Note: Descriptions are shown in the official language in which they were submitted.
l~ t~ LD-6077
The invention relates to high pre~sure metal halide arc
discharge lamps and is especially applicable to such lamps
containing sodium iodide.
High pressure metal halide discharge lamps generally
comprise a tubular fused silica arc tube containing a ioniza-
ble fill and having a pair of main thermionic electrodes in
the ends. The electrodes are supported by inleads which in-
clude a thin molybdenum ribbon portion extending hermetically
through a pinch seal in the end of the lamp. Generally a
starter electrode is disposed in the arc tube adjacent one
of the main electrodes to facilitate starting. A diæcharge
can be ignited across the short gap between the starter elec-
trode and it~ adjacent main electrode at a much lower voltage
than is required to ignite a discharge across the longer gap
between the two main electrodes. Once the discharge has
ignited, the ionized gas decreases the resistance between
the twv main electrodes and the discharge changes over into
an arc between them.
- m e ionizable fill of metal halide lamps comprises
mercury, an inert starting gas such as argon, plus one or
more metal halides having useable vapor pres~ures and a de-
sirable emission in the visible spectrum. One well-known
metal halide charge comprises the iodides of sodium, thallium,
and indium Another w~ll-known charge comprises the iodides
of sodium, scandium, and thorium. ~he addition of sodium
has from the beginning been troublesome because sodium ions
; migrate out of the arc tube during lamp operation. As sodium
is selectively lost and freed iodine is left behind, the lamp
spectrum deteriorates through loss of the sodium radiation and
the operating voltage rises, ultimately resulting in lamp
failure.
. . .
lO~lZ~E~
LD-6077
It was found that sodium loss could be gxeatly reduced
by changing the msunt frame or harness which ~upports the
arc tube within the outer envelope from the conventional
side rod construction to a divided mount construction. In
the former, the so-called side rods of the frame carry
current ~o the electrode at the opposite end of the arc tube
from the lamp base. In the latter which i9 a reversion to
an integral mount disclosed in U.S. patent No. 2,888,585
dated May 26, 1959 - Martt et al , the long side rods are
eliminated and the mount is in two sections, one extending
from the stem of the outer envelope to the arc tube and
the other extending from the dome end of the outer envelope
to the arc tube. In Electric Discharge Lamps, MIT Press
1971, John F. Waymouth explains the improvement as follows.
Since the lamp operates on alternating current, the side
:i
rods alternately have a positive and a negative potential
with respect to the surface of the arc tube The side rods
are bathed in ultraviolet light from the arc tube and as a
result emit photoelectrons When the side rods are negative,
some of these photoelectrons drift to the outer surface of
the arc tube, charging it up negatively. On the other half
cycle, when the side rods are positive, there is no return
photo current because silica is a very poor photo emitter.
The resulting negative charge on the arc tube causes posi-
tively charged sodium ions to move through the fused silica
-to the outer surface where they evaporate off. When the side
rods of the arc tube mount are eliminated, a major source of
electrons is removed.
;However even with the divided mount construction, an
appreciable loss of sodium continues to take place. The object
:
LD 6077
10~
of the invention is to further reduce the loss of sodium from
metal halide lamps.
SUMM~RY OF THE INVENTION
Sodium loss from the fused silica envelopes of metal
halide lamps can be reduced by means of a collector wire with-
in the outer envelope having a control potential thereon. Pref-
erably the collector wire is located close to the current car-
rying lead wire in order to effectively shadow it and collect
electrons that otherwise would drift to the art tube and pro-
mote sodium electrolysis.
We have found that in the divided mount construction,
an important source of photoelectrons is the long lead wire
that extends from the stem of the outer envelope to the elec-
trode at the far end of the arc tube. In order to reduce the
photoelectric effect, we add a collector wire to the lamp cir-
cuit and make it extend along the long lead wire and as close
to it as practicable. The collector wire is connected elec-
; trically to the opposite side of the lamp circuit from the
long lead wire. On the half cycle when the collector wire is
- 20 positive with respect to the long lead wire, it collects a
large fraction of the photoelectric current from the long
lead wire. In the next half cycle the polarities are re-
~ versed and the long lead wire collects the photoelectron
;~ c~rrent from the collector wire. In this way the photoelec-
tron current to the arc tube is greatly reduced even though
the primary photoelectron current passing back and forth
~ between long lead wire and collector wire may be approxi-
- mately doubled.
In a base-down metal halide lamp the starter elec-
trode is mounted at the dome end so as to be uppermost along
with the bimetal switch, and two long lead wires extending
. .
iO gl ~ ~ ~D 6077
from the stem to the dome end are provided, one running to
the starter electrode and the other to the main electrode.
In accordance with the invention, these wires are extended
closely parallel to each other and the starter resistor is
mounted at the starter end of the wire. Thus during opera-
tion the wires are at opposite polarity and a substantial
reduction in sodium loss results.
