Heat insulation for single-ended discharge lamp

The invention relates to a single-ended discharge lamp the light source of which is a discharge tube consisting of at least two tube legs, the said single-ended discharge lamp has a cap part for mechanical and electrical connection to a lampholder, the cap part is, on its side being closer to the discharge tubes, bordered by a separator wall separating it from the space of the light source. According to the invention, cavities (24) accepting the ends of the tube legs are formed in the separator wall (20), and openings (26) for passing inleads are formed at the bottom of the cavities (24) at the places of current feedthroughs.

The subject of the invention is a single-ended discharge lamp having a 
light source and a cap part connecting to a lampholder mechanically and 
electrically. The invention relates to the heat insulation between the 
light source and the inner space of the cap part. 
There is a widespread knowledge about this kind of discharge lamps. e.g., 
in a patent application No. P 94 03360 disclosed with No. T/73090 in the 
Szabadalmi Kozlony es Vedjegyertesito (Hungarian Patent and Trademark 
Bulletin), No. 6, 1996, a single-ended discharge lamp is described in 
which openings for passing the legs of the discharge tube are formed in a 
separator wall separating the cap part from the space of the light source. 
The legs of the discharge tube pass through these openings and protrude 
into the space of the cap part to a small extent. The portions of the legs 
protruding into the space of the cap part are fixed to the separator wall, 
which also serves as a tube support, by means of a bonding material or an 
adhesive. An electronic ballast and a starting circuit which are known in 
themselves and may also comprise several semiconductor parts may also be 
placed in this space of the cap part. 
This solution has the disadvantage that the tube legs of the discharge tube 
protruding into the tube support to a small extent conduct a part of the 
heat produced in them into the space of the cap part, thus heat the parts 
of the ballast circuit. In addition to the conducted heat, radiated heat 
and light also reach the inside of the cap part which contributes to the 
heating of parts during operation and the light getting into the cap part 
causes some loss of illumination and is responsible for a reduction of the 
luminous efficiency of the lamp. 
We have set ourselves the objective with this invention to improve the 
discharge lamp outlined above in the way that a better heat insulation is 
provided between the space of the light source and the inner space of the 
cap part to reduce the extent of heating of the cap part or the ballast 
circuit parts being optionally inside the cap part. At the same time, the 
luminous efficiency of the lamp can be enhanced by adding the light 
emitted towards the cap part to the useful light emission. 
According to the invention, the objective set was achieved with a discharge 
lamp outlined in the introduction by providing cavities in the separator 
wall which cavities fit to and accept the ends of the tube legs, and the 
bottoms of the said cavities have openings for passing the inleads at the 
places of current feedthrough. 
This solution has the advantage that the tube legs of the discharge tube 
protrude into the cavities formed in the separator wall and not into the 
inner space of the cap part. Owing to this, the discharge tube is 
separated from the inner space of the cap part, practically no heat will 
be radiated into it, and also heat conduction will only take place through 
the separator wall made of an insulator material which means a 
significantly smaller extent of heat conduction. 
The openings for passing the inleads at the places of current feedthrough 
at the bottom of the cavities have small cross-sections which allows only 
an insignificant extent of heat convection to occur along the inleads. 
In the event that the discharge tube also has tube legs provided with 
exhaust tube(s) at their ends close to the separator wall, openings for 
passing the exhaust tube(s) are formed in the bottom of the cavities. 
Although the exhaust tube(s) will conduct only a small amount of heat into 
the inner space of the cap, the opening(s) for passing the exhaust tube(s) 
is (are) closed with cover(s) fitting to the shape of and surrounding the 
exhaust tube and protruding into the space of the cap part in accordance 
with a further preferred embodiment of our invention. This results in an 
additional improvement of heat insulation of the cap part. 
In a possible embodiment, the opening(s) for passing the exhaust tube(s) 
and the openings for passing the inleads are formed in the bottom of one 
and the same cavity. 
In a further possible embodiment, the ends of the tube legs are fixed in 
the cavities by means of a bonding material or adhesive. 
In order to enhance luminous efficiency, at least the surface of that side 
of the separator wall closer to the light source, is light reflecting. 
In the following, the discharge lamp according to the invention will be 
described in more details by means of an example of an embodiment also 
illustrated in the attached drawing figures where 
FIG. 1 is a side view of a compact discharge lamp, 
FIG. 2 is a perspective drawing of the separator wall separating the space 
of the light source from the cap part, 
FIG. 3 is the bottom view of the separator wall viewed from the cap part, 
FIG. 4 is the section of the separator wall shown in FIG. 3, taken along 
the broken line A--A and 
FIG. 5 is the section of a further possible embodiment of the separator 
wall, taken along the line B--B shown in FIG. 3.

FIG. 1 shows a compact discharge lamp known in itself and having a 
discharge tube 10 consisting of tube legs 12, 14, 15, 16 in series 
connection which discharge tube serves as the light source of the lamp. 
The discharge lamp has eight tube legs of which only four tube legs 12, 
14, 15, 16 are seen in the side view of the figure. It is practical to 
place the tube legs along a circle in top view, thus the longitudinal axis 
of each of the eight tube legs according to the example are in the 
vertices of a regular octagon. Of course, discharge tubes consisting of 
four or six tube legs are also used. 
In the lower part of FIG. 1, cap part 18 of the discharge lamp is seen. 
This cap part 18 is illustrated only schematically. In the example shown, 
this cap part 18 has a conventional screw part for screwing it into a 
lampholder to ensure electrical and mechanical connection, and the upper 
part of the screw part fits to a conical housing, in the inside of which 
an electronic ballast circuit is placed. This conical housing of the cap 
part 18 is separated from the space of the light source represented by the 
discharge tube 10 by a separator wall 20. In the example shown, the 
discharge tube 10 consisting of the tube legs 12, 14, 15, 16 is fixed 
mechanically by this separator wall 20. 
Perspective view of the separator wall 20, its bottom view from the cap 
part 18, and its section taken over the broken line A--A are shown in FIG. 
2, FIG. 3, and FIG. 4, respectively. A circular rim 22 extending downward 
from and in essential perpendicularly to the separator wall 20 is placed 
over the perimeter of the said separator wall 20, for a mechanical joint 
between the separator wall 20 and the housing of the cap part 18. This rim 
22 may have guides, slots, snap-in shoulders, etc. for fitting and fixing 
it to the housing. 
The separator wall 20 of a compact discharge lamp having a discharge tube 
10 consisting of eight tube legs is seen in this example. In contrast to 
the known solution, in the separator wall according to the invention, the 
end of each tube leg 12, 14, 15, 16 is placed in a cup-shaped cavity 24, 
the shape and size of which fit to the end of the corresponding tube leg. 
In accordance with this, the bottom of the cavities 24 has usually the 
shape of a hemisphere. The tube legs 12, 14, 15, 16 of the discharge tube 
10 may be fixed by means of an adhesive placed in one or more cavities 24 
of the separator wall 20. 
The inner discharge spaces of the tube legs 12, 14, 15, 16 are connected in 
series with each other by means of bridging portions. Current inleads pass 
through the ends of the two outer tube legs being closer to the cap part 
to supply energy required by the discharge at the ends of the series 
connected spaces. In order that the current inleads can pass through the 
separator wall 20, an opening 26 is provided at the bottom of each cavity 
24 belonging to the corresponding two tube legs. In the event that an 
adhesive for fixing the tube legs 12, 14, 15, 16 is brought also in these 
cavities 24, this adhesive will surround also the current inleads and can 
also close the opening 26. 
In order to provide an appropriate fill in the discharge space, an exhaust 
tube is placed at the ends of some tube legs being closer to the cap part 
18. This exhaust tube is tipped off to be sealed after exhausting the air 
from and adding the appropriate fill to the discharge space. The exhaust 
tube may be placed at the end of the tube legs having the electrodes for 
the discharge, adjacent to the current inleads or at the ends of separate 
tube legs. These tipped-off exhaust tube studs protrude to a small extent 
downward from the tube legs, therefore opening(s) 28 is (are) formed at 
the bottom of the cavity (cavities) 24 for these studs. In the event that 
an adhesive for fixing the tube legs is also brought in these cavities 24, 
this adhesive will also surround the current inleads and close the opening 
28. 
In FIG. 5 which is a section taken along the line B--B shown in FIG. 3, it 
is seen that the separator wall 20 is provided with covers 30 connecting 
to the openings 28. These covers 30 protrude into the inner space of the 
cap part 18 and their shape and size fit to those of the exhaust tube stud 
protruding from the end of the corresponding tube leg, and the covers 30 
surround the exhaust tube studs. This results in that the heat conducted 
by the exhaust tube studs is insulated from the inner space of the cap 
part 18 by the covers 30. 
The separator wall 20 is preferably made of an insulator material. This is 
preferable for two reasons: firstly, a better heat insulation can be 
provided for the electronic ballast circuit and secondly, no separate 
electric isolation is needed at the current feedthroughs. It is practical 
if the insulator material is a plastic that withstands the operating 
temperature of the lamp without deformation. 
It is preferable if at least the side of the separator wall 20 being closer 
to the light source has a light reflecting surface. This is preferable for 
two reasons: firstly, the light reflecting surface reflects the heat 
radiation leaving the discharge tube during operation, thereby increasing 
the heat insulation for the parts of the electronic ballast circuit placed 
in the housing 18, and secondly, it also reflects the light rays reaching 
it, thereby enhancing the luminous efficiency of the lamp. In accordance 
with this, the separator wall may be made of a light reflecting white 
material. A metal layer applied to the side of the separator wall being 
closer to the light source is also conceivable.