The present invention relates to a compact fluorescent tube of the kind which comprises one or more U-shaped main parts forming a closed discharge chamber having electrodes located at each end of the chamber and which is coated internally with a fluorescent substances, each of said main parts comprising two straight tubular legs of circular cross-section and a connecting tube which connects the tubular legs at one end thereof. The inven¬ tion also relates to a method of producing such tubes or lamps, comprising heating and bending straight glass tubes into a U-shaped configuration to form said main parts, and blow moulding the connection between the legs of each said main part.

Compact fluorescent lamps of this kind have been known to the art for several years. A conventional embodiment of such lamps is illustrated and described in UK patent 2050046. In the case of this lamp, the tube connecting legs of the main tube parts consists of an additional tube of small diameter which is produced by local melt fusion of the outer peripheral surfaces of the legs. The relatively simple construction of the individual straight tubular parts of the lamp enables the lamp to be manufactured quickly and in a simple fashion in rational production plants. The pronounced restriction in the gas discharge path, however, has a negative effect on light yield and also increases the ignition voltage, thereby influencing, among other things,, the willingness of the lamp to ignite at low temperatures. Another drawback with this known method of manufacture is that the glass mass is distributed unevenly at the junction between the tubular parts, therewith creating a

risk of tension build-up and fracturel at this location. Furthermore, the narrow connecting tube is also liable to crack, both during manufacture and when in use, and not least when fitting and removing lamps to and from 5. their respective fittings.

In the case of another known kind of compact fluorescent lamp, the fluorescent tubes have a U-bend basic con¬ figuration and the connecting part between the legs of 0 the U-shaped lamp is re-formed in a manner to achieve the best possible conditions for good light yield and good ignition characteristics. One example of this kind of lamp is described and illustrated in USA-patent 4481442, according to which the actual U-bend is blow 5 moulded in a manner to form extended and pointed corner parts. These corner parts are intended to maintain a desired low temperature in use, so as to enable the mercury vapour to condense. The cooling zones in the corners, however, are often not sufficiently effective 0 in comparison with the tower-like extensions formed in the first mentioned type of lamp comprising a separate intermediate or connecting tube. Consequently, the aforesaid lamp types are roughly equivalent with respect to their properties in operation. The blow moulded 5 tubes, however, are more difficult to manufacture in a rational manner. Because of their fragility, the tubes are also difficult to handle, both manually and by mechanical means, due to the excessive thinness of the glass at the extended corner parts of the tubes. 0

Consequently, one object of the present invention is to provide a compact fluorescent lamp or tube whose opera¬ ting characteristics are an improvement on the operating characteristics of the earlier known lamps or tubes and 5 which have a greater mechanical strength and toughness

in manufacture, transportation and installation. Another object of invention is to provide a method for the manufacture of blow moulded compact fluorescent or tubes which can be carried out more rationally than earlier known methods and which provides stronger lamps or tubes having a more uniform glass thickness. Further objects of the invention and advantages afforded thereby will be apparent from the following description and from the characterising clauses of the following claims.

The invention is based on the realization that the provision of good cooling zones must be effected without the aid of separate constrictions, and that the cooling zones may not be configured in a manner such that the lamp or tube will not be susceptable to breakage when handled in manufacture and in use. Accordingly, the legs of the inventive tube maintain their tubular basic shape along their whole lengths, up to the top or connecting region of the main part. This top is configured to present a completely flat end surface, such as to form at least four marked cooling zones at the juncture between the cylindrical peripheral surface and the flat end surface. The connecting tube extending between the legs has a rectangular cross-section, with one side of the tube coinciding with said flat end surface. This creates conditions for a durable and readily handled lamp or tube.

A suitable method of manufacture is one which will permit sufficient control of glass thickness, stresses and strains, cracking, etc., such as to provide lamps or tubes of high and uniform quality. This is achieved by heating and bending appropriate glass tubes around a first mould part to produce a U-shaped basic form, and by forming the outer peripheral surface of the

glass tube together with the connecting part on a second, complementary mould part. The two forming parts are brought together so that the second dish-shaped part surrounds the connecting part. Molten glass can there- 5. with be readily taken up in this forming part, such that sufficient glass will be available for the subsequent blow moulding process.

The invention will now be described in more detail with 0 reference to the accompanying drawings, in which

Fig. 1 is a side view, partly in section, of a compact fluorescent tube constructed in accordance with the invention;

Fig. 2 is a longitudinal sectional view taken on the 5 line 2-2 in Fig. 1;

Fig. 3 is a perspective view of the upper part of the fluorescent tube illustrated in Fig. 1;

Figs. 4-6 are schematic sideviews, partly in. section, of glass tubes and mould parts and are illustrative of the 0 manufacturing process; and

Fig. 7 is a top view of a closed mould»

The compact fluorescent lamp illustrated in Fig. 1 comprises solely one U-shaped main tube part 11, which 5 is mounted on a base 12 in a conventional manner and provided with cathodes 13. The main part 11 has two straight legs 14 of circular cross-section and a con¬ necting tube 15 of rectangular cross-section. The tube legs 14 and the connecting tube 15 form a coherent gas 0 discharge chamber and are coated on their respective inner surfaces with a fluorescent substance for conver¬ sion of the generated UV-radiation to visible light. The connecting tube 15 is connected to the circular periphe¬ ral surface of respective legs at their outer free ends. 5 The surfaces 16 are completely flat and extend perpen-

dicularly to the longitudinal axis of the legs. Conse¬ quently, these end surfaces and one side 17 of the connecting tube will together form a flat coherent end surface or top 18 on the fluorescent tube. The connect- 5 _. ing tube will normally have a length of only some few millimeters and the underside thereof can therefore be configured suitably with an appropriate bend 20 towards the inner peripheral surface of the legs 14. The free cross-sectional area in the connecting tube will prefer- 0 ably be equal to or somewhat larger than the free cross- sectional area of the tubular legs, such as to ensure that the discharge current between the cathodes will not be obstructed by any form of constriction. In other respects, the connecting tube will be configured so that its width b is markedly smaller than the diameter a of the tubes, and the vertical extension c will, accord¬ ingly exceed said diameter. Suitable relationships in this respect are b=0.45-0.65 x a and c=1.2-l..θ x a.

