On the topic of examining vehicle light bulbs, you may hear:

“You don’t need to engage a light bulb Expert; examination of light bulbs is easy”.

  • “If the filament has distorted the bulb was On at the time of the crash”
  • “If it hasn’t distorted it was Off”

Whilst it might be true in some, and maybe many, instances it certainly is not a reliable universal guide.  If you take that approach you will arrive at the wrong conclusions in some instances.  Not good for justice, and not good for any party involved in litigation as those conclusions may well be challenged and found to be wanting.  Forensic investigation of light bulbs can yield valuable insight into crashes and performance of the light bulb at the time.

Some of the problems that may be encountered when examining vehicle light bulbs:

Old-age burn out

  • Filaments thin in localised areas owing to ‘evaporation’ (technically ‘sublimation’) of the tungsten metal.
  • It can lead to blackening of the glass if it persists for a long time.
  • It will also lead to a thinner area that will overheat and eventually burn out, arcing as it does so.
  • The broken filament may flail when the vehicle is in use and the ends may touch and weld together if power is present, albeit the weld may not be good enough to last for a long time. Alternatively, one end may touch the other filament (in a twin filament bulb) or the shield and weld to that.


  • The filament may have little or no distortion if the impact it received was below the threshold necessary to cause distortion of an illuminated filament. That threshold will differ depending on the wattage of the filament.  The threshold means that bulbs remote from the point of impact, such as rear bulbs in a frontal collision, often do not exhibit distortion even in violent collisions.
  • The filament may have distorted as a result of an impact a very short while after the light was extinguished; the circumstances of the accident and ideally other bulbs from the vehicle need to be considered to determine if this might be an issue.
  • In other instances, the filament may have some distortion from manufacture; this will depend on the type of bulb and the shape of the filament,. Whilst slight distortion may be expected in some types of bulbs of ‘premium’ manufacture, cheap unnamed bulbs can have larger amounts of distortion.
Stop/tail bulb with both filaments distorted by a violent impact whilst hot

Stop/tail bulb with both filaments distorted by a violent impact whilst hot


  • The structure of a vehicle light bulb filament changes as it ages, which alters its behaviour and which can cause great difficulty in examination. The filament will progressively experience recrystallisation and grain growth, which can weaken it considerably; this may create small areas where the diameter of the filament has reduced.  This weakening may lead to the filament failing at a time when a newer filament would not be affected.  The broken ends may arc and re-join by welding when switched on, or one end may attach to another area and sometimes it may even attach to an adjacent filament.  Such a weak, heavily‑aged, filament may shatter into many fragments and crucial detail may be missed if not examined in great detail.
SEM view of an old filament that has suffered recrystallisation and grain growth

SEM view of an old filament that has suffered recrystallisation and grain growth

Oxidation (NB not ‘oxidisation’ – that word doesn’t exist!)

  • If air enters the glass envelope of the bulb, the filament will oxidise when hot, but what do the various colours mean, and what does it mean if both filaments in a bulb are discoloured? Only careful microscopic examination is likely to provide the answer.


In many instances, examination of a vehicle light bulb filament by an Expert using a conventional stereo-zoom microscope will be sufficient.  However, in some instances, particularly with thinner filaments, examination using a scanning electron microscope is desirable or sometimes essential (see photos) but be prepared, it will cost you more – they aren’t cheap machines!