Patent Publication Number: US-6707246-B1

Title: Low-pressure mercury vapor discharge lamp with improved auxiliary amalgam

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a low-pressure mercury vapor discharge lamp comprising a discharge vessel, which discharge vessel encloses a discharge space containing a filling of mercury and an inert gas in a gastight manner, and said discharge vessel ( 10 ) comprising tubular end portions, which each have a longitudinal axis, electrodes being arranged in the discharge space for generating and maintaining a discharge in the discharge space, and at least an auxiliary amalgam being provided on a carrier in the discharge vessel in the proximity of at least one of the electrodes. 
     In mercury vapor discharge lamps, mercury constitutes the primary component for (efficiently) generating ultraviolet (UV) light. An inner surface of the discharge vessel may be coated with a luminescent layer comprising a luminescent material (for example a fluorescent powder) for the conversion of UV to other wavelengths, for example to UV-B and UV-A for tanning purposes (sunbed lamps) or to visible radiation for general lighting purposes. As a result, such discharge lamps are also referred to as fluorescent lamps. The discharge vessel of low pressure mercury vapor discharge lamps generally is tubular in shape with a circular cross-section and comprises both elongated and compact embodiments. In general, the tubular discharge vessel of so-called compact fluorescent lamps comprises a collection of relatively short straight parts having a relatively small diameter, which straight parts are connected to each other, on the one hand, by means of bridge parts and/or, on the other hand, by means of curved parts. Compact fluorescent lamps are generally provided with an (integrated) lamp cap. 
     In the description and the claims of the current invention, the designation “nominal operation” is used to indicate operating conditions where the mercury vapor pressure is such that the radiant efficacy of the lamp is at least 80% of that during optimum operation, i.e. operating conditions where the mercury vapor pressure is optimal. Furthermore, in the description and the claims, the “initial radiant efficacy” is defined as the radiant efficacy of the discharge lamp 1 second after switching on the discharge lamp, and the “run-up time” is defined as the time required by the discharge lamp to reach a radiant efficacy of 80% of that during optimum operation. 
     A low-pressure mercury vapor discharge lamp as mentioned in the opening paragraph is known from U.S. Pat. No. 5,204,584. Said known low-pressure mercury vapor discharge lamp comprises a suitable combination of a main amalgam and an auxiliary amalgam, the latter being provided on one of the current supply conductors which extend from the electrodes through a so-called stem in the tubular end portion and, subsequently, issue from the discharge vessel to the exterior. In this known lamp the main amalgam is dosed so that it controls the mercury vapor pressure discharge space except for the starting period. FIG. 5 shows this known lamp, having a main amalgam  131  and auxiliary amalgams  133  and  135 . 
     In general, a low-pressure mercury vapor discharge lamp containing an auxiliary amalgam with sufficient mercury has a relatively short run-up time. Upon switching on the lamp, the auxiliary amalgam is heated by the electrode, so that the auxiliary amalgam relatively rapidly delivers a substantial part of the mercury it contains. It is desirable that, prior to switching on the lamp, said lamp has been out of operation for a sufficiently long time to enable the auxiliary amalgam to absorb sufficient mercury. If the lamp has been out of operation for a relatively short period of time, the run-up time reduction is only small and, in addition, the initial radiant efficacy is (even) lower than that of a lamp comprising only a main amalgam because the auxiliary amalgam sets the mercury vapor pressure in the discharge space at a relatively lower value. In addition, relatively long lamps exhibit the drawback that relatively much time goes by before the mercury delivered by the auxiliary amalgam has spread throughout the discharge vessel, so that after switching on such lamps there is a relatively bright zone near the auxiliary amalgam and a relatively dark zone at some distance from the auxiliary amalgam, which zones last a few minutes. 
     A drawback of the known low-pressure mercury vapor discharge lamp resides in that the run-up time is relatively long in spite of the application of an auxiliary amalgam. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a lamp of the type described in the opening paragraph which, when regularly used, has a relatively short run-up time. To achieve this, the low-pressure mercury vapor discharge lamp is characterized in accordance with the invention in that a part of the carrier is arranged in a plane transverse to the longitudinal axis. 
