GB2092822A

GB2092822A - High Pressure Sodium Vapour Lamp

Info

Publication number: GB2092822A
Application number: GB8200046A
Authority: GB
Original assignee: Egyesuelt Izzolampa es Villamossagi Rt
Current assignee: Egyesuelt Izzolampa es Villamossagi Rt
Priority date: 1981-01-13
Filing date: 1982-01-04
Publication date: 1982-08-18
Also published as: HU181262B; FR2498014B3; RO84270B; FR2498014A1; NL8200077A; ES508622A0; SE8200110L; DD201955A5; ES8303816A1; JPS57145263A; DE3200698A1; RO84270A; YU8382A; BE891763A; IT8247530A0; AU7944582A

Abstract

A high pressure sodium vapour lamp consists of a lamp cap 3 hermetically sealed to a light- transmitting envelope 1 containing a tubular discharge vessel 4 of alumina, having a maximum internal diameter of 5.5 mm., and a filling of a mixture of an inert gas and metallic additives. Current is supplied via lead-in wires 5, 6 to the electrodes of the discharge tube 4, the distance between the tips of which is at most 5.5 times the maximum internal diameter of the tube 4. Such high pressure sodium vapour lamps satisfy requirements for interior lighting: their colour temperature exceeds 2000 DEG K and their colour rendition index exceeds 20. They consist of components that can be assembled to a finished product on traditional high-speed mass-production lamp machinery, the small discharge tube 4 replacing the incandescent coil filament. <IMAGE>

Description

SPECIFICATION High Pressure Sodium Vapour Lamp The invention relates to a high pressure sodium vapour lamp consisting of a lamp base (cap) electrically connectible either to a lampholder or to a light fixture, a light-transmitting outer envelope hermetically sealed from the ambient atmosphere and an essentially tubular discharge vessel arranged inside the outer envelope with a maximum internal diameter of 5.5 mm the wall of which is transparent or translucent and is made of alumina, the vessel containing a filling of a mixture of a noble (inert) gas and metal additives, particularly sodium, cadmium and/or mercury, and being fitted with electrodes connected via electric current lead-in wires to the base (cap) of the lamp.The high pressure sodium vapour lamps according to the invention are built up from components which can be assembled to a finished product by means of traditional incandescent lamp-making machinery and their colour rendition and colour temperature are equivalent to those of incandescent lamps.

High pressure sodium vapour lamps of high efficiency are widely used for industrial and outdoor illumination purposes. However, their very low colour temperature (approximately 20000 K) and poor colour rendition (their general colour rendition index being approximately 20) have not permitted their general application in the field of interior lighting. A further limiting factor in this respect is that high pressure sodium vapour lamps of output, external dimensions, construction and optical properties approximately equivalent to those of incandescent lamps could not hitherto be produced similarly to incandescent lamps, i.e. on existing manufacturing machinery for incandescent lamps.

To render the high pressure sodium vapour lamps competitive with the customary incandescent lamps by using the same or similar mass-production machinery, a discharge vessel has to be developed which is suitable for replacing the incandescent coil of traditional incandescent lamps, i.e. these two types of light source should differ only in that the incandescent coil is replaced by a discharge vessel. The generally employed tubular discharge vessels have an internal diameter of 6-8 mm, but their outer dimensions render them unsuitable for this purpose. Only tubular discharge vessels with an internal diameter of at most 5.5 mm can be taken into consideration. It is also expedient at least partially to substitute the mercury in the metal additives by cadmium to render the emitted light whiter.

US-PS 4,146,813 describes a high pressure sodium vapour lamp in which it is proposed that the internal diameter of the discharge vessel should not exceed a maximum value of 5 mm.

The luminous efficiency of this light source is good but, due to the selected distance between the peaks or tips of the electrodes, the requirements of general lighting service cannot be met.

A well-known measure for improving the index of the colour temperature and colour rendition is the application of overload wherein, due to the increased operational pressure in the discharge vessel, a small decrease of the luminous efficiency must be accepted. Also, this solution cannot provide suitable high pressure sodium vapour lamps to meet the requirements of special purpose colour applications because the maximum permissible temperature of the discharge vessel is 1 2500C. At higher temperatures the rate of evaporation of the alumina material becomes so high that a deposit is formed on the inside wall of the outer envelope, whereby gradually to darken or render opaque the transparent material of the outer envelope.In the vicinity of the discharge arc the temperature of the wall of known discharge vessels comes very close to the mentioned critical wall temperature.

For the production of high pressure sodium vapour lamps of the desired service life and with good colour rendition as well as a pleasant hue of the emitted light, US-PS 3,898,504 proposes discharge vessels with an internal diameter exceeding that prescribed or determined for such lamps. In numerous embodiments this minimum diameter is, however, higher than the traditional value of 6-8 mm. Although a high pressure sodium vapour lamp comprising such a discharge vessel achieves its objective, it cannot be massproduced in the manner of traditional incandescent lamps.

