Oxygen protected electric lamp

A double jacket arc lamp is disclosed. A double jacket arc lamp may be formed with an electric lamp capsule having an inner light transmissive envelope with electrical leads extending from the inner envelope through a first seal, and a second light transmissive envelope sealed to the inner envelope at a second seal to substantially surround the electric lamp capsule and form an enclosed volume between a portion of the exterior of the inner envelope and the interior of the outer envelope, and leaving the leads unenclosed by the outer envelope. The lamp leads then pass through only one seal, and have no portions exposed in the intermediate insulating volume. The intermediate volume of the double jacket arc lamp may include a fill gas including a noninert component. In particular, the fill gas may include oxygen which is thought to have useful gettering and glass preservation attributes. The use of oxygen in the fill gas allows the lamp to operate at a higher temperature, and therefore more efficiently.

CROSS REFERENCE 
Basic aspects of this invention are disclosed in a simultaneously filed, 
copending application, Ser. No. 287,952, Double Jacket Lamp, by Merle A. 
Morris, Michael L. Martin, Louis D. Neff and George B. Kendrick. 
1. Technical Field 
The present invention relates to electric lamps, and electric lamps with 
double envelopes. In particular, the present invention relates to fill 
gases present in the intermediate volume in double envelope electric 
lamps. 
2. Background Art 
Arc discharge lamps generate light by passing an electric current through 
an ionized gas. The discharge gas, including various dopants selected for 
spectral features, is thereby heated to yield light. The surrounding 
envelope is necessarily heated at the same time which helps vaporize any 
solid or liquid dopants. The dopants evaporate to a degree generally 
determined by the coldest temperature of the inside envelope surface, 
which therefore affects the lamp spectrum. Temperature control in the 
envelope is then important to proper lamp function. One method of 
controlling temperature is to jacket the lamp envelope with an outer 
envelope. The outer envelope allows the inner envelope to be heated 
uniformly to a higher temperature. The outer jacket reduces thermal 
gradients in the inner envelope, conserves the arc tube heat, and shields 
the inner envelope from exterior temperature changes. The outer jacket may 
also be used to filter a certain portion of the energy spectrum, for 
example, ultraviolet radiation, lower the temperature of the outer exposed 
surface, dampen any corona effect at high frequencies, or simply to 
contain any lamp failures that may occur in the capsule. 
The known method of jacketing the inner envelope is to form the inner lamp 
as an ordinary lamp with a first seal and the various appended leads. The 
inner lamp is then inserted in an outer envelope and held in place with 
internal support structures. An intermediate volume is then formed between 
the inner and outer envelops which may be evacuated, or filled with an 
inert gas. The support structures are typically composed of wire rods, and 
metal straps; but may include glass beads, ceramic rods, or other 
nonconductive insulative elements. The metal leads from the inner lamp are 
then lead out through the intermediate volume to a second seal made with 
the outer envelope. 
The existing wire structures are expensive to make. The internal support 
structures are expensive, not so much for the cost of materials, but for 
the cost of assembling, aligning, testing, and similar labor costs. Also, 
the outer envelope must be large enough to enclose the internal 
positioning structures, in addition to providing an adequate insulating 
volume. The large exterior envelope is an additional material cost. The 
large exterior envelope further requires a large supporting base, and 
therefore a large socket, and therefore a large lamp housing and so on. It 
is a general design advantage to make a lamp small and thereby reduce all 
the subsequent related lamp costs. There is then a need to provide a 
double jacketed lamp with a small size, and a similar need for a double 
jacketed lamp without expensive metal positioning equipment. 
The existing wire support structures may also be difficult to align. The 
seals made in press sealing an envelope are commonly made by pressing the 
surrounding molten envelope material against molybdenum foils. The foils 
can expand and contract with the envelope during normal thermal cycling of 
the lamp. Unfortunately, the foils have little rigidity and may be 
twisted, or flexed during assembly. The result can be a misalignment of 
the inner envelope with respect to the outer envelope. Anticipating such 
misalignment accounts for some of the expensive internal positioning 
hardware seen in double jacketed lamps. There is then a need to provide a 
sealing mechanism for double jacketed lamps that reduces the possibility 
of misalignment. 
