Abstract:
A luminescent brightness compensation of sub-pixels of tri-electrode based field-emission display. The cathode conductive layers corresponding to sub-pixels constituting a pixel are arranged at various levels according to the respective luminescent efficiencies thereof. Thereby, the color with lower or higher luminescent efficiency obtains a stronger or weaker electric field between anode and cathode, respectively. Therefore, the different luminescent efficiency of three sub-pixels with primary color is compensated.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates in general to a field-emission display, and more particularly, to compensation of luminescent brightness of sub-pixels of a field-emission display. 
   The field-emission display (FED) is a very newly developed technology. Being self-illuminant, such type of display does not require a back light source like the liquid crystal display (LCD). In addition to the better brightness, the viewable angle is broader, power consumption is lower, responding speed is faster (no residual image), and the operation temperature range is larger than currently available flat displays. The image quality of the field-emission display is similar to that of the conventional cathode ray tube (CRT) display, while the dimension of the field-emission display is much thinner and lighter than the cathode ray tube display. Therefore, it is foreseeable that the field-emission display will replace the liquid crystal display and plasma display panel in the future. Further, the fast growing nanotechnology enables nano-material to be applied in the field-emission display, such that the technology of field-emission display will be commercially available in the near future. 
     FIG. 1  shows a cross sectional view of a basic tri-electrode based field-emission display essentially consisting of an anode plate  10 , a cathode plate  20  and a gate layer  25 . The anode plate  10  and the cathode plate  20  are supported by a spacer  14 . The anode plate  10  includes an anode substrate  11 , an anode conductive layer  12  and a phosphor layer  13 . The cathode plate  20  includes a cathode substrate  21 , a cathode conductive layer  22 , an electron-emission source layer  23  and a dielectric layer  24 . The gate layer  25  is apart disposed between the anode plate  10  and the cathode plate  20 . The anode plate  10  is subjected to a potential difference to drain electron beams emitted from the electron-emission source layer  23 . The voltage provided by the gate layer  25  accelerates the electron beams to impinge the phosphor layer  13  of the anode plate  10 , so as to generate visible light. 
   The display includes a plurality of pixels composed of red, blue and green cathode and anode units. One anode unit with one of the three primary colors can be called “sub-pixel”. The different composition of the phosphor layer  13  provides three primary colors; however, the lights with different color emitted by the phosphors have different luminescent efficiencies. As a result, although the electron beams emitted from the electron-emission source layer carry the same kinetic energy, the brightness efficiencies of different colored phosphors are different. Thus, the brightness of the different colored lights emitted from the phosphor layer are substantially different. Typically, the brightness ratio of the red, blue and green colored light is about 2:1:7. Therefore, color or brightness distortion at one pixel or on whole screen always occurs. In order to solve this problem, conventional FEDs use a complex control circuit to compensate the inconsistent luminescent efficiencies. But this solution costs a lot. It is thus very uneconomic. 
   Another approach to resolve the discrepancies in luminescent efficiencies is to adjust the thickness or area size of the phosphor layer  13 . The drawback of such approach is that it is very difficult to make the thickness or area size of the phosphor layer  13  for the same colored sub-pixels maintain the identicality among different pixels because of extremely numerous pixels in a display to be processed. 
   BRIEF SUMMARY OF THE INVENTION 
   A luminescent brightness compensating structure for a field-emission display is provided to allow the differences in luminescent efficiencies for three primary colors within each pixel of the display to be compensated under the same voltages provided between anode, cathode and gate electrode. In addition, the compensating structure does not require complex circuit or process, such that the cost is greatly reduced. 
