Plasma light bulb and method for making a plasma light bulb

Abstract

 A plasma light bulb, comprising a hollow sphere (1) of a translucent material, a substance (3) within the hollow sphere, wherein the substance can be excited electromagnetically so as to form a plasma, which radiates visible light, and a stem (5) protruding from the hollow sphere, wherein the hollow sphere is tip- or tail-less, and the stem is bonded to the hollow sphere.

Description

[0001] The invention is directed to a plasma light bulb and also to a method for making a plasma light bulb. The bulb can be mounted in a plasma light lamp or system, wherein the lamp or system is also a subject-matter of the invention. Also concerned is a bulb as such provided for integration in a plasma light lamp or system.

[0002] Plasma light systems are of growing interest for commercial and industrial applications. Sulphur plasma lamps or systems, which are of particular concern for the invention, radiate over the complete range of visible light and resemble sun light closer than other artificial light sources. Sulphur plasma and also selenium plasma light is particularly useful for lighting fabrication or other working facilities, for growing plants and lighting aqua cultures, also for the photo industry and further lighting or processing purposes. The bulb is rotated in many applications to cool it and to generate homogeneous plasma.

[0003] Plasma light systems including bulbs of the type the invention is concerned with are known from e.g. WO 92/08240 Al and EP 0 942 457 A2. The bulbs are comprised of a hollow sphere made of glass having a fill which can be excited electromagnetically so as to form a plasma which radiates light, and a stem protruding from the hollow sphere. The stem serves as shaft for driving the hollow sphere rotationally to cool it and also to stabilize a uniform discharge of the excited fill. Despite all the measures taken in the prior art, the durability of the bulbs need to be increased.

[0004] It is an object of the invention to increase the durability i.e. the operational life time of plasma light bulbs.

[0005] A plasma light bulb, preferably an electrode less bulb, according to the present invention, comprises a hollow sphere of a translucent material, a substance within the hollow sphere, and a stem which protrudes from the hollow sphere. The sphere envelopes the substance which is of the type that can be excited electromagnetically so as to form a plasma which radiates visible light. The light is emitted in the visible range directly upon excitement or after absorption and re-emission. The substance is preferably sulphur or a sulphur compound. Selenium and tellurium as well as compounds thereof are also preferred candidates for the excitable substance. The translucent material is preferably glass.

[0006] According to the invention, and in contrast to the prior art bulbs, the hollow sphere is tip-less or tail-less, in German "zipfelfrei", except for the stem which, however, is not regarded as an imperfection but serves the purpose to mount the bulb in a plasma light system. The stem preferably defines a shaft for rotationally driving the bulb. In principle, however, the stem may serve only the purpose for supporting the bulb stationary. The stem can be bonded to the hollow sphere, i.e. the sphere and the stem are formed separately and jointed by means of a substance bond. This facilitates forming the sphere with a perfectly spherical inner and outer surface and of universally constant wall thickness, except of course for the location at which the stem is jointed to the sphere. In embodiments in which the stem defines a shaft for rotationally driving the bulb, the stem advantageously defines an axis of rotation extending through the sphere, preferably through the centre of the sphere.

[0007] The prior art spherical bulbs, in practice, all comprise a tip or tail, in German "Zipfel", opposite the location at which the stem reaches the sphere's surface. Such tips or tails are of course not shown in the figures of the prior art documents, but are nevertheless present in all bulbs which have been produced hitherto. Although bulb technique has been matured to such a level that the bulbs can be operated over thousands of hours, the invention attains a considerable increase of operational lifetime. The invention has recognized that the tail or tip of the prior art bulbs constitutes a hot spot prone to fracture during operation. Eliminating such an imperfection does accordingly increase the average operational lifetime of the bulbs thereby lowering the operating expenses of plasma light systems.

[0008] In some embodiments, the stem comprises a thermally insulating axial section between the hollow sphere and the connector, wherein the isolating axial section is preferably made of ceramic. In some embodiments, the bulb is mounted in a plasma light system comprising a diaphanous resonator which is arranged to couple electromagnetic energy into the internal volume of the hollow sphere, and the thermally isolating section can protrude from outside into the resonator.

[0009] The invention is also directed to an intermediate product for manufacturing a plasma light bulb. The intermediate product comprises a hollow sphere of a translucent material. The hollow sphere has an opening. A tube is fixed to the hollow sphere, wherein the tube and the opening form a passage into the hollow sphere. In some embodiments, a substance that is electromagnetically excitable so as to form a plasma which radiates visible light can be provided in the inner volume of the hollow sphere. In other embodiments, the hollow sphere can be empty. To form a plasma light bulb, an inert gas can be introduced into the interior volume of the hollow sphere through the tube and the opening. In embodiments wherein the intermediate product is provided with an empty hollow sphere, an electromagnetically excitable substance can be introduced into the hollow sphere through the tube before the interior volume of the hollow sphere is filled with the inert gas. After introducing the inert gas and, optionally, the electromagnetically excitable substance through the tube, the opening can be sealed, and the tube can be fused or cut to length to from a stem protruding from the hollow sphere.

