Patent Publication Number: US-2007109321-A1

Title: Method for compensating a brightness error of a flat panel display

Description:
FIELD OF THE INVENTION  
      The present invention relates to a method for compensating a brightness error of a flat panel display, and more particularly to a method applying to a flat panel display for creating a look up table according to actual output brightness and ideal output brightness and compensating the brightness error of the flat panel display by using the look up table, enabling the flat panel display to output a gray level brightness close to the idea output brightness of the flat panel display and effectively solve the problem having an error between the actual output brightness and the ideal output brightness of the flat panel display.  
     BACKGROUND OF THE INVENTION  
      In general, a plasma display panel (PDP) controls the brightness of each discharge cell of its display panel by the discharge number, and thus a relation of the linear function exists between the discharge number and the brightness as shown in the following formula: 
 
Brightness=k 2 ×Discharge Number×Brightness Per Discharge 
 
      Where, k 2  is a variable representing the number of gray levels of the plasma display panel. For example, if the number of gray levels of the plasma display panel equals to 256, then k2=256. From the above formula, it is known that the more the discharge number of the plasma display panel, the larger is the brightness. However, there is a difference between the number of display gray levels and the actual measured brightness of the plasma display panel due to the following reasons and thus causing unexpectedly poor performance and affecting the quality of displayed images.  
      (1) Trature Effect &amp; Fluorescent Characteristics  
      The display gray level of a plasma display panel is comprised of a finite number of sub-fields (SF) and sustain pulse number ratio (which is known as the brightness ratio) as shown in Table 1.  
                       TABLE 1                                      One Display Field = 8 SF                                                     SF1   SF2   SF3   SF4   SF5   SF6   SF7   SF8                         Sustain Pulse Ratio (Brightness Ratio)                                                     1   2   4   8   16   32   64   128                     Display Output   ◯ indicates SF is ON for this gray level,       Gray Level   and X indicates SF is OFF.                                                          0   X   X   X   X   X   X   X   X        1   ◯   X   X   X   X   X   X   X        2   X   ◯   X   X   X   X   X   X        3   ◯   ◯   X   X   X   X   X   X       . . .   . . .   . . .   . . .   . . .   . . .   . . .   . . .   . . .       127   ◯   ◯   ◯   ◯   ◯   ◯   ◯   X       128   X   X   X   X   X   X   X   ◯       129   ◯   X   X   X   X   X   X   ◯       . . .   . . .   . . .   . . .   . . .   . . .   . . .   . . .   . . .       253   ◯   X   ◯   ◯   ◯   ◯   ◯   ◯       254   X   ◯   ◯   ◯   ◯   ◯   ◯   ◯       255   ◯   ◯   ◯   ◯   ◯   ◯   ◯   ◯                  
 
      A flat display panel displaying an 8-bit gray level and having 8 sub-fields for the arrangement is used for illustrating the relation between sustain pulse number ratio of its sub-field and the arrangement of sub-fields of the output gray level. Assumed that the sustain pulse number is linearly proportional to the brightness, the brightness of the fluorescent powder of three color discharge units R(red), G(green), and B(blue) of any pixel having different sustain pulse numbers of the flat panel display is measured in this experiment, and the results are shown in  FIG. 1 . In the results as shown in  FIG. 1 , if the three color discharge units are normalized by using the sustain pulse number m as the standard, the results will be the same as those shown in  FIG. 2 . Obviously, a non-linear proportional relation exists between the brightness and the gray level (which is the sustain pulse number), and the non-linear proportional relation between the brightness and gray level varies as the discharge units of different colors being made of different fluorescent materials. Non-linear phenomenon is caused by Temperature Effect—discharge cause temperature increasing and attenuate fluorescent light emitting. And further, temperature effect of different color fluorescent is different.  
      (2) Display Ratio &amp; Sustain Frequency  
      The display ratio (DR, which is an average display gray level of the screen) of the plasma display panel varies with different display screens. For different DR, the sustain pulse number ratio of each sub-field is also different as shown in Table 2. This will cause Sustain Frequency (fs) and pulse number different of the same gray level in different display screens and affect the proportional of gray level and brightness.  
                                                       TABLE 2                                                           Total           Display                                   Pulse       Ratio   SF1   SF2   SF3   SF4   SF5   SF6   SF7   SF8   Number   fs (kHz)                                                                            0   0   0   0   0   0   0   0   0   0   0        1%   3   7   14   28   56   112   224   448   892   53.52        2%   3   7   14   28   56   112   224   448   892   53.52       . . .   . . .   . . .   . . .   . . .   . . .   . . .   . . .   . . .   . . .   . . .       10%   3   6   12   25   51   102   204   409   812   48.72       . . .   . . .   . . .   . . .   . . .   . . .   . . .   . . .   . . .   . . .   . . .       20%   2   5   11   22   44   89   179   358   710   42.6       . . .   . . .   . . .   . . .   . . .   . . .   . . .   . . .   . . .   . . .   . . .       40%   2   4   8   16   32   64   128   256   510   30.6       . . .   . . .   . . .   . . .   . . .   . . .   . . .   . . .   . . .   . . .   . . .       80%   1   2   4   8   16   32   64   128   255   15.3       . . .   . . .   . . .   . . .   . . .   . . .   . . .   . . .   . . .   . . .   . . .       100%    1   1   2   4   8   20   45   80   161   9.66                  
 
