Patent Publication Number: US-6664726-B2

Title: Fluorescent lamp with a by-pass means in the discharge space resulting in low starting voltage

Description:
BACKGROUND OF INVENTION 
     This invention relates to fluorescent lamps of the type comprising an elongated discharge vessel. The discharge vessel encloses a discharge path. The discharge vessel contains a gas fill which is excitable from a non-excited state into an excited state by a discharge arc in the operating state of the lamp. 
     The invention further relates to a method for starting a discharge arc through the elongated discharge path in the discharge vessel of such fluorescent lamp. 
     Low pressure discharge lamps are well known in the art. These lamps contain a gas fill which radiates UV light when excited by a discharge arc. The UV light is converted to visible light by a suitable light powder on the surfaces of the discharge vessel which is made by glass in most cases. The discharge arc is generated by a suitable voltage applied to a pair of electrodes at the two ends of the discharge path. The achieved light output is a function of the length of the discharge path, and it is sought to make the discharge path as long as possible. 
     However, a long discharge path requires a relatively high starting voltage between the electrodes of the lamp. In turn, the high voltage requires special electronic circuits because the high voltage must be applied only during the start-up phase of the discharge. As the discharge arc develops and the gas fill is excited, the overall impedance across the discharge arc drops, and a relatively low voltage level is sufficient to maintain the discharge process in the lamp. 
     Therefore, when the discharge across the discharge path has stabilized, the built-in electronics in the lamp detects the current level through the discharge path, and reduces the voltage applied to the electrodes. These electronics are not only expensive, but also bulky. The electronics system could be significantly simplified if the starting voltage across the electrodes of the discharge vessel could be lower. 
     A flat compact fluorescent lamp is disclosed in U.S. Pat. No. 5,767,618. This lamp contains a gas fill which is enclosed in a discharge vessel. A spiral-shaped discharge path is formed in the discharge vessel which latter is constituted by a bottom and top panel. The discharge path contains adjacent sections which are separated from each other by a convoluted wall. The wall is a part of the bottom panel, and an edge of the wall is in close proximity to the top panel, so a narrow gap exists between the wall and the top panel. It is recognized in U.S. Pat. No. 5,767,618 that arching may develop across the gap between the wall and the top panel. This arching is regarded as a negative effect. The U.S. Pat. No. 5,767,618 describes a lamp structure where the arching is suppressed. It is not recognized or implied that the cross-arching between the adjacent sections of the lamp could be put to use during the start-up phase of the lamp. 
     Therefore, there is a particular need for a method for starting the discharge arc in fluorescent lamps with a relatively reduced voltage, so that the electronics of the lamp could be made simpler and cheaper, or some parts of the electronics could be dispensed with completely. Also, there is a need for a fluorescent lamp which would require lower starting voltage, and which at the same time may be manufactured in a simple manner with existing technologies. 
     SUMMARY OF INVENTION 
     In an embodiment of the present invention, there is provided a fluorescent lamp comprising an elongated discharge vessel which encloses a discharge path. The discharge vessel contains a gas fill. The gas fill is excitable from a non-excited state into an excited state by a discharge arc in the operating state of the lamp. The discharge path has a first impedance in a non-excited state. The gas fill is of the type where the impedance of the gas is lower in the excited state than in the non-excited state. The discharge vessel has at least two sections located adjacent to each other, and electrode means for generating a discharge arc across the discharge path in the discharge vessel. There is bypass means for providing a bypass path for the gas discharge during a startup of the lamp between the two adjacent sections of the discharge vessel. The bypass path results in a short-cut across the impedance of at least a portion of the discharge path when the gas in said portion is in the non-excited state. 