[0010] The invention is also directed to a method for making a plasma light bulb. In the method, a substance which can be excited electromagnetically so as to form a plasma which radiates visible light is filled into a hollow sphere made of a translucent material, preferably glass. The hollow sphere can be formed to have an outer spherical surface without any tails or tips and which is flawless in this sense, i.e. tail-less and tip-less, except for a single opening through which the substance is introduced. A tube is positioned at the opening of the pre-fabricated hollow sphere, an axial end of the tube opposite the opening of the sphere, and jointed to the hollow sphere so as to surround the opening in a gas-proof substance joint. The excitable substance can be introduced into the sphere through the tube or is preferably introduced before jointing the tube to the sphere. After the tube has been bonded to the hollow sphere the sphere is filled with an inert gas through the tube and the sphere's opening. The inert gas can in particular be argon or xenon. When filling is completed the tube is sealed at the opening so as to make the hollow sphere gas-proof, and a stem protruding from the hollow sphere is formed, wherein the stem is connected to the hollow sphere at the location of the sealed opening.

[0011] In some embodiments, the hollow sphere can be positioned for jointing and during the jointing process by holding it directly at its spherical outer surface. The sphere can be held in position relative to the tube, preferably stationary, by a holding device which contacts, e. g. sucks or clamps the hollow sphere at its spherical outer surface. In other embodiments, the hollow sphere can be positioned for jointing by means of a glass support tube melted to the hollow sphere.

[0012] The stem can be fused to the hollow sphere. A fusion bond necessitates that the hollow sphere and the stem are made of materials which melt at approximately the same temperatures. Preferably the sphere and the stem are made of the same material which can in particular be glass, preferably quartz glass. Alternatively to the preferred fused bond the stem and the sphere can be jointed by gluing, i.e. by a glued bond. Care will have to be taken that the glued bond can withstand the temperatures of bulb operation and, if the bulb is rotated, also the torque and any other mechanical load associated with rotation.

[0013] In some embodiments the stem comprises a ceramic over at least a part of its axial length. The ceramic can advantageously extend over the major part of the axial length of the stem. The stem preferably consists of the ceramic material over that part, i.e. it comprises an axial stem section consisting of the ceramic. The ceramic axial stem section can reach up to and can be bonded directly to the spherical surface of the hollow sphere e. g. by gluing. The stem can in total be comprised of the ceramic material. More preferred, however, is a stem which comprises a first axial section made of a material identical or similar to the translucent material of the hollow sphere, preferably a glass material, which protrudes from the sphere and is bonded, preferably fused to the hollow sphere, and a second axial section which comprises or is made solely of the ceramic and extends the first axial section axially and away from the hollow sphere. The first axial section and the second axial section are preferably jointed by bonding, optionally in addition by a form fit or force fit which can in particular be a form or force fit for transmitting torque, i.e. for transmitting a force in the circumferential direction of the stem.

[0014] In some embodiments, the tube can be fused off or cut to length to form a peg protruding from the hollow sphere.

[0015] In some embodiments, a ceramic structure, for example a rod, can be jointed to the peg or the tube, for example by bonding, such that the ceramic structure extends the peg or tube in an axial direction and, together with the peg or tube, constitutes a stem for rotationally driving the hollow sphere.

[0016] The stem can comprise a connector for connecting it to a support. It should not be excluded that the bulb, if mounted, is unmovable in relation to the plasma light system in which it is integrated. It is regarded to be advantageous, however, if it comprises a connector for moving the bulb during its operation e. g. for cooling purposes. Most preferred is a connector for driving the bulb rotationally about a rotational axis defined by the stem. In a plasma light system with a rotary drive which can in particular be an electric motor, the bulb is connected to the drive by means of the connector. In principle the connector can be a connector for bonding the stem to the support which can in particular be an output shaft of a rotary drive. More preferred is a connector for connecting the stem by a form or force fit to the support, or by screwing. The word "or" is used in its usual logical meaning, i.e. it is an "inclusive or" covering the meaning of "either ... or" as well as the meaning of "and", as long as the specific context does not rule out one of these meanings without any doubt. In the form or force fit embodiment the connector may therefore be a connector for only a form fit or only a force fit with the support, or it can be a connector for connecting the stem to the support in a combined form and force fit. In embodiments in which the bulb is moved during operation the form or force fit is such that it transmits the force necessary for the movement. A form or force fit connector can optionally in addition be connected to the support by bonding, however, a joint constituted purely by form or force fit, without a substance bond, is preferred.

[0017] The stem of the bulb can comprise an axial stem section which provides for a thermal isolation of the hollow sphere from the support when the bulb is mounted in a plasma light system. The thermally isolating section can be disposed between the hollow sphere and the connector mentioned above, if the stem comprises such a connector. The thermally isolating stem section can in particular be made of ceramic. The ceramic stem section mentioned above can be the thermally isolating section. Ceramics have a lower heat conductivity and lower brittleness than glass. The ceramic can be reinforced e.g. by embedded filaments or reinforcing elements of some other shape. A reinforced ceramic can in particular be composed of a ceramic matrix with embedded ceramic reinforcing elements e.g. ceramic filaments. The ceramic can, but need not be reinforced. Providing the bulb with a thermally isolating stem section is useful in particular for applications in which the bulb is moved during operation. The thermal isolation prevents overheating the drive, for example the rotary bearing(s) of a rotary drive. Decoupling the hollow sphere from the support, thermally, contributes to increase the operational lifetime and availability of the plasma light system in which the bulb is integrated, in particular in cases in which the bulb is mounted movably. To minimize disturbance of the electromagnetic field it is advantageous if the thermally isolating stem section does not contain a material which would be excited to a practically relevant extend by the electromagnetic waves for exciting the fill of the sphere.

