GB1600269A - High pressure electric discharge lamps - Google Patents

Description

(54) IMPROVMENTS IN OR RELATING TO HIGH PRESSURE ELECTRIC DISCHARGE LAMPS (71) We, THE GENERAL ELEC TRIC COMPANY LIMITED, of 1 Stanhope Gate, London W1A 1EH, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:: This invention relates to high pressure electric discharge lamps of the type (hereinafter referred to as the type specified) including a discharge envelope which is formed of fused silica, has mounted within it a pair of tungsten electrodes between which an electric discharge passes in operation of the lamp, and contains a filling consisting of mercury, one or more metal halides which are vaporisable at the operating temperature of the lamp, and a small amount of rare gas for facilitating starting of the discharge. The term "high pressure", as used herein with reference to the lamps concerned, is to be understood to mean that the total vapour pressure developed within the discharge envelope in normal operation of the lamp is at least one atmosphere.

It is well known to include various combinations of metal iodides in the fillings of high pressure mercury vapour discharge lamps, in order to produce lamps having high luminous efficacy and desired colour appearance and colour rendering properties, one particularly advantageous combination of such compounds consisting of sodium iodide and scandium tri-iodide. However it has been found that the lumen maintenance of lamps of this type is variable, the luminous efficacy being reduced, after varying periods of operation, as a result of blackening of the discharge envelope wall, which appears to be due to deposition of tungsten evaporated from the electrodes: this effect is particularly marked when a scandium halide is present.

It is an object of the present invention to provide, in lamps of the type specified which include scandium bromide and/or iodide in the filling, an improved filling composition whereby such blackening of the discharge envelope wall can be reduced, with consequent improvement in the lumen maintenance of the lamps.

According to the invention, in a high pressure electric discharge lamp of the type specified, the filling of the discharge envelope consists of mercury, rare gas, one or more metal iodides and one or more metal bromides, including scandium iodide and/or scandium bromide, the percentage atomic ratio of the total amount of bromine in said metal bromide or bromides to the total amount of iodine in said metal iodide or iodides being in the range of 20:80 to 60:40.

It will be understood that the filling constituents specified above are those which are initially introduced into the discharge envelope during the manufacture of the lamp: thus the metal halides present are of stoichiometric composition, the initial filling including no free metal (apart from mercury) and no excess halogen, although metal and/or halogen atoms or ions will be liberated subsequently as a result of reactions and/or dissociation occurring during operation of the lamp. We have found that the presence of one or more metal bromides, in admixture with one or more metal iodides, with a total bromine to total iodine ratio in the range specified, reduces or prevents blackening of the discharge envelope wall in operation of the lamp, so that the lumen maintenance, and therefore the useful operational life, of the lamp is increased.

The metal halide constituents of the filling preferably include the iodide and/or bromide of sodium in addition to the iodide and/or bromide of scandium. Thus some preferred discharge envelope fillings for lamps in accordance with the invention consist of mercury and rare gas together with sodium bromide and scandium iodide, or with sodium iodide and scandium bromide, or with sodium iodide and scandium iodide and sodium bromide, or with sodium iodide and scandium iodide and scandium bromide, in each case with or without one or more additional metal iodides and/or bromides, for example thallium iodide, indium iodide, or caesium iodide.

