1. Field of Invention
The present invention relates to a Mercury-free flat light source structure, and more particularly, to a Mercury-free flat light source structure capable of enhancing and adjusting brightness, maintaining stable and uniform discharge, and improving luminous efficiency, a large flat light source apparatus using the same Mercury-free flat light source structure as a unit cell in order to adjust brightness and cause local discharges in selective areas, and a driving method thereof.
2. Description of the Related Art
In general, a flat or planar light source apparatus has a wider range of applicability compared to a line light source apparatus as the back lights of passive type display apparatus such as liquid crystal display (LCD) units or lightings. However, there is a difficulty in constructing an optimized flat light source apparatus with sufficient luminance and luminous efficiency. For such a reason, a flat light source apparatus has been typically implemented by overlapping a plurality of linear light source apparatuses such as fluorescent lamp or light emitting diode (LED) and/or a plurality of point source apparatuses, and by using additional optical parts such as diffusion plate or reflection plate, thereby changing the linear light source and/or the point light source into a flat light source. This kind of flat light source apparatuses are disadvantageous in that light efficiency decreases greatly and manufacturing cost becomes higher due to a lot of additional parts required for converting the line and/or point light source apparatuses to a flat light source apparatus. Further, since the flat light source apparatus is implemented by assembling a plurality of light source apparatuses, it is difficult to partially emit light from selected areas or difficult to adjust their brightness. Accordingly, there is still a need to develop a flat light source apparatus capable of light-emitting over the entire area by itself with high luminance and luminous efficiency, capable of allowing active adjustment such as adjustment of brightness in a short time of one TV frame time (e.g. 16.67 milliseconds) of TV image signal, and capable of emitting lights with different brightness levels from selected areas.
Conventional flat light sources such as the fluorescent light sources used to use a Mercury-containing gas as the discharge gas. The reason of such is that the fluorescent light source with a Mercury containing gas shows excellent discharge characteristics and wide driving voltage margin. However, a light source based on Mercury discharge is disadvantageous in that Mercury is expected to be restricted in use for lightings because it is classified as an environmentally hazardous material, and the light source becomes difficult to operate and its luminous efficiency decreases when the lamp temperature becomes low, which necessitates a Mercury-free discharge gas. Accordingly there is still a need to develop a flat light source apparatus using a Mercury-free discharge gas.
FIG. 1 schematically illustrates a conventional flat light source using a Mercury-free discharge gas. A conventional flat light source 100 comprises an upper substrate 10, a lower substrate 20 and barrier ribs 30, and generates plasma in a discharge space defined by the upper substrate 10, the lower substrate 20 and the barrier ribs 30 when an appropriate voltage waveform is supplied to a pair of electrodes disposed near the discharge space to induce electric field. As the ultra violet (UV) rays emitted from the plasma excite the phosphors 18 and 24 coated on the inner-side the discharge space, the visible light rays 5 are emitted. Particularly, as shown in FIG. 1, the light source having electrodes X and Y covered with a dielectric layer 16 and disposed in the discharge space is called a dielectric barrier discharge (DBD) type, and the light source 100 is driven with driving waveforms applied to the electrodes X and Y where the polarities of the driving waveforms are changed periodically. FIG. 2 illustrates a typical driving waveform for driving the flat light source 100.
In order to manufacture the flat light source 100, the first barrier ribs 30 are formed between the upper substrate 10 and the lower substrate 20 which are typically made of glass or silica, then the upper substrate 10 and the lower substrate 20 are hermetically sealed to each other, the discharge space is evacuated, and a discharge gas is injected and finally sealed off. The electrodes X and Y are formed to have their ends protruded from the end of the light source 100 so as to be easily electrically connected to an external driving circuit.
Before the upper substrate 10 and the lower substrate 20 are put together, phosphor layers 18 and 24 are formed at proper positions. If it is necessary, a reflection layer 22 is formed so as for light to be emitted toward one direction. The discharge gas usually contains Xe, which emits vacuum ultra violet rays when excited. The discharge gas can be a mixture gas further containing various gases such as He, Ne, Ar, Kr, etc.
Here, a voltage pulse to be applied to the electrodes to initiate a discharge is determined mainly by the distance between the electrodes and the gas pressure. Given that a discharge gap, the distance between the electrodes of a flat light source is sufficiently long as much as a positive column area which is effective in a glow discharge is utilized enough, the discharge breakdown voltage is determined by the discharge gas pressure and composition as follows:
      V    f    =      Bpd                  ln        ⁡                  (          pd          )                    +              ln        (                  A                      ln            ⁡                          (                              1                +                                  1                  γ                                            )                                      )            
where A and B are constants determined by the kinds of gases, pd is a value of discharge gas pressure p multiplied by discharge gap (distance) d, and γ is the coefficient of secondary electrons generated by ions on the cathode surface.
