Motivation and objective of this thesis

(a) original image (b) CBU image Fig. 1-9 (a) An original image and (b) a CBU image.

1.4 Motivation and objective of this thesis

In a world that is increasingly aware of its ecological footprint and the harm that is causing to our planet, there is a great incentive to reduce the power dissipation of state-of-the-art LCDs. LED backlight technologies are of advantageous of above requirements. However, these LED applications still exist several issues that need to be improved.

For the CBU issue, it has been widely studied in the context of digital light processing (DLP) projectors [40]-[42]. Since DLPs employ color wheels, most traditional solutions involve either straightforward field rate increases [43] or the

switched very rapidly, the response time of liquid crystal prevents the implementation of many of the above methods for large-screen FSC-LCDs. In spite of the emergence of new fast panel technologies, it is highly unlikely that affordable panels with frame rates higher than 240 Hz will be widely available in the foreseeable future. Cennini et al. [46] proposed a display solution based on spatio-temporal color synthesis that employs two color filters in combination with two color fields thus striking a compromise between conventional spatial color mixing and field-sequential technology. Although its benefits in terms of light efficiency and screen resolution are moderate compared to those achieved by full field-sequential displays, perception tests have confirmed that the visibility of CBU is drastically reduced.

To promote field sequential color technology by developing new processing algorithms to mitigate the effects of color breakup is the objective of this thesis. The performance of newly developed CBU reduction techniques will be evaluated and benchmarked against existing methods. The architecture of this thesis can be divided into three parts for CBU suppression on small- to large-sized LCDs as shown in Fig.

1-10. The methods have progressed from consistent color order to adaptive color field and the 2D-dimming control.

mobile laptop TV

(Basic CBU suppression) (Further CBU suppression)

(Design Recommendation)

(Basic CBU suppression) (Further CBU suppression)

(Design Recommendation)

Fig. 1-10 The thesis architecture.

For mobile-sized one, we proposed an effective method, color field arrangement (CFA) [47]-[49], to reduce the CBU of FSC-LCDs. According to the relation between time and location of motional image, the adequate color field orders of three consecutive frames were found to compensate with each primary color. The CBU mechanism and perceived images on the retina were simulated when eyes followed the motional image. In addition, the flicker phenomenon, which occurs in both temporal variation of the luminance (brightness) and chromaticity (color) [50], has been considered and prevented. A 4-CFA method repeating on color fields was proposed to suppress the flicker phenomenon as well as the CBU.

To lighten the primary color fields by redistributing the LC gray levels to the fourth color field is also proposed [51]. The visibility of the CBU artifact is reduced while the intensity of color separation is decreased. We proposed the RGBW method, condensing the noticeable fields on a white field and reducing the intensities of R, G, and B fields [52][53]. However, the consistent color sequence is not suitable for all kinds of image contents. Therefore, we chose the cyan and yellow as the fourth color field in order to lighten the most sensitive green field. A full color image alternately consists of RGBC and RGBY color field sequences, adapted to image contents. It is obvious that the image intensities of primary colors are lightened. By redistributing gray levels, the noticeable fields are condensed to a primary and mix-colored one. The algorithm of arrangement the color field was studied for laptop-sized LCDs.

Further reductions in CBU visibility can be achieved by intelligently adapting the colors and intensities of the individual fields to the incoming video content [54].

This is generally achieved by focusing as much image energy in one temporal field. A common approach is to flash an adaptive image in one frame and use the remaining frames to display the color residuals. An effort was made in order to find the adaptive

color according to the image content in each frame. The algorithm of feedback determination of adaptive color and LC/BL signals was studied for the real time application, followed by the implementation and experimental results of CBU evaluation. With rearranging LC/BL signals dynamically to optimize color backlight, our results successfully demonstrate that the proposed method is a practical way to suppress the CBU in field sequence color LCD applications.

On large-sized LCDs, the local adaptation seems proper for the presentation of colorful images as local adaptation can concentrate more intensity to the adaptive field [55], a combination of locally controlled backlight and FSC concept. In each segment, the backlight can adjust for only a corresponding part of image. This backlight modulation is primarily determined by the number of addressable segments and the spatial extent of the optical profile of these segments [56]. We considered a boundary-free backlight pattern in this local adaptive backlight. In other words, the brightness variation at the boundary between segments should be indistinguishable.

The threshold of boundary-free image was found according to human contrast sensitivity function. The criteria of insensible variations and the recommended profile and size of segment were determined [57].