Light Bulbs and Filaments: Examination of

When road traffic accidents occur at night time, the question whether the lights of one of the vehicles were on or off at the time of the accident is sometimes raised. In such instances, the questioned light bulb(s) may be forensically examined and it may be possible to determine whether the bulb was energized or not at the time of the glass breakage and/or the filament rupture. A complete forensic examination of light bulbs must also include the accident scene and vehicle examinations. The circumstances around the case must be clearly integrated in the interpretation of the results, otherwise a wrong conclusion may be reached. The electrical circuit of the vehicle must be understood and evaluated. Then, the light bulb assemblies are observed and, whenever significant, collected as evidence. The rest of the examination takes place at the crime laboratory, where the light bulb is electrically tested (resistance measurement) and observed under magnification with an optical and possibly an electronic microscope. The criminalist must comprehend the failure mechanisms of a tungsten filament in order to properly interpret the evidence observed on the filaments. By following a clear interpretation scheme, an educated conclusion can be reached in most instances. Keywords: road traffic accident; light assemblies; tungsten oxide; glass bead; rupture; fracture surface; glass breakage; halogen; quartz