Electrodeless low-pressure discharge lamp having a straight exhaust tube fixed on a conical stem

Electrodeless low-pressure discharge lamp having a lamp vessel sealed in a gas-tight manner and filled with a metal vapor and a rare gas. The lamp vessel comprises a glass envelope which is connected in a gas-tight manner to the edge of a conically widening collar at the end of a tubular part of a sealing member also made of glass. The tubular part accommodates a rod-shaped core of a magnetic material by means of which a discharge is generated in the lamp vessel during operation of the lamp. An exhaust tube extends parallel to the longitudinal axis of the tubular part terminating at the wall of the conical collar. The end of the exhaust tube engages round an orifice in a planar surface in a cam-shaped recess provided in the wall of the conical collar. The planar surface extends substantially at right angles to the longitudinal axis of the tubular part of the sealing member.

BACKGROUND OF THE INVENTION 
The invention relates to an electrodeless low-pressure discharge lamp 
having a lamp vessel which is sealed in a gas-tight manner and is filled 
with a metal vapor and a rare gas. The lamp vessel comprises a glass 
envelope which is connected in a gas-tight manner to the edge of a 
conically widening collar at the end of a tubular part of a sealing member 
also made of glass. The tubular part accommodates a rod-shaped core of a 
magnetic material by means of which a discharge is generated in the lamp 
vessel during operation of the lamp. The known lamp has an exhaust tube 
which extends parallel to the longitudinal axis of the tubular part of the 
sealing member and terminates at the wall of the conical collar. A lamp of 
this type is known from "Neues aus der Technik", 1986, 1. 
The sealing member in the known lamp is in the form of a stem which is 
connected in a gas-tight manner by means of sealing glass to the 
envelope-shaped part of the lamp vessel. The use of a stem in itself is 
quite conventional in tubular low-pressure mercury discharge lamps 
provided with electrodes and in incandescent lamps. In these lamps the 
stems are provided with electrodes and a tubular exhaust tube which 
terminates in a so-called pinch which is located on or near the 
longitudinal axis of the stem. An exhaust tube is required for exhausting 
the lamp vessel, that is to say, filling the lamp vessel with a desired 
rare gas atmosphere, degassing of electrodes, the wall and other 
components of the lamp vessel, etc. 
However, due to the presence of the rod-shaped magnetic core there is no 
space for an exhaust tube in the centrally located tubular part of the 
sealing member formed as a stem in an electrodeless lamp. However, these 
lamps also require exhausting of the lamp vessel. 
In the known lamp the exhaust tube is therefore connected to the conical 
collar of the sealing member. For the purpose of satisfactory progress of 
exhausting in a bulk manufacturing process it is desirable that an exhaust 
tube extends in the direction of the longitudinal axis of the lamp vessel 
(the same direction as the longitudinal axis of the tubular part). When 
the end of an exhaust tube engages a wall which extends obliquely with 
respect to the said longitudinal axis, it is necessary that prior to or 
subsequent to fixation to the said wall part the exhaust tube is bent 
until the correct position (in the direction of the longitudinal axis) for 
exhausting is reached. This bending process is an additional 
time-consuming step in the manufacturing process creating a ready risk of 
breakage of the sealing member or the exhaust tube. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide an electrodeless low-pressure 
in a bulk manufacturing process in which the above-mentioned drawbacks of 
the known lamps are obviated. 
According to the invention, an electrodelss low-pressure discharge lamp of 
the type described in the opening paragraph is therefore characterized in 
that the wall of the conical collar is provided with a cam-shaped recess 
having a bottom part which is a planar surface perpendicular to the 
longitudinal axis of the tubular part of the sealing member. The end of an 
exhaust tube is connected to the perpendicular surface and the exhaust 
tube is aligned with the longitudinal axis of the tubular part. An orifice 
in the perpendicular surface allows communication between the exhaust tube 
and the inner space of the lamp vessel. 
The advantage of the lamp according to the invention is that the exhaust 
tube can be fixed in the correct position to the conical collar without 
extra operations during the manufacturing process. It has been found that 
the risk of breakage in the glass wall of the collar around the point of 
fixation of the exhaust tube is small during manufacture of the lamps. 
In the manufacture of the lamp the cam-shaped recess is firstly provided in 
the conical wall part by locally heating the glass over a small surface 
area. The recess having the perpendicular planar surface is formed by 
means of an abutment. Subsequently the end of the exhaust tube is 
connected to the bottom part by softening the end of the exhaust tube and 
an opening is provided in the bottom part. The exhaust tube is then 
positioned in the vertical direction, that is to say, parallel to the 
longitudinal axis of the lamp. Subsequently the exhausting process can be 
carried out. 
In a practical embodiment of the lamp according to the invention two 
parallel exhaust tubes located diametrically opposite each other are fixed 
to the conical collar. The exhausting process can then be carried out in 
an efficient manner by introducing a quantity of gas into the lamp vessel 
via a first exhaust tube and by removing unwanted gases via the second 
exhaust tube. 
In a special embodiment one of the exhaust tubes is provided with an 
amalgam to control the mercury vapour pressure during operation of the 
lamp. In order to prevent the amalgam from reaching the lamp vessel, the 
opening in the bottom art of the recess associated with that exhaust tube 
is relatively small. Due to the substantially horizontally extending shape 
of the bottom art the size of the opening can be adjusted in a simple 
manner, for example, by punching a softened spot in the bottom part with a 
pin having a defined diameter.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The lamp according to FIG. 1 comprises a glass lamp vessel 1 which is 
sealed in a gas-tight manner and is filled with mercury and a rare gas. 
The lamp vessel has a glass envelope 2 whose inner wall is provided with a 
luminescent coating (not shown) and whose lower side has a recess with a 
circular raised edge 2a. The edge of a conically widening collar 3 is 
secured at the area of the edge 2a in a gas-tight manner by means of 
sealing glass ("solder glass"), which collar is located at the end of a 
tubular part 4 which is sealed at its other end (the upper side)5. The 
tubular part 4 and the collar 3 form one integral assembly, sometimes 
referred to as the stem. This stem is actually the sealing member 6 (see 
FIG. 2) for the envelope-shaped part 2 of the lamp vessel. The tubular 
part 4 accommodates a rod-shaped core 7 of ferrite. A winding 8 having a 
number of copper wire turns is provided around this core 7. The supply 
wires 9 and 10 of the winding 8 are electrically connected to a 
high-frequency supply unit which is accommodated in the housing 11. During 
operation of the lamp an electric field is generated in the lamp vessel 
causing a discharge. The housing 11 is surrounded by a synthetic material 
wall part 12 which is secured to the lamp vessel 1 at one end and is 
provided with an Edison lamp cap 13 at its other end. 
Cam-shaped recesses 14 and 15 by means of which two tubular exhaust tubes 
16 and 17, respectively, are fixed to the collar wall 3 by means of a 
fusing process are provided in the wall of the conical collar 3 in two 
positions which are located diametrically opposite each other. To this end 
each cam-shaped recess is provided with a flat bottom part extending at 
right angles to the longitudinal axis 22 of the tubular part of the 
sealing member 6. This is shown in detail in FIG. 2. As is evident from 
FIG. 2, axis 22 is substantially the longitudinal axis of the lamp. 
The bottom part of recess 14 is denoted by 14a (see FIG. 2) and that of 
recess 15 is denoted by 15a. The end of an exhaust tube is positioned 
against each bottom part. By means of an orifice in each bottom part the 
bores of the exhaust tubes are connected to the space within the lamp 
vessel. 
In the embodiment shown the lamp vessel is exhausted via exhaust tube 16. 
(In the drawing this exhaust tube is shown with an open end). A relatively 
large orifice 18 in the bottom part 14a is present in the collar. The 
exhaust tube 17 is shown already sealed and provided with a 
vapour-pressure controlling amalgam 20 (In-Bi-Hg). To prevent this amalgam 
(which in solid form has the shape of a rod) from reaching the lamp 
vessel, the orifice 19 in bottom art 15a is much narrower than orifice 18 
in the bottom part 14a. (The diameter orifice 19 is approximately 0.5 mm; 
the internal diameter of the exhaust tube is approximately 3.2 mm). 
Furthermore a metal plate-shaped mercury capsule 21 which is fixed in the 
exhaust tube in known manner (see GB-PS No. 1,475,458,) is present in the 
exhaust tube 16. After exhausing, this exhaust tube 16 is sealed and the 
mercury is released from capsule 21. Furthermore the lamp vessel contains 
an In-starting amalgam 23 (see FIG. 1) on a gauze-shaped holder. 
The diameter of orifice 19 is preferably between 0.3 and 0.7 mm. For 
smaller diameters insufficient mercury vapour diffuses to the lamp vessel. 
If the diameter is too large, there is a risk that amalgam 20, which 
preferably is a pellet with a diameter of about 1 mm in a ductile state, 
could pass into the lamp vessel through opening 19 during operation of the 
lamp. 
The lamp according to FIG. 1 has a luminous efficiency of approximately 
1200 lumen at a power supply of 17 W. The luminescent coating consists of 
a mixture of green-luminescing terbium-activating cerium magnesium 
aluminate and red-luminescing yttrium oxide activated by trivalent 
europium. A transparent conducting layer (not shown) of fluorine-doped tin 
oxide was provided between this luminescent coating and the wall of the 
lamp vessel to reduce interference currents in the supply mains during 
operation of the lamp.