Field Sequential Color Liquid Crystal Display (FSC-LCD) removes the color-filter. In order to generate a colorful image, the FSC-LCD flashed each color component in sequence using synchronously pulsed colored backlights. In RGB driving method, the FSC-LCD shows red, green, and blue images sequentially as Fig.
2-1 shows. Based on the temporal color mixing phenomena, human vision system will combine those images and feel a colorful image.
The FSC-LCD driving scheme includes three main parts: the TFT addressing, the LC orientation, and the backlight flashing [6]. A frame is divided into three fields. The frame rate is 60Hz the same as the conventional LCD to present people from feeling flicker. Thus, the field rate will be 3 times higher than the frame rate is. In each field, the display has to complete loading data for the whole panel, orientating liquid crystal
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to the right position, and flashing backlight as shown in Fig. 2-2. After showing the red, green, and blue fields, the inherent temporal color mixing mechanism in human vision system mixes the three fields, and generates a full-color image as Fig. 2-3 shows.
Fig. 2-1 The RGB driving method of FSC-LCD.
Fig. 2-2 The FSC-LCD driving scheme.
Fig. 2-3 Temporal color mixing.
The display is driven in 60 Hz frame rate
RED field GREEN field BLUE field
RED field
GREEN field
BLUE field Full-colorimage
Sequentially displayed three field time
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2.1.1 Color Break-up Phenomenon
FSC-LCD is a color filter-less display, it achieves three times higher optical throughput than a conventional LCD. The way FSC-LCD forms a colorful image, sequentially displaying fields, brings itself a serious issue, Color Break-up (CBU).
CBU looks as a rainbow blur on the object edge in the image as Fig. 2-4 shows. CBU strongly reduces the image quality and bothers people when they watch TV.
CBU can be typically divided into two types. One is a stationary type which occurs in stationary image. When people watch a display, the human eye performs a sequence of saccades [7] and fixations. Saccade is an action of scanning quickly, as Fig.
2-5 shows, but fixation is an action of gazing at something. The white lines in Fig. 2-5 describe the randomly eye saccade movements. The stationary CBU mechanism is shown in Fig. 2-6 [8][9]. The gray line in Fig. 2-6 (a) is an eye saccade movement.
When the eye moves along the gray line, the CBU phenomenon occurs, as shown in Fig. 2-6 (b). The other type of CBU is the dynamic CBU. A dynamic CBU occurs when the eye traces the smoothly moving object. The mismatch of the object’s digital motion and the eye’s analogic motion makes the red, green, and blue fields locate on different places on the retina. The mechanism of CBU is illustrated in Fig. 2-7 [10].
When a white bar moves from left side to right side in a panel, the red, green, and blue fields in the same frame locate at the same horizontal position. However, tracing the white bar is a smooth and analogic motion for the eye, as the blue block in Fig. 2-7 shows. Thus, the edges of the white bar lose some colors and look colorful here as shown in the bottom of Fig. 2-7. The colorful edges are the dynamic CBU.
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(a) The original image (b) CBU image
Fig. 2-4 A comparison between the original image and CBU image: (a) the original image and (b) CBU image.
Fig. 2-5 The saccade phenomena.
(a) (b)
Fig. 2-6 The stationary CBU mechanism. (a) The image displayed on the FSC panel and the path of a saccade movement (gray line), and (b) static CBU phenomenon.
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Fig. 2-7 The mechanism of dynamic CBU.
2.1.2 Methods to Suppress CBU
CBU is a serious issue in FSC. Many methods were reported to suppress CBU such as RGBRGB, RGBCY, and RGBKKK [10]. RGBRGB increases the field rate to shrink the colorful edges. RGBCY inserts cyan and yellow fields to reduce color differences among each field and the CBU visibility. RGBKKK which gave the best CBU suppression increases the field rate and inserts black fields to shrink the colorful edges. Both RGBRGB and RGBKKK shrink the colorful edges produced by fields.
However, the total CBU area is thinner in RGBKKK. All the methods can suppress CBU effect. However, all of them increased the field rate and reduces the LC response time a lot.
The analogic motion of the eye
Horizontal position
Time
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2.1.3 Two Field Method
The FSC-LCD drives LC cells three times or even higher faster than the conventional LCD does. However, the LC cells on the market are hard to response and orient in such a short period. Thus, a method, Two Field method, utilizing the least fields to generate a colorful image was proposed. The LED backlight and local color dimming backlight technique accomplished the Two Field method [11].
The lack of the third primary color field prevents FSC-LCD from showing a full-color image. Thus, the Two Field method has to incorporate the local color dimming backlight technique to show three primary colors in two fields. The replacement of the LED backlight achieves the local color dimming backlight technique. In the first field, the Two Field method shows red information and part of blue information by locally controlling and lighting up red and blue LEDs. In the second field, Two Field method shows green information and the remaining blue information as Fig. 2-8 shows. Therefore, the Two Field method can merely use two fields to show the three primary colors. The reduced number of fields reduces a field time and gains more time for LC.
The CBU visibility comparison is compared in Fig. 2-9. Conventional FSC show strong CBU phenomena. The color band edges in convention FSC, as shown in Fig.
2-9 (a), are induced by the primary colors which are easy to be observed. However, the color band edges in Two Field method are mixed colors. Human vision system is less sensitive for the mixed colors as shown is Fig. 2-9. Thus, the CBU visibility of the Two Field is lower than the convention FSC.
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Fig. 2-8 RGB driving method versus Two Field method.
(a) (b)
Fig. 2-9 CBU comparison (a) conventional FSC, and (b) Two Field method.