Note: Descriptions are shown in the official language in which they were submitted.
8~
1 PMN 8981
The invention relates to an electrical
high-pressure metal vapour discharge lamp comprising
an elongate discharge tube provided near each of its
two ends with a respective internal main electrode,
the wall of the discharge tube consisting substantially
wholly of aluminium oxide, a strip-shaped coating being
present over at least a portion of the outer surface of
the wall of the discharge tube, this coating containing
a first electrically conductive elementary material as
well as a second material counteracting evaporation of
the first material, and in which at least part of the `
first material is in direct contact with the aluminium
oxide, the strip-shaped coating being fastened over sub-
stantially its whole length to the discharge tube and
forming in use, part of a starting auxiliary device for
starting a discharge between the main electrodes.
A prior art electrical high-pressure metal
vapour discharge lamp of the type defined above is, for
example, disclosed in British Patent 1,421,406 - Egyesult
20 Izzolampa - April 4, 1973. A drawback of that known
high-pressure metal vapour discharge lamp is that, for
promoting the starting of a discharge between the main
electrodes, the strip-shaped wall coating is connected
to an internal auxiliary electrode of the discharge tube.
This requires an additional electric feed-through through
the wall of the discharge tube.
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It is an object of the invention to
provide a lamp of the type defined in the preamlle in
which the strip-shaped wall coating, provided on the
outer side of the discharge tube wa1l, is the auxiliary
electrode oP the discharge tube, but that this auxiliary
electrode does not substantially intercept visible light
generated in the discharge tube.
An electrical high-pressure metal vapour
discha~ge lamp according to the invention comprising al7
elongate discharge tube provided near each of its two
ends with a respective internal main electrode, the wall
of the discharge tube consisting substa.ntially wholly
of aluminium oxide, a strip-shaped coating oeing present
.~ over at least a portion of the outer surface of the dis-
charge tube wa]l, this coating containing a first elec-
trically conductive elementary material as well as a
second material counteracting evaporation of the first
material of that coating and in which at least part of
the fi.rst material is in direct contact with the alun1iniulr.
oxide, th.e strip-shaped coating being fastened over
substantially its whole length to the discharge tube arld
forming in use, part of a starting auxiliary device for
starting a discharge between the main electrodes, i5
characterized in t.hat the strip-shaped coating extends
.5 along substantially the whole path between the rnain
electrodes, and in. that this strip-shaped coating has a
width smaller tha~ 0.5 mm over at least 9O~ of its length.
An advantage of this lamp is that the
strip-shaped coating is here an external auxiliary
electrode. The narrow width, over the greater portion
of the ].ength, of thi.s auxiliary electrode implies that
the auxiliary electrode intercepts the l,.gh~ generated
in rhe di.scharge tube to a verv sma:Ll extent on7y.
The strip-shaped coating ~nay have a width
exceeding O.5 mm for a short 3ength, for exanple wllere
there is a connect ng termiIlal f`or e3.ectrically connecting
thc strip shaped coating to thc circuit of the lamp
684
22 . 9 . 7~ - 3 PHN. 8981
The strip-shaped coating of an e]ectrical
high-pressure metal vapour discharge lamp accordillg to
the invention consists, for example~ of a solid metal
conductor, the second mat0rial being in the form of a
protective coating protecting the so]id conductor.
In an embodiment of an elecbrical high-
pressure metal vapour discharge lamp according to the
invention the first material of the strip-shaped coating
consists for 16 to 90% by volume of an element ~rom the
group molybdenum, tungsten, tantalum, niobium and carbon,
and the second material of the strip-shaped coating
consists of one or more oxides which, at a temperature
of 1500 Kelvin, have vapour pressures below 10 torr.
An advantage of this embodiment is that more
of the first material can be included in the strip-shaped
coating than would be possible in the case of a soiid
strip on the discharge tube. In addition, it is a good
electrically-conducting material. The following should
be noted by way of explanation. With the narrow width
of not more than 0.5 mm of the strip-shaped coating,
a sufficiently low ohmic resistance thereof per running
centimetre of length can - in the case of a solid metal -
only be realised with a fairly thick layer of that metal.
However, a very thick layer results in a poor adhesion
f that metal to the discharge tube wall which mainly
consists of aluminium oxide. This is caused by differences
in the coefficients of expansion. By also including in
the strip-shaped coating oxides which have a low vapour
pressure at 1500 Kelvin, a sufficiently low ohmic re-
3o sistance of the strip-shaped coati.rlg - which also remains
low hecausc the evaporation during life of the lamp is
relatively low - can be combined with a p~oper adhesion
to the discharge tube wall.
In an improvement of a lamp according to
3S the above-mentioned embodiment, the first matariai of
the strip-shaped coating is molybdenum or carbon, this
first material being ur~iformly distributed in the second
material of the strip-shaped coating. An advantage oi` this
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22.9.78 4 PHN.89~1
i.mprovement is that a .1.ow degree of evaporation c~n be
oomb ned with a high electrical conductivity of the
strip-shaped coa.tiIlg.
