Video Evidence (World of Forensic Science)
The use of surveillance cameras and closed-circuit television (CCTV) for security and crime prevention has been growing in recent years. Shopping malls, car parks, offices, airports, and many other
When collecting video evidence, the investigators must take as much care as they would in collecting any other form of evidence. Videotapes can readily be wiped or recorded over, so the first task is to preserve the evidence from a camera by preventing this. For analog video evidence, the record tab must be removed or moved to a saved position. For digital video evidence, write protection has to be in place. The chain of custody of the evidence, from collecting the tape from the camera to its receipt in the processing lab, must be carefully adhered to, because questions may be asked in court about whether the video evidence could have been tampered with. Storage should be in a climate-controlled room, because extremes of temperature can damage a video tape.
The images from a surveillance camera or closed-circuit TV system are often blurred, grainy, and of low resolution. Lighting conditions, tape wear, and deficiencies in the camera system all contribute to poor quality pictures. Enhancing such images, without altering them, is challenging. The effort may, however, be well worthwhile if a crucial car number plate or a suspect may thereby be identified.
The video analysis lab will contain a monitor that can produce large images from the tape, a playback deck, a printer, and equipment that can digitize the signal from the original tape so that it can be processed by a computer. Before any analysis is actually carried out, the integrity of the tape should be reviewed and careful notes made of any damage. The video evidence must be protected throughout from external hazards such as magnetic fields or static electric charges that may harm it. It is also important not to over-play the tape, as this can also impair its quality.
There are various software packages that can enhance an image from a video camera and present them either as video tape, still images, or prints for the court. There are many image formats that can be used to do this work, but one of the most popular is the tagged image file format (.tif file). Everything the forensic video analysis technician does to the image must be carefully recorded, because this is sure to be questioned in court. Computer images can be readily manipulated and so everything that has been done to the evidence must be accounted for so that its integrity is preserved.
If a suspect has been detained, video evidence can be used to help identify them. They can be taken back to the original location of the camera and rerecorded standing or walking in the same position. This second image can be compared with the original and an identification or an elimination can often be usefully made. The original image can also be used to give an idea of the actual height and size of a suspect.
Video evidence played an important role in the investigation of the murder of two-year-old James Bulger in Liverpool, England, in 1993. A surveillance camera in a shopping mall clearly shows the child being separated from his mother and then being led to his death by the two boys who were later convicted of the killing. The tape was repeatedly shown on television and its poignancy has helped fix this especially tragic case in the memory of the British public. In another case, video footage from cameras in a West London shopping mall was intensively studied by police to solve the doorstep shooting of TV presenter Jill Dando in 1999. Although the key suspect did not appear in these images, they were a powerful aid to reconstruction of the crime as they provided sharp, clear images of much of the last hour of Miss Dando's life. Another famous piece of video evidence is the recording of Diana, Princess of Wales, leaving a hotel in Paris just minutes before the car accident that was to end her life in 1997.
SEE ALSO Digital imaging.