The discharge envelope is usually in the form of a fused silica tube, closed at each end by a pinch into which is sealed a strip of molybdenum foil supporting an electrode extending into the adjacent region of the en velope and a lead wire extending to the exterior of the pinch. The electrodes may each consist of a coil of tungsten wire, if desired with an integral linear extension at the inner end of the coil, for facilitating starting and stabilisation of the discharge, or may be of any other desired form, and usually contain an electron emissive activator material, the preferred activator material being scandium oxide.If the lamp is designed to be operated with the longitudinal axis of the discharge envelope disposed vertically, or to produce a discharge of the wall stabilised form, the electrodes are sealed into the centres of the respective pinches so as to extend substantially along the discharge envelope axis; alternatively, in a lamp which is de signed to be operated with the longitudinal axis of the discharge envelope disposed hori zontally, and to produce a discharge of the constricted arc type, the electrodes are pre ferably sealed into the pinches at positions offset from the centres thereof, so that the discharge path between the electrode tips is substantially parallel to the discharge en velope axis.In the latter case the lamp is mounted, for operation, with orientation such that the electrodes lie below the envelope axis: this arrangement prevents overheating of the upper part of the discharge envelope wall as a result of upward bowing of the arc, and reduces the temperature difference occurring between the upper and lower parts of the discharge envelope wall in operation of the lamp, thus enabling a relatively high vapour pressure to be maintained in the en velope, with resulting high luminous efficacy of the lamp.

The reduction or prevention of blacken ing of the discharge envelope wall, resulting from the use of a mixture of bromides and iodides in the envelope filling, may be due to the promotion of a tungsten-halogen re generative reaction cycle whereby tungsten evaporated from the electrodes is redeposited thereon, which cycle does not take place in a lamp of the type specified in which metal iodides are the only halides present in the filling.Alternatively, or in addition, we be lieve that where an oxide activator material is incorporated in the lamp electrodes any hydrogen present in the envelope will com bine with oxygen released from the activator during operation of the lamp, causing blacken ing of the envelope due to the well known water-tungsten cycle, and that the bromine of the metal bromide or bromides in the filling acts as a getter for the hydrogen and thus prevents the blackening.

One form of lamp in accordance with the invention will now be described by way of example, with reference to the diagrammatic drawing accompanying the Provisional Specification, which drawing shows the discharge envelope of the lamp in elevation.

The lamp of the example is designed for operation with the longitudinal axis of the discharge envelope disposed horizontally. The discharge envelope consists of a fused silica tube 1, containing a filling of mercury, rare gas, one or more metal iodides, and one or more metal bromides, including scandium iodide and/or scandium bromide, and dosed at each end by a pinch 2, into which is sealed an assembly consisting of a molybdenum foil strip 3, a molybdenum lead wire 4, and an electrode located between the envelope axis and the envelope wall.Each electrode consists of a single coil 5 of tungsten wire with a linear extension of the wire at each end of the coil, the extension 6 at the outer end of the coil being attached to the inner end of the molybdenum foil, and the extension 7 at the inner end of the coil extending into the envelope substantially parallel to the envelope axis; the interiors of the electrode coils are filled with activator material consisting of scandium oxide. The construction of the lamp is completed by mounting the discharge envelope in known manner within a tubular outer envelope of borosilicate glass fitted with a bipin cap at each end, each of the lead wires 4 being connected to one of the pins which constitute the lamp terminals for connection to a source of electric current supply for operation of the lamp. (The outer envelope is of a well known form and is not shown in the drawing).

For the manufacture of the lamp described above with reference to the drawing, the electrode coils are first filled with an aqueous paste of scandium oxide powder, and the electrodes are heated to a temperature of 32000C in a flowing argon atmosphere, by means of an eddy current heating coil coupling into a tungsten cylinder surrounding the electrode, to cause the scandium oxide to melt so that, on cooling, it fonns a solid mass adhering to the coil. The lead wires 4 and electrode extensions 6 are welded to the respective molybdenum foils in the position shown in the drawing, and the discharge envelope is formed in conventional manner by pinching the ends of a suitable length of silica tubing over the foil-electrode-lead assemblies. The envelope is baked at 10000C for 7.5 hours in vacuum, and the desired filling is introduced into the envelope via side tubes, which are then sealed off to leave pips 8; it is necessary to ensure that no moisture or elemental halogen is introduced into the discharge envelope during or subsequently to the baking of the envelope or during the introduction of the filling. Finally the discharge envelope is mounted in the outer envelope and the latter is sealed and capped, in known manner.

