GB2521666A

GB2521666A - Extended life discharge lamp

Info

Publication number: GB2521666A
Application number: GB1323093.3A
Authority: GB
Inventors: Graham Harry Moss; Martin Kavanagh
Original assignee: Digital Projection Ltd
Current assignee: Digital Projection Ltd
Priority date: 2013-12-27
Filing date: 2013-12-27
Publication date: 2015-07-01
Also published as: GB201323093D0

Abstract

A system and method for operating a discharge lamp monitors the lamp voltage while operating the lamp at a normal power level. When the voltage passes a threshold value the input power level is changed to a regeneration power level for a first period of time, operation at which power level may at least partially restore electrode shape of the lamp. For different types of lamp the change to the regeneration power level may be made when the voltage exceeds a threshold voltage, or drops below a threshold voltage, as appropriate. The regeneration power may be higher or lower than the normal power, as appropriate. Which options are appropriate will depend upon the characteristics of the lamp used.

Description

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EXTENDED LIFE DISCHARGE LAMP

TECHNICAL FIELD

This invention relates to discharge lamps. In particular, the invention relates to the automatic reconditioning of electrode elements within such a lamp.

BACKGROUND

In discharge lamps, a light source is formed by applying a voltage and current waveform to two spaced apart electrodes within an envelope, to create a light emitting plasma between the electrodes.An order to do this a power supply provides a high voltage waveform to the lamp, generally under software command. The space within the envelope contains a mix of gases, compounds and elements to provide a medium for a plasma discharge. The shape of the plasma is determined by the envelope design, and parameters such as operating pressure, IS voltage, current, electrode shape and electrode separation.

Ouring operation, the main parameters that can vary in the lamp are the electrode shape and electrode separation. One possible mechanism varying the electrode separation is that as the lamp ages, the electrodes burn back so that the gap between them gets wider. This effect may be caused by the continuous loss of electrode material by evaporation from the tips of the electrodes, which are generally domed or pointed. This tends to cause a transition from electrodes with domed or pointed tips to electrodes with flat or blunt tips, the transition also causing the gap between the electrodes to increase. This in turn tends to make the plasma shape between the electrode tips get longer and thicker.

The lost electrode material will generally eventually be deposited elsewhere in the lamp, including on parts of the electrodes away from the electrode tips.

The increase in size of the plasma may cause a major loss of overall efficiency in an optical system linked to the lamp and a gradual reduction in the useable optical output power of the lamp as the lamp ages.

Another possible mechanism is that the loss of the electrode material from the tips of the electrodes may change the profile of the tips of one or both electrodes to a profile that does not support a stable arc. This may result in flickering of the arc.

Another possible mechanism is that spikes or whiskers or electrode material may grow from the tips of the electrodes. This may cause the intensity of the light emitted by the arc to be reduced and ultimately form a short circuit, causing the lamp to fail.

Therefore, a mechanism to at least partially restore electrode shape, and to mitigate the effects of erosion, is desired. Further, it is desired that such partial I 5 restoration of electrode shape be automatic without requiring human intervention.

Such restoration may advantageously extend the usefW life of the lamp.

SUMMARY OF THE INVENTION

In one aspect, the invention provides.a method of operating a gas discharge lamp, the method comprising: measuring an operating voltage of the lamp at a first input power level, comparing the measured operating voltage to a first threshold value, and based on the result of the comparison, either setting the input power to the lamp to a second input power level different from the first for a first period of time, and then setting the input power to the lamp back to the first power level, or maintaining the input power at the first power level.

In another aspect, the invention provides a control system for operating a gas discharge lamp, the control system configured to perform at least the following steps: measure an operating voltage of the lamp at a first input power level; compare the measured operating voltage to a first threshold value; and, based on the. result of the comparison; either set the input power to the lamp to a second power level different from the first for a first period of time; and then set the input power to the lamp back to the first power level; or maintain the input power at the first power level.

In another aspect the invention relates to. operating a discharge lamp at a normal power while monitoring the voltage. When the voltage passes a threshold value the power is changed to a regeneration power level. For differenLtypes of lamp the change to regeneration power may be made when the voltage exceeds a threshold voltage, or drops below a threshold voltage, as appropriate. The regeneration power may be higher or lower than the normal power, as appropriate.

Which options are appropriate will depend upon the characteristics of the lamp IS used.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. I is a cross-sectional view of the core of a typical discharge lamp.

FIG. 2A is a cross-sectional view of discharge lamp electrodes prior to erosion.

FIG. 2B is a cross-sectional view of discharge lamp electrodes after erosion.

FIG. 3 shows components of a system that uses the electrode regeneration of the present invention.

FIG. 4 shows components of the control system of the present invention.

FIG. 5A is a flowchart of a method for automatic electrode regeneration, according to a first embodiment.

FIG. SB is a flowchart of another method of automatic electrode regeneration, according to a second embodiment.

FIG. 6A is a graph showing the variation of lumen output over time of a discharge lamp that uses the principles of the present invention.

FIG. 6B is a graph showing the variation of lamp operating voltage over time of a discharge lamp that uses the principles of the present invention.

FIG. 6C is a graph showing the variation of lamp input power over time of a discharge lamp that uses the principles of the present invention.

FIG. 7 is a graph showing maximum output luminance of a discharge lamp over time, both when electrode regeneration is used and when it is not used.

FIG. 8 shows components of a typical projector system including a discharge lamp with the automatic electrode regeneration of the present invention.

DETAILED DESCRIPTION

FIG. I is a cross-sectional view of the core part of a typical alternating current (AC) discharge lamp 100. This part of the lamp 100 comprises an envelope 110 which may, for example, be formed from glass, ceramic, quartz or sapphire. The space 140 within the envelope 110 cothains a mix of gases, compounds and elements to provide a medium for a plasma discharge. The lamp 100 further comprises two electrodes, 120a and l2Ob, extending into the envelope 110, generally from opposing sides of the envelope 110. Prior to use, in the illustrated example, the electrodes I 20a and I 20b are elongate, narrow and have tapered tips.

