FLASHLIGHT WITH HIGH-PRESSURE MERCURY DISCHARGE LAMP

FIELD OF THE INVENTION

The current invention is related to a flashlight comprising a high-pressure mercury discharge lamp and method for operation such a flashlight.

BACKGROUND OF THE INVENTION

The maximum intensity and minimum beam divergence of a light beam spotted by a flashlight determine the range of the flashlight. Both intensity and beam divergence are determined by means of the lighting device and the reflector. The smaller the beam divergence at a given intensity is the higher the resulting range of the flashlight. In DE 10123541 Al a flashlight with a housing, a power supply integrated in the housing, further a reflector with a lighting device coupled to the power supply integrated in the housing and the lighting device is a High Intensity Discharge (HID) lamp further coupled to an ignition device for switching on the HID lamp is disclosed. The proposed flashlight has an optimum range at a beam divergence of around 6°. A further reduction of the beam divergence causes disproportional reduction of the available intensity reducing the range of the flashlight.

SUMMARY OF THE INVENTION

It is an objective of the current invention to provide an improved flashlight.

The objective is achieved by means of a flashlight comprising: a high-pressure mercury discharge lamp; an electrical power supply electrically connected to the high-pressure mercury discharge lamp; - an ignition device for switching on the high-pressure mercury discharge lamp and the high-pressure mercury discharge lamp comprising an envelope of a

material capable of withstanding high temperatures with a discharge vessel and two electrodes extending from two sealing sections into the discharge vessel, said electrodes having an electrode gap (d _e ) smaller than or equal to 2.5 mm, preferably smaller than or equal to 1.5 mm, wherein the discharge vessel contains a filling which essentially comprises the following substances: rare gas , oxygen, halogen comprising chlorine, bromine, iodine, or a mixture thereof, as well as mercury in a quantity greater than or equal to 0.15 mg/mm ³ .

An arc is ignited for generating light between the two electrodes of the high-pressure mercury vapor discharge lamps. Such lamps are referred to as short-arc lamps because of their small electrode gap. The mercury evaporates in operation and with a quantity of 0.15 mg/mm ³ usually provides a mercury vapor pressure of approximately 150 bar in the lamp. An example of a high-pressure mercury vapor discharge lamp of such a type -but with a still higher mercury portion - is described in DE 381 34 21 Al. Such lamps having mercury vapor pressures above 100 bar produce a high luminance and a relatively continuous spectrum. Therefore, these high-pressure mercury vapor discharge lamps are often denoted UHP lamps, wherein UHP means "Ultra High Pressure" because of the high pressure or "Ultra High Performance" because of the high luminance. A major field of application of these lamps is the use in projection systems. However, the high electrode load of the lamps has the effect that the tungsten evaporates from the electrodes and is deposited on the wall of the discharge vessel. This leads to a blackening of the envelope, as a result of which the latter is strongly heated, which may give rise to an explosion of the envelope, particularly at high mercury vapor pressures. With the aforementioned lamps, this is yet compounded by the relatively small dimensions of the envelope or the discharge vessel. Therefore, such a blackening of the wall should be avoided at all cost. As a measure to avoid the blackening of the wall by tungsten transport, the high-pressure mercury discharge lamp comprises, as mentioned, a small quantity of at least one of the halogens chlorine, bromine, or iodine. These halogens create a tungsten transport cycle by which the tungsten deposited on the wall of the discharge vessel is transported back to the electrodes. Surprisingly it has been found that the high-pressure mercury discharge lamps or UHP-lamps does have the advantage in comparison to HID lamps described in the cited prior art that the arc length can be reduced down to 0.8 mm - lmm essentially

without reducing the intensity at a given input power. This extremely short arc length results in a beam divergence of less than 3° enabling a flashlight with increased range at a given input power. Alternatively the input power can be reduced resulting in a comparable range as a flashlight with an HID lamp but improved operating time or with a comparable operating time but reduced size and weight since the size of the electrical power supply (batteries or rechargeable batteries) can be reduced. The reduction of the size and weight results in enlarged application area of the flashlight.

In a further embodiment of the flashlight according to the current invention the electrodes of the high-pressure mercury discharge lamp are rod- shaped here and designed such that at the latest after a definite period of operation they each have at their tip a nipple extending in the longitudinal direction of the electrode.

The electrodes in high-pressure mercury vapor discharge lamps generally comprise a thin tungsten rod having a thick, solid electrode head or a coil at its front end, which coil is wound around the tungsten rod. Alternatively, the tungsten rod itself may be helically wound at the end. DE 381 3421 Al cited above shows examples of this. Such a relatively thick electrode head serves to ensure that electrode stability is guaranteed over a wide current range, i.e. during starting-up and during operation of the lamp, and that radiation cooling is improved. A typical diameter of such an electrode head in classical UHP lamps is between 800 μm for IOOW UHP lamps and 2000 μm for 275W UHP lamps.

