Patent Publication Number: US-7586250-B2

Title: Lamp having an arc tube protected from breakage

Description:
This application is based on applications Nos. 2004-203954 and 2005-34894 filed in Japan, the contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a flat double-spiral lamp. 
     2. Related Art 
       FIG. 1  shows a lamp  600  having an arc tube  610  in which a discharge path turns in a double spiral, as one well-known type of compact lamps (German Patent No. 871927). This lamp  600  is a single-based lamp, with both tube ends  611  of the arc tube  610  being held by a holder  630  equipped with a base  620  and a pair of electrodes (not illustrated) sealed at both tube ends  611  being electrically connected to the base  620 . 
       FIG. 2  shows a lamp  700  having an arc tube  710  in which a discharge path turns in a double spiral substantially on one plane (disclosed in Japanese Patent Application Publication No. H09-45283. Note that this type of lamp has not yet been put in practical use). This lamp  700  is thinner than the lamp  600  shown in  FIG. 1 , because the arc tube  710  is substantially a disc in outside shape whereas the arc tube  610  is substantially a cone in outside shape. Such a lamp  700  allows a lamp fixture to be made thinner, and so is suitable as a light source of a downlight or a wall light recessed in a ceiling or a wall. Also, the lamp  700  is a surface light source having a circular light emitting surface, and so exhibits a favorable light distribution on an irradiated surface. Hence the lamp  700  is suitable for store or home lighting. 
     As shown in  FIG. 2 , the lamp  700  is a double-based lamp that is equipped with separate bases  720   a  and  720   b  at both tube ends  711   a  and  711   b  of the arc tube  710 . This being so, if an excessive force is applied when inserting the bases  720   a  and  720   b  into sockets of a lamp fixture, the arc tube  710  may be broken near the tube end  711   a  or  711   b . Besides, to attach the lamp  700  to the lamp fixture, the arc tube  710  needs to be held by hand, which causes a sense of worry about breaking the arc tube  710 . 
     To assume a circular light emitting surface, the arc tube  710  is designed so as to minimize a gap  712  between adjacent turns in a spiraled portion between the tube ends  711   a  and  711   b . This being the case, when an inner part of the spiraled portion vibrates due to an external impact, adjacent turns in that part may collide with each other, as a result of which the arc tube  710  can be broken. This tends to occur particularly during transportation. With a conventional transportation container that holds only outer edges of the arc tube  710 , such vibrations of the inner part of the spiraled portion cannot be prevented. 
     SUMMARY OF THE INVENTION 
     In view of the above problems, the present invention aims to provide a flat double-spiral lamp in which an arc tube is protected from breakage during attachment or transportation. 
     The stated aim can be achieved by a lamp including: an arc tube including a pair of electrodes respectively at both tube ends and for forming, between the pair of electrodes, a discharge path which turns in a double spiral substantially on one plane; and a holder holding the arc tube in a state of fixing the tube ends of the arc tube and at least one part of the arc tube between the tube ends. 
     According to this construction, even if an excessive force is applied when inserting a base of the lamp into a socket of a lamp fixture, there is no danger of breaking the arc tube near the tube ends, unlike a conventional flat double-spiral lamp having two bases. Also, the holder can be held by hand instead of the arc tube when attaching/detaching the lamp to/from the lamp fixture. Accordingly, the lamp can be used safely as a light source of a lighting unit that is located relatively high, such as a downlight or a wall light. Furthermore, at least one part of the arc tube between the tube ends is fixed by the holder. This prevents an inner part of a spiraled portion of the arc tube from vibrating due to impact during transportation. Therefore, there is no danger that adjacent turns of the arc tube collide with each other and as a result the arc tube is broken. Hence the lamp can be safely transported using a conventional transportation container that holds only outer edges of the arc tube. 
     Here, the lamp may further include a base provided on the holder so as to be attachable to a lamp fixture. 
     According to this construction, the lamp can be attached to or detached from the lamp fixture more easily. 
     Here, the holder may include fitting parts for fixing the tube ends of the arc tube. 
     According to this construction, the lamp can be assembled easily. 
     Here, guide grooves for guiding the tube ends of the arc tube into the fitting parts may be formed in the holder. 
     According to this construction, the tube ends can be easily fitted into the fitting parts, with it being possible to further ease the assembly of the lamp. 