An aim of the invention is to provide a high pressure sodium vapour lamp with the essential features and construction of a general lighting service (GLS) lamp, which makes it possible to employ the required high pressures in order to achieve a colour temperature higher than 20000K and an index of general colour rendition higher than 20, while at the same time the temperature of the wall of the discharge vessel remains below the maximum permissible temperature.

The invention is based on the discovery that high pressure sodium vapour lamps suitable for GLS duties may be produced with discharge vessels of maximum internal diameters not exceeding 5.5 mm provided that the distance between the tips of its electrodes is reduced below suitable limits. In this way the discharge is changed over from the so-called 'wall-stabilized' mode into the so-called 'electrode-stabilized' mode, whereby the temperature gradient in the vicinity of the alumina wall becomes less steep, and so the heat transfer to the wall is lower.In this case the arc between the electrodes is stabilized in the discharge space by the circumstance that the arc-voltage increases as a function of the magnitude of the deviation from the straight line connecting the electrodes, whereas in the case of a wall-stabilized discharge arc the relatively high gradient of temperature in the vicinity of the wall of the discharge-tube is the stabilizing factor for the arc discharge. The wall temperature may in this way be considerably reduced by the application of an electrodestabilized mode of discharge, but the rate of reduction cannot be arbitrary, because the maintenance of a suitable wall temperature is also important to ensure the required operational pressure in the discharge vessel.The vapour pressure of the metal additives is practically always suitable for generating the required pressure at temperatures below 1 2500C.

Based on the above-described discovery, there has been developed a high pressure sodium vapour lamp provided with a lamp cap that can be connected to a lamp holder or to light fixture, an outer envelope made of light-transmitting material hermetically sealed from the ambient atmosphere and an essentially tubular discharge vessel arranged within the outer envelope, the wall of the vessel being made of transparent or translucent alumina and which contains a fill of noble gas and metal additives, particularly sodium, cadmium and/or mercury, the vessel being fitted with electrodes connected via electric current lead-in wires to the lamp cap and wherein, according to the invention, the distance between the centres or tips of the electrodes is at most substantially 5.5 times the maximum internal diameter of the tubular discharge vessel.

With this solution it can be achieved that in operation the wall temperature of the discharge vessel does not exceed 1 2500C while the value of the colour temperature exceeds 20000K and the value of the general colour rendition index exceeds 20. The electrode-stabilized mode of arc discharge is particularly evident in variants of the high pressure sodium vapour lamps according to the invention wherein the power consumption is at most 100 watts. Such high pressure sodium vapour lamps can be mounted in the customary incandescent lamp bulbs of rotationally symmetrical shape and a maximum diameter of 70 mm. The essentially tubular discharge vessel is positioned at and perpendicularly to the longitudinal axis of the outer bulb.

The high pressure sodium vapour lamps according to the invention may include currentlimiting and, if required, ignition impulse generating circuit elements, particularly e.g. a high frequency semi-conductor converter advantageously fitted to the outer envelope and in given cases incorporated in a compact operating unit built into the outer envelope and/or into the lamp cap.

The invention is further described, purely by way of example, with reference to preferred embodiments illustrated in the accompanying schematic drawing, wherein: Fig. lisa diagrammatic view of the high pressure sodium vapour lamp according to a first embodiment of the invention in a so-called 'transversal mounting' of the tubular discharge vessel, the outer bulb of which is partly provided with a reflective coating, the latter being partially removed for the sake of better illustration, and Fig. 2 is a diagrammatic view of another embodiment of the high pressure sodium vapour lamp according to the invention, containing a tubular discharge vessel positioned essentially coaxially to the longitudinal axis of the outer bulb.

Fig. 1 illustrates a high pressure sodium vapour lamp according to the invention, wherein a tubular discharge vessel 4 is fixed to a lamp stem 2 sealed into a traditional mushroom-shaped bulb 1 as generally used for crypton-filled GLS lamps; the bulb is provided with a cap 3 which in this case is a screw (Edison) type. One part of the interior of the neck of the bulb 1 extending from a predetermined sectional plane to the plane of maximum bulb diameter is provided with a light and heat reflecting mirror coating.The tubular discharge vessel 4 has a transparent or translucent wall, is mounted perpendicularly to the longitudinal axis of the bulb 1 and of the lamp and is fixed to the end portions of current lead-in wires 5, 6 by means of straps 41, 42 embracing the region of the end seals of the vessel and by current conductors 43, 44 welded to the lead-in wires 5, 6 or to the straps 41, 42. The electric current lead-in wires 5, 6 are embedded in the stem 2 in a manner known per se. The maximum internal diameter cf the tubular discharge vessel 4 is 5.5 mm; the length of the tube is 24 mm; the overall length of the discharge vessel, inclusive of the terminal ranges of the wires 43, 44 protruding from the ends of the discharge tube 4, is 31 mm.

