Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
11366~0
- Background of the Invention
This invention relates to high pressure gaseous
discharge devices and in particular to improvements
in the envelope structure and window seal for high
pressure arc lamps.
In any arc lamp, various portions of the lamp
structure will be at different potentials corresponding
to one or another electrode. It is necessary, therefore,
to provide insulation between these portions in order
to maintain their mutual electrical isolation. Prior
art high pressure arc lamps, for example, as described
in our U.S. patents 3,876,908 ~pril 8, 19~51; 3,852,629
(Decembe~ 3~ 19741, and 3~808~496 CApril 30r 1~741, employ
ceramic cylinders separating the lamp envelope into anode
and cathode portions with a circumferential surface of
the insulator as part of the lamp envelope exterior. It
will be apparent that as a consequence of th s constru~-tion,
the lamp design, operation and usage are subject to a
number of critical specifications.
One such critical specification is related to the
internal pressure and temperature to which the ceramic
and the ceramic to metal bonds are subject. Ceramic
cylinders or rings so employed are primarily under tensile
stresses due to the high pressure gas filling of the lamp
which tends to push apart the anode and cathode portions
of the lamp envelope. The brazed metal to ceramic bonds
of these prior art lamps are likewise under tensile or
shear stress. Due to the properties of the brazing alloys
and the metallic members, the stresses within the joint
must be maintained within safe tolerances. These stresses
are thermally dependent and thus place 11mits on the 1amD
operation and associated cooling system.
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1 1136690 . I
1 ¦ Another critical consideration is the ambient
2 ¦ environme~t wherein the lamp will be operated. Moisture
3 ¦ dirt, grease, fingerprints and the like can seriously
4 ¦ degrade the insulation characteristics of the ceramic,
5 ¦ especially for high voltage starting conditions. Moeeover,
6 ¦ the exposed ceramic is subject to impact which can initiate
7 ¦ a fracture thereof.
8 ¦ It will be clear also that failure of the ceramic
9 ¦ or the metal-ceramic seals of a high pressure lamp can
lO ¦ be hazardolls to personnel or equipment nearby.
ll ¦ Another prior art arc lamp, described in our U.S. patent
12 ¦ 3,715,613 ~eb~uar~ 6, 1973~ employed a ceramic cylinder
13 ¦ ~ith a recessed plane sur~ace ~ox accepting a metal disk which
14 ¦ in turn supported the anode structure. The outside surface o~
15 ¦ the ceramic was brazed to the inner surface of the envelope
16 ¦ neck. Thus, the metal ring exerted compressive and shear
17 ¦ stresses at one end of the ceramic due to the internal
gas pressure while a brazement of the outer circumferential
19 ¦ wall of the ceramic to the inner cylindrical surface
20 ¦ of the envelope neck resulted in shear stresses due to
21 ¦ the internal aas pressure.
22 ¦ An arc lamp is also characterized by an optical
23 ¦ window which must withstand a very great pressure differential
24 ¦ between the interior and exterior of the lamp. Prior
25 ¦ art window seals in such lamps have normally employed
26¦ a window disc of sapphire, metallized around iis circumference
271 and then ~razed to one leg of a U-shaped metal flange,
28¦ which was subsequently brazed into the window-cathode
29 ¦ portion assembly, as shown for example in said U.S. patent
30l 3,852,629. This window sealing techniaue resulted in an
31 ¦ im~roved seal but one which is not as good as the seal
32 ¦ to be hereinafter describe~ wlth~respect to this in~ention.
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" 1~3~690
It is an object of this invention to produce a
safer and more rugged lamp capable of both higher perform-
ance and production at reduced manufacturing cost.
Accordingly, the present invention provides an
envelope for an arc lamp, said envelope being capable of
containing a gas therewithin at a pressure higher than
atmospheric pressure, said envelope comprising:a first
metallic member having an inwardly projecting flange por-
tion; a second metallic member having an outwardly pro-
jecting flange portion; the outwardly projecting flange
portion of said second metallic member being received with-
in said first metallic member, the inwardly projecting
flange portion of said first metallic member being dispos-
ed to overlap the outwardly projecting flange portion of
said second metallic member, said overlapping flange por-
tions of said first and second metallic members being
spaced apart to define a generally annular region between
said first and second metallic members; a ceramic member
disposed in said annular region,and means for hermetically
20 sealing said ceramic member to said inwardly and outwardly
projecting flange portions so that said ceramic member is
under compression when said envelope contains a gas at a
pressure higher than atmospheric pressure.
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1136690
Brief Descriptioh of th'e Drawings
FIG. 1 is a section of an embodiment of the high
pressure arc lamp.
FIG. 2 is a detail of an alternate embodiment of
the main body seal.
FIG. 3 is a detail of an alternate embodiment of
the window seal.
Detailed Description of the Embodiments
FIG. 1 is illustrative of a high pressure sealed
beam short arc lamp of the present embodiment. One elec-
trode shown as cathode 10 is mounted in the forward part
of the làmp adjacent to window 11. Cathode 10 is supported
by metal struts 12 which are centrally attached to cathode
10. A getter 13 is provided to remove contaminants from
the gas filling of the lamp. The struts 12 are attached
to metal ring 14 adjacent the outer ends of the struts.
