High-pressure discharge lamp with torsionally wound electrode structure

The high pressure discharge lamp has refractory electrodes each having a top and a winding of refractory metal wire, which has turns and is, in an area thereof remote from the top, secured to the electrode. The turns of the winding are made to have torsion. Thereby an accurately defined position with respect to the top of the electrode is obtained. The direction of the torsion is opposite to the direction of the turns, which causes neighbouring turns to press one against the other.

BACKGROUND OF THE INVENTION 
The invention relates to a high-pressure discharge lamp provided with: 
a lamp vessel which is closed in a vacuumtight manner and which contains an 
ionizable filling; 
a pair of heat-resistant electrodes each with a tip in the lamp vessel and 
each connected to a respective current conductor which issues from the 
lamp vessel to the exterior, 
a winding of turns of heat-resistant metal wire being present around a 
free-end portion of each electrode, which winding is fixed in a location 
remote from the electrode tip, said turns having a winding direction. 
Such a high-pressure discharge lamp is known from U.S. Pat. No. 4,952,841. 
The electrode winding may have the object of realizing a desired 
temperature gradient during operation, or also of accommodating an emitter 
for reducing the work function of the electrodes. 
A winding of a certain type is desired for a lamp of a certain type, i.e. a 
winding of one or several layers of turns, with a predetermined number of 
turns per layer, with wire of a chosen thickness, and with a predetermined 
distance to the electrode tip. Such a winding is known in detail from U.S. 
Pat. No. 4,929,863. 
The winding may be made beforehand on a winding mandrel which has a 
thickness greater than the electrode, whereupon it is passed over the 
electrode and fixed thereon. One or several turns may be flattened for 
this purpose so that the winding clamps itself around the electrode, or a 
welded joint between electrode and winding may be made, for example a 
resistance weld or laser weld. 
Alternatively, however, the winding may be made directly on the electrode. 
It is usually necessary also in this case, however, to secure the winding 
because winding stresses result in the turns becoming wider. 
Electrodes of the type mentioned are suitable for use in high-pressure 
discharge lamps of various types and power ratings. 
It is known from U.S. Pat. No. 5,001,397 and U.S. Pat. No. 4,783,611 to 
provide a knot in the winding which prevents winding stresses from making 
the turns wider such that the winding can shift. An additional fixation 
through, for example, a weld is not necessary in the case of a knotted 
winding. The winding is not universally applicable, however, because a 
knot can only be made when the winding has at least two layers of turns. 
The making of the winding in addition requires a complicated equipment. 
The application copending U.S. application Ser. No. 08/431,881 filled May 
1, 1995 not previously published describes an electrode which has 
deformations causing the electrode to have unround cross-sections where 
wire has been coiled around it. Owing to these unround cross-sections, the 
winding hooks itself around the electrode and the turns are incapable of 
widening. It was found to be difficult, however, to manufacture the 
electrodes in an automated process for lamps of comparatively high power 
ratings, above-approximately 150 W. 
It was found to be a disadvantage that the distance of the electrode 
winding of the lamp described in the opening paragraph to the tip of the 
electrode is badly reproducible. The distance of the winding to the 
electrode tip is found to vary strongly after the lamp has been operating 
without the fastening of the winding to the electrode having been broken. 
This holds, too, in the case of lamps manufactured with windings from one 
batch which were accurately provided at a predetermined distance from the 
tips of the electrodes, for example by means of a jig, and which were 
fixed in a location remote from the tips. Some windings had become 
comparatively much, others comparatively little longer. The result is that 
some electrodes project with their tips comparatively little beyond the 
windings, others comparatively far. 
Changes in the distance between the electrode tip and the winding may be 
ascribed to stresses in the winding caused by the winding process. Changes 
as such can be taken into account for each type of lamp during mounting of 
the winding. Fluctuations in these changes, which are found to be in fact 
unpredictable, cannot. Nevertheless, the distance from the winding to the 
electrode tip has a major influence on lamp properties, i.e. whether this 
distance is positive: freely projecting tip; negative: tip surrounded by 
winding; or nil. The distance influences inter alia (i) the stability of 
the discharge, for example whether the discharge arc leaps from one point 
to the other, which determines whether the lamp flickers, (ii) the 
temperature of the coldest spot in the lamp vessel, which may change the 
color of the generated light or the voltage drop across the lamp, and thus 
the light output, and (iii) lumen maintenance during lamp life. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a high-pressure discharge lamp 
of the kind described in the opening paragraph in which the winding 
occupies an accurate, predetermined position on the electrode also after 
the lamp has been operating. 
According to the invention, this object is achieved in that the turns are 
made with torsion in the metal wire, which torsion has a direction opposed 
to the winding direction of the turns, whereby adjoining turns press 
against one another. 
Owing to the torsion in the turns, the wire has a tendency to deform such 
that the torsional stress in the wire becomes less. Given a torsion 
direction opposed to the winding direction of the wire, this manifests 
itself in that the distances between adjoining turns are minimized and the 
turns are pressed together. The winding has the tendency to become 
shorter, and thus thicker. This has the result that a well-defined 
distance to the electrode tip is retained when the winding relieves itself 
at operational temperature. Not only do the turns press against one 
another laterally, but the fact that the metal wire was wound with 
rotation, which introduced torsion into the turns, also has the result 
that grooves in the wire surface arising from the manufacture of the wire 
in a drawing die do not extend in axial direction along the wire but 
revolve around the wire at an angle to the axis. 
