GB1587988A - Low pressure gas and/or vapour discharge lamp - Google Patents

Description

(54) LOW PRESSURE GAS AND/OR VAPOUR DISCHARGE LAMP (71) We, N.V. PHILIPS' GLOEILAMPEN FABRIEKEN, a limited liability Company, organised and established under the laws of the Kingdom of the Netherlands, of Emmasingel 29, Eindhoven, the Netherlands 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: The invention relates to a low-pressure gas and/or vapour discharge lamp having a discharge vessel provided with means for selectively increasing the temperature in one or more minor portions of the discharge vessel with respect to the remaining major portion thereof.

In most low-pressure discharge lamps, the temperature during operation is lower in given minor portions of the discharge vessel than in other portions thereof. This is, for example, the case in the curved portion of a U-shaped discharge vessel of a lowpressure sodium vapour discharge lamp.

When producing such a lamp the sodium is distributed in a plurality of bulges provided in the wall of the discharge vessel. As the curved portion has a relatively low operating temperature, the sodium in the discharge vessel tends to migrate in the direction of the cooler curved portion during operation of the lamp. As a consequence, the optimum sodium vapour pressure (i.e. the vapour pressure at which the conversion of electric power into useful radiation is as efficient as possible) cannot be maintained and the efficiency of the lamp is adversely affected.

In order to solve the above-mentioned problem it has been proposed to increase the temperature of the relatively cool portion of the charge vessel, such as the curved portion, during operation of the lamp by providing this portion selectively with a coating which reflects the heat radiation of the discharge. Such a coating may consist of a silver coating applied to the outer wall surface of the curved portion of the discharge vessel. However, such a solution has the drawback that the emission of light from the portion of the discharge vessel coated with said coating is impeded.

It is an object of the invention to provide a lamp wherein means is provided, in at least one usually relatively cool portion of the discharge vessel, which locally increases the operating temperature without impeding the emission of light.

The invention accordingly provides a lowpressure gas and/or vapour discharge lamp having a discharge vessel provided with a means for selectively increasing the temperature in one or more minor portions of the discharge vessel with respect to the remaining major portion thereof, wherein the means is disposed in the discharge vessel in the or each minor portion and comprises a filamentary material having a thinlydistributed structure which is transmissive to the discharge, the said major portion of the vessel either containing no such filamentary material or containing a body of filamentary material whose distribution density is lower than that of the said means.

By providing said means in the portion of the discharge vessel which normally has a relatively low temperature, the power dissipation in that portion increases and, consequently, the local temperature of the wall of the discharge vessel also increases.

As a result the metal, for example sodium, remains substantially evenly distributed over the entire discharge space during the life of the lamp.

Further, the portion of the discharge vessel where the means is provided contributes towards the luminous efficiency of the lamp.

The most suitable temperature which should prevail in said portion is obtained by a suitable choice of density and/or distribution over the discharge vessel of the body material.

The means may consist of thinly distributed glass wool and, for low-pressure sodium vapour discharge lamps, of gehlenite glass wool which is resistant to the action of sodium. Alternatively, the means may comprible thinly-distributed electrically-conducting filamentary material such as wire wool.

The said means can be used in low pressure gas and/or vapour discharge lamps in general. The use of the means is particularly advantageous, however, when applied in the curved portion of the U-shaped discharge vessel of low-pressure sodium vapour discharge lamps.

An embodiment of the invention will now be described, by way of example, with reference to the sole Figure of the accom panying drawing.

In the drawing a U-shaped discharge vessel 1 of a low-pressure sodium vapour discharge lamp comprises two legs 2 and 3 provided with respective bulges 2a and 3a in which the sodium is distributed during fabrication of the lamp. Besides sodium, the discharge vessel contains a small quantity of a rare gas or a mixture of rare gases.

The discharge vessel is enveloped by an outer bulb 4 whose inner wall is provided with a heat-reflecting coating 4a (for example of indium-oxide). The space between the discharge vessel and the outer bulb is exhausted. In addition, the lamp is provided with a socket 5 and a sleeve Sa.

Electrodes 6 and 7 are respectively disposed at the ends of each of the legs 2 and 3 of the discharge vessel. Thinly-distributed gehlenite glass wool 9 having a filament thickness of approximately 15 microns and a distribution density of approximately 2.5 mg/cm3 is disposed in the curved portion 8 of the discharge vessel. Owing to the presence of the gehlenite glass wool, the temperature in the portion 8 is higher during operation of the lamp than in the two legs 2 and 3. The luminous efficiency (in lumens per Watt) of the described lamp is higher than that of an identical lamp not having glass wool in the curved portion.

