Patent Publication Number: US-6906460-B2

Title: Device and method for retaining mercury source in low-pressure discharge lamps

Description:
BACKGROUND OF INVENTION 
   This invention relates to a device and a method for retaining a mercury source in the discharge space of a low-pressure discharge lamp. The invention also relates to a lamp equipped with the device. 
   A wide variety of low-pressure discharge lamps are known in the art. These lamps contain small doses of mercury, which radiates under the influence of the discharge arc. The mercury may be introduced into the discharge space of the lamp in a number of ways. One possible method is the introduction of an amalgam, typically containing bismuth, e.g. a Biln or BiSnPb compound. The mercury vapour necessary for the operation of the lamp is released from the amalgam. The amalgam is optimally positioned near a cold spot of the lamp, for example near a tip of the discharge tube. Another method uses a so-called pellet, which contains liquid mercury. The mercury is released from the pellet after the sealing of the discharge space with the help of a heat treatment of the pellet. Both an amalgam or a pellet must be prevented from rolling freely about in the discharge space, as it may collide with the electrodes and it could scratch off the light emitting layer from the internal surface of the discharge vessel. 
   A known method to position the amalgam is to insert it into an exhaust tube of the discharge vessel. The amalgam is then held in a predetermined location with various methods. In the method disclosed in U.S. Pat. Nos. 5,629,584 and 5,434,482, the amalgam is held in place with indentations on the exhaust tube and glass balls before and after the amalgam. However, this structure has certain disadvantages. The tube section of the discharge vessel must be held in a vertical position, otherwise the glass balls and the amalgam will not remain in the desired location during the so-called tip-off, i. e. when the exhaust tube of the lamp is sealed and the remaining excess length of the tube is removed. In certain production lines, this is not always feasible, and there is a need for an amalgam retaining method where the amalgam is held in place irrespective of the orientation of the tube, which receives the amalgam. 
   A discharge lamp with an amalgam container is disclosed in U.S. Pat. No. 6,201,347. In this known discharge lamp, the container is held in place with the help of a resilient, coiled wire, which is attached to the container with the amalgam. The container and the coiled wire are pushed into a tube within the discharge space of the discharge lamp. The coiled wire acts as a clamping means, which substantially prevents the movement of the container within the tube. 
   Another discharge lamp with an amalgam container is disclosed in U.S. Pat. No. 6,137,236. In this known discharge lamp the container is held in place with the help of a resilient body, which surrounds the container with the amalgam. The resilient body is provided with radially extending portions, which press against a wall of a tube within the discharge space of the lamp. The extending portions of the resilient body keep the container in a predetermined location within the tube. When the container is not inserted in the resilient body, the radially extending portions of the body are somewhat retracted, and the resilient body may be inserted into the tube with ease. The extending portions spread when the container is pushed into the resilient body. 
   Though the retaining methods disclosed in U.S. Pat. Nos. 6,137,236 and 6,201,347 are practicable in any orientation of the discharge vessel, other problems remain. For various reasons, it is desirable to insert the mercury source into the discharge space only after an evacuation of the discharge vessel, and only shortly before the final sealing of the discharge vessel. However, the containers with the amalgam, as disclosed in U.S. Pat. Nos. 6,137,236 and 6,201,347, require relatively complicated equipment, if the containers must be fed into the tube in the evacuated state of the tube. Further, the containers need to be inserted into the tube in a predetermined position (orientation) relative to the tube. This requires further specialised positioning means in the feeding equipment, which must operate in vacuum. Such an equipment is complicated, hence expensive[007]Therefore, there is a need for a method for retaining a mercury source, which allows the insertion of the mercury source into the discharge space in vacuum, and which does not require complicated manufacturing facilities, and which may be integrated into all types of existing production lines in a simple manner. 
   SUMMARY OF INVENTION 
   In an exemplary embodiment of the present invention, a device for retaining a mercury source in the discharge space of a low-pressure discharge lamp comprises a holder with an inner space. The inner space of the holder is in communication with the discharge space. The holder further comprises a receiver opening for receiving a mercury source, and resilient clamping means for clamping the holder in a tubular space segment of the discharge space. The holder also comprises resilient retaining means. The function of the resilient retaining means is to block the receiver opening, at least partially. The retaining means are adapted for allowing a passage of the mercury source in a direction towards the inner space of the holder, and blocking the movement of the mercury source through the receiver opening in a direction out of the holder. 
