WO2010007557A1

WO2010007557A1 - Method of driving a gas-discharge lamp

Info

Publication number: WO2010007557A1
Application number: PCT/IB2009/052945
Authority: WO
Inventors: Pavel Pekarski; Jens Pollmann-Retsch; John-John P. J. Van Den Bergh
Original assignee: Philips Intellectual Property & Standards Gmbh; Koninklijke Philips Electronics N.V.
Priority date: 2008-07-14
Filing date: 2009-07-07
Publication date: 2010-01-21

Abstract

The invention describes a method of driving a gas-discharge lamp (1) enclosing a pair of electrodes (3, 4), wherein, during a normal mode of operation, the lamp (1) is driven according to a stabilisation driving scheme, which stabilisation driving scheme comprises a repeating sequence (SAB) of driving schemes (A, B), wherein each driving scheme (A, B) causes a particular shape change of the electrodes (3, 4), and the driving scheme sequence (SAB) is compiled such that the electrodes (3, 4) of the lamp (1) are subject to repetitive shape changes. During an ancillary mode of operation in which an alteration in light output of the lamp (1) is essentially imperceptible to an observer, the lamp (1) is driven according to a remodelling driving scheme (C) to extensively remodel the electrodes (3, 4). The invention further describes a driving unit (10) for driving such a gas-discharge lamp (1), and a projection system (22) with such a lamp (1) and driving unit (10).

Description

METHOD OF DRIVING A GAS-DISCHARGE LAMP

FIELD OF THE INVENTION

The invention describes a method of driving a gas-discharge lamp, a driving unit for driving a gas-discharge lamp and a projection system with such a lamp and driving unit.

BACKGROUND OF THE INVENTION

In gas discharge lamps such as HID (High Intensity Discharge) and UHP (Ultra-High Pressure) lamps, a bright light is generated by a discharge arc spanning the gap between two electrodes disposed at opposite ends of a discharge chamber of the lamp. In short-arc and ultra-short-arc discharge lamps, the electrodes in the discharge chamber are separated by only a very short distance, for example one millimetre or less. The discharge arc that spans this gap during operation of the lamp is therefore also short, but of intense brightness. Such lamps are useful for applications requiring a bright, near point source of white light, for example in image projection applications or in automotive headlights.

When such a lamp is driven using alternating current (AC), each of the electrodes functions alternately as anode and cathode, so that the discharge arc alternately originates from one and then the other electrode. Ideally, the arc would always attach to the electrode at the same point, and would span the shortest possible distance between the two electrode tips. Maintaining a stable light flux ultimately means maintaining a favourable arc-length for prolonged times. Because of the high temperatures that are reached during AC operation at high voltages, the electrodes of a gas-discharge lamp are subject to physical changes, i.e. an electrode tip may melt or burn back, and structures may grow at one or more locations on the electrode front face at the point where the arc attaches to the tip. Physical alterations to the electrode can adversely affect the brightness of the arc, since the arc may become longer or shorter, leading to fluctuations in the light output and collectable flux of the lamp.

The known relationship between arc-length and lamp voltage/lamp current is used in some approaches to the problem, for example by applying a dedicated lamp driving scheme for a particular lamp type in an attempt to control the electrode topology during operation of that lamp type. A state of the art driver for such a lamp is described in WO 2005/062684 Al which is incorporated herein by reference and which describes a method in which a target voltage is predefined and the lamp driver uses the predefined value to decide when to switch between driving schemes or modes of operation with specific combinations of different current wave-shapes and operating frequencies, for instance whenever the observed operating voltage of the lamp crosses the target voltage value or deviates by a predefined amount from the target voltage value. A controlled growing of structures on the lamp's electrodes is achieved by application of a driving scheme in which, directly preceding a commutation of the current, current-pulses are superimposed on the rectangular wave shape of the lamp current. A controlled melting back of the electrode front faces is achieved by driving the lamp at a higher frequency, and without such a current-pulse superimposed on the current wave shape.

However, these methods may not always be effective and ultimately may not suffice to inhibit electrode growth, since the power curve and therefore the maximum current of a lamp driver is limited. If the lamp voltage drops below a certain limit, the nominal power of the lamp cannot be reached. The result of the decreased electrode gap is an undesirable drop in light output of the lamp.