The electrode end at the other side, i.e. the end of the electrode connected to a molybdenum foil and sealed within the respective sealing section, should be as thin as possible for two reasons. First, a thinner electrode end reduces the heat transfer into the sealing section and thus prevents a quartz re-crystallization when the usual quartz glass is used for the lamp envelope. Second, it is very difficult to produce pressure-resistant vacuum-tight quartz sealing sections for electrode diameters above 500 μm owing to the large difference in the thermal coefficients of expansion of tungsten and quartz. For these reasons, so far all existing high-pressure mercury vapor discharge lamps either have a rod winding or electrodes having solid heads. Another essential element of the electrode design is the exact shape of the electrode head. Typical solid electrodes with thick heads have conical tips, whereas the rod winding electrodes usually have tips extending forward at the front of the coil (for

example, see the cited DE 381 3421 Al). Such a tip is essential in stabilizing the arc discharge for two reasons:

During operation of the discharge lamp, temperatures above the melting point of tungsten should be obtained in the region of the electrode tip in order to obtain a sufficient thermionic emission from the electrode surface. It is clear that in case of electrodes having definitely shaped thin tips, this tip region can be brought to the necessary melting point temperature with a smaller input power than in the case of flat electrode surfaces. Furthermore, the electrode tip defines a stable position for establishing the arc discharge, whereas a flat electrode surface provides a plurality of possible contact points for an arc discharge, so that jumping of the arc discharge (fluttering) will constitute a substantial problem.

However, the manufacture of such electrodes having solid heads or a defined coil with an accurately designed tip involves a significant cost, which increases the total price of the high-pressure mercury vapor discharge lamps. The simple, rod-shaped electrodes can be manufactured in a relatively economical way. However, it has been proven in further experiments that a suitable design, i.e. the selection of suitable dimensions of such a rod-shaped or cylindrical electrode is readily capable of achieving that it has the desired nipple at the latest after said definite period of operation. The length of such a nipple may reach the order of magnitude of approximately the diameter of the relevant electrode, while the dimensions of the rod-shaped electrode are selected such that the nipple is formed for its length, shape and position at the electrode tip to be essentially stable - that is, viewed on a long- term scale, apart from the usual brief fluctuations. Using such lamps with simple electrodes reduce the costs for the flashlight. In the flashlight comprising the high-pressure mercury vapor discharge lamp there are various options for the precise design of the electrodes such that they have nipples at their tips after the specified period of operation according to the invention:

First, simple rod-shaped electrodes may be used, whose diameter and whose free electrode length- which is defined by the distance from the exit point of the respective electrode from the sealing section, i.e. the point of contact of the electrode with, for example, quartz glass, to the tip of the relevant electrode- are selected such

that the nipples are formed automatically during lamp operation at the latest within the specific period of operation. In experiments, it has surprisingly been found that with a suitable selection of the diameter and electrode length under definite temperature conditions, i.e. for definite operating currents, simple rod-shaped electrodes have a strong growth of such nipples at the electrode tips, and the nipples stay sufficiently stable during the entire life span of the lamp. In addition, this growing nipple guarantees arc stability and reduces the power input into the electrode per current unit and thus the heat flow in the direction of the sealing sections compared with the original rod- shaped geometry. Preferably, rod-shaped electrodes are used here whose electrode diameters are < 600, preferably < 500 μm, and particularly preferably < 450 μm. Preferably, the electrode diameters are > 200, particularly preferably > 300 μm. Moreover, it has been found that a suitable selection of such electrode diameters at the tip of the rod-shaped electrode directly behind the nipple causes tungsten accumulated there to form a swelling during operation. This results in an increase in the rod diameter at the tip directly behind the nipple and in addition a wrinkling of the electrode surface, thus providing an intensified radiation cooling of the electrode.

Preferably, the electrode is designed such that the growth of the nipple at the electrode tip takes place in the first 30 hours of operation of the lamp, the strongest part of the growing process already taking place in the first 10 hours of operation. At the same time, the growing process is accompanied by a decrease in the operating voltage by more than 5 V in the high-pressure mercury vapor discharge lamp within the first 30 hours of operation.

In order to circumvent this growing process in the first hours - i.e. in order to provide appropriately formed electrodes immediately - it is also possible to produce the nipples by radiation with a laser at the tips of the rod-shaped electrodes already during manufacture, for example before inserting the rod-shaped electrode. However, the diameter of the rod-shaped electrodes and the free electrode length of the relevant electrode (including the nipple) should then be selected such that during operation of the high-pressure mercury vapor discharge lamp the corresponding nipples remain sufficiently stable. For this purpose, the dimensions should be selected exactly as described above. The laser treatment merely ensures that the electrodes have nipples

with the desired shape form from the outset. Such a laser treatment is an additional process step during lamp manufacture, but the cost of this step is not comparable to the expense of manufacturing the electrodes mentioned above with thickened heads or helical electrodes. Thus a significantly more economical manufacture of the lamps is also possible with such electrodes.

Furthermore, preferably adjusted parameters are the wall load of the lamp, which preferably should be > 0.7 W/mm ² , particularly preferably >1 W/mm ² . The halogen quantity, for which bromine is preferably used, advantageously lies between 10 ^{" 5} μmole/mm ³ and 2 x 10 ^~4 μmole/mm ³ . Particularly advantageously, the flashlight comprises a high-pressure mercury vapor discharge lamps, which have an operating nominal wattage between 20 and 60 Watts, preferably a nominal wattage of approximately 40 or approximately 50 Watts. Here, it relates to relatively small lamps, which could also be denoted as miniaturized high-pressure mercury vapor discharge lamps. Additionally, the lower input power enables longer operating times and/or size of the flashlight.

