EP0000842B1

EP0000842B1 - Low pressure metal vapour discharge lamp

Info

Publication number: EP0000842B1
Application number: EP78300262A
Authority: EP
Inventors: Yoshio Watanabe; Mikiya Yamane; Teruichi Tomura
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1977-08-10
Filing date: 1978-08-07
Publication date: 1982-12-15
Also published as: US4335331A; US4438373A; DE2862125D1; EP0037589B1; EP0000842A1; EP0037589A3; EP0037589A2

Description

The present invention relates to low pressure metal vapour discharge lamps and, more particularly, to such a lamp having an outer glass envelope defining a closed discharging space, an inner glass tube disposed within said envelope and having an open end and a closed end, a cathode disposed within said tube and a plurality of anodes disposed within said envelope outside said tube.
The conventional low pressure metal vapour discharge lamp, such as the common fluorescent strip lamp (which is a low pressure mercury vapour discharge lamp), has an elongate glass tube provided at both ends with electrodes and containing a rare gas at a few Torr pressure and a small amount of a metal such as mercury. This type of lamp has a considerable length for a given lighting power. For instance, a straight tube fluorescent lamp of this type may be 120 cm long for an electric input of 40W.
The size of this double-base type of fluorescent lamp (i.e. a lamp having two ends with electrodes) makes it inconvenient or unsuitable for some uses. There is thus a demand for a smaller fluorescent lamp having a reduced tube-length, while still providing the required brightness.
In view of this demand, Japanese Patent Publication No. 35796/1974 discloses a novel fluorescent lamp having only one base. This lamp has a double-tube structure consisting of a fully closed outer glass bulb and an inner glass tube within the outer glass bulb, the inner tube being open at its one end but closed at its other end. A single cathode is disposed within the inner tube, while a single anode is disposed outside the inner tube. With this arrangement, the discharge path formed between the two electrodes bends at the open end of the inner tube, so that a sufficient length of discharge path can be obtained with a relatively short lamp. Furthermore, the luminous efficiency is improved, because the surface area of the glass surrounding the discharging space, to which phosphor material paint is applied, is increased.
This known double-tube type discharge lamp, however, has the disadvantage that it is difficult to distribute the discharge plasma uniformly over the entire discharging space between the inner tube and the outer bulb. Instead, the discharge plasma outside the inner tube is concentrated in the region which exhibits the smallest resistance to the discharge current, and is not spread uniformly over the entire discharge space. This local concentration of the discharge plasma cannot be avoided even by use of a ring-shaped anode disposed around the inner tube. In such a lamp, the luminous intensity is high only at the region where the plasma is locally concentrated, while only a low luminous intensity is obtained at regions of the lamp where the plasma is not distributed. Thus, it is difficult to obtain uniform luminous intensity distribution over the entire lamp body.
In addition, in this double-tube type lamp, the discharge plasma, which is locally concentrated in a portion of the discharging space, changes its position irregularly, causing flickering.
In order to overcome this problem of local concentration of the plasma in the double-tube type fluorescent lamp, U.S.A. Patents Nos. 3,609,436, and 3,849,689 of Campbell and an article by the same Campbell in the Journal of the Illuminating Engineering Society (Vol. 2, No. 2, October 1972, pages 3 to 7) have proposed an improved lamp in which a plurality of anodes are disposed around the inner glass tube. These anodes are described in USA Patent No. 3,609,436 as being operated by sequential switching. Sequential switching controlledly rotates the locally concentrated plasma at a high speed around the inner glass tube, thereby to achieve on average with time a uniform luminous intensity over the entire lamp. This switching however requires a relatively complicated and expensive transistor switching circuit for high-speed switching of the voltage around the anodes and is therefore not practical either technically or economically.
Campbell appears to concentrate on this sequential switching arrangement as being the only practical one, but further states that a multi-anode lamp is "capable of operation on ac at any frequency or on dc continuous, pulsing or on sequential switching". No' circuit for operation in any manner except sequential switching is given, but in the IES article he describes how individual ballasting of each arc to provide equal current is impractical for a finished product but was achieved in experimental models.
It should be mentioned also that the IES article discloses the use of a fluted inner tube, to avoid the merging of adjacent arcs by confining the ionized paths.
It is therefore the main object of the present invention to provide a practical and inexpensive double-tube type low pressure metal vapour discharge lamp in which local concentration of the discharge is avoided. The aim is to achieve a highly uniform light output distribution over the whole of the lamp, without a complicated and expensive switching circuit such as is proposed in the lamp last described above.
This problem is solved according to the invention by the construction set out in the claims. The invention is based on the discovery that stable multi-channel arcs can exist if the circuit is such as to allow an anode oscillation current to flow between the anodes. The anode oscillation at each anode is therefore influenced by that at the other anode or anodes (e.g. if there are two anodes, they oscillate alternatingly). Overall current is limited by the ballast means. No external switching is required, so that a simple and economic circuit is feasible.
In order to eliminate the possibility of unstable location of the plasmas, thereby to achieve a stable non-flickering light output, means are preferably provided for fixing the locations of the paths of the discharge currents corresponding to the respective plasma lines, so as to prevent fluctuation of the positions of the plasma lines. Any construction which exclusively defines the paths for the respective discharge currents can be used as the means for fixing the locations of the discharge current paths. A number of such constructions are described below. The effect of such means is to define, for each of the plasma lines, a stable current path through which the discharge current can flow, the path specifically having a lower resistance than adjacent regions.
This arrangement affords a substantially uniform luminous intensity distribution over the whole part of the lamp, because of the presence of a plurality of plasma lines corresponding to respective anodes and disposed around the inner glass tube. In addition, flickering of the output light is avoided because irregularfluctua- tion of the discharge plasmas does not occur.
Preferred embodiments of the invention will now be described by way of example with reference to the accompanying drawings.
In the drawings:

