Note: Descriptions are shown in the official language in which they were submitted.
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TITLE: E7LECTRIC TAMP WITH
CONDENSATE RESERVOIR AND METHOD OF OPERATION THEREOF
TECHNICAL FIELD
The present invention relates to an electric lamp, and method
of operation thereof~ having a lamp envelope that is useful
in controlling the melt temperature of the fill material
within such envelope. The present invention is particularly
of interest regarding a metal halide Tamp having such an
improved lamp envelope.
BACKGROUND ART
Lamp manufacturers are constantly searching for ways to
improve their products. One such improvement would be the
removal of mercury from discharge lamps. However, mercury is
beneficial in discharge lamps and leads to lamp systems with
high efficiency.
As an example, high intensity discharge (HID) headlamps are
an emerging application for mercury in automobiles. These
headlamps offer improved visibility, longer life and use less
energy than standard tungsten halogen headlamps. Each HID
light source contains approximately 0.5 mg of mercury and
passes the Federal TCLP test for hazardous waste. The
European Union ELV (end-of life vehicles? directive exempts
mercury-containing bulbs from its ban on mercury in vehicles.
The usage of HID headlamps is expected to increase as
introduction of less expensive, higher volume model cars
continues. In 2000, about 3.5 million HID headlamps were
used in the production of new cars worldwide. This amounts
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to less than 4 pounds of mercury. 'vJhile this amount of
mercury pales in comparison with the metric tons of mercury
used in automotive switch applications, it is desirable to
eliminate this source of mercury from the waste stream, if
possible.
Considerable effort has been expended in recent years to
produce Hg free lamps that operate at high voltages so they
can be used as retrofits with existing ballasts. Exarnples
where high doses of metal additives are used to elevate the
voltage are described by Ishigami et al. in EP 0 883 160 A1,
by Takeda et al. in EP 1 032 010 A1 and Uemura et al. i.n EP
1 150 337 A1. Examples of other voltage enhancing additives
are described by Takahashi et al. in EP 1 172 839 A2, and by
Takahashi et al. in United States Patent No. 6,265,827.
Examples of high efficacy fills of a corrosive or toxic
nature are described by Kaneko et al, in EP 1 172 840 A2.
In considering the elimination of mercury in the manufacture
of an electric lamp, an acceptable alternate fill material is
required. One problem involved in making such a selection is
that during operation of the lamp, fill condensate in the arc
stream region between opposing lamp electrodes tends to wet
the inner wall adjacent the arc stream region and cause a
film of such condensate on such wall thereby coating the
light transmitting portions of the lamp envelope and impeding
light transmission. Another problem is that the presence of
such condensate in the arc stream regior: tends to provide a
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less than desirable color stable source. A further problem
is that movement of such condensate in the arc stream region
during lamp operation causes the lamp to flicker. Further,
some replacement fill materials are so volatile that they
extinguish the arc during lamp start-up. Although voltage
within the lamp may be enhanced using fill materials having
easily vaporized chemistries, the doses of such materials to
produce acceptable voltage drop for lamp operation tend to
cause unstable operation in quartz lamp prototypes.
For demanding optical applications, such as a headlamp or
medical illumination system, transparent material for the arc
tube body is preferred. Fused silica is corarnonly used now,
but ceramics are also possible, and indeed necessary for
operation at higher temperatures or with certain reactive
chemistries. The scattering nature of polycrystalline
alumina, a perfectly good material for general illumination,
reduces the arc luminance and adversely affects the system
etendue. The best optical coupling of ceramic metal halide
lamps to reflectors or fiber systems will be achieved with
transparent ceramic vessels.
United States Patent ~lo. 5,621,275 discloses a sapphire arc
tube enclosed with a polycrystalline alumina (PCA) cap
through an interference (sintering shrinlcage) of the PCA cap
against the sapphire arc tube, for an electrodeless arc
discharge lamp. PCA arc tubes enclosed with PCA caps
through the direct joint are also described in the same
patent.
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International patent application WO 99/41761 describes a
monolithic seal for a sapphire ceramic metal halide lamp.
