Introduction

ESDA stands for Electrostatic Detection Apparatus. It is used to detect and to assist in the decipherment of indented impressions of handwriting on paper.

When writing is produced on a sheet of paper which is resting on other sheets of paper, impressions of the writing may be indented into the surface of the sheets underneath. If these indentations are deep (caused, for example, by use of heavy pen pressure) they may be visible to the naked eye - sometimes with the assistance of a light source shone at an oblique angle to the paper surface. If the indentations are too shallow to see clearly, then the ESDA is used.

The examination of documents for impressions can be helpful in a variety of investigations. For example, in determining the origin of anonymous documents, the linking of apparently unconnected documents and, in some circumstances, in determining the sequence in which a series of pages were written or when they were written.

The Technique
The sheet of paper is placed on the ESDA and a thin plastic film is placed over it, held in close contact by means of vacuum suction. The surface of the plastic film has an electrostatic charge applied to it. The surface of the paper causes a different pattern of charging in those areas where there are indentations. This charge difference is visualised by applying an oppositely charged black toner. The result is that toner tends to adhere to those areas corresponding to indentations. A sticky, transparent plastic film is placed across the ESDA trace to preserve it.