Fig. 1 is a schematic longitudinal sectional view of a first low pressure metal vapour discharge lamp embodying the present invention, showing its internal structure and its ignition circuit;
Figs. 2 and 3 are longitudinal sectional views of end portions of two different inner glass tubes which may alternatively be incorporated in the lamp of Fig. 1, showing different forms of notches formed in the tube end;
Figs. 4A and 4B illustrate the operation of the lamp of Fig. 1;
Figs. 5A and 5B are respectively a plan view and a sectional view of a modified form of the lamp of Fig. 1, showing specifically a partition plate at the open end of the glass inner tube;
Fig. 6 is a sectional view of part of yet another modification of the lamp of Fig. 1;
Figs. 7A, 7B and 7C are schematic views of another low pressure metal vapour discharge lamp embodying the invention, being respectively a transverse section, a first longitudinal section and a second longitudinal section on a plane orthogonal to that of the first;
Figs. 8A and 8B are a plan view and a longitudinal sectional view of a part of another lamp of the invention, specifically showing a partition plate provided on the outer surface of the inner glass tube;
Figs. 9A, 9B and 9C are schematic views respectively corresponding to the views of Figs. 7A, 7B and 7C of yet another lamp embodying the invention;
Figs. 1 OA, 1 OB and 1 OC are schematic views, also corresponding to the views of Figs. 7A, 7B and 7C respectively of a further lamp embodying the invention;
Figs. 1-1, 12 and 13 are transverse-sectional views respectively of three more forms of the lamp of the invention.