Particularly for these preferred high-pressure mercury vapor discharge lamps, rod-shaped electrodes are preferably used whose diameter is between 220 μm and 420 μm and whose free electrode length is selected between a minimum free electrode length and a maximum free electrode length. Here, the minimum free electrode length may be calculated as:

L _mm = 6.4 - 8-d + (15-d - 7) - I (1)

and the maximum electrode length may be calculated as:

L _max = 8.4 - 8-d + (15-d - 7) - I (2)

i.e. both the minimum electrode length and the maximum electrode length are determined in dependence on the diameter d of the electrodes and an average operating current I of the high-pressure mercury vapor discharge lamp. In the equations (1) and (2), the current I is expressed in the unit A and the electrode diameter d, the minimum free electrode length L _min , and the maximum free electrode length L _max in the unit mm.

The average operating current I is the RMS value (Root Mean Square Value) and not the maximum current, which may be substantially higher in the case of pulsed operation. The high-pressure mercury vapor discharge lamp is preferably structured such that the sealing sections of the lamp envelope have a cross-sectional area of between 6 mm ² and 20 mm ² , particularly preferably of approximately 10 mm ² . This has significant advantages for the thermal balance and thus the entire design of the lamp. For this purpose, the heat conduction mechanisms and the temperature conditions in the lamp are described briefly below:

As common knowledge, the length of the sealing section of the UHP lamps is determined by the temperature at the location where the metal parts of the lamp, i.e. the supply lines to the electrodes, come into contact with the ambient air. In order to avoid a fast oxidation of these parts, which usually comprise molybdenum, this location should be such that it does not become hotter than 350 ⁰ C to 400 ⁰ C. It is clear that the temperature decreases along the longitudinal direction of the sealing sections away from the main heat source, which is the hot discharge vessel in the center of the lamp envelope. The main mechanism heating the ends of the sealing sections is the heat conduction through the material of the sealing sections in outward directions from the center of the lamp. In addition, the molybdenum foil contributes to the thermal conduction by approximately 10 to 20%. Apart from this main heating mechanism, there is a second substantial heating mechanism with which energy from the discharge vessel is transported to the ends of the sealing sections. Part of the radiation generated in the lamp does not leave the lamp envelope, but is guided within the sealing sections by total internal reflection (TIR) as in an optical waveguide. This radiation is absorbed by the metal parts within the sealing sections and contributes substantially to the heating of the ends of the sealing sections.

The sealing sections are cooled through the radiation of the hot material, for example in the case of a quartz glass envelope of the hot quartz material, as well as by heat conduction to the ambient air. According to the Stefan-Boltzmann law, both mechanisms provide the best cooling when as large a temperature difference to air as possible is present. However, the sealing sections are subject to additional heat, which is reflected back by radiation within a reflector or by the optical system. Here, there are

three main mechanisms:

If a metal reflector is used, heat radiation is reflected by the hot discharge vessel against the sealing sections. This is boosted by the fact that the reflector is designed for a point source, whereas the hot discharge vessel has a larger expansion, and hence radiation is reproduced not only in the desired focus. As this radiation originates from the hot material of the discharge vessel, it is re-absorbed correspondingly well by the sealing sections. Even in the case of dichroic reflectors (similar to a metal oxide vaporized mirror), in which only visible light is reflected and IR and UV radiation is transmitted as much as possible, radiation from the arc discharge strikes the sealing section ends of the lamp. This radiation then originates from the rear part of the reflector. Likewise, as the arc discharge is an expanded radiation source, the ray is not accurately focused, but part of it also strikes the ends of the lamp. This radiation is not absorbed by the envelope material, as it was already transmitted by the same material in the region of the discharge vessel, but is absorbed by the metal parts in the sealing sections.

It was found in a plurality of experiments that the heating mechanism acting on the optical path within the sealing sections mentioned above as the second mechanism accounts for a very significant portion of the heating of the ends of the sealing sections. The advantageous arrangement of the envelope such that the cross- sectional area lies below 20 mm ² can considerably reduce this portion of the heat conduction - in contrast to known UHP lamps, which have at least a 25 -mm ² cross- section, or as a rule well above it. This renders it possible to design the sealing sections to be significantly smaller. Nevertheless, the temperatures in the end region of the sealing sections, where the metal comes into contact with air, remain below the desired 350 to 400 ⁰ C. This renders possible the design of smaller lamps and thus of correspondingly smaller reflectors, as a result of which the optical structure in a flashlight according to one embodiment of the current invention needs less space altogether. Hence, not only the total size of the flashlights, but advantageously also their cost can be greatly reduced.

Despite the relatively small cross-sectional area of the sealing sections, it is advantageously ensured that the wall thickness of the discharge vessel at the thickest location of the discharge vessel, the so-termed equator of the lamp, is greater than or

equal to 1.3 mm, preferably greater than or equal to 1.6 mm, and particularly preferably greater than or equal to 1.7 mm.

In a particularly preferred embodiment, the outer diameter of the discharge vessel at the thickest location is approximately 7.1 mm and the inner diameter there is approximately 3.5 mm.

The lamp envelope is preferably formed from a tube with an outer diameter of only approximately 4.1 mm and an inner diameter of approximately 2 mm.

Until now, such lamps were customarily formed from significantly thicker tubes. The tube sections may be formed into the sealing sections at the discharge vessel by means of pressing or fusing, as described above. Here, fusing is the preferred method, as a greater compression strength can be achieved thereby. Sealing sections are thus produced which have an essentially round diameter. Preferably, it is then ensured that the diameter of the sealing sections lies between 2.5 mm and 5 mm- preferably at approximately 3.6 mm- in order to obtain the desired cross-sectional area. The length of the metal strip sections, which should be fully embedded in the sealing sections, is preferably < 12 mm.

