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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.〔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.〕 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.〔 〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Electrostatic detection device」の詳細全文を読む スポンサード リンク
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