EP1416518A2

EP1416518A2 - Short arc high intensity mercury discharge lamp

Info

Publication number: EP1416518A2
Application number: EP03256860A
Authority: EP
Inventors: Mahomed Hanif Girach; Barry The Arch House Preston; Michael Justin Vulliamy; Stephen Harold Howe
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 2002-10-30
Filing date: 2003-10-30
Publication date: 2004-05-06
Also published as: JP2004152768A; GB0225254D0; CN100401452C; CN101165847A; CN1505092A; EP1416518A3

Abstract

The lamp (40) has an operating circuit acting on electrodes and/or lamp to ensure that a mercury dose globule does not form between the electrodes at shutdown or restarting of the lamp. The operating circuit includes a lamp driver (42), controller (46) and ignition circuit (44). The operating circuit produces a phased reduction of lamp power on shutdown so that the envelope cools before the arc is extinguished.

Description

This invention relates to a short arc high intensity mercury discharge lamp. Such a lamp is particularly, although not exclusively, useful as a light source for an image projection apparatus.

For use in an image projection apparatus such as a liquid crystal projector, the ideal lamp has a light source which is as close as possible to being a point source, as well as being of high intensity. An ultra high pressure mercury arc lamp comes close to meeting this need, especially when the arc gap is very short, preferably less than about 1.5 mm.

In a typical ultra high pressure mercury arc lamp, a generally spherical quartz envelope forms the arc discharge chamber and contains the spaced-apart tungsten electrodes defining a discharge path, the electrodes being connected to current conductors which extend from the lamp to the exterior. The discharge chamber also contains an inert gas such as argon at a pressure of the order of 10-100 kPa; 10^-12 - 10^-8 moles per cubic millimetre of halogen (chlorine, bromine or iodine); and a dose of mercury of at least 0.15 mg per cubic millimetre. A lamp of this kind is described in US-A-2002/0000777. During operation of the lamp, the mercury is vaporised, with a typical vapor pressure of 15 to 25 MPa. When the lamp is inoperative, the mercury condenses, and generally forms one or more globules within the discharge chamber.

A problem which occurs in ultra high pressure high intensity discharge lamps with very short arc gaps (of the order of 1 mm), is that the mercury dose globule diameter may exceed the arc gap. In such lamps, there is a possibility, observed in practice, that following switch off, dose globules form on the electrode tips, grow towards each other and merge into one. The now short circuited lamp will not re-ignite, as a potential difference cannot be established between the electrode tips. A range of performance constraints prevent either the reduction of the dose or the lengthening of the arc gap to solve this problem.

In US-A-2002/0000777, the solution offered is to dislocate the longitudinal axes of the electrodes, as shown in its Fig. 2. It is claimed that the action of the surface tension forces in the mercury globules now makes the stable formation of a shorting globule less likely. Providing the diameters of the dose balls which form on each electrode as the lamp cools do not combine to exceed the shortest distance between the head of one of the electrodes and the head of the other electrode, distance d (see Figure 2 of the US Patent publication), the problem is solved. However, for high power, ultra high mercury pressure designs, the necessary axis offset and resultant effective arc gap may be unacceptably large, since the arc length must be kept to an absolute minimum to achieve satisfactory light collection.

It is an object of the present invention to ensure that immediately before power is re-applied to the lamp, a substantial fraction of the mercury dose rests on the arc tube wall. As a result, the volume of dose situated at the electrode tips is too small to bridge the gap between the electrodes and short circuit the lamp. Thus successful lamp operation is assured.

According to the present invention, there is provided a high pressure arc discharge lamp apparatus comprising a lamp and operating means therefor, the lamp comprising an envelope containing a dose of mercury and a pair of electrodes with their tips spaced apart from one another to define an arc gap, and the operating means including means for acting on the electrodes and/or the lamp to ensure that a dose globule does not exist between the electrodes on restarting the lamp.

In one embodiment of the invention, the operating means includes means for producing a phased reduction of lamp power on shutdown of the lamp.