     According to another embodiment of the invention, there is provided a method for starting a discharge arc through an elongated discharge path in a discharge vessel of a fluorescent lamp, where the discharge vessel contains a gas fill which is excitable from a non-excited state into an excited state by the discharge arc in the operating state of the lamp. The method is applicable with such gas fills where the impedance of the gas fill is lower in the excited state than in the non-excited state. The method comprises the following steps. A voltage is applied between two electrodes across the discharge path in the discharge vessel, where the gas fill in the discharge path between the electrodes has a first impedance in a non-excited state. A bypass path is provided between two ends of a bypassed part of the discharge path, thereby dividing at least a portion of the discharge path into a bypassed part and at least one remaining part. The combined impedance of the bypass path and the associated bypassed part is selected to be lower than the impedance of the associated bypassed part of the elongated discharge vessel, when the gas in at least a part of said bypassed part is in the non-excited state. Thereby a relatively increased voltage is provided across the remaining part of the discharge path. The gas fill in the remaining part of the discharge path is excited into the excited state with the help of the increased voltage across the remaining part. As a result, the impedance of the remaining part is lowered and the voltage across the bypassed part is relatively increased. Finally, the gas fill is excited in at least a part of the bypassed part by the relatively increased voltage across the bypassed part. 
     The suggested fluorescent lamp thus requires a lower starting voltage on the electrodes because the available starting voltage needs not generate the discharge arc across the full length of the discharge path. Instead, the discharge path is effectively divided into shorter sections which are started after each other. The shorter sections have a lower impedance, and may be excited by a lower starting voltage. 
     The method and the fluorescent lamp implementing the method ensures a gradual, softer startup of the discharge arc, while the lamp may be manufactured at a lower cost. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The invention will now be described with reference to the enclosed drawings. 
     FIG. 1 is a side view of the discharge tube having four parallel, vertically oriented discharge tube sections. 
     FIG. 2 is a schematic folded-out view of the discharge tube of the discharge lamp shown in FIG.  1 . 
     FIG. 3 illustrates the equivalent impedance circuit of the discharge tube shown in FIG.  2 . 
     FIG. 4 is a perspective view of a flat compact fluorescent lamp with a plate-shaped discharge vessel having a planar double spiral shaped discharge path. 
     FIG. 5 is a cross-section of the discharge vessel of the lamp shown in FIG. 4, taken along the plane V—V. 
     FIG. 6 is a partial cross-section of the discharge vessel of the lamp shown in FIG. 4, taken along the plane VI—VI of FIG.  5 . 
     FIG. 7 is a perspective view of a wall section in the discharge vessel of the lamp shown in FIG. 4, with a bypass opening across the wall section. 
     FIG. 8 is a partial cross-section of the wall section shown in FIG. 7, taken along the plane VIII—VIII of FIG.  7 . 
     FIG. 9 illustrates the equivalent impedance circuit of the discharge vessel shown in FIG.  5 . 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIGS. 1 and 2, there is shown a low pressure arc discharge lamp  1 . The lamp  1  has a discharge vessel in the form of a discharge tube  2 , the ends  31  of which are inserted into a lamp housing  4  or base terminal. The lamp  1  of FIG. 1 has four straight discharge tube sections  21 ,  22 ,  23  and  24 , which are interconnected through bent sections  32 ,  33 ,  34  at the upper and lower ends of the tube sections  21 - 24 . It is noted that the proportions of FIG. 3 are not to scale, and the straight tube sections  21 - 24  are normally longer than they appear in FIG. 2, and in this respect FIG. 2 is a schematic drawing only, illustrating the operating principle of the discharge tube  2 . 
     The discharge tube  2  is mechanically supported by the lamp housing  4 . The lamp housing  4  surrounds the ends  31  of the discharge tube  2 . More precisely, the sealed ends  31  of the tube sections  21 ,  24  are within the lamp housing  4 while the major part of the tube sections  21 - 24  is external to the lamp housing  4 . Electrodes  41 ,  42  are placed in the discharge tube  2  at the ends  31 . The electrodes  41 ,  42  act as means for generating a discharge arc across the discharge path in the discharge tube  2 . The lamp  1  is of a type where light is emitted by a phosphor layer deposited on the inner surface of the discharge tube  2 . Such a discharge lamp arrangement is known by itself. In a typical embodiment, the lamp housing  4  is equipped with a screw terminal  8  which fits into a standard screw socket (not shown). 
