GB2137407A

GB2137407A - Low pressure discharge lamps

Info

Publication number: GB2137407A
Application number: GB08308104A
Authority: GB
Inventors: Michael Wilfred Kirby
Original assignee: Thorn EMI PLC
Current assignee: Thorn EMI PLC
Priority date: 1983-03-24
Filing date: 1983-03-24
Publication date: 1984-10-03
Also published as: GB2137407B; GB8308104D0

Abstract

A low pressure sodium discharge lamp has a U-shaped tube in an outer envelope (11). The lamp's cool spot tends to be at the bend of the U since the straight leg portions (30, 31) are mutually self-heating. The temperature at the bend can be too low for optimum efficiency and to increase the cool spot temperature the lamp includes a heat shield (47) within the outer envelope reflecting heat from the tube back to the bend. The heat shield is preferably an aluminised disc resiliently supported on the outer envelope. <IMAGE>

Description

SPECIFICATION Improvements in low pressure discharge lamps This invention relates to low pressure metal vapourdischarge lamps wherein a folded discharge tube is enclosed within an outer envelope. The invention is particularly concerned with low pressure sodium discharge lamps. Such lamps are particularly useful in street lighting applications and are generally contained within a fitting or luminaire designed to give a light distribution transverse to the longitudinal axis of the lamp. Due to the folded construction elongate straight leg portions of the discharge tube are formed which lie in a single plane. In most cases as well as being in the same plane these leg portions are parallel. In use these straight leg portions provide a mutual self heating effedtalong their length.At the folded end of the discharge tube, however, this no longer applies and consequently there is a drop in temperature in the region of the bend compared with the temperature along the straight portions of the discharge tube. This temperature difference affects the efficiency of the lamp.

An object of this inventiom is to provide a low pressure discharge lamp designed to reduce this temperature difference.

According to one aspect of this invention there is provided a low pressure metal vapour discharge lamp having a discharge tube, folded to provide two leg sections joined by a joining section, enclosed within an outer envelope, the discharge tube having a respective electrode at each end thereof, the lamp being arranged to have a coolspot in the discharge tube and further a heat shield supported between the joining section of the discharge tube and the end of the envelope to reflect heat from the discharge tube towards the joining section.

According to another aspect of the invention there is provided a low pressure sodium discharge lamp having a discharge tube, folded to provide two legs sections joined by a joining section, enclosed within an outer envelope, the discharge tube having a respective electrode at each end thereof and the lamp being arranged to have a coolspot within the length of the discharge tube, the end of the outer envelope including an annular shoulder member and a heat shield resiliently held against the shoulder member, the heat shield being disposed to reflect heat from the discharge tube towards the joining section in the direction of the coolspot.

Preferably the heat shield comprises an aluminium disc or, a disc covered with aluminium material, such as foil since aluminium is a good reflector of infra-red radiation. Preferably, also, the disc is supported by a shoulder member formed on the end of the outer envelope and in a particularly preferred embodiment the disc is held against the shoulder member by a resilient clip member.

A low pressure mercury vapour discharge lamp is known having a folded discharge tube enclosed within an outer envelope. Such a lamp, disclosed in Specification GB.2 033 653A has an appendix formed on the discharge tube which provides the coolspot for the lamp. A heat shield is provided to protect the appendix from the heat being radiated from the discharge tube. By contrast the heat shield of the present invention is designed to reflect heat back in the direction of the coolspot to heat the discharge tube in the area of the section joining the two leg sections.

One embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings wherein: Figure 1 is a schematic perspective view looking down on a lamp according to the invention and Figure 2 is a part sectional view of the upper of the lamp of Figure 1 looking on 2-2 of Figure 1.

One embodiment of a lamp according to the invention is shown generally at 10 in Figure 1. This comprises a soft glass outer envelope 11 containing a folded tube 12, also of soft glass, and having respective electrodes 13,14 at each end 15, of the discharge tube 12. The envelope 11 is necked down at 17 and sealed within a brass collar 18 surrounding a ceramic plug 19 which carries bayonet pins 20.

