Electrostatic detection device

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.

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.[1] 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.[2] This led to models like the 'Thickness Variation Theory' and the 'Surface Variation Theory'.[3] 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[4] and 1999[5] 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"[4] and is appropriately called the 'charge transport model'.

Subsequent testing[6],[7] 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:

  1. 'pure' indentations caused by impact printing, for example, may not produce good EDD results
  2. indirect secondary 'impressions' may appear that are caused by lateral relative motion between two sheets of paper when the source sheet bears significant embossing.[8]

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.[9]

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:[10]

  1. 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.[11] 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.[12]
  2. 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.
  3. 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.[13]
  1. Humidification: an EDD works best when the relative humidity is less than 60%.[11] Some studies have also suggested that absolute humidity is important.[14] 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.[15]
  2. 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.[16] 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.
  1. Type of document: as noted above, an EDD works well with a single sheet of paper up to a legal-sized sheet.[18] 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.[11]
  2. 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.[19]
    • If indentations are developed from the FFU test sample, the EDD can be considered as functioning correctly.
    • If no indentations developed from the FFU test sample, the EDD is not working.
    • If the FFU test was positive and no indentations are developed in the area of the questioned document, then the assessment will be that no detectable indentations have been developed. It is important to note that this does not mean the questioned document was never in contact with other items of interest. The absence of detectable indentations may be due to one or more limitations causing the indentations to not be detected.
    • Note that the presence of indentations in the document area is prima facie evidence that detectable indentations exist such that the FFU test is redundant.

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[24] on paper.

Manufacturers

An EDD is a highly specialized device and there are only a few manufacturers:

  1. Foster + Freeman Ltd manufactures and markets the ESDA® (from Electrostatic Detection Apparatus) which is the original such device developed in 1978.[25] It was designed to meet the requirements of the UK Home Office and is still in production today in two different forms: the ESDA2®[26] and ESDA-Lite® (portable)[27]
  2. Projectina manufactures the Docustat DS-220
  3. Lightning Powder Company, Inc. manufactures two units: the Electrostatic Vacuum Box[28] and Vacu-Box™ for Indented Writing[29]
  4. Kinderprint (now CSI Forensic Supply[30]) used to manufacture the IMEDD® (from Indentation Materializer Electrostatic Document Device) but their website no longer lists this device in their product line