In order to achieve the necessary wall thickness in the central region of the discharge vessel and at the same time a correspondingly small cross-sectional area in the region of the sealing sections, the tube for forming the discharge vessel in the thickest location of the discharge vessel is compressed in axial direction by more than

250%, preferably by more than 300%, under simultaneous radial expansion. A method of implementing such a compression is described below.

In another embodiment of a flashlight according to the current invention the flashlight further comprises: a system control unit being arranged in a way that the power supply of the high-pressure mercury vapor discharge lamp can be regulated at least between two different power level; a regulator, and the regulator is arranged in a way that the power supply of the high-pressure mercury vapor discharge lamp can be regulated by means of the regulator via the system control unit.

The regulator in combination with the system control unit can be used to

enable signalling with flashlight comprising a high-pressure mercury vapor discharge lamp. As already discussed above results the high electrode load of a high-pressure mercury vapor discharge lamp in evaporation of tungsten from the electrodes. The tungsten is then deposited on the wall of the arc tube, leading to a very undesirable blackening of the arc tube. Such a blackening of the wall must be avoided, otherwise the wall temperature of the arc tube increases during the operational life time of the arc tube, due to increased absorption of thermal radiation, ultimately destroying the arc tube. In an attempt to avoid such wall blackening due to tungsten transport, precise amounts of oxygen and halogen, preferable bromine, have been added to the gas in the arc tube. Such additives to the lamp atmosphere prevent the tungsten, that evaporates from the electrodes, from the deposition on the bulb wall, since, in the cooler regions of the bulb close to the bulb wall, the tungsten atoms react chemically to form volatile oxyhalide molecules which are transported, e.g. through convection, to the hotter regions of the lamp near the electrodes, where the molecules dissociate. In this way, tungsten atoms are returned to the lamp electrodes in a regenerative manner. This transport cycle is usually called the "regenerative cycle".

A problem arises if the lamp is driven with an operational power much below the nominal power of the lamp. Below a certain power level, the mercury condenses, with the result that the halogen, e.g. bromine, is dissolved into the liquid mercury. The regenerative cycle is thus no longer effective.

However, for high-pressure mercury vapor discharge lamp with input power above IOOW it has been shown that alternating operation between extreme dimmed level and nominal power-i.e. with and without a regenerative cycle results in a cleaning of the quartz wall. In other words, by constantly monitoring the periods in which the lamp is operated in the saturated and unsaturated operation regimes, and by judiciously switching between these states, it can be ensured that no significant blackening of the inner walls arises during the total operation time of the lamp. The term "saturated operation regime" describes the operating regime of the lamp in which so much mercury condenses in the arc tube to interrupt the regenerative cycle, normally resulting in a significant blackening of the walls of the arc tube. On the other hand, the term "unsaturated operation regime" describes that operating regime in which the mercury has evaporated to such an extent that the regenerative cycle remains essentially

undisturbed. During signalling with a flashlight the periods of working in the saturated operation regime are rather short (some seconds). In general the time period working in the saturated operation regime can be directly regulated by means of the regulator via the system control unit. The system control unit regulating the power supply of the high- pressure mercury vapor discharge lamp has only to monitor whether a defined time limit (in general several minutes) is reached in order to prevent irreversible wall blackening. In the case this time limit is reached the mercury vapor discharge lamp is switched back by means of the system control unit to the unsaturated operation regime by increasing the power level. The power supply of the flashlight can be one or more batteries respectively rechargeable batteries. The regulator can be a simple switch or a combination of two switches, a first one for switching on and off of the flashlight and a second one for regulating the light output of the flashlight. Whereby special embodiments of the switch are a button or a rotary switch. Further the regulator can be a more sophisticated device for controlling the functions of the system control unit as discussed below. In a further embodiment of a flashlight according the current invention the system control unit is arranged in a way that the power supply of the mercury vapor discharge lamp in at least one power level is timed by the system control unit. Using high-pressure mercury vapor discharge lamp with an input power much lower than the nominal power a cleaning of the quartz wall of the mercury vapor discharge lamp doesn't happen. At a power level of around 40 W and less the halogen cycle does not function properly for a lamp with nominal power of 5OW. Consequently a blackening of the walls has to be prevented at all. This problem is solved by a technical adjustment in the system control unit. The system control unit is arranged in such a way that by activating the regulator the driver dims the lamp but not longer than a maximum allowed time period determined by the thermal inertia of the lamp since the temperature of the lamps determines when the saturated operation region starts. If the maximum allowed time period is reached the system control unit automatically increases the lamp power to its minimum value at which the unsaturated operation region is reached. In this way, the averaged power of the lamp is high enough to ensure that the halogen cycle functions. Hence, wall blackening will be prevented at all, and signalling will not have harmful consequences for the lifetime of the lamp. Frequently switching between the two power levels, even on very short time intervals, does not have any harmful consequences for the

lamp, and does not lead to a premature lamp failure. Further the system control unit can be specified in a way that the power supply of the mercury vapor discharge lamp can be regulated between two power levels, a high power level and a low power level and the time period of the low power level is timed by the system control unit. In addition the system control unit can provide two timing periods those can be regulated by means of the regulator. That means the regulator e.g. a button can activate a short predetermined time period of low power level via the system control unit by shortly pressing the button and additionally a predetermined longer time period (being maximum as long as the maximum allowed time period) of low power level via the system control unit by holding the button. All these measures can also be used together with mercury vapor discharge lamps with an high nominal input power above IOOW less suitable for flashlights (depending on available battery capacity).