   The luminescent brightness compensating structure includes a cathode conductive layer with different levels in height according to the luminescent efficiency of the different colored phosphor layer, such that the distance between the electron-emission layer of the cathode plate and the gate layer for three primary colors is adjusted to be different. As a result, different electric fields are driven for the cathode and anode units according to the color of the phosphor layer. Therefore, the discrepancies of luminescent efficiencies for different colors can be compensated. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objects and advantages of the present invention will be become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
       FIG. 1  is a cross sectional view of a conventional tri-electrode based field-emission display; and 
       FIG. 2  is a cross sectional view of an embodiment of a tri-electrode based field-emission display according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 2 , as provided, the field-emission display of a preferred embodiment according to the present invention is based on a tri-electrode topology, essentially including an anode plate  30 , a cathode plate  40  and a gate layer  45  apart disposed between the anode plate  30  and the cathode plate  40 . The anode plate  30  includes an anode substrate  31 , an anode conductive layer  32  form on the anode substrate  31  and a phosphor layer  33  formed on the anode conductive layer  32 . The phosphor layer  33  is composed of a plurality of red, green and blue anode units  33 R,  33 G,  33 B. Each anode unit  33 R,  33 G,  33 B forms a sub-pixel with one primary color. One red anode unit  33 R, one green anode unit and one blue anode unit  33 B constitute a pixel of the display. A spacer  43  is disposed between anode plate  30  and gate layer  45 . The cathode plate  40  includes a cathode substrate  41 , a cathode conductive layer  42  formed on the cathode substrate  41  and an electron-emission layer  43  formed on the cathode conductive layer  42 , The electron-emission layer  43  is composed of a plurality of cathode units  43 R,  43 G,  43 B. A dielectric layer  44  is disposed between gate layer  45  and cathode plate  40 . As shown, each anode unit  33 R,  33 G,  33 B is aligned with a cathode unit  43 R,  43 G,  43 B. 
   A person skilled in the art must know that the luminescent efficiency ratio for the green, red and blue light emitted by the anode unit  33 G,  33 R,  33 B under identical electric field is about 7:2:1. Theoretically and ideally, if the distance between the electron-emission source layer  43  and the green, red and blue anode units  33 G,  33 R,  33 R sets to 7:2:1, such that the distinct electric field strength ratio for the couples of anode and cathode units for green, red and red colors under identical voltage between anode and cathode can be adjusted to a 1/7:½:1, that is, 2:7:14. (According to the relationship E=V/D, where E is electric field strength, D is distance and V is potential) Thereby, the luminescent efficiencies ratio for the green, red and blue colors will become 1:1:1. As a result, colors of the phosphor layer  33 , and a uniform brightness of the sub-pixels with primary color is achieved. The difference in luminescent efficiencies is thus compensated. Stronger electric fields are driven for the colors such as blue and red having lower luminescent efficiencies, the brightness of the blue and red colors is thus enhanced. In reality, however, the above-mentioned distance ratio can not be realized by currently available technology due to the very short distance between anode plate  30  and cathode plate  40 . So an alternative way must be found out. 
   According to the result of experiments the inventors made, the luminescent efficiency ratio for the sub-pixels can approach to 1:1:1 by adjusting the distance ratio between the gate layer  45  and the green, red and blue cathode unit  43 G,  43 R,  43 B to about 2:1:1 for green, red and blue colors. A preferred embodiment is shown in  FIG. 2 . The gate layer  45  is a uniform plane and the thickness of the cathode conductive units  42 G,  42 R,  42 B aligned with different colored anode units  33 G,  33 R,  33 B are different. By this, an almost perfect brightness compensation of sub-pixels with primary color can be reached under the condition of limited distance between anode plate  30  and cathode plate  40 . 
   In the embodiment as shown in  FIG. 2 , the levels of the cathode unit  43 G,  43 R,  43 B are adjusted by forming the cathode conductive units  42 G,  42 R,  42 B with various heights or thickness. The method for fabricating the cathode conductive layer  42  with various thicknesses can be achieved by various processes. 
   For example, the thick-film process can be applied. By screen-printing multiple layers of silver paste, the cathode conductive layer  42  can be formed with a thickness determined by the number of layers of the silver paste. 
   Another example for forming the cathode conductive layer  42  includes photolithography process. A photosensitive silver paste is used as the material for forming the cathode conductive layer  42 . By performing exposure on the silver paste with different exposure time, the height or thickness of the resulting cathode conductive layer  42  can be adjusted. 
   By either the thick-film process or photolithography process, the thickness or height of the cathode conductive layer can be precisely controlled. Therefore, the complex control circuit or complex process is not required. The brightness compensation can be achieved with the least cost. 
   While the present invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those of ordinary skill in the art the various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the appended claims.