Low-pressure gas discharge lamp

FIELD OF THE INVENTION:

The invention relates to a low-pressure gas discharge lamp.

BACKGROUND OF THE INVENTION: Unless otherwise indicated, the term "light" is used in its broadest sense to encompass the visible-light, ultra-violet (UV) and infra-red (IR) ranges.

Hot cathode fluorescent lamps (HCFL) are well known for use in backlight display devices, such as liquid crystal displays (LCD), and for other general applications. Typically, a high frequency voltage with a frequency ranging from between 20 kHz to 100 kHz for instance is supplied to a discharge space within a discharge vessel or tube of the

HCFL, forming a discharge resulting in the generation of electromagnetic radiation as a result of which a display device can be illuminated. A HCFL, however, requires that its hot cathode is kept at an increased temperature permanently, even when the HFCL is temporarily turned off, in order to secure instantaneous correct functioning of the lamp after switching it on again. The need to continuously power the HCFL is unfavorable from an energy preservation point of view. To overcome this problem it is preferred nowadays to use cold cathode fluorescent lamps (CCFL) or alternatively external electrode fluorescent lamps (EEFL). These do not require continuous powering during a temporary standby state of the lamp, as a result of which an LCD can be illuminated relatively economically. An EEFL usually comprises a discharge vessel of a suitable glass material, which vessel is provided at its ends with conductive coatings. The conductive coatings function as capacitive electrodes, between which a discharge extends, during lamp operation, along the axial distance between both ends. The EEFL achieves an energy saving relative to the HCFL by applying duty-cycle dimming to its alternating supply, and thus no extra heating is required compared to HCFL.

These capacitive lamps are already available in EEFL backlights for use in, for example, LCD screens. In this application the lamp currents are relatively low. To use an EEFL for general lighting, much higher lamp currents are required to achieve the required output intensity. One solution is to supply the EEFL at higher frequencies. However, this

may lead to electromagnetic interference (EMI) problems, and to higher costs for the lamp drivers in the supply. Also, at higher electric currents, the end losses at the electrode regions increase substantially, reducing the luminous efficacy of the lamp.

SUMMARY OF THE INVENTION:

It is an object of the invention to provide a low-pressure discharge lamp which has an improved luminous efficacy.

According to a first aspect of the invention, the object is achieved by providing a light-transmitting discharge vessel comprising a filling, the filling being configured to emit a discharge of light when excited; a supply of electrical energy; a first and a second electrode, disposed outside the discharge vessel, the electrodes being configured for supplying the electrical energy to the filling to excite the filling between the electrodes, the first electrode comprising a plurality of electrical supply channels; and a current controller, configured to control the portion of the electrical energy supplied by each channel to the filling.

The low-pressure discharge lamp may be configured to supply the electrical energy to the filling by capacitive coupling. In this case, each electrical supply channel comprises a conducting surface enclosing a section of the discharge vessel, the conducting surface being disposed such that the electrical energy is transferable by capacitive coupling to the filling.

While not wishing to be bound by any theory, it is believed that the capacitive coupling of energy into the discharge vessel is carried by a very thin sheath of ionized gas at the inside wall of the discharge vessel. It is believed that the transport of these ions through this sheath causes most of the end losses. Measurements have shown that the current of the electrical energy is coupled into the filling through a small area, close to the edge of the conducting surface which covers the outer wall of the discharge vessel. By providing a plurality of channels for supplying the electrical energy to the filling, the total current of the electrical energy supplied is divided into several smaller electrical currents. The invention is based upon the insight that differences, such as in impedance, between the channels may result in an uneven current distribution over the electrode channels. It is believed that this causes the arc of discharge inside the discharge vessel to only attach to a small part of the total electrode surface, resulting in an increase in end losses. By controlling the distribution of the electrical supply current through the channels, imbalances may be corrected, resulting

in the attachment of the discharge arc to a larger part of the total electrode surface, and thus also to reduced power losses.