In this embodiment of the invention, the distance between centres or tips of the non-illustrated electrodes of the discharge vessel 4 is 1 6 mm.

The tubular discharge vessel 4 is filled with xenon and a sodium-cadmium alloy containing 8% by weight of sodium. The light source constructed in this way has a colour temperature of 25000K and a luminous flux of 2300 lumens, an ignition (operating) voltage of 55 Volts and an electrical power consumption of about 35 watts. The luminous efficiency of this lamp is approximately four times higher than the efficiency of a 40 watt GLS incandescent lamp of essentially identical external appearance.

Fig. 2 illustrates an embodiment of a high pressure sodium vapour lamp according to the invention which as regards its main structural elements, is very similar to the embodiment of the invention according to Fig. 1. In this embodiment the tubular discharge vessel 4 is fixed by welding to current lead-in supports 5, 6 by means of straps 41,42 and is arranged within and coaxially with the longitudinal axis of the bulb 1 and thus of the lamp, the bulb being a standard incandescent lamp bulb of a spherical dome shape of a maximum diameter of approximately 60 mm. In this arrangement the incandescent coil (filament) of the customary incandescent lamp is replaced by the discharge vessel, the manufacturing steps or sequence being the well-known one of traditional lamp manufacture, starting with the assembly of a lamp mount comprising the stem 2 and the discharge vessel 4, sealing the mount into the bulb 1, then evacuating the air from the bulb 1, optionally filling it with an inert gas and finishing the process with the fixing of the lamp cap 3 to the bulb 1 by means of a hardenable or thermosetting adhesive capping cement. In this embodiment of the invention the tubular discharge vessel 4 with an internal diameter of 4.7 mm and a length of 30 mm is produced from a polycrystalline alumina tube and the distance between the tips or centres of the hermetically sealed-in electrodes is selected to be 20 mm.The tubular discharge vessel 4 contains a xenon gas filling with a pressure of 20 Torr at room temperature and a mixture of additives consisting of 25% by weight sodium, 55% by weight cadmium and 20% by weight mercury. This high pressure sodium vapour lamp according to the invention, has an operating voltage of 100 V, a luminous flux of 2700 lumens and a colour temperature of 25500 K. Thus its luminous efficiency is 67.5 lumens per watt (Im/w).

It proved advantageous in both embodiments of the invention to use getters in the bulb 1 to ensure or enhance the purity of the inert gas filling or of the vacuum.

The lamps according to the invention may be operated with the traditional series-resistance ballasts and ignition units generally used with high pressure gas discharge lamps. However, design solutions may also be developed wherein compact lighting units comprising these operational elements are either detachably joined to the lamps or as common inseparable structural elements, the units expediently containing semiconductor high frequency converters and/or other current limiting and, in certain cases, ignition impulse-supplying circuitry, and being fitted as structural elements to suit the dimensions and shape of the lamp. These structural elements may be arranged partly or wholly around the neck of the lamp and/or in the lamp cap, or fitted and connected to the lamp cap in some other way.

Claims

1. A high pressure sodium vapour lamp including a lamp cap electrically connectible to a iampholder or a lighting fitting, a lighttransmitting outer envelope hermetically sealed from the ambient atmosphere and an essentially tubular discharge vessel arranged inside the outer envelope and having an internal diameter of at most 5.5 mm, the wall of the vessel being translucent or transparent and made of alumina, the vessel containing a filling of a mixture of a noble gas and metallic additive(s) e.g. sodium, cadmium and/or mercury, and being provided with electrodes which are electrically connected to the lamp cap by current lead-in wires, wherein the distance between centres or tips of the electrodes is at most 5.5 times the maximum internal diameter of the tubular discharge vessel.

2. A high pressure sodium vapour lamp according to claim 1, wherein the discharge vessel is arranged within the envelope coaxially with or transversely to the longitudinal axis of the envelope.

3. A high pressure sodium vapour lamp according to claim 1 or 2, wherein circuitry is fitted to the envelope for current-limiting and/or ignition impulse-generating purpose(s).

4. A high pressure sodium vapour lamp according to claim 3, wherein the circuitry includes a high frequency converter containing semi-conductor elements.

5. A high pressure sodium vapour lamp according to any preceding claim, wherein the stabilized operating power input of the discharge vessel is at most 100 watts.

6. A high pressure sodium vapour lamp according to any preceding claim, wherein the maximum cross-sectional diameter of the envelope perpendicular to its longitudinal axis is 70 millimetres.

7. A high pressure sodium vapour lamp substantially as herein described with reference to and as shown in Fig. 1 or Fig. 2 of the accompanying drawing.