The monolithic seal employs the PCA cap approach of United
States Patent No. 5,621,275, except that electrode
feedthroughs that are frit-sealed to capillaries are
included.
DISChOSURE OF THE INVENTION
It is an object of the present invention to provide an
improved electric lamp, and method of operating same.
It is another object of the present invention to obviate the
disadvantages of the prior art by providing an improved
electric lamp, and method of operating same.
A further object of the present invention is to provide an
economical, efficient and high quality electric lamp, and
method of operating same.
Another object of the present invention is to provide an
electric lamp wherein excess condensate of the fill material
within the lamp envelope is removed from the arc stream
region during lamp operation, and method of operating same.
Yet a further obj ect of the present invention is to provide
an electric lamp having reduced color shifting and flicker,
and method of operating same.
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A further object of the present invention is to provide an
electric lamp having a well-defined temperature zone in which
chemical fill condensate resides during lamp operation, and
method of operating same.
Yet a further object of the present invention is to provide
an electric lamp wherein the arc is not extinguished during
start-up, and method of operating same.
Another object of the present invention is to provide an
electric lamp having easily vaporizable fill chemistries that
do not cause unstable lamp operation, and method of operating
same.
Another object of the present invention is to provide an
improved metal halide lamp, and method of operating same.
Another object of the present invention is to provide an
electric lamp having a ceramic envelope which can be dosed at
a higher salt level relative to a conventional electric lamp
having a silica envelope thereby permitting lamp operation at
relatively higher voltages without the need for mercury, and
method of operating same.
Yet a further object of the present invention is to provide
an improved electroded transparent ceramic mercury free lamp,
and method of operating same.
This invention achieves these and other objects by providing
an electric lamp comprising a sealed envelope having a wall
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defining an enclosed volume. At least a portion of the wall
is a substantially clear light transmissive window. The
enclosed volume comprises one cavity open to at least one
other cavity. A fill material is contained in the enclosed
volume. At least one electrode is provided, the electrode
being sealed through the wall and extending from a first
electrode end within the one cavity to a. second electrode end
exterior of the envelope for electrical contact. The
enclosed volume is so structured and arranged, and the fill
material is of such a chemical composition, that in an
operational mode of the lamp, fill material vaporizes in the
one cavity and excess fill material condenses in the other
cavity. The other cavity provides a cooler region within the
enclosed volume than the one cavity during the operational
mode. A method of operating the electric lamp is also
provided comprising the steps of initiating energization of
the lamp in a lamp initiation mode; vaporizing the fill
material in the one cavity; and condensing excess fill
material in the other cavity.
BRIEF DESCRIPTION OF THE DRAWINGS
This invention may be clearly understood by reference to the
attached drawings in which like reference numerals designate
like parts and in which:
FIG. 1 is an illustration of one embodiment of an electric
lamp of the present invention;
FIG. 2 is an illustration of another embodiment of an
electric lamp of the present invention;
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FIG. 3 is an illustration of one embodiment of an end cap
useful in the present invention;
FIG. 4 is an illustration of another embodiment of an end cap
useful in the present invention;
FIG. 5 is an illustration of one of two identical ends of a
further embodiment of a lamp of the present invention.
FIG. 6 is another view of the embodiment of the lamp of the
present invention illustrated in FIG. 2; and
FIG. 7 is a graph illustrating spectral output of a lamp
according to the present invention.
MODE FOR CARRYING OUT THE TNVENTxON
For a better understanding of the present invention, together
with other and further objects, advantages and capabilities
thereof, reference is made to the following disclosure and
appended claims taken in conjunction with the above-described
drawings.
FIG. 1 is an illustration of one embodiment cf a lamp of the
present invention. In. the embodiment of FIG. 1, an electric
lamp 2 is provided which comprises a sealed envelope 4.