      From above, the correct gray level output of the display device such as a plasma display panel using the pulse number modulation to produce the gray levels will be affected by the discharge characteristic, the light emitting characteristic of the fluorescent object, the temperature and brightness attenuation effect, and the sustain frequency (fs), etc, and thus we cannot be guaranteed to obtain an accurate brightness output. To overcome this problem, U.S. Pat. No. 5,943,032 issued to Fujitsu proposed a method of adjusting the sustain pulse number, and U.S. Pat. No. 6,088,009 issued to LG proposed a method of adding a pseudo pulse for changing the relation between the gray level and the brightness, such that the gray level is linearly proportional to the brightness. However, the actual effects of these patented technologies are very limited and cannot surely and effectively overcome the foregoing problems.  
      In a digital video processing, it is common to increase the bit number of the image processing appropriately to improve the processing precision, but a general display can show an integral number of the gray levels only. Referring to  FIG. 3  for the traditional method that eliminates the profile contouring phenomenon of the plasma display panel at its low gray level range due to insufficient number of the gray levels for image signals, the image signal of the plasma display panel is generally compensated by an error diffusion circuit  20  with an error diffusion method, so as to compensate the error caused by the difference between the processing bit number and the outputted bit number. However, such method only takes the gray level error into consideration without considering the error between the actual brightness and the ideal brightness of the flat panel display and still cannot surely and effectively overcome the foregoing problem.  
      Therefore, finding a way of compensating the inputted image signal of a flat panel display for the brightness error occurred at different sustain frequencies of the flat panel display in a digital video signal processing to overcome the gray level error as well as the error between the actual output brightness and the ideal brightness of the flat panel display becomes the subject that demands immediate attentions and solutions, in hope of achieving the object of obtaining an actual brightness very close to the ideal brightness.  
     SUMMARY OF THE INVENTION  
      In view of the shortcomings of the prior art that cannot surely and effectively overcome the error between the actual output brightness and the ideal brightness of a flat panel display, the inventor of the present invention based on years of experience to conduct extensive researches and experiments, and finally invented a method for compensating a brightness error of a flat panel display in accordance with the present invention.  
      Therefore, it is a primary objective of the present invention to obtain actual output brightness Y mea  (fs, Gray) by measuring output brightness after a series of gray level signals (Gray) being inputted to a flat panel display under a fixed sustain frequency (fs), and calculate the ideal brightness Y ideal  (fs, Gray) corresponding to the input gray level according to the linear proportional relation between the input gray level and the ideal output brightness, and create a look up table according to the actual output brightness Y mea  (fs, Gray) and the ideal output brightness Y ideal  (fs, Gray). The look up table includes a series of converted gray levels (Gray′) and the actual output brightness Y mea  (fs, Gray′) corresponding to each converted gray level (Gray′), and the actual output brightness Y mea  (fs, Gray′) corresponding to each converted gray level is very close to the ideal output brightness Y ideal  (fs, Gray) of each input gray level. Thus, when a gray level is going to be inputted to the flat panel display, a corresponding converted gray level will be looked up from the look up table first, and the converted gray level will then be inputted to the flat panel display, so that the flat panel display can output a gray level brightness close to the idea output brightness Y ideal  (fs, Gray) of the flat panel display to effectively solve the problem having an error between the actual output brightness and the ideal output brightness of the flat panel display.  
      Another objective of the present invention is to use an error diffusion method to compensate the input gray levels under a fixed sustain frequency by an error diffusion compensation circuit. After the compensated input gray levels are inputted to the flat panel display, the brightness will be outputted and measured to obtain an actual output brightness Y mea  (fs, Gray) and the ideal output brightness Y ideal  (fs, Gray) corresponding to the input gray level is calculated according to the linear proportional relation between the input gray level and its ideal output brightness, and then the look up table is created according to the actual output brightness Y mea  (fs Gray) and the ideal output brightness Y ideal  (fs, Gray), such that the look up table can overcome the gray level error as well as the error between the actual output brightness and the ideal output brightness of the flat panel display.  
      A further objective of the present invention is to create a brightness to gray look up Table (Brightness to Gray LUT) according to the actual output brightness Y mea  (fs, Gray) and the ideal output brightness Y ideal  (fs, Gray), and the look up table includes a first converted gray level LUT BG  (fs, Gray), and each actual output brightness Y mea  (fs, Gray′) corresponds to the first converted gray level LUT BG  (fs, Gray). Each actual output brightness Y mea  (fs, Gray′) corresponding to the first converted gray level LUT BG  (fs, Gray) is a brightness closest to the ideal output brightness Y ideal  (fs, Gray) of each input gray level, so as to effectively overcome the gray level error of the flat panel display as well as effectively overcome the error between the actual output brightness and the ideal output brightness of the flat panel display.  
      A further objective of the present invention is to use the actual output brightness Y mea  (fs, Gray) as the base and the following formula to interpolate and calculate the brightness error compensation value LUT BE  (fs, Gray) required by the ideal output brightness Y ideal  (fs, Gray), assumed that the frequency fs=f and the gray level Gray=g:  
         if   ⁢           ⁢       Y   ideal     ⁡     (     f   ,   g     )         ≥       Y     mea   .       ⁡     (     f   ,   g     )           
           LUT   BE     ⁡     (     f   ,   g     )       =           Y   ideal     ⁡     (     f   ,   g     )       -       Y     mea   .       ⁡     (     f   ,   g     )               Y     mea   .       ⁡     (     f   ,     g   +   1       )       -       Y     mea   .       ⁡     (     f   ,   g     )               
         if   ⁢           ⁢       Y   ideal     ⁡     (     f   ,   g     )         &lt;       Y     mea   .       ⁡     (     f   ,   g     )           
           LUT   BE     ⁡     (     f   ,   g     )       =           Y   ideal     ⁡     (     f   ,   g     )       -       Y     mea   .       ⁡     (     f   ,   g     )               Y     mea   .       ⁡     (     f   ,   g     )       -       Y     mea   .       ⁡     (     f   ,     g   -   1       )               
 