According to an aspect of the invention, it is advantageous to provide a capacitively coupled electrode in which the length of the edges of the conducting surface are increased by: the edges of the conducting surface comprising undulations or serrations, and/or the conducting surface comprising perforations.

The sheath resistance Rsh of each channel is distributed along the edge of the external conducting surface. Therefore Rsh may be reduced by increasing the length of this edge. The length of the edge may be increased by adding undulations or serrations. Perforation of the conducting surface will also increase the effective edge length.

According to an aspect of the invention, it is advantageous to provide a discharge lamp wherein: the section of the discharge vessel is cylindrical; the conducting surface covers a perimeter of the section; and the conducting surface has a trapezoidal cross- section.

If an electrode ring is used around a cylindrical discharge vessel, the ring may be made with a trapezoidal cross-section, which increases the edge length compared to a ring with a rectangular cross-section. Note that "cylindrical" in this context means any 3- dimensional shape suitable as a discharge vessel. In particular, the term embraces shapes that do not have a circular transverse cross-section.

According to an aspect of the invention, it is advantageous to provide a discharge lamp wherein: the section of the discharge vessel is cylindrical; a perimeter of the section covers the conducting surface; and the conducting surface has a trapezoidal cross- section.

If an electrode ring is used surrounded by a section of a cylindrical discharge vessel, the ring may be made with a trapezoidal cross-section, which increases the edge length compared to a ring with a rectangular cross-section.

According to an aspect of the invention, the current controller comprises a plurality of impedances connected electrically in the plurality of electrical supply channels such that each channel has its current controlled by at least one impedance. Impedances per channel provide a direct method of controlling the current per channel, and allow a degree of independence in setting the current.

According to an aspect of the invention, the plurality of impedances are connected electrically in the plurality of electrical supply channels such that a first and a second channel have their current controlled by a common impedance. In some applications, it may be advantageous to allow an increase of the degree of dependence between channel currents. Such a configuration also allows use to be made of impedances integrated into the electrode.

According to an aspect of the invention, each impedance is a capacitor; each capacitor comprises a first and a second conducting surface separated by an insulating surface; each first conducting surface is disposed adjacent to the discharge vessel to transfer the electrical energy of the channel to the filling; and each second conducting surface is connected electrically to the electrical energy supply. The use of capacitors as impedances is advantageous because they are relatively simple to construct, using two conducting surfaces sandwiching an insulator, and the dimensions and properties may be varied to achieve the desired impedance. According to an aspect of the invention, the second electrode comprises a further plurality of electrical supply channels; and the current controller is further configured to control the portion of the electrical energy supplied by each of the second plurality of channels to the filling. This allows the end losses at more than one location to be improved by the invention.

BRIEF DESCRIPTION OF THE DRAWINGS:

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 through the longitudinal axis of a discharge lamp according to the invention,

Figure 2A shows a longitudinal cross-sectional view and Figure 2B shows a transverse cross-sectional view of a discharge lamp according to the invention,

Figure 3A shows a longitudinal cross-sectional view and Figure 3B shows a transverse cross-sectional view of a discharge lamp according to the invention,

Figure 4 shows a longitudinal cross-sectional view of a discharge lamp according to the invention,

Figure 5 shows a schematic electrical circuit diagram of the electrical connections to a discharge lamp according to the invention,

Figure 6 shows a schematic electrical circuit diagram of the electrical connections to a discharge lamp according to the invention,

Figure 7 shows a schematic electrical circuit diagram of the electrical connections to a discharge lamp according to the invention, Figure 8 shows cross-sectional views of discharge lamp electrode rings according to the invention,

Figures 9A, 9B and 9C depict cross-sectional views of discharge lamp electrode rings according to the invention,

Figures 1OA, 1OB and 1OC depict cross-sectional views of discharge lamp electrode rings according to the invention,

Figure 11 depicts cross-sectional views of discharge lamp electrode rings according to the invention, and

Figure 12 depicts cross-sectional views of discharge lamp electrode rings according to the invention.

