Plasma flat lamp

A plasma flat lamp includes an upper plate, a lower plate separated a predetermined distance from the upper plate, a wall portion for forming a sealed discharge space between the upper and lower plates, a discharge gas filled in the discharge space, a first pair of electrodes including a first upper plate electrode and a first lower plate electrode arranged to face each other on each of the upper and lower plates with the discharge space interposed therebetween, and a second pair of electrodes including a second upper plate electrode separated a predetermined distance from the first upper plate electrode and a second lower plate electrode separated a predetermined distance from the first lower plate electrode arranged to face each other on each of the upper and lower plates with the discharge space interposed therebetween. Thus, the plasma flat lamp according to the present invention has a stable discharge feature which is a merit of the conventional electrodes discharge flat lamp and a high luminance of light emission which is a feature of the facing surfaces discharging type, while not having a low luminance and unstable discharge feature of the conventional surface discharge type flat lamp and facing electrodes discharging flat lamp, respectively.

BACKGROUND OF THE INVENTION

This application claims the priority of Korean Patent Application No. 2001-73017 filed 22, Nov. 2001, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.

1. Field of the Invention

The present invention relates to a plasma flat lamp, and more particularly, to a plasma flat lamp having a high luminance, a high efficiency of light emission, and a uniform distribution of luminance.

2. Description of the Related Art

Flat lamps mainly used as back-lights of LCDs have been developed into surface discharge type or facing electrodes discharge type plasma lamps, in which the entire space under a light emitting surface makes a discharge space considering the efficiency of light emission and uniformity of luminance of light emission, from the conventional edge-light or direct-light type plasma lamps using a cold cathode fluorescent lamp.

In general, the surface discharge type is advantageous in that a discharge feature is stable compared to the facing electrodes discharge type. However, the overall luminance of the surface discharge type is lower than that of the facing electrodes discharge type. As an example of a conventional surface discharge flat lamp, there is a lamp in which the overall discharge space is divided into fine discharge areas to prevent local concentration of discharge. This lamp can discharge stably. However, since the uniformity of the overall luminance of light emission is inferior due to a difference in the luminance of light emission between the fine discharge areas and a gap between the fine discharge areas, a diffusing paper or diffusing plate is needed to uniformly diffuse light (M. Ilmer et al., Society for Information Display International Symposium Digest of Technical Papers 31, 931 (2000)).

FIG. 1shows another example of the conventional surface discharge type flat lamp. A discharge space filled with a discharge gas is formed between an upper plate1and a lower plate2separated by a wall portion7. Electrodes3and4for discharge are formed at both sides of an inner surface of the lower plate2. A dielectric layer5is formed on each of the electrodes3and4. A fluorescent layer6is formed on each of the inner surfaces of the upper and lower plates1and2. The conventional surface discharge type flat lamp having the above structure has been known to have a low luminance due to its discharge characteristic (Y. Ikeda et al., Society for Information Display International Symposium Digest of Technical Papers 31, 938 (2000)).

FIG. 2shows an example of the conventional facing electrodes discharge type flat lamp. An upper plate1aand a lower plate2aare separated a predetermined distance from each other by a wall portion7aand a discharge space is provided therebetween. Electrodes3aand4afor discharge are formed on inner surfaces of the upper and lower plates1aand2ato face each other. A fluorescent layer6ais formed on each of the electrodes3aand4a. The upper and lower plates facing electrodes discharge type flat lamp having the above structure has a high luminance compared with the surface discharge type flat lamp shown inFIG. 1, but has a low efficiency of discharge and an unstable discharge due to excessive current (J. Y. Choi et al., Proceedings of the 1stInternational Display Manufacturing Conference, 21 (2000)).

FIG. 3shows yet another example of the conventional facing electrodes discharge type flat lamp. Electrodes3band4bare formed to face each other on inner surfaces of a wall portion7bfacing each other. Each of the electrodes3band4bis protected by a dielectric layer5b. Upper and lower plates1band2bare separated by the wall portion7b. A discharge space in which a discharge between the electrodes3band4bis induced is provided between the electrodes3band4b. A fluorescent layer6bis formed on each of inner surfaces of the upper and lower plates1band2b. The side wall portion facing electrodes discharge flat lamp having the above structure can prevent excessive current. However, discharge is unstable, in particular, discharge tends to concentrate on a local point.