     Here, the lamp may further include a wire embedded in the holder to electrically connect the pair of electrodes to the base. 
     According to this construction, a wiring operation can be conducted easily, which eases the assembly of the lamp. Such an assembly can be automated without difficulty. Also, the lamp can be made thinner. 
     Here, a positioning groove may be formed on a surface of the holder facing the arc tube, along a tube axis of the arc tube, wherein a part of the arc tube is fitted in the positioning groove. 
     According to this construction, the inner part of the spiraled portion of the arc tube is prevented from vibration more reliably. 
     Here, at least one part of the arc tube may be bonded to the holder using an adhesive. 
     According to this construction, the arc tube can be fixed more firmly. 
     Here, the holder may be a substantially rectangular platelike member positioned in parallel with the plane, wherein the tube ends of the arc tube are fixed to both ends of the holder in a longitudinal direction, and a tube center of the arc tube is fixed to a center of the holder. 
     According to this construction, the lamp can be made thinner while maintaining the protection of the arc tube from breakage. Also, the holder has a simple structure, which contributes to higher industrial productivity. 
     Here, the holder may be a substantially cross-shaped platelike member positioned in parallel with the plane, wherein the tube ends of the arc tube are fixed to two opposite ends of the holder, and a tube center of the arc tube is fixed to a center of the holder. 
     According to this construction, the lamp can be made thinner while maintaining the protection of the arc tube from breakage. Also, the holder has a simple structure, which contributes to higher industrial productivity. 
     Here, the lamp may further include a cover attached to the holder so as to cover at least outer edges of the arc tube. 
     According to this construction, the outer edges of the arc tube are protected by the cover, thereby making the arc tube more resistant to breakage. 
     Here, the lamp may further include a cover attached to the holder so as to cover at least outer edges of the arc tube and a holder side of the arc tube, and having an optical reflection plane on a surface facing the arc tube. 
     According to this construction, a luminous flux can be efficiently extracted from inside the cover. Therefore, even when the lamp is attached to a simple lamp fixture which does not include a reflection plate, a relatively high illuminance can be attained. 
     The appearance of the lamp can be improved by setting a gap Gb between adjacent turns of the arc tube at each of the tube ends to be equal to a gap Ge between adjacent turns of the arc tube in a predetermined part of a spiraled portion between the tube ends. Ge needs to be set small to downsize the lamp. If Gb is made equal to such a small Ge, there is a possibility that, in an electrode sealing step, an adjacent part to any of the tube ends is affected by heat from a burner or a sealing jig for sealing the electrodes to the tube ends cannot be used for lack of a sufficient space. Therefore, when Ge is relatively small, it is preferable to set Gb such that the adjacent part to the tube end will not be affected by heat from the burner and the sealing jig can be used in the electrode sealing step, while also taking factors such as the lamp appearance and the holder structure into consideration. 
     In view of this, a gap between adjacent turns of the arc tube may be larger at each of the tube ends than in a predetermined part of a spiraled portion between the tube ends, the gap being substantially uniform in the predetermined part. 
     According to this construction, a space for fixing the tube ends to the holder can be ensured. Also, the tube ends can be heated sufficiently by the burner in the electrode sealing step, so that a crack in the electrode sealing part and a leakage of mercury or buffer gas from the arc tube due to such a crack can be prevented. Also, the electrodes can be kept from being displaced and contacting the inner wall of the arc tube. This contributes to a higher yield of nondefective units. 
     Here, the ranges 0.5≦Ge≦2.0 and 3.0≦Gb≦10.0 may be set where Ge denotes the gap in the predetermined part in mm and Gb denotes the gap at each of the tube ends in mm. 
     According to this construction, a nondefective unit yield of 99.5% at the maximum can be achieved. 
     Here, the gap may increase from Ge in a part of the spiraled portion in which a tube axis extends more closely toward each of the tube ends than in the predetermined part, with a straight line that connects a tube center of the arc tube and a point at which the gap begins to increase being substantially orthogonal to a straight line that connects the tube center and the tube ends. 
     According to this construction, the appearance of the lamp can be further improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrate a specific embodiment of the invention. 