   In an exemplary embodiment of another aspect of the invention, a method for retaining a mercury source at a predetermined location in a discharge space of a low-pressure discharge lamp is provided. In this method, a retaining device as described above is inserted into the discharge space of the discharge lamp. The retaining device is clamped at the predetermined location in the discharge space. This is followed by the insertion of the mercury source into the holder through the receiver opening and past the retaining means. 
   In an embodiment of still another aspect of the invention, a low-pressure discharge lamp comprises a discharge space, a discharge electrode and a mercury source located in a predetermined location of the discharge space. In the lamp, the mercury source is retained in a retaining device as described above. 
   The resilient retaining means of the retaining device makes it possible to insert the retaining device into the discharge space in an early stage of the production, while the mercury source itself may be fed into the retaining device in the very last moment before the discharge space is sealed. In this manner, no or a negligible amount of mercury vapour escapes from the discharge vessel during production, and mercury contamination of the production equipment remains low. 
   As a further important advantage, the suggested retaining device remains in its position—practically in an exhaust tube of the discharge vessel—, in an arbitrary orientation of the exhaust tube. This advantage may be exploited especially at horizontal manufacturing of linear fluorescent lamps, which in turn results in increased productivity of the manufacture. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     The invention will now be described with reference to the enclosed drawings where 
       FIG. 1  is a perspective view of a low-pressure discharge tube manufactured according to the method. 
       FIG. 2  is an enlarged cross section of an end portion of the lamp shown in  FIG. 1 , with an embedded electrode assembly, taken along the plane II—II of FIG.  1 . 
       FIG. 3  is an enlarged view of an exhaust tube in the end portion shown in  FIG. 2 , with the inserted retaining device and the mercury source within the retaining device, 
       FIG. 4  is a cross section of the exhaust tube and a top view of the retaining device, seen in the plane IV—IV in FIG.  3 . 
       FIG. 5  is another cross section of the exhaust tube and a bottom view of the retaining device, seen in the plane V—V in FIG.  3 . 
       FIG. 6  illustrates a ball-formed mercury source being inserted in the retaining device in a view similar to FIG.  6 . 
       FIG. 7  is a perspective view of another embodiment of the retaining device. 
       FIG. 8  shows a cross-section of the exhaust tube with the retaining device of  FIG. 7  being inserted, in a view similar to FIG.  6 . 
       FIG. 9  is a perspective view of yet another embodiment of the retaining device. 
       FIG. 10  illustrates the insertion of the retaining device into the exhaust tube of the discharge lamp. 
       FIG. 11  is an enlarged view of a part of FIG.  10 . 
       FIG. 12  illustrates a first step during the insertion of a mercury source into the retaining device, in partial cross-section. 
       FIG. 13  illustrates a subsequent step in the insertion of a mercury source into the retaining device, following the step shown in FIG.  12 . 
       FIG. 14  illustrates another insertion method for the insertion of the mercury source into the retaining device. 
       FIG. 15  is a cross-section of the end of the discharge tube with inserted mercury source and the sealed exhaust tube. 
   

   DETAILED DESCRIPTION 
   Referring now to  FIGS. 1  to  3 , there is shown a low-pressure discharge lamp  1  in the form of a straight light tube. The lamp  1  has a sealed discharge vessel  2 . A cap  4  covers the ends  22  and  24  of the discharge vessel  2 , and also holds the electric contacts  8  of the lamp. The contacts  8  are mechanically supported by an insulating plate  6 , which latter is embedded in the cap  4 . The contacts  8  are welded to the ends of lead-through wires  10  and  12 . The wires  10 , 12  connect to a filament  14 . 
   The discharge vessel  2  of the low-pressure discharge lamp  1  encloses a discharge space  16 . The filament  14  functions as a discharge electrode, which is located in the discharge space  16 . For the proper operation of the discharge lamp  1 , a mercury source  18  is also provided in the discharge space  16 . In the shown embodiment, the mercury source  18  is an amalgam, for example made of a BilnPb compound, which is capable of forming an amalgam alloy with mercury. 
   The mercury source  18  is located in a predetermined location of the discharge space  16 . In the shown embodiment, the mercury source  18  is located in an end of an exhaust tube  20 . The exhaust tube  20  connects to a stem  26  supporting the discharge electrode, i. e. the filament  14 . This arrangement of the stem  26  and the exhaust tube  20  at the ends of the discharge vessel  2  is well known in the art, and needs no further explanation. 
   In order to retain the mercury source  18  in the predetermined location of the discharge space  16 , the discharge lamp  1  comprises a retaining device  30 , which will be explained in detail below. The mercury source  18  is retained in the retaining device  30 , and in this manner it permanently remains in the predetermined location. 
