Accurate Color Capture

344 • SID 03 DIGEST P-38: Driving Scheme for Improving Color Performance of LCD’s:Accurate Color CaptureSeung Woo Lee*, Jong Seon Kim, Jun SoukLCD R&D , AMLCD Div, Samsung Electronics, Kiheung, Kyunggi-Do, KoreaAbstractThis paper describes a novel driving scheme, ACC (Accurate Color Capture), for improving color accuracy of LCD's. ACC changes RGB gamma curves separately to reduce the color shift with respect to the gray level. ACC dramatically reduced correlated color temperature shift from 3300K to 75K through 255-64 gray range with 2.4 gray gamma value. As experimental results, ACC is little affected by gamma value variation of 2.0-2.8.1. IntroductionRecently, performance of TFT-LCD's has been improved significantly in the area of brightness, contrast ratio, response time, and color saturation. As performance of LCD is improved, color accuracy of TFT-LCD's has become an important issue. Many studies about color of LCD's have been carried out than ever [1], [2]. Most LCD's, however, have color shift phenomenon [2]. The color shiftis caused by the different transmittance characteristics of liquid crystal molecules with respect to red, green, and blue light. In this paper, we propose a novel driving scheme to eliminate color shift in LCD's.2. Color shift in liquid crystal displaysFig.1 shows the color shift phenomenon in typical LCD. The correlated color temperature (CCT) rises up to 10,000K at 64-gray from 6,500K at white. Actually, gray image appears to be bluish on the LCD. The tristimulus values, x and y, are shifted by 0.03 and 0.04, respectively as shown in Fig. 2. The color shift is caused by the relation of transmittance of LCD with respect to wavelength of light. That is, the higher transmittance is, the shorter wavelength is. Fig. 3 shows gamma curves of red, green, and blue light. As shown in Fig. 3, blue gamma curve is located above green and red curves. It is believed that the relative higherportion of blue gives a bluish image, that is, a color shiftedFig.1 Color shift phenomenonFig. 2 Color coordinate of white to 64-grayISSN/0003-0966X/03/3401-0344-$1.00+.00 © 2003 SIDimage. The key idea of ACC for improving color shift is to reduce blue portion and to increase red portion. To do so, blue and red gamma curves have to be moved independently downward and upward, respectively.3. ACC (Accurate Color Capture)ACC changes RGB gamma curves separately to reduce color shift. Fig. 4 shows how to change the original gamma curve to a new target gamma curve. For example, to move a gamma curve to lower location, we can find a new image data to get the luminance of the target gamma curve. As shown in Fig. 4, 122 and 123 input data must be changed to 120.3 and 121.5, respectively. The only way to express 120.3 and 121.5 is extension of the data bit to 9 or 10 bit data. Therefore, selection of 256 data with extended data bit is the first step in ACC. Because driver IC has 6 or 8 bit data input, 9 or 10 bit data must be transformed to 6 or 8 bit data. The bit reduction is carried out through the temporal and spatial averaging (FRC: frame rate control). As a consequence, ACC comprises a data expansion step for changing gamma curves adaptively and a bit reduction step for data format of drive IC.4. Experimental resultsColor shift enhancement: After selecting ACC data for the panel with 2.4 gray gamma shown in Fig. 1, we obtained a almost constant CCT (|△CCT| < 100K) from white to 64-Fig. 3 Gamma curves of red, green, and blue121.5120.3Fig. 4 Schematic diagram of gamma curvemodificationSID 03 DIGEST • 345346 • SID 03 DIGESTgray level as shown in Fig. 5. The measured gamma curves of RGB were different from those before ACC (Fig. 6). Blue gamma curve was located at the lowest position after ACC. And red gamma curve is located at similar position of green. As mentioned before, red portion was increased and blue portion was reduced so that color shift might beminimized. Fig. 7 shows that ACC gives a little color shift from white to 64-gray. Before ACC, there were color coordinate shifts of 0.033 and 0.041 in △x and △y, respectively, as shown in Fig. 8. But after ACC, the maximum shifts were 0.001 and 0.003 in △x and △y, respectively.Application to various gamma values : ACC is a technique to change original gamma curve to a new gamma curve. Therefore, it is necessary to evaluate the effect of ACC application when original gamma curve varies. Optimum ACC data was selected in LCD with the original gamma value of 2.4. We applied the same ACC data to LCD with various gamma values from 2.0 to 2.8. Fig. 9 shows application results of ACC to various gamma values. As shown in the figure, almost constant CCT can be obtained even though gamma values varies from 2.0 to 2.8. In addition, there were a little variation in △x and △yafter applying same ACC data as shown in Table 1.Fig. 6 Gamma curves of red, green, and blueafter ACCFig. 7 Comparison of color coordinate before andafter ACCFig. 8 Color coordinate x and y with respect to gray data before and after ACC (□: before ACC, ■ after ACC)SID 03 DIGEST • 347Table 1. Application results of ACC to various gamma values (all were selected as maximum data in the range between 255 and 64-gray)5. ConclusionsWe proposed a new driving method for improving the color performance of LCD's. We call it ACC (Accurate Color Capture). ACC changes RGB gamma curves separately to reduce the color shift. ACC comprises a data expansion step for changing gamma curves adaptively and a bit reduction step for drive IC data format. ACC dramatically reduced correlated color temperature variation from ~3300K to ~75K through 255-64 gray range. Asexperimental results, ACC is little affected by gamma value variation. ACC is a very simple way to improve color shift in LCD's.6. References[1] H. Sugiura, et al., "LCD module adopting a color conversion circuit", SID '02, pp. 288-291, 2002.[2] Y. Utsumi, et al., "Development of authentic-color IPS LCD without color tracking," pp. 820-823, 2002.Fig. 9 Measured CCT with respect to gamma values after ACC。