In some approaches, an abrupt increase in thermal load is applied to the electrodes, for example by applying a DC current for a certain length of time. However, all known measures result in an abrupt increase of the electrode distance and therefore in a sudden change of light intensity, an effect which may be perceived by an observer and which is evidently undesirable, particularly during image rendering.

Therefore, it is an object of the invention to provide a method of driving a gas-discharge lamp, and a driving unit, such that a stable light output can be maintained while avoiding the problems mentioned above. SUMMARY OF THE INVENTION

The object of the invention is achieved with a method of driving a gas- discharge lamp according to claim 1, and a driving unit according to claim 11. Accordingly, the present invention describes a method of driving a gas- discharge lamp, wherein, during a normal mode of operation, the lamp is driven according to a stabilisation driving scheme, which stabilisation driving scheme comprises a repeating sequence of driving schemes wherein each driving scheme causes a particular shape change of the electrodes and the driving scheme sequence is compiled such that the electrodes of the lamp are subject to repetitive shape changes. During an ancillary mode of operation in which an alteration in light output of the lamp is essentially imperceptible to an observer, the lamp is driven according to a remodelling driving scheme to extensively remodel the electrodes.

The stabilisation driving scheme serves, as far as possible, to control the shape of the electrodes. Since the electrodes are separated by a small gap, the stabilisation driving scheme essentially serves to keep this separation as constant as possible, so that, in turn, the length of the discharge arc is also kept essentially constant. The electrode stabilisation driving scheme can be, for example, the technique described in WO 2005/062684 Al. The term 'normal mode of operation' is used to refer to the usual mode of operation of the lamp, for example, when a lamp for image rendering purposes is driven at a nominal operating power level for a duration of time.

The 'remodelling driving scheme', also referred to as 'safety brake' in the following, is a driving scheme in which a high thermal load is placed on the electrodes in order to subject these to a radical remodelling involving a burning back of the electrode front surfaces in order to increase the gap between the electrodes. After the remodelling driving scheme has completed, the usual stabilisation driving scheme can be applied again to good effect.

The term 'ancillary mode of operation' is used to refer to any other mode of operation such as a secondary or non-critical mode of operation in which an alteration in light output will be insignificant or inconspicuous. An obvious advantage of the method according to the invention is that, since the remodelling driving scheme is applied during the ancillary mode, any fluctuation in light output that will probably arise due to the extensive reshaping of the electrodes will not be noticeable to an observer. Observations made during development of the method according to the invention have shown that it is sufficient to perform the electrode remodelling during such a non-critical phase, since the amount of light collected in etendue-limited projection systems increases when the electrode distance (and therefore the arc size) decreases. As a result, the reduction in the screen lumens caused by a decrease in lamp power is partially compensated for by the increased collection efficiency due to shorter electrode gap. Furthermore, it has been observed that, as the electrode gap decreases, the rate of tip growth also slows down.

These observations suggest that prolonged lamp operation below the power limit is possible without a visible effect on the projection screen. This in turn means that a remodelling of the electrodes does not necessarily have to be performed immediately when the lamp voltage drops below some critical value, nor does it have to be performed at regular time intervals, as is the case in some state of the art approaches. When the electrodes are remodelled, a fluctuation in light output will arise as a result of the increase in the gap between the electrodes as these are burned back. In the method according to the invention, in contrast to state of the art approaches, remodelling is advantageously carried out in an ancillary mode of operation, so that any abrupt change in luminous flux that might arise will therefore not be visible to the user. An appropriate driving unit for driving a gas-discharge lamp comprises a switching unit for switching between a stabilisation driving scheme applied during a normal mode of operation, which stabilisation driving scheme comprises a repeating sequence of driving schemes, wherein each driving scheme causes a particular shape change of the electrodes, and the driving scheme sequence is compiled such that the electrodes of the lamp are subject to repetitive shape changes; and a remodelling driving scheme applied to extensively remodel the electrodes during an ancillary mode of operation in which an alteration in light output of the lamp is essentially imperceptible to an observer. The dependent claims and the subsequent description disclose particularly advantageous embodiments and features of the invention.