     As a result of the fact that said part of the carrier is arranged in a plane transverse to the longitudinal axis, the auxiliary amalgam is better irradiated by the heat generated in the electrode when the low-pressure mercury vapor discharge lamp is started. As a result, relatively more mercury is loosened relatively more rapidly from the auxiliary amalgam upon starting the discharge lamp. By virtue of the measure in accordance with the invention, the run-up time of the low-pressure mercury vapor discharge lamp is relatively short. In low-pressure mercury vapor discharge lamps, the electrode is generally arranged transversely to the longitudinal axis. 
     In the known discharge lamp, the auxiliary amalgam is provided on one of the current supply conductors, which extend from the electrodes through a so-called stem in the tubular end portion and, subsequently, issue from the discharge vessel to the exterior. As a result, the auxiliary amalgam is arranged so as to be parallel to the longitudinal axis, thus rendering the auxiliary amalgam relatively insensitive to radiant heat emitted by the electrodes upon starting the low-pressure mercury vapor discharge lamp. 
     Relative to the known discharge lamp, the auxiliary amalgam in the low-pressure mercury vapor discharge lamp in accordance with the invention is provided so as to be rotated through 90° as it were, so that the major part of the carrier is positioned so as to extend parallel to the electrode. Without being obliged to give any theoretical explanation, the inventors hold the opinion that the reduction of the run-up time is caused by the fact that a part of the mercury, which is loosened from the auxiliary amalgam on the carrier, travels more toward the interior of the discharge vessel of the low-pressure mercury vapor discharge lamp. As a result, in a short period of time after starting the discharge lamp, the back diffusion of mercury to the cold locations at the side of the electrodes facing away from the discharge space takes longer than in the known lamp. By virtue thereof, more mercury is available at locations in the discharge space which heat up relatively rapidly, thereby giving rise to an improved run-up behavior. 
     An embodiment of the low-pressure mercury vapor discharge lamp is characterized in accordance with the invention in that a stem in the tubular end portion carries the electrode, and in that the stem comprises a supporting body on which the carrier is provided. As will be explained in greater detail hereinbelow, the supporting body is formed by an extended exhaust tube or by a supporting wire provided in the stem. In an alternative modification, the carrier is provided directly on the stem. In the known discharge lamp, the auxiliary amalgam is provided on one of the current supply conductors, which extend from the electrodes through a so-called stem in the tubular end portion and, subsequently, issue from the discharge vessel to the exterior. Particularly in so-called cold-start low-pressure mercury vapor discharge lamps, this causes material to be sputtered off the auxiliary amalgam and deposited on the tubular end portions of the discharge vessel. Without being obliged to give any theoretical explanation, the inventors hold the opinion that the above problem is caused by the cold ignition of the discharge lamp, whereby, shortly after starting the lamp, the discharge acts on the (still) cold emitter and on the auxiliary amalgam, resulting in a discharge on the emitter and on the auxiliary amalgam. The discharge on the auxiliary amalgam causes the amalgam to be sputtered off the carrier and gives rise to blackening of the tubular end portions and a poor run-up behavior. Sputtering as well as blackening are undesirable phenomena. Furthermore, the service life of the lamp is reduced if any amalgam originating from the auxiliary amalgam lands on the electrode. The effect is enhanced by the relatively high cathode drop which is characteristic of cold-ignition low-pressure mercury vapor discharge lamps. 
     In a preferred embodiment of the low-pressure mercury vapor discharge lamp in accordance with the invention, the carrier is arranged at a side of the electrode facing away from the discharge space. In this embodiment, the carrier is situated between the stem and the electrode. By virtue thereof, a simple construction is obtained having a relatively short supporting wire supporting the carrier. 
     In an alternative, favorable embodiment of the low-pressure mercury vapor discharge lamp in accordance with the invention, the carrier is electrically insulated with respect to the electrode. As the carrier with the auxiliary amalgam is electrically insulated from the current supply conductor connected to the electrode, said auxiliary amalgam is at the same potential as its surroundings and will not attract ions present in the discharge. As a result, the run-up behavior of the discharge lamp is improved and blackening of the tubular end portions of the low-pressure mercury vapor discharge lamp is substantially precluded. The auxiliary amalgam on the carrier is heated by the discharge in a manner comparable to that in induction lamps. 
