The Kilpatrick Group, PA

Forensic Engineering | Accident Reconstruction | Product Liability Analysis

Lamp Filament Analysis

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Some motor vehicle accidents take place during the night time hours when natural sun light is not present. Questions may arise as to whether or not lamps were burning during a night time traffic accident. A driver may have failed to turn on the vehicle’s lights, one or more lamps could be burned out, the vehicle’s electrical system may be malfunctioning to the extent that it can not energizing the lamps to burn.

We have the technology and experience to perform a lamp filament analysis for a client if there are questions about a vehicle’s night time visibility and whether or not the vehicle’s lamps were burning.

We perform a Lamp Filament Analysis by observing the filament or filaments in a motor vehicle’s lamp after a collision has taken place. A lamp’s filament is manufactured into a straight and uniformly shaped linear coil that is made of Tungsten which has a melting point of approximately 6170°F. The filament’s tails are clamped to filament supports on either side of the filament. The filament and its supports are sealed inside of a glass bulb which is evacuated of ambient air and oxygen and is partially filled or pressurized with an inert gas, depending on the type of lamp and will not react with the tungsten filament. See the photograph above.

During a motor vehicle’s collision, impact forces are highest in the immediate vicinity of the collision contact area of a vehicle’s body. If this contact area is on or very near a lamp, the lamp’s filament will experience these high impact forces. A filament being a mass itself has inertia and will react to these forces. If the impact forces are large enough, the filament can be distorted or fractured and pulled apart. The glass bulb can also be fractured as well.

When a lamp is not energized, the tungsten filament is in a cold and brittle state. The cold and brittle fracture of a lamp’s filament is a phenomenon called cold shock. If the glass bulb is in-tact after a collision, the filament pieces will be captured inside of the glass bulb and will simply slide around the inside of the bulb when one spins or changes the bulbs orientation. The filament break points will have sharp and jagged edges indicating a cold brittle fracture which is evidence that the filament was not energized during the collision.

If the lamp is energized and burning, the filament becomes ductile due to its very high temperature. If the filament has ductile distortion where it is deformed into a significant bow shape or has been stretched or pulled to one side or the other between its supports where the filament coil is no longer straight and uniform and the bow or stretch is in the opposite direction of a vehicle’s collision force, then this phenomenon is called hot shock and indicates that the lamp’s filament was energized and burning during the collision. See the photograph above.

If the collision is severe enough, the impact forces can tear apart the filament into one or more pieces. The filament coil pieces themselves will have a stretched appearance and will usually be necked down near the break points due to soft stretching. The tip ends of the filament break points may also have a circular ball formation of molten tungsten that has cooled and solidified after its fracture. This is caused by an electric arc that jumps between the filament pieces during a filament tear. This arc creates intense heat which is hot enough to melt the tungsten forming the small molten tungsten balls on the tip ends of the filament which is another indication that the lamp’s filament was energized and burning during a collision. See the three photographs below. The hot filament pieces can contact the inner surface of the glass bulb, melt and stick or imbed itself into the glass material.

If the lamp is energized during a collision and the glass bulb is fractured, ambient air will enter the glass bulb, and the hot tungsten filament will oxidize immediately creating a film of yellowish-white tungsten oxide that will be deposited inside the glass bulb and can also be deposited on other inner bulb components such as another filament, the stem, filament supports or the seal and is very visible to the naked eye. The filament itself however will be burned black.

In some instances, if the glass bulb fractures while the lamp’s filament is energized, glass shards from the bulb can impact, stick to and melt to the filament. This can be seen by the naked eye and is again another indication that the lamp’s filament was energized during a collision.