Electric lamp and discharge devices – With gas or vapor – Having electrode shield
Reexamination Certificate
1999-02-09
2001-04-24
Font, Frank G. (Department: 2879)
Electric lamp and discharge devices
With gas or vapor
Having electrode shield
C313S243000
Reexamination Certificate
active
06222318
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a low-pressure mercury vapor discharge lamp provided with a discharge vessel which encloses a discharge space provided with a filling of mercury and a rare gas in a gastight manner, the low-pressure mercury vapor discharge lamp comprises discharge means for generating and maintaining a discharge in the discharge space, the discharge vessel being translucent to radiation generated in the discharge space and said discharge vessel having a first and a second end portion.
A low-pressure mercury vapor discharge lamp of the kind mentioned in the opening paragraph is known from U.S. Pat. No. 4,544,997. The inner surface of the tubular wall of the discharge vessel of the known lamp is provided with a translucent layer of a metal oxide. The metal oxide layer serves to counteract that the wall of the discharge vessel is attacked owing to interactions with mercury and thus has a favorable influence on the maintenance of the radiation output of the lamp.
The mercury consumption of the lamp, i.e. the quantity of mercury bound to lamp components during lamp operation and thus no longer available for the operation of the lamp, is comparatively low owing to the metal oxide layer as compared with that in lamps which lack such a metal oxide layer. Nevertheless, a comparatively large mercury dose is necessary also for the known lamp if a sufficiently long lamp life is to be realized. This forms an environmental hazard in the case of inexpert waste disposal after the end of lamp life.
SUMMARY OF THE INVENTION
To achieve this object, the low-pressure mercury vapor discharge lamp to reduce mercury consumption, shielding means are present in the discharge vessel for counteracting that UV radiation generated in the discharge space will reach the end portions of the discharge vessel.
Mercury forms the primary component for the efficient generation of ultraviolet (UV) light in mercury vapor discharge lamps. A luminescent layer comprising a luminescent material (for example a fluorescent powder) may be present on the inner surface of a wall of the discharge vessel for the purpose of converting UV into other wavelengths, for example into UV-B and UV-A for suntanning purposes (sun couch lamps), or into visible radiation. Such discharge lamps are accordingly also called fluorescent lamps. It is observed that mercury is absorbed in the glass during the operation of low-pressure mercury vapor discharge lamps. It is found in general that mercury is chemically bound to the glass, with the result that the quantity of mercury available for the discharge decreases, which adversely affects lamp life. Further experiments have shown that the mercury enters into a complicated (chemical) bond with the glass surface. To counteract such a mercury loss during lamp life, a comparatively high mercury dose is necessary in the lamp, which is undesirable from an environmental point of view. The inventors have gained the insight that UV radiation (originating from the discharge) creates reactive spots at the surface of the end portions. Mercury is bound to such reactive spots. In contrast to the inner wall of the discharge vessel, where the luminescent layer present considerably reduces the sensitivity to attacks by UV radiation, the surfaces of the end portions are not protected against the influence of UV radiation. A luminescent layer is absent in some discharge lamps; for example, a Y
2
O
3
layer may be used in such lamps. The application of shielding means in the discharge vessel prevents at least substantially that the UV radiation generated by the discharge reaching the end portions of the discharge vessel. The mercury consumption of the discharge lamp is limited by a considerable reduction in the degree to which UV radiation reaches the end portions.
A preferred embodiment of the low-pressure mercury vapor discharge lamp according to the invention is characterized in that the shielding means reflect UV radiation. Reflection of the ultraviolet (UV) radiation prevents this radiation from being lost (for example through absorption) and improves the luminous efficacy of the lamp. An alternative possibility for preventing UV radiation from reaching the end portions is by guiding away of the undesirable radiation, for example through the use of light waveguides.
If as little as possible UV radiation useful for the generation of visible light is to be lost in the discharge space, it is desirable for the shielding means to be situated not in the region between the discharge in the discharge vessel and the luminescent layer, but at a side of the respective electrode (discharge means) facing away from the discharge space. Accordingly, a preferred embodiment of the low-pressure mercury vapor discharge lamp according to the invention is characterized in that the discharge means comprise a first electrode arranged in the discharge space and supported by the first end portion and a second electrode arranged in the discharge space and supported by the second end portion, and in that the shielding means are present between the first electrode and the first end portion and/or between the second electrode and the second end portion.
A further embodiment of the low-pressure mercury vapor discharge lamp according to the invention is characterized in that the shielding means comprise a screen which is provided with a UV-repelling coating or layer at least at a surface which faces the discharge space. The screen acts as a shield against the UV radiation. The shape of the screen is preferably adapted to the shape of the discharge vessel.
A particularly preferred embodiment of the low-pressure mercury discharge lamp according to the invention is characterized in that the discharge means comprise a first electrode arranged in the discharge space and supported by the first end portion and a second electrode arranged in the discharge space and supported by the second end portion, in that current supply conductors extend from the respective electrodes through the end portions to outside the discharge vessel, and in that each screen is fastened to the corresponding current supply conductor. Screens fastened to the current conductors can be incorporated in the existing lamp in a simple manner without further adaptations. Said screens are preferably electrically insulating.
The screen itself must not absorb mercury. A further embodiment of the low-pressure mercury vapor discharge lamp according to the invention is characterized in that the material of the layer on the screen comprises at least an oxide of at least one element from the series formed by magnesium, aluminum, titanium, zirconium, yttrium, and the rare earths.
REFERENCES:
patent: 4308650 (1982-01-01), Hernandez et al.
patent: 4544997 (1985-10-01), Seuter et al.
patent: 5272407 (1993-12-01), Tillman et al.
patent: 5646487 (1997-07-01), Ikedo et al.
patent: 5686795 (1997-11-01), Sakoske et al.
patent: 5702179 (1997-12-01), Sidwell et al.
Geerdinck Leonie M.
Oomen Emmanuel W. J. L.
Van Der Pol Adrianus J. H. P.
Faller Brice
Font Frank G.
Lauchman Layla
U.S. Philips Corporation
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