Plasma display panel

Abstract: A plasma display panel has spaced apart and parallel first and second substrates substrate. A base glass layer is provided on one surface of the second substrate confronting to the first substrate. A plurality of spaced apart parallel ribs are positioned on the base glass layer and between first and second substrates. Each rib defines a channel with another neighboring rib. A plurality of phosphor capable of emitting light are provided, each of which being positioned in the channel. In particular, the base glass layer and/or ribs are made of material substantially impermeable to light.

Description: FIELD OF THE INVENTION The present invention relates to a display element and plasma display panel preferably used for an image display of television and computer. BACKGROUND OF THE INVENTION A conventional AC plasma display panel is illustrated in part in Fig. 5. As shown in Fig. 5, a plurality of scanning electrode 4 and sustaining electrode 5 that are parallel to each other are formed on a first insulative substrate 1. The scanning electrodes 4 and the sustaining electrode 5 are covered with a dielectric layer 2 and a protection layer 3. On a second insulative substrate 6, which is facing the first insulative substrate 1, a plurality of data electrodes 7 that are parallel to each other are formed orthogonally to the scanning electrodes 4 and the sustaining electrodes 5. The data electrodes 7 are covered with a base glass layer 8 made of white material. A plurality of ribs 9 also made of white material are formed on the base glass layer 8. The ribs 9 are arranged so that each cooperates with the neighboring rib to define a channel adjacent to and along the data electrode 7. A phosphor 10 is provided in each channel to cover opposing side walls of the neighboring ribs and a part of the base glass layer exposed between the neighboring ribs, so that a discharge chamber 11 is defined on and along phosphor 10. As can be seen in the drawing, the neighboring scanning and sustaining electrodes, 4 and 5, are paired so that an electric discharge can be occurred with an aid of the data electrode 7 in a restricted region of the discharge chamber 11. The discharge generates ultraviolet light that excites an adjacent part of the phosphor 10. The excited portion of the phosphor 10 emits visual light for displaying an image. In this manner, each area where the data electrode 7 crosses the pair of scanning and sustaining electrodes, 4 and 5, defines a discharge cell 12 as hatched in Fig. 5. As shown in the drawing, three neighboring phosphors 10, each separated by the ribs 9, constitute red phosphor 10R, green phosphor 10G, and blue phosphor 10B, respectively, in this order. Selected and used for the white material of both the base glass layer 8 and ribs 9 is, for example, a white glass reflective to the visual light. Preferably, a thickness of the base glass layer 8 is as small as possible to minimize a voltage for driving the data electrodes 7. For this reason, typically, the base glass layer has a thickness of about 10 to 15 micrometers. Also, a thickness of the ribs 9 is as small as possible to maximize an opening area of the discharge chamber 11. For this reason, typically, each of the ribs 9 has a thickness of about 20 to 60 micrometers. Referring to Fig. 6 illustrating a cross sectional view taken along lines VI-VI in Fig. 5, descriptions will be made to functions of the base glass layer 8 and ribs 9. Fig. 6 shows that only the green phosphor 10G is excited for emitting green light. For clarity of the drawing and illustrative purpose, only a few light passes are illustrated in the drawing, which passes may not be depicted correctly from an optical standpoint. As shown in the drawing, ultraviolet light generated by the discharge between the scanning and sustaining electrodes, 4 and 5, with an aid of the data electrode 7 excites the green phosphor 10G. This causes the green phosphor 10G to emit green light to be projected through the first insulative substrate 1 as depicted by the arrows in Fig. 6, displaying a corresponding image. At this moment, part of the green light emitted from a surface of the green phosphor 10G is reflected at surfaces of the base glass layer 8 and ribs 9 and then transmitted through the first substrate 1 for displaying. This is because the base glass layer 8 and the ribs 9 are made of white material, for example, white glass, reflective to the visual light. With the arrangement, a brightness of the display panel is increased to a certain extent. However, the white material can reflect only about 50 to 60 percent of the visual light. Then, the structure does not improve the brightness of the panel so much. In addition, remaining 40 to 50 percent of the light is transmitted into the white material where it may be damped. Disadvantageously, several to several tens percent of the light can pass the white material, which may provide adverse effect. Fig. 7 is a graph illustrating a relationship between thickness of the phosphor and brightness of light from the display panel. In this graph, thickness-brightness characteristic curve A is for the base glass layer 8 and rib 9 having reflectance