GB2087141A - Discharge lamps - Google Patents

Abstract

A vapour discharge electric lamp incorporates a pump stem (5) extending through an aperture in an arc tube envelope (8, 6) and a discrete annular member (10) disposed around and secured to the pump stem (5) to locate the latter in said aperture by abutment against the outside of said envelope. By this means the pump stem (5) which incorporates the cold spot may be accurately located in the arc tube envelope (8, 6), perpendicular to the aperture therein, during assembly and subsequently sealed to the envelope in the desired position. The location of the cold spot enables the colour rendering index of a high pressure sodium vapour lamp, for example, to be controlled. <IMAGE>

Description

SPECIFICATION Improvements in or relating to vapour discharge lamps This invention relates to a method of increasing the colour rendering index of vapour discharge lamps, particularly but not exclusively of high pressure sodium vapour lamps. The colour rendering index of a vapour discharge lamp is dependent on the vapour pressure therein. For example conventional high pressure sodium vapour lamps have a colourtemperature of between 2000 and 2100K and a colour rendering index of between 20and30. If the vapour pressure of sodium in the arc increases the colour temperature rises. At colour temperatures around 2350K the colour rendering index exceeds 80 but at colour temperatures greater than 2500K the colour rendering index decreases.

The vapour pressure of a discharge lamp having a pump stem, usually of metal, extending through part of the wall of the arc tube may be controlled by varying the cold spot temperature in the pump stem.

This cold spot is changed by varying the amount of metal extending through the wall of the arc tube.

It is important that the cold spot temperature is accurately maintained at an optimum value and it is an object of this invention to provide an improved design for vapour discharge lamps which meets this requirement.

According to this invention a vapour discharge electric lamp incorporates a pump stem extending through an aperture in an arc tube envelope and an annular member disposed around and secured to the pump stem to locate the latter in said aperture by abutment against said envelope. By this means the pump stem which incorporates the cold spot may be accurately located in the arc tube envelope, perpendicular to the aperture therein, during assembly and subsequently sealed to the envelope in the desired position.

An annular member in accordance with this invention conveniently comprises a wire ring. Use of a wire ring or other annular member is preferred to a previously known arrangement in which the pump stem is provided with ridges which abut in use against the envelope since such ridges may only be manufactured before the lamp electrode is attached to the pump stem. Consequently use of an annular member allows a greater accuracy of assembly of the lamp.

The ring may comprise a wire disposed around and welded to the pump stem, and may be composed of molybdenum, niobium, tungsten ortantalum.

The invention will now be further described by way of an example with reference to Figures 1 and 2 of the accompanying drawings.

Figure 1 shows a cross-sectional view of a prior art lamp assemblage, Figure 2 shows a cross sectional view of an assemblage in accordance with this invention.

The assemblage shown in Figure 1 illustrates a major disadvantage of the prior art in which passage of the pump stem 1 through the aperture in the arc tube 2,3 is limited by abutment of a piece of wire 4 welded longitudinally to the pump stem 1 against the arc tube. It is possible, as shown in the drawing, that the pump stem is inserted non-perpendicularly into the aperture with consequent innaccurate positioning of the cold spot.

The arrangement in accordance with the invention shown in Figure 2 comprises a niobium pump stem 5 inserted into an aperture in an end section 6 of a polycrystalline alumina envelope 8. The pump stem 5 carries a tungsten lamp electrode 7 held by a pinch in the stem. The polycrystalline alumina envelope 8 carries a sealing frit 9.

During assembly of the lamp the pump stem is inserted through the apertures in the envelope 6 and frit 9. Perpendicular insertion of the stem 5 is ensured by a molybdenum wire ring 10 which is welded onto the stem 5 and abuts against the envelope 6 when the stem is correctly positioned.

The positioned stem is then sealed in place in the envelope in a known manner. The location of the cold spot (not shown) is therefore accurately determined and the colour rendering index of the lamp is increased.

1. A vapour discharge lamp incorporating a pump stem inserted into an aperture in an end section of a light-transmitting envelope wherein a stop carried by the pump stem abuts against the outside of the end section, characterised in that the stop is a discrete member which circumferentially surrounds, and is attached to, the pump stem.

2. A vapour discharge lamp according to Claim 1 in which the stop is annular.

3. A vapour discharge lamp according to Claim 1 or Claim 2 in which the stop is made from a different material to the pump stem.

4. A vapour discharge lamp according to any of Claims 2,3 or 4 in which the stop consists of a wire ring.

5. A process for the manufacture of a vapour discharge lamp comprising the steps of attaching a pump stem to an electrode, attaching a stop to the outside of the pump stem and inserting the joined electrode and pump stem into a light-transmitting envelope until the stop abuts against the outside of an end section of the envelope, characterised in that the stop is attached to the pump stem after the electrode has been attached to the pump stem.

