Ruth A. Mendonsa
Any "whodunit" fan is familiar with the scene where the sleuth uncovers a vital clue from a pad of paper by rubbing it with a lead pencil to bring out the impressions of handwriting from the previous page. If only it were always that simple.
Although this method of detection is still in use, today the suspect document is processed with an electrostatic detection apparatus that uses photocopier toner to develop the areas of indentation after a document has been covered with an electrically charged plastic film. In the best of cases, when the document is processed with the electrostatic device and the toner image is developed, the writing impressions are clearly defined and highly contrasted. If the developed writing is to be used in an investigation or to demonstrate results for the courts, the examiner must turn to electronic image processing technology for further enhancement.
The digital domain
The Los Angeles Sheriff's Department crime lab images a lot of threatening letters and forgeries. It has brought the electrostatic detection apparatus technique into the digital domain with CLAB-2000, a mobile video-to-digital workstation from Mideo Systems of Huntington Beach, Calif. These two instruments allow superior visualization of latent indented writing impressions.
The CLAB-2000 is useful in the examination of alterations, obliterations, credit card embossers and rubber stamps. The workstation has an articulated arm with two flexible high-intensity fiber optic light pipes. At the end of the arm is a charge-coupled device (CCD) video camera with a manually adjustable macro zoom lens. The camera's output is sent into a desktop computer's frame grabber board, digitized, saved as a TIFF and stored on the disk. In those cases where the image needs digital enhancement, it is exported to an image processing software program. What follows is a labor-intensive job of processing the image with image processing software that requires a skilled computer operator.
Streamlining the process
The crime lab has streamlined the process and increased the efficiency and accuracy of the document examiner's efforts with the DEI-750 three-CCD camera from Goleta, Calif.-based Optronics Engineering Inc. Besides spatial resolution of 750 lines, the camera provides real-time video-rate digital image processing and manual control over the exposure time. Controlled by the camera's keyboard, it allows a document examiner to perform in minutes the same level of processing that once took hours. In addition, the manual control over exposure time allows the examiner to focus optimal illumination in the area of interest.
The crime lab staff said that the DEI-750 has significantly increased the speed and ease with which its images are processed. It plans more testing and evaluation of the Mideo CLAB-2000 workstation with this camera configuration for ballistics, trace evidence and serology for viewing fluids, fibers, hair and other hard-to-work-with samples.