In a base-up lamp, reduced sodium loss is achieved
by running a collector wire, alongside the long lead wire, to
a dummy terminal at the far end of the end and connecting it
to the opposite a.c. potential. The effectiveness of the col-
lector wire can be further improved by means of a rectifying
diode connected in such manner as to keep the collector wire
positive with respect to the arc tube throughout the a.c.
cycle.
., .
~, DESCRIPTION OF DR~WINGS
. . .
In the drawings wherein like symbols denote corre-
sponding parts in the several figures:
FIG. 1 represents the geometrical relationships
between a tubular arc tube and a pair of conductors extend-
ing parallel thereto.
~ FIG. 2 is a front view of a base-down metal halide
; lamp embodying the invention.
~A .,
FIG. 3 shows diagrammatically the location of the
, 25 starting resistor in prior art base-down lamps.
FIG. 4 shows diagrammatically the relocation of
. .
the resistor according to the invention.
FIG. 5 is a simplified front view of a base-up
, ~
- lamp embodying the invention.
FIG. 6 is a simplified front view of another lamp
embodying a variant of the invention wherein a positive bias
:
:
- 4 -
,:
~ .......... . .
lV9~ LD 6077
is applied to a collector wire.
DETAILED DESCRIPTION
.
Considering a lamp comprising an inner arc tube
within an outer jacket, if approximating assumptions regard-
ing the boundary conditions determining the electric field
in the outer jacket are made, it is possible to determine,
by means of a current analog, the fraction f of photoelec-
tron current collected by the arc tube as a function of the
arc tube surface potential normalized to the potential of
the collector wire. For the geometry illustrated in FIG. 1
wherein the two wires A and B are located at 2-1/2 arc tube
diameters from the arc tube axis 0 and subtend an angle
at the axis, the estimated average photocurrent to the arc
tube per unit length of wire per emitted electron is the in-
` 15 tegral from 0 to 1 of f. The following values have been de-
termined for the case where wires A and B are at differing
,,
potentials.
; Angle Integral
180 .27
` 20 90 .22
:
~ 45 .17
:,
',: 11 .10
, .
The foregoing indicates that if the estimated
. .
average photoelectric current is 0.5 for the case of a single
wire extending along the arc tube and having an a.c. potential
thereon, it will be 0.27 with two wires having opposite po-
tentials and located 180 apart on diametrically opposite
, .
sides of the arc tube, 0.22 when the wires subtend 90 at
. the arc tube, and 0.10 when they subtend 11. If the two
, 30 wires located 180 apart have the same potential, the esti-
mated average photoelectric current is l.
5 -
: ~ , ~ . - ......
LD 6077
1091Z8~
FIG. 2 illustrates a metal halide divided mount lamp
for base-down operation utilizing the foregoing principles to
further reduce sodium loss. The lamp 1 comprises a vitreous
outer envelope or bulb 2 and a fused silica inner arc tube 3,
the outer envelope having a screw base 4 at its lower end.
The arc tube contains a quantity of mercury which is substan-
tially completely vaporized in operation, sodium iodide in ex-
cess of the quantity vaporized, and other suitable metal ha-
lides, for instance smaller amounts of thallium iodide and
indium iodide or scandium iodide and thorium iodide. An in-
ert rare gas at a low pressure, for instance argon at 25 torr,
is included in the arc tube to facilitate starting and warm
up. A pair of main arcing electrodes, 5 at the lower end
and 6 at the upper end plus an auxiliary starting electrode
7 at the upper end are sealed into the arc tube. The elec-
trodes are supported on inleads which include intermediate
thin molybdenum foil sections 8 extending through the pinch-
sealed ends 9, 10 of the tube. Main electrodes 5, 6 each
comprise a tungsten wire helix wrapped around a core wire
and may include activating material such as thorium oxide
filling the interstices between turns.
The arc tube is supported within the outer envelope
- by a divided mount comprising a wire frame 11 at the base end
and another wire frame 12 at the dome end which include metal
straps 13, 14 respectively encompassing the pinch seals 9, 10.
- The neck of the outer envelope is closed by a re-entrant stem
15 through which extend stiff inlead wires 16, 17 connected
~ at their outer ends to the screw shell and center contact of
; base 4. Lower wire frame 11 and connector 18 provide circuit
continuity from inlead 16 to main electrode 5~ A long, fine,
and resilient curving lead wire 19 extends from inlead 17 to
: .
- 6 -
,
lV~ 1. Z~ LD 6 () 7 7
main electrode 6 at the upper end of the arc tube. A s~cond
curving lead wire 20 extends through frame 11 from inlead 16
to a starting resistor 21 connected to auxiliary starting elec-
trode 7. Long curving wires 19 and 20 serve as electrical con-
ductors only and provide substantially no physical support of
the arc tube. A bimetal switch 22 is mounted on the inlead of
main electrode 6 and is arranged to engage the inlead of aux-
; iliary electrode 7 and short-circuit electrodes 6 and 7 to- gether after the lamp has warmed up.
