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White Balance In Gamut Correction

White Balance In Gamut Correction

Nov 16, 2017

  

The color coordinates and uniformity of white field are the important quality indicators of LED display, and also an important standard to measure the color fidelity. Therefore, the red, green and blue primary colors of the LED display screen must be treated with white balance. It requires that the 3 primary colors are still pure white [10] at the same gray level.


Without using the color gamut correction system LED display, only red, green, blue three primary color LED white balance processing can be [11-12]. The white balance in the gamut correction process, in addition to the accuracy of white field coordinates, must also consider whether the luminous intensity of LED can meet the requirements of gamut correction, i.e., formula (10), and avoid new display defects due to insufficient display ability.


In this paper, the color coordinate (0.313,0.329) of D65 standard light source is regarded as the target of white field color coordinate. Therefore, only the target brightness value Yaim after white balance can be obtained, and the target three stimulus values Xaim and Zaim can be obtained.


Type (11) describes the process of the white balance, color correction after obtained after the new red, green and blue color of M, N, l don't multiply the three stimulus values obtained for white m, N, three stimulate value L for white balance coefficients of 3 primary colors, each color is used to adjust the intensity finally, the same gray level the same color of the LED light for the synthesis of pure white light, color coordinates of (0.313, 0.329).

  16.png


 In the formula (12), the unknown quantities are Yaim, Xaim and Zaim, which can be obtained by the color coordinate formula. Obviously, different Yaim values calculate different m, N, L. In order to ensure the high brightness of the LED display, the greater the value of Yaim, the better the brightness of the white field, and the brightness of the red LED (10) is further expressed as LED:

17.png

 

In formula (13), when mkr1+nkg1+lkb1>1, the red LED brightness can not meet the maximum brightness of the "white field", LED display defect red component insufficient will appear in the highest brightness "white field", the corresponding LED display brightness and color pixels will be different with the normal LED display pixel, influence great display quality.


Therefore, for the red, green, blue LED of each display pixel in the LED display, there will be a restrictive condition (14). When the formula (14) is satisfied, gamut correction and white balance can be realized simultaneously.

18.png

In order to meet the type (14), must choose the appropriate Yaim value is difficult to direct calculation, the most reasonable algorithm through the bisearch algorithm, Y=YR+YG+YB, on the Yaim value of bisearch between 0~Y, will continue to put the Yaim into (12) is calculated, and the type of pixels to judge (14) show all of the type (14), to meet the maximum Yaim value is the white balance when the target brightness value.


For LED in Table 1, when Yaim=0.875X (YR+YG+YB), after the gamut correction, the white field color coordinates before the white balance are W1 points in Figure 4, and after white balance, the white field color coordinates are changed to W2, while the white balance target color coordinates are (0.313,0.329),


According to the graph, the difference between the color coordinates of any display pixel and the target color coordinates is less than 0.003[3],

19.png

 

This algorithm can complete white balance very well. As shown in Figure 4, "white field" coordinates is determined, and before the three primary color coordinates after correction with LED, all the pixels of the color gamut will converge, the color coordinates of all pixels of each color LED can display will be determined, so as to achieve precise control of color LED display.


When the white balance target value is Yaim, the correction factor corresponding to the current display pixel can be arranged as (15):


20.png


 At this time, a LED display pixel of red, green, blue LED gray level is I1, I2, I3, after the gamut correction, the output of the true gray level is:

21.png

 conclusion


The mathematical model of LED display color is established, and the gamut correction algorithm is used to improve the inconsistency of LED display color gamut. Furthermore, the processing methods of the defects introduced by correction in white balance are discussed further.


The experimental results show that after color correction, LED display pixel brightness and color consistency increased, color color coordinates of each display pixel and the "white" color coordinates are highly close to the target color correction, color coordinates can achieve 0.005 error, "white" color coordinate error 0.003 brightness difference is less than 3% LED, can greatly improve the display effect of the display screen.