of 60 percent, and curve B for base glass layer and ribs having reflectance of zero percent. This graph shows that if the thickness is lower than about 15 micrometers the brightness increases with the thickness due to the reflected light from the base glass layer 8 and the ribs 9 and if the thickness is greater than about 25 micrometers the brightness no longer increase too much. This means that it is effective to increase the thickness of the phosphor for increasing the brightness of the panel. Next, referring to Fig. 8 which is a cross sectional view taken along lines VI-VI in Fig. 5, another descriptions will be made to the adverse effect of the transmitted light through the base glass layer and ribs. Fig. 8 illustrates that the discharge cells 12 of the red and green phosphors, 10R and 10G, are excited to emit respective light, but the discharge cell 12 of the blue phosphor 10B is not excited. For clarity of the drawing and illustrative purpose, only a few light passes are illustrated in the drawing, which passes may not be depicted correctly from an optical standpoint. In this instance, as shown by solid lines or arrows, the red and green visual light emitted from the surfaces of red and green phosphors, 10R and 10G, respectively, is transmitted through the first substrate 1 for display. Likewise, as shown by dotted lines or arrows, red and green visual light emitted into the interior of the elements is reflected at the surfaces of the base glass layer 8 and ribs 9 and then transmitted through the elements and then the first substrate 1 for display. Contrary to this, as shown by phantom lines or arrows, the visual light transmitted through the red and green phosphors, 10R and 10G, is further transmitted through the base glass layer 8 and ribs 9. A portion of light transmitted through the base glass layer and ribs may travel through neighboring phosphors for different colors. In this instance, the display light from the red phosphor 10R can merge with that from the green phosphor 10G. This will degrade purity of respective colors. In addition, another portion of light transmitted through the base glass layer and ribs may travel through a portion of the first substrate 1 facing to the discharge cell 12 not excited, which disadvantageously serves as an additional light. for display. In this instance, green light from the green phosphor 10G is emitted through the discharge cell of blue phosphor 10B not excited, which is referred to as "halation". The above-described problems, i.e., degradation of color purity and halation, are essential for the conventional plasma display panels irrelevant to the number of discharge cells or display colors from discharge cells. US 5,541,479 discloses a plasma display device having a face plate and a rear plate spaced apart from each other. Barrier ribs are disposed to define discharge gas spaces adjacent to crossovers of parallel electrodes. At least some of the barrier ribs are transparent barrier ribs made of a light-permeable material, so that, particularly when the rear plate is covered with a reflective layering, light which might otherwise leak from the rear plate is usefully saved and caused to radiate through the face plate, whereby the use ratio of the emitted light increases. US 5,701,056 discloses a plasma display panel which includes a first substrate, a second substrate, a plurality of sets of electrode pairs, a partition wall structure formed overlapping the sets of electrode pairs and third partition walls. The first partition walls have a width greater than a width of the third partition walls. Advantageously, this construction of the plasma display panel permits the panel to exhibit improved luminance and contrast characteristics, and permits the display to constitute a high grade display. EP 0 712 148 discloses a plasma display panel for plasma display systems having barrier ribs forming a cell having wavelength-selective reflective filters for reflecting only a light of wavelength needed to display and absorbing the other lights coated thereon. The wavelength-selective reflective filters have phosphors coated thereon. A color filter is inserted between a first face panel and second front glass panel for feeding out the light in a sandwich state. SUMMARY OF THE INVENTION The present invention proposes a display panel as claimed in claim 1. A plasma display panel according to the present invention has spaced apart and parallel first and second substrates substrate. A base glass layer is provided on one surface of the second substrate confronting to the first substrate. A plurality of spaced apart parallel ribs are positioned on the base glass layer and between first and second substrates. Each rib defines a channel with a neighboring rib. A plurality of phosphor capable of emitting light are provided, each of which being positioned in the channel. In particular, the base glass layer and ribs are made of material capable of absorbing visible light. With the arrangement, due to the material substantially impermeable to visual light improves color purity and halation, allowing to display a high quality image. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an enlarged partial perspective view of a plasma display panel of the present invention; Fig. 2 is an enlarged cross sectional view of the plasma display panel taken along lines II-II in Fig. 1; Fig. 3A is a front view of the plasma display panel in which all the discharge cells in left half are turned on and all in the right half are turned off; Fig. 3B is a graph showing a distribution of brightness along a central horizontal line in the panel shown in Fig. 3A; Fig. 4 is a graph showing a relationship between a distance from a boundary of left and right halves and brightness measure; Fig. 5 is an enlarged partial perspective view of a prior art plasma display panel; Fig. 6 is an enlarged cross sectional view of the plasma display panel taken along lines VI-VI in Fig. 5, showing passes of visual light from a discharge cell for green; Fig. 7 is a graph showing a relationship between thickness of a phosphor and brightness of visual light emitted; and Fig. 8 is another enlarged cross sectional view of the plasma display panel taken along lines VI-VI in Fig. 5, showing passes of visual light from discharge cells for red and green. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to the drawings, a plasma display panel according to the present invention will be described in detail hereinafter. Fig. 1 illustrates a partial enlarged perspective view of the plasma display panel of the present invention, generally indicated by reference numeral 20. As can be seen from the drawing, a structure of the display panel 20 of the present invention is substantially identical to the plasma display panel 20' illustrated in Fig. 5. Therefore, descriptions of the structure of the plasma display panel 20 are omitted, and like reference numerals are provided for like parts and assemblies performing the same function in Figs. 1 and 5. Differences between the plasma display panels, 20 and 20', are materials of base glass layer and ribs. Specifically, although the material of base glass layer 8 and ribs 9 of the prior art plasma display panel 20' has white color, the corresponding base glass layer 18 and ribs 9 of the plasma display panel 20 of the present invention is made of material substantially impermeable to light. For this purpose, the base glass layer 18 and ribs 19 of the plasma display panel 20 are made of material having dark color, preferably black color. Examples of such materials are manganese (Mn), chromium (Cr), cobalt (Co) and nickel (Ni). It is to be understood that the base glass layer 18 and ribs 19 may include any one or more such materials. With reference to Fig. 2, functions and resultant effects of the base glass layer 18 and ribs 19 made of black material will be described in detail hereinafter. Fig. 2 illustrates that red and green phosphors, 10R and 10G, of a discharge cell 12 are excited while a blue phosphor 10B thereof is not excited. In this instance, the red and green phosphors, 10R and 10G, excited by ultraviolet light due to a maintaining discharge between scanning and sustaining electrodes, 4 and 5, emit red and green light, respectively. Red and green light emitted from surfaces of red and green phosphors, 10 R and 10G, to exterior thereof is transmitted through a first substrate 1 for displaying an image. On the other hand, red and green light emitted from the surfaces of red and green phosphors to interior thereof is absorbed, rather than reflected, at surfaces of base glass layer 18 and ribs 19, adjacent to the phosphors, and is prevented from returning through respective phosphors and then travelling through a first insulative substrate 1. This is because that, as described above, the base glass layer 18 and ribs 19 are made of black material capable of absorbing light. Also, no red or green light traveled through corresponding phosphors 10R/10G is transmitted through the base glass layer 18 or ribs 19. This prevents red or green light from being transmitted into the adjacent discharge cell 12 where it could otherwise be served as display light. Therefore, according to the present invention, one light emitted from one phosphor (e.g., 10R) of one discharge cell 12 will never merge or mix with another light emitted from phosphor (e.g., 10G and 10B) of the adjacent another discharge cell 12. This ensures that each light emitted from corresponding phosphor appears to have correct color, for example, pure red, green, and blue. In addition, no light (e.g., red light) is transmitted into the adjacent discharge cell for different color (e.g., green and blue) where it would otherwise be used for display. This ensures that the discharge cell whose phosphor (e.g., 10B) is not excited would never emit visual light. This further ensures that no halation would appear. Although both the base glass layer and ribs are made of material impermeable to light, either the base glass layer or ribs may be made of black material substantially impermeable to light. When the base glass layer 18 is made of black material, it will absorb light which would otherwise be transmitted through the base glass layer into adjacent discharge cell. When on the other hand the ribs 19 are made of black material, they will absorb light which would otherwise