[0018] In a plasma light system the thermally isolating stem section can be at least partially located in a cavity or cage which surrounds the hollow sphere and couples the electromagnetic energy, preferably microwaves, into the sphere's interior. The cavity is a resonator for the electromagnetic waves for exciting the fill of the sphere, but diaphanous. If the support is arranged outside the cavity, the thermally isolating stem section can protrude from outside into the cavity.

[0019] In applications in which the bulb is rotated during operation the stem constitutes the drive shaft and defines an axis of rotation of the bulb. Although the stem can have a circular outer contour in cross-section all over its length, the stem can also be rotationally asymmetrical with respect to the axis of rotation. When rotated, the rotationally asymmetrical stem vortexes the surrounding fluid, in most applications air, and thereby creates or at least assists in cooling the hollow sphere at its outer surface by convection. The rotating stem creates a rotational flow of the fluid surrounding the stem and can also create turbulences near the surface of the hollow sphere. The stem, although defining the rotational axis of the bulb, can comprise one or more section(s) which is or are inclined with respect to the rotational axis, this section or sections can be straight or curved. The stem as a whole can, for example, meander in its longitudinal direction or zigzag. Preferably, however, the stem is axially straight and has a non-circular cross-section over at least part of its axial length, preferably over a predominant part of its axial length. The non-circular cross-section can be oval, for example elliptic with a long and a short principal axis.

[0020] Even more preferred is a cross-section with at least one flat, expediently planar outer surface. A polygonal cross-section, for example a triangular or in particular a rectangular or trapezoidal cross-section is most advantageous. The non-circular cross-section can expediently be formed in or by the ceramic stem section or thermally isolating stem section of some other material.

[0021] A non-circular cross-section is also advantageous for coupling the stem to a rotary drive. The non-circular cross-section can serve to establish a form fit connection with the drive, either directly with an output shaft of the drive or with a connector by which the bulb can be connected to and disconnected from the drive easily and quickly.

[0022] The invention is also concerned with a plasma light system comprising the bulb. The system furthermore comprises a radiator for radiating electromagnetic energy, preferably a magnetron, a resonator for coupling the electromagnetic energy into the hollow sphere to excite the fill, and a waveguide for connecting the radiator to the resonator. The system preferably furthermore comprises a rotary drive to which the bulb is connected or can be connected, for example by means of the connector disclosed above, for driving the bulb rotationally.

[0023] A stem which consists of or at least comprises a ceramic, including embodiments with only one ceramic as well as embodiments with a compound of different ceramics, can advantageously be provided in any type of a plasma light bulb or some other type of a preferably electrodeless bulb forming an envelope containing an electromagnetically excitable fill to radiate light. A ceramic stem or stem section is advantageous on its own, for example in combination only with the features a) to c) of claim 1, although it is most advantageous in combination with the bulb specified in claim 1. The same is true with respect to thermally decoupling the hollow sphere from a support, either a rigid support or a bearing or a drive shaft. A stem which is rotationally asymmetrical although it defines an axis of rotation of the bulb is also advantageous on its own and not only in combination with the invention claimed here. The envelope for the excitable fill can in particular be a hollow sphere but can in principle also have some other shape.

[0024] Advantageous features are also disclosed in the dependent claims and combinations of the same. The features disclosed in the dependent claims and the embodiments described above also complement each other reciprocally.

[0025] Example embodiments of the invention are explained below on the basis of figures. Features disclosed by the example embodiments, each individually and in any combination, advantageously develop the subjects of the claims and the embodiments described above. In the figures:

Figure 1

shows a pre-formed hollow sphere and a tube positioned for jointing to form a plasma light bulb,

Figure 2

shows the hollow sphere and the tube jointed by substance bond,

Figure 3

shows the hollow sphere sealed off and the tube fused or cut to length to form a bulb of a first embodiment,

Figure 4

shows a plasma light bulb according to the first embodiment,

Figure 5

shows a bulb of a second embodiment comprising the hollow sphere of the first embodiment and a stem with a ceramic axial section,

Figures 6a to 6d

show a process for forming the hollow sphere,

Figure 7

shows on half of a mould which can be used to form the hollow sphere and

Figure 8

shows how the hollow sphere is positioned and fixed in a jointing position.

[0026] Figures 1 to 3 show a sequence of a jointing process for producing a plasma light bulb from a hollow sphere 1 made of glass and a tube 4 also made of glass.