Referring first to Fig. 1, the lamp shown therein has an elongated bulbous outer glass bulb 1, a cylindrical tubular inner glass tube 2 and a stem 3. The bulb 1 is fusion-welded at its lower end to the stem 3, so as to constitute a completely closed discharge vessel, while the tube 2 is bonded or fusion-welded at its lower end to the stem 3, and is thereby held at the center of the space within the outer bulb 1 and coaxial therewith. The tube 2 is open at its upper end but is closed at its lower end by the stem 3.
A sole cathode 4 (a filament electrode coated with an electron-emitting substance) is disposed within the inner tube 2 near its lower end. A plurality of anodes each in the form of a separate rod-shaped electrode are disposed in the lower part of the discharging space defined by the outer surfaces of the tube 2 and the inner surface of the bulb 1, in the region of the discharging space close to the closed end of the tube 2.
In this specific embodiment, two anodes 5, 5' are disposed diametrically opposite each other, i.e. at positions symmetrical with respect to the axis of the tube 2. The inner peripheral surface of the bulb 1 is coated with a film (not shown) of a fluorescent material. The rim 2' of the tube 2 is notched at portions 6, 6' respectively axially aligned with the anodes 5, 5'.
In the manufacture of the lamp, the space inside the discharge vessel 1 is evacuated via an . exhaust tube (not shown) provided in the stem 3, and is then charged with a rare gas at low pressure (e.g. argon gas of a few Torr) and a small amount of metal such as mercury. Finally, this space is sealed at the end of the exhaust tube.
A single base (not shown) is attached to the lower end portion, i.e. to the stem portion, of this double-tube type discharge lamp, which is thus a single-base type lamp.
In use, the lamp is connected to an A.C. source 13 through an ignition circuit including a ballast 7, a diode bridge rectifier circuit 8 and a glow lamp 9.
The two anodes 5, 5' are in this embodiment directly connected to each other without impedance between them. This ignition circuit may be produced separately from the lamp body, or may be incorporated in the base attached to the lamp.
When the lamp is connected to the A.C. source 13 through the ignition circuit, the A.C. voltage is rectified by the diode bridge rectifier circuit 8 the output from which ignites the glow lamp 9. Since the contact is made in the glow lamp 9, preheating current flows through the cathode 4. When the contact is broken in the glow lamp 9 after the cathode 4 has been sufficiently heated, a high voltage pulse is applied between the anodes 5, 5' and the cathode 4, at the instant at which the contact is broken, so as to cause electric discharges between the cathode 4 and respective anodes 5, 5', bringing the lamp into operation.
The inner tube 2 is disposed coaxially at the center of the outer bulb 1, and the two anodes 5, 5' are symmetrical with respect to the axis of the inner glass tube 2, being mounted similarly. The voltage levels applied to both anodes by the rectifier circuit 8 through the common single ballast in the ignited condition are equal. In this condition, anode oscillations take place alternatingly at the two anodes.
Two paths for the discharge current are formed: one being between the anode 5 and the cathode 4 via the notch 6 in the rim 2' of the tube 2, while the other is between the anode 5' and the cathode 4 via the other axial notch 6'. These two discharge currents each include the anode oscillation current which oscillates at high frequency, due to the anode oscillation taking place alternatingly at the anodes 5, 5'. The anode oscillation current is able to pass between the anodes because they are directly connected together. Fig. 4B described below illustrates this anode oscillation current.
Fig. 4A is a transverse cross-sectional view of the lamp of Fig. 1 at a plane including the notches 6, 6' showing the axial alignment of the notches 6, 6' and the anodes 5, 5' and indicating how the two discharge currents generate two plasmas 11, 11' outside the tube 2. The wave forms of the current of the plasma 11 of one of the anodes and its luminescence over a half cycle of the rectified A.C. of the power source are shown in Fig. 4B. The current includes an oscillation component at a frequency of 1.5 KHz, as does the luminous intensity. This current oscillation is due to the anode oscillation current flowing between the two anodes. It can be seen in Fig. 4B that, in the half cycle of the power supply, the anode current is not reduced to zero by the oscillations. The wave forms of the current of the plasma 11' of the other anode and its luminescence are similar to but in inverse relation to those of Fig. 4B. Within the tube 2, as indicated at 12 the two plasmas are substantially united. At the outside of the tube the two plasmas 11, 11' are separate from each other but each considerably spreads laterally.
It is thus possible to spread the plasma over almost entire region of the discharging space outside the inner tube, by forming two plasmas and permitting a high frequency self-excited anode oscillation current to flow between the anodes. It is true that there are some areas outside the inner tube which are not filled with the plasma but this does not matter because the ultraviolet radiation radiated from the plasmas 11, 11' in all directions is applied uniformly to the entire area of the fluorescent coatings (not shown) provided on both faces of the tube 2 and on the inner surface of outer bulb 1, so that visible radiation is radiated substantially uniformly from all portions of the fluorescent coatings.
In the embodiment of Fig. 1, the notches 6, 6' in the rim 2' of the tube 2 fix the path of each discharge current at a constant position on the circumference of the rim 2' thereby stabilizing the position at which each plasma is formed. If there were no notches 6, 6' each discharge current would tend to select a path of the shortest length, i.e. the path of least resistance. However, since there is no substantial difference in path length, whichever part of the rim 2' the path may pass over, the discharge current does not always select the same discharge path, but rather fluctuates over a selected region irregularly. Consequently, the position of each plasma changes irregularly to cause flickering of the output light from the lamp.
