[0001] The present invention relates to high pressure sodium vapor high intensity discharge
lamps, and more particularly to the support structure for a high pressure sodium discharge
light source within the lamp.
[0002] High pressure sodium discharge lamps are comprised of a discharge device mounted
in an evacuated outer envelope. The discharge device is typically a ceramic discharge
vessel comprised of alumina or sapphire and having conductive terminals for receiving
an operating voltage. The conductive terminals are niobium which is used because its
coefficient of thermal expansion matches that of alumina and because it is resistant
to sodium vapor. Titanium solder is used in connections to the niobium.
[0003] The outer envelope is evacuated in order to thermally isolate the discharge device,
and to avoid reactions of any gas within the outer envelope with the discharge device.
Nitrogen, which is used in the outer envelope of other types of high intensity discharge
lamps, cannot be used in high pressure sodium lamps because of its reactivity with
niobium and titanium at high temperature.
[0004] The evacuated outer envelope of high pressure sodium lamps must be strong and able
to withstand severe mechanical impacts without breaking. If the lamp outer envelope
were to break, it would implode scattering glass fragments and create a safety hazard.
[0005] It has been the practice to manufacture high pressure sodium lamps with evacuated
outer envelopes, and to make those envelopes sufficiently strong to avoid breakage.
However, high envelope strength is not feasible in the case of many reflector lamps.
Reflector lamp envelopes have a large face that merges with the envelope side walls
at an edge portion having a small radius of curvature. The atmospheric pressure acting
on the evacuated envelope causes high stress concentrations in the edge portion and
makes it susceptible to breakage. Moreover, reflector lamps have thin blown glass
envelopes and cannot be strengthened by making them substantially thicker. Incandescent
reflector lamps having blown glass envelopes uniformly contain a fill gas with an
internal pressure of about one atmosphere. With the inner and outer pressures acting
on the envelope being approximately equal, no implosion will occur if the envelope
breaks and there is less apt to be flying glass fragments.
[0006] There has been some consideration of gas filled high pressure sodium lamps. U.S.
patent 3,932,781 issued to Jozef C.I. Peeters et al discloses a high pressure sodium
lamp having an outer envelope that is gas filled to inhibit evaporation of the alumina
discharge tube. This reduces the deposition of alumina on the outer envelope and the
attendant reduction in light output. The results of experiments involving such a lamp
are also disclosed in the article by R.J. Campbell et al, "Evaporation studies of
the sintered aluminum oxide discharge tubes used in high pressure sodium (HPS) lamps",
Journal of the IES, July 1980, pages 233-239.
[0007] The introduction of a fill gas into the outer envelope of a high pressure sodium
discharge lamp presents the problem of voltage breakdown through the gas. These lamps
have closely spaced metal parts having a potential difference of around 4000 volts
during lamp operation. In the high vacuum of conventional high pressure sodium lamps
electrical breakdown between the lamp parts was not a problem. A fill gas has the
potential of ionizing and providing a conductive path between the internal lamp parts
at the different potentials and electrical breakdown can occur.
[0008] Accordingly, it is an object of the invention to provide a high pressure sodium discharge
lamp having a gas filled outer envelope in which electrical breakdown through the
fill gas is prevented.
[0009] It is another object of the invention to provide a high pressure sodium lamp that
is practicable to be operated in any orientation without electrical breakdown through
the gas filled in the outer envelope.
[0010] It is another object of the invention to provide support structure for the discharge
device of a lamp that will operate in a rare gas atmosphere without electrical breakdown
through the rare gas.
[0011] According to the invention a high pressure sodium lamp is comprised of an outer envelope
containing a rare gas at a pressure of approximately one atmosphere. A metallic reflective
layer is disposed on a portion of the outer envelope for defining a reflector. Mounting
means mounts the discharge device of the lamp within the outer envelope and is comprised
of a pair of conductors for providing a conductive path to the discharge device. The
pair of conductors are configured to have a breakdown voltage between them greater
than a certain value, and to maintain the breakdown voltage between the conductors
and the metallic reflector greater than the certain value.
[0012] In a preferred embodiment of a high pressure sodium lamp according to the invention,
the lamp comprises means within the outer envelope connected across conductors supplying
voltage to the discharge device for exhibiting a high impedance below a certain applied
voltage and a low impedance above a certain applied voltage. The voltage at which
the impedance changes is selected to be lower than the breakdown voltage through the
gas atmosphere within the outer envelope.