A further embodiment of the flashlight according the current invention further comprises an adjustable filter being arranged in a way that the illumination of the flashlight can be controlled independently from the power supply of the high-pressure mercury vapor discharge lamp by means of the regulator via the system control unit. The adjustable filter can be a mechanical device as an adjustable aperture or alternatively a transparent substrate placed at the aperture of the flashlight whereby the transparency of the transparent substrate can be adjusted. The transparent substrate can e.g. be a glass or polymer substrate with a thin layer whereby the transmission of the layer can be electrically regulated, a glass or polymer substrate with electrically adaptable absorption or an LCD device as used for LCD displays. Further polymer-dispersed liquid crystals (PDLC) offer the possibility to adjust the transparency. The adjustable filter can be used to provide an independent control of the light output of the flashlight by adding e.g. an independent switch to the regulator or the adjustable filter is synchronized with the regulation of the power supply of the mercury vapor discharge lamp. In the latter case the transparency of the transparent substrate is reduced at the same time when the power supply of the mercury vapor discharge lamp is reduced. This measure can be used to increase the ratio between the maximum available light output and the minimum available light output in order to improve the signal quality. Further this measure can be used to increase the temperature of the mercury vapor discharge lamp in the low power level by reflecting and/or absorbing the light by means of the transparent substrate or the

adjustable aperture. This reduces the condensation of the mercury and enables even lower minimum power level of the high-pressure mercury vapor discharge lamp or longer time periods of the mercury vapor discharge lamp at the low power level without causing blackening of the Quartz wall. In another embodiment of the flashlight according to the current invention the system control unit is arranged in a way that at least one defined sequence of signals implemented in the system control unit can be activated via the regulator. The regulator can e.g. comprise an additional switch in order to activate a predefined sequence of signals as e.g. SOS. Further the regulator comprises a more sophisticated interface offering several predefined sequences of signals those can be activated via the regulator. Additionally the possibility of defining a sequence of signals by the user can be implemented.

It is further an objective of the current invention to provide a method for operation of flashlight comprising a high-pressure mercury vapor discharge lamp. The objective is achieved by means of a method for operation of a flashlight comprising a high-pressure mercury vapor discharge lamp comprising the steps of: supplying the high-pressure mercury vapor discharge lamp with electrical power at a high power level; - reducing the electrical power supply of the high-pressure mercury vapor discharge lamp for a predefined time period by means of a regulator to a lower power level and automatically switching back the electrical power supply of the high- pressure mercury vapor discharge lamp to the high power level after the predefined time period.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be explained in greater detail with reference to the figures, in which the same reference signs indicate similar parts, and in which:

Fig. 1 shows a cross-sectional view of one embodiment of the current invention.

Fig. 2 shows an exploded view of the exterior components of one embodiment of the invention. Fig. 3 shows an exploded view of the internal components of one embodiment of the current invention. Fig. 4 shows a principal sketch of a further embodiment of the current invention. Fig. 5 shows a longitudinal section through a high-pressure mercury vapor discharge lamp according to a first embodiment, before the first start-up, Fig. 6 shows a longitudinal section through the high-pressure mercury vapor discharge lamp of Fig. 5, but after the start-up, with an enlarged schematic representation of the electrode tips, Fig. 7 shows a radiograph of an embodiment of a high-pressure mercury vapor discharge lamp according to the invention, after a few minutes of operation,

Fig. 8 shows a radiograph of an embodiment of a high-pressure mercury vapor discharge lamp according to the invention after a period of operation of approximately 200 hours,

Fig. 9 shows the dependence of the temperature T of the electrode on the distance from the electrode tip for different free electrode lengths and different electrode diameters, Fig. 10 is a graph showing the advantageous regions for selecting the free electrode length L in dependence on an average operating current

I of the lamp for different electrode diameters, Fig. 11 is a graph showing the voltage drop across a high-pressure mercury vapor discharge lamp as a function of the period of operation,

DETAILED DESCRIPTION OF EMBODIMENTS

In Fig. 1 a cross-sectional view of a first embodiment of the current invention is depicted. A high-pressure mercury vapor discharge lamp 1 is connected to a

system control unit 118. The system control unit 118 drives the mercury vapor discharge lamp 1 by means of an electrical power supply 126. The system control unit 118 is regulated via a regulator comprising a first button 107 and a optional second button 108. The first button 107 is used to switch the flashlight on and off. The second button 108 regulates the power supply of the high-pressure mercury vapor discharge lamp 1 by means of e.g. decreasing or increasing the power applied to the high-pressure mercury vapor discharge lamp 1 via the system control unit 118.

Fig. 2 shows an exploded view of the external components of a flashlight according to the current invention. Essentially a front cap 101 with the aperture of the flashlight, a front screen 104, a reflector 105, the reflector housing 106, the battery housing 109 the first switch 107 and the second switch 108, the end cap 110 and a protective cap 111 are shown. The focal point of the flashlight can be regulated by means of the reflector housing 106 being in a fixed connection with the reflector 105.

