Electrodeless low-pressure discharge lamp

Abstract: The electrodeless low-pressure discharge lamp has a lamp vessel (1) having a cavity (2) at an end portion (3) thereof. An electric coil (4) surrounding a plastic tube (5), wherein a liquid-filled tubular container (7) surrounded by a soft-magnetic core (6) is present, is accommodated in the cavity (2). The plastic tube (5) is substantially filled up with an elastic polymer. Additionally, the tube (5) may be enveloped by an elastic polymer (9). The elastic polymer ensures a relatively low operating temperature of the members inside the cavity (4).

Description: The invention relates to an electrodeless low-pressure discharge lamp comprising a lamp vessel which is sealed in a vacuumtight manner, contains ionizable metal vapour and rare gas, and has a cavity at an end portion of said vessel, an electric coil around a sleeve of synthetic material in the cavity of the lamp vessel, a core of soft magnetic material in the sleeve of synthetic material, a tube containing a liquid in the core of soft magnetic material, which tube projects to outside the cavity and has a flange there. Such a lamp is known from EP-0 384 520. The lamp is provided with a tube containing liquid in order to discharge heat generated during operation of the lamp, to prevent inter alia that the core of soft magnetic material will assume too high a temperature. This is because the specific magnetic losses of the core increase with increasing temperature, whereas the magnetic permeability starts to decrease from a raised temperature. It is the object of the cooling by means of the tube containing liquid to suppress these factors, which adversely affect the luminous efficacy of the lamp. There is a risk that the temperature of the core of the known lamp is comparatively high. The invention has for its object to provide a lamp of the kind described in the opening paragraph which is of a simple construction and in which nevertheless the temperature of the core can be relied on to have a comparatively low value. This object is achieved in that the sleeve is at least substantially filled with an elastic polymer such that in particular a good thermal connection is obtained between the core and the tube. The core and the tube each have their own thermal coefficient of expansion. As a result of this and of the tolerances which have to be accepted with regard to the dimensions of these bodies, it is difficult to achieve a close contact between the tube and the core and also to prevent stresses to occurr owing to which the core is cracked. It is true that one can aim at a close fit of the tube in the core during operation, but even then heat transfer must take place through a slit between the tube and the core. Since the tube, being the innermost body, has the smaller surface, a comparatively great heat flow must still run per unit area. Since the space inside the sleeve in the lamp according to the invention is at least substantially filled with an elastic polymer, there is a close connection between the core and the tube. The sleeve and the core may be made closely fitting, for example, in that the core is ground to the correct diameter. There is a good heat transfer between the sleeve and the core then, also because the core has a comparatively large outer surface. Alternatively, there may be a clearance between the sleeve and the core which is filled up with the elastic polymer. A better heat transfer to the tube, and from there to the surroundings of the lamp, is achieved by the measure according to the invention. In a favourable embodiment, the sleeve is not only filled with, but also enveloped in the elastic polymer. A lower heat resistance is realised in this way, so that the cavity of the lamp vessel forms a cooler ambience for the core. An enveloped sleeve also has the advantage that the coil remains fixed around the sleeve. Expansion of the coil at operating temperature could cause the coil to sag in the long run, turns getting a greater pitch. It is favourable for easy manufacture of the lamp if the lamp vessel on the one hand and the assembled body in the cavity on the other hand are separate sub-assemblies. In view of the tolerances which must be permitted for the dimension of the cavity in the lamp vessel, a body which consists mostly of glass, it is not safeguarded then that the enveloped sleeve is in contact with the lamp vessel all around. The surface of the envelope of the sleeve, being the outermost surface, however, is much greater than the surface of the tube. The heat flow per unit area is therefore much smaller, so that a less close contact between the lamp vessel and the envelope is of minor influence. The use of an elastic polymer is also advantageous in that differences in coefficient of expansion between the materials on which the various bodies are manufactured, especially those of the tube and the core, can be easily accommodated. Frequently used materials are: glass for the lamp vessel; synthetic material, for example liquid crystalline polymer, for the tube; ferrite, for example Philips 4C6, for the core; metal, for example copper, for the tube; and, for example, rubber, such as silicone rubber, as an elastic polymer. In addition, the lamp is better resistant to shocks and vibrations, for example, during transport. This and other more detailed aspects of the invention are described and explained with reference to the drawing in which a lamp is shown partly in cross-section, partly in elevation. The electrodeless low-pressure discharge lamp has a lamp vessel 1 which is closed in a vacuumtight manner, is made of, for example, lime glass, contains ionizable metal vapour and rare gas, and comprises a cavity 2, for example of lead glass, at an end portion 3 of said vessel. An electric coil 4 around a sleeve 5 of synthetic material is present in the cavity 2. A core 6 of soft magnetic material is present in the sleeve 5 of synthetic material. A tube 7 containing liquid is present in the core 6, projects to outside the cavity 2, and has a flange 8 there. In the embodiment drawn, the lamp vessel 1 contains a rare gas and mercury as an ionizable metal and is coated with a fluorescent powder 1a. The sleeve 5 is filled with an elastic polymer 9. The polymer fills the gap between the tube 7 and the core 6. In the embodiment drawn, the polymer also envelops the sleeve 5. The tube 7, the core 6 and the sleeve 5 with the coil 4 in the drawing together with the polymer 9 form a sub-assembly which is provided as such in the cavity 2. The sub-assembly 4, 5, 6, 7, 9 is thus removable from the cavity 2. Silicone rubber is used as the polymer 9. In the drawing, the sleeve 5 is fastened with a snap connection 12, 13 to a support 14 of synthetic material which has a flange 15, on which are present hooks 16, at a free end. The flange 8 of the tube 7 and the flange 15 of the support 14 are fastened to one another and a foil 17 of synthetic material, for example silicone rubber, is provided against the flange 8. As a result, the lamp may be mounted against a metal support without the risk of a galvanic element being formed by this support and the flange 8. A collar 19 of synthetic material, which is held by the hooks 16, is mounted to the lamp vessel 1, for example with silicone compound 18. A cable leading to an electric supply may be connected to contacts 20 at the flange 15, to which the coil 4 is connected.