The Figures are purely diagrammatic and not drawn to scale. Particularly for clarity, some dimensions are exaggerated strongly. Similar components in the Figures are denoted by the same reference numerals as much as possible.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS :

Figure 1 shows very schematically a cross-sectional view of an embodiment of a discharge lamp 10. The gas discharge lamp 10 comprises a light transmitting discharge vessel 20 which encloses a discharge space 50 in a gas-tight manner. The discharge vessel 20 comprises a gas filling 60. The gas discharge lamp 10 further comprises a first electrode 30 and a second electrode 40, connected via electrical connections 5 to a supply of alternating electrical energy 80. The electrodes 30, 40 capacitively couple energy into the filling 60 to excite it, and to cause and maintain a discharge in the filling 60 between the two electrodes 30, 40.

For capacitive coupling, the electrodes 30, 40 are conducting surfaces usually attached to the outer wall of the discharge vessel 20. In Figure 1, the discharge vessel 20 is cylindrical, and the capacitive electrodes 30, 40 are conducting plates attached to the end of the discharge vessel 20. By applying an alternating electrical potential difference between the two electrodes 30, 40 a discharge is initiated. This discharge is generally located between the two electrodes 30, 40 and is indicated in Figure 1 as the discharge space 50.

Figure 3 A very schematically shows a longitudinal cross-sectional view of an embodiment of a discharge lamp 14. Figure 3B shows a transverse cross-sectional view through the electrode 34 and the discharge vessel 24. The gas discharge lamp 14 according to the invention comprises a light transmitting discharge vessel 24 which encloses a discharge space 54 in a gas-tight manner. The discharge vessel 24 comprises a gas filling 60. The gas discharge lamp 14 further comprises a first electrode 34 and a second electrode of similar type (not shown), disposed in suitable recesses in the wall of the discharge vessel 24, and connected via electrical connections 5 to a supply of alternating electrical energy (not shown). The electrode 34 is a conductive ring which covers a section of the discharge tube 24. To ensure efficient capacitive coupling of electrical energy into the filling 60, the outer conducting surface of the ring electrode 34 contacts and closely follows the outer wall of the discharge vessel 24 within the recess.

If the gas discharge lamp 10,17 according to the invention is configured for low-pressure discharge, a molecular gas discharge takes place which emits radiation comprising the characteristic lines of the filling. In general, light generation in a low-pressure gas discharge lamp is based on the principle that charge carriers, particularly electrons but also ions, are accelerated by an electric field applied between the electrodes 30,40 in the discharge vessel 20. Collisions of these accelerated electrons and ions with the gas atoms or molecules in the gas filling in the discharge vessel cause these gas atoms or molecules to be dissociated, excited or ionized. When the atoms or molecules of the gas filling return to the ground state, a more or less substantial part of the excitation energy is converted to radiation.

The emission spectrum of the low-pressure gas discharge lamp 10, 17 is determined by the gas filling 60, together with, for example, the pressure and temperature inside the discharge vessel 20,27. The filling typically comprises an inert gas, for example, helium, neon, argon, krypton and/or xenon, and different metal compounds such as metal atoms and molecules which all contribute to the emission spectrum of the low-pressure gas discharge lamp 10, 17 according to the invention.

Figure 2A very schematically shows a longitudinal cross-sectional view of an embodiment of a discharge lamp 12. Figure 2B shows a transverse cross-sectional view through the electrode 32 and the discharge vessel 20. The gas discharge lamp 12 comprises a light transmitting discharge vessel 20 which encloses a discharge space 52 in a gas-tight manner. The discharge vessel 20 is substantially circular in transverse cross-section, and comprises a gas filling 60. The gas discharge lamp 12 further comprises a first electrode 32 and a second electrode of similar type (not shown), connected via electrical connections 5 to

a supply of alternating electrical energy (not shown). The electrode 32 is a conductive ring, also substantially circular in cross-section, which covers a section of the outer wall of the discharge vessel 20. To ensure efficient capacitive coupling of electrical energy into the filling 60, the inner conducting surface of the ring contacts and closely follows the outer wall of the discharge vessel 20.