The etendue of a projection system is an indication of the extent to which the light, generated by the near-point light source (e.g. an ultra-high-pressure gas- discharge lamp) can be transmitted through the projection system. Factors which influence the etendue of a projection system are, for example, the choice of light source, or the type of collimator used. In an etendue-limited projection system, the light-flux on the projection screen is determined by the product of the luminance of the light source and the smallest etendue in the projection system, which is typically the etendue of the micro-display which might be an LCD (Liquid Crystal Display), LCoS (Liquid Crystal on Silicon), or DMD (Digital Micro -mirror Device) used in the projector. Hence, for any given light source, the usable light-flux is limited by the etendue of the projection system. In the following, but without limiting the invention in any way, it will be assumed that the lamp is used in an etendue-limited projection system. It has been observed that the arc length in a high-pressure lamp of the type described above is related to the operating voltage of the lamp. As already indicated, a higher voltage across the electrodes is associated with a melting of the electrode tips, so that the separation between the electrodes (which face each other from opposite ends of the glass envelope) and therefore also the arc-length, increases. Similarly, a lower voltage across the electrodes is associated with the growing of structures or tips on the electrode faces, so that the distance between the electrode tips is effectively decreased, and the arc length decreases accordingly.

Therefore, when the lamp is driven in normal mode using the repeating sequence of driving schemes, a switch-over between driving schemes is triggered according to a relationship between an observed lamp parameter value and a threshold level. In a preferred embodiment of the invention, the lamp parameter is the lamp voltage, and the threshold value is referred to in the following as a 'target voltage'. In the normal mode of operation, a switch-over between driving schemes occurs when the lamp voltage drops below or rises above this target voltage, depending on the driving scheme being applied at any one time. The term 'normal mode of operation' refers to a mode of operation in which images are rendered, regardless of whether the lamp is being driven at nominal power or at a dimmed power level.

A target voltage can therefore be regarded as a kind of threshold level used to trigger a switch between driving schemes of the electrode stabilisation driving scheme. Whenever the operating voltage crosses the target voltage, the lamp driver triggers a driving scheme switch-over. If the operating voltage drops below the target voltage, implying that the arc length has become too short, a first driving scheme may be used in which the frequency of the lamp current can be sufficiently high so that the electrode tips melt back slightly. If the operating voltage increases above the target voltage, implying that the arc length is becoming too long, a second driving scheme may be used in which the lamp current wave-shape includes a pulse that causes a tip to grow again on the front face of the electrode. In a normal mode of operation, as long as the gap between the electrodes is not too short, a stable discharge arc can be achieved by switching between driving schemes in this way.

As outlined in the introduction, the electrodes can slowly grow over time, thus decreasing the gap, and the lamp voltage drops accordingly. Below a certain level, the stabilisation driving scheme becomes ineffective. Therefore, in a particularly preferred embodiment of the invention, a lamp parameter is monitored during a normal mode of operation of the lamp, and a remodelling indicator is set if the monitored lamp parameter indicates that a radical re-shaping or correction of the electrodes is necessary. A suitable lamp parameter can be any parameter that can easily be measured and that provides information about the momentary status of the lamp, for example lamp voltage, lamp current, or lamp power. If the observed lamp parameter drops below or exceeds an acceptable threshold level, depending on the lamp parameter being observed, the remodelling indicator, which may be a bit or a flag, is set, for example to T. For instance, if the observed lamp parameter is the lamp voltage - a preferred lamp parameter for observation - and this drops below an acceptable threshold level, the remodelling indicator is set. The lamp driver can evaluate the remodelling indicator continually, and apply the remodelling driving scheme during a later ancillary mode.

As mentioned above, the observed lamp parameter can comprise a lamp voltage value. Obviously, the lamp voltage can drop below this threshold, i.e. can reach a minimum voltage value for that lamp, while the lamp is being driven in an ancillary mode of operation anyway, for example during a shut-down phase. In such a simple case, the remodelling driving scheme can be applied immediately so that the electrode shape can be corrected. Any alteration in light flux will not be visible to the user, since, for example, the shut-down phase generally occurs after an image rendering has been halted. However, of course, the lamp voltage can drop below the threshold while the lamp is being driven in a normal mode of operation, for example during image rendering. In this case, applying the remodelling immediately would lead to a visible alteration in light flux. Therefore, in a preferred alternative embodiment, the remodelling indicator is set when the lamp voltage reaches or drops below the threshold, so that remodelling is postponed until the next ancillary mode of operation.

