Patent Publication Number: US-6657375-B2

Title: Fluorescent lamp and method for manufacturing the same

Description:
TECHNICAL BACKGROUND/FIELD OF THE INVENTION 
     The present invention relates to a fluorescent lamp including two electrodes and 2n glass tubes, where n is at least 2. the tubes being connected by respective bridge parts to form a discharge path through the tubes between the two electrodes. The invention further relates to a method for manufacturing such a fluorescent lamp. 
     A method for manufacturing a fluorescent lamp with eight tubes is described in UK Patent Application GB 2 335 538. The lamp is assembled from four dual tubes that are previously manufactured. Dual tubes are manufactured from two parallel glass tubes each having one end sealed. The tubes are connected to each other by means of a bridge near the sealed tube ends. The distance created between the bridge and the sealed tube ends is of such a dimension that a cold spot is created in both tube end parts. These cold spots are intended for mercury vapor pressure control and, as mercury vapor pressure determines the lamp lumen output, the effectiveness of the lamp. In the finished lamp the number of cold spots equals the number of tubes, e.g. in the lamp according to GB 2 335 538 there are eight cold spots. The creation of more cold spots than one being needed for mercury vapor pressure control is at the expense of light-emitting tube area, resulting in a lower luminous flux or longer lamp length. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to provide an improved fluorescent lamp for which above-mentioned drawback does not arise. 
     This object is achieved in accordance with the invention with a fluorescent lamp defined in the introduction which is characterized in that the first bridge part of the first dual shaped lamp part is located at a distance d 1  from the sealed ends of the tubes, and that the second bridge part of the second dual shaped lamp part is located at a distance d 2  from the sealed ends of the tubes, wherein the distance d 2  is shorter than the distance d 1  such that only a cold spot is formed in a tube end part of the first dual shaped lamp part. Only one dual shaped lamp part has a cold spot, while the or each other dual shaped lamp part fully utilizes its length for the gas discharge, light-emitting area, which will result in a higher lumen output or reduced total lamp length. 
     The invention also relates to a method for manufacturing a fluorescent lamp as defined in claim  2 . 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention also relates to a method for manufacturing a fluorescent lamp, the lamp comprising two electrodes and 2n glass tubes, where n is at least 2, the tubes being connected by respective bridge parts to form a discharge path through the tubes between the two electrodes, the method comprising the steps of: 
     a) manufacturing a first dual shaped lamp part by disposing two substantially straight tubes parallel to each other, each tube being sealed at one end, and joining the tubes near the sealed ends with a first bridge part, 
     b) manufacturing at least a second dual shaped lamp part by disposing two substantially straight tubes parallel to each other, each tube being sealed at one end, and joining the tubes near the sealed ends with a second bridge part, and 
     c) mutually connecting the first and second dual shaped lamp parts with bridge parts near the ends of the tubes opposite to the sealed ends thereof, characterized in that during step a) the first bridge part is formed at a distance d 1  from the sealed ends of the tubes, and that during step b) the second bridge part is formed at a distance d 2  from the sealed ends of the tubes, said distance d 2  being shorter than said distance d 1 , whereby cold spots are only formed near the sealed ends of the tubes of the first dual shaped lamp part. 
     FIG. 1 illustrates schematically four dual shaped lamp parts for an eight tube lamp according to one embodiment of the invention; 
     FIGS. 2 and 3 respectively are a top and front view of the eight tube lamp, and 
     FIG. 4 is a sectional view according to line IV—IV in FIG.  2 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows schematically one first dual shaped lamp part  1  and three second dual shaped lamp parts  2  for an eight tube lamp. The first dual shaped lamp part  1  has two substantially straight tubes  3 , which are parallel to each other. Each tube has one sealed end  4 . Near the sealed ends  4 , the tubes  3  are joined by means of a first bridge part  5 . The first bridge part  5  is located at a distance d 1  from the sealed ends  4  of the tubes  3 . The second dual shaped lamp parts  2  each have two tubes  6 , which are parallel to each other and have sealed ends  7  at one side. The tubes  6  are connected mutually by second bridge parts  9 , located at distance d 2  from the sealed ends  7 . The distance d 2  is shorter than the distance d 1  such that only a cold spot  8  is formed in a tube end part of the first dual shaped lamp part  1 . 
     In FIGS. 2 and 3 respectively a top and front view of a eight tube lamp manufactured from the dual shaped lamp parts  1 , 2 . In manufacturing the lamp from dual shaped lamp parts  1 , 2 , these are mutually connected by bridge parts  10  at the end of the tubes  3 ,  6  opposite to the sealed ends  4 , 7  thereof. The assembly of tubes  3 , 6  are finally placed in a lamp base  11  provided with screw thread  12 . 
     Finally, FIG. 4 shows a sectional view according to line IV—IV in FIG.  2 . The cold spots  8  are located in the first dual shaped lamp part  1  at the right of FIG.  4 . Because the tubes  3 , 6  are mutually connected by bridge parts  5 , 9 , 10  a discharge path is formed through the tubes  5 , 9 , 10  between two electrodes  13 , which are provided in one tube each. As only the first dual shaped lamp part  1  has cold spots  8 , the tubes  6  of the three other dual shaped lamp parts  2  use their full length for the gas discharge, light-emitting area, which results in a higher lumen output compared to an eight tube lamp known from the prior art which has eight cold spots, one in each tube.