Light Bulb Analysis
Light Bulb Analysis
Light Bulb Analysis


Determining the operational status of vehicle lamps during a collision can be pivotal in both criminal prosecutions and subsequent civil litigation. Scenarios requiring analysis may involve, being able to objectively show that the required obligatory lamps of a vehicle were operational during the hours of darkness in pedestrian collisions, the activation of brake lamps on approaches to junctions, the use of hazard warning lights in broken down and subsequently struck vehicles or the use of indicator lamps during turning manoeuvres all have significant implications for all involved.

We use high powered microscopes to analyse vehicle lamps to determine their use of at the time of a collision. Although automotive lighting is rapidly progressing to HID, LED and laser technologies, the majority of vehicle lamps still in use have single or dual coiled tungsten filaments suspended on a metallic support. The construction of the lamp and in particular the characteristics properties of tungsten, the inert gas within the envelope and the 'getter' all offer scientifically backed opportunities to act as objective witnesses to a lamp's state.

Determining the operational state of regulated vehicle lamps at the time of a collision is of fundamental importance to collision investigation and those tasked to form judgements on the culpability of the involved road users.

At present, there is no definitive method to determine the operational state of LED and laser light technologies at the time of a collision. These are the only automotive lights we, and all others, are unable to examine. Importantly, we can determine the operational state of indicator bulbs even if the collision occurred during the 'off state' of their cycle.

CI Forensics has conducted over one hundred individual lamp examinations under known conditions during our in-house testing. We use high-powered optical microscopy and when required, scanning electron microscopy, to determine the operational states of the lamps we examine.

As a result of our examinations we provide one of the following determinations:

  • On
  • Probably on
  • Off
  • Probably off
  • On but failed as a result of burnout
  • On but unable to determine if collision related
  • We examine each bulb to determine electrical continuity, oxidisation, hot shock, cold shock, overload and ageing/thermal burnout.


    Bulb Construction

    Conventional incandescent lighting bulbs comprise a tungsten filament mounted between two or more molybdenum posts, within a glass envelope. An inert gas, a combination of gases or a vacuum environment fills the glass envelope to prevent the rapid combustion of the tungsten filament. Other bulb components include a getter designed to collect any gaseous impurities, and various designs of stage, upon which the filament posts are mounted. A steel sleeve affixes the bulb envelope assembly, within which is mounted the stage and through which passes the electrical connections to the two filament posts.



    Tungsten has the required operational characteristic properties to form the filament of vehicle lamps. When electrical currents flows through the tungsten, it causes an increase in thermal energy which produces white light. Often stabilising additives, such as oxides of aluminium, potassium, and silicon (AKS) are alloyed with the tungsten to improve its grain structure to prevent against excessive stretching.

    When a lamp is in its cold state, the grains of the tungsten are short, whereas once a tungsten filament exceeds its incandescent threshold, in the region of 2500° Celsius, the grain structure transitions from small to long grains; we refer to this change point as the Brittle to Ductile Transition Temperature (BDTT). Above this BDTT the grain structure alters the tungsten's coil ductability, from a brittle cold state, to a malleable and ductile hot state.

    A cold state refers to when a lamp is off. Alternatively, a hot state refers to when a lamp is on.

    Importantly, tungsten coils may fracture when they are either cold or hot.

    When a tungsten coil is neither conducting nor adjacent to a heat source, i.e. in a cold state, the coil is brittle and non-ductile with short crystalline grains. When brittle tungsten coils are subject to collision forces, they often fail by 'brittle fast failure'. When this occurs, the tungsten coils are usually bright and shiny, with evenly spaced coils. Another term for brittle fast failure is 'cold shock'. Under high power microscopy examination, the fracture faces of the two filament ends are seen to be clean and sharp.

    Alternatively, when a tungsten coil is incandescent in a hot state, the coil has long grains and is both malleable and ductile. When ductile tungsten coils are subject to significant collision forces, they often stretch as their grains elongate. Stretching increases the spacing between the coils and is often readily observable to the naked eye. The residual distortion and stretching of the filament represents the direction of the collision forces. If the collision forces are significant, an incandescent coil may fracture with the ductility of the tungsten demonstrating necking with no clean and sharp break.