[0013] In another preferred embodiment the lamp comprises thermal control means for controlling
the relative thermal dissipation of the ends of the discharge device for rendering
its operating voltage insensitive to the orientation of the lamp during the lamp orientation.
Thus also the risk of voltage breakdown through the gas filled outer envelope is further
reduced. In one embodiment the thermal control means is comprised of a heat shield
on the end of the discharge device closest to the lamp base. In another embodiment
the thermal control means is comprised of different length of the electrode mounting
means mounting the discharge electrode, resulting in the one electrode being closer
to the adjacent end wall than the other electrode to its adjacent end wall. In yet
another embodiment the thermal control means is comprised in that the discharge device
has end walls of different thickness. The thicker end wall dissipating more heat than
the thinner end wall and thus operating at a lower temperature then the thinner end
wall.
[0014] Embodiments of the invention will be explained with reference to accompanying drawings,
in which:
Fig. 1 is a partial vertical section of an HPS reflector lamp with blown glass envelope
according to the invention;
Fig. 2 is an isometric view of the discharge tube support structure shown in Fig.
1;
Fig. 3 is a partial cross section of the support structure shown in Fig. 2;
Fig. 4 is a partial vertical section of an HPS reflector lamp with a blown glass envelope
in which the discharge tube has thermal control structure;
Fig. 5 is a vertical section of a high pressure sodium discharge tube having unsymmetrical
structure for thermal control;
Fig. 6 is a partial vertical section of an HPS reflector lamp according to the invention
having structure for preventing internal electrical breakdown;
Fig. 7 is a partial vertical section of an HPS reflector lamp like that shown in Fig.
1 and having structure for preventing internal electrode breakdown; and
Fig. 8 is a graph illustrating the relative magnitudes of different voltages that
characterize the lamp operation.
[0015] Fig. 1 illustrates a high pressure sodium reflector lamp having a blown glass envelope.
The envelope has a transparent or translucent front dome 1 from which light is emitted
during lamp operation. A mid-section 2 converges toward a narrow neck 3 which terminates
at the base end of the lamp envelope. A lamp base 4 is mounted on the base end of
the envelope opposite the front dome 1.
[0016] A reflective layer 5 is disposed over at least a portion of the converging mid-section
2 of the lamp envelope. It is illustrated extending up to the edge of the dome 1 of
the lamp envelope, and down onto a part of the narrow neck 3. The reflective layer
5 is typically metallic aluminum which is vapor deposited on the inner surface of
the envelope. A high pressure sodium discharge device 10 is mounted axially symmetrically
within the envelope and emits light which is incident on the reflective layer 5. The
convergence of the envelope mid-section 2 having the reflective layer 5 is effective
to reflect light from the light source 10 in a forward direction through the dome
end of the envelope so as to concentrate the light and give it directivity.
[0017] The high pressure sodium discharge device 10 has a translucent body 11 and a pair
of terminals 12, 13 each extending from a respective end of the tubular body 11. When
a sufficiently high voltage is applied across the terminals 12 and 13, an electrical
discharge is established between a pair of spaced internal electrodes (not shown)
within the tubular body 11 and intense visible light is emitted.
[0018] The discharge device 10 is mounted within the envelope by a frame structure which
also comprises conductors for applying an operating voltage to the discharge device.
The base end of the envelope is closed by a stem 7 which is terminated at a pinch
seal 8. A pair of rigid support conductors 14, 15 emerge from the pinch seal 8 and
extend longitudinally of the envelope toward the dome end 1. The shorter conductor
14 has a free end which is connected to the terminal 13 of the discharge device by
a conductive link 21. Similarly, the free end of the longer conductor 15 is attached
to the terminal 12 by the conductive link 22. Each of the support conductors 14, 15
extend into the pinch seal 8 and are connected by respective conductive leads to the
lamp base 4, in a conventional manner. Consequently, a voltage applied across the
lamp base 4 is developed across the terminals 12, 13 of the high pressure sodium discharge
device 10 for energizing it to emit light.
[0019] In order to avoid the danger of implosion upon breakage of the outer envelope 1,
the outer envelope contains rare gas at a fill pressure of about 700 torr at room
temperature. At the lamp operating temperature, the rare gas pressure is greater than
one atmosphere (760 torr), in one example 930 torr. So there is no substantial pressure
difference across the wall of the lamp envelope, as the occuring range of pressure
difference is of the same magnitude as barometric variations. Consequently, if the
envelope is broken there will be no substantial pressure difference to accelerate
glass fragments and cause flying fragments of the broken envelope. The rare fill gas
within the outer envelope thus makes it safe to use thin blown glass outer envelopes
in high pressure sodium reflector lamps.