   In the embodiment shown in  FIGS. 3  to  6 , the retaining device  30  is made as double wire coil  31  as best seen in FIG.  3 . The central windings and the ends of the coil  31  act as a holder, which surrounds the mercury source  18 . In this manner the holder of the retaining device  30  comprises an inner space, which communicates with the discharge space  16 . This is necessary to allow an unhindered passage of the mercury vapours from the mercury source  18  into the discharge space  16 . 
   The holder of the mercury source  18  also has a receiver opening  32  for receiving the mercury source  18  as will be explained with reference to  FIGS. 12  to  14 . In the embodiment shown in  FIGS. 3  to  5 , the receiver opening  32  is defined as the opening surrounded by the last windings and the two ends  34 , 36  of the coil  31 . The receiver opening  32  is best seen in  FIG. 6 , which shows the retaining device  30  from the ends  34 , 36  of the coil  31 . As it is apparent from  FIG. 5 , the distance between the ends  34 , 36  of the coil  31  are only slightly smaller than the diameter of the ball-shaped mercury source  18 . As a comparison, the tip  38  of the coil  31 , where the two strands of the coil  31  are joined, substantially closes the inner space in the holder of the retaining device  30 , and prevents any passage of the mercury source  18  between the windings of the coil  31 . 
   The retaining device  30  is equipped with resilient clamping means. These serve to clamp the mercury source holder in a tubular space segment of the discharge space, typically in the exhaust tube  20  as shown in  FIGS. 2 and 3 . In the embodiment where the retaining device  30  is made as the double coil  31 , the central windings  40 , 42  of the coil  31  act as the resilient clamping means. In the non-stressed state of the coil  31 , the external diameter of the central windings  40 , 42  is slightly larger than the internal diameter D of the exhaust tube  20 . In this manner, when the coil  31  is inserted into the exhaust tube  20 , the central windings  40 , 42  are compressed, and press against the internal surface  44  of the exhaust tube  20 . Due to the friction between the coil  31  and the wall of the exhaust tube  20 , the retaining device  30  remains at the location where it has been inserted. 
   The retaining device  30  is further equipped with resilient retaining means. In the embodiment shown in  FIGS. 3  to  6 , the retaining means is embodied by the ends  34  and  36  of the coil  31 . The ends  34  and  36  are folded back, so they partly turn towards a central axis of the coil  31 . In this manner, the retaining means, i.e. the ends  34  and  36  are at least partially blocking the receiver opening  32 , as best seen in FIG.  5 . The retaining means are adapted for allowing a passage of the mercury source  18  in a direction towards the inner space of the holder. At the same time, the retaining means are blocking the movement of the mercury source  18  through the receiver opening  32  in a direction out of the holder. In the embodiment shown in  FIGS. 3  to  6 , this works as follows: the flexible resistance of the ends  34 , 36  is relatively easily surmounted, and the ends  34 , 36  yield to the external force and spread, when the mercury source  18  is pushed in the inner space of the retaining means  30  between the two ends  34 , 36  of the coil. This is shown in  FIG. 6 , which shows the ends  34 ,  36  as they spread while the mercury source  18  passes between them. However, when the mercury source  18  would move out of the retaining device  30 , for example under the force of gravity, or because of its inertia, the retaining means, i. e. the folded ends  34 ,  36  of the coil  31  show sufficient resistance for preventing the movement of the mercury source  18  out of the inner space of the retaining device  30 . It is assumed that the mercury source  18  inserted into the retaining device  30  is itself not capable of exerting a force that is large enough to press it again out from the retaining device  30 . 
   In the embodiment shown in  FIGS. 3  to  6 , the retaining device  30  is made of resilient wire material, typically made of stainless steel, molybdenum, tungsten or nickel. As explained above, in this case the mercury source holder of the retaining device is constituted by the double coil  31  itself, where the ends  34 , 36  of the coil are folded back, and turned at least partly towards a central axis of the coil  31 . In this manner, the ends  34 , 36  act as the retaining means of the retaining device  30  embodied by the coil  31 . 
   Another embodiment of the retaining device  30  is shown in  FIGS. 7 and 8 . This retaining device  30  also comprises a holder part with an inner space and receiver opening, resilient clamping means for clamping the holder in a tube of the discharge space  16 , and resilient retaining means at least partially blocking the receiver opening. 