      A brightness error look up table (Brightness Error LUT) is created accordingly. Thus, when a gray level is going to be inputted to the flat panel display, a brightness error compensation value LUT BE  (fs, Gray) is looked up from the look up table first, and the gray level plus the brightness error compensation value LUT BE  (fs, Gray) is obtained as a second converted gray level value. After the second converted gray level value is inputted to the flat panel display, such that the flat panel display can output a brightness Y BE  (fs, Gray) close to the ideal output brightness to overcome the gray level error as well as the error between the actual output brightness and the ideal output brightness of the flat panel display. Another further objective of the present invention is to create both panel brightness gray level look up table and brightness error look up table according to the actual output brightness Y mea  (fs, Gray) and the ideal output brightness Y ideal  (fs, Gray), such that the input gray level is able to utilize the first converted gray level value and the second converted gray level value and then inputted to the flat panel display, enabling the flat panel display to output the output brightness close to the ideal output brightness.  
      The above and other objects, features and advantages of the present invention will become apparent from the following detailed description taken with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a curve showing a proportional relation between a normalized discharge number of the three color discharge units in each pixel and the normalized brightness (corresponding to maximum brightness of all color) of a prior art plasma display panel;  
       FIG. 2  is a curve showing a proportional relation between a normalized discharge number of the three color discharge units in each pixel and the normalized brightness (corresponding to the brightness of m pulse number of each color) of a prior art plasma display panel;  
       FIG. 3  is a flow chart of an input gray level processed by an error diffusion method according to a prior art plasma display panel;  
       FIG. 4  is a flow chart of an input gray level being corrected according to a first preferred embodiment of the present invention;  
       FIG. 5  is a flow chart of an input gray level being corrected according to a second preferred embodiment of the present invention;  
       FIG. 6  is a flow chart of an input gray level being corrected according to a third preferred embodiment of the present invention;  
       FIG. 7  is a curve showing a relation between the brightness of the input gray level and the compensated input gray level according to the first and second preferred embodiments of the present invention; and  
       FIG. 8  is a curve showing a relation between the brightness errors of the input gray level and the compensated input gray level according to the first and second preferred embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Since most of the present flat panel displays are 8-bit gray level displays, the actual image processing is carried out by a processing over 8 bits to improve the accuracy of the image processing. If a high bit (i.e. the integral gray level) of the computation result is used directly for the output, the computational accuracy will be sacrificed and the image processing effect will be affected adversely. For the low bit gray level (i.e. the decimal gray level) that cannot be outputted directly for the display, an error diffusion or a spacial error diffusion method is used for diffusing the low bit gray level to the peripheral pixels as shown in  FIG. 3 , so as to solve the problem produced by a low output accuracy. Since the error diffusion method has been used in many prior arts, including U.S. Pat. No. 6,774,873, and thus the details of the error diffusion method will not be described here.  
      Referring to  FIG. 1 ,  FIG. 2 , Table 1 and Table 2 for the relation between the brightness and the sustain frequency (fs) of the flat panel display, the relation does not show a linear proportion due to the characteristics of different fluorescent materials in the discharge unit. In the first preferred embodiment of the present invention, a series of gray level signals are inputted to each discharge unit of the flat panel display under the fixed sustain frequency (fs) before the brightness of the flat panel display is compensated. In  FIG. 3 , the input gray level is compensated by an error diffusion compensation circuit  20  by using an error diffusion method, After the compensated input gray level is inputted to the flat panel display, the output brightness is measured to obtain an actual output brightness Y mea  (fs Gray). The ideal output brightness Y ideal  (fs, Gray) corresponding to the input gray levels is calculated according to the linear proportional relation between the gray level and the ideal brightness. Therefore, the first preferred embodiment creates a brightness to gray look up table (Brightness to Gray LUT) according to the actual output brightness Y mea  (fs, Gray) and the ideal output brightness Y ideal  (fs, Gray), and the look up table includes a first converted gray level LUT BG  (fs, Gray), and the first converted gray level LUT BG  (fs, Gray) can map an input gray to the corresponding gray level which is the closest to the ideal output brightness Y BG  (fs, Gray), and the brightness error E BG  between the output brightness Y BG  (fs, Gray) and the ideal output brightness Y ideal  (fs, Gray) can be calculated by the following formula: 
 