Figure 4 very schematically shows a longitudinal cross-sectional view of a discharge lamp 13 according to the invention. The gas discharge lamp 13 comprises a light transmitting discharge vessel 20 which encloses a discharge space 56 in a gas-tight manner. The discharge vessel 20 is substantially circular in transverse cross-section.. The gas discharge lamp 13 further comprises a first electrode 32 and a second electrode 42, connected via electrical connections 5 to a supply of alternating electrical energy 80. The second electrode 42 is of the type depicted in Figures 2A and 2B. The first electrode 32 comprises four conductive rings 32a,32b,32c,32d which are also substantially circular in cross-section, covering a section of the outer wall of the discharge vessel 20. Each electrode ring 32a,2b,32c,32d is connected via an electrical connection 5a,5b,5c,5d and via an impedance 70a,70b,70c,70d to the electrical energy supply 80, to form four parallel channels for the current supplied to the filling 60. The current through each channel may be adjusted by varying the impedance 70a,70b,70c,70d electrically connected in that channel - however, it will be apparent to the skilled person that the parallel nature of the connections means that varying one impedance 70a,70b,70c,70d may imply an adjustment of the current in more than one channel.

In Figure 4, the impedances 70a,70b,70c,70d are comprised in the electrical connections 5a,5b,5c,5d of the electrode rings 32a,32b,32c,32d. However, it will be apparent to the skilled person that the impedances may also be comprised in the electrical energy power supply 80. The impedances 70a,70b,70c,70d may be discrete components; they may be integrated into the electrical wiring and the connections 5a,5b,5c,5d or a combination thereof.

It has been observed that the end losses vary with the current of the electrical energy supplied to the filling. While not wishing to be bound by any theory, it is believed that the capacitive coupling of energy into the discharge vessel 20 is carried by a very thin sheath of ionized gas at the inside wall of the discharge vessel 20. It is believed that the transport of these ions through this sheath causes most of the end losses. The resistance of this sheath of ionized gas may be termed sheath resistance Rsh, and may be used to model the end losses.

Power losses in the sheath resistance resistor are P-loss = I ² * Rsh. In this equation, I is the total current of the electrical energy supplied to the filling.

Measurements have shown that the current of the electrical energy is coupled into the filling through a small area, close to the edge of the conducting surface which covers the outer wall of the discharge vessel 20.

By providing a plurality of channels for supplying the electrical energy to the filling 60, the total current of the electrical energy supplied I is divided into several smaller electrical currents.

For example, if four identical channels are provided, each channel would supply an equal portion Ic of the total electrical current I, where I = 4 * Ic. The current supplied by each channel Ic flows through its own sheath, but the sheath resistance for each channel remains the same, namely R _SH - The total end losses for an electrode with four channels is therefore

4 * ( Ic ² * Rsh) compared to ( 4*Ic) ² * Rsh for the case of a single channel. Therefore, the potential reduction in power loss can be up to a factor of 4.

In practice, however, the reduction achieved may be lower due to the more complex coupling of the electrical energy into the filling through the plurality of channels. Additionally it has been observed that Rsh increases with decreasing current I, although it is believed that at very low currents, Rsh may be independent of I - for the example given above, the improvement may then approach said factor of 4 at these low currents. It is also believed that the filling and the conditions inside the discharge vessel 20 may also influence Rsh. However, for general lighting purposes, even a reduction in power loss approaching 10% is considered substantial and advantageous.

