Note: Descriptions are shown in the official language in which they were submitted.
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P~ 10.934 l 04-03-1985
The invention relates to a ]ow-pressure mercury vapour dischar-
ge lam? con~rising a discharge vessel which is sealed in a vacuum-tight
n~nner and has at least two parallel tuke parts arranged beside each
other and connected to each other at or near their one ends, there being
present in the discharge vessel duriny operation of the lamp a discharge
path between electrodes provided at the other ends of the tube parts,
discharge path which is curved at at least one area, the discharge ves-
sel being shaped near the connection between the tube parts in a rnanner
such that during operation of the lamp in situ a part of the inner wall
of the discharge vessel has a comparatively low temperature.
Such a lamp is kncwn from British Patent Specification 2050046.
The known lamp is comparatively compact and serves as an alter-
native to incandescent lam?s for general illumination ?urposes.
In the case of an increase of the electrical energy supplied to
such a lamp or if the lamp is operated in a space in which the tempera-
ture is higher than the normal room te~mperature (for example in a closed
luminaire of small dimensions~, the light output nevertheless remains
comparatively high. The mercury vapour pressure in the discharge vessel
is in fact stabilized at an optim~ ~ roximately 10 2 torr) due to the
20 presence of the cool area in the discharge vessel near the end of a tube
part.
However, it has been found that condensation of mercury is lia-
ble to occur at the cool area in the discharge vessel under nornnal ope-
rating conditions. This is especially disadvantageolls ifthe lamp is ar-
25 ranged in a vertical operating position, in which the electrodes are si-
tuated at the lower end of the lamp. They are then located substantially
perpendicularly below the com?aratively cool wall portior~ There is then
a risk that the mercury drips d~nwards along the wall of the discharge
vessel, as a result of which a luminescent layer present on the inner
30 wall of the discharge vessel is at-tacked. rrhis is no-t only detrimental
to the light output, but rr~oreover the ap?earance of the lamp is spoilt.
F~1rthermore there is also a risk that the liquid mercury falls dcwnwards
in the form of drops and impinges on the filament spiral of an electrode.
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P~ 10.994 2 04-03~1985
This adversely affects the operation of the lamp. Moreover, with the oc~
curence of this phenomenon, fluctuations in the light output are obt~ined
and the light intensity distribution over the discharge vessel is inho-
mogeneous
The invention has for its object to provide a lamp having a high
light output, in which, however, the aforementioned disadvantageous phe-
nomena are avoided.
According to the invention, a low-pressure mercury vapour dis-
charge lamp of the kind mentioned in the opening paragraph is for this
purpose characterized in that the discharge vessel contains an amalgam
for controlling the rnercury vapour pressure in the discharge vessel du-
ring operation of the lamp.
In the lamp according to the invention, the aforementioned unde-
sirable phenomena do not occur. The mercury vapour pressure is controlled
in the discharge vessel under normal operating conditions by the amalgam,
as a result of which no condensation of mercury takes place at the cool
area in the discharge vessel. The mercury vapour pressure during operation
of the lamp is rnoreover stabilized at a value of approxirnatRly 10 torr
over a wide temperature range~ The light output of the lamp then remains
as high as possible even if the lamp is operated in a comparatively warm
area, such as in a luminaire.
The amalgam is preferably situated at an area behind one of the
- electrodes. A suitable area is, for example, the hase of the so-called
mount. Such a mount is provided not only with an electrode, but, for
example, also with wires for this electrode as well as with an exhaust
tuhR located behind the electrode which is used for pumping the discharge
vessel
In a preferred er~odiment of the lamp according to the invention,
the arnalgam is present in the exhaus-t tU~R. It has bRen found that such
an area is favourable for a satisfactory vapour pressure stahilization.