The expression "direction of the torsion" is here understood to mean: the 
direction in which the free end of the wire viewed by an observer looking 
along the wire towards the winding mandrel has been revolved about its 
axis during winding: clockwise or counterclockwise. In that case, drawing 
lines on the wire will revolve in counterclockwise and clockwise 
direction, respectively, around the wire, as seen by this observer. The 
expression "winding direction of turns" is understood to mean: the 
direction in which the wire in the turns moves away from the observer in a 
layer of turns around the electrode. 
It is noted that a high-pressure discharge lamp is known from U.S. Pat. No. 
4,847,534 in which the electrodes have windings of which some turns were 
made with torsion in the wire. The torsion direction, however, is the same 
as the winding direction here. According to this document, this has the 
result that the winding indeed has a tendency to become longer, and thus 
thinner, in order to reduce torsional stresses. Such windings manufactured 
on a winding mandrel can be removed therefrom with difficulty only in 
order to pass them over a thinner electrode. Such windings when made 
directly on an electrode are found to result in an uncertainty as to the 
degree to which the winding will become longer, partly also in dependence 
on unintentional variations in the degree of torsion. The distance from 
the winding to the electrode tip is not accurately defined as a result, 
and the torsion is counterproductive to the object of the present 
invention. 
In the lamp according to the invention, by contrast, the position of the 
winding can be determined with high accuracy, for example through assembly 
in a jig, while in addition the degree of torsion is of little importance 
in a given electrode because the turns will indeed press more strongly 
against one another in the case of a greater torsion, but will not lie 
closer together. The minimum torsion to be provided for obtaining 
reproducible electrodes may be readily ascertained for each type of 
electrode in a small test series. 
Is obvious that it is immaterial to the invention in which type of 
high-pressure discharge lamp the electrodes are to be used. Thus, for 
example, the lamp vessel may be made, for example, of quartz glass or 
ceramic material, the ionizable filling may comprise besides rare gas also 
metal halides and/or mercury. The electrodes may be made, for example, 
from tungsten, whether or not doped with an emitter such as, for example, 
Y.sub.2 O.sub.3, HfO.sub.2, while the tip may be formed from or with the 
aid of a different material. The winding may in particular be made from 
tungsten wire. It is an advantage of the winding that it surrounds the 
electrode with clearance, so that it can enclose comparatively much 
emitter material together with the electrode when used in lamps which 
require such material. The winding is universal in the sense that it may 
comprise one, two or more layers of turns and that it may leave exposed or 
surround the electrode tip.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The high-pressure discharge lamp of FIG. 1 has a lamp vessel 1 which is 
closed in a vacuumtight manner, is made of ceramic material in the Figure, 
and is filled with an ionizable gas, for example with rare gas, mercury, 
and sodium. A pair of heat-resistant, or refractory, electrodes 2, 
substantially made of tungsten in the Figure, with respective tips 3 are 
arranged in the lamp vessel and connected to current conductors 4 which 
issue from the lamp vessel to the exterior. A winding 5 with turns 6 of 
heat-resistant, or refractory, metal wire, tungsten wire in the Figure, is 
present around a free-end portion of each electrode 2. The winding is 
secured with a resistance weld 9 in a location of the electrode remote 
from the tip. The turns have a winding direction. In FIG. 1, the lamp 
vessel 1 is accommodated in a closed outer envelope 10 which supports a 
lamp cap 11. 
The turns 6 are manufactured with torsion in the metal wire, see FIG. 2, 
which torsion has a direction opposed to the winding direction of the 
turns 6. As a result, adjoining turns 6 press against one another. 
The winding 5 has a first layer 7 of turns 6 which merges near the tip 3 
into a second layer 8 of turns 6. Emitter 14, for example barium 
tungstate, is accommodated in the winding 5. 
The turns 6 of the second layer 8 were made in counterclockwise direction 
as seen by an observer A. As seen by an observer B, the wire from which 
the turns were made has a torsion in clockwise direction, i.e. opposed to 
the winding direction. Drawing lines 6 as a result enclose an angle with 
the axis of the wire and revolve themselves around the wire in 
counterclockwise direction. The electrode was provided with emitter 
material in that it was immersed in a suspension so as to fill up the 
space between electrode and winding as well as spaces between turns, and 
its surface was cleaned by brushing after drying. 
A winding of tungsten wire of 0.6 mm thickness was provided around an 
electrode of 1.2 mm diameter. The coiling wire was given a torsion of 
0.040 Nm against the winding direction during winding. The winding was 
fixed in a jig at a predetermined distance from the tip. The windings were 
found to have the same distance to the tip after 100 burning hours in 
lamps provided with electrodes of this kind. The electrode can be 
manufactured in an automated process and may be used for a lamp consuming 
a power of approximately 400 W.