Without limitation, envelope 4 may be fabricated from a
ceramic material. Envelope 4 includes a wall 6 that defines
an enclosed volume 8. At least a portion 10 of the wall 6 is
a substantially clear light transmissive window 12 through
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which light may be emitted from within the enclosed volume 8,
the remaining portion being translucent or opaque. In one
alternate embodiment, the wall 6 may be transparent
throughout its length. The enclosed volume 8 comprises one
cavity that forms a main portion of the enclosed volume open
to at least one other cavity that provides a subportion of
the enclosed volume. For example, in the embodiment
illustrated in FIG. l, enclosed volume 8 comprises one cavity
formed by wall 14 open to two cavities 16, 18, one at each
end of the lamp 2. Each cavity 16, 18 is open to the cavity
formed by wall 14 at a respective end of the cavity formed by
wall 14. In the embodiment illustrated in FIG. 1, each
cavity 16, 18 is a recessed subportion formed by flanged
portions 20 of the wall 6, the flanged portions extending
circumferentially about axis 22 of the envelope 4. As
explained in more detail herein, each recessed subportion 16,
18 provides a reservoir that is remote to the lamp discharge
volume located in the cavity 14.
At least one electrode is provided sealed through the wall
which forms the sealed envelope 4, the electrode extending
from one electrode end within the cavity formed by wall 14 to
a second electrode end exterior of the envelope for
electrical contact in a conventional manner. For example, in
the embodiment illustrated in FIG. 1, two opposed electrodes
24 are sealed through the wall 6 at respective wall ends 26
and 28 of the envelope 4. Respective ends 30 of the two
opposed electrodes 24 face each other within the cavity 14
and are separated by an arc stream region or gap 32 which
provides the lamp discharge volume between the electrodes in
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the conventional manner. The arc stream region 32 is
adjacent the window 12, and during lamp operation emits light
through the window, the arc stream region being the hottest
region of the lamp.
The lamp 2 includes a fill material 34 within the enclosed
volume 8. In the preferred embodiment, the fill material is
mercury free and highly volatile. The enclosed volume 8 is
structured and arranged such that in an operational mode of
the lamp, the fill material 34 vaporizes in the cavity formed
by wall 14, excess fill material gravitating to and
condensing in the cavities 16, 18. To this end, each section
of wall 6 adjacent the recessed subportions 16, 18 is
structured and arranged to provide sufficient heat radiation
to maintain a lower temperature in the recessed subportions
16, 18 than in the arc stream region 32 where heating of the
plasma is localized between electrode tips during normal lamp
operation. For example, each. wall section of wall 6 adjacent
the recessed subportions 16, 18 is provided ~_n such a manner
as to (a) form adequate volume to contain the condensed
excess chemical fill and (b) be located at a relatively
greater distance in comparison to the window 12 from the arc
stream region 32, to provide a lamp cold spot to which such
condensate can migrate during lamp operation. As a practical
matter, in this manner there is enhanced condensation of
excess fill material in the recessed subportions 16, 18
relative to the arc stream region 32.
FIG. 2 illustrates another embodiment o:E the present
invention. In the embodiment illustrated in FIG. 2, a 1_amp
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100 is provided which comprises a sealed envelope 102 having
a wall that defines an enclosed volume 104. In this
embodiment, the wail which forms the sealed envelope 102
comprises a tubular portion 106, having a first end portion
108 and an opposite second end portion 110, a first cap 112
attached to the first end portion, and a second cap 114
attached to the second end portion. A first electrode 116
extends through the cap 112 at 118, and a second electrode
i16 extends through the cap 114 at 120.
In the embodiment illustrated in FIG. 2, the enclosed volume
104 includes one cavity, within the tubular portion 106,
formed by wall 122 of the tubular portion, a second cavity
124 between the tubular portion and first cap 112, and a
third cavity 126 between the tubular portion and the second
end cap 114. Cavities 124 and 126 perform the same function
as cavities 16 and 18 of the embodiment of FIG. 1. The
volume of the cavities 124 and 126 may be controlled by cap
configuration and shrinkage of each cap during fabrication of
the lamp 100 as explained herein. Each r_avity 124 and 126 is
located between the tubular portion 106 and each respective
cap 112, 114 at a respective end of the tubular portion, In
an operational mode of the lamp 100, a mercury-free fill
material 128 contained within the enclosed volume 104
vaporizes in the cavity formed by wall 122, excess fill
material migrating to and condensing in 'the cold spots
provided at cavities 124 and 126. As in the embodiment of
FIG. 1, cavities 124 and 126 provide a cooler region within
the enclosed volume 104 than the cavity formed by wall 122,
during the operational mode.