[0020] The use of a rare fill gas in the outer envelope of a high pressure sodium lamp has
certain consequences for the lamp's characteristics. These in turn dictate that the
lamp incorporate certain structural features.
[0021] A major and substantial consequence of the use of the rare fill gas is the lowering
of the breakdown voltage between internal lamp components. The American National Standards
Institute (ANSI) recommends that the lamp be able to withstand an a.c. voltage of
4,000 volts peak. Commercially available high pressure sodium lamp starters produce
a voltage pulse of up to 4000 volts having a duration of one millisecond. Conventional
high pressure sodium lamps have a high internal vacuum of less than 10⁻⁴ torr in their
outer envelope. As a result, internal metal components, such as discharge device mounting
frame parts, can be as close as about three millimeters without a breakdown occurring
at 4000 volts applied to the lamp.
[0022] The higher pressure rare gas fill increases the probability of internal voltage breakdown
being caused by the 4,000 volt starting pulse. In order to avoid breakdown from occurring,
the metallic components of the discharge device mounting structure are shaped to maximize
the distance between the support conductors 14 and 15 that have an electrical potential
between them during lamp operation.
[0023] As shown in Fig. 2, the discharge device 10 is positioned on the lamp center line,
and the short straight conductor 14 is on one side of the center line. The conductor
15 emerges from the pinch seal 8 on the opposite side of the lamp center line, and
after a short length 16 it is bent perpendicular to the conductor 14. The section
17 of the conductor 15 extends perpendicularly away from the conductor 14, and is
bent to define a portion 18 extending parallel to the conductor 14. The next portion
19 extends away from the imaginary plane defined by the conductor 14 and the portions
16 and 17 of the conductor 15. The next section 20 again extends parallel to the lamp
longitudinal direction, and the successive section 21 extends back toward the original
line of direction of the section 18. The last section 22 of the conductor 15 extends
along the same line of direction as the section 18. This structure allows sufficient
separation between the conductors 14 and 15 and at the same time avoids the conductor
15 from coming too close to the reflective layer 5, which is typically a metallic
and conductive layer such as aluminum.
[0024] Section 16 of the conductor 15 is the part that is closest to the conductor 14. This
is where electrical breakdown is most likely to occur. In order to reduce the likelihood
of breakdown, a glass sleeve 33 covers the portion of the conductor 14 opposite the
section 16 of the conductor 15. The glass sleeve 33 increases the breakdown voltage
between the conductors 14 and 15. The gas krypton was used in a reflector lamp having
the glass sleeve 33 and did not break down. Thus, krypton fill gas provides a practicable
way of eliminating the implosion problem.
[0025] In order to establish the effectiveness of the glass sleeve 33, high pressure sodium
reflector lamps were made which were identical except that some had the sleeve and
some did not. The lamps had 70 watt HPS discharge devices mounted in an RL-38 outer
envelope filled with krypton at a pressure of 700 torr. The space between the conductor
14 and the section 16 of the conductor 15 was eight millimeters. After the lamp reached
normal operating temperature, and power was interrupted, the application of a 4,000
volt one microsecond pulse caused arcing between the conductors 14 and 15, in the
lamp without a glass sleeve. For the lamp with the glass sleeve 33, no arcing occurred
as long as the terminal 13 of the HPS discharge device 10 was at least 13 millimeters
from the conductor 15.
[0026] To further improve the breakdown characteristics of the lamp internal structure,
all metallic parts are configured to eliminate sharp points and edges. Sharp points
create regions of electric field concentration and may facilitate localized ionization
of the rare fill gas which could initiate a breakdown between the conductors 14 and
15. In HPS lamps the discharge device is frequently attached to the supporting conductors
by thin metallic ribbons or straight rigid rods. In the present invention, connectors
30 and 31 are made from wire having a circular cross section and are wrapped around
the respective discharge device terminal and support conductor in the manner shown
in Fig. 3. This eliminates the sharp edges or ends inherent in the prior art structure
and avoids any attendant reduction in breakdown voltage. In a lamp having argon at
700 torr in the outer envelope, the curved connectors 30, 31 increased the breakdown
voltage by 1000 peak a.c. volts relative to straight rod connectors.