In order to determine the next ancillary mode, i.e. the next favourable time at which to apply the remodelling driving scheme or 'safety brake', information pertaining to the status of the driving unit of the lamp, or the projection system, can be analysed. For example, a suitable signal of the driving unit can indicate when the lamp is to be operated in a run-up or run-down phase. A signal provided by the projection system could indicate a period of time in which image content is not available, for example, due to a change of the source of image content source. Any of these times can be particularly suitable for applying a safety brake, since the resulting change in light output would not interfere with any image rendering. Therefore, in a projection system according to the invention, the ancillary mode of operation of the lamp is a mode of operation during which image content is not rendered by the projection system.

A projection system according to the invention, comprising a gas- discharge lamp and a driving unit for driving the lamp using the described method, therefore also preferably comprises an information signal, related to the image rendering status, to inform the lamp's driving unit of the opportunity to employ a remodelling driving scheme in an upcoming ancillary mode of operation.

Evidently, it may be necessary to apply a safety brake remodelling scheme during a time when image content is being rendered. In such a situation, control software of a projection system can 'look ahead' to determine a time in which the remodeling scheme has no visible effect on the image rendering. Therefore, in a projection system according to the invention, the ancillary mode of operation of the lamp preferably comprises a changeover phase from a first image rendering mode of operation to a second image rendering mode of operation. For example, the first image rendering mode can be a nominal mode, i.e. a mode in which the lamp is driven at its nominal power, while the second image rendering mode can be a dimmed mode of operation, in which the lamp is operated at a lower power level.

A driving unit according to the invention preferably comprises a monitoring unit for monitoring a lamp parameter during a normal mode of operation of the lamp, and a setting unit for setting a remodelling indicator if the monitored lamp parameter indicates that a remodelling of the electrodes is necessary at a later, more suitable time. To store the remodelling indicator value or flag, the driving unit according to the invention also preferably comprises some type of storage element such as a nonvolatile memory. An analogue storage element could also be used.

The electrodes of the lamp could be shaped as simple rods. However, experimental results have shown that a greater mass concentrated in the front part of the electrode is beneficial to the electrode modelling process. Therefore, preferably, the electrodes comprise rod-sphere electrodes, which differ from the usual electrode shape in that they have a large, heavier tip with more mass.

A number of approaches are possible to remodel the electrode tips. Usually, the frequency of the lamp current is decreased, so that, basically, each electrode has the opportunity to act as an anode for a (relatively) long period of time, in which time the surface of the electrode can be melted back and reshaped. In a preferred embodiment of the invention, therefore, the remodelling driving scheme comprises an alternating current at a frequency in a range between 10Hz and 40Hz, more preferably in a range between 15Hz and 25Hz. These low frequencies, and therefore long periods, give the electrodes enough time to be melted back.

Taking this concept even further, in another preferred embodiment of the invention, the lamp current of the remodelling driving scheme is applied as a direct current for a predefined time interval. Under direct current, one electrode acts as anode and is burned back. In the method described, the voltage can be reversed so that each electrode can act as anode, so that both electrodes get remodelled to an equal extent. Evidently, these two approaches could be combined, by taking turns to apply first an AC current and then a DC current. The combination could also be repeated. Since it takes a certain length of time for the electrodes to be remodelled completely, the remodelling process is preferably carried out over a time interval in a range between 0.1 s and 20 s, preferably in a range between 0.1 s and 1 s. This length of time has been observed to be sufficient to remodel the electrodes of a gas-discharge lamp of the type usually used for projection purposes.

In yet another preferred embodiment, the remodelling scheme could start with a Io w- frequency AC mode, with a frequency in the upper region of the range given above, for example 40 Hz. Only if this mode is not sufficient to remodel the electrodes within a certain time, say 0.5 s, the frequency will subsequently be reduced, for example to half of the previous operating frequency, in this case to 20 Hz. The lamp is operated at this reduced frequency, again for a limited time, say 0.5 s. By repeating this process until a sufficient remodelling of the electrodes has been achieved, the total duration of the remodelling driving scheme can dynamically be adapted to the actual behaviour of the electrodes.

Yet another way to remodel the electrodes of the lamp during operation is to apply a current higher than the one needed for the normal mode of operation, for example in the form of a current-pulse, again for a limited time. As already indicated, the higher heat-load to the electrodes will lead to a rapid remodelling of their shapes. In an approach similar to the frequency-ramp described above, the lamp current could also be ramped up slowly, instead of being abruptly increased, until a sufficient remodelling of the electrodes has been achieved.

