Low-pressure mercury vapor discharge lamp with mercury amalgam

The invention relates to a compact low-pressure mercury vapor discharge lamp consisting of a glass inner member which is closely surrounded by a glass cylindrical outer member, one end of which is hemispherical. The edge at the other end of the outer member is sealed in a gas-tight manner to the inner member. The discharge path between the electrodes may be folded as shown. The discharge is present in a groove, formed in the wall of one of the members. Near the said sealed edge there is an amalgam which is accessible to the discharge space via a narrow channel.

The invention relates to a low-pressure mercury vapor discharge lamp 
comprising a glass hollow inner member which closely is surrounded with a 
discharge-tight fit by a predominantly cylindrical sealed glass outer 
member one end of each member being closed, the two members being sealed 
in a gas tight manner at their other ends, the lamp also comprising two 
electrodes between which a discharge takes place during operation of the 
lamp, said discharge being present in a discharge space formed by a groove 
in the wall of at least one of the members and the wall portions of the 
other member which are located opposite this groove. 
A discharge-tight fit is defined for present purposes as a fit wherein the 
two members are dimensioned so that the discharge in the groove is 
enclosed in a discharge-tight manner, that is to say, the discharge does 
not leave the groove, so that no short circuit can occur between parts of 
the discharge path present in two adjacent groove-portions. 
A lamp of the type described in the opening paragraph is disclosed in U.S. 
Pat. No. 4,095,135. 
This United States patent specification describes compact low-pressure 
mercury vapor discharge lamps wherein the discharge path between the 
electrodes has been folded. Provided they comprise a suitable lamp base 
(wherein there are, for example, an electric stabilization ballast and a 
starter) and a cap, (screw or bayonet type), these lamps can be used in 
luminaires for incandescent lamps for general lighting purposes. 
During operation of these known compact lamps circumstances may exist, for 
example poor ventilation, or when the lamp is operated in places having a 
relatively high temperature which cause the mercury vapor pressure in the 
lamp to increase to such a value that the critical mercury vapor pressure 
for an optimum conversion of electric power into ultra-violet radiation 
may be easily exceeded. Particularly in compact lamps wherein the electric 
stabilization ballast in included in the lamp base, the mercury vapor 
pressure in the discharge space may be too high for the most advantageous 
conversion efficiency, owing to the relatively high temperature of the 
ballast during operation of the lamp. The optimum mercury vapor pressure 
for this purpose is approximately 0.8 Pa. With a higher vapor pressure the 
luminous flux of the lamp decreases for the same applied power. 
The object of the invention is to provide a low-pressure mercury discharge 
lamp for which measures have been taken to stabilize the mercury vapor 
pressure at a relatively high operating temperature. 
This object is accomplished with a low-pressure mercury vapor discharge 
lamp of the type described in the opening paragraph which according to the 
invention is characterized in that near the sealed edge there is an 
amalgam present on or adjacent to a wall portion, which is not in but 
accessible to the discharge space via a channel defined by facing surfaces 
of the two members. 
It was found that particularly in a lamp according to the invention, 
comprising a lamp base including an electric stabilization ballast this is 
the most advantageous position to apply the amalgam for the most 
advantageous conversion efficiency. 
The invention can be advantageously used in lamps wherein the inner member 
is enveloped by the outer member with a small clearance. The mercury 
amalgam is then not only in connection with the portion of the groove 
which is closest to it but also with the entire space between the two 
members. Consequently, the mercury vapor pressure can then be stabilized 
by the amalgam in a simple and rapid manner, also in, even in the more 
remote portions of the groove (the groove may, for example, have a folded 
shape). 
It should be noted that it is known, for example in highly loaded tubular 
low-pressure mercury vapor discharge lamps, to dispose an alloy in the 
discharge vessel which forms an amalgam with mercury in order to control 
the mercury vapor pressure in the discharge vessel. Such an amalgam is 
then often applied to the inside of the discharge vessel wall, for example 
in a place behind the electrode. In a lamp according to the invention, 
however it was found that, as the lamp is very poorly cooled only owing to 
its compactness, such a place behind the electrode or a different place of 
the wall of a groove where the discharge is present assumes a temperature 
which is still too high for a satisfactory vapor pressure stabilization. 
The amalgam may be applied in different manners to the wall of one of the 
members, for example by means of a bonding agent. In a lamp according to 
the invention, the amalgam may be located in a recessed portion in the 
wall of one of the members, preferably in the wall of the inner member. 
The recess is accessible to the discharge space, for example via a channel 
which is defined by the facing walls of the two members. This embodiment 
has the advantage that the amalgam is bondable to the wall on a larger 
portion of its surface area. Furthermore, a relatively large quantity of 
the amalgam can be applied to a comparatively small portion of the wall. 
In a further embodiment of a lamp according to the invention the amalgam is 
present in a metal container having an opening. Measures should be taken 
during the production or operation of the lamp to prevent the amalgam or 
the amalgam-producing alloy from becoming detached from the wall and from 
moving to any random position in the lamp. The container is preferably 
disposed in the above-mentioned recess in the wall of the inner member. 
The container is kept in place by the adjoining wall of the outer member.

