Case of the Month: Hot Shock Light Bulb Examination

Jun 1, 2017 by

It was a dark morning before sunrise. One car rear-ended another. The allegation was that the lead car was driving with its lights off, so the striking car could not see the lead car until it was too late. GEI was brought in to shed some illumination upon the situation.

Our expert inspected the lead vehicle. It sustained major damages from a rear impact.

The trunk lid, rear panel and floor, taillight assemblies, rear quarter panels, and rear bumper assembly were heavily damaged.

The left and right side taillight assemblies each included three bulbs per side for turn signal, brake, running light, and backup functions.

There is a general principle for determining if a bulb with a filament was on or off at the moment of impact. If a light is off then the spring shaped filament is cold. If a cold filament is subjected to a violent shock, such as a collision, the hard brittle filament will break sharply and will generally not be bent or deformed from its own weight. In contrast, if the light is on, the filament will be hot. A hot filament is much softer, more flexible, and more pliable. If a hot filament is subjected to a violent shock, the filament will elongate and stretch under its own weight. When it cools, it does not return to its original shape, it cools in that stretched or elongated shape.

The left taillight lens and housing were examined. The left side outer bulb had an orange globe and the orange paint was deteriorating. The bulb was still intact with white discoloration on the inner surface.

This discoloration was a result of the water on the damaged filaments. The condition of the filaments indicated that the bulb was illuminated at the time of impact.

The left side middle bulb was broken. The lower of the two filaments was mostly missing; the break pattern appeared that the filament was cold (not illuminated) at the time of impact. The upper filament was found distorted, indicating it was illuminated at, or shortly after, its glass globe was broken.

The left inner bulb (back-up light) was found with its globe intact and showed no distortion, indicating it was not illuminated at the time of impact. This was an indication that the lead vehicle was not backing up when it was struck.

The right taillight lens and housing were also examined.

The right side outer bulb was heavily damaged. The lower filament (brake or turn filament) had broken off.

It could not be determined if the lower filament was illuminated at the time of impact. The upper filament was distorted and stretched, which indicated that it was illuminated at the time of impact. The filament stanchion (or base) was found broken and melted.

The right side middle bulb was missing, as was the right inner bulb (back-up light).

The Center High Mount Stop Light (CHMSL) bulbs were removed by an unknown party and were not available for inspection.

The headlight switch and wiring were inspected and appeared to be in working order. The switch, fuses, and wires were found in good condition with the wiring connected properly. A National Highway Traffic Safety Administration recall, related to the possibility of a problem within the headlight switch circuit breaker that could cause the headlights to flash on and off, did not cause or contribute to the incident. The manufacturer’s website showed no open recalls on the vehicle related to the rear taillights.

In summary,

1) Some of the bulbs were removed prior to inspection.

2) Some of the bulbs were not illuminated at the time of the crash.

3) Some of the bulbs were illuminated at the time of the crash.

Therefore the lead vehicle taillights were on at the time of the impact and the lead vehicle driver did not contribute to the accident by driving in the dark without lights.

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