Electrodeless low-pressure discharge lamp

Abstract: The electrodeless low-pressure discharge lamp comprises a tube (11) which is closed at one end, which surrounds a body (7) of soft magnetic material with clearance, and which is enclosed with clearance in the cavity (6) in the outer bulb (5), which cavity enters the cavity (4) in the discharge vessel (1). The tube (11) provides a good insulation of the body (7) from the heat of the discharge in the discharge vessel (1).

Description: The invention relates to an electrodeless low-pressure discharge lamp comprising a discharge vessel with a discharge space containing an ionizable vapour and rare gas, the discharge vessel having a cavity at an end portion, an evacuated outer bulb surrounding the discharge vessel, which outer bulb has a cavity which enters the cavity of the discharge vessel, a body of soft magnetic material enclosed in the cavities, which body has an end near the end portion of the discharge vessel and is supported at said end by a thermal insulator, an electric coil inside the cavity of the discharge vessel around said body of soft magnetic material, a heat-repelling envelope of electrically insulating material between the body of soft magnetic material and the discharge space. Such a lamp is known from EP-A-0 298 538. Lamps of the said type with, for example, sodium or a metal halide as the ionizable vapour have a relatively high optimal operating temperature. Thus the optimal lowest temperature of the discharge vessel is 260° C if sodium vapour is the ionizable vapour. This means that the lamp requires a good thermal insulation of the discharge vessel. Thermal losses of the lamp may be reduced by restricting the flow of heat through the body of soft magnetic material. Soft magnetic materials of low retentivity, however, have a low heat resistance. The specific magnetic losses increase with increasing temperature, while at an increased temperature, moreover, the magnetic permeability of said materials starts to decrease. In proportion as the body of soft magnetic material is thermally better insulated from the discharge space, the heat insulation at the end of the body may have a higher thermal resistance without a critical temperature in said body being exceeded. A higher thermal resistance of the heat insulation leads to a better luminous efficiency of the lamp. Materials which are suitable for use as a heat-repelling envelope, such as aerogels of Al₂O₃ or SiO₂, are expensive, as are synthetic materials having a very high thermal resistance. Such a heat-repelling envelope has a considerable influence on the cost price of the lamp. The invention has for its object to provide a lamp of the kind described in the opening paragraph which has an inexpensive and effective heat-repelling envelope which is easy to manufacture. According to the invention, this object is achieved in that a tube which is closed at one end, which surrounds the body of soft magnetic material with clearance and is enclosed with clearance in the cavity in the outer bulb, and which is open near the end portion of the discharge vessel, is used as a heat-repelling envelope. It has been shown that such a tube with clearance provides a simple, inexpensive and effective insulation. The tube may be of, for example, glass or ceramic material, for example of white ceramic for a good light reflection, for example Al₂O₃. Such tubes can be manufactured in a simple manner. Thanks to the presence of the tube and its clearance the soft magnetic body is surrounded by two gaps. It has been shown that there occurs no or substantially no convection in these gaps, if they are filled with air and are narrower than 5 mm. Since it is favourable to construct the lamp as compact as possible, gaps which are not wider than 1 mm will usually be chosen. The gap between the tube and the cavity in the outer bulb may be evacuated for an even better thermal insulation. The use of a glass tube is very attractive because of its ready availability. Moreover, such a tube can be easily provided with bulges and dimples distributed over its surface, which separate the tube from the soft magnetic body and the cavity in the outer bulb, respectively. A further increase in the heat insulation of the soft magnetic body may be achieved through the use of an infrared-reflecting coating between the discharge space and the low-retentivity magnetic body. This coating may consist of one or several interference layers of dielectric material, or of electrically conducting material, for example doped tin oxide or, for example, indium oxide doped with tin, or of a metal such as, for example, gold, silver, aluminium. If an electrically conducting material is used, it is advisable to provide the coating with interruptions which extend in the direction of the soft magnetic body. This serves to suppress eddy currents in the coating. An embodiment of the lamp according to the invention is shown in the drawing in lateral elevation, partly cut away. The electrodeless low-pressure discharge lamp of Fig. 1 has a discharge vessel 1 with a discharge space 2 containing an ionizable vapour, sodium in the lamp drawn, and rare gas, for example argon, and a cavity 4 at an end portion 3. An evacuated outer bulb 5 is arranged around the discharge vessel 1. The outer bulb 5 has a cavity 6 which enters the cavity 4. A body 7 of soft magnetic material is enclosed in the cavities 4, 6. The body 7 has an end 8 near the end portion 3 of the discharge vessel 1, which end is supported by a thermal insulator 9. An electric coil 10 is present around the body 7 of soft magnetic material, for example made of ferrite, such as 4C6 ferrite, inside the cavity 4 of the discharge vessel 1. The lamp has a heat-repelling envelope of electrically insulating material between the body 7 of soft magnetic material and the discharge space 2. A tube 11, closed at one end, which surrounds the body 7 of soft magnetic material with clearance and which is enclosed with clearance by the cavity 6 in the outer bulb 5, and which is open near the end portion 3 of the discharge vessel 1, is used as a heat-repelling envelope of the body 7. The tube shown is made of glass and has dimples 12 and bulges 13 which keep said tube separated from the body 7 and the cavity 6 in the outer bulb, respectively, so that an insulating air gap, of approximately 1 mm in the drawing, is present inside and outside the tube. The outer bulb is provided with an IR-reflecting coating 14 of, for example, indium oxide doped with tin. The discharge vessel 1 is mounted in the outer bulb 5 by means of a glass plate 15 which is provided with an IR-reflecting coating 16, an aluminium plate 17 and glass rings 18. Inside the outer bulb 5 is a holder 19 for an evaporable getter, such as, for example, barium. The lamp is fixed in a shell 20 in which there is a supply unit 21, which has an output frequency of at least 1 MHz. Conductors 22 connected to said unit 21 extend to the coil 10. The shell carries a lamp cap 23 provided with contacts 24 which are connected to the unit 21. The thermal insulator 9 is supported by a mounting plate 25. In a modification of the lamp shown, the space between the tube 11 and the cavity 4 in the discharge vessel 1 is evacuated. The body of soft magnetic material of a lamp as shown in the drawing had a temperature of 290° C during operation. In a similar lamp, in which the tube 11 was absent, and which was operated with the same load on the discharge vessel wall (0,118 w/cm²), the temperature of the body was 320° C. The considerable temperature decrease caused by the measure according to the invention renders it possible to limit the thermal losses of the lamp caused by a heat flow in the direction of the lamp cap by giving the thermal insulator a higher heat resistance, for example, by making it longer and/or thinner.

