EP0104594B1

EP0104594B1 - Double-enveloped lamp

Info

Publication number: EP0104594B1
Application number: EP83109346A
Authority: EP
Inventors: Philip J. White; Robert S. White, Jr.; William Keefe
Original assignee: GTE Products Corp
Current assignee: Osram Sylvania Inc
Priority date: 1982-09-23
Filing date: 1983-09-20
Publication date: 1989-01-25
Also published as: AU570012B2; DE3379089D1; CA1237111A; EP0104594A2; AU1950783A; EP0104594A3

Description

This invention relates to double-enveloped lamps comprising an outer envelope, a light-source capsule and a light transmissive hollow shield which is open at least at one end, said shield being mounted within the outer envelope by means of support wires and said capsule being contained within said shield.
There is a small probability that a metal halide lamp or a tungsten halogen lamp will burst during operation of the lamp, hereinafter called a "containment failure" of the lamp. When a lamp containment failure occurs, the sequence of events internal to the lamp is as follows: The arc tube or the tungsten halogen capsule bursts causing fragments of glass or shards to be propelled against the outer envelope; these shards shatter the outer envelope of the lamp. The external result is that the lamp bursts. It is this type of lamp failure that is the subject of this disclosure.
The causes of these infrequent lamp failures are varied and unpredictable. Although occurrence of the failure is rare, nevertheless it could present a safety hazard to a person in the immediate vicinity of the lamp. Where such failures can be anticipated, lamp manufacturers notify users by means of warnings on packages and other descriptive materials and by suggested precautions in specifications. This hazard may be avoided by operating the lamp in a fixture designed to contain such a failure. The requirement that the lamp be operated in a protective fixture is frquently employed in commercial usage. However, this procedural safeguard is less acceptable for consumer usage. For reasons of safety, economy, and convenience in both commercial and consumer usage, it may be desirable to incorporate a reliable containment device as part of the lamp itself.
In GB-A-20 56 760 there is disclosed a double-enveloped lamp as mentioned in the first paragraph above. The shield made of glass forming a sleeve is suggested as being useful as a means to protect against a containment failure of the lamp, notwithstanding the fact that the main function of the glass sleeve is to prevent sodium loss from the arc by trapping ultraviolet light and by shielding the arc tube against photoelectrons. Protecting the outer envelope is only a secondary function and while such sleeve may be effective in containing some arc tube bursts, it has been found that in a substantial percentage of cases the sleeve itself is shattered by the burst and containment failure of the lamp follows. Thus, the sleeve taught in this patent offers only limited protection against lamp-containment failures, and such protection is especially tenuous in lamps having light-source capsules in which operating pressures may be as high as 20.27 to 30.4 bars) (20 or 30 atmospheres).
From FR-A-852 426 an incandescent lamp is known having a single envelope which is surrounded by a mesh for containment of shards produced by a burst of the lamp. The mesh or net of this prior art protects the surroundings from glass splinters in the event of a burst of the bulb of the lamp. Therefore, the net must have a mesh or stitch or opening size fine enough to contain splinters of virtually any size. This reduces the luminous efficacy of the lamp.
From FR-A-849 331 a double-enveloped incandescent lamp comprising a mesh surrounding the inner envelope is known. The mesh of this lamp has the function of supporting the inner capsule but not of protecting the environment in case of a containment failure of the lamp. Said protective function is to be accomplished by the outer envelope.
The object of the present invention is to provide a double-enveloped lamp of the type mentioned in the first paragraph above in which the operating safety characteristics are iproved.
This object is accomplished by a stem for mounting the capsule within the outer envelope, by a mesh surrounding the shield and restricting shards of the capsule and of the shield from shattering said outer envelope in case of a capsule burst, the size of the openings in the mesh being such that those shards of the capsule and of the shield with mass large enough to cause a rupture of the outer envelope are restrained from passing through the mesh and impacting the outer envelope, the mesh being formed from light-reflecting metal wire and causing a minimal reduction in the light transmissiveness of the shield, and by mesh mounting means for mounting mesh on the shield.
With the invention the surroundings are protected as long as the outer envelope remains intact. The containment mesh need not restrain all shards of the capsule and the shield from passing through the mesh; it need restrain those shards having a mass large enough to rupture the outer envelope. Thus, the containment mesh and the outer envelope cooperate to protect the surroundings. As a result of this cooperation, the stitch or opening size of the mesh of the instant invention may be larger providing for greater lamp efficacy.
Advantageous developments of the invention are subject of the subclaims.
As used herein, the term "light-source capsule" denotes an arc tube of an arc discharge lamp, a halogen capsule of a tungsten halogen lamp, or- any light-emitting capsule within the outer envelope of a lamp where the possibility of a lamp-containment failure exists.
The terms "efficacy" or "luminous efficacy" used herein are a measure expressed in lumens per watt of the total luminous flux emitted by a light source over all wavelengths divided by the power input of the source.
Lamps with light-source capsule containment devices constructed in accordance with the foregoing description will exhibit substantially improved operating safety characteristics when compared to lamps of the prior art.
The invention and preferred embodiments thereof are described by means of the accompanying drawings. Therein is