     These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a first embodiment of the low-pressure mercury vapor discharge lamp in accordance with the invention; 
     FIG. 2 is a perspective view of a detail of the discharge lamp shown in FIG. 1, 
     FIG.  3  and FIG. 4 are perspective views of a corresponding detail of, respectively, a second and a third embodiment of the low-pressure mercury-vapor discharge lamp in accordance with the invention, and 
     FIG. 5 is a cross-sectional view of a known lamp having a main amalgam and auxiliary amalgams. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The Figures are purely diagrammatic and not draw to scale. Particularly for clarity, some dimensions are exaggerated strongly. In the Figures, like reference numerals refer to like parts whenever possible. 
     FIG. 1 shows a first embodiment of a low-pressure mercury vapor discharge lamp in accordance with the invention, which is provided with a (glass) discharge vessel  10  having tubular end portions  11 ;  11 ′. The discharge vessel  10  encloses, in a gastight manner, a discharge space  18  containing an ionizable filling comprising less than 3 mg mercury and an inert gas, for example a 75/25 mixture of argon and neon. In the embodiment shown, the discharge vessel  10  comprises two tube portions  13 ;  13 ′ each having a tubular end portion  11 ;  11 ′ with a longitudinal axis  12 ;  12 ′. The end portions  11 ;  11 ′ are jointly fixed in a lamp cap  50 , which is shown very diagrammatically. In an alternative embodiment, a so-called integrated lamp cap is employed wherein a copper-iron ballast or an electronic gear control is situated, and which lamp cap is further provided with, for example, so-called E14 or E27 connection means. At tube ends  14 ;  14 ′ situated opposite to the lamp cap  50 , the tube portions  13 ;  13 ′ are in communication with each other via a channel  15 . The discharge vessel may alternatively be embodied so as to be a single elongated or (multiple-) bent tube, for example a tube bent in the form of a hook. The discharge vessel  10  is provided, at a side facing the discharge space  18 , with a luminescent layer  16 . In each end portion  11 ;  11 ′, an electrode  20 ;  20 ′ is arranged on a so-called stem  21 ,  21 ′ in the discharge space  18 . The electrode  20 ;  20 ′ is preferably arranged transversely to the longitudinal axis. In an alternative embodiment of the low-pressure mercury vapor discharge lamp, the electrode is axially mounted in the end portion. In addition, in a further alternative embodiment of the low-pressure mercury vapor discharge lamp, an external electrode may be provided at an end portion of the discharge vessel to bring about a capacitive coupling with a lamp power supply. Current supply conductors  30 A,  30 B;  30 A′,  30 B′ extend from the electrodes  20 ,  20 ′ through the stem  21 ;  21 ′ in the end portion  11 ;  11 ′ and issue from the discharge vessel  10  to the exterior. At least one stem  21 ;  21 ′ carries an auxiliary amalgam (not shown in FIG. 1) which is provided on a carrier  25 ;  25 ′, which carrier  25 ;  25 ′ is provided in the stem  21 ;  21 ′ by means of a supporting wire  23 ;  23 ′. In the embodiment shown, both stems  21 ;  21 ′ carry an auxiliary amalgam. In accordance with the invention, (a part of) the carrier  25 ;  25 ′ is arranged in a plane transverse to the longitudinal axis  12 ;  12 ′. 
     FIG. 2 is a perspective view of a detail of the discharge lamp shown in FIG.  1 . The auxiliary amalgam  27 , in this example lead-tin-mercury, is provided on a part  25 A of the carrier  25 . This part  25 A of the carrier  25  is arranged, in accordance with the invention, in a plane transverse to the longitudinal axis  12 . In the example shown in FIG. 2, the carrier  25  comprises a further part  25 B which is arranged in a plane parallel to the longitudinal axis  12 . The supporting wire  23  is attached to this further part  25 B, which supporting wire is anchored in the stem  21 . In the example shown in FIG. 2, the supporting wire  23  in the stem  21  is symmetrically placed with respect to the current supply conductors  30 A,  30 B. In an alternative embodiment, the supporting wire is asymmetrically arranged with respect to the current supply conductors (FIG.  1 ). The carrier  25 ,  25 ′ may be in the form of a plate and/or comprises, preferably, a network of mesh-woven wire on which the auxiliary amalgam is deposited. In an alternative embodiment, the carrier  25 ;  25 ′ of the auxiliary amalgam is made from a dense piece of strip material wherein small cuts are made, whereafter the strip is stretched so as to form an open structure. The parts  25 A and  25 B of the carrier are very diagrammatically shown, and both parts  25 A and  25 B may be covered with the auxiliary amalgam Preferably, only the part  25 A, which extends transversely to the longitudinal axis  12  and parallel to the electrode  20 , is provided with the auxiliary amalgam  27 . In the example shown in FIG.  1  and FIG. 2, the carrier  25 ;  25 ′ is arranged at a side of the electrode  20 ;  20 ′ facing away from the discharge space  18 . In this embodiment, the carrier is situated between the stem  21 ;  21 ′ and the electrode  20 ;  20 ′. In this manner, a simple construction is obtained having a relatively short supporting wire  23 ;  23 ′ which supports the carrier  25 ;  25 ′. 