     In this manner, the fluorescent lamp  1  comprises an elongated discharge vessel enclosing a discharge path. The discharge tube  2  constituting the discharge vessel contains a gas fill the composition of which is known per se. In the operating state of the lamp, the gas fill is excited from a non-excited state into an excited state by a discharge arc sustained by the electrodes  41 ,  42  in the ends  31  of the tube  2 . During an initial stage of the excitation, the gas is ionized and the number of charge carriers in the gas increases. When density of the charge carriers reaches a certain threshold, the ionization suddenly increases through collisions between the gas particles which results in an increased current through the gas, though the voltage across the discharge path does not increase. When this state has been reached, the increased discharge current across the discharge path remains even if the voltage across the discharge path is lowered. The discharge vessel behaves initially as an Ohmic impedance and the discharge path has a first impedance in a non-excited state. But, as the discharge current increases without the corresponding increase of the electrode voltage, it may be also interpreted so that the impedance of the gas is lower in the excited state than in the non-excited state. 
     As best seen in FIG. 2, the tube sections  21  and  22  of the discharge tube  2  are located adjacent to each other. Bypass means are inserted between the tube section  21  and  22 . In the lamp  1  shown in FIG. 1, the bypass means are realized as a lead-through connection  45  between the tube sections  21  and  22  of the discharge tube  2 . The lead-through connection  45  provides a bypass path for the gas discharge during a startup of the lamp  1  between the two adjacent tube sections  21  and  22  of the discharge tube. The startup of the lamp  1  is the short time interval between the switching on of the electric power to the lamp and the time when the discharge arc in the discharge vessel has stabilized. The bypass path through the lead-through connection  45  results in a short-cut across the impedance of that portion of the discharge path which is constituted by the tube section  21 , the bent section  32  and the tube section  22 . As will be explained below, this short-cut is effective only when the gas in the relevant portion of the discharge path is in the non-excited state. 
     Due to the bypass path, the discharge arc in the discharge tube  2  may be started with a lower starting voltage applied to the electrodes  41 ,  42 . This is explained with reference to FIGS. 2 and 3. 
     When the discharge arc is to be generated, a starting voltage Us is applied between the two electrodes  41 ,  42 . Apparently, this starting voltage Us is effective across the discharge path in the discharge tube  2 . The discharge path between the electrodes  41 ,  42  has a certain first impedance in a non-excited state of the gas fill. This impedance is a sum of the impedances of the straight tube sections  21 - 24  and the bent tube sections  32 - 34 . The total impedance of the tube  2  may be considered as the sum of impedances Z 1  and Z 2  connected in series as shown in FIG. 3 where impedance Z 1  is a sum of the impedance of tube sections  21 ,  32  and  22 , while impedance Z 2  is a sum of the impedance of tube sections  34 ,  23 ,  33  and  24 . As mentioned above, the lead-through connection  45  behaves as a bypass path between two ends of a bypassed part of the discharge path in the discharge tube  2 . Apparently, the bypassed part of the discharge path corresponds to the tube sections  21 ,  32  and  22  in FIG. 2 because these sections are bypassed by the lead-through connection  45 . In this manner, a portion of the discharge path in the discharge tube  2  is divided into a bypassed part—the tube sections  21 ,  32  and  22 —and a remaining part, namely tube sections  34 ,  23 ,  33  and  24 . The impedance of the bypass path, i.e. that of the lead-through connection  45 , is denoted as the impedance B and the total impedance of the discharge tube  2  may be treated as the impedance circuit shown in FIG.  3 . 
     Now the combined impedance of the bypass path and the associated bypassed part is selected to be lower than the impedance of the associated bypassed part. With other words, the value of the impedance B is selected so that the impedance between the nodes  51  and  52  is lower than the impedance Z 1  itself. Though this will be true for most values of the impedance B when the gas in at least a part of the bypassed part is in the non-excited state, it is preferred that the impedance of the bypass path, i.e. the value of the impedance B is selected to be substantially lower than the impedance of the associated bypassed part, i.e. the impedance Z 1  . 