Other objects and features of the present invention will become apparent from the following detailed descriptions considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for the purposes of illustration and not as a definition of the limits of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a very simplified representation of the electrodes in a high- pressure gas discharge lamp as time progresses during lamp operation; Fig. 2 shows a graph of lamp voltage against burning time for a high- pressure gas discharge lamp without remodelling of the electrodes; Fig. 3 shows a graph of lamp flux for a UHP lamp, calculated with and without correction for etendue-limited light collection; Fig. 4 shows a graph of lamp voltage over time, and a temporal representation of the remodelling indicator flag and the corresponding driving schemes applied when driving the lamp using the method according to the invention;

Fig. 5 a shows graphs of lamp voltage, remodelling flag indicator and driving scheme for a UHP lamp driven beyond a voltage limit using the method according to the invention; Fig. 5b shows graphs of lamp voltage, remodelling flag indicator and driving scheme for the lamp of Fig. 5a driven in a following run-up phase using the method according to the invention; Fig. 6 shows a gas-discharge lamp and a block diagram of a possible realisation of a driving unit according to the invention; Fig. 7shows a possible realisation of a projection system with a lamp controlled by a driving unit according to the invention.

In the drawings, like numbers refer to like objects throughout. Objects in the diagrams are not necessarily drawn to scale.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Fig. 1 shows a very simplified representation of the electrodes 3, 4 in a high-pressure gas discharge lamp as time progresses during lamp operation. From an initial shape (I), in which the front faces of the electrodes 3, 4 are separated by a sufficiently large gap, the contours of the electrode tips (II, III) alter as structures grow and melt back on the front faces. As long as the gap G between the electrodes 3, 4 remains sufficiently wide, the lamp can be driven using an electrode stabilisation driving scheme in which the lamp frequency and current are periodically altered to control the growth of the structures. In a last stage (IV), the structures on the electrode front faces are so large that the gap G' between the front faces of the electrodes 3, 4 has shortened to a point where the lamp current can no longer be increased to compensate for the low lamp voltage. At this stage, a radical remodelling of the electrodes 3, 4 is necessary. After remodelling, the electrodes 3, 4 once again have their initial shape (I) and the gap between the front faces has widened again to the original length G.

The reason for the necessity of burning back or remodelling of the electrodes can be seen in Fig. 2, which shows a graph of lamp voltage against burning time for a 150W high-pressure gas discharge lamp. After start-up, the lamp voltage remains at about the nominal level of 50V for a few hours, but then starts to drop noticeably. After a burning time of 10 hours, without remodelling the electrodes at any time, the lamp voltage is only about 20V. At this low level, the lamp current can hardly be increased any further to compensate, leading to a drop in screen lumens.

As explained already, the drop in screen lumens as a result of the shorter discharge arc is compensated for by the fact that, in an etendue-limited projection system, the amount of light collected increases with shorter distance between electrodes. This is illustrated with the aid of Fig. 3, which shows graphs of light flux in lumen (Im) plotted against lamp voltage (V) for a 132W lamp with maximum current of 3 A and for a projection system etendue of 6mm² sr. When the lamp voltage drops below the power curve, i.e. the lamp power is limited by the maximum current, the light flux from the lamp decreases, as indicated by the solid line. However, the lower voltage is associated with a shorter arc, so that a higher collection efficiency in the projection system is obtained. This in turn leads to a higher collected light-flux (dashed line). Altogether, the drop in light flux at voltages below the power curve of the driver is less severe than to be expected from the low lamp power alone. This fact is put to good use in the method according to the invention, which proposes carrying out the electrode remodelling in intervals that are non-critical from an image-rendering point of view. This is shown in Fig. 4, which shows a graph of lamp voltage over time, and a temporal representation of the remodelling indicator RMI and the corresponding driving schemes applied during operation of the lamp. During lamp operation, the lamp voltage decreases from its rated voltage V_rated and ultimately drops below a 'safety brake voltage' Vsb. When this happens, a remodelling indicator is set, in this case the remodelling indicator is a bit value which can either be O' ('FALSE') or T ('TRUE'). The bit RMI is set to T to indicate that the lamp voltage is too low and that a remodelling is due at the next opportunity. The lamp driver continues to drive the lamp using the usual electrode stabilisation scheme S_AB until, at some later point, an ancillary mode of operation is applied which can be, for instance, a period of time in which no image rendering occurs, or a switch-over from operation at a normal power level to a dimmed power level. In this ancillary mode, since the remodelling flag value is set to T, the driving unit applies the remodelling driving scheme C for a brief time (in the diagram, the relative widths of the driving schemes C, S_AB are not drawn to scale). The driving unit causes the remodelling indicator value to be reset to '0', and continues to drive the lamp using the usual electrode stabilisation driving scheme S_AB- Eventually, the lamp voltage will drop to below the safety brake lamp voltage level again, and the process will be repeated.

