A low pressure gas discharge lamp

Abstract

 By surrounding a gas discharge lamp, such as a fluorescent tube (2), with a tubular glass envelope (8), the discharge chamber of the fluorescent tube (2) is thermally insulated. This enables a sufficiently high temperature to be maintained in the fluorescent tube (2) for, e.g., mercury in the discharge chamber to be brought to a partial pressure at which good illuminating efficiency is obtained even at very low ambient temperatures around the lamp. The tubular enve­lope (8) may be fuzed at the ends thereof to the fluorescent tube (2), or otherwise fixed securely around the tube, in a manner to obtain a tubular space (10) of constant uniform width between the fluorescent tube and the surrounding envelope.

Description

[0001] The present invention relates to a gas discharge lamp of the tubular kind filled with gas or vapour at low pressures, e.g. a fluorescent tube. Because of its construction, the gas discharge lamp has properties which render it especially suited for use in low ambient temperatures. The lamp is therefore particularly suited for outdoor use in the Nordic winter climate, and also for illuminating cold storage and freeze storage facilities. Fluorescent lamps are widely used in the open, because the fluorescent lamp gives light more efficiently than an incandescent lamp. In addition to street lighting, fluorescent lamps have thus been used to illumi­nate road signs, as canopy lighting, e.g. in railway stations, to illuminate loading piers and gasoline stations, and to an increasing extent as a means of illumination in both freestanding and surface-mounted advertising signs. When fluorescent lamps, or tubes, are used to illuminate signs, and not only to illuminate such signs from within, it is desirable that the luminous flux is uniform throughout the sign, irrespective of the ambient air temperature.

[0002] Because up to 80% of the energy supplied to a fluorescent tube is converted into heat, a fluorescent tube which is mounted in known kinds of lamp casings or enclosures will, to some extent, be self-heating, since the air present in the casing is able to conduct heat away from the actual fluorescent tube only to a very limited extent. This problem applies to fluorescent tubes incorporated in advertising signs (company name signs) and road signs (traffic), such as overhead lane-destination signs of partially translucent design, and also to fluorescent tubes which are mounted in enclosed lamp fittings. With the ambient air stationary and the air temperature beneath 0°C, this self-heating effect will result in a surface temperature of +15°C on the coldest part of the fluorescent tube. At an air temperature of -20°C the effect is so small as to produce hardly any increase in luminous flux. At such low temperatures, the lamp casings or enclosures absorb all of the increase in luminous flux achieved by self-heating of the fluorescent tube. Conse­quently, the great majority of fluorescent lamps for outdoor use are of the kind which have reflectors fitted over the fluorescent tube, but which lack the provision of a casing. The purpose of encasing fluorescent lamps is to protect the fluorescent tubes from damage through mechanical causes, and the lighting requirement has been made secondary to the need of protecting the lamp.

[0003] Although in the case of advertising signs the need to obtain a high luminous yield is not equally as important as in the case of lamps which are intended for street lighting, the rising price of electrical energy will influence the future design of such sign illumination. The lamps used to illumi­nate such signs will also be required to have a higher luminous efficiency, which means that the lamps must sustain higher temperatures at the coldest point on the envelope surface of the lamp. In order to achieve optimum luminous yield, this point on the lamp envelope needs to be heated to close to 40°C.

[0004] Tunnels are another area of use in which fluorescent tubes or lamps can be subjected to the effects of low ambient temperatures. The air flow through tunnels, even when the tunnels have a length of several hundred meters, is so large that any heat which may be radiated from the surrounding rock or earth is unable to supplement heating of the sur­faces of the fluorescent tube. Thus, when used for the aforesaid purpose the luminous flux will decrease exponen­tially with falling air temperatures. This can have a serious consequence, for instance, on a cold sunny winter's day, a car driver will see the road with an illumination strength of close to 100 000 lux. When this driver enters an illuminated tunnel, his eyes must adjust to an illuminance which is far below 100 lux. Road safety and the driver's own feeling of security are assisted by the fact that the fluorescent lamps in the tunnel maintain a practically normal luminous flux, even in very cold weather conditions.