     As a result of the lower impedance between the nodes  51  and  52 , the proportion of the starting voltage Us falling on the impedance Z 2  between the nodes  52  and  53  will increase. As explained above, the impedance Z 2  equals the impedance of the remaining part of the discharge path in the discharge tube  2 , corresponding to the tube sections  34 ,  23 ,  33  and  24 . With other words, a relatively increased voltage appears across the remaining part of the discharge path. 
     If the impedance B of the bypass is much lower than the impedance Z 2 , practically the total starting voltage Us will fall across the impedance Z 2 . If the impedances Z 1  and Z 2  are approximately equal, it means that the starting voltage falling on the tube sections  34 ,  23 ,  33  and  24  has doubled as a result of the bypass. 
     The gas fill in the remaining part of the discharge path is subsequently excited into the excited state with the help of the increased starting voltage across the remaining part. With other words, a discharge arc is generated in the remaining part of the discharge path, i.e. through the tube sections  34 ,  23 ,  33  and  24 . The discharge will have initially a limited current, the limitation mainly caused by the physical dimensions of the bypass, i.e. the lead-through connection  45 . 
     However, as the discharge arc in the tube sections  34 ,  23 ,  33  and  24  develops, the impedance of the remaining part decreases due to the physical processes described above. This decrease of the impedance Z 2  between the nodes  52  and  53  will result in a decrease of the voltage proportion falling on the impedance Z 2 , i.e. the impedance of the remaining part. As the voltage across the remaining part, i.e. the tube sections  34 ,  23 ,  33  and  24  decreases, the proportion of the starting voltage Us across the bypassed part will relatively increase, i.e. the tube sections  21 ,  32  and  22  will be subjected to an increasing proportion of the starting voltage Us. 
     The impedance of the tube sections  34 ,  23 ,  33  and  24  decreases quite significantly when the discharge arc is established and the discharge current starts to flow through the tube sections  34 ,  23 ,  33  and  24 . The decrease in impedance may be several orders of magnitude. This means that practically the total starting voltage Us will now fall across the bypassed part. 
     Finally, the gas fill will be excited into the excited state by the increased voltage in the bypassed part as well. This means that the discharge arc will be established in the complete discharge path in the discharge tube  2 . As the current limiting effect caused by the bypass disappears, the discharge current increases until it is again limited by density of the charge carriers, the dimensions of the discharge tube and the exciting voltage across the electrodes  41  and  42 . With appropriate dimensioning of the bypass and the discharge tube, the impedance B of the bypass may be selected to be larger than the impedance Z 1  of the bypassed part when the gas is in the excited state, so that a major part of the discharge current will be conducted over the bypassed part when the discharge arc have been generated in the bypassed part as well. 
     When the startup of the lamp is thus completed, the discharge through the lamp is largely unaffected by the presence of the bypass. A small portion of the discharge current may flow through the bypass, but the light output of the lamp will be generated by the majority of the current flowing through the main discharge path. The narrow dimensions of the bypass will act as a current limiter, which will in effect result in an increased impedance B of the bypass. Thus as the discharge current through the tube sections  21 - 24  and  32 - 34  gradually increases until a stationary discharge is reached, simultaneously the bypass current through the bypass path decreases and stabilizes on a low level. 
     In this manner, it is seen that the value of the starting voltage Us need not be larger than the voltage which is necessary for starting a discharge arc through a part of the discharge path only. In the above example, the starting voltage Us is only slightly higher than half of the voltage which otherwise would be needed to start the lamp  1  without the bypass. 
     Another embodiment of a compact fluorescent lamp with a bypassed discharge vessel will be now explained with reference to FIGS. 4 to  9 . 
