1.  Field of the Invention
The present invention relates to a flat fluorescent lamp having a plurality of serpentine shaped discharge channels, and more particularly, to a flat fluorescent lamp for minimizing a deviation of discharge between discharge channels, thereby reducing a discharge initiation voltage, and enhancing luminance uniformity. 
2.  Description of the Related Art
In general, a liquid crystal display (LCD) among flat display devices employs a backlight source unit such as a cold cathode fluorescent lamp (CCFL), an external electrode fluorescent lamp (EFFL), a flat fluorescent lamp (FFL), and a light emitting diode (LED). 
The CCFL already tested in reliability for a long time is being much used for a thin film transistor LCD (TFT LCD). 
As a backlight type using the CCFL, there are a direct type CCFL and an edge type CCFL. 
The direct type CCFL uses tens of lamps.  Therefore, it has a drawback in that the used lamps become an obstacle in securing a reliability of the LCD and increase an assembly cost, thereby deteriorating economy. 
The edge type CCFL is disposed at an end of a light guide plate and irradiates light.  Therefore, it has a limitation in providing a necessary luminance to a large-sized liquid crystal display panel since the number of lamps used is limited. 
Because of the above drawback of the CCFL, in recent years, it is being positively considered that the flat fluorescent lamp is employed as the backlight unit.  The flat fluorescent lamp is advantageous of simultaneously satisfying a luminance and a reliability of lamp while improving an optic efficiency and reducing a manufacture cost of a liquid crystal display apparatus. 
In general, the flat fluorescent lamp is classified as a CCFL type flat fluorescent lamp and an EEFL type flat fluorescent lamp. 
In the CCFL type flat fluorescent lamp, discharge channels are all partitioned using a barrier rib while being extended as one serpentine shaped channel, and internal electrodes are disposed to face at a start and an end of the discharge channel, and a phosphorous film is coated within the long discharge channel. 
However, the CCFL type flat fluorescent lamp requires a high discharge initiation voltage in proportion to a length of the discharge channel due to the very long discharge channel.  In other words, the CCFL type flat fluorescent lamp requires a high voltage of tens of kilo volts for lighting, thereby increasing an output voltage of an inverter and causing a power loss resulting from an interference of electromagnetic wave and a leakage voltage.  Due to this drawback of the CCFL type flat fluorescent lamp, it is difficult to use at a home a liquid crystal display apparatus employing the CCFL type flat fluorescent lamp serving as the backlight unit. 
On the contrary, in the EEFL type flat fluorescent lamp, an electrode is positioned outside both ends of a glass substrate including a discharge channel, thereby making it possible to perform a discharge at a shorter distance than in the CCFL type flat fluorescent lamp.  Therefore, the EEFL type flat fluorescent lamp can perform the discharge at a low voltage to seek a stable discharge.  Further, in the EEFL type flat fluorescent lamp, the electrode can be also installed with a great convenience. 
However, in the EEFL type flat fluorescent lamp using the external electrode, a wider electrode area should be secured to allow flow of a sufficient current, thereby providing a desired luminance.  Therefore, a dead space of the lamp is increased, thereby deteriorating an appearance of the lamp.  Further, the EEFL type flat fluorescent lamp includes a plurality of horizontal discharge channels.  Therefore, the EEFL type flat fluorescent lamp has a drawback in that an excessive power is consumed to obtain a proper current density at each discharge channel.  Further, in case where the discharge channel is reduced in section area to obtain the proper current density, the discharge channel is increased in number and a barrier rib is increased in width.  If the number of the discharge channels is increased as above, power consumption is increased, and if the barrier rib is increased in width, a dark region caused by the barrier rib is increased.  Further, in order to remove the dark region, diffusion plates should be spaced apart at an upper end of the lamp and therefore, there occurs a serious drawback of increasing a thickness of a backlight unit. 
