Safety filament assembly for double-enveloped arc discharge lamp

An electric lamp assembly includes a sealed outer envelope, an arc discharge tube mounted within the outer envelope, a safety filament connected in series with the arc discharge tube for extinguishing the arc tube within a predetermined time after the outer envelope is broken, and an electrically-insulating sleeve disposed around the safety filament. The insulating sleeve suppresses emission of electrons from the safety filament and thereby prevents a reduction in frame potential which would increase sodium loss from the arc tube. The insulating sleeve includes openings which permit air to reach the safety filament when the outer envelope is broken. A mounting arrangement includes a pair of mounting tabs which provide electrical connections for the safety filament and which approximately centers the safety filament in the insulating sleeve.

CROSS REFERENCE TO RELATED APPLICATION 
This application discloses, but does not claim, subject matter that is 
claimed in application Ser. No. 538,549 filed Jan. 15, 1990 concurrently 
herewith and assigned to the assignee of this application. 
FIELD OF THE INVENTION 
This invention relates to arc discharge lamps and, more particularly, to 
double enveloped arc discharge lamps which contain a low wattage safety 
filament that is designed to rapidly oxidize and extinguish the lamp when 
the outer envelope of the lamp is broken. 
BACKGROUND OF THE INVENTION 
High intensity arc discharge lamps such as metal halide lamps and mercury 
lamps typically include a quartz arc tube mounted within a glass outer 
envelope. In some cases, the region between the arc tube and the outer 
envelope is filled with an inert gas such as nitrogen, while in other 
cases this region is evacuated. The radiation generated by arc discharge 
lamps contains potentially harmful ultraviolet radiation which is blocked 
by the glass outer envelope. 
In one failure mode, the arc tube bursts, thereby terminating emission of 
radiation. Various techniques have been disclosed in the prior art for 
containing the fragments of an arc tube which bursts, and for insuring 
that the outer envelope remains intact. Such techniques include the use of 
a thick walled outer envelope and the use of a light-transmissive shroud 
between the arc tube and the outer envelope. 
In another failure mode, the outer envelope of the arc discharge lamp is 
broken by an external impact. In this case, the arc tube may continue 
operating and emitting potentially harmful ultraviolet radiation which is 
no longer blocked by the outer envelope. Frequently, arc discharge lamps 
are operated in enclosed fixtures which contain fragments of a shattered 
outer envelope and absorb ultraviolet radiation. However, in other 
applications, it is desirable to operate arc discharge lamps in open 
fixtures which are generally less expensive than enclosed fixtures and may 
be preferable for technical and/or aesthetic reasons. 
To prevent operation of the arc tube in the event of an outer envelope 
failure, a low wattage, easily-oxidized safety filament is sometimes 
included in the lamp. The safety filament is located within the outer 
envelope and is electrically connected in series with the arc tube. If the 
outer envelope is broken, the safety filament is rapidly oxidized when it 
comes in contact with the oxygen in the air, thereby interrupting the 
electrical circuit of the arc tube and extinguishing the lamp. This 
technique is disclosed in European Patent Application No. 0,326,079. 
Techniques for extinguishing arc discharge lamps when the outer envelope 
is broken are also disclosed in U.S. Pat. No. 4,013,919 issued Mar. 22, 
1977 to Corbley, U.S. Pat. No. 4,013,920 issued Mar. 22, 1977 to Petro, 
U.S. Pat. No. 4,208,614 issued June 17, 1980 to Strauss et al and U.S. 
Pat. No. 4,629,939 issued Dec. 16, 1986 to Jaworowicz et al. 
It has been observed that the inclusion of a safety filament in a metal 
halide arc discharge lamp of the type disclosed in U.S. Pat. No. 4,888,517 
issued Dec. 19, 1989 to Keeffe et al and containing sodium iodide and 
scandium iodide, considerably reduces the operating life of the lamp. The 
lamp disclosed in the Keeffe et al patent includes a shroud surrounding 
the arc tube and a frame for mechanically supporting the shroud and the 
arc tube. When a safety filament is included in such a lamp, excessive arc 
tube voltage rise and changes in the color temperature of the lamp are 
observed in a relatively short time. It is desirable to overcome such 
problems and to provide an arc discharge lamp having a safety filament for 
protection in the event of outer envelope breakage, and having a long 
operating life. 
