High intensity discharge lamp having safety device with pyrophoric material

A high intensity arc discharge lamp has an arc tube the emission of which includes some harmful UV radiation. An outer envelope surrounds the arc tube and substantially blocks the harmful UV radiation. The lamp includes means to render the arc tube inoperative when the outer envelope becomes cracked or punctured sufficiently to admit air therewithin, the means including a pyrophoric material to initiate combustion of a current carrying portion of the lamp circuit.

THE INVENTION 
This invention relates to high intensity arc discharge (HID) lamps of the 
type having an arc tube within an outer envelope, the arc tube emission 
including some UV radiation. The outer envelope is generally made of a 
glass, such as nonex, that blocks passage of harmful UV radiation. This 
invention is particularly related to such lamps having a safety device to 
terminate arc tube current when the outer envelope is broken. Examples of 
such lamps are shown in U.S. Pat. Nos. 4,013,920 and 4,032,816. However, 
the mechanically operated safety devices in said patents are not activated 
by a puncture, say, about 1.0 cm in diameter, in the outer envelope that 
leaves the rest of the envelope intact. Another type is shown in U.S. Pat. 
No. 4,013,919 in which a normally closed switch is at an elevated 
temperature during normal operation but cools off and opens when the outer 
envelope is broken, thereby passing the arc tube current through a fuse 
heater which oxidizes in the presence of air and opens the circuit. A 
problem with this lamp is that the switch can take a thermal set after 
several thousand hours of operation and not open when the outer envelope 
breaks. Still another type, such as in U.S. Pat. No. 4,090,105, has a fuse 
heater in series with the arc tube and which operates at a high enough 
temperature, for example, at incandescence, to oxidize rapidly in the 
presence of air and open the circuit. This device wastes I.sup.2 R power. 
A purpose of this invention is to overcome the disadvantages of the prior 
art lamps. A lamp in accordance with this invention utilizes a pyrophoric 
conductor electrically in series with the arc tube. Upon exposure to air, 
the conductor ignites at a relatively low temperature and opens the arc 
tube circuit. In one embodiment, the pyrophoric conductor comprises a yarn 
of loosely wound filaments of a combustible metal coated with a powdered 
reaily ignitable material.

As shown in the drawing, one example of a lamp in accordance with this 
invention includes a generally tubular outer bulbous envelope 1 having a 
bulbous central portion and a conventional base 14 attached to the bottom 
thereof. Extending inwardly from the base and inside of the envelope 1 is 
a mount 15 having a pair of stiff lead-in wires 12 and 16 in electrical 
conducting relation with the base 15. Disposed upon one of the stiff 
lead-in wires 12 is a lower, U-shaped support 8 welded thereto. U-shaped 
support 8 comprises a pair of vertical wires 23 and 24 rising from a 
horizontal base wire 25. The upper ends of lower U-shaped support 8 are 
welded together with a lower strap 7 which in turn supports an arc tube 2. 
Preferably, the lower strap includes two sections abutting against either 
side of arc tube 2 thereby holding it firmly in place. They touch only the 
press seal of the arc tube and not the body. Generally, both sides of the 
lower strap 7 can be of identical constuction. A pair of bumpers 26 are 
welded to lower U-shaped support 8 and abut against the tubular portion of 
walls of outer bulbous envelope 1 thereby stabilizing the structure within 
the lamp. Preferably, these bumpers are made of a reslient material so 
that if the lamp is jarred they will absorb much of the shock. 
Since lower U-shaped support 8 is electrically connected to stiff lead-in 
wire 12, support 8 forms part of the circuit in the device. Current passes 
from base 14 into lower U-shaped support 8 and thence to lead-in wire 21 
which in turn is connected to a cathode 4 in the arc tube. It is sometimes 
desirable to place an insulating shield about lead-in wire 21 to prevent 
arcing within the lamp and between the various elements. Current passes 
from lead-in wire 21 to cathode 4 through an intermediary molybdenum foil 
section 6. 
The other side of the circuit is formed through stiff lead-in wire 16 which 
is preferably bent out of place so that parts on one side of the line are 
insulated from those on the other side. A resistor 13 is attached to stiff 
lead-in wire 16 through a lead-in wire associated therewith and thence to 
a connector 27 which is attached to starting probe 5. Bimetal 22 is biased 
open when the lamp is turned off but when the lamp starts, it biases 
closed against the lead-in to probe 5 thereby establishing the same 
current potential at probe 5 and cathode 4. Such closing prevents 
electrolysis between the probe and cathode. 
At the other end of arc tube 2, an upper support 10 is mounted within the 
tubular portion of bulbous envelope 1. Support frame 10 includes a 
horizontal section 18 having vertical supports 17 and 19 depending 
downwardly therefrom and attached at the free ends to an upper strap 11 
which surrounds the press seal of arc tube 2 and rigidly holds it in 
place. Preferably, the construction and disposition of upper strap 11 is 
similar to lower strap 7. A pair of upper bumpers 9 are mounted upon 
vertical sections 17 and 19 of upper support 10 and resiliently abut 
against the sides of the tubular portion of bulbous envelope 1. Such 
disposition prevents breakage of the lamp if the arc tube is shaken or 
dropped. 
A lead-in wire 28 extends to the outside or arc tube 2 and is attached at 
its inner end to a molybdenum foil section 6 and thence to a cathode 3. A 
pyrophoric conductor 30 is electrically connected in series between 
cathode 3 and lead-in wire 16. Pyrophoric conductor 30 is physically 
connected between lead-in wire 28 and upper strap 11 by means of support 
wires 31 and 32. Electrical connection to upper cathode 3 is established 
by a thin conducting wire 20 connected at its lower end to lead-in wire 16 
and at its upper end to vertical support 19 which is connected to upper 
strap 11. Support wire 32 is connected to upper strap 11 and support wire 
31 is connected to lead-in wire 28. 
In one example, pyrophoric conductor 30 was made by twisting together 
loosely about a hundred filaments 33 of 1" to 2" long shredded aluminum 
foil of the type used as the combustible material in photoflash lamps. The 
ends of the loose yarn formed were clamped in support wires 31 and 32. The 
yarn was then coated with a slurry of finely powdered zirconium metal 
dispersed in water, the powdered zirconium being, for example, of the type 
used as a primer in photoflash lamps. The powdered zirconium 34 easily 
penetrated into the loose yarn. After drying, the zirconium was securely 
retained in pyrophoric conductor 30. 
Pyrophoric conductor 30 should have sufficiently low electrical resistivity 
so that there are substantially no I.sup.2 R losses therein during normal 
lamp operation. Conductor 30 also should be sufficiently pyrophoric so 
that it will spontaneously ignite in the presence of air at a relatively 
low temperature, say, at or somewhat above room temperature. 
In operation, pyrophoric conductor 30 is mounted in a lamp so that it is 
maintained, during normal lamp operation, at a temperature above its 
ignition temperature. In the example shown in the drawing, a 400 watt 
lamp, the temperature of conductor 30 during normal lamp operation was 
about 200.degree. C. to 250.degree. C. The fill inside jacket 1 is either 
a vacuum or an inert gas. When jacket 1 cracks or breaks, and admits air, 
the powdered zirconium of conductor 30 spontaneously ignites and generates 
sufficient heat to ignite the aluminum filaments. In a short time, the 
aluminum is consumed and the circuit is opened. 
Examples of other combustible metals which will burn readily in air if 
heated sufficiently and which can carry the lamp current are magnesium, 
zirconium and scandium.