An electrostatic detection device, or EDD, is a specialized piece of equipment commonly used in questioned document examination to reveal indentations or impressions in paper which may otherwise go unnoticed. It is a non-destructive technique (will not damage the evidence in question) thus 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.
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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 (e.g., an offender's notepad). 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.
Additionally, indentations can sometimes not develop very well even though they are clearly visible. This may occur, for example, with very heavy or deep impressions or with impressions in glossy heavy-weight paper such as that used for brochures. In such situations, it is possible to use digital scanning and image-processing to document such writing impressions.
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:
- Assess adequacy of material for examination: an EDD works best when used on a single sheet of clean, smooth paper (that is, paper without wrinkles, creases or stains). The presence of any of these factors may preclude effective examination or, at the minimum, mean that the results will be limited. Heavy-weight, coated or treated papers often produce poor results. The purported age of the document is not generally a factor of concern. Latent indentations have been developed on documents older than fifty years.
- Examination using side (oblique) lighting: as a general rule, all items should be examined using side lighting. This is important because visual examination may detect deep indentations even though the EDD may fail to develop those indentations properly.
- Documents that have been subjected to high levels of humidity will not generally retain indentations. As a result, other forensic examinations involving the wetting of a document, such as fingerprint development using a ninhydrin solution, should not be done until after examination for indentations.
- Humidification: an EDD works best when the relative humidity is less than 60%. Some studies have also suggested that absolute humidity is important. To help ensure proper humidification, most manufacturers provide a 'humidification chamber' that can be used to prepare a document for examination. It is important to note that over-humidification is also possible so care must be taken to not actually wet the document.
- Fitness-for-use (FFU) test; AKA, control sample: since an EDD may fail to detect indentations when they are actually present (for a variety of reasons), it is important to check on the equipment's performance using a control sample (commonly called a FFU test) known to have indentations. Recommendations include using a FFU test sample at the beginning and end of a series involving the examination of multiple questioned documents, or placing a small FFU test sample adjacent to each questioned document on the platen. The FFU test can be prepared by hand using a similar writing instrument and similar paper. Alternatively, a special device call the 'Gradient®' can be used to produce control indentations that vary in depth in a known manner thereby permitting the examiner to determine not only that the EDD is working but also the degree to which it is sensitive. It is not essential to precisely match the questioned document in terms of paper type or pen type since the objective is to show that the device is functioning properly.
- Type of document: as noted above, an EDD works well with a single sheet of paper up to a legal-sized sheet. However, if there are concerns about biological or chemical contamination, due to the condition of the questioned document, it is possible to use a second sheet of clean paper, e.g. copier paper, known to be free of indentations as a barrier between the platen and questioned document. Books and other multipage items can also be examined in some instances but these require additional preparation. Ideally, individual pages should be removed from such items but this is not always possible. As an alternative, a sheet of conductive material (e.g., aluminum foil) can be used to contact the conductive surface.
- Covering with charging film: As noted above, the charging film should cover the document completely and contact the platen all the way around the document. The charging film serves two purposes: it protects the document from subsequent toner application and it takes the electrostatic charge required to 'develop' any latent indentations on the document. The film should be laid over the document taking care to avoid wrinkles or excessive stretching of the film. Most EDD devices have a roll of bulk film positioned adjacent to the device for ease in application. The charging film will adhere to the platen due to suction applied through the surface of the platen.
Aside from visualizing indentations, the method can be used to detect the presence of fresh fingerprints, fiber disturbance on the paper surface (e.g. relating to mechanical erasure of information), or the presence of footwear impressions on paper.
Manufacturers
An EDD is a highly specialized device and there are only a few manufacturers:
- Foster + Freeman Ltd manufactures and markets the ESDA® (from Electrostatic Detection Apparatus) 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® (portable)
- Projectina manufactures the Docustat DS-220
- Lightning Powder Company, Inc. manufactures two units: the Electrostatic Vacuum Box and Vacu-Box™ for Indented Writing
- Kinderprint (now CSI Forensic Supply) used to manufacture the IMEDD® (from Indentation Materializer Electrostatic Document Device) but their website no longer lists this device in their product line