Further examples are given by Fig. 5a and Fig. 5b which show operating voltage against time, a remodelling indicator flag, and a driving scheme sequence for a 200W UHP lamp with a current limit of 4 A and a nominal operating voltage of 56V. In Fig. 5a, the 'safety brake' voltage, defined for this lamp to be 52V, is reached after a time Tl of projector operation. A remodeling indicator flag RMI is set in the nonvolatile memory of the driver, shown in the center part of the Figure. During subsequent lamp operation, the lamp voltage drops, and, at about time T2, reaches a value of only 48 V, corresponding to a lamp power of only 192W. However, because of the increased collection efficiency in the etendue-limited projection system, the corresponding small reduction in screen lumens is not perceptible to the user. Therefore, although the lamp voltage has dropped below a voltage limit for that lamp, the driving unit continues to apply the usual electrode stabilisation driving scheme, even though the associated electrode re-shaping is insufficient to widen the gap between the electrodes. This is indicated by the AB sequence in the driving scheme S_AB shown in the lower part of the Figure. At some point, the lamp is turned off. The remodelling indicator value RMI is stored in a non- volatile memory of the driving unit. During the next run-up of the projector, indicated in Fig. 5b, the lamp- driver checks the status of the remodelling indicator flag RMI and, finding the flag set to T or TRUE, applies a safety brake driving scheme C for 2 seconds, 35 seconds after switch-on (note that the timescale of this diagram is different from that of Fig. 5a). Here, the safety brake driving scheme comprises driving the lamp at 20Hz with a lamp current of 4A. The remodelling indicator flag RMI is reset to ¹O', or FALSE. After completion of the run-up phase, in this example 50s after switch-on, the lamp voltage has increased to 56V, which is well above the 'safety brake' voltage. Normal operation of the lamp can continue with the electrode stabilisation schemes A, B, as usual.

Fig. 6 shows a gas discharge lamp 1 and a block diagram of one embodiment of a driving unit 10 according to the invention. The system as shown can be used, for example, as part of an etendue-limited projection system.

The circuit shown comprises a power source P with a DC supply voltage, for example, 380V for a down converter unit 2. The output of the down converter unit 2 is connected via buffer capacitor CB to a commutation unit 6, which in turn supplies an ignition stage 5 by means of which the lamp 1 is ignited and operated. When the lamp 1 is ignited, a discharge arc is established between the electrodes 3, 4 of the lamp 1. The frequency of the lamp current is controlled by a frequency generator 7, and the wave shape of the lamp current is controlled by a wave forming unit 8. A control unit 11, whose function will be explained in more detail below, supplies control signals 70, 80 to the frequency generator 7 and wave forming unit 8 respectively so that the amplitude, frequency and wave-shape of the lamp voltage and current can be controlled according to the momentary requirements.

The voltage applied to the buffer capacitor C_B is additionally fed via voltage divider Ri, R₂ to a voltage monitoring unit 12 in the control unit 11. The voltage monitoring unit 12 monitors the operating voltage of the lamp 1. The operating voltage can be measured at predetermined time intervals given by a timer or clock, not shown in the diagram. Measured values are compared to values of target voltage V_T and safety brake voltage V_SB stored in a non-volatile memory 16. A setting unit 17, shown here as part of the voltage monitoring unit 12, checks whether the voltage drops below the safety brake voltage VSB- As described in WO 2005/062684 Al, when the lamp is being driven in a normal mode of operation according to an electrode stabilisation driving scheme comprising two different driving schemes A, B, and the lamp voltage exceeds or drops below a target voltage level V_T, a switch-over is made from one driving scheme to another. In this way, the growth of the electrode tips can be controlled to some extent. Possible driving scheme parameters (wave-shape, frequency etc.) for the electrode stabilisation driving schemes A, B are described in WO 2005/062684 Al.

