Whether a vehicle’s lights were on or off can affect liability for a nighttime car crash. Fortunately, there is often physical evidence of whether a vehicle’s lights were on or off at the time of a collision.
An incandescent lightbulb emits visible light when an electric current is passed through the finely coiled wire filament suspended inside its glass bulb. The dense tungsten-based filament softens at hot incandescent temperatures and can be stretched by the filament’s own inertia when the bulb is abruptly accelerated during a crash. This stretching phenomenon is called “hot shock” and its presence indicates that a bulb was on at the time of impact. If instead the bulb were off at impact, the filament would be cold (less soft and deformable), which can lead to a broken filament rather than a stretched filament, or no damage at all to the filament.
The figure below shows digital microscope images of a bulb with hot shock compared to an undamaged exemplar bulb. This examination was done non-destructively by carefully looking at the filament through the glass envelope of the bulb. The stretching of the filament and uneven coil spacing show that this filament was energized at the time of impact. In contrast, the undamaged bulb has uniform coil spacing and no evidence of stretching.
Figure 1: Digital microscope images of a bulb with hot shock (Bulb A, Inset A) compared to an undamaged exemplar bulb (B, Inset B). The white arrows show regions of filament stretching in the hot shocked bulb.
Some caution and experience are needed to properly interpret bulb filaments following a crash. Hot shock can be confused with “age sag”, where the filaments of old bulbs can droop downward and stretch after prolonged use. Also, the lack of hot shock does not necessarily mean that a bulb was not on at impact: it may simply mean that the acceleration was not large enough to deform the filament.
This type of forensic lightbulb analysis can be performed on headlights, brakelights and other incandescent bulbs with filaments. Some modern lighting systems that include light emitting diodes (LEDs) do not have heavy coiled filaments and therefore do not yield the same kind of forensic information as incandescent bulbs. Lightbulb evidence is also very fragile and should be captured early by examining and/or carefully extracting the bulbs soon after the crash. Attempts to energize a car’s electrical system to test other vehicle or safety systems can spoil lightbulb filaments and therefore should not be done until the lightbulb evidence has been preserved.
 JS Baker, LB Fricke, KS Baker, TL Aycock, Lamp Examination for ON or OFF in Vehicle Collisions,Northwestern University Center for Public Safety, (2003)