       In the drawings: 
         FIG. 1  shows a conventional lamp, where  FIG. 1A  is a bottom view,  FIG. 1B  a top view,  FIG. 1C  a front view, and  FIG. 1D  a side view; 
         FIG. 2  shows another conventional lamp, where  FIG. 2A  is a top view,  FIG. 2B  a front view, and  FIG. 2C  a side view; 
         FIG. 3  shows a lamp to which an embodiment of the present invention relates, where  FIG. 3A  is a top view,  FIG. 3B  a bottom view,  FIG. 3C  a front view, and  FIG. 3D  a side view; 
         FIG. 4  shows an arc tube in the lamp shown in  FIG. 3 , where  FIG. 4A  is a sectional top view and  FIG. 4B  a front view; 
         FIG. 5  shows a holder in the lamp shown in  FIG. 3 , where  FIG. 5A  is a top view,  FIG. 5B  a front view, and  FIG. 5C  a side view; 
         FIG. 6  shows a lamp to which a first modification of the present invention relates, where  FIG. 6A  is a top view,  FIG. 6B  a sectional view taken along line A-A given in  FIG. 6A ,  FIG. 6C  a front view, and  FIG. 6D  a side view; 
         FIG. 7  shows a lamp to which a second modification of the present invention relates, where  FIG. 7A  is a top view,  FIG. 7B  a front view, and  FIG. 7C  a side view; 
         FIG. 8  shows a holder in the lamp shown in  FIG. 7 , where  FIG. 8A  is a top view,  FIG. 8B  a side view,  FIG. 8C  an exploded front view, and  FIG. 8D  an exploded side view; 
         FIG. 9  shows a lamp to which a third modification of the present invention relates, where  FIG. 9A  is a top view,  FIG. 9B  a bottom view,  FIG. 9C  a front view, and  FIG. 9D  a side view; 
         FIG. 10  shows a lamp to which a fourth modification of the present invention relates, where  FIG. 10A  is a top view,  FIG. 10B  a bottom view,  FIG. 10C  a front view, and  FIG. 10D  a side view; and 
         FIG. 11  shows a lamp to which a fifth modification of the present invention relates, where  FIG. 11A  is a top view,  FIG. 11B  a bottom view,  FIG. 11C  a front view, and  FIG. 11D  a side view. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     (Embodiment) 
     The following describes a lamp to which an embodiment of the present invention relates, with reference to drawings. 
     (Construction of the Lamp) 
       FIG. 3  shows a lamp  1  according to the embodiment of the present invention. This lamp  1  is a fluorescent lamp used, for example, for a downlight or a wall light directly mounted on a ceiling or a wall of a store, a house, and the like. The lamp  1  is roughly made up of an arc tube  10  and a holder  20  which holds the arc tube  10 . 
     (1) Arc Tube  10   
       FIG. 4  shows the arc tube  10  before being attached to the holder  20 . As shown in the drawing, the arc tube  10  is shaped as a flat double spiral. This being so, a discharge path which, centered on a midway portion thereof, turns in a double spiral substantially on one plane is formed between one pair of electrodes  30  provided at both tube ends  11   a  and  11   b  of the arc tube  10 . The tube ends  11   a  and  11   b  are opposite to each other with a tube center  12 , which corresponds to the midway portion of the discharge path, in between. 
     A gap between adjacent turns of the arc tube  10 , i.e. a gap between adjacent turns of a glass tube  13  which constitutes the arc tube  10  together with the pair of electrodes  30 , is as follows. The gap between adjacent turns of the glass tube  13  is substantially uniform in a spiraled portion between the tube ends  11   a  and  11   b , except the tube center  12  and its neighboring part and neighboring parts of the tube ends  11   a  and  11   b  (this substantially uniform gap is denoted by Ge). The gap between adjacent turns of the glass tube  13  increases as the tube ends  11   a  and  11   b  approach, in the neighboring parts of the tube ends  11   a  and  11   b . As a result, the gap between adjacent turns of the glass tube  13  at the tube ends  11   a  and  11   b  (this gap is denoted by Gb) is larger than the gap Ge. 
     A straight line L 1  which connects the tube center  12  and a point P at which the gap begins to increase forms an angle of about 90° with a straight line L 2  which connects the tube center  12  and the tube ends  11   a  and  11   b . In other words, the straight line L 1  is substantially orthogonal to the straight line L 2 . In this case, the neighboring parts of the tube ends  11   a  and  11   b  are such parts that extend inwardly from the tube ends  11   a  and  11   b  to points forming an angle of about 45° with the tube ends  11   a  and  11   b . By heating the glass tube  13  at and near the point P to soften and processing the glass tube  13  so as to widen the gap between adjacent turns from the point P, the arc tube  10  with the tube ends  11   a  and  11   b  spreading out can be obtained without compromising appearance. 