In another embodiment, the operating means includes means for re-igniting the arc for a period during cooling of the lamp following shutdown.

In a further embodiment, the operating means includes means to vibrate the lamp during cooling of the lamp following shutdown, and/or before ignition of the arc.

Using the invention disclosed herein, the arc length may be reduced beyond what was previously considered the practical lower limit.

Embodiments of a short arc discharge lamp in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 is a cross-sectional view of a known type of short arc mercury discharge lamp;

Fig. 2 is a view similar to Fig. 1 showing how the electrodes become shorted by the mercury dose on cooling of the lamp after operation;

Fig. 3 is a diagrammatic representation of a short arc mercury discharge lamp operated in accordance with the present invention;

Fig. 4 is a graph of lamp voltage vs. time as the lamp power is smoothly reduced over a two minute period, in accordance with a first embodiment of the invention;

Fig. 5 is a graph of lamp voltage vs. time for a second embodiment of the invention;

Fig. 6 is a diagrammatic representation of a short arc mercury discharge lamp operated in accordance with a third embodiment of the present invention; and

Fig. 7 is a graph of lamp voltage vs. time (upper curve) and solenoid plunger position vs. time (lower curve) for the third embodiment of the invention.

Referring to Fig. 1, a short arc mercury discharge lamp of known kind comprises a quartz envelope 10 which has a generally spherical central discharge chamber 12. Sealing

arms

14, 16 extend from opposite sides of the discharge chamber 12, sealing the chamber 12. The

arms

14, 16 also contain and support

electrodes

18, 20, as well as

metal foil connectors

22, 24 and lead-in

wires

26, 28. The discharge chamber 12 contains a rare gas such as argon, at a pressure of the order of 10⁴ - 10⁵ Pa at room temperature, a small amount (10^-13 - 10^-8 moles per cubic millimetre) of a halogen, and a dose, of at least 0.15 mg per cubic millimetre, of mercury. Typically, the halogen may be bromine at a density of 10^-12 - 10^-9 moles per cubic millimetre. The

electrode tips

32, 34, are spaced about 1 mm apart, and a wire coil may be wound around each electrode tip to improve cooling of the electrodes. In operation of the lamp, an electrical supply is connected through the lead-in

wires

26, 28, the

foil connectors

22, 24 and the

electrodes

18, 20 to establish an electrical potential difference between the

electrode tips

32, 34.

In operation, as will be described below with reference to Fig. 3, an electrical potential difference applied between the

electrode tips

32, 34 causes an arc to be established. On shutdown of the lamp, the arc is extinguished, and the lamp cools down from its operating temperature, gradually approaching room temperature. Because of the higher thermal conductivity of the electrodes compared with the quartz envelope, and the fact that there is a heat conductive path through the

foils

22, 24 and the lead-in

wires

26, 28, the electrodes cool more rapidly than the walls of the discharge chamber 12. The mercury dose thus tends to condense preferentially onto the

electrode tips

32, 34, and forms a bridging globule 36 between them, as seen in fig. 2. Such a bridging globule, of course, forms a direct short between the electrodes, and prevents subsequent operation of the lamp.

Referring to Fig. 3, a lamp 40 of the kind described with reference to Fig. 1 is operated by a lamp driver circuit 42. An arc is initially struck by means of an igniter circuit 44. Both the driver circuit 42 and the igniter circuit 44 are controlled by a control unit such as a microprocessor 46. On operating a lamp switch 48, the microprocessor sends a signal to the igniter circuit to strike the arc, which is then maintained, under the control of microprocessor 46, by the driver circuit 42.

Three different methods by which the object of the invention can be achieved will now be described:

1. Phased removal of lamp power before lamp extinction optionally combined with increased force cooling of the lamp. This may be termed a "soft" shutdown.

2. A brief re-ignition of the lamp after extinction. The re-ignition is timed to occur before sufficient mercury to form a bridge has condensed on the electrode tips. This invention is especially suitable for enclosed, non-vented mirror types.

3. Mechanical shock to and/or vibration of the lamp module after extinction and/or before attempted ignition.

1. Phased removal of lamp power before lamp extinction

Referring to Figs. 3 and 4, when the lamp switch 48 is operated to turn off the lamp, the microprocessor 46 causes the driver circuit 42 to reduce the power supplied to the lamp over a period of time. Other control means may be used instead of a microprocessor, including well known electrical or electromechanical devices. Fig. 4 shows a suitable lamp voltage reduction profile. In Fig. 4, the lamp is initially operating at normal operating voltage V_n. On the 'lamp turn off' command, the microprocessor 46 reduces the lamp power in a smooth fashion, for example as shown, until the lamp voltage is below the level, V_max, at which the lamp temperature has cooled to an extent such that there is insufficient mercury vapour left in the discharge chamber to form a globule large enough to bridge the inter-electrode gap. Alternatively, the lamp power may be reduced in other ways, for example in a stepwise fashion, or by using pulse width modulation techniques. At this point, the lamp power is reduced to zero. Optionally, the lamp cooling rate is increased during the lamp shutdown cycle; for example the speed of a cooling fan 50 may be increased by the microprocessor 46.

For a 132W short arc projector lamp, an average operating temperature, T_bar, for the mercury vapour has been calculated. First, the operating mercury vapour pressure was calculated using the relationship given by given by W Elenbaas in Chapter 9 of the book entitled "High Pressure Mercury Vapour Lamp", published by North Holland in 1951. Plamp (atmospheres) = (10 x E-100)3

The electric field in the positive column of the arc, E, (measured in volts per mm) is given by E = Vpc d where the inter-electrode gap = d (mm) and the arc positive column voltage is V_pc

The measured lamp voltage V _lamp is approximately given by V lamp = (Vpc + 15) volts, since a total of approximately 15V is dropped in the cathode and anode fall regions.

For a typical 132W lamp, V_lamp = 73 volts, electrode gap = 1.1 mm, and thus E = 53V/mm, assuming the electrode falls total 15V. According to equation 1, the operating vapour pressure is then 143 atmospheres. Since 1 atmosphere is approximately 10⁵ Pa, the vapour pressure in the operating lamp, P_lamp, is approximately 1.43 x 10⁷ Pa.

The Universal Gas Law is then used to calculate a mean mercury vapour temperature, T_bar. The mercury dose for this lamp type is 0.21 mg/mm³, or 210 kg/m³. Then, Plamp x Volumelamp = n x R x Tbar where n is the Mole fraction, R the Universal gas Constant (8314 joules per kg mole per K), and Volume_lamp is the arc tube volume (in cubic metres). Thus Tbar = 200.6 x Plamp 8314 x 210 = 1643K Note The atomic weight of mercury is 200.6

The lamp voltage V_max at which there is only just enough mercury left in the vapour state to form a sphere that shorts the electrodes can be determined as follows.
If an inter-electrode gap, d (mm) is specified, the diameter of a mercury sphere that just fits between the electrodes when the lamp has cooled down is clearly d mm. Given the arc tube volume and an average temperature, the universal gas law can be used to estimate the operating pressure corresponding to this amount of mercury in the vapour state. This pressure can then be used to estimate the arc electric field at this condition, and hence the lamp voltage. The lamp may be safely switched off when the lamp voltage falls below this value. Simplifying, equations 1 to 4 above, and assuming an average vapour temperature of 1643K, Vmax =1463 x d4 Volumelamp + 10 x d + 15 (Note: The constant term 1463 carries the average temperature 1643K.)

This strategy can potentially work well down to gaps of 0.7mm or even less, since lamps will operate at voltages down to 25V.

In response to a request by the operator to turn the lamp off, the electronic control gear manages the reduction in lamp power (and hence lamp voltage) to the critical level V_max and then switches the lamp power off. Concurrently, the degree of forced cooling of the lamp may be increased by the controlling circuitry or software in order to shorten the power-down process. Since there is now insufficient mercury to bridge the arc gap when in liquid form, the arc gap cannot be bridged as the lamp cools to room temperature.

In laboratory experiments, a sample of ten typical 132W short arc lamps intended for video projection applications was studied. Phased removal of lamp power before lamp extinction was achieved by manually controlling power-down in the manner described above. Using a random sample of ten 132W short arc video projection lamps of current design, the effect of increasing the impedance in series with the lamp was studied while operating the lamp on a square wave current at 170Hz with a constant 300V RMS open circuit voltage.

It was found that suddenly increasing the resistance in series with the lamp often resulted in lamp instability. However, when the series resistance was smoothly increased, it was found that the lamps operated in a stable fashion down to approximately 20% normal power when the power was reduced uniformly over a period of 120 seconds to the point when V_max was reached. (The lamp supply frequency was 170Hz and the current waveform was essentially square.) A substantial deposit of mercury was seen on the wall of the lamp where the jet of cooling air from the fan impinged, demonstrating that the method was working well.

Unstable lamp behaviour is indicated by scattered voltage readings at times after the occurrence of the voltage minimum. For a typical 132W lamp, V_max is calculated to be 67V or 92% of the lamp voltage (73V). Experimental data shows that the desired voltage reduction of 8% may be achieved in less than 20 seconds with no evidence of lamp malfunction.

A limited number of controlled power-down runs was conducted using one sample lamp. In these tests, the effects of lamp operating frequency and cooling fan voltage were investigated. The results indicate that cool down times of 20 seconds are possible under favorable conditions.

This operating sequence can be achieved in practice by the combination of additional software and/or hardware to the control unit.

2. Re-ignition following extinction

For lamp designs in which no ventilation apertures in the mirrors are provided, the method described above for the first embodiment may take too long, as the arc chamber wall cools only slowly when power is removed. (Note - approximately 25% of input power is required to maintain the wall temperature in its normal operating range of 1150-1350C). For such designs, the object of the invention - which is to avoid electrode bridging - may be achieved by re-igniting the lamp for a brief interval after the initial extinction. The process keeps the electrode tips hot, while the remainder of the lamp cools down. Any mercury beginning to condense on the fast cooling tips is removed by the forces unleashed in the hot re-ignition process.

Referring to Figs. 3 and 5, this operating sequence can be achieved by the combination of additional software and/or hardware to the control unit.

At the 'lamp turn off signal, the lamp voltage is reduced to zero. Shortly after turn-off, the igniter is energised (V_p1) with a high voltage pulse for a brief period so as to briefly re-ignite the arc. This may be repeated one or more times (V_p2, V_p3) to cause the mercury dose to remain vaporised until the walls of the discharge chamber have cooled sufficiently (relative to the electrodes) to promote condensation of the dose on the wall rather than on the electrodes. Once there is insufficient mercury vapour remaining to form a bridging globule, the igniter is turned off.

3. Mechanical shock to and/or vibration of the lamp module

A third method whereby the object of the invention may be achieved is by the use of mechanical shock and/or vibration applied to the lamp in such a direction or range of directions as to ensure that any globule of mercury lodged between the electrodes is bodily removed. This excitation may be applied during the cool down period when the mercury is least viscous, and / or immediately before an ignition attempt when the lamp is cold. Referring to Fig. 6, a solenoid 60, which operates a reciprocating or vibrating plunger 62, is activated by a solenoid actuator circuit 64 under the control of the microprocessor 46. The plunger 62 acts on a suitable mounting 66 for the lamp 40, and upon actuation moves in a direction which tends to dislodge any mercury globule bridging the electrodes. A series of actuating pulses may be applied to the solenoid 60 during the cooling down period after the lamp has been switched off, as shown in Fig. 7. Additional hardware and/or software for controlling the solenoid actuator circuit is incorporated in the microprocessor.

The invention represents a major breakthrough in the development of short arc projection lamps with arc gaps of the order of 1.5 mm or less since:

1. a major source of lamp unreliability in current lamp designs has been effectively eliminated; and

2. a strong lamp design constraint has been removed permitting the development of higher performance types featuring mercury pressures in excess of 20 MPa and arc gaps below 1 mm.

The electronic control gear required for the invention is readily implemented by control gear suppliers with minimal modifications to existing designs.

The invention removes a major source of unreliability in the operation of short arc video projection lamps characterised by arc gaps of the order of 1.5 mm or less and mercury pressures exceeding 15 MPa.

In addition, an extremely limiting lamp design constraint has been removed, since the diameter of the dose globule can now be considerably larger than the arc gap. Reliable ultra high pressure mercury lamps with sub-millimetre arc gaps for optical efficiency can now be designed using the present invention.

The invention does not require a reconfiguration of the electrodes such as that described in US-A-2002/0000777 cited above. This eliminates all the manufacturing difficulties associated with the assembly of arc tubes with precisely oriented electrodes such as those described therein. Additionally, any degradation in the efficiency of light collection caused by the offsetting and/or angling of the electrodes with respect to one another is completely avoided.

The implementation of that aspect of the invention which employs a gradual reduction of lamp power before final power removal requires only additional logic in the electronic control gear. For microprocessor controlled types, this may be achieved by the addition of just a few lines of software code. For hardware-controlled types, additional circuitry will be required. In either case, the changes are small
and straightforward, not impairing the operation, life or efficiency of the lamp unit.

The implementation of that aspect of the invention which employs hot re-ignition following power removal requires additional logic in the electronic control gear. For microprocessor controlled types, additional software only may be required. For other types, some additional circuitry will be required. In either case, the changes are small and straightforward, not impairing the operation, life or efficiency of the lamp unit.

The implementation of that aspect of the invention which employs mechanical shock and/or vibration of the lamp before ignition is attempted will involve only minor modifications to the equipment containing the lamp. These may be simply extra lines of software code or a simple timer circuit in addition to a modified lamp module mounting means incorporating a suitable transducer. Vibration in a direction approximately orthogonal to the line joining the electrode tips could be applied to the lamp using an inexpensive transducer such as those used in mobile telephones. Alternatively, shock in a similar direction could be applied using suitably damped solenoid action.

Claims

A high pressure arc discharge lamp apparatus comprising a lamp and operating means therefor, the lamp comprising an envelope containing a dose of mercury and a pair of electrodes with their tips spaced apart from one another to define an arc gap, and the operating means including means for acting on the electrodes and/or the lamp to ensure that a dose globule does not exist between the electrodes on restarting the lamp.
The lamp apparatus of claim 1 wherein the envelope is a generally spherical quartz envelope, and the electrodes are of tungsten and are connected to current conductors which extend through the envelope to the exterior.
The lamp apparatus of claim 1 or claim 2 wherein the envelope contains an inert gas at a pressure between 10 and 100 kPa, 10^-13 to 10^-8 moles of halogen per cubic millimetre, and said dose of mercury comprises at least 0.15 mg per cubic millimetre.
The lamp apparatus of claim 3 wherein said halogen is bromine at a density of 10^-12 to 10^-9 moles per cubic millimetre.
The lamp apparatus of any one of claims 1 to 4 wherein the operating means includes means for producing a phased reduction of lamp power on shutdown of the lamp.
The lamp apparatus of claim 5 wherein the operating means is arranged to gradually reduce the power so that the envelope cools before the arc is extinguished.
The lamp apparatus of any one of claims 1 to 6 wherein the operating means includes means to increase forced cooling of the envelope on shutdown of the lamp.
The lamp apparatus of any one of claims 1 to 4 wherein the operating means includes means for re-heating the electrodes for a period during cooling of the lamp following shutdown.
The lamp apparatus of claim 8 wherein the means for re-heating the electrodes includes means for re-igniting the arc one or more times.
The lamp apparatus of any one of claims 1 to 4 wherein the operating means includes means to vibrate the lamp during cooling of the lamp following shutdown, and/or before ignition of the arc.