Forensic Lamp Examination
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.