The amalgam present in the discharge vessel of the larnp according
to the invention consists, for example, of an alloy of indium and mercu-
ry or of an alloy of indium, bismuth and mercury (see, for exarnple,
British Patent SpRcification 1503636). Favourable results were obtained
with an amalgam consis-ting of bismuth, lead, tin and mercury ~see, for
example, British Patent Specification 1572657).
The discharge vessel of the lamp according to the invention may
have differen-t shapes. The discharge vessel may comprise, for example,
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PHN 10.994 3 04-03-1985
two tuke parts which are connected to each o-ther by a transverse connec-
tion, as descril~ed in the aforementioned British Paten-t Specification
2050046. However, it may alternatively comprise four such interconnected
tube parts. Favourable results were also obtained with a lamp having a
5 discharge vessel of the kind described in German Offenlegungsschrift
3112878. In this k~mp, a cool wall portion is ob-tained by shaping a dis-
charge tuke which is in the form of a U at the area of the curved part
in a manner such that a wall portion is obtained whose cross-section ex-
tends substan-tially at right angles to the direction of the axis.
An em~cdiment of the invention will now be described more fully
with reference to -the accompanying drawing. In the drawing, a l~pressure
mercury vapour discharge lamp according to the invention is shown, partly
in elevation and par-tly in sectional view.
The lamp comprises a discharge vessel which is sealed in a va-
15 cuum-tight manner (filled with m~ercury and a rare gas, such as argon) com-
prising two parallel tuke parts 1 and 2, which are arranged keside each
other and are connected to each other through a transverse connection 3.
The ilmer wall of the tube parts and the transverse connection is coated
with a lumincescent layer. The transverse connection is provided, for
20 example, by means of a method as described in GB-PS 2048562. This trans-
verse connection is located at a certain distance from the one ends 4 and
5 of the tube parts 1 and 2, respectively. These ends are in the form of
end faces arranged at right angles to the longitudinal axes of the tuke
parts. The electrodes 6 and 7 are arranged beside each other, each at the
25 other end of a respective tube part. ~uring operation of -the lamp, a U-
shaped discharge path is present in the tube parts 1 and 2 between these
electrodes.
A lamp cap 8 is secured at the ends of the tube parts ~1,2), at
which the electrodes are located, and this cap preferably consists of syn-
30 thetic ma-terial. The lamp cap comprises a projec-ting wall portion 9, in
which a starter is provided. Current connection pins 10 and 11 are arran-
ged on either side of the projecting wall portion.
During operation of -the lamp, portions of the inner wall of the
discharge vessel located near the one ends of the tube parts (especia]ly
35 the wall portions ~1 and 5) are cor~?aratively cool. Without additional
steps being taken, condensation of mercury can occur at these areas, which
has the aforementioned disadvantageous consequences, especially if the
lamp is arranged in vertical position.
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PHN 10.994 4 04-03-1985
In order to counteract these phenomena, an amalgam 13 for con-
trolling the vapour pressure in the discharge vessel during operation of
the lamp is present in the exhaust tube 12 behind the electrode 6 forming
part of the mount associated with the electrode 6, the end of the exhaust
tube 12 remote from the electrode 6 being sealed. Condensation of mercury
at the aforementioned ends is then avoided. The alloy forming with mercu~
ry an amalgam preferably consists of the elements lead, bismuth and tin.
In a practical embcdiment of the aforementioned lamp, the length
of the tube parts is approximately 13 cm and -the inner diameter is appro-
ximately 10 mm. The luminescent layer on the inner wall is a mixture of
two phosphors, i.e. green luminescing terbium-activated cerium magnesium
aluminate and red luminescing yttrium oxide activated by trivalent euro-
pium. 180 mg of an alloy with a ration in atoms of lead,bismuth and tin
equal to 16 : 36.5 : 47.5 is present in the exhaust tube. Furthermore,
lS approximately 6 mg of mercury is present. With a rare gas filling of ar-
gon (3 torr), the light output was approximately 600 lumen with a pcwer
supplied to the lamp of approximately 9 W.