[0027] In a first step, shown in figure 1, the pre-formed hollow sphere 1 and the tube 4 are positioned for jointing, one relative to the other, in a cleanroom. The hollow sphere 1 has a spherical shell being thin as compared to an internal spherical volume enveloped by the shell. This glass shell has no macroscopic tips or dents and is macroscopically smooth throughout its inner and outer spherical surface, except for a single small opening 2. The opening 2 is the only opening in the otherwise macroscopically flawless spherical and smooth inner and outer surface of the shell. The opening 2 has the shape of a simple through-bore with a bore diameter in the range of the thickness of the sphere's shell. It serves the purpose of introducing a substance 3 into the empty internal volume and also to fill this volume with an inert gas. The substance 3 is of the type that can be excited electromagnetically so as to form a plasma which emits light, preferably as in the example embodiment, visible light. The substance 3 can in particular be sulphur, selenium, tellurium or a compound containing at least one of sulphur, selenium and tellurium. In their condensed, not excited state such substances are granular or powdery. The substance 3 has been filled in before jointing the hollow sphere 1 and the tube 4.

[0028] Tube 4 is an axially elongated straight tube with an inner cross-section similar or identical to the cross section of the opening 2. For jointing, the hollow sphere 1 is positioned in the clean-room in a jointing position and fixed in the jointing position by means of a holding device which comprises a receptacle and a clamping means for fixing the hollow sphere 1 in the jointing position relative to the receptacle. In the jointing position the hollow sphere is advantageously positioned such that it is oriented horizontally, i.e. that a radial axis R extending through the centre of the opening 2 and the centre M of the hollow sphere 1 is horizontal. The sectional plane of the figures 1 to 4 is the horizontal including this axis R.

[0029] With the hollow sphere 1 in its jointing position the tube 4 is also positioned and fixed in a jointing position in which a central longitudinal axis of the inner axial passage of the tube 4 is in axial alignment with said sphere's radial axis R. With the sphere 1 and the tube 4 in their respective jointing position the tube 4 abuts the outer surface of the hollow sphere 1 such that the shell of the tube 4 surrounds the opening 2.

[0030] The hollow sphere 1 and the tube 4 are jointed while being held in their jointing position by creating a substance bond of the tube and the sphere directly. A gas-proof joint is created by fusing the tube 4 with the shell of the hollow sphere 1 locally all around the opening 2. The hollow sphere 1 and the tube 4 being jointed to each other form an intermediate product that can be used for manufacturing a plasma light bulb.

[0031] Figure 2 shows the hollow sphere 1 and the tube 4 in the jointed state in which the tube 4 and the opening 2 form a passage through which an inert gas, preferably argon or xenon, is introduced into the hollow sphere 1. The inert gas assists in starting the discharge when charging the sphere's internal volume with the electromagnetic energy.

[0032] When filling is completed the hollow sphere 1 is sealed off gas-proof by closing the opening 2, as is shown in figure 3. Sealing is accomplished by heating the tube 4 locally near the opening 2 to melting temperature thereby closing the opening 2 with molten glass. After the sphere 1 has been sealed the tube 4 is fused or cut to length to form a stem 5 protruding from the hollow sphere 1. Thus, the stem 5 is connected to the hollow sphere 1 at the location of the sealed opening 2. The stem 5 serves the purpose to mount the bulb to a support, which can in particular be an output shaft of a rotary drive of a plasma light system.

[0033] Since the substance 3 and the inert gas are introduced into the hollow sphere through the opening 2, and the stem 5 is provided at the location of the sealed opening, no tip or tail is formed at the hollow sphere 1 in the manufacturing of the plasma light bulb. Thus, problems of prior art plasma light bulbs, which may comprise a tip or tail at a position opposite the stem, such as a fracture during operation can advantageously be avoided.

[0034] Figure 4 shows a view of a plasma light bulb according to the first embodiment in a later stage of manufacturing. In this embodiment, the stem 5 can substantially have the shape of a solid cylinder. A stem 5 being a solid cylinder can be obtained by fusing the tube 4 along the entire length of the tube 4. In other embodiments, the tube can be cut off adjacent the sealed opening 2, and a piece of a transparent material, for example silica, substantially being a solid cylinder, can be attached to the hollow sphere 1 at the location of the sealed opening 2, for example by means of fusing techniques known to persons skilled in the art, to form the stem 5.

[0035] The present invention is not limited to embodiments wherein the stem 5 is a solid cylinder. In other embodiments, a portion of the stem 5 distal of the hollow sphere 1 can be a hollow cylinder. In some of these embodiments, the hollow portion of the stem 5 can be provided in form of a portion of the tube 4 that is not fused in the manufacturing of the plasma light bulb.

[0036] At an end 103 of the stem 5 opposite the hollow sphere 1, a connector 8 can be fixed to the stem 5. The connector can comprise a ferrule enclosing the end 103 of the stem 5, and can have a thread 104 for attaching the plasma light bulb to other components of a plasma light source.

[0037] In some embodiments, a portion 105 of the stem 5 can be comprised of a non-transparent material, for example fused silica. Thus, conduction of light and infrared radiation through the stem 5, which might lead to an undesirable heating of the connector 8 and/or components of a plasma light source connected thereto, can advantageously be avoided. In other embodiments, the portion 105 of non-transparent material can be omitted.

