Rapid-start high-pressure discharge lamp, and method of its operation

To provide for pre-heating of low-power small metal halide high-pressure charge lamps (1), at least one of the electrodes (16) is formed as a heater wire by introducing a thin tungsten wire, in V shape, into one end of the elongated bulb (2), and carrying out each one of the legs of the V, separately, and electrically insulated within an end press seal (4) by parallel foils (10, 11) externally of the bulb. Continuous heater current is caused to flow through the V-shaped wire electrode, thus heating the discharge vessel, to complete vaporization of the fill. To start the lamp, the two legs of the V, of the heater electrode, are connected in parallel and across the lamp operating voltage and then a high-voltage pulse is applied between the V-shaped heater electrodes, for example between one or both of the external conductors and the other electrode (9). Light output, upon starting, is substantially accelerated from a lamp of this type with respect to non-preheated lamps.

Reference to related patent, the disclosure of which is hereby incorporated 
by reference: 
U.S. Pat. No. 3,519,872, Ward. 
The present invention relates to high-pressure discharge lamps, and more 
particularly to high-pressure discharge lamps which include a metal 
halide, and which require a high voltage pulse to initiate a discharge 
between two electrodes. 
BACKGROUND 
Metal halide high-pressure discharge lamps require starting pulses to 
initiate the discharge across spaced electrodes. The efficiency of energy 
conversion to light obtainable from such lamps is high, with good color 
temperature. These lamps do not, however, provide light immediately upon 
energization but, rather, require some starting time. 
A metal halide high-pressure discharge lamp has been describefd in U.S. 
Pat. No. 3,519,872, Ward, in which a fine heating wire made of coil-coiled 
tungsten is serially connected with the main electrode. The respective 
free ends of the electrode and of the heating coil are connected to a 
current supply, each, which is, electrically insulated from each other, 
conducted through a pinch or press seal or melt seal of the discharge 
vessel. The arrangement improves the ignition characteristics of the lamp 
since heat which can be obtained from the heating coil heats the discharge 
electrode. Fill condensates which possibly precipitated on the electrode 
are thereby vaporized. The fine heating wire, typically a fine tungsten 
wire, cannot, in high-power lamps, heat the entire discharge vessel. Thus, 
other than facilitating ignition, due to more rapid stabilization of the 
discharge, no further advantages can be obtained thereby. 
Metal halide high-pressure lamps are used more and more for general purpose 
illumination and, also, have been proposed as headlamps for vehicular use. 
The power rating of such lamps is low--100W and less, and, especially, 50W 
and less. The color temperature of such lamps must be acceptable and 
pleasing to users; additionally, the light output to be obtained from the 
lamps should rapidly occur upon energization of the lamps. 
The discharge vessel of low-power lamps of this type is very small. A 
typical volume is in the order of about 0.03 cm.sup.3. It is not possible 
to introduce heater coils or heater windings in such small discharge 
vessels. Lamps with high power, and having substantial discharge vessels, 
may have heater wires placed therein. 
THE INVENTION 
It is an object to provide a metal halide high-pressure discharge lamp, and 
especially a metal halide high-pressure discharge lamp of small power, 
which provides a high percentage of rated light output rapidly, after 
energization of the lamp. 
Briefly, the lamp is a double-ended lamp of small volume. At least one of 
the electrodes is shaped and dimensioned to have a narrow V form, with the 
apex of the V facing the other electrode of the lamp, and each leg of the 
V being separately carried through the pinch or press seal of the 
respective end of the lamp, insulated from each other. A molybdenum foil 
connection is preferably used. 
The second electrode of the lamp may be conventional or, in accordance with 
a feature of the invention, may likewise be a V electrode with the two 
legs thereof carried out of the respective other pinch or press seal, 
separately. The lamp is operated by first pre-heating the filament which, 
preferably, is an uncoiled single tungsten wire which is pre-heated by 
applying a current at low voltage thereto. If it is desired to start the 
lamp, a high voltage pulse is applied between the spaced, opposite 
electrodes and, as soon as the lamp has fired, the current supply leads to 
the two legs of the V are disconnected from the current supply and, 
rather, connected in parallel and to the ordinary current supply for the 
lamp, for example through a ballast. 