     In an alternative embodiment, the carrier is arranged in the discharge space at a side of the electrode facing away from the stem in the tubular end portion of the discharge lamp. In this embodiment, the auxiliary amalgam is placed in the discharge in a similar manner as auxiliary amalgams in electrode less discharge lamps, which are also referred to as induction lamps. 
     FIG. 3 relates to a perspective view which corresponds to that shown in FIG. 2, and corresponding parts bear the same reference numerals. In FIG. 3, the supporting body is not formed by the supporting wire  23 ,  23 ′ provided in the stem  21 ,  21 ′, but by an exhaust tube  26  which extends at least partly in the discharge space  18  almost up to the electrodes  20 ,  20 ′. The carrier  25 ,  25 ′ with the amalgam is clamped on to the end of the exhaust tube  26  extending between the current-supply conductors  30 A,  30 B. In this case, the carrier  25 ,  25 ′ comprises four wing-shaped portions which are bent around the end portion of the extended exhaust tube  26  so as to be a tight fit thereon, thereby forming a portion  25 A which extends in a plane transverse to the longitudinal axis  12  and four portions  25 B which extend in a plane parallel thereto. As shown in FIG. 4, in an alternative embodiment the carrier  25 ,  25 ′ can be directly provided on the stem  21 ,  21 ′ so as to be a tight fit thereon, for example by means of pinching. This is achieved in the same manner as described with respect to the embodiment of FIG. 3 by bending, in this case two, wing-shaped portions of the carrier  25 ,  25 ′ around the stem  21 ,  21 ′ so as to be a tight fit thereon, whereby, also in this case, a portion  25 A extending in a plane transverse to the longitudinal axis  12  and, in this case, two portions  25 B extending in a plane parallel to said axis are formed. 
     It will be clear to those of ordinary skill in the art that, to provide the quick release of mercy described above, the significant property of the arrangement is that the auxiliary amalgam extends in both orthogonal directions in the plane transverse to the longitudinal axis  12 , and be close to the electrode. Therefore at least a portion of the carrier on which the auxiliary amalgam exists is aligned, parallel to the longitudinal axis of the end portion, with the nearby electrode. As a result the auxiliary is effectively irradiated by the heat generated in the electrode when the lamp is started. 
     The particular advantage of the modifications as shown in FIGS. 3 and 4 is that in both cases the carrier  25 ,  25 ′ can be provided so as to be a tight fit after the stem  21 ,  21 ′ has been brought to a desired cooling temperature in a controlled process using a heating arrangement wherein said stem is accommodated, said cooling operation being carried to preclude stresses in the glass due to (too) rapid cooling in the temperature range below 1000° C. This is advantageous, in particular, because the auxiliary amalgam  27  on the carrier  25 ,  25 ′ cannot withstand said high temperatures. 
     In FIGS. 2,  3  and  4 , the current supply conductors  30 A,  30 B comprise a first segment  31 A,  31 B of iron wire having a thickness of 0.6 mm, a second segment  32 A,  32 B of NiFeCuMn wire having a thickness of 0.35 mm, and a third segment  33 A,  33 B of CuSn wire having a thickness of 0.4 mm, which segments extend substantially, respectively, in the discharge vessel  10 , in a wall  22  of the stem  21 , and outside the discharge vessel  10  (see FIGS. 2,  3  and  4  wherein the second segments  32 A,  32 B are represented by means of dashed lines). At the end portion  11 ′, the lamp is similarly constructed (not shown in FIGS. 2,  3  and  4 ). 
     The electrode  20 ;  20 ′ is a winding of tungsten which is covered with an electron-emitting substance, in this case a mixture of barium oxide, calcium oxide and strontium oxide. The electrode  20 ;  20 ′ comprises a winding which is clamped at both ends  21 A,  21 B in a bend  36 A,  36 B of a current supply conductor  30 A,  30 B, respectively. 
     In the embodiments shown in FIGS. 2,  3  and  4 , both end portions  11 ,  11 ′ of the discharge vessel  10  comprise an auxiliary amalgam  27  which is provided on a carrier  25 ;  25 ′ which is connected to the stem  21 ;  21 ′ via a supporting wire  23 ;  23 ′ (FIG.  2 ), on an extended exhaust tube  26  (FIG. 3) or directly on the stem  21 ,  21 ′ (FIG.  4 ). For clarity, the construction of the end portions is not shown in detail in FIG.  1 . 
     In FIGS. 2,  3  and  4 , the auxiliary amalgam  27  is situated at a distance d from the electrode  20 ;  20 ′, where d&gt;0. The distance d is measured, as is shown in FIGS. 2,  3  and  4 , from the surface of the amalgam  27  to the center of the electrode  20 ;  20 ′. In accordance with a favorable embodiment of the invention, the distance d meets the relation: 
     
