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TM 9-8000
b.  Use of Prismatic Lens.  The light beam is
(1)  A superior headlight that has been adopted
is the sealed-beam headlamp unit.  Not only does it
distributed over the road by means of a prismatic lens.
provide far better and more powerful illumination than
The effect of a prismatic lens fitted to a parabolic
previous lamps, but it maintains its initial brilliancy with
reflector is shown in figure 16-3. The lens bends the
only a slight loss throughout its life. This is because the
paralleled rays from the reflector so that the light is
lens is permanently sealed to the reflector, barring
distributed over the road. The vertical flutes of the lens
moisture (which corrodes the reflector) and preventing,
spread the light rays so that the beam is flattened, with
the entrance of dust and dirt.
the edges thrown out toward the side of the highway.
c. Combinations of Beams. Many combinations of
(2)  When a filament burns out, the whole unit
must be replaced. However, it has a greater filament life
light beams are possible. One combination of headlight
than other types of lamps and requires no maintenance
beams that has been used commonly is shown in figure
to keep it in good condition.
16-4. The beam from the right headlight is projected
high to the right side of the road and low to the left side,
(3)  The sealed-beam headlamp unit is made in
and the beam from the left headlight is projected high to
two types: one with a silver-plated metal reflector and the
the left side and low to the right side. Portions of the
other with an aluminum-surfaced glass reflector.  The
beam are deflected lower than other portions because of
metal type contains a conventional double-filament lamp
the design of the lens. When the right and left beams
that is sealed in the unit mechanically, whereas the glass
are not the same, the lenses for right and left headlights
type is its own lamp because the lens and reflector are
are not interchangeable. These beams combine to give a
fused together, forming a gas tight unit with the filaments
nearly symmetrical beam for driving.
With some
sealed into the reflector.
headlights, the left light illuminates the right side of the
road, while the right light illuminates the left side of the
(4)  Two filaments are provided in the sealed-
road. Both lights together give a symmetrical beam.
beam headlight lamp unit: one provides an upper beam
for country driving, and the other gives a downward
beam for passing or city driving. With the upper beam in
use, current sealed-beam lamp units provide 50 percent
a. General(Fig. 16-5).
more light than previous 32-candlepower lamps, and
also distribute the light more effectively.  The upper
(1)  In headlights of the older type, means are
beam filament requires 40 to 45 watts, and the
provided for focusing and directing the light. Focusing
depressed beam filament requires 30 to 35 watts, which
means bringing the filament of the lamp to the focal point
is more current than that required by the 32-candlepower
of the reflector; aiming means pointing or directing the
lamps. Directing the headlight to the roadway is the only
light properly.
adjustment required on sealed-beam headlamps.
(2)  Later developments brought into general
(5)  The sealed-beam headlamp is mounted
use a 2-filament lamp having its position fixed with
with long self-locking screws and springs. The screws
respect to its mounting socket at the rear of the reflector
serve to aim the headlamps, depending on their position.
so that the filaments remain fixed at the proper focus (fig.
16-5). It is necessary only to direct the light to Improve
16-5.  Road Illumination (Fig.
the lighting of the roadway.
development has brought about a radical change in what
is considered good road illumination. The high-intensity
(3)  The most common headlamp configuration
beam of light has given way to the principle of more
in modern automotive use is the sealed beam.
illumination and lower general intensity.
The 32-
b. Sealed Beam(Fig. 16-6).


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