[0027] A getter support 40 is attached to the section 22 at the free end of the conductor
15. This position maximizes the distance of the getter support 40 from the conductor
14 and also avoids reducing the internal breakdown voltage of the mounting frame structure.
[0028] The rare fill gas also contributes to dissipation of heat developed in the discharge
device 10 during lamp operation. HPS discharge devices have minimum operating temperatures.
If they are not sufficiently heated during operation their internal sodium vapor pressure
will be too low and the light output will be substantially reduced. In order to compensate
for thermal losses through the rare fill gas, the discharge device 10 is physically
smaller than a discharge device for the same wattage used in an evacuated HPS lamp.
The lamps described herein have a discharge device lenght of 41.8 millimeters as compared
to the standard 48.0 millimeter length, and a 4.0 millimeter inside diameter as compared
to the 4.8 millimeter standard. The smaller physical size reduces the area of the
discharge device through which heat can transfer to the rare fill gas so that the
discharge device operates at the correct temperature even though substantial amounts
of thermal energy can be transferred through the rare gas.
[0029] The smaller HPS discharge device 10 results in a lamp for which the beam spread is
substantially determined by the position of the discharge device along the center
line of the lamp. This is shown by the data in the following Table I. The beam spread
of the lamp can be set between 15 and 96 degrees by selecting the position of the
discharge device within an interval of 15 millimeters. This broad range in beam spread
was achieved with an RL-38 outer envelope.
TABLE I
Mount Height (mm) |
Beam Spread (deg.) |
ANSI Notation |
72 |
15 |
NSP |
74 |
23 |
SP |
82 |
53 |
WFL |
87 |
96 |
VWFL |
[0030] The RL-38 bulb has a seal length (the distance from the base of the stem 7 to the
dome 1) of 130 mm. The mount height is measured from the base of the stem 7 to the
center of the discharge device 11. The lamps for which data is reported in Table I
had a discharge device 41.8 mm in length, with an arc length of about 21 mm.
[0031] In the case of very wide flood lamps the HPS discharge device 10 is relatively closer
to the dome end of the lamp envelope 1. This results in the lamp voltage being strongly
dependent upon the orientation of the lamp during operation. When the lamp is operated
in a base-up orientation the cooler end of the discharge device 10 will be at the
dome end of the discharge envelope. Consequently, the sodium amalgam within the discharge
device will condense at that end. On the other hand, when the lamp is operated in
a base-down orientation the colder end of the discharge device will be at the bas
end of the discharge device 10 and that is where the sodium amalgam will condense.
[0032] In the base-up orientation, the lamp voltage will too high because of excessive reflected
heat back onto the end of the discharge device which elevates the discharge device
temperature. It was found that for the 70 watt lamp, the lamp voltage was 49.6 volts
in the base-down orientation and 62.6 volts in the base-up orientation. The discharge
device may be made unsymmetrical in order to eliminate the lamp voltage sensitivity
to lamp operating position.
[0033] Fig. 4 illustrates an HPS reflector lamp having a discharge device 10′ with a heat
reflector 35 at its end closest to the lamp base. The heat reflector is effective
for reflecting internally generated heat back into the discharge device 10′ and maintaining
the end of the discharge device 10′ with the heat reflector 35 at a higher temperature.
Those elements of the lamp shown in Fig. 4, which correspond to the elements of the
lamp shown in Fig. 1 have the same reference numerals.
[0034] An alternative to the use of a heat reflector is the asymmetrical discharge device
10˝ shown in Fig. 5. A pair of discharge electrodes 36, 37 are mounted internally
at the ends of connectors 12 and 13, respectively. The distance from an electrode
tip to an end wall of the discharge device 10 affects the end temperature of the discharge
device; the shorter the distance the higher the temperature. A discharge device 10˝
with an electrode tip to end wall distance for the electrode 36 of 7.75 millimeters
and the tip to wall dimension for the electrode 37 of 7.25 millimeters was used in
a reflector lamp with an RL-38 outer envelope. As shown in Table II, the 0.5 millimeter
shorter distance reduced the variation in operating voltage to less than one volt.
TABLE II
Electrode configuration |
lamp voltage base down |
lamp voltage base up |
Δv |
asymmetrical |
48.4 |
49.2 |
0.8 |
symmetrical |
49.6 |
62.6 |
13.0 |
[0035] An asymmetrical discharge device can also be realized with equal electrode tip to
end wall distances for both electrodes but with end walls of different thicknesses.