A fluorescent tube having the aforesaid geometrical configuration will have effective cooling zones located in the upper circle-segment shaped part 21 formed be¬ tween an imaginary extension of the connecting tube and the tubular legs 14. The discharge current will thus flow essentially in the direction indicated by the connecting tube, and consequently the corners of said circle-segment 21 will be subjected to a lower surface load and will therewith function as cooling zones. Not fewer than four cooling zones are created in this manner, which ensures that the lamp will function in mutually different, external operating conditions _, , e.g. with respect to its position in the lamp fitting, temperature. Furthermore, the aforesaid geometric con¬ figuration provides a lamp in which all glass surfaces are active with respect to illumination. Thus, both the

connecting tube 15 and the end surface 18 are coated with the same fluorescent substance as the lamp in general, and since the discharge current passes close to these surfaces the fluorescent substance coated thereon will be activated in a conventional manner. This is achieved, inter alia, by restricting the vertical exten¬ sion c of the connecting tube 15 to a maximum of 1.6 times the diameter a, such that the discharge current will be forced up into contact with the end surface 18.

In addition to the advantages afforded from a purely illuminating technical aspect, the illustrated basic configuration of two tubes of circular cross section connected to a tube of rectangular cross-section also affords advantages from the aspect of mechanical strength and manufacture. The circular-cylindrical configuration thus provides a mechanically strong and readily gripable lamp top part, in comparison with the previously known more refined U-bends. Furthermore, the tube of rectangular cross-section provides a more dur¬ able connection than earlier known mutually fused tube parts. The lamp also has no sharply jutting corner parts which are liable to be damaged when handling the lamp or in operation, and there is very little risk in the case of the chosen configuration for given glass portions to be weakened as a result of thin glass or unfavourable stress distribution. The glass thickness will preferably not be less than 0.6 mm in any region of the finished lamp.

A preferred method of manufacture of the afore-described and similar compact fluorescent tubes or lamps will now be described with reference to Figs. 4-7. As illustrated in Fig. _r manufacture is commenced, in a conventional manner, with straight glass tubes 24 cut into appro-

priate lengths. These tubes are heated in their centre regions with the aid of a burner 25, whereafter the tube is compressed through some millimeters in the direction 5. of the arrows F, such as to thicken the glass in the heated zone. Heating is continued until the glass reaches its melting point and begins to run against the undersurface of the tube. The tube is then bent around a first mould part 26, which has formed therein a 0 channel-like abutment surface 27 adapted to the shape of the tube. Thus, in the illustrated embodiment, this surface has a semi-circular cross section in the longitudinal part 29, whereas the end part 30 is con¬ figured to impart an appropriate appearance to the connecting part 15. This first mould part 26 thus forms the inner peripheral surface of the glass tube located between the legs, i.e. in sustantially half the glass tube. The external peripheral surface of the tube, remote from the U-bend, is formed with the aid of a second mould part 31. This mould part is also provided with longitudinally extending, semi-circular channel¬ like abutment surfaces 32, and includes a dished bottom part 33 for shaping the outer parts of the connecting tube 15. The mould parts are positioned vertically, so that molten glass will be concentrated in the bend 34, and said mould parts are intended to be moved towards one another in the direction of the arrows A, into sealing abutment with one another. The glass tube is now fixated in the mould and the end parts of the tubes and of the connecting part can be finally shaped by blow moulding, which is effected in a conventional manner by introducing air under pressure into the glass tube, such as to press the soft tube walls out against the walls of the mould. In order to facilitate removal of the moulded glass tube from the mould, the first mould part 26 is

divided into two first sections 36, 37 and the second mould part 31 is also divided into two sections 38, 39. The mould parts can be divided along respective lines 40 in the directions of the arrows B. The gas discharge 5. tube can then be further processed, in a conventional manner, to provide a finished compact fluorescent tube in accordance with Fig. 1.

The afore-described method of manufacture will ensure

10 that sufficient glass mass is distributed to all parts of the tube, without risk of other parts becoming exces¬ sively thick or without the occurrence of troublesome, thick moulding joins. This is achieved by the fact that the molten glass mask 34 can collect in the dish-shaped

Tff lower mould part 31. Consequently, a large quantity of glass can be permitted to exist in a liquid state in comparison with prior known methods in which the mould parts are brought together in the plane of the U-bend. Thus, when practicing the inventive method, it is always

20 ensured that the end part 18 will obtain a sufficient glass thickness. The glass mass available for melting can also be adjusted readily by the introductory axial compression of the tube. The moulding joins are placed in the juncture region between the inner and the outer

.25. peripheral surfaces of the U-bent tube, and consequently the join will be located in a part of the tube which is much, less sensitive than the pointed or corner- region of the tube. The method is particularly suitable for the manufacture of compact fluorescent tubes having _^ flat

30 tops 18, since the glass mass can be distributed over a large flat bottom surface, without risk of being con¬ centrated in given recesses or indentations. The method is not restricted to this embodiment thereof, but can be applied generally with other types of compact fluore-

35 scent tubes having an external diameter of between 9-15 mm.