The tips of the electrodes I 20a and 1 20b face each other and there is a small gap between the electrode tips. Other shapes of electrode are possible and may be used according to the present invention. In other examples, the electrodes I 20a and I 20b may have a domed or pointed tip, or may not have a tapered tip.

In the illustrated example, the electrodes l2Oa and I20b are made from tungsten.

Other materials may be used for the electrodes I 20a and I 20b as will be readily apparent to a person of skill in the an. Further, in the illustrated embodiment, the discharge lamp 100 is a mercury arc lamp. Accordingly, in operation the space within the envelope 110 contains vaporised mercury. Other examples may utilise other gases to fill the envelope 110, as will be understood by a person of skill in the art. The gas or gases used to fill the envelope will be depend on the type of discharge lamp. In some examples the envelope 110 may contain a noble

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gas or gases such as Argon or Xenon. In the illustrated embodiment, the gas within the envelope 110 would typically be at a pressure of around I mBar when the lamp 100 is not in use.

In the illustrated example the lamp is an AC discharge lamp. Accordingly, the electrodes 120a and 120b are identical. In other examples the lamp may be a DC discharge lamp, and in such examples the electrodes may have different shapes or profiles.

During operation of the lamp 100, a light source is formed by applying a voltage and current AC waveform to the two electrodes I 20a and I 20b. The voltage and current may typically be provided by a power control unit. Application of the voltage and current to the electrodes I 20a and 1 20b creates an arc or light emitting plasma 130 between thetips of the electrodes I 20a and I 20b, the arc or plasma between the electrode tips comprising an intense localised source of light.

The illustrated example is an AC discharge lamp. The present invention may also be applied to DC discharge lamps. In such examples a controlled current and voltage may be applied to the two electrodes, instead of an AC waveform.

FIGS. 2A and 2B comprise cross-sectional views of two electrodes 220a, 220b, 240a, 240b before and after electrode erosion. As describcd above, in the illustrated example erosion occurs where the temperature of the plasma is high, causing evaporation or loss of the tungsten at the tips of the electrodes. In the illustrated example of a mercury arc lamp with tungsten electrodes, electrode erosion occurs when the electrode surface in contact with the arc becomes sufficiently hot that tungsten atoms can be removed from the electrode by energetic ions from the arc striking the electrode surface. The evaporated tungsten is deposited elsewhere in the envelope 110, coating any surface other than the electrode tips, including cooler parts of the electrodes. This causes a gradual reshaping of the electrodes over time. In many discharge lamps the contents of the envelope are selected and arranged so that Tungsten is chemically scavenged and redeposited on the electrodes near the tips. FIG. 2A illustrates the electrodes 220a and 220b prior to erosion. As can be seen, here the electrodes 220a and 220b are elongate and narrow and have tapered tips. Further, the shape of the plasma 230 between the electrodes 220a and 220b is relatively, short and narrow. FIG. 28 illustrates the electrodes 240a and 240b after erosion. Here, the electrodes 240a and 240b are shorter and thicker with substantially flat tips. The erosion also causes the gap between the electrodes to increase. The plasma shape is also changed. After electrode erosion, the plasma 260 tends towards a rectangular form, and is longer and wider.

A further effect of electrode erosion is that as the gap between the electrodes increases, the operating voltage required to drive the lamp also increases.

In developing the present invention, it was discovered that by changing the lamp power the electrodes may be regenerated and at least partially returned towards their original shape. For example, by reducing the lamp power temporarily, the electrodes 240a and 240b of FIG. 2B which are shortand thick with flat tips may beat least partially restored towards the shape of the electrodes 220a and 220b of FIG. 2A, which are elongate and narrow with domed tips. In this example, electrode regeneration while the lamp 100 is in the changed power mode occurs because the change in the input lamp power has the effect of changing the size and shape of the arc plasma.

In the example of Fig.2A and Fig. 28, the change in size and shape of the plasma arc has the effect of moving a condensation site of the electrode material nearer to the centre of the electrode. This reverses the electrode erosion shaping effect so that an elongate and narrow electrode with domed tips is rebuilt as electrode material condenses closer to the centres of the electrode tips. Such electrode regeneration causes, advantageously, the size of the light emitting plasma to be reduced, typically by reducing both the length and the axial diameter of the light emitting plasma.

There are a number of possible mechanisms which can be used to regenerate the electrodes.

In examples where the electrodes are tungsten a halogen cycle may be used where the lamp contains a halogen scavenger material, such as bromine or iodine.

Provided the operating temperature of the lamp is high enough any tungsten deposited on internal surfaces of the lamp will combine chemically with the halogen scavenger to form a halogenous gaseous compound which will circulate within the lamp until i enters a region hot enough to decompose the halogenous compound and deposit the scavenged tungsten metal onto the hot surface.

Another example of a regeneration mechanism is that the halogenous gaseous compound can be decomposed by the heat in the arc and the resulting tungsten metal ions are deposited from the arc onto the electrodes. This regeneration mechanism may be particularly effective at lower lamp power levels where the lower temperature of the electrodes may be cool enough to effectively condense the tungsten metal ions and deposit the tungsten metal onto the tips of the electrodes to regenerate them.

In general these and other examples or regeneration mechanisms can be controlled to deposit electrode material onto desired locations by varying the operating power of the lamp.

In developing the present invention, it was realised that the instantaneous operating voltage of an arc lamp is a good measure of the extent of electrode erosion that has taken place during operation of the device.

