Hot Shock SEM Case Study

A client wanted to know whether or not a car’s headlights were on or off at the time of a crash in order to determine their liability in a car crash case, and came to us for hot shock analysis.

When an electric current is passed through the  coiled wire filament suspended inside a glass bulb, an incandescent lightbulb emits visible light. The tungsten-based filament will soften at high incandescent temperatures and will be stretched by the filament’s own inertia when the bulb is abruptly accelerated during a crash. This observable stretching phenomenon is called “hot shock” and discovering it indicates that a bulb was on at the time of impact. If the bulb were off at the time of a crash, the filament would be cold and rigid, which more often leads to a broken filament instead of a stretched filament, or no damage all to the filament. This is all observable by ocular microscopy or even the naked eye. 

At Vista Engineering, we take this a step further. When examined with a scanning electron microscope, beyond looking at the filament itself, glass or plastic particles that shatter on impact actually melt onto the filament. If glass or plastic particles are observed melted onto the filament, it is further proof that the filament was hot and the headlights were on. 

We need to know for sure whether or not headlights of a vehicle were on or off at the time of a crash. 

Analyze materials of a headlight under a scanning electron microscope to discover the answer. 

The client collected the evidence shortly after the accident and did not allow the vehicle to be powered back up before the headlamps were collected so the evidence was preserved.

Vista examined the tungsten coil just visually, with an ocular microscope, and with a scanning electron microscope.

Photos were taken and explanation provided describing how we found the evidence displayed signs of the headlights being on when the vehicle crashed.