Physical Evidence Handbook

Forensic Imaging Unit

Forensic ImagingThe Forensic Imaging unit provides analysis of photographic materials and other multimedia evidence using photographic and electronic imaging techniques.

The Imaging unit differs from the other laboratory units in that much of its work is in support of other units, as opposed to cases received directly from submitters. The Imaging unit documents evidence as needed to illustrate analytical findings. This includes images depicting the condition of an object at the time of receipt (e.g., clothing, vehicles, etc.), documenting the location and condition of items of interest (e.g., transfers left behind on a hit-and-run vehicle), or recording the results of analytical investigation (e.g., developed latent prints, etc.).

Digital Imaging is currently available to law enforcement agencies and more evidence is being captured with this technology. The forensic imaging unit also uses digital technology to analyze images captured with digital and analogue systems and submitted to the unit for imaging processing.

Approximately 90% of the Imaging case submissions are provided by the Identification unit. Fingerprint lifts, inked fingerprint cards, footwear impressions and items processed with ninhydrin and other chemical latent print development techniques are representative of the kinds of items received.

Ninhydrin-developed print

However, there is no limit as to what can be touched--and therefore could contain a print. Sometimes the method used to make the print visible introduces special challenges to imaging. Fingerprints developed with silver powder or super glue (cyanoacrylate) which have white ridge detail can require color reversal for comparison to standard ink fingerprint cards. Transparent media bearing fingerprints (glass, plastic bags) require special lighting techniques.

Transmitted Light Photograph
transmitted light photography

All of the photographs produced of fingerprints, palm prints and footwear must include a scale to facilitate reproduction at one to one or life size for comparison. Court charts are prepared to support expert testimony by fingerprint analysts.

Document Unit Support
Record photographs are taken of document evidence to show its original condition prior to chemical or other potentially destructive examinations. In addition, record photographs are made of items which may be the subject of court presentation. Enlargements at twice life size (2X) are used to illustrate the expert testimony of the document examiner.

Sometimes an attempt is made to alter or obliterate writing by over-writing with ink or using "Wite·Out" (correction fluid) to change the content or meaning of a document. The original writing may be restored by photography using infrared, ultraviolet or laser light photography.

Obliterated Writing
Luminescence photograph reveals obliterated writing "hidden" by correction fluid

The Imaging unit also gives technical support on digital imaging using the VSC-1 system. This system provides capabilities for infrared, infrared luminescence and ultraviolet videography. Images are captured using a video camera and the appropriate filters for the technique utilized. A digital frame grabber is used to capture the images and these images are used to produce photographic prints as needed.

Videotape Evidence
Of the types of cases that the Imaging unit receives directly from submitters, the largest number are requests for videotape examination. Tapes are examined using time-lapse videotape recorders to produce VHS videotape copies. These copies can be played on standard videotape recorders or used as court exhibits or investigative aids in criminal cases. Preserving the original by working from a copy can be crucial to preserving the record. In several cases, prior to submission the important part of a tape (i.e., the crime) has been so repeatedly viewed that its quality has been badly degraded--occasionally to the point of rendering the tape of little value. By immediately making one or more copies and using them to view the action of interest, the original is protected from damage.

A frame grabber can also be used to capture individual "still" images from videotapes. After adjusting contrast and sharpening images using industry-standard commercial software, the final product is printed to paper or film.

Patterned Injury Evidence
The Imaging unit is requested by the Medical Examiner to photograph patterned injuries, such as bite marks, at point of autopsy. The photographic reproductions are produced at life size, one to one scale, on color print and color transparency materials. These photographic exhibits are examined by a forensic odontologist to determine if the injury is a bite mark. When a suspect is identified, the Imaging unit will assist the forensic odontologist in doing a photographic work-up on the suspect's teeth. If a positive match occurs after the dental work-up, the Imaging unit will provide court displays as required.

Photographs Provided By The Crime Laboratory (Past and Present)
Photographs taken by the Crime Laboratory prior to 2007 in Wausau and 2010 in Madison and Milwaukee were captured on film. The crime lab photographers bracketed exposures when capturing images on film and only printed a representative sample of the photographs believed to document the relevant part(s) of the crime scene. Non crime scene related photographs taken by the Crime Laboratory Bureau were distributed to our submitters only upon request. The reason for these practices was the high cost and time involved in printing the individual photographs especially those containing bracketed exposures of the same image.

Starting in 2007 in Wausau and 2010 in Madison, the conversion to digital imaging changed distribution of crime scene photographs taken by the Crime Laboratory. With implementation of digital photography, all crime scene photographs are currently written to a CD/DVD in a PDF file unless the submitter states no photographs are needed. In addition to the CD/DVD, a printed set of index prints is provided. If additional photographs are needed including enlargements for court, please contact the Crime Laboratory in your service area at least 30 days in advance. This lead time is especially important if the photographs were originally taken on film which would require scanning in the negatives to create a digital file for printing.