Fig. 3 shows an exploded view of the internal components of a flashlight according to the current invention. It is depicted how the high-pressure mercury vapor discharge lamp 1, the system control unit 118 and the power supply 126 can be fixed in the housing of the flashlight. Further a connector 130 is shown that can be used for recharging a rechargeable battery being part of the power supply 126 and optionally to provide a data connection to the system control unit 118 in order to reconfigure or add functions of the system control unit 118.

Fig. 4 shows a principal sketch of a further embodiment of the current invention. The light output of the flashlight is additionally regulated by means of the front screen 104 being arranged as an adaptable filter. The adaptable filter comprises a layer of polymer dispersed liquid crystals (PDLC) sandwiched between two glass layers with transparent (ITO) electrodes. As long as a voltage is applied between the transparent electrodes the adaptable filter is transparent. If no voltage is supplied the PDLC layer scatters the light reducing the maximum light intensity per solid angle. The transparency of the adaptable filter is regulated by means of the regulator via the system control unit 118 synchronously to the power supplied to the high-pressure mercury vapor discharge lamp 1. That means, if maximum power is supplied to the high-pressure mercury vapor discharge lamp 1 the transparency is maximized and if minimum power is supplied to the high-pressure mercury vapor discharge lamp 1 the transparency is

minimized, maximizing the relation between maximum and minimum light output of the flashlight for improved signalling. The connector 130 is directly connected to the system control unit 18 in order to program or reprogram signalling sequences that can be activated by means of the regulator. The latter can be done by e.g. rotating switch 107 (without affecting the on state of the flashlight) from a first neutral position to second position corresponding to a first sequence of signals (e.g. SOS) and activating or deactivating the sequence of signals by means of pushing the second switch 108. In a neutral position of switch 107 the signalling can be done manually by means of the second switch 108. The high-pressure mercury vapor discharge lamp 1 schematically shown in Figs. 5 and 6 and the high-pressure mercury vapor discharge lamp shown in Figs. 7 and 8 have a rated power of approximately 50 W enabling long time operation of the flashlight.

In a usual manner, the lamps 1 comprise an envelope 2 of quartz glass with a centrally arranged discharge vessel 3 and two sealing sections 4 arranged at opposite sides of the discharge vessel. Electrodes 5, 6 extend into the discharge vessel 3 from the sealing sections 4. These electrodes 5, 6 are connected inside the sealing sections 4 to respective molybdenum foil sections 8, which in their turn are connected at the other ends to the supply lines 9, usually molybdenum wires. The electrode gap d _e , i.e. the distance between the tips of the mutually facing electrodes 5 and 6, is approximately 1.5 mm.

The discharge vessel 3 is filled not only with a rare gas, but in the present case also with argon having a pressure of 200 mbar, with oxygen, mercury, and a halide, here bromine. The oxygen is present in only a very small quantity. Generally, the oxygen quantity introduced into the lamp by the surface oxidation of the metal parts will be sufficient. The bromine quantity is approximately 1 x 10 ^"4 μmole/mm ³ . The mercury is present in a quantity of more than or equal to 0.15 mg/mm ³ and less than or equal to 0.35 mg/mm ³ . In this particular preferred embodiment, the total mercury quantity is 6 mg (this corresponds to approximately 0.17 mg/mm ³ ). The wall load is more than 0.7 W/mm ² in this lamp.

The lamp envelope is manufactured from a quartz glass tube having an outer diameter of 4.1 mm and an inner diameter of 2 mm. The shaping of the discharge

vessel 3 takes place in a glass lathe, in which the tube is held at both ends in a headstock and a tailstock. The tube is heated in its central region, whereupon the headstock and the tailstock are brought together in order to compress the material in the central region, at the thickest location of the discharge vessel. At the same time, the tube is radially widened in the heated areas by a positive pressure from the inside, for example by injecting an inert gas, so as to achieve the desired shape of the discharge vessel. The exact shape of the discharge vessel may be determined from the outside through pressure by a negative mold. Such methods are known to those skilled in the art from US 4,389,201, for example. In order to obtain as large a compression of the material in the central region of the discharge vessel 3 as possible, the compression and expansion processes preferably have at least two stages, i.e. compression takes place, then stretching, then compression again, and finally stretching again. This process may be carried out for a long time until the desired shape has been obtained. The finished discharge vessel then has, at its thickest location, an envelope outer diameter d _a of 7.1 mm and an envelope inner diameter U ₁ of 3.5 mm. The wall thickness d _w is thus approximately 1.7 mm. This corresponds to a compression of approximately 300% with respect to the original wall thickness of the glass tube.

Subsequently, for example, the electrode 5, fastened at one side to the molybdenum foil and to the lead wire 9, is supplied. Then the discharge vessel 3 is filled with mercury in the form of a mercury droplet. This usually happens in an inert gas atmosphere. In addition, the second electrode 6 is then inserted. Thereafter, the glass tube section is sealed at one side in order to produce the sealing section that is to seal off the discharge vessel 3 at this side. Subsequently, the discharge vessel 3 is filled from the yet open side with the desired halogen, for example in the form of methyl bromide as described in DE 38 13 421 Al, and is filled with the desired rare gas, and finally the second seal is provided, whereby the discharge vessel 3 is completely sealed. These methods are also known to those skilled in the art from US 4,389,201, for example. The electrodes are preferably positioned with the help of a monitoring system in order to maintain the exactly specified electrode gap d _e . The small thickness of the initial glass tube ensures on the one hand that the diameter of the sealing section 4 or the seals is only 3.6 mm, i.e. the cross-sectional area of the seal is approximately 10 mm ² . On the other hand, the strong compression

process in forming the discharge vessel ensures that the wall thickness in the region of the discharge vessel is sufficient for withstanding high mercury vapor pressures of 200 bar and more.