The arrangement has the advantage that, even with discharge vessels or 
bulbs which have a volume as small as about 0.03 cm.sup.3, and incapable 
of retaining a separate heating filament, the tungsten wire can be used to 
provide the necessary pre-heating. The wire, preferably, is a single 
tungsten wire, bent in narrow V configuration and, for example, directly 
welded to molybdenum foils which are pinch-sealed in the respective end or 
ends of the bulb or discharge vessel. The usual pinch or pinch seal is big 
enough to accept two current supply leads, insulated from each other next 
to each other. The voltage beteeen these two separate current supply leads 
is low, and thus wide separation is not needed. During pre-heating, only 
heating current flows through the current supply leads from external leads 
through the molybdenum foils and to the V tungsten wire. After firing or 
ignition of the lamp, both current supply leads, upon disconnection of the 
heater supply current, are connected in parallel to carry the operating 
current in parallel. 
The pre-heated metal halide high-pressure discharge lamps provide light 
output substantially faster than prior art lamps. For example, 30% of 
light output is obtained 5 times as fast; 50% light output 4 times as 
fast; 90% of the light output is obtained 21/2 times as fast as prior art 
lamps, or as similar lamps which are cold-started, that is, without 
pre-heating. 
In accordance with a feature of the invention, both electrodes are 
constructed as V-shaped heater electrodes; this permits even faster 
starting and obtaining of light from the lamp; or to permit use of a bulb 
or discharge vessel having a greater volume, and thus permitting higher 
light output from the lamps.

DETAILED DESCRIPTION 
Referring first to FIG. 1: The metal halide high-pressure discharge lamp 1 
has an elongated bulb or discharge vessel 2 made of quartz. Opposite ends 
of the quartz are formed with pinch or press seals 3, 4. A sealing foil 5 
of molybdenum is located in pinch seal 3, connected to an external current 
supply lead 6. The current supply lead 6 is connected through the 
molybdenum foil 5 with an internal current supply lead 7, of tungsten, 
which terminates in a ball-shaped electrode 9. Electrode 9 is located 
within the discharge or arc space 8 of the bulb 2 of lamp 1. 
The other end of bulb 2 is formed with a pinch seal 4 which retains two 
narrower sealing foils 10, 11, also of molybdenum. Foils 10, 11 are 
electrically insulated from each other, and, as best seen in FIG. 1, 
spaced from each other, and embedded in parallel relationship in the pinch 
seal. External current supply leads 12, 13 are connected to the respective 
foils 10, 11. 
In accordance with a feature of the invention, a heater electrode 16, 
having within the discharge space 8 a V shape with internal current supply 
leads 14, 15 forming part of the legs of the V, is connected, for example 
by welding, to the foils 10, 11. Thus, the internal current supply leads 
14, 15 are welded to foils 10, 11, as schematically shown in FIG. 1. 
The discharge space 8 has a volume of about 0.03 cm.sup.3. The V-shaped 
portion of the heater electrode 16 extends into the discharge space 8 by 
about 2.5 mm. The heater electrode 16, itself, is formed of an uncoiled 
straight--except for the V bend--wire of tungsten of about 0.15 mm 
diameter. The distance or spacing between the electrode 9 and the apex of 
the electrode 16 is about 4.5 mm. A fill of about 1 mg mercury, as well as 
sodium halide, tallium halide and tin halide of, overall, about 0.3 mg, 
and argon, is filled into the arc tube space 8. The argon forms the 
starting gas; the operating pressure of the lamp is about 40 bar. In 
operation, the color temperature of the lamp with the fill as given is 
about 3600 K. 
