Electrostatic detection device
An electrostatic detection device,
' or ESDA''', is a specialized piece of equipment commonly used in questioned document examination to reveal :wikt:impression|indentations or impressions in paper that may otherwise go unnoticed. It is a non-destructive technique, allowing further tests to be carried out. It is a sensitive technique capable of detecting indentations on pages several layers below the top sheet and many years after the indentations were created.
EDD equipment and investigative techniques were central to overturning a number of convictions in the United Kingdom, as it was possible to demonstrate that witness statements had been altered or signed as blank pages in reverse order to the main notes. This was central in a number of cases investigated at the West Midlands Serious Crime Squad that were appealed. The allegations of tampering with evidence and witness statements led to the unit being disbanded, and over 60 convictions being quashed, many of the appeals relying on EDD evidence.
How it works
When writing is fashioned on a sheet of paper resting upon other pages, the indentations or impressions produced are transferred to those below. These transferred impressions can be detected using an EDD.In some situations, a questioned document such as a ransom note, or an extortion letter, may exist which can be determined to be the source of indentations detected on another piece of paper. Alternatively, indentations detected on a business contract might match information present on another such document. In some situations this would be an entirely innocent finding; however, if the two businesses are supposed to be operating independent of one another, then the finding could be significant. Decipherable indentations may also provide valuable information even when a second document is not present or cannot be located. For example, an anonymous letter may bear impressions of writing that relate to some mundane activity of the offender which could ultimately lead an investigator to a particular suspect.
The electro-physical basis whereby an EDD actually works is complex. The original theory suggested that the paper sandwiched between the grounded platen and the mylar charging film acted as a type of capacitor with the change in capacitance being due to differing compression of the paper. This led to models like the 'Thickness Variation Theory' and the 'Surface Variation Theory'. However, it turns out that a detectable 'indentation' is not due to the physical pressure applied to the writing instrument as one might expect. Rather, Seward in 1998 and 1999 proposed an alternative theory explaining the detection capability of an EDD as being due to a surface charge effect created by paper-to-paper friction specifically in the area where a writing instrument is pressed down into the top-most sheet of paper. Seward's model was based upon "charge transport through the Mylar-paper-platen structure" and is appropriately called the 'charge transport model'.
Subsequent testing, demonstrated that the charge transport model, while not perfect, is sound. This testing also clarified that areas of indentation are less negatively charged than surrounding areas. It is this relative difference in potential that causes the toner to be attracted to the areas of indentation, rather than other areas on the mylar surface.
Seward's model also helps to explain two unusual phenomena sometimes observed when using an EDD:
- 'pure' indentations caused by impact printing, for example, may not produce good EDD results
- indirect secondary 'impressions' may appear that are caused by lateral relative motion between two sheets of paper when the source sheet bears significant embossing.
Practical use
Despite the complexity of the underlying theoretical mechanism, the practical use of an EDD is straightforward. Most devices are similar in their operation. The following are the key steps in using an EDD to visualize indentations:- Evaluation of material
- Preparation
- Placement on platen: the goal is to place the questioned document flat on the platen surface with few wrinkles or distortions. The document should not extend beyond the platen edges and enough room should be present around the document to allow the charging film to seal against the platen surface.
- Charging of surface: the top surface of the charging film must be electrostatically charged. To achieve this most EDDs use a shielded hand-held unit containing a high-tension corona wire which is turned on and literally waved over the entire surface of platen for several seconds criss-crossing the platen area. The objective is to produce an evenly distributed electrostatic charge in the air above the surface of the charging film. After passing the unit over the platen, the examiner waits for a few minutes until the charge has been passed to the surface of the charging film.
- Application of toner to develop indentations. The EDD is able to visualize indentations because the surface is differentially charged depending upon whether or not an indentation is present. The negatively charged toner particles are attracted to areas where there are indentations in the paper surface. Once an electrostatic charge has been applied to the charging film, the latent indentations can be developed. This is done using a powder toner very much like toner found in any electrophotographic printing device. There are various methods used for this purpose.
- * Cascade method: Some EDDs have a platen that is hinged on one edge to permit it to be tilted. A mixture of toner and small carrier beads is then poured over the surface of the document from one side to the other. The beads, and excess toner, are captured on the lower edge of the device to be reused. This process can be repeated several times in order to develop all areas of the document.
- * Aerosol spray method: Some EDDs provide the ability to spray toner over the surface of the charging film. In these systems a protective shell or chamber is fitted that covers the entire platen. At one end there is a nozzle connected to a reservoir of toner. The examiner then manually activates a pump that sprays toner into the chamber creating a cloud of fine toner that settles down onto the surface of the charging film. Ideally the cloud is randomly distributed but often this method produces a relatively distinctive pattern of toner on the film.
- * Brush method: With some EDDs the toner is applied using a magnetic brush such as would be used for development of latent fingerprints. This is a manual process that is quite tedious and requires meticulous control of the brush. But the method works well for selective development of subtle indentations.
- * Toner pad method: Similar to the brush method described above but uses a special pad with a built-in toner reservoir. The toner pad is placed on the surface of the charging film and moved across the surface by hand.
- Assessment of results. After the development process is complete the examiner must assess the results of the test process.
- * FFU test results: since the questioned document may or may not have indentations, the correct method to assess the functionality of the EDD is through inspection of the FFU test sample.
- Preservation of results. It is important to record and/or preserve the results of a test, especially if the test is positive. This can be done in at least two ways.
- * Adhesive lift: in this method, a sheet of fixing film is used to preserve the developed indentations. The fixing film is usually a relatively rigid sheet of self-adhesive, transparent polymer film. It is applied by removing the backing material and carefully laying the fixing film directly onto the charging film, thus trapping and fixing the toner in place between the films. Pressure is then carefully applied to the top of the fixing film by hand or with a small roller. The resulting sandwich is referred to simply as a "lift", or EDD lift. Interpretation of the developed indentations is generally done using the lift after removal from the document because this permits the examiner to see the developed indentations on their own without interference from any visible markings on the document.
- * Photography/scanning: the results may also be captured using a camera, either directly from the charging film or from the lift after removal from the document. Scanning is also an option but only when using the lift itself.
Manufacturers
An EDD is a highly specialized device and there are only a few manufacturers:- Foster + Freeman Ltd manufactures and markets the ESDA® which is the original such device developed in 1978. It was designed to meet the requirements of the UK Home Office and is still in production today in two different forms: the ESDA2® and ESDA-Lite®
- Lightning Powder Company, Inc. manufactures two units: the Electrostatic Vacuum Box and Vacu-Box™ for Indented Writing
- Kinderprint used to manufacture the IMEDD® but their website no longer lists this device in their product line