For operation, the lamp is mounted in a pair of lampholders and is disposed horizontally and so orientated that the inner extensions 7 of the electrodes lie substantially vertically below the longitudinal axis of the discharge envelope.

Some specific lamps of the form shown in the drawing will now be described in the following examples.

Example 1.

The discharge envelope 1 has an internal diameter of 12 mm, the molybdenum foils 3 are 5 mm wide, and the extensions 7 of the electrodes are positioned 3 to 4 mm from the interior surface of the adjacent part of the discharge envelope wall, and thus 2 to 3 mm below the envelope axis when the lamp is in the correct horizontal operating position.

In each electrode, the coil 5 consists of three turns of 0.5 mm diameter tungsten wire, the internal diameter of the coil being 0.6 mm, and the inner extension 7 is 2 mm long; the horizontal length of the discharge path between the tips of the said extensions is 21 mm. The discharge envelope filling consists of 21 mg of mercury, 20 mg of sodium iodide, 10 mg of scandium tri-iodide, 7 mg of sodium bromide, and argon at a room temperature pressure of 35 torr. In normal operation this lamp dissipates 175 watts at a tube voltage of 120 volts, has an initial luminous efficacy of 103 lumens per watt, with a colour temperature of 35000 to 40000K, and shows 92% lumen maintenance after 1000 hours operation. By comparison, a lamp which is identical in all respects with the exception that the sodium bromide is omitted from the filling shows a lumen maintenance of 80% after 1000 hours operation.

Example 2.

The lamp of this example has a discharge envelope of the same dimensions, and electrodes of the same dimensions and positions, as those described in Example 1, and has a discharge envelope filling consisting of 21 mg of mercury, 10 mg of sodium iodide, 10 mg of scandium tri-iodide, 7 mg of sodium bromide, and argon at a room temperature pressure of 35 torr. This lamp, in normal operation, dissipates 175 watts at a tube voltage of 120 volts, has an initial luminous efficacy of 104 lumens per watt with a colour temperature of 40000 to 45000K. and shows 85 Ó lumen maintenance after 1000 hours operation. By comparison, a lamp which is identical apart from the omission of sodium bromide from the filling shows a lumen main tenance of 70 . after 1000 hours operation.

Example 3.

In the lamp of this example, the discharge envelope dimensions and the electrode dimensions and positioning are as described in Example 1, and the discharge envelope filling consists of 21 mg of mercury, 21 mg of sodium iodide, 5 mg of scandium tri-iodide, 3 mg of thallium iodide, 0.6 mg of indium iodide, 9 mg of sodium bromide, and argon at a room temperature pressure of 35 torr. In normal operation this lamp dissipates 175 watts at a tube voltage of 120 volts, has an initial luminous efficacy of 92 lumens per watt with a colour temperature of 40000 to 45000K, and shows 90% lumen maintenance after 1000 hours operation.By comparison, a lamp of the same dimensions and operating characteristics, and with the same filling except that sodium bromide is omitted, shows lumen maintenance of 80% after 1000 hours operation.

WHAT WE CLAIM IS:- 1. A high pressure electric discharge lamp of the type specified, wherein the filling of the discharge envelope consists of mercury, rare gas, one or more metal iodides and one or more metal bromides, including scandium iodide and/or scandium bromide, the percentage atomic ratio of the total amount of bromine in said metal bromide or bromides to the total amount of iodine in said metal iodide or iodides being in the range of 20:80 to 60:40.

2. A lamp according to Claim 1, wherein the said filling includes sodium iodide and/or sodium bromide.

3. A lamp according to Claim 1 or 2, wherein an electron emissive activator material consisting of scandium oxide is incorporated in the electrodes.

4. A lamp according to Claim 1, substantially as shown in and hereinbefore described with reference to the drawing accompanying the Provisional Specification.

