Field of invention:
The present invention relates to a plasma display panel (hereinafter referred to as PDP) with a reflective coating on the rear side of the PDP to enhance its luminance.
Background of the invention:
Last few years has seen PDP emerging as a strong contender in flat panel display arena. PDP is composed of a matrix of discharge cells that are formed by partitions made by barrier ribs between a pair of glass substrates. Three color (Red, Blue and Green) phosphors are provided on the surface of the respective barrier ribs, and the cell volume is filled with a gas mixture of Neon and Xenon. During PDP operation, the gas discharge takes place between electrodes in the cells that result in the generation of VUV photons with a wavelength of 147 nm and 172 nm. The phosphors absorb these VUV photons and emit visible light to display a picture including characters and graphics.
The luminance efficacy of an AC-PDP depends on the four successive conversion factors. These are Generation of VUV photons, Capturing the VUV photon by phosphor layer, Conversion of VUV to visible light and transmission of visible light through front glass substrate and reflection of the visible light through the rear glass substrate. The luminance efficacy of the PDP is low in comparison to other display devices. This luminance efficacy can be improved through the enhancement of the luminance. A significant amount of the visible light produced after the phosphor excitation is lost at various stages due to geometrical structure of the PDP.
In order to increase the luminance of the PDP, various approaches have been
applied.
According to the approach mentioned in US Patent No.610033, phosphor
particles having a particle size of at most 1.5 µM should be used for forming a
phosphor layer on rear glass substrate. The reduced phosphor particle size will
allow less wastage of the visible light formation and thereby increases the
luminance of AC-PDP.
According to US patent No 72295722, the thickness of the phosphor layer should
be gradually increased towards the front glass substrate to increase the
luminance of the AC PDP. The layer thickness can not be increased beyond a
certain value above which the emission of the phosphor reduces.
According to Japanese patent No. 2000-11885; a reflective layer consisting of Titanium oxide (Ti02) should be formed below the phosphor layer. This reflective titanium coating reflects the light rear towards the front side of the panel thereby increasing the luminance of AC-PDP. However the firing temperature of Ti02 is higher than the firing temperature of the reflective dielectric layer thus the co-firing of the barrier rib & reflective dielectric leading to barrier rib structure deformation due to high temperature. The presence of the TiO2 material over the

top of the barrier ribs creates the path for charge leakage leading to the cross talk between the cells. Hence reflecting coating (TiO2) layer has to be removed over the barrier rib wall leading to complicated printing process.
According to Japanese patent no. 1999-60272 and Japanese patent no 1998-74455 the reflectivity of the dielectric layer can be increased by adding white additives like Ti02, AI203 etc. These white additives when added to the reflective dielectric paste increase the reflectance of the dielectric layer. This leads to the deterioration of the dielectric strength of the reflective dielectric layer and causes breakdown of the dielectric layer during the voltage application for creating the discharge.
However, any of these schemes do not account for resolving the problem of loss of light by transmission through the reflective di-electric layer covering the address electrodes. It has been seen that 10-20% of the total light output is lost due to transmission through the phosphor and reflective layer and does not contribute for PDP luminance in the conventional PDP. This transmitted light through the rear glass substrate can be used to increase the luminance by re-reflecting it towards front glass substrate. For this purpose we are proposing a reflective coating (reflectivity 90-95%) on the rear side of the rear glass substrate of PDP to reflect the transmitted light by multiple reflection. The reflected light exhibits cumulative effect for increasing the luminance as well as the luminous efficacy of the PDP.
Object of the invention:
A significant amount of visible light is lost because of transmission through the phosphor and the reflective dielectric layer. The reflectivity of the Reflective Dielectric layer on the rear substrate ranges from 60-80%. The transmission of light through rear substrate ranges from 10-20% while the residual 10-20% of the light is absorbed by reflective dielectric layer and/or phosphor layer. So the 10-20% of the total light output from R, G, B phosphor, is lost through transmission from the phosphor & reflective di-electric layer of the rear substrate of PDP thus reducing both luminance & luminance efficacy.
The principal object of the invention is to enhance the luminance of AC-PDP by forming a coating on the back side of the rear glass substrate of AC PDP which reflects the residual transmitted light through phosphor and reflective dielectric layer again by multiple reflections towards the front substrate.
Yet another object of the present invention is to improve the luminous efficacy of the AC-PDP.
Yet another object of the present invention is to provide a conductive and reflective layer to act as a heat equalizer of the AC-PDP.

Statement of the invention:
Accordingly the invention provides a Plasma Display Panel comprising, a pair of front and rear glass substrate and gas enclosed between the substrates, pairs of scan-sustain display electrodes extending in second direction covered with transparent dielectric layer, provided with an electron emissive layer over the said transparent dielectric layer on the front glass substrate, plurality of address electrodes extending in the first direction on the rear glass substrate, a reflective dielectric layer is provided to cover the plurality of address electrodes, plurality of barrier ribs structure is provided in first direction parallel to address electrodes on rear glass substrate that are oriented orthogonal to scan-sustain display electrodes, each address electrode is enclosed between rib wall structure, the R, G and B phosphors emitting red, green and blue light, respectively are provided in the barrier rib channel spaces, which is characterized in that , a coating is provided on the back side of rear glass substrate outside of PDP.
Brief Description of the Drawings:
Figure (1) illustrates the cross-sectional view of conventional AC-PDP with straight barrier ribs.
Figure (2) illustrates the cross sectional view of the conventional AC-PDP along with the reflective coating at rear side of the rear glass substrate according to the present invention.
Figure (3) illustrates the optical ray diagram for multiple light reflections from this reflective coating and transmission through rear substrate.
Detailed description of the invention:
In the present invention, figure 1 illustrates the cross-sectional view of AC-PDP with straight barrier ribs.
As in figure 1, the front glass substrate (1) and rear glass substrate (2) are shown. In the front glass substrate (1), display electrodes are made of transparent ITO sheet (3). To reduce the resistance of the display electrode, opaque electrically conducting bus electrodes (4) are made over the ITO electrodes. The display electrode is covered with a transparent dielectric layer (5) to limit the discharge current. Then the electron emissive layer (6) is deposited over the transparent dielectric layer (5). On the rear glass substrate (2), a plurality of address electrodes (7) are formed with one address electrode (7) is formed in each sub-pixel. The address electrodes (7) are covered with a dielectric layer (8) to limit the discharge current and for light reflection. The