Notes

  1. The sourcing/matching of indentations must be interpreted in the context of the investigative scenario. That is, indentations and source writing that 'match' indicate only that pieces of paper were in physical juxtaposition at the time the writing occurred.
  2. Seward, GH (1999), "Practical Implications of Charge Transport Model for Electrostatic Detection Apparatus (ESDA)", Journal of Forensic Sciences 44 (4), doi:10.1520/JFS14560J, Foster and Morantz attributed the ESDA image of indented writing to a change in capacitance of the Mylar-paper-platen structure (2). This change in capacitance was theoretically due to the compression of the paper.
  3. Wanxiang, L; C. Xiaoling (1988), "A study of the principle of the electrostatic imaging technique", Journal of Forensic Sciences 28: 237–242, doi:10.1016/s0015-7368(88)72841-8 Cite uses deprecated parameter |coauthors= (help)
  4. 1 2 Seward, GH (1998), "Model for electrostatic imaging of forensic evidence via discharge through Mylar-paper path", Journal of Applied Physics 83 (3): 71–76, doi:10.1063/1.366849, ISSN 0021-8979
  5. Seward, GH (1999), "Practical Implications of Charge Transport Model for Electrostatic Detection Apparatus (ESDA)", Journal of Forensic Sciences 44 (4), doi:10.1520/JFS14560J
  6. Yaraskavitch, Luke; Matthew Graydon, Tobin Tanaka, Lay-Keow Ng (2008), "Controlled electrostatic methodology for imaging indentations in documents", Forensic Science International 177 (2): 97–104, doi:10.1016/j.forsciint.2007.11.004, In contrast with Seward's findings, a period of charge decay before image development is not required when operating in this optimal regime. Cite uses deprecated parameter |coauthors= (help)
  7. Tanaka, TA (2000), "An Evaluation of the Methods for Optimal Development of Indented Writing as Suggested by the Seward Method", Proceedings of 58th Annual Meeting ASQDE (Ottawa, Ontario) line feed character in |title= at position 57 (help)
  8. Strach, Steven J.; Graeme M. McCormack, Robert W. Radley, Paul D. Westwood (1995), "Secondary impressions of writing detected by ESDA", Forensic Science International 74 (3): 193–204, doi:10.1016/0379-0738(95)01761-7, Experiments have been performed which show conclusively that, under certain conditions, ‘impressions’ of writing can be detected by ESDA (ElectroStatic Detection Apparatus) on paper which has been placed in contact with the reverse of paper bearing writing which has significantly embossed the back. Such indirect ‘secondary’ impressions are caused by some degree of lateral relative motion of the two sheets of paper. Cite uses deprecated parameter |coauthors= (help)
  9. Strach, Steven J. (Sep 2007), "Writing impressions revealed by scanners", Forensic Science, Medicine, and Pathology 3 (3): 210–216, doi:10.1007/s12024-007-0015-4
  10. 1 2 ASTM Standard E 2291 – 03, 2003, "Standard Guide for Indentation Examinations," ASTM International, West Conshohocken, PA, 2003, www.astm.org.
  11. 1 2 3 4 Foster + Freeman ESDA-2 User Manual 14-06, July 2008 (UK/US)
  12. Horan, G.J.; Horan, J.J. (November 1988). "How long after writing can an ESDA image be developed?". Forensic Science International 39 (2): 119–125. doi:10.1016/0379-0738(88)90084-9.
  13. MacVicar, R. (December 6, 2007). "Examination for Indentations Information" (PDF). Investigators & Submitters, Technical Information Sheets. Centre of Forensic Sciences, Ministry of Community Safety and Correctional Services, Ontario, Canada. Retrieved January 20, 2011. Fingerprinting using ninhydrin and other solvents inhibits the development of indentations
  14. Pearse, M.L.; J.S. Brennan (2 December 1996). "Importance of absolute humidity in the operation of the electrostatic detection apparatus". Forensic Science International 83 (2): 121–131. doi:10.1016/s0379-0738(96)02026-9. However, we have further shown that relative humidity alone is not the primary determinant of the quality of ESDA results; at high levels of relative humidity and low temperatures there is a significant reduction in the quality of results and at very low relative humidity and high temperatures acceptable results can be obtained. These data strongly suggest that it is the absolute humidity of the atmosphere used that determines the quality of the ESDA results.
  15. The Foster + Freeman ESDA2 user manual states "Over-humidification may have the same effect as wetting the document with a liquid. The ESDA image may be degraded or obliterated."
  16. Purdy, Dan (2005). "Gradient® - a device for producing uniform EDD test strips". Proceedings of the 2005 ASQDE Annual Conference. Montreal, Quebec: American Society of Questioned Document Examiners. This poster describes a device capable of producing control impressions in paper. Each impression varies in depth from 0.000" to 0.002". These controls or test strips not only verify when equipment and experimental conditions are satisfactory but they can also be used to quantify EDD results, optimize experimental conditions or develop protocols that optimize results and ensure critical evidence is not overlooked. line feed character in |quote= at position 82 (help)
  17. Some manufacturers refer to this as an "imaging film"
  18. The platen size varies by device. As an example, the platen on the Foster + Freeman ESDA is approximately 24cm x 42 cm
  19. Many systems use a sintered brass plate for the platen but any porous and conductive surface would, in theory, work for this purpose
  20. A single strand of wire through which a high voltage (-8 kV) electrical charge is passed similar to a corona wire used in some electrophotographic devices. Some manufacturers refer to this as a 'corona wand'.
  21. The carrier material and toner composition is carefully designed based upon their triboelectric properties to ensure proper deposition.
  22. Some manufacturers have built in a 'catch tray' for this purpose.
  23. Some manufacturers refer to this as a toner application device, or TAD.
  24. Craig, C.L.; Hornsby, B.M.; Riles, M. (July 2006). "Evaluation and Comparison of the Electrostatic Dust Print Lifter and the Electrostatic Detection Apparatus2 on the Development of Footwear Impressions on Paper". Journal of Forensic Sciences 51 (4): 819–826. doi:10.1111/j.1556-4029.2006.00173.x.
  25. Foster, D.J.; Morantz, D.J. (1979), "An Electrostatic Imaging Technique for the Detection of Indented Impressions in Documents", Journal of the Forensic Science Society 13: 51–54, doi:10.1016/0379-0738(79)90262-7
  26. "ESDA2". Foster + Freeman Ltd. Retrieved 17 January 2011.
  27. "ESDA-Lite". Foster + Freeman Ltd. Retrieved 17 January 2011.
  28. "Redwop: Electrostatic Vacumm Box". Lightning Powder Company Inc. Retrieved 17 January 2011.
  29. "Redwop: Vacu-Box™ for Indented Writing". Lightning Powder Company Inc. Retrieved 17 January 2011.
  30. "About Us". CSI Forensic Supply. Retrieved 17 January 2011.

External links

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