1.1 Field-Sequential-Color Liquid Crystal Display (FSC LCD)
Liquid Crystal Display (LCD) has been globally popularized for displaying various kinds of information, because it is thinner and lighter than the Cathode Ray Tube (CRT) display.
Furthermore, the power consumption of LCD is also lower than that of CRT. The configuration of conventional LCD using spatial color formation mechanism, as shown in Fig.
1(a), to represent color has been developed using a combination of Liquid Crystal (LC) cell, Color Filter (CF), and Cold Cathode Fluorescent Lamp (CCFL) backlight. A single pixel consists of three sub-pixels, red (R), green (G), and blue (B) in conventional LCDs. Under suitable viewing conditions, each sub-pixel is indistinguishable and the light from each sub-pixel is seen as emitting form a single pixel. However, conventional LCDs have some drawbacks in regards light efficiency. Conventional LCD is only one third of the total light transmittance from backlights. Additional high power light sources must be used in order to increase system luminance.
However, images have become higher quality and higher resolution, thus LCDs require higher resolution to maintain image quality. In the Field-Sequential-Color (FSC) LCD, when using three-fields, each frame is temporally divided into R, G, and B. By sequentially displaying R, G, and B fields fast enough, a full color image can be perceived. Therefore, the temporal color formation mechanism is widely used in all kinds of information displays to achieve high resolution [1-3]. The architecture of FSC LCD is shown in Fig. 1(b).
(a) (b)
Fig. 1 The features of (a) color TFT LCD and (b) color FSC LCD.
Unfortunately, FSC LCDs are required to have a much faster response time for LC molecules and backlight than in the conventional LCDs. Therefore, in order to drive FSC displays, a driving schedule for scanning data, LC response, and backlight flashing time are important. The timing chart is shown in Fig. 2. FSC display has a data loading time (tTFT) and an illumination time (tBL) for each of color field. An additional setting time (tLC) related to the characteristic response time of liquid crystal is also required. As shown in Eq. (1), scanning the whole panel, LC response, and the backlight flash must be completed within one field time 1/3f.
1
3f = tTFT + tLC + tBL (1) where f is the frame frequency, tTFT is the scanning time for the whole panel, and tLC is LC
response time, and tBL is the backlight flash time.
Fig. 2 Timing chart in the FSC LCD with TFT addressing.
However, the fluorescent lamp is normally difficult to use for pulse operations, because mercury vapor pressure drops during the blanking duration and light output is decreased as a result. The fluorescence decay times for R and G phosphors used in a conventional fluorescent tube are not fast enough for the FSC display. Therefore, LEDs are used in FSC LCDs because they satisfy faster response times for color sequential operation. In addition, the emission spectra of LEDs are suitable for displaying color purity.
Using the current FSC platform, a color-filter-less LCD with R, G, and B LEDs as light sources was developed [4]. Optical Compensation Band (OCB) mode LC was proposed to achieve a field rate of 180Hz for a fast response. The 32-inch screen with resolution 1366*768 pixels and LEDs were used in 20*12 array layout. Finally, the maximum brightness was 407.78 cd/m2and the color gamut was 125% compared with the NTSC standard.
1.2 Color Break-up Phenomenon
Color Break-up (CBU), or the rainbow effect, is known as a latent artifact in FSC LCDs in degrading visual quality [5-9]. An illustration of CBU is shown in Fig. 3. CBU occurs when there is a difference in the relative motion between the object within the image and the observer’s eyes.
(a) (b)
Fig. 3 (a) A stationary image and (b) the perceived image with CBU due to relative motion in the FSC display.
1.3 Motivation and Objective
Since CBU is an intrinsic visual artifact, it degrades the image quality and causes visual discomfort. Therefore, topics related to CBU in FSC displays have been an important research themes in both the academic and industrial communities. The major subjects of CBU phenomenon can be classified into formation mechanism, quantitative analysis, and resolution.
While formation mechanism and resolution have gained numerous results, a methodology or an index for defining CBU quantitatively has not been commonly acknowledged.
Using either the CIELAB color difference or the CIEDE2000 value, as a trial metric for quantifying CBU [9-10], is regarded inadequate for two reasons. Firstly, both color difference formulas are fitted based on data sets from color-patch samples, not complex images.
Moreover, the linearity of perception difference is merely effective in small range differences [11]. That is, large difference values, due to large hue difference from separate R, G, and B fields, may fail to reflect perceptions moderately.
The difficulty in quantifying CBU is at least two-fold. First, no physical and identical stimuli of CBU can be controlled for repeated analysis. Second, the human visual system has complex responses to flash stimuli [12]. Therefore, the objective of this thesis is to build up a
methodology that can simulate CBU stimuli correctly and derive an index matching that of psychometric evaluation.
1.4 Organization
This thesis is organized as following. In Chapter 2, the properties of the human color vision will be introduced. Then, the prior arts of CBU quantification methods will be briefly summarized. In Chapter 3, the proposed methodology will be presented. Then, the proposed model for psychophysical evaluation of stationary images will be described in Chapter 4.
Moreover, the sensitivity of the CBU fringe will be computed by using the proposed model.
After that, psychophysical evaluations of stationary images will be described. In Chapter 5, the proposed index will be verified in other multi-primary color fields, and the application will be presented. Finally, conclusions and future works will be summarized in Chapter 6.