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EVERY PICTURE TELLS A STORY - DON'T IT?

People who investigate traffic accidents must rely on a variety of information. The optimum would be to personally examine the vehicles and site shortly after the incident, documenting and relevant details with measurements and photographs. At the same time, the attending police documentation and photographs could be obtained, as well as driver and independent witness statements. The minimum often appears to change with each new incident, but usually some information regarding vehicle damage and/or vehicle positions is available.

Technology has played a major role in the preservation and distribution of evidence. The most recent advancement is a prevalent use of colour laser copies. This has allowed the fast and wide distribution of the information shown on colour photographs. Colour laser copying works by first scanning the photograph, and then digitally mapping the colour and patterns, which are then put onto the paper by laser or bubble jet, etc. The higher quality copiers have scanners with a resolution of 400 + dots per inc. (DPI) meaning that each square inch is broken up into 400 dots, and the colour of each dot is read. The more advanced copies utilize "24 bit" colour, meaning that the scanner can capture images at 256 gradations of each colour, yielding 16 million colours. The colour information is digitally recorded as a specific combination of black, magenta (red), cyan (blue), and yellow. To output the image, the digital colour information regarding each dot is relayed to the laser, etc., to reproduce the colour map onto the output paper. The result is normally very good copies, which the copier manufacturer's indicate as "near photographic quality".

However, for some legal purposes "near" may not be sufficient. In order to investigate this, we conducted an informal test consisting of placing a series of dark and light colour "x's" on a roadway surface. Photographs were taken and then developed in 4x6 and 5x7 inch sizes which, in turn, were used to produce laser copies. We then evaluated the amount of information visible in each picture or copy respectively. The results demonstrated that the larger photographs displayed the most information (82.3%), with the smaller photographs slightly worse (79.3%), the larger laser colour copy slightly worse yet (75.1%), and the smaller laser colour copy the worst of all (70.3%). Note that the numbers are applicable for this specific test only, however, the general trend is clear.

The results indicate that colour laser copies would be acceptable for general images of overall vehicle damage and scene information, however, only original or reprinted photographs should be used for examination of scene evidence for skidmarks, road scrapes, etc., and for examining vehicles for very subtle areas of contact such as paint transfer.

Even before these tests, this lesson was brought home to me by a fellow employee. He was reviewing the opposing side's engineering report, which relied heavily on scene evidence to position the vehicles after the impact. The report contained laser copies of the original report photographs, and our engineer wanted the original photographs to examine. The other side refused. A court order followed. Soon after, the original photographs arrived at our office. Shortly after, the engineer ran out of his office yelling "Eureka"! The original photographs revealed a skidmark not visible on the laser copies. The skidmark was not consistent with the vehicle position indicated in the opposing report, providing our engineer with the evidence he required. The lesson to be learned is that even a colour laser copy can tell a story, however, in some cases it may turn out to be a short story.

 
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Walters Forensic Engineering | 277 Wellington Street West, Suite 800 | Toronto, ON M5V 3H2
Information contact: engineering@waltersforensic.com