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Is it difficult to judge the stage of the head light prior prior to accident ( ON
Or OFF )
HEADLIGHTS WAS ON OR OFF, WHO WILL DECIDE?
As we have noticed during night driving with putting head light off that This
occurs Frequently when a driver travels from a lighted area, such as a convenience
store, And forgets to turn on the lights. Drivers also neglect to use their headlights
under Reduced visibility conditions of fog, dusk, and rain. But then, on the other
hand, Other drivers just simply don't see a well-lighted vehicle. Following a collision
it is Often possible to determine whether a vehicle was driven with its headlights
Illuminated. First, it is usually easier to demonstrate that the lights were in
use than it is to Demonstrate that they were off. Usually, one positive indication
that a headlight Was illuminated is sufficient to conclude that the headlights were
"on". However, This conclusion must be exercised with some caution, since it is
possible for some Of the lights to be burned out, missing, or otherwise damaged
prior to the collision. To demonstrate that the headlights were off requires either
very positive evidence Or an accumulation of "off signals" from a number of bulbs.
Headlights, or headlamps as they are technically referred to, generally are of two
Basic types, sealed beam or halogen. Both consist of a coiled tungsten filament
Surrounded by a glass envelope. The glass envelope of the sealed beam lamps is large,
4 to 6 inches across and shaped like a small TV picture tube. The air inside is
removed and the glass envelope is "sealed" so that the tungsten filament is heated
in a vacuum. In contrast, the filaments in halogen headlamps are surrounded by a
small cylindrical glass envelope, approximately ½ inch in diameter and 1 inch long.
They are called halogen headlamps, because the glass is filled with an inert halogen
gas. The halogen bulbs also have an outer glass or plastic covering which provides
additional protection and focuses the emitted light. In the sealed beam headlamps,
all of these functions are provided by the glass envelope. When a headlamp is illuminated,
the coiled tungsten filament is heated by electrical resistance to a temperature
of 3000 degrees F. At this temperature, the filament emits radiation in the form
of white light. If the glass covering is broken during a collision, several things
are likely to happen. The "shock" or impact which breaks the glass will cause the
headlamp to move violently. When this occurs, there will be inertial loads imposed
on the filament coils. If the filament is cold, it will sustain very high inertial
loads without deforming the filament coils. However, when the filament is white
hot, the metal is relatively malleable and most impacts which break the glass will
also deform the filament. This results in a filament which has bends, bows or other
non-uniformity in the coils. When the glass covering breaks, in a sealed beam bulb,
the vacuum is lost and in a halogen bulb, the halogen gas dissipates. In either
case, the hot filament is exposed to the atmosphere which contains oxygen. Oxygen
will chemically combine with the hot tungsten to form tungsten oxide on the surface
of the filament. This changes the color of the filament from a shiny silver color
to a blue/black color. If the filament remains energized, the filament metal will
continue to combine with oxygen, producing a yellow green powder of tungsten oxide.
The filament will continue to decompose until it separates and the electric circuit
is broken. Where it separates, the ends of the filament will taper down to the separation.
When the glass envelope is broken, the breaking glass will generate many small fragments.
These small fragments are swirled around and may contact the tungsten filament.
If the filament is cold, the glass particles simply bounce away. However, if the
filament is hot, the glass melts onto the surface of the filament, forming little
glass nodules. This effect is usually more pronounced with a sealed beam headlamp,
because the air rushes inward to fill the vacuum carrying the glass Particles toward
the filament at the center. Hot Filament Deformed by Inertial Forces Generated During
Impact Notice the Adjacent Unreformed Filament Microscopic view of glass melted
onto a filament In some situations, the headlamps may be turned on after the collision.
These Filaments will exhibit oxidation but none of the other conditions. Careful
Examination will usually allow a determination of whether the lights were on, off,
Or turned on after the collision.