In sharp constrast to this, when the notches 6, 6' are provided, the discharge paths passing through these notches are much shorter paths passing over other portions of the rim 2', so that the discharge currents always flow through the corresponding notches. This stabilizes the positions of the plasmas, avoiding undesirable flickering of the output light.
The notches 6, 6' have any suitable shape. For instance, each can be rectangular as illustrated in Fig. 2 or have a valley-like shape with gentle slopes as shown in Fig. 3, or even a V-shape. The width and depth of the notch can easily be suitably selected. All that is required is to fix the paths of the respective discharge currents of the respective anodes.
A set of principal dimensions typical for the lamp of Fig. 1 are shown below purely by way of example.
A discharge lamp having these particulars has been experimentally manufactured. Its luminous condition, when ignited by commercial A.C. 100V power supply through the ignition circuit shown in Fig. 1 confirmed that the plasmas are made substantially unitary with each other within the tube 2 where they are stabilized by the wall of the tube 2. The two plasmas separate from each other as they clear the open end of the tube 2, and lead to the respective anodes.
In each anode, an anode oscillation of a frequency of 1 to several KHz was observed. It was also confirmed that the anode oscillations of the two anodes take place alternatingly.
Since the discharging space available outside the tube 2 is ample, the two plasmas are sufficiently diffused and spread laterally, so that almost the whole of the discharging space is uniformly filled with the plasma. It is thought that the two plasmas are stabilized in this discharging space by the walls of the inner tube and the outer bulb.
It was found desirable to minimize the surface area of each anode exposed to the discharging space, in order that the discharge plasma is maintained stably in the vicinity of the anode. Two plasmas could be maintained relatively stably even when there are no notches 6, 6' in the rim 2' of the tube 2, provided that the respective parts of the lamp are arranged in correct symmetry with respect to the lamp axis. However, the demand for the symmetry is not so strict when the notches are formed in the rim of the tube 2.
The means for stabilizing the plasmas by fixing the paths of the respective discharge currents may be other than the notches so far discussed. For instance, these means may be constituted by a partition plate secured to the open end of the tube 2 in such a manner as to isolate the discharge plasmas from one another. An example of this arrangement is shown in Figs. 5A and 5B. It will be seen that a partition plate 14 extends diametrically across the open end of tube 2, perpendicularly to the line connecting the anodes 5, 5'.
With this arrangement, the two plasmas are physically separated from each other by the partition plate 14 as they clear the open end of the tube. In addition, they are prevented from changing their positions by the presence of the partition plate 14 and are therefore held in a stable manner.
Fig. 6 shows a partition wall at the open end of the tube 2 provided by a downward projection or ridge 1' of the top of the bulb 1 extending to within the open end of the tube 2. This ridge 1' extends perpendicularly to the plane of the drawing, so as to isolate the plasmas from one another. This arrangement produces the same effect as the plate 14 of Figs. 5A and 5B. Needless to say, it is possible to use the notches 6, 6' in combination with either the partition plate 14 or the plate-like projection 1'.
In the embodiment of Figs. 7A, 7B and 7C an elongated fin-shaped partition plate 15 is fixed to the tube 2 instead of the notches 6, 6' or the partition plates already described. This plate 15 consists of a portion 1 5' which divides the discharging space around the open end 2' of the tube 2 and portions 15" which extend along the tube 2 for more than half its length and divide the discharging space around the external surface of the tube 2. This partition plate 15 extends diametrically at a right angle to the line connecting the anodes 5, 5' in a plane which contains the axis of the tube 2.
Both surfaces of the tube 2, the inner surface of the bulb 1 and the surface of the partition plate 15 are coated with fluorescent films 16. The ignition circuit for this lamp may be that of Fig. 1.
In operation, two plasmas are formed simultaneously in the discharging space around the tube 2, by the two discharge currents flowing along the paths between the respective anodes 5, 5' and the common cathode 4. These two plasmas are stabilized by the partition wall 15. More specifically, the plasmas, which are unitary within the tube 2, are separated from each other as they pass over the rim 2' and are stabilized in the area around the opening end of the tube 2, because they are completely isolated from each other by the upper portion 15' of the plate 15. Thus, no irregular fluctuation of the plasmas around the rim 2' takes place. At the same time, winding, snaking and irregular shifting of the plasmas are largely avoided also in the regions between the rim 2' and the respective anodes, because they are stabilized by the portions 15" of the plate 1 5. In this way stable plasmas are generated over the whole length of the lamp.
A typical set of principal dimensions of the lamp of Figs. 7A. 7B and 7C is given by way of example below:
In the operation of this lamp, the two plasmas are not always spread over the entire volume of the discharging space. However, the paths of the two discharge currents are fixed in respective portions of the discharging space, so that the resulting two plasmas are held extremely stably. The ultraviolet radiation from both plasmas is converted into visible radiation by the fluorescent films 16 within the lamp. This visible radiation is scattered and reflected repeatedly within the lamp, so that an essentially uniform luminous intensity distribution is obtained over the entire surface of the lamp. It will be understood that the partition plate 15 not only stabilizes the paths of the discharge currents over almost the whole length of the lamp, but also increases the area of the wall surfaces in the lamp, i.e. the area of the fluorescent films, thereby improving luminous efficiency.
Figs. 8A and 8B show another means for stabilizing the discharging plasmas outside the tube 2. This is a flange-like annular plate 17 secured to the outside of the tube 2 a short distance from the rim 2'. This plate 17 has peripheral notches 18, 18' and extends perpendicularly to the axis of the tube 2. The notches 18, 18' are diametrically opposite each other, i.e. in symmetry with respect to the axis of the tube 2, such that each notch 18, 18' corresponds to one of the anodes 5, 5'.