       
         0.5≦ d≦ 8 mm. 
       
     
     A particularly suitable value of the distance d is 1≦d≦3 mm. In this manner, a compact discharge lamp is obtained. 
     Known low-pressure mercury vapor discharge lamps, wherein the auxiliary amalgam is provided on one of the current supply conductors extending parallel to the longitudinal axis of the tubular end portion, and low-pressure mercury. vapor discharge lamps in accordance with the invention, wherein an auxiliary amalgam is provided on a carrier, and said auxiliary amalgam is arranged substantially parallel to the electrode, and the carrier is electrically insulate with respect to the current supply conductors, are subjected to life tests. All tests are carried out using so-called PLE-T 20 Watt at a rated voltage of 230 V (at a mains voltage of 50 Hz). The switching cycle is such that the lamp is alternately 1 minute in the on-state and 3 minutes in the off-state, the lamps burning in the so-called “base-up” position for 8 hours; the switch-off time was 16 hours. At zero, 1000, 2000 and 3000 switching operations, the run-up time was measured for eight lamps of each series. The time necessary to reach the rated light intensity of the discharge lamp is expressed by means of the so-called “run-up” time, which describes the time period within which the discharge lamp reaches 80% of its maximum light output. Table I shows the results. Also the standard deviations of the run-up times are listed in the Table. 
     
       
         
           
               
             
               
                 TABLE I 
               
             
            
               
                   
               
               
                 Run-up time 
               
            
           
           
               
               
               
            
               
                   
                 Run-up time (s) 
                   
               
            
           
           
               
               
               
            
               
                   
                   
                 discharge lamp in 
               
               
                   
                   
                 accordance with the 
               
               
                   
                 known discharge lamp: 
                 invention: auxiliary 
               
               
                 number of 
                 auxiliary amalgam 
                 amalgam on an electrically 
               
               
                 switching 
                 on current 
                 insulated carrier parallel 
               
               
                 operations 
                 supply conductor 
                 to the electrode 
               
               
                   
               
            
           
           
               
               
               
            
               
                 0 
                 177 ± 13 
                 112 ± 27 
               
               
                 1000 
                 168 ± 8  
                 105 ± 27 
               
               
                 2000 
                 174 ± 18 
                  95 ± 29 
               
               
                 3000 
                 162 ± 39 
                 106 ± 25 
               
               
                   
               
            
           
         
       
     
     The known discharge lamps exhibit a much longer run-up time than the discharge lamps in accordance with the invention. By virtue of the measure in accordance with the invention, a low-pressure mercury vapor discharge lamp of the type described in the opening paragraph is provided which reaches its rated light intensity more rapidly. By electrically insulating the auxiliary amalgam with respect to the current supply conductors, blackening on the tubular end portion of cold-start low-pressure mercury vapor discharge lamps in accordance with the invention is effectively precluded. 
     It will be clear that, within the scope of the invention, many variations are possible to those skilled in the art. 
     The scope of protection of the invention is not limited to the above examples. The invention is embodied in each novel characteristic and each combination of characteristics. Reference numerals in the claims do not limit the scope of protection thereof. The use of the verb “to comprise” and its conjugations does not exclude the presence of elements or steps other than those stated in the claims. The use of the article “a” or “an” preceding an element does not exclude the presence of a plurality of such elements.