Note: Descriptions are shown in the official language in which they were submitted.
9 2-1-116 PATENT ~: ~
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1!5ETAL HALII)E ARC l,AMP HAVING `
~LASS CONTAINMENT SHROUD
Field of the Invention
This invention relates to electric lamps wherein a
light-source capsule is mounted within an outer
envelope and, more particularly, to a glass shroud for :
containing fragments of the light-source capsule in
the event that the capsule bursts.
Backqround of Invention
Double-enveloped lamps, such as metal halide arc
lamps, include a light-source capsule, such as an arc
tube, and a sealsd outer enve~ope surrounding the
light-source capsule. The arc tube typically operates
at high temperature and high pressure. In such lamps, :;~
there is a small probability that the light-source
capsule will burst. ~hen this occurs, hot fragments
of~fused quartz from the arc tub~ are forceably
propelled against the outer envelope. If the outer
envelope al~o shatter~, there is a potential safety
hazard to persons or property in the immediate :~
surroundings. A failure of the outer envelope is : .
k.nown as a containment failure. -~
United States Patent No. 5,122,706, issued June
16, 1992 to Parrott et al., discloses a metal halide
arc lamp wherein a generally cylindrical fused quartz .~
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shroud ncircles an arc tube. Typically, the shroud : ~:
is made of fused quartz and is at least about 2.0 mm
~0.098 inches) thick. The shroud may be open at both
ends, or may be open at one end and have a domed
configuration a~ the other end. The shroud is
typically mounted within the lamp by ~lips or straps
a~tached to a frame member. Fu~ed quartz has been
used to abricate prior art shrouds because it is a
reliable material which has a ~ery low water content
and is resistant to high operating temperatures.
Furthermore, the arc tube which the shroud surrounds
is usually fused guartz.
Shrouds for metal halide arc discharge lamps are
also disclosed in U.~. Patent Nos. 5,023,505, issued
June 11, 1991 to Ratliff et al.; 4,812,714, issued
March 14, 1989 to Keeffe et al.; 4,888,517, issued
December 19, 1989 to ~eeffe et al.: 4,79~,334, issued
December 13, 198~ to Ke~ffe et al.; 4,721,876, issued
January 26, 1988 to White et al.; 4,~09,184, issued -~
November 24, 1987 to Xeeffe et al.: 4,625,141, issued ~ :
November 25, 1986 to Keeffe et al.; 4,620,125, issued
October 28, 1986 to Reeffe et al.; and 4,499,396,
issued February 12, 1985 to Fohl et al. It is also . ~:
known to reinforce a shroud by using a mesh of
substantially non-conducting fiber, as disclosed in
U.S. Patent No. 4,942,330, issued July 17, 1990 to
Karlotski et al.
In addi~ion to containment, the ~hroud has other
beneficial effects on lamp operation. In lamps with a ~:
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gas filled outer envelope, the shroud reduces
convective heat losses from the arc tube and thereby
improves the luminous output and the color temperature
of the lamp. In lamps with an evacuated outer
envelope, the shroud helps to equalize the temperature
of the arc tube.
It would be desirable to replace the fused quartz
shroud with a different material which could be easily
manufactured, which would provide the benefits of a -~
fused quartz shroud, but which would require less
material. ~ -
Accordingly, it is an object of the pr~sent - -~
invention to provide an improved shroud for containing
an arc tube in the event that the arc tube bursts.
It i~ another object of the present invention to
provide a shroud which is lighter in weight than the
fused quartz shroud, but which will contain fragments
of a burst light-source capsule.
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Summary of the Invention ~ m
Accordingly, these and other objects and
advantages are achieved in a double-enveloped lamp -~
assembly which includes a light-source ~apsule
enclosed in a sealed, liqht-transmissive outer
envelop~. A ~ontainment means is mounted within the ~ -
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outer envelope and substantially surrounds the -
light-source capsule. The containment means comprises
a shroud made of a glass which has a strain point - -
greater than the operating temperature of the shroud.
The strai~ point is d~fined as the temperature at
which stress in the glass is substantially removed ~-
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in about four hours when the glass is maintained at
tha~ temperature.
The lamp assembly is typically a metal halide arc
lamp, and the light-source capsule comprises a metal
halide arc tube. The glass is preerably an
aluminosilicate glass which has a strain point of
about 600C or greater. The aluminosilicate glass
preerably has a low water content in order to
minimize outgassing of water vapor when the lamp is at
operating temperature. The shroud preferably
comprises an open-ended cylindrical tube, and for
lamps with a power rating of about 150 watts or less,
the wall thi~kness is about 1 mm.