Fig. 1 a perspective view of one embodiment of a light-source capsule containment device;
Fig. 2 a perspective view of a second embodiment of a light-source capsule containment device;
Fig. 3 an elevational view of an arc discharge lamp employing an embodiment of the invention
Fig. 4 an enlarged partial view of one embodiment of a mesh forming part of a containment device;
Fig. 5 an enlarged partial view of an alternate embodiment of a mesh..

For a better understanding of the present invention, together with other and further objects, advantages, and capabilities thereof, reference is made to the following disclosure and appended claims taken in conjunction with the above- described drawings.
Referring to the drawings with greater particularity, Fig. 1 shows a light-source capsule containment device 2 comprising shield 1, containment means 3, and mounting means 5. Shield 1 is a light-transmissive cylinder. Containment means 3 is a loosely woven wire mesh 4 which is wrapped around and substantially surrounds shield 1. Mesh 4 may be sewn or constructed such that the mesh forms a continuous cylinder around the exterior of shield 1. Mounting means 5 comprises two metal straps 6 which secure mesh 4 to shield 1. Metal straps 5 may be bound by appropriate means, such as welding or clamping.
Fig. 2 shows an alternate embodiment of light-source capsule containment device 2 comprising shield 9, containment means 11, and mounting means 13. Shield 9 comprises light-transmissive cylinder 15 and dome 17 fused into a single light-transmissive body. There is a circular opening 19 at the base of shield 9. Containment means 11 is a loosely-woven wire mesh 12 which surrounds shield 8 laterally and about dome 17. Mesh 12 may be sewn or constructed such that the mesh forms a continuous enclosure around the exterior of shield 9 everywhere except in the vicinity of opening 19. Mounting means 13 comprises a metal strap 14. Mesh 12 may be securely mounted to shield 9 with strap 14. Strap 14 may be be bound by appropriate means, such as welding or clamping.
Fig. 3 shows a lamp 8 with light-source containment device 2. Lamp 8 comprises outer envelope 21, light-source capsule 23, stem 25, shield 1, containment means 3, and mounting means 5. In the embodiment of Fig. 3, lamp 8 is an arc discharge lamp and light-source capsule 23 is a double-ended arc tube 24. Containment device 2, comprising shield 1, containment means 3, and mounting means 5, is the identical embodiment shown in Fig. 1 above. Containment device 2 is mechanically mounted within outer envelope 21 to lamp mount 33 by means of support wires 31 such that containment device 2 substantially surrounds arc tube 24 laterally. Strap 10, being the closer of the two straps of mounting means 5 to lamp mount 33, is rigidly fastened to support - wires 31. Support wires 31 are rigidly fastened to lamp mount 33. Lamp mount 33 is mounted on stem 25: Rigid mechanical fastening may be accomplished by welding, clamping, or other suitable means.
In the event arc tube 24 bursts, shield 1 will absorb a substantial portion of the shards and energy emanating from such burst. Frequently, shield 1 will be shattered by the effects of the burst. At this point, mesh 4 is critical. Mesh 4 will substantially restrict shards of shield 1 and shards of arc tube 24 from shattering outer envelope 21. Thus, the protection against the possibility of a lamp-containment failure has been significantly improved.
Fig. 4 shows an enlarged partial view of mesh 4. The stitching of the mesh 4 is approximately square. As shown in the diagram, distance x is size of the stitch.
Fig. 5 shows an enlarged partial view of another embodiment of containment means 3 comprising - a rigid mesh 35. This type of mesh or screen is well known.
In some embodiments, the containment means is a loosely woven wire mesh. In other embodiments, the containment means may be a rigid mesh. In some embodiments, the loosely woven mesh may be preferred because of its superior energy-containing capability. In other embodiments, the rigid mesh may be preferred because of its tractability or greater transmissiveness.
It is desirable that the mesh be highly transmissive of visible light so that there will be a minimal effect on the efficacy of the lamp. A certain percentage of light will be reflected by the mesh on the light's first pass through the shield. A portion of the reflected light will be unobstructed by the mesh on the reflected light's subsequent pass or passes through the shield. Thus, the net reduction in luminous efficacy is less than would be expected by estimating the percentage of the area of the shield covered by the mesh. In all observed cases, efficacy was reduced by less than 5% due to the presence of the mesh. Because of the partial diffusion caused by the mesh, lamps with mesh-containment devices tend to have less glare.
