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Dimensional Review of Scales for Forensic Photography

NCJ Number
Massimiliano Ferrucci; Theodore D. Doiron; Robert Thompson; John P. Jones II; Susan M. Ballou; Janice A. Neiman
Date Published
August 2013
48 pages
This study evaluated the quality of commercially available photo scales that provide a geometrical reference in the photographic documentation of evidence, documents manufacturing processes, and suggests procedures for establishing standards for forensic photo scales that will ensure accuracy and user confidence.
The results from each test indicate that all vendors satisfied the length scale specification as written in the ABFO No. 2 technical note; however, the measurements of internal and external circle diameters and center-to-center distance showed a lack of adherence to specifications. Still, the results for each vendor were consistent. The testing for perpendicularity of the two legs showed that more than half of the rulers tested were off from the ABFO No. 2 specification. In addition, there was inconsistency between rulers of the same vendor. The 4-year revisit found minimal changes in length scale, internal and external circle diameters, and center-to-center distance. Perpendicularity, on the other hand, exhibited significant change. Having a master rule with the marks within 0.1 mm of their nominal position is a reasonable requirement. There are also a number of accredited laboratories that can calibrate these master rules to the required accuracy. Plastic scales cannot be expected to hold this accuracy in actual use. A new standard could be used to better define the materials allowed for scales, criteria for the dimensions and legibility of markings, and optimizing targets for photography. 3 notes, 34 figures, and 7 tables