[0005] According to statistics, November is the month in which the majority of road accidents occur in the Nordic countries. These accidents occur mostly in the dark hours and to a large extent are the result of poor street lighting. When this street lighting comprises lamp fittings with low-pressure mercury vapour discharge lamps, the luminous flux from these lamps is halved at temperatures between +10°C and 0°C, when conventional fluorescent tubes with an external diameter of 38 mm are used. In recent years there has been a change from tubes of this diameter to tubes of 26 mm in diameter, these latter tubes having been given a 10% lower power output than the former. This decrease in power output has resulted in an energy saving when the tubes are in operation, although there is no appreciable reduction in the luminous flux of such tubes at ambient temperatures of +20°C. The conditions engendered when the ambient temperature falls from +20°C to 0°C in the case of a 58W tube cause a decrease in luminous flux from 4700 to 1400 lumens. In the case of a 26 mm fluorescent tube, the luminous flux is reduced to a third of its original value when the ambient temperatures lies within a range of +10°C to 0°C. The matter is made more serious by the fact that the luminous flux of a 26 mm tube at +10°C is 20% lower than the luminous flux of a 38 mm tube of corresponding power.

[0006] Because the narrower tubes are being used to an ever increasing extent and now practically dominate all demand, the majority of fluorescent tube manufacturers have ceased to produce the 38 mm tube. Cathodes and other lamp compo­nents have therewith been fully adapted to tubes of 26 mm diameter.

[0007] Now that the drawbacks of the narrower tubes have been observed, it should be possible simply to restart the manufacture of components for tubes of 38 mm diameter. This is not the case, however, since the production lines would need to be adjusted to the tubes of larger diameter in several respects and at heavy costs.

[0008] The object of the present invention is to solve the problems which are associated with the use of narrow fluorescent tubes in freezing temperatures and to provide a lamp which has high illuminance at low temperatures. This object is realized in accordance with the invention with a narrow fluorescent tube which is surrounded along the whole of its length by a fixed transparent outer tube, for instance a glass tube. Other characteristic features of the inventive solution are set forth in the following claims.

[0009] The invention is illustrated in the accompanying drawings; in which

Figure 1 is a partly cut-away view of an inventive fluores­cent tube; and

Figures 2, 3 and 4 are graphs which show the ratio of ambient temperature, in °C, to illuminance strength, in lumens (Lm) in respect of fluorescent tubes of 18/20W, 36/40W, 58/65W respectively.

[0010] In accordance with the invention, a gas-discharge lamp in the form of a tube is surrounded by a glass tube or envelope, which may be transparent or opalescent. The ends of this envelope are fixed to the cathode-containing ends of the glass tube of the discharge lamp, such as to leave a tubular space of constant width between the envelope and the tube. Although this space may be allowed to communicate with the ambient air, such communication may result in condensa­ tion problems. Consequently, it is preferred to provide seals at the ends of the lamp, between the envelope and tube. These seals may have the form of polymer sealing rings, or may otherwise comprise aging-resistant gas-­impermeable material. From the aspect of manufacture, it is a simple matter to flange or neck-down the ends of the envelope while heating the same, so that these ends fuze together with the inwardly lying glass tube, suitably before fitting end caps to the tube.

[0011] Irrespective of how the envelope surrounding the tube is fixed thereto, an advantage is afforded when the space between the tube and envelope is filled with a pure gas, so that no light losses will occur. The gas most preferred in this respect is dry, dust-free air, although in particular cases the gas may comprise a noble gas or a mixture of such gases.

[0012] The gas in the aforesaid space may be kept at atmospheric pressure, although in combination with the tube wall, which is normally less than 2 mm thick, and in order to increase the heat insulating ability, the gas is preferably held at a pressure beneath atmospheric. The insulating ability is also dependent on the width of the space, which width may be from 2-10 mm, depending on the intended lamp application. When the fluorescent tube has an external diameter of 26 mm and is surrounded by a glass envelope whose outer diameter is 38 mm, the tubular space will have a width of 5 mm. In the case of very narrow tubes and spaces in excess of 10 mm, an exchange of heat-transporting air may take place between the other surfaces of the inwardly located fluorescent tube and the inner surface of the tubular envelope. This will increase convection and part of the advantage afforded by the invention will be lost. An excessively narrow tubular space will not give the desired effect, unless the space is completely evacuated. An optimum space width has therefore been judged to be from 4 to 8 mm.

[0013] In the case of the embodiment in which the tubular envelope is fuzed to the inwardly located fluorescent tube and the tubular space is filled with noble gas, the inner surface of the envelope may be coated with one or more fluorescent substances. This coating will convert to visible light any ultraviolet light that penetrates through the fluorescent layer of the fluorescent tube and the glass wall thereof. These fluorescent substances on the envelope may be selected to provide a desired colour complement in the light emitted.

[0014] In order to utilize the light emitted still further, the light may be directed positively from the lamp fitting, normally downwards. Since the invention is also intended for use in conjunction with very simple lamp fittings, the inner surface of the tubular envelope is coated with a reflective material, through an angle of arc of up to 180°C. In addition to increasing the light strength in the visible wave-lengths, this embodiment affords the further advantage of reflecting heat rays back to the discharge chamber of the lamp. The resultant increase in the temperature of the discharge chamber corresponds to an increase in illumination strength of more than 20% when the ambient temperature is beneath +10°C. Fluorescent tubes of this construction can also be turned through 180°C in reflector-fitted lamp fittings, resulting in a type of top-reflection. This gives a very soft light and promotes self-heating of the lamp.