    Filament Coil Deformation

    When a tungsten coil lamp is incandescent, and the collision forces are significant, the coil may deform, and the spacings between the coil turns elongate. It is of crucial importance to understand that there is a wide variance of spacings and overall filament geometry between lamps from the same manufactured batch. Unless evident beyond manufacturer inconsistencies, CI Forensics cannot stress enough that basing a determination of a lamp's operational status on filament coil deformation alone is significantly flawed.

    Some vehicle lamps contain more than one filament in the bulb; for example dip and full beam headlamps, and rear position and stop lamps. For these lamps it is essential to consider the individual levels of coil deformation as they will either be of a similar magnitude or one coil may be more deformed than the other. When one coil deforms more than the other, it may indicate that one filament was incandescent and its thermal radiation emitted to the other, which partially increased its neighbouring coil ductility. Although we always consider other factors, such as wire gauge, the filament with the greater deformation typically indicates the coil was in the on state.

    Our clients must understand that the apparent lack of distortion in a bulb filament is not indicative of an unlit filament at the time of a collision, as there may have been insufficient force conducted to the filament for distortion to be sustained. The relative locality of the principle direction of force to the vehicle lamps significantly influences the transmission of collision forces to the filaments. If the main collision forces occur away from the vehicle lamps, then there is likely to be insufficient high force accelerations/decelerations for a filaments inertia to cause deformation.


    Coil Oxidation

    The inert gas or vacuum in the bulb envelope prevents the tungsten coil from oxidising during its on state. However, if the integrity of the bulb envelope is compromised, the nitrogen and oxygen in the air replace the inert gas, and if the coil is incandescent, the tungsten coil oxidises. This oxidisation is a conclusive witness to the lamp being in the on state during the demise of the structural integrity of the bulb envelope. If the coil is hot yet not incandescent, the tungsten may appear tinted with a range of colours; however if the lamp was lit, the oxides would tend to form around the filament, supports and even on the bulb envelope. It is essential to understand that the coils may have oxidised before the collision due to historical failings.


    Frequently Asked Questions

    1 - Can you determine the operational state of a lamp during a collision if it was damaged as a result and not on prior, but subsequently turned on afterwards?

    Comment - If a lamp was in an off state pre-collision, then when electricity subsequently flows through the tungsten coils it rapidly heats up and oxidises due to the lack of envelope integrity. The oxide forming on the tungsten coil and surrounding structures leaves definitive differing compositional levels of oxide during this process, which are the direct opposite of oxides forming as a lamp goes from on to off. However, the determination of the compositional levels requires the use of auger electron microscopy, which due to its high cost and specific operating requirements is rarely justifiable.

    2 - Can you determine the operational state of LED bulbs?

    Comment - At present there exists no definitive methodology to determine an LED's operational state from only the direct examination of the bulb.

    3 - Can you determine the operational state of HID bulbs?

    Comment - At present there is limited testing surrounding the examination of HID bulbs. However, there are reported characteristics to determine a HID's operational status such as colourless residues vs read/yellow residues; traces vs no traces of mercury on electrodes; ductile deformation vs brittle fracture of electrodes; and grey-blue vs silver colour of electrodes.

    4 - Can you determine the operational state of halogen bulbs?

    Comment - Yes, through similar methodologies employed for low-cost non-halogen tungsten filament bulbs.

    5 - Can you determine the operational state of internal bulbs?

    Comment - Yes, although due to the distance the instrument cluster and occupant courtesy lights are from the direct collision forces they less readily exhibit witness signs of their operational status.

    6 - Can you determine if an indicator bulb had been flashing during a collision?

    Comment - Yes, providing the bulb was close enough to the impact forces. Testing has shown that providing the indicator lamp is flashing between the required rates of 60 to 120 flashes per minute, then the tungsten coil remains above the DBTT and may show signs of coil deformation and oxidisation.



    Light Bulb Analysis
    Light Bulb Analysis
    Light Bulb Analysis
    Light Bulb Analysis


    Fees & Lead Times

    Our service level agreements for fees and lead times for our lamp analysis services are fixed, please contact us to discuss.

    This service is part of our all inclusive investigative reconstruction package.

    Please contact us to discuss your requirements.


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