Note: Descriptions are shown in the official language in which they were submitted.
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This invention is in the field of short-arc high
intensity gas discharge lamps, such as are used in certain
photographic projectors. It is common practice to manufacture
integral lamp and reflector combinations employing such a type
of lamp.
A typical short-arc high-intensity discharge lamp
comprises a quartz envelope having a thick-wall bulb-like arc
chamber, and a pair of elongated electrodes are sealed into
stems extending from the envelope at opposite ends and along
a common axis. The envelope includes elongated stems extend-
ing along and around portions of the electrodes. The arc
length between the inner ends of the electrodes is about 2 or
3 millimeters, and the overall bulb diameter is about 8 to
10 millimeters, and overall length (including stems) is about
5 centimeters, for a 300-watt metal halide gas lamp. U.S.
Patent No. 3,379,868 issued April 23, 1968 to Taillon dis-
closes a short-arc lamp mounted in a reflector laterally of
the projected light's optical axis, and U.S. Patent No.3,700,881
issued October 24, 1972 to Slomski discloses a short-arc lamp
mounted in a reflector along the optical axis. U.S. Patent
No. 3,780,342 issued December 18, 1973 to Grimshaw et al
discloses a ballast circuit for a short-arc lamp, which
applies a relatively high starting voltage pulse to the lamp
electrodes, followed by a relatively lower operating voltage.
Canadian patent application Serial No. 281,234 filed June
23, 1977 by Fridrich and Bergman, titled "Short-Arc Discharge
Lamp With Starting Device" and assigned to the present
assignee discloses a short-arc lamp having a starting
aid comprising a conductor connected electrically to one
of the electrodes and extending to the vicinity of the
stem containing the other electrode.
The principal object of the invention is to improve
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the starting characteristics of short-arc lamps so they will
start more reliably and/or at a lower value of starting volt-
age, and to accomplished this in an inexpensive manner.
The invention comprises, briefly and in a preferred
embodiment, a short-arc type of lamp having a bulb portion,
a pair of elongated electrodes extending into the bulb portion
and sealed through stems extending outwardly from the bulb
portion and along and surrounding portions of the electrodes,
and an external conductive starting device extending from the
vicinity of one sealing stem of the vicinity of the other
sealing stem. Preferably, the inner surfaces of the stems
are slightly spaced from the surfaces of the electrodes be-
tween the bulb and junction points at which the stems make
contact against the electrodes, and the starting device com-
prises conductive bands respectively surrounding the stems
at their junction points with the electrodes.
In drawings which illustrate embodiments of the invention:
FIG. 1 is a front view of a lamp and reflector combination
in accordance with a preferred embodiment of the invention.
FIG. 2 is a side view of FIG. 1.
FIG. 3 is a cross-sectional view taken on the line
3-3 of FIG. 1.
FIG. 4 is an axial view of the starting aid shown in
FIGS. 1 and 3.
FIG. 5 is a front view of a lamp and reflector com-
bination in accordance with another preferred embodiment
of the invention.
FIG. 6 is a cross-sectional view taken on the line
6-6 of FIG. 5.
FIG. 7 is an axial view of the starting aid shown
in FIG. 6.
Referring to FIGS. 1-4 of the drawings, there is illustrated
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a projection lamp unit 1 comprising a reflector portion 2,
preferably made of glass and molded to an ellipsoidal shape,
having a light source in the form of an arc tube 3 at its
focus fl. The ellipsoidal shape of the reflector con- --
centrates the light at the far focus which conveniently
may be located in front of the rim seating plane of the
light unit at a distance about equal to the diameter across
the rim. The reflector is provided with two flange segments 4,
5 at its rim by which the lamp is supported against a co-
operating seating shoulder 6 in a support member 7 as shown
in FIG. 2. By using a glass reflector, dimensional stability
is assured. Also, it may be coated on its inner surface
with a reflecting coating 8 consisting of a known type of
multiple layer interference film which is highly reflective
of visible light but transmissive of heat or infrared radi-
ation. A metal refelctor with a conventional mirror finish
may also be used.