As described above, in the conventional flat lamps, it is disadvantageous that either luminance is low while discharge is stable or luminance is high while discharge is unstable.

SUMMARY OF THE INVENTION

To solve the above-described problems, it is an object of the present invention to provide a plasma flat lamp having a high luminance and a stable discharge feature.

To achieve the above object, there is provided a plasma flat lamp comprising an upper plate, a lower plate separated a predetermined distance from the upper plate, a wall portion for forming a sealed discharge space between the upper and lower plates, a discharge gas filled in the discharge space, a first pair of electrodes including a first upper plate electrode and a first lower plate electrode arranged to face each other on each of the upper and lower plates with the discharge space interposed therebetween, and a second pair of electrodes including a second upper plate electrode separated a predetermined distance from the first upper plate electrode and a second lower plate electrode separated a predetermined distance from the first lower plate electrode arranged to face each other on each of the upper and lower plates with the discharge space interposed therebetween.

DETAILED DESCRIPTION OF THE INVENTION

Referring toFIGS. 4A and 4B, in a plasma flat lamp according to a first preferred embodiment of the present invention, an upper plate10and a lower plate20are separated a predetermined distance from each other and a discharge space80filled with a discharge gas is formed therebetween.

A fluorescent layer61is formed on each of the upper and lower plates10and20. Pairs of first electrodes31and32and second electrodes41and42are provided on the outer surfaces of the upper and lower plates10and20. The pairs of electrodes31and32, and41and42are arranged to face each other with respect to the discharge space80. The first upper electrode31and the first lower electrode32facing each other maintain the same electric potential so that discharge is not induced therebetween. Also, the second upper electrode41and the second lower electrode42facing each other maintain the same electric potential so that discharge is not induced therebetween. A predetermined difference in electric potential is present between the first electrode pair31and32and the second electrode pair41and42so that discharge is induced between the electrode pairs in a direction parallel to the upper plate10or the lower plate20. The first upper plate electrode31and the first lower plate electrode32constituting the first electrode pair31and32are electrically connected. The first upper plate electrode31and the first lower plate electrode32are directly and electrically connected, or are connected by an electric connection unit30which can prevent electric interference therebetween and simultaneously maintain the same electric potential.

The second electrode pair41and42are connected in the same manner as the first electrode pair31and32. That is, the second upper plate electrode41and the second lower plate electrode42constituting the second electrode pair41and42are electrically connected. The second upper plate electrode41and the second lower plate electrode42are directly and electrically connected, or are connected by an electric connection unit40which can prevent electric interference therebetween and simultaneously maintain the same electric potential.

The plasma flat lamp shown inFIGS. 4A and 4Bhas the electrodes31,32,41, and42that are not exposed to the discharge space. Electric power applied to the first and second electrode pairs is an alternating voltage or direct current pulse voltage. Since a pure direct current voltage cannot sustain discharge, it cannot be applied to the plasma flat lamp shown inFIGS. 4A and 4B.

FIG. 5shows a plasma flat lamp according to a second preferred embodiment of the present invention.

The flat lamp of the present invention shown inFIG. 5is different from the flat lamp shown inFIGS. 4A and 4Bin the position of electrodes. In the flat lamp shown inFIGS. 4A and 4B, the electrodes are formed on the outer surfaces of the upper and lower plates10and20. In the second preferred embodiment of the flat lamp ofFIG. 5, the electrodes are formed on the inner surfaces of the upper and lower plates10and20.

Referring toFIG. 5, a discharge space80filled with a discharge gas is formed between the upper and lower plates10and20which are separated a predetermined distance from each other by the wall portion70. The fluorescent layer61is formed on each of the inner surfaces of the upper and lower plates10and20. The pairs of the first electrodes31and32and the second electrodes41and42are provided on the inner surfaces of the upper and lower plates10and20, close to the wall portion70. Dielectric layers50are formed on each of the electrodes31,32,41, and42. The dielectric layers50are optional in the structure of the present preferred embodiment. The structure of driving voltage for the plasma flat lamp according to the second preferred embodiment of the present invention shown inFIG. 5is the same as that described with reference toFIGS. 4A and 4B.

