Note: Descriptions are shown in the official language in which they were submitted.
This lnvention relates to PAR (parabolic,
aluminized reflector) lamp~ and, in particular to an
all plastic headlamp having a sealed inner bulb as
the light source.
In this era of sealed beam lamps it is some-
times difficult to remember that non-sealed beam headlamps,
ie headlamps having an inner lamp, date back almost to
the invention of the automobile. However, the
disadvantages of such a structure ~emain, eg. deter-
iorating performance or failure caused by dirt, water,
corrosion, and condensation. The all-glass, sealed
beam PAR lamp alleviates these problems but is no
panacea.
Because an all-glass design is used in
current sealed beam headlamps, the lamps are heavier
than they might be otherwise. Also, there are limits
on the mold designs for glass that do no exist for molds
for plastic. Further, considering the equipment invest-
ment which must be made by a manufacturer, plastic
is more easily handled and does not wear moulds out as
quickly as glass.
Desiring an alternative to an all-glass
construction is not the same as finding a practical
design. For examplel in U. S. Reissue Patent 25,017 -
D.P. Cooper Jr. dated August 8, 1961 an all-plastic
beam lamp is described. While a lamp built as described
therein may be made to operate, the result is not
necessarily a commercially viable headlamp. For
example, the exposed filament cannot survive the
three hundred or so hours lamp operating life required
of automotive headlamps. Also, so far as is known, no
plastic is impervious to water vapor or other gases over
~6~ii7~
the life of the lamp. (In all glass PAR lamps, the fusion
of the lens to the reflector renders the lamp impervious to
water vapor or other gases.) Further, in use, the "hot spot"
on a headlamp, herein defined as the region in the reflector
above the filament may easily exceed the 80C discussed in
the patent. Similarly, particularly if the lamp is dirty,
a second "hot spot", herein defined as a region on the lens
approximately one-fourth the distance down -from the top of
the lamp, may also exceed 80C, depending upon ambient conditions.
In view of the foregoing, it is therefore an
object of the present invention to provide a practical plastic
PAR lamp.
Another object of the present invention is to
provide a plastic PAR lamp suitable for use in vehicle
lighting.
A further object of the present invention is to
provide an all-plastic lamp capable of dissipating wattages
comparable to all-glass PAR lamps.
Another object of the present invention is
to provide a plastic lamp in which the atmosphere within
the plastic envelope need not be devoid of water vapor or
other gases.
The foregoing objects are achieved in the
present invention wherein the PAR lamp comprises a high
(greater than 130Celsius) heat distortion temperature plastic,
such as polycarbonate copolymers or polysulfone, having a
loading of from 4.5% to 7.8%, which can be alternatively
expressed as 0.045 to 0.078 watts per cc. As used herein,
"loading" is defined as the watts dissipated in a given
volume and has the units watts per cubic centimeter (W/cc.).
A more complete understanding of the present
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73
invention can be obtained by considering the following
detailed description in conjunction with the accomp~nying
drawing, in which:
The FIGURE illustrates a preferred embodiment
of a PAR lamp in accordance with the present invention.
Ia ~p
The FIG~E illustrates a ~p 10 in accordance
with the present invention, in this particular example
having the configuration of a rectangular headlamp.
Specifically, lamp 10 comprises a lens 11 suitably attached
to a reflector 12 having a specular coating 15 on the
interior thereof. Sealed within lamp ~2 is an inner
bulb 15 which preferably comprises a sealed halogen
cycle lamp suitably attached to leads 19 and 20 which
exit the rear or bottom of reflector 12 and are each
connected to a suitable terminal, such as terminal 21.
Positioned above inner bulb 16 is a heat shield,
preferably comprising a metal disc 17 attached to a
conductive lead 18, which serves to interfere with
the convection heating of the hot spot on lamp 10,
which forms directly above inner bulb 16 in the flat
portion of re~lector 12. The convection currents in
the atmosphere within lamp 10 are spread by shield 17,
which causes the atmosphere to mix thereby diffusing
the heat from inner bulb 16 into a greater volume
of the atmosphere within lamp 10. To some extent,
shield 17 acts as a sink by absorbing heat and
redistributing it over a larger area, thus reducing
the temperature of the hot spot.
It has been found that plastic lamps can be
made from plastics having a heat distortion temperature
in excess of 130C and a thickness of 50-120 mils if the
volume of the lamp and the wattage dissipated by the
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~,
filament are such that no more than 7.8 x lO 2 watts per
cc are dissipated within the lamp. For example, lamps
having a life in excess of approximately 300 hours have
been made comprising an inner bulb dissipating
approximately 50-65 watts in lamps having an interior
volume of 830-llO0 cc's. These lamps are dimensionally
the same as the inner lamps in a four-lamp rectangular
headlamp lighting system. In operation, the lamp
breathes, ie. the plastic is not impervious to the
ambient atmosphere. As the lamp is turned on and off,
the atmosphere within the lamp heats and cools, eventually
causing some of the ambient atmosphere to permeate the
plastic. Since an inner bulb is used, the filament
is protected from the change in the atmosphere within
lamp lO and, in turn, the atmosphere within lamp lO is
protected from the extremely high temperature of the
filament such that chemical reactions are not caused
thereby. Since inner bulb 16 is permanently attached
within the reflector, the permeation of the ambient
atmosphere is sufficiently slow that sufficient
contaminants cannot accumulate within the lamp
atmosphere prior to the expiration of the life of
bulb 16 to cause deterioration of the lamp.
Suitable plastics for use in the present
invention include, by way of example only, polycarbonate
copolymers, such as sold under the trade name "Lexan"
by General Electric Company, and polysulfone. Further,
depending upon the plastic chosen, an abrasion resistant
coating comprising for example acrylate resins, melamine
resins, or siloxane resins, may be applied to the outer
surface of lens ll. While polycarbonate copolymers
and polysulfones are generally transparent, opaque
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polymers haviny a heat distortion temperature in excess
of 130C, may be utili~ed for the reflector, such as
filled phenolic resins, polyimide resins, and poly-
phenylene oxide type resins.
There is thus provided by the present invention
a practical all-plastic PAR lamp suitable for use in
vehicle lighting systems. Further, the present invention
enables one to consider alternative designs in plastic
which were heretofore impractical in glass; for example,
overall lamp shapes and lens configurations.
Having thus described the invention it will
be apparent to those of skill in the art that various
modifications can be made within the spirit and scope
of the present invention. For example, heat reflecting/
light transmitting coatings, known in the art, may be
applied within the lamp to assist the plastic in
tolerating the heat from the filament.