     For example, in the case of a 27 W lamp, the glass tube  13  has an outside diameter of 9.0 mm, an inside diameter of 8.0 mm, and a total length of 850 mm. Also, a distance between the electrodes  30  is 820 mm, and a diameter of a substantial circle defined by the arc tube  10  is 130 mm. Further, the gap Ge is 1.0 mm, the gap Gb is 5.0 mm, and a number of turns of the glass tube  13  is 4.1. The glass tube  13  is made of barium-strontium silicate glass (soft glass with a softening point of 675° C.) as one example. 
     To make the lamp  1  compact, it is preferable to limit the inside diameter of the glass tube  13  in a range of 3.0 mm to 20.0 mm, and a tube input in a range of 6 W to 80 W. 
     When the gap Ge is small, a large gap Gb is preferable to maintain a high yield of nondefective units in a step of sealing the electrodes  30  at the tube ends  11   a  and  11   b . In detail, when the gap Ge is in a range of 0.5 mm to 2.0 mm, the gap Gb is preferably in a range of 3.0 mm to 10.0 mm. 
     If the gap Gb is no smaller than 3.0 mm, it is possible to sufficiently heat the tube ends  11   a  and  11   b  using a burner in the electrode sealing step. This suppresses defects such as the sealed portions being cracked and as a result leaking or the electrode coils touching the inner walls of the tube ends  11   a  and  11   b , so that a higher yield of nondefective units can be achieved. 
     A gap Gb exceeding 10 mm is basically undesirable, since processability decreases and also the appearance of the lamp  1  is ruined thereby causing a decrease in commercial value. Depending on the shape and size of the lamp  1 , however, these problems may not arise even if the gap Gb is over 10 mm. 
     An opposite surface of the tube center  12  to the holder  20  slightly protrudes in an expanded condition, thereby forming a bulge  14 . This bulge  14  is a coldest spot of the lamp  1  at the time of lighting. For example, if the inside diameter of the glass tube  13  is in a range of 3 mm to 16 mm, a favorable luminous efficiency can be achieved by designing the lamp  1  so that the temperature of the bulge  14  is in a range of 40° C. to 50° C. during lighting. 
     Each of the electrodes  30  includes a filament coil  31  made of tungsten and one pair of lead wires  32 . The electrode  30  is hermetically sealed in a corresponding one of the tube ends  11   a  and  11   b  at the pair of lead wires  32  according to a glass bead mounting method, in a state where the filament coil  31  is contained inside the arc tube  10 . Also, an exhaust pipe  15  is hermetically sealed in the tube end  11   a , in a state where a tip of the exhaust pipe  15  is sealed inside the arc tube  10 . The exhaust pipe  15  is used for introducing a rare gas into the arc tube  10 . 
     The electrode sealing step is explained in detail below, taking an example of the tube end  11   a . The tube end  11   a  in which the pair of lead wires  32  and the exhaust pipe  15  are inserted is heated to soften by a burner from opposite sides of the plane on which the discharge path of the arc tube  10  lies. The softened portion is then crushed from the opposite sides using a pinch block which serves as a sealing jig, thereby sealing the tube end  11   a . The resulting sealed portion has a surface that coincides with the plane on which the discharge path of the arc tube  10  lies. In this way, the tube ends  11   a  and  11   b  can be easily sealed without being obstructed by adjacent parts of the spiraled portion to the tube ends  11   a  and  11   b , which contributes to a higher yield of nondefective units. 
     A phosphor layer (not illustrated) of a rare-earth phosphor composed of a red phosphor (Y 2 O 3 :Eu), a green phosphor (LaPO 4 :Ce, Tb), and a blue phosphor (BaMg 2 Al 16 O 27 :Eu, Mn) is formed on the inner wall of the arc tube  10 . Note here that the phosphor layer may be composed of phosphors other than the above. Also, the present invention includes not only a lamp which has a phosphor layer but also a lamp which does not have a phosphor layer. 