FIG. 6shows a plasma flat lamp according to a third preferred embodiment of the present invention in which the structures according to the second preferred embodiment shown inFIG. 5are arranged in an array.

As shown inFIG. 6, the entire lamp p is divided into unit discharge areas P1, P2, P3, and P4, each having an independent discharge space. Each of the unit discharge areas P1, P2, P3, and P4has separate discharge electrodes and the structure of each area is similar to the structure shown in FIG.5.FIG. 7is a sectional view taken along line A—A of FIG.6. Referring toFIG. 7an upper plate10aand a lower plate20aare separated a predetermined distance from each other by a wall portion70aand a discharge space is provided therebetween. A spacer71ais provided in the discharge space to section the discharge space into unit discharge spaces80aand80band prevent a lamp from being broken due to a difference in pressure between the inside and the outside of the lamp.

Each of the unit discharge spaces80aand80bhas an independent discharge structure. That is, the discharge spaces80aand80bfilled with a discharge gas and maintaining a predetermined distance from each other by the wall portion70aand the spacer71aare formed in each of the discharge spaces P1and P2between the upper and lower plates10aand20a. The fluorescent layer61is formed on each of the inner surfaces of the upper and lower plates10aand20aof each of the discharge spaces80aand80b. A first pair of electrodes31aand32aand a second pair of electrodes41aand42aare provided on the inner surfaces of the upper and lower plates10aand20ain each of the discharge spaces80aand80b, close to the wall portion70aor the spacer71a. A dielectric layer50ais formed on each of the electrodes31a,32a,41a, and42a. The above structure is useful when a large scale of illumination is needed. In the above structure, the second electrodes41aon the upper plate10aare electrically connected and the second electrodes42aon the lower plate20aare electrically connected. As shown inFIG. 8, each of the second electrodes41aon the upper plate10aand the second electrodes42aon the lower plate20acan be formed integrally as upper and lower common electrodes41a′ and42a′ and extended toward each of the unit discharge spaces80aand80bwith respect to the spacer71disposed therebetween.

FIG. 9is a sectional view of a fourth preferred embodiment of the present invention in which the first preferred embodiment shown inFIGS. 4A and 4Bis applied to the array structure of the flat lamp shown inFIGS. 6 and 7. Referring toFIG. 9, the upper plate10aand the lower plate20aare maintained with a predetermined distance and a discharge space is provided therebetween. The spacer71ais provided in the discharge space to divide the discharge space into the unit discharge spaces80aand80band prevent the lamp from being broken due to a difference in pressure between the inside and outside of lamp. The discharge spaces80aand80bfilled with a discharge gas and maintaining a predetermined distance by the wall portion70aand the spacer71aare formed in the discharge areas P1and P2between the upper and lower plates10aand20a. The fluorescent layer61is formed on each of the inner surfaces of the upper and lower plates10aand20a. A first pair of electrodes31band32band a second pair of electrodes41band42bare provided on the outer surfaces of the upper and lower plates10aand20acorresponding to the respective discharge spaces80aand80b, close to the wall portion70aor the spacer71a. A separate protective layer can be formed on each of the electrodes31b,32b,41b, and42b. The above structure is useful when a large scale of illumination is needed. In the above structure, the second electrodes41bon the upper plate10aare electrically connected and the second electrodes42bon the lower plate20aare electrically connected. As shown inFIG. 8, each of the second electrodes41bon the upper plate10aand the second electrodes42bon the lower plate20acan be formed integrally.

In the above-described preferred embodiments, a reflection layer (not shown) can be interposed between a lower plate (not shown) and the fluorescent layer on the lower plate to reflect light proceeding toward the lower plate back to the upper plate, thus improving luminance. The reflection layer may also be formed at a portion where improvement of luminance is expected, for example, the wall portion. In the above-described where the electrode is formed in the discharge space, when the dielectric layer is not formed on the electrode, the lamp can be driven by a direct current pulse voltage. When there is dielectric and the electrodes are formed on the outer surfaces of the upper and lower plates, the lamp can be driven by an alternating current voltage or an alternating or a direct current pulse voltage.

