The filaments of some vehicle light bulbs can be deformed by the extremely large accelerations involved in collisions. For incandescent light bulbs, a filament becomes more prone to deformation when the bulb is lit because the filament is extremely hot. We can examine the filaments of headlights, brake lights, turn signals and running lights to see if they were affected by the accident. A light bulb filament can be undeformed, mildly deformed, extensively deformed, etc. The amount of deformation is affected by proximity of the light bulb to the area of direct impact, the type of bulb, the amount of time the bulb was lit, etc. A lack of deformation may not be definitive that a light bulb was not lit when the vehicle was struck, but a bulb that is extensively deformed is a clear indication that a bulb was lit during the collision.