[0038] As already mentioned above, the hollow sphere 1 can be tip-or tail-less, and/or can be flawlessly spherical. In Fig. 4, reference numeral 106 denotes a point on an outer surface of the hollow sphere 106. The point 106 is located on a calotte 101 of the hollow sphere 1. The calotte 101 is located opposite the stem 5, such that the center M of the hollow sphere 1 is between the calotte 101 and the stem 5. A circumference of the calotte 101 is lying in a plane 107 that is substantially perpendicular to a longitudinal axis 102 of the stem 5. The longitudinal axis 102 of the stem 5 can be substantially identical to the central longitudinal axis of the inner axial passage of the tube 4 that is connected to the hollow sphere 1 in the manufacturing of the plasma light bulb as described above with reference to Figs. 1 to 3, and can also substantially coincide with the axis R of the hollow sphere 1 shown in Figs. 1 and 2.

[0039] Lines 108, 109 connecting the circumference of the calotte 101 with the center M of the hollow sphere 1 include an angle amax with the longitudinal axis 102 of the stem 5. For each point 106 on the outer surface of the hollow sphere 1 located on the calotte 101, an angle a between the longitudinal axis 102 of the stem and a line 110 connecting the center M of the hollow sphere 1 with the point 106 is less than the angle amax.

[0040] For each point 106 on the outer surface of the hollow sphere 1 located on the calotte 101, an absolute value of a difference between the distance r' of the point 106 from the center M of the hollow sphere 1 and the outer radius r of the hollow sphere 1 can be smaller than an upper boundary dmax. The upper boundary dmax is a measure of the maximum allowed deviation of the shape of the portion of the outer surface of the hollow sphere 1 on the calotte 101 from a mathematically exact spherical shape.

[0041] If the hollow sphere 1 would comprise a tip or tail, such as in plasma light bulbs according to the state of the art, an absolute value of the difference between the distance of portions of the outer surface of the hollow sphere lying on the tip or tail from the center of the hollow sphere and the outer radius r of the hollow sphere would be relatively large. Conversely, in embodiments of the present invention wherein the hollow sphere is tip- or tail-less and/or flawlessly spherical, absolute values of the difference between the distance r' of points 106 on the outer surface of the hollow sphere 1 within the calotte 101 from the center M and the outer radius r are relatively small. Hence, the absolute value of the difference between the distance of each point 106 on the calotte 101 from the center M of the hollow sphere and the outer radius r of the hollow sphere 1 can be smaller than the upper boundary dmax for relatively small values of the upper boundary dmax, for example for values of the upper boundary dmax of 3 mm, 2 mm, 1 mm, 0,5 mm, 0,25 mm, 0,1 mm or 0,05 mm.

[0042] Additionally, or alternatively, the upper boundary dmax can be expressed as a percentage of the radius of the hollow sphere 1. For example, the upper boundary dmax can have a value of 20%, 12,5%, 6%, 3%, 1,5%, 0,6%, or 0,3% of the outer radius r of the hollow sphere 1. In some embodiments, the outer radius r of the hollow sphere 1 can have a value of approximately 16 mm. In other embodiments, the outer radius r can have a different value.

[0043] The angle amax defining the extension of the calotte 101 can have a value of 5°, 10°, 20°, 40°, or 45°.

[0044] The absolute value of the difference between the distance of points on the outer surface of the hollow sphere 1 outside the calotte 101 and the outer radius r of the hollow sphere can also be smaller than the upper boundary dmax. In some embodiments, this may be the case for the entire outer surface of the hollow sphere 1, or at least for the outer surface of the hollow sphere 1 with the exception of portions adjacent the joint between the hollow sphere 1 and the stem 5.

[0045] The above-described properties concerning the deviation of the shape of the hollow sphere from a mathematically exact sphere can also be fulfilled during the manufacturing of the plasma light source, for example for the intermediary product shown in Fig. 2.

[0046] Figure 5 shows a plasma light bulb of a second embodiment. In this embodiment the bulb comprises a hollow sphere 1 containing an excitable fill as in the first example embodiment, and a stem 5 protruding from the hollow sphere 1, also as in the first embodiment. The second embodiment differs from the first in that the stem 5 is composed of a ceramic over a predominant part of its axial length. Only a small peg 6 has been left from the glass stem 5 of the first embodiment. Stem 5 of the second embodiment comprises a first axial section 6 which is the glass peg 6 protruding from the hollow sphere 1, and a second axial section 7 made of the ceramic. The ceramic has a heat conductance value lower than that of the glass material of the first section 6. Section 7 isolates the sphere 1 thermally from its support when the bulb is mounted in a plasma light system.

[0047] In some embodiments, the first axial section 6 can have a length of about 20% of the total length of the stem 5 or less, for example a length in a range from about 10% to about 20% of the total length of the stem 5. The second axial section 7 can have a length of about 80% of the total length of the stem 5 of more, for example a length in a range from about 80% to about 90% of the total length of the stem 5. In some embodiments, the total length of the stem 5 can be approximately 10 cm.