The light source or arc tube 3 comprises a quartz
envelope having a generally spherical central portion or
bulb 10 provided with generally cylindrical outwardly ex-
tending extensions or stems 11, 12 having outer diameters
considerably less than that of the bulb 10. The electrodes
13, 14 consist of lengths of tungsten wire which are welded
to molybdenum foils 15, 16, the foils in turn being welded
to inleads 17, 18. The foils are wetted by the quartz of
the stems to provide hermetic seals. The illustrated lamp
is intended for direct-current operation and the anode 13
is of tungsten wire larger in diameter and longer than the
cathode 14. In a lamp for a-c operation, the two electrodes
would be of the same size and the stems would be equal in
length. The lamp operates with the electrodes close to the
melting point of tungsten and may operate with substantially
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molten tips resulting in the rounding and balling of the
electrode ends during operation. The lamp contains an
ionizable filling which includes an inert gas such as argon
and halogen or metal halide such as indium iodide. By way
of example, the overall length of the arc tube quartz body
may be 5 centimeters, and the outside diameter of the bulb
portion may be 9 millimeters with the internal diameter of
the discharge space being about 2.5 millimeters and the
arc length between the ineer ends of the electrodes being
about 2.5 millimeters. The inner surfaces of the stems 11
and 12 are spaced slightly from the outer surfaces of the
electrodes 13 and 14, as shown, for example about one milli-
meter or less, from the bulb 10 outwardly along the stems to
junction points lla and 12a after which the stems make cont-
act against the electrodes (but not hermetic seal contact because
the quartz stems cannot "wet" the tungsten electrodes).
Segments are cut away from the sides of the reflector
2 leaving flat vertical side walls 19, 20 which are chord-
like in end view as seen in FIG. 1 and tapered rearward
slightly towards the axis in plan section as seen in FIG. 3.
In the front half of the reflector, the chord-like side walls
19, 20 cut into the normal elliptical curve of the reflector
surface, but in the rear half, they are extended into shoulders
21, 22 defining generally wedge-like expansions within the
reflector. Within the shoulder expansions, the glass is
built up into collars 23, 24 about lateral apertures 25, 26
through the chord walls; the apertures are centered on a line
passing through focus fl and transverse to the optical axis.
Arc tube 3 is mounted laterally of the reflector's
optical axis with stem 11 projecting into aperture 25 and
set in a glassy cement 27 which fills the volume of the aperture
and of a ceramic cap 28 which is placed over the opening. An
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insulated wire lead 29 is welded to the end of inlead 17 and
emerges from cap 28 through a small side aperture at the
rear. The stem 12 at the cathode end of the arc tube does
not penetrate into aperture 26; a snug-fitting sleeve 31 is
provided around inlead 18 and the sleeve projects into lateral
aperture 26 and is there set in cement 27. An insulated wire
lead 32 is connected to the end of sleeve 31 and emerges from
cap 33 through a side aperture to the rear. Before cement
27 sets hard, arc tube 3 is adjusted to optically center the
arc at the near focus fl while the lamp unit as a whole is
accurately located relative to the projection system by the
flange segments 4, 5. Preferably, a cement is used which
sets quickly under heat and bonds to both the glass re-
flector and the quartz arc tube. One suitable cement com-
prises primarily fine alumina and calcined kaolin along
with minor additions of disodium phosphate and trialuminum
phosphate mixed with phosphoric acid to form a paste.
By setting one end of arc tube 3 in cement so that it
is rigidly fixed to reflector 2, a projection lamp unit
results wherein the arc tube is accurately located in an
optical reference system. When subsequently the unit is
inserted into a socket properly accommodating flange segments
4, 5, it will provide the desired light at the film gate with-
out further adjustment. At the other end of the arc tube,
the inlead is slidably engaged in sleeve 31. This permits
differential expansion of the quartz arc tube having a low
coefficient of expansion and of the glass reflector having a
relatively high coefficient of expansion, without subjecting
the parts to excessive strain. At the same time, the dis-
placement of the interelectrode gap relative to the reflectorfocus as a result of differential thermal expansion is too
slight to be of any consequence in the optics of the system.
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The projector lamp thus far described is similar to that dis-
closed in the above-referenced Taillon patent. As mentioned
above, the lamp 3 requires a ballast circuit which applied a
relatively high value of starting voltage (about 8,000 to
10,000 volts).