     FIG. 4 depicts a so-called flat compact fluorescent lamp  100 . The lamp  100  has a substantially disk shaped lamp head  102 . The lamp head  102  comprises the discharge vessel. The discharge vessel of the lamp  100  is constituted by a flat double spiral shaped discharge channel. The spiral shaped discharge channel is formed by two convoluted walls  111 ,  112  between a top panel  105  and bottom panel  104 . At the periphery of the panels  104 ,  105 , an external ring wall  113  encloses the discharge volume, and the ends  131 ,  132  of the discharge channel are closed by end wall sections  114 ,  115 . The convoluted walls  111 ,  112 , the ring wall  113  and the end wall sections  114 ,  115  are integral with one of the panels, in the shown embodiment with the bottom panel  104 . 
     The top and bottom panels  104 ,  105  are bonded to each other along a sealing  116 . The sealing  116  also provides a gas-tight sealing of the discharge vessel. Such a sealing may be provided between walls  111 ,  112  and the top panel  105  as well. 
     Electrodes  141 ,  142  are located at the ends  131 ,  132  of the discharge channel. As best seen in FIG. 5, due to the spiral form of the discharge channel, several sections of the discharge vessel are located adjacent to each other, separated by the walls  111 ,  112  only. 
     In the discharge vessel of the lamp head  102 , the bypass means between the adjacent sections of the discharge channel are formed as openings  151 - 154  in the convoluted walls  111 ,  112 . As will be shown below, these openings function as a bypass path for the gas discharge between two adjacent sections of the discharge vessel during a startup of the lamp. The bypass path created by the openings  151 - 154  results in a short-cut across the impedance of certain a portions of the discharge path, when the gas in these portions is in the non-excited state. 
     As it will be apparent from the explanation of FIG. 9, with the spiral-shaped discharge vessel of the lamp  100 , certain bypassed parts of the discharge path themselves comprise further bypassed parts and associated bypass paths. For example, as illustrated in FIG. 5, the complete double spiral shaped discharge channel of the lamp head  102  may be divided into five channel sections  121 - 125  by the bypass openings  151 - 154 . The first channel section  121  extends from the first electrode  141  until the bypass openings  153  and  154 . The second channel section  122  extends from the bypass openings  153  and  154  until the bypass opening  152 . The third, central channel section  123  extends from the bypass openings  152  until the bypass opening  153 . The fourth channel section  124  extends from the bypass opening  153  until the bypass openings  151  and  152 . Finally, the fifth channel section  125  extends from the bypass openings  151  and  152  until the second electrode  142 . The impedances of the channel sections  121 - 125  are represented by the impedances R 1 -R 5 , respectively, and the impedances of the bypass openings  151 - 154  are represented by the impedances B 1 -B 4 , respectively. As best perceived from FIG. 9, the impedance B 1  corresponding to bypass opening  151  bypasses the impedances R 1 -R 4  which in turn correspond to the channel sections  121 - 124 . At the same time, channel sections  122  and  123  are themselves bypassed by the bypass opening  153 . With other words, the sections of the discharge vessel are arranged adjacent each other, and multiple bypass openings  151 - 154  acting as bypass paths are provided between the adjacent sections. 
     The impedance of the bypass openings  151 - 154  is influenced by the dimensions of opening between the adjacent discharge channel sections  121 - 124 . Typically, the width D of the walls  111 ,  112  may be between 0.5-2 mm, while the width d of the openings  151 - 154  is preferably not larger than 0.4 mm. Typically, the width D of the walls  111 ,  112  is less than five times the width d of the openings  151 - 154 . The openings  151 - 154  have an almost square area, and they are conveniently as an incision in the walls  111 ,  112 , as best seen in FIG.  1 where the shape of the incision forming the opening  152  is shown in perspective. In order to direct the discharge current into the discharge channel instead of the bypass, the area Ad of the cross-section of the discharge path preferably is at least ten times the area Ab of the cross-section of the bypass openings  151 - 154 . Other cross-section proportions are also suitable for limiting the discharge current in the discharge channel. 