When the lamp voltage drops below a safety brake voltage level V_SB, a remodelling indicator value RMI is set by the setting unit 17 and stored in the nonvolatile memory 16.

On the basis of a control output from the voltage monitoring unit 12, a driving scheme switching unit 14 decides on driving scheme A, B, C to apply and on the wave shape and frequency with which the lamp 1 is to be driven during that driving scheme. Accordingly, it supplies the appropriate signals 70, 80 to the frequency generator 7, which drives the commutation unit 6 at the appropriate frequency, and to the wave-shaping unit 8, which, using the down converter 2, ensures that the correct current/pulse wave shape is generated for that chosen driving scheme A, B, C.

The control unit 11 itself knows when the lamp 1 is being driven in an ancillary mode such as a shut-down or switch-on phase. When the remodelling indicator RMI is set, therefore, the control unit 11 can cause the remodelling driving scheme C to be applied. Furthermore, a signal 13 from the projection system (not shown) informs the control unit 11 of the current status of an image rendering application, so that, even during operation of the projection system, the remodelling driving scheme C can be applied, for example in an ancillary mode of operation between operating at normal and dimmed power levels or during a content-source switch., For example, in a home entertainment system, such a content switch might take place when a change-over is made to show image content from, say, a satellite receiver instead of from a DVD (digital versatile disc) player. In a business environment, for example during a presentation, such a content switch might take place when presenters change, for example when one laptop is disconnected from the projection system and another laptop is connected, or when any other event takes place indicating a change in presenter. In this short space of time when the content switch is being made, the electrode remodelling can be carried out. When the driving unit 10 shown is used in a projection system, a synchronisation signal S is supplied from the projection system (not shown) to the driving unit 10, and is distributed to the frequency generator 7, the wave-shaping unit 8 and the control unit 11, so that the lamp driver 10 can operate synchronously with, for example, a display unit or a colour generation unit of the projection system.

In the diagram, the memory 16, the driving scheme switching unit 14, the voltage monitoring unit 12, and the timer 15 are all shown as part of the control unit 11. Evidently, this is only an exemplary illustration, and these units could be realised separately if required. The control unit 11 or at least parts of the control unit 11, such as the driving scheme switching unit 14, can be realised as appropriate software that can run on a processor of the driving unit 10. This advantageously allows an existing lamp driving unit to be upgraded to operate using the method according to the invention, provided that the driving unit is equipped with the necessary wave-shaping unit and frequency generator. The driving unit 10 is preferably also equipped with a suitable interface (not shown in the diagram) so that values of target voltage, safety brake voltage and any other required parameters can be loaded into the memory 16 at time of manufacture or at a later time, for example when a different lamp type is substituted or a different performance is desired.

Fig. 7 shows a possible realisation of a projection system 22 with a lamp 1 mounted in a reflector 18 and controlled by a driving unit 10 as described above. Light emitted by the lamp 1 is cast in the usual manner at a display 20 and projected onto a screen 21 for viewing. An image rendering control module 19 of the projection system 22 controls the display 20 and supplies the driving unit 10 with a synchronisation signal S and an information signal 13 so that the driving unit 10 can apply the driving scheme most suited to the momentary operating status of the lamp 1.

The invention can preferably be used with all types of ultra-short-arc UHP lamps that can be driven with the method described above in applications requiring a stable arc (both axial and lateral). Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention. For example, the electrode stabilisation driving scheme mentioned is not limited to the two different driving schemes A and B mentioned in the description. Any number of appropriate driving schemes could be implemented, as appropriate. Similarly, the method according to the invention is not limited to the use of a single remodelling driving scheme, but could conceivably apply one of several different remodelling driving schemes, according to the requirements of the lamp.

It is also conceivable that a lamp driver could manage several different target and/or safety-brake voltages for a lamp, and can apply particular voltage thresholds according to the conditions under which the lamp is being driven at any one time. For example, the safety-brake voltages could be different in nominal and dimmed operation.

For the sake of clarity, it is to be understood that the use of "a" or "an" throughout this application does not exclude a plurality, and "comprising" does not exclude other steps or elements. A "unit" or "module" can comprise a number of units or modules, unless otherwise stated.