[0048] In a method for making the bulb of the second embodiment, the steps from preforming the hollow sphere 1 to sealing-off the sphere's internal volume can be the same as for the first embodiment. Producing the bulb of the second embodiment includes, in addition, forming the ceramic section 7 and jointing the ceramic section 7 and the glass section 6 to form the assembled stem 5. The bulb of the second embodiment can be derived from that of the first embodiment by fusing or cutting the stem 5 of the first embodiment to a shorter length to form the relatively short first axial section 6, and then jointing the ceramic section 7 by bonding it to the section 6 such that it is in axial alignment therewith extending the section 6 axially and away from the hollow sphere 1.

[0049] The ceramic section 7 can be an axially elongated straight rod having a rectangular cross-section over its entire length. As preferred, the cross-section can be a square. In other embodiments, the ceramic section 7 can be cylindrical. The ceramic section 7 can be solid in cross-section. It can be hollow alternatively, a passage can for example extend from one axial end to the other.

[0050] In preferred embodiments in which the bulb is mounted rotatably and is rotationally driven during operation, the stem 5 constitutes a drive shaft and thereby defines a rotational axis of the bulb, which may be substantially identical to the longitudinal axis 102 of the stem 5. One function of the ceramic section 7 is to thermally decouple the rotary drive from the hollow sphere 1. In deviating from a circular-cross-section, generally speaking in deviating from rotational symmetry with respect to the axis 102, the section 7 fulfills a further function in that, upon rotation, it creates a rotational flow of the surrounding fluid, this flow increasing the cooling effect of the fluid surrounding the bulb, in particular the sphere 1.

[0051] Stem 5 of the first embodiment can also comprise a ceramic section or a section of some other material of a lower thermal conductivity than glass. Such a thermally isolating axial section preferably also extends coaxially with the longitudinal axis 102 of the stem 5. With a glass section, which is long as compared to the first section 6 of the second embodiment, and which may, for example, have a length of approximately 50% of the total length of the stem 5, the joint of the glass section 5 and the ceramic section can include or even consist of a force or more preferably a form fit in that such sections overlap axially over an adequate length. In the overlapping portion, the two sections can additionally be jointed by substance bond, for example a glued bond.

[0052] Similar to the first embodiment described above with reference to Figs. 1 to 4, a connector 8 can be fixed to the stem 7 of the plasma light bulb according to the second embodiment. The connector 8 can comprise a ferrule enclosing an end 103 of the stem 5, and can have a thread 104 for attaching the plasma light bulb to other components of a plasma light source.

[0053] In applications in which the plasma light bulb according to any of the embodiments described above is driven rotationally about the longitudinal axis 102 of the stem 5, a nut and screw connection to the output shaft of a rotary drive can be established by means of the connector 8. The output shaft can be directly the rotor shaft of a rotary motor or the output shaft of a transmission gear coupled between a rotary motor and the stem 5. The connector 8 of the stem 5 can form either the screw or the nut of the connection. The connection can in principle be of any type of a coupling, it can even be a substance bond, preferred are however couplings which allow for easily connecting and disconnecting the bulb to and from the drive, preferably a coupling for connecting and disconnecting by hand without tools.

[0054] A preferred process for forming the hollow sphere 1 is illustrated in Figures 6a to 6d. These figures show in detail the steps for producing the hollow sphere 1 from a glass tube 8 and attaching of the hollow sphere 1 to the tube 4. As shown in Figure 6a, the base material which the hollow sphere 1 is made of is a glass tube 8 which has been closed semi-circularly on one side. Before the process of forming the hollow sphere 1 starts, a glass support tube 9 is melted onto the closed end of the glass tube 8. In Figure 6b, the closed end of the tube 8 has been heated in an oven (not shown), or with an open flame, to a temperature at which the glass of the glass tube 8 is still solid but is deformable. A tool 16 is then brought into contact with the deformable closed end of the tube 8. When the tool 16 is rotated around the closed end of the tube 8, or the closed end of the tube 8 is rotated in the tool 16, said end is formed into the hollow sphere 1, which is positioned between the support tube 9 on the one side and a formed contraction of the tube 8. Once this forming process has been completed, a predetermined breaking point is formed, somewhere on the contracted part of the tube 8, preferably at the narrowest point, on the connection between the tube 8 and the hollow sphere 1, with the aid of a special tool 17, for example a flat carbon, as shown in Figure 6b. The hollow sphere 1 is cut off the tube 8 using a special glass knife (not shown), such that the hollow sphere 1 is free at the cut-off end, but still connected to the support tube 9, as shown in Figure 6c. Finally, in Figure 6d, the hollow sphere 1 is connected, for example melted or glued, to the tube 4. The tube 4 and the outer diameter of the contracted part of the tube 8 which sticks out from the hollow sphere 1 can thus have essentially the same diameter, and the two elements can be attached to each other end to end. Equally, the tube 4 may have an outer diameter which is smaller or wider than the inner or outer diameter, (respectively), of the tube rest which sticks out from the hollow sphere 1, resulting in an overlapping region which allows for a safer connection of the two elements. Once the connection between the hollow sphere 1 and the tube 4 is completed, the support tube 9 is separated from the hollow sphere 1 in a final step, for example melted off, cut off or simply knocked off. After separation, the outer surface of the hollow sphere 1 can be polished or finished in order to smooth the outer surface of the region where the support tube 9 was connected to.

