Patent Publication Number: US-6222318-B1

Title: Low-pressure mercury vapor discharge lamp

Description:
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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a longitudinal sectional view of a low-pressure mercury vapor discharge lamp according to the invention; 
     FIG. 1B shows a detail from FIG. 1A, and 
     FIG. 1C shows an embodiment of the screen according to the invention. 
     The Figures are purely diagrammatic and not drawn true to scale. Some dimensions are particularly exaggerated for the sake of clarity. Similar components have been given the same reference numerals in the Figures as much as possible. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1A shows a low-pressure mercury vapor discharge lamp provided with a glass discharge vessel  10  with a tubular portion  11  which is translucent to radiation generated in the discharge vessel  10  and with a first and a second end portion  12   a ;  12   b . In this example, the tubular portion  11  has a length of 120 cm and cm. The discharge vessel  10  encloses a discharge space  13  in a gastight manner, which space is provided with a filling of 1 mg mercury and a rare gas, for example argon. The wall of the tubular portion is coated with a luminescent layer comprising a luminescent material (for example a fluorescent powder) which converts the ultraviolet (UV) radiation generated through ionization of the mercury, usually into visible light. The end portions  12   a ;  12   b  each support an electrode  20   a ;  20   b  which is arranged in the discharge space  13 . Current supply conductors  30   a ,  30   a ′;  30   b ,  30   b ′ extend from the electrodes  20   a ;  20   b  through the end portions  12   a ;  12   b  to outside the discharge vessel  10 . The current supply conductors  30   a ,  30   a ′;  30   b ,  30   b ′ are connected to respective contact pins  31   a ,  31   a ′  31   b ,  31   b ′, which are fastened to a lamp cap  32   a ;  32   b . An electrode ring is arranged around each electrode  20   a ;  20   b . FIG. 1B shows such an electrode ring  21   a  surrounding the electrode  20   a  (the electrode  20   a  is not shown in FIG.  1 B). A glass capsule  22 , with which mercury was dosed, is clamped on the electrode ring  21   a . A metal wire  23  tensioned over the glass capsule  22  was for this purpose inductively heated in a high-frequency electromagnetic field, such that the capsule  22  was cut through and the mercury to be dosed was released from the capsule  22  into the discharge space  13 . 
     Shielding means, in the example of FIGS. 1A and 1B in the form of a screen  15   a ;  15   b , whose shape is adapted to that of the tubular portion  11 , are present between the electrode  20   a ;  20   b  and the relevant end portion  12   a ;  12   b . The screen  15   a ;  15   b  is so shaped that the screen  15   a ;  15   b  will cause substantially no scratches in the luminescent layer (not shown in FIG. 1A) provided on the inner surface of the discharge vessel  10  when the end portions  12   a ;  12   b  are being inserted into the tubular portion. The screen  15   a ;  15   b  is provided with a layer  16   a ;  16   b  on a surface which faces the discharge space, the material of these layers  16   a ,  16   b  includes at least an oxide of at least one element from the series formed by magnesium, aluminum, titanium, zirconium, yttrium, and the rare earths. 
     The screen  15   a ;  15   b , which may or may not be provided with a coating layer  16   a ;  16   b , acts as a UV-radiation-repelling shield which substantially prevents UV radiation from reaching the end portions  12   a ;  12   b . The shape of the screen  15   a ;  15   b  is preferably adapted so as to fit the shape of the discharge vessel  10 . For example, if the discharge vessel  10  comprises a tubular portion  11  with a longitudinal axis  22  at the areas of the end portions, the screen  15   a ;  15   b  is a preferably situated in a plane transverse to the longitudinal axis  22  of the tubular portion  11  for obtaining an optimum shielding effect. If the discharge vessel  10  comprises a tubular portion  11  at the areas of the end portions  12   a ;  12   b , the screen  15   a ;  15   b  is preferably at least substantially circular in shape and has a diameter which is smaller than or almost as large as the inner diameter of the tubular portion  11  of the discharge vessel  10 . The screen  15   a ;  15   b  need not be flat, but may have, for example, bent or flared edges which facilitate mounting of the screens  15   a ;  15   b  in the discharge vessel  10 . It is also possible for openings to be provided in the screen  15   a ;  15   b . FIG. 1C shows an embodiment of the screen  15   a ;  15   b  according to the invention where a slot  25  is provided in the round glass plate  15   a  for allowing the current supply conductors  30   a ,  30   a ′;  30   b ,  30   b ′ to pass. The shape of the screen  15   a ;  15   b  is adapted so as to fit the shape of the discharge vessel  10 . The layer  16 a;  16   b  is not shown in FIG.  1 C. 
     For comparison, a lamp not according to the invention was manufactured, which lamp does not have the screens  15   a ;  15   b  between the electrodes  20   a ;  20   b  and the end portions  12   a ;  12   b , but which corresponds to the lamp according to the invention in all other respects. 
     The lamps were subjected to an endurance test of 5000 hours. After the endurance test, the quantity of mercury bound to the end portions was ascertained by means of a wet-chemical analysis. The results (in μg) are shown in Table 1 for the lamp according to the invention (I) and the lamp not according to the invention (II). 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                  quantity of bound mercury in μg in lamp according to 
               
               
                 the invention I and lamp not according to the invention II 
               
            
           
           
               
               
               
            
               
                   
                 I 
                 II 
               
               
                   
                   
               
               
                   
                 10 
                 57 
               
               
                   
                   
               
            
           
         
       
     
     The measure according to the invention leads to a strong reduction in the quantity of mercury bound to the end portions  12   a ;  12   b . It was found that the amount of mercury consumed during the first burning hours of the lamp is substantially independent of the presence of the shielding means. Apparently it takes some time before UV radiation originating from the discharge has attacked the surface of the end portions  12   a ;  12   b  to such an extent that an increased binding of mercury will take place, whereupon the mercury is bound (chemically) to the surface of the glass of the end portions  12   a ;  12   b . A considerable reduction in the mercury consumption was found for low-pressure mercury vapor discharge lamps fitted with screens  15   a ;  15   b  between the electrodes  20   a ;  20   b  and the respective end portions  12   a ;  12   b , which screens are made from glass (for example pyrex glass) or mica fitted with a coating layer  16   a ;  16   b  of aluminum oxide and/or yttrium oxide. 
     It will be obvious that many variations are possible to those skilled in the art within the scope of the invention. The shape of the discharge vessel need not necessarily be elongate and tubular, but may differ therefrom. In particular, the discharge vessel may have a bent or meandering shape. The shape of the shielding means is adapted soas to fit the shape of the discharge vessel at the areas of the end portions. The discharge means may alternatively be situated outside the discharge vessel, for example in the case of a discharge lamp which is operated inductively. In that case, again, the shielding means must be positioned between the UV radiation generated by the discharge means and the end portions. Preferably, the shielding means are situated as close as possible to the end portions.