     The discharge vessel in the lamp head  102  also contains a gas fill which is excitable from a non-excited state into an excited state by the discharge arc in the operating state of the lamp  100 . This gas fill has similar properties as the known gas fills for fluorescent lamps, and particularly, the impedance of the gas fill is lower in the excited state than in the non-excited state. Therefore, the starting of the lamp  100  may be also performed with a lower starting voltage Us applied to the electrodes  141 ,  142 , as compared with a similar flat compact lamp without the bypass openings of the invention. 
     The startup process of the lamp  100  will be explained with reference to FIG.  9 . Firstly, the starting voltage Us is applied between the two electrodes  141 ,  42  across the discharge path in the spiral-shaped discharge channel of the lamp head  102 . The total impedance of the discharge channel corresponds to the combined impedance of the serially connected impedances R 1 -R 5  of FIG.  9 . The bypass openings  151 - 154  act as the bypass impedances B 1 -B 4 . For example, the impedance B 4  provides a bypass path between the nodes  162  and  163 . As apparent from FIG. 9, nodes  162 ,  163  are the two ends of a bypassed part of the circuit, where the bypassed part contains the impedance R 5  connected in series with the circuit between the nodes  163  and  164 . 
     As explained above, the channel sections  121 - 125  divide the elongated discharge vessel of the lamp head  102  into multiple sections. As it is best seen in FIG. 9, the channel sections  121 - 125  have impedances in a non-excited state equaling a fraction of the total impedance over the discharge path, because the impedances R 1 -R 5  are connected in series. Accordingly, only a fraction of the starting voltage Us would fall on each channel sections  121 - 125  in the absence of the bypass openings. Assuming the impedance of the channel sections  121 - 125  to be largely equal, only about one fifth of the starting voltage Us would fall across each channel section. 
     However, the bypass openings  151 - 154  divide portions of the discharge path into bypassed parts and associated remaining parts. Four such bypasses may be identified in FIG.  9 . The combined impedance of each of the bypass paths and the associated bypassed parts is selected to be lower than the impedance of the respective associated bypassed part, when the gas in at least a part of the corresponding bypassed part is in the non-excited state. For example, the combined impedance of the bypass impedance B 1  and the impedance of the circuit between the nodes  162  and  163  is certainly lower than the impedance of the circuit between the nodes  162  and  163  by itself, for any finite value of the impedance B 4 , since the impedance B 4  and the circuit between the nodes  162  and  163  are connected in parallel. 
     In most cases, the impedance of the bypass path, i.e. the value of the bypass impedance B 4  itself is selected to be lower than the impedance of the associated bypassed part. In a practical realization, the impedance of a channel section is in the order of 100 Mohm, which falls to approx. 20 ohm when the discharge arc develops in the channel section. The value of the bypass impedances B 1 -B 4  is also in the order of 100 Mohm when the gas fill is in the non-excited state, and the final impedance of the bypass is in the order of 200 ohm when the discharge current through the bypass has stabilized. In the present example, the impedance of the bypassed part corresponds to the impedance of the circuit between the nodes  162  and  163 , when the gas in the bypassed part is in the non-excited state. 
     It may be practical if the ratio between the impedance of the bypassed part of the discharge path and the impedance of the associated bypass path in the non-excited state of the gas fill is selected to be between 1 and 10. In this manner, assuming the value of the bypass impedances B 1 -B 4  to be at least an order of magnitude lower than the value of the impedances R 1 -R 5 , the total impedance of the circuit between the nodes  163  and  162  will be much larger than the value of impedance B 4 , as it is apparent from the layout of the circuit in FIG.  9 . With other words, the impedance between the nodes  163  and  162  will be determined by the value of the bypass impedance B 4 , which, as mentioned above, is an order of magnitude-smaller than the impedance R 1 . 
     As seen in FIG. 9, impedance R 1  is connected in series with the bypass impedance B 4 . Accordingly, a major portion of the starting voltage Us will fall between the nodes  161  and  163 . This will result in a relatively increased starting voltage Us across the remaining part of the circuit, i.e. the impedance R 1 . With other words, instead of a fraction of the starting voltage Us only, almost the total starting voltage Us will fall across the impedance R 1 . Since the impedance R 1  corresponds to the first channel section  121 , the gas fill will be excited in this channel section  121  into the excited state with the help of the starting voltage Us, and a discharge arc will develop in the channel section  121 . 
