Electrostatic detection analysis
ESDA stands for Electrostatic Detection Apparatus. This instrument is used to produce a permanent 'lift' from paper which provides a visual map of indented impressions in that paper. (See also indented impressions above) ESDA has the advantage of being non-destructive so that the paper under examination remains in exactly the same state, and is still available for other examinations. It is also extremely sensitive meaning that indentations found up to seven sheets below the page where the original writing was made may be visualised.
How it works
ESDA works by stretching a Mylar film (like Clingfilm) over the document being examined. This Mylar film is then electrostatically charged using a 'wand' containing a fine wire charged to 7KV. Where the paper is smooth the charging is generally uniform, however where the fibres of the paper have been disturbed by paper-paper contact caused by indentations, the electrostatic charge is different to the background. This creates a latent image. In order to visualise this latent image, black toner similar to that used in laser printers is applied to the surface of the Mylar. The toner preferentially sticks where the electrostatic charge congregates - i.e. in the areas of the indentations. The result is an image with a grey toner background and darker toner traces in the areas where there are indented impressions. The image is made permanent as a 'lift' by placing clear 'sticky backed plastic' over the toner, producing a fixed transparent image.
There are a large number of variables which affect the quality of ESDA lifts. These include the type of paper, the type of pen or stylus used, the number of sheets of paper between the writing and the sheet holding the impressions, the humidity at the time the impressions were made, the humidity at the time the ESDA lift was made, the method by which the toner is put onto the Mylar film*, and the amount and direction of the electrostatic charge put onto the Mylar film. ESDA is not suitable for the examination of loose fibred paper such as newspaper or very glossy paper such as magazine covers. If a document gets wet by any liquid this will completely destroy the ESDA impression. This means that if fingerprint treatment is required then this should be done AFTER any ESDA examination.
* Toner can be put onto the Mylar film using three methods: by cascading tiny glass beads coated with toner over the surface, by puffing an aerosol of toner powder over the surface or by using a device similar to a powder puff to apply the toner.