The existing structure may be subject to internal electrical faults. The 
support structure between the inner envelope and the outer envelope is 
usually metal, and therefore conductive. In arc discharge lamps, high 
voltages are used to strike the arc, and the nearby metal support 
structures are likely sources for voltage leakage. Also, the inner, or 
outer envelope may leak gas, allowing the changing gas composition to 
alter the electrically insulating aspects of the intermediate volume. A 
secondary glow may result between the leads passing through the 
intermediate volume, altering the quality of the lamp, and possibly 
leading to early lamp failure. Getters, insulating separators and similar 
protective structures may then be required in the intermediate volume, but 
increasing the lamp cost. There is then a need for a double jacketed lamp 
that reduces the possibility of electrical fault in the intermediate 
volume. 
Arc discharge lamps often use high voltage, high frequency pulses to start 
or restart the arc. Operating at high frequencies and high voltages offers 
the possibility of a glow discharge or corona around both metal and glass 
parts in the lamp. The corona drains the useful energy supplied to the 
lamp making it more difficult to ignite, and may cause other problems in 
lamp operation. The outer lamp leads may be potted with high dielectric 
cements to prevent the glow discharge or corona affect, but around the 
inner lamp capsule potting is not possible. There is then a need to 
prevent, suppress or make the corona glow harmless in the intermediate 
volume. 
In its clear state, glass is an amorphous or non-crystalline material. At 
elevated temperatures, such as the operating temperatures of electric 
lamps, glass becomes more chemically active. With increasing temperature, 
glass tends to devitrify, that is, become crystalline, due either to the 
loss of oxygen or the introduction of contaminants at the surface. The 
devitrifying surface reactions may start as a single crystal structure 
which in time grows to greater depths in the glass. Crystallization causes 
the glass to loose transparency and become more brittle. Lamps in general, 
but arc lamps in particular operate more efficiently at higher 
temperatures. To avoid devitrification, lamps are therefore usually 
operated at a temperature less than that causing devitrification. The 
lower temperature is then at the expense of lamp efficiency. There is then 
a need to prevent devitrification while allowing lamps, and particularly 
arc discharge lamps, to be operated at higher, more efficient 
temperatures. 
Often it is desirable to coat certain portions of a lamp with reflective, 
heat absorbing, filtering or similar coatings. The coatings are generally 
oxides of active metals such as zirconium, tantalum, or magnesium but the 
use of other coatings is known. At high temperatures, the metal oxide 
coatings may break down giving up oxygen, which is then free to combine 
elsewhere, or otherwise interfere with the lamp chemistry or composition. 
The coating may also be attacked by contaminants, or in breaking down, 
react with the envelope. The coating, in breaking down, also is less 
functional and may fail to be adequate in extended service. There is then 
a need to protect lamp coatings, and particularly metal oxide coatings, 
without damaging the lamp chemistry, or composition. 
Metal exposed to hot oxygen gas normally deteriorates. In particular, the 
oxygen in the intermediate volume with the various included metal parts as 
shown in the prior art is considered a problem to be avoided. In 
particular, the oxygen may track along the sealed leads and oxidize the 
seal components, such as foils. As the seal components oxidize, the seal 
may open up because of a thermal mismatch in the new compounds, and 
because the conductivity of the leads may be reduced, resulting in a 
hotter running lead. The hotter lead may cause further deterioration of 
the seal. Corroded seals are a problem to be avoided, so the use of only 
inert fill gas is though to be standard practice. 
