into a filament. This indicates that the
filament was incandescent at the time of
In some incidents, the glass fractures
and the filament remains intact, but is
heavily oxidized. The oxidized tungsten
surfaces are a result of high temperatures,
indicating that the filament was incandescent at one time after the glass fractured.
There are two possibilities in this case.
Either the filament was incandescent
at the time of the accident, or someone
turned on the light switch after the accident. To determine if the light was on in
this case, one has to eliminate the possibility of spoliation of the evidence by the
intervention of someone turning on the
Another type of lamp now installed in
modern automobiles is the high-inten-
sity discharge (HID) lamp as shown in
Figure 7. The bulb contains an arc tube
with electrodes. The arc tube contains xe-
non gas, mercury and metal halide salts.
When voltage is applied, an arc develops
providing a bright light.
Figure 8 is a close-up of the arc tube on
a new lamp. The metal halide salts have a
yellowish color as indicated by the yellow
arrow above the arc tube. While in oper-
ation, the metal halide salts are vaporized
and distributed throughout the arc tube.
If the arc tube is fractured from the ac-
cident, then these gaseous components
escape. Upon cooling, there is very little
evidence of deposits on the inside of the
fractured arc tube, which suggests the
light was on at the time of impact.
Oxidized electrodes (greyish blue in
color) and/or ductile deformations are also
an indicator that the light was on at the
time of impact. Figure 8 also shows mer-
cury deposits on the electrodes of a new
lamp. If the lamp is powered at the time
the arc tube is fractured, mercury escapes
and there is little left on the electrodes.
We have presented several examples of
post-accident, lamp examinations. After
an accident, the lamps in an automobile
often contain evidence as to whether the
lights were illuminated at the time of the
accident. This evidence will certainly aid
in deciding issues regarding whether or
not the lights were on at the time of an
Charles C. Roberts, Jr., Ph.D., P.E.,
( firstname.lastname@example.org) is president of C.
Roberts Consulting Engineers, Inc.,
which provides professional engineering
services in accident reconstruction,
failure analysis, fire causation, explosion
analysis, and biomechanics.
Figure 7 Figure 6