- Lights On, Lights Off
- General Aspects of Forensic Lamp Examinations for Private
- by Christopher C Voeglie, LPI
- President of Forensic Associates, Inc.
- Many of us have encountered through the investigation of personal injury
cases involving a traffic accident, the question inevitably arises whether
one of the vehicles did or did not in fact have its lights on at the time
of collision. This presents us with the issue of whether or not the driver,
pedestrian or bicyclist could have perceived the hazard of the approaching
vehicle in order to take action to avoid a collision. This article is not
intended to encompass the entire subject of lamp examinations, but rather
to assist you in understanding some of the basics as well as to help you
determine whether or not to employ the services of a qualified accident
reconstructionist or expert in lamp examinations.
- Many of you have heard of, or been asked about lamp examinations in
the past. The procedure is conducted, but it does have its limits. Before
we go yanking apart lamp assemblies we must first understand what it is
we're looking for. It is imperative that we get to the vehicle as soon
as possible as this type of physical evidence is quickly ruined or destroyed
inadvertently. It is also important that you don't inadvertently destroy
the same evidence you're looking for by making the common mistake of simply
turning the lights on to see if they work. By turning the lights on you
may have just destroyed your clients only physical evidence. Depending
on the attorney or adjuster involved you may be given the case when he
or she is retained, or it may be months or even years before you even see
the case file. Insurance companies don't like to spend money storing vehicles
that aren't worth repairing, so in order to avoid costly storage expenses
they tend to destroy these vehicles making them unavailable for later expert
examination. To understand what takes place during the collision we must
first understand how a lamp operates. There are two basic types of automotive
lamps, incandescent and halogen.
- Incandescent lamps are basically the same as your average everyday
light bulb. The filament is a tightly coiled wire that is strung across
two support posts. The filament is made of tungsten which is a very hard
material and has a bright luster appearance. Filaments may be arched or
straight between the support posts. There may also be more than one filament
enclosed within the glass housing. This allows the bulb to posses multiple
functions. Lamps with more than one filament will have one filament that
is larger than the other. The larger of the two is used for the brake and
turn signals with the other functioning as the parking or taillights. When
lamps are in a state of incandescence, electrical current is past through
the filament which raises its temperature to the point that it glows brightly,
(incandescence is approx. 4000 degrees Fahrenheit) this produces the white
light of the bulb. Although tungsten is a hard material, it will oxidize
very quickly if it is exposed to the atmosphere while in this state of
incandescence. For this purpose air is removed from the lamp, producing
a vacuum, and is replaced with nitrogen or other inert gas. This allows
the filament to remain lit without burning out or oxidizing. When heated
to incandescence tungsten will evaporate which weakens the element. In
this bulb type the tungsten settles on the cooler glass surface, which
darken the bulb as it ages. This is more evident in smaller bulbs.
- The majority of the vehicles on our highways today are equipped with
halogen lamps. These lamps are similar in construction to the incandescent
lamp. As does the Incandescent lamp, Halogen lamps also contain a gas called
halogen. This is the same element used in various fire suppression systems
as it will not allow, or support combustion by removing oxygen from the
atmosphere. The air is removed from the bulb and replaced with the halogen
gases pressurizing the bulb 4 to 8 times that of the atmosphere. The halogen
may have iodine, bromine or chloride added to the gas. In the halogen bulb,
the boiled off tungsten combines with these gases in the cooler part of
the bulb near the glass. Instead of depositing on the glass as in the incandescent
lamp, the halogen tungsten vapor is re-circulated toward the filament where
its high heat breaks it into atoms.
- The tungsten is then deposited back to the filament and the halogen
back toward the glass. This continuous cycle requires the glass temperature
to be approximately 480 degrees Fahrenheit and may exceed 900 degrees.
These high operating temperatures require the glass to be made of quartz
or heat resistant glass, hence the Quartz Halogen Lamp. Newer model cars
have a multiple piece lamp housing while some of the remaining older models
with earlier halogen lamps have one large lamp assembly.
- Normal Lamps will posses the same basic qualities and appearance as
when it was originally manufactured. The glass housing will be intact and
free of any darkening or film on the inside, the filament will exhibit
a bright luster and is composed of many evenly spaced tight coils, and
the base will be free from corrosion. Changes in appearance will develop
as the bulb ages, including the darkening of the bulb that was discussed
earlier. The filament will also begin to pit and will appear rough under
magnification. Filaments may also begin to sag, this is most evident in
long, thin elements used in bulbs which are mounted horizontally.
- We have covered the basics of operation and construction. We can now
discuss the signs and conditions of actual lamp failure. Sooner or later
a lamp will inevitably fail. A lamp that has failed due to normal burnout
will have a bright filament that will most probably be parted with rounded
or ball ends at the part or break, and may exhibit a darkening of the glass.
Normal burnout occurs when a filament is weakened and thinned by the normal
evaporation of tungsten and pitting. A filament in this condition lacks
its original resistance and will heat to its approximate melting point
of 6,100 degrees. When the filament parts, an electrical arc will jump
across the gap and a bright flash will be visible until the space between
the filament ends is to great to hold the current. The effects of a collision
on a lamp filament will vary greatly depending on the conditions of the
lamp at the time of impact. Hot and cold lamps will produce different results.
Factors including impact severity, filament size, temperature, age, as
well as whether or not the glass was broken, will play a role in the amount
and type of deformation a filament will experience. This also determines
what evidence will be recovered for examination.