The discharge currents flowing from the rim 2' of the tube 2 to the anodes 5, 5' pass through the respective notches 18, 18'. Thus the paths of the discharge currents are fixed so as to stabilize the discharge plasmas. The flange-like plate 17 may be single as shown or may be repeated at a suitable axial spacing. Also, it is possible to use this plate 17 in combination with a longitudinal partition plate 15 such as shown in Figs. 7A, 7B and 7C. These plates 15, 17 may be used in combination with the notches 6, 6' shown in Fig. 1.
In the embodiment of Figs. 9A, 9B and 9C solid fillers 19 such as glass fibre or glass wool are provided around the tube 2, in place of the plate 15 secured to the outer surface of the tube 2, so as to fix the paths of the discharge currents, thereby stabilizing the discharge plasmas. More specifically, portions of the discharging space which divide the whole discharging space into two sections corresponding to the two anodes 5, 5' are filled with solid fillers 19 such as glass wool at a relatively high density, whereas the space corresponding to the anodes 5, 5' are not filled with the fillers 19 at all or are charged with the fillers only at a low density. The fillers 19 are bundled so as to have a high density at a region extending across the rim 2' of the tube 2 at a right angle to the line intersecting the anodes 5, 5'. The fillers 19 are secured to and suspended from the rim 2'.
The inner surface of the bulb 1 and both surfaces of the tube 2 are coated with fluorescent films 16, while the space inside the discharge vessel has been evacuated and filled with mercury and a rare gas at a pressure of a few Torr. This lamp can be ignited also using the ignition circuit shown in Fig. 1.
In operation, the channels of the discharge currents of the two anodes are fixed by the presence of the solid fillers 19 so that the plasmas are generated and maintained in a stable manner. This is because the discharge currents avoid the regions where the density of the fillers is relatively high, i.e. regions having higher resistance, and flow only through the regions where no filler is provided or where the filler is at only a low density, i.e. only through the regions where the resistance is relatively low.
By way of example, the density of the fillers in the region of higher density is preferably 10-⁴ to 10-³ (volume ratio), while in the region of low density it is preferably one-third to about one fiftieth of that of the region of higher density.
These fillers 19 may be coated with the fluorescent paint, so as to increase the efficiency of conversion of the ultraviolet radiation radiated from the plasmas into visible radiation.
The fillers are effective not only in forming stable paths for the discharge currents, but also in increasing the loss of charged particles in the plasmas, thereby enhancing the operating voltage of the lamp. It is therefore possible to operate the lamp to a high voltage, even when the size of the lamp is small. Thus, this embodiment can be effectively used as a light source for general illumination making use of commercially available A.C. 100V power.
The glass wool constituting the fillers 19 may be substituted by fine glass tubes having much greater diameter than the glass wool, or by insulating material other than glass, achieving similar effects.
Figs. 10A, 10B and 10C show a further embodiment of the invention, in which, in order to stabilize the discharge plasmas, the inner glass tube 2 has a flattened cross-sectional shape. More specifically, the tube 2 is of flattened shape such that its major transverse axis extends perpendicularly to the line connecting the anodes 5, 5'. Consequently, the discharging space around the tube 2 is wider where the anodes 5, 5' are provided and narrower at the areas remote from both anodes 5, 5'. Consequently the discharge currents flow through the wider regions of the discharging space where resistance is relatively small, avoiding the narrowed areas where resistance is relatively high. The paths of the discharge currents are thus fixed, leading to stable generation and maintenance of the plasmas.
A typical set of principal dimensions for this embodiment is given below, merely by way of example.
It is well known that higher efficiency is obtainable using a glass tube of non-circular cross section in a straight-tube type large output fluorescent lamp. The lamp of this embodiment may be considered as being effectively a large output lamp, because it can be a small- sized double-tube type lamp having a high lamp temperature and high pressure of mercury vapour.
The flattened shape of the inner glass tube offers the additional advantage of high efficiency of conversion of the ultraviolet radiation to visible radiation and, accordingly, provides a higher lamp efficiency. In addition, the flattened shape of the inner glass tube does not pose problems in practical use as a general lighting source, because it does not cause any change of the external appearance and design of the lamp.
Instead, of flattening the sides of the inner glass tube 2 corresponding to the anodes 5, 5' the tube 2 can have an oval shape as shown in Fig. 11. Alternatively, it is possible and leads to effective results to form fins 20, 20' at the sides of the inner glass tube 2, as shown in Fig. 12. Furthermore, the same result can be achieved by means of a tube 2 having a rectangular cross-section as shown in Fig. 13.
Although in the specific embodiments illustrated herein two discharge plasmas are formed by two anodes in the lamp, it is possible to provide three or more anodes suitably spaced apart (preferably uniformly) and make these anodes maintain their plasmas, by applying substantially equal voltage simultaneously to the anodes in the manner described above, while permitting the anode oscillation current to flow between the anodes. In such a case, the means for fixing the discharge paths, such as notches, partition plates, masses of fillers, are selected to correspond in number to the number of anodes.
It will be appreciated from the above that the lamp of the invention can be used most conveniently for general illuminating purposes. In particular the lamp has a single base and can be small in size on account of the double-tube structure. A stable light output is achieved, and the plasmas occupy most of the volume of the envelope.