     Five milligrams of mercury  16  is enclosed in the arc tube  10 . In addition, a gas mixture of argon and neon (Ar+25% Ne) is enclosed in the arc tube  10  as a buffer gas, at a pressure of about 400 Pa. The mercury  16  enclosed may be a simple substance or a mercury amalgam with zinc, tin, or bismuth-indium. The gas enclosed may be a gas mixture of argon, neon, krypton, and the like. 
     (2) Holder  20   
       FIG. 5  shows the holder  20 . In the drawing, the holder  20  is a long rectangular platelike member made of a resin. The holder  20  includes a holder body  21  which supplies power to the arc tube  10 , and a fixing member  22  which fixes the tube ends  11   a  and  11   b  of the arc tube  10 . 
     The holder body  21  and the fixing member  22  are formed in one piece, by fitting tips of stoppers  24  formed at both ends of the holder body  21  into holes  23  formed at both ends of the fixing member  22 . 
     Two pairs of electrode terminals  25  for connecting to the lead wires  32  of the arc tube  10  are partially embedded respectively at both ends of a surface of the holder body  21  facing the arc tube  10 . 
     Also, a base  40  having four power connecting terminals  41  is provided at a center on an opposite surface of the holder body  21  to the arc tube  10 . The four power connecting terminals  41  are connected with pin-type connecting terminals  42 , which are connected with the electrode terminals  25  via wires  26  embedded in the holder body  21 . In this way, the electrodes  30  of the arc tube  10  and the base  40  are electrically connected to each other. 
     Meanwhile, fitting parts  27   a  and  27   b  used for fixing the arc tube  10  are formed at both ends on a surface of the fixing member  22  facing the arc tube  10 . Each of the fitting parts  27   a  and  27   b  has a circular hole  28  that conforms to the outside diameter of the arc tube  10 . Also, a guide groove  29  which is substantially semicircular in cross section is formed on a tube end entering side of the fitting part in front of the hole  28 , to guide a corresponding one of the tube ends  11   a  and  11   b  into the hole  28 . 
     (Method of Assembling the Lamp  1 ) 
     The lamp  1  having the above construction is assembled in the following manner. Having applied an adhesive  50  to a substantially middle portion on the surface of the fixing member  22  facing the arc tube  10 , the tube ends  11   a  and  11   b  of the arc tube  10  are fitted into the holes  28  of the fixing member  22 . Following this, in a state where the fixing member  22  is positioned substantially in parallel with the plane on which the discharge path of the arc tube  10  lies, the tube center  12  of the arc tube  10  is pressed against the adhesive  50  so that the arc tube  10  is secured to the fixing member  22 . Furthermore, an adhesive (not illustrated) is poured into gaps between the inner walls of the holes  28  and the tube ends  11   a  and  11   b  so that the tube ends  11   a  and  11   b  are secured to the fitting parts  27   a  and  27   b . Lastly, after connecting the lead wires  32  of the arc tube  10  to the electrode terminals  25  of the holder body  21 , the holder body  21  and the fixing member  22  are integrated to complete the lamp  1 . 
     The adhesive used here may be silicone resin, epoxy resin, acrylic resin, or cement. 
     (Evaluations of the Lamp  1 ) 
     The assembly operation of the lamp  1  of this embodiment was evaluated. As a result, the lamp  1  showed a same level of processability as a conventional single-based compact lamp. This indicates the assembly operation of the lamp  1  can be automated without difficulty. 
     Also, the ease of attachment/removal of the lamp  1  of this embodiment was evaluated. As a result, the lamp  1  was able to be attached and removed more easily than a conventional double-based lamp. In addition, since the arc tube  10  is fixed by the holder  20 , the lamp  1  can be handled without worrying about breaking the arc tube  10 . The arc tube  10  was actually unbroken when the lamp  1  was attached to and removed from a lamp fixture. 
     Also, the endurance of the lamp  1  of this embodiment during transportation was evaluated. As a result, the lamp  1  showed a low breakage ratio even when a transportation container such as the one for conventional compact lamps was used. 
     Furthermore, various properties of the lamp  1  of this embodiment were evaluated through experiment. In detail, the lamp  1  was lit using an electronic ballast dedicated for high frequencies, and various properties of the lamp  1  were measured. Here, the tube input of the lamp  1  was 27 W. As a result, the lamp  1  showed excellent lamp properties with a luminous flux of 2210 lm and a luminous efficiency of 81.9 lm/W. Also, a rating life of the lamp  1  was about 10000 hours on average. 