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Brief Des r ption of the Drawing
~ or a better understanding of the present
invention together with other and further objects, : -~
advantages, and capabilities thereof, reference is ~-~
made to the accompanying drawing which is incorporated
her~in by referenc~ and which is a partially cut-away, -~ ~:
side view of a doubIe-enveloped lamp assembly
a~cording to the present invention. :~
: Description of the Preferred Embodiments
A metal halide arc discharge lamp 10 is shown in
the drawing. A sealed outer envelope 12 (also called . -
an outer jacket) encloses a generally cylindrical
shroud 14. The shroud 14 surrounds an arc tube 16
which encloses two electrodes (not shown) and a fill ~
material for supportin~ an arc discharge. Suitable ~ :
fill materials are well known to those skilled in the
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art, Each elec~rode is coupled through a press seal
36 that hermetically seals the arc tube 16.
Elec~rical energy is coupled from a lamp base 2& .
through a lamp stem 24 and electrical leads 32 and 34
to the electrodes in the arc tube 16.
A mounting assembly 13 mechanically supports the
arc tube 16 and the shroud 14. The assembly 18
includes a support rod 20 which is coupled to the stem
24 and to a dimple 22 in the upper end of the envelope
12. The rod 20 is coupled to the shroud 14 with a
pair of clips 26. The structure of the lamp is
described in more detail in U.S. Patent No. 5,122,706 ~
which is hereby incorporated by reference. In that :.:
patent, the shroud is fabricated of fused quartz and ~;
has a thickness of 2.0 mm or 2.5 mm, depending on the
embodiment.
The shroud 14 according to the present invention
is made of glass, preferably an aluminosilicate ;.~;
glass. Corning 1724 is generally preferred, but other
glasses such as GE 177, GE 179, GE 180, Scho~t 8252,
or Schott 8253 can be used. The shroud 14 is .
typically formed as a cylindrical tube open at both
ends. In other embodiments, the shroud is closed at
one end by a dome shaped portion. Th~ glass shroud ~;
preferably has a wall thickness of 2 mm or less, and
in a pre~erred embodiment has a wall thickness of 1 mm
~0.039 inches) when it is used in metal halide arc ~ ~
lamps in a range of 15 wa~ts to 150 watts. The glass ~-
shroud has the ability to contain fragments nf the arc
tube, if it bursts, at least as well as a fused quartz
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shroud with a substantially thicker wall. In the
preferred embodiment for metal halide lamps in the
above wa~tage range, the shroud has an inner diameter
of 20 mm and an outer diameter of 22 mm, and the
length of the shroud ranges from about 46 mm to s6
mm. Other lengths and wall thicknesses can be used in
different lamps.
The aluminosilicate glass shroud material
preferably has a SiO2 content o~ 57% to 63%, and an
A1203 content of 14% to 17%. The remainder can
include one or more of the following compounds in -~
differing quantities: B203, MgO, CaO, BaO, SrO,
ZrO2, Sb203, and P205; and may include
certain impurities. ~ ~-
A suitable glass should have a strain point which -~
is gréater than the temperature of the shroud when the ~
lamp is operating. The strain point, which is defined ~ -
by the American Society for Testing of Materials
standard terminoloqy of glass and glass products
(C162), is the temperature at which a stress, such as
tension or compression, can be substantially relieved ~:
in a matter of hours. Typically, the shroud
temperature is about 500~C during operation. Assuming
a 100C safety margin, the strain point should be at
least 600~. The safety margin is maintained to
insure that the strain pattern in the shroud does not
change with lamp operating life. The glass should
also have high resistivity to maintain an electrical
: charge on the shroud. The charge inhibits sodium loss
from th~ arc tube.