In developing the present invention, it was further recognised that the fact that electrode erosion can be measured using the lamp operating voltage and the fact that electrode shape may be restored by a change in power supplied to the lamp could be used as a basis to develop a feedback control system for regeneration of discharge lamp electrodes, and in particular a control system for automatic regeneration.

The illustrated example is a mercury arc lamp with tungsten electrodes. The problem of electrode erosion may occur in other types of lamp, and for other electrode materials, and the approach of reducing the power applied to the lamp is order to regenerate the electrodes may be applied to these other types of lamp and electrode materials.

Another example is lamps which suffer from flicker. One example of a type of lamp which suffers from flicker is a Xenon lamp. Flicker can occur when electrode erosion causes the shape of an electrode to change so that the electrode tip no longer provides a stable arc seat able to stably support an arc. In Xenon lamps, this is generally caused by erosion changing the shape of the cathode tip.

In lamps which may suffer from flicker electrode regeneration may for example be carried out by increasing the lam? power temporarily. This increase in lamp power may melt the electrode tips, causing them to be reshaped into a form providing a more stable arc seat. When the temporary increase in the lamp power ends and the lamp power is reduced, the electrode tips may resolidifS' in the new shape providing a more stable arc seat.

Another example is the problem that spikes or whiskers of electrode material may grow from the tips of the electrode. The problem of spike growth can occur in arc lamps in some circumstances. This problem may occur, for example, in mercury arc lamps, including lamps with tungsten electrodes.

In such examples the spikes can cause the intensity of the light emitted by the plasma of the arc to be reduced, and may eventually form a short circuit, causing the lamp to fail.

A further effect of the spike growth is that as the spikes get larger, the operating voltage required to drive the map may decrease. This may lead to higher operating currents as the power supply driving the lamp attempts to maintaiii the lamp power at a constant value. If the operating current reaches the maximum that the power supply is able to provide, further reductions in the operating voltage may produce a reduction in the lamp power and light output.

In lamps with electrodes suffering from spike growth, the electrodes may be regenerated by increasing the lamp power temporarily. The increased lamp power may reduce the size of any spikes or eliminate them.

FIG. 3 is a diagram showing the components of a system 300 for automatic regeneration of eroded electrodes, such as electrodes 240a and 240b, in a discharge lamp 330.

Power supply unit (PSU) 310 supplies an input voltage to the power control unit 320. The power control unit 320 receives an input voltage from the PSU 310 and supplies a controlled voltage and power to the discharge lamp 330. The power control unit 320 interacts with a control system 340 to determine the level of input power to supply to the discharge lamp 330. In normal conditions, wherein the electrodes 220a and 220b are not eroded beyond a predetermined threshold level of erosion, the power control unit 320 provides a nonnal level of power to the lamp 330. This normal level of power generally corresponds to the desired or rated optical power output of the lamp 330.

When the electrodes have been eroded by mote than the threshold level of erosion, by providing a changed input power to the discharge lamp, the control system 340 may cause the eroded electrodes 240a and 240b of the lamp 100 to be regenerated as described above.

The control system 340 functions as follows. The control system 340 receives as an input from the power control unit 320 a voltage signal indicating the current operating voltage of the discharge lamo 100. If the operating voltage is lower than a predetermined threshold value, the control system 340 causes the power control unit 320 to continue to provide the normal level of power to the lamp 100. If the operating voltage is higher than the predetermined threshold value, the control system 340 causes the power control unit 320 to change the input power to the lamp 100. As is explained above, this changed input power causes the electrodes to regenerate.

In the illustrated embodiment, the PSU 3 10, power control unit 320 and discharge lamp 330 are separate components and interact with the control system 340 in the manner shown in Fig. 3. The control system 340 is capable of receiving signals from and controlling the power control unit 320, which in turn provides electrical power to the discharge lamp 100.

FIG. 4 is a diagram showing the principal components of the control system 340.

The control system 340 receives the operating voltage level from the power control unit 320 and outputs a signal that causes the power control unit 320 to provide a controlled level of power to the discharge lamp 100.

The control system 340 comprises a voltage monitor 420, control logic 430 and a switch 410 for setting the power control unit 320 to either a regeneration power mode or the normal mode. The voltage monitor 420 receives a voltage signal from the power control unit 320 and provides data or signals to the control logic 430 representing the current operating voltage of the discharge lamp 100. The control logic 430 determines, based on the current operating voltage, whether the power control unit 320 should provide the normal level of power to the lamp 100 or whether it should provide the regeneration level of power.

In embodiments where the regeneration power level is lower than the normal power level, the regeneration power level may be a lamp power that is the lowest power at which the lamp can safely operate. The lowest power at which the lamp can safely operate is sometimes referred to as the Eco power. In some examples, thisJs approximately 86% of the normal full power for a high-power bulb and approximately 80% of the normal thil power for other bulbs. These figures are examples only and are not intended to be limiting.

The control logic 430 outputs a control signal that indicates the desired power setting, determined by the control logic 430, that isto be supplied to the lamp 100.

This control signal is provided to switch 410 which in turn provides an input to the power control unit 320 indicative of the desired power.

In the illustrated embodiment, the control system utilises the dedicated control logic 430 to control the power control unit 320. In some examples the control logic 430 may be implemented by either using dedicated circuitry or a general-purpose microprocessor coupled to a memory, wherein the memory comprises instructions which when executed by the microprocessor, causes the microprocessor to perform the functions of the control logic 430.

In the illustrated embodiment the changed regeneration power level is lower than the normal power level. In other examples, the regeneration power level may be higher than the normal power level. For example, in lamps which may suffer from flicker or lamps which may suffer from spike growth the regeneration power level may be higher than the normal power level in order to reshape the electrodes, as described previously.