In a further improvement OI' that lamp
the strip-shaped coating consists, by volume~ of
approximately ~0% Molybdenum ~ 1 Oc/o Al203 ~ 10% CaO.
With such a strip-.shaped coating it is possible to obtain,
next to the above-mentioned advantages of a low evaporation
and a high electrical conductivity, a very good adhesion
to the di.scharge tube wall.
In a further embodiment of a h.igh-pressure
metal vapour discharge lamp accordingto the invention,
the first materiai of the strip-shaped coating is molyb-
~ denum and the second material is tullgsten~ the tungsten
being applied as a top coating o~rer the molybdenum.
An advantage of this embodiment is that the strip-shaped
coating can be applied in a si.mple manner to the dis-
charge tube. This is done by, for example, vacuum
deposition or by means of a sputteri.ng techrique, for
example b~r magnetron sputtering.
The di.scharge lampis, for exaMple, a
high--pressure sodium vapour discharge lamp the discharge
tube of which also contains a starting gas,-for e~ample
xenon, at a filling pressure of 1ess than 20 torr;
In an embodiment of an electrical high-
pressure metal vapour discharge lamv according to thc
invention~ whi.ch is implemented as a high-pressure sodium
vapour discharge lamp, the discharge tube contains sodium
~e na~
~ and xenon, the filling pressure Or the sodium cxceeding
3G 50 torr, the c.ircumference of a transverse cross-section
through the disch3.rge tube being between 10 and 40 mm.
An advantage of thi.s embodi.ment is that it may furnish
a lamp which combines a high lumillous effi.cacy with a
relatively low starting voltage and wherein the inter-
3S ceptioll of ].ight by +he strip--shaped coating is ve.ry
small only.
A l~igh-pressure dlscharge lamp according
to the in~rerlti.on may comprise a discharge tube,
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22.9.78 5 PHN.898l
the strip-shaped coating of which is noi electricall.y
connected to the lamp circuit. In that case the strip-
shaped coating has a floating potential in use, preferably,
however, in a high-pressure metal vapour discharge lamp
according to the invention, the strip-shaped coating
is clectrically connected to one of the main electrodes
of the discharge tube. A.n advantage of this is that
during the starting procedure of the lamp the difference
in potential between the strip-shaped coating - which
then operates as an auxiliary electrode - and the main
electrode of the discharge tube not connected thereto
can be substantially equal to the nmains voltage. As a rule
this promotes the creation of ions and electrons in the
.~............ discharge tube which facilitates the creation of a dis-
charge between the main electrodes of the discharge tube.
.ll this implies that the mini.mum voltage between the
maln electrodes for starting a discharge between these
electrodss i5 then relatively low.
Some erllb~di.ments of lamps according to
the invent:io~ wi.ll now be further explai.ned with refer-ence
to a drawing i.n which:
Figure 1 shows an eievational view,
partly in Gross-secti.on, of a high-pressure sodium
va.pour discharge lamp according to the invention;
2; ~igure 2 shows a cross-section not to
scale - througll a variant of a discharge tube for the
lamp of Figure 1.
In Figure 1 reference numeral 1 denotes
a dischargc tube whose wall consi.sts mainly o~ densely
sintered alumilllum oxide. This tube is located in an
outcr bulb 2. ~.efsrence numeral 3 denotes a base of
the lanlp. Tlle di.scharge tubc 1 comprises two internal
n1ain electxodes 4 and 5, respectively, located near
the ends of this discharge -tubs. V~a a feed-throu~h 6
the main electrode 4 is connected. to a metal conductor 7
cornected to a pole wire 8 which is cu.rved arouMd the
discharge tube 1. Thi.s form of the pole wire 8 has the
ad~ran.tage that annoyi.ng shadows are avoided.
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6~4
6 PHN 8981
Pole wire 8 is electrically connected to a contact of the
base 3 of the lamp. An extended portion 9 of the pole
wire 8 serves, together with a support 10, for supporting
and centring the discharge tube 1 in the outer bulb 2.
Vla a tubular feed-through 11 the main electrode 5 is
mechanically fastened to a metal conductor 12. The elec-
trical connection is effected via a metal conductor 13.
The other end of the conductor 12 is connected to a fur-
ther contact in the base 3 of the lamp.