 E   BG ( fs , Gray)= Y   ideal ( fs , Gray)− Y   BG ( fs , Gray) 
 
      To make the design concept of the first preferred embodiment clearer and more specific, the present invention specially uses the gray level values of 0˜20 as the series of input gray levels as shown in the data of Table 3 to illustrate the method and process of creating the brightness to gray LUT. In  FIG. 3 , when a gray level value of an input signal is equal to 4 and compensated by the error diffusion compensation circuit  20  and inputted to the flat panel display, then the actual measured output brightness Y mea  (fs, 4)=2.18 and the ideal output brightness Y idea  (fs, 4)=3.00. Therefore, the error E ORI  (fs, 4) between the ideal output brightness Y ideal  (fs, 4) and the actual output brightness Y mea  (fs, 4) is up to 0.82. After that, when a gray level value of a signal equal to 5 is inputted, as referring to Table 3, the actual measured output brightness Y mea  (fs, 5)=3.26, and the error is just −0.26 by comparing Y mea  (fs, 5) with the ideal output brightness Y ideal  (fs, 4)=3.00. In other words, Y mea  (fs, 5) is closer than Y mea  (fs, 4) to the corresponding ideal output brightness Y ideal  (fs, 4). When the present invention creates the brightness to gray look up table, the gray level 5 is used as the first converted gray level LUT BG  (fs, 4) for the input gray level 4 to fill up the look up table, and the first converted gray level LUT BG  (fs, 4) is inputted to the flat panel display to provide the output value of the measured brightness Y BG  (fs, 4)=Y mea  (fs, 5)=3.26. Similarly, in  FIG. 3 , after a gray level value 5 of an input signal is inputted to the flat panel display, the actual measured output brightness value Y mea  (fs, 5)=3.26, and the ideal output brightness value Y ideal  (fs, 5)=3.75. Therefore, the error E ORI  (fs, 5) between the ideal output brightness Y ideal  (fs, 5) and the actual output brightness Y mea  (fs, 5) is 0.49. After that, when a signal with a gray level value 6 is inputted, as referring to Table 3, the actual measured output brightness value Y mea  (fs, 6)=3.83, the error is just −0.08 by comparing Y mea  (fs, 6) with the corresponding ideal output brightness Y ideal  (fs, 5)=3.75. In other words, Y mea  (fs, 6) is closer than Y mea  (fs, 5) to the corresponding ideal output brightness Y ideal  (fs, 5). Therefore, the gray level 6 is used as the first converted gray level LUT BG  (fs, 5) for the input gray level 5 to fill up the look up table, and the first converted gray level LUT BG  (fs, 5) is inputted to the flat panel display to produce the output value of the actual brightness Y BG  (fs, 5)=Y mea  (fs, 6)=3.83, and so on. The brightness to gray look up table can be created successfully by the corresponding first converted gray level LUT BG  (fs, Gray) for each input gray and the brightness closest to the ideal output brightness Y BG  (fs, Gray).  
                           TABLE 3                          Input   Ideal   Original   Brightness to Gray LUT                                         Gray   Y ideal     Y mea     E ORI     LUT BG     Y BG     E BG         (g)   (f, g)   (f, g)   (f, g)   (f, g)   (f, g)   (f, g)       (unit)   (cd/m 2 )   (cd/m 2 )   (cd/m 2 )   (gray)   (cd/m 2 )   (cd/m 2 )                                                 0   0.00   0.00   0.00   0   0.00   0.00       1   0.75   0.00   0.75   2   0.57   0.18       2   1.50   0.57   0.93   3   1.62   −0.12       3   2.25   1.62   0.63   4   2.18   0.07       4   3.00   2.18   0.82   5   3.26   −0.26       5   3.75   3.26   0.49   6   3.83   −0.08       6   4.50   3.83   0.67   7   4.85   −0.35       7   5.25   4.85   0.40   8   5.42   −0.17       8   6.00   5.42   0.58   9   6.47   −0.47       9   6.75   6.47   0.28   9   6.47   0.28       10   7.50   7.15   0.35   10   7.15   0.35       11   8.25   7.93   0.32   12   8.47   −0.22       12   9.00   8.47   0.53   13   9.01   −0.01       13   9.75   9.01   0.74   14   9.57   0.18       14   10.50   9.57   0.93   15   10.57   −0.07       15   11.25   10.57   0.68   16   11.14   0.11       16   12.00   11.14   0.86   17   11.92   0.08       17   12.75   11.92   0.83   18   12.94   −0.19       18   13.50   12.94   0.56   18   12.94   0.56       19   14.25   14.43   −0.18   19   14.43   −0.18       20   15.00   14.97   0.03   20   14.97   0.03                  
 
