Electrostatics in Forensics

Indentations on the surface of a paper, created when two or more sheets of paper are resting on top of each other during the writing act are called "latent writing impressions." The underlying sheets contain the latent impressions of what was being written above. These can be of very valuable to the document examiner by offering significant information, such as phone numbers, names etc. It can be a source of identification in anonymous note cases, and is an invaluable investigation procedure when medical records are suspected of alteration -- a writing addition to a record or file can be revealed by what has been transferred to the page below, if it is different from the document in question.

Indented writing is normally recovered by one of two methods: photographically using oblique light, or by use of an apparatus commonly referred to as ESDA, for Electrostatic Detection Apparatus.

  Oblique lighting

Oblique light, or simply lighting at an angle, is applied to the surface of the paper to reveal any furrows of indented writing. A photo is them taken of the shadowed indentation. Moving the light source and taking multiple exposures should fill in most available indentations with shadow, and hopefully reproduce the indented writing. While oblique lighting techniques are often quite accepted in court, they're unable to recover invisible microscopic indentations, which may occur three or four pages down. They also have an inherently lengthy processing time.

Electrostatic Detection (ESDA)

ESDA provides a method of detecting and permanently recording latent writing impressions on the surface of a paper. Indentations are created when two or more sheets of paper are resting atop one another during the writing act. The underlying pages contain the latent impressions that the ESDA is designed to record. Using ESDA, it's possible to image indented writing three, four, or more pages below the original writing. Some documents are unsuited for this process -- documents previously processed for latent fingerprints, made of thick cardboard, or which have been saturated with fluids fall into this category.

The page suspected of bearing indentations is covered with a cellophane material which is then pulled into firm contact with the paper by a vacuum drawn through a porous bronze plate. This serves to "fasten" the document, and cellophane covering, to the plate. The cellophane covering prevents damage to the original document. The document and cellophane are then subjected to a repeated high voltage static charge.

This results in a variably charged surface, with the heavier static charge remaining within any impressions, even microscopic ones. Black toner (similar to that used in dry-process photocopy machines), is then cascaded over the cellophane surface. The areas of the document containing the higher static electric charge retain the black toner, resulting in a deposit of toner in the indentations in the paper. These developed indentations may be photographed and then preserved by means of placing an adhesive backed clear plastic sheet over the cellophane while it is still being held in place by the vacuum of the ESDA.

A sheet of paper taken from a notepad in a bank robbery suspect's home was processed using the Electrostatic Detection Apparatus. The result shown on the left matches the writing on the note on the right, which was given to the bank teller.

If the recovered indented handwriting is of a high enough quality, it can associate somebody to the questioned document, as in the case of the man who was receiving rambling anonymous letters which referred to his personal life. The handwriting, however, didn't seem to belong to anyone associated with him. A ESDA version of the notes revealed previous drafts of the letters, written in his wife's handwriting. She had been authoring the notes, and giving them to a friend to write.

The advantages of electrostatic detection are two-fold: ESDA is extremely sensitive, and it's also non-destructive. The indentations are revealed on the protective cellophane surface, and the original document remains unharmed throughout the process.