In the particular embodiment described, the temperature-increasing means is located in the curved portion of a U-shaped discharge tube. The invention is, of course, equally applicable to other forms of discharge lamp and the means may be located in one or more selected portions of the discharge tube.

In order further to increase the luminous efficiency per unit volume of the lamp, a body of thinly-distributed filamentary material transmissive to the discharge may be located in a portion or portions of the discharge tube other than that in which said temperature-increasing means is located. In order to increase the operating temperature in the minor portion of the tube occupied by the temperature-increasing means to a greater extent than in the portion(s) occupied by the said body, the distribution density of the body is made less than that of the said means.

WHAT WE CLAIM IS: 1. A low-pressure gas and/or vapour discharge lamp having a discharge vessel provided with a means for selectively increasing the temperature in one or more minor portions of the discharge vessel, with respect to the remaining major portion thereof, wherein the means is disposed in the discharge vessel in the or each minor portion and comprises filamentary material having a thinly-distributed structure which is transmissive to the discharge, the said major portion of the vessel either containing no such filamentary material or containing a body of filamentary material having a thinly-distributed structure transmissive to the discharge whose distribution density is lower than that of the said means.

2. A low-pressure discharge lamp as claimed in Claim 1 or 2, wherein the means is thinly distributed glass wool.

3. A low-pressure discharge lamp as claimed in Claim 1 or 2, wherein the means is gehlenite glass wool.

4. A low-pressure discharge lamp as claimed in any previous Claim, wherein the means is disposed in the curved portion of a U-shaped discharge vessel.

5. A low-pressure discharge lamp substantially as hereinbefore described with reference to the accompanying drawing.

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

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. pressure gas and/or vapour discharge lamps in general. The use of the means is particularly advantageous, however, when applied in the curved portion of the U-shaped discharge vessel of low-pressure sodium vapour discharge lamps. An embodiment of the invention will now be described, by way of example, with reference to the sole Figure of the accom panying drawing. In the drawing a U-shaped discharge vessel 1 of a low-pressure sodium vapour discharge lamp comprises two legs 2 and 3 provided with respective bulges 2a and 3a in which the sodium is distributed during fabrication of the lamp. Besides sodium, the discharge vessel contains a small quantity of a rare gas or a mixture of rare gases. The discharge vessel is enveloped by an outer bulb 4 whose inner wall is provided with a heat-reflecting coating 4a (for example of indium-oxide). The space between the discharge vessel and the outer bulb is exhausted. In addition, the lamp is provided with a socket 5 and a sleeve Sa. Electrodes 6 and 7 are respectively disposed at the ends of each of the legs 2 and 3 of the discharge vessel. Thinly-distributed gehlenite glass wool 9 having a filament thickness of approximately 15 microns and a distribution density of approximately 2.5 mg/cm3 is disposed in the curved portion 8 of the discharge vessel. Owing to the presence of the gehlenite glass wool, the temperature in the portion 8 is higher during operation of the lamp than in the two legs 2 and 3. The luminous efficiency (in lumens per Watt) of the described lamp is higher than that of an identical lamp not having glass wool in the curved portion. In the particular embodiment described, the temperature-increasing means is located in the curved portion of a U-shaped discharge tube. The invention is, of course, equally applicable to other forms of discharge lamp and the means may be located in one or more selected portions of the discharge tube. In order further to increase the luminous efficiency per unit volume of the lamp, a body of thinly-distributed filamentary material transmissive to the discharge may be located in a portion or portions of the discharge tube other than that in which said temperature-increasing means is located. In order to increase the operating temperature in the minor portion of the tube occupied by the temperature-increasing means to a greater extent than in the portion(s) occupied by the said body, the distribution density of the body is made less than that of the said means. WHAT WE CLAIM IS:

1. A low-pressure gas and/or vapour discharge lamp having a discharge vessel provided with a means for selectively increasing the temperature in one or more minor portions of the discharge vessel, with respect to the remaining major portion thereof, wherein the means is disposed in the discharge vessel in the or each minor portion and comprises filamentary material having a thinly-distributed structure which is transmissive to the discharge, the said major portion of the vessel either containing no such filamentary material or containing a body of filamentary material having a thinly-distributed structure transmissive to the discharge whose distribution density is lower than that of the said means.

2. A low-pressure discharge lamp as claimed in Claim 1 or 2, wherein the means is thinly distributed glass wool.

3. A low-pressure discharge lamp as claimed in Claim 1 or 2, wherein the means is gehlenite glass wool.

4. A low-pressure discharge lamp as claimed in any previous Claim, wherein the means is disposed in the curved portion of a U-shaped discharge vessel.

5. A low-pressure discharge lamp substantially as hereinbefore described with reference to the accompanying drawing.