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In the example illustrated in FIG. 2, the caps 112 and 114
each include extended capillary sections 132 and 134,
respectively, which form capillaries through which respective
electrodes 116 extend. The caps 112 and 114 fit onto the
tubular portion 106 and are sintered thereto to provide a
hermetic arc tube that forms the body of lamp 100. The
capillary sections 132 and 134 extend away from enc:Losed
volume 104. Ir_ the embodiment illustrated in FIG. 2, each
electrode 116 includes a length 136 of tungsten, a length 138
of molybdenum and a length 140 of niobium. The electrodes
116 are inserted through the end caps 112 and 114 at the
respective capillary sections 132 and 134, such that
respective electrode ends 142 and 144 face each other.
The arc stream region between the ends 142 and 144 provides
the lamp discharge volume 146. The elect rodes 116 are sealed
into the capillary sections 132 and 134 with a frit glass 148
in a conventional manner. It should be noted that the end of
each capillary section 132 and 134 adjacent respective
cavities 124 and 126 is open to the enclosed volume 104.
Therefore, some of the condensate formed during lamp
operation will migrate into the capillaries formed by the
capillary sections 132 and 134, However, the volume and
location of such capillaries is such that the capillaries do
not provide a satisfactory cold spot for collection of excess
fill condensate. To the contrary, in the absence of cavities
124 and 126, the fill condensate will be distributed randomly
and will tend to ooze back into the arc tube body, that is,
the volume provided by the surface 122, .and cause corrosion.
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This results from th.e fact that the melt pool is spatially
extended over a region where a temperature gradient and hence
solubility gradient exists. The cavities 124, 126, on the
other hand, act as a receptacle for the fill condensate that
~ would ordinarily ooze into the arc tube body, the condensate
being trapped within the "moat-like" cavities.
Prior to final sealing, the lamp is dosed with the chemical
fill material, filled with inert gas and hermetically sealed
in a conventional manner. Some examples of the fill material
and inert gas are discussed herein.
In a preferred embodiment of FIG.
2, the lamp 100 is a metal
halide lamp that is made from three pieces: a transparent
cylindrical tubular portion 106, a:nd i:wo translucent
polycrystalline molded end caps 112 and 114. The end caps
112 and 114 are sintered onto 'the cylindrical portion 106.
The cylindrical portion 106 is a substantially transparent
ceramic material such as a single crystal fully dense
sapphire tube. Such material is
readily available
commercially. Without limitation,
other transparent ceramic
materials such as yttrium aiumina
garnet (YAG) could also be
used. The caps 112 and 114 are PCA. In the manufacturing
of
the lamp 100, the caps 112 and 114 are structured and
arranged such that during sintering
of th.e caps to the
tubular portion 106, shrinkage of the caps increases the
volume of cavities 124 and 126 and .affixes the caps to the
tubular portion. This results from the facts that during
sintering the PCA capa 124 and 126 shrink as they densify,
but the ceramic tubular portion 106, being fully dense, does
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not. During operation of the lamp 100, the cavities 124 and
126 hold the excess condensed fill material. In essence, the
cavities 124 and 126 act as a constant temperature reservoir
of the condensed fill material. By manipulating the shape
and degree of shrinkage of the cap to control the
configuration of the cavities 124 and 126, the volume of the
cavities 124 and 126 can be controlled t.o contain the desired
amount of the excess condensed fill material produced during
lamp operation. Similarly, by adjusting the thickness of the
cap walls, or by the addition of exterior heat sinking, or
radiating features on the cap, the caps can function as heat
sinks to further adjust the temperature of the condensate
reservoirs. For example, FIG. 3 illustrates a cap 250
similar to caps 112 and 114 wherein the cap 150 includes a
surface coating 152, which promotes thermal radiation.
