GB2080020A

GB2080020A - Electrical Light Source with a Metal Halide Discharge Tube and a Tungsten Filament Connected in Series with the Discharge Tube

Info

Publication number: GB2080020A
Application number: GB8117489A
Authority: GB
Original assignee: Egyesuelt Izzolampa es Villamossagi Rt
Current assignee: Egyesuelt Izzolampa es Villamossagi Rt
Priority date: 1980-07-03
Filing date: 1981-06-08
Publication date: 1982-01-27
Also published as: JPS5787060A; HU181148B; NL8103106A; DE3124802A1

Abstract

A high pressure discharge lamp comprising a metal halide discharge tube (1) provided with an ignition electrode or with a preheatable electrode has tungsten coil(s) (2) connected in series therewith which we located so that the heat from the tungsten coil(s) effects the setting of the optimal temperature for the cold spot of the discharge tube (usually adjacent its ends) and its operating point. A tungsten coil may be provided at only one or at both ends of the discharge tube 1, and the coil(s) are incandescent so that they add to the light output of the lamp. A further incandescent coil may also be provided to give light when the discharge tube is inoperative. Heat reflecting layers 13, 14 of zirconium oxide may also be provided at the ends of the tube; alternatively the quartz body of the tube 1 may have its entire surface provided with an infrared reflecting layer. In a further embodiment, Fig. 5 (not shown), instead of a tungsten coil an incandescent halogen lamp is used at one end of the discharge tube. <IMAGE>

Description

SPECIFICATION Electrical Light Source with a Metal Halide Discharge Tube and a Tungsten Filament Connected in Series with the Discharge Tube The invention concerns electrical light source with a metal halide discharge tube and a tungsten filament connected in series with the discharge tube.

In the light source according to the invention there is provided a light-transparent envelope sealed by vacuum technology, a high-pressure metal halide discharge tube provided with an ignition electrode or a preheatable cathode, furthermore a tungsten coil or filament connected in series with the latter and optionally a further coil or filament with which a switching element is connected in series.

As is known, in metal halide lamps with quartz envelopes an arc discharge arises between the opposed electrodes under the effect of an external voltage source and a current-limiting element connected in series with the lamp. In the arc discharge mercury and halides of various metals are comprised in a buffer or sustaining gas atmosphere. As is known, the spectral distribution of the light of the discharge is a function of the metals used.

Of the metals used, one or more metals are dosed for preventing the metals from fully going over into a vapour phase during operation under working temperature conditions, so that a portion remains in a liquid condition on the wall of the discharge space. In this way the (vapour) pressure of the metal or metals, and thereby the spectral distribution of the light, is determined by the coldest point of the wall of the discharge vessel, i.e. the temperature is approximately 5006000C, while when rarer earth metals, e.g.

dysprosium is used, this temperature lies between 600-7000C. In general, the temperature of a discharge vessel is at its lowest in the vicinity of the cathode space. Consequently, with the aim of increasing the temperature of the cathode space heat insulating aluminium oxide or zirconium oxide layers are used. Such a solution is described inter alia in Hungarian Patent Application No. EE2551. (US PS 3374 377).

The influencing of the temperature is also achieved by the goemetrical configuration of the cathode space. The setting of the "cold point" (cathode space) is particularly difficult for lamps of low output. In these lamps the setting of this temperature is effected mainly by reducing the dimensions. However, this measure has a disadvantageous effect on the achievable lifetime of the lamp since, in order to heat up the cathode space, more power must be used, which is very disadvantageous in terms of the luminous efficiency of the lamp, which becomes significantly poorer. Proposals have become known wherein the outer quartz envelope or bulb is brought into the discharge space, whereby a part of the heat is radiated back.Such a solution is known inter alia from German Published Patent Application No. 2 840 771 (=British Published Patent Application No. 2 035 679, Application No. 79.32259).

In most cases it is necessary however to arrange the discharge tube in an evacuated envelope.

Metal halide lamps are mostly operated by an ignition unit producing a high voltage impulse.

However, solutions are also known in which an internal ignition electrode is arranged next to the cathode, as is also customary in mercury lamps.

In this case, however, care must be taken that the cathode space provided with the ignition electrode should be cooled down later (more slowly) than other locations of the discharge space, e.g. than the other cathode space, when the lamp is switched off.

When this does not happen in this way the metal halides solidifying out from the vapour phase can deposit on the cathode and on the ignition electrode whereby re-ignition of the lamp is hindered.

However, the methods used to eliminate these phenomena considerably restrict the position or orientation of the lamp-setting operation.

In discharge lamps it is problematic that one must use a special current-limiting element. This is an inductive member or an electronic means arranged externally of the lamp. In this way the construction becomes more complicated and the installation becomes more expensive.