 the actual output brightness and the ideal output brightness of the flat panel display can be obtained by the curves showing the relation between the measured brightness value and the brightness error before and after compensation as shown in  FIGS. 7 and 8 . Obviously, the output brightness error E BG  (fs, Gray) is much smaller than the original error E ORI  (fs, Gray) and thus the gray level and brightness error of the flat panel display can be corrected and compensated to give an excellent effect. In the first preferred embodiment, the brightness to gray look up table is used to produce a panel brightness gray level correction circuit  31  to operate together with a prior art error diffusion compensation circuit  30  as shown in  FIG. 4 . After the error diffusion compensation circuit  30  compensates the input gray level by the error diffusion method, the panel brightness gray level correction circuit  31  corrects the signals inputted to the flat panel display, so as to overcome the gray level error as well as the error between the actual output brightness and the ideal output brightness of the flat panel display. 
 
      In the second preferred embodiment of the present invention, a frequency brightness error compensation function is added to correct the error of the output brightness of the flat panel display caused by the sustain frequency. The method is to input a series of gray level signals to each discharge unit of the flat panel display at a fixed sustain frequency. Again referring to  FIG. 3 , after the gray levels are inputted, an error diffusion compensation circuit  20  will compensate the gray levels by using an error diffusion method, and the compensated gray level will be inputted to the flat panel display, and the output brightness will be measured to obtain the actual output brightness Y mea  (fs, Gray). The ideal output brightness Y ideal  (fs, Gray) corresponding to the input gray level value is calculated according to the ideal linear proportional relation between the gray level and the brightness. Therefore, the invention uses the actual output brightness Y mea  (fs, Gray) as a base to interpolate and calculate the brightness error compensation value LUT BE  (fs, Gray) required for the ideal output brightness Y ideal  (fs, Gray), if the frequency at the time is fs=f, and the gray level value Gray=g.  
         if   ⁢           ⁢       Y   ideal     ⁡     (     f   ,   g     )         ≥       Y     mea   .       ⁡     (     f   ,   g     )           
           LUT   BE     ⁡     (     f   ,   g     )       =           Y   ideal     ⁡     (     f   ,   g     )       -       Y     mea   .       ⁡     (     f   ,   g     )               Y     mea   .       ⁡     (     f   ,     g   +   1       )       -       Y     mea   .       ⁡     (     f   ,   g     )               
         if   ⁢           ⁢       Y   ideal     ⁡     (     f   ,   g     )         &lt;       Y     mea   .       ⁡     (     f   ,   g     )           
           LUT   BE     ⁡     (     f   ,   g     )       =           Y   ideal     ⁡     (     f   ,   g     )       -       Y     mea   .       ⁡     (     f   ,   g     )               Y     mea   .       ⁡     (     f   ,   g     )       -       Y     mea   .       ⁡     (     f   ,     g   -   1       )               
 
      After a series of gray level signals are inputted to the flat panel display at different sustain frequencies, the above formula is used to calculate the brightness error compensation value LUT BE  (fs, Gray) of each input gray level at different frequencies, and a brightness error look up table (Brightness Error LUT) is created as shown in Table 4. Thus, after an input gray level is used to look up a corresponding brightness error compensation value in the brightness error look up table, the original input gray level plus the brightness error compensation value are used to obtain a second converted gray level value. After the second converted gray level value is inputted to the flat panel display, the flat panel display outputs a brightness close to the ideal output brightness Y BE  (fs, Gray). The brightness error E BE  between the actual output brightness and the ideal output brightness Y ideal  (fs, Gray) is given by the following formula: 
 
 E   BE ( fs , Gray)= Y   ideal ( fs , Gray)− Y   BE ( fs , Gray) 
 
     
       
         
           
               
               
               
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                   
               
               
                 Input 
                 Ideal 
                 Original 
                 Brightness Error LUT 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Gray 
                 Y ideal   
                 Y mea   
                 E ORI   
                 LUT BE   
                 Y BE   
                 E BE   
               
