The invention relates to a compact low-pressure mercury vapor discharge lamp for illumination of a display device. The invention also relates to a display device comprising at least one low- pressure mercury vapor discharge lamp. In mercury vapor discharge lamps, mercury constitutes the primary component for the (efficient) generation of ultraviolet (UV) light. A luminescent layer comprising a luminescent material may be present on an inner wall of the discharge vessel to convert UV to other wavelengths, for example, to UV-B and UV-A for tanning purposes (sun panel lamps) or to visible radiation for general illumination purposes or for illumination display devices. Such discharge lamps are therefore also referred to as fluorescent lamps. The discharge vessel of a low-pressure mercury vapor discharge lamp is usually circular and comprises both elongate and compact embodiments. Generally, the tubular discharge vessel of compact fluorescent lamps comprises a collection of relatively short straight parts having a relatively small diameter, the straight parts being connected together by means of so-called bridge parts or via so-called bent parts. Generally, means for maintaining a discharge in the discharge space are electrodes arranged in the discharge space.

Patent US-A 0 589,467 discloses a low-pressure mercury vapor discharge lamp in which the lead-in wires or the stems of the electrodes are sealed in radial direction directly to the longitudinal wall of the envelope without the enclosure of side tube elements protruding from the envelope permitting the end spaces of the envelope to participate in the radiation conversion. A drawback of the known low-pressure mercury vapor discharge lamp is that the light emitted by the known low-pressure mercury vapor discharge lamp is not sufficiently uniform to illuminate a display device. The invention has for its object to eliminate the above disadvantage wholly or partly. According to the invention, a compact low-pressure mercury vapor discharge lamp for illumination of a display device comprising: a dual-shaped lamp part comprising a first tubular portion, a second tubular portion and a third tubular portion, the second and third tubular portion substantially being in line with each other and the second and third tubular portion being provided substantially alongside the first tubular portion, a first end portion of the first tubular portion and a first end portion of the second tubular portion being interconnected via a first tubular interconnection means, a second end portion of the first tubular portion and a first end portion of the third tubular portion being interconnected via a second tubular interconnection means, a second end portion of the second tubular portion and a second end portion of the third tubular portion facing each other, a discharge path being formed through the tubular portions between a first and a second electrode, the first electrode being provided in a first electrode portion of the second tubular portion, said first electrode portion being provided in the vicinity of the second end portion of the second tubular portion, said first electrode portion extending in radial direction to the third tubular portion, the second electrode being provided in a second electrode portion of the third tubular portion, said second electrode portion being provided in the vicinity of the second end portion of the third tubular portion, said second electrode portion extending in radial direction to the second tubular portion. The compact low-pressure mercury vapor discharge lamp according to the invention provides a more homogenous and a more uniform illumination of the display device. A very compact design is achieved by providing a dual-shaped lamp part with a first tubular portion and a second and third tubular portion, the second and third tubular portion lying substantially alongside the first tubular portion, the second and third tubular portion being substantially in line with each other and end portions of the second and third tubular portion facing each other. The uniformity of the light emitted by the compact low-pressure mercury vapor discharge lamp according to the invention is further improved by providing the first electrode (in the first electrode portion of the second tubular portion) in the vicinity of the second end portion of the second tubular portion and by providing the second electrode (in the second electrode portion of the third tubular portion) in the vicinity of the second end portion of the third tubular portion, wherein the first and the second electrode portion extend in radial direction to the second and third tubular portion, respectively. Another advantage is that the length of the compact low-pressure mercury vapor discharge lamp according to the invention is approximately twice as long as compared to corresponding straight tubular fluorescent lamps, and give twice as much light as a straight tubular fluorescent lamps. If the same number of compact low-pressure mercury vapor discharge lamp is applied as the number of straight fluorescent discharge lamps, a display device with an improved luminance is obtained. A preferred embodiment of the low-pressure mercury vapor discharge lamp according to the invention is characterized in that the first and second electrode portion