[0055] The hollow sphere 1 can also be formed in a mould consisting essentially of two identical halves 18, each of the two mould halves 18 having an inner semicircular surface with a diameter which corresponds to the outer diameter of the hollow sphere 1 and an orifice for receiving half of the tube 4, as shown in Figure 7. In order to form the hollow sphere 1, a piece of solid glass material or a pre-formed hollow glass blank and the tube 4 are placed in the mould, wherein the glass blank and the tube 4 can already be connected. The glass blank and the tube 4 can also be separate parts, in which case the glass blank has an opening 2 which the tube 4 abuts when the mould is closed. At least the glass blank is heated to the deformation temperature, either prior to being inserted in the mould or inside the mould. Air or gas with a temperature high enough to keep the glass blank in the deformable state is forced into the glass blank through tube 4 and the opening 2, such that the deformable glass blank expands against the inner surface of the mould, thus forming the hollow sphere 1 with a perfect inner surface. The mould, or at least its inner surface, is made of a material which the glass will not stick to. During the forming process, the hollow sphere 1 is also connected to the tube 4, for example melted onto it. Once the expanding process of the glass blank is finished, the mould and/or the hollow sphere 1 can be cooled down until the material of the hollow sphere 1 becomes rigid again. Preferably, the pressure of the air or gas inside the hollow sphere 1 is maintained until the hollow sphere 1 has cooled down, thus preventing the hollow sphere 1 from partially collapsing inside the mould due to the force of gravity. Cooling can be achieved or supported by blowing cool air or gas into the already formed hollow sphere 1. After cooling down, the mould can be opened and the finished product consisting of the tube 4 and the hollow sphere 1 can be taken out of the mould and can be finished, for example a burr formed on the outer surface of the hollow sphere 1 where the two mould halves abut can be removed.

[0056] A tool consisting of only one half of the mould can also be used as a receptacle 18 for assembling the hollow sphere 1 and the tube 4. The hollow sphere 1 can be attached to a holding and handling device which transports the hollow sphere 1 from an interim storage facility to the receptacle 18 and places the hollow sphere 1 in the receptacle 18. The tube 4 is also fed in a receptacle 18 from another machine or storage facility, and the two parts are connected to each other as already described. Once the two parts have been connected, the finished product can be taken out of the receptacle 18 by the handling device which is still connected to the hollow sphere 1 and can be transported to a processing station for the next production process or to a storage facility. The receptacle 18 can be used for quality control purposes, since a hollow sphere 1 having a form which does not fit into the receptacle 18 can be removed from further production.

[0057] The handling device can be a robot arm which picks up the support tube 9 melted onto the outer surface of the hollow sphere 1, or it can for example be a sucker which holds the hollow sphere 1 while it is separated from the support tube 9 and subsequently.

[0058] Figure 8 illustrates how the hollow sphere 1 is positioned and fixed in its jointing position. The sphere 1 is accommodated in a receptacle 18 formed as a spherical half of a calotte. The receptacle 18 supports the sphere 1 from below and thereby gravity assisted. A holding device for positioning and fixing the sphere 1 furthermore comprises a clamping means 19 for adjusting the position of the sphere 1 in the receptacle 18 with respect to the orientation of the opening 2. The clamping means 19 contacts the sphere 1 at its spherical outer surface merely by abutment without any substance bond.

Reference signs:

[0059] 

1

hollow sphere

2

opening

3

substance

4

tube

5

stem

6

first axial section

7

second axial section

8

connector

101

calotte

102

longitudinal axis of the stem

103

end of the stem

105

portion of the stem comprised of a non-transparent material

106

point on the outer surface of the hollow sphere

107

plane of the circumference of the calotte

108, 109

lines connecting the center of the hollow sphere with the circumference of the calotte

110

line connecting the center of the hollow sphere with the point 106

9

glass support tube

16

tool

17

special tool

18

receptacle

19

clamping device

a,

amax angles

r

outer radius of the hollow sphere

M

center of the hollow sphere

R

axis connecting the center of the hollow sphere and the opening of the hollow sphere

Claims

1. A plasma light bulb, comprising

a) a hollow sphere (1) of a translucent material,

b) a substance (3) within the hollow sphere (1), wherein the substance is electromagnetically excitable so as to form a plasma, which radiates visible light, and

c) a stem (5) protruding from the hollow sphere,

wherein

d) the hollow sphere (1) is tip- or tail-less, and

e) the stem (5) is bonded to the hollow sphere (1).

2. The plasma light bulb according to the preceding claim, wherein the stem (5) is fused to the hollow sphere (1).

3. The plasma light bulb according to any one of the preceding claims, wherein the stem (5) consists of a ceramic over at least an axial section (7).

4. The plasma light bulb according to any one of the preceding claims, wherein the stem (5) comprises a connector (8) for connecting the stem (5), preferably by a form or force fit, or by screwing, to a support, preferably to a drive for moving the hollow sphere (1) during operation.