Claims: An electrodeless low-pressure discharge lamp comprising a discharge vessel (1) with a discharge space (2) containing an ionizable vapour and rare gas, the discharge vessel having a cavity (4) at an end portion (3), an evacuated outer bulb (5) surrounding the discharge vessel (1), which outer bulb (5) has a cavity (6) which enters the cavity (4) of the discharge vessel (1), a body (7) of soft magnetic material enclosed in the cavities (4, 6), which body (7) has an end near the end portion (3) of the discharge vessel (1) and is supported at said end by a thermal insulator (9), an electric coil (10) inside the cavity of the discharge vessel (1) around said body (7) of soft magnetic material, a heat-repelling envelope of electrically insulating material between the body (7) of soft magnetic material and the discharge space (2), characterized in that a tube (11) closed at one end, which surrounds the body (7) of soft magnetic material with clearance and which is enclosed with clearance in the cavity (6) in the outer bulb (5), and which is open near the end portion (3) of the discharge vessel (1), is used as a heat-repelling envelope. An electrodeless discharge lamp as claimed in Claim 1, characterized in that dimples (13) and bulges (12) distributed over the surface of the tube (11) separate the tube (11) from the cavity (6) in the outer bulb (5) and from the body (7) of soft magnetic material. An electrodeless discharge lamp as claimed in Claim 1, characterized in that the gap between the tube (11) and the cavity (6) in the outer bulb (5) is evacuated.

Inventors: KROES ANTHONY, KROES, ANTHONY
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1991
Main classification: H01J65
IPC codes: H01J65