foπn an integral part of the second and third tubular portion, respectively, and in that the first and second electrode substantially extend in radial direction to the second and third tubular portion, respectively. In this manner the compactness of the low-pressure mercury vapor discharge lamp is further improved. An alternative, preferred embodiment of the low-pressure mercury vapor discharge lamp according to the invention is characterized in that the first and second electrode portion comprises a first and a second electrode tubular portion provided with the first and second electrode, respectively, and in that the first and second electrode tubular portions extend in radial direction to the second and third tubular portion, respectively. This construction simplifies the mounting of the first and second electrode. Standard tubular portions provided with an electrode mounted in the tubular portion in the standard manner can be used in this alternative embodiment. A further alternative, preferred embodiment of the low-pressure mercury vapor discharge lamp according to the invention is characterized in that the first and second electrode portion comprises a first and a second electrode tubular portion provided with the first and second electrode, respectively, in that the electrode tubular portions extend substantially parallel to the second and third tubular portion, respectively, in that the first and second electrode tubular portions are connected to the second and third tubular portion via first and second bridge portions, respectively, and in that the first and second bridge portions extend in radial direction to the second and third tubular portion, respectively. This construction further simplifies the mounting of the first and second electrode. Standard tubular portions provided with an electrode mounted in the tubular portion in the standard manner can be used in this alternative embodiment. In addition, standard bridge portions can be applied. In a favorable embodiment of the low-pressure mercury vapor discharge lamp according to the invention the distance between the second end portion of the second tubular portion and the second end portion of the third tubular portion facing each other is of the order of less than 5% of the length of the first tubular portion. In this manner the second end portion of the second tubular portion and the second end portion of the third tubular portion are practically in a kind of abutting relationship with only a relatively small space (air gap) between the end portions. In general there are two embodiments of the low-pressure mercury vapor discharge lamp. In one embodiment of the low-pressure mercury vapor discharge lamp the tube interconnection means comprises a so-called bridge portion. In another embodiment of the low- pressure mercury vapor discharge lamp the tube interconnection means comprises a bent portion.

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. In the drawings: Figure 1 shows a cross-sectional view of a first embodiment of the compact low-pressure mercury- vapor discharge lamp in accordance with the invention; Figure 2 shows a cross-sectional view of a second embodiment of the compact low-pressure mercury vapor discharge lamp in accordance with the invention; Figure 3 shows a cross-sectional view of a third embodiment of the compact low-pressure mercury vapor discharge lamp in accordance with the invention, and Figure 4 shows a display device comprising two compact low-pressure mercury vapor discharge lamps according to the invention. The Figures are purely diagrammatic and not drawn to scale. Notably, some dimensions are shown in a strongly exaggerated form for the sake of clarity. Similar components in the Figures are denoted as much as possible by the same reference numerals.

Figure 1 schematically shows a cross-sectional view of a first embodiment of the compact low-pressure mercury-vapor discharge lamp in accordance with the invention. The compact low-pressure mercury vapor discharge lamp for illumination of a display device comprises a dual-shaped lamp part. The dual-shaped lamp part comprises a first tubular portion 11, a second tubular portion 12 and a third tubular portion 13. The second and third tubular portion 12, 13 are arranged substantially in line with each other. In addition, the second and third tubular portion 12, 13 are provided substantially alongside the first tubular portion 11. A first end portion l la of the first tubular portion 11 and a first end portion 12a of the second tubular portion 12 are interconnected via a first tubular interconnection means 14. In addition, a second end portion 1 Ib of the first tubular portion 11 and a first end portion 13a of the third tubular portion 13 are interconnected via a second tubular interconnection means 15. Inthe embodiment as shown in Figure 1 the first and second tubular interconnection means 14, 15 comprise so-called bridge portions. In an alternative embodiment the tube interconnection means comprise so-called bent portions. Preferably, the first and second tubular interconnection means 14, 15 are provided as close as possible to the first end portion l la of the first tubular, portion 11 and a first end portion 12a of the second tubular portion 12. In order to enhance the homogeneity and the uniformity