Without limitation, coating 152 may be a graphite, refractory
metal or metal oxide end paint. In another example
illustrated in FIG. 4r a cap 154 similar to caps 112 and 114
includes projections 156 along the cap surface 158 to promote
thermal radiation.
The recessed cavities 124 and 126 are illustrative of one
configuration of recessed subportions that provide cold spots
for condensed excess fill material during lamp operation.
FIG. 5 illustrates another embodiment of a lamp of the
present invention identical to the embodiment of FIG. 2 with
the exception of the configuration of the inner wall of the
end caps, and recessed. cavities formed thereby, only one end
cap being illustrated. In particular, in FIG. 2 an inner
wall of each end cap 112, 114 is meniscus {dish) shaped at
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walls 160 and 162. I:n contrast, in the embodiment of FIG. 5,
the inner walls 164 and 166 of end ca~> 168 of lamp 170 are
flat. The embodiment of FIG. 5 is identical to the
embodiment of FIG. 6 with the exception of the inner walls
164 and 166.
Referring once again to FIG. 2, the reservoirs formed at
cavities 124 and 126 control the melt temperature within the
lamp 100. The cavities 124 and 126 are closer to the lamp
discharge volume, and therefore the lamp arc, than are the
capillaries formed by the capillary sections 132 and 134, and
as such are the hottest reservoirs provided for the salt
condensate thereby controlling the vapor pressure and
composition of the gases within the lamp during lamp
operation. As a result of the migration of the fill material
condensate from the arc stream region to the cooler
reservoirs 124 and 126, the condensate does not wet the inner
wall 122 and cause a film of salt on the interior of the arc
chamber. Consequently, vapor material for the plasma within
the enclosed volume 104 may be provided at constant pressure,
but without condensate coating the light emitting portions of
the clear sapphire and impeding light transmission. This
provides a more color stable source and one substantially
free of flicker which is important for optical applications
such as use of the metal halide lamp as a headlight or
projector source. A source of lamp f7_icker is introduced
when the film of salt moves during lamp operation.
It should be noted that some chemistries are so volatile that
they extinguish the arc during lamp start-up. Easily
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vaporized chemistries of some fill materials such as gallium
halides are often used as voltage enhancing additives in Hg
free lamps. The doses of such fill material needed to
produce acceptable voltage drop for lamp operation cause
unstable operation is quartz lamp prototypes. The current
art of producing quartz lamps leaves no reservoir.for the
salt, that is, the arc chamber is the only salt repository.
With the present invention, the fill condensate is localized
away from the arc stream region and turbulent fluid flow
IO around the electrodes, and reduced heating of the condensate
contributes to a stable, well-behaved ignition and warm up in
similarly dosed lamps. In this way the lamp can be overdosed
with salts, while functionally appearing to be minimally
dosed.
One method of fabricating the electric lamp of the present
invention will now be described with reference to the
electric lamp 100, illustrated in FIGS. 2 and 6. FIG. 6 is
identical to FIG. 2 and has been included so that the lamp
dimensions can be clearly shown.
A single-crystal aluminum oxide (sapphire) cylindrical
tubular portion 106 was obtained having a 3.15 millimeters
outer diameter 172 and a 1.5 millimeters inner diameter 174.
Tubular portions of this type are available from Saphikon,
Inc. The tubular portion was cut into 10 millimeter lengths
176. Polycrystalline alumina end caps 112 and 114 were
formed using high purity aluminum oxide powder (CR6,
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Baikowski) (less than 500 ppm impurities) doped with 200 ppm
Mg0 + 20 ppm Y2O3 -~- 400 ppm Zr02 as sintering aids. The doped
alumina powder was mixed with a wax binder and molded to form
the caps 112 and 114, including the capillary sections 132
and 134. The shape of the caps 112 and 1148 and therefore
the shape of the cavities 124 and 126, was determined by the
shape of the mold used for forming the caps. The caps so
formed were fired in air to 1000 degrees Celsius to remove
the binder and strengthen and maintain the shape of the raps.