Hydrogen may also be a problem element an inner lamp capsule, and 
particularly in an arc discharge capsule. Hydrogen in an arc discharge 
lamp has undesirable affects on both lamp starting and lamp life. Hydrogen 
is a small, and chemically active element. Hydrogen is small enough to 
easily migrate through glass, and at elevated temperatures, such as those 
found in electric lamps, hydrogen migration may be acute. There is then a 
need to prevent hydrogen activity in the inner lamp capsule, and to 
prevent migration of hydrogen to the inner lamp capsule. 
U.S. Pat. No. 4,754,195 to Rasch et al. for High Pressure Discharge Lamp, 
and Method of its Manufacture shows an inner high pressure discharge lamp 
with a press seal positioned in an outer envelope also sealed with a press 
seal. The outer press seal is made with the lamp leads, so the inner lamp 
floats on exposed lamp leads completely internal to the outer envelope. 
U.S. Pat. No. 4,717,852 to Dobrusskin et al. for Low-Power, High-Pressure 
Discharge Lamp shows an inner arc discharge lamp enclosed in an outer 
envelope where the outer envelope seals on the lamp leads leaving the 
inner envelope free floating. 
DISCLOSURE OF THE INVENTION 
A fill gas including a noninert component may be used to protect an 
included lamp capsule. In Particular, oxygen may be used in an 
intermediate volume in a double jacketed lamp. A double jacket arc lamp 
may formed as an electric lamp capsule having an inner light transmissive 
envelope with electrical leads extending from the inner envelope through a 
first seal, a second light transmissive envelope sealed to the inner 
envelope at a second seal to substantially surround the electric lamp 
capsule and form an enclosed volume between a portion of the exterior of 
the inner envelope and the interior of the outer envelope with the leads 
extending to the exterior of the outer envelope, and a fill gas may be 
included in the intermediate volume including a noninert component.

BEST MODE FOR CARRYING OUT THE INVENTION 
FIG. 1 shows a preferred embodiment of an axial cross section of a 
preferred embodiment of a double jacket lamp. The double jacket lamp 10 
includes an inner lamp capsule 12 having a means of generating light from 
electric energy. The preferred inner lamp capsule 12 is an arc discharge 
lamp capsule, but the lamp structure and gas fill discussed here may be 
equivalently extended to filamentary lamps with ordinary engineering. The 
lamp capsule 12 includes an inner envelope 14 with an outer surface 16, 
and a seal. The seal may be any conventional seal for the type of inner 
lamp capsule 12 used. Arc discharge and tungsten halogen lamp capsules are 
typically press sealed, and the preferred seal is a press seal 18. 
Adjacent the point of the inner lamp capsule 12, the press seal 18 
includes a capsule side Portion 20. The press seal 18 next includes a 
central portion 22 where the envelope material is pressed to form the 
seal. The press seal 18 then includes an outer portion 24 where two leads 
26 emerge to the exterior. 
In the preferred embodiment, the inner lamp capsule 12 is formed by closing 
one end of a tube, and press sealing an opposite end to capture the inner 
electrode leads, foils, and outer leads 26. The preferred press seal 18 is 
made at a point intermediate the tube ends so the enclosed inner lamp 
capsule 12 is on one side of the press seal 18, and on the opposite side 
of the press seal 18 is a radial extension of the inner capsule envelope 
material, offset from the outer leads 26. The radial extension then forms 
a skirt 28. The skirt 28 preferably has sufficient diameter and length to 
conveniently seal with. The skirt 28 may be conveniently formed as an 
unpressed, residual portion of the original tubing, and having an outer 
diameter approximately equal to the inner envelope diameter. The skirt 28 
may also be formed with a tool, or added as a separate piece. 