   In the retaining device  30  of  FIG. 7 and 8 , the mercury source holder is a substantially cylindrical capsule  130 . The capsule  130  is made of a sheet material formed in an essentially cylindrical shape. In order to facilitate the insertion of the retaining device  30 , i. e. the capsule  130  into the exhaust tube  20 , the external diameter of the capsule  130  at the closed end  132  is positively smaller than the internal diameter D of the exhaust tube  20 . As best seen in  FIG. 7 , the cylindrical holder of the capsule  130  comprises cylinder segments  134  and  136 . In the shown embodiment, one cylinders segments  134  are relatively wide, while other segments  136  are somewhat narrower. The cylinder segments  134 , 136  are separated with slits  138 . The slits  138  are substantially parallel with a central axis of the cylinder. 
   In the embodiment shown in  FIGS. 7 and 8 , the clamping means of the retaining device  30  is constituted by the wide cylinder segments  134 . In the non-stressed state of the capsule  130 , the segments  134  are tilting radially outward. When the capsule  130  is inserted into the exhaust tube  20 , the segments  134  press against the internal surface of the exhaust tube  20 , and thereby hold the capsule  130  in place. 
   At the same time, the resilient mercury source retaining means of the capsule  130  are constituted by the free ends  140  of the narrow cylinder segments  136 . These free ends  140  are folding radially inward, toward a central axis of the capsule  130 . In this manner the receiver opening  32  of the mercury source holder is surrounded by the free edges  142  of the cylinder segments  134 , and the ends  140  protrude into the receiver opening  32 , at least partly blocking it. The ends  140  of the segments  134  are folded slightly towards the inner space of the capsule  130 , and the ends  140  also act as resilient retaining means which are adapted for allowing a passage of the mercury source  18  through the receiver opening  32  in a direction towards the inner space of the holder. At the same time, the ends  140  are capable of blocking the movement of the mercury source  18  through the receiver opening in a direction out of the capsule  130 . 
   Similarly to the coil  31 , the capsule  130  may be manufactured of stainless steel, molybdenum, tungsten, nickel, or any other material which is suitably resilient, and which does not destroy the discharge atmosphere in the discharge space  16 . 
   Another embodiment of the mercury source retaining device  30  is shown in FIG.  9 . Here, the mercury source holding part of the retaining device  30  is formed as a substantially frusto-conical barrel  230 . As with the capsule  130 , the retaining device  30  constituted by the barrel  230  is made of a resilient sheet material. The clamping of the barrel  230  in the tubular segment of the discharge space  16  is ensured by the flexibility of the external shell of the barrel  230 . A longitudinal slit  232  is formed substantially along a generatrix of the barrel  230 , which means that the circumference and thereby the diameter of the barrel  230  may decrease when the barrel  230  is inserted into the exhaust tube  20  of the discharge vessel  2 . 
   The retaining means of the retaining device  30  constituted by the barrel  230  are formed as tongues  240 . The tongues  240  extend radially inwards from an edge  242  of the barrel  230 , substantially towards the principal central axis of the barrel  230 . The tongues  240  function substantially in the same manner as the folded ends  140  of the segments  134  of the capsule  130 . This means that the receiver opening  32  of the barrel  230  is defined by the surrounding edge  242 , and this receiver opening  32  is partly blocked by the tongues  240 , because the diameter of an included circle between the tips  244  of the tongues  240  is smaller then the external diameter of a ball-shaped mercury source  18  (not shown in FIG.  9 ). However, the tongues  240  also yield to an external pressing force when a ball-shaped mercury source  18  is pressed into the inner space of the barrel  230  between the tongues  240 . 
   The mercury source retaining device  30  is suitable for retaining a mercury source  18  at a predetermined location in the discharge space  16  of the low-pressure discharge lamp  1 . The method, in which the retaining device  30  is used, is explained with reference to  FIGS. 10  to  15 . These illustrate the use of a retaining device  30  formed as a double-ended coil  31 , but the other embodiments of the retaining device  30  are used in a similar manner. 
   In a first step, as shown in  FIG. 10 , the retaining device  30  is inserted into the discharge space  16 . More precisely, the retaining device  30  is inserted into its final position, in the shown embodiment into that end of the exhaust tube  20 , which is closer to the stem  26  holding the filament  14 . In this manner, the mercury source  18  is located in a relatively cold place, which is sufficiently far from the discharge arc and also far from the thermal load which arises when the other end of the exhaust tube  20  is sealed. 