The flat lamp of the present invention is operated in a driving method which is well known. A plasma discharge is generated by a voltage applied between the electrodes in the discharge space filled with a discharge gas and the plasma discharge is sustained. Here, high temperature electrons to excite neutral gas atoms and molecules are generated. Ultraviolet rays generated as the atoms and molecules in excited states excited by the electrons fall to the ground state excite the fluorescent layer coated in the discharge space to generate visible rays. To prevent the electrodes coated on the upper plate from being seen by an observer, the upper plate electrodes and the dielectric are formed of substance exhibiting a high light transmittance or a diffuser sheet can be added on the upper plate. To prevent a discharge contraction and encourage a stable discharge, as shown inFIG. 10A, the electrode10can be formed such that a portion having a narrow width and a portion having a great width are repeated periodically or, as shown inFIG. 10B, the heights of the electrode31and the dielectric layer of the upper plate10or the lower plate20change periodically. When the electrodes are formed at the outer surfaces of the upper and lower plats, the upper and lower plates work as dielectric. Thus, making the surface of glass uneven has the same effect of giving a change in the height of the dielectric layer.

FIGS. 11A through 14Bshow the results of simulation of a discharge feature of the flat lamp according to the present invention.

FIGS. 11A and 11Bshow distribution of electric potential generating under the same conditions with respect to the surface discharge flat lamp in which electrodes are formed on the lower plate like the conventional flat lamp shown in FIG.1and the flat lamp of the present invention shown in FIG.5. InFIGS. 11A and 11B, the distribution of electric potential around the electrodes are curved similarly so that contraction of discharge is less by far compared to a general opposed electrode flat lamp and an effect of decreasing in life span by sputtering is small. However, since the distribution of electric field at the central portion of the flat lamp shown inFIG. 11Bis similar to that of the opposed electrode flat lamp, instead of that for the surface discharge flat lamp inFIG. 11A, that is, the electric field is distributed around the center of the lamp.

FIGS. 12A and 12Bshow distribution of density of electrons with respect to the conventional surface discharge flat lamp of FIG.1and the flat lamp according to the present invention of FIG.5. In the flat lamp according to the present invention shown inFIG. 12B, it can be seen that electrons are mainly distributed at the center compared to the density of electrons of the conventional flat lamp ofFIG. 12A, so that loss of diffusion, that is, electrons or ions hit the wall portion and disappear, is less.

FIGS. 13A and 13Bshow the distribution of energy used to excite Xe with respect to the conventional surface discharge flat lamp and the flat lamp according to the present invention. When the excited Xe atoms or molecules fall to the ground state, ultraviolet rays are generated and the ultraviolet rays excite fluorescent substance to generate visible rays. Thus, in the same structure, comparison of energy used to excite Xe actually corresponds to comparison of luminance of light emission. In the flat lamp of the present invention, like the case of electron distribution, the energy is mainly distributed at the center so that the density of energy is higher than that of the conventional surface discharge flat lamp of FIG.1.

FIGS. 14A and 14Bshow the use of input energy by percentage in the conventional flat lamp and the flat lamp according to the present invention, respectively. Since the energy which contributes to actual light emission is the energy used to excite neutral particles by electrons, the efficiency of light emission can be compared by comparing the ratio of the energy used for the excitation with respect to the overall energy. The ratio of the energy used for the excitation with respect to the overall energy is referred to as a UV efficiency. As shown inFIGS. 14A and 14B, under the same conditions, the UV efficiency of the flat lamp of the present invention ofFIG. 14Bis 5% which is higher than 2% of the conventional flat lamp shown in FIG.14A.

FIG. 15shows the result of a test of actual products of the conventional flat lamp and the flat lamp according to the present invention having the same size with respect to a mixed gas of Xe(4%)/Ne. A driving frequency is 15.2 kHz, the flat lamps are driven by AC pulse, and the peak voltage of a driving pulse is 2.8 kV. The luminance and efficiency of the flat lamp of the present invention are higher than those of the conventional flat lamp.

As described above, the plasma flat lamp according to the present invention has a stable discharge feature which is a merit of the conventional surface discharge flat lamp and a high luminance of light emission which is a feature of the facing electrodes discharge type, while not having a low luminance and unstable discharge feature of the conventional surface discharge type flat lamp and facing electrodes discharge flat lamp, respectively.