Substantially surrounding the inner envelope 14, but allowing the lamp 
leads 26 to pass to the exterior for electrical connection is an outer 
envelope 30 with an inside surface 32. The material used in forming the 
inner envelope 14 may be repeated in the outer envelope 30, for example 
both envelopes may be made from quartz. Using a different outer envelope 
material allows the lamp spectrum to be sculpted for chosen purposes. In 
particular, a quartz inner envelope 14 transmits the ultraviolet light 
typical of a mercury doped arc lamp. Portions of the ultraviolet light may 
be cut off at different wavelengths, according to the formulation of the 
outer envelope material. For example, an ozone generating lamp, or an 
ozone free germicidal lamp may be made according to different glass 
formulations. For example, an ozone free quartz may be made by doping 
quartz with titanium or vanadium. Other dopants and formulations similarly 
alter the spectral transmission. The inside surface 32 may be clear, or 
may be etched, coated or roughened to provide a diffused source image. 
The inside surface 32 may also be dichroicly coated to alter the spectrum 
for safety, color temperature, heat control, color rendering, or similar 
purposes. The typical surface coatings include layers of metal oxides but 
many others are known. The inside surface 32 may be phosphor coated or 
similarly treated, since no chemical interaction occurs between the 
internal chemistry of the inner lamp capsule 12, and the inside surface 
32. The inside surface 32 is similarly protected from the exterior. 
The geometric extent of the inner envelope 14 is for the most part smaller 
than the enclosing outer envelope 30. It is also possible for the inner 
envelope 14 to contact portions of the outer envelope 26, and in 
particular, such contacts may be useful in initial positioning the inner 
lamp capsule 12 with respect to the outer envelope 30. Formed between the 
inner envelope 14 and the outer envelope 30 is then an insulating 
intermediate volume 34. The inner lamp capsule 12 is sealed in the region 
of the press seal 18 to the outer envelope 30. The seal with the inner 
lamp capsule 12 may be made on the capsule side portion 20, or the central 
portion 22. While both the capsule side portion 20, and central portion 22 
are possible seal point, sealing to these areas is felt to likely stress, 
or distort the inner lamp capsule, and therefore require more care. The 
preferred sealing point is on the outer portion 24, and in particular to 
an extended skirt 28. 
With the inner lamp capsule 12 sealed to the outer envelope 30, a large 
portion of the inner lamp capsule 12, is then enclosed and insulated by 
the intermediate volume 34. The intermediate volume 34 may be a vacuum, or 
may be filled with an inert fill gas 36, such as argon, or nitrogen. Since 
the intermediate volume 34 is segregated from the internal light 
generating mechanisms of the inner lamp capsule 12, and the exterior, 
there is no reason why noninert gases may not also be used. The 
intermediate volume may then be filled with a noninert fill gas such as 
oxygen mixed with an inert gas, or even pure oxygen. Other active gases or 
combinations thereof may be formulated for inclusion in the intermediate 
volume. 
The inner envelope 14 should be thermally matched to the outer envelope 30 
to avoid cracking or separation during normal thermal cycling of the lamp. 
The preferred method is to thermally match the materials of the inner 
envelope 14 or at least the portion coupling with the outer envelope 30 
such as skirt 28. The simplest method is to use the same materials for 
both envelopes. Quartz is a relatively expensive material to make lamp 
envelops from, but quartz has good high temperature qualities. Glasses on 
the other hand are less expensive, provide adequate protection from the 
exterior, and can be formulated for spectral transmission. There is then 
an advantage to using quartz with a thermal expansion of about 
5.5.times.10.sup.-7 cm/cm degree C. as the inner envelope 14, and a glass, 
such as a borosilicate with a thermal expansion of about 30 to 
50.times.10.sup.-7 cm/cm degree C. as an outer envelope 30. An alternative 
method of thermal matching is to include a spacer ring 40 between the 
inner envelope 14 and outer envelope 30 in the zone where the seal is made 
between the two envelopes such as between the skirt 28 and envelope 30. 
The spacer ring 40 may be a tubular section sized to fit between the 
coupling envelope portions and made of a material with an intermediate 
coefficient of thermal expansion thereby forming a graded seal. The spacer 
ring 40 seals on one side to the inner envelope 14, and seals on an 
opposite side to the outer envelope 30. FIG. 2 shows an axial cross 
section of an alternative embodiment of a double jacketed arc lamp with a 
spacer ring in the seal area. Other graded seal methods, such as coating 
intermediate materials, may be used in the inner envelope to outer 
envelope seal. 