The lamp shown in FIG. 1 comprises a glass cylindrical outer member 1 
having a hemispherical end 1a. It encloses with a discharge-tight fit 
(with a clearance of, for example, 0.1 to 1.5 mm) a glass inner member 2. 
At the other end the members are bonded together around the edge of member 
1 in a gas-tight manner with glass enamel 3. The lamp comprises two 
electrodes between which a discharge takes place during operation of the 
lamp. The drawing shows only one of these electrodes (4). The discharge 
path is extended by folding. The discharge space is defined by a folded 
groove formed in the wall of the inner member and by the outer member wall 
portions located opposite this groove. This visible portions of the groove 
are denoted by the numerals 5, 6 and 7. As viewed in the drawing, the 
discharge path extends upwards from electrode 4 via portion 5, reverses 
thereafter to continue its way via a bridge portion, (not shown) on the 
back to the top of groove portion 6 and then via portion 6, bridge portion 
6a and portion 7 to the other electrode (not shown) at the back of the 
lamp. A mercury amalgam 8 is present near the connecting edge 3. This 
amalgam is applied to a wall portion which is accessible to the discharge 
space (see FIG. 2). The lamp further comprises a lamp base 9 with screw 
cap 10. The lamp base includes an electric stabilization ballast 11 and a 
starter 12. 
FIG. 2 is a cross-sectional view through the plane II--II of the lamp shown 
in FIG. 1. Corresponding components have been given the same reference 
numerals. The amalgam (consisting, for example, of indium, bismuth and 
mercury) is present in a metal container 13 having opening 14. This 
container is disposed in a recess 15 formed in the wall of the inner 
member 2. The recess is accessible to the discharge space via a narrow 
channel 16, which is defined by the facing walls of members 1 and 2 and 
which opens into groove portion 6a. The channel is automatically provided 
when members 1 and 2 are assembled in close relationship during production 
due to their configurations. The clearance between 1 and 2 should be 
sufficiently narrow to avoid the risk that the discharge may be 
short-circuited during operation of the lamp, for example so that the 
discharge cannot jump from portion 5 to portion 6. 
In one embodiment of a lamp according to the invention a layer of 
luminescent material is applied to the inside of the groove wall and to 
the portion of the outer member wall facing the groove. The remaining 
portions of the members are free from luminescent material. The 
luminescent material consists of a mixture of two phosphors, namely 
green-luminescing, terbium-activated cerium magnesium aluminate and a 
red-luminescing, trivalent europium-activated yttrium oxide. In this 
embodiment the discharge path consists of six parallel, interconnected 
groove portions, the discharge path having a length of 40 cm, the distance 
from the seal 3 to the top of the glass outer member being approximately 8 
cm. The outer diameter of member 1 is approximately 6 cm. The average 
spacing between the two members 1 and 2 other than in the region of the 
grooves is about 0.5 mm. In addition to approximately 8 mg of mercury the 
lamp contains a quantity of approximately 80 mg of an indium-bismuth alloy 
(50-50% by weight) as the amalgam forms alloy. With a rare gas filling of 
argon at a pressure of 400 Pa, the luminous flux of the lamp was 1000 
lumen at an applied power to the lamp together with the 19 W electric 
ballast. (Power 19 W). 
In the embodiment described with reference to the drawing, the discharge 
path is defined by a groove formed in the wall of the inner member 2 and 
the ungrooved wall of the outer member 1. It is of course, alternatively 
possible to provide such a groove in the outer member 1 only or to provide 
corresponding grooves in both members.