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The glass should be dry ~have a low water content)
so that there is minimal outgassing of water vapor and
other gasses, when the lamp is at operating
temperature. A measure of the water content is known
as beta, whi~h is a measure of the infrared absorption
of light in the material due to structurally bonded
water. Beta is commonly measured at the absorption
peak found at 2.7 - 2.9 microns, and is normalized to
a 1 mm path length. Consequently, beta values are
expressed in mm 1, A lower value of the beta
indicates ~hat a glass is dryer than another glass
with a higher beta. The specified measurement
wavelength varies for different types of materials, so ~-
~he beta for an aluminosilicate glass is not directly -~
comparable to the beta for fused guartz. The
preferred Corning 1724 glass has a beta of 0.31
mm 1, Other types of glass have a lower beta, e.g.,
the bsta for GE 180 is 0.22 mm 1 In general, the ;~
value of beta for the glass shroud is preferably less ~/ -
than about 0.4 mm . ~- -
Referring to Table I below, ~ontainment tests were
performed on glass and fused quart~ shrouds and the ~ -
results were compared. The test was similar to the
Underwriters Laboratories Standard UL1572-57A, which
descr~bes containment criteria for a fixture. In
these tests, a 100 watt metal halide arc lamp with a
Corning 1724 glass shroud was mounted in a ceramic ~-
socket rated for 400 V pulses. A layer of dry, -
absorbent cotton was positioned about twelve inches ~-
below th~ lamp. The arc tube was exploded by charginq
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a 30 microfarad capacitor connected to the ara tube to
between 1500 and 2000 volts. If the cotton was
scorched or if it was ignited by the fragments when
the arc tube capsule exploded, the result was ~
considered a containment failure. :;
TABLE I -~
0.098" walled Fused Quartz (2.5 mm) -
Percentage Quantity -
Contained, with no outer jacket cracks65% 88
Contained, with cracked outer jacket 30% 40 ~:
Contained, with small hole in outer jacket 5% 7
Did Not Contain 0% o :~
0.039" walled Fused Quartz (1.0 mm) -
Contained, with no outer jacket cracks23% 12 m-~
Contained, with cra~ked outer jacket 42% 22
Contained, with small hole in outer jacket 8% 4
Did Not Contain 27% 14
O.079" Aluminosilicate Glass (2.0 mm) ~:
Contained, with no outer jacket cracksgl~ 29
Contained, with cracked outer jacket 9% 3 ~ -
Contained, with small hole in outer jacket 0% o
Did Not Contain o% 0
O.049" Aluminosilicate Glass (1.25 mm)
Contained, with no outer jacket cracks71~ 20
Contained, with cracked outer jacket 29% 8
Contained, with small hole in outer jacket 0% o
Did Not Contain Dt 0
O.039" Aluminosilicate Glass (1.O mm)
Contained, with no outer jacket cracks71% 52
Contained, with cracked outer jacket 2g% ! 21
Contained, with small hol~ in outer jacket 0~6 o
.. Did Not Contain o~ o
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92-1-116 PATENT
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The ~.5 mm fused guartz shrouds contained in every
instance, but in a number of cases, a small hole was formed :: ~:
in the envelope. ~he 1.0 mm fused quartz shroud did not ~: ~
contain in 27% of the cases -- an unacceptable result. A ~ :
surprising result was that the aluminosilicate glass
shrouds with thicknesses of 2.0 mm, 1.25 mm, and 1.0 mm
contained in every instance. In no case was even a small ::~
hole formed in the outer envelope.
Referring to Table II below, fused quartz shrouds and -~
glass (Corning 1724) shrouds were tested and cnmpared for
impact resistance. The test shroud was clamped against a --.
wall. A weighted pendulum was raised to a specified angle
above the sample and was dropped so that the weight:would s~ -~
strike the shroud at about the middle of its length. If ~ .
the sampl~ did not break, the pendulum was raised by an ~
additional 5 and was dropped again. The angle was : ~-
increased until the sample shattered. Note that the angles
are relati~e and are measured by the pendulum device, and
are not absolute with respect to a horizontal or vertical
plane. As shown in Tabl~ II, the 1.0 mm fused quartz
shrouds required pendulum angles of 60 and 75~ to shatter,
while the glass shrouds with the same thickness re~uired
pendulum angles of 115 and 85 to shatter. This test
demonstrates that the aluminosilicate glass is more impact :~ .
resis.ant than the fused quartz of the same thlckness.
Since the aluminosilicate glass is more impact resistant, --
less material can be used in the shroud.
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92-1-116 PATENT
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ABLE II
0.039" Fused Quartz Tube
Sample ~I required 60 to shatter~
Sample ~2 required 75 to shatter*
0.039" Alu~inosilicate Glass Tube :
Sample #1 required 115 to shatter* -~
Sample #2 required 85 to shatter*
O.049" Aluminosilicate Glass Tube
Sample Kl required 145 to shatter* ~: ;
Sample ~2 required 175 to shatter*
*The angle, in degrees, is not the angle above horizontal,
but is the angle recorded from the impact device.
Other types of glass may also be used to fabricats the
shroud. For example, a heat treated, dry glass, such as
VYCOR (available from Corning) can be used. This glass :~
contains about 96% sio2, the remainder including boron,
alumina, and alkali. It has a strain point of about ~90C
and a beta which is less than 0.4.
By using conventionally melted glass instead of fused
quartz to manufacture the shroud, the amount of material,
the weight, and the cost are all reduced without reducing
containment performance. The tests described above show -:~
that a glass shroud contains as well as a fused quartæ
shroud having a wall thickness that ~s two and one half
times greater. The lighter weight of the glass shroud
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reduces shipping costs and potential damage during shipment
and handling. . .-
While there have been shown and described what are at
present considered the preferred embodiments of the present .
invention, it will be obvious to those skilled in the art -~
that various changes and modification may be madQ therein
without departing from the scope of the invention as ~ :
defined by the appended claims.
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