FIGS. 3(a) through 3(e) illustrate the change of discharge status according to time in a conventional flat light source. If a voltage is applied between the electrodes protected by dielectric layers, a local discharge is started as shown in FIG. 3(a), initial discharge paths having an elongated band shape are then formed between the two electrodes as shown in FIG. 3(b) after a predetermined time in which the voltage is applied continuously. After that, referring to FIG. 3(c), if the applied voltage increases further, the discharge paths expand in a space between the electrodes in the vertical direction. As the discharge paths expand, referring to FIG. 3(d), the discharge paths are combined with neighboring discharge paths to fill the discharge space, thereby generating a discharge uniform over the entire area.
The discharge is typically generated by way of process steps including (a) inducement of electric field in the discharge space by application of a voltage to the electrodes, (b) acceleration of charged particles by the electric field, (c) generation of Townsend discharge, (d) progress that a neutral gas turns into plasma from an area where the density of charged particles is high, (e) formation of initial discharge paths according to the direction of the electric field, (f) acceleration of charged particles of the plasma toward opposite polarity electrodes, (g) formation of a wall potential by the charged particles accumulated on electrode surfaces after one cycle of a driving voltage signal, (h) formation of a wall voltage by the wall potential, (i) application of a voltage to the opposite electrode by a pulse with the reversed polarity, and (j) formation of the high electric field as the applied voltage is added to the wall voltage. Continuous polarity reversal of the applied voltage results in the stable, diffused, glow discharge.
However, the conventional flat light source apparatus as shown in FIG. 3(e) has a disadvantage that the discharge contraction (filamentation) is easily caused. For example, if the application power abruptly increases, if the discharge gas condition is not proper, or if non-uniformity is induced due to the structure of a discharge vessel, the discharge is concentrated partially as shown in FIG. 3(e), and for example, the abrupt increase of a discharge current takes place. In case that such a phenomenon appears, the brightness of the area where the discharge is concentrated abruptly enhances and a uniform brightness over the entire surface cannot be obtained. In the status of foregoing, if the application voltage increases, the width of a discharge area slightly widens but an abrupt increase of a discharge current is accompanied thereto. Consequently, it can be regarded as a discharge mode change, and such a local discharge concentration is thought to be caused by the plasma instability. The reasons thereof are diverse but the main reason is thought to be a non-uniform distribution of charged particles and thermal instability in the discharge space.
The local discharge concentration is caused by the procedure including the steps of (a) local increase of electron density, (b) local increase of resistive heating at an area where the electron density is high, (c) local increment of gas temperature, (d) reduction of neutral particle density due to the increase of gas temperature, (e) increase of electron temperature due to E/N (electric field to gas density ratio) enhancement at the corresponding area, and (f) further increase of electron density.
Repetition of the above procedures results in strong concentration of discharge along the line of electric force. If the discharge mode changes due to the discharge contraction, the current abruptly increases and the discharge is contracted, and as a result, a uniform, whole surface discharge cannot be obtained. According to the facts known recently, many factors affect the discharge concentration including the applied voltage, composition and partial pressure of discharge gases, the frequency and the duty ratio of a driving pulse applied, and the structural variable such as the cross-sectional shape of the discharge space.
In the above-described conventional flat light source, there exists a very narrow operational voltage margin or area where a discharge can be generated stably on the whole surface of a panel, avoiding the discharge contraction. In the respect of a voltage in the operational margin, the voltage is greater than the discharge breakdown voltage and less than the discharge contraction voltage (Vfiring<normal driving voltage<Vcontraction). On the other hand, in the respect of the composition of a discharge gas, there exists a content limit of a gas in determining the content of a specific gas (for example, content of Xe). In the respect of the discharge gas pressure, application voltage, frequency of the application voltage and duty ratio (Ton min<operation pulse width<Ton contraction), there is a certain operational range where the flat light source operates stably. Further, as the operational range becomes wider, the flat light source becomes more stable and efficient.
Further, a conventional flat light source includes the phosphor layers 24 and 18 on the surfaces of the lower substrate 20 and the upper substrate 10, respectively as shown in FIG. 1. The phosphor layers are formed on the upper substrate 10 as well as the lower substrate 20 to maximize the efficiency of use of the vacuum ultra violet rays since the vacuum ultra violet rays emitted from the plasma generated in the discharge space are emitted in all directions. However, in order to obtain high brightness and efficiency, the upper substrate 10 should be capable of self-emitting as well as transmitting the visible light emitted from the phosphor layer 24 on the lower substrate 20 with high transmittance. Accordingly, the thickness and structure of the phosphor layers should be optimized taking into account of the brightness and efficiency.
Further, the conventional flat light source 100 shown in FIG. 1 is disadvantageous in that the phosphor layers adhere very weekly to the upper and lower substrate and can be easily separated from the surface of the substrates.