Lead wires 21,22 from end 16 and lead wires 23,24 from end 16 and lead wires 23,24 from end 15 are led through glass stem portion 25 to respective conductive members 26,27 at the bottom of the ceramic plug 19. The stem portion 25 is sealed to necked down portion 17 and carried a mica disc 28 which is slotted to receive and support the ends 15, 16 of the discharge tube 12. The discharge tube 12 is folded over at one end to form a curved end 29 and as a result elongate straight leg portions 30,31 are formed. These leg portions 30,31 are parallel and lie in a single plane as best seen in Figure 2. The end 32 of the outer envelope 11 is shaped to form an annular shoulder in member 33 which is transverse to the longitudinal axis 34 of the lamp. A generally spherical portion 35 completes the end 32.The inside portion 36 of the curved end 29 of the discharge tube is necked slightly to surround and support a hollow ceramic bridge member 37. This bridge member, in turn, supports a resilient clip member 38 best seen in Figure 2 which clip member supports and centres the curved end 29. The clip member is made of resilient strip material, such as for example, stainless steel and has lower leg portions 39, 40 extending through the bridge member 37 outwardly to form upwardly extending side portions 41,42 respectively which resiliently bear on the envelope 11. The side portions 41,42 are bent over at 43,44 and then depend downwards to be attached to a upport plate 45 iocated by the exhaust tip 46 of the discharge tube 12.The bent ends 43,44 bear on a heat shield 47 and urge it against the annular shoulder member 33 formed on the end of the envelope 11 thereby resiliently supporting the heat shield 47. The heat shield can comprise an annular disc of aluminium of a disc covered with alumihium material, such as, for example, aluminium foil. Aluminium is preferred because it is a good reflector of infra-red radiation. Although the heat shield will obscure light being emitted from the end of the envelope this is not important since the fittings with which these lamps are used are designed to transmit the light transverse to the lamp's longitudinal axis 34.

The clip member 38 includes a member 48 which extends transversely to legs 39, 40 as best seen from Figure 1, and has depending portions 49,50 respectively which bear the envelope 11 to provide further centreing supportforthe curved end 29 of the discharge tube 12.

In Figure 1 reference numerals 51, 52 indicate the coolspot area where sodium metal tends to condense out on the inside of the discharge tube 12. The inside of the glass outer envelope 11 includes a layer of indium oxide doped with tin which reflects infra-red radiation but passes visible radiation.

When the lamp is operating there is a mutual self heating effect along the length of the straight leg portions 30, 31 however this does not apply at the curved orfolded end 29 of the discharge tube 12so there is a drop in temperature at this area. The use of a heat shield in accordance with the invention reflects heat back towards this area.

In a practical embodiment of a lamp in accordance with the invention a low pressure sodium lamp has a gas fill comprising a mixture of neon and argon (99% neon, 1% argon) at 5 torr and a pressure of sodium of approximately 1 millitorr. The diameter of the discharge tube was 15 mm while the diameter of the outer envelope was nominally 50 mm with an overall length of 310 mm maximum. The width of the annular shoulder was 5 mm nominally and the distance between the centre lines of parallel legs was nominally 22 mm. Such a lamp had a wattage of 26 but, of course a range of wattages is contemplated, for example, between 26 and 100. Likewise the gas fill pressures could range between 4 and 9 torr. The phrase folded discharge tube in this specification is meant to include the case where tubes are joined by one or more straight portions as well as the curved portion described hereinbefore.

Claims

1. A low pressure metal vapour discharge lamp having a discharge tube, folded to provide two leg sections joined by a joining section, enclosed within an outer envelope, the discharge tube having a respective electrode at each end thereof, the lamp being arranged to have a coolspot in the discharge tube and further including a heat shield supported between the joining section of the discharge tube and the end of th envelope arranged to reflect heat from the discharge tube towards the joining section.

2. A low pressure sodium discharge lamp having a discharge tube, folded to provide two legs sections joined by a joining section, enclosed within an outer envelope, the discharge tube having a respective electrode at each end thereof and the lamp being arranged to have a coolspot within the length of the discharge tube, the end of the outer envelope including an annularshoulder member and a heat shield resiliently held against the shoulder member, the heat shield being disposed to reflect heat from the discharge tube towards the joining section in the direction of the coolspot.

3. A lamp according to Claim 2 in which the heat shield is held against the shoulder member by a resilient clip.

4. A lamp according to any preceding Claim in which the heat shield is a disc formed at least in part of infra-red reflective material.

5. A lamp according to Claim 4 in which the disc is formed of aluminium.

6. A lamp according to Claim 4 in which the disc is covered with aluminium material.

7. A lamp according to Claim 6 in which the disc is covered with aluminium foil.

8. A low pressure metal vapour discharge lamp substantially as herein described with reference to the accompanying drawings.