     Equally excellent lamp properties were observed when the same experiment was conducted on lamps other than the 27 W tube input type, over a wide range of tube inputs from 6 W to 80 W (3.0 mm to 16.0 mm in inside diameter). 
     (Modifications) 
     The present invention has been described by way of the above embodiment, though it should be obvious that the present invention is not limited to the above. Example modifications are given below. 
     (1) First Modification 
       FIG. 6  shows a lamp  100  to which a first modification of the present invention relates. As shown in the drawing, the lamp  100  is roughly made up of an arc tube  110  and a holder  120 . 
     The arc tube  110  is of the same type as the arc tube  10  in the lamp  1  of the above embodiment. In detail, the arc tube  110  is shaped as a flat double spiral. This being so, a discharge path which, centered on a midway portion thereof, turns in a double spiral substantially on one plane is formed between one pair of electrodes (not illustrated) provided at both tube ends  111   a  and  111   b  of the arc tube  110 . The tube ends  111   a  and  111   b  are opposite to each other with a tube center  112 , which corresponds to the midway portion of the discharge path, in between. 
     The holder  120  is a long rectangular platelike member made of a resin, and includes a holder body  121  and a fixing member  122 . 
     The holder body  121  is of the same type as the holder body  21  in the lamp  1  of the above embodiment. In detail, electrode terminals  125  are partially embedded at both ends of a surface of the holder body  121  facing the arc tube  110 , and a base  140  having power connecting terminals  141  which are connected to the electrode terminals  125  via wires (not illustrated) is provided at a center on an opposite surface of the holder body  121  to the arc tube  110 . 
     Fitting parts  127   a  and  127   b  for fixing the arc tube  110  are formed at both ends on a surface of the fixing member  122  facing the arc tube  110 . Each of the fitting parts  127   a  and  127   b  has a hole  128 . Also, a guide groove  129  for guiding a corresponding one of the tube ends  111   a  and  111   b  into the hole  128  is formed on a tube end entering side of the fitting part in front of the hole  128 . 
     Furthermore, positioning grooves  160  are formed on the surface of the fixing member  122  facing the arc tube  110 , along a tube axis of the arc tube  110 . The positioning grooves  160  are shaped substantially semicircular in cross section in accordance with an outside diameter of a glass tube  113  which constitutes the arc tube  110  together with the pair of electrodes. The arc tube  110  is positioned such that the holder  120  side of the glass tube  113  is fitted in these positioning grooves  160 , and then fixed to the fixing member  122 . This keeps adjacent turns of the glass tube  113  from approaching each other. Accordingly, an inner part of a spiraled portion of the glass tube  113  is prevented from vibration, with it being possible to protect the arc tube  110  from breakage. 
     The arc tube  110  is fixed to the fixing member  122  at the tube center  112  and the tube ends  111   a  and  111   b , using an adhesive  150 . An adhesive may also be poured into gaps between the arc tube  110  and walls of the positioning grooves  160 , to fix the arc tube  110  to the fixing member  122  more firmly. 
     (2) Second Modification 
       FIG. 7  shows a lamp  200  to which a second modification of the present invention relates. In the drawing, the lamp  200  is roughly made up of an arc tube  210  and a holder  220 .  FIG. 8  shows the holder  220 . 
     The arc tube  210  is of the same type as the arc tube  10  in the lamp  1  of the above embodiment. In detail, the arc tube  210  is shaped as a flat double spiral. This being so, a discharge path which, centered on a midway portion thereof, turns in a double spiral substantially on one plane is formed between one pair of electrodes (not illustrated) provided at both ends  211   a  and  211   b  of the arc tube  210 . The tube ends  211   a  and  211   b  are opposite to each other with a tube center  212 , which corresponds to the midway portion of the discharge path, in between. 
     The holder  220  includes a holder body  221  which is a long rectangular platelike member made of a resin with slightly wider ends, and one pair of end covers  222  attached to both ends of the holder body  221 . Electrode terminals  225  are partially embedded at both ends of a surface of the holder body  221  facing the arc tube  210 . Also, a base  240  having power connecting terminals  241  is provided at a center on an opposite surface of the holder body  221  to the arc tube  210 . These power connecting terminals  241  are connected with pin-type connecting terminals  242 , which are connected with the electrode terminals  225  via wires  226 . 