               
                 (g) 
                 (f, g) 
                 (f, g) 
                 (f, g) 
                 (f, g) 
                 (f, g) 
                 (f, g) 
               
               
                 (unit) 
                 (cd/m 2 ) 
                 (cd/m 2 ) 
                 (cd/m 2 ) 
                 (gray) 
                 (cd/m 2 ) 
                 (cd/m 2 ) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 0 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
               
               
                 1 
                 0.75 
                 0.00 
                 0.75 
                 1.32 
                 0.81 
                 −0.06 
               
               
                 2 
                 1.50 
                 0.57 
                 0.93 
                 0.89 
                 1.45 
                 0.05 
               
               
                 3 
                 2.25 
                 1.62 
                 0.63 
                 1.12 
                 2.20 
                 0.05 
               
               
                 4 
                 3.00 
                 2.18 
                 0.82 
                 0.76 
                 3.01 
                 −0.01 
               
               
                 5 
                 3.75 
                 3.26 
                 0.49 
                 0.86 
                 3.84 
                 −0.09 
               
               
                 6 
                 4.50 
                 3.83 
                 0.67 
                 0.65 
                 4.51 
                 −0.01 
               
               
                 7 
                 5.25 
                 4.85 
                 0.40 
                 0.70 
                 5.29 
                 −0.04 
               
               
                 8 
                 6.00 
                 5.42 
                 0.58 
                 0.55 
                 6.02 
                 −0.02 
               
               
                 9 
                 6.75 
                 6.47 
                 0.28 
                 0.41 
                 6.69 
                 0.06 
               
               
                 10 
                 7.50 
                 7.15 
                 0.35 
                 0.45 
                 7.52 
                 −0.02 
               
               
                 11 
                 8.25 
                 7.93 
                 0.32 
                 0.60 
                 8.25 
                 0.00 
               
               
                 12 
                 9.00 
                 8.47 
                 0.53 
                 0.99 
                 9.02 
                 −0.02 
               
               
                 13 
                 9.75 
                 9.01 
                 0.74 
                 1.31 
                 9.70 
                 0.05 
               
               
                 14 
                 10.50 
                 9.57 
                 0.93 
                 0.93 
                 10.48 
                 0.02 
               
               
                 15 
                 11.25 
                 10.57 
                 0.68 
                 1.20 
                 11.20 
                 0.05 
               
               
                 16 
                 12.00 
                 11.14 
                 0.86 
                 1.10 
                 12.08 
                 −0.08 
               
               
                 17 
                 12.75 
                 11.92 
                 0.83 
                 0.81 
                 12.76 
                 −0.01 
               
               
                 18 
                 13.50 
                 12.94 
                 0.56 
                 0.38 
                 13.53 
                 −0.03 
               
               
                 19 
                 14.25 
                 14.43 
                 −0.18 
                 −0.12 
                 14.31 
                 −0.06 
               
               
                 20 
                 15.00 
                 15.00 
                 0.03 
                 0.00 
                 15.00 
                 0.00 
               
               
                   
               
            
           
         
       
     
      To make the design concept of the second preferred embodiment clearer and more specific, the present invention specially uses a series of gray level value 0˜20 for the input gray levels to operate with the data as shown in Table 4 to illustrate the method and process of creating the brightness error look up table. As referring to  FIG. 3  again, when the gray level values 4 and 5 of the input signals have been processed by the error diffusion compensation circuit  20  through an error diffusion method for compensating the errors thereof and are inputted to the flat panel display, the actual measured brightness values Y mea  (fs, 4) and Y mea  (fs, 5) are 2.18 and 3.26, and the ideal output brightness values Y ideal  (fs, 4) and Y ideal  (fs, 5) are 3.00 and 3.75 respectively. Since Y mea  (fs, 4)&lt;Y ideal  (fs, 4), therefore the foregoing brightness error compensation formula is used to obtain the brightness error compensation value LUT BE  (fs, 4)=0.76. Now, the original input gray level 4 plus the brightness error value LUT BE  (fs, 4) are used to produce a second converted gray level. After the second converted gray levels are inputted to the flat panel display, the actual measured output brightness Y BE  (fs, 4)=3.01 of the flat panel display is measured and the E BE  (fs, 4) between the actual output brightness Y BE  (fs, 4)=3.01 and the ideal output brightness Y ideal  (fs, 4)=3.00 is just −0.01, which is closer to the ideal output brightness Y ideal  (fs, 4), and so on. In the second preferred embodiment, the brightness error compensation value LUT BE  (fs, Gray) corresponding to each input gray level and the brightness closest to the ideal output brightness Y BE  (fs, Gray) are found for creating the brightness error look up table successfully.  
      From Table 4, after the gray levels inputted to the flat panel display have been corrected by the brightness error look up table, in view of curves shown in  FIGS. 7 and 8  for showing the relation between the brightness values and the brightness error before and after compensation, the error E BE  (fs, Gray) between the actual output brightness and the ideal output brightness of the flat panel display is obviously much smaller than the original error E ORI  (fs, Gray), and the brightness error look up table definitely produces an excellent correction and compensation effect to the brightness error caused by the sustain frequency. Therefore, the second preferred embodiment can produce a brightness error compensation circuit  41  according to the brightness error look up table as shown in  FIG. 5  to operate with a prior art error diffusion compensation circuit  40 . Before the error diffusion compensation circuit  40  uses the error diffusion method to compensate the input gray levels, the brightness error compensation circuit  41  corrects the signals inputted to the flat panel display in advance, so as to overcome the gray level error as well as the error between the actual output brightness and the ideal output brightness of the flat panel display.  
      The third preferred embodiment of the present invention bases on the first and second preferred embodiments of the invention to measure the actual output brightness value Y mea  (fs, Gray) and the ideal output brightness value Y ideal  (fs, Gray) to create a brightness to gray look up table and a brightness error look up table respectively as shown in Table 5, wherein the first converted gray level LUT BG  (fs, Gray) in the brightness to gray look up table maps the original input gray level to a corresponding brightness closest to the ideal output brightness Y BG  (fs, Gray), and the brightness error E BG  between the output brightness Y BG  (fs, Gray) and the ideal output brightness Y ideal  (fs, Gray) can be calculated according to the following formula: 
 