FIG 5A is a flowchart 500 of one embodiment of the invention, indicating the operations performed by the control logic 430 described above. As mentioned S above, in some examples the control logic 430 may be implemented by either using dedicated circuitry or by a microprocessor coupled to a memory.

In a first step 510, the control logic 430 monitors the current operating voltage of the discharge lamp I 00. It does this by means of a voltage signal provided by the power control unitS20 as described above.

At step 520, the control logic 430 determines whether the current operating voltage Vop of the lamp 100 is greater than a predetermined threshold value, VTHRESHOLD. If the operating voltage is greater than the threshold value, this indicates that the electrodes have been eroded to such an extent that regeneration is desirable. Therefore, if the operating voltage is greater than the threshold, at step 530, the input power to the lamp 100 is changed to the regeneration power level. This will cause the lamp 100 to operate in the regeneration power mode, which in turn will cause electrode regeneration.

In the illustrated embodiment the value of VTHRESHOLD is chosen to correspond with a level of erosion of the electrodes that reduces the lumen output to below a certain level. This is not necessarily a critical level below which operation is not possible, but may, for example, be a level at which the reduction in output becomes noticeable or inconvenient for users. In other examples the value of VTHRESHOW may be chosen to correspond to a voltage which if exceeded would result in lamp failure.

After setting the lamp 100 to the regeneration power mode, the control logic 430 at step 540 waits for a predetermined amount of time, T, to elapse The time TE is chosen to be sufficient to allow the electrodes to regenerate so that they are restored as closely as possible to their normal shape and size. In some examples TE may be about four hours.

After regeneration during time TE, at step 550 the control logic 430 sends a signal to the power control unit 320 indicating that the lamp 100 should once again be provided with a normal level of power. The control logic 430 then returns to step 510 of flowchart 500 and continues to monitor the voltage.

On the other hand if at step_520 the control logic 430 determines that the operating voltage is below the threshold, at step 560 the control logic 430 will not modify the power level of the lamp 100. In other words at step 560, as the operating voltage is below the threshold, the control logic 430 leaves the power at the normal value.

The process then returns to step 510 where the control logic 430 continues to monitor the operating voltage of the lamp 100.

In the illustrated embodiment the input power to the lamp is changed to the regeneration power level when the operating voltage of the lamp is greater than a threshold value. In other examples, the input power to the map is changed when the operating voltage of the lamp is lower than a threshold value. For example, in lamps which may suffer from spike growth the input power may be increased to reshape the electrodes when the operating voltage of the lamp drops below a threshold. In such examples the threshold value may correspond to a level of spike growth that lowers the light intensity of the lamp to such an extent that regeneration is desirable.

FRI SB is a flowchart 600 of a second, more complex embodiment of the invention, indicating the operations performed by the control logic 430. As before, in some examples the control logic 430 may be implemented either using dedicated circuitry or by a general-purpose microprocessor coupled to a memory.

In this embodiment automatic regeneration of the electrodes 240a and 240b when they have been eroded to an extent where regeneration is desirable is made optional under user control and a further operating voltage threshold is introduced beyond which the system should not proceed as this is an operating voltage beyond which the lamp will reach a driver limit at which the lamp will automatically be closed down by the power control unit, or other driver. In this embodiment the operating voltage threshold corresponds to the electrodes being eroded to an extent where regeneration is desirable, while the further operating voltage threshold corresponds to the electrodes being eroded to an extent where regeneration is required. In some examples the driver limit voltage corresponds to the lamp being in a condition so far from its original condition that it is unreliable or unsafe to operate. In some examples the driver limit may be the 1 5 highest voltage the driver is able to provide.

Allowing the option of selectively disabling automatic regeneration of electrodes ensures that, for example, a user can ensure that a discharge lamp 100 does not automatically change power to the changed regeneratiQn power mode when this would be undesirable. This may for example be undesirable when the discharge lamp 100 is being used in a projector for a presentation to a live audience. This may also be undesirable when the discharge lamp 100 is being used in a projector producing a part of a large image, with other projectors producing other parts of the image, so that a change in lamp power will render the illumination of different parts of the large image non-uniform. In general, even when the operating voltage threshold has been reached, continuing to operate the lamp for a short period without regeneration will not cause any problems.

However, if the further operating voltage threshold is reached it is generally better to accept the possible effects of changing to the regeneration power rather than risk the iamp shutting down entirely if the driver limit is reached.

In a first step 610, the control logic 430 monitors the operating voltage of the lamp 100 by means of the voltage signal provided by the power control unit 320.

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At step 620, the control logic 430 determines whether or not automatic regeneration of the electrode has been enabled by the user.

If at step 620 it is determined that automatic regeneration of the electrodes is enabled by the user, the control logic 430 at step 630 determines whether the operating voltage Vop is greater than the threshold voltage value VTHRESHOLD, as described above.

If it is determined that the operating voltage exceeds the threshold value, then at step 640 the control logic sends a signal to the power control unit 320 indicating that the lamp 100 should run at the regeneration power, that is, in the changed, regeneration power mode.

After this, at step 670, the control logic 430 waits for a predetermined amount of time, TE to elapse. This allows the electrodes enough time to regenerate to the required level.

After regeneration during time TE, at step 660 the control logic 430 sends a signal to the power control unit 320 indicating that the lamp 100 should once again be provided with a normal leve! of power. The control logic 430 then returns to step 610 of flowchart 600 and continues to monitor the voltage.

If in step 630 it is determined that the operating voltage Vop does not exceed the threshold voltage VTHRESHOLD, then it is assumed that the electrodes have not yet been eroded sufficiently to warrant regeneration. In this case, the process progresses to step 680 where the input power remains at its normal value. The process then returns to step 610 where the control logic 430 continues to monitor the operating voltage of the lamp 1 00.