be transmitted through the ribs into adjacent discharge cell. Therefore, even when either base glass layer or ribs are made of black material, both color purity and halation are improved to some extent. However, the present invention is limited to the can that both the base glass layer and ribs are made of a material capable of absorbing visible light. Descriptions will be made to an evaluation of halation effect of the panel according to the present invention. Assume that each of the discharge cells in the left half of the panel are turned on, i.e., excited to emit light, and each of the discharge cells in the right half of the panel are turned off, i.e., not excited. In this instance, ideally, i.e., if no halation exists in the panel, the left half of the panel represents white (i.e., 100 percent brightness) and the right half represents black (i.e., zero percent brightness), so that the brightness changes from 100 to zero percent at a boundary between the left and right halves without any gradation, which is shown by a dotted line in Fig. 3B. Actually, however, due to halation caused by various reasons, as shown in Fig. 3B, although the brightness in the left half has 100 percent brightness, it decreases gradually from 100 to zero percent in a zone adjacent to the left half in the right half. An increase of a distance from the boundary to a position where the brightness would reach substantially zero makes a boundary or edges of white black images unclear, which degrades a contrast and purity of two colors. In the actual evaluation, three panels were prepared; first panel (Prior-art) having base glass layer and ribs of white material, second panel (Embodiment 1) having base glass layer of black material and ribs of white material, and third panel (Embodiment 2) having base glass layer and ribs of black material. Thickness of the base glass layer and ribs were set to 10 micrometers and 20 micrometers, respectively. For each panel, brightness distribution was measured along a central horizontal line. The measurement result is graphed in Fig. 4, which shows a relationship between a distance (L) from the boundary of left and right halves and relative brightness measured. The graph shows that halation reduces as the brightness decreases rapidly. Also, the second and third panels (Embodiments 1 and 2) in which base glass layer or/and ribs are black provides less halation than prior art first panel in which both base glass layer and ribs are white. Further, the third panel (Embodiment 2) in which both base glass layer and ribs are black according to the present invention provides less halation than the second panel (Embodiment 1) in which only ribs are black. It can be said that the same results can be attained without depending upon the number of discharge cell, color emitted from the discharge cell, or thickness of the phosphor, and color purity and halation will be improved according to the present invention. Although in the previous embodiment the base glass layer is a single layer, the present invention can equally be applied to a multi-layered base glass layer. In this instance, at least one layer of the base glass layer may be made of material substantially impermeable to visual light. Also, although the present invention has been fully described in light of one specific AC plasma display panel, it can equally be applied to another AC and DC plasma display panels. In this instance, the base glass layer and ribs may be made material substantially impermeable to light, which improves color purity and possible halation. In view of above, with the present invention in which base glass layer and ribs are made of black or dark material, obtained is a high quality plasma display panel having improved color purity of the displayed image and further having little halation.

Claims: A display panel (20) for emitting light, comprising: a first substrate (1); a second substrate (6) parallel to and spaced from said first substrate (1); a base glass layer (18) on a surface of said second substrate (6) that faces said first substrate (1), with said base glass layer (18) being from 10µm to 15µm in thickness and also being substantially non-reflective of visible light; at least two spaced apart parallel ribs (19) on said base glass layer (18) and between said first and second substrate (1, 6), with each of said at least two spaced apart parallel ribs (19) being from 20µm to 60µm in width and cooperating with an adjacent rib to define a channel therebetween; and at least one phosphor (10) that is capable of emitting light, with each said at least one phosphor (10) being greater than 25µm in thickness and positioned within a respective said channel; wherein each of said base glass layer (18) and said at least two spaced apart parallel ribs (19) is made of glass material having black color, with said glass material including at least one of manganese, chromium, cobalt and nickel and being capable of absorbing said visible light.

Inventors: AOTO KOJI, HIRAO KAZUNORI, TAHARA YOSHIHITO, AOTO, KOJI, HIRAO, KAZUNORI, TAHARA, YOSHIHITO
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1999
Main classification: H01J17
IPC codes: H01J17, H01J11