- If a lamp is broken while in the state of incandescence the element
is exposed to air and will oxidize rapidly and blacken. If an incandescent
lamp is positioned close enough to direct impact the filament will stretch,
uncoil, tangle or break due to its inertia during impact. This condition
is known as hot shock. For this to occur the filament must be hot, although
not necessarily incandescent and the glass remain intact during the impact.
A filament may also show signs of deformation if the lamp was incandescent
just prior to impact. The time required for a filament to cool depends
on its size. The larger the filament and the more deformation observed
the better the indication the lamp was on at impact. For example, a tail
light filament that was turned off four seconds prior to impact will show
a slight deformation. A turn signal filament with the same conditions will
show medium deformation. In these types of multi-element bulbs one filament
may be deformed more than the other. This would indicate which element
was hotter at the time of impact. If the other element is not deformed
as much as the first or only shows a slight deformation, this can be attributed
to the radiant heat from the incandescent one. As discussed earlier, a
filament exposed to air while incandescent, as well as cooling, will oxidize
quickly. The amount of oxidation will also give evidence of the temperature
of the filament when exposed to air. These stages of oxidation are easily
recognized by color, starting with pale yellow, to greenish, to purple
and finally to black. Each color variation represents thin layers of oxidation.
When a filament oxidizes, it produces tungsten oxides which rise from the
filament as white smoke. A white dust or powder appearance may be deposited
on nearby surfaces, including the adjacent filaments, pieces of the bulb,
supports, stems etc. The presence of oxide film, and filament blackening
exhibits positive evidence that the lamp was incandescent when the bulb
broke, or that current was applied to the lamp after the bulb was broken.
There may also be signs of fused glass particles which adhered to the incandescent
filament due to it's intense heat when the glass broke. These particles
can be seen under magnification and have the appearance of glass droplets
or fine dust. Large pieces will also adhere to an element, these are normally
visible to the naked eye. The absence of the signs of hot shock does not
mean the bulb was not lit. There may not have been enough impact to effect
the filament. The amount of impact force needed to deform a hot filament
is substantial, although less than that of cold shock.
- When a bulb is broken and the lamp is cold there will be no effect
to the pre-impact appearance of the filament barring any intrusion by contact
damage. If the bulb is not broken and the filament is of normal appearance
do not assume that the lamp was off. It could have been lit and not received
enough change in velocity to deform the incandescent filament. If the filament
displays damage in the form of clean, sharp, breaks and the bulb remains
undamaged the lamp was definitely not lit. Cold fracture may also occur
when a bulb is broken and the filament is exposed to handling and environmental
conditions. Cold shock or fracture with an intact bulb and an otherwise
normal filament was subjected to substantial impact. The amount of force
necessary to produce cold shock to a lamp is much greater than that necessary
to produce hot shock. A cold lamp needs to be directly involved in, or
very close to the contact damaged areas.
- Just how much force is needed to produce profound abnormalities to
a filament? Although conditions vary, stopping a lamp from 20miles an hour
in a quarter of an inch will do. Tests have found the acceleration rate
necessary to produce deformation to be between 400 and 900 times the acceleration
rate of gravity. That's 400 to 900 g's. Do not confuse these acceleration
factors with those used in accident reconstruction. Low impact speeds can
be adequate to produce deformations. How can we use this information? If
you're confronted with the issue of whether your client, or the other party
had their headlamps on at the time of impact, we know what to look for.
If you have the opportunity to inspect the vehicles in question and observe
any of the signs discussed here you can do one of two things. One is to
carefully remove the lamps in question, photograph them, record who removed
them and have them examined by a qualified expert who will render a conclusive
report. Make sure you maintain and document the chain of custody. Two,
have a qualified expert do an independent supplemental investigation regarding
the lamp examination. A hint of advise is to make sure you have the legal
right, or consent to remove anything from the inspected vehicles. You should
also advise your client that the majority of the time a fairly strong determination
can be established with the right conditions, but there is always the chance
that a detailed and flawless investigation will be indeterminate.
- Keep in mind the scope of lamp examination is extensive, so unless
your qualified to do so, I highly recommend you contact someone who is.
A supplemental publication on lamp examinations consisting of 45 pages
with good photos of the many conditions discussed within this article,
is available from the Northwestern University Traffic Institute for twelve
dollars. It can be obtained through their web page listed below or by mail.
Their address is Northwestern University Traffic Institute, 405 Church
St., PO Box 1409, Evanston IL 60204. Their toll free number is 1 800 323
4011. For those on the Internet point your browser at http://www.nwu.edu/traffic/
This is the web site for the Northwestern University Traffic Institute.
Reference materials, as well as a wealth of additional information can
be acquired from this site. This site also houses several links to state
and federal agencies regarding traffic safety issues and highway engineering.
I also personally recommend acquiring the Accident Investigation manual
from Northwestern, at a cost of $55 it would be an excellent addition to
any investigator's library.
- You might want to also check out the Traffic Accident Reconstructionists
search site at http://www.TARSearch.com.
This site is to assist attorneys in locating a Traffic Accident Reconstruction
expert. If you'd like to locate one in your area, you can start there.
Most of the listed experts have links to their home pages or online curriculum
vitea, resume or professional profiles. One additional site of interest
is that of the Institute of Police Technology and Management in Jacksonville
Florida. This facility along with Northwestern and a select few others
represent the industry standard in training. Additional reference materials
and links are available at this site as well. Their address is http://www.unf.edu/iptm.
- RETURN TO NEWSLETTER INDEX