On or off?

Occasionally, a valuable piece of information during a crime scene investigation is whether or not a light was on at the time of the incident. This is commonly required during traffic accident investigations. For instance, one vehicle collides with the rear of another vehicle at night. The driver of the rear vehicle states that the taillights of the vehicle in front weren’t working. Another scenario is when a vehicle pulls out in front of another vehicle at night and the driver claims that the other vehicle did not have its headlights on.

There is a way to tell if the lights were on or not without having to depend on the statements of people who may be trying to hide their own fault. It involves examining the filaments under magnification. It doesn’t require a lot of magnification and can usually be done with a camera alone, depending on the lenses available for that camera. I prefer to use a macro lens which allows for very detailed close-up photography. But first, it helps to understand how light bulbs work. Very simply stated, a light bulb is a wire encased in a glass container that is filled with a gas. As electricity is passed through the wire, the wire becomes hot and glows. The gas is used to remove oxygen and prevents the filament from burning up. Most modern lights use coiled wire. When a light bulb is impacted while it is illuminated, the filament is malleable and will bend. If the light bulb is impacted while it is not illuminated, it is inflexible and will break if the impact is strong enough. It’s a lot like plastic in that when force is applied to cold plastic, it breaks. If force is applied to hot plastic, it bends.

This photograph is from a headlight that has two settings: low beam and high beam. Notice that there is a separate filament for each setting. On an undamaged bulb, both of these filaments should be straight and shiny. In this case, one of the filaments is a straight coil while the other is bent. This bulb is from the headlight assembly of a vehicle that struck another vehicle. If you’ve ever been involved in an accident, you know that your body keeps moving in the direction you were going (inertia) while the car is stopping suddenly from the impact. It feels like you are being thrown forward in your seat. The force that you feel has the same effect on every piece of the vehicle including the headlight filaments. One of the filaments was hot enough to bend, the other wasn’t.

This photograph is from the other headlight on the same car. Notice that the filaments are similar to the last photo in that one is bent and one is not. But in this photograph, the bent filament is discolored. This is caused by oxidation. If the glass on a light bulb breaks during the impact, an illuminated filament will essentially combust when it contacts the oxygen in the air causing the wire to become oxidized making it appear discolored or even burned. If the filament is not illuminated when the glass breaks, nothing happens. The discoloration on the bent filament is a result of the tip of the glass breaking just enough to allow the gas to escape which let oxygen in. In some cases, you will even be able to see small round spots on the filament where tiny fragments of the glass landed on the filament while it was still hot causing the glass to melt to it.

As always, interpretation of the evidence should be done by an expert. There are certain cases where it can be tricky. For instance, what if the light had been on for some time but then turned off shortly before the accident? The filament will still be hot enough to bend even though the light was not on at the time of the impact. There are several factors that need to be collectively considered before making a final conclusion.