In one embodiment, the double jacket arc lamp may be manufactured by first 
forming an arc lamp capsule with an extended portion radiating from the 
seal. The inner lamp capsule 12, and outer envelope 30 may then be merged 
to form a hermetic seal at a necked down portion of the outer envelope 30. 
The inner lamp capsule 12 may then be said to cork the outer envelope 30. 
With the inner lamp capsule 12 sealed to the outer envelope 30 around the 
seal area, the intermediate volume 34 is then evacuated, or filled with a 
fill gas 36. The opposite, second end of the outer envelope 30, the end 
away from the lamp leads 26 is then sealed. The second outer envelope seal 
may be a press seal, but the preferred method is to tip off the outer 
envelope 30. The inner envelope 14 is then substantially thermally 
insulated from the exterior. The inside surface 32 of the outer envelope 
may be safely treated with coatings to alter the light produced without 
interference from the inner envelope chemistry. No metal, or other voltage 
carrying elements are required in the intermediate volume 34, and no metal 
or conductive elements are required to be positioned near the lamp leads 
26. The inner lamp envelope 14 is aligned without internal metal hardware. 
In an alternative embodiment, the inner lamp capsule may be made as a 
double ended inner lamp capsule 42, with lamp leads 26 emerging at 
opposite ends of the inner lamp capsule. Two skirts 28 may be formed on 
opposite ends, with two press seals 18 intermediately positioned. An 
enclosing outer double ended envelope may be slipped over the inner lamp 
capsule 22 and coupled at each end to the two skirts 28. FIG. 3 shows a 
preferred embodiment of an axial cross section of an of a double jacketed, 
double ended arc lamp. 
To make the overall lamp structure small, its is felt to be preferable to 
neck the outer envelope 30, 40 down to the inner lamp capsule 12, 42. The 
decreasing diameter of the necked portion may then be captured in a 
cemented type base (FIG. 8). Alternatively, a screw type base may be 
preferred. It is then convenient that the lamp base not neck down but 
maintain a fairly constant radial extension to securely meet with a screw 
type base (FIG. 4). 
FIGS. 4A-F show a series of cross sectional views of the stages of 
manufacture of an alternative single ended double jacketed lamp, for 
example, one intended for use in a standard screw type base. The outer 
envelope need not be necked down to be joined to the inner lamp capsule. 
The skirt portion may in one alternative be flared out to meet the outer 
envelope. FIG. 4A shows a tube 50 of a meltable, light transmissive 
material of the type used to make filamentary, or arc discharge lamps. The 
end of the tube 50 may be heated by, for example flames, to soften the 
end. A tool may be inserted in the tube cavity, adjacent the softened tube 
end, and used to flare the softened tube ends outwards. The flare may be 
made in sections, approximately coaxial with the tube 50. For example, a 
first section may be a funnel shaped portion 52, leading with further 
curvature to a disk portion 54. The disk portion 54 need not have a large 
radial extension 56, but may be limited to the radial depth of the 
intermediate volume 34 to be created. FIG. 4B shows a tube with a flared 
end. 
The flared end tube may then receive a light generating means 58, such as a 
filament, or arc discharge electrode structure, positioned in the inner 
tube cavity, and the outer leads projecting from the flared tube end. The 
flared tube may then be press sealed 60 by methods known in the art to 
capture the filament or electrode structure. The press seal 60 is 
preferably made axially adjacent the flared end of the tube. Press sealing 
the flared end may thin portions of the envelope material to an 
unacceptable degree, while press sealing offset from the flared end is 
thought to needlessly lengthen the lamp structure. FIG. 4C shows a flared 
tube, equipped with an electrode structure 58, and press sealed 60 
adjacent the flared end of the tube. 
