Welcome to this activity forensic lamp examination.
lamps can have a single filament or dual filaments. any or all of these filaments
can display evidence to suggest whether or not the lamp was on. when examining a
vehicle that was involved in a collision evidence in automotive lamps can help
you to assess whether or not the lights were being used. in particular it is the
internal filament that may show evidence of use
the filament which is made of tungsten is supported by two posts inside the
bulb.
When an automotive bulb is new the tungsten filament appears bright silver
in color. then as the lamp is used throughout the vehicle's lifetime it
starts to appear rough and pitted however it will remain silver even as it
ages. when power is provided to a filament the
filament heats to about 4,000 degrees and gives off heat and light. as it heats
up the filament becomes ductile that means an outside force could change the
filaments form without the filament breaking.
if a lamp is on when a vehicle is involved in a crash
the filament may stretch and deform. when the car or truck stops suddenly the bulb
itself also stops however the filament may keep going
until it stretches out
when the bulb is later examined the filaments will appear stretched and
distorted however it will still maintain its silver color
when a vehicle is slowed or stopped suddenly from a high-speed crash event a
cold filament may break apart as it continues forward.
the filament will be in one or more pieces that can usually be found in the
bulbs glass envelope. all components will be silver in color
when examining the pieces there will be no sign of melting or heat. the
breakpoints will likely be jagged.
as the cold filament continues forward when the vehicle stops suddenly in a
crash it may break away from its post. the color will remain silver and pieces
may be found in the glass envelope when the lamp is later examined small pieces
of the filament may be seen in the bulb. again the filament will maintain its
silver color.
in an air evacuated glass envelope oxygen is removed and replaced with an
inert gas. if the envelope breaks with a hot or
incandescent filament the new presence of oxygen will also leave evidence.
it should be noted that in both of these examples the glass envelope surrounding
the filament remained intact. this envelope keeps oxygen away from the
filament. if the envelope breaks during a crash in
the lamp is on the onrush of oxygen will cause the filament to burn.
note the black discolored brake filament and the yellowish powdery substance on
nearby surfaces including the tail lamp filament.
The Burning filament will produce tungsten oxide which is a yellowish
powdery substance that will stick to nearby surfaces.
the filament will immediately turn black in stark contrast to its usual silver
color. in addition to the blackened filament
and presence of tungsten oxide, glass pieces from the envelope may fuse to the
hot filament. while the microscope is often needed to view this evidence it is
sometimes visible to the naked eye.
if the glass envelope breaks on a lamp that is not on, the filament will remain
silver. the filament may also be pulled or
stretched by striking components. but it will not be stretched in the same manner
as a hot filament. again it is the silver color that helps
to designate a cold filament.
when examining lamp evidence never turn on the lights to see if they work.
this could cause filament evidence to be destroyed
rather examine each bulb individually for evidence of hot shock, cold shock, hot
break, and cold break. these observations will assist in determining whether or
not the vehicle's lamps were being used at the time of the collision.
You have completed this activity, forensic lamp examination.