The caps 112 and 114 were then placed onto respective ends
108 and 110 of the tubular portion 106 and fired vertically
at 1330 degrees Celsius in air causing partial densification
and shrinkage, thereby locking the caps onto the tubular
portion. The assembled sapphire tubular portion 106 with end
caps 112 and 114 attached thereto were then final-sintered in
flowing nitrogen with 8o hydrogen at 1890 degrees Celsius for
one hour. As the end caps 112 and 114 were sintered onto the
sapphire tubular portion 106, a significant amount of
dimensional shrinkage and densification occurred in the PCA
caps, while the fully dense sapphire tubular portion remained
unchanged. In this manner, a circumferential hermetic seal
was formed between the sapphire and the PCA where the caps
112 and 114 were previously locked onto the tubular portion
106, and the cavities 124 and 126, which form the respective
salt reservoirs, grew at the end of the tubular portion. In
particular, in the embodiment illustrated in FIGS. 2 and 6,
prior to sintering, the length 178 of the end caps 112, 114
was 21.4 millimeters and the thickness 180 was 0.85
millimeters. The diameter 182 of each respective cavity 124,
126 was 3.9 millimeters and the depth 184 was 0.7
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millimeters. Upon completion of sintE;ring, the length 178
was 16.3 millimeters, the thickness 180 was 0.65 millimeters,
the diameter 182 was 3.15 millimeters and the depth 184 was
0.5 millimeters. It will be apparent to those skilled in the
art that the predetermined shape and material of the caps
112, 114 and the degree of shrinkage thereof will determine
the configuration and volume of the cavities 124, 126. It
will further be apparent to those skilled in the art that by
varying processing parameters such as the sintering
temperature and time, the degree of shrinkage can be
controlled. The degree of shrinkage and hence the final
volume of the cavities 124, 126 will depend upon the volume
of fill condensate the cavities will be expected. to
accommodate to prevent condensate interference with lamp
operation. ~nlithout limitation, in lamps of the type
illustrated in FIGS. 2 and 6, the depth 184 will be about 0.1
to 0.25 times the diameter 172 of the sapphire tube 106,
preferably 0.1 times such diameter. Since the depth 184 is
so small, the thermal gradient across the hottest melt pool
is reduced. Consequently, the solubility gradient is reduced
and corrosion should be reduced. In addi,vion, since the
gradient is reduced, the vapor pressure above the salt is
more precisely defined, and the lamp is rr~ore color stable.
The electrodes 116 were inserted through the capillary
sections 132 and 134, respectively and sealed in place using
the glass frit 148. Electrodes 116 were 5 millimeters in
length and 0.25 millimeters in diameter. The length of the
lamp discharge volume 146 was 4.2 millimeters nominal. Prior
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to final sealing, the lamp was dosed in a conventional manner
with a mercury-free highly volatile chemical fill material
128 and filled with xenon, an inert gas. Other rare gases
and mixtures may be used. The lamp 100 was then hermetically
sealed in a conventional manner.
The chemical fill of the lamp of the present invention will
typically be a highly volatile fill material by which is
meant that during lamp operation fill material vaporizes in
the arc stream region, and excess fill material migrates to
and condenses in the recessed subportion(s) of the enclosed
volume of the lamp. V~7ithout limitation, the chemical fill of
the present invention can include gallium, indium, thallium
and aluminum halides, as for example, GaI3, InI, InI3, AlI3
and TlI. Rare earth halides may also be used. Although the
lamp of the present invention is particularly useful as a
mercury-free lamp, mercury can be included in the chemical
fill if desired. An. example would be the use of mercury
halides. One or more of the foregoing fill materials may be
combined with other salts such as scandium halides or rare
earth halides. The present invention is not limited to any
particular fill material so long as the fill material
vaporizes in the main portion of the lamp and condenses in
the recessed subportion as described herein.