It is a general object of the present invention to provide improved arc 
discharge lamps. 
It is another object of the present invention to provide double enveloped 
arc discharge lamps having a safety filament and having a long operating 
life. 
It is a further object of the present invention to provide double-enveloped 
arc discharge lamps having a safety filament provided with an insulating 
sleeve for suppressing emission of electrons. 
It is still another object of the present invention to provide arc 
discharge lamps wherein migration of sodium and other alkali metal ions 
from the arc tube is suppressed, thereby extending the operating life of 
the lamp. 
It is a further object of the present invention to provide arc discharge 
lamps that are safe in the event of outer envelope breakage. 
It is another object of the present invention to provide double enveloped 
arc discharge lamps that are extinguished within a prescribed time after 
the outer envelope is broken. 
SUMMARY OF THE INVENTION 
According to the present invention, these and other objects and advantages 
are achieved in an electric lamp assembly comprising a sealed outer 
envelope, a lamp capsule mounted within the outer envelope for generating 
light upon application of electrical energy, the lamp capsule containing a 
fill material including an alkali metal or an alkali metal compound, means 
for coupling electrical energy through the outer envelope to the lamp 
capsule, the coupling means including a safety filament mounted within the 
outer envelope for extinguishing the lamp capsule within a predetermined 
time after the outer envelope is broken and the safety filament is exposed 
to air, the safety filament being connected in series with the lamp 
capsule, an electrically insulating sleeve disposed around the safety 
filament, and means for mounting the safety filament in the sleeve, the 
mounting means including means for positioning the safety filament in the 
sleeve without contact between the safety filament and the sleeve. 
The lamp capsule typically comprises a metal halide arc tube containing one 
or more sodium halides. It has been found that thermionic emission of 
electrons from the safety filament causes a reduction in positive 
potential on frame elements located within the outer envelope. The 
reduction in positive potential in turn permits migration of positive 
sodium ions through the arc tube and causes a reduction in its operating 
life. By suppressing emission of electrons from the safety filament, the 
frame elements maintain a positive potential and suppress emission of 
sodium ions from the arc tube. 
The insulating sleeve typically comprises a generally cylindrical, open 
ended tube surrounding the safety filament. The open ends of the tube 
permit air to reach the safety filament when the outer envelope is broken. 
The means for positioning the safety filament in the sleeve typically 
comprises mounting tabs at each end of the sleeve. The safety filament is 
electrically and mechanically attached to the mounting tabs such that the 
filament is approximately centered in the sleeve. In a preferred 
embodiment, the mounting tabs each comprise a generally flat element 
having a first portion which extends into the sleeve and a second portion 
which remains outside the sleeve. The first portion of the mounting tab 
has a width that is slightly less than the inside diameter of the sleeve. 
The second portion of the mounting tab has a width that is at least 
slightly larger than the inside diameter of the sleeve. Electrical leads 
which provide mechanical support for the filament and the sleeve are 
attached to the mounting tabs. The mounting tabs permit circulation of air 
to the safety filament and position the safety filament relative to the 
insulating sleeve. The mounting arrangement provides uniform and 
repeatable oxidation of the filament and extinguishing of the lamp capsule 
within a predetermined time after the outer envelope is broken.

DETAILED DESCRIPTION OF THE INVENTION 
A prior art metal halide arc discharge lamp assembly 10 incorporating a 
safety filament is shown in FIG. 1. The lamp assembly 10 includes an outer 
envelope 12 and an arc tube, or lamp capsule 14, mounted within outer 
envelope 12 by mounting means 16. The arc tube 14 is positioned within a 
light transmissive shroud 20, which is supported by the mounting means 16. 
