Forensic Trace Photography

Introduction

Photographically securing visible or chemically treated fingerprints on the surface of cylindrical, curved, or reflective evidence presents a number of difficulties and therefore often involves considerable effort.

Cylindrical exhibits in particular, such as cartridge cases, glasses, bottles, ampoules, etc., pose a challenge because the depth of field of the photographic image decreases as the magnification increases. The conversion of a curved surface into a two-dimensional “photographic” image inevitably results in a geometrically incorrect image.

Conventional photographic recording methods can only capture such trace images in sections and with geometric errors. Therefore, comparing individual features in several images does not provide an optimal basis for the subsequent dactyloscopic evaluation of a trace.

Fingerprints or grip marks that extend over a large section of a cylinder shell cannot, by their very nature, be viewed continuously when viewed from the front, because the part of the mark facing away from the viewer is obscured by the trace carrier itself on the circular arc.

Examples

The ForensiScan Trace Scanner

The ForensiScan is a system specially developed for forensic evidence collection that allows visible traces on any surface to be captured photographically. Particular attention is paid to securing traces, such as fingerprints, on cylindrical objects such as glasses, bottles, and cartridge cases, as well as on reflective surfaces such as chip cards, mirrors, or CDs.

In order to capture fingerprints or other traces on cylindrical surfaces geometrically correctly, the ForensiScan scans the surface of an object line by line with high precision. The required accuracy of the scanning process is achieved by stepping motor-controlled rotation of the object being examined in the focal plane of the ForensiScan camera. The ForensiScan software automatically combines the line images to form a geometrically accurate representation of the scanned surface.

In addition to cylindrical objects, the ForensiScan can also be used to examine flat surfaces. To do this, the to be examined evidence is moved along the camera’s focal plane using a linear feed. This method is particularly suitable for photographing highly reflective or transparent objects such as credit cards, CDs, or knife blades.

Precise positioning of the object and camera, as well as the fully automated scanning process, ensure productive working methods with excellent results and high photographic resolution.