 E   BG ( fs , Gray)= Y   ideal ( fs , Gray)− Y   BG ( fs , Gray) 
 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                   
               
               
                 Input 
                 Ideal 
                 Original 
                 Brightness to Gray LUT 
                 Brightness Error LUT 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Gray 
                 Y ideal   
                 Y mea   
                 E ORI   
                 LUT BG   
                 Y BG   
                 E BG   
                 LUT BE   
                 Y BE   
                 E BE   
               
               
                 (g) 
                 (f, g) 
                 (f, g) 
                 (f, g) 
                 (f, g) 
                 (f, g) 
                 (f, g) 
                 (f, g) 
                 (f, g) 
                 (f, g) 
               
               
                 (unit) 
                 (cd/m 2 ) 
                 (cd/m 2 ) 
                 (cd/m 2 ) 
                 (gray) 
                 (cd/m 2 ) 
                 (cd/m 2 ) 
                 (gray) 
                 (cd/m 2 ) 
                 (cd/m 2 ) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 0 
                 0.00 
                 0.00 
                 0.00 
                 0 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
               
               
                 1 
                 0.75 
                 0.00 
                 0.75 
                 2 
                 0.57 
                 0.18 
                 0.17 
                 0.70 
                 0.05 
               
               
                 2 
                 1.50 
                 0.57 
                 0.93 
                 3 
                 1.62 
                 −0.12 
                 −0.11 
                 1.45 
                 0.05 
               
               
                 3 
                 2.25 
                 1.62 
                 0.63 
                 4 
                 2.18 
                 0.07 
                 0.06 
                 2.20 
                 0.05 
               
               
                 4 
                 3.00 
                 2.18 
                 0.82 
                 5 
                 3.26 
                 −0.26 
                 −0.24 
                 2.98 
                 0.02 
               
               
                 5 
                 3.75 
                 3.26 
                 0.49 
                 6 
                 3.83 
                 −0.08 
                 −0.14 
                 3.73 
                 0.02 
               
               
                 6 
                 4.50 
                 3.83 
                 0.67 
                 7 
                 4.85 
                 −0.35 
                 −0.35 
                 4.49 
                 0.01 
               
               
                 7 
                 5.25 
                 4.85 
                 0.40 
                 8 
                 5.42 
                 −0.17 
                 −0.30 
                 5.19 
                 0.06 
               
               
                 8 
                 6.00 
                 5.42 
                 0.58 
                 9 
                 6.47 
                 −0.47 
                 −0.45 
                 5.89 
                 0.11 
               
               
                 9 
                 6.75 
                 6.47 
                 0.28 
                 9 
                 6.47 
                 0.28 
                 0.41 
                 6.72 
                 0.03 
               
               
                 10 
                 7.50 
                 7.15 
                 0.35 
                 10 
                 7.15 
                 0.35 
                 0.27 
                 7.44 
                 0.06 
               
               
                 11 
                 8.25 
                 7.93 
                 0.32 
                 12 
                 8.47 
                 −0.22 
                 −0.16 
                 8.25 
                 0.00 
               
               
                 12 
                 9.00 
                 8.47 
                 0.53 
                 13 
                 9.01 
                 −0.01 
                 −0.01 
                 8.95 
                 0.05 
               
               
                 13 
                 9.75 
                 9.01 
                 0.74 
                 14 
                 9.57 
                 0.18 
                 0.18 
                 9.72 
                 0.03 
               