In mercury lamps of mixed light it is a wellknown solution to use as the current-limiting element an incandescent tungsten filament arranged within the same envelope as the discharge tube and is electrically connected in series therewith. Such a filament is in general in the middle of the quartz body, i.e. is relatively far from the quartz body. A solution is known from US Patent Specification No. 3 445 719 wherein the heat effect of a serial dropping (ballast) resistor is exploited for increasing the temperature of the quartz body. In this solution the series or dropping filament or a plurality of filaments are arranged next to the quartz body and parallel therewith on both sides and the temperature at the end of the quartz body is controlled by a heat-reflecting layer i.e. the temperature is set in this way.A disadvantage of this method lies in that the whole quartz body is thus heated up, particularly also its central part which is already at a higher temperature. This high temperature, which in the sense of the invention lies between 900--12000C, disadvantageously influences the operation of the lamp and causes it to have a short lifetime. In the sense of West German Published Patent Application No. 2 915 208 (=British Published Patent Application No. 2 019 084, Application No. 79.12905), a filament is divided into two parts at the two ends of the mercury discharge tube to simplify the manufacturing technology, to increase the lifetime and to achieve a lower risk of flashover. However, here the heating effect of the filament is not utilized because for mercury vapour lamps this is not required.

An aim of this invention is to provide a construction by means of which the good properties of mixed light lamps and metal halide lamps may be combined and moreover in such a manner that these good properties are optimal in every respect.

The invention concerns an electric light source having a light-transparent envelope sealed in a vacuum-technical manner and containing a high pressure metal halide discharge tube pro'vided with an ignition electrode or with a pre-heatable electrode and a coil or coils connected in series with the electrodes; the novelty of the invention consists in that the optimal temperature of the "cold point" of the discharge tube (of its ends) is set by the heat generated by the operation of the discharge and by the heat dissipated in the coil(s) connected in series with the discharge tube (but not by the working current of the discharge tube).

The invention is described with reference to the illustrated preferred embodiment shown in the drawings, wherein Figure 1 is a circuit diagram of the simplest example of the lamp according to the invention; Figure 2 is a simplest circuit diagram of the lamp according to Claim 8 of the invention; Figure 3 is a sectional view illustrating the principle of the lamp according to the invention; Figure 4 is a lamp according to the invention within a reflector-type outer envelope; Figure 5 is a lamp according to the invention in an embodiment wherein instead of a tungsten filament, a halogen lamp is being used.

In the arrangement according to the invention the filament used for limiting the current is so arranged that, in contrast with mixed light lamps, either both cathodes or one cathode or the cold space of one cathode is intensively heated. In this way it is made possible significantly to raise the temperature of the "cold point". This effect is particularly important for lamps of lower output because then the loss output appearing at the filament is exploited. In this way the luminous efficiency of the metal halide lamps is significantly increased.

The heating effect of the filament exerted on the ends of the quartz body has a consequence that the dimensions of the heat-reflecting layer at the ends may be reduced, whereby an appreciable increase in luminous flux can be achieved.

The thermal asymmetry to achieve a later or subsequent cooling of the cathode space of the discharge tube provided with the ignition electrode is set either by using an asymmetric heat-insulating layer or by arranging the currentlimiting incadescent tungsten filament around only one of the cathode spaces.

In this way it is made possible to achieve a light source that is expediently so constructed that it can be connected directly to the mains and, as in the case of incandescent lamps, the realization of metal halide lamps of lower output provided with ignition electrodes can be manufactured.

An additional advantage of a light source made in this way is that after ignition of the lamp, light is immediately emitted by it and the spectrum of the light radiated by the incandescent filament is offset in the direction of lower colour temperature. This effect is particularly true for metal halide lamps dosed with the more rare earth metals (Dy, Ho, etc.) where the colour is offset in the neutral direction for daylight lamps so that in this way light sources which are more favourably usable in interiors and which have excellent colour rendering can be obtained.

A further advantage lies in the reduction, after switching on, of the time required for a higher voltage to arise at the series connection of the coils due to the lower operating temperature of the discharge tube. This improved output heats the end or ends of the quartz body more intensively, whereby the metal or metal halide dose in the quartz body evaporates more quickly and the ignition voltage increases more rapidly to reach the predetermined value. It follows from this that in this system the operating voltage or the operating point has a stabilizing effect because in the case where the operating voltage is increased a lower voltage is applied to the series filament(s), the heat output is reduced and the "cold point" temperature of the quartz body decreases, a portion of the additive condenses whereby also the operating voltage is reduced.In this way the lifetime of the lamp is significantly prolonged.

At the same time the disadvantages of the per se known arrangement with mixed light are obviated and in particular the fact that the additional filaments are in general arranged in the centre of the quartz body so that the quartz body is heated at the very position where the temperature is already very high. This circumstance very unfavourably influences the lifetime.

In order to achieve that the quartz body should be ignited through mains voltage either an ignition electrode may be used or a preheatable cathode, as is the case in conventional mercury lamps. The utilization of noble gas mixtures may also facilitate ignition.

A disadvantage of high-pressure discharge lamps lies in that they cannot be re-ignited after switching off for a shorter or longer time. An exception to this are those special types which are the so-called immediately re-ignitable types, but these require a high frequency impulse. This problem is solved in the sense of the invention by means of an additional filament which functions with the aid of a switch whenever no current flows through the discharge tube. Such a case could for instance be the re-ignition in a hot state, as already mentioned above, or damage to the quartz body.