5. The plasma light bulb according to any one of the preceding claims, wherein the stem (5) constitutes a drive shaft with a rotational axis (R) for rotationally driving the hollow sphere (1) and is rotationally asymmetrical with respect to this axis (R) for cooling the hollow sphere (1) by vortexing a surrounding fluid.

6. The plasma light bulb according to claim 5, wherein the stem (5) deviates from a circle in a cross-section and fulfills at least one of the following features:

-- it is straight over the axial length of this deviating cross-section;

-- the deviating cross-section is constant over at least a predominant part of the axial length of the stem (5);

-- the stem (5), in deviating from the circle, comprises at least one flat outer surface;

-- the deviating cross-section extends over a predominant part of the axial length of the stem (5), preferably up to and including the connector (8) according to claim 4.

7. The plasma light bulb according to any one of the preceding claims, wherein for each point (106) on an outer surface of said hollow sphere (1) for which an angle (a) between a longitudinal axis (102) of said stem (5) and a line (110) connecting a center (M) of said hollow sphere (1) and said point (106) is less than 5°, said point (106) being located on a calotte (101) of said hollow sphere (1) opposite said stem (5), an absolute value of a difference of a distance (r') of said point (106) from the center of the hollow sphere (1) and the outer radius (r) of the hollow sphere (1) is less than3 mm.

8. The plasma light source according to any one of the preceding claims, wherein for each point (106) on an outer surface of said hollow sphere (1) for which an angle (a) between an axis (102) of said stem (5) and a line (110) connecting a center (M) of said hollow sphere (1) and said point (106) is less than 5°, said point (106) being located on a calotte (101) of said hollow sphere (1) opposite said stem (5), an absolute value of a difference of a distance of the point (106) from the center (M) of the hollow sphere (1) and the outer radius (r) of the hollow sphere (1) is less than 20 percent of the outer radius (r) of said hollow sphere (1).

9. Intermediate product for manufacturing a plasma light bulb, comprising:

a hollow sphere (1) of a translucent material, the hollow sphere (1) having an opening (2); and

a tube (4) fixed to said hollow sphere (1), wherein the tube (4) and the opening (2) form a passage into said hollow sphere (1).

10. Intermediate product according to claim 9, wherein for each point (106) on an outer surface of said hollow sphere (1) for which an angle (a) between a central longitudinal axis of an inner axial passage of the tube (4) and a line (110) connecting a center (M) of said hollow sphere (1) and said point (106) is less than 5°, said point (106) being located on a calotte (110) of said hollow sphere (1) opposite said opening (102), an absolute value of a difference of a distance (r') of the point (106) from the center (M) of the hollow sphere (1) and the radius (r) of the hollow sphere (1) is less than 3 mm.

11. Intermediate product according to any of claims 9 and 10, wherein for each point on an outer surface of said hollow sphere (1) for which an angle (a) between a central longitudinal axis of an inner axial passage of the tube (4) and a line (110) connecting a center (M) of said hollow sphere (1) and said point (106) is less than 5°, said point (106) being located on a calotte (110) of said hollow sphere (1) opposite said opening (2), an absolute value of a difference between a distance of the point (106) from the center (M) of the hollow sphere (1) and the radius (r) of the hollow sphere (1) is less than 20 percent of the radius (r) of the hollow sphere (1).

12. A method for making a plasma light bulb, preferably the plasma light bulb according to any one of the preceding claims, wherein

a) a substance (3) is filled into a hollow sphere (1) having a single opening (2), said hollow sphere (1) being made of a translucent material, wherein the substance (3) is filled into the hollow sphere through the opening (2) and wherein the substance (3) is electromagnetically excitable, so as to form a plasma, which radiates visible light,

b) a tube (4) is positioned at the opening (2) and jointed to the hollow sphere (1), so as to surround the opening in a gas-proof substance joint,

c) the hollow sphere (1) is filled with an inert gas through the tube (4) and the opening (2),

d) the tube (4) is sealed at the opening (2), preferably by fusing, so as to make the hollow sphere (1) gas-proof; and

e) a stem (5) protruding from the hollow sphere (1) is formed, wherein the stem (5) is connected to the hollow sphere (1) at the location of the sealed opening (2).

13. The method according to claim 12, wherein the hollow sphere (1), which is made of glass, is blown to form the spherical surface universally flawless, except for the opening (2), the tube (4) preferably also be made of glass and preferably being jointed with the sphere (1) by fusing.

14. The method according to any one of claims 12 and 13, wherein the hollow sphere (11) is shaped from molten glass (10) in a spherical cavity (13) confined by a spherical internal surface (11a, 12a) of a mold (11, 12) by feeding a pressurized fluid into the cavity (13), preferably into the molten glass (10), and blowing the molten glass (10) against the internal surface (11a, 12a).

15. The method according to the claim 14, wherein the internal surface (11a, 12a) is flawlessly spherical except for a single port (15) through which the fluid is introduced into the cavity (13), and the molten glass (10) is blown to wet the spherical surface (11a, 12a) throughout up to and surrounding the port (15).

Drawing