-' Reflector 16 is suspended from ring ~4. The exterior of
the forward portion of the lamp is a cylindrical shell
18 made of a high strength metallic alloy, such as stain-
less steel.
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2 Window 11, pre~erably of sapphire, is face brazed
with a knife edge braze process to annular sealing ring
3 21, the latter preferably of Kovar*.
Although the window 11 remains subject to the same
6 bending moment across its plane surface (face) as in prior
art lamps, the outer peripheral region or rim of the window,
7 together with the window to metal bond, is compressively
9 loaded, with consequent reduction of tensile stresses.
Since the ultimate yield strength for sapphire in compression
far exceeds that in tension!this seal has been found to
2 withstand much higher internal pressures than the prior
art peripheral seals for equal thickness sapphire windows.
Whereas the prior art peripheral seals have been tested
to seal failure at 1200 psi,the compressive window seal
of the present invention has not been observed to fail
6 at 1900 psi. It is conjectured that exposure of the
18 circumferential surface 19 of the window to internal gas
19 pressure of the lamp results in a compressive stress across
the diameters of the window, resulting in the higher observed
21 resistance to fracture. It is also noted that the braze
22 bond between the window 11 and the sealing ring 21 is ¦
in compression due to the high internal gas pressure.
23 Thus higher pressures may be achieved, or alternatively,
24 a thinner sapphire window may be employed to maintain
the same structural integrity as characterized by the
26 prior art.
27 A second electrode, here anode 22, is supported
28 in the rear of the lamp. Surrounding and attached to
anode 22 is a high strength metal alloy shell 28. Rear
31 ~ and front shells comprising the lamp envelope are connected
¦ by attaching metal shells 18 and 28 to ~1 insulating ring 2~ oE h~gh
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_ 1136690
l¦ strength ceram1c. Sealing rings 29 of, for example,
21 Kovar,* are used to seal the ceramic ~in~ 25 to anode shell
3 ¦ 28 and cathode shell 18, thereby forming a hermetic envelope.
4 ¦ The cathode shell 18 and the anode shell 28 axe ~utuall~ conigured
5¦ to place ceramic ring 25 in compression and to shield cera~ic
6 ¦ ring 25 from contamination and impact. This is acco~plished
7 ¦ by forming on cathode shell 18 an inward projecting flange
8 ¦ 30, and by forming on anode shell 28 an outward pro~ecting flange
9 ¦ 32. These flanges overlap each other to intercept a common
lO ¦ radial region defined from center line 34;and it is this
ll ¦ common radial region,which is occupied by ceramic ring
i 12 1 25. The internal pressure thus tends to hold the envelope
13 ¦~ together. The exterior overlap of cathode shell 18 over
14 ¦ the ceramic ring 25 tends to thus protect the cer~mic ring 25 from
l5 ¦ contamination,which could promote short circuits,and from
16 ¦ impacts,which could damage it. Should the ceramic ring 25 ~ail,
17 l fragments are likewise prevented from causing damage to
18 ¦ nearby personnel or apparatus. Moreover should the ceramic
19¦ insulator ring 25 or its seals fail, the cathode and anode shells
201 18 and 28 are prevented from separating by the flanges
21¦ 30 and 32.
22 The ratio of compressive strength to tensile strength
231 for a typical ceramic as employed here is in excess of
241 10:1; therefore the ceramic insulator can withstand much
251 higher compressive stresses arising from the high pressure
26¦ gas filling of the lamp than would be the case were the
271 ceramic to be tensile loaded.
28¦ It will be observea that preparation of ceramic ring
291 25 for brazing reauires only grinding of the end ~lane
301 surfaces rather than the more elaoorate reauilements
3il¦ fcr ?reparation of radial surfaces ^or brazing. Similarly,
32ll only plane surfaces (of flanges 30 and 32) need be prepared
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113~i690
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l ¦ for brazing rather than circumferential surfaces of
2 ¦ shells 18 ar.d 28. The seals are knife-edge brazements
3 ¦ which are subject to substantially purely compressive
4 ¦ stresses.
5 ¦ FIG. 2 is an alternate seal wherein two coaxial
6 ¦ rings 29' and 29" are employed in place of the
¦ single ring 29. .Similarly, dual sealing rings are
8 ¦ illustrated in FIG. 3 for an alternate arrangement to,
9 ¦ seal the window 11 to the assembly ring 14.
lO ¦ Assembly of the lamp of FIG. 1 is accompIished
ll ¦ by first performing the brazing operations required .
12 ¦ to seal cathode shell 18 to anode shell 28. The assembly
13 ¦ ring 14 supporting the window ll and the cathode 10 is brazed to the
14 ¦ cathode shell 18 as the last braze. The lamp is then .
15 ¦ filled to high pressure through port 36, preferably
16 ¦ with xenon, and the po~t 36 is then sealed o~.
17 ¦ Sïnce changes could be made in particular details
18 ¦ of the embodiment of the invention disclosed herein
1~ ¦ without departing from the scope of the invention, it is
20 ¦ intended that the above description and accompanying drawings
~¦ be interpr ted as illustrative onl- and not as lir iting.
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