Silent witness – forensic collision investigation

Materials World magazine
1 Dec 2009

Optical microscopy is being used in determining the causes of road traffic collisions in the UK. Reita Smith reports

Based near the A55 road, which runs from Chester to Holyhead in the UK, The North Wales Police Forensic Collision Investigation Unit comprises seven investigators and three vehicle examiners. The unit attends all life threatening and fatal road traffic collisions in the North Wales area. This amounts to approximately 200-250 collisions per year, of which around 50 are fatal.

The fatalities are always fully investigated, as are a lot of the other incidents, as the unit’s primary purpose is to produce a detailed impartial report to support the prosecuting authority in cases of careless or dangerous driving, as well as reporting to coroners, and the Crown and Civil Courts with regards to insurance claims.

All members of the unit are registered with the forensic authorities and authorised, as expert witnesses, to give opinion in court. Their role is to investigate and gather evidence at the scene of collision and to examine vehicles to determine any mechanical or other issues that may or may not have contributed to the cause of the accident, such as brakes, lights and steering.

Whether or not a car light (indicator, headlamp, side and rear light) was on or off prior to a road traffic collision or impact can often be cited as a direct cause of the incident. In traffic accidents at night, it is important to establish whether headlamps and rear lights were being used at the moment of impact. Daytime accidents can raise questions as to whether the driver was indicating. This is especially important when there are no independent witnesses, only interested parties. The problem for investigators is how to determine if lights were on, particularly if they were damaged.

Illuminating information

Optical microscopy has proved invaluable in aiding forensic investigation of car light bulbs in road traffic collisions. By understanding the basic metallurgy of the components that make up the filament and the manufacture of such bulbs, investigators can reveal what happened in an accident.

After attending a short course on Forensic Examination of Light Bulbs at Sheffield Hallam University, UK, Sergeant Colin Dobbins, a senior collision investigator, and Gary Roberts, a vehicle examiner, both with the North Wales Unit. realised that the force would benefit from owning its own stereomicroscope. This would allow them to examine most of the bulbs in-house rather than sending them off for further investigations at a cost of £350 per sample.

The microscope has been useful in several situations. ‘Everything we used to look at under the magnifying glass we now look at under the microscope,’ explained Roberts. ‘Its principal use is in examining the various physical characteristics which can determine the on/off status of tungsten car light bulbs.’

These bulbs comprise a glass enclosure (the envelope or bulb) with a filament or coil of tungsten wire inside the bulb, through which the electric current passes. The bulb is filled with an inert gas or partial vacuum to reduce evaporation of the filament. The tungsten filament becomes hot when illuminated and relatively cold when the light is off. Analysis is possible since the mechanical and chemical behaviour of a hot filament is markedly different from the behaviour of a cold filament during a collision.

When a filament is on, it becomes ductile so if the bulb experiences a strong impact the coil may deform. Collision speeds above 20km/hr (about 12.5mph) will cause a hot tungsten filament to deform, and the greater the speed, the more apparent this becomes. Hot tungsten wire coils stretch easily and permanently. By contrast, a cold filament will appear as a uniform spring. ‘We have found extruded filament coils can bow more than 10mm,’ commented Roberts.

On occasion, the force of impact will be so great that one or both ends of the filament break away from their posts. A hot illuminated filament will form a small ball of tungsten at the end of the failed filament. This can be observed with the naked eye or through a low power optical microscope. The presence of the solidified globule indicates the light was on.

If, on the other hand, the ends of the filament are jagged, show no change in diameter and have no globule, then the bulb was off when the break occurred. ‘We are also looking for arcing,’ Roberts explains, ‘where, if the white hot filament breaks it arcs between two parts with globules of tungsten at both or either end.’ Sometimes the glass bulb shatters during impact, allowing oxygen to reach the filament. If the filament is hot (illuminated), the oxygen will react with it to form tungsten oxide on the surface of the filament, visible as a yellowish-white powder. However, if the glass breaks when the filament is cold, the tungsten coil remains clean and shiny. There are times after the bulb has been broken when arcing occurs, between the mounting posts. This causes discolouration.

When the glass shatters, small particles and grains will implode into the bulb. If the filament is hot, these glass particles adhere to the filament while it cools down. Their presence also indicates that the light was illuminated (see image above, right). By contrast, glass particles will not adhere to a cold filament. Therefore, if the filament is clean when viewed under a microscope, the light was probably off when the bulb shattered.

By searching for hot stretch, cold breaks, discolouration and fused glass particles, an investigator can determine whether the bulb was on or off when the collision occurred. According to Dobbins, ‘We have now improved our investigations immeasurably, progressing from 6x [views] – where you can see a deformed coil, to 20-60x where you can see the end of a coil and take good images of [it and] the deformed coil for forensic support.’

Belting up

In addition, the unit uses the microscope to examine seatbelts for evidence of friction marks on the plastic backing material, support and anchorage points and on the seatbelt webbing material. The transfer marks where the two materials interact become visible under magnification, so it is obvious if the seatbelt was being worn or not at the time of a collision.

Microscopy is also useful in identifying stolen vehicles. ‘We can look for tooling marks to see whether the chassis number has been tampered with,’ explains Roberts. ‘These are difficult to see with the naked eye, but are [highly] visible under the microscope!’.

The equipment has already shown itself to be cost effective, offsetting the price of submitting samples to a laboratory. ‘The [microscope] has certainly made a big difference – it can be pivotal in some cases,’ adds Dobbins. By linking a camera to the microscope, ‘we can produce excellent high resolution images for reports which provide more conclusive irrefutable evidence. We were recently able to use it to identify a vehicle from debris very quickly’.