Electric discharge lamp er 'tting ultraviolet light, particularly solarium lamp and methods for the manufacture of such lamps
Technical field
This invention relates to electric discharge lamps emitting ultraviolet (UV) light, particularly solarium lamps which contain -dumina or yttrium oxide in the luminescent coating on the inner surface of the envelope. The invention also relates to the manufacture of such lamps.
Background art
Electric discharge lamps are filled with an ionizable medium containing mercury vapour and have luminescent coating on the inner surface of a glass envelope generally having the shape of an elongated tube. Due to various factors, e.g. formation of mercury compounds and their deposition on the luminescent material, the light flux emitted during operation is not constant. According to GB 2 091 936, this problem can be reduced by covering the surface of the luminescent material with a layer of -dumina of the specific weight of at least 160 μg/cm2.
To avoid greying of the inner wall of the envelope which is due to the interaction of mercury with glass, in US 4 544 997 the use of yttrium oxide layer between the luminescent layer and the glass wall is described. This layer is
obtained by rinsing the inner wall of the glass envelope with a liquid containing yttrium acetyl acetonate, followed by drying and sintering.
Disclosure of invention
In solarium lamps the radiation generated by the discharge is converted into UV A light by means of the luminescent material. One of the most important characteristics of the solarium lamps is the degree of efficiency, i.e. the amount of the emitted UV A light at constant output.
The most suitable luminescent material for the generation of UV A light is BaSi2U5:Pb. Duiing the operation of the lamp this luminescent material binds mercury to a much greater extent than the halophosphate luminescent materials used in fluorescent lamps. Therefore, the lifetime of the solarium lamps is much shorter than that of the fluorescent lamps. The lifetime of the known solarium lamps is under 500 hours and towards the end of the lifetime typically drop-off in light output occurs.
Accordingly, the object of this invention is to provide electric discharge lamps, particularly solarium lamps which have high UV A output, are of longer lifetime than the known lamps and have improved lumen maintenance.
Improvements in the above characteristics of the electric discharge lamps, particularly solarium lamps according to this invention are achieved by covering the inner surface of the envelope with a luminescent material containing alumina or yttrium oxide. The lun inescent material used in accordance with electric discharge lamps of the invention is BaSi2θ5:Pb.
Alumina mixed into luminescent material as adhesion promoter has aheady been used in mercury discharge lamps, mainly in the amount of 2 to 3 weight per cent. However, it was a surprise not expected by a person skilled in the art to find
that urnina mixed into luminescent material in the amount of 5 to 15 weight per cent improves the degree of efficiency of the solarium lamps and reduces the dropoff in light output.
Hence, the lamp according to the present invention is an electric discharge lamp eimtting ultraviolet light, particularly solarium lamp having a glass envelope containing an ionizable medium which includes mercury vapour, the envelope having electrodes sealed into its ends, the envelope further having on its interior surface a layer of luminescent material of BaSi2Os:Pb containing umina or yttrium oxide, wherein the amount of the alumina or yttrium oxide in the luminescent material is 5 to 15 weight per cent.
Modes for carrying out the invention
A possible embodiment of the invention is illustrated in the accompanying drawings. Fig. l. shows a side-view of the lamp, partially in longitudinal section,
Fig. 2. shows the cross-sectional view of the lamp of Fig. 1. The mercury vapour discarge lamp 1 has a glass envelope 4, into both ends of which mount stems 5 are sealed. Pairs of electrodes 7 are led through both mount stems 5 with vacuum-tight metal-glass binding, the electrodes 7 inside the lamp are connected to each other by a filament 6. The inner surface of the envelope 4 is coated with a lurriinescent material 2 of BaSi2Os:Pb containing alumina or yttrium oxide covered by a layer 3 of Alon C. Both ends of the mercury vapour discharge lamp 1 are equipped with a base 8 whereby the lamp 1 can be inserted into a standard lamp-socket (not shown) for normal operation. The electrodes 7 are led out from the interior of the lamp and are connected to plugs of the bases 8. fn a practical embodiment the alumina or yttrium oxide does not form a separate layer. Instead, the aluminium oxide or yttrium oxide is included in the luminescent layer itself.