The inner lamp capsule may then be filled with a gas fill and closed by 
known methods to complete the inner lamp capsule 62 structure. FIG. 4D 
shows a completed inner lamp capsule 62 with a flared base extending to a 
disk portion 54. The disk portion 54 is functionally equivalent to the 
skirt 28 for sealing purposes. 
The next step of assembly is to enclose the inner lamp capsule 62 with an 
outer envelope 64. The inner lamp capsule 62 may be enclosed by a straight 
tubular section with a first end 66 and second end 68, so the extended 
disk portion 54 and first end 66 are butted together. The junction of the 
extended disk portion 54, and the first end 66 may be heated and allowed 
to fuse, thereby forming a hermetic seal. FIG. 4E shows an inner lamp 
capsule 62 with a flared base mated to an enclosing straight tube outer 
envelope 64. 
The inner lamp capsule 62 and the enclosing, but offset outer envelope 64 
form an intermediate volume 70 which may be filled with a selected fill 
gas 72. The second end 68 may then be closed by know methods, such as 
press sealing, or tipping off. FIG. 4F shows an inner lamp capsule 62 with 
a flared base mated to an enclosing straight tube outer envelope 64 with 
the second end 68 of the outer envelope tipped off. 
A similar method of construction may be used in forming a double ended 
lamp. FIGS. 5A-D show a series of cross sectional views of the stages of 
manufacture of an alternative double ended double jacketed lamp. A tube 78 
may be flared at both ends 80, 82, FIG. 5B, forming flared skirt like 
portions, for example disk portions 84, 86 extending radially from the 
ends of the tube. The first disk portion 84 may have a diameter 88 some 
what larger than the second disk 86 portion's diameter 90. The first end 
80 and second end 82 may receive electrode structures 92, 94 and be press 
sealed 96, 98 adjacent the flared portions to capture electrodes 92, 94 in 
the press seals 96, 98, leaving outer leads 100, 102 axially projecting 
from the flared tube ends. FIG. 5G shows a double ended inner lamp capsule 
formed with flared ends having radially extending disk portions. 
Electrodes 92, 94 are captured in the press seals 96, 98. 
A straight tubular section 104 with a first end 106 and second end 108 may 
be chosen with an inside diameter less than the radial extension 88 of the 
first disk portion 84, and slightly greater than the radial extension 90 
of the second disk portion 86. The straight tubular section 104 with an 
exhaust tube may be slipped over the inner lamp capsule, so the first disk 
section 84 and first tube end 106 are adjacent, and the second disk 
section 86 is adjacent the inside surface of the tube 104. The first disk 
section 84 and first tube end 106 may be fused in a flame to form a 
hermetic seal, and the second disk section 86 and tubular section 104 may 
be similarly fuse to seal off the enclosed intermediate volume 110. An 
inner lip may be formed on the straight tube to assist in making the 
second seal. The intermediate volume 110 may then be evacuated, or filled 
through the exhaust tube and then sealed by closing the tube 112. FIG. 5D 
shows a double ended, double jacketed lamp with a straight tubular outer 
envelope. 
FIGS. 6A-D show a series of cross sectional views of the stages of 
manufacture of an alternative single ended double jacketed lamp. The 
radial skirt portion need not be initially formed as a portion of the 
press seal, but alternatively may be added subsequently in a separate 
step. The separate addition of the radial skirt portion is less preferred, 
but is considered functional. FIG. 6A shows a standard press sealed lamp 
capsule to be adapted as an inner lamp capsule 120. The inner lamp capsule 
120 includes an envelope 122, with a filament or electrode structure 124 
captured in a press seal 126, with outer lead(s) 128 extending for 
electrical connection. FIG. 6B shows a disk 130 of envelope material with 
an aperture 132 adapted to receive and mate with a portion of the inner 
lamp capsule 120, for example the axial end of the press seal 126. The 
inner lamp capsule 120 is mated with the disk 130 so the outer leads 128 
extend through the disk aperture 132. The inner lamp capsule 120 and disk 
130 may then be fused to form a seal. FIG. 6C shows an inner lamp capsule 
120 mated to and sealed with a disk 130 thereby forming a press seal with 
a radial skirt portion The inner lamp capsule 120 may then be enclosed 
with an outer envelope 134. FIG. 6D shows the inner lamp capsule with the 
attached disk portion coupled with a straight tubular outer jacket seal 
with a plate end with a tipped off tubulation 136. 