5. A lamp according to Claim 4, substantially as described in one of the foregoing specific Examples 1, 2 or 3.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. capped, in known manner. For operation, the lamp is mounted in a pair of lampholders and is disposed horizontally and so orientated that the inner extensions 7 of the electrodes lie substantially vertically below the longitudinal axis of the discharge envelope. Some specific lamps of the form shown in the drawing will now be described in the following examples. Example 1. The discharge envelope 1 has an internal diameter of 12 mm, the molybdenum foils 3 are 5 mm wide, and the extensions 7 of the electrodes are positioned 3 to 4 mm from the interior surface of the adjacent part of the discharge envelope wall, and thus 2 to 3 mm below the envelope axis when the lamp is in the correct horizontal operating position. In each electrode, the coil 5 consists of three turns of 0.5 mm diameter tungsten wire, the internal diameter of the coil being 0.6 mm, and the inner extension 7 is 2 mm long; the horizontal length of the discharge path between the tips of the said extensions is 21 mm. The discharge envelope filling consists of 21 mg of mercury, 20 mg of sodium iodide, 10 mg of scandium tri-iodide, 7 mg of sodium bromide, and argon at a room temperature pressure of 35 torr. In normal operation this lamp dissipates 175 watts at a tube voltage of 120 volts, has an initial luminous efficacy of 103 lumens per watt, with a colour temperature of 35000 to 40000K, and shows 92% lumen maintenance after 1000 hours operation. By comparison, a lamp which is identical in all respects with the exception that the sodium bromide is omitted from the filling shows a lumen maintenance of 80% after 1000 hours operation. Example 2. The lamp of this example has a discharge envelope of the same dimensions, and electrodes of the same dimensions and positions, as those described in Example 1, and has a discharge envelope filling consisting of 21 mg of mercury, 10 mg of sodium iodide, 10 mg of scandium tri-iodide, 7 mg of sodium bromide, and argon at a room temperature pressure of 35 torr. This lamp, in normal operation, dissipates 175 watts at a tube voltage of 120 volts, has an initial luminous efficacy of 104 lumens per watt with a colour temperature of 40000 to 45000K. and shows 85 Ó lumen maintenance after 1000 hours operation. By comparison, a lamp which is identical apart from the omission of sodium bromide from the filling shows a lumen main tenance of 70 . after 1000 hours operation. Example 3. In the lamp of this example, the discharge envelope dimensions and the electrode dimensions and positioning are as described in Example 1, and the discharge envelope filling consists of 21 mg of mercury, 21 mg of sodium iodide, 5 mg of scandium tri-iodide, 3 mg of thallium iodide, 0.6 mg of indium iodide, 9 mg of sodium bromide, and argon at a room temperature pressure of 35 torr. In normal operation this lamp dissipates 175 watts at a tube voltage of 120 volts, has an initial luminous efficacy of 92 lumens per watt with a colour temperature of 40000 to 45000K, and shows 90% lumen maintenance after 1000 hours operation.By comparison, a lamp of the same dimensions and operating characteristics, and with the same filling except that sodium bromide is omitted, shows lumen maintenance of 80% after 1000 hours operation. WHAT WE CLAIM IS:-

1. A high pressure electric discharge lamp of the type specified, wherein the filling of the discharge envelope consists of mercury, rare gas, one or more metal iodides and one or more metal bromides, including scandium iodide and/or scandium bromide, the percentage atomic ratio of the total amount of bromine in said metal bromide or bromides to the total amount of iodine in said metal iodide or iodides being in the range of 20:80 to 60:40.

2. A lamp according to Claim 1, wherein the said filling includes sodium iodide and/or sodium bromide.

3. A lamp according to Claim 1 or 2, wherein an electron emissive activator material consisting of scandium oxide is incorporated in the electrodes.

4. A lamp according to Claim 1, substantially as shown in and hereinbefore described with reference to the drawing accompanying the Provisional Specification.

5. A lamp according to Claim 4, substantially as described in one of the foregoing specific Examples 1, 2 or 3.