The invention is based upon the insight that differences between the channels may result in an uneven current distribution over the plurality of electrode channels. These differences may be due to factors such as the construction of the electrodes; the electrical connections; the properties of the filling 60 inside the discharge vessel 20,27 during operation; and the position where the energy is coupled into the filling 60 by the electrode in relation to the discharge. The differences may result in a difference in impedance and/or sheath resistance Rsh, which causes the uneven current distribution. It is believed that these differences cause the arc of discharge inside the discharge vessel to only attach to a small part of the total electrode surface, resulting in an increase in end losses. By controlling the portion of the electrical energy supplied by each channel to the filling, imbalances in the arc

attachment may be corrected, resulting in the attachment of the discharge arc to a larger part of the total electrode surface, and thus also to reduced power losses.

Figure 5 shows a schematic electrical circuit diagram of the electrical connections in the channels depicted in Figure 4. Note that the electrical connections to the second electrode 42 are not shown. Each electrode ring 32a,32b,32c,32d is connected to the electrical energy supply 80, and each connection comprises an impedance 70a,70b,70c,70d. The distribution of the electrical supply current to the electrode 32 - that is the portion of the total current that flows through each channel - may be adjusted by varying the impedance 70a,70b,70c,70d, however, it will be apparent to the skilled person that the parallel nature of the connections means that varying one impedance 70a,70b,70c,70d may imply an adjustment of the current in more than one channel.

The impedances may be resistors, capacitors, inductors or any combination of these components. Preferably, these impedances should be lossless to reduce the power loss due to the impedances 70a,70b,70c,70d. It may be advantageous to make the current through a particular channel more directly dependent on the current flowing through another channel. Figure 6 depicts a schematic electrical circuit diagram in which the impedances are configured to do this. The electrical connections to the second electrode 42 are not shown. The impedances comprised in each channel may be summarized as: - the channel of electrode ring 32a comprises a single impedance 70a the channel of electrode ring 32b comprises a single impedance 70b the channel of electrode ring 32c comprises the impedances 70c and 70b the channel of electrode ring 32d comprises the impedances 7Od, 70c and 70b. In this case, the channels of electrode rings 32c and 32d have two common impedances, namely 70b and 70c, and the channels of electrode rings 32b and 32c have a common impedance, namely 70b.

The skilled person will appreciate that many other combinations of single and common impedances are possible - the invention provides a very flexible solution to provide the most optimum current distribution during lamp operation. It may also be advantageous to configure the current controller to vary the current distribution through the channels depending upon the mode of operation. For example, it may be desired to reduce the luminous output of the discharge lamp (dimming) by varying the duty cycle. The current controller may do this by switching between different impedance networks, or by employing

appropriate common impedances or some combination of these. This provides considerable flexibility to control the end losses in different modes of operation.

It may be advantageous to implement the current controller and its components within the electrical discharge supply 80, to configure the electrode rings 32a,32b,32c,32d appropriately, to configure the electrical connections 5a, 5b,5c,5d appropriately, or some suitable combination of these. Components and connection may also be integrated into a lamp cap, which also provides a safety cover.

Figure 8 shows some examples of how impedances may be implemented in the electrode 32. Electrode ring 32e is an electrode ring of the type depicted in Figure 2A, but comprises an impedance connected between the electrode ring 32e and the electrical connection 5e. The impedance is a capacitor comprised of an insulator 132 sandwiched between a conducting region of the electrode ring 32e and a conducting region of the electrical connection 5e. As will be apparent to the skilled person, the value of the capacitance is determined by the areas of the conducting regions, the permittivity of the insulating material 132 and the distance between the conducting regions. The channel of electrode ring 32e thus comprises a capacitor in its electrical connection to the electrical discharge supply.

Electrode rings 32f,32g are of a similar type to electrode ring 32e. Here a capacitor is formed between the channel of electrode ring 32f and the channel of ring 32g, by sandwiching an insulating ring 131 between conducting electrode ring 32f and conducting electrode ring 32g. In this way, the current portions through the channels of electrode rings 32f and 32g have a common impedance.