This applicant has ever made various efforts to solve the above drawback of a reduction of efficiency of a surface discharge type flat fluorescent lamp.  As a result, this applicant has filed applications for inventions relating with the flat fluorescent lamp, such as Korean Patent Application No.  1020040005829 (2004 Jul. 26) entitled “flat fluorescent lamp with improved discharge efficiency”, Korean Patent Application No.  1020040058291 (2004 Jul. 26) entitled “flat fluorescent lamp with improved discharge efficiency”, Korean Patent Application No.  1020040072846 (2004 Sep. 11) entitled “thin flat fluorescent lamp”, and has ever proposed a structure employing a plurality of respective independent serpentine-shaped discharge channels by improving the EEFL type flat fluorescent lamp, for increasing a current density of a discharge channel to improve an efficiency of discharge and a luminance, improving an electrode structure to reduce a discharge initiation voltage, and solving a drawback of a non-emission region caused by an external electrode through a design of an electrode space having a greater width than the discharge channel. 
Hereinafter, a construction of the “flat fluorescent lamp with improved discharge efficiency” will be described with reference to the drawing. 
FIG.  1 is a perspective view illustrating a construction of the conventional flat fluorescent lamp having a serpentine shaped discharge channel. 
Referring to FIG.  1, the flat fluorescent lamp includes a front substrate 10 and a rear substrate 12. 
The front substrate 10 includes a power supply unit 50, and two external electrodes, that is, a first external electrode 42 and a second external electrode 44 connected with the power supply unit 50. 
The rear substrate 12 includes two external electrodes, that is, a third external electrode 46 and a fourth external electrode 48, a sidewall 14, a barrier rib 16, a discharge channel 20, an exhaust channel 22, a connection unit 24, and a frit glass 340. 
The front substrate 10 and the rear substrate 12 are coupled by the sidewall 14 formed at an end of the rear substrate 12 as shown in FIG.  1. 
A reflective layer (not shown) such as Al2O3 can be coated under the rear substrate 12. 
The discharge channel 20 and the exhaust channel 22 are defined using the sidewall 14 and the barrier rib 16, and the front substrate 10 are adhered onto upper surfaces of the sidewall 14 and the barrier rib 16. 
For description convenience, a first barrier rib 16-1 denotes a long-axis barrier rib for forming the barrier rib 16 in a serpentine shape, and a second barrier rib 16-2 denotes a short-axis barrier rib. 
The discharge channel 20 has a serpentine shape that is defined by the sidewall 14 serving as a frame of the flat fluorescent lamp, and a plurality of the barrier ribs 16 that are comprised of the first barrier ribs 16-1 perpendicularly alternately connected to the upper/lower sidewall 14, and the second barrier ribs 16-2 each integrated with and perpendicularly alternately connected to both of the first barrier ribs 16-1 in an opposite direction to be spaced apart from each other.  The serpentine shaped discharge channel 20 is connected at its end with the vertical exhaust channel 22 arranged at the sidewall 14, through the connection unit 24.  The respective ends of the discharge channels 20 are used as electrode spaces of the exhaust channels 22 arranged in mutually opposite directions. 
In detail, the first and second external electrodes 42 and 44 are disposed outside the front substrate 10, and the third and fourth external electrodes 46 and 48 are disposed outside the rear substrate 12 to have a band shape.  The first external electrode 42 of the front substrate 10 and the third external electrode 46 of the rear substrate 12 are bound together, and the second external electrode of the front substrate 10 and the fourth external electrode 48 of the rear substrate 12 are bound together, to receive an alternating current from the power supply unit 50 and alternately generating a discharge initiation voltage depending on a frequency of the alternating current source.  In other words, at both ends of the discharge channels, that is, at ends of the discharge channels disposed closely to the external electrodes 42 and 46, and 44 and 48, the discharge initiation voltage is generated depending on the frequency of the alternating current source.  However, as going to a center of the discharge channel, the discharge initiation voltage gets lower.  In other words, the discharge channel has a greater discharge initiation voltage at both ends than at a center. 
As described above, in the conventional flat fluorescent lamp, the plurality of serpentine shaped discharge channels arranged within the lamp have the greater discharge initiation voltage, as going to the both ends, than at the center.  Therefore, the conventional flat fluorescent lamp has a drawback of locally decreasing brightness as going to the both ends. 
Accordingly, in the conventional flat fluorescent lamp, the greater discharge initiation voltage than in a normal condition should be applied in order to obtain the stable discharge.  Therefore, there is a drawback of complicating a circuit construction and having a difficulty in obtaining greater luminance uniformity.