The length of the molybdenum foils in the present case is just below 12 mm, the length of the sealing sections is only approximately 15 mm. Thus, with a length of the discharge vessel of approximately 7 mm, it is possible to design a lamp envelope 2 having a total length of only approximately 36 to 38 mm. The selected lamp dimensions, particularly the small diameter d _s of the sealing sections 4 and the associated smaller cross-sectional area, achieve that the temperatures at the outer ends of the sealing sections 4 are below the permissible level of 400 ⁰ C also with the sealing sections 4 shorter than in the known lamps.

A dramatic temperature reduction at the outer ends of the sealing sections can indeed be achieved with this structure in experiments. Thus, for comparison, UHP lamps were constructed in the usual manner from glass tubes with a diameter of approximately 6 mm and were compared with the UHP lamps manufactured from 4-mm glass tubes as shown in Fig. 5. The sealing sections of the lamps from the 4 mm tubes have half the cross-sectional area of the lamps manufactured from the 6-mm tubes. This reduction in the cross-section led to a temperature that is 100 K lower at the ends of the sealing sections. Moreover, in order to be able to manufacture the lamp as economically as possible, simple rod-shaped electrodes are used, but the electrode diameter d and the free electrode length L from the tip of the electrode 5, 6 to the quartz glass exit point of the sealing section 4 were selected in dependence on the average operating current I such that in the course of the operation, preferably in the first 10 hours of operation, a substantially stable nipple 7 is formed at the electrode tip. This is shown in Fig. 6, which additionally shows an enlarged schematic cutout of the lamp 1 in the region of the electrode tips. The nipples 7 achieve that the electrodes 5, 6 have at their outermost tips, i.e. in the region of the nipples 7, a sufficiently high temperature above the melting point of mercury for ensuring a sufficient electron emission. At the same time, these nipples 7 guarantee a stable position for the arc discharge, so that fluttering of the arc is avoided.

Figs. 7 and 8 show radiographs of a further prototype of the lamp according to the invention. Fig. 7 shows the lamp after an operation of some minutes and

Fig. 8 shows the same lamp after an operation of approximately 200 hours. Here, the electrode gap is approximately 0.9 mm.

The lamp was operated at a nominal wattage of 50 W. Such a lamp may be operated with customary drivers in pulsed operation. Descriptions of advantageous current waveforms favorable for forming sturdy nipples, as well as appropriate drivers for operating the lamp can be found, for example, in WO 95/35645, WO 00/36882, and WO 00/36883.

The operation was interrupted for producing the radiographs each time, and the radiograph was generated at the cold lamp. As the radiograph of the yet almost unoperated lamp in Fig. 7 shows, the electrodes are at first simple rod-shaped electrodes. This may be recognized particularly well at the left electrode 5. The almost spherical dot 11 is due to mercury which condenses in the cooled-off state of the lamp, usually precipitates in a drop form at the electrodes 5, 6, and evaporates again immediately after the start-up of the lamp. The right electrode 6 is equally rod- shaped as the left electrode 5, but the rod shape cannot be recognized so well here owing to different mercury deposits 12.

By comparison, Fig. 8 clearly shows how the desired nipples 7 are formed at the tips of the electrodes 5, 6 during operation. At the same time, tungsten deposits directly behind the tip 7 cause a swelling 10 of the electrodes 5, 6. Herein, the diameter in this location increases by approximately 10%. At the same time, the electrode surface in this region becomes wrinkled. The radiation cooling of the electrode 5, 6 is substantially improved by this swelling and wrinkling of the surface.

The remaining apparent swellings 13, 14 at the electrodes 5, 6 are again formed by condensed mercury, which deposits at the electrodes 5, 6 in the cold state of the lamp and evaporates again during operation.

In order to achieve the desired growth of the nipples 7 at the rod-shaped electrodes 5, 6, arbitrary rod-shaped electrodes obviously cannot be selected, but it is to be heeded according to the invention that the diameter d and the free electrode length L are suitably selected in dependence on the desired average operating current I. If the electrodes 5, 6 are too long, they will become very hot in the transition region during operation and as a rule break down already during the start-up of the lamp. Very short electrodes 5, 6 lead to a strong jumping of the discharge arc and in addition to a re-

crystallization in the sealing section owing to a too strong heat transfer into the sealing sections 4.

In order to show the dependence of the electrode temperature on the free electrode length L and the electrode diameter d, results of a simulation implemented for finding the suitable dimensions are shown in Fig. 9. The electrode temperature T in K along the electrode is plotted against the distance from the electrode tip in μm. The melting temperature T _m of the electrode material of 3680 K is also shown. The line drawn topmost shows the temperature gradient for an electrode having a diameter d of 300 μm with a free electrode length L of 3,000 μm. The dashed curve below it shows the temperature gradient for the same electrode, but with a free electrode length L of only 2,500 μm. The third, dotted curve shows the temperature gradient for an electrode having a diameter d of 400 μm with a free electrode length L of 3,000 μm, and the lowest, dot and dash curve shows the temperature gradient for a corresponding electrode having a diameter of 400 μm and a free electrode length of 2,500 μm. For this simulation the average operating current I was assumed to be 0.8 A in each case. The power input to the electrode here is approximately 8 W/A. These simulations clearly show that both the electrode diameter d and the free electrode length L affect the temperature gradient along the electrode. It is understood that the operating current I also has an influence on the temperature gradients in that the stronger the average operating current I, the higher the temperature. This, however, is not shown in Fig. 9 for the sake of clarity.