The lamp of the present invention and conventional silica
lamps dosed with high concentrations of easily vapor~_zed
salts were tested and v~he results compared. All of the lamps
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were tested on a 500 Hz square wave ballast capable of
developing 500 VOC a:nd delivering more than 2 amperes. The
fills in two of the conventional silica lamps tested were 1
mg GaI3, 0.34 mg of Type 4 rare earth chemistry (19.5% DyI3,
19 . 5 o HoI3, 19 . 5 o TmI3, 32 . 5% NaI and 9. 0% T7_I by weight ) and
8 bar Xenon. The fill of a third silica lamp tested was 1 mg
GaI3, 0.8 mg InI, 0.24 mg of the same Type 4 rare earth
chemistry and 8 bar Xenon. The volume of each silica lamp
tested was about 23 mm3.
In testing the foregoing conventional silica lamps, each lamp
would start at room temperature, but th.e Gallium and Indium
halides would vaporize too rapidly. The vaporized fill had
no place to go except into the vapor state, there being no
colder region to allow for re-condensing of the vaporized
fill. As a result, lamp voltage rose rapidly due to wild and
uncontrolled impedance changes in the lamp, causing the lamp
to extinguish and leave salt residue all over the interior
surface of the arc chamber. Repeated attempts to sustain
discharge in each of these silica lamps failed. It was noted
that the salt splattered over the entire inner surface area,
which is indicative of an abrupt, uncontrolled interruption
of lamp operation.
A lamp of the present invention of the type illustrated in
FIGS. I and 6, was fabricated using the method and dimensions
described above. Whereas the volume of the silica lamps
tested was about 23 mm3, the volume of the lamp of the
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present invention was smaller than about 19.5 mm3. Yet, the
lamp of the present invention was dosed with a chemical fill
of 4 mg of InI, 1 mg of NaI and 5 bar of Xenon. The average
density of salt within the enclosed volume 104 was about
5g/cc or 5 mg/mm3. The volume of each cavity 124 and 126 was
about 0.5 mm3. Therefore, each cavity 124 and 126 could
contain roughly half of the salt dose amount, or the full
amount in both. Although some salt vaporized as the lamp
heated up, the salt zone migrated to the cavities 124 and
126, which provided remote colder regions for the salt to re-
condense in. It is in this manner that the salt zone was
removed from the arc stream region 146 allowing the main
discharge chamber to heat less rapidly than in the silica
lamp. This avoided the depositing of salt residue on the
interior surface of the arc chamber. In addition, the lamp
operated in a stable fashion for hours. Although some of the
salt condensed in the capillaries formed by the capillary
regions 132, 134, the temperature distributian was such that
the salt in the cavities 124, 126 was at a higher temperature
than the salt in the capilaary regions, such higher
temperature salt controlling the vapor pz:essure inside of the
lamp.
The lamp of the present invention allows for the use of at
least 6 to 7 times as much salt on a per-volume basis in the
enclosed volume of th.e lamp than in a conventional silica
lamp. This ability to dose at a higher salt level ultimately
permits operation of the lamp at a higher voltage without the
need for mercury, although mercury can be ~_ncluded in the
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fill if desired. In addition, the higher salt density in the
vapor, which can be achieved in a stable fashion, provides
improved radiation properties.
The spectral output of the foregoing tested lamp of the
present invention is illustrated in FIG. 7.
The voltages seen ir. the mercury free conventional silica
lamps with voltage enhancing additive are about 42V. Higher
voltages may be achieved with reduced lamp efficacy at the
onset of instability. In the lamp of the present invention,
voltages on the order of 60V with stable operation are
routinely seen. The higher voltage translates into less
amperage for the required power levels, the lamp having the
characteristics illustrated in FIG. 7 being 35W. This means
that electrodes developed for use in mercury containing lamps
may be used without fear of meltback or evaporation. The
lower voltage silica lamps require about twice the steady
state current and may have problems with excessive wall
darkening due to elevated electrode tip temperature. For
example, a mercury containing 35W headlamp operates at about
82V with 0.44 A.
The embodiments which have been described herein are but some
of several which utilize this invention and are set forth
here by way of illustration but not of limitation. It is
apparent that many other embodiments which will be readily
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CA 02422433 2003-03-18
D 02-1-835 PATENT APPLICATION
apparent to those skilled in the art may be made without
departing materially from the spirit and scope of this
invention.
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