     The end covers  222  are used for holding and fixing the tube ends  211   a  and  211   b  of the arc tube  210 . The end covers  222  form fitting parts  227   a  and  227   b  together with both ends of the holder body  221 . In a state where the tube ends  211   a  and  211   b  of the arc tube  210  are positioned at both ends of the holder body  221 , the end covers  222  are attached to the holder body  221  so as to cover the tube ends  211   a  and  211   b.    
     The tube center  212  of the arc tube  210  is fixed to the holder  220  using an adhesive  250 . Also, the tube ends  211   a  and  211   b  of the arc tube  210  are fixed to the fitting parts  227   a  and  227   b  of the holder  220  using an adhesive (not illustrated). 
     According to this modification, the fitting parts  227   a  and  227   b  are formed by the end covers  222  and the ends of the holder body  221 . This makes it unnecessary to provide a fixing member such as the fixing member  22  in the lamp  1  of the above embodiment. Hence the lamp  200  can be made slimmer. 
     (3) Third Modification 
       FIG. 9  shows a lamp  300  to which a third modification of the present invention relates. In the drawing, the lamp  300  is roughly made up of an arc tube  310 , a holder  320 , and a cover  360 . 
     The arc tube  310  is of the same type as the arc tube  10  in the lamp  1  of the above embodiment. In detail, the arc tube  310  is shaped as a flat double spiral. This being so, a discharge path which, centered on a midway portion thereof, turns in a double spiral substantially on one plane is formed between one pair of electrodes (not illustrated) provided at both ends  311   a  and  311   b  of the arc tube  310 . The tube ends  311   a  and  311   b  are opposite to each other with a tube center  312 , which corresponds to the midway portion of the discharge path, in between. 
     The holder  320  includes a holder body  321  and a fixing member  322 . The holder body  321  and the fixing member  322  are respectively of the same types as the holder body  21  and the fixing member  22  in the lamp  1  of the above embodiment. In detail, a base  340  having power connecting terminals  341  is provided at a center on an opposite surface of the holder body  321  to the arc tube  310 , and fitting parts  327   a  and  327   b  for fixing the arc tube  310  are formed at both ends on a surface of the fixing member  322  facing the arc tube  310 . 
     The tube center  312  of the arc tube  310  is fixed to the holder  320  using an adhesive  350 . Also, the tube ends  311   a  and  311   b  of the arc tube  310  are fixed to the fitting parts  327   a  and  327   b  of the holder  320  using an adhesive (not illustrated). 
     The cover  360  is a short tubular member, and is attached to the holder  320  so as to cover outer edges of the arc tube  310 . In detail, the cover  360  is attached to the fixing member  322  so as to contain the arc tube  310  and part of the fixing member  322 . 
     According to this modification, the outer edges of the arc tube  310  are protected by the cover  360 , thereby making the arc tube  310  more resistant to breakage. 
     (4) Fourth Modification 
       FIG. 10  shows a lamp  400  to which a fourth modification of the present invention relates. In the drawing, the lamp  400  is roughly made up of an arc tube  410 , a holder  420 , and a cover  460 . 
     The arc tube  410  is of the same type as the arc tube  10  in the lamp  1  of the above embodiment. In detail, the arc tube  410  is shaped as a flat double spiral. This being so, a discharge path which, centered on a midway portion thereof, turns in a double spiral substantially on one plane is formed between one pair of electrodes (not illustrated) provided at both tube ends  411   a  and  411   b  of the arc tube  410 . The tube ends  411   a  and  411   b  are opposite to each other with a tube center  412 , which corresponds to the midway portion of the discharge path, in between. 
     The holder  420  includes a holder body  421  and a fixing member  422 . The holder body  421  and the fixing member  422  are respectively of the same types as the holder body  21  and the fixing member  22  in the lamp  1  of the above embodiment. In detail, a base  440  having power connecting terminals  441  is provided at a center on an opposite surface of the holder body  421  to the arc tube  410 , and fitting parts  427   a  and  427   b  for fixing the arc tube  410  are formed at both ends on a surface of the fixing member  422  facing the arc tube  410 . 