Claims

CLAIMS:

1. A method of driving a gas-discharge lamp (1) enclosing a pair of electrodes (3, 4), wherein during a normal mode of operation, the lamp (1) is driven according to a stabilisation driving scheme, which stabilisation driving scheme comprises a repeating sequence (S_AB) of driving schemes (A, B), wherein each driving scheme

(A, B) causes a particular shape change of the electrodes (3, 4), and the driving scheme sequence (S_AB) is compiled such that the electrodes (3, 4) of the lamp (1) are subject to repetitive shape changes; and, during an ancillary mode of operation in which an alteration in light output of the lamp (1) is essentially imperceptible to an observer, the lamp (1) is driven according to a remodelling driving scheme (C) to extensively remodel the electrodes (3, 4).

2. A method according to claim 1, wherein - a lamp parameter is monitored during a mode of operation of the lamp (1), and a remodelling indicator (RMI) is set if the monitored lamp parameter indicates that a remodelling of the electrodes (3, 4) is necessary.

3. A method according to claim 1 or claim 2, wherein the observed lamp parameter comprises a lamp voltage value, and the lamp (1) is driven according to a remodelling driving scheme (C) when the lamp voltage reaches a minimum voltage value for that lamp (1) while the lamp (1) is being driven in an ancillary mode of operation; or the remodelling indicator (RMI) is set when the lamp voltage reaches a minimum voltage value for that lamp (1) while the lamp (1) is being driven in a normal mode of operation.

4. A method according to any of claims 1 to 3, wherein the remodelling driving scheme (C) comprises an alternating current at a frequency in a range between 10Hz and 40Hz, preferably in a range between 15Hz and 25Hz.

5. A method according to any of claims 1 to 3, wherein the remodelling driving scheme (C) comprises a direct current, which direct current is applied for a predefined time interval to the electrodes (3, 4) of the lamp (1).

6. A method according to any of the preceding claims, wherein the remodelling driving scheme (C) according to claim 4, and, optionally, according to claim 5 is applied over a time interval in a range between 0.1s and 20s, preferably in a range between 0.1s and Is.

7. A method according to any of claims 4 to 6, wherein, while the lamp (1) is being driven using a remodelling driving scheme (C), the frequency of the lamp current is successively decreased, and/or the direct current is successively increased.

8. A method according to any of the preceding claims, wherein, when the lamp (1) is driven in normal mode using the repeating sequence (S_AB) of driving schemes

(A, B), a switch-over between driving schemes (A, B) is triggered according to a relationship between an observed lamp parameter value and a threshold level (V_T).

9. A method of driving a projection system (22) comprising a gas-discharge lamp (1), which lamp (1) is driven according to any of claims 1 to 8, wherein a time during which image content is not rendered is allocated to be a time during which the lamp (1) can be driven in the ancillary mode of operation.

10. A method of driving a projection system (22) comprising a gas-discharge lamp (1), which lamp (1) is driven according to any of claims 1 to 8, wherein a changeover phase from a first image rendering mode of operation to a second image rendering mode of operation is allocated to be a time during which the lamp (1) can be driven in the ancillary mode of operation.

11. A driving unit (10) for driving a gas-discharge lamp (1) enclosing a pair of electrodes (3, 4) and comprising a switching unit (14) for switching between a stabilisation driving scheme applied during a normal mode of operation, which stabilisation driving scheme comprises a repeating sequence (S_AB) of driving schemes (A, B), wherein each driving scheme (A, B) causes a particular shape change of the electrodes (3, 4), and the driving scheme sequence (S_AB) is compiled such that the electrodes (3, 4) of the lamp (1) are subject to repetitive shape changes; and a remodelling driving scheme (C) applied to extensively remodel the electrodes (3, 4) during an ancillary mode of operation in which an alteration in light output of the lamp (1) is essentially imperceptible to an observer.

12. A driving unit (10) according to claim 11 , comprising a monitoring unit (12) for monitoring a lamp parameter during a normal mode of operation of the lamp (1), and a setting unit (17) for setting a remodelling indicator (RMI) if the monitored lamp parameter indicates that a remodelling of the electrodes (3, 4) is necessary.

13. A driving unit (10) according to claim 11 or claim 12, comprising a nonvolatile memory (16) for storing the remodelling indicator value (RMI).

14. A projection system (22) comprising a gas-discharge lamp (1) and a driving unit (10) according to any of claims 11 to 13 for driving the lamp (1).

15. A projection system (22) according to claim 14, comprising an information signal (13) to inform the switching unit (14) of an ancillary mode of operation.