Lights On, Lights Off
General Aspects of Forensic Lamp Examinations for Private Investigators
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 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 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