The degree to which the currents are dependent on each other is dependent upon the value of the capacitance, and the way in which the electrode rings 32f and 32g are connected by their electrical connections 5f, 5g to the electrical energy supply 80. It may be advantageous to connect only one of the electrode rings 32f, 32g to the electrical power supply, thus creating a total dependence of one channel current on another. This flexibility is available in any situation where an impedance is formed between two electrodes.

Electrode rings 32h,32i are of a similar type to electrode ring 32e. Here a capacitor is formed between the channel of electrode ring 32h and the channel of ring 32i, by sandwiching an insulator 134 between a conducting area of the electrode ring 32h and a conducting area of electrode ring 32i. In this way, the current portions through the channels of electrode rings 32h and 32i have a common impedance.

It is particularly advantageous to configure the first and second electrodes 32, 42 in a substantially identical way. This is shown in Figure 7, which depicts a schematic diagram of the electrical connections from the electrical energy supply 80 to the first electrode 32 and the second electrode 42. The first electrode 32 comprises three channels, each comprising an electrode ring 32a,32b,32c and an impedance 70a,70b,70c. The second electrode 42 comprises three channels, each comprising an electrode ring 42a,42b,42c and an impedance 90a,90b,90c. For simplicity, only three channels are shown for each electrode and each channel comprises a single impedance. However, it will be apparent to the skilled person that the configurations of the invention described in relation to the first electrode 32 may also be applied to the second electrode 42. By configuring the first and second electrodes 32,42 in the same way, the reduction in end losses achieved by the invention may be applied to both ends. Although the schematic representation of Figure 7 depicts an identical layout of impedances and connections, in practice the impedances and connections may need to be implemented differently to achieve a substantially identical current distribution.

Alternatively, it may be advantageous to implement different current distributions for the first and second electrodes 32,42. In certain applications, an asymmetric light distribution may be desired. For example, the inner wall of the discharge vessel 20, 27 may be provided with different phosphors, such as red and blue, at different positions. By varying the degree of asymmetry by controlling the current distributions, the color of the lamp may be changed.

Figures 9A and 9B depict a further example of an electrode comprising a plurality of channels. The electrode 36 comprises conductive electrode rings 36a,36b,36c and insulating rings 136b, 136c disposed between the electrode rings 36a, 36b and 36b,36c respectively. Each electrode ring 36a, 36b,36c is provided with a direct electrical connection 5a, 5b, 5c, for connection to the electrical energy supply 80, although it is not required that all connections are used. In a similar way to the electrode rings 32f and 32g depicted in Figure 8, the current portion through the channels of electrode rings 36a and 36b has a common impedance formed by the insulating ring 136b, and the current portion through the channels of electrode rings 36b and 36c has a common impedance formed by the insulating ring 136c. Each electrode ring 36a,36b,36c is constructed to have two regions with different diameters and an intermediate region, such that the electrode regions may interlock and partially overlap. By placing the insulating rings 136b, 136c between the electrode rings

36a,36b,36c in the regions of overlap, a simple and cost-effective manner is provided to form capacitors in the electrode 36.

Figures 1OA and 1OB depict a further example of an electrode comprising a plurality of channels. The electrode 38 comprises conductive electrode rings 38a,38b,38c and a contiguous insulating ring 138 covering the electrode rings 38a,38b,38c. The electrode 38 further comprises an electrical connection 6 which comprises a conductive ring covering the insulating ring 138. The electrical connection 6 is connected to the electrical energy supply 80. In this way, capacitors are formed between the electrode rings 38a,38b,38c and the conducting ring of the electrical connection 6. The value of the capacitors is influenced in particular by the area of the outer conducting surface of each electrode ring 38a,38b,38c.

It may be advantageous to provide the conductive ring of the electrical connection 6 with perforations to obtain the desired value of capacitance.

