Bulb Examination for On/Off in Traffic Accidents


In many traffic accidents each year, the probative question may be whether one or both vehicles involved had their headlamps illuminated at the time of the crash. Most incandescent bulbs, including automotive headlamps, turn signals and brake lamps utilize a tungsten filament to create illumination. A current is passed between the two ends of the tungsten filament causing the filament to heat up and become incandescent. The physical properties of tungsten filaments allow forensic examiners to make a determination whether a bulb was on or off at the time of an impact. These properties include: ductility and oxidation.


Tungsten filaments are very brittle when cold and pliable when hot. A bulb that is off at the time of an impact near the bulb will often display a filament with a jagged fracture pattern. A bulb that is on at the time of an impact near the bulb will be ductile and may show deformation of the filament


Tungsten filaments, when incandescent, oxidize readily on exposure to air. The purpose of the glass envelope surrounding the filament in an incandescent bulb is to prevent such oxidation. If a bulb is off at the time of the glass envelope being cracked or broken, no filament oxidation will occur. If a filament is incandescent at the time the glass envelope is cracked or broken, the filament will begin to oxidize, forming tungsten oxide.

The scientists at Forensic Science Consultants have considerable experience and have been admitted as expert witnesses numerous times in the examination of bulbs for on/off in traffic accidents. We utilize state-of-the-art techniques such as scanning electron microscopy and photo-stereomicroscopy to document filament deformation and oxidation.



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