               
                 14 
                 10.50 
                 9.57 
                 0.93 
                 15 
                 10.57 
                 −0.07 
                 −0.07 
                 10.45 
                 0.05 
               
               
                 15 
                 11.25 
                 10.57 
                 0.68 
                 16 
                 11.14 
                 0.11 
                 0.15 
                 11.25 
                 −0.00 
               
               
                 16 
                 12.00 
                 11.14 
                 0.86 
                 17 
                 11.92 
                 0.08 
                 0.08 
                 12.03 
                 −0.03 
               
               
                 17 
                 12.75 
                 11.92 
                 0.83 
                 18 
                 12.94 
                 −0.19 
                 −0.19 
                 12.71 
                 0.04 
               
               
                 18 
                 13.50 
                 12.94 
                 0.56 
                 18 
                 12.94 
                 0.56 
                 0.38 
                 13.48 
                 0.02 
               
               
                 19 
                 14.25 
                 14.43 
                 −0.18 
                 19 
                 14.43 
                 −0.18 
                 −0.12 
                 14.26 
                 −0.01 
               
               
                 20 
                 15.00 
                 15.00 
                 0.00 
                 20 
                 15.00 
                 0.00 
                 0.00 
                 15.00 
                 0.00 
               
               
                   
               
            
           
         
       
     
      The brightness error look up table includes a second converted gray level equal to the original input gray level plus the brightness error compensation value LUT BE  (fs, Gray) as given below:  
         if   ⁢           ⁢       Y   ideal     ⁡     (     f   ,   g     )         ≥       Y   BG     ⁡     (     f   ,   g     )           
           LUT   BE     ⁡     (     f   ,   g     )       =           Y   ideal     ⁡     (     f   ,   g     )       -       Y   BG     ⁡     (     f   ,   g     )               Y   BG     ⁡     (     f   ,     g   +   1       )       -       Y   BG     ⁡     (     f   ,   g     )               
         if   ⁢           ⁢       Y   ideal     ⁡     (     f   ,   g     )         &lt;       Y   BG     ⁡     (     f   ,   g     )           
           LUT   BE     ⁡     (     f   ,   g     )       =           Y   ideal     ⁡     (     f   ,   g     )       -       Y     mea   .       ⁡     (     f   ,   g     )               Y   BG     ⁡     (     f   ,   g     )       -       Y   BG     ⁡     (     f   ,     g   -   1       )               
 
      After the second converted gray level is inputted to the flat panel display, the flat panel display will output a brightness close to the ideal output brightness Y BE  (fs, Gray), and the brightness error E BE  between the output brightness Y BE  (fs, Gray) and the ideal output brightness Y ideal  (fs, Gray) can be calculated according to the following formula: 
 
 E   BE ( fs , Gray)= Y   ideal ( fs , Gray)− Y   BE ( fs , Gray) 
 
      From Table 5, after the gray levels inputted to the flat panel display are corrected by the brightness to gray look up table and the brightness error look up table, the error E BE  (fs, Gray) between the actual output brightness and the ideal output brightness of the flat panel display is much smaller than the original error E ORI  (fs, Gray) and the error after brightness to gray LUT E BG  (fs, Gray), and thus the correction and compensation of the brightness error produce a certain effect. The third preferred embodiment can use the brightness to gray look up table and the brightness error look up table to produce a panel brightness gray level correction circuit  52  and a brightness error compensation circuit  51  as shown in  FIG. 6  to operate with a prior art error diffusion compensation circuit  50 . Therefore, before the error diffusion compensation circuit  50  can compensate the input gray level by using an error diffusion method, and the brightness error compensation circuit  51  can correct the signals inputted to the flat panel display in advance. After the error diffusion compensation circuit  50  compensates the input gray level, the panel brightness gray level correction circuit  52  corrects the signals inputted to the flat panel display to overcome the gray level error as well as the error between the actual A output brightness and the ideal output brightness of the flat panel display.  
      It is worthy to note that the first, second, and third preferred embodiments of the present invention are adopted for overcoming the foregoing error, but these preferred embodiments are not intended for limiting the hardware circuits, and those skilled in the arts can accomplish the same purpose by software according to the foregoing design concept of the invention. In addition, the invention is not limited to be used for plasma display panels only. For any flat panel display having an unexpected result on the proportional relation between the gray level and the brightness of the flat panel display, the method disclosed in the foregoing preferred embodiments can be used, and the actual measured output brightness and the calculated ideal output brightness of the flat panel display can be used to create a look up table. Before a gray level is inputted to the flat panel display, the look up table is used to find a corresponding converted gray level, and the brightness error compensation gray level value corresponding to the converted gray level will be inputted to the flat panel display to allow the flat panel display to output a gray level brightness closest to the ideal output brightness Y ideal  (fs, Gray), and all of the above are intended to be covered in the patent claims of this invention.  
      While the invention herein disclosed has been described by means of specific embodiments, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope and spirit of the invention set forth in the claims.