Reasons to challenge digital evidence and electronic photography


	Print or Email This Page Informal Opinion July 2003, Page p42 Reasons to challenge digital evidence and electronic photography By Michael Cherry If digital evidence can be incorrectly altered or enhanced1 by newly trained personnel, and digital cameras2 and printers3�are not equal to their film counterparts in quality and color, what does that say about the quality of today’s forensic evidence which is transitioning to digital? Examples of digital weaknesses include: • Digital cameras do not accurately represent color. This can be important at a crime scene. � • Dye-sublimate digital printers can even confuse imaging experts. They cannot produce the highly accurate photographic images that film does, but their images appear to be photographs. They produce color and negative prints on photographic style paper that mimics the look and feel of photographs. � • In many instances, the digital printer used is not as accurate as the digital camera used, and therefore crime scene details and fingerprint minutiae is lost. • Even unsophisticated image enhancements can render some crime scene details and fingerprint minutiae unprintable. Dodge-and-burn, the selective lighting and darkening of areas within an image, can place details outside of the threshold of a digital printer’s range of light and dark printing capabilities. • The forensic community is incorrect regarding the acceptability of using traditional darkroom enhancement techniques on digital images: Traditional enhancement techniques are techniques that have direct counterparts in traditional darkrooms. They include brightness and contrast adjustment, color balancing, cropping, and dodging and burning. These traditional and acceptable forensic techniques are used to achieve an accurate recording of an event or object.”4�The transition to digital images requires a brand new level guidelines and training. In addition to the examples stated, there is a relevant challenge for any form of digital image or digital enhancement associated with audio, video and fingerprint images. The Iowa International Association for Identification (IAI) Web site highlights State v. Hayden, 950 P.2d 1024 (Wash. App. 1998),where the Washing-ton court of appeals noted experts’ claims that digital photographs are superior to regular film photographs because digital photographs can pick up and differentiate between many more colors and shades of gray than film photographs.5 Unfortunately this is not true, forensic quality film offers at least as many colors and more�shades of gray then digital images. I did not realize how rapidly the criminal forensics community was transitioning to the use of digital technology until I watched CBS News 60 Minutes II the Hidden Clue, “Detectives now have a new tool for cracking even the toughest of cases,” Jim Stewart reported. “Known as digital fingerprint enhancement,6 it’s become the silver bullet among police forensic units all across the country.” As a voting member of the evidentiary committee of The Association for Information and Image Management (AIIM)7�and a pioneer in image management and digital photography going back to early NASA days, I know it’s very difficult to perform a proper enhancement, particularly a fingerprint en-hancement. Digital enhancement is highly controversial within the imaging community. The product of a digital enhancement is a new image which is identical to old one, except for its altered characteristics. For example,�the red car image now has a twin, a blue car image. In the courtroom enhanced digital images are original images that have undergone some computer changes, and it falls to the discretion of a trial judge as to whether they are admissible as duplicates. I would like to classify enhanced digital images as enhanced digital images and not as originals or duplicates. I would also like to number enhanced images to readily identify their lineage. An image derived from the source image would be a first-order enhancement, an image derived from that image would be a second-order enhancement and so on. As two or more enhanced images can be spawned from the source image, I would like to see them alphabetized, e.g., image 2a and image 2b. Mathematical-enhancements include:�magnification, color substitution, or even more controversial, such as the “removal” of a thin nylon stocking or mask covering the features of a persons face. After removal, the nose shape, chin type and the presence or absence of a mustache or beard can often be determined. Mathematical enhancements can be quite powerful. Unfortunately, it is not uncommon for mathematical enhancements to be performed incorrectly as we are in a transitional period from analog to digital. Artistic enhancements can be used to do anything: selectively darken or lighten areas within an image (dodge-and-burn), place a person within a picture or video, as well as to modify conversations within an audio or video recording. Artistic enhancement can lead to pure fantasy. All enhancements should be challenged as they require very precise steps and newly trained personnel may find them difficult to understand or implement. This is particularly true of the audio, video and fingerprint enhancements. In addition it is not uncommon to see artistic and mathematical enhancements on the same newly created image. On the positive side, scientists are having some success in enhancing low quality video tape commonly found in gas stations and convenience stores. While not necessarily ready for the courtroom, the results of these improvements can be very useful in determining the probable absence or presence of a specific person. Some concluding thoughts: �• Are there known quality problems associated with the digital printer, scanner or camera used? � • If enhanced images are introduced, were the enhancements correctly done? Can they be repeated using a different person? � • Enhancements of enhancements should be not be allowed as they are unnecessarily misleading. • Digitally enhanced pictures should be identified as such. • If digital images are compressed, was a loss-free method used? If not, why not? � • Digital images should be challenged when the original image is not available for comparison. • During today’s cost-driven transition period to digital, both the quality of the images and the experts tend to be inferior to their analog counterparts. � • Text files that describe the evidence should be properly safeguarded. Notes 1. The product of a digital enhancement is a new image which is identical to old one, except for its altered characteristics. For example, the red car image now has a twin, a blue car image. 2. Kodak T-MAX 100 can resolve approximately 200 line pair per mm TOC 1000:1, Kodak Web Site, 200 pixels/mm at 35mm resolution 3620024200 = 34,560,000 or 34 Megapixels In conventional digital cameras systems color filters are applied to a single layer of photo-detectors in a tilted mosaic pattern. The filters let only one wavelength of light - red, green or blue - pass through to any given pixel,* allowing it record only one color. As a result, typical mosaic sensors capture 50% of the green and only 25% of each of the blue and red light. The approach has inherent drawbacks, no matter how many pixels a mosaic-based image sensor might contain. Since they only capture one third of the color, mosaic-based image sensors must rely on complex processing to the two-thirds they miss. Not only does this slow down the speed of image rendering, interpolation also leads to color artifacts and a loss of image detail. Some cameras even intentionally blur pictures to reduce color artifacts. http: //www.pctechguide.com/. 3. Computer printers print at 300 to 2400 dots per inch. Film requires at least 8000 dots per inch. Sales terminology can be misleading; 4800 and 5760 optimized dots per inch (dpi) are used to describe printers that improve the appearance of basic 1200 x 1200 dpi images. These printers can not accurately print 4800 or 5760 dpi input images. Some drum scanners can scan 35mm film at 11,000 dpi. Bob Myers, Heidelberg USA, Inc. 4.Forensic Science Communications,�January 2003 Volume 5 Number 1, Standards and Guidelines, Recommendations and Guidelines for the Use of Digital Image Processing in the Criminal Justice System Scientific Working Group on Imaging Technologies (SWGIT) Version 1.2, June 2002. 5.Iowa Division International Association for Identification (Iowa IAI) in describing State of Washington v. Hayden (Wash Ct App, DivI, 2/17/98). http: //www.geocities.com/cfpdlab/perspect.htm 6. The product of a digital enhancement is an identical twin image, except for its altered characteristics. Examples include a red gun instead of a blue gun or the removal of an extraneous pattern, such as the weave of a bed sheet, to make the new fingerprint image more apparent. 7. AIIM holds the secretariat for International Standards Organization (ISO) ISO/TC 171 SC2, Document Imaging Applications, Application Issues. AIIM is also the administrator for the U. S. Technical Advisory Group (TAG) to ISO TC 171, Document Imaging Applications that represents the United States at international meetings. Over 80 of AIIM’s standards, recommended practices and technical reports have been drafted and approved by the American National Standards Institute (ANSI).
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