FIGS. 7A, B show a double jacketed single ended double press sealed lamp 
140, and a double jacketed, double ended, double press seal lamp 142. The 
lamps portrayed in FIG. 7A, and 7B, use the known press seal designs for 
the inner lamp capsules 144, 146. The outer envelope is then added to 
substantially surround the inner lamp capsule by forming a single press 
seal 148, in the case of FIG. 7A, to couple with the single press seal 150 
of the inner lamp capsule 144. Similarly, in the case in FIG. 7B, two 
press seals are used to couple the outer envelope to the two press seals 
152, 154 of the double ended inner lamp capsule. The two designs are 
theoretically possible; however, to thermally accommodate the union, the 
inner lamp capsules need to be heated, which is thought to invite a break 
down of the original seals, over pressurize the inner lamp capsules 144, 
146, lead to misalignment, or have other undesirable results. The designs 
shown in FIG. 7A and FIG. 7B are then thought to be functional, but are 
felt to require more care in completing, and are therefore less preferred. 
In the present double jacketed configuration, no metal support or 
electrical conductors are required in the intermediate volume between the 
inner lamp capsule and outer envelope. The inner lamp capsule leads extend 
directly to the outside of the inner lamp capsule and outer envelope 
assembly without passing through the intermediate volume. As a result, the 
intermediate volume may contain a fill gas mixture including noninert 
gases, and in particular oxygen containing mixtures or even pure oxygen. 
The addition of chemically active gases, and in particular an oxygen 
atmosphere, to the intermediate volume of outer envelope of electric lamps 
may have a number of advantages, such as glass and coating stabilization, 
corona suppression, and hydrogen gettering and similar functions depending 
on the particular lamp. 
A preferred fill gas for the intermediate volume includes oxygen. Pure 
oxygen is preferred for lamp performance. Oxygen fill helps stabilize 
glass at high temperature to slow or prevent devitrification If the glass 
surface gives up oxygen, oxygen from the fill gas is available to replace 
the lost oxygen. The more active oxygen may also preferential bond with 
silicon to the exclusion of contaminants. The more active oxygen may also 
bond directly with the contaminant to prevent the contaminant from 
combining with the glass. Damage to the glass surface is then either 
delayed, prevented or even healed by oxygen in the fill gas. As a result, 
the inner lamp capsule may be operated at a higher temperature with a 
reduced occurence of devitrification as to the higher temperature. Pure 
oxygen enhances fires, so for safety, an inert carrier gas, such as xenon 
may be mixed with the oxygen. 
The fill gas preferably has a less than atmospheric pressure at normal 
temperature. Thermal conduction between the inner lamp capsule and outer 
envelope is increased with greater fill gas pressure. Reducing the fill 
gas pressure helps thermally insulate the inner lamp capsule, and preserve 
the heat energy of the inner lamp capsule. On the other hand, the surface 
preservation, coating protection, and gettering aspects of a fill gas, 
such as oxygen are reduced with lower pressure. A balancing of priorities 
is then made. Applicants suggest a fill gas pressure of 160 torr at normal 
temperature. 
Having an oxygen rich atmosphere around the coatings stabilize and extends 
the coatings operating temperature range. In a similar fashion, an oxygen 
or oxygen and inert fill gas in the intermediate volume may be used to 
preserve and protect surface coatings such as metal oxides and other 
oxides, made along the surfaces adjacent the intermediate volume. Oxygen 
in the intermediate fill gas also protects doped quartz, such as titanium 
and vanadium doped quartz. An oxygen rich atmosphere in the intermediate 
volume is particularly useful in arc discharge lamps. Oxygen is a good 
insulating or arc depressing gas. Even at a low pressure of 150 torr, 
oxygen is able to quench corona glow. Since the corona is quenched, power 
is not lost in starting, operating or restarting the lamp. 
