveal unexpected evidence of writings from indentations:
- Writing indentations may incriminate the writer of anonymous letters;
- Writing indentations may link a questioned document to its source, which would otherwise have remained unknown;
- Indentations may indicate that a written entry is inconsistent with being written at the time it was claimed; diary, journal or notebook entries.
The ESDA is manufactured in the UK by Foster & Freeman Ltd.