Operation: The heater electrode 16 is pre-heated by applying to current 
supply leads 12, 13 a voltage of about 2.8V, resulting in a current of 
about 5.5A. This current is supplied to the lamp before it is started, and 
may flow for some time before the lamp is intended to be lit. The 
discharge vessel 2, thus, is continuously held at a temperature in which 
the fill components are at least partly vaporized. To obtain light from 
the lamp, the lamp 1 is ignited or started by applying a pulse of about 16 
kV between the electrodes 9 and 16. The pulse is applied to the electrode 
9 by connecting it to the external lead 6 and to one or, via a transformer 
T (FIG. 4) to both of the external leads 12', 13'. As soon as the lamp is 
started, the heater circuit for the V electrode 16 is interrupted. An 
operating current of about 0.35A is then conducted, in parallel, to the 
external leads 12, 13 and hence to the electrode 16. At an arc voltage of 
about 100V, electric power consumption of the lamp is about 35 W. 
FIGS. 2 and 3 illustrate, graphically, the comparison of starting speed and 
light output of a lamp operated conventionally, that is, without 
pre-heating and without applying heater current to the external leads 12, 
13 (FIG. 2) and, in FIG. 3, to operating the lamp by pre-heating, as 
described. 
FIGS. 2 and 3 illustrate the difference, and clearly show that with the 
pre-heated lamp (FIG. 3), after about 2.3 seconds, about 30% of light 
output flux is obtainable. This corresponds to about 1000 lumens, which is 
approximately the light output of a conventional H4 halogen incandescent 
lamp. After only about 3.7 seconds, 50% of the rated light output of the 
high-pressure discharge lamp is obtained; 90% of rated light output is 
obtained after about 11.9 seconds, at which the final light output of 
about 2650 lumens, corresponding to 100% of rated light, is reached. In 
contrast, and for comparison, a lamp which is an all respects similar 
except that it does not have the pre-heated electrode 16--that is, for 
example two identical electrodes 9 or an electrode 16 to which no heater 
current has previously been applied--requires 11.8 seconds to reach 30% of 
light output, 14.6 seconds to reach 50% light output, and 28.7 seconds, 
that is, almost half a minute, to obtain 90% of light output. The color 
temperature of the lamp with the fill, as noted, is about 3600K. 
FIG. 4 illustrates a double-heated lamp. The lower half of the lamp of FIG. 
4 is identical to that shown in FIG. 1; the upper half is the mirror image 
of the lower half, and reference numerals used in FIG. 1 have been used 
identically, or, respectively, with prime notations, where appropriate, to 
show the components forming the mirror image of the lower part of FIG. 1. 
FIG. 4 also illustrates the circuit. A pulse transformer T having two 
parallel wound secondary windings L1 and L2 with low ohmic resistance is 
connected to the respective external current supply leads 12', 13'; its 
primary winding L3 permits application of a high voltage starting pulse to 
the electrode 16'. Heater current can be applied through heater current 
terminals 17, 17' and through switch S1, with its contacts a, b, c being 
in the heating position. Prior to the firing of the lamp 1, the switch S1 
is moved to its operating position, thereby removing the heater current 
energy from terminals 17, 17' and short-circuiting the external supply 
leads 12, 13 and 12', 13'. Operating voltage can then be connected between 
terminals 18, 18', Capacitors C1 and C2 are included to short-circuit high 
voltage or high frequency respectively for safety of the operating voltage 
device. Other circuit arrangements may be used, as appropriate, for 
example by connecting the starting pulse to both of the opposed external 
current supply leads 12, 13 and 12', 13' of the lamp 1. 
Various changes and modifications may be made, and features described in 
connection with one of the embodiments may be used with the other, within 
the scope of the inventive concept. 
Pre-heating of lamps is particularly effective for small metal halide 
high-pressure discharge lamps up to about 50W capacity, which means a 
vessel having an internal volume in the order of about 0.03 cm.sup.3. The 
invention is applicable, however, with lamps having different internal 
bulb volumes, and especially small bulbs with internal volumes to about 1 
cm.sup.3, or even above, and, especially, in any arrangement where it 
would be difficult or inconvenient to provide separate pre-heater wires 
and where the electrode structure for pre-heating can be readily formed 
and dimensioned to provide for pre-heating of the interior space of the 
bulb, with reasonable current consumption, to an extent sufficient to 
vaporize the fill content within the bulb.