The selection of construction materials for the shield and containment means is heavily influenced by the environment within the outer envelope during operation of the lamp and immediately following a burst of the light-source capsule. During lamp operation, the ambient temperature about the shield and containment means may be in excess of 300°C. Upon the burst of the light-source capsule, the ambient pressure about the shield and containment means may be in the neighbourhood of 30.4 bars (30 atmospheres). Because of their excellent high-temperature properties and transmissiveness, quartz fused silica, Pyrex (a registered trade mark), and hard glass would be suitable materials for the construction of the shield. Stainless-steel wire with a high chromium content is a preferred material for the construction of the containment mesh and mounting strap or straps because of its superior high-temperature properties, relatively low coefficient of thermal expansion, good resistance to oxidation and corrosion, and high tensile strength.
In embodiments employing a containment mesh, the mesh size, i.e., the number of stitches per cm, should be selected such that the mesh will contain shards with mass large enough to be likely to cause a rupture of the outer envelope in the event of a burst of the light-source capsule. Thus, the selection of mesh size is dependent on many factors, such as the type of lamp, the properties of the light-source capsule, the atmosphere within the capsule, the type of mesh, the diameter and tensile strength of the wire in the mesh, etc.
In Fig. 3, there is shown an arc discharge lamp having a doube-ended arc tube and a cylindrical containment device. There are equally feasible embodiments where arc discharge lamps with double-ended arc tubes employ domed containment devices like the device shown in Fig. 2. There are also feasible embodiments where arc discharge lamps with single-ended art tubes employ domed containment devices. All of the benefits and variations of embodiments that have been disclosed with respect to arc discharge lamps apply with equal effectiveness and validity to tungsten halogen lamps and more generally to any electric lamps having a light-source capsule within an outer envelope where the possibility of lamp-containment failure exists.
Several example lamps were constructed. Each lamp employed a quartz shield. The containment means was a loosely woven wire mesh surrounding the shield. Mesh sizes ranged from 3 to 8 stitches per cm) (8 to 20 stitches per inch). The mesh was constructed of stainless-steel wire having a diameter of approximately (0,1217 mm (.005 inches). The mounting means comprises one or two stainles-steel straps.
In some embodiments of. the invention, the mounting means for securing the containment . means to the shield may be inherent in the shield or containment means. For another embodiment the containment means is secured to-the shield by means of elastic or adhesive forces exerted by the containment means itself.
Thus, there is provided a light-source capsule containment device and a lamp employing such a device which provides substantially improved operational safety characteristics.

Claims

1. A double-enveloped lamp (8) comprising an outer envelope (21), a light-source capsule (23) and a light-transmissive hollow shield (1) which is open at least at one end, said shield being mounted within the outer envelope by means of support wires (31) and said capsule being contained within said shield, characterized by a stem (25) for mounting the capsule (23) within the outer envelope, by a mesh (3) surrounding the shield (1) and restricting shards of the capsule (23) and of the shield (1) from shattering said outer envelope (21) in case of a capsule burst, the size of the openings in the mesh being such that those shards of the capsule (23) and of the shield (1) with mass large enough to cause a rupture of the outer envelope (21) are restrained from passing through the mesh (3) and impacting the outer envelope (21), the mesh (3) being formed from light-reflecting metal wire and causing a minimal reduction in the light transmissiveness of the shield (1), and by mesh-mounting means (5) for mounting the mesh o.n the shield (1).

2. A lamp according to claim 1, characterized in that the mesh (3) is loosely woven.

3. A lamp according to claim 1, characterized in that the sleeve (1) is made of quartz glass.

4. A lamp according to claim 1, characterized in that the mounting means (5) includes elastic forces exerted by the mesh (3) itself on the shield (1