The thicker end wall will dissipate more heat than the thinner end wall and thus operate
at a lower temperature than the thinner end wall. By making the discharge device end
wall that is closer to the envelope dome thicker than the more distant end wall, the
heat reflected back from the envelope dome will be dissipated and the sensitivity
of lamp operating voltage to position will be diminished.
[0036] Another approach to preventing electrical breakdown between the internal support
conductors is to provide a circuit path within the lamp that will become conductive
before unintentional breakdown occurs. The lamp shown in Fig. 6 includes an HPS discharge
device 50 mounted within a lamp envelope by support conductors 51, 52 in the manner
previously described. A voltage across the conductors 51, 52 is the voltage which
is applied to the discharge device 50 for operating it. The lamp outer envelope contains
the rare gas argon at a pressure of the order of 700 torr.
[0037] A switching device 60 is incorporated in the lamp to define a circuit path having
a selected breakdown voltage which is lower than the breakdown voltage between the
conductors 51 and 52. The circuit path is isolated from the argon atmosphere in the
lamp envelope and has a normally high impedance. When the voltage between the support
conductors 51 and 52 exceeds a certain threshold voltage a low impedance circuit path
is established between the conductors 51, 52 through the switching device 60.
[0038] The switching device 60 is a spark gap device comprised of a non-conductive cylindrical
wall 61 and conductive end closures 62, 63 and having an internal chamber. Internal
electrodes 64, 65 are each mounted on a respective one of the conductive end closures
62, 63. Lead 66 extends from the conductive end closure 62, and lead 67 extends from
the conductive end closure 63. The leads 66 and 67 are each connected to a respective
one of the conductors 52, 51 so that the potential applied across the discharge device
50 is also applied across the spark gap device 60. The chamber of the spark gap device
60 has a gas fill selected to establish a particular breakdown voltage.
[0039] The voltage difference between the conductors 51 and 52 is applied through the leads
66 and 67 to the respective conductive end closures 62 and 63. Consequently, the voltage
difference between the conductors 51 and 52 exists between the internal electrodes
64, 65. When that voltage difference exceeds the selected breakdown voltage of the
spark gap device 60, the gas fill within the spark gap device 60 ionizes and a discharge
or spark occurs between the internal electrodes 64 and 65. The spark gap device 60
has a low impedance and is conductive, and the voltage difference between the conductors
51 and 52 is short circuited before breakdown of the argon fill gas within the lamp
outer envelope can occur.
[0040] When the voltage between the conductors 51 and 52 decreases below the switching device
threshold voltage, the discharge through the gas fill within the device 60 stops and
its impedance increases to the normal high impedance value. The switching device 60
is a self-restoring device and can be repeatedly switched to its low impedance conductive
state and each time it will return to its high impedance condition after the applied
voltage decreases below its threshold voltage.
[0041] Fig. 7 illustrates a reflector lamp having a discharge switching device like that
incorporated in the lamp of Fig. 6. The controlled and isolated discharge path provided
by the switching device is particularly advantageous in a reflector lamp. The reflector
lamp includes a reflective layer such as metallic aluminum which is conductive. The
metallic reflective layer can provide part of a breakdown path between the conductors
51 and 52. For example, an electrical breakdown could occur through the argon fill
gas between the conductor 51 and the reflective layer, and between the reflective
layer and the conductor 52. The metallic conductive layer would thus provide part
of the breakdown path between the conductors.
[0042] Fig. 8 illustrates the relationship among the various voltage magnitudes which define
the modes of operation of the invention. The starting voltage V
s of the discharge device 10 is typically around 2500 volts for a high pressure sodium
lamp; the 70 watt discharge device used in the lamps made and discussed herein have
a starting voltage of less than 1800 volts. The maximum voltage V
max that the lamp should withstand is nominally 4,000 volts. The controlled breakdown
voltage V
c of the spark gap device is selected to have a value between V
s and V
max.
[0043] Both V
s and V
max change as the temperature of the lamp increases during lamp operation. As the lamp
heats, the breakdown voltage of the argon gas within the lamp outer envelope decreases.
This was an unexpected result because the breakdown voltage should have been independent
of pressure at the constant gas density expected in a sealed lamp. The decrease in
breakdown voltage was measured in a lamp having an outer envelope filled with argon
at 700 torr and a stem like that shown in Fig. 2 but without the glass sleeve 33.