Figure 1 shows the simpiest circuit connection of the principle of the invention. An ignition electrode of a metal-halide discharge tube is connected in series with a filament 2 which has the task of limiting the current and to provide for heating of the cold space. The ignition electrode of the discharge tube is fed via an ignition resistor 3.

Figure 2 shows a further possible embodiment according to the invention in which the circuit is shown diagrammatically. In addition to the abovementioned elements this system has a further incandescent filament 4 which glows when the discharge tube is inoperative, e.g. when the discharge tube has become damaged or when the system is switched on again in a hot condition and therefore the discharge tube cannot ignite.

The element 5 senses the current in the discharge tube and when the current flows through this element the switch 6 is opened. The circuit formed in this way assures that the lamp may always be lit.

Figure 3 shows the construction in principle of the lamp. As may be seen from the drawing, an ignition electrode 1 of a metal-halide discharge tube is connected in series with an incandescent filament 2 which has the task of limiting the current and to provide for the heating of the cold space. The ignition electrode receives its voltage through an ignition resistor 3. A switch 4 in the form of a bimetallic member connects the ignition electrode with the main electrode when the lamp is hot, the two electrodes being side by side to prevent electrolysis. The auxiliary filament 5 is connected via a switch 6 in parallel with the system should no current flow through the sensor.

The current supply is provided by stem electrodes 8 and 9 provided within an envelope 10 having a cap 12, sealed in a gas-tight manner. The outer envelope is either evacuated or is filled with an inert gas. A getter 11 ensures the purity of the inert gases. The discharge tube is provided with a heat-reflecting layer 13 or 14 which may, for instance, be made from zirconium oxide. The quartz body may be provided over its entire surface with an infared-reflecting layer.

Figure 4 shows a variant of an embodiment according to the invention for envelopes of the reflector type (PAR). The reference numbers in Figure 4 are identical with those of Figure 3. In the outer envelope is either a vacuum or a noble gas filling. In this way the two ends of the quartz body are heated by a divided series or protective filament.

Figure 5 shows an embodiment of the invention wherein instead of a tungsten coil, an incandescent halogen lamp is used at one end of the quartz body. This lamp may expediently operate "cap up" but may also be realized by the use of a heat-reflecting layer at the other end of the quartz body.

In this case one must impede the exit of alkali ions through the quartz wall of metal halide lamps using alkali metals in the manner known per se (by decreasing the secondary emission of the fittings, by using an internal gas filling in the outer envelope etc.) The invention is further described with the aid of the following, non-limiting Examples.

Example 1 In an embodiment according to Figure 5 the quartz body is made from a tube with an inner diameter of 1 3 mm and an electrode spacing of 25 mm. The quartz body contains a fill of Dy, Ho, TI, I, Hg and Ar, as is described in Hungarian Patent Specification No. 1 772 230. As a supplement an incandescent halogen lamp is built in at the height of the lower pinch of the quartz body. The power requirement at a mains supply of 240 V is 500 W, the luminous efficiency is 35 ImfW, the colour temperature is 45000K and the general colour rendering index is 94.

Example 2 In this embodiment, similarly to that in Example 1, the quartz body is made from a tube with an internal diameter of 80 mm and an electrode gap of 1 5 mm. The quartz body is filled with Dy, Ho, TI, I, Hg and Ar gas. The power requirement of the lamp is 250 W, the luminous efficiency is 35 Im/W, the colour temperature is 46000K and the general colour rendering index is 93.

Example 3 In an embodiment according to Figure 4 the quartz body is produced from a tube with an internal diameter of 6 mm and an electrode spacing of 13 mm. The quartz body contains a gas filling of Dy, Ho, TI, I, Hg and Ar. The power requirement of the lamp is 1 50 W, the luminous efficiency is 23 lm/W, the colour temperature was 45000K and the general colour rendering index was 93.

Claims

1. Electrical light source with a metal halide discharge tube which in its vacuum-technically sealed envelope contains a high pressure metal halide discharge tube provided with an ignition electrode or with a reheatable electrode, the discharge tube being connected in series with tungsten coil(s), wherein the means for setting the optimal temperature of the cathode space of the discharge tube and the working point are constituted by the tungsten coil(s) connected in series with the discharge tube having regard to the temperature (heat) produced by the operating current of the discharge lamp and the current flowing therethrough.

2. Electrical light source according to Claim 1 wherein the lamp envelope is provided with a light-diffusing coating.

3. Light source according to Claim 1 wherein the light source has a reflector bulb.

4. Light source according to Claim 1 wherein the means for setting the cathode space temperature is constituted by one side of the tungsten coil.

5. Electric light source according to Claim 1 wherein instead of the tungsten coil an incandescent halogen lamp is used.

6. Light source according to claim 1 wherein the outer glass envelope has an Inert gas filling and the temperature setting means of the cathode space is the heat radiation, heat fiow, and heat conduction of the tungsten filament.

7. Electrical light source according to Claim 1 wherein the outer envelope and/or the discharge tube is or are coated with a heat-reflecting layer.

8. Electrical light source according to any preceding claim wherein in the inoperative condition of the discharge tube a further filament is connected to the light source.