Claims: An electrodeless low-pressure discharge lamp comprising a lamp vessel (1) which is sealed in a vacuumtight manner, contains ionizable metal vapour and rare gas, and has a cavity (2) at an end portion (3) of said vessel, an electric coil (4) around a sleeve (5) of synthetic material in the cavity (2) of the lamp vessel (1), a core (6) of soft magnetic material in the sleeve (5) of synthetic material, a tube (7) containing a liquid in the core (6) of soft magnetic material, which tube (7) projects to outside the cavity (2) and has a flange (8) there, characterized in that the sleeve (5) is at least substantially filled with an elastic polymer (9), such that in particular a good thermal connection is obtained between the core and the tube. An electrodeless low-pressure discharge lamp as claimed in Claim 1, characterized in that the sleeve (5) is enveloped in the polymer (9). An electrodeless low-pressure discharge lamp as claimed in Claim 1 or 2, characterized in that the enveloped sleeve (5) with the coil (4), the core (6), and the tube (7) form a sub-assembly which is separate from the lamp vessel (1). An electrodeless low-pressure discharge lamp as claimed in Claim 1, 2 or 3, characterized in that the polymer (9) is silicone rubber.

Inventors: EGGINK HENDRIK JAN, FRIEDERICHS WINAND HENDRIK ANN, VAN GENNIP NICASIUS GERARDUS T, EGGINK, HENDRIK JAN, FRIEDERICHS, WINAND HENDRIK ANNA MARIA, VAN GENNIP, NICASIUS GERARDUS TIELEMANUS
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1992
Main classification: H01J65
IPC codes: H01J65, H01J61