At the lamp operating temperature, the internal breakdown voltage will decrease by
about 500 volts to 3500 volts. At the same time, the internal pressure of the sodium
vapor within the discharge device 10 increases substantially and the starting voltage
increases. In fact, the starting voltage may increase to a value greater than the
controlled breakdown voltage V
c of the arc gap device. The breakdown voltage V
c must therefore be selected less than the lowered maximum voltage V
max that the lamp can withstand, but it should be higher than V
s so that the lamp can be restarted without having to first cool down completely. A
good nominal value for V
c is around 3,000 volts.
[0044] The use of the switching device 60 is not limited to reflector lamps. It can also
be applied to high pressure sodium lamps having conventional envelopes but which have
a rate gas fill rather than a high vacuum. Such lamps might use the rare gas to limit
discharge device material evaporation as discussed above. The problem of internal
electrical breakdown through the rare gas could also be solved with the switching
device as it is in reflector lamps.
1. In a high pressure sodium discharge reflector lamp, comprising a sealed outer lamp
envelope having a reentrant stem and an axis of symmetry through said stem and defining
a lamp centerline, a high pressure sodium discharge device comprising an elongate
body having a pair of terminals each extending from a respective end of said elongate
body for receiving thereacross an electrical potential to energize said discharge
device to emit light, and mounting means for mounting said discharge device within
said sealed envelope on the lamp centerline and for defining a conductive circuit
to said pair of terminals to permit energization of said discharge device, the lamp
further having the features;
said sealed outer envelope containing a quantity of rare gas such that the pressure
of said rare gas is approximately one atmosphere when the lamp is at its operating
temperature; and
said mounting means comprising a pair of upstanding support conductors extending from
said stem, one shorter and one longer than the other, and extending generally parallel
to said lamp axis, transverse conductive links connected between said support conductors
and said discharge device terminals for mounting said discharge device on the lamp
centerline and establishing a conductive path for energizing said discharge device,
and means for establishing a relative high electrical breakdown voltage between said
pair of support conductors to avoid electrical breakdown through said rare gas in
said outer envelope when said discharge device is energized during lamp operation.
2. In a high pressure sodium discharge lamp according to Claim 1, one of said support
conductors comprising a straight length of wire extending from said stem, and said
means for establishing the relative high electrical breakdown voltage comprising a
sleeve of non-conductive material covering a substantial length of said one support
conductor.
3. In a high pressure sodium discharge lamp according to Claim 2, wherein said sleeve
of non-conductive material is a glass sleeve to said stem and extending from said
stem with said one support conductor extending through said sleeve and into said stem.
4. In a high pressure sodium discharge lamp according to Claim 1, said transverse
conductive links are each comprised of a length of wire having a circular cross and
respective end portions wound substantially around one of said terminals and one of
said support conductors for defining a connecting link free of protrusions and mounting
said discharge device on said support conductors.
5. In a high pressure sodium discharge lamp having an outer envelope, a high pressure
sodium discharge device disposed in said outer envelope, and mounting means for mounting
said discharge device within said outer envelope and for defining a conductive circuit
to said discharge device to permit energization of said discharge device, comprising:
a metallic reflective layer disposed on a portion of said outer envelope for reflecting
and imparting directivity to light emitted from said discharge device;
a rare gas atmosphere within said outer envelope having a fill pressure at room temperature
of the order of one atmosphere; and
said mounting means having first and second conductors which define said conductive
circuit to said discharge device, a first of said conductors to said discharge device,
and a second of said conductors following a non-linear path spaced from said first
conductor for establishing an electrical breakdown voltage through said rare gas atmosphere
between said conductors above a certain value and spaced from said reflective layer
for establishing the electrical breakdown voltage through said rare gas atmosphere
between said second conductor and said reflective layer above said certain value.
6. A high pressure sodium discharge lamp according to one or more of the preceding
Claims, characterized in that the lamp incorporates means in the outer envelope connected
across the conductors and exhibiting a high impedance below a certain applied voltage
and a low impedance above said certain applied voltage.
7. A high pressure sodium discharge lamp according to Claim 6, characterized in that
the said means comprise a self-restoring threshold switch for establishing a low impedance
circuit path between the conductors when the voltage between the conductors exceeds
the threshold voltage of the threshold switch, the threshold voltage being greater
than the starting voltage of the discharge device and being less than the breakdown
voltage of the inert fill gas between the conductors.
8. A high pressure sodium discharge lamp according to any of the foregoing Claims,
characterized in that the lamp comprises thermal control means for controlling the
relative thermal dissipation of said ends of said discharge device for rendering the
operating voltage thereof insensitive to the orientation of the lamp during lamp operation.