The outer envelope 12 is sealed and is filled with an inert gas, such as 
nitrogen, at a pressure on the order of 300 torr to 400 torr. The outer 
envelope 12 may be clear or may have a phosphor coating on its inside 
surface. A base 22 is attached to the lower end of the outer envelope 12. 
Electrical leads 24 and 26 are connected to base 22 and pass through a 
lamp stem 28 to the interior of the outer envelope 12. 
The mounting means 16 includes a generally U shaped support frame 30 
attached to lamp stem 28 by a strap 32. The arc tube 14 is mechanically 
attached to support frame 30 by straps 34 and 36, and the shroud 20 is 
retained by annular ring clips 35 and 37. The mounting means 16 further 
includes spacers 38 which center the support frame 30 in the outer 
envelope 12. 
Electrodes 40 and 42 are sealed in arc tube 14 and are connected through 
press seals at opposite ends of the arc tube 14 to inleads 44 and 46, 
respectively. The arc tube 14 is typically fabricated of quartz. A 
startinq electrode 48 adjacent to electrode 42 is connected through the 
press seal to an inlead 50. The arc tube 14 typically contains mercury, a 
starting gas and one or more metal halides including a sodium halide. 
Electrical lead 26 is connected to inlead 46 by an electrical lead 52. 
Electrical lead 24 is connected by electrical leads 54 and 56 to a first 
end of a safety filament 60. A second end of safety filament 60 is 
connected to inlead 44. The first end of safety filament 60 is supported 
by a dummy lead 62 in the press seal of arc tube 14 adjacent to electrode 
40. Thus, the safety filament 60 is electrically connected in series with 
the arc tube 14. Lead 54 is connected through a resistor 64 to inlead 50 
of the starting electrode 48. 
During normal operation, the operating current for arc tube 14 passes 
through safety filament 60. In the event that the outer envelope 12 is 
broken, oxygen in the atmosphere causes rapid oxidation of the safety 
filament 60. When the safety filament 60 burns out, the continuity of the 
electrical circuit of the arc tube 14 is interrupted, and the arc tube 14 
is extinguished. The safety filament 60 is selected to oxidize and burn 
through in a time of about 15 minutes or less. For a 400 watt arc tube 14, 
safety filament 60 typically comprises a coiled, nonsag tungsten filament 
that is about 6 mm in length and is made of 0.2 mm diameter wire with a 
coil diameter of 1 mm. The filament operates at a temperature of 
approximately 1300.degree. C. and a power dissipation of 6 watts. 
As indicated above, metal halide arc discharge lamps constructed as shown 
in FIG. 1 and described hereinabove have exhibited excessive increases in 
operating voltage and undesired changes in color temperature during early 
life as compared with similar discharge lamps not including a safety 
filament. The excessive increases in operating voltage and the changes in 
the color temperature exhibited by lamps with a safety filament are 
indicative of sodium loss from the arc tube 14. 
The reason for the sodium loss was investigated by measuring the floating 
potential of the shroud 20 and the support frame 30 of 400 watt double 
enveloped metal halide lamps with and without the safety filament 60. 
Lamps having the safety filament 60 were constructed as shown in FIG. 1 
and described hereinabove. Lamps not having a safety filament were 
constructed as shown in FIG. 1 except that the safety filament 60 was 
omitted and lead 56 was connected directly to inlead 44. The potential of 
the support frame 30 with respect to leads 52 and 56 as a function of the 
power applied to the lamp is shown in FIG. 1 for the case with no safety 
filament 60. Curve 70 shows the potential between support frame 30 and 
grounded lead 56, while curve 72 shows the potential between support frame 
30 and high voltage lead 52. The lamp without a safety filament displays 
the expected result that the potential increases with increasing lamp 
power and approaches 100 volts near the nominal lamp operating power of 
A high positive frame potential is required to inhibit the sodium ions in 
the discharge within the arc tube 14 from drifting and diffusing through 
the quartz wall of the arc tube under the influence of the time dependent 
electric field between the arc tube 14 and the support frame 30 and shroud 
20. Sodium ions which migrate through the wall of arc tube 14 are 
subsequently attracted to the support frame 30 where they are neutralized. 
