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Many people do not realize that it is possible to determine whether or not the headlights on a vehicle were on or off prior to an accident. Sometimes the issue can be critical to affix blame for an accident. Motorists often claim that the motorcycle they struck was not visible since the headlight was off. Then there are night-time car accidents in which one of the drivers may feel the other vehicle did not have its light illuminated. Sometimes, there is a claim that the one vehicle had its high beams illuminated, causing the driver in the oncoming vehicle to be blinded, lose control and crash.

Headlight analysis can be used to resolve these claims. The evidence indicating whether the light was on or off is clear and conclusive. The purpose of this article is to reveal how headlights are analyzed in determining whether or not they were on prior to a collision.

Sealed Beams vs. Quartz Halogen

The two most common headlights used today are sealed beam and quartz halogen. Sealed beam headlights are similar to ordinary light bulbs. A tungsten coil, called a filament, is braze welded to two posts inside a vacuum glass envelope. The axis of the filament is perpendicular to the direction of travel. Quartz halogen headlights, by contrast, are relatively newer and brighter than sealed beams. The high and low beam are mounted inside a small cylindrical bulb filled with halogen gas. The bulb is mounted inside a larger glass envelope. The axis of the filaments is parallel to the direction of travel. Instead of glowing white hot like the filaments inside sealed beam headlights, the filaments inside quartz halogen bulbs heat up the halogen gas which causes the gas to glow brightly.

Despite their differences, headlight analysis can be performed on both sealed beam and quartz halogen bulbs. In both types of headlights, a tungsten filament is extremely hot when illuminated and relatively cold (at ambient temperatures) when the headlight is off. Headlight analysis is possible since the mechanical and chemical behaviour of a hot filament is markedly different from the bahaviour of a cold filament during a collision.

The Analysis

When the investigative engineer examines a headlight, he will begin by examining the glass. Has the glass cracked or shattered? Has the inside surface of the glass blackened? If the glass has shattered, care must be exercised to preserve the condition of the tungsten filament. If the glass has blackened, then arcing between the post and filament has occurred. This blackening is more commonly seen in smaller bulbs such as brake lights and running lights. Its presence indicates that the filament broke from the post well before the accident and was, therefore, not working; or, at best, working intermittently prior to the collision.

Next, the examination of the filament is performed. Collision velocities greater than 20 km/hr will cause a hot tungsten filament to deform. The greater the speed, the greater the deformation. Hot tungsten wire coils stretch easily and permanently. By contrast, a cold filament will appear as a uniform spring. As a general rule, only lights near the point of impact undergo a filament deformation during an accident.

On occasion, the force of impact will be so great that one or both ends of the filament will break away from its post. A hot illuminated filament will form a small ball of tungsten at the end of the filament or post. This can be observed with the naked eye or through a low power optical microscope. The presence of the solidified ball or even a "fat" wire indicated the headlights were on. If, on the other hand, the ends of the filament were jagged, show no change in diameter and have no round, irregular, or shiny areas, then the filament was not illuminated when the break occurred. At times, the filament will have shattered into small pieces, indicating that it had previously burned out.

Sometimes the glass bulb shatters during impact, allowing oxygen to reach the filament. If the filament is hot (illuminated), the oxygen will react with the filament, forming a yellowish white powder on the surface of the filament. This powder is called tungsten oxide. Sometimes, an illuminated filament will turn a dull grey colour when the glass breaks. However, if the glass breaks when the filament is cold, the tungsten coil does not react with the in-rushing oxygen and the coil remains clean and shiny. This observation indicates that the light was off at impact.

When the glass shatters, small glass particles and grains will implode into the bulb. If the filament is hot, these glass particles will adhere to the filament while it cools down. The presence of these glass particles when viewed under an optical microscope indicate that the light was illuminated. By contrast, glass particles will not adhere to a cold filament. Therefore, if the filament is clean when viewed under a microscope, then the light was off when the light bulb shattered.

Research has shown that lamp age, lamp manufacturer and electrical disruption at impact has no significant effect on the analysis.

Helpful Hints to the Adjuster

To assist the investigative engineer with the analysis, the headlights should be handled with care from the accident site to the laboratory. Since the investigative engineer is not always present at the scene, there are several steps taken by the individual attending the scene to assist with the ensuing headlight analyses. These are as follows:

  1. If the glass envelope has broken, photograph the filaments at close range.
  2. Make notes on the condition of the filaments if the glass envelope has shattered. Make reference to the colour and linearity of the filament.
  3. Carefully remove the headlamp from the vehicle, labelling which side the lamp came from i.e., "driver side" or "passenger side".
  4. Individually enclose and seal headlamps in plastic bags to prevent loose parts from being lost.
  5. If the headlamp is wet from rain, try to dry it without physically touching the filament or subjecting it to excessive heat.
  6. Ensure nothing will make contact with the filaments during transport.



The technique of headlamp analysis can be a useful tool in accident reconstruction. Unfortunately, many people are unaware that such a technique exists. Others are unaware of how clear and straightforward the analysis can be. Often the analysis can be performed in a short time yielding conclusive results. This information can, at times, be very important to the insurance company when affixing blame for an accident.

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