The starting electrode is always located at the end
of the arc tube which is uppermost in order to minimize elec-
trolysis in the fused silica about the inleads. This means
~ that the bimetal shorting switch 22 is likewise mounted at
; the upper end, that is at the dome end in the base-down lamp
of FIG. 2. Curving lead wire 19 carrying the arc current is
made of fine tungsten wire in order to have minimum inter-
ception of ultraviolet and blue light rays which cause photo-
electron emission. For the same reason, the wire is curved
away from the arc tube and made to lie close to the wall of
the outer envelope. Current flow through the wire, particu-
larly at starting, causes it to heat up considerably and the
choice of tungsten enables the wire to retain its resiliency
and maintain its shape under these conditions. The second
curving lead wire 20 matches first lead wire 19 in physical
~25 characteristics.
The common practice prior to our invention was to
mount starting resistor 21, which limits the current to the
- starting electrode, at the stem or base end of the lamp. It
was attached to stiff inlead 16, and the second curving lead
wire 20 extended from the resistor to the inlead of starting
electrode 7. The arrangement is i~lustrated schematically
iO~ lZ~ LD 6077
in FIG. 3 and was favored because if the two curving lead
wires 19 and 20 should accidentally come into contact as a
result of bumping or jarring the lamp, the starting resistor,
typically of 40,000 ohms, would limit the current to a fraction
of a milliampere and prevent any damage. The foregoing con-
ventional arrangement produced the result that when the lamp
attained operating temperature and bimetal switch 22 closed,
both curving lead wires 19 and 20 were placed at the same po-
tential, namely that of inlead 17. The fact that this is al-
most the worst possible case for photoelectron current was
either not appreciated or overlooked.
In accordance with our invention, we obtain a sub-
stantial reduction in the photoelectron current by the very
simple expedient of moving starting resistor 21 up to the
starter electrode end of curving lead wire 20, as illustrated
in FIGS. 2 and 4. In other words, starting resistor 21 is
now attached to the inlead of starter electrode 7 at the up-
per end of the arc tube, and second curving lead wire 20 ex-
tends from inlead 16 to the starting resistor. This arrange-
ment has the effect of leaving curving lead wires 19 and 20
at opposite polarities during operation. This change made a
very substantial reduction in sodium loss and in the voltage
rise associated therewith. We have found that over a 1,000
; hour life test, changing the position of the starting resistor
`25 as described reduced the voltage rise from 9.1 to 3.0 volts.
Since the life of a metal halide lamp containing sodium is
an inverse function of voltage rise, lamp life is correspond-
ingly improved.
In the case of a lamp la for base-up operation, the
starting electrode 7 and the bimetal switch 22 are located at
the base end of the lamp in order to be uppermost, as illus-
- trated in FIG. 5. The practice prior to our invention was to
LD 6077
run a single long, fine curving wire l9 from inlead 17 at
the stem to the inlead of main electrode 5. Wire 19 was
curved away from the arc tube and made to lie close to the
outer envelope wall in order to minimize photoelectrode cur-
rent to the arc tube. However, we have found that a substan-
tial photoelectron current remains. In accordance with our
invention, a further reduction is achieved by providing a
; second curving lead wire 20 which now extends from inlead 16
at the stem to a dummy pin 23 which is sealed into the pinch-
` lO ed end 9 of the arc tube. The two wires l9 and 20 have oppo-
site a.c. potentials during operation and subtend an angle of
about ll degrees or less at the arc tube axis. This means
that the photoelectron current factor is reduced from 0.5 to
0.1 or less, resulting in a substantial reduction in sodium
loss and voltage rise with life.
The effectiveness of curving wire 20 can be further
increased by maintaining it positive with respect to the arc
tube at all times throughout the a.c. cycle. A convenient
arrangement for so doing is illustrated in the variant of
the illustration shown in FIG. 6. In lamp lb otherwise simi-
lar to that of FIG. 5, the stem 15 is modified to include a
dummy inlead 24. The second curving lead wire 20 extends
from dummy inlead 24 of the stem to dummy pin 23 of the arc
; tube at the dome end of the lamp, and both wires 19 and 20
~25 extend parallel and close together and curve away from the
arc tube. A pair of solid state diodes 25, 26 are connected
from inleads 16, 17, respectively, to dummy inlead 24, the
polarity connections biasing inlead 24 and lead wire 20 posi-
tive. The positive potential on wire 20 is even more effective
in collecting photoelectrons from wirP 19 than the a.c. con-
nection of FIG. 5 and further reduces sodium loss and voltage
- .. .
iO~ LD 6077
rise. The use of two diodes is preferred to a single diode in
order to maintain a positive potential on wire 20 through both
half cycles, but a single diode can suffice if adequate capac-
itance is provided.
-- 10 --