The lower the frame potential, the greater will be the loss of sodium from 
the arc tube 14 due to the higher electric field between the arc tube 14 
and the shroud 20 or support frame 30 on each positive half cycle of the 
applied lamp voltage. 
The potential between the support frame 30 and the electrical leads 52 and 
56 is shown in FIG. 3 for a lamp that is identical to the lamp tested in 
FIG. 2 except for the inclusion of a safety filament 60 as shown in FIG. 
1. The potential between frame 30 and grounded lead 56 is shown by curve 
74, while the potential between frame 30 and high voltage lead 52 is shown 
by curve 76. In this case, the potential on the support frame 30 and the 
shroud 20 increases with increasing lamp wattage only up to about 350 
watts. At higher lamp power, the potential on the support frame 30 
actually decreases with further increases in lamp wattage. The potential 
on the support frame 30 with respect to the high voltage lead 52 becomes 
negative at power levels above about 430 watts. 
The reason for the result shown in FIG. 3 is that at higher lamp wattages, 
the temperature of the safety filament 60 is sufficiently high that the 
filament 60 thermionically emits electrons. The electrons are attracted to 
the support frame 30 on each negative half cycle of the applied lamp 
voltage and drive the average frame potential to a negative voltage with 
respect to the arc tube 14. A low positive voltage or a negative voltage 
on the shroud 20 and the support frame 30 with respect to the arc tube 14 
causes increased sodium ion loss from the arc tube 14. As a result, the 
discharge voltage across the arc tube 14 increases, thereby leading to 
early lamp failure. 
The above described problem can be reduced or eliminated by inhibiting or 
suppressing the emission of electrons from the safety filament 60. We have 
found that a way to reduce and possibly eliminate the emission of electrons 
from the filament 60 is to enclose the filament in an insulating sleeve 
that is capable of withstanding the operating temperature of about 
1000.degree. C. to 1500.degree. C. in the immediate vicinity of the 
filament 60. A lamp assembly in accordance with the invention is shown in 
FIG. 4. The lamp assembly shown in FIG. 4 is the same as the lamp assembly 
shown in FIG. 1 except that an insulating sleeve 80 surrounds the safety 
filament 60. 
As indicated above, the sleeve 60 can be fabricated of any insulating 
material that is able to withstand the operating temperature near the 
filament 60 without melting or cracking. Such materials include quartz, 
high temperature glasses, ceramics, alumina and boron nitride. An 
enlarged, cross-sectional view of the safety filament 60 and the 
insulating sleeve 80 is shown in FIG. 5. In a preferred embodiment, the 
sleeve 80 comprises a cylindrical, open ended tube. The sleeve 80 cannot 
be sealed and must have at least one opening of sufficient size to permit 
oxygen in the atmosphere to reach the filament 60 and oxidize it-within a 
prescribed time after the outer envelope 12 is broken. However, the sleeve 
80 must sufficiently enclose the filament 60 to prevent a significant 
number of electrons from escaping and reaching the support frame 30 and 
shroud 20. 
The above requirements are met by a cylindrical sleeve 80, as shown in FIG. 