Hydrogen is known to be attracted or active with oxygen. In an oxygen 
atmosphere or oxygen containing fill gas, particularly at warm atmosphere, 
hydrogen reacts readily with the oxygen to form water. Once the hydrogen 
is tied up as a water molecule it is unlikely to be freed, and is too 
large to permeate the inner envelope. The hydrogen can therefore escape 
from the inner lamp capsule, but cannot migrate from the outside into the 
inner lamp capsule. Hydrogen gettering by oxygen reduces the amount of 
hydrogen in the inner lamp capsule thereby improving lamp life and 
function. 
In a working example a doubled jacketed arc discharge lamp was made. The 
structure and dimensions were approximately as follows: The arc discharge 
was designed to be formed between two generally side by side electrodes. 
The electrodes were positioned in a 7.0 mm diameter quartz tube and press 
sealed at one end to entrain the inner electrode leads, sealing conductive 
foils and outer connection leads in a 1.35 cm long press seal. A 7.0 mm 
section of tube extended beyond the press seal encompassing the outer 
leads as a skirt. The opposite end of the inner envelope tube was tipped 
off creating an inner lamp capsule section approximately 1.0 cm long, and 
0.7 cm in diameter. The electrode containing arc discharge capsule was 
then positioned coaxially in a 1.5 cm diameter ozone free quartz outer 
tube, so the skirt was overlapped by the end of the outer glass tube. The 
lamp capsule, and outer envelope tube were then synchronously rotated in a 
flame playing on the outer envelope tube. The outer envelope tube was then 
necked down to close on the skirt portion of the press seal. The skirt and 
necked down portion of the outer envelope tube fused forming a hermetic 
seal. The necked down portion of the outer envelope was about 1.5 cm 
diameter. The outer envelope extended about 1.5 cm at 1.5 cm diameter over 
the inner envelope. The outer envelope then enclosed an intermediate 
volume of about 3.0 mm depth around the inner lamp capsule. The 
intermediate volume was filled with nitrogen and tipped off. The skirt 
portion of the inner capsule then acted as a sort of cork for the necked 
down portion of the outer envelope. The ozone free quartz blocked 
ultraviolet light below 200 nanometers, but allowed the 253.7 nanometer 
mercury line to pass thereby forming a germicidal spectrum that did not 
generate ozone. The combined inner envelope and outer envelope were 
positioned in a bayonet type mount. One lamp lead was welded to the 
bayonet housing while a second lead was mounted centrally in a ceramic 
contact holder. The bayonet mount and the cavity formed by the skirt 
surrounding the lamp leads were then filled with a high dielectric cement 
to hold the fused inner and outer envelopes and leads in place while 
insulating the leads. The portion of the inner lamp capsule not shielded 
by the outer envelope was still shielded by the cement, and bayonet mount. 
Nonetheless, since the cement, and bayonet mount were not particularly 
thermally conductive, the inner envelope remained substantially thermally 
insulated. The lamp was rated as 100 watt, 65 volt, with a color 
temperature of 4200.degree. K., a color rendering index of 80+ and a lumen 
output of 7500+. The disclosed dimensions, configurations and embodiments 
are as examples only, and other suitable configurations and relations may 
be used to implement the invention. FIG. 9 shows an axial cross section of 
the example lamp made. 
In an alternative example, the intermediate volume was filled with pure 
oxygen at a pressure of 160 torr then closed. The lamp was operated with 
no detrimental affects notice due to the oxygen in the intermediate 
volume. 
While there have been shown and described what are at present considered to 
be the preferred embodiments of the invention, it will be apparent to 
those skilled in the art that various changes and modifications can be 
made herein without departing from the scope of the invention defined by 
the appended claims.