Thus, it can be seen that when automatic regeneration is enabled, the operation of the second embodiment is identical to that of the first embodiment.

In the second embodiment, however, there is a further automatic operation to that described in the embodiment represented by FIG. 5A. In this embodiment, the control logic 430 also determines whether the operating voltage has exceedecLa..

further threshold value higher than the threshold value. The further voltage threshold value is different to the voltage threshold value described above. The further voltage threshold value corresponds to the electrodes being eroded to an extent where regeneration is required, whereas the threshold value corresponds to regeneration being only desirable. As was discussed above, regeneration being required corresponds to a maximum voltage limit beyond which, if the operating voltage were to be set any higher, the operation of the bulb would be automatically closed down.

If at step 620, it is determined that automatic regeneration of the electrode has been disabled by the user, the process progresses to step 650. For example, a user might wish to ensure that during a presentation to a live audience, his projector does not automatically transition to the changed, regeneration power mode upon detection of a certain level of electrode erosion where regeneration is desirable.

Such a transition may adversely affect the delivery of the presentation. For example, where the regeneration power level is lower than the normal power level the change to the regeneration power mode may noticeably reduce the brightness of the projector display. Similarly, where the regeneration power is higher than the normal power level the change may noticeably increase the display brightness.

At step 650, the control logic 430 checks whether the current operating voltage of the lamp 100 exceeds the further threshold value, VMAX, described above.

If, at step 650 it is determined that the current operating voltage indeed exceeds the further threshold value, the process progresses to step 640, in which, as described above, the control logic 430 sends a signal to the power control unit 320 to provide the lamp 100 with the changed, regeneration level of power. That is, the lamp is set to the regeneration mode. Subsequent to setting the power to the regeneration level, the control logic 430 then waits for a second predetermined time TE to expire. This gives time for the electrodes to be restored sufficiently close to their original shape and size.

At step 660, after time TE has expired, so that the electrodes have been regenerated, the power is set back to its normal value. The process then returns to step 610 and the voltage monitor resumes monitoring of the operating voltage of the lamp 100.

If at step 650, it is determined that the current operating voltage does not exceed the further threshold value, control logic 430 determines that no action is to be taken. Therefore, the process returns to step 610 where the voltage monitor resumes monitoring of the operating voltage of the lamp 100.

Note that there is no need to check if the operating voltage is greater than VMAX if the system has ascertained that V0p is less than VTHRESMOLD. This is because VM is higher than VTHRESHOLD and therefore if Vop is less than VTHRESUOLD then Vop will also be less than VMAX. As explained, VMAX corresponds to an operating voltage level beyond which further electrode erosion would cause the lamp to be closed down.. VTIIRESHOLD on the other hand is concerned with a level of erosion of the electrodes that reduces the lumen output to below a certain (not necessarily critical) level. It is clear that therefore, VMAX will be greater than VTHRESHOLD.

In both of the above embodiments the time TE may be about four hours.

In some examples of both of the above embodiments, the threshold value may be about 11 5V. In some examples of the embodiment of figure SB, the further threshold value may be about 145V. In some examples of both embodiments, the operating voltage of the discharge lamp 100 at the start of the lamp's life is in the range of 80-bOy.

In the embodiment of figure SB described above, the system waits for the same amount of time TE in each of two conditions. The first condition is where the operating voltage is found to be higher than the voltage threshold value and where automatic regeneration of the electrodes is enabled. The second condition is where the operating voltage is found to be higher than the further voltage threshold value and where automatic regeneration of the electrodes is enabled. In other examples, the system may wait for a time TE in the first condition, and a time TM in the second condition, where the times TE and TM are different. In some examples the time TM is longer than the time TE.

In the illustrated second embodimen: the regeneration lower level is lower than the normal lower level. Similarly to the first embodiment, in other examples the regeneration power level may be higher than the normal power level.

In the illustrated second embodiment when automatic regeneration is enabled, the operation is identical to the first embodiment. Similarly to the first embodiment, in other examples the input power to the lamp may be changed when the operating voltage of the lamp is lower than a first threshold value.

Further in such examples, when automatic regeneration is disabled by the user, the input power to the lamp may be changed to the regeneration power level when the generating voltage of the lamp is lower than a second threshold value. This second threshold value may be lower than the first threshold value.

For example, in lamps which may suffer from spike growth the second threshold value may correspond to a level of spike growth beyond which the lamp may be short circuited.

In some examples where automatic regeneration of the electrodes may be disabled by a user, the user selection to disable automatic regeneration may be automatically cancelled in response to events or the passage of a set period of time. In some examples the user disabling of automatic regeneration may be automatically cancelled when the lamp is switched off and subsequentlyswitched on. This may help to prevent a user forgetting, or not noticing, that they or a previous user had disabled automatic regeneration and unintentionally continuing operation of the lamp without regeneration until the fbrther voltage threshold value is reached.

In other examples automatic representation of the electrodes may remain disabled until a user manually re-enables automatic regeneration.

FIGS. 6A, fIB and 6C show three graphs which illustrate the functioning of an embodiment of the present invention that is capable of automatic regeneration of electrodes in discharge lamps. The graphs of FIGS. 6A, 6B and 6C illustrate an example where the changed, regeneration power level is lower than the normal power level so that the bulb power level is reduced to carry out electrode regeneration. FIG. 6A is a graph showing the variation of lumen output over time of a lamp 100 using the control system 340 of the present invention. It can be seen that as the electrodes erode and regenerate the lumen output fluctuates. FIG. ÔB is a graph showing the variation of the lamp operating voltage over time. FIG. 6C is a graph showing the variation of lamp input power over time. As such, the three graphs 6A, 6B and 6C illustrate the changes in these three variables over time for the same lamp.

In the graphs, prior to time T1, the lamp 100 has been functioning normally for S some time. Prior to time T1, as time progresses, the electrodes of the lamp 100 are gradually eroded as described earlier. This causes the operating voltage to gradually rise. The operating voltage is illustrated in the graph of FIG. 6B, and it can be seen that prior to time T1, the operating voltage gradually rises with increasing electrode erosion. As described, electrode erosion makes the plasma shape get longer and wider. The increase in size of the plasma causes a loss of efficiency in the system and a gradual reduction in output optical power. Thus, as depicted in the graph of FIG. GA, prior to time T1, the lumen output of the lamp gradually falls. However, prior to time T1, the input electrical power t the lamp 100, depicted in the graph of FIG. 6C, is at its normal level, indicated by the label "100%".

At time Ti, the operating voltage reaches the threshold value discussed above. As described above, the power control unit 320 provides a signal to the control system 340 indicating the current operating voltage of the lamp 100. The control system 340 then provides a signal to the power control unit 320 indicating that the lamp should be set to a reduced pewer regeneration mode. The mechanism governing the control system 340 has been described above. The control logic 430 in the control system 340 will cause the lamp power to be at the reduced power regeneration level for a certain amount of time, TE, as discussed above. This is depicted in the graph of FIG. 6C as the time elapsed between T1 and T2.

When the lamp power is reduced, electrode regeneration starts as described above. As the electrodes regain their &iape, the operating voltage required to drive the electrodes falls. Further, due to the reduction in input electrical power, the lumen output of the lamp immediately falls, as depicted in the graph of FIG. 6A.

This is a temporary fall in lumen output. The lumen output will then gradually increase as the electrodes are regenerated.

At time T2, the electrodes are deemed to have been adequately regenerated because sufficient time, TE, has been spent with the lamp 100 in the reduced power regeneration mode. Note that in practice the electrodes will usually never regain their original shape completely. Each time the electrodes are eroded and regenerated, the electrodes usually lose some of their material permanently.

At time T2, as the electrodes have been sufficiently regenerated, the operating voltage of the lamp 100 has fallen as depicted in the graph of FRi. 6B at time T2.

After time TE has elapsed, the control system 340 provides a signal to the power control unit 320 indicating that the lamp 100 should be run under frill power, indicated by "100%", once again. The lamp 100 is then switched to frill power and the lumen output of the lamp 100 is increased. However, as mentioned above, the electrodes do not entirely regain their former shape and therefore the lumen output does not necessarily go back entirely to its previous level under normal power.

Subsequent to time T2, because the lamp 100 is restored to frill power, the electrodes start eroding once again, thereby causing the operating voltage to again increase as can be seen between times T2 and T3. Due to the electrode erosion, the lumen output falls just as described above in relation to the time prior to T1. Once again, when the operating voltage reaches the threshold voltage, the control system 340 causes the power control unit 320 to provide the lamp 100 with the reduced regeneration power, just as described above in relation to the time prior to T1. This then results in electrode regeneration again.

The cycle above repeats. In this example, each time the cycle is repeated, the electrodes lose some of their shape permanently and therefore the operating voltage does not fall back to it original value but rather, after each cycle, the minimum value of the operating voltage increases slightly. Therefore, although the useful life of the lamp 100 may be increased significantly it cannot be extended indefinitely.

In some examples, the time TE at which the system is maintained at changed, regeneration power is of the order of four hours. In othcr examplcs, the control system, instead of waiting for a fixed amount of time Tr to elapse before switching the lamp 100 back to normal power, the control system 340 instead waits for the operating voltage to go back to a predetermined voltage level before switching the lamp 100 back to normal power.

FIG. 7 depicts the variation of the maximum luminance of a discharge lamp 100 over time, with and without the electrode regeneration of the present invention. It is can be seen that by providing electrode regeneration by changing input power according to the present invention, *the useful life of the lamp 100 is greatly increased and the luminance values decline more slowly over time than when the electrode regeneration of the present invention is not utilised.

In some examples, the electrode regeneration does not take place automatically using a feedback control system but rather takes place under human control. For example, a human user may observe an indicator light which represents the operating voltage being below the threshold VTHRESHOLD. The human user may then manually set the discharge lamp to function at changed, regeneration power (that is, to function in regeneration mode). The human user may then reset the device to function at normal levels of power when another indicator light, representing the operating voltage being at acceptable levels, is activated, or after a predetermined time has passed.

EXEMPLARY A?PLICATION OF THE INVENTION FIG. 8 is an illustration of a projection system 800 that utilises the discharge lamp with electrode regeneration of the present invention.

Projection systems normally employ a number of optical and electronic systems to successfiuly translate an electronic signal containing image information into a projected image on a screen.

In the system of FIG. 8, a single lamp source 812 is used to illuminate a projection system. In other examples, multiple lamps may be combined into one effective lamp with auxiliary optics.

The lamp 812 illustrated in FIG. 8 is a discharge lamp 100 which, when controlled by the control system 340 of the present invention, is capable of electrode regeneration.

Light from the lamp, or multiple lamp system, is focussed onto an integrator and relay optical system 816 by means of a condenser optical system 810, or by the reflector of the lamp, or by some combination of the two. The integrator and relay optical system 816 provides the light to modulator 820. In this example, the modulator 820 comprises three reflective light modulators, one for each colour.

There is usually a filter system 814 employed in the optical path to remove unwanted wavebands such as infra-red and ultra-violet light. The integrator system changes the spatial format of the beam to suit the format required by the modutator 820, converts the illumination angle to one suitable for the modulator 820 operation and arranges the conjugates of the modulator and condenser optical system to be in convenient locations within the projector system 800.

In alternative configurations, it is possible to use three lamp sources to individually illuminate three modulators 820, and then combine the output.

As mentioned, a useful concept for analysing optical system efficiency is the etendue. This is an optical parameter of the optical system, defined as the product of the convergence solid angle and image size at any conjugate in the system.

There is usually a limiting etendue defined by some component within an optical system 800. In this case the limiting etendue is defined by the modulator 820. The relay-integrator system 814 converts the modulator 820 angle-image size product into a mote convenient angle-image size product at the lamp-condenser system focus by means of its magnification factor. Both products are the same and equal to the modulator etendue.

The efficiency of this system depends on many elements within the optical system. One element of interest is the coupling between the lamp-condenser optical system 810, 812 and the relay-integrator system 816. This is normally described in teims of the etendue of the lamp-condenser system 810, 812, and the etendue of the relay-integrator system 816. In general terms, it is necessary to keep the image size and convergence angle product of the lamp-condenser system 810, 812 smaller than or equal to the object size and acceptance angle product of the relay-integrator system 816 to ensure no losses are introduced at this interface.

The image size and acceptance angle product of the relay-integrator system 816 is fixed and determined by the modulator 820 etendue and the magnification of the relay integrator system 816. Therefore any factor which increases the convergence angle or image size of the lamp-condenser system 810, 812 beyond the limits set by the relay-integrator system 816 will reduce system efficiency.

The output convergence angle and image size of the lamp-condenser system 810, 812 is determined by the size of the light source and the magnification of the beam forming optics that image the source onto the relay-integrator system 816.

In this case the light source is formed by a discharge lamp 100 controlled by the control system of the present invention which is capable of causing automatic electrode regeneration.

The beam forming optics capture the radiation from the plasma arc of the discharge iamp 100 using the control system 340 of the present invention and transforms it into a configuration suitable for transfer to the relay-integrator system 816. This typically takes the form of a coaxial reflector, sometimes with additional focussing elements, which concentrates the radiation onto the relay-integrator system 816 with a known convergence angle and focal spot size. The nominal values of the plasma shape, and condenser factors, are used during the design of the system andiiny variation in the values can lead to loss of efficiency.

Any increase in the size of the plasma caused by electrode erosion tends to increase the etendue of the lamp-condenser system 810, 812 either by increasing the convergence angle, the image size, or both. This can cause a major loss of efficiency in the system and a gradual reduction in output power of the projector 800 as the lamp 100 ages.

As described above, the plasma shape depends on electrode erosion. Therefore, by using the electrode regeneration of the present invention, the plasma shape may be maintained closer to its original shape for a longer period of time than would be possible without the principles of the present invention. By using the control system 340 of the present invention with existing projectors the etendue of a lamp-condenser system 810, 812 within a projector 800 may be maintained within an acceptable range for a longer period of time. This in turn means that the acceptable lamp life of a lamp 100 used in a projector device 800 that utilises the control system 340 of the present invention is longer. By using this method, projectors have demonstrated a more than doubled lamp lifetime.

Apart from projector systems, other applications of the control system 340 of the prescnt invention, used with prior art discharge lamps, are possible. Indeed, any application requiring the use of discharge lamps may use the control system 340 provided in the present inventiOn to lengthen the useftul life of the lamp 100.

Another application where the present invention can be used is in the street lighting, for example in mercury vapour street lights. Other applications of discharge lamps equipped with the present control system 340 will be readily apparent to a person of skill in the art.

The illustrated embodiments are alternating current (AC) discharge lamps. The present invention may also be applied to direct current (DC) discharge lamps. In DC lamps the erosion and/or reshaping_of the electrodes may be asymmetric.

The control system of the invention may be provided using digital or analogue signals and components. In some examples the control system may comprise digital components. I5

Further, as mentioned, in some embodiments the control logic 430 of the control system 340 of the present invention, instead of being implemented by dedicated logic circuitry, may be implemented by a general purpose microprocessor coupled to a memory. The memory may contain software comprising data or instructions and the microprocessor may execute said instructions using said data to perform the method that would otherwise be performed by the control logic 430.

Thus, insofar as embodiments of the inventive control system 340 described above are implementable, at least in part, using a software-controlled programmable processing device such as a general purpose processor or special-purposes processor, digital signal processor, microprocessor, or other processing device, data processing apparatus or computer system it will be appreciated that a computer program for configuring a programmable device, apparatus or system to implement the foregoing described methods, apparatus and system is envisaged as an aspect of the present invention. The computer program may be embodied as any suitable type of code, such as source code, object code, compiled code, interpreted code, executable code, static code, dynamic code, and the like. The instructions may be implemented using any suitable high-level, lowdevel, object-oriented, visual, compiled and/or interpreted programming language, such as C, C++, Java, BASIC, Per!, Matlab, Pascal, Visual BASIC, JAVA, ActiveX, assembly language, machine code, and so forth. A skilled person would readily understand that term "computer" in its most general sense encompasses programmable devices such as referred to above, and data processing apparatus and computer systems.

The computer program may be stored on a carrier medium in machine readable form, for example the carrier medium may comprise memory, removable or non-removable media, erasable or non-erasable media, writeable or re-writeable media, digital or analog media, hard disk, floppy disk, Compact Disk Read Only Memory (CD-ROM), Company Disk Recordable (CD-R), Compact Disk Rewriteable (CD-RW), optical disk, magnetic media, magneto-optical media, removable memory cards or disks, various types of Digital Versatile Disk (DVD) subscriber identify module, tape, cassette solid-state memory. The computer program may be supplied from a remote source embodied in the communications medium such as an electronic signal, radio frequency carrier wave or optical carrier waves. Such carrier media are also envisaged as aspects of the present invention.

Claims

CLAIMS1. A method of operating a gas discharge lamp, the method comprising: measuring an operating voltage of the lamp at a first input power level; comparing the measured operating voltage to a first threshold value; and based on the result of the comparison; either setting the input power to the lamp to a second input power level different from the first for a first period of time; and then setting the input power to the lamp back to the first power level; or maintaining the input power at the first power level.
2. The method of claim I wherein, if the operating voltage is eater than the first threshold value; setting the input power to the lamp to the second power level different from the first for the first period of time; and then setting the input power to the lamp back to the first power level; or if the operating voltage is not greater than the first threshold value maintaining the input power at the first power level.
3. The method of claim I wherein, if the operating voltage is lower than the first threshold value; setting the power to the lamp to the second input power level different from the first period of time; and then setting the input power to the lamp back to the first power level; or if the operating voltage is not lower than the first threshold value, maintaining the input power at the first lower level.
4. A method of operating a gas discharge lamp, the method comprising: receiving an input that indicates whether the lamp is operating in a first mode or a second mode; if the input indicates that the lamp is operating in the first mode, performing the method of claim 2; and if the input indicates that the lamp is operating in the second mode: measuring the operating voltage of the lamp at the first input power level; comparing the measured operating voltage to a second threshold value; wherein, if the operating voltage is greater than the second threshold value: setting the input power to the lamp to the second power level for a second period of time; and then setting the input power to the lamp back to the first power level; or -1.0 if the operating voltage is not greater than the second threshold value, maintaining the input power at the first power level.
5. The method of any one of claims 1, 2 or4 wherein the first threshold value is an operating voltage beyond which erosion of electrodes of the discharge lamp exceeds a threshold level.
6. The method of any preceding claim wherein the first threshold value is about 115V.
7. The method of any preceding claim where the second power level is lower than the first power level.
8. The method of any one of claims 1 to 6 wherein the second power level is higher than the first power level.
9. The method of claim 4 wherein the first period of time and the second period of time are the sarne.
10. The method of claim 4 or claim 9 wherein the second threshold value is an operating voltage beyond which the discharge lamp would fail due to electrode erosion.
11. The method of any one of claims 4, 9 or 10 wherein the second threshold value is about 145V.
12. The method of any preceding claim wherein said first period of time is about four hours.
13 The method of claim 4 wherein said second power level is in the range of 80%-86% of the first power level.
14. The method of any preceding claim wherein the operating voltage of the gas discharge lamp at the start of the lamp's life is in the range of 80-IQOY.
15. The method of any preceding claim wherein the gas within the envelope of the gas discharge lamp comprises mercury vapour.
16. The method of any preceding claim wherein the gas within the envelope of the gas discharge lamp comprises a noble gas.
17. The method of any preceding claim wherein the electrodes of the gas discharge lamp comprise tungsten.
18. A control system for operating a gas discharge lamp, the control system configured to perform at least the following steps: measure an operating voltage of the lamp at a first input power level; compare the measured operating voltage to a first threshold value; and, based on the result of the comparison; either set the input power to the lamp to a second power level different from the first for a first period of time; and then set the input power to the lamp back to the first power level; or maintain the input power at the first power level.
19. The control system of claim 18 wherein, if the operating voltage is greater than the first threshold value; setting the input power to the lamp to the second power level different from the first for the first period of time; and then setting the input power to the lamp back to the first power level; or if the operating voltage is not greater than the first threshold value maintaining the input power at the first power level.
20. The control system of claim 18 wherein, if the operating voltage is lower than the first threshold value; setting the power to the lamp to the second input power level different from the first period of time; and then setting the input power to the lamp back to the first power level; or if the operating voltage is not lower than the first threshold value, maintaining the input power at the first lower level.
21. A control system according to any of claims 18 to 20 wherein the control system is further configured to: receive an input that indicates whether the system is in a first mode or a second mode; if the input indicates that the system is in the first mode, perform the steps defined in claim 19; and if the input indicates that the system is in the second mode: measure an operating voltage of the lamp at the first input power level; compare the measured operating voltage to a second threshold value; wherein, if the operating voltage is greater than the second threshold value: set the input power to the lamp to the second power level for a second period of time; and then set the input power to the lamp back to the first power level; or if the operating voltage is not greater than the second threshold value, maintain the input power at the first power level.
22. The control system of any one of claims 18, 19 or 21 wherein the first threshold value is an operating voltage beyond which erosion of electrodes of the discharge lamp exceeds a threshold level.
23. The control system of any one of claims 18 to 22 wherein the first threshold value is about 11 5V.
24. The control system of any one of claims 18 to 23 wherein the second level is lower than the first power level.
25. The control system of any one of claims 18 to 23 wherein the second power level is higher than the first power level.
26. The control system of claim 21 wherein the first period of time and the second period of time are the same.
27. The control system of claim 21 or claim 26 wherein the second threshold value is the operating voltage beyond which the discharge lamp would fail due to electrode erosion.
28. The control system of any of claims 21, 26 or 27 wherein the second thrcsholcl value is about 145V.
29. The control system of any of claims 1 8 to 28 wherein said first period of time is about four hours.
30. The control system of claim 24 wherein said second power level is between 8O%-86% of the first power level.
31. The control system of any of claims 18 to 30 wherein the operating voltage of the gas discharge lamp at the start of the lamp's life is in the range of 80-boy.
32. The control system of any of claims 18 to 31 wherein the gas within the envelope of the gas discharge lamp comprises mercury vapour.
33. The control system of any of claims 18 to 32 wherein the gas within the envelope of the gas discharge lamp comprises a noble gas.
34. The control system of any of claims 18 to 33 wherein the electrodes of the gas discharge lamp comprise tungsten.
35. An optical projection system comprising: the control system of any of claims 18 to 34 arranged to control the power supply to a gas discharge lamp of the optical projection system.