It will be obvious to the skilled person that the arrangements for forming capacitors may be modified to form other impedance types. For example, if a resistive material is used instead of an insulating one, resistors will be formed. It will also be obvious that more than one type of impedance may be formed at the same location.

Although only one layer has been depicted, it will also be obvious that more conducting, insulating and/or resistive layers may also be employed. In practice, a working system may comprise one or more combinations of the impedance types described. It will be apparent to the skilled person that the invention may also be employed to control the current distribution to any discharge lamp, and that it is not limited to lamps employing capacitive coupling to couple the electrical energy into the filling 60. It is therefore within the skill of those in the art to modify the described embodiment to employ a different coupling method. It will be apparent to the skilled person that the embodiments described using electrode rings may be modified to use electrodes such as those depicted in Figures IA and 3A. For an electrode employing capacitive coupling to the filling, the only requirement is that a conducting surface must be present at the outer wall of the discharge vessel 20. Although described as such, it is not essential that the invention be implemented with channels providing a symmetrical conducting surface to the discharge vessel 20. The ability to control the current supplied through each channel means that electrodes of any shape may be employed. This allows different light effects to be generated at the electrode regions. For example, the electrode conducting surfaces 35a,35b,35c,35d,35e depicted in Figure 12 may be useful. Each surface is provided with an electrical connection to

the discharge supply, either directly as depicted via the electrical connections 5a,5b,5c,5d,5e, or via an indirect capacitive connection as depicted in Figures 1OA and 1OB, or via some combination of connections.

As previously mentioned, measurements have shown that the current of the electrical energy is coupled into the filling through a small area, close to the edge of the conducting surface which covers the outer wall of the discharge vessel. The sheath resistance Rsh of each channel is therefore distributed along the edge of the external conducting surface. This means that Rsh may be further reduced by increasing the length of this edge. Some examples of techniques which may be employed to increase the edge length are depicted in Figure 10. Figure 10 depicts modified electrode rings 233, 234, 235, 237 which are similar in type to the electrode rings 32a, 32b, 32c, 32d in Figure 5. Electrode ring 233 has an edge comprising undulations, electrode ring 234 has an edge comprising serrations, and electrode ring 235 comprises perforations; all these measures increase the length of the edge presented to the outer wall of the discharge vessel, compared to the electrode rings of Figure 5.

Alternatively, the length of the edge may be increased by making external conducting surfaces that are not cylindrically symmetric, but slanted, as is depicted for electrode ring 237. For optimal capacitive coupling, the inner conducting surface of the ring fully remains in contact with the outer wall of the discharge vessel. The longitudinal axis of the ring 237 therefore lies at a non-perpendicular angle to the longitudinal axis of the discharge vessel 20, while the ring still makes contact over the whole perimeter with the outer wall of the discharge vessel 20. In other words, the cross-section of the ring has a trapezoidal shape instead of a rectangular one. Figure 9C depicts a slanted version 37a of the ring 36a depicted in Figure 9B. Similarly, Figure 1OC depicts a slanted version 39b, 39c of the rings 38b,38c depicted in Figure 1OB.

Alternatively, if an internal ring is used similar to the electrode depicted in Figure 3 A, the longitudinal axis of the ring would lie at a non-perpendicular angle to the longitudinal axis of the discharge vessel 20 while the outer conducting surface of the ring would still make contact over the whole perimeter with the outer wall of the discharge vessel 20. In other words, the cross-section of the ring would have a trapezoidal shape instead of a rectangular one.

In summary, a low-pressure discharge lamp is provided with an electrode comprising a plurality of said channels, and a current controller for controlling the current distribution through these channels. Current control may be provided by using a suitable

network of impedances in the electrical connections to the electrode. This permits a high degree of control over how the electrical energy is coupled into the gas filling, enabling the end losses to be reduced. Additionally, the current controller may also be used to create a wide range of light distributions emitted by the discharge lamp.