6. A process according to Claim 5 in which the stop is welded to the pump stem.

7. A process according to Claim 6 in which the stop is made of molybdenum, niobium, tungsten or tantalum.

8. A process according to Claim 7 in which the pump stem is made of niobium.

9. A high pressure sodium vapour discharge lamp produced by a process according to any of claims 5 to 8.

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

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Improvements in or relating to vapour discharge lamps This invention relates to a method of increasing the colour rendering index of vapour discharge lamps, particularly but not exclusively of high pressure sodium vapour lamps. The colour rendering index of a vapour discharge lamp is dependent on the vapour pressure therein. For example conventional high pressure sodium vapour lamps have a colourtemperature of between 2000 and 2100K and a colour rendering index of between 20and30. If the vapour pressure of sodium in the arc increases the colour temperature rises. At colour temperatures around 2350K the colour rendering index exceeds 80 but at colour temperatures greater than 2500K the colour rendering index decreases. The vapour pressure of a discharge lamp having a pump stem, usually of metal, extending through part of the wall of the arc tube may be controlled by varying the cold spot temperature in the pump stem. This cold spot is changed by varying the amount of metal extending through the wall of the arc tube. It is important that the cold spot temperature is accurately maintained at an optimum value and it is an object of this invention to provide an improved design for vapour discharge lamps which meets this requirement. According to this invention a vapour discharge electric lamp incorporates a pump stem extending through an aperture in an arc tube envelope and an annular member disposed around and secured to the pump stem to locate the latter in said aperture by abutment against said envelope. By this means the pump stem which incorporates the cold spot may be accurately located in the arc tube envelope, perpendicular to the aperture therein, during assembly and subsequently sealed to the envelope in the desired position. An annular member in accordance with this invention conveniently comprises a wire ring. Use of a wire ring or other annular member is preferred to a previously known arrangement in which the pump stem is provided with ridges which abut in use against the envelope since such ridges may only be manufactured before the lamp electrode is attached to the pump stem. Consequently use of an annular member allows a greater accuracy of assembly of the lamp. The ring may comprise a wire disposed around and welded to the pump stem, and may be composed of molybdenum, niobium, tungsten ortantalum. The invention will now be further described by way of an example with reference to Figures 1 and 2 of the accompanying drawings. Figure 1 shows a cross-sectional view of a prior art lamp assemblage, Figure 2 shows a cross sectional view of an assemblage in accordance with this invention. The assemblage shown in Figure 1 illustrates a major disadvantage of the prior art in which passage of the pump stem 1 through the aperture in the arc tube 2,3 is limited by abutment of a piece of wire 4 welded longitudinally to the pump stem 1 against the arc tube. It is possible, as shown in the drawing, that the pump stem is inserted non-perpendicularly into the aperture with consequent innaccurate positioning of the cold spot. The arrangement in accordance with the invention shown in Figure 2 comprises a niobium pump stem 5 inserted into an aperture in an end section 6 of a polycrystalline alumina envelope 8. The pump stem 5 carries a tungsten lamp electrode 7 held by a pinch in the stem. The polycrystalline alumina envelope 8 carries a sealing frit 9. During assembly of the lamp the pump stem is inserted through the apertures in the envelope 6 and frit 9. Perpendicular insertion of the stem 5 is ensured by a molybdenum wire ring 10 which is welded onto the stem 5 and abuts against the envelope 6 when the stem is correctly positioned. The positioned stem is then sealed in place in the envelope in a known manner. The location of the cold spot (not shown) is therefore accurately determined and the colour rendering index of the lamp is increased. CLAIMS

1. A vapour discharge lamp incorporating a pump stem inserted into an aperture in an end section of a light-transmitting envelope wherein a stop carried by the pump stem abuts against the outside of the end section, characterised in that the stop is a discrete member which circumferentially surrounds, and is attached to, the pump stem.

2. A vapour discharge lamp according to Claim 1 in which the stop is annular.

3. A vapour discharge lamp according to Claim 1 or Claim 2 in which the stop is made from a different material to the pump stem.

4. A vapour discharge lamp according to any of Claims 2,3 or 4 in which the stop consists of a wire ring.

5. A process for the manufacture of a vapour discharge lamp comprising the steps of attaching a pump stem to an electrode, attaching a stop to the outside of the pump stem and inserting the joined electrode and pump stem into a light-transmitting envelope until the stop abuts against the outside of an end section of the envelope, characterised in that the stop is attached to the pump stem after the electrode has been attached to the pump stem.

6. A process according to Claim 5 in which the stop is welded to the pump stem.

7. A process according to Claim 6 in which the stop is made of molybdenum, niobium, tungsten or tantalum.

8. A process according to Claim 7 in which the pump stem is made of niobium.

9. A high pressure sodium vapour discharge lamp produced by a process according to any of claims 5 to 8.