It has been found that, in order to achieve the desired growth of the nipples 7 at the electrode tips to an ideal shape, the free electrode length L should be chosen within definite fixed limits in dependence on the operating current and on the electrode diameter D. The upper limit value, i.e. the maximum free electrode length L _max , and the lower limit value, i.e. the minimum free electrode length L _min , can be calculated from equations (1) and (2) given above in dependence on the diameter D of the electrode and the desired average operating current I.

Fig. 10 once again shows the upper and lower limit values L _max , L _min thus calculated for the free electrode length L in dependence on the current I for different electrode diameters. The free electrode length L is plotted in mm against the current I in A. The drawn curves show the upper and lower limits for the free electrode length L

with an electrode diameter d of 300 μm, the dashed curves show the limit values for an electrode diameter d of 350 μm, and the dotted curves show the values for an electrode diameter d of 400 μm. Fig. 10 also shows that a definite electrode diameter d should be preferably selected for definite operating currents I, so that a particularly good growing process is ensured. Thus, an electrode diameter d of 300 μm may be selected in a current range of approximately 0.6 A to approximately 1 A, an electrode diameter d of 350 μm preferably in a current range of approximately 0.8 A to approximately 1.2 A, and an electrode diameter d of 400 μm in a range of approximately IA to approximately 1.4 A.

The values or ranges indicated in the following Table are ideal values derived from the experiments, below which an optimum growth of the desired nipples at the electrode tips can be achieved:

A plurality of examples of configurations are indicated below, which result, for example, from the equations (1) and (2), from Fig. 10, or from the above table:

1) A first high-pressure mercury vapor discharge lamp having a spherical discharge vessel is operated at 50 W and has an electrode gap of 1.3 mm. The operating current is 62.5 V and the average operating current is 0.8 A. Rod electrodes having a diameter of preferably 0.3 mm and a free electrode length of 2.5 mm should then be selected.

2) A second high-pressure mercury vapor discharge lamp having a spherical discharge vessel is operated at 50 W and has an electrode gap of 1 mm. The operating current is 50 V and the average operating current is 1 A. Rod electrodes having a diameter of preferably 0.35 mm and a free electrode length of 2.8 mm should then be

selected.

3) A third high-pressure mercury vapor discharge lamp having an elliptical discharge vessel is operated at 40 W and has an electrode gap of 1.5 mm. The operating current is 67 V and the average operating current is 0.6 A. Rod electrodes having a diameter of preferably 0.3 mm and a free electrode length of 3.1 mm should then be selected.

4) A fourth high-pressure mercury vapor discharge lamp having an elliptical discharge vessel is operated at 40 W and has an electrode gap of 1.35 mm. The operating current is 60 V and the average operating current is 0.66 A. Rod electrodes having a diameter of preferably 0.28 mm and a free electrode length of 2.9 mm should then be selected.

The exact growing process of the nipples at the electrode tips may best be followed via a measurement of the operating voltage in dependence on the operating time. Given the same pressure and the same power, the voltage is determined by the electrode gap, and the growth of the desired nipples at the electrode tips leads to a reduction of the electrode gap, with the result that a voltage drop also indicates the growing process. This is shown in Fig. 11. Here, the operation voltage in volts is plotted against the period of operation in hours. The lamp was operated here -in order to simulate as realistic an operation as possible- for two hours each time and then cooled down again for 15 minutes. The electrode gap at the beginning of the experiment was 1.25 mm, i.e. as yet without nipples at the rod-shaped electrodes. As is to be seen from this Figure, the voltage drops already by more than 10 V in the first 10 hours of operation and then drops further in the first 30 hours of operation. This demonstrates that the desired nipples are formed already in the first hours of operation of the lamp. The Figure also shows that -apart from the customary fluctuations-the nipples remain very stable when viewed on a long-term scale, i.e. the electrode gap does not change as significantly any more during the further life span of the lamp as in the first 30 hours of operation.

The calculations and tests carried out clearly show that it is surprisingly possible to manufacture a UHP lamp having excellent operating properties with simple rod electrodes that is extraordinarily economical to manufacture. It should merely be ensured that the suitable dimensions of the electrodes are selected in dependence on the

subsequent operating current. Besides, the lamps manufactured by the methods described above have the advantage that they are extraordinarily short. This small size of the high-pressure mercury vapor discharge lamp 1 has the advantage that the entire flashlight can be made shorter. The present invention will be described with respect to particular embodiments and with reference to certain drawings, but this is not to be construed in a limiting sense, as the invention is limited only by the appended claims. Any reference signs in the claims shall not be construed as limiting the scope thereof. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn to scale for illustrative purposes. Where the term "comprising" is used in the present description and claims, it does not exclude other elements or steps. Where an indefinite or definite article is used when referring to a singular noun, e.g. "a" or "an", "the", this includes a plural of that noun unless specifically stated otherwise. Furthermore, the terms first, second, third and the like in the description and in the claims are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances, and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

Moreover, the terms top, bottom, first, second and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein.