High pressure metal vapor discharge lamp

A high pressure metal vapor discharge lamp comprising a discharge tube disposed in an outer jacket; said discharge tube having electrodes and containing a filling comprising a starting rare gas composed primarily of xenon at a pressure from 40 to 200 Torr; means for starting said discharge tube comprising a glow starter comprising a pair of contacts spaced apart not more than 2.5 mm from each other, and containing a gas composed primarily of argon at a pressure of at least 7 Torr; the pressure in said glow starter (P.sub.g) being related to the pressure in the discharge tube (P.sub.i), according to the expression ##EQU1## and a starting electric conductor adapted to contact the discharge tube on starting thereof and to separate from the discharge tube after starting.

BACKGROUND OF THE INVENTION 
The present invention relates to a high pressure metal vapor discharge 
lamp, and more particularly to a high pressure sodium vapor discharge lamp 
with improved starting characteristics. 
High pressure sodium vapor discharge lamps generally have a higher 
efficiency, i.e., a higher lumen output per watt, than high pressure 
mercury vapor discharge lamps, metal halide discharge lamps or the like. 
However, sodium vapor lamps require a specially designed ballast for 
starting and stable operation because a high starting voltage is needed. 
The expense of the special ballast hinders general use of high pressure 
sodium vapor discharge lamps. 
A high pressure sodium vapor lamp has been developed which has in its outer 
jacket a discharge tube and a starting device comprising a heating 
filament and a thermally responsive switch. Such sodium vapor lamps can be 
started and stably operated with a ballast for a high pressure mercury 
vapor discharge lamp. In the starting operation, the thermally responsive 
switch is operated by the heating filament so that the switching voltage 
of the thermally responsive switch is converted into high voltage pulses 
by the induction of the choke coil of the ballast, and the high voltage 
pulses are impressed upon the electrodes of the discharge tube so that the 
lamp may be started. Accordingly, such a high pressure sodium vapor 
discharge lamp does not require any special external high voltage pulse 
generating device so that it can be used in place of a high pressure 
mercury vapor discharge lamp in lighting devices equipped with 
conventional mercury vapor lamp ballast while enjoying the advantage that 
an intensity of illumination may be attained which is twice as bright as 
that of a high pressure mercury vapor discharge lamp. 
However, a high pressure sodium vapor discharge lamp having such a starting 
device in its outer jacket generates a pulse voltage as high as 4000 volts 
in the starting operation due to the action of the thermally responsive 
switch. When such high voltage pulses are generated, there is a 
possibility that a dielectric breakdown may occur between the choke coils 
of the ballast, between the socket and the screw base of the sodium vapor 
discharge lamp or between other points in the lamp circuit. This 
possibility is especially high in circuits of mercury vapor lamp lighting 
devices in which the insulation has deteriorated after years of use. 
Particularly in a lamp circuit in which a ballast having a short circuit 
current of 0.9 to 1.7 amperes, such as a ballast for an 80 watt mercury 
vapor lamp, is used with 70 to 90 watt sodium vapor lamps, the high 
pressure sodium vapor discharge lamps have small size bases of the E26 or 
E27 type so that the possibility of dielectric breakdown occurring at the 
fitting between the base and the socket during a high voltage starting 
pulse is quite high. Hence, it is necessary to reduce the pulse voltage at 
the start. However, if the pulse voltage is reduced, the lamp cannot be 
started smoothly and the desired lamp operation cannot be attained. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
improved high pressure metal vapor discharge lamp having a starting device 
comprising a glow starter. 
Another object of the invention is to provide a high pressure metal vapor 
discharge lamp having improved starting characteristics. 
A further object of the invention is to provide a high pressure metal vapor 
discharge lamp wherein the nature and properties of the filling in the 
glow starter and the discharge tube are improved. 
Still another object of the invention is to provide a high pressure metal 
vapor discharge lamp which may be started with a low voltage starting 
pulse. 
An additional object of the invention is to provide a high pressure metal 
vapor discharge lamp which avoids dielectric breakdown. 
Yet another object is to provide a high pressure metal vapor discharge lamp 
which may be smoothly started with an external ballast having short 
circuit current of 0.9 to 1.7 amperes. 
These and other objects are achieved by providing a high pressure metal 
vapor discharge lamp having an outer jacket with a lamp base at one end 
thereof, a discharge tube disposed in said outer jacket, said discharge 
tube being provided with a pair of spaced electrodes and containing a 
filling comprising a starting rare gas composed primarily of xenon at a 
pressure from 40 to 200 Torr, means for starting the discharge tube 
comprising a glow starter disposed in said outer jacket containing a gas 
composed primarily of argon at a pressure of at least 7 Torr, the pressure 
in the glow starter (P.sub.g) being related to the pressure in the 
discharge tube (P.sub.i) according to the expression 
##EQU2## 
said glow starter being provided with a pair of contacts spaced at most 
2.5 mm from each other, and a starting electric conductor connected to one 
of the electrodes, said starting electric conductor being adapted to 
contact the discharge tube on starting and to separate from the discharge 
tube after starting.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
FIG. 1 shows a preferred lamp embodiment comprising a vitreous outer jacket 
10 having one end sealed to a stem 12 and a discharge tube 14 disposed in 
the center of said jacket. 
As shown in FIG. 2, discharge tube 14 comprises a transparent ceramic tube 
or bulb 16 having both its ends sealed with caps 18 of, for example, 
ceramic material. A pair of electrode supporting conductors 20, which act 
as exhaust tubes, are hermetically extended through caps 18, and a pair of 
electrodes 22 are connected to the inner ends of electrode supporting 
conductors 20. Electrodes 22 are prepared by fitting helically coiled 
tungsten wire electrode portions 24 upon electrode stems 26. Although not 
shown in the drawing, coiled portions 24 are coated with an electron 
emitting substance comprising at least one material selected from the 
group consisting of barium oxide, calcium oxide and yttrium oxide. 
Discharge tube 14 contains predetermined quantities of mercury, sodium and 
xenon which serves as a starting gas. The pressure (P.sub.i) of the xenon 
gas may range from 40 to 200 Torr. 
The electrode supporting conductors 20 of discharge tube 14 are supported 
at their outer end portions by conductive tube holders 28 and 30. Tube 
holders 28 and 30 are in turn electrically and mechnically connected to 
and supported by conductive supports 32 and 34. Conductive tube holder 30 
is insulated from conductive support 32 by means of a ceramic tube or 
sleeve 35 through which the conductive support extends. Supports 32 and 34 
are connected by lead-in wires 42, and 44, respectively, to the screw base 
36 and to the eyelet terminal 38 of an E26 or E27 type base 40 attached to 
the sealed end of the outer jacket 10. The lead-in wires are sealed in the 
stem 12. A starting electric conductor 46 made of tungsten or molybdenum 
is arranged in outer jacket 10 substantially in parallel with the 
discharge tube 14. One end of starting conductor 46 is supported by a 
bimetal member 48 fixed on the support 32. The other end of starting 
conductor 46 is rotatably mounted on holder 28. Starting conductor 46 lies 
in close contact with the outer surface of discharge tube bulb 16 when the 
discharge lamp has been off and the lamp is cool, but moves away from 
discharge tube bulb 16 after the lamp has been turned on because the 
bimetal member 48 warps, i.e., assumes a curved configuration, as it 
receives heat from the discharge tube 14. 
A starting device 50, shown in greater detail in FIG. 3, is connected 
between the conductive supports 32 and 34 in parallel with the discharge 
tube 14. A glow lamp starter 52 is connected in series with a normally 
closed type bimetal switch 54 having a contact 54A. The resulting series 
circuit is connected between the supports 32 and 34 in parallel with the 
discharge tube 14. An insulating support 56 is provided for solidly 
mounting bimetal switch 54 between glow starter 52 and conductive support 
34. 
Glow starter 52 is constructed, as shown in FIG. 4, with a pair of bimetal 
members 58 held facing each other within an enclosure 60 by conductive 
holders 61 and with a pair of contacts 62 made of tungsten rods welded to 
the adjacent faces of bimetal members 58. Conductive holders 61 are 
connected to conductive support 32 and bimetal switch 54, respectively. 
The spacing between the contacts 62 in glow starter 52 is 2.5 mm or less. 
Argon gas is confined in the glow starter enclosure 60. The pressure of the 
argon gas (P.sub.g) is at least 7 Torr and is related to the pressure of 
the xenon gas (P.sub.i) according to the following expression: 
##EQU3## 
The discharge lamp is connected via a ballast 64 with a power source 66, 
shown schematically in FIG. 1, by screwing base 40 into an appropriate 
socket (not shown). Ballast 64 is a single choke type ballast designed for 
high pressure mercury vapor discharge lamps and has a short circuit 
current ranging from 0.9 to 1.7 amperes. 
The discharge lamp of the invention operates as follows: When the power 
source 66 is connected, the glow starter 52 is supplied with the secondary 
no-load voltage of the ballast 64, because the normally closed bimetal 
switch 54 is closed, so that the glow starter 52 is operated. Starting 
conductor 46 is initially in contact with the outer surface of discharge 
tube 14 because bimetal member 48 is in its comparatively straight 
configuration. In this condition, a pulse voltage is generated by opening 
of the contacts 62 after the contacts have been closed by glow discharge 
of glow starter 52. The pulse voltage is superimposed upon the secondary 
voltage of the ballast 64 and is impressed upon the discharge tube 14. 
Consequently, when the aforementioned pulse voltage is impressed upon the 
discharge tube 14, a weak discharge passage is formed within the discharge 
tube between a portion of the surface of one electrode 22 and the inner 
surface of the discharge tube 14 adjacent where the tube is contacted by 
starting conductor 46, and between portions of the surfaces of the two 
electrodes 22 by the dielectric action of the starting conductor 46. The 
resulting weak discharge path accelerates electrons and ions so that it is 
extended into the discharge tube 14 while repeating collisions, 
ionizations and so on until it induces the arc discharge. 
In this way, the discharge tube 14 is started. The temperature of the 
discharge tube increases as its lighting state becomes more stable. 
Bimetal member 48 is thus heated by the discharge tube and is warped or 
caused to assume a curved configuration whereby starting conductor 46 is 
moved away from discharge tube 14 so that emitted light is not blocked and 
so that sodium is not lost by absorption into the wall of the discharge 
tube 14. Normally closed bimetal switch 54 is also heated by the discharge 
tube 14 so that it is opened to disconnect the power supply circuit from 
glow starter 52 and maintain the glow starter 52 in an inoperative 
condition. 
The pulse peak value of the glow starter 52 is influenced by the nature and 
pressure of the gas confined therein and by the size of the spacing 
between the contacts 62. Since argon is generally used as the confined 
gas, argon was used in the following experiments. The effects of changes 
in the argon pressure and changes in the inter-contact spacing on the 
pulse peak values obtained using a ballast having a short circuit current 
of 1.3 amperes were determined experimentally and are tabulated in Table 
1: 
TABLE 1 
______________________________________ 
Inter-contact 
Argon Gas Spacing 
Pressure (P.sub.g) 
0.8 mm 1.3 mm 2.5 mm 3.5 mm 
______________________________________ 
20 Torr 1000 V 1200 V 1300 V 1400 V 
15 Torr 1200 1400 1500 1700 
10 Torr 1700 1900 2000 2300 
7 Torr 2400 2500 2600 3000 
5 Torr 2600 3000 3100 3300 
______________________________________ 
As noted above, it is desired that the pulse voltage be 3,000 volts or less 
in order to guard against dielectric breakdown. It has been found that 
there is no likelihood of dielectric breakdown at pulse peak values of 
3,000 volts or less even if the discharge tube 14 does not start despite 
the operation of glow starter 52. In order to maintain the pulse peak 
value at 3,000 volts or less, it is desirable if the inter-contact spacing 
is at most 2.5 mm and if the argon gas pressure (P.sub.g) is at least 7 
Torr. 
It should be noted that the pulse peak value can be held to a maximum of 
3,000 volts at argon gas pressure as low as 5 Torr if the inter-contact 
spacing is 0.8 mm, and at inter-contact spacings as large as 3.5 mm if the 
argon gas pressure (P.sub.g) is at least 10 Torr. Such pressures and 
spacings are considered less desirable because the glow discharge time 
(T.sub.G) from the instant when a voltage is impressed upon the glow 
starter in order to start the glow discharge between the contacts 62 to 
the instant when these contacts 62 contact each other increases as the 
confined gas pressure decreases. Such conditions are also undesirable 
because the gas deterioration increases as the gas pressure decreases if 
the glow starter of the lamp is used for a long time. 
Experiments conducted with a glow starter identical to that used in the 
experiments reported in Table 1 have shown that the T.sub.G was initially 
1 to 2 seconds but increased to the values tabulated in Table 2 after 6 to 
7 hours of continuing operation. 
TABLE 2 
______________________________________ 
Argon Gas 
Inter-contact Spacing 
Pressure 0.8 mm 1.3 mm 2.5 mm 3.5 mm 
______________________________________ 
10 Torr 3-4 sec. 3-5 sec. 5-15 sec. 
10-300 sec. 
7 Torr 3-5 sec. 5-10 sec. 
6-20 sec. 
20-240 sec. 
5 Torr 30-300 sec. 
40-300 sec. 
50-480 sec. 
20-600 sec. 
______________________________________ 
It is apparent from Table 2 that glow starters having an argon gas pressure 
P.sub.g of 5 Torr and an inter-contact spacing of 3.5 mm are not desired 
because the glow discharge time (T.sub.G) increases substantially and 
becomes highly variable. The T.sub.G after 6 to 7 hours corresponds to the 
summation of the glow discharge time periods if it is assumed that the 
glow starter is subjected to the glow discharge for 10 seconds to 
initially light the discharge lamp, that the initial lighting operation of 
the discharge lamp is continued for 5 hours and that the life of the 
discharge lamp is 12,000 hours. 
Although Tables 1 and 2 report only results obtained when the short circuit 
current of the ballast is 1.3 amperes, it has been found that the pulse 
peak value can be controlled to be 3,000 volts or less when the short 
circuit current of the ballast is from 0.9 to 1.7 amperes if the 
inter-contact spacing is at most 2.5 mm and if the argon gas pressure 
P.sub.g is at least 7 Torr. Results of experiments by which this was 
determined are tabulated in Table 3: 
TABLE 3 
______________________________________ 
Argon Gas Pressure (P.sub.g) 
10 
5 Torr 7 Torr Torr 
Inter-contact Spacing 
2.5 mm 
1.3 mm 2.5 mm 1.3 mm 
0.8 mm 
2.5 mm 
______________________________________ 
Short 0.9A 2700 2600 2200 2100 2000 1600 
Circuit 
1.3A 3100 3000 2600 2500 2400 2000 
Cur- 1.6A 3300 3200 -- -- -- -- 
rent of 
1.7A -- -- 2900 2800 2700 2250 
Ballast 
2.0A 3600 3500 3100 3000 2900 2400 
______________________________________ 
As can be seen from Table 1, the pulse peak value may vary greatly even 
though the inter-contact spacing of the glow starter is at most 2.5 mm and 
the argon gas pressure P.sub.g is at least 7 Torr. It is, therefore, 
necessary that the glow starter 52 start without fail even with such 
fluctuations in pulse peak value. 
With this in mind, the invention regulates the pressure (P.sub.i) of the 
xenon gas confined in the discharge tube 14. Experiments regarding the 
difficulty in starting a high pressure sodium vapor discharge lamp using 
the aforementioned glow starter 52 as the pulse generating starter have 
shown that when a number of relatively low voltage pulses are generated 
using the aforementioned glow starter, the starting characteristics of the 
discharge lamp should be evaluated at two stages. In the first stage, weak 
discharge paths are discontinuously formed within the discharge tube 14 
between one electrode 22 and the inner surface of the wall of the 
discharge tube 14 in the vicinity of the starting conductor 46 and between 
the two electrodes 22 by the impression of the pulse voltage. In the 
second stage, the weak discharge accelerates the charged particles, i.e., 
electrons or ions, to enlarge the discharge path as a result of 
collisions, ionizations, etc., until the discharge path shifts to arc 
discharge between the electrodes 22. 
Initially, it is necessary that the weak discharge path be formed and 
maintained without fail as a result of the action of starting conductor 
46. The results of experiments to determine the probability that the weak 
discharge path of the first stage will be formed within 10 seconds using 
the glow starter used in the experiments reported in Table 1 are tabulated 
in Table 4: 
TABLE 4 
______________________________________ 
Xe Gas Pressure 
in Discharge Tube 
Kind of Glow 40 100 200 250 300 
Starter Torr Torr Torr Torr Torr 
______________________________________ 
Pulse Peak Value: 
100% 100% 100% 60% 20% 
1,000 volts 
Inter-contact Spacing: 
0.8 mm 
Ar Gas Pressure: 20 Torr 
Pulse Peak Value: 
100 100 100 70 30 
1,900 volts 
Inter-contact Spacing: 
1.3 mm 
Ar Gas Pressure: 10 Torr 
Pulse Peak Value: 
100 100 100 70 40 
2,600 volts 
Inter-contact Spacing: 
2.5 mm 
Ar Gas Pressure: 7 Torr 
______________________________________ 
It can be seen from Table 4 that the probability of formation of the weak 
discharge path decreases, when the zenon gas pressure in the discharge 
tube exceeds 200 Torr, so that starting within 10 seconds is not assured. 
Since a high xenon gas pressure in the discharge tube generally lowers the 
restriking voltage thereby lowering the extinguishing voltage, the time 
period to the instant when the lamp is extinguished after it has been lit 
for a long time is increased so that the life of the discharge lamp is 
extended and so that the luminous efficiency is improved. However, the 
starting characteristics deteriorate if the pressure of the xenon gas is 
raised. Table 4 verifies that the starting characteristics of the 
combination with the glow starter deteriorate. The discharge tube used in 
the experiments reported in Table 4 was an alumina tube having an internal 
diameter of 4.0 mm and an inter-electrode spacing of 29 mm. The ballast 
used in these experiments had a short circuit current of 1.3 amperes and a 
secondary no-load voltage of 220 volts. 
Next, the shift to reliable arc discharge may not be effected in the second 
stage even when the weak discharge path is formed without fail at the 
indicated gas pressure. When the weak discharge path is formed, glow 
discharge also occurs between the contacts of the glow starter, and the 
so-called secondary voltage of the ballast is shunted between the weak 
discharge path and the glow discharge path of the glow starter. As the 
current flowing through the glow tube increases, the number of charged 
particles in the discharge tube is reduced so that the weak discharge path 
cannot be sufficiently widened. Instead, when the pressure of the xenon 
gas in the discharge tube is high, the charged particles cannot attain 
sufficient energy to cause the needed collisions, ionizations and so on, 
so that the discharge is interrupted. 
FIG. 5 is a graph of the results of experiments to determine whether or not 
the arc discharge occurred in the discharge lamp within 10 seconds from 
starting of discharge depending on changes in the argon gas pressure in 
the glow starter and on changes in the xenon gas pressure in the discharge 
tube. A glow starter as used in the experiments reported in Table 1 having 
an inter-contact spacing of 0.8 mm was used in the experiments. In FIG. 5, 
the portion appearing to the left of and below curve A corresponds to the 
range within which the discharge is ensured and which is approximately 
defined by the expression: 
##EQU4## 
Similar experiments conducted using glow starters having inter-contact 
spacings of 1.3 mm and 2.5 mm have shown that the curve A has a tendency 
to be shifted similarly to curves A' and A", but that the start is ensured 
if the determining point at least falls in the range below the curve A. 
The gas pressure P.sub.i of the xenon in the discharge lamp should be at 
least 40 Torr. When a glow starter is used as the starter, a number of 
pulses are applied to the discharge tube at each start. This invites the 
possibility that the end portions of the discharge tube will be blackened 
as a result of splashing of the substance from which the electrodes are 
made. The possibility of blackening is greater at lower xenon gas 
pressures. The blackening of the end portions invites reduction in the 
flux of light maintaining percentage. 
Experiments have been conducted using a ballast having a short circuit 
current of 1.3 amperes, a glow starter having an inter-contact spacing of 
1.3 mm and containing argon at a pressure of 10 Torr, and a discharge tube 
having an internal diameter of 4.0 mm and an inter-electrode spacing of 29 
mm. FIG. 6 is a graph of the results of flashing tests of lighting the 
discharge lamp for 15 minutes and extinguishing the same for 20 minutes 
conducted to attain such flux of light maintaining percentages for 
different xenon pressures in the discharge tube. As can be seen from FIG. 
6, when the xenon gas pressure is lower than 40 Torr, the flux of light 
maintaining percentages is reduced undesirably. 
In accordance with the hatched range shown in FIG. 5, therefore, the xenon 
gas pressure (P.sub.i) in the discharge tube should lie within the range 
defined by the expression: 40.ltoreq.P.sub.i .ltoreq.200 (Torr); the argon 
gas pressure (P.sub.g) in the glow starter should lie within the range 
defined by the expression: 
##EQU5## 
and the inter-contact spacing of the glow starter should be at most 2.5 
mm. 
Desirably, the inter-contact spacing of the glow starter will be at least 
0.8 mm to minimize possible trouble with the brazed contacts while taking 
into account production tolerances. 
The argon gas in the glow starter need not be limited to pure argon. The 
invention includes the use of mixtures of argon with, for example, up to 
about 30 mole percent neon and/or helium. Since neon or helium has a 
lighter mass than argon, the thermal conduction loss due to diffusion is 
increased to raise the pulse peak value. For example, the pulse peak 
voltage for a mixture containing up to about 30 mole percent neon or 
helium diluent rises about 10% higher than that for pure argon. 
Nevertheless, the invention can be practiced because the pulse peak 
voltage does not exceed 3,000 volts if the inter-contact spacing is at 
most 2.5 mm and if the argon gas pressure is at least 7 Torr. 
Similarly, the starting rare gas in the discharge lamp need not be limited 
to pure xenon. However, the starting gas should be composed primarily of 
xenon. No difficulties arise if up to about 30 mole percent of the xenon 
is replaced with krypton and/or argon. 
A preferred high pressure sodium vapor lamp according to the invention may 
be constructed as follows. A transparent alumina tube having an internal 
diameter of 4.0 mm was used as the discharge tube, and coil electrodes 
were arranged therein facing each other at a spacing of 29 mm. A barium, 
calcium or yytrium emitter was applied to the electrodes. In the discharge 
tube were confined 3 mg sodium, 17 mg mercury, and xenon gas at a pressure 
of 70 Torr. A molybdenum wire having a diameter of 0.3 mm was arranged, as 
shown in FIG. 1, as the starting conductor along the outer surface of the 
discharge tube. The glow starter had an inter-contact spacing of 1.3 mm 
and contained argon at a pressure of 12 Torr. The glow starter contacts 
were prepared from pieces of 1.0 mm diameter tungsten rod and welded to 
bimetal members having a length of 10 mm, a width of 2 mm, a thickness of 
0.15 mm, a warping modulus of 12.times.16.sup.-6 /.degree. C. and an 
elastic modulus of 175,000 kg/mm.sup.2. The resulting glow starter was 
connected in series with a normally closed type bimetal switch and was 
assembled into the lamp as shown in FIG. 1. The outer jacket of the lamp 
was attached to an E26 type screw base, which was then fitted in a socket. 
The discharge lamp was used in combination with a ballast having a short 
circuit current of 1.3 amperes. The discharge lamp achieved stable 
lighting operation at a lamp voltage of 100 to 120 volts and a lamp 
current of 0.75 to 0.95 amperes. A pulse peak voltage of 1,700 volts was 
attained when the glow starter was operated. As a result, the discharge 
lamp started without fail within 5 seconds. 
The foregoing description has been set forth merely to illustrate the 
invention and is not intended to be limiting. Since modifications of the 
disclosed embodiments incorporating the spirit and substance of the 
invention may occur to persons skilled in the art, the scope of the 
invention is to be limited solely with respect to the appended claims and 
equivalents.