5, which extends beyond each end of the filament 60 by a distance L that is 
sufficient to prevent a significant number of electrons from escaping from 
the sleeve 80. However, the inside diameter D of the sleeve 80 must be 
large enough and the distance L short enough that oxygen from the 
surrounding atmosphere is able to diffuse into the sleeve 80 and oxidize 
the filament 60 within a prescribed time on the order of about 15 minutes 
or less. It is believed that the ends of the sleeve 80 which are cool 
relative to the central portion adjacent to filament 60 build up a 
negative charge and prevent electrons from escaping through the open ends 
of the sleeve 80. Preferably, the ratio between the distance L and the 
inside diameter D is in the range of about 2 to 5. By way of example, a 
safety filament 60 for a 400 watt metal halide lamp has a length of 6 mm, 
a wire diameter of 0.2 mm and a coil diameter of 1.1 mm. A suitable quartz 
insulating sleeve 80 has an overall length A of 15 mm, an inside diameter D 
of 1.9 mm and a wall thickness of 1.0 mm. In another example, a safety 
filament 60 for a 400 watt metal halide lamp has a length of 7 mm and a 
coil diameter of 1.5 mm. A suitable quartz insulating sleeve 80 has an 
overall length A of 27 mm, an inside diameter D of 5 mm and a wall 
thickness of 1.0 mm. The second example meets the above described 
requirements regarding the ratio between the distance L and the inside 
diameter D and provides more clearance between the safety filament 60 and 
the insulating sleeve 80. As described hereinafter, the safety filament 60 
preferably does not contact the insulating sleeve 80. 
The frame potential of a lamp of the type shown in FIG. 4 wherein the 
safety filament 60 is enclosed in a quartz sleeve 80, is shown in FIG. 6. 
Curve 84 shows the potential between support frame 30 and grounded lead 
56, while curve 86 shows the potential between support frame 30 and high 
voltage lead 52. In this case, the potential on the support frame 30 and 
shroud 20 as a function of lamp power is very similar to that of the lamp 
which did not include a safety filament. The frame potential of a lamp 
without a safety filament is shown in FIG. 2. Thus, FIG. 6 indicates that 
filament emission has been considerably reduced, thereby allowing the 
positive frame potential to be maintained. As a consequence, the life of 
this lamp is expected to be similar to the life of lamps that do not 
include a safety filament. 
The results of tests of several lamps with and without the present 
invention are shown in Table 1 below. In Table 1, the dimension A is the 
total length of the quartz sleeve 80, dimension D is the inside diameter 
of the quartz sleeve and L is the distance between each end of the 
filament 60 and the end of the quartz sleeve. These dimensions are 
illustrated in FIG. 5. The frame potential is the average voltage measured 
between the support frame 30 and the high voltage lead 52. Each of the 
lamps tested was a 400 watt metal halide lamp similar to a Type MP400. 
TABLE 1 
______________________________________ 
FRAME 
LAMP A D L RATIO POTENTIAL 
NO. (mm) (mm) (mm) L/D (VOLTS) 
______________________________________ 
1 no filament -- 85 
2 no sleeve -- 22 
3 no sleeve -- 16 
4 15 1.9 4 2.1 73 
5 14 2.4 3 1.25 21 
6 16.6 5 5 1.0 46 
7 16.6 5 5 1.0 46 
8 16.6 5 5 1.0 22 
9 12.5 1.9 1.5 0.8 26 
10 12.5 5 3 0.6 41 
11 12.5 5 3 0.6 33 
12 12.5 5 3 0.6 33 
13 12.5 5 3 0.6 24 
______________________________________ 
Lamp 1 contained no safety filament. Thus, the frame potential was 
relatively high. Lamps 2 and 3 contained a safety filament without an 
insulating sleeve, as shown in FIG. 1. For lamps 2 and 3, the frame 
potential was a factor of about 5 or more smaller than that of lamp 1. 
Lamps 4-13 included a quartz sleeve having the dimensions indicated. Lamp 
4 had the highest frame potential and is expected to have the longest 
operating life. Lamps 5-13 had L/D ratios less than 2. As a consequence, 
the frame potentials were a factor of 2-5 smaller than desired. 
It has been found that the mounting arrangement for the safety filament and 
the insulating sleeve can affect operation of the safety filament. The 
sleeve can be loosely mounted relative to the safety filament. In this 
case, it is quite likely that at least a portion of the sleeve will rest 
against the filament. Tests of such a configuration have indicated that 
when the sleeve and the safety filament are in contact, heat is conducted 
away from the filament, and it operates at a lower temperature than when 
the filament and sleeve are not in contact. Thus, when the outer envelope 
is broken and the filament is in contact with the sleeve, the filament 
oxidizes slowly, and the time required for the safety filament to burn 
through is extended, sometimes beyond the required time for extinguishing 
the lamp. When the safety filament is not in contact with the sleeve, the 
filament burns through more rapidly. 