     Simultaneously, the discharge arc will also develop in the channel section  125 , because the same considerations apply to the impedance R 5  and the bypass impedance B 1  as above, these impedances being symmetrically situated relative to the impedances R 1  and B 4  in the circuit shown in FIG.  9 . As a consequence, the impedance of the remaining parts of the discharge channel, i.e. the impedance of the channel sections  121  and  125  decreases. This, in turn, relatively increases the voltage across the bypassed part. Clearly, as the impedances R 1  and R 5  diminish in value, the starting voltage Us will appear on the nodes  163  and  164 . 
     Now the same considerations may be repeated with respect to the impedances R 4 , R 3  and R 2 . These are bypassed by the bypass impedances B 2  and B 3 , respectively. As B 3  bypasses the impedances R 2  and R 3 , the starting voltage Us on the nodes  163  and  164  will appear between the nodes  165  and  164 , i.e. the starting voltage Us will fall on the impedance R 4 , and, due to the symmetry of the circuit, also on the impedance R 2 . Accordingly, the corresponding channel sections  124  and  122  will be excited by the starting voltage Us, and the discharge arc develops in the channel sections  124  and  122  as well. 
     As the discharge current starts to flow through the channel sections  124  and  122 , their corresponding impedances R 2  and R 4  also decrease, and the starting voltage Us will now fall on the last remaining channel section  123 . Finally, the gas fill is excited by the starting voltage Us and the discharge current flows in all sections of the discharge vessel. 
     It is noted that in this final stage, if the bypass impedances B 2 , B 3  are much smaller than the impedances R 2 -R 4 , almost the total starting voltage Us falls on the central impedance R 3  as well, and this means that the central channel section  123  may be excited almost at the same time or even before as the channel sections  122  and  124 . However, in order to ensure that channel sections  122  and  124  excite before central channel section  123 , the impedance of the central channel section  123 , i.e. the value of the impedance R 3  should be at least as large as the impedances R 2 -R 4 . In this manner, the voltage falling on the impedances R 2 -R 4  will be larger than the voltage falling on the impedance R 3 , and therefore the discharge arc will develop in the channel sections  122  and  124  before developing in the central channel section  123 . If the value of the bypass impedances B 2 , B 3  is negligible relative to the impedances R 2  and R 4 , almost the total starting voltage Us is utilised for exciting the channel sections  122 - 124 , and subsequently the channel section  123 . In this manner, it is understood that the method significantly reduces the necessary voltage for the starting of the discharge arc in the discharge vessel. 
     As in the above example, the current across the bypass paths, i.e. the discharge current across the bypass openings  151 - 154 , is limited by the effective area of the cross-section of the bypass path. For example, the area Ad of the cross section of the discharge channel may be approx. 80 mm 2 , while the area Ab of the cross section of the bypass opening may be approx. 8 mm 2 . The ratio between the impedance of a bypassed part of the discharge path and the impedance of the associated bypass path in the excited state of the gas fill may be between 0.01 and 0.1. Therefore, as the current increases in the channel sections  121 - 125 , the value of the bypass impedances B 1 -B 4  will relatively increase. This impedance increase becomes significant when all sections of the discharge channels are excited, and the discharge current starts flowing unhindered between the electrodes  141 ,  142  through the full length of the discharge channel. Therefore, when the discharge arc has stabilized, only a relatively small part of the discharge current will flow through the bypass openings. 
     It is clear for those skilled in the art that the same principle may be applied to discharge vessels with even more spiral turns, and with similar bypass paths between the turns of the spiral, and an even more significant reduction in the starting voltage may be achieved. 
     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. For example, the proposed discharge arc starting method and the bypassed discharge vessel is applicable not only with straight or flat compact fluorescent lamps, but also with other types of discharge lamps having different coil-like or arbitrary other shapes, as long as a suitable bypass can be realized between adjacent sections of the discharge vessel of the lamp.