In addition, the discharge tube is provided
with a strip-shaped external auxiliary electrode 20 in
the form of a local coating on the outer surface of the
discharge tube 1. The auxiliary electrode 20 extends
over substantially the whole distance between the main
electrodes 4 and 5. The auxiliary electrode is approx-
imately 0.2 mm wide, its composition by volume is: a mix-
ture of 80% Molybdenum, 10% A12O3 and 10% CaO. The two
mentioned oxides A12O3 and CaO have, at a temperature of
20 1500 Kelvin, a vapour pressure below 10 6 Torr. Near the
main electrode 5 the discharge tube 1 comprises a conduct-
ing nickel sleeve 21 which also extends over the auxiliary
electrode 20 and is in electrical contact therewith. A
strip 22, also of nickel, connects the sleeve 21 to an
electrode of a capacitor 23, disposed in the space between
the discharge tube 1 and the outer bulb 2. The other elec-
trode of capacitor 23 is constituted by a portion of the
metal conductor 13 which provides the electrical connection
between the main electrode 5 and the conductor 12.
The discharge tube 1 contains both sodium
and mercury as well as xenon. The xenon pressure at 300
Kelvin is approximately 200 torr. The space between the
discharge tube 1 and the outer bulb 2 is evacuated.
The described lamp is, for example ignited
by means of a starter (not shown) provided with a thyris-
35 tor, for example as disclosed in Canadian Patent 896,070 -
Moerkene - March 21, 1972 (PHN 3905).
In the operating condition of the lamp
shown in Figure 1 the lamp is connected through an
22.9-7~ 7 PHN.8981
inductive stabilisation impedance of approximately
0.3 Henry to an a.c. mains supply of approximately
220 Volts, 50 Hertz. Further details of the described
lamp are included in the following table. The temperature
of the coldest spot in ~e discharge tube 1 is - in the
operating condition of the lamp according to the invention-
approximately 1000 Kelvin. A sodium vapour pressure in
the discharge tube 1 of approximate]y 130 torr corresponds
therewith. The average temperature within the discharge
tube 1 in the operating condition of the lamp is approxi-
mately 2400 Kelvin. The average temperature of the wall
of the discharge tube is approximately 1500 Kelvin.
At this temperature of 1500 Kelvin the vapour pressure
'- of the cxides of the strip-shaped auxiliary electrode
is - as already mentioned above - small. So there is
only a low degree ~f evaporation of said oxides.
Lamp according to
the invention
. _~ . _.
Power (in watts) 150
~ _
Operating voltage
(in volts) 100
_ _ .
Inside diameter of
the discharge tube
(in mm) 4.5
_ _ . .
Circumference of cross-
section (in mm) of the
discharge tube approx. 20
_ .
Main electrode spacing
(in mm) 63
Weight of the amalgam
(in mgram) 10
. _ .
l~eight ratio mercur~r/
~odium 2.7
_ _ __ ~
Xellon pressure cold ~
(in t;orr) 1 200
Xenon pressure during
operation (in torr) ¦ 1600
Luminous e~ficacy
(lumens/watt 11$
.__ ~ _ ~
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22.9.7~ 8 P~lN.~981
The voltage required between the main
electrodes 4 and 5 for starting the discharge is in
the present case approxiMately 2 k~olts.
Without the strip 20 the voltage requirecl
between the main electrodes 4 and 5 - for igniting the
discharge - would be much greater, namely: approximately
6 kVolts.
The strip-shaped coating 20 of Figure 1
is approximately 35 micron3 thick. Its ohmic resistance
per running centimetre is approximately 0.1 kOllm.
The strip-shaped coating 20 is applied
to the wall of the discharge tube by means of a pen.
For this purpose this pen is first dipped into a sus
-~ pension of 80% molybdenum powder with 10% aluminium
oxide and 10% calcium oxide in butyl acetate. After the
coating has been applied a firing operation is performed
at 1~00 l~elvin for 30 minutes in a reducing atmosphere.
This results in a proper adhesion to the wall of the
discharge tu~e 1 over the full length of the strip-
shaped coating 20.
In a case where the first material ofthe strip-shaped coating is carbon, that strip-shaped
coating can, for exarnple, also be applied to the dis-
charge twbe by means of a pen.
~5 Also other processes of reali~ing the
strip~shaped coating on the wall of the discharge tube
are conceivable.
Figure 2 shows a perpendicular CI'OSS-
section 100 of a second c~ischarge tube~ enlarged three
ti~les relative to that of the discharge tube 1 o~
Figure 1. Al50 tube 100 consists mainly of aluminium
oxide. A ~trip-shaped molybdenum coating 101 is provided
on tube 100. A tungsten top coating 102 is app:Lied ove-
~coating 101. The full width of the strip is approxi-
~5 Mately 0.2 ,nm. To indicate tl1e various ]ayers~ the stripis not dra-.~n to scale in ~igure 2.
A31 advantagc of` the described strip-
shaped coatings in a la~np a-cording to the iI1ventio
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22.9,78 9 PHN.8981
-as opposed to starting wires - is that the strip-shaped
coatings are always very close to the main electrode
path without requiring an addltiorlal measure such as
subjecting it to a tenqile load - as is the case with
starting wires.
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