of the light emitted by the compact low-pressure mercury vapor discharge lamp, a second end portion 12b of the second tubular portion 12 and a second end portion 13b of the third tubular portion substantially face each other. In this manner the second end portion 12b of the second tubular portion 12 and the second end portion 13b of the third tubular portion 13 are practically in a kind of abutting relationship leaving only a relatively small space (air gap) between the end portions. Preferably, the distance between the second end portion 12b of the second tubular portion 12 and the second end portion 13b of the third tubular portion 13 facing each other is of the order of less than 5% of the length of the first tubular portion 11. Preferably, the gap between both second end portions 12b, 13b is less than 2 mm. In a favorable embodiment the gap is approximately 0.5 mm. In Figure 1 a discharge path between a first 22 and a second 23 electrode is formed from the second tubular portion 12, via the first tubular portion 11 to the third tubular portion 13. The first electrode 22 is provided in a first electrode portion of the second tubular portion 12, said first electrode portion being provided in the vicinity of the second end portion 12b of the second tubular portion 12, said first electrode portion extending in radial direction to the second tubular portion 12. In a similar manner, the second electrode 23 is provided in a second electrode portion of the third tubular portion 13, said second electrode portion being provided in the vicinity of the second end portion 13b of the third tubular portion 13, said second electrode portion extending in radial direction to the third tubular portion 13. The first and second electrode 22, 23 can be oriented in various ways in the respective end-portion. In an advantageous embodiment the electrode body is arranged radially in the tube. In the example of Figure 1 the first and second electrode portion form an integral part of the second and third tubular portion 12, 13, respectively. In addition, the first and second electrode 22, 23 substantially are arranged in a direction radial to the second 12 and third tubular portion 13, respectively. The compact low-pressure mercury vapor discharge lamp as shown in Figure 1 provides a more homogenous and a more uniform illumination of the display device. Figure 2 schematically shows a cross-sectional view of a second embodiment of the compact low-pressure mercury vapor discharge lamp in accordance with the invention. In Figure 2, the first electrode portion comprises a first electrode tubular portion 32 provided with the first electrode 22. The first electrode tubular portion 32 extends in radial direction to the second tubular portion 12. In addition, the second electrode portion comprises a second electrode tubular portion 33 provided with the second electrode 23. The second electrode tubular portion 33 extends in radial direction to the third tubular portion 13. Figure 3 schematically shows a cross-sectional view of a third embodiment of the compact low-pressure mercury vapor discharge lamp in accordance with the invention. In Figure 3, the first electrode portion comprises a first electrode tubular portion 42 provided with the first electrode 22. The first electrode tubular portion 42 extends substantially parallel to the second tubular portion 12. The first electrode tubular portion 42 is connected to the second tubular portion 12 via a first bridge portion 44. The first bridge portion 44 extends in radial direction to the second tubular portion 12. In addition, the second electrode portion in Figure 3 comprises a second electrode tubular portion 43 provided with the second electrode 23. The second electrode tubular portion 43 extends substantially parallel to the third tubular portion 13. The second electrode tubular portion 43 is connected to the third tubular portion 13 via a second bridge portion 45. The second bridge portion 45 extends in radial direction to the third tubular portion 13. Figure 4 very schematically shows a display device comprising two compact low-pressure mercury vapor discharge lamps according to the invention. The display device 1 is for instance a flat picture display device, for instance a liquid crystal display device or a plasma-activated display device. Very schematically, two compact low-pressure mercury vapor discharge lamps 2; 2' are arranged adjacent the display device 1 forming a so-called (direct) backlight illumination system. Each of the compact low-pressure mercury vapor discharge lamps 2, 2' for illumination the display device 1 comprises a dual-shaped lamp part. Each of the dual-shaped lamp parts comprises a first tubular portion 11 ; 11', a second tubular portion 12; 12' and a third tubular portion 13; 13'. The second and third tubular portion 12, 13; 12', 13' are arranged substantially in line with each other. In addition, the second and third tubular portion 12, 13; 12', 13' are provided substantially alongside the first tubular portion 11, 11' . By arranging the compact low-pressure mercury vapor discharge lamps 2; 2' in the manner as shown in Figure 4, the display device 1 is illuminated by a light source with a relatively high homogeneity and a relatively high uniformity. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.