[0015] The inventive lamp is believed to afford a good solution to the illuminating requirements expressed by those who work on oil rigs in arctic climates. In addition to giving a much higher light yield than hitherto known discharge lamps at the low temperatures which prevail during the six dark months of the year, the outer envelope of the inventive lamp will also afford protection against mechanical damage.

[0016] Should the tubular envelope break, the inwardly located fluorescent tube is likely to remain intact and the lamp to continue to give-out light, without risk of sparking between the cathodes igniting gas located around the oil platform or rig. The inventive lamp thus provides in this instance a safety lamp which will reduce the explosion hazards of oil platforms and rigs.

[0017] The exemplifying embodiment of the inventive fluorescent lamp 1 illustrated in Figure 1 comprises a fluorescent tube 2 which has a diameter of 26 mm and which is fitted at both ends with lamp bases 3 having connector pins 4. The tube 2 also has cathodes placed on a terminal foot 5 in the usual manner, the cathodes in this case being surrounded by elec­trode screens 6. The electrical contact pins extend through the foot, or base, 5 to the cathode current distributor 7.

[0018] The fluorescent tube 2 of the illustrated embodiment is surrounded by a tubular glass envelope 8 which is transparent and has an outer diameter of 38 mm and the ends of which are drawn or necked slightly inwards. The ends of the tubular envelope 8 are inserted into ring-shaped grooves in polymer rings 9 which are press-fitted onto the bases 3. When the tubular envelope is fitted with the aid of polymer rings 9 in a chamber which is under a partial vacuum and to which only dry, filtered air is introduced, the air present in the tubular space 10 between the fluorescent tube 2 and the tubular envelope 8 will be free from dust. Furthermore, the application of atmospheric pressure will assist in holding the polymer rings 9 tightly and sealingly between the envelopes 8 and respective lamp bases 3. The polymer rings 9 will conveniently incorporate cavities for accommodating silica gel or some other powdered hygroscopic material.

[0019] For advertising purposes, or for the purpose of otherwise meeting particular desiderata with regard to a given wave­length composition of the light emitted, the inner surfaces of the envelope of the inventive lamp may be coated with substances which will filter out undesired light. This technique enables critical ultraviolet lines to be further reduced with the aid of light-absorbing or fluorescent sub­stances.

Claims

1. A low-pressure gas discharge lamp of tubular form for use in low ambient temperatures, characterized by a glass fluorescent tube (2) which is provided with cathodes and has one or more fluorescent substances coated on the inner surfaces thereof and which is hermetically sealed at its ends and forms a gas-filled discharge chamber, and by a tubular envelope (8) which surrounds the fluorescent tube (2) along the whole of its length such as to define a tubular space (10) with said tube, and the ends of which envelope are fixed permanently to respective ends of the fluorescent tube.

2. A lamp according to Claim 1, characterized in that the tubular envelope (8) is permeable to light.

3. A lamp according to Claim 1 or Claim 2, characterized in that the tubular envelope (8) is fixed to the fluorescent tube (2) with the aid of sealing polymer rings (9).

4. A lamp according to Claims 1, 2 or 3, characterized in that the tubular space (10) defined between the fluorescent tube (2) and the tubular envelope (8) is filled with dry, dust-free air.

5. A lamp according to Claims 1, 2 or 3, characterized in that the tubular space (10) defined between the fluorescent tube (2) and the tubular envelope (8) contains one or more noble gases.

6. A lamp according to any of the preceding Claims, charac­terized in that the tubular space (10) defined between the fluorescent tube (2) and the tubular envelope (8) has a width of 2-8 mm, preferably 4-5 mm.

7. A lamp according to Claim 5, characterized in that the inner surface of the tubular envelope (8) is coated with fluorescent substance.

8. A lamp according to Claim 1 or 2, characterized in that the tubular envelope (8) is fuzed at the ends thereof with the inwardly located fluorescent tube (2).

9. A lamp according to any of the preceding Claims, charac­terized in that the inner surface of the tubular envelope (8) is coated with a reflective material through an angle of arc of up to 180°C.

10. A lamp according to any of the preceding Claims, charac­terized in that the whole of the inner surface of the tubu­lar envelope (8) or part of said surface is coated with one or more substances which change the colour of the light radiating through the fluorescent tube (2).

Drawing