straight channel barrier ribs (9) are formed over the dielectric layer (8). The R (10a), G (10b), B (10c) phosphor layers are formed in the barrier rib (9) channel spaces. The Red (R) (11), Green (G) (12) and Blue (B) (13) sub-pixels are formed with the combination of front and rear substrate structure to form a pixel. One PDP cell or sub-pixel comprises of one pair of display electrodes (3), (4) and an address electrode (7).
Figure 2 shows the cross sectional view AC PDP provided with a reflective coating on back side of the rear glass substrate (2) according to the present invention. To utilize the transmitted light at rear glass substrate (2) a layer of reflective material (11) of high reflectivity (75-95%) is formed on the back side of the rear glass substrate (2) of PDP. This reflective layer (11) reflects light towards the front glass substrate (1) by multiple reflections thereby increasing the luminance of the PDP as well as the luminous efficacy.
Figure 3 shows the optical ray diagram for multiple reflections at the interface of the reflective layer (8) and rear glass substrate (2). The dielectric constant (e1) of the reflective dielectric layer (8) is more than the dielectric constant (e2) of the rear glass material. Due to the light transmission through the reflective dielectric layer (8) a portion of light enters in to the rear glass substrate (2). When the incident light (i) enters from denser medium i.e. reflective dielectric layer (8) to the rarer medium i.e. glass, at an incident angle, the light is refracted and shifts away from the normal. This refracted light (rf1) now incident on the reflective layer (11) coated at rear side of the rear glass substrate (2). Being highly reflective (reflectivity 90-95%), most of the incident light (rf1) is reflected rear to the reflective dielectric layer (8) as indicated by r1. A part of the r1 is reflected rear to the reflective coating (11) (indicated by rf2) due to reflective nature of the reflective dielectric layer (8) and other part (T1) is transmitted through the reflective dielectric layer (8) by refraction process and contributes for PDP luminance. This process continues and the transmitted light (T1, T2, T3 ....) keep on adding for enhancing PDP luminance.
The reflective coating (11) having thickness with in range of 1 urn to 500 urn and having reflectivity 75-95% is provided on the back side of the rear glass substrate (2) of PDP can be formed by screen printing method, spray method, E-Beam method or roller process. Aluminum is a suitable option for making this reflective coating (11) however other materials like silver, gold, titanium oxide or any other material having reflective properties can also be used. The reflective coating (11) can also be applied in the form of aluminum sheet which can be pasted on the back side of rear substrate of PDP by applying adhesive agents.
During normal PDP operation a significant amount of heat is produced. Flow of the discharge current through the resistive display electrodes is one of the causes of the heat generation. Static pattern with gray scale variation causes the localized hot spots that leads to the degradation of the phosphor and can also induce thermal stress which can crack the panel.

The reflective layer (11) is also a good conductor for heat along with its reflective nature. By adding this layer at back side of the rear glass substrate (2), the produced heat can be spread along the panel surface. Hence there is a great possibility that the reflective layer (11) can also work as heat equalizer.
To make this PDP functional, it is provided with the drive control system (not shown) consists of an analog-to-digital converter, a frame memory, a scan control portion, a sustain electrode driving circuit, a scan electrode driving circuit and an address electrode driving circuit. The drive control system is electrically connected to the sustain, scan and address electrodes respectively via a flexible printed circuit board (not shown).
Although the invention been described with reference to specific embodiments, this description is not meant to embodiments, as well as alternate embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore contemplated that such modifications can be made without departing from the spirit or scope of the present invention as defined.

We Claim:

1. A Plasma Display Panel comprising

a pair of front and rear glass substrate and gas enclosed between the substrates, pairs of scan-sustain display electrodes extending in second direction covered with transparent dielectric layer, provided with an electron emissive layer over the said transparent dielectric layer on the front glass substrate,

plurality of address electrodes extending in the first direction on the rear glass substrate, a reflective dielectric layer is provided to cover the plurality of address electrodes, plurality of barrier ribs structure is provided in first direction parallel to address electrodes on rear glass substrate that are oriented orthogonal to scan-sustain display electrodes, each address electrode is enclosed between rib wall structure,

the R, G and B phosphors emitting red, green and blue light, respectively are provided in the barrier rib channel spaces,

Wherein, a coating is provided on the back side of rear glass substrate outside of
PDP.

2. The Plasma Display Panel as claimed in claim 1, wherein said coating is made up of aluminum, silver, gold or any other material having reflective prope dies.

3. The Plasma Display Panel as claimed in claim 1, wherein said coating act as heat equalizer.

4. The Plasma Display Panel as claimed in claim 1, wherein said coating increases the luminance of the PDP.

5. The Plasma Display Panel as claimed in claim 1, wherein said coating increases the luminance efficacy of the PDP.

6. A plasma display panel here in substantially described with reference to the accompanying drawings.