Claims

1. A low pressure metal vapour discharge lamp having an outer glass envelope (1) defining a closed discharging space, an inner glass tube (2) disposed within said envelope (1) and having an open end (2') and a closed end, a cathode (4) disposed within said tube (2) a plurality of anodes (5, 5') disposed within said envelope (1) outside said tube (2), and circuit means (7, 8, 9) for simultaneously applying voltage to the anodes, said circuit means including ballast means (7) to provide current stabilization, characterized in that in said circuit means the connection between the anodes (5, 5') is such as to allow an anode oscillation current, which is due to anode oscillation occurring at each of the anodes and is included in the discharge current of each respective anode, to flow between the anodes (5, 5') thereby to provide a plurality of stable discharge plasmas in the discharge space.

2. A discharge lamp according to claim 1 wherein the anodes (5, 5') are substantially directly connected to each other and in that said ballast means (7) is a single ballast common to all the anodes (5, 5').

3. A discharge lamp according to claim 1 wherein the structure of the lamp is such that the locations of the discharge paths to the respective anodes are respectively fixed in order to prevent fluctuating movement of the discharge plasmas.

4. A discharge lamp according to claim 3, wherein means are provided in order to fix the locations of the said discharge paths, such means comprising one or more of the following:

(a) notches (6, 6') in the rim (2') of the tube (2) at its open end;

(b) a partition member (1'; 14; 15') at or adjacent the open end of the tube (2);

(c) a plurality of plate elements (15"; 20) projecting outwardly from and extending longitudinally along the outer wall of the tube (2):

(d) at least one flange-like element (17) projecting outwardly from the outer wall of the tube (2) normally to the axis of the tube (2) and having a plurality of notches or gaps (18, 18') spaced circumferentially;

(e) solid filler material (19) located in the discharging space, the density of the solid filler material being low or zero at regions where said discharge paths are to form and relatively high at regions where such paths are not to form.

5. A discharge lamp according to claim 4 wherein said solid filler material (19) is provided in the form of glass wool.

6. A discharge lamp according to any one of the preceding claims wherein the tube (2) is located centrally of the envelope (1) and the tube (2) and envelope are substantially coaxial.

7. A discharge lamp according to any one of the preceding claims wherein the said anodes (5, 5') are two in number.

8. A discharge lamp according to claim 7 wherein the tube (2) has an elongate or flattened shape as seen in transverse section (Figs. 10 to 13).

9. A discharge lamp according to any one of the preceding claims wherein the anodes are uniformly spaced circumferentially around the tube (2).

10. A discharge lamp according to any one of the preceding claims wherein the cathode (4) is located in the tube (2) adjacent said closed end of the tube (2) and the anodes (5, 5') are located outside the tube (2) adjacent said closed end of the tube (2).