This invention relates to a lighting unit comprising an electrodeless low-pressure discharge lamp and a supply for said lamp, which lamp is provided with a discharge vessel having a radiation-transmitting enveloping portion and a recessed portion, which discharge vessel surrounds a discharge space in a gastight manner and contains an ionizable filling, the lamp being further provided with a coil arranged in the recessed portion for generating a high-frequency magnetic field so as to maintain an electric discharge in the discharge space, while the enveloping portion of the discharge vessel is provided with a radiation-transmitting, electrically conducting layer on a surface facing the discharge space, the discharge vessel having a tubular projecting portion where a lead-through member connected to the electrically conducting layer issues to the exterior.
The invention further relates to an electrodeless low-pressure discharge lamp and to a discharge vessel for use in the lighting unit.
A lighting unit of the kind described in the opening paragraph, wherein the supply is accommodated in a holder which is fastened to the discharge vessel and which also supports a lamp cap, is known from U.S. Pat. No. 4,940,923. The lamp of the known lighting unit is operated at high frequency, i.e. at a frequency higher than approximately 20 kHz, in that case a frequency of approximately 3 MHz. The discharge vessel of the lamp supports a radiation-transmitting, electrically conducting layer, also called a conducting layer hereinafter, of fluorine-doped tin oxide. A metal wire electrically connected to the conducting layer issues to the exterior through the fused free end of a glass tubular projection of the discharge vessel, thus at the same time forming the lead-through member.
During operation of the lighting unit, the conducting layer is connected to a pole of the power mains via the metal wire, the lead-through member formed thereby, and a contact of the lamp cap. Interferences in the power mains and in the area surrounding the lamp caused by the high-frequency operation of the lamp thus remain limited.
It is comparatively easy during the manufacture of the known lamp to close the glass projection around the lead-through member in a gastight manner, provided cleaning and filling of the discharge vessel take place through a separate channel. Indeed, if the tubular projection is used for this purpose and is subsequently sealed up around the lead-through member, deformations will arise in the molten glass as a result of the pressure difference between the inside and outside of the discharge vessel. These deformations of the glass of the projection adjoining the lead-through member result in inadmissible mechanical stresses which give rise to fractures. The presence of a separate channel, however, renders it difficult to achieve a compact discharge vessel construction.