     The tube center  412  of the arc tube  410  is fixed to the holder  420  using an adhesive  450 . Also, the tube ends  411   a  and  411   b  of the arc tube  410  are fixed to the fitting parts  427   a  and  427   b  of the holder  420  using an adhesive (not illustrated). 
     The cover  460  is an umbrella-like member that is attached to the holder  420  so as to cover both the outer edges of the arc tube  410  and the holder  420  side of the arc tube  410 . In detail, the cover  460  is attached to the holder  420  so as to cover the arc tube  410  and the fixing member  422  from the holder body  421  side. An optical reflecting plane  461  is formed on a surface of the cover  460  facing the arc tube  410 . 
     According to this modification, the outer edges and the holder  420  side of the arc tube  410  are protected by the cover  460 , which makes the arc tube  410  more resistant to breakage. Also, even when attached to a simple lamp fixture which does not include a reflecting plate, the lamp  400  exhibits a relatively high illuminance. 
     It should be noted here that the shape of the cover  460  is not limited to an umbrella. For example, by shaping the cover  460  like a closed-bottom tube having only one end opened, the lamp  400  can be used as a spotlight. Even when the cover  460  is shaped like such a closed-bottom tube, a luminous flux can be efficiently extracted from inside the cover  460 , since the optical reflecting plane  461  is formed inside the cover  460 . 
     (5) Fifth Modification 
       FIG. 11  shows a lamp  500  to which a fifth modification of the present invention relates. In the drawing, the lamp  500  is roughly made up of an arc tube  510  and a holder  520 . 
     The arc tube  510  is of the same type as the arc tube  10  in the lamp  1  of the above embodiment. In detail, the arc tube  510  is shaped as a flat double spiral. This being so, a discharge path which, centered on a midway portion thereof, turns in a double spiral substantially on one plane is formed between one pair of electrodes (not illustrated) provided at both ends  511   a  and  511   b  of the arc tube  510 . The tube ends  511   a  and  511   b  are opposite to each other with a tube center  512 , which corresponds to the midway portion of the discharge path, in between. 
     The holder  520  is a cross-shaped platelike member made of a resin. The holder  520  includes a long rectangular holder body  521  and a cross-shaped fixing member  522 . The holder body  521  is of the same type as the holder body  21  in the lamp  1  of the above embodiment. In detail, a base  540  having power connecting terminals  541  is attached to the holder body  521 . 
     Fitting parts  527   a  and  527   b  for fixing the tube ends  511   a  and  511   b  of the arc tube  510  are formed at two ends  524   a  and  524   b  of the fixing member  522  that oppose to each other with a cross center  523  in between as shown in  FIG. 11B . On the other hand, no fitting parts are formed at remaining two ends  524   c  and  524   d  which oppose to each other with the cross center  523  in between. The fitting parts  527   a  and  527   b  each have a hole  528 . Also, a guide groove  529  for guiding a corresponding one of the tube ends  511   a  and  511   b  into the hole  528  is formed on a tube end entering side of the fitting part in front of the hole  528 . 
     The tube center  512  of the arc tube  510  is fixed to the holder  520  using an adhesive  550 . Also, the tube ends  511   a  and  511   b  of the arc tube  510  are fixed to the fitting parts  527   a  and  527   b  of the holder  520  using an adhesive (not illustrated). Furthermore, the arc tube  510  is fixed to the ends  524   c  and  524   d  of the fixing member  522  at which no fitting parts are provided, using an adhesive (not illustrated). 
     According to this modification, the four ends  524   a  to  524   d  of the fixing member  522  extend outward over the outer edges of the arc tube  510 . Accordingly, the outer edges of the arc tube  510  are protected by the ends  524   a  to  524   d  on four sides. This makes the arc tube  510  more resistant to breakage. 
     Also, the arc tube  510  is fixed to the holder  520  at a total of five locations, namely, the tube center  512 , the tube ends  511   a  and  511   b , and the portions corresponding to the ends  524   c  and  524   d  of the fixing member  522  where no fitting parts are provided. This suppresses vibrations of an inner part of a spiraled portion of the arc tube  510 , with it being possible to protect the arc tube  510  from breakage during transportation and the like. 
     The present invention is applicable to a fluorescent lamp having a flat double-spiral arc tube in which a discharge path, centered on a midway portion thereof, turns in a double spiral substantially on one plane. 
     Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. 
     Therefore, unless such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.