To alleviate the above problem and to provide more uniform and predictable 
operation, it has been found desirable to mount the safety filament within 
the insulating sleeve such that it does not contact the insulating sleeve. 
Preferably, the safety filament is approximately centered in the 
insulating sleeve. The mounting arrangement must not conflict with the 
above described requirements that the insulating sleeve suppresses 
emission of electrons from the safety filament and is sufficiently open to 
permit oxygen in the atmosphere to reach the safety filament when the outer 
envelope is broken. 
A preferred mounting arrangement is shown in FIGS. 7 and 8. The assembly is 
mounted in the location of filament 60 and sleeve 80 shown in FIG. 4. Leads 
64 and 66 of safety filament 60 are attached to mounting tabs 90 and 92, 
respectively, at opposite ends of insulating sleeve 80. Electrical leads 
62 and 44 from arc tube 14 (see FIG. 4) are also attached to mounting tabs 
90 and 92, respectively. Mounting tabs 90 and 92 position the safety 
filament 60 relative to the insulating sleeve 80 and also provide 
electrical connections to the safety filament 60. 
Each of the mounting tabs 90 and 92 comprises a roughly T shaped sheet of 
nickel plated steel, stainless steel or other suitable conductor having a 
thickness of about 0.015 inch to 0.020 inch. Each of the mounting tabs 90 
and 92 includes a first portion 94 having a width that is slightly smaller 
than the inside diameter of the sleeve 80 and a second portion 96 that is 
wider than the inside diameter of sleeve 80. The first portion 94 of each 
of the mounting tabs 90 and 92 extends into the sleeve 80, and the ends of 
the sleeve 80 abut against edges 98 and 99 of second portion 96. The leads 
64 and 66 of the safety filament 60 are resistance welded to the mounting 
tabs 90 and 92, respectively, such that the safety filament 60 is 
approximately centered within the sleeve 80. The connecting leads 62 and 
44 are also resistance-welded to the mounting tabs 90 and 92, 
respectively. During assembly, one lead of the safety filament 60 is 
welded to one of the mounting tabs 90, 92. Then, the safety filament 60 is 
inserted into sleeve 80 such that the end of sleeve 80 abuts against edges 
98 and 99. Then, the other mounting tab is inserted into the opposite end 
of sleeve 80, and the other lead of the safety filament 60 is welded to 
the other mounting tab. 
It will be understood that a variety of mounting tab configurations are 
included within the scope of the present invention. The requirements of 
the mounting tabs are that (1) the mounting tabs position the safety 
filament 60 relative to the sleeve 80 such that the safety filament 60 
does not contact the sleeve 80, and (2) the mounting tabs permit 
circulation of air through the sleeve and around safety filament 60 such 
that the arc tube is extinguished within a prescribed time after the outer 
envelope is broken. Thus, for example, the first portion 94 of the mounting 
tabs shown in FIGS. 7 and 8 can be replaced with a ring, collar or cap that 
matches either the inside diameter or the outside diameter of the sleeve 
80. The ring, collar or cap is attached to a flat portion to which the 
safety filament leads and interconnecting leads are attached. In another 
configuration, the mounting tabs are generally L shaped so that only one 
edge of the portion outside the sleeve 80 abuts against the end of the 
sleeve 80. The portions of the mounting tabs outside the sleeve 80 can be 
extended, if desired, to provide more convenient electrical and mechanical 
connections. 
The present invention has been described in connection with metal halide 
arc discharge lamps containing sodium halides. It will be understood that 
the invention can be utilized in any arc discharge lamp in which the arc 
tube fill material contains an alkali metal or an alkali metal compound, 
and migration of alkali metal ions through the arc tube is a problem. 
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 modifications may be 
made therein without departing from the scope of the invention as defined 
by the appended claims.