   The retaining device  30  is pushed into the exhaust tube  20  by a suitably formed tool, e.g. a rod  50  with a positioning pin  52  at the end thereof. The diameter of the rod  50  and that of the pin  52  is selected to ensure a loose fit in the exhaust tube  20  and in the retaining device  30  during insertion. In this manner the rod  50  is easily withdrawn from the exhaust tube  20  and also from the retaining device  30 , while the latter remains in the exhaust tube. As the retaining device  30  is inserted, the wall of the exhaust tube  20  slightly compresses the windings  40  and  42  of the coil  31 . If necessary, the rod  50  and the coil  31  may be rotated during insertion in order to make the compression of the coil  31  even easier (in the shown embodiment the rotation is counter-clockwise). For this purpose, the rod  50  may comprise suitable extensions to cause the simultaneous rotation of the coil  31 . Thereby the coil is “screwed” into the exhaust tube. 
   The retaining device  30  is pushed into the exhaust tube  20  in a position where the receiver opening  32  of the retaining device  30  turns towards an outer end of the exhaust tube  20 . This means that in the shown embodiment, the receiver opening  32  is to the right, and the positioning pin  52  of the pushing rod is inserted into the retaining device  30  through the receiver opening  32 . When retaining devices in the form of the capsule  130  or the barrel  230  are to be inserted, the positioning pin  52  may comprise suitable grooves, which loosely receive the ends  140  of the segments  134  or the tongues  240 , without positively engaging those. In this manner the rod  52  may be withdrawn, without pulling out the capsule  130  or the barrel  230  from the exhaust tube  20  while the retaining device  30  is clamped at the predetermined location of the discharge space  16 . 
   Advantageously, the retaining device  30  is inserted in the discharge space  16  before the discharge space  16  is evacuated. This means that the equipment, which feeds the retaining devices  30  into the production line and onto the rod  50 , need not be in vacuum. This makes the feeding and positioning of the retaining devices  30  easier. 
   Following the insertion of the retaining device  30 , the mercury source  18  is inserted into the holder of the retaining means  30 . The mercury source  18  is inserted through the receiver opening  32  and past the retaining means, i. e. past the ends  34 ,  36  of the coil  31  in the shown embodiment. This may also take place before evacuation, but it is preferred to insert the mercury source  18  in the holder of the retaining device  30  after evacuating the discharge space. Thereby the emission of mercury vapours into the ambient atmosphere is minimized. 
   The mercury source  18  may be pushed through the receiver opening  32  of the retaining device  30  with another, suitably formed pushing rod  60 . For the sake of proper positioning and feeding of the mercury source  18 , the pushing rod  60  may comprise an external sheath or sleeve  62 , the end  64  of which snugly receives the ball-shaped mercury source  18 . The sleeve  62  and the rod  60  are pushed until the unit reaches the retaining device  30 . Thereafter the rod  60  pushes the mercury source  18  out from the end  64  of the sleeve  62 , and into the retaining device  30  through its receiver opening  32 . 
   In another version of the method, the mercury source insertion process utilises the energy of a filling gas, such as argon. After evacuation of the discharge vessel  2 , which is symbolised with the flange  70  of the evacuating equipment, the filling gas is fed into the discharge space  16  before the latter is sealed. The mercury source  18  is inserted into the input end of the exhaust tube  20 , and thereafter the mercury source  18  is blown through the receiver opening  32  with the filling gas. This is illustrated in FIG.  14 . For this purpose, the mercury source  18  needs to develop sufficient inertia to surmount the resistance of the resilient retaining means, which block the receiver opening  32 . 
   Finally, as illustrated in  FIG. 15 , the evacuated discharge space  16  is sealed at the outer end  28  of the exhaust tube  20  after the insertion of the mercury source  18  into the retaining device  30 . The sealing is done in a known manner, by melting the outer end  28  of the exhaust tube  20 . 
   In the above embodiments, the mercury source  18  was an amalgam. However, the retaining device and method is also applicable if the applied mercury source is a so-called pellet, which contains liquid mercury. Such pellets are activated after the sealing of the discharge space. The carrier materials of such pellets—e.g. zinc—are known in the art. The release of the mercury from the pellet is normally activated with a short thermal pulse. With suitable adjustment of the production equipment, the thermal pulse may be delivered during the sealing of the exhaust tube. 
   The invention is not limited to the shown and disclosed embodiments, but other elements, improvements and variations are also within the scope of the invention. It is clear for those skilled in the art that the same principles may be applied to other types of low-pressure discharge lamps, and not only to straight light tubes such as shown in FIG.  1 . For example, the proposed mercury source retaining device is applicable with all types of mercury vapour lamps