In accordance with the present invention, the start-
ing voltage of the lamp 3 is reduced and/or the lamps start
more reliably by providing a starting aid device 36 comprising
an elongated electrically conductive member extending from
the vicinity of one stem 11 to the vicinity of the other
stem 12. In the preferred embodiment shown, the starting
device 36 comprises a wire 37 welded or otherwise attached
at one end thereof to a metal strap 38 which fits tightly
around and encircles the stem 11. The wire 38 extends along
and behind the lamp 3, between the lamp and the rear surface
of the reflector, and is welded or otherwise attached to
a metal strap 39 which fits tightly around and encircles
the stem 12 of the anode electrode 13. This starting
device 36 is thus held in place and supported at both ends
thereof, and, being positioned behind the lamp 3, has no
appreciable effect on the light output of the lamp-
reflector combination. The starting device 36 is not
directly connected electrically, and thus is electrically
"floating." The wire 37 can be spaced away from the dis-
charge bulb.
Preferably, the starting aid straps 38 and 3~ are
positioned on the stems 11 and 12 respectively at and sur-
rounding the locations of the junction points lla and 12a
where the stems meet and make contact with the electrodes
13 and 14. Such a location of the starting aid straps is
found to improve the effectviness of the starting aid by
causing the lamps to start more reliably at lower starting
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voltages. To aid in optimum positioning of the starting
aid bands lla and 12a, they may be made sufficiently wide
so as to axially flank the junction points lla and 12a to
help insure that they will be at these junction points, as
shown in the drawings.
By using the starting device 36 as shown and described
above, the voltage for starting the lamp 3, which is known
commercially as the General Electric Company MARC 300 pro-
jector lamp, was reduced to a value of about 4,000 to
6,000 volts, whereas without the starting device 36, the
starting voltage was about 8,000 to 12,000 volts, for typical
groups of production lamps, the starting pulse being a fast-
rising d-c pulse having its positive polarity applied to the
anode and its negative polarity applied to the cathode, and
having a rise time of about one microsecond. Instead of
providing the straps 38 and 39, the ends of the wire 37 can
be bent around the stems 11 and 12.
In the embodiment of FIGS. 5, 6, and 7, the lamp 3'
is the same as or similar to the lamp 3 described above,
and is mounted along the optical axis of a concave reflector
2' having a circular configuration in all planes thereof
perpendicular to the optical axis. The anode stem 11' of
the lamp extends into a hollow collar 48 at the rear of
the reflector, where it is cemented to the reflector by
cement 49 which also holds a ceramic end cap 51 in place.
A connector wire 29' is attached to the inlead 17' of the
anode electrode, and a connector wire -32' is attached to
an end of a stiff wire 52, curved as shown and with its
other end connected to the cathode inlead 18'. The connector
3Q wires 29' and 32' are brought out of the end cap 51 through
an opening. The embodiment of FIGS. 5 and 6, as thus far
described, is similar to the projector lamp disclosed in the
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above-referenced Slomski patent.
In accordance with the present invention, the start-
ing aid 36' comprises a wire 37' connected at an end thereof
to a strap 38' which fits tightly around and encircles the
anode stem 11', the other end of wire 37' being connected to
a strap 39' which fits tightly around and encircles the
cathode stem 12'. Preferably, these straps are at the
junction points ll'a and 12'a where the stems come into
contact against the elctrodes, similarly as described above
for the embodiment of FIGS. 1-4. The wire 37' preferably
is aligned between the lamp 3' and connector wire 52 so as
not to cause a shadow in addition to that cuased by the
connector wire 52.
The starting aid wire 37 or 37' may be shaped in various
convenient configurations, such as straight as shown in
FIG. 3 or curved as shown in FIG. 6.
The invention achieves its objects of improving the
starting of short-arc lamps, and in an inexpensive manner.
The lamp starting aid of the present invention improves the
lamp starting as well as the starting aid disclosed in the
above-referenced patent application, and has the advantage
thereover of not being connected to a lamp electrode. Such
connection to an electrode is somewhat difficult to arrange
in certain types of lamps such as the embodiment of FIGS.
1-4 in which there is a very small length of exposed in-
lead wire 18 between the stem 12 and cement 27. Another
advantage of the invention over that of the above-reference
patent application is a reduction of electrical shock hazard,
and reduced possibility of malfunctioning of the lamp in case
of shorting of the starting aid against an object, the for-
going advantages being achieved by the starting aid not being
connected to carry the high voltage of a lamp electrode.
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While preferred embodiments and modifications of the
invention have been shown and described, various other
embodiments and modifications thereof will become apparent
to those skilled in the art and will fall within the scope
of the invention as defined in the following claims.