In an alternative embodiment of the invention, the luminescent material coating of BaSi205:Pb containing alumina in the amount of approximately 8 weight per cent is overcoated with a layer of alumina. This second layer has a specific weight of maximum 80 μg per square centimeter (80 μg/cm ), i.e. the specific weight of the alumina layer is not only in itself but also together with the amount of umina of the first layer lower than that of what is described in GB 2 091 936.
The improved characteristics of the lamps according to the invention are illustrated in Tables 1-3. The luminescent material of the control lamp contained alun ina in the amount of 1 weight per cent to promote its adhesion only.
Table I
UV radiation values (mW/cm ) at 100 W output
Table II ,
UV radiation values (mW/cm2) at 160 W output
UV radiation values (mW/cm2) at 100 W output
It is seen from the Tables that the light output of the lamps according to the invention is much higher than that of the control lamp both at the beginning of the operation and throughout the operation of the lamp.
Moreover, while the known lamps show approximately 30-40 % drop-off in light flux after 500 hours of operation, with the lamps according to the invention the degree of the drop-off may remain under 20 %. The nominal lifetime of the lamps according to the invention is 800 hours.
The invention further provides a method for the m-mufacture of electric discharge lamps emitting ultraviolet light comprising the following steps: preparing an aqueous dispersion of BaSi2Os:Pb lurninescent material containing 5 to 15 weight per cent of - umina or yttrium oxide, applying this dispersion to the inside surface of an elongated glass envelope, baking the binding material and processing said envelope to form said lamp in a manner known per se.
Accordingly, the lamps of the invention can be manufactured in a very comfortable way, the -dumina or yttrium oxide is simply mixed into the luminescent material, consequently, the use of these oxides for the improvement of
the properties of electric discharge lamps does nor require any additional technological step.
In another process according to the invention the method for the manufacture of electric discharge lamps emitting ultraviolet light comprises the following steps: preparing an aqueous dispersion BaSi2Os:Pb mminescent material contai-ning 5 to 15 weight per cent of -dumina, applying this dispersion to the inside surface of an elongated glass envelope, baking the binding material, applying a second layer of alumina of the specific weight of maximum 80 μg/cm in the form of an aqueous dispersion, baking the binding material again and processing said envelope so as to arrive at a lamp in accordance with known techniques.
Example 1
An aqueous dispersion of lumdnescent material of BaSi2Os:Pb of quality
Degussa C containing 8 weight per cent alumina was prepared. The particle size of the -diimina was smaller than 1 μm, the dumina was prepared by flame hydrolysis where the size of the primary particles was 20 nm and their density was 60 g/1. The dispersion obtained was applied in the specific amount of 3,2 mg/cm2 to the inside surface of the glass envelope of a solarium lamp. After coating the binding material was baked at 600 °C for 150 s.
Example 2
An aqueous dispersion of luminescent material of BaSi2Os:Pb of quality
Degussa C containing 8 weight per cent dumina was prepared. The particle size of the dumina was .smaller than 1 μm, the alumina was prepared by flame hydrolysis where the size of the primary particles was 20 nm and their density was 60 g/1. The
dispersion obtained was applied in the specific amount of 3,2 mg/cm2 to the inside surface of the envelope of a solarium lamp. After coating the binding material was baked at 600 °C for 150 s. Onto this first layer of luminescent material a second layer of diimina was applied in the form of an aqueous dispersion. The specific weight of the second layer was 80 μg/cm2. After the second coating the bmding material was baked again.
Example 3
A dispersion of luminescent material of BaSi2θ5:Pb of quality Degussa C containing 8 weight per cent yttiium oxide with water was prepared. The particle size of the yttrium oxide was smaller than 1 μm, the yttrium oxide was prepared by flame hydrolysis where the size of the primary particles was 20 nm and their density was 60 g/1. The dispersion obtained was applied in